Jiwei Fan

Deep level transient spectroscopy of SnO2-based varistors

Deep level transient spectroscopy measurements were performed to investigate the effect of Nb2O5 and Cr2O3 on the electronic states of SnO2-based varistors. Two electron traps, Ec−0.30(±0.01)eV and Ec−0.69(±0.03)eV, were identified in both SnO2–CoO–Nb2O5 and SnO2–CoO–Nb2O5–Cr2O3 varistors. These two traps could be associated with the second ionization energy of oxygen vacancies VO∙∙ or impurities on host lattice site CoSn′ or NbSn∙. The two trap levels are not associated with chromium doping, since Cr2O3 doping only changes the donor density and trap densities.

Phase development in ZnO varistors

Abstract Based on a typical ZnO varistor composition (97·0 mol.-% ZnO, 1·0 mol.-% Bi2O3, 1·0 mol.-% Sb2O3, 0·5 mol.-% MnO and 0·5 mol.-% Co3O4), phase development of the ZnO varistor during sintering has been investigated using in situ high temperature X-ray diffraction up to 900°C, and conventional ambient X-ray diffraction for samples sintered at 900°C to 1250°C. The results indicate that α-Bi2O3 can be detected until 700°C; the pyrochlore phase can be detected in the samples heat treated at 700°C and up to 1250°C; the spinel phase is present at and >900°C. However, the main phases in the varistor are established by 950°C. By this temperature, the essential microstructure features are formed, and the varistors exhibit non-linear electrical properties, with a non-linear coefficient α of 35 and breakdown field of 8000 V cm−1. With increasing sintering temperature, both the α value and breakdown field decrease.

Investigation of the effect of different dopants on the trap states of ZnO-based and SnO2-based varistors

The electrical properties of ZnO varistors and novel SnO2 varistors have been attributed to their grain boundary barriers. In this report, the techniques of capacitance-voltage (C-V) measurement and deep level transient spectroscopy (DLTS) were employed to investigate the effect of different dopants on the electronic states of ZnO-based and SnO2-based varistors. Two trap levels were determined in typical ZnO varistors: trap L1 at Ec-0.15±0.01eV and trap L2 at Ec-0.25±0.01eV. The different dopants affected the parameters of those traps significantly. Two deeper trap levels, trap S1 at Ec-0.30±0.03eV and trap S2 at Ec-0.69±0.03eV, were identified in both SnO2-CoO-Nb2O5 and SnO2-CoO-Nb2O5-Cr2O3 varistors. The variations in the donor density and trap density appear to be associated with the addition of trivalent Cr3+. The features of these trap levels and the defect theory related to the SnO2 varistors need to be further studied.