Yanqing Hou,Z hifeng Nie ,Gang Xie,3,Rongxing Li,Xiaohua Yu,Plant A.Ramachandran

1State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization,Kunming University of Science and Technology,Kunming 650093,China

2Faculty of Metallurgical and Energy Engineering,Kunming University of Science and Technology,Kunming 650093,China

3Kunming Metallurgical Research Institute,Kunming 650031,China

4Department of Energy,Environmental and Chemical Engineering,Washington University in St.Louis,St.Louis,MO 63130,USA

Keywords:

A B S T R A C T

1.Introduction

Research into alternative energy sources for electricity production is of considerable value globally in view of the ever-increasing cost and diminishing supply of petroleum fuels.As a result,high-efficiency solar energy conversion has become a subject receiving attention due to being environmentally friendly process and having no impact on the thermal balance of the planet[1,2].Polycrystalline silicon is presently one of the best materials for application in photovoltaic energy conversion.Other materials may be found as the primary material instead of silicon in the photovoltaic industry in the next at least 50 years[3,4].The cost of raw material-polycrystalline silicon-amounts to about 20%of the cost of a silicon solar cell[5-8].Therefore,the cost of polycrystalline silicon has a considerable influence on the cost of the solar cell.Solar energy power technology has developed quickly,which results in the cost of polycrystalline silicon increasing quickly in recent years.Therefore,the photovoltaic industry was inhibited[9].The modified Siemens process is the primary technology for manufacturing polycrystalline silicon production at present[10].However,the technology is controlled by seven companies around the world,having a silicon yield of 76.7%of the total[11].The modified Siemens method has some advantages,for example safety,mature technology,and high purity of product.However,the cost is high with the current technologies.At present,intensive research efforts have been concentrated on reducing the cost of polycrystalline silicon with this method.However,efficient results have not been achieved presently[12,13].Therefore,it is very important to find new technologies for silicon production for high-efficiency solar cells to lower the manufacturing cost of high purity silicon.Although the gas phase zinc reduction method had been developed over nearly 50 years,it was displaced by the Siemens method due to the purity of silicon being not up to electro-grade.However,Battelle's Columbus laboratories restudied the method upon an urgent request to curb the cost of polycrystalline silicon.The results show that the efficiency of web-dendrite solar cells made from the material by gas phase zinc reduction process is indistinguishable from that of those made from semiconductor-grade silicon[14].A thorough study into the process was started by an SST company in Japan and the purity of silicon above 6N(99.9999%)silicon was obtained[15].Presently,the industrialization research on the process is being developed by three companies in Japan,achieving silicon purity up to 8N(99.999999%)[16].From the analysis above,it is obvious that the zinc reduction process has been developed as an important method for manufacturing a low-cost polycrystalline silicon.The main side reactions are SiCl4(g)+ Zn(g)→ SiCl2(g)+ ZnCl2(g)and SiCl4(g)+Si(s)→SiCl2(g)reported by Hou[17].It is noted that SiCl2is the main by-product in zinc reduction process and it,as we know,can decompose into SiCl4and ZnCl2in gaseous zinc atmosphere.The process is a consumption of material and power.So the thermodynamic behavior of SiCl2is very important in gas phase zinc reduction of SiCl4process for manufacturing polycrystalline silicon.Thereby,the thermodynamic behavior of SiCl2is studied in the paper and the interesting results of the optimum conditions,under which the by-product SiCl2can be restricted,are hoped to be obtained.

2.Thermodynamic Characters in Zinc Reduction Process

Hou[17]has reported that there may be 10 kinds of components when the reactions,which are present in the gas phase zinc reduction of SiCl4process for manufacturing polycrystalline silicon,reach equilibrium,for example,Si(s),ZnCl2(g),SiCl4(g),Zn(g),SiCl2(g),Zn2Cl4(g),ZnCl(g),SiCl3(g),SiCl(g),Cl(g)and Cl2(g).Among the above components,the first five are the main ones.The other components are scarce and can be neglected in the equilibrium gas phase composition analysis.And the reactions,which may occur in gas phase zinc reduction of SiCl4for manufacturing polycrystalline silicon,have been obtained as following[17]:

It is noted that SiCl2is the main product in the Side-reactions(2)and(3).SiCl4is converted into SiCl2in Reduction reaction(2)by gas phase zinc.It is a waste of SiCl4.And the solid silicon is eroded by SiCl4in Reaction(3)in which SiCl2is the only product.It is noted that SiCl2can decompose into SiCl4and ZnCl2in gaseous zinc atmosphere and the process is a consumption of material and power.Thus,it not only is a waste of material SiCl4but also increases the power consumption when the Reactions(2)and(3)occur.Furthermore,the productive ratio of silicon decreases since the side-reactions occur in the process.Therefore,the thermodynamic behaviors of SiCl2have a great effect on gas phase zinc reduction of SiCl4process for manufacturing polycrystalline silicon.

