Inversion boundary induced grain growth in ZnO ceramics: from atomic-scale investigations to microstructural engineering

Abstract

Abstract. In semiconducting materials special boundaries play the key role in crystal growth. They introduce an abrupt structural and chemical anisotropy, which is readily reflected in an unusual microstructure evolution, whereas their local structure affects the physical properties of semiconducting materials. These effects, however, can be exploited to tailor the electronic and optical properties of the materials, as demonstrated in this review. The presented topic fits in the field of preparatory stage of phase transformations, manifested through evolution of chemically induced structural faults. In the noncentrosymmetric structure of ZnO, inversion boundaries (IBs) are the most common type of planar faults that can be triggered by the addition of specific spinel-forming dopants (Sb 2 O 3 , SnO 2 , TiO 2 ). In addition to conventional HRTEM techniques several new methods were developed to resolve crystallography and atomic-scale chemistry of IBs. The absolute orientation of the polar c-axes on both sides of the IB was determined by a novel quantitative microdiffraction method, providing a reliable identification of crystal polarity in noncentrosymmetric crystals. To determine sub-monolayer quantities of dopants on the IB, we developed a special technique of analytical electron microscopy using concentric electron probe (CEP) in EDS or EELS mode, providing more accurate and precise results than any other available technique. Knowing the local crystal chemistry of IBs we were able to design experiments to identify their formation mechanism. IBs nucleate in the early stage of grain growth as a dopant-rich topotaxial 2D reaction product on Zn-terminated surfaces of ZnO grains. Soon after their nucleation, ZnO is epitaxially grown on the inherent 2D phase in an inverted orientation, which effectively starts to dictate anisotropic growth of the infected crystallite. In very short time the grains with IBs dominate the entire microstructure in ZnO ceramics via IB-induced exaggerated grain growth mechanism. This phenomenon was used to design physical properties of ZnO-based varistor ceramics, whereas the bottom-up approach demonstrated here provides the basic tool for microstructural engineering of functional materials in virtually any system that is prone to formation of special boundaries.

Evaluation of Chip Formation Mechanisms in the Turning of Sintered ZnO Electro-Ceramics

“…In order to enhance the threshold voltage and the surge current withstanding capability, the number of intergranular layers in series and parallel, respectively, have to be enhanced. Therefore, for high surge protection applications, a finer ZnO grain size and a high grain boundary surface area are prerequisites [24][25][26][27][28][29] This difference in the acceleration and the frequency FFT signals for the top and bottom parts further required microscopical characterization to determine the plausible changes in the machined surfaces as a result of the variation in feed rates, which is discussed in the ensuing section.…”

“…In order to enhance the threshold voltage and the surge current withstanding capability, the number of intergranular layers in series and parallel, respectively, have to be enhanced. Therefore, for high surge protection applications, a finer ZnO grain size and a high grain boundary surface area are prerequisites [24][25][26][27][28][29] [22,[26][27][28][29], which in a pragmatic sense defines the high voltage transient protection of MOVs [25].…”

“…Within the composition, Sb 2 O 3 is added to suppress ZnO grain growth and to enhance the solubility of Zn ions in the Bi 2 O 3 -rich phase at 740 • C eutectic conditions. The presence of Sb 2 O 3 inclines the transformation thermodynamics towards the Zn 7 Sb 2 O 12 spinel phase above 900 • C and this formation is catalyzed by the decomposition of the Bi 3 Zn 3 Sb 3 O 14 pyrochlore phase above this temperature range during sintering [27,28]. The spinel phase appears as light greyish regions surrounded by the bright Bi 2 O 3 -rich intergranular phase as indicated by the 'S' phase in Figure 9a,b, whereas the pyrochlore phase can be present as scattered whitish precipitates along the grain boundaries and is partially decomposed in the spinel phase.…”

Synergistic Effects of Pr6O11 and Co3O4 on Electrical and Microstructure Features of ZnO-BaTiO3 Varistor Ceramics

“…The employment of dopant is significant since the varistor performance is sensitive to the presence of dopant additives, even though in a small proportion of their composition [ 14 , 15 ]. Some dopants have their properties in modifying varistor microstructures by either inhibiting the grain growth in the application of high voltage causing it to have a smaller average grain size, or exhibit grain growth and induced a larger average grain size suitable for low voltage application [ 16 ].…”

Microstructure and Electrical Properties of Low-Voltage Barium Titanate Doped Zinc Oxide Varistor Ceramics

“…Such notion is due to the varistor performance is very sensitive to the presence of some dopants even though their content is very low [2]. The additive plays a vital role in inhibit grain growth in the application of high voltage varistor and exhibit grain growth for low voltage varistor [3]. Previously, the varistor composition has been developed by Matsuoka [4], with five additive oxides to improve the nonlinear electrical characteristics of Bi 2 O 3 -based ZnO varistors.…”