With thermally stimulated depolarization currents, we researched the relaxations of crosslinked polyethylene as it is used in medium-voltage cable insulation. Through conventional polarization two heteropolar peaks stand up in the spectra, at 80 and 105°C. As the sample is annealed, a homopolar peak is developed at about 99°C. With window polarization, our results indicated that the 80°C peak is a structured peak related to polar crosslinking subproducts and impurities. The 105 and 99°C peaks are fitted to the general kinetic-order model because the 105°C peak is related to free-charge detrapping at the crystalline phase, in the bulk and maybe at the amorphous-crystal interphases, and the peak that is observed at 99°C is due to injected charge. Annealing at high temperatures promotes the creation of traps in the material. Charge trapping at T Ͻ 70°C seems to be related to the increased insulator resistivity with annealing time.

We studied the space charge behavior in crosslinked polyethylene cable insulation with thermally stimulated depolarization current (TSDC). The role of the semiconducting layers of the cable in space charge formation was determined with several experiments carried out with two different cables. TSDC spectra were obtained through the exchange of the semiconducting layers of each cable with aluminum or silver coatings. Heteropolar current peaks at 80 and 105 °C were observed in the TSDC spectrum. As a sample was annealed at high temperatures, the creation of traps in the material was promoted, and a homopolar contribution at 98 °C developed. This homopolar mechanism was linked to the diffusion, with annealing, of ionic components from the semiconducting layers into the insulation bulk. The penetration depth of these components was studied with Fourier transform infrared in sections obtained at several depths starting at the outer surface of the cable insulation. These diffused components remained on the crystal surfaces, and they became trapping centers for free charge. Charge injection from the semiconducting electrodes and homopolar trapping of injected charges on these components took place afterwards during polarization. This process tended to saturation. Besides this mechanism, the continuous dissociation of molecules took place in the material with thermal annealing. This ionic charge generation resulted in an increase in the heteropolar contribution of the TSDC spectrum. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4164–4174, 2004

A uniaxial compressive stress significantly modifies the dielectric and piezoelectric response of Pb(Zr,Ti)O3 (PZT)‐based piezoceramics. In soft PZT, the pre‐stress decreases both the piezoelectric coefficient and its increment with increasing dynamical stress amplitude. However, an increase of both quantities is shown in hard PZT. Similar effects are observed in the dielectric response. It is proposed that the differences between such behaviors are due to the possibility of domain coalescence in the materials whose domain walls have high mobility. Furthermore, the enhancement of the response in hard PZT could be explained by change of the interaction energy between complex defects and domain walls induced by the compression.

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Several semiconducting (SC) shields used in medium voltage cables were analyzed. Besides, four experimental SC shields were prepared in the laboratory and were studied as well. PEA measurements were performed in LDPE films, in contact with the SC shields, with applied fields up to 140 MV/m. These measurements show the presence of a threshold electric field, above 100 MV/m, from which charge packets formation is observed. Packed size and its evolution, however, is quite different depending on the SC shield. In four cases the packed charge size show a decrease as it propagates, not observed in the other cases. Propagation speed is as well notably dependent on the SC shield with transit speeds from 1.4xl0-7 m/s to 4x10-7 m/s. With regard of packed size, the biggest charge packet observed is almost five times bigger than the smallest one. Obtained TSDC spectra are in good agreement with all these results. Some correlation is obtained between the 'packet generation ability' and the nature and composition of the SC shields. Break-down tests in three of the original cables show good correlation with free charge measurements. The presence of free charge shows up as the main cause of poor breakdown results. Infrared spectroscopy show an increase of diffusion of some components from the SC shield into the LDPE in the case of the poorest test results.