Based on the thermodynamic data for the related pure substances listed in Table 1,the thermodynamic behaviors of SiCl2can be analyzed in the gas phase zinc reduction of SiCl4process for manufacturing polycrystalline silicon.

Table 1 Thermodynamic data for related pure substances(Sp.);all data are taken from compilation of Ye and Hu[18];enthalpies and entropies are refereed to stand conditions(i.e.,0.1 MPa and 298 K)and are expressed in J/mol and in J/mol K:specific heats can be calculated from the reported values as:Cp=Cp,m=A1+A2 × 10J·mol-1·K-13T+A3 × 105T-2

3.Thermodynamic Behaviors of SiCl2

SiCl2generation ratio(x),which can be calculated from the following equation,is employed to describe the degree of Side-reactions(2)and(3).

where x is the SiCl2generation ratio,nSiCl2-Eqis the molar content of SiCl2when the reactions reach equilibrium and nSiCl4-inis the feed molar content of SiCl4.

SiCl2generation ratio can indicate the degree of side reactions in gas phase zinc reduction process.Smaller SiCl2generation ratio,which shows the side reactions are restricted,stands for the higher polycrystalline silicon yield ratio.Thus,SiCl2generation ratio can also been employed to describe the polycrystalline silicon yield ratio.

3.1.Influence of temperature on SiCl2generation ratio

SiCl2generation ratio was analyzed while changing the temperature with the pressure and the molar ratio unchanged.Because the reactant must be in the gaseous state,the minimum temperature for analysis should be up to the boiling point of zinc(1184 K).Diagrams of SiCl2generation ratio x versus temperature can be plotted under 0.1 MPa,0.3 MPa and 0.6 MPa,and feed molar ratiosof 1:2,1:4 and 1:8 as shown in Figs.1 and 2,respectively.

SiCl2generation ratio increases with increasing temperature when pressure and feed molar ratio of Zn to SiCl4are kept as a constant as shown in Figs.1 and 2.And SiCl2generation ratio increases slowly when the temperature is lower and becomes more quick when the temperature is higher.The reasons for that should be addressed from equilibrium constant(KpΘ).Based on the thermodynamic data for the related pure components listed in Table 1,the relations of standard free energy(ΔrGmΘ)versus temperature for Reactions(1)-(3)can be calculated,as shown in the following equations:

where T is the temperature.

Fig.1.Variation of SiCl2generation ratio with temperature at 0.1 MPa(a),0.3 MPa(b)and 0.6 MPa(c),n:1(n=1-10)notes feed molar ratios of gas phase zinc to SiCl4.

Fig.2.Variation of SiCl2generation ratio with temperature at

It is noted that the numbers(1,2,3)in the brackets on the left hand of Eqs.(5)-(7)stands for Reactions(1),(2)and(3),respectively.In order to describe the influence of temperature on the reactions more clearly,the diagram of KPΘ-T has been plotted,as shown in Fig.3.

Fig.3.Diagram of KPΘ-T for Reactions(1)-(3)versus temperature,---Reaction(1),Reaction(2),—Reaction(3).

The values of KPΘfor reaction(1),which is the main reaction in gas phase zinc reduction process,decrease sharply with increasing temperature.Therefore,the silicon yield ratio decreases with increasing temperature,which is accord to the results reported by Hou[19].The values of KPΘfor both side Reactions(2)and(3)increase with increasing temperature.Thus,SiCl2generation ratio increases with increasing temperature as shown in Figs.1 and 2,since SiCl2is the product in reactions(2)and(3).However,it is noted that the values of KPΘfor Side reaction(2)increase slightly against quickly for Side reaction(3)with increasing temperature.Therefore,SiCl2generation ratio increases mainly by eroding silicon,which is the product in the process.That must cause the next problems:(1)The polycrystalline silicon yield ratio decreases;(2)the power consumption increases because Reactions(2)and(3)are endothermic reactions;(3)the zinc consumption increases because SiCl2,as we know,can decompose into SiCl4and ZnCl2by reacting with zinc in gaseous zinc atmosphere;and(4)the price must increase.Thus,the temperature must be low in the gas phase zinc reduction process.

When temperature is lower,the values of KPΘincreases lowly against quickly when the temperature is higher.Thus,SiCl2generation ratio increases slowly when the temperature is lower and becomes more quick when the temperature is higher,as shown in Figs.1 and 2.It is noted that the main reaction converts into Reaction(3)when the temperature is higher than 1460 K under 0.1 MPa,which indicates that SiCl2generation ratio is very high as shown in Fig.1(a).

