Precision wire resistors with vitreous coating - and nonfusing intermediate layer over 180oc surface

摘要

The resistor wire is first given a non-fusing intermediate coating before the vitreous final coating is applied, to make it suitable for working temperatures exceeding 180 C on the surface. The intermediate layer is a silicone cement which is applied by immersion, spraying or rolling, baked at 150 to 200 C to ensure a network structure for the silicones and then converted at 400 to 800 C into an oxide-ceramic frit. The inert fillers used for the silicon cement are aluminium oxide, mica and/or beryllium oxide. Compounds on an aluminium-organic base are used as hardeners. Advantages are the elimination of any depassivation and the narrow tolerances (plus-or-minus 0.5-2%) obtainable in the resistance. The cushion action of the silicone frit compensates for any difference in thermal coefficient of expansion. Two to three coatings of vitreous enamel are sufficient to ensure a breakdown strength of at least 5 kV. The good thermal conductivity of the silicone ensures a better heat equalisation.