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The ultrasound attenuation in langasite crystals increases rapidly at about 800 K with increasing temperature for reasons that are not well understood. In this paper, the attenuation quantified as $Q^{-1}$ of the langasite-type materials La3Ga5SiO14 (LGS) and La3Ta0.5Ga5.5O14 (LGT) was studied from room temperature to 1653 and 1608 K, respectively, using resonant ultrasound spectroscopy. Two to three attenuation peaks can be seen. A change of the magnitudes of the largest two attenuation peaks in LGT was correlated with the changing color of an LGT sample, which is related to its oxygen vacancy concentration. Thus, the attenuation likely involves oxygen vacancies. The observed $Q^{-1}$ can be explained well by a model based on the anelastic relaxation of two to three noninteracting point defects causing Debye peak-like attenuation maxima. The activation energies of the largest two relaxation peaks match the activation energies for different conductivity mechanisms in LGS and LGT. Thus, the oxygen movement-based conductivity and the relaxation processes seem to involve the exchange of ions and vacancies on the same positions. The largest two attenuation peaks are probably caused by the movement of ions induced by two different phenomena, the deformation of the lattice (point-defect relaxation) on the one hand and the electric field via the piezoelectric effect (piezoelectric/carrier relaxation) on the other hand. [ABSTRACT FROM AUTHOR]

In this paper, to understand the insulation recovery phenomenon of water-tree cables when an applied voltage is removed, the insulation recovery mechanism of the water tree cable is analyzed. A water-needle electrode is used in an accelerated water-tree aging experiment for XLPE (Cross Linked Polyethylene) cables. The dielectric loss factor (tan?) increases with aging time, but it gradually returns to a lower level once the applied voltage is removed for some time, which means that the insulation recovery of the water tree cables possibly occurs during the process. Depending on the geometric size by micro observations of water-tree slices, a watertree model is constructed for electric field simulation, which includes a series of water-filled micro voids and interconnected channels. According to analysis of the electric field force and the mechanical properties of water tree region, the force generated by the elastic deformation of the molecular chains results in the shrinking of the channels. The water is gradually squeezed out of the channels, which causes the gradual decrease of conductivity and relative dielectric constant in channels. According to the computed results of tan? at different states of water tree, the reduction of tan? can be mainly caused by the decrease of conductivity and relative dielectric constant in channels. The time needed for insulation recovery may be determined by the internal electric field in the water tree, the mechanical relaxation time of XLPE, and the diffusion of water. [ABSTRACT FROM PUBLISHER]