title: Nonlinear Lock-In Infrared Microscopy: A Complementary Investigation Technique for the Analysis of Functional Electroceramic Components.
abstract: Using lock-in infrared microscopy as a tool for current detection on the micrometer scale in AC-driven specimens in combination with iterative grinding procedure allows preparation of current dominating microstructure regions on well-polished surfaces. This technique is applied successfully on varistor components based on specially doped ZnO-based varistor ceramics. This peculiar electroceramic material exhibits exceptional high nonlinear current-voltage (I-V) characteristics, described by a power law according I~V(α), caused by double Schottky barriers at the grain boundaries. As a novelty the thermographic response is used to evaluate local electrical properties, namely the nonlinearity coefficient α, on basis of higher order harmonics with respect to the basic electrical driving AC-frequency. To correlate the observed electrical properties to the microstructure, the polar crystal orientation of the relevant ZnO grains is determined by combining electron backscatter diffraction and orientation-dependent patterns as a result of a chemical etching procedure. These findings support a modified new model for describing the grain boundary controlled current flow in a varistor microstructure including orientation-dependent barrier properties. Hence, the experimentally observed current direction-dependent behavior can be described consistently.
authors.affiliation: 1Institut für Struktur- und Funktionskeramik,Montanuniversitaet Leoben,8700 Leoben,Austria.
authors.forename: Michael
authors.initials: M
authors.lastname: Hofstätter
authors.affiliation: 1Institut für Struktur- und Funktionskeramik,Montanuniversitaet Leoben,8700 Leoben,Austria.
authors.forename: Nadine
authors.initials: N
authors.lastname: Raidl
authors.affiliation: 2Materials Center Leoben Forschung GmbH,Roseggerstraße 12,8700 Leoben,Austria.
authors.forename: Bernhard
authors.initials: B
authors.lastname: Sartory
authors.affiliation: 1Institut für Struktur- und Funktionskeramik,Montanuniversitaet Leoben,8700 Leoben,Austria.
authors.forename: Peter
authors.initials: P
authors.lastname: Supancic
keywords: Schottky barriers; electroceramic materials; thermography; varistor; zinc oxide