SI20689A - Steatite material and process of its manufacture - Google Patents

Abstract

A composition of steatite material is described containing in mass % with regard to the total mass of the composition: from 68 to 75 % of microcrystalline talc containing less than 0,6 % CaO and less than 1 % Fe2O3; from 10,5 to 12 % of barium carbonate containing more than 98 % BaCO3; from 7,5 to 9,0 % kaolin containing less than 0,4 % Fe2O3, less than 0,3 % K2O and less than 0,1 % Na2O; from 6,0 to 8,0 % finely ground zirconium silicate containing less than 0,4 % Fe2O3 and less than 0,3 % Na2O; from 1,5 to 2,5 % bentonite containing less than 3 % Fe2O3, less than 2 % CaO, less than 2 % Na2O and less than 2 % K2O; a combination of 0,2 to 0,4 % medium-polymerised polyvinyl alcohol and 0,3 to 0,6 % high-polymerised polyethylenglycol as a plasticizer; and a deflocculant usual for steatite suspensions. The composition is applicable for the manufacture of steatite products having a low dielectric loss factor and low leakage currents for devices where no leakage currents should appear as well as a substitute for usually available steatite materials.

Description

Steatite material and process for its manufacture

The field of technology

The present invention relates to a new low dielectric loss steatite material and to a process for its manufacture.

A technical problem

There was a need for a steatite material, which, due to its composition and simple manufacturing process, would be more suitable for the field of high-frequency electrical engineering and thermal engineering and not more expensive than conventional low-alkaline steatites, but for sintering u would have as excellent mechanical properties as the said ceramic material for for the same purpose, while lowering the dielectric loss factor and lower discharge currents at elevated temperature.

The state of the art

Electrical ceramics include a wide range of compositions and structures. Most ceramic materials can be used as electrical insulators or for other electrical purposes.

Steatite compositions are a general class of dielectrics containing steatite or talc as the main constituent. Steatite is a massive sub-microscopic talc aggregate that is highly regarded in the ceramic industry for its low contraction during firing. Steatite compositions are widely used for high frequency insulators due to their good strength, relatively high dielectric constant and low dielectric losses.

Most of the components or raw materials used to make steatite materials are of natural origin and are therefore relatively inexpensive compared to synthetic material based dielectric compositions. Steatite compositions for high-frequency insulators and high-temperature electrical insulators (heaters insulators, thermostat carriers, thermal element mounts) are usually included as components for the production of natural raw materials, e.g. talc, clay, bentonite, and synthetic bath carbonate. Since the listed raw materials contain a certain amount of alkali, calcium oxide and iron oxide, after sintering, the steatite compositions have a higher value of the dielectric loss factor and a lower volume resistance, expressed in Qm, at elevated temperature in proportion to these contents.

Steatite products are usually made using the technology of dry compression of the granulate from the spray tower. In the process of spray and drying on the drying tower, the water evaporates from the suspension and granules are formed, the size of which depends on the properties of the suspension, the number of spray nozzles and the pressure on the nozzles. Suspensions of bentonite steatite compositions have increased viscosities, so conventional plasticizers based on medium and highly polymerized polyvinyl alcohols, added in an amount of more than 0.4% by weight, are mixed into the suspension after grinding is completed in solution-shaped ball mills. Energy-intensive mixers are used to mix plasticizer solutions because polyvinyl alcohol-based plasticizers, especially medium and high polymerisers, thicken the suspension. The introduction of the plasticizer into the slurry enables the production of plasticized granulate with increased strength and better flowability, which is required in the automated many batch production of pressed products and for the production of those products which are rotated or rotated after compression.

Description of solution to a technical problem

The invention solves the problem of the selection of raw materials and additives used for the production of steatite in the form of granules by producing a plasticized steatite granulate by simple technological processes, from which pressing technology followed by sintering in an oxidizing atmosphere at a temperature of 1220 to 1290 ° C, made by steatite products. After sintering, the material has a low dielectric loss factor and low runoff at elevated temperatures.

The first object of the invention is a composition for steatite material, characterized in that it contains, by weight, based on the weight of the total composition, from 68 to 75% of microcrystalline talc containing less than 0.6% CaO and less than 1% Fe ₂ O ₃ ;

from 10.5 to 12.0% of barium carbonate containing more than 98% of BaCO ₃ ;

from 7.5 to 9.0% kaolin containing less than 0.4% Fe ₂ O ₃ , less than 0.3% K ₂ O and less than 0.1% Na ₂ O;

from 6.0 to 8.0% of finely ground zirconium silicate containing less than 0.4% Fe ₂ O ₃ and less than 0.3% Na ₂ O;

from 1.5 to 2.5% bentonite containing less than 3% Fe ₂ O ₃ , less than 2% CaO, less than 2% Na ₂ O and less than 2% K ₂ O;

a combination of 0.2 to 0.4% of medium polymerized polyvinyl alcohol and 0.3 to 0.6% of highly polymerized polyethylene glycol as a plasticizer; and a deflocculant common to steatite suspensions.

