| As a novel and excellent preparation technology of solar grade polysilicon,metallurgical method has been widely studied because of its advantages of low cost consumption,green,and simple process.As a new process method in the metallurgical process,the alloy solidification refining method has become a research hotspot due to its advantages of low temperature refining and high efficiency of impurity removal.The main principle of the alloy solidification refining method is that impurities in silicon will occur segregation phenomenon during the recrystallization of silicon,and the impurities mainly concentrate in the liquid melting to achieve the purpose of purifying primary Si.However,the primary Si is precipitated discretely in the melting,so it is difficult to recover.In this paper,aiming at the above-mentioned problems in the alloy solidification refining,it is proposed that it grew bulk Si directionally in the Si-Al alloy by the temperature gradient zone melting(TGZM)method.At the same time,it achieved the separation and purification of the primary Si entirely.Moreover,it solved the recovery problem of the primary Si in the process of alloy solidification refining.A Si source was added to the upper end of the alloy to ensure precipitation of primary Si continually and rapidly,meanwhile,it increased the growth rate of bulk Si and achieved high efficient separation of primary Si.During the experiment,the mechanism of crystal nucleation growth of bulk Si was explored,and various factors affecting the growth morphology and growth rate of bulk Si were identified.Finally,the distribution and removal efficiency of impurities were analyzed.And the main conclusions are as following:(1)The growth process of bulk Si by TGZM was under a negative temperature gradient.The Si source dissolved at the high temperature side,and Si atoms diffused in the alloy melt,and primary Si precipitated continuously at the bottom low temperature side.The bulk Si was successfully grown in the Si-Al alloy by controlling the experimental conditions such as temperature gradient,initial temperature,lower rate,and addition of Si source.(2)The growth of bulk Si was mainly influenced by a variety of factors.Slower lower rate and larger temperature gradient could effectively increase the growth rate of bulk Si.When the temperature gradient was increased from 1.79K/mm to 3.97K/mm,the growth rate of bulk Si was increased from 1.9?10~-55 mm/s mm/s to 2.47?10~-44 mm/s.Addition of Si source and increasing of initial temperature could increase the content of Si in the alloy,which increased the growth rate of bulk Si.(3)The nucleation growth mechanism of bulk Si was a continuous growth mode,so the growth of bulk Si was affected by the solute transport.Theoretical studies shown that the alloy thickness has a great influence on the bulk Si growth within a certain range.However,in this experiment,when the alloy thickness was in the range of 3≤l≤10mm,there was almost no effect on the growth rate of bulk Si.(4)When the holding time was 1h,primary Si began to grow nucleated.When the holding time was increased to 5 h,the bulk Si grown continuously and the growth process was not intermittent.The constitutional supercooling would intensify with the increase of holding time.Quenching would cause internal stress between the bulk Si and the alloy melting,which there was a cracks in the interface.(5)After TGZM,the impurities mainly accumulated in the liquid phase melting,thereby greatly reducing the impurities in the bulk Si.The temperature gradient has a great influence on the content of impurities in the bulk Si,and the greater the temperature gradient,the less efficient the removal of impurities in the bulk Si.When the temperature gradient was 1.81K/mm,Fe impurity content in bulk Si decreased from 1106.85ppmw to 1.02ppmw,B element content decreased from 14.14ppmw to 4.07ppmw,and P element content decreased from 15.25 ppmw to 1.49ppmw.The removal rates of the elements reached 99.91%,90.23%,and 71.22%,respectively. |
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