The effects of heat treatment on B 2 O 3 -contained ZnO varistor
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The effects of annealing on B2O3-contained ZnO varistors with particular emphasis on the degradation behavior, nonlinear coefficient, trap density, donor density, and Schottky energy barrier were investigated. The thermal stability of ZnO varistors was considerably improved by the thermal annealing process.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
Zinc oxide (ZnO) ceramics with several additives are employed as varistor materials because of their highly nonohmic properties and excellent surge withstanding capability. 12 Unfortunately, ZnO varistor materials are subject to various stresses (electrical, thermal, chemical, and mechanical), which can affect their performance and useful life.3 The ZnO ceramics have a tendency of increasing in current with increasing time. One of the most serious problems for the application is, therefore, their degradation at bias voltage. The degradation phenomena of ZnO have been studied under ac, dc, and pulse electric field. The mechanisms that have been proposed to account for this phenomena include electron trapping, dipole orientation, ion migration, and oxygen desorption.2'4 The model of grain-boundary defects comprising the Schottky barriers is probably the most generally accepted.5 Iga et al. have reported that heat-treated nonohmic ZnO ceramics exhibited less current creep.14 It has been shown that when an otherwise unstable varistor was heated for several hours in an oxidizing atmosphere, preferably at 600 °C to 800 °C, the varistor became stable with respect to time.6'7 Gupta and Carlson8 suggested that this is because the zinc interstitials comprising the metastable components of the barrier were out-diffused via thermal annealing, and permanently removed by chemical reaction at the grain boundary, thus producing a grain boundary junction that was more stable after annealing than before.8 The primary purpose of this study was to investigate the effects of annealing on varistors with particular emphasis on the degradation behavior, nonlinear coefficient, trap density, donor density, and Schottky energy barrier. Since boron oxide has been reported to have substantial influence in breakdown voltage and degradation properties,8'9 its effect on the degradation behaviors was also investigated.
A. Preparation of samples
1526 http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 6, Jun 1994 Downloaded: 14 Mar 2015
Powder mixtures were prepared with the compositions as shown in the following formula: (100 - X)ZnO +
X/l
(Bi 2 O 3 + 2Sb 2 O 3 + 2CoO + Cr 2 O 3 + MnO) (1) Raw materials comprising ZnO, Bi 2 O 3 , Sb 2 O 3 , CoO, and Cr 2 O 3 , and MnO were mixed in de-ionized water and milled with high purity alumina balls for 4 h. They were then calcined at 800 °C for 2 h and ground using a mortar and pestle. Boron oxide (up to 5 wt. %) and poly vinyl alcohol (3 wt. %, used as binder) were added in, milled for another 24 h, and pelletized by sieving through a 100 mesh screen. The pellets were then pressed into a disk shape with 12.8 mm in diameter and 1.5 mm in thicknes
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