Carmo Lança

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 950ºC to 1150ºC 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.

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)).

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.

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.