GB1596303A

GB1596303A - Production of silicon carbide bodies

Info

Publication number: GB1596303A
Application number: GB1025377A
Authority: GB
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1978-03-02
Filing date: 1978-03-02
Publication date: 1981-08-26

Description

(54) IMPROVEMENTS IN OR RELATING TO PRODUCTION OF SILICON CARBIDE BODIES (71) We, UNITED KINGDOM ATOMIC ENERGY AUTHORITY, London, a British Authority, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed. to be particularly described in and by the following statement: This invention relates to the production of silicon carbide bodies and, in particular, bodies formed of self-bonded silicon carbide by reaction sintering of a coherent mixture of silicon carbide and carbon in the presence of molten silicon. Such reaction sintering is hereinafter referred to as "siliconising".

One method of siliconising is described in British Patent Specification No. 1,180,918.

In the production of self-bonded silicon carbide bodies according to the invention, by reaction sintering of a coherent mixture of silicon carbide and carbon in the presence of molten silicon, the coherent mixture is formed by casting a slip consisting essentially of carbon and silicon carbide particles dispersed in a liquid medium.

Slip-casting is a well-established technique in which solid particles are mixed with a suitable liquid medium to form a suspension or slip which is put into a mould porous to the liquid. The liquid is absorbed by the mould and a solid cast is formed by deposit of the particles on the wall of the mould.

When the required cast thickness has been obtained surplus slip is poured away and the casting is removed, dried and usually sintered. To form self-bonded silicon carbide bodies fro a slip-casting of a mixture of silicon carbide and carbon in accordance with the invention the casting is siliconised by reaction sintering in the presence of silicon as already explained.

Because there is a molecular volume change when the carbon in the casting is converted to silicon carbide it is possible to obtain self-bonded silicon carbide bodies of much higher density (and also of greater strength) than can be achieved by slipcasting silicon carbide alone. Indeed advantage is taken in carrying out the invention of the fact that slip-casting in general produces green bodies of low density and that the two component C/SiC system, by selective grading of particle size, products castings in the region of 60% of the theoretical density. Thus a body containing 1 part carbon to 4 parts silicon carbide must contain 33% porosity if it is to yield 90% dense silicon carbide on siliconising and one containing 1 part carbon to 2 parts silicon carbide must contain 42% porosity if it is to yield 90% dense silicon carbide on siliconising.

These are the densities normally desired for self-bonded silicon carbide bodies.

To encourage build up of a deposit on the wall of the mould as liquid is absorbed, rather than of a loose sediment at the base of the mould, the slip is generally stabilised so as to delay settling. This is conveniently done by electrically charging the solid particles so that they tend to repel one another.

In practice an "electric double layer" on the surface of the solid particles is formed by, for example, adding an electrolyte to the suspension so that ions with one charge are strongly absorbed on the surface of the particles and ions of opposite charge form a cloud about each particle. Such an electrolyte is termed a deflocculent and long-chain fatty acids are commonly used, often as a proprietary material of which a variety is available. We have found the deflocculant known as Dispex G40 to be suitable with water as the liquid suspension medium for the mixture of silicon carbide and carbon required to produce self-bonded silicon carbide bodies. Dispex is a Trade Mark and Dispex G40 is produced and sold by Allied Colloids Ltd. Bradford, England. It is stated to be a salt of a polymeric carboxylic acid.

In slip-casting the size, shape and surface condition of the solid particles is important in ensuring that they interlock and adhere properly when compressed. Attention must therefore be given to these features when slip-casting mixtures of silicon carbide and carbon particles in accordance with the invention. Satisfactory results have however geen obtained with alpha silicon carbide which is angular - up to 1200 BSS mesh size and also with beta silicon carbide which can be produced by pyrolysis of silanes as approximate spheres of sub-micron size.

Segregation of carbon and silicon carbide during casting has not been noted. It is rather unexpected that it does not occur.

The following is an example of one way of carrying the invention into effect.

