Improvement on the degradation of microwave sintered ZnO varistors by postannealing
- PDF / 305,132 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 15 Downloads / 203 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Improvement on the degradation of microwave sintered ZnO varistors by postannealing Chang-Shun Chen Department of Mechanical Engineering, HWA-HSIA College of Technology and Commerce, Taipei 235, Taiwan, Republic of China
Cheng-Tzu Kuo Institute of Materials Science and Engineering, National Chiao-Tung University, Hsinchu 30050, Taiwan, Republic of China
I-Nan Lin Materials Science Center, National Tsing-Hua University, Hsinchu 30043, Taiwan, Republic of China (Received 25 September 1996; accepted 7 July 1997)
The microwave sintering process not only densified the ZnO materials in a higher rate, but also resulted in significantly better varistor characteristics. Large nonlinear coefficient and low leakage current density were attained by cooling the samples under a rate of 80 ±Cymin after sintering, followed by 600 ±C postannealing for 60 min under oxygen atmosphere. Inappropriate annealing deteriorated the varistor characteristics that can either be attributed to the insufficient reoxidation along grain boundaries when annealed in N2 (or air) or loss of Zn species in these regions when annealed at 750 ±C (900 ±C). By contrast, the degradation behavior of these materials can be improved by the annealing process regardless of the annealing atmosphere or temperature.
I. INTRODUCTION
The ZnO-based ceramics containing donors, sintering aids, and microstructural stabilizers1–3 have been widely used as transient surge suppressors for protecting electrical circuits,4,5 because of their highly nonlinear voltage (V)-current (I) characteristics and excellent surge-withstand capability. The non-ohmic properties of these materials are sensitive to the postsintering process even when the samples have been properly sintered,6 –8 which was attributed to the modification on the defect chemistry of the depletion region of the materials. The degradation of varistor characteristics described by the increase in leakage current density with operation time is the behavior of prime concern in device applications, besides the non-ohmic property of the ZnO varistors.9,10 Among the models proposed to account for the degradation mechanisms, the most acceptable one is due to Gupta and Carlson,11 based on the migration of zinc interstitials present in the depletion region under the action of electric stress. Several approaches8,11–15 have been attempted to improve the degradation behavior of ZnO varistors. Among them, heat treatment after sintering can effectively improve the stability of the materials,16 –18 which was assumed to result from the out diffusion of interstitial zinc (Zni ).19 In this work, the donor-doped ZnO materials were densified by the microwave sintering process, since this process can enhance the densification rate for ceramic materials.20 –22 The effect of postsintering process, including the cooling rate and annealing conditions, on 1560
http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 6, Jun 1998
Downloaded: 11 Mar 2015
their varistor characteristics, especi
Data Loading...
