Treatment for improving the stability of silicon carbide heating elements

#There are several types of elements materials commonly used for electrical heating (see Table 1). In addition to those which are discussed below, heating elements made of graphite, aluminum and boron nitride are also sometimes used as electric heaters. Even less common are Lanthanum Chromite heating elements.

#The two types of metallic elements are nickel/chrome (NiCr) and iron-chromium-aluminum (Fe—Cr—Al). The metallic elements are the least expensive, however, they have the lowest use temperatures. Ni—Cr is limited to use at 1100° C. whereas the iron-chromium aluminum alloys (sometimes referred to in the industry as Kanthal A-1) can be used to 1300° C. Nichrome, although limited to the lower temperatures, has very good hot strength so the elements can be self-supporting whereas the Fe—Cr—Al elements with poorer hot strength must be supported usually by ceramic tubes.

#Nichrome elements are 80% nickel and 20% chrome which is the most common composition. Both types of metallic elements have the advantage that their electrical resistance stays constant with time so that as the elements age it is not necessary to make compensations for changing resistance. Also, the metallic elements have more or less constant resistance at all temperatures. As a result of the fact that the resistances are constant, inexpensive on/off controls can be used with metallic elements.

#We have invented a treatment where the stability and other performance measures such as emissivity of the silicon carbide is greatly enhanced. Stability is defined as the lack of change or very little change in the electrical properties and power draw with time during use at a high temperature. To the best of our knowledge there is no previous claim to a treatment which claims improved stability and performance in a silicon carbide containing heating element. Both alpha and beta Silicon carbide elements may be improved in performance by the proposed technique and examples of use are given below.

#The treatment consists of applying an adherent mixture of silica (a binder) and compounds of molybdenum silicon i.e. molybdenum suicides (in powder or short fiber form, both forms herein called powder) to a SiC heating element prior to use in a furnace. Since all Silicon Carbide heating elements are porous, the mixture is normally well absorbed by the heating element. One or more layers can be applied by brushing or spraying or dipping in an aqueous mixture of Silica and molybdenum silicon compounds. The silica can be in any suitable form, which causes adherence of the treatment materials (all forms of crystalline and/or amorphous oxides of silicon are expected to be encompassed by the word silica). The silica may be a gel or colloid or in the form of powder which is mixed into the Molybdenum silicon compound. Both the binder material and the molybdenum silicon material could be either powder ranging from nanometer size particles to millimeter size particles or finely dispersed in a fluid such as water or common organic solvents such as kerosene or alcohol. The silica could also be obtained by using clay as the binder (Clay is an alumino-silicate). When the word silica powder is used in this application and claims it encompasses colloidal dispersed silica or gel or silica compounds in the various dispersed forms possible or by themselves. When coating of the a silicon carbide element is mentioned it is assumed that this coating is possible for new as well as partially used silicon carbide elements.

#Although molybdenum disilicide, is considered in the experiment given below, for the molybdenum silicide compound of choice, we recognize that all forms of molybdenum silicides or tungsten silicides or other common silicides are encompassed even if claimed separately). The words molybdenum disilicide, MoSi2 or molybdenum silicide are expected to cover all such variations.