CS198809B1 - Method of producing self-binding compact silicon carbide - Google Patents

Description

The invention relates to a process for the production of self-binding dense silicon carbide (SiC) by compressing the starting composition at a pressure of 100-140 MPa to obtain compacts and firing these compacts in the presence of crushed silicon at a temperature of 2000-2200 ° C.

Dense polycrystalline silicon carbide is a top-quality material in the field of silicon carbide-based materials. At present, one of the ways leading to its preparation is the method of reaction sintering, where the binding component of the body is formed by chemical reaction during firing. The heat treatment of a suitably selected starting composition results in a secondary silicon carbide which joins the primary grains of the starting mixture into a dense whole. High silicon carbide content, small amount of impurities give the product top quality. An important role in the reaction sintering process is currently the appropriate composition of the starting mixture. In addition to the binder, substances which serve as a carbon source for reaction with silicon to form a secondary silicon carbide forming a binding component are introduced into the silicon carbide mixture of different granulometric composition.

The nature of these substances and the way in which they are introduced into the mixture determines the further processing of the blanks. Some carbon sources require chemical treatment (polycondensation of furfural) or recrystallization firing (when the carbonaceous material is fed to the preform in the liquid state) that precedes its own high-temperature reaction sintering, increasing the cost of producing dense silicon carbide. Methods are known to address these complications

198 809

198 809. However, they use a fairly high percentage of a carbon source (graphite, furfural, diesel coke), which results in a higher residual carbon content in the body and often leads to surface defects, particularly the cracking of the product. Another disadvantage is the fact that when carbonaceous substances are dosed in liquid form, it is necessary to use double firing at a high temperature of up to 2200 ° C, which makes production more expensive.

These drawbacks of the prior art are overcome by the invention, which comprises using as a starting composition a mixture consisting of 47 to 53% by weight of silicon carbide with a grain size of 630 to 800 µm and / or silicon carbide with a grain size of 60 to 400 and 33 to 35% by weight of ground silicon carbide having a sieve residue of less than 5% on a mesh of 0.063, and 8 to 11% by weight of crystalline graphite of which at least 50% of the particles are less than 5 µm and 100% of the particles are smaller. and from 4 to 9% by weight of a 5% aqueous solution of sulphite lye or an 8% aqueous solution of polyvinyl alcohol. ¹

An advantage of the invention is that the mass containing a relatively small amount of carbonaceous material (up to 15% graphite) has a smaller percentage of surface defects and requires only a one-time heat treatment, the material obtained having the same peak properties as those obtained by the above processes.

The invention will be described and explained in more detail on two examples of possible embodiments.

Example 1

In a particular embodiment of the invention, a masterbatch was prepared comprising:

silicon carbide with a specific grain size of 630 to 800 µm 47.4 weight% silicon carbide with a specific grain size of 250 to 315 µm 4.3 weight% of silicon carbide ground 34.5 weight% graphite CR - 5 (ie 100% particles * 12 µm carbon content 99.5% minimum, ash content 0.5% maximum) 8.6% by weight

5% aqueous solution of sulphite liquor 5.2% by weight

Roller blanks with a diameter of 15 mm and a height of 15 mm and beams of 7 x 7 x 70 mm were formed by pressing the above composition at 140 MPa, embedded in graphite sleeves and covered with a layer of crushed silicon of sufficient purity (at least 98%). The layer of silicon exceeded the height of the moldings by at least 5 mm. Silicon then became the second component of the reaction during firing

The firing was carried out in an inert nitrogen atmosphere (which may also be argon) at a temperature of 2000 ° C by a suitably selected mode and held at a final temperature of 30 to 60 minutes.

By this process dense polycryetallic silicon carbide 8 was obtained with high physicochemical and mechanical properties, as follows:

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density 3.1 g / cm @ 2 absorbability 0% mechanical bending strength HOMPa mechanical compressive strength 49OMPa thermal conductivity 42 W / m.K specific heat 590 J / kg.K resistance to sudden changes 1300 ° C - - > 20 ° C temperature 10 cycles 1,500 ° C - -15 ° C mean coefficient of thermal linear expansion 4.03 ° C ^_1 . IO ⁶

Example 2

In a specific example according to the invention, a base mixture having the following composition was prepared:

silicon carbide having a specific grain size of 200 to 250 µm 51.2 weight% silicon carbide having a specific grain size of 63 to 80 µm 12.8 weight% ground carbide 21.3 weight% graphite CR-5 (see Example 1) 7.7 weight% %

8% aqueous polyvinyl alcohol solution 7% by weight

The procedure was the same as in the first example. The resulting material had the following characteristics:

bulk density 3,02 g / cm ^ absorbability 0% mechanical flexural strength 90MPa mechanical compressive strength 795MPa thermal conductivity 30 W / m.K specific heat 690 J / kg.K

Dense self-bonding silicon carbide finds use in a number of applications that utilize its high strength, thermal conductivity, resistance to thermal shock and corrosion. These include heat exchanger components, main heating tubes, molds for metal casting, protective sleeves for pyrometers, various abrasion-resistant components such as nozzles, pump pistons and ball bearings. The use of dense silicon carbide in generators seems to be promising.

Claims (1)

A process for producing self-bonded dense silicon carbide by molding the starting composition at a pressure of 100 to 140MPa to obtain compacts and firing these compacts in the presence of crushed silicon at a temperature of 2,000-2,200 ° C, characterized in that a starting material composition consisting of 47-53 % by weight of silicon carbide with a specific grain size of 630 to 800 µm and / or silicon carbide with a specific grain size of 60 to 400 µm, 33 to 35% by weight of ground silicon carbide, the net residue of which is 0.063 less than 5%, 8 to 11% by weight of graphite crystalline, of which at least 50% of the particles are less than 5 µm 198,809 and 100% of the particles are less than 12%, and 4 to 9% by weight of a 5% aqueous solution of sulfite lye or an 8% aqueous solution of polyvinyl alcohol.