IMPROVED ELECTRICAL PROPERTIES

The object of the invention is a process for producing non-alkaline steatite ceramics having controlled electrical properties for electrotechnology. Non-alkaline steatite ceramics of the invention belongs to a group of magnesium silicates of C C221 type. Steatite ceramics is an electrical insulator with low electrical conductivity at increased temperature, low dielectric constant and small dielectric losses.

It is used in electrotechnology as an electrical insulator. It is used for casings, housings of thermostats and fuses. Steatite ceramics properties are defined in Standard IEC 60672-3, Ceramic and glass-insulating materials. Steatite ceramics belongs to a subgroup of magnesium silicates C 200. The invention discloses a material from subgroup C 221 which is dense non-alkaline magnesium silicate based steatite ceramics with low dielectric losses. The standard requires the values of dielectric constant 6, dielectric losses at 1 MHz 1.2 x 10 ^"3 , specific electrical resistance at 200 °C is 1 x 10 ⁹ Ohm m, at 600 °C it is 1 x 10 ⁵ Ohm m. An extrapolated value at 400 °C is 1 x 10 ⁷ Ohm m. The steatite ceramics of C 221 type has requirements for higher electrical resistance and for more reproducible and controlled fabrication of final ceramic products.

Due to low price and its good electrical properties steatite ceramics is widely used, however, the available latest literature contains but few articles that discuss the problem from this field. Preparation of steatite ceramics was intensely searched in the 1950s; nowadays only solutions to a cheaper, higher quality and more reproducible production of products on an industrial scale are looked for.

Steatite ceramics products are mostly fabricated from natural materials that are ground and homogenised in water and then formed in their raw state by using various ceramic technological processes and finally blanks are fired at high temperatures. Firing is performed at temperatures around 1300 °C. During a firing process, chemical reactions, phase conversions take place, a glass phase and crystalline phases occur. The quantity of magnesium silicate which is the main crystalline phase in steatite ceramics depends on the materials used, the temperature and time of firing. Mineralogical composition of fired steatite ceramics is magnesium silicate and glass phase, whereas flintstone and other crystalline forms of silicon oxide can also be present in small quantities. Usual materials for the preparation of steatite ceramics of C 221 type are natural materials such as talc, clay components (clays, bentonites, kaolins) and synthetic barium carbonate. Clays, bentonites and kaolins are the components that allow for a plastic forming of material. During the firing the talc is disintegrated to magnesium silicate, and silicon oxide, clays, bentonite and barium carbonate form a liquid phase which coats magnesium silicate granules and provide for densification of steatite ceramics. The liquid phase may contain impurities, such as alkaline oxides or iron oxides, which are introduced into the system with natural materials. The presence of these oxides reduces the electrical properties of steatite ceramics and it might not meet the required values according to standard IEC 60672-3.

The task and goal of the invention is a method for the production of such non-alkaline steatite ceramics of C 221 type that will have controlled and reproducible electrical properties, i. e. electrical resistance higher than required by the standard and other standard-required characteristics.

The task of the invention is solved by a method for producing non-alkaline steatite ceramics, in which the influence of iron oxide impurity is reduced by an addition of calcium oxide or magnesium oxide or a mixture of both in any weight ratio, thus the electrical resistance of steatite ceramics is increased. Calcium oxide and magnesium oxide can be added in the form of hydroxides, carbonates, nitrates or other salts, such as calcium carbonate - marble, calcium magnesium carbonate - dolomite, calcium hydroxide, magnesium hydroxide, magnesium hydroxy carbonate, magnesium or calcium nitrate.

The starting material, i. e. a mixture for the preparation of non-alkaline steatite ceramics of the invention, contains the following ingredients in % by weight:

- 80 to 88 % of talc

- 5 to 1 5 % of clay components

- 5 to 10 % of barium carbonate

- 0.1 to 10 % of calcium oxide or magnesium oxide or a mixture of both in the form of hydroxides, carbonates, nitrates or other salts - additives, such as surfactants e. g. polyphosphates, polyacrylates, lignosulphonates, and binders e. g. polyvinyl alcohol, polyethylene glycol, polyvinyl butyral.

