GB1391203A - Heating element - Google Patents

Abstract

1391203 Resistance heating elements AGENCE NATIONALE DE VALORISATION DE LA RECHERCHE 18 May 1972 [19 May 1971] 23329/72 Heading H5H An oxidation resistant heating element is made of a refractory material which is non- conducting when at a temperature below the operating temperature, the element being generally parallelepiped in shape and having between the two major parallel faces F 1 , F 2 a non-homogeneous area which reduces heat transfer between faces F 1 , F 2 such that when in operation edge F 1 is electrically conducting and edge F 2 is electrically insulating. The non- homogeneity may be formed by through holes 2 and channels 3, or by variation in the porosity between parts A and B of the structure, Fig. 4. The temperature gradient between the ends of edge F 1 may be varied by shaping the channel portions, altering the cross-section of the element or varying the shape of the portion to be heated. Individual heaters may be assembled with spacers to form various furnace shapes, Figs. 7-10 (not shown). The element may be partially or entirely of "stabilized" zirconia (cubic form), obtained by the addition of lime, yttrium oxide, rare earth oxides, magnesia &c. The element may comprise a compound based on Zr 2 T 2 O 5 , T being a metal, preferably a rare earth. The terminal plates, e.g. 6, Fig. 1, may be platinum, bonded under pressure or moulded. Grooves 7b in the edge F 1 reduce the chance of element failure by cracking and the lattice structure absorbs stress &c. The ratio of cavity to solid should be in the range of 0À1-3À0. In the embodiments relating to Fig. 4 arrangements, the portion A of low porosity may be of stabilized zirconia, as above, and the high porosity portion B may alternatively be a ZrO 2 -Al 2 O 3 eutectic, alumina or another suitable refractory material. Part A may be attached to part B by; for example, projection melting using a plasma torch. Part A will have a density of the order of 4À5 and Part B of 2.