DD161055A3 - METHOD FOR PRODUCING AN IMPRESSION SOLUTION FOR A GLAZED PROTECTION LAYER ON SIC FORMKOERPER - Google Patents

Abstract

The invention relates to a method for producing a Impraegnierloesung for a glassy protective layer on SiC moldings. The invention makes it possible to provide a solution which contains all glaze-forming components in homogeneous distribution and is stable. According to the invention, the solution is prepared by slowly adding a stabilized silica sol with intensive mixing in an aqueous solution of alkali, alkaline earth and aluminum salts until a pH of 3.5-3.7 has been established. After impregnation of the shaped bodies, a thermal treatment takes place.

Description

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Process for the preparation of an impregnating solution for a glassy protective layer on SiC shaped bodies

The invention relates to a method for producing a solution for the production of a uniformly composed protective layer on SiC molded bodies, in particular SiC heating conductors.

Due to the manufacturing process, silicon carbide molded bodies are porous. Use at high temperatures in an oxidizing atmosphere leads to gradual oxidation of the silicon carbide to SiO.sub.2. This process is particularly disadvantageous for silicon carbide heating elements because, as oxidation proceeds, the electrical resistance increases considerably and leads to the failure of Eizizstäbe · '.

It is known to provide such SiC moldings with a vitreous coating for protection against aggressive substances. "Methods are known according to which protective effects are achieved by spraying suspensions with subsequent thermal treatment.

According to DE-PS 1 671 053 a method is known in which the solution for impregnating the SiC shaped bodies consists of a mixture of a water-soluble salt of aluminum and an alkaline earth metal. "The aqueous solution is applied to the shaped body, the shaped body dried and oxidized at 165Q ⁰ C · '

The disadvantage of this invention is that the solution contains no glass-forming components, whereby the resulting oxide film is porous and does not protect sufficiently against oxidation.

A two-stage coating process of SiC molded articles to obtain a glaze is described in DD-WrVbIl. The disadvantage of this process is that the glaze components have to be applied to the molding separately one after the other. A protective coating is formed between the two phases SiO ₂ . and the salt layer as a result of surface diffusion processes, and not as an overall reaction of the components used. The resultant protective layer is consequently inhomogeneous, not reproducible in its composition and can not be calculated from its effect.

Object of the invention

The aim of the invention is to provide a solution for producing a reproducible and homogeneous protective layer on SiC moldings, which makes it possible to reduce the technological complexity and to predetermine the application properties.

Essence of the invention

The invention is based on the object to provide a method for producing a solution containing all glaze-forming constituents in homogeneous distribution and is stable. "According to the invention, this object is achieved in that a stabilized silica sol with intensive mixing slowly into an aqueous solution Alkaline, alkaline earth and aluminum salts is added until a pH of 3.5-3.7 has been established. This solution is used to impregnate the SiC molded bodies, after which they are dried and then heated. The silica sol used is a stabilized silica sol a Si0 ₂ molar ratio of 600-900, a pH of 9 to 10

and a mean particle size of the colloidal silica of 15-20 nm suitable ·

embodiment

The complete protective coating of the entire accessible surface of porous SiC moldings with silicate glazes of the desired composition by means of a one-step impregnation process with an aqueous system which already contains all the glaze-forming constituents in dissolved or colloidal form has hitherto not been accessible. It is known that it is extremely difficult to produce relatively stable acidified silica sols with high metal salt contents. Surprisingly, this is easily achieved by using stabilized silica sol having an average SiO 2 particle size of 15-20 nm and introducing this silica sol into the highly concentrated acidic aqueous solution of one or more metal salts with a pH of 3.5 - 3.7 must be set ·. Even after 250 days, the stable metal salt-containing silica sol produced in this way has virtually no gelling or no remarkable granulation. Such stability of the impregnated solution is an essential prerequisite for the technical feasibility of the protective coating.

The molar composition of the glaze-forming constituents, based on the impregnation solution, is calculated as oxides (Segergleichung)

: 0.4: 0.6: 0.5

: 4.0 · ·

K ₂ O, N ₂ O CaO, MgO Al ₂ O ₃ SiO ₂

For this purpose, 4 403 g of a 50.3% aqueous salt solution whose pesticide content is composed

6.9% K ₂ CO ₃

3.4% Na ₂ CO ₃

5.3% MgCO ₃

6.2% CaCO ₃

78.2% Al (NO ₃ ) ₃ . 9H ₂ O,

5 597 g of commercially available silica sol type KS 30 K are slowly added with vigorous stirring. File adjusts to a pH of 3.5 - 3.7.

Claims (2)

Erfindungsanspräehe A process for the preparation of a glassy protective layer impregnation solution on SiC bodies, in particular SiC heating conductors using a stabilized silica sol and an aqueous solution of alkali, alkaline earth and aluminum salts, characterized in that the stabilized silica sol is slowly mixed with intensive mixing the aqueous solution is added from alkali metal salts, alkaline earth metal salts and aluminum salts until a pH of 3.5-3.7 has been established. 2. ' Process according to item 1, characterized in that the stabilized silica sol used is a sol having a SiO 2 molar ratio of 600 to 900, a pH of 9 to 10 with a mean particle size of the colloidal silica of 15 to 20 nm.