Substrate with low-permeability coating for the solidification of silicon

Abstract

A substrate characterised in that it is at least partially surface-coated with a coating containing at least one so-called barrier layer having silica and one or more material(s) X selected from among SiC, Si, Si.sub.3N.sub.4, in which layer the amount of X varies between 25-wt. % and 50.-wt. % in relation to the total weight of the barrier layer, the barrier layer being formed by grains of one or more materials X covered at least partially in a silica shell, and the barrier layer being in direct contact with the substrate.

Claims

1. A crucible wherein it is at least partially surface-coated with a coating containing at least one layer, termed barrier layer, comprising silica and one or more material(s) X chosen from SiC, Si and Si.sub.3N.sub.4, in which layer the weight amount of X ranges from 25% to 50% relative to the total weight of the barrier layer, said barrier layer being formed from grains of one or more materials X covered at least partially with a silica shell, the barrier layer being placed in direct contact with the crucible, the barrier layer having a thickness between 10 m and 100 m, said crucible being made of a material selected from the group consisting of silicon carbide SiC, silicon nitride Si.sub.3N.sub.4, composites comprising graphite and silicon carbide or comprising graphite and silicon nitride, and siliconized graphite.

2. The crucible as claimed in claim 1, wherein the barrier layer has an open porosity ranging from 0 to 5%.

3. The crucible as claimed in claim 1, wherein the barrier layer has a permeability of less than 10.sup.15 m.sup.2.

4. The crucible as claimed in claim 1, wherein the barrier layer is in the form of a silica matrix into which are incorporated grains of one or more material(s) X.

5. The crucible as claimed in claim 1, wherein the barrier layer comprises between 50% and 75% of silica by weight relative to the total weight thereof.

6. The crucible as claimed in claim 1, wherein the specific surface area of the barrier layer is between 5 cm.sup.2/g and 5 m.sup.2/g.

7. The crucible as claimed in claim 1, wherein the coating also comprises, at the surface of the barrier layer, a release layer.

8. The crucible as claimed in claim 1, wherein it is a crucible for solidifying silicon.

9. A process for preparing a crucible as claimed in claim 1, said crucible being at least partially surface-coated with a coating forming a gas barrier, wherein said process comprises at least the steps: a) providing a fluid medium comprising at least one or more material(s) X chosen from SiC, Si and Si.sub.3N.sub.4; b) applying said fluid medium to the surface of the crucible in a sufficient amount to form a deposit thereon, c) treating said deposit under an oxidizing atmosphere, at a temperature of between 1000 C. and 1300 C. and under conditions sufficient to form a barrier layer comprising silica and one or more material(s) X chosen from SiC, Si and Si.sub.3N.sub.4, in which layer the weight amount of X ranges from 25% to 50% relative to the total weight of the barrier layer, said barrier layer being formed from grains of one or more materials X covered at least partially with a silica shell, the barrier layer being placed in direct contact with the crucible, the barrier layer having a thickness between 10 m and 100 m, said crucible being made up of a material chosen from silicon carbide SiC; silicon nitride Si3N4; composites comprising graphite and silicon carbide or comprising graphite and silicon nitride; and siliconized graphite.

10. The process as claimed in claim 9, wherein the fluid medium also comprises silica.

11. The process as claimed in claim 9, wherein it comprises, between step b) of forming a deposit and step c) of treating under an oxidizing atmosphere, at least one step of drying at a temperature of less than 50 C.

12. The process as claimed in claim 9 wherein the step of treating under an oxidizing atmosphere is carried out at a temperature ranging from 1100 C. to 1300 C.

13. The process as claimed in claim 9, wherein step c) of treating under an oxidizing atmosphere is carried out for a period ranging from 1 hour to 5 hours.

14. The process as claimed in claim 9, comprising, after the treatment under an oxidizing atmosphere, a step of treating in the presence of a neutral gas at a temperature of between 1400 C. and 1500 C.

15. The process as claimed in claim 9, wherein step b) is repeated several times before step c) is carried out.

16. The process as claimed in claim 9, wherein the fluid medium used in step a) comprises one or more material(s) X in an amount ranging from 15% to 35% by weight relative to the total weight of said fluid.

Description

EXAMPLE

(1) A slip, consisting of 23% of a mixture of Si.sub.3N.sub.4 powder mixture, 4% of polyvinyl alcohol PVA and 73% of water, as weight percentages, is passed through a planetary mill filled with silicon carbide balls or agate balls.

(2) Since the objective of the silicon carbide balls or agate balls is only to reduce the powder agglomerates, silicon nitride balls may also be envisaged, the risk of pollution with nitrogen being very limited.

(3) The fluid medium thus formed is then sprayed with a spray gun (compressed air pressure of 2.5 bar, 0.4 mm nozzle placed about thirty centimeters from the substrate) onto the entire inner surface of a crucible to be coated.

(4) The deposit thus obtained is dried with hot air at a temperature below 50 C.

(5) A sublayer with a thickness of about 50 m, consisting of PVA-bound powders, is thus obtained.

(6) This layer is then subjected to a hold of 3 h at 1100 C. under air for debonding and oxidation of the powders.

(7) Once this oxidation treatment has been carried out, the weight fraction of oxygen in the coating is 29% measured by the IGA (Interstitial Gas Analysis) technique. This technique, well known to those skilled in the art, makes it possible to establish that the corresponding volume fraction of silica is 64%, which corresponds to a silica content of 56% by weight relative to the total weight of the coating.

(8) The weight amount of Si.sub.3N.sub.4 relative to the total weight of the mixture of silica and Si.sub.3N.sub.4, is therefore 44%.

(9) Presented in FIG. 1 is the coating obtained at the end of example 1. This coating is in the form of a matrix of SiO.sub.2 into which are incorporated nonoxidized grains of Si.sub.3N.sub.4.