The effect of sintering temperature on the microstructure, density and nonlinear electrical properties of 0.1 mol% Nb 2 O 5 added ZnO-V 2 O 5 varistor ceramics had been investigated. Density measurement, scanning electron microscopy (SEM), X-ray diffraction (XRD) pattern analysis characterized the sintered samples. The electric current and capacitance measurements of the varistor samples were done in varying voltage over a wide range. Image analysis by SEM shows an increase in the average grain size from 2.8 to 7.2 µm when the sintering temperature increases from 850 to 975 °C. The varistor sample, sintered at 850 °C, exhibits breakdown field of 4258 V/cm with nonlinear exponent value of 24. Degradation in breakdown field (1440 V/cm) and nonlinear exponent (9) have been observed with an increase in sintering temperature up to 975 °C. Barrier height estimated from the electric field-current density (E-J) and the capacitance-voltage (C-V) measurements follow an inverse relationship with sintering temperature.

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SUMMARYA ®nite dierence method of solving problems of free convection in a parallel plate channel with or without internal obstructions is presented. It uses the steady-state governing equations including the axial diusion terms, and it does not require pressure matching or conditions to obtain the solution. The buoyancy-induced upward velocity is self-generated without requiring any extra eort or strategy over that normally used for iterative ®nite dierence solution of elliptic equations. The method simply uses the average upward velocity from one iteration to the next until convergence. Comparison and a few illustrations are included to indicate the validity and usefulness of the method.

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