ISSN:
1551-2916
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
,
Physics
Notes:
The effect of Y2O3 addition on the hydrogen-sensing properties of two series of porous ZnO-based varistors has been investigated; the two varistor series are with and without 5.0 mol% Cr2O3, but both series contain 1.0 mol% Bi2O3 and small amounts of other additives essential for realizing nonlinear current-voltage characteristics. The variation in H2 sensitivity, which is defined as the shift in breakdown voltage upon exposure to H2 balanced with air, is discussed in terms of microstructural and crystallographical changes of the varistors. The addition of Y2O3 to the base ZnO varistors without Cr2O3 results in a decrease in ZnO grain size and an enhancement in H2 sensitivity, with the highest sensitivity being achieved with an addition of 0.25 mol% of Y2O3. The addition of 5.0 mol% of Cr2O3 to the base ZnO varistor also has been confirmed to be effective for enhancing the H2 sensitivity, which is accompanied by stabilization of the δ-Bi2O3 phase to room temperature and a decrease in ZnO grain size. The H2 sensitivity is further enhanced by the simultaneous addition of Y2O3 to the ZnO-based varistors that contain 5.0 mol% Cr2O3, whereas the ZnO grain size remains almost unchanged. This series of varistors also exhibits the highest sensitivity with 0.25 mol% of Y2O3. The enhanced sensitivity has been anticipated to be related to the decrease in ZnO grain size, i.e., the increase in the number of H2-sensitive grain boundaries, and the sensitization of grain boundaries themselves. The sensitization of grain boundaries has been suggested to occur not only via the decrease in ZnO grain size but also via stabilization of the δ-Bi2O3 phase that exhibits high oxygen-ion conductivity. Thus, the present study confirms that the mobility of excess oxygen ions and the reactivity of excess oxygen ions and oxygen adsorbates determine the H2 sensitivity of porous varistors; this study also shows that the existence of such oxygen species is essential for the formation of a double Schottky barrier of the varistors.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1151-2916.1998.tb02525.x
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