METHOD FOR THE PRODUCTION THEREOF

The object of the invention is a process for producing alkaline steatite ceramics having controlled electrical properties for electro tech no logy. Alkaline steatite ceramics of the invention belongs to a group of magnesium silicates of C 220 type. Steatite ceramics is an electrical insulator with a relatively low electrical conductivity at increased temperature.

It is used in electrotechnology for fabrication of parts of electrical devices, such as housings of switches, thermostats and fuses. The ceramics must therefore exhibit good mechanical properties and a relatively high electrical resistance at increased temperature. 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 200 which is dense magnesium silicate based steatite ceramics. Standard required values for specific electrical resistance at 200 °C are 1 x 10 ⁸ Ohm m, at 600 °C 1 x 10 ³ Ohm m. An extrapolated value at 400 °C is 2.9 x 10 ⁵ Ohm m. Requirements for steatite ceramics of C 220 type are more reproducible and controlled fabrication of ceramics and improved properties, especially higher electrical resistance.

Alkaline steatite ceramics are produced from natural materials. These materials do not contain substances that are harmful to the environment but the properties of the materials that are important for the production of ceramics are not constant and depend on the place of finding, excavation site and subsequent processing of materials such as crushing, grinding and sieving. The method for the production of ceramics must be adapted accordingly in order to obtain the required and reproducible properties.

The goal of the invention is to produce steatite ceramics from natural materials that would have reproducible and high electrical resistance at an increased temperature. The method of the invention is used to produce steatite ceramics from natural materials with electrical resistance that corresponds to the predetermined value for non-alkaline steatite ceramics according to standard C 221 which is produced from natural and synthetic materials. An example of a synthetic material is barium carbonate which is an environmentally harmful substance. This ceramics has a standard required specific electrical resistance at 200 °C 1 x 10 ⁹ Ohm m, at 600 °C 1 x 10 ⁵ Ohm m, the extrapolated value at 400 °C is 1 x 10 ⁷ Ohm m. Available literature from the field of preparation and properties of steatite ceramics contains few data that would describe the influence of natural materials on electrical properties of ceramics. Patent US 4687749 discloses preparation of steatite based glass-ceramics. Crystallization of steatite glass at different temperatures was studied. They reported on mechanical properties of the obtained glass-ceramics with a predominantly steatite phase. Patent US 5108665 discloses a method for the preparation of pure and dense steatite ceramics. Powder was prepared by a thermal decomposition of organometallic compounds and sintered at a temperature up to 1450 °C. The influence of various parameters on density of steatite ceramics was studied. Synthetic materials were an origin in both mentioned patents. Grinding of materials in water for the preparation of steatite ceramics is disclosed in patents CN 103952121 and CN 103923606. The first patent discloses a grinding method, wherein the quantity of water needed for grinding is reduced by an addition of suitable additives which results in energy saving for drying of the material. The second patent reports on a reduced time for grinding materials in water. The additives reduced the grinding time by one third. Influence of additives on the preparation of steatite ceramics is described in patent SI 20689 A. The patent discloses a material and a method for the production of steatite ceramics from predominantly natural materials having a low factor of dielectric losses and low leakage currents. None of the patents reports on the influence of a chemical composition of steatite ceramics on electrical properties.

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, 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 220 type are natural materials such as talc, clay components (clays, bentonites) and claystone. Clay components provide for plastic forming of the suspension. During firing process the talc is disintegrated to fnagnesiurn silicate and silicon oxide. The clay components and claystones form the liquid phase which coats magnesium silicate granules and causes densification of steatite ceramics.

The task and goal of the invention is a method for producing such steatite ceramics of C 220 type that will have the characteristics required by standard C 220 and controlled and reproducible high electrical resistance, i. e. the electrical resistance as required by the standard for non-alkaline ceramics of C 221 type.

The task is solved by the invention by steatite ceramics and a production method, in which bentonite is partly or completely replaced by kaolin, wherewith electrical resistance of steatite ceramics is controlled and increased.

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

- 78 to 88 % of talc

- 0.1 to 10 % of a bentonite-kaolin mixture

- 1 to 10 % of clay components

- 0 to 8 % of dolomite

- 0 to 7 % of 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 1.5 % by weight. In the bentonite-kaolin mixture, 1 to 100 % of bentonite is replaced by kaolin.

According to the method of the invention, the starting material is ground in a ceramic mill coated with A1 ₂ 0 ₃ or Zr0 ₂ ceramic coating and containing beads from the same material as the ceramic coating. Grinding is performed for 1 to 20 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, or the suspension is dried in a drier at 105 °C. 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 1340 °C for 1 hour to 10 hours, so that 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.

Alkaline steatite ceramics with bentonite without added kaolin has specific electrical resistance at 400 °C 1.4 x 10 ⁶ Ohm m. Steatite ceramics of the invention, in which bentonite is partly or completely replaced by kaolin, has a higher specific electrical resistance at 400 °C, specific electrical resistance at 400 °C is more than 1 x 10 Ohm m when the complete content of bentonite is replaced by kaolin.

Embodiment 1

Ingredients that contain natural humidity: talc (2409 g), clays (210 g), bentonite (148 g) and dolomite (145 g) are added to a grinder, water and additives (surfactants, binder) (73 g) are added, such that the final suspension has a dry matter/water mass ratio 62/38. The mixture is ground for 6 hours or until an average granule size of 6 micrometres is reached. 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 1319 °C for 2 hours. The fired test blanks have a bending strength of 150 MPa and specific electrical resistance at 400 °C 1.4 x l0 ⁶ Ohm m.

Embodiment 2

Ingredients that contain natural humidity: talc (2424 g), clays (181 g), kaolin (149 g), bentonite (91 g) and dolomite (58 g) are added to a grinder, water and additives (surfactants, binder) (84 g) are added, such that the final suspension has a dry matter/water mass ratio 62/38. The mixture is ground for 6 hours or until an average granule size of 5.8 micrometres is reached. 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 1319 °C for 2 hours. The fired test blanks have a bending strength of 155 MPa and specific electrical resistance at 400 °C 4.2 x 10 ⁶ Ohm m. Embodiment,3

Ingredients that contain natural humidity: talc (2424 g), clays (181 g), kaolin (149 g) and dolomite (149 g) are added to a grinder, water and additives (surfactants, binder) (84 g) are added, such that the final suspension has a dry matter/water mass ratio 62/38. The mixture is ground for 6 hours or Until an average granule size of 5.8 micrometres is reached. 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 1306 °C for 2 hours. The fired test blanks have a bending strength of 147 MPa and specific electrical resistance at 400 °C more than 1 x 10 Ohm m.

Embodiment 4

Ingredients that contain natural humidity: talc (83.5 g), clays (6.2 g), kaolin (5.15 g) and dolomite (5.14 g) are added to a grinder and water is added so that the final suspension has a dry matter/water mass ratio 50/50. The mixture is ground for 2 hours or until an average granule size of 4.5 micrometres is reached. After the grinding, the suspension is dried at a temperature of 105 °C. Test blanks are formed from the mass and then fired at a temperature 1300 °C for 2 hours. The fired test blanks have specific electrical resistance at 400 °C more than 1 x 10 ⁸ Ohm m. .