Cosputtered metal and SiO2 layers for use in thick-film MISiC NH3 sensors

High-temperature metal-insulator-silicon-carbide (MISiC) sensors are currently under development for use as NH3 sensors in selective-catalytic-reduction (SCR) systems in diesel engines or non-SCR (NSCR) systems in boilers. The detection of NH3 by these sensors requires the presence of triple points where the gas, the metal, and the insulator meet. These triple points have traditionally been located at the interface between the insulator and a porous metal. However, to facilitate the long-term stability of the devices when used in a harsh environment, a nonporous gate material would be preferred. Here, the behavior of the samples where such triple points have been introduced in a dense film through cosputtering of the insulator (SiO2), and either Pt or Ir is studied. The NH3 sensitivity of the materials was found to be in accordance with the earlier investigations on Si-based samples with cosputtered gate materials. Several metal-to-insulator ratios for each of the metals Pt and Ir were studied. The sensitivity of the layers as well As their selectivity to different concentrations of NH3 at temperatures ranging from 150 degrees C to 450 degrees C was investigated. The films containing 60%-70% Pt or Ir were found to give a high sensitivity toward NH3. These samples were shown to be sensitive also to propylene and H-2 but were rather insensitive to NO and CO.