Improved stability of the ZnO varistor via donor and acceptor doping at the grain boundary
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(Received 11 September 1987; accepted 29 February 1988) The ZnO varistor degradation has been attributed to the field-assisted, temperature-activated diffusion of interstitial zinc in the depletion layer. To improve stability, one approach is to reduce the formation of interstitials, and then further, to prevent their migration through empty interstitial sites. Based on this concept, an amphoteric dopant, such as Na or K, has been incorporated in the ZnO varistor grain boundary wherein a dopant is substituted both in the lattice and in the interstitial sites. A grain boundary defect model has been developed for this dual mode of substitution, with the dopant acting as an acceptor at the lattice site and as a donor at the interstitial site. Under these conditions, and given a desired neutrality range, the concentration of zinc interstitial is indeed shown to be reduced and stability greatly improved. The experimental data presented here validate the grain boundary defect model presented in this and in an earlier paper [ J. Mater. Sci. 20,3487 (1985) ].
I. INTRODUCTION
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Disordered layer Grain
Grain Negative surface charge layer
Atomic defect model
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FIG. 1. The atomic defect model proposed for the Schottky barrier at the grain boundary; also shown in the figure is the analogy with band model (Ref. 1).
0003-6951 /88/040745-10$01.75
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Positive depletion layer
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J. Mater. Res. 3 (4), Jul/Aug 1988
Schottky barrier model
Mill
A varistor's life1"3 is determined by a degradation phenomenon4"9 that is strongly influenced by several materials and design parameters. 10 This phenomenon has been attributed to the migration of ions in the depletion layer11 in the region of the grain boundary. The predominant migrating ion has been concluded to be the zinc interstitial, Zn), formed and "frozen-in" in the depletion layer during cooling from the fabrication temperature. 1213 Based on this concept, a grain boundary defect model has been developed1 to explain the stability/instability in the ZnO varistor. It has been shown that the instability arises as a result of a field-assisted diffusion of zinc interstitials in the depletion layer followed by chemical interaction with the grain boundary defects—a process that leads to a decreased barrier height and an increased leakage current with time.14 A large body of experimental data was presented to support this concept. Furthermore, it has been shown that when the zinc interstitials in the depletion layer are permanently out-diffused via thermal anneal, the varistors show improved stability.l Again, a series of quasichemical reactions were derived to illustrate the diffusion and chemical interaction that accompany thermal anneal. Supporting experimental evidences were presented to substantiate the model. For quick reference, this grain boundary
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