Electrical and thermal properties of a varistor at cryogenic temperatures

A ZnO-based varistor composition has been found whose I-V characteristic is essentially unaffected by temperature (4.2–300 K) or intense magnetic fields at 4.2 K (up to 10 T). Electrical measurements are reported at various temperatures for current densities in the range 10−11–10−3 A/cm2 . Specific-heat and thermal conductivity data are reported at 1.7–300 K. The specific heat below 10 K and above 100 K is dominated by defect modes, and the data can be understood based on simple defect models. The thermal conductivity displays a broad maximum at about 80 K (0.65 W cm−1 K−1 ), and it is concluded that the defect modes do not carry heat. The small temperature dependence of the prebreakdown conductivity is found to be in excellent agreement with a variable-range-hopping model, suggesting that this composition is just above the percolation threshold.