All ingredients are commercially available.

From the above ingredients selected according to the invention, in the final sintered bark, a low content of alkali and iron and calcium is obtained, and at the same time an increased content of barium and zirconium, which in the sintering process produces steatite with low dielectric losses, water absorption of 0.0% and high mechanical strength.

Another object of the invention is a process for the manufacture of a steatite granulate, characterized in that deflocculant, kaolin, bentonite and barium carbonate are dissolved in the mill in water, rotate the mill for a short time, then add talc and a combination of flakes in the solid state and grind to suspensions to a mulch from 0.4 to 1.5% of the residue per sieve of 0.063 mm, and then the suspension is spray dried to form a granulate.

Unlike the known process, where conventional plasticizers based on medium-polymerized polyvinyl alcohol in an amount of more than 0.3% by weight are mixed into the suspension after grinding, in the form of solutions, the present invention uses the aforementioned solid-state combination of plasticizers in flakes. Therefore, strong post-mixing, which is otherwise required in the known process, is eliminated. Thus, in the process according to the invention, the grinding and homogenization of all ingredients is carried out in one step only. However, much more plasticizer can be added, which is very desirable, but so far has not been economically justified by the complicated process management.

The process is carried out by dosing all raw materials and water into the mill, e.g. mill with ceramic balls and ceramic liners, before commencing grinding. After the grinding is complete, the suspension is pumped into a pool under a spray tower and dried into granules. The residue on the sieve is determined in ceramic technology according to the usual rinsing procedure with water.

Another object of the invention are steatite products with low dielectric loss factor and low discharge currents, characterized in that they are prepared from the steatite granulate of the invention.

The subject of the invention is also a process for the preparation of steatite products with low dielectric loss factor and low discharge currents, characterized in that the steatite granulate according to the invention is compressed dry and then sintered in an oxidizing atmosphere at a temperature of 1220 to 1290 ° C. Due to its composition according to the invention, the granulate is therefore burned at lower temperatures compared to the prior art. One skilled in the art will determine the sintering temperature based on its compressed density based on his / her general knowledge.

The following Table 1 gives some of the properties of commercially available low loss steatite from the C 221 steatite group (international standard EN 60 672, Part 3, defines low alkaline steatites under code C221) compared to the steatite material of the invention (code NW - 12) .

Table 1

Features Symbol Unit C 221 NW- 12 1 open porosity Well vol.% 0.0 0.0 2 volume density Well Mgm ' ³ 2.7 2.92 3 flexural strength σ _ή MPa 140 140 4 mean linear a (30 ° C IO ' ⁶ K' ¹ 7-9 5.9 expansion coefficient up to 300 ° C) from (30 ° C IO ' ⁶ K' ¹ 7-9 6,9 up to 600 ° C) a (30 ° C to 1000 ° C) 1θ-6 _K -1 8-10 7.9 5 relative dielectric e _t 6 7.1 constant 6 dielectric loss factor at 1 MHz, max tandiM IO ' ³ L2 0.7 7 Drainage currents (240 ° C, 1260W) mA 1,2-1,6 0.67 - 0.92

As can be seen from Table 1, the new material of the invention has the same mechanical properties as the commercially available steatite material, a lower coefficient of expansion and, therefore, a higher resistance to temperature changes, a lower dielectric loss factor and lower elevated discharge currents.

It is a further object of the invention to use steatitmh products with low dielectric loss factor and low discharge currents for devices where no discharge currents may occur, e.g. for insulation of heaters and fireboxes, and as a substitute for commonly available steatite materials.

The invention will be explained by the following embodiments, which are by no means limiting.

Example 1

Preparation of steatite granulate

Ingredients used containing natural moisture: crushed Australian talc (1402 kg), grain size 0 to 15 mm, low alkali kaolin (184.8 kg), synthetic barium carbonate (236 kg), zirconium silicate 325 mesh (146 kg) ), low alkaline, Ca and Fe bentonite (53.9 kg), conventional deflocculant (28.7 kg), medium polymerized polyvinyl alcohol (7 kg) and highly polymerized polyethylene glycol (9 kg) for steatite suspensions.