The total content of iron oxide impurity in the starting material is up to 3 % by weight.

According to the process of the invention, the starting material is wet ground in a grinder that is coated with a ceramic coating made from a material having hardness identical to AI2O or higher and contains corundum ceramic balls containing from 90 % by weight to 94 % by weight of AI2O3. The dry matter/water weight ratio is 40/60 or rather such that grinding is performed in a ball mill up to an average particle size below 10 micrometres. Grinding is performed for 4 to 24 hours to reach an average particle size below 10 micrometres. Particle size is controlled by a laser granulometer. After the grinding, part of the water is removed from the suspension, the suspension is poured onto a dry plaster surface where it is left to dry up to a water content of 15 to 35 % by weight. After drying, the water content in the ceramic material should be 15 to 35 % by weight to allow for forming of products in an extruder. The blanks are dried in a drier at 105 °C for 10 to 24 hours to reach the final content of water of 1 % by weight or less. The blanks are fired at a temperature between 1280 and 1350 °C for 1 hour to 10 hours, so that non-alkaline steatite ceramics of the invention is obtained. While fired, the blanks are in a vertical or horizontal position. After the firing is completed, the products have their final shape and dimensions.

Non-alkaline steatite ceramics without an additional of calcium oxide and magnesium oxide has specific electrical resistance at 400 °C 0.6 x 10 ⁷ Ohm m. In case when calcium and magnesium oxides are added, the specific electric resistance of the ceramics is increased and amounts to 1 .4 x 10 ⁷ Ohm m at 400 °C.

Embodiment 1

ingredients that contain natural humidity: talc (2453 g). clays ( 121 g), kaolin (58 g). bentonite (92 g) and barium carbonate ( 189 g) are added to a grinder, water, surfactants (50 g) and binder ( 1 7 g) are added, such that the final suspension has a dry matter/water weight ratio 61 /39. The mixture is ground for 17 hours and 30 minutes or until an average granule size of 5.7 micrometres is obtained. After the grinding is completed, the suspension is poured onto a dry plaster surface. Test blanks are formed from the mass, they are dried at a temperature of 105 ⁰ for a sufficient time to contain less than 1 % of humidity and then fired at a temperature of 13 19 °C for 2 hours. The fired test blanks have a mechanical bending strength of 135 MPa and specific electrical resistance at 400 °C 0.6 x 10 ⁷ Ohm m.

Embodiment 2

Ingredients that contain natural humidity: talc (2409 g), clays ( 121 g), kaolin (58 g), bentonite (92 g), barium carbonate ( 1 89 g), dolomite (43 g) are added to a grinder, water, surfactants (50 g) and binder ( 1 7 g) are added, such that the final suspension has a dry matter/water weight ratio 61/39. The mixture is ground for 8 hours or until an average granule size of 6.3 micrometers is obtained. After the grinding is completed, the suspension is poured onto a dry plaster surface. Test blanks are formed from the mass, they are dried at a temperature of 105 ° for a sufficient time to contain less than 1 % of humidity and then fired at a temperature of 13 16 °C for 2 hours. The fired test blanks have a mechanical bending strength of 133 MPa and specific electrical resistance at 400 °C 1 .4 x 10 ⁷ Ohm m.

Embodiment 3

Ingredients that contain natural humidity: talc (2402 g), clays (1 19 g), kaolin (57 g), bentonite (92 g), barium carbonate ( 186 g), marble (28.4 g) and dolomite (28.4 g) are added to a grinder, water, surfactants (50 g) and binder ( 1 7 g) are added, such that the final suspension has a dry matter/water weight ratio 61 /39. The mixture is ground for 7 hours and 30 minutes or until an average granule size of 6.4 micrometres is obtained. After the grinding is completed, the suspension is poured onto a dry plaster surface. Test blanks are formed from the mass, they are dried at a temperature of 105 ⁰ for a sufficient time to contain less than 1 % of humidity and then fired at a temperature of 13 16 °C for 2 hours. The fired test blanks have a mechanical bending strength of 141 MPa and specific electrical resistance at 400 °C 1 .4 x 10 ⁷ Ohm m.