Abstract
The effects of the grain size of 
on 
sensor for 
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 
of 
were varied in the range of 16 to 57 nm. The sensitivities of the 
sensor elements to 
(10 ppm) as well as to 
(200 ppm) remained independent of 
down to 
, below which the sensitivities increased steeply with a decrease in 
, reaching at 
the sensitivity values three to four times as large as those for 
.