Effect of Three-Stage Cooling Process on the Purification of Solar-Grade Polysilicon by Solvent Refining in Al-30%Si Alloy

doi:10.16553/j.cnki.issn1000-985x.2025.0033

Abstract

Abstract: During solvent refining for solar-grade polysilicon， the cooling process significantly influences the nucleation， crystal growth and impurity segregation of primary silicon. This study investigated the cooling process of Al-30%Si alloy over a temperature range of 900 ℃ to 600 ℃ and compared the effects of single-rate cooling and three-stage cooling processes on the morphology and impurity content of primary silicon. The main objective is to optimize the solvent refining process. The results indicate that， under the single-rate cooling process， a cooling rate of 1 ℃/min can significantly increase the primary silicon grain size， reduce the impurity content in the primary silicon， and improve the impurity removal rate. In contrast， under the three-stage cooling process， a nodal temperature close to the high-temperature region results in a larger grain size and improves the purity and yield of primary silicon. The best purification effect can be achieved when the nodal temperature is set at 700 ℃. In this case， the grain size of primary silicon is slightly lower， while the impurity content is comparable to that before optimization. At the same time， the optimized process achieves a 62.3% reduction in cooling duration and a 23.3% decrease in energy consumption. This study provides a potential strategy to fabricate solar-grade polysilicon using Al-Si solvent refining， which is of significant experiment accumulation.

Key words: solar grade polysilicon; solvent refining in Al-Si alloy; cooling rate; nodal temperature; process optimization

CLC Number:

O782

TANG Hong, DI Jiahui, YANG Pingping, LI Shaomeng, SHI Yujie, HE Zhanwei, ZHAO Ziwei, GAO Mangmang. Effect of Three-Stage Cooling Process on the Purification of Solar-Grade Polysilicon by Solvent Refining in Al-30%Si Alloy[J]. Journal of Synthetic Crystals, 2025, 54(8): 1454-1462.

Figures/Tables 10

Table 1 Impurity content in raw materials

Impurity	Al	Fe	Cu	B	P	Ti	Mg	Ca
Al	—	926	189.86	50.48	28.05	12.92	2.17	48.82
Si	213.00	136.40	81.90	10.51	55.50	4.51	7.07	20.14

Table 2 Heat treatment process of alloy samples

Sample	Stage-1	Stage-2		Stage-3
Sample	Cooling rate/（℃·min^-1）（900~850 ℃）	Cooling rate/（℃·min^-1）（900~600 ℃）	Cooling rate/（℃·min^-1）（850 ℃~T）	Cooling rate/（℃·min^-1）（T~600 ℃）
N₁	—	0.5	—	—
N₂		1
N₃		4
N₄		7
N₅		11
N₆		15
N₇	Rapid cooling to 850 ℃	—	11	1
N₈
N₉
N₁₀

Fig.1 Alloy morphology. （a） ~ （e） Cooling rates of 1， 4， 7， 11， 15 ℃/min；（f） ~ （i） nodal temperatures of 625， 650， 675 and 700 ℃；（j） micro-morphology at a cooling rate of 1 ℃/min

Fig.2 Average length and width curves （a）， FN and yield curves （b） of primary Si at different cooling rates

Fig.3 Curves of impurity content of primary Si at different cooling rates

Fig.4 Average length and width curves （a），and FN and yield curves （b） of primary Si at different nodal temperatures

Fig.5 Curves of impurity content of primary Si at different nodal temperatures

Table 3 Impurity content of primary Si under different cooling rates and process optimizations

Sample	Al	Fe	Cu	B	P
N₂	387.9	1	0.71	5.03	10.99
N₃	387.9	4	2.97	6.88	12.87
N₄	468.7	9	7.71	10.13	14.99
N₅	565.8	24	10.89	13.24	16.58
N₆	485.4	90	12.58	17.39	16.89
N₇	386.1	18	4.92	20.30	20.61
N₈	305.9	20	2.86	15.00	19.76
N₉	285.4	14	1.15	10.10	16.22
N₁₀	278.8	4	0.65	6.40	11.38

Table 4 Comparison of different properties of primary silicon before and after optimization

Property		1 ℃/min	T=700 ℃
Size of Silicon/mm	Length	1.56	1.26
Size of Silicon/mm	Width	0.55	0.42
FN		0.66	0.65
Yield/%		58.93	58.41
Refining time/min		300	113
Specific energy consumption/kW		5.46	4.19

Fig.6 Distribution of primary silicon grains before and after process optimization

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