Coating composition for a mould
A coating system for a mould for the directional solidification of silicon. The coating is applied as an aqueous slurry of silicon nitride particle, carbon black and microsilica.
1. A coating composition for a mould for the directional solidification of silicon, which comprises silicon nitride particles, a carbon and an oxide and/or silicon powder, wherein the carbon represents from 1 to 20 wt %, the oxide represents 0.5 to 48 wt %, the silicon powder represents 1 to 20 wt % and silicon nitride represents at least 51 wt % and is the balance of the composition.
2. The composition of claim 1 , wherein the oxide is SiO 2 .
3. The composition of claim 2 , wherein the SiO 2 is amorphous microsilica.
4. The composition of claim 2 , wherein the SiO 2 is amorphous fumed silica.
5. The composition of claim 1 , wherein the carbon is carbon black.
6. The composition of claim 1 , wherein the silicon nitride particles fall in the size range 0.1 to 2 microns.
7. The composition of claim 6 , wherein the silicon nitride particles fall in the size range 0.4 to 1 microns.
8. The composition of claim 1 , wherein the carbon represents 1 to 15 wt % of the composition.
9. The composition of claim 1 , wherein the oxide represents 2 to 25 wt % of the composition.
10. The composition of claim 1 , wherein the silicon powder represents 2 to 15 wt % of the composition.
11. The composition of claim 1 , dispersed in a liquid.
12. The composition of claim 11 , wherein the liquid is water.
13. The composition of claim 12 , wherein the composition is an aqueous dispersion and further includes a binder.
14. The composition of claim 13 , wherein the binder is present as up to 46 wt %, based on the weight of dry solids.
15. The composition of claim 1 , further comprising a dispersant.
16. The composition of claim 15 , wherein the dispersant is present as 0.1 to 5 wt % based on the weight of dry solids.
17. The composition of claim 16 , wherein the dispersant is present as 0.2 to 1 wt % based on the weight of dry solids.
18. A coating composition for a mould for the directional solidification of silicon, which comprises silicon nitride particles, a carbon and an oxide and/or silicon powder, wherein the carbon represents 2 wt % carbon black and the oxide represents 10% microsilica, the silicon nitride represents at least 51 wt % and is the balance of the composition.
19. A coating composition for a mould for the directional solidification of silicon, which comprises silicon nitride particles, a carbon and an oxide and/or silicon powder; the carbon represents from 1 to 20 wt %, the oxide represents 0.5 to 47 wt %, the silicon powder represents 1 to 20 wt % and silicon nitride represents at least 51 wt % and is the balance of the composition, wherein the composition is an aqueous dispersion and further include a binder, and the binder is polyvinylalcohol.
20. A coating composition for a mould for the directional solidification of silicon, which comprises silicon nitride particles, a carbon and an oxide and/or silicon powder; the carbon represents from 1 to 20 wt %, the oxide represents 0.5 to 47 wt %, the silicon powder represents 1 to 20 wt % and silicon nitride represents at least 51 wt % and is the balance of the composition, wherein the composition is an aqueous dispersion and further include a binder, and the binder is polyethylene glycol.
21. A coating composition for a mould for the directional solidification of silicon, which comprises silicon nitride particles, a carbon and an oxide and/or silicon powder; the carbon represents from 1 to 20 wt %, the oxide represents 0.5 to 47 wt %, the silicon powder represents 1 to 20 wt % and silicon nitride represents at least 51 wt % and is the balance of the composition; wherein the composition further comprises a dispersant and the dispersant is a polyacrylate salt of ammonium or sodium.