Kinetic field approach to study liquid phase sintering of ZnO based ceramics Friedrich Raether * , Mohammad Lutful Arefin Fraunhofer-Institut Silicatforschung (ISC), Neunerplatz 2, 97082 Wu ¨rzburg, Germany Received 28 December 2009; received in revised form 10 January 2010; accepted 2 February 2010 Available online 1 March 2010 Abstract Liquid phase sintering kinetics in the system ZnO–Bi 2 O 3 –Sb 2 O 3 was studied using closed crucibles and an optical dilatometer. A modified kinetic field technique was applied for the first time to investigate the densification rates. The values obtained were assessed with existing liquid phase sintering models. Grain growth data were derived from the kinetic field diagram and compared to those obtained from microstructure analysis of quenched samples. Good agreement was obtained between both techniques. Values for both the activation energies (activation energies for grain growth and densification) were also reported for the ZnO–Bi 2 O 3 –Sb 2 O 3 system for the first time. In the initial sintering stage mechanisms were identified which retard densification and are essentially unaffected by temperature. It was shown how the position and slope of the iso-strain lines in the modified kinetic field diagram can be used for a qualitative understanding of the interaction of coarsening, liquid redistribution and densification during sintering. # 2010 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: E. Varistors; Liquid phase sintering; Optical dilatometer; Kinetic field analysis 1. Introduction Since the early seventies, when it was first reported by Matsuoka [1], ZnO has become the most popular industrial ceramic for metal oxide varistor (MOV) fabrication. The varistor functionality results from the semiconductive ZnO grains, separated by a dielectric grain boundary layer [2]. It has been agreed that varistor functionality in MOVs generally result from Schottkey barriers formed at the grain boundaries [3,4]. The most important component for varistor activity across the ZnO-grain boundaries is bismuth oxide which creates a thin layer in the nano to micron range around ZnO grains. The voltage at the grain boundary where the switching from linear current–voltage characteristic to break down takes place is approximately 3.0–3.6 V—almost independent of the composition of the varistor [5,6]. Therefore, the macroscopic break down voltage of a varistor depends on the number of grain boundaries, i.e. its grain size and its length. A homogenous grain size distribution is aimed at to avoid break down along any favoured current paths which reduces the lifetime of varistors [7]. A zinc-antimonate spinel (Zn 7 Sb 2 O 12 ) [8,9] is typically used to reduce grain growth in ZnO based varistor ceramics [2]. The spinel is formed from antimony oxides added as sintering aid. The grain growth controlling mechanism was related to the Zener drag process involving spinel particles located at the ZnO grain boundaries [2]. The ternary system ZnO–Bi 2 O 3 –Sb 2 O 3 shows a number of microstructural and phase changes during sintering. Bi 2 O 3 forms a liquid by eutectic melting with ZnO at 738 8C [10], which enables liquid phase sintering. Sb 2 O 3 , Bi 2 O 3 and ZnO form a pyrochlore phase at temperatures between 500 and 700 8C [8]. The pyrochlore phase decomposes at higher temperatures (>1000 8C) to form the spinel and additional melt phase [11]. The most important densification mechanisms during liquid phase sintering are contact flattening and Ostwald ripening. Both mechanisms are active during solution and reprecipitation in the intermediate sintering stage. They are described by equations of identical structure [12,13]:  DL L 0 ¼ CD l g l G n T   1=3 t 1=3 ; D ¼ D 0 exp  E D RT   ; G m ¼ G m 0 þ k 0 exp  E G RT   t; (1) where DL/L 0 = sintering shrinkage, C = constant depending on sintering state, D l = diffusion coefficient of atoms of solid www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 36 (2010) 1429–1437 * Corresponding author. Tel.: +49 0931 4100 200; fax: +49 0931 4100 299. E-mail address: friedrich.raether@isc.fraunhofer.de (F. Raether). 0272-8842/$36.00 # 2010 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2010.02.002