APPLICATION OF METAL TUNGSTATE-CARBONATE COMPOSITE TO NITROGEN-OXIDESSENSOR OPERATIVE AT ELEVATED-TEMPERATURE

Metal tungstates and their composites with carbonates have been invest igated for application to semiconductor-type sensors capable of detect ing nitrogen oxides at elevated temperature. Among the metal tungstate s tested at 500 degrees C, MgWO4 and ZnWO4 are highly sensitive to NO2 only, while BaWO4 is remarkably sensitive to both NO and NO2. The sen sitivity to NO and NO2 is greatly promoted when BaWO4 is added to BaCO 3. The resulting composite, BaWO4-BaCO3 (2:1 in molar ratio) exhibits the most excellent sensing characteristics: its signal output is the s ame to both NO and NO2 and increases almost linearly with increasing g as concentration in the range 0-400 ppm NO and 0-200 ppm NO2 at 600 de grees C.