@article {Tavares2024, title = {Bioactive Hydroxyapatite Aerogels with Piezoelectric Particles}, journal = {Biomimetics 2024, Vol. 9, Page 143}, volume = {9}, number = {3}, year = {2024}, note = {

n/a

}, month = {feb}, pages = {143}, publisher = {Multidisciplinary Digital Publishing Institute}, abstract = {

Open-cell foams based on hydroxyapatite (HAp) can mimic the extracellular matrix (ECM) to better replace damaged hard tissues and assist in their regeneration processes. Aerogels of HAp nanowires (NW) with barium titanate (BT) particles were produced and characterized regarding their physical and chemical properties, bioactivity, and in vitro cytotoxicity. Considering the role of piezoelectricity (mainly due to collagen) and surface charges in bone remodeling, all BT particles, of size 280 nm and 2 and 3 {\textmu}m, contained BaTiO3 in their piezoelectric tetragonal phase. The synthesized nanowires were verified to be AB-type carbonated hydroxyapatite. The aerogels showed high porosity and relatively homogeneous distribution of the BT particles. Barium titanate proved to be non-cytotoxic while all the aerogels produced were cytotoxic for an extract concentration of 1 mg/mL but became non-cytotoxic at concentrations of 0.5 mg/mL and below. It is possible that these results were affected by the higher surface area and quicker dissolution rate of the aerogels. In the bioactivity assays, SEM/EDS, it was not easy to differentiate between the apatite deposition and the surface of the HAp wires. However, a quantitative EDS analysis shows a possible CaP deposition/dissolution cycle taking place.

}, keywords = {Aerogel, barium titanate, Bioactivity, Hydroxyapatite, piezoelectricity, solvothermal synthesis}, issn = {2313-7673}, doi = {10.3390/BIOMIMETICS9030143}, url = {https://www.mdpi.com/2313-7673/9/3/143/htm https://www.mdpi.com/2313-7673/9/3/143}, author = {Tavares, Catarina and Vieira, T{\^a}nia and Silva, Jorge C and Borges, Jo{\~a}o PMR and M. Carmo Lan{\c c}a} } @article {Dias2023a, title = {Hydroxyapatite-Barium Titanate Biocoatings Using Room Temperature Coblasting}, journal = {Crystals 2023, Vol. 13, Page 579}, volume = {13}, number = {4}, year = {2023}, note = {

n/a

}, month = {mar}, pages = {579}, publisher = {Multidisciplinary Digital Publishing Institute}, abstract = {

The use of orthopaedic and dental implants is expanding as a consequence of an ageing population and also due to illness or trauma in younger age groups. The implant must be biocompatible, bioactive and interact favourably with the recipient{\textquoteright}s bone, as rapid osseointegration is key to success. In this work, Ti-6Al-4V plates were coated using the CoBlastTM technique, with hydroxyapatite (HAp) and HAp/BaTiO3 (barium titanate, BT) non-piezoelectric cubic nanopowders (HAp/cBT) and piezoelectric tetragonal micropowders (HAp/tBT). The addition of BT, a piezoelectric ceramic, is a strategy to accelerate osseointegration by using surface electric charges as cues for cells. For comparison with commercial coatings, plates were coated with HAp using the plasma spray technique. Using XRD and FTIR, both plasma spray and CoBlastTM coatings showed crystalline HAp and no presence of by-products. However, the XRD of the plasma-sprayed coatings revealed the presence of amorphous HAp. The average surface roughness was close to the coatings{\textquoteright} thickness (≈5 $μ$m for CoBlastTM and ≈13 $μ$m for plasma spray). Cytotoxicity assays proved that the coatings are biocompatible. Therefore, it can be concluded that for HAp-based coatings, CoBlastTM is a viable alternative to plasma spray, with the advantage of facilitating room temperature addition of other ceramics, like piezoelectric BaTiO3.

}, keywords = {barium titanate, CoBlastTM, Hydroxyapatite, plasma spray}, issn = {2073-4352}, doi = {10.3390/CRYST13040579}, url = {https://www.mdpi.com/2073-4352/13/4/579/htm https://www.mdpi.com/2073-4352/13/4/579}, author = {Dias, In{\^e}s J. G. and P{\'a}dua, A. Sofia and Pires, Eduardo A. and Borges, Jo{\~a}o PMR and Silva, Jorge C and M. Carmo Lan{\c c}a} } @article {DeOliveira2023, title = {Biocomposite Macrospheres Based on Strontium-Bioactive Glass for Application as Bone Fillers}, journal = {ACS Materials Au}, volume = {3}, number = {6}, year = {2023}, note = {

n/a

}, pages = {646{\textendash}658}, publisher = {American Chemical Society}, abstract = {

Traditional bioactive glass powders are typically composed of irregular particles that can be packed into dense configurations presenting low interconnectivity, which can limit bone ingrowth. The use of novel biocomposite sphere formulations comprising bioactive factors as bone fillers are most advantageous, as it simultaneously allows for packing the particles in a 3-dimensional manner to achieve an adequate interconnected porosity, enhanced biological performance, and ultimately a superior new bone formation. In this work, we develop and characterize novel biocomposite macrospheres of Sr-bioactive glass using sodium alginate, polylactic acid (PLA), and chitosan (CH) as encapsulating materials for finding applications as bone fillers. The biocomposite macrospheres that were obtained using PLA have a larger size distribution and higher porosity and an interconnectivity of 99.7\%. Loose apatite particles were observed on the surface of macrospheres prepared with alginate and CH by means of soaking into a simulated body fluid (SBF) for 7 days. A dense apatite layer was formed on the biocomposite macrospheres{\textquoteright} surface produced with PLA, which served to protect PLA from degradation. In vitro investigations demonstrated that biocomposite macrospheres had minimal cytotoxic effects on a human osteosarcoma cell line (SaOS-2 cells). However, the accelerated degradation of PLA due to the degradation of bioactive glass may account for the observed decrease in SaOS-2 cells viability. Among the biocomposite macrospheres, those composed of PLA exhibited the most promising characteristics for their potential use as fillers in bone tissue repair applications.

}, keywords = {Bioactive glass, bone defects, encapsulation, macrospheres, strontium}, issn = {26942461}, doi = {10.1021/acsmaterialsau.3c00048}, url = {https://pubs.acs.org/doi/full/10.1021/acsmaterialsau.3c00048}, author = {de Oliveira, Ivone Regina and dos Santos Gon{\c c}alves, Isabela and Wallace dos Santos, Kennedy and Lan{\c c}a, Maria Carmo and Vieira, T{\^a}nia and Silva, Jorge Carvalho and Cengiz, Ibrahim Fatih and Reis, Rui Lu{\'\i}s and Oliveira, Joaquim Miguel and Miranda Ribeiro Borges, Jo{\~a}o Paulo} } @article {16684, title = {Extensive Investigation on the Effect of Niobium Insertion on the Physical and Biological Properties of 45S5 Bioactive Glass for Dental Implant}, journal = {International Journal of Molecular Sciences}, volume = {24}, year = {2023}, pages = {5244-5263}, author = {Hammami, Imen and Gavinho, S{\'\i}lvia R and P{\'a}dua, Ana Sofia and Lan{\c c}a, Maria do Carmo and Jo{\~a}o Paulo Borges and Silva, Jorge Carvalho} } @conference {Dias2023, title = {TSDC and surface potential measurements of charged hydroxyapatite/BaTiO 3 biocoatings deposited by CoBlast}, booktitle = {ISE19-19th Int Symp on Electrets, 18-22 Sept.}, year = {2023}, note = {

n/a

}, pages = {77}, publisher = {Johannes Kepler Univ.}, organization = {Johannes Kepler Univ.}, address = {Linz, Austria}, abstract = {

n/a

}, keywords = {contact charging, corona charging, hard tissue engineering, Hydroxyapatite, tsdc}, author = {Dias, I.J.G. J G and P{\'a}dua, A.S. S and Pires, E and Borges, J.P.M.R. and Silva, Jorge C and MC Lan{\c c}a} } @proceedings {Madeira2022, title = {Piezoelectric Calcium Modified Barium Titanate for Bone Regeneration}, journal = {Materials Proceedings 2022, Vol. 8, Page 121}, volume = {8}, number = {1}, year = {2022}, note = {

n/a

}, month = {jul}, pages = {121}, publisher = {Multidisciplinary Digital Publishing Institute}, address = {Basel Switzerland}, abstract = {

Solid state reaction was used to produced barium titanate modified with calcium (BCT) showing the presence of the piezoelectric tetragonal phase after sintering at 1350 \°C. Bioglass 45S5 (BG) was synthetized by sol-gel route. From these two materials and commercial hydroxyapatite (HAp) were obtained composites. The BG produced showed some cytotoxic character that was weakened by passivation. All other materials were non-cytotoxic. Contact polarization at constant temperature was chosen composites polarization. Electric/dielectric properties were evaluated by thermally stimulated depolarization currents (TSDC). The material showed bioactivity with the composite with BCT/BG/HAp 90/5/5 (wt\%) showing increased bioactivity. In vitro test showed high proliferation rates for the composites.

}, keywords = {Bioceramics, bone regeneration, piezoelectrics}, issn = {2673-4605}, doi = {10.3390/MATERPROC2022008121}, url = {https://www.mdpi.com/2673-4605/8/1/121/htm https://www.mdpi.com/2673-4605/8/1/121}, author = {Madeira, Ricardo MD and Vieira, T{\^a}nia and Silva, Jorge C and Oliveira, Ivone R and Borges, Jo{\~a}o P and Lima, M. M. R. A. and M. Carmo Lan{\c c}a} } @article {Oliveira2022, title = {Properties of strontium-containing BG 58S produced by alkali-mediated sol-gel process}, journal = {Ceramics International}, year = {2022}, note = {

n/a

}, month = {jan}, publisher = {Elsevier}, abstract = {

n/a

}, issn = {02728842}, doi = {10.1016/j.ceramint.2022.01.002}, url = {https://linkinghub.elsevier.com/retrieve/pii/S0272884222000025}, author = {Oliveira, IR and Barbosa, AM and Santos, KW and MC Lan{\c c}a and Lima, M. M. R. A. and Vieira, T. and Silva, JC and J.P. Borges} } @article {Teixeira2021, title = {Nanostructured LiFe5O8 by a Biogenic Method for Applications from Electronics to Medicine}, journal = {Nanomaterials}, volume = {11}, number = {1}, year = {2021}, note = {

n/a

}, month = {jan}, pages = {193}, publisher = {MDPI AG}, abstract = {

The physical properties of the cubic and ferrimagnetic spinel ferrite LiFe5O8 has made it an attractive material for electronic and medical applications. In this work, LiFe5O8 nanosized crystallites were synthesized by a novel and eco-friendly sol-gel process, by using powder coconut water as a mediated reaction medium. The dried powders were heat-treated (HT) at temperatures between 400 and 1000 {\textdegree}C, and their structure, morphology, electrical and magnetic characteristics, cytotoxicity, and magnetic hyperthermia assays were performed. The heat treatment of the LiFe5O8 powder tunes the crystallite sizes between 50 nm and 200 nm. When increasing the temperature of the HT, secondary phases start to form. The dielectric analysis revealed, at 300 K and 10 kHz, an increase of $ε$' (≈10 up to ≈14) with a tan$δ$ almost constant (≈0.3) with the increase of the HT temperature. The cytotoxicity results reveal, for concentrations below 2.5 mg/mL, that all samples have a non-cytotoxicity property. The sample heat-treated at 1000 {\textdegree}C, which revealed hysteresis and magnetic saturation of 73 emu g-1 at 300 K, showed a heating profile adequate for magnetic hyperthermia applications, showing the potential for biomedical applications.

}, keywords = {Cellular viability, cellularviability, Coconut water powder, coconutwaterpowder, Dielectric spectroscopy, dielectricspectroscopy, Lithium ferrite, lithiumferrite, Magnetic hyper-thermia, magnetichyperthermia, Proteic route, proteicroute, Specific absorption rate, specificabsorptionrate}, issn = {2079-4991}, doi = {10.3390/nano11010193}, url = {https://www.mdpi.com/2079-4991/11/1/193 https://doi.org/10.3390/nano11010193}, author = {Teixeira, Silvia Soreto and Gra{\c c}a, Manuel PF and Lucas, Jos{\'e} and Valente, Manuel Almeida and Soares, Paula I. P. and Lan{\c c}a, Maria Carmo and Vieira, T{\^a}nia and Silva, Jorge Carvalho and Jo{\~a}o Paulo Borges and Jinga, Luiza-Izabela and Socol, Gabriel and Mello Salgueiro, Cristiane and Nunes, Jos{\'e} and Costa, Lu{\'\i}s C.} } @article {Gavinho2019, title = {{Nontoxic glasses: Preparation, structural, electrical and biological properties}}, journal = {International Journal of Applied Ceramic Technology}, year = {2019}, note = {n/a}, month = {apr}, pages = {ijac.13243}, publisher = {John Wiley {\&} Sons, Ltd (10.1111)}, abstract = {n/a}, keywords = {antibacterial properties, Bioactive glass, biomaterial, cytotoxicity, electrical properties}, issn = {1546-542X}, doi = {10.1111/ijac.13243}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/ijac.13243}, author = {Gavinho, S{\'\i}lvia R and Prezas, Pedro R and Ramos, Diogo J and S{\'a}-Nogueira, Isabel and Borges, Joao P and M. Carmo Lan{\c c}a and Silva, Jorge C and Henriques, C{\'e}lia MR and Pires, Eduardo and Kumar, Jakka Suresh and Gra{\c c}a, M. P. F.} } @article {12234, title = {TSDC and impedance spectroscopy measurements on hydroxyapatite, β-tricalcium phosphate and hydroxyapatite/β-tricalcium phosphate biphasic bioceramics}, journal = {Applied Surface Science}, volume = {424}, year = {2017}, pages = {28-38}, abstract = {

Bone grafting and surgical interventions related with orthopaedic disorders consist in a big business, generating large revenues worldwide every year. There is a need to replace the biomaterials that currently still dominate this market, i.e., autografts and allografts, due to their disadvantages, such as limited availability, need for additional surgeries and diseases transmission possibilities. The most promising replacement materials are biomaterials with bioactive properties, such as the calcium phosphate-based bioceramics group. The bioactivity of these materials, i.e., the rate at which they promote the growth and directly bond with the new host biological bone, can be enhanced through their electrical polarization.In the present work, the electrical polarization features of pure hydroxyapatite (Hap), pure β-tricalcium phosphate (β-TCP) and biphasic hydroxyapatite/β-tricalcium phosphate composites (HTCP) were analyzed by measuring thermally stimulated depolarization currents (TSDC). The samples were thermoelectrically polarized at 500. {\textdegree}C under a DC electric field with a magnitude of 5. kV/cm. The biphasic samples were also polarized under electric fields with different magnitudes: 2, 3, 4 and 5. kV/cm. Additionally, the depolarization processes detected in the TSDC measurements were correlated with dielectric relaxation processes observed in impedance spectroscopy (IS) measurements.The results indicate that the β-TCP crystalline phase has a considerable higher ability to store electrical charge compared with the Hap phase. This indicates that it has a suitable composition and structure for ionic conduction and establishment of a large electric charge density, providing great potential for orthopaedic applications.

