GB802731A - Improvements in or relating to ª‡-aluminium oxide - Google Patents

Abstract

An electrically non-conducting a -alumina powder contains at least one of the following components in the specified proportion by weight of the aluminium: 0.05-0.5 per cent silica, 0.01-0.1 per cent alkali metal oxide, 0.1-5 per cent calcium oxide, 0.1-5 per cent glass or other silicate containing less than 0.01 per cent of alkali metal. Where alkali metal oxide is present, at least 5 parts by weight of silica must also be present per part by weight of alkali metal. In a first embodiment, an alumina powder containing 0.3 to 0.8 per cent of sodium oxide and 0.05-0.5 per cent of silica, which has been calcined at about 1100 DEG C., is heated at a temperature between 1300 DEG and 1700 DEG C. until a substantial part of the sodium content of the powder is vaporized, and the product is washed with hot hydrochloric acid and hot water until its sodium oxide content is further reduced to 0.01 to 0.1 per cent by weight. The resultant powder may be ground together with 0.2 to 1 per cent of alkali free hard glass powder in dilute hydrochloric acid and then washed with hot hydrochloric acid and hot water and suction filtered or may be contacted with an aqueous solution of aluminium sulphate, chloride or nitrate or with sodium chloride and the product dried at 150 DEG to 650 DEG C. for subsequent use in coating electrical elements. In Example 1 alumina is heated at 1450 DEG C. and milled in the presence of aqueous hydrochloric acid with powdered hard glass; the mass is heated with nitric acid, filtered, washed with hot hydrochloric acid and hot water, dried, stirred with aluminium sulphate solution, filtered, dried and heated at 600 DEG C. Finally the powder is shaken with acetone and amyl alcohol, filtered, dried and screened for use in the coating of electrical elements by electrophoresis followed by firing in hydrogen at 1600 DEG C. In a second embodiment, aluminium hydroxide is precipitated from a solution of silica-free aluminium salt, mixed with sufficient calcium nitrate to provide a content of 0.1 to 0.5 per cent of calcium oxide in the finished product and then heated at 1300 DEG to 1600 DEG C. Example 2 describes a process in which aluminium hydroxide is precipitated from a solution of aluminium chloride with ammonia, the mixture boiled and filtered and the precipitate washed with aqueous ammonium nitrate made alkaline with ammonia. The precipitate is heated at 1000 DEG C., the product mixed with calcium nitrate solution and the mixture dried and then heated at 1500 DEG C. The cake thus formed is milled in the presence of water and extracted with hot hydrochloric acid and hot water; a solution of sodium chloride is added to the air-dry powder and the mixture dried, heated at 500 DEG C., ground and screened for use in the formation of coatings on electrical elements as in the first example. It is stated that where materials are prepared from alumina with ground glass, beryllium or magnesium oxides may also be added to counteract the effect of any sodium present. In addition to coating by electrophoresis (see Group XXXVI) an electrical element, e.g. a tungsten spiral, may be coated with or dipped into a suspension of the alumina in a binding agent, e.g. an amyl-acetate-acetone mixture containing collodion or methanol containing collodion. The electrical element is subsequently fired to form the final coating. The alumina may also be used for coating refractory articles such as ceramic crucibles and combustion boats.ALSO:A refractory body or coating for refractory articles such as combustion boats or ceramic crucibles may be prepared from a -alumina powder containing at least one of the following components in the specified proportion by weight of the aluminium: 0,05-0,5 per cent silica, 0,01-0,1 per cent alkali metal oxide, 0,1-5 per cent calcium oxide, 0,1-5 per cent glass or other silicate containing less than 0,01 per cent of alkali metal; where alkali metal oxide is present at least 5 parts by p weight of silica must also be present per part by weight of alkali metal oxide.