Cimon Anastasiadis

Low Temperature Dielectric Relaxations in ZnO Varistor

The main purpose of this work is to study the behaviour of ZnO varistor by means of dielectric relaxation spectroscopy at the low temperature range. The complex frequency spectrum of dielectric losses has been studied in detail and analysed carefully by fitting of a sum of Havriliak–Negami expression. Three relaxation processes are studied here, exhibiting a very strong Cole–Davidson behaviour. The faster relaxation process shows an unusual thermal behaviour. Initially, its relaxation time increases as the temperature rises but later it becomes thermally activated. The transition temperature of thermal behaviour of this process was found very close to the characteristic value of 0.4θD=160 K, where θD is the Debye temperature of ZnO. The de-activation energy of this process has been estimated by using the Arrhenius type relation to be 78 meV, a value very close to the higher longitudinal optical (LO) phonon energy in ZnO, 72 meV. This is a strong indication that the predicted Holstein transition between large to small-polaron motion should be observed in ZnO. The two remaining relaxations are electronic processes, the slower one is associated with the doubly ionised zinc interstitial Zni••, while the other should be related to defects induced by the dopants, or of complexes of intrinsic native defect with dopant impurities.

Modelling of electric signals stimulated by bending of rock beams

Experimental laboratory work was conducted to study the electric signals of marble beams that have been subjected to three-point bend tests. The specimens suffered 12 abrupt loading steps up to failure and the corresponding Bending Stimulated Current (BSC) was recorded by means of copper electrodes placed on the lower surface of the sample. Alongside the application of abrupt loading to the samples, BSC peaks were observed and the current followed an exponentially decreasing relaxation process. In this paper the BSC peaks, the total charge and the characteristic relaxation time variables are correlated with each other and are analysed towards the applied loading. The ultimate purpose of this work is the evaluation of the fatigue of the sample using simple criteria.

Probing the electrical properties of the Si nitride/Si interface

The present publication employs Dielectric Relaxation Spectroscopy for the examination of the relaxation mechanisms in silicon nitride MIS structures. These results are combined with capacitance and conductance measurements in order to give a more complete picture of the dielectric behaviour of silicon nitride. The analysis concludes with a method based on Dielectric Relaxation Spectroscopy for the calculation of the depletion layer width.

Rock damage estimation with dielectric loss (tan δ) measurements

In addition to existing non-destructive testing methods a novel technique is presented here. It is based on Dielectric Spectroscopy (DS) methodology for the verification of mechanical damages in geomaterial structures. Such techniques do not influence the material structure or properties, thus it can be re-examined with various methods. In this work damages are introduced in a geomaterial by uniaxial stress isothermally. The damage caused to marble samples and its influences on the dielectric loss angle were examined relative to stress. Results indicate that when stress produces microcracks then tan δ increases and a loss peak appears in the studied frequency range.