}, author = {Prezas, P. R and Melo, B. M. G and Costa, L.C. and Valente, M. A. and MC Lan{\c c}a and Ventura, J. M. G. and Pinto, LFV and Gra{\c c}a, M. P. F.} } @article {7847, title = {Space Charge and Dipolar Charge Contribution at Polar Polymers Polarization}, journal = {Ieee Transactions on Dielectrics and Electrical Insulation}, volume = {22}, number = {3}, year = {2015}, note = {n/a}, pages = {1419-1426}, type = {Journal Article}, abstract = {n/a}, issn = {1070-9878}, doi = {10.1109/tdei.2015.004870}, url = {://WOS:000356516500016}, author = {Neagu, E. R. and Lanca, M. C. and Dias, C. J. and Marat-Mendes, J. N.} } @booklet {6330, title = {Electrical characterization of biphasic ceramics used in hard tissue replacement}, year = {2013}, note = {n/a}, pages = {217}, type = {Peer reviewed}, chapter = {217}, address = {Coimbra Portugal}, abstract = {

Hydroxyapatite [Hap; Ca10(PO4)6(OH)2) and b-tricalcium phosphate [b-TCP; Ca3(PO4)2] are biocompatible calcium phosphates used in skeletal surgery. The natural HAp is one of the main components of bone and, as a synthetic material, has been widely used for bone replacement presenting good bioactivity. Nevertheless synthetic HAp presents a slow in vivo degradation rate which is disadvantageous for bone\’s reparative process. b-TCP has also good osteogenic characteristics presenting the ability to form strong bonds with the bone however, its degradation rate is too fast [1]. Therefore, a composite combining these two ceramics is valuable as it exhibits a suitable degradation rate. Because of the piezoelectric properties of bone it is known that electrical polarization of calcium phosphates can enhance the bioactivity and biointegration of implants [2]. Previous studies have already showed that HAp/b-TCP ceramics can be electrically polarized and that electrical polarization enhances osteogenesis in the early stage of the implantation process. However further studies are required to understand, optimize and improve the polarization technique [1]. In this work a commercial biphasic ceramic powders were pressed in a mold at 200 MPa to produce disc shaped samples. Afterwards, the samples were sintered at temperatures from 950\ºC to 1150\ºC and the influence of the heat treatment in the electrical polarization and subsequent bioactivity was investigated. The samples were polarized under a high DC electric field at relatively lower temperature (200oC) compared to previous studies and the stability of polarization was tested using TSDC (thermally depolarization currents) measurements. It was studied the influence of the water, initially present in the material, in the total charge deposited during polarization, its stability and its relation with heat treatment after pressing. The influence of the addition of b-TCP on sample\’s stored charge was also evaluated. Finally bioactivity tests in a simulated body fluid solution were made taking into account the signal of the charge in each surface of the disc samples so that the results could be compared to previous ones.

}, author = {Pedrosa AS and Borges JPMR and Neagu E and Marat-Mendes JN and Lan{\c c}a MCH} } @inbook {7816, title = {Water Content Control To Improve Space Charge Storage in a Cork Derivative}, booktitle = {Advanced Materials Forum Vi, Pts 1 and 2}, series = {Materials Science Forum}, volume = {730-732}, year = {2013}, note = {

Gil, Luis/B-2908-2013; Dias, Carlos /J-4947-2013; Lanca, M. Carmo/A-4437-2013 Dias, Carlos /0000-0003-3042-5029; Lanca, M. Carmo/0000-0001-9258-4899 6th International Materials Symposium (MATERIALS 2011)/15th Meeting of SPM Apr 18-20, 2011 Univ Minho, Guimaraes, PORTUGAL Portuguese Mat Soc; Univ Minho, Sch Engn; Portuguese Fdn Sci \& Technol; Ctr Mech \& Mat Technol; Inst Polym \& Composites; 3Bs Res Grp Biomaterials Biodegradables \& Biomimet; Minho Univ, Ctr Phys; Um, Sch Sci; TecMinho; Town Hall Guimaraes; Beneq; Celoplas; Filsat; Izasa; Leica MicroSystems; Mario Costa Martins \& Filho Lda; Panalytical; ScienTec 0255-5476

}, pages = {395-400}, type = {Book Section}, abstract = {n/a}, isbn = {*****************}, doi = {10.4028/www.scientific.net/MSF.730-732.395}, url = {://WOS:000315545200066}, author = {Lanca, M. C. and Cunha, I. and J. P. Marques and Neagu, E. R. and Gil, L. and Dias, C. J. and Marat-Mendes, J. N.}, editor = {Pinto, AMP and Pouzada, AS} } @conference {Cunha_In{\^e}s:2012, title = {Water Content Control to Improve Space Charge Storage in a Cork Derivative}, series = {Materials Science Forum}, year = {2012}, note = {n/a}, month = {Jan}, pages = {395-400}, type = {Peer reviewed}, abstract = {n/a}, author = {Cunha In{\^e}s and Marques Jo{\~a}o Paulo and Gil Lu{\'\i}s and Neagu E. R. and Dias C. J. and Marat-Mendes J. N. and Lan{\c c}a M. C.} } @conference {6332, title = {Water Content Control to Improve Space Charge Storage in a Cork Derivative}, booktitle = {Materiais2011}, series = {Materials Science Forum}, year = {2012}, note = {n/a}, type = {Peer reviewed}, abstract = {n/a}, doi = {10.4028/www.scientific.net/MSF.730-732.395}, author = {Cunha In{\^e}s and Marques Jo{\~a}o Paulo and Gil Lu{\'\i}s and Neagu E. R. and Dias C. J. and Marat-Mendes J. N. and Lan{\c c}a M. C.} } @article {7837, title = {Water Content Control to Improve Space Charge Storage in a Cork Derivative}, volume = {730-732}, year = {2012}, note = {n/a}, pages = {395-400}, type = {Journal Article}, abstract = {n/a}, author = {M.C., Lan and I., Cunha and Paulo, Marques Jo and GIL L. and NEAGU E.R. and DIAS C.J. and N., Marat-Mendes Jose} } @conference {Neagu_E._R.:2011, title = {Discrimination between Space Charge and Dipolar Contributions in Ferroelectric Polymers}, series = {International Symposium on Electrets}, year = {2011}, note = {n/a}, month = {Jan}, pages = {145-146}, type = {Peer reviewed}, abstract = {n/a}, author = {Neagu E. R. and Dias, C. and Lan{\c c}a M. Carmo and Igreja R. and Inacio P. and Marat-Mendes J. N.} } @article {LAN{\c C}A_M.C.:2011, title = {Electrical properties of cork and derivatives}, volume = {23}, year = {2011}, note = {n/a}, month = {Jan}, pages = {64-70}, type = {Peer reviewed}, abstract = {n/a}, author = {LAN{\c C}A M.C. and NEAGU E.R. and DIAS C.J. and GIL L. and MARAT-MENDES J.N.} } @conference {Pedrosa_A.:2011, title = {Influence of Polarization on the Bioactivity of Nanopowders of Hydroxyapatite}, series = {International Symposium on Electrets}, year = {2011}, note = {n/a}, month = {Jan}, pages = {55-56}, type = {Peer reviewed}, abstract = {n/a}, author = {Pedrosa A. and LAN{\c C}A M.C. and Borges J.P. and NEAGU E.R. and DIAS C.J. and Marat-Mendes and {J.N.} Varandas} } @article {Neagu_ER:2011, title = {The use of the final thermally stimulated discharge current technique to study the molecular movements around glass transition}, volume = {354}, year = {2011}, note = {n/a}, month = {Jan}, pages = {385-390}, type = {Peer reviewed}, abstract = {n/a}, author = {Neagu ER and Dias CJ and Lan{\c c}a MC and Igreja R and Inacio P and Marat-Mendes J. N.} } @conference {6338, title = {Discrimination between Space Charge and Dipolar Contributions in Ferroelectric Polymers}, booktitle = {14th International Symposium on Electrets}, series = {International Symposium on Electrets}, year = {2011}, note = {n/a}, type = {Peer reviewed}, abstract = {

The final thermally stimulated discharge current method allows a better selection of the experimental conditions for sample polarization. By decreasing the ratio between the charging time and the discharging time, the apparent peak is of the same order of magnitude as the genuine peaks and there is only a partial overlap between then. Two peaks have been identified for polyamide 11, one associated with the glass transition around 60 \°C and the second associated with the Curie transition around 96 \°C.

}, isbn = {978-1-4577-1025-4}, doi = {10.1109/ISE.2011.6085024}, author = {Neagu E. R. and Dias, C. and Lan{\c c}a M. Carmo and Igreja R. and Inacio P. and Marat-Mendes J. N.} } @book {7845, title = {Discrimination between Space Charge and Dipolar Contributions in Ferroelectric Polymers}, year = {2011}, note = {

Neagu, E. R. Dias, C. J. Lanca, M. C. Igreja, R. Inacio, P. Marat-Mendes, J. N. ISE 14th International Symposium on Electrets (ISE) AUG 28-31, 2011 Montpellier, FRANCE

}, pages = {145-146}, type = {Book}, abstract = {

The final thermally stimulated discharge current method allows a better selection of the experimental conditions for sample polarization. By decreasing the ratio between the charging time and the discharging time, the apparent peak is of the same order of magnitude as the genuine peaks and there is only a partial overlap between then. Two peaks have been identified for polyamide 11, one associated with the glass transition around 60 degrees C and the second associated with the Curie transition around 96 degrees C.

}, isbn = {978-1-4577-1025-4}, url = {://WOS:000298852900073}, author = {Neagu, E. R. and Dias, C. J. and Lanca, M. C. and Igreja, R. and Inacio, P. and Marat-Mendes, J. N. and Ieee} } @article {6336, title = {Electrical properties of cork and derivatives}, volume = {23}, year = {2011}, note = {n/a}, type = {Peer reviewed}, abstract = {n/a}, author = {LAN{\c C}A M.C. and NEAGU E.R. and DIAS C.J. and GIL L. and MARAT-MENDES J.N.} } @conference {6337, title = {Influence of Polarization on the Bioactivity of Nanopowders of Hydroxyapatite}, booktitle = {14th International Symposium on Electrets}, series = {International Symposium on Electrets}, year = {2011}, note = {n/a}, type = {Peer reviewed}, abstract = {n/a}, isbn = {978-1-4577-1025-4}, doi = {10.1109/ISE.2011.6084979}, author = {Pedrosa A. and LAN{\c C}A M.C. and Borges J.P. and NEAGU E.R. and DIAS C.J. and Marat-Mendes and {J.N.} Varandas} } @article {7858, title = {Influence of Polarization on the Bioactivity of Nanopowders of Hydroxyapatite}, journal = {2011 14th International Symposium on Electrets (Ise)}, year = {2011}, note = {n/a}, pages = {55-56}, type = {Journal Article}, abstract = {n/a}, issn = {978-1-4577-1025-4}, author = {Pedrosa, A. and Lanca, M. C. and J.P. Borges and Neagu, E. R. and Dias, C. J. and Marat-Mendes, J. N. and Ieee} } @book {7859, title = {Influence of Polarization on the Bioactivity of Nanopowders of Hydroxyapatite}, year = {2011}, note = {

Pedrosa, A. Lanca, M. C. Borges, J. P. Neagu, E. R. Dias, C. J. Marat-Mendes, J. N. ISE 14th International Symposium on Electrets (ISE) AUG 28-31, 2011 Montpellier, FRANCE

}, pages = {55-56}, type = {Book}, abstract = {n/a}, isbn = {978-1-4577-1025-4}, doi = {10.1109/ISE.2011.6084979}, url = {://WOS:000298852900028}, author = {Pedrosa, A. and Lanca, M. C. and J.P. Borges and Neagu, E. R. and Dias, C. J. and Marat-Mendes, J. N. and Ieee} } @article {6335, title = {The use of the final thermally stimulated discharge current technique to study the molecular movements around glass transition}, volume = {354}, year = {2011}, note = {n/a}, type = {Peer reviewed}, abstract = {

During electric polarization charge is injected into the material. The structure is decorated with space charge and during the subsequent heating an apparent peak and the genuine peaks that are related to dipole randomization and charge detrapping are observed. The method is used here to analyze the molecular movements in polyimide in the temperature range from 293 to 623K. Two weak relaxations have been observed around 337K and around 402K. The electrical conductivity changes with temperature in agreement with the Arrhenius law only below (W=(0.84\±0.03) eV ) and above ( W=(0.82\±0.03) eV) the temperature range where the \β relaxation is observed. The variation of the electrical conductivity with temperature, in the range of the \β relaxation, is controlled by the variation of the charge currier mobility with temperature and it shows a non-Arrhenius behavior. We suggest that the \β1 sub-glass relaxation is related to the rotation or oscillation of phenyl groups and the \β2 sub-glass relaxation is related to the rotation or oscillation of the imidic ring. At higher temperatures an apparent peak was observed. The relaxation time of the trapped charge, at 573K, is high than 8895s.

}, doi = {10.1016/j.jnoncrysol.2010.05.095}, author = {Neagu ER and Dias CJ and Lan{\c c}a MC and Igreja R and Inacio P and Marat-Mendes J. N.} } @article {7840, title = {The use of the final thermally stimulated discharge current technique to study the molecular movements around glass transition}, journal = {Journal of Non-Crystalline Solids}, volume = {357}, number = {2}, year = {2011}, note = {

Neagu, E. R. Dias, C. J. Lanca, M. C. Igreja, R. Inacio, P. Marat-Mendes, J. N. Sp. Iss. SI

}, pages = {385-390}, type = {Journal Article}, abstract = {

During electric polarization charge is injected into the material. The structure is decorated with space charge and during the subsequent heating an apparent peak and the genuine peaks that are related to dipole randomization and charge detrapping are observed. The method is used here to analyze the molecular movements in polyimide in the temperature range from 293 to 623 K. Two weak relaxations have been observed around 337 K and around 402 K. The electrical conductivity changes with temperature in agreement with the Arrhenius law only below (W= (0.84 +/- 0.03) eV) and above ( W (0.82 +/- 0.03) eV) the temperature range where the beta relaxation is observed. The variation of the electrical conductivity with temperature, in the range of the beta relaxation, is controlled by the variation of the charge currier mobility with temperature and it shows a non-Arrhenius behavior. We suggest that the beta(1) sub-glass relaxation is related to the rotation or oscillation of phenyl groups and the beta(2) sub-glass relaxation is related to the rotation or oscillation of the imidic ring. At higher temperatures an apparent peak was observed. The relaxation time of the trapped charge, at 573 K, is high than 8895 s. (C) 2010 Elsevier B.V. All rights reserved.

