Grain-boundary characterization of ZnO varistors by positron annihilation spectroscopy

Abstract

Positron annihilation spectroscopy (PAS) was performed in a Doppler mode to characterize the negatively charged grain-boundary defect, V′Zn, in ZnO varistor. The PAS study was conducted as a function of annealing treatment. As the annealing temperature increases from 400 to 600 °C the concentration of VZn increases, at intermediate temperatures from 600 to 800 °C the V′Zn concentration decreases, and finally, beyond 800 °C it increases again. These results are explained in terms of a grain-boundary defect model presented earlier [T. K. Gupta and W. G. Carlson, J. Mater. Sci. 20, 3487 (1987)]. The effect of quenching on PAS response was also explained in terms of a defect model.

Role of defects in tailoring structural, electrical and optical properties of ZnO

“…Quenching from high temperature is the process where we can freeze the high temperature disordered state in a material. As we discussed earlier in this section that for polycrystalline ZnO, this process induces a different defective state within the grain and in the grain boundary [115,164]. Here, it is also evident from the reduced stain in the quenched Nano-ZnO-III sample with respect to 4 h annealed ZnO (Figs.…”

“…This indeed indicates that the process of quenching involves a grain size-dependent defect structure. Gupta et al [115] predicted that such phenomena might occur due to different cooling rate (and hence different equilibrium defect concentration) of the grain interior and the grain boundary regions. Interestingly, Olsson et al [164] have found the absence of insulating barrier at the grain surfaces due to quenching from high temperature.…”

“…Suitable choice of temperature [66,[109][110][111], time [112,113] and annealing ambient environment [76,111,114], rate of heating and cooling of annealing [22,115,116] has been proved beneficial to monitor material properties in a controllable fashion. However, an effective control over thermally generated unwanted defects could not be established till date.…”

Influence of Sm2O3 on electrical performance of (Nb, Si)-doped TiO2-based varistor

“…The varistor action observed in ZnO ceramics is explained by the presence of Schottky type energy barrier [21] at the grain boundaries. In analogy to the model for a ZnO based varistor [22], the potential barriers for TiO 2 -based varistor created from the phase segregations. A depletion layer is formed by positively charged defects induced by the incorporation of niobium.…”

Improvement in electrical stability of ZnO varistors by infiltration of molten Bi2O3

“…Several mechanisms have been proposed to account for the degradation, including electron trapping, dipole orientation, ion migration and oxygen desorption, among which the ion migration mechanism has been found supportive on the basis of experimental evidences [9][10][11]. It was believed that the degradation is associated with the migration of the positively charged zinc interstitial ions Zn i • in the depletion layers [12][13][14][15][16]. According to ion migration mechanism, efforts have previously been made to improve the electrical stability of ZnO varistor by annealing processes at temperatures from 600 to 800 • C [9], and Gupta and Carlson [17] reported the sample heat-treated at 600 • C exhibited the best electrical stability.…”