Cermet surface-distributive heaters based on nickel-titanium powder

Cermet surface-distributive heaters based on powdered metals and obtained by firing on a glass-enamel substratum are being increasingly used. The structure consists of steel glass-enamel insulation-resistive layer-glass enamel sealing components. The properties of heaters based on Ni-Ti powders were investigated by determining the specific surface resistance of the heaters and its relationship with the composition. An insulating enamel was applied to steel plates. The resistive layer (a suspension of Ni-Ti powder, clay and glass frit milled to 0.1 mm) was sprayed on to the glass enamel substratum and dried and fired. The contact brass areas were then sprayed onto the resistive layer. The sealant enamel layer was then sprayed on, followed by drying and firing. It was shown that the insulating and sealant layers have sufficient continuity which was unaffected by changes in firing temperature and time. The optimum composition of the resistive layer was shown to be 38-73% Ni- Ti powder, 7-52% glass frit, and 5-20% clay. Heaters with a specific resistance of 2.5 ohm/quadrant have a negative temperature coefficient of resistance. In the 20-200 C range it is -0.27%/C, which is lower than that of ferrosilicon-based heaters (-0.54%/C). 3 refs.