}, issn = {0022-3093}, doi = {10.1016/j.jnoncrysol.2010.05.095}, url = {://000287640800025}, author = {Neagu, E. R. and Dias, C. J. and Lanca, M. C. and Igreja, R. and Inacio, P. and Marat-Mendes, J. N.} } @article {E._R._Neagu:2010, title = {The analysis of isothermal current in terms of charge injection or extraction at the metal-dielectric contact}, volume = {356}, year = {2010}, note = {n/a}, month = {Jan}, pages = {833-837}, type = {Peer reviewed}, abstract = {n/a}, author = {Neagu, E. R. and Neagu, R. M. and Dias, C. J. and M. Carmo Lan{\c c}a and Marat-Mendes, J. N.} } @conference {Neagu_ER:2010, title = {Charge Carriers Injection/Extraction at the Metal-Polymer Interface and Its Influence in the Capacitive Microelectromechanical Systems-Switches Actuation Voltage}, booktitle = {JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY}, series = {JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY}, volume = {10}, year = {2010}, note = {n/a}, month = {Jan}, pages = {2503-2511}, publisher = {Univ Nova Lisboa}, organization = {Univ Nova Lisboa}, abstract = {n/a}, author = {Neagu ER and Dias CJ and Lanca MC and Igreja R and Inacio P and Marat-Mendes JN} } @article {E._R._Neagu:2010, title = {The determination of the pull-in voltage from the condition of bridge stability}, volume = {5}, year = {2010}, note = {n/a}, month = {Jan}, pages = {139-151}, type = {Peer reviewed}, abstract = {n/a}, author = {Neagu, E. R. and Neagu, R. M. and Dias, C. J. and M. Carmo Lan{\c c}a and Marat-Mendes, J. N.} } @conference {M._Carmo_Lan{\c c}a:2010, title = {Dielectric spectra of natural cork and derivatives}, series = {International Conference on Dielectric Spectroscopy and Its Applications}, volume = {356}, year = {2010}, note = {n/a}, month = {Jan}, pages = {763-767}, type = {Peer reviewed}, abstract = {n/a}, author = {M. Carmo Lan{\c c}a and Neagu, E. R. and Dias, C. and Marat-Mendes, J.} } @conference {Neagu_ER:2010, title = {Electrical Method to Study the Weak Molecular Movements at Nanometric Scale in Low Mobility Materials}, series = {Materials Science Forum}, volume = {636-637}, year = {2010}, note = {n/a}, month = {Jan}, pages = {430-436}, type = {Peer reviewed}, abstract = {n/a}, author = {Neagu ER and Neagu RM and Dias CJ and Lanca MC and Inacio P and Marat-Mendes JN} } @conference {Neagu_ER:2010, title = {Medium Electric Field Electron Injection/Extraction at Metal-Dielectric Interface}, series = {Materials Science Forum}, number = {636-637}, year = {2010}, note = {n/a}, month = {Jan}, pages = {437-443}, type = {Peer reviewed}, abstract = {n/a}, author = {Neagu ER and Dias CJ and Lanca MC and Igreja R and Marat-Mendes JN} } @conference {Neagu_ER:2010, title = {Medium Electric Field Electron Injection/Extraction at Metal-Dielectric Interface}, series = {Materials Science Forum}, volume = {636-637}, year = {2010}, note = {n/a}, month = {Jan}, pages = {437-443}, type = {Peer reviewed}, abstract = {n/a}, author = {Neagu ER and Dias CJ and Lan{\c c}a MC and Igreja R and Marat-Mendes JN} } @conference {Neagu_ER:2010, title = {On the width of the thermally stimulated discharge current peak}, series = {IEEE International Conference on Solid Dielectrics-ICSD}, year = {2010}, note = {n/a}, month = {Jan}, type = {Peer reviewed}, abstract = {n/a}, author = {Neagu ER and Dias CJ and Lanca MC and Igreja R and Inacio P and Marat-Mendes JN} } @conference {Neagu_ER:2010, title = {The study of the molecular movements in the range of glass transition by the final thermally stimulated discharge current technique}, series = {IEEE International Conference on Solid Dielectrics-ICSD}, year = {2010}, note = {n/a}, month = {Jan}, type = {Peer reviewed}, abstract = {n/a}, author = {Neagu ER and Dias CJ and Lanca MC and Igreja R and Inacio P and Marat-Mendes JN} } @article {7850, title = {The analysis of isothermal current in terms of charge injection or extraction at the metal-dielectric contact}, journal = {Journal of Non-Crystalline Solids}, volume = {356}, number = {11-17}, year = {2010}, note = {

Neagu, E. R. Neagu, R. M. Dias, C. J. Lanca, M. Carmo Marat-Mendes, J. N. 5th International Conference on Dielectric Spectroscopy and Its Applications AUG 26-29, 2008 Lyon, FRANCE

}, pages = {833-837}, type = {Journal Article}, abstract = {

The measured isothermal charging and discharging currents are analyzed either in terms of polarization mechanisms or in terms of charge injection/extraction at the metal-dielectric interface and the conduction current through the dielectric material. We propose to measure the open-circuit isothermal charging and discharging currents just to overpass the difficulties related to the analysis of the conduction mechanisms through the dielectric materials. Besides a polarization current, there is a current related with charge injection or extraction at the metal-dielectric contact and a reverse current related to the charge trapped into the superficial trap states of the dielectric and that can jump at the interface in a reverse way. By fitting the experimental data, two important parameters can be determined (i) the highest value of the relaxation time for the polarization mechanisms still involved into the transient current and (ii) the height W-0 of the potential barrier at the metal-dielectric interface immediately after the step voltage is applied. Only the initial part of the measured isothermal charging or discharging current can be used to obtain information about the polarization processes. By transforming the time-domain data into the frequency domain, a maximum for the imaginary part of the dielectric permittivity is obtained, in good agreement with the data obtained from AC dielectric measurements and the finally thermally stimulated discharge current measurements. (C) 2009 Elsevier B.V. All rights reserved.

}, issn = {0022-3093}, doi = {10.1016/j.jnoncrysol.2009.04.080}, url = {://000276665500063}, author = {Neagu, E. R. and Neagu, R. M. and Dias, C. J. and Lanca, M. C. and Marat-Mendes, J. N.} } @article {7839, title = {Charge Carriers Injection/Extraction at the Metal-Polymer Interface and Its Influence in the Capacitive Microelectromechanical Systems-Switches Actuation Voltage}, journal = {Journal of Nanoscience and Nanotechnology}, volume = {10}, number = {4}, year = {2010}, note = {

Neagu, E. R. Dias, C. J. Lanca, M. Carmo Igreja, R. Inacio, P. Marat-Mendes, J. N. 2nd International Conference on Advanced Nano Materials 2008 Aveiro, PORTUGAL

}, pages = {2503-2511}, type = {Journal Article}, abstract = {

Opposite results concerning the sign of the parasitic charge accumulated at the metal dielectric contact in RF microelectromechanical systems (MEMS) capacitive switches are found in the literature. The mechanism concerning charge injection/extraction at the metal-dielectric contact and its influence on the pull-in voltage needs to be further clarified. A model-switch, for which only one dimension is in the microns range, is used to study the behaviour of a capacitive RF MEMS switch. The aim is to analyze how the electric charge is injected/extracted into or from the dielectric material under the applied field and to obtain realistic data to understand how this parasitic charge influences the pull-in voltage V-pi and the pull-off voltage V-po. A triangle voltage is employed to measure V-pi and V-po by measuring the isothermal charging/discharging currents. Our results demonstrate that V-pi is strongly dependent on the injected/extracted charge on the free surface of the dielectric. The charge injected/extracted at the bottom side of the dielectric has no influence on the actuation voltage. The charge injected/extracted on the free surface of the dielectric determines an increase of the modulus of V-pi and, eventually, the switch can fail to actuate. An estimation of the charge stored into the material was obtained (i) by measuring the charging current and the discharging current and (ii) from the value of the V-pi. The parasitic charge necessary to keep the bridge stick to the insulator is 5.3 x 10(-4) cm(-2) for our experimental conditions. The modification of the V-pi determined by the stored charge in the dielectric is analyzed. An increase of the relative dielectric permittivity by a factor of 2 produces a decrease of the actuation voltage of 10\%. A variation of 30\% in the elastic constant determines a variation of about 20\% in the V-pi. A voltage threshold for charge injection/extraction was not observed.

}, issn = {1533-4880}, doi = {10.1166/jnn.2010.1405}, url = {://000273984900033}, author = {Neagu, E. R. and Dias, C. J. and Lanca, M. C. and Igreja, R. and Inacio, P. and Marat-Mendes, J. N.} } @article {7815, title = {Dielectric spectra of natural cork and derivatives}, journal = {Journal of Non-Crystalline Solids}, volume = {356}, number = {11-17}, year = {2010}, note = {

Lanca, M. C. Brandt, M. Neagu, E. R. Dias, C. J. Marat-Mendes, J. N. 5th International Conference on Dielectric Spectroscopy and Its Applications AUG 26-29, 2008 Lyon, FRANCE

}, pages = {763-767}, type = {Journal Article}, abstract = {

Cork is a cellular biomaterial that has unique characteristics that make it suitable for many types of applications. Since it is also an electrical insulator, the study of its electrical and dielectric properties can lead to new interesting applications. The moisture present in cork and derivatives has a very important role on the dielectric properties. In this work a composite made of both recycled cork and TetraPak (R) used containers was studied and compared with other cork products. The dielectric relaxation spectra of natural cork (as received), commercial cork agglomerate and of a composite cork/Tetrapak (R) was investigated in the temperature range of -50 to 120 degrees C and in the frequency range of 10(-1) Hz-2 MHz. For some samples of the composite a small amount of paraffin was added. The highest values for the imaginary part of the dielectric permittivity were found for the commercial material and the composite without paraffin. The lowest was found for the cork/TetraPak (R)/paraffin composite. The influence of humidity content was investigated for the composite with wax. Natural cork shows a peak around 80 degrees C (not seen in the derivative materials). The commercial agglomerate and the cork/TetraPak (R)/paraffin composite show a peak around 40-50 degrees C. In the composite this peak becomes smaller as humidity is removed. (C) 2009 Elsevier B.V. All rights reserved.

}, issn = {0022-3093}, doi = {10.1016/j.jnoncrysol.2009.09.042}, url = {://000276665500049}, author = {Lanca, M. C. and Brandt, M. and Neagu, E. R. and Dias, C. J. and Marat-Mendes, J. N.} } @inbook {7848, title = {Electrical Method to Study the Weak Molecular Movements at Nanometric Scale in Low Mobility Materials}, booktitle = {Advanced Materials Forum V, Pt 1 and 2}, series = {Materials Science Forum}, volume = {636-637}, year = {2010}, note = {

Neagu, E. R. Neagu, R. M. Dias, C. J. Lanca, M. C. Inacio, P. Marat-Mendes, J. N. 5th International Materials Symposium/14th Conference of the Sociedade-Portuguesa-de-Materiais APR 05-08, 2009 Lisbon, PORTUGAL

}, pages = {430-436}, type = {Book Section}, abstract = {

For the characterization of the new materials and for a better understanding of the connection between structure and properties it is necessary to use more and more sensible methods to study molecular movement at nanometric scale. This paper presents the experimental basis for a new electrical method to study the fine molecular movements at nanometric scale in dielectric materials. The method will be applied for polar and non-polar materials characterization. Traditionally, the electrical methods used to study the molecular movements are based on the movements of the dipoles that are parts of the molecules. We have proposed recently a combined protocol to analyze charge injection/extraction, transport, trapping and detrapping in low mobility materials. The experimental results demonstrate that the method can be used to obtain a complex thermogram which contains information about all molecular movements, even at nanoscopic level. Actually during the charging process we are decorating the structure with space charge and during the subsequent heating we are observing an apparent peak and the genuine peaks that are related to charge de-trapping determined by the molecular movement. The method is very sensitive, very selective and allows to determinate the parameters for local and collective molecular movements, including the temperature dependence of the activation energy and the relaxation time.

}, isbn = {0255-5476}, doi = {10.4028/www.scientific.net/MSF.636-637.430}, url = {://000280763200066}, author = {Neagu, E. R. and Neagu, R. M. and Dias, C. J. and Lanca, M. C. and Inacio, P. and Marat-Mendes, J. N.}, editor = {Rosa, L. G. and Margarido, F.} } @inbook {7846, title = {Medium Electric Field Electron Injection/Extraction at Metal-Dielectric Interface}, booktitle = {Advanced Materials Forum V, Pt 1 and 2}, series = {Materials Science Forum}, volume = {636-637}, year = {2010}, note = {

Neagu, E. R. Dias, C. J. Lanca, M. C. Igreja, R. Marat-Mendes, J. N. 5th International Materials Symposium/14th Conference of the Sociedade-Portuguesa-de-Materiais APR 05-08, 2009 Lisbon, PORTUGAL

}, pages = {437-443}, type = {Book Section}, abstract = {

The isothermal charging current and the isothermal discharging current in low mobility materials are analyzed either in terms of polarization mechanisms or in terms of charge injection/extraction at the metal-dielectric interface and the conduction current through the dielectric material. We propose to measure the open-circuit isothermal charging and discharging currents just to overpass the difficulties related to the analysis of the conduction mechanisms in dielectric materials. We demonstrate that besides a polarization current there is a current related to charge injection or extraction at the metal-dielectric interface and a reverse current related to the charge trapped into the shallow superficial or near superficial states of the dielectric and which can move at the interface in the opposite way that occurring during injection. Two important parameters can be determined (i) the highest value of the relaxation time for the polarization mechanisms which are involved into the transient current and (ii) the height of the potential barrier W-0 at the metal-dielectric interface. The experimental data demonstrate that there is no threshold field for electron injection/extraction at a metal-dielectric interface.

