Method for synthesizing ultrahigh-purity silicon carbide

1. A method of forming polycrystalline SiC material comprising the steps of:

(a) positioning bulk carbon at a first location inside of a graphite crucible, wherein the bulk carbon is gas-permeable and vapor-permeable;

(b) positioning a mixture comprised of elemental silicon (Si) and elemental carbon (C) at a second location inside of the graphite crucible;

(c) following steps (a) and (b), removing adsorbed gas, or moisture, or volatiles or some combination of adsorbed gas, moisture and volatiles from the mixture and the bulk carbon positioned inside of the graphite crucible by heating the mixture and the bulk carbon positioned inside of the enclosed crucible to a first temperature which is below the melting point of the elemental Si;

(d) following step (c), forming as-synthesized silicon carbide (SiC) inside of the crucible by heating the mixture positioned inside of the enclosed crucible to a second temperature sufficient to initiate a reaction between the elemental Si and the elemental C of the mixture that forms the as-synthesized SiC inside of the crucible, wherein during each of steps (c) and (d) a vacuum pump evacuates at least the inside of the enclosed crucible; and

(e) following step (d), forming polycrystalline SiC material inside of the gas-permeable and vapor-permeable bulk carbon by heating the as-synthesized SiC and the bulk carbon in the presence of a temperature gradient sufficient to cause the as-synthesized SiC to sublime and produce vapors that migrate under the influence of the temperature gradient into the bulk carbon where the vapors condense on the bulk carbon and react with the bulk carbon forming the polycrystalline SiC material, wherein a lowest temperature of the temperature gradient is a third temperature.

2. The method of claim 1 , wherein the mixture consists essentially of elemental Si and elemental carbon C.

3. The method of claim 1 , wherein following steps (a) and (b), part of the mixture contacts part of the gas-permeable and vapor-permeable bulk carbon inside of the graphite crucible.

4. The method of claim 1 , wherein:

the gas-permeable and vapor-permeable bulk carbon is at least 99.9999% pure C;

the elemental Si is at least 99.9999% pure Si; and

the elemental C is at least 99.9999% pure C.

5. The method of claim 1 , wherein:

the gas-permeable and vapor-permeable bulk carbon is at least 99.9999% pure C;

the elemental Si is at between 99.99999% and 99.9999999% pure Si; and

the elemental C is at least 99.99999% pure C.

6. The method of claim 1 , wherein:

the elemental Si is comprised of lumps or granules of polysilicon, with each lump or granule having a maximum linear dimension of 1 to 7 mm; and

the elemental C is a carbon powder.

7. The method of claim 1 , wherein:

the gas-permeable and vapor-permeable bulk carbon is carbon black, carbon beads or pelletized carbon black; and

the gas-permeable and vapor-permeable bulk carbon has a density between 0.3-0.5 g/cm 3 .

8. The method of claim 1 , wherein step (e) is carried out in the presence of either a vacuum or a pressure of inert gas between 1 and 50 Torr.

9. The method of claim 8 , wherein the inert gas is argon.

10. The method of claim 1 , wherein the first temperature is less than the second temperature, and the second temperature is less than the third temperature.

11. The method of claim 1 , wherein step (d) occurs for a period of time sufficient to complete the reaction between the elemental Si and the elemental C.

12. The method of claim 1 , wherein step (e) occurs for a period of time sufficient to substantially complete the sublimation of the as-synthesized SiC and the formation of polycrystalline SiC material inside of the gas-permeable and vapor-permeable bulk carbon.

13. The method of claim 1 , wherein the mixture of step (b) has a C:Si atomic ratio between 1:1 and 1.2:1.

14. The method of claim 1 , comprising one or more of the following:

the first temperature is between 1300° C. and 1400° C.;

the second temperature is between 1550° C. and 1800° C.; and

the third temperature is between 2200° C. and 2400° C.

15. The method of claim 1 , wherein the polycrystalline SiC material comprises:

a mixture of alpha (hexagonal) SiC polytypes;

particle sizes between 0.2 and 2 mm;

a concentration of nitrogen <8·10 15 cm −3 ;

a concentration of boron <6·10 15 cm −3 ;

a concentration of aluminum below 1·10 15 cm −3 ;

a concentration of iron below 3·10 14 cm −3 ; and

a concentration of titanium below 3·10 14 cm −3 .

16. The method of claim 1 , comprising one or more of the following:

step (d) immediately follows step (c);

the vacuum in step (c) runs between 10 −3 -10 −4 Torr near the beginning of step (c) to between 10 −5 -10 −6 Torr just prior to step (d);

step (e) immediately follows step (d);

the vacuum in step (d) runs between 10 −2 -10 −3 Torr near the beginning of step (d) to less than 10 −4 Torr just prior to step (e); and

the vacuum in step (e) is less than 10 −4 Torr.

17. The method of claim 1 , wherein following steps (a) and (b) and prior to step (c), the method further including the step of: outgassing the mixture and the gas-permeable and vapor-permeable bulk carbon positioned inside of the graphite crucible via the vacuum pump evacuating at least the inside of the enclosed crucible at ambient temperature.