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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.

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.

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.

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.

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.

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.

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.

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).

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).

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)).