}, isbn = {0255-5476}, doi = {10.4028/www.scientific.net/MSF.636-637.437}, url = {://000280763200067}, author = {Neagu, E. R. and Dias, C. J. and Lanca, M. C. and Igreja, R. and Marat-Mendes, J. N.}, editor = {Rosa, L. G. and Margarido, F.} } @article {7841, title = {On the Width of the Thermally Stimulated Discharge Current Peak}, journal = {Proceedings of the 2010 Ieee International Conference on Solid Dielectrics (Icsd 2010)}, year = {2010}, note = {n/a}, type = {Journal Article}, abstract = {n/a}, issn = {1553-5282;978-1-4244-7944-3}, author = {Neagu, E. R. and Dias, C. J. and Lanca, M. C. and Igreja, R. and Inacio, P. and Marat-Mendes, J. N. and Ieee} } @inbook {7842, title = {On the Width of the Thermally Stimulated Discharge Current Peak}, booktitle = {Proceedings of the 2010 Ieee International Conference on Solid Dielectrics}, series = {IEEE International Conference on Solid Dielectrics-ICSD}, year = {2010}, note = {

Neagu, E. R. Dias, C. J. Lanca, M. C. Igreja, R. Inacio, P. Marat-Mendes, J. N. ICSD 2010 International Conference on Solid Dielectrics ICSD JUL 04-09, 2010 Potsdam, GERMANY

}, type = {Book Section}, abstract = {

The Thermally Stimulated Discharge Current (TSDC) method is a very sensitive technique to analyze the movement of dipoles and of space charge (SC). To increase the selectivity of the method we have proposed a variant of the TSDC method, namely the final thermally stimulated discharge current (FTSDC) technique. The experimental conditions can be selected so that the FTSDC is mainly determined by SC de-trapping. The aim of this paper is to analyze if the elementary peaks obtained by using the two methods can be assumed as elementary Debye peaks and to determine the best experimental conditions to obtain a narrow experimental peak which means to increase the selectivity of the method.

}, isbn = {1553-5282 978-1-4244-7944-3}, url = {://000287375600170}, author = {Neagu, E. R. and Dias, C. J. and Lanca, M. C. and Igreja, R. and Inacio, P. and Marat-Mendes, J. N. and Ieee} } @article {7843, title = {The Study of the Molecular Movements in the Range of Glass Transition by the Final Thermally Stimulated Discharge Current Technique}, journal = {Proceedings of the 2010 Ieee International Conference on Solid Dielectrics (Icsd 2010)}, year = {2010}, note = {n/a}, type = {Journal Article}, abstract = {n/a}, issn = {1553-5282;978-1-4244-7944-3}, author = {Neagu, E. R. and Dias, C. J. and Lanca, M. C. and Igreja, R. and Inacio, P. and Marat-Mendes, J. N. and Ieee} } @inbook {7844, title = {The Study of the Molecular Movements in the Range of Glass Transition by the Final Thermally Stimulated Discharge Current Technique}, booktitle = {Proceedings of the 2010 Ieee International Conference on Solid Dielectrics}, series = {IEEE International Conference on Solid Dielectrics-ICSD}, year = {2010}, note = {

Neagu, E. R. Dias, C. J. Lanca, M. C. Igreja, R. Inacio, P. Marat-Mendes, J. N. ICSD 2010 International Conference on Solid Dielectrics ICSD JUL 04-09, 2010 Potsdam, GERMANY

}, type = {Book Section}, abstract = {

The electrical methods used to study the molecular movements are based on the movement of the dipoles under DC or AC electric field. We have proposed recently a combined measuring protocol to analyze charge injection/extraction, transport, trapping and de-trapping in polar or non-polar dielectric materials. The method is used here to analyze the molecular movements in polyimide in the temperature range from 293 to 572 K. A strong relaxation was observed around 402 K and a very weak relaxation around 345 K. This is the beta relaxation which is quite complex. As concern the behavior at high temperatures, above the beta relaxation, a high peak was observed that shifts continuously to higher temperatures as the charging temperature and/or the charging field increase. The maximum current of the peak increases and the temperature corresponding to the maximum current increases as the charging temperature and/or the charging field increase, given a direct observation of the so called cross-over effect related to current decay for sample charged at high fields and/or high temperatures.

}, isbn = {1553-5282 978-1-4244-7944-3}, url = {://000287375600166}, author = {Neagu, E. R. and Dias, C. J. and Lanca, M. C. and Igreja, R. and Inacio, P. and Marat-Mendes, J. N. and Ieee} } @article {E_R_Neagu:2009, title = {The study of molecular movements in dielectrics using isothermal and non- isotehermal current measurements}, volume = {183}, year = {2009}, note = {n/a}, month = {Jan}, pages = {{\textendash}-}, type = {Peer reviewed}, abstract = {n/a}, author = {Neagu, E. R. and Dias, C. J. and MC Lan{\c c}a and Marat-Mendes, J. N.} } @article {Neagu_ER:2008, title = {The determination of the metal-dielectric interface barrier height from the open-circuit isothermal charging current}, journal = {Journal of Applied Physics}, volume = {104}, year = {2008}, note = {n/a}, month = {Jan}, abstract = {n/a}, url = {://000258493900111}, author = {Neagu ER and Neagu RM and Dias CJ and Lanca MC and Marat-Mendes JN} } @conference {Lanca_MC:2008, title = {Electrical Properties Studies of a Cork/TetraPak (R)/Paraffin Wax Composite}, booktitle = {Advanced Materials Forum Iv}, series = {Materials Science Forum}, volume = {587-588}, number = {587-588}, year = {2008}, note = {n/a}, month = {Jan}, pages = {613-617}, abstract = {n/a}, url = {://000259898200122}, author = {Lanca MC and Peuckert S and Neagu ER and Gil L and Silva PC and Marat-MendeS J}, editor = {Marques AT, Silva AF, Baptista APM, Sa C, Alves F, Malheiros LF, Vieira M} } @conference {Neagu_RM:2008, title = {New Experimental Facts Concerning the Thermally Stimulated Discharge Current in Dielectric Materials}, booktitle = {Advanced Materials Forum Iv}, series = {Materials Science Forum}, volume = {587-588}, number = {587-588}, year = {2008}, note = {n/a}, month = {Jan}, pages = {328-332}, abstract = {n/a}, url = {://000259898200066}, author = {Neagu RM and Neagu ER and Lanca MC and Marat-Mendes JN}, editor = {Marques AT, Silva AF, Baptista APM, Sa C, Alves F, Malheiros LF, Vieira M} } @article {7849, title = {The determination of the metal-dielectric interface barrier height from the open-circuit isothermal charging current}, journal = {Journal of Applied Physics}, volume = {104}, number = {3}, year = {2008}, note = {

Neagu, E. R. Neagu, R. M. Dias, C. J. Lanca, M. Carmo Marat-Mendes, J. N.

}, type = {Journal Article}, abstract = {

There is a sustained interest both from theoretical and from practical points of view to understand the isothermal charging and the isothermal discharging currents in dielectrics. The measured currents are analyzed either in terms of polarization mechanisms or in terms of charge injection/extraction at the metal-dielectric interface and the conduction current through the dielectric material. As long as we do not know the nature of the origin of the current, it is not clear what information we can get by analyzing the experimental data. We propose to measure the open-circuit isothermal charging and discharging currents just to overpass the difficulties related to the analysis of the conduction mechanisms in dielectric materials. We demonstrate that besides a polarization current, there is a current related with charge injection or extraction at the metal-dielectric contact and a reverse current related to the charge trapped into the superficial trap states of the dielectric and that can jump at the interface in a reverse way. An analytical expression for the current is proposed. By fitting the experimental data to this analytical equation, two important parameters can be determined: (i) the highest value of the relaxation time for the polarization mechanisms still involved into the transient current and (ii) the height of the potential barrier W-0 at the metal-dielectric interface at the initial time when the step voltage is applied. The value obtained for Al-polyethylene terephthalate interface is (0.43 +/- 0.02) eV. For a charging voltage of 220 V there are 6x10(14) trapped electrons/m(2). (c) 2008 American Institute of Physics.

}, issn = {0021-8979}, doi = {034102 10.1063/1.2952015}, url = {://000258493900111}, author = {Neagu, E. R. and Neagu, R. M. and Dias, C. J. and Lanca, M. C. and Marat-Mendes, J. N.} } @article {7851, title = {The determination of the metal-dielectric interface barrier height from the open-circuit isothermal charging current}, journal = {Journal of Applied Physics}, volume = {104}, number = {3}, year = {2008}, note = {n/a}, type = {Journal Article}, abstract = {n/a}, issn = {0021-8979}, doi = {10.1063/1.2952015}, author = {Neagu, E. R. and Neagu, R. M. and Dias, C. J. and Lanca, M. Carmo and Marat-Mendes, J. N.} } @inbook {7832, title = {Electrical Properties Studies of a Cork/TetraPak (R)/Paraffin Wax Composite}, booktitle = {Advanced Materials Forum Iv}, series = {Materials Science Forum}, volume = {587-588}, year = {2008}, note = {

Lanca, M. Carmo Peuckert, Stefan Neagu, Eugen R. Gil, Luis Silva, Paulo C. Marat-Mendes, Jose 13th Conference of the Sociedade-Portuguesa-de-Materiais/4th International Materials Symposium APR 01-04, 2007 Oporto, PORTUGAL

}, pages = {613-617}, type = {Book Section}, abstract = {

Lately the electrical and dielectric properties of cork and some cork-based materials (commercial and non-commercial) have been studied in order to understand their ability to store electrical charge. The main problem found so far is related to the water content in cork, only of a few \% weight. but large enough to influence greatly the conductivity of cork and, consequently, the charge storage capability. To overcome this problem cork has been combined with hydrophobic materials. In this work a commercial wax (paraffin wax) was used to produce a cork/paraffin composite by hot pressing. After milled and mixed natural cork. TetraPak (R) containers waste and paraffin were pressed to make plaques of a new composite. Different concentrations of cork. TetraPak (R) and paraffin, different granules sire, different temperature and pressure were used to produce the samples. The electrical properties of the new composite were measured by the isothermal charging and discharging current method and the results compared to previously ones obtained for natural cork and other derivative products. The new composite has shown to have lower conductivity than the commercial agglomerate. which makes it a better material for charge storage.

}, isbn = {0255-5476}, url = {://000259898200122}, author = {Lanca, M. C. and Peuckert, S. and Neagu, E. R. and Gil, L. and Silva, P. C. and Marat-Mendes, J.}, editor = {Marques, AT and Silva, AF and Baptista, APM and Sa, C and Alves, FJLA and Malheiros, LF and Vieira, M} } @article {7833, title = {Electrical Properties Studies of a Cork/TetraPak (R)/Paraffin Wax Composite}, journal = {Advanced Materials Forum Iv}, volume = {587-588}, year = {2008}, note = {

Lanca, M. Carmo Peuckert, Stefan Neagu, Eugen R. Gil, Luis Silva, Paulo C. Marat-Mendes, Jose Marques, AT Silva, AF Baptista, APM Sa, C Alves, FJLA Malheiros, LF Vieira, M 13th Conference of the Sociedade-Portuguesa-de-Materiais/4th International Materials Symposium APR 01-04, 2007 Oporto, PORTUGAL

}, pages = {613-617}, type = {Journal Article}, abstract = {

Lately the electrical and dielectric properties of cork and some cork-based materials (commercial and non-commercial) have been studied in order to understand their ability to store electrical charge. The main problem found so far is related to the water content in cork, only of a few \% weight. but large enough to influence greatly the conductivity of cork and, consequently, the charge storage capability. To overcome this problem cork has been combined with hydrophobic materials. In this work a commercial wax (paraffin wax) was used to produce a cork/paraffin composite by hot pressing. After milled and mixed natural cork. TetraPak (R) containers waste and paraffin were pressed to make plaques of a new composite. Different concentrations of cork. TetraPak (R) and paraffin, different granules sire, different temperature and pressure were used to produce the samples. The electrical properties of the new composite were measured by the isothermal charging and discharging current method and the results compared to previously ones obtained for natural cork and other derivative products. The new composite has shown to have lower conductivity than the commercial agglomerate. which makes it a better material for charge storage.

}, issn = {0255-5476}, url = {://WOS:000259898200122}, author = {Lanca, M. C. and Peuckert, S. and Neagu, E. R. and Gil, L. and Silva, P. C. and Marat-Mendes, J.} } @inbook {7835, title = {Electrical Properties Studies of a Cork/TetraPak (R)/Paraffin Wax Composite}, booktitle = {Advanced Materials Forum Iv}, series = {Materials Science Forum}, volume = {587-588}, year = {2008}, note = {

Times Cited: 1 13th Conference of the Sociedade-Portuguesa-de-Materiais/4th International Materials Symposium APR 01-04, 2007 Oporto, PORTUGAL

}, pages = {613-617}, type = {Book Section}, abstract = {

Lately the electrical and dielectric properties of cork and some cork-based materials (commercial and non-commercial) have been studied in order to understand their ability to store electrical charge. The main problem found so far is related to the water content in cork, only of a few \% weight. but large enough to influence greatly the conductivity of cork and, consequently, the charge storage capability. To overcome this problem cork has been combined with hydrophobic materials. In this work a commercial wax (paraffin wax) was used to produce a cork/paraffin composite by hot pressing. After milled and mixed natural cork. TetraPak (R) containers waste and paraffin were pressed to make plaques of a new composite. Different concentrations of cork. TetraPak (R) and paraffin, different granules sire, different temperature and pressure were used to produce the samples. The electrical properties of the new composite were measured by the isothermal charging and discharging current method and the results compared to previously ones obtained for natural cork and other derivative products. The new composite has shown to have lower conductivity than the commercial agglomerate. which makes it a better material for charge storage.

}, url = {://WOS:000259898200122}, author = {Lanca, M. Carmo and Peuckert, Stefan and Neagu, Eugen R. and Gil, Luis and Silva, Paulo C. and Marat-Mendes, Jose}, editor = {Marques, AT and Silva, AF and Baptista, APM and Sa, C and Alves, FJLA and Malheiros, LF and Vieira, M} } @inbook {7836, title = {Electrical Properties Studies of a Cork/TetraPak (R)/Paraffin Wax Composite}, booktitle = {Advanced Materials Forum Iv}, volume = {587-588}, year = {2008}, note = {n/a}, pages = {613-617}, type = {Book Section}, abstract = {n/a}, isbn = {0255-5476;*****************}, author = {Lanca, M. Carmo and Peuckert, Stefan and Neagu, Eugen R. and Gil, Luis and Silva, Paulo C. and Marat-Mendes, Jose and Marques, AT and Silva, AF and Baptista, APM and Sa, C and Alves, FJLA and Malheiros, LF and Vieira, M} } @inbook {7855, title = {New Experimental Facts Concerning the Thermally Stimulated Discharge Current in Dielectric Materials}, booktitle = {Advanced Materials Forum Iv}, series = {Materials Science Forum}, volume = {587-588}, year = {2008}, note = {

Neagu, R. M. Neagu, E. R. Lanca, M. Carmo Marat-Mendes, J. N. 13th Conference of the Sociedade-Portuguesa-de-Materiais/4th International Materials Symposium APR 01-04, 2007 Oporto, PORTUGAL

}, pages = {328-332}, type = {Book Section}, abstract = {

The thermally stimulated discharge current (TSDC.) method is a very sensitive and a very selective technique to analyze dipole disorientation and the movement of de-trapped space charge (SC). We have proposed a variant of the TSDC method, namely the final thermally stimulated discharge current (FTSDC) technique. flee experimental conditions can be selected so that the FTSDC is mainly determined by the SC de-trapping. The temperatures of the maximum intensity of the fractional polarization peaks obtained at low temperature, in the range of the local (secondary) relaxation, are in general about 10 to 20 K above the poling temperature. Measurements of the FTSDC in a wide temperature range demonstrate the existence of an apparent peak at a temperature T-ma shifted with about 10 to 30 K above the charging temperature T-c. The shift of T-ma with respect to T-c depends on the experimental conditions. The peak width at the half maximum intensity decreases as T-c increases and the thermal apparent activation energy increases. The variations are not monotonous revealing the temperature range where the molecular motion is stronger and consequently the charge trapping and de-trapping processes are affected. Our results demonstrate that there is a strong similarity between the elementary peaks obtained by the two methods, and the current is mainly determined by SC de-trapping. Even the best elementary peaks are not fitted very well by the analytical equation, indicating that the hypothesis behind this equation have to be reconsidered.