Servacarb carbon black which is a carbon powder of particle size less than 1 micron produced and sold by Philblack Ltd., London, England and commercially available green gnt silicon carbide (97% of particle size less than 7 microns and a mean particle size of 3 microns) are dry blended in a ball mill for about two hours in the ratio 1:2 by weight.

tune dry mix is added to water containing 0.1% Dispex G40 deflocculant to give a slurry which contains 70% by weight solids (45% by volume solids). The mixture is ball milled for a further 2 hours to break up agglomerates and to form a uniform slip.

The slip is passed through a coarse sieve to remove any lumps which remain, and then it is held under vacuum for a period of 5-10 min to de-airit.

The slip is poured into a plaster mould which has been previously coated with sodium alginate (to reduce casting rate and to prevent wrinkles forming). After about 10 min (the time depends upon the thick ness of the cast required), the excess slip is poured away and the cast is allowed to stand in the mould for 24 hours, to dry slowly.

(The approximate moisture content on removal from the mould should be about 16% by weight).

The cast is trimmed, allowed to dry slowly at room temperature for several hours and then siliconised.

WHAT WE CLAIM IS: 1. The production of self-bonded silicon carbide bodies by reaction sintering of a coherent mixture of silicon carbide and carbon in the presence of molten silicon, the coherent mixture being formed by casting a slip consisting essentially of carbon and silicon carbide particles dispersed in a liquid medium.

2. The production of self-bonded silicon carbide bodies as claimed in Claim 1 wherein the slip is stabilised by the presence of a deflocculent which is a salt of a polymeric carboxylic acid.

3. The production of self-bonded silicon carbide bodies as claimed in Claim 1 or Claim 2 wherein the silicon carbide particles are in the alpha form and pass a 1200 BSS mesh.

4. The production of self-bonded silicon carbide bodies as claimed in Claim 1 or Claim 2 wherein the silicon carbide particles are in the beta form.

5. The production of self-bonded silicon carbide substantially as hereinbefore described in the example.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

can be produced by pyrolysis of silanes as approximate spheres of sub-micron size.

Segregation of carbon and silicon carbide during casting has not been noted. It is rather unexpected that it does not occur.

The following is an example of one way of carrying the invention into effect.

Servacarb carbon black which is a carbon powder of particle size less than 1 micron produced and sold by Philblack Ltd., London, England and commercially available green gnt silicon carbide (97% of particle size less than 7 microns and a mean particle size of 3 microns) are dry blended in a ball mill for about two hours in the ratio 1:2 by weight.

tune dry mix is added to water containing 0.1% Dispex G40 deflocculant to give a slurry which contains 70% by weight solids (45% by volume solids). The mixture is ball milled for a further 2 hours to break up agglomerates and to form a uniform slip.

The slip is passed through a coarse sieve to remove any lumps which remain, and then it is held under vacuum for a period of 5-10 min to de-airit.

The slip is poured into a plaster mould which has been previously coated with sodium alginate (to reduce casting rate and to prevent wrinkles forming). After about

10 min (the time depends upon the thick ness of the cast required), the excess slip is poured away and the cast is allowed to stand in the mould for 24 hours, to dry slowly.

(The approximate moisture content on removal from the mould should be about 16% by weight).

The cast is trimmed, allowed to dry slowly at room temperature for several hours and then siliconised.

WHAT WE CLAIM IS: 1. The production of self-bonded silicon carbide bodies by reaction sintering of a coherent mixture of silicon carbide and carbon in the presence of molten silicon, the coherent mixture being formed by casting a slip consisting essentially of carbon and silicon carbide particles dispersed in a liquid medium.
2. The production of self-bonded silicon carbide bodies as claimed in Claim 1 wherein the slip is stabilised by the presence of a deflocculent which is a salt of a polymeric carboxylic acid.
3. The production of self-bonded silicon carbide bodies as claimed in Claim 1 or Claim 2 wherein the silicon carbide particles are in the alpha form and pass a 1200 BSS mesh.
4. The production of self-bonded silicon carbide bodies as claimed in Claim 1 or Claim 2 wherein the silicon carbide particles are in the beta form.
5. The production of self-bonded silicon carbide substantially as hereinbefore described in the example.