Piezotronischer Effekt in Zinkoxid-Varistoren

Translated title of the contribution: Piezotronic effect in Zinc oxide varistors

Thomas Billovits

Research output: ThesisMaster's Thesis

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Abstract

Varistors are electrical components whose resistance depends on the applied voltage. When a certain electrical switching voltage is exceeded, the conductivity increases by several orders of magnitude within nanoseconds. This behavior is made possible by a doped, polycrystalline zinc oxide, which forms the base material for varistors. A specially adjusted structure forms negatively charged electrical barriers at the grain boundaries of the zinc oxide, which are referred to as Double Schottky Barriers. If the applied electrical voltage exceeds the switching voltage, electrons can overcome these barriers. On the other side of the barrier, the electrons accelerate and generate electron hole pairs through impact ionization. By recombining these holes with the negative grain boundary charge, an avalanche-like effect sets in and the a strongly non linear current-voltage behavior sets in. In addition, zinc oxide has piezoelectricity. As a result, additional electrical charges can be generated at the grain boundaries by mechanical tension, which change the switching behavior of the varistor. One speaks here of the so-called piezotronic effect, the influencing of electronic properties by mechanical pressure. The aim of this work is to determine the change in electrical behavior under the influence of mechanical tension. The experiments are carried out on two variants of zinc oxide varistor ceramics, one with praseodymium and one with bismuth. First, the macroscopic behavior of the entire varistor under uniaxial pressure is examined. For this purpose, the sample is clamped in two different configurations in order to record the direction-dependent effect of the mechanical tension on the current voltage characteristic. In order to investigate the piezotronic effect on a microscopic level, the current voltage characteristics of individual grain boundaries are recorded under biaxial pressure. For this purpose, a thin section of the structure is produced and glued to a piezo actuator, which acts as a strain controlled carrier. Two adjacent grains are contacted by means of micromanipulators and the I-V characteristic is recorded when the piezo actuator is activated to different extents. The grains examined are analyzed by EBSD in order to be able to correlate the shift of the I-V characteristic with the orientations of the grains to one another.
Translated title of the contributionPiezotronic effect in Zinc oxide varistors
Original languageGerman
QualificationDipl.-Ing.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Supancic, Peter Hans, Supervisor (internal)
Award date1 Jul 2020
Publication statusPublished - 2020

Bibliographical note

embargoed until null

Keywords

  • varistor
  • zinc oxide
  • I-V charakteristics
  • piezotronic

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