}, isbn = {0255-5476}, url = {://000259898200066}, author = {Neagu, R. M. and Neagu, E. R. and Lanca, M. C. and Marat-Mendes, J. N.}, editor = {Marques, AT and Silva, AF and Baptista, APM and Sa, C and Alves, FJLA and Malheiros, LF and Vieira, M} } @inbook {7856, title = {New Experimental Facts Concerning the Thermally Stimulated Discharge Current in Dielectric Materials}, booktitle = {Advanced Materials Forum Iv}, series = {Materials Science Forum}, volume = {587-588}, year = {2008}, note = {

Times Cited: 0 13th Conference of the Sociedade-Portuguesa-de-Materiais/4th International Materials Symposium APR 01-04, 2007 Oporto, PORTUGAL

}, pages = {328-332}, type = {Book Section}, abstract = {

The thermally stimulated discharge current (TSDC.) method is a very sensitive and a very selective technique to analyze dipole disorientation and the movement of de-trapped space charge (SC). We have proposed a variant of the TSDC method, namely the final thermally stimulated discharge current (FTSDC) technique. flee experimental conditions can be selected so that the FTSDC is mainly determined by the SC de-trapping. The temperatures of the maximum intensity of the fractional polarization peaks obtained at low temperature, in the range of the local (secondary) relaxation, are in general about 10 to 20 K above the poling temperature. Measurements of the FTSDC in a wide temperature range demonstrate the existence of an apparent peak at a temperature T-ma shifted with about 10 to 30 K above the charging temperature T-c. The shift of T-ma with respect to T-c depends on the experimental conditions. The peak width at the half maximum intensity decreases as T-c increases and the thermal apparent activation energy increases. The variations are not monotonous revealing the temperature range where the molecular motion is stronger and consequently the charge trapping and de-trapping processes are affected. Our results demonstrate that there is a strong similarity between the elementary peaks obtained by the two methods, and the current is mainly determined by SC de-trapping. Even the best elementary peaks are not fitted very well by the analytical equation, indicating that the hypothesis behind this equation have to be reconsidered.

}, url = {://WOS:000259898200066}, author = {Neagu, R. M. and Neagu, E. R. and Lanca, M. Carmo and Marat-Mendes, J. N.}, editor = {Marques, AT and Silva, AF and Baptista, APM and Sa, C and Alves, FJLA and Malheiros, LF and Vieira, M} } @inbook {7857, title = {New Experimental Facts Concerning the Thermally Stimulated Discharge Current in Dielectric Materials}, booktitle = {Advanced Materials Forum Iv}, volume = {587-588}, year = {2008}, note = {n/a}, pages = {328-332}, type = {Book Section}, abstract = {n/a}, isbn = {0255-5476;*****************}, author = {Neagu, R. M. and Neagu, E. R. and Lanca, M. Carmo and Marat-Mendes, J. N. and Marques, AT and Silva, AF and Baptista, APM and Sa, C and Alves, FJLA and Malheiros, LF and Vieira, M} } @article {Lanca_MC:2007, title = {Influence of humidity on the electrical charging properties of cork agglomerates}, journal = {Journal of Non-Crystalline Solids}, volume = {353}, year = {2007}, note = {n/a}, month = {Jan}, pages = {4501-4505}, abstract = {n/a}, url = {://000251476300050}, author = {Lanca MC and Wirges W and Neagu ER and Gerhard R and Marat-MendeS J} } @article {Lanca_MC:2007, title = {Space charge analysis of electrothermally aged XLPE cable insulation}, journal = {Journal of Non-Crystalline Solids}, volume = {353}, year = {2007}, note = {n/a}, month = {Jan}, pages = {4462-4466}, abstract = {n/a}, url = {://000251476300043}, author = {Lanca MC and Fu M and Neagu E and Dissado LA and Marat-MendeS J and Tzimas A and Zadeh S} } @article {6334, title = {Influence of humidity on the electrical charging properties of cork agglomerates}, journal = {Journal of Non-Crystalline Solids}, volume = {353}, year = {2007}, note = {n/a}, abstract = {

Cork is a natural cellular and electrically insulating material which may have the capacity to store electric charges on or in its cell walls. Since natural cork has many voids, it is difficult to obtain uniform samples with the required dimensions. Therefore, a more uniform material, namely commercial cork agglomerate, usually used for floor and wall coverings, is employed in the present study. Since we know from our previous work that the electrical properties of cork are drastically affected by absorbed and adsorbed water, samples were protected by means of different polymer coatings (applied by spin-coating or soaking). Corona charging and isothermal charging and discharging currents were used to study the electrical trapping and detrapping capabilities of the samples. A comparison of the results leads to the conclusion that the most promising method for storing electric charges in this cellular material consists of drying and coating or soaking with a hydrophobic, electrically insulating polymer such as polytetraflouroethylene (Teflon (R)).

}, isbn = {0022-3093}, doi = {10.1016/j.jnoncrysol.2007.03.037}, url = {://000251476300050}, author = {Lanca MC and Wirges W and Neagu ER and Gerhard R and Marat-MendeS J} } @article {7834, title = {Influence of humidity on the electrical charging properties of cork agglomerates}, journal = {Journal of Non-Crystalline Solids}, volume = {353}, number = {47-51}, year = {2007}, note = {

Lanca, M. C. Wirges, W. Neagu, E. R. Gerhard, R. Marat-Mendes, J. 4th Conference of the International-Dielectric-Society/9th International Conference on Dielectric and Related Phenomena SEP 03-07, 2006 Poznan, POLAND

}, pages = {4501-4505}, type = {Journal Article}, abstract = {

Cork is a natural cellular and electrically insulating material which may have the capacity to store electric charges on or in its cell walls. Since natural cork has many voids, it is difficult to obtain uniform samples with the required dimensions. Therefore, a more uniform material, namely commercial cork agglomerate, usually used for floor and wall coverings, is employed in the present study. Since we know from our previous work that the electrical properties of cork are drastically affected by absorbed and adsorbed water, samples were protected by means of different polymer coatings (applied by spin-coating or soaking). Corona charging and isothermal charging and discharging currents were used to study the electrical trapping and detrapping capabilities of the samples. A comparison of the results leads to the conclusion that the most promising method for storing electric charges in this cellular material consists of drying and coating or soaking with a hydrophobic, electrically insulating polymer such as polytetraflouroethylene (Teflon (R)). (c) 2007 Elsevier B.V. All rights reserved.

}, issn = {0022-3093}, doi = {10.1016/j.jnoncrysol.2007.03.037}, url = {://000251476300050}, author = {Lanca, M. C. and Wirges, W. and Neagu, E. R. and Gerhard, R. and Marat-Mendes, J.} } @article {7822, title = {Space charge analysis of electrotherinally aged XLPE cable insulation}, journal = {Journal of Non-Crystalline Solids}, volume = {353}, number = {47-51}, year = {2007}, note = {

Lanca, M. C. Fu, M. Neagu, E. Dissado, L. A. Marat-MendeS, J. Tzimas, A. Zadeh, S. 4th Conference of the International-Dielectric-Society/9th International Conference on Dielectric and Related Phenomena SEP 03-07, 2006 Poznan, POLAND

}, pages = {4462-4466}, type = {Journal Article}, abstract = {

Cross-linked polyethylene (XLPE) is currently widely used as an insulating material for power cables due to its good physical properties, however when in use it undergoes an electrical ageing process. Its ability to trap electric charge can give rise to space charge accumulation in the bulk of the polymer and produce localised electric stresses that can lead to cable failure, since the electric field will be increased above the design stress in some regions favouring the initiation of degradation there. In this work the PEA (pulsed electro-acoustic) method was used to compare the charge dynamics in three samples (XLPE cable peelings) aged in different ways (electrothermally in the laboratory, field aged in service and thermally aged in the laboratory). Very different transient behavior was found depending upon the ageing history. This is related to differences in the migration of chemical species in the insulation layer, which are known to act as charge traps. All materials showed heterocharge peaks when the space charge reached stability, the magnitude of which seems to be related to the severity of the ageing. (c) 2007 Elsevier B.V. All rights reserved.

}, issn = {0022-3093}, doi = {10.1016/j.jnoncrysol.2007.03.035}, url = {://000251476300043}, author = {Lanca, M. C. and Fu, M. and Neagu, E. and Dissado, L. A. and Marat-Mendes, J. and Tzimas, A. and Zadeh, S.} } @article {6333, title = {Space charge analysis of electrothermally aged XLPE cable insulation}, journal = {Journal of Non-Crystalline Solids}, volume = {353}, year = {2007}, note = {n/a}, abstract = {

Cross-linked polyethylene (XLPE) is currently widely used as an insulating material for power cables due to its good physical properties, however when in use it undergoes an electrical ageing process. Its ability to trap electric charge can give rise to space charge accumulation in the bulk of the polymer and produce localised electric stresses that can lead to cable failure, since the electric field will be increased above the design stress in some regions favouring the initiation of degradation there. In this work the PEA (pulsed electro-acoustic) method was used to compare the charge dynamics in three samples (XLPE cable peelings) aged in different ways (electrothermally in the laboratory, field aged in service and thermally aged in the laboratory). Very different transient behavior was found depending upon the ageing history. This is related to differences in the migration of chemical species in the insulation layer, which are known to act as charge traps. All materials showed heterocharge peaks when the space charge reached stability, the magnitude of which seems to be related to the severity of the ageing.

}, isbn = {0022-3093}, doi = {10.1016/j.jnoncrysol.2007.03.035}, url = {://000251476300043}, author = {Lanca MC and Fu M and Neagu E and Dissado LA and Marat-MendeS J and Tzimas A and Zadeh S} } @conference {Lanca_MC:2006, title = {Space charge studies in XLPE from power cables using combined isothermal and thermostimulated current measurements}, booktitle = {Advanced Materials Forum Iii, Pts 1 and 2}, series = {Materials Science Forum}, volume = {514-516}, number = {514-516}, year = {2006}, note = {n/a}, month = {Jan}, pages = {935-939}, abstract = {n/a}, author = {Lanca MC and Neagu ER and Dissado LA and Marat-MendeS J} } @conference {Lanca_MC:2006, title = {Study of electrical properties of natural cork and two derivative products}, booktitle = {Advanced Materials Forum Iii, Pts 1 and 2}, series = {Materials Science Forum}, volume = {514-516}, number = {514-516}, year = {2006}, note = {n/a}, month = {Jan}, pages = {940-944}, abstract = {n/a}, author = {Lanca MC and Neagu ER and Silva P and Gil L and Marat-MendeS J} } @inbook {7825, title = {Space charge studies in XLPE from power cables using combined isothermal ans thermostimulated current measurements}, booktitle = {Advanced Materials Forum Iii, Pts 1 and 2}, series = {Materials Science Forum}, volume = {514-516}, year = {2006}, note = {

3rd International Materials Symposium/12th Meeting of the Sociedad-Portuguesa-da-Materials (Materials 2005/SPM) MAR 20-23, 2005 Aveiro, PORTUGAL

}, pages = {935-939}, type = {Book Section}, abstract = {

Cross-linked polyethylene (XLPE) peelings from aged power cables from three different sources were studied using a combined procedure of isothermal and thermo-stimulated current measurements. Different parameters, such as electric field, temperature, charging/discharging times, can be selected in order to make an analysis of the space charge characteristics (such as, relaxation times and activation energies). Three different cables peelings were analyzed: A - electrically aged in the laboratory at high temperature, B - service aged for 18 years and C - thermally aged in the laboratory at high temperature. The results were compared for the different types of samples and also with previous results on laboratory aged and produced films of low-density polyethylene (LDPE) and XLPE.

}, isbn = {0255-5476}, url = {://000238056400186}, author = {Lanca, M. C. and Neagu, E. R. and Dissado, L. A. and Marat-Mendes, J.}, editor = {Vilarinho, P. M.} } @inbook {7831, title = {Study of electrical properties of natural cork and two derivative products}, booktitle = {Advanced Materials Forum Iii, Pts 1 and 2}, series = {Materials Science Forum}, volume = {514-516}, year = {2006}, note = {

3rd International Materials Symposium/12th Meeting of the Sociedad-Portuguesa-da-Materials (Materials 2005/SPM) MAR 20-23, 2005 Aveiro, PORTUGAL

}, pages = {940-944}, type = {Book Section}, abstract = {

Electrical properties of natural cork, commercial cork agglomerates (for floor and wall coverings) and a recently developed composite of cork/TetraPak (R) were studied. Measurements of isothermal charge and discharge currents were made for natural cork samples in different directions (axial, radial and tangential cuts). The isothermal current characteristics and the samples conductivity were investigated under different conditions (electric field, temperature and environmental conditions: in air at ambient relative humidity (RH), dry air and vacuum), also the samples could be or not conditioned (dried in vacuum or in a P2O5 atmosphere at room temperature). From these results the influence of water on the electrical properties of natural cork could be seen. In order to compare the three different cork materials a preliminary study was made. Isothermal charge and discharge currents and conductivity after 1h charging were measured and compared for different electric fields and temperature in air at ambient RH.

