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Zinc Oxide Varistor Gas Sensors: II, Effect of Chromium(III) Oxide and Yttrium Oxide Additives on the Hydrogen‐Sensing Properties

1998; Wiley; Volume: 81; Issue: 6 Linguagem: Inglês

10.1111/j.1151-2916.1998.tb02525.x

1551-2916

Yasuhiro Shimizu, Feng‐Cang Lin, Yuji Takao, Makoto Egashira,

Analytical Chemistry and Sensors

The effect of Y 2 O 3 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% Cr 2 O 3 , but both series contain 1.0 mol% Bi 2 O 3 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 upon exposure to H 2 balanced with air, is discussed in terms of microstructural and crystallographical changes of the varistors. The addition of Y 2 O 3 to the base ZnO varistors without Cr 2 O 3 results in a decrease in ZnO grain size and an enhancement in H 2 sensitivity, with the highest sensitivity being achieved with an addition of 0.25 mol% of Y 2 O 3 . The addition of 5.0 mol% of Cr 2 O 3 to the base ZnO varistor also has been confirmed to be effective for enhancing the H 2 sensitivity, which is accompanied by stabilization of the δ‐Bi 2 O 3 phase to room temperature and a decrease in ZnO grain size. The H 2 sensitivity is further enhanced by the simultaneous addition of Y 2 O 3 to the ZnO‐based varistors that contain 5.0 mol% Cr 2 O 3 , whereas the ZnO grain size remains almost unchanged. This series of varistors also exhibits the highest sensitivity with 0.25 mol% of Y 2 O 3 . The enhanced sensitivity has been 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 grain boundaries has been suggested to occur not only via the decrease in ZnO grain size but also via stabilization of the δ‐Bi 2 O 3 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 H 2 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.