Physics of zinc-oxide cryovaristors in high power switching applications

Recent work has shown that a ZnO ceramic used for a cryovaristor has a high thermal conductivity and enthalpy at liquid nitrogen temperatures (77 K), yet, unlike other varistor materials, retains the characteristic back-to-back Zener diode I-V relationship at 80 K. Measurements show that the thermal conductivity of the ZnO cryovaristor has a maximum value of 0.65 W/cm/K at 80 K and a value of 0.24 W/cm/K at 320 K. The enthalpy of the ZnO ceramic was determined to be 1186 J/cm/sup 3/ at 80 K, with an upper adiabatic limit of 1215 J/cm/sup 3/. Computational analysis shows that a variable-range-hopping model can explain the small temperature dependence of the prebreakdown.<
>