}, isbn = {0255-5476}, url = {://000238056400187}, author = {Lanca, M. C. and Neagu, E. R. and Silva, P. and Gil, L. and Marat-Mendes, J.}, editor = {Vilarinho, P. M.} } @conference {Neagu:2005, title = {Identification of an apparent peak by use of the final thermally stimulated discharge current technique}, series = {12th International Symposium on Electrets}, year = {2005}, note = {n/a}, month = {Jan}, pages = {296-299}, type = {Peer reviewed}, abstract = {n/a}, author = {Neagu and E. R. ;Neagu and R. M. ;Lanca and M. C. ;Vassilikou-Dova and A. ;Marat-Mendes and J. N.} } @conference {Lanca_MC:2005, title = {Space charge studies of aged XLPE using combined isothermal and thermostimulated current measurements}, booktitle = {Cross-Disciplinary Applied Research in Materials Science and Technology}, series = {Materials Science Forum}, volume = {480-481}, number = {480}, year = {2005}, note = {n/a}, month = {Jan}, pages = {501-505}, abstract = {n/a}, author = {Lanca MC and Neagu ER and Marat-Mendes JN} } @conference {Neagu:2005, title = {The time as a parameter to investigate the landscape of the apparent activation energies in the final thermally stimulated discharge current measurements}, series = {12th International Symposium on Electrets}, year = {2005}, note = {n/a}, month = {Jan}, pages = {292-295}, type = {Peer reviewed}, abstract = {n/a}, author = {Neagu and E. R. ;Neagu and R. M. ;Lanca and M. C. ;Marat-Mendes and J. N.} } @book {7821, title = {Comparative study of space charge in the polymeric insulation of power cables using PEA, isothermal and non-isothermal currents measurements}, year = {2005}, note = {

ISE 12 12th International Symposium on Electrets (ISE 12) SEP 11-14, 2005 Salvador, BRAZIL

}, pages = {284-287}, type = {Book}, abstract = {

An understanding of space charge build-up in the polymeric insulation of power cables is important in determining how aging occurs and progresses and, also in predicting cable lifetime. In this investigation electric-field induced space charge in peelings from XLPE (cross-linked polyethylene) cables was measured using two different methods: the pulsed electro-acoustic technique (PEA) and the combined procedure of isothermal and non-isothermal charging/discharging currents (FTSDC). These two methods allow the study of space charge in highly insulating materials. Also, since electric fields of different orders of magnitude are applied to the sample in the two methods, it is possible to analyze different characteristics of the space charge traps. Prior to the measurements the samples were subjected to conditioning to remove volatiles. Cable peelings from various brands aged under different conditions (including field aged and thermally aged samples) were studied as received from the manufacturers. Some of the samples have undergone further ageing in AC electric field (50Hz) for 1000h to see the influence of further ageing on space charge build-up. The results for the different types of samples are compared in an attempt to correlate different ageing parameters.

}, isbn = {0-7803-9116-0}, url = {://000236764500074}, author = {Lanca, M. C. and Fu, M. and Neagu, E. and Dissado, L. A. and Marat-Mendes, J. and Tzimas, A. and Zadeh, S.} } @book {7854, title = {Identification of an apparent peak by use of the final thermally stimulated discharge current technique}, year = {2005}, note = {

ISE 12 12th International Symposium on Electrets (ISE 12) SEP 11-14, 2005 Salvador, BRAZIL

}, pages = {296-299}, type = {Book}, abstract = {

The final thermally stimulated discharge current (FTSDC) technique can be used to analyze charge trapping and transport in insulating materials. The experimental conditions can be selected so that the FTSDC is mainly determined by the space charge detrapping. Measurements of the FTSDC in a wide temperature range including the local (secondary) beta relaxation and the non-local (primary) cc relaxation, for different polymers, demonstrate the existence of an apparent peak. The shift of peak temperature T-m with respect to the charging temperature T-p is analyzed. The interval T-m - T-p decreases from about 25 K to zero, as T-p approaches the glass transition T-g. T-m - T-p is lower for materials of lower conductivity. The peak width at the half maximum intensity decreases as Tp increases and the thermal apparent activation energy increases. The variations are not monotonous revealing the temperature range where the molecular motion is stronger and consequently the charge trapping and detrapping processes are affected by the strong thermal motion.

}, isbn = {0-7803-9116-0}, url = {://000236764500077}, author = {Neagu, E. R. and Neagu, R. M. and Lanca, M. C. and Vassilikou-Dova, A. and Marat-Mendes, J. N.} } @inbook {7828, title = {Space charge studies of aged XLPE using combined isothermal and thermostimulated current measurements}, booktitle = {Cross-Disciplinary Applied Research in Materials Science and Technology}, series = {Materials Science Forum}, volume = {480}, year = {2005}, note = {

1st International Meeting on Applied Physics OCT 13-18, 2003 Badajoz, SPAIN

}, pages = {501-505}, type = {Book Section}, abstract = {

Space charge in electrically aged cross-linked polyethylene (XLPE) was studied using a procedure combining isothermal and non-isothermal measurements of charge and discharge currents. Aging is carried out using an AC field while immersing the disk-shaped samples in an ionic aqueous solution at constant temperature. After aging the samples were isothermally DC charged and discharged. Next a non-isothermal experiment with constant heating rate was performed (FTSDC). Finally the sample was kept at the highest temperature in order to completely discharge the polymer. The space charge introduced in the XLPE during aging can be analyzed from the study of the FTSDC spectra. The thermogram (FTSDC) shows a very broad peak. The peak is attributed to trapped space charge in traps with long relaxation times. It is possible to decompose it into three or four individual peaks and obtain the corresponding activation energies. The results were compared with previous ones obtained for LDPE (low density polyethylene) aged under similar conditions.

}, isbn = {0255-5476 0-87849-962-8}, url = {://000228157300082}, author = {Lanca, M. C. and Neagu, E. R. and Marat-Mendes, J. N.}, editor = {MendezVilas, A.} } @book {7852, title = {The time as a parameter to investigate the landscape of the apparent activation energies in the final thermally stimulated discharge current measurements}, year = {2005}, note = {

ISE 12 12th International Symposium on Electrets (ISE 12) SEP 11-14, 2005 Salvador, BRAZIL

}, pages = {292-295}, type = {Book}, abstract = {

The experimental results obtained in a wide range of temperatures, for polyethylene terephthalate, demonstrate that the apparent activation energy changes when the charging (polarization) time or the isothermal discharging time, prior to the final thermally stimulated discharge current measurement, are used as variable parameters. Consequently, the charging and/or discharging time can be used as a variable parameter to investigate the landscape of the apparent thermal activation energies. A continuous distribution of the traps in the range from 0.4 to 3 eV was observed. The experimental results demonstrate that there is a range of experimental conditions for which the thermal activation energy is independent of the experimental parameter values. This is the activation energy value which should be used to characterize a certain mechanism.

}, isbn = {0-7803-9116-0}, url = {://000236764500076}, author = {Neagu, E. R. and Neagu, R. M. and Lanca, M. C. and Marat-Mendes, J. N.} } @article {7853, title = {The time as a parameter to investigate the landscape of the apparent activation energies in the final thermally stimulated discharge current measurements}, journal = {12th International Symposium on Electrets (ISE 12), Proceedings}, year = {2005}, note = {

Neagu, ER Neagu, RM Lanca, MC Marat-Mendes, JN Faria, RM Oliveira, ON Faria, RM 12th International Symposium on Electrets (ISE 12) SEP 11-14, 2005 Salvador, BRAZIL IEEE Dielectr \& Elect Insulat Soc, IEEE Ultrason \& Frequency Control Soc 0-7803-9116-0

}, pages = {292-295}, type = {Journal Article}, abstract = {

The experimental results obtained in a wide range of temperatures, for polyethylene terephthalate, demonstrate that the apparent activation energy changes when the charging (polarization) time or the isothermal discharging time, prior to the final thermally stimulated discharge current measurement, are used as variable parameters. Consequently, the charging and/or discharging time can be used as a variable parameter to investigate the landscape of the apparent thermal activation energies. A continuous distribution of the traps in the range from 0.4 to 3 eV was observed. The experimental results demonstrate that there is a range of experimental conditions for which the thermal activation energy is independent of the experimental parameter values. This is the activation energy value which should be used to characterize a certain mechanism.

}, url = {://WOS:000236764500076}, author = {Neagu, E. R. and Neagu, R. M. and Lanca, M. C. and Marat-Mendes, J. N.} } @conference {Lanca:2004, title = {Comparative study of space charge in aged low-density polyethylene and crosslinked polyethylene}, series = {Proceedings of the 2004 Ieee International Conference on Solid Dielectrics, Vols 1 and 2}, year = {2004}, note = {n/a}, month = {Jan}, pages = {209-212}, type = {Peer reviewed}, abstract = {n/a}, author = {Lanca and M. C. ;Neagu and E. R. ;Marat-Mendes and J. N.} } @conference {Lanca:2004, title = {Comparative study of space charge in aged low-density polyethylene and crosslinked polyethylene}, series = {Proceedings of the 2004 Ieee International Conference on Solid Dielectrics, Vols 1 and 2}, year = {2004}, note = {n/a}, month = {Jan}, pages = {209-212}, type = {Peer reviewed}, abstract = {n/a}, author = {Lanca and M. C. ;Neagu and E. R. ;Marat-Mendes and J. N.} } @conference {Lanca_MC:2004, title = {Dielectric breakdown statistics of polyethylene for progressively-censored data}, booktitle = {Advanced Materials Forum Ii}, series = {Materials Science Forum}, volume = {455-456}, number = {455-456}, year = {2004}, note = {n/a}, month = {Jan}, pages = {602-605}, abstract = {n/a}, author = {Lanca MC and Marat-MendeS J} } @article {Lanca_MC:2004, title = {Space charge studies in LDPE using combined isothermal and non-isothermal current measurements}, journal = {IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION}, volume = {11}, year = {2004}, note = {n/a}, month = {Jan}, pages = {25-34}, publisher = {Univ Wales, Univ Nova Lisboa}, abstract = {n/a}, author = {Lanca MC and Neagu ER and Neagu RM and Dias CJ and Marat-Mendes JN and Das-Gupta DK} } @book {4194, title = {Comparative study of space charge in aged low-density polyethylene and crosslinked polyethylene}, year = {2004}, note = {

8th IEEE International Conference on Solid Dielectrics JUL 05-09, 2004 Toulouse, FRANCE

}, pages = {209-212}, type = {Book}, abstract = {

Polyethylene is one of the most widely used polymeric insulators in medium and high voltage power cables. However the importance of space charge distribution and its influence on the electrical aging in this polymer is not fully understood. The very good insulating properties of the material implying very long relaxation times (few days and even longer are usual) and low currents (few pA or below) make individual measurements of isothermal charge/discharge currents and thermostimulated currents difficult to analyze and reproduce. A single type of measurements does not take into account the space charge that remains trapped for long times. A combined procedure of isothermal and non-isothermal current measurements developed for high insulating polymers was used for low density polyethylene (LDPE) and crosslinked polyethylene (XLPE) films electrically aged. The press-molded LDPE and XLPE films were electrically aged under similar conditions using an AC electric field while immersed in a sodium chloride aqueous solution at constant temperature (electro-thermal aging). The use of the combined procedure for current measurement allowed obtaining information about space charge traps, activation energies and relaxation times for both LDPE and XLPE. This data was used to compare electrical aging under similar conditions for the two types of polyethylene.

}, isbn = {0-7803-8348-6}, url = {://000223357100052}, author = {Lanca, M. C. and Neagu, E. R. and Marat-Mendes, J. N. and Ieee} } @book {7829, title = {Comparative study of space charge in aged low-density polyethylene and crosslinked polyethylene}, year = {2004}, note = {

8th IEEE International Conference on Solid Dielectrics JUL 05-09, 2004 Toulouse, FRANCE

}, pages = {209-212}, type = {Book}, abstract = {

Polyethylene is one of the most widely used polymeric insulators in medium and high voltage power cables. However the importance of space charge distribution and its influence on the electrical aging in this polymer is not fully understood. The very good insulating properties of the material implying very long relaxation times (few days and even longer are usual) and low currents (few pA or below) make individual measurements of isothermal charge/discharge currents and thermostimulated currents difficult to analyze and reproduce. A single type of measurements does not take into account the space charge that remains trapped for long times. A combined procedure of isothermal and non-isothermal current measurements developed for high insulating polymers was used for low density polyethylene (LDPE) and crosslinked polyethylene (XLPE) films electrically aged. The press-molded LDPE and XLPE films were electrically aged under similar conditions using an AC electric field while immersed in a sodium chloride aqueous solution at constant temperature (electro-thermal aging). The use of the combined procedure for current measurement allowed obtaining information about space charge traps, activation energies and relaxation times for both LDPE and XLPE. This data was used to compare electrical aging under similar conditions for the two types of polyethylene.

}, isbn = {0-7803-8348-6}, url = {://000223357100052}, author = {Lanca, M. C. and Neagu, E. R. and Marat-Mendes, J. N. and Ieee} } @inbook {4190, title = {Dielectric breakdown statistics of polyethylene for progressively-censored data}, booktitle = {Advanced Materials Forum Ii}, series = {Materials Science Forum}, volume = {455-456}, year = {2004}, note = {

2nd International Materials Symposium APR 14-16, 2003 Caparica, PORTUGAL

}, pages = {602-605}, type = {Book Section}, abstract = {

The dielectric breakdown of thin films of low-density polyethylene (LDPE) electrically aged in an aqueous solution of NaCl under an AC electric field was investigated. A two-parameter Weibull function was used for the dielectric breakdown time to failure. The probability of failure for a sample was obtained by the White method for progressively censored data. Samples aged at different temperatures were compared. The results show that initially the samples aged at lower temperature (approximate to25degreesC) are more prone to fail, while those aged at higher temperature (50degreesC) fail at longer times. This was attributed to a competition between oxidation and diffusion.

}, isbn = {0255-5476 0-87849-941-5}, url = {://000222018500129}, author = {Lanca, M. C. and Marat-Mendes, J.}, editor = {Martins, R. and Fortunato, E. and Ferreira, I. and Dias, C.} } @inbook {7823, title = {Dielectric breakdown statistics of polyethylene for progressively-censored data}, booktitle = {Advanced Materials Forum Ii}, series = {Materials Science Forum}, volume = {455-456}, year = {2004}, note = {

2nd International Materials Symposium APR 14-16, 2003 Caparica, PORTUGAL

}, pages = {602-605}, type = {Book Section}, abstract = {

The dielectric breakdown of thin films of low-density polyethylene (LDPE) electrically aged in an aqueous solution of NaCl under an AC electric field was investigated. A two-parameter Weibull function was used for the dielectric breakdown time to failure. The probability of failure for a sample was obtained by the White method for progressively censored data. Samples aged at different temperatures were compared. The results show that initially the samples aged at lower temperature (approximate to25degreesC) are more prone to fail, while those aged at higher temperature (50degreesC) fail at longer times. This was attributed to a competition between oxidation and diffusion.

