Cosputtered Metal and Layers for Use in Thick-Film MISiC 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 (SiO ₂), 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 degC to 450 degC 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 H₂ but were rather insensitive to NO and CO