A catalytic detector element for flammable gases comprises contact areas 5, 6 on either side of a zig-zag strip-form heater 4 which is coated with catalyst pad or bead 7. Current is passed through the heater in the usual way and a flammable gas is detected by combustion in the catalytic bead 7 thus raising the temperature of the heater and changing its resistance. Prior heater coils were fragile but the heater 4 and contacts 5 and 6 are photolithographically deposited in plantinum on a layer 2 of silicon nitride (or oxide) on a silicon chip, overcoming this problem. To assist thermal isolation of the heater an air gap 9 is formed beneath the silicon nitride by etching through minute perforations which have been themselves etched in the silicon nitride layer. The bead 7 may be of alumina containing a catalyst. IMAGE
展开▼