}, isbn = {0255-5476 0-87849-941-5}, url = {://000222018500129}, author = {Lanca, M. C. and Marat-Mendes, J.}, editor = {Martins, R. and Fortunato, E. and Ferreira, I. and Dias, C.} } @article {4195, title = {Space charge studies in LDPE using combined isothermal and non-isothermal current measurements}, journal = {Ieee Transactions on Dielectrics and Electrical Insulation}, volume = {11}, number = {1}, year = {2004}, note = {n/a}, pages = {25-34}, type = {Journal Article}, abstract = {

Using a recently developed procedure combining isothermal and non-isothermal current measurements space charge trapping and transport in LDPE was successfully studied. Unaged, thermally and electrically aged samples were investigated. The samples were conditioned before each measurement in order to obtain reproducible results. In the non-isothermal measurements appeared a broad peak (40degreesC to 50degreesC) that was possible to decompose into two or three peaks (35, 45 and 65degreesC). At even higher temperature another peak was sometimes present (85degreesC) depending on the prior sample conditioning. The space charge is trapped near the surface in deep traps (maximum depth of approximate to 15 mum). Relaxation times, mobilities and activation energies have been calculated for different charging/discharging conditions. For unaged samples the reproducibility of the results was poor while for the aged polyethylene it was quite good, meaning that aging helps conditioning. In the electrically aged LDPE there is a decrease of conductivity and the broad peak of the non-isothermal spectra shows a slight shift towards higher temperatures when compared with the data found in the thermally aged polymer.

}, issn = {1070-9878}, url = {://000189389500005}, author = {Lanca, M. C. and Neagu, E. R. and Neagu, R. M. and Dias, C. J. and Marat-Mendes, J. N. and Das-Gupta, D. K.} } @article {7830, title = {Space charge studies in LDPE using combined isothermal and non-isothermal current measurements}, journal = {Ieee Transactions on Dielectrics and Electrical Insulation}, volume = {11}, number = {1}, year = {2004}, note = {n/a}, pages = {25-34}, type = {Journal Article}, abstract = {

Using a recently developed procedure combining isothermal and non-isothermal current measurements space charge trapping and transport in LDPE was successfully studied. Unaged, thermally and electrically aged samples were investigated. The samples were conditioned before each measurement in order to obtain reproducible results. In the non-isothermal measurements appeared a broad peak (40degreesC to 50degreesC) that was possible to decompose into two or three peaks (35, 45 and 65degreesC). At even higher temperature another peak was sometimes present (85degreesC) depending on the prior sample conditioning. The space charge is trapped near the surface in deep traps (maximum depth of approximate to 15 mum). Relaxation times, mobilities and activation energies have been calculated for different charging/discharging conditions. For unaged samples the reproducibility of the results was poor while for the aged polyethylene it was quite good, meaning that aging helps conditioning. In the electrically aged LDPE there is a decrease of conductivity and the broad peak of the non-isothermal spectra shows a slight shift towards higher temperatures when compared with the data found in the thermally aged polymer.

}, issn = {1070-9878}, url = {://000189389500005}, author = {Lanca, M. C. and Neagu, E. R. and Neagu, R. M. and Dias, C. J. and Marat-Mendes, J. N. and Das-Gupta, D. K.} } @conference {Lanca:2003, title = {Comparative study of dielectric relaxation spectra of electrically and thermally aged low density polyethylene}, series = {2003 Annual Report Conference on Electrical Insulation and Dielectric Phenomena}, year = {2003}, note = {n/a}, month = {Jan}, pages = {161-164}, type = {Peer reviewed}, abstract = {n/a}, author = {Lanca and M. C. ;Dias and C. J. ;Das Gupta and D. K. ;Marat-Mendes and J.} } @book {4187, title = {Comparative study of dielectric relaxation spectra of electrically and thermally aged low density polyethylene}, year = {2003}, note = {

Annual Conference on Electrical Insulation and Dielectric Phenomena (CEIDP) OCT 19-22, 2003 ALBUQUERQUE, NM

}, pages = {161-164}, type = {Book}, abstract = {

Low-density polyethylene (LDPE) films were thermally aged in a sodium chloride aqueous solution at constant temperature (thermal aging). Some of the samples were simultaneously immersed in solution and subjected to an electric AC field (electrical aging). The dielectric relaxation spectra at 30 degreesC in the range of 10(-5) Hz to 10(5) Hz were obtained for unaged and aged samples. For the low frequency (LF) region (10(-5) Hz to 10(-1) Hz) the time domain technique was used. A lock-in amplifier was used for the 10(-1) Hz to 10(1) Hz medium frequency (MF) region. While for the high frequency (HF), 10(-1) Hz to 10(5) Hz, RLC bridge measurements were performed. The main differences can be seen between electrically, thermally aged and unaged LDPE in the HF and LF regions. The LF peak is a broad peak related to localized space charge injection driven by the electric field. For electrically aged samples this peak increases in an earlier stage of electrical aging, decreasing afterwards. While in thermally aged samples the peak amplitude always increases with aging time. Finally the HF shows the beginning of a peak due to the gamma and beta transitions. This peak decreases with aging disappearing for the most aged samples.

}, isbn = {0-7803-7910-1}, url = {://000186828000038}, author = {Lanca, M. C. and Dias, C. J. and Dasgupta, D. K. and Marat-Mendes, J. and Ieee, Ieee} } @book {7818, title = {Comparative study of dielectric relaxation spectra of electrically and thermally aged low density polyethylene}, year = {2003}, note = {

Annual Conference on Electrical Insulation and Dielectric Phenomena (CEIDP) OCT 19-22, 2003 ALBUQUERQUE, NM

}, pages = {161-164}, type = {Book}, abstract = {

Low-density polyethylene (LDPE) films were thermally aged in a sodium chloride aqueous solution at constant temperature (thermal aging). Some of the samples were simultaneously immersed in solution and subjected to an electric AC field (electrical aging). The dielectric relaxation spectra at 30 degreesC in the range of 10(-5) Hz to 10(5) Hz were obtained for unaged and aged samples. For the low frequency (LF) region (10(-5) Hz to 10(-1) Hz) the time domain technique was used. A lock-in amplifier was used for the 10(-1) Hz to 10(1) Hz medium frequency (MF) region. While for the high frequency (HF), 10(-1) Hz to 10(5) Hz, RLC bridge measurements were performed. The main differences can be seen between electrically, thermally aged and unaged LDPE in the HF and LF regions. The LF peak is a broad peak related to localized space charge injection driven by the electric field. For electrically aged samples this peak increases in an earlier stage of electrical aging, decreasing afterwards. While in thermally aged samples the peak amplitude always increases with aging time. Finally the HF shows the beginning of a peak due to the gamma and beta transitions. This peak decreases with aging disappearing for the most aged samples.

}, isbn = {0-7803-7910-1}, url = {://000186828000038}, author = {Lanca, M. C. and Dias, C. J. and Dasgupta, D. K. and Marat-Mendes, J. and Ieee, Ieee} } @article {Lanca_MC:2002, title = {Combined isothermal and non-isothermal current measurements applied to space charge studies in low-density polyethylene}, journal = {Journal of Physics D-Applied Physics}, volume = {35}, year = {2002}, note = {n/a}, month = {Jan}, pages = {L29-L32}, abstract = {n/a}, author = {Lanca MC and Neagu ER and Marat-Mendes JN} } @conference {Lanca_MC:2002, title = {Dielectric properties of electrically aged low density polyethylene}, booktitle = {Advanced Materials Forum I}, series = {Key Engineering Materials}, volume = {230-2}, number = {230-232}, year = {2002}, note = {n/a}, month = {Jan}, pages = {396-399}, abstract = {n/a}, author = {Lanca MC and Dias CJ and Das Gupta DK and Marat-MendeS J} } @conference {Mateo_J:2002, title = {Infrared spectroscopy studies of aged polymeric insulators}, booktitle = {Advanced Materials Forum I}, series = {2003 Annual Report Conference on Electrical Insulation and Dielectric Phenomena}, volume = {230-2}, year = {2002}, note = {n/a}, month = {Jan}, pages = {384-387}, abstract = {n/a}, author = {Mateo J and Lanca MC and Marat-MendeS J} } @conference {Lanca:2002, title = {Studies of space charge in electrically aged low density polyethylene}, series = {11th International Symposium on Electrets}, year = {2002}, note = {n/a}, month = {Jan}, pages = {19-22}, type = {Peer reviewed}, abstract = {n/a}, author = {Lanca and M. C. ;Neagu and E. R. ;Marat-Mendes and J. N.} } @article {4192, title = {Combined isothermal and non-isothermal current measurements applied to space charge studies in low-density polyethylene}, journal = {Journal of Physics D-Applied Physics}, volume = {35}, number = {8}, year = {2002}, note = {n/a}, pages = {L29-L32}, type = {Journal Article}, abstract = {

A new experimental procedure combining usual isothermal DC charging and discharging with non-isothermal current measurements has been recently proposed. It is mainly suitable for very high insulating polymers and it was successfully applied to the study of space charge trapping and transport in low-density polyethylene. The analysis of the isothermal currents revealed the presence of different traps whose characteristic (de)trapping times can be deduced. The isothermal procedures allowed the selective charging of the sample. By choosing the charging field and the ratio of charge/discharge times, non-isothermal analysis permitted the differentiation of three or four peaks (at approximate to50degreesC, approximate to65degreesC, approximate to70degreesC and approximate to85degreesC) associated with charge detrapping from surface or near-surface (\<20 mum) traps. These traps have activation energies between 0.21 and 1.54 eV. The mobility at 30degreesC is around 5 x 10(-16) m(2) V-1 s(-1). Samples had to be conditioned before each experiment in order to obtain reproducible results.

}, issn = {0022-3727}, url = {://000175435200002}, author = {Lanca, M. C. and Neagu, E. R. and Marat-Mendes, J. N.} } @article {7826, title = {Combined isothermal and non-isothermal current measurements applied to space charge studies in low-density polyethylene}, journal = {Journal of Physics D-Applied Physics}, volume = {35}, number = {8}, year = {2002}, note = {n/a}, pages = {L29-L32}, type = {Journal Article}, abstract = {

A new experimental procedure combining usual isothermal DC charging and discharging with non-isothermal current measurements has been recently proposed. It is mainly suitable for very high insulating polymers and it was successfully applied to the study of space charge trapping and transport in low-density polyethylene. The analysis of the isothermal currents revealed the presence of different traps whose characteristic (de)trapping times can be deduced. The isothermal procedures allowed the selective charging of the sample. By choosing the charging field and the ratio of charge/discharge times, non-isothermal analysis permitted the differentiation of three or four peaks (at approximate to50degreesC, approximate to65degreesC, approximate to70degreesC and approximate to85degreesC) associated with charge detrapping from surface or near-surface (\<20 mum) traps. These traps have activation energies between 0.21 and 1.54 eV. The mobility at 30degreesC is around 5 x 10(-16) m(2) V-1 s(-1). Samples had to be conditioned before each experiment in order to obtain reproducible results.

}, issn = {0022-3727}, url = {://000175435200002}, author = {Lanca, M. C. and Neagu, E. R. and Marat-Mendes, J. N.} } @inbook {4186, title = {Dielectric properties of electrically aged low density polyethylene}, booktitle = {Advanced Materials Forum I}, series = {Key Engineering Materials}, volume = {230-2}, year = {2002}, note = {

1st International Materials Symposium (Materials 2001) APR 09-11, 2001 COIMBRA, PORTUGAL

}, pages = {396-399}, type = {Book Section}, abstract = {

Low density polyethylene (LDPE) films kept in a sodium chloride aqueous solution, were aged under a high AC electrical field. The films were prepared from press moulding of LDPE pellets with small amounts of antioxidants. The dielectric spectra at 30 degreesC in the range of 10(-5) Hz to 105 Hz were obtained prior and after ageing. Three different experimental techniques were used to obtain the full spectrum. For the low frequency (LF) region (10(-5) Hz to 10(-1) Hz) the time domain technique was used (charge and discharge currents were also measured). The measuring device used for the 10(-1) Hz to 10(1) Hz medium frequency (MF) region was a lock-in amplifier. While for the high frequency (HF), 10(-1) Hz to 10(5) Hz, RLC bridge measurements were performed. Differences can be seen between aged and unaged PE. The region showing less changes with ageing is the MF region where the peak of the unaged samples seems to become less defined with ageing time. This peak is probably due to additives and impurities (such as antioxidants) that will tend to slowly diffuse out with time. The LF peak is a broad peak related to localised space charge injection driven by the electric field. This peak increases in an earlier stage of ageing decreasing afterwards possibly when the polymer becomes more conductive. Finally the HF shows the beginning of a peak due to gamma and beta transitions. The later is related to dipolar rotation of carbonyl groups in amorphous polymer regions, while the former is associated to crankshaft motions in the main polymer chain. This peak decreases with ageing disappearing for the most aged samples. This could also be explained if the sample becomes more conductive.

}, isbn = {1013-9826 0-87849-905-9}, url = {://000179553200091}, author = {Lanca, M. C. and Dias, C. J. and Dasgupta, D. K. and Marat-Mendes, J.}, editor = {Vieira, T.} } @inbook {7817, title = {Dielectric properties of electrically aged low density polyethylene}, booktitle = {Advanced Materials Forum I}, series = {Key Engineering Materials}, volume = {230-2}, year = {2002}, note = {

1st International Materials Symposium (Materials 2001) APR 09-11, 2001 COIMBRA, PORTUGAL

}, pages = {396-399}, type = {Book Section}, abstract = {

Low density polyethylene (LDPE) films kept in a sodium chloride aqueous solution, were aged under a high AC electrical field. The films were prepared from press moulding of LDPE pellets with small amounts of antioxidants. The dielectric spectra at 30 degreesC in the range of 10(-5) Hz to 105 Hz were obtained prior and after ageing. Three different experimental techniques were used to obtain the full spectrum. For the low frequency (LF) region (10(-5) Hz to 10(-1) Hz) the time domain technique was used (charge and discharge currents were also measured). The measuring device used for the 10(-1) Hz to 10(1) Hz medium frequency (MF) region was a lock-in amplifier. While for the high frequency (HF), 10(-1) Hz to 10(5) Hz, RLC bridge measurements were performed. Differences can be seen between aged and unaged PE. The region showing less changes with ageing is the MF region where the peak of the unaged samples seems to become less defined with ageing time. This peak is probably due to additives and impurities (such as antioxidants) that will tend to slowly diffuse out with time. The LF peak is a broad peak related to localised space charge injection driven by the electric field. This peak increases in an earlier stage of ageing decreasing afterwards possibly when the polymer becomes more conductive. Finally the HF shows the beginning of a peak due to gamma and beta transitions. The later is related to dipolar rotation of carbonyl groups in amorphous polymer regions, while the former is associated to crankshaft motions in the main polymer chain. This peak decreases with ageing disappearing for the most aged samples. This could also be explained if the sample becomes more conductive.

