Characterization of ZnO varistor degradation using lifetime positron-annihilation spectroscopy

Positron-annihilation spectroscopy (PAS) was used in the lifetime mode to study changes in the grain-boundary defect equilibrium associated with dc-bias-induced degradation of a ZnO varistor. The PAS lifetime spectra were collected while the sample was under an applied bias ranging from 100 V (approximately 400 V/cm) to 500 V (approximately 2000 V/cm). The current through the sample was continuously monitored. The simple trapping model was used to interpret the lifetime PAS results and to obtain an estimate for the positron-capture rate. The experimental results show that an increase in the bias voltage results in a decrease in the positron-trap density and an increase in the positron lifetime associated with the dominant positron trap. These results are explained on the basis of a decrease in the concentration of negatively charged zinc vacancies at the grain boundary. The PAS results support the ion-migration model for degradation, which suggests that the bias-induced migration of positively charged zinc interstitials to the grain boundary reduces the concentration of negatively charged zinc vacancies at the boundary. This results in a reduction in the barrier height (degradation) and is consistent with the PAS data.

Title	Characterization of ZnO varistor degradation using lifetime positron-annihilation spectroscopy
Creator	Schaffer, James P.
	Rohatgi, A.
	Ramanachalam, M. S.
	Gupta, T. K.
Publisher	Journal of Applied Physics
Academic Department	Chemical & Biomolecular Engineering
Division	Engineering
Organization	Lafayette College
Date Issued	June 15, 1991
Date Available	2013-01-22T20:03:22Z
Type	Article
Language	English
Bibliographic Citation	Ramanachalam, M. S., et al. (1991) "Characterization of ZnO varistor degradation using lifetime positron-annihilation spectroscopy." Journal of Applied Physics 69 (12): 8380-8386.
Standard Identifier	DOI 10.1063/1.347402
Standard Identifier	Handle 10385/1049
Permalink	http://hdl.handle.net/10385/1049
Rights Statement	In Copyright
Rights Holders	American Institute of Physics