The effects of the grain size of WO 3 on WO 3 ‐ based sensor for NO x were investigated. By hydrolyzing ammonium paratungstate with hot nitric acid in solution and calcining the resulting precipitate in air at prescribed temperatures in the range of 300 to 600°C, the mean crystallite sizes ( D ) of WO 3 were varied in the range of 16 to 57 nm. The sensitivities of the WO 3 sensor elements to NO 2 (10 ppm) as well as to NO (200 ppm) remained independent of D down to D = 33 nm , below which the sensitivities increased steeply with a decrease in D , reaching at D = 25 nm the sensitivity values three to four times as large as those for D > 33 nm .
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Tamaki et al. (1994) studied this question.