}, isbn = {1013-9826 0-87849-905-9}, url = {://000179553200091}, author = {Lanca, M. C. and Dias, C. J. and Dasgupta, D. K. and Marat-Mendes, J.}, editor = {Vieira, T.} } @inbook {4196, title = {Infrared spectroscopy studies of aged polymeric insulators}, booktitle = {Advanced Materials Forum I}, series = {Key Engineering Materials}, volume = {230-2}, year = {2002}, note = {

1st International Materials Symposium (Materials 2001) APR 09-11, 2001 COIMBRA, PORTUGAL

}, pages = {384-387}, type = {Book Section}, abstract = {

Thin films of low density polyethylene (LDPE) and crosslinked polyethylene (XLPE) were aged under an AC electric field while kept in sodium chloride aqueous solution. After aging the samples showed water trees (localized damaged with the appearance of hydrophilic ramified structures whose size ranges from a few microns to I mm). Some of the samples suffered dielectric breakdown showing small channels (1-2 mm. diameter) crossing the film and sometimes also signs of carbonization. In order to identify the oxidation mechanisms contributing to aging, FTIR was used to analyze both unaged and aged specimens. Comparing between unaged and aged LDPE an increase in the FTIR spectrum for bands at 1720 cm(-1), 1640 cm(-1) and 1590 cm(-1) was visible for the aged samples. The first region corresponds to carbonyl groups (C=O bonds) resulting from oxidation (most probably ketones). While the second one is related to carbon double bonds formed due to chain scission. Finally the third one is due to carboxylates. For the XLPE the analysis is more difficult. Besides aging it needs to be taken into account the by-products of crosslinking that will tend also to diffuse out with time. The main effect of aging is an increase in the concentration of 1640 cm(-1) band (C=C bonds). For the water treed regions dry and wet samples were compared. In the wet ones the absorbance is larger for the 3380 cm(-1) exhibiting, as expected, water absorption in the water treed regions (hydrophilic characteristics were increased).

}, isbn = {1013-9826 0-87849-905-9}, url = {://000179553200088}, author = {Mateo, J. and Lanca, M. C. and Marat-Mendes, J.}, editor = {Vieira, T.} } @inbook {7838, title = {Infrared spectroscopy studies of aged polymeric insulators}, booktitle = {Advanced Materials Forum I}, series = {Key Engineering Materials}, volume = {230-2}, year = {2002}, note = {

1st International Materials Symposium (Materials 2001) APR 09-11, 2001 COIMBRA, PORTUGAL

}, pages = {384-387}, type = {Book Section}, abstract = {

Thin films of low density polyethylene (LDPE) and crosslinked polyethylene (XLPE) were aged under an AC electric field while kept in sodium chloride aqueous solution. After aging the samples showed water trees (localized damaged with the appearance of hydrophilic ramified structures whose size ranges from a few microns to I mm). Some of the samples suffered dielectric breakdown showing small channels (1-2 mm. diameter) crossing the film and sometimes also signs of carbonization. In order to identify the oxidation mechanisms contributing to aging, FTIR was used to analyze both unaged and aged specimens. Comparing between unaged and aged LDPE an increase in the FTIR spectrum for bands at 1720 cm(-1), 1640 cm(-1) and 1590 cm(-1) was visible for the aged samples. The first region corresponds to carbonyl groups (C=O bonds) resulting from oxidation (most probably ketones). While the second one is related to carbon double bonds formed due to chain scission. Finally the third one is due to carboxylates. For the XLPE the analysis is more difficult. Besides aging it needs to be taken into account the by-products of crosslinking that will tend also to diffuse out with time. The main effect of aging is an increase in the concentration of 1640 cm(-1) band (C=C bonds). For the water treed regions dry and wet samples were compared. In the wet ones the absorbance is larger for the 3380 cm(-1) exhibiting, as expected, water absorption in the water treed regions (hydrophilic characteristics were increased).

}, isbn = {1013-9826 0-87849-905-9}, url = {://000179553200088}, author = {Mateo, J. and Lanca, M. C. and Marat-Mendes, J.}, editor = {Vieira, T.} } @book {4193, title = {Studies of space charge in electrically aged low density polyethylene}, year = {2002}, note = {

ISE 11 11th International Symposium on Electrets (ISE 11) OCT 01-03, 2002 MELBOURNE, AUSTRALIA

}, pages = {19-22}, type = {Book}, abstract = {

Space charge in electrically aged LDPE was studied using a recently developed technique combining isothermal charging and discharging with non-isothermal measurements. Samples were aged in a NaCl aqueous solution at 40degreesC for 1500h under an AC field of 6MV/m (50Hz). The samples were then isothermally DC charged and discharged (both currents recorded). Next a non-isothermal experiment with constant heating rate was performed. Finally the sample was kept at the highest temperature and the final isothermal discharge current registered. The last step has to be carried on for long time to ensure an almost complete discharge of the remnant charge so that results become reproducible and possible to analyze. Selective charging (careful choice of the field, temperature and the ratio of charging/discharging times) revealed the presence of different trapping sites. From the analysis of the isothermal and non-isothermal data the relaxation times and activation energies could be obtained.

}, isbn = {0-7803-7560-2}, url = {://000179971700006}, author = {Lanca, M. C. and Neagu, E. R. and Marat-Mendes, J. N.} } @book {7827, title = {Studies of space charge in electrically aged low density polyethylene}, year = {2002}, note = {

ISE 11 11th International Symposium on Electrets (ISE 11) OCT 01-03, 2002 MELBOURNE, AUSTRALIA

}, pages = {19-22}, type = {Book}, abstract = {

Space charge in electrically aged LDPE was studied using a recently developed technique combining isothermal charging and discharging with non-isothermal measurements. Samples were aged in a NaCl aqueous solution at 40degreesC for 1500h under an AC field of 6MV/m (50Hz). The samples were then isothermally DC charged and discharged (both currents recorded). Next a non-isothermal experiment with constant heating rate was performed. Finally the sample was kept at the highest temperature and the final isothermal discharge current registered. The last step has to be carried on for long time to ensure an almost complete discharge of the remnant charge so that results become reproducible and possible to analyze. Selective charging (careful choice of the field, temperature and the ratio of charging/discharging times) revealed the presence of different trapping sites. From the analysis of the isothermal and non-isothermal data the relaxation times and activation energies could be obtained.

}, isbn = {0-7803-7560-2}, url = {://000179971700006}, author = {Lanca, M. C. and Neagu, E. R. and Marat-Mendes, J. N.} } @article {Lanca_MC:2001, title = {The fractal analysis of water trees - An estimate of the fractal dimension}, journal = {IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION}, volume = {8}, year = {2001}, note = {n/a}, month = {Jan}, pages = {838-844}, publisher = {Univ Nova Lisboa, Univ Leicester}, abstract = {n/a}, author = {Lanca MC and Marat-Mendes JN and Dissado LA} } @article {4191, title = {The fractal analysis of water trees - An estimate of the fractal dimension}, journal = {Ieee Transactions on Dielectrics and Electrical Insulation}, volume = {8}, number = {5}, year = {2001}, note = {n/a}, pages = {838-844}, type = {Journal Article}, abstract = {

Water trees result from ac electrical aging of the polymeric insulation of medium and HV power cables in a humid or wet environment. As suggested by their name, they arise from penetration of water in the polymer. Visual observation with the help of an optical microscope shows tree (bush) type structures. This suggests that water trees might be fractal objects. Calculation of the fractal dimension from experimental samples may confirm the fractal characteristics and also give information on the damage caused to the polymer. In this work images of water trees taken under the optical microscope, dyed by methylene blue and etched for scanning electron microscopy (SEM), were studied in order to estimate the fractal dimension using a box-counting algorithm. The photographs, made using an optical microscope (scale of 100 mum), of the dyed samples were obtained from laboratory-aged low-density polyethylene (LDPE) specimens using accelerated techniques. Different field amplitude and frequency and also time of aging were used and the dimension values were compared. SEM images resulting from aged cross-linked polyethylene (XLPE) cables revealed a structure at a different scale (similar to 3 mum). Each photograph was analyzed to compare regions with and without water trees.

}, issn = {1070-9878}, url = {://000171826800014}, author = {Lanca, M. C. and Marat-Mendes, J. N. and Dissado, L. A.} } @article {7824, title = {The fractal analysis of water trees - An estimate of the fractal dimension}, journal = {Ieee Transactions on Dielectrics and Electrical Insulation}, volume = {8}, number = {5}, year = {2001}, note = {n/a}, pages = {838-844}, type = {Journal Article}, abstract = {

Water trees result from ac electrical aging of the polymeric insulation of medium and HV power cables in a humid or wet environment. As suggested by their name, they arise from penetration of water in the polymer. Visual observation with the help of an optical microscope shows tree (bush) type structures. This suggests that water trees might be fractal objects. Calculation of the fractal dimension from experimental samples may confirm the fractal characteristics and also give information on the damage caused to the polymer. In this work images of water trees taken under the optical microscope, dyed by methylene blue and etched for scanning electron microscopy (SEM), were studied in order to estimate the fractal dimension using a box-counting algorithm. The photographs, made using an optical microscope (scale of 100 mum), of the dyed samples were obtained from laboratory-aged low-density polyethylene (LDPE) specimens using accelerated techniques. Different field amplitude and frequency and also time of aging were used and the dimension values were compared. SEM images resulting from aged cross-linked polyethylene (XLPE) cables revealed a structure at a different scale (similar to 3 mum). Each photograph was analyzed to compare regions with and without water trees.

}, issn = {1070-9878}, doi = {DOI:}, url = {://000171826800014}, author = {Lanca, M. C. and Marat-Mendes, J. N. and Dissado, L. A.} } @article {Judeinstein_P:2000, title = {Pore dimension of water trees in PE: NMR studies}, journal = {POLYMER}, volume = {41}, year = {2000}, note = {n/a}, month = {Jan}, pages = {8151-8154}, publisher = {Univ Paris 11, Univ Nova Lisboa, Univ Paris 11, Univ Paris 11}, abstract = {n/a}, author = {Judeinstein P and Lanca MC and Marat-MendeS J and Rault J} } @article {4185, title = {Pore dimension of water trees in PE: NMR studies}, journal = {Polymer}, volume = {41}, number = {22}, year = {2000}, note = {n/a}, pages = {8151-8154}, type = {Journal Article}, abstract = {

In PE films aged under electric field the crystallisation of water (and melting of ice) has been studied by quadrupolar NMR, this technique allows one to determine the concentration of water as low as 10(-4). It is shown that the pore dimensions of the tracks forming the water trees of the order of 2.5 nm, are independent of the ageing time. The mobility of water in these water trees and in porous glass, of similar pore dimensions, are compared. (C) 2000 Elsevier Science Ltd. All rights reserved.

}, issn = {0032-3861}, url = {://000088312500027}, author = {Judeinstein, P. and Lanca, M. C. and Marat-Mendes, J. and Rault, J.} } @article {7814, title = {Pore dimension of water trees in PE: NMR studies}, journal = {Polymer}, volume = {41}, number = {22}, year = {2000}, note = {n/a}, pages = {8151-8154}, type = {Journal Article}, abstract = {

In PE films aged under electric field the crystallisation of water (and melting of ice) has been studied by quadrupolar NMR, this technique allows one to determine the concentration of water as low as 10(-4). It is shown that the pore dimensions of the tracks forming the water trees of the order of 2.5 nm, are independent of the ageing time. The mobility of water in these water trees and in porous glass, of similar pore dimensions, are compared. (C) 2000 Elsevier Science Ltd. All rights reserved.

}, issn = {0032-3861}, url = {://000088312500027}, author = {Judeinstein, P. and Lanca, M. C. and Marat-Mendes, J. and Rault, J.} } @book {4188, title = {Study of fractal properties in Lichtenberg figures}, year = {1995}, note = {

3rd IFIP Working Conference on Fractals in the Natural and Applied Sciences (FRACTAL 95) 1995 MARSEILLE, FRANCE

}, pages = {133-144}, type = {Book}, abstract = {n/a}, isbn = {0-412-71020-X}, url = {://A1995BE14K00014}, author = {Lanca, M. C. and Domingues, J. and Franco, I.} } @book {7819, title = {Study of fractal properties in Lichtenberg figures}, year = {1995}, note = {

3rd IFIP Working Conference on Fractals in the Natural and Applied Sciences (FRACTAL 95) 1995 MARSEILLE, FRANCE

}, pages = {133-144}, type = {Book}, abstract = {n/a}, isbn = {0-412-71020-X}, url = {://A1995BE14K00014}, author = {Lanca, M. C. and Domingues, J. and Franco, I.} } @book {4189, title = {SIMULATION OF THE FERROELECTRIC SWITCHING OF PVDF AND ITS COPOLYMERS}, year = {1992}, note = {

4TH INTERNATIONAL CONF ON CONDUCTION AND BREAKDOWN IN SOLID DIELECTRICS JUN 22-25, 1992 SESTRI LEVANTE, ITALY

}, pages = {62-66}, type = {Book}, abstract = {n/a}, isbn = {0-7803-0129-3}, url = {://A1992BX58U00013}, author = {Lanca, M. C. and Franco, I. and Mendes, J. M. and Ieee} } @book {7820, title = {SIMULATION OF THE FERROELECTRIC SWITCHING OF PVDF AND ITS COPOLYMERS}, year = {1992}, note = {

4TH INTERNATIONAL CONF ON CONDUCTION AND BREAKDOWN IN SOLID DIELECTRICS JUN 22-25, 1992 SESTRI LEVANTE, ITALY

}, pages = {62-66}, type = {Book}, abstract = {n/a}, isbn = {0-7803-0129-3}, url = {://A1992BX58U00013}, author = {Lanca, M. C. and Franco, I. and Mendes, J. M. and Ieee} } @article {4184, title = {CODES TO COMPUTE RELEVANT GAMMA-GAMMA AND GAMMA-X TRUE-COINCIDENCE LINES IN ABSOLUTE COUNTING OF GAMMA-RAYS WITH A LEPD}, journal = {Biological Trace Element Research}, volume = {26-7}, year = {1990}, note = {n/a}, pages = {33-41}, type = {Journal Article}, abstract = {n/a}, issn = {0163-4984}, url = {://A1990EM70100006}, author = {Freitas, M. C. and Lanca, M. C. and Carvalho, A. M. and Decorte, F.} } @article {7813, title = {CODES TO COMPUTE RELEVANT GAMMA-GAMMA AND GAMMA-X TRUE-COINCIDENCE LINES IN ABSOLUTE COUNTING OF GAMMA-RAYS WITH A LEPD}, journal = {Biological Trace Element Research}, volume = {26-7}, year = {1990}, note = {n/a}, pages = {33-41}, type = {Journal Article}, abstract = {n/a}, issn = {0163-4984}, url = {://A1990EM70100006}, author = {Freitas, M. C. and Lanca, M. C. and Carvalho, A. M. and Decorte, F.} }