1. 1 Challenges of carbothermic route of solar silicon synthesis M.A. Arkhipov, A.B.Dubovskiy, A.A. Reu, V.A. Mukhanov, S.A. Smirnova Quartz Palitra Ltd. 1, Institutskaya St., Alexandrov, Vladimir Region 601650, Russia Email: arkh8@yahoo.comarkh8@yahoo.com

2. 2 Traditional route for silicon synthesis MG: SiO 2 + 2C = Si+ 2CO 2N, B, P = 20-40 ppm Si + 3HCl = SiHCl 3 + H 2 SiHCl 3 + H 2 = Si + 3HCl 9N, B, P = 0.001– 0.1 ppm SOLAR & SEMI

3. 3 World production of solar grade silicon Production: 25 000 -30 000 tonnes/year Demand: over 50 000 tonnes/year Booking up to Y 2019 Main drawbacks Ecoligical threats – due to chlorine use Machinery - absence of “turnkey” suppliers.

4. 4 Alternative route SiO 2 + 2C = Si + 2CO 4N, B, P ~ 1 ppm Purification by Direct Solidification and Chemical etching to 6N, B, P = 1 ppm

5. 5 MG carbo process Solar carbo process Quartz Quartzite 2N-3NQuartz 4N5 Carbon Charcoal, coke 2N-3N Thermal black 4N Electrode Carbon 4NGraphite 4N

6. 6 Si SiC Si drops Electrode Arc furnace before stocking Raw material Oxide lining Carbon lining

7. 7 1. SiO 2 + C = SiO + CO 2. SiO + 2C = SiC + CO 3. SiC + SiO = 2Si + CO 4. 2SiO = SiO 2 + Si 5. 2SiC + SiO 2 = 3Si +2CO 6. 2SiO 2 + SiC = 3SiO + CO

8. 8 Equilibrium SiO pressures after Schei, Tuset and Tveit.

9. 9 SiO +2C = SiC +CO 2SiO = SiO 2 +Si

10. 10 For carbon important: pores, surface area diffusivity Ideal : upper zone SiC formation lower zone SiC → Si

11. 11 SiO 2 + C(1+x) = x Si + (1-x)SiO + (1+x)CO x – yield x = 0.8-0.9 for MG silicon x = 0.6-0.85 for solar silicon

12. 12 Silicon move in high temperature zone T X Si Energy stored in liquid-solid surface is decreased strongly with temperature rise

13. 13 Si SiC SiC + quartz charge Arc is strong Silicon is collected under electrode

14. 14 Si SiC SiC + quartz current Too big concentration of SiC or too high conductivity of charge Uniform heating Silicon remains at sintering place

15. 15 AC arc DC arc t 1 – arc absent because of low voltage

16. 16 + _ High electrode consumption and contamination

17. 17 High purity materials Low reaction ability SiC formation near bottom Solution Catalyst that can be removed during process

18. 18 Carbon-powder Charcoal-foam use glue Briquette: quartz, carbon, glue Quartz 10% - 75% weight

19. 19 Reaction in briquette (upper zone) 1. SiO 2 + C = SiO + CO 2. SiO + 2C = SiC + CO Sources SiO: a) reaction#1 b) from bottom zone

20. 20 Optimum gas flow inside briquette Stage 1: SiC formation Stage 2: binder lose cementing ability

21. 21 Weak cementing force or low density briquette C C C SiO 2 SiO

22. 22 Strong cementing force or high density briquette C C SiO 2 C SiC C C SiO SiC

23. 23 150 kW DC arc furnace V = 28-65 V I = 1500-3600 A Graphite lining Graphite electrode

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30. 30 Average batch purity: 99.98% B = 0.4 ppm P = 2 ppm Na = 20 ppm Al = 60 ppm Ca = 10 ppm Ti = 15 ppm Fe = 50 ppm Mn = 1 ppm Mg =1.5 ppm Cu = 1.5 ppm Zr = 2 ppm Main impurities

31. 31 Maximum batch weight: 15 kg Energy consumption: 35 kW*h/kg

32. 32 CONCLUSIONS: 1. Carbothermic arc technology presuppose SiC sintering below 1900 °C.To meet the requirement with high purity components efficient to use catalyst.

33. 33 2. DC arc furnace is more efficient than AC: a) less electrode consumption (if electrode is cathode) b) less contamination c) less loss of energy through electrode

34. 34 3.Binder (cement), chemical composition of briquette and method of its preparation are to guarantee: a) SiC formation in upper zone b) High resistivity

35. 35 4. After SiC formation it’s important to avoid losing SiO by reaction: SiC + 2SiO 2 = 3SiO + CO

36. 36 5. Important to keep top of furnace “cold” and bottom “hot” to provide condensation of SiO gas to get capsulation of crater.

37. 37 The present work was done under the contract with Big Sun Energy Technology Co., Ltd.