Challenges of carbothermic route of solar silicon synthesis

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.com

2. Traditional route for silicon synthesis MG: SiO2 + 2C = Si+ 2CO 2N, B, P = 20-40 ppm Si + 3HCl = SiHCl3 + H2 SiHCl3 + H2 = Si + 3HCl 9N, B, P = 0.001– 0.1 ppm SOLAR & SEMI

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. Alternative route SiO2 + 2C = Si + 2CO4N, B, P ~ 1 ppm Purification by Direct Solidification and Chemical etching to 6N, B, P = 1 ppm

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

7. SiO2 + C = SiO + CO • SiO + 2C = SiC + CO • SiC + SiO = 2Si + CO • 2SiO = SiO2 + Si • 2SiC + SiO2 = 3Si +2CO • 2SiO2 + SiC = 3SiO + CO

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

9. SiO +2C = SiC +CO2SiO = SiO2 +Si

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

11. SiO2 + C(1+x) = x Si + (1-x)SiO + (1+x)COx – yieldx = 0.8-0.9 for MG siliconx = 0.6-0.85 for solar silicon

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

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

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

15. AC arc DC arc t1 – arc absent because of low voltage

16. _ + High electrode consumption and contamination

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

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

19. Reaction in briquette (upper zone) 1. SiO2 + C = SiO + CO 2. SiO + 2C = SiC + CO Sources SiO: a) reaction#1 b) from bottom zone

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

21. Weak cementing force or low density briquette C C C SiO SiO SiO2 SiO2

22. Strong cementing force or high density briquette C SiO2 C C SiO2 SiC SiO2 SiO C SiC C

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

30. Average batch purity: 99.98% Main impurities 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

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

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

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

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

35. 4. After SiC formation it’s important to avoid losing SiO by reaction: SiC + 2SiO2 = 3SiO + CO

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

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