Ceramic heating plug using embedded ceramic resistor - has improved resistance to thermal cycling as result of material selection and dimensions

摘要

A green sheet is prepd. consisting of an organic binder and electrically conducting ceramic, pref. nitrides or carbides of the elements Ti, Zr, Hf, V, Nb, Ta, Cr, Mo or W. A suitable resistor shape is stamped out, pref. as a U-shape. The resistor is embedded in ceramic powder based on Si-nitride. The product is then compressed with a suitable pressure in a direction parallel to the width of the resistor and sintered. Also calimed is a product made by compression during sintering which reduces the distance between the parallel tracks (22-25) of the resistor. The product claimed pref. has a cylindrical shape or is a sheet. The main resistor cross section area is pref. 1/10,000-1/38 of that of the ceramic end-product. In addn. the total area of the resistor cross ection is less than 1/6 of that of the ceramic end product. Also claimed is a min. distance between the centre of the resistor tracks and the outside surface of the element which is more than 0.15 times the element dia. or thickness, but less than 0.5 times this dimension. USE/ADVANTAGE - The element uses better matched parts than currently available elements making it more resistant to thermal cycling. The dimensions are chosen to produce acceptable stresses. As a result the heating element survives 20,000 cycles, without suffering cracks, of switching on and off at 1200 deg C followed by fast cooling with an airstream. The rapid heating elements are used in diesel engines and other applications requiring a fast temp. rise.