Precise determination of phonon constants in lead-free monoclinic (K0.5Na0.5)NbO3 single crystals

A polarized Raman analysis of ferroelectric (K0.5Na0.5)NbO3 (KNN) single crystals is presented. The Raman modes of KNN single crystals are assigned to the monoclinic symmetry. Angular-dependent intensities of A', A\", and mixed A' + A\" phonons have been theoretically calculated and compared with the experimental data, allowing the precise determination of the Raman tensor coefficients for (non-leaking) modes in single-domain monoclinic KNN. This study is the basis for non-destructive assessments of domain distribution by Raman spectroscopy in KNN-based lead-free ferroelectrics. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License

Comparison of biaxial strength measured with the Ball-on-Three-Balls- and the Ring-on-Ring-test

The Ball-on-Three-Balls-test (B3B) and the Ring-on-Ring-test (RoR) were conducted on alumina discs and compared based on Weibull-Theory. The influence of various intermediate layers was evaluated. To support these findings, Finite-Element-Analysis was conducted to analyze the effects of deviations from ideal loading conditions. The influence of friction between sample and fixture and the effects of an inhomogeneous load distribution on the maximum stress were investigated. The experiments demonstrated that measuring corresponding strength values with both testing methods is possible. To properly asses the strength using the RoR-test, intermediate layers must be used. Teflon-foils or adhesive tapes are considered suitable. If no intermediate layer is used, the materials strength will be underestimated. Finite-Element-calculations show that this effect cannot be explained by the influence of friction and is rather caused by a non-homogeneous load distribution along the load-ring. Fractography supports these findings

A modelling approach to describe the DC current-voltage behaviour of low-voltage zinc oxide varistors

Zinc oxide varistors are among the most important surge arresters. The current-voltage (I–V) behaviour of these devices can be approximated by a model of electrical networks of grain boundaries arranged in 3D space, where nonlinear resistors mimic the effective grain boundary behaviour of the varistor microstructure. A simplified version of the network model, namely parallel circuits of chains of serially connected resistors of different length (path model), is derived from a fully 3D model of the polycrystalline microstructure. A comparison of the path model with a complex resistor network based on the 3D model demonstrates that the network of highly nonlinear resistors has a very similar DC I–V characteristic as parallel paths of these resistors connected in series. The path model, whose parameters were adapted from microscopic I–V measurements, is able to reproduce the macroscopic I–V behaviour of real low-voltage zinc oxide varistors

Varistor piezotronics: Mechanically tuned conductivity in varistors

The piezoelectric effect of ZnO has been investigated recently with the goal to modify metal/semiconductor Schottky-barriers and p-n-junctions by application of mechanical stress. This research area called “piezotronics” is so far focused on nano structured ZnO wires. At the same time, ZnO varistor materials are already widely utilized and may benefit from a piezotronic approach. In this instance, the grain boundary potential barriers in the ceramic can be tuned by mechanical stress. Polycrystalline varistors exhibit huge changes of resistivity upon applied electrical and mechanical fields and therefore offer descriptive model systems to study the piezotronic effect. If the influence of temperature is contemplated, our current mechanistic understanding can be interrogated and corroborated. In this paper, we present a physical model based on parallel conducting pathways. This affords qualitative and semi-quantitative rationalization of temperature dependent electrical properties. The investigations demonstrate that narrow conductive pathways contribute to the overall current, which becomes increasingly conductive with application of mechanical stress due to lowering of the barrier height. Rising temperature increases the thermionic current through the rest of the material with higher average potential barriers, which are hardly affected by the piezoelectric effect. Hence, relative changes in resistance due to application of stress are higher at low temperature