ZINC-OXIDE VARISTOR GAS SENSORS - II, EFFECT OF CHROMIUM(III) OXIDE AND YTTRIUM-OXIDE ADDITIVES ON THE HYDROGEN-SENSING PROPERTIES

The effect of Y2O3 addition on the hydrogen-sensing properties of two series of porous ZnO-based varistors has been investigated; the two va ristor series are with and without 5.0 mol% Cr2O3, but both series con tain 1.0 mol% Bi2O3 and small amounts of other additives essential for realizing nonlinear current-voltage characteristics. The variation in H-2 sensitivity, which is defined as the shift in breakdown voltage u pon exposure to H-2 balanced with air, is discussed in terms of micros tructural and crystallographical changes of the varistors. The additio n of Y2O3 to the base ZnO varistors without Cr2O3 results in a decreas e in ZnO grain size and an enhancement in H(2 )sensitivity, with the h ighest sensitivity being achieved with an addition of 0.25 mol% of Y2O 3. The addition of 5.0 mol% of Cr2O3 to the base ZnO varistor also has been confirmed to be effective for enhancing the H-2 sensitivity, whi ch is accompanied by stabilization of the beta-Bi2O3 phase to room tem perature and a decrease in ZnO grain size. The H-2 sensitivity is furt her enhanced by the simultaneous addition of Y2O3 to the ZnO-based var istors that contain,5.0 mol% Cr(2)O(3 )whereas the ZnO grain size rema ins almost unchanged. This series of varistors also exhibits the highe st sensitivity with 0.25 mol% of Y2O3. The enhanced sensitivity has be en anticipated to be related to the decrease in ZnO grain size, i.e., the increase in the number of H-2-sensitive grain boundaries, and the sensitization of grain boundaries themselves. The sensitization of gra in boundaries has been suggested to occur not only via the decrease in ZnO grain size but also via stabilization of the delta-Bi2O3 phase th at exhibits high oxygen-ion conductivity. Thus, the present study conf irms that the mobility of excess oxygen ions and the reactivity of exc ess oxygen ions and oxygen adsorbates determine the H(2 )sensitivity o f porous varistors; this study also shows that the existence of such o xygen species is essential for the formation of a double Schottky barr ier of the varistors.