Water, deflocculant, kaolin, bentonite and barium carbonate are dosed into a ball mill of 82% A1 ₂ O ₃ . Rotate the mill for about 5 minutes, then add talc and plasticizer. The water is dosed so that the ratio of raw materials without moisture to water is 61.5 / 38.5. The mass ratio of raw materials without moisture to the condensed balls is 1 / 2.1. Grind for 20 hours or. to ground before weeding into the pool: dry residue of 0.4-0.6% on a sieve of 0.063 mm.

After grinding, the suspension is poured into the mixer pool via a vibrating sieve of 0.350 mm mesh. In the pool, the suspension is sprayed with slow stirring for at least one day, specifically 2 days, before being sprayed. The suspension is dried into granules of less than 0.630 mm according to the usual procedure in a spray-spray tower. The particle size below 0.1 mm is less than 0.5%. The moisture content of the granulate after dispersion is below 2%.

The following Table 2 shows the typical chemical composition (oxides in%) of raw materials based on ICP (mductive coupled plasma) analysis.

Table 2

Suro- wines SiO ₂ A1 ₂ O ₃ Fe ₂ O ₃ MgO CaO At ₂ O K ₂ O TiO ₂ BaO ZrO ₂ LOI * 1) Australian talk 59.03 0.99 0.78 32.03 0.47 0.08 0.10 0.03 6.4 2) kaol- and 55.31 29.74 0.3 0.18 0.2 0.01 0.17 0.16 13,4 3) barium carbonate Solvay 76.14 21.9 4) Zirconium silicate 325 mesh 30.38 1,27 0.2 0.17 0.19 0.06 0.10 66.01 0.7 5) Bentonite 56.7 22,92 2,33 2.54 1,75 1.60 1.10 0.22 10.4

* LOI = heat loss

Table 3 shows the typical chemical composition of the steatite granulate of the invention for dry compression made from the ingredients of the invention according to ICP analysis.

Table 3

SiO ₂ A1 ₂ O ₃ Fe ₂ O ₃ MgO CaO At ₂ O K ₂ O TiO ₂ BaO ZrO ₂ LOI granulate 52.26 3.87 0.64 21.19 0.36 0.14 0.07 0.07 8.09 3.85 9,4

Example 2

Preparation of sintered product

On a single-axis press for dry pressing, products (tablets, tiles, binders, beads) are formed by compressing the granulate in a steel die. The compressed product density is 2.15 to 2.20 g / cm ³ .

The granulate of Example 1 is sintered at a temperature of 1240 ° C in an electric or gas furnace for 60 minutes in an oxidizing atmosphere.

The physical properties of the sintered steatite obtained are:

dielectric properties at 1 MHz:

C (capacity) = 9.1 pF k '(relative dielectricity) = 7.1 tanb = 7. IO ⁴ drain currents (240 ° C, 1260 W). from 0,7 - 0,9 mA volume density = 2,92 g / cm ³

Claims (5)

Composition for steatite material, characterized in that it contains, by weight, based on the weight of the total composition, from 68 to 75% of microcrystalline talc containing less than 0.6% CaO and less than 1% Fe ₂ O ₃ ; from 10.5 to 12.0% of barium carbonate containing more than 98% of BaCO ₃ ; from 7.5 to 9.0% kaolin containing less than 0.4% Fe ₂ O ₃ , less than 0.3% K ₂ O and less than 0.1% Na ₂ O; from 6.0 to 8.0% of finely ground zirconium silicate containing less than 0.4% Fe ₂ O ₃ and less than 0.3% Na ₂ O; from 1.5 to 2.5% bentonite containing less than 3% Fe ₂ O ₃ , less than 2% CaO, less than 2% Na ₂ O and less than 2% K ₂ O; a combination of 0.2 to 0.4% of medium polymerized polyvinyl alcohol and 0.3 to 0.6% of highly polymerized polyethylene glycol as a plasticizer; and a deflocculant common to steatite suspensions. 2. A process for the production of a steatite granulate of the composition of claim 1, characterized in that defoculant, kaolin, bentonite and barium carbonate are dissolved in the mill, rotate the mill for a short time, then add talc and a solid state plasticizer combination in flake and grind to a slurry until grinding from 0.4 to 1.5% of the residue on a sieve of 0.063 mm, then spray dry the slurry to form a granulate. -1010 3. Steatite product with low dielectric loss factor and low discharge currents, characterized in that it is prepared from the steatite granulate obtained according to claim 2. Process for the preparation of steatite products with low dielectric loss factor and low discharge currents, characterized in that the steatite granulate obtained according to claim 2 is pressed dry and then sintered in an oxidizing atmosphere at a temperature of 1220 to 1290 ° C. 5. Use of steatite products with low dielectric loss factor and low discharge currents for devices where no discharge currents may occur and as a substitute for commonly available steatite materials.