Furthermore,SiCl2generation ratio increases more slowly with increasing temperature when the pressure increases from 0.1 MPa to 0.6MPa by comparing(a),(b)and(c)in Fig.1.Thus,the higher pressure has a positive effect on the gas phase zinc reduction process,which will be addressed in detail later.The changing trend of the curves in(a),(b)and(c)of Fig.2is very similar.The difference is that SiCl2generation ratio increases more slowly with increasing temperature when the feed molar ratio of Zn to SiCl4increases.It is noted that SiCl2generation ratio almost keeps as a constant,which is a small quantitative value,when the pressure and feed molar ratio of gas phase zinc to SiCl4are higher.Therefore,the higher pressure and feed molar ratio of gas phase zinc to SiCl4can restrict the SiCl2generation ratio.

It can be concluded that the temperature must be low in the zinc reduction process.However,zinc should be in the gaseous state in the process.In order to maintain solely the zinc gaseous state,the temperature should be slightly higher than 1184 K.The temperature can be controlled at about 1200 K in practical production.

3.2.Influence of pressure on SiCl2generation ratio

The production ratio of silicon has also been analyzed while changing the pressure when the temperature and the molar ratio of reactant remain unchanged.The relations of SiCl2generation ratio(x)and pressure(P)are plotted at 1200 K,1300 K and 1400 K,and feed molar ratios/of 1:2,1:4 and 1:8,as shown in Figs.4 and 5,respectively.

SiCl2generation ratio decreases quickly firstly and then becomes smoothly with increasing pressure as shown in Figs.4 and 5.The increasing pressure almost has no effect on SiCl2generation ratio when the pressure is higher than 0.2 MPa at 1200 K.When the temperature is kept at 1300 K,SiCl2generation ratio keeps as a constant under the pressure of higher than 0.6 MPa.However,the pressure has a great effect on SiCl2generation ratio when the temperature is 1400 K.Furthermore,SiCl2generation ratio increases more quickly when pressure changes from 0.1 MPa to 0.8 MPa at higher temperature.Therefore,the pressure has a larger effect on SiCl2generation ratio when the temperature is higher.Since the gaseous amount decreases when the reaction goes for main reaction(1),the value of KPΘincreases with increasing pressure,as shown in Fig.6.For reaction(2),the gaseous amount keeps unchanged.So,the value of KPΘkeeps unchanged with increasing pressure.The pressure has no effect on Reaction(2).However,for side Reaction(3),the gaseous amount increases,which causes the value of KPΘdecreases with increasing pressure as shown in Fig.7.

The value of KPΘfor Reaction(1)increases meanwhile the value of KPΘfor Reaction(3)decreases with increasing pressure,which gives an evidence for that SiCl2generation ratio decreases with increasing pressure,as shown in Figs.4 and 5.Furthermore,the value of KPΘfor Reaction(1)keeps as a constant which is a small quantitative value meanwhile that for Reaction(3)increases sharply with increasing pressure at higher temperature.That provides an interpretation why the pressure has a larger effect on SiCl2generation ratio at higher temperature as mentioned above.

Therefore,the higher pressure can restrict the SiCl2generation ratio,which indicates that the polycrystalline silicon yield ratio increases with increasing pressure.However,SiCl2generation ratio keeps as a constant when the pressure is higher than 0.2 MPa at 1200 K,which is the optimum temperature obtained above.Furthermore,the technical indexes for the related equipment are higher and they fail more easily with higher operation pressure,a result of which the cost of zinc reduction process rises.Therefore,the optimum operation pressure should be about 0.2 MPa in actual production.

3.3.Influence of feed molar ratio of reactants on SiCl2generation ratio

Finally,the influence of the feed molar ratio of gas phase zinc to SiCl4on SiCl2generation ratio(x)has been studied when the temperature and pressure remain unchanged.The graphs of x versus the feeding molar ratio nZn/nSiCl4can be plotted under 0.1 MPa,0.3 MPa and 0.6 MPa,and 1200 K,1300 K and 1400 K,as shown in Figs.8 and 9,respectively.

Fig.4.SiCl2generation ratio with pressure at 1200 K(a),1300 K(b),1400 K(c),n:1(n=1-10)notes feed molar ratios of gas phase zinc to SiCl4.

Fig.5.SiCl2generation ratio with pressure under nZn/nSiCl4of 2(a),4(b)and 8(c).

Fig.6.The Diagram of KPΘfor reaction(1)versus temperature under different pressure,1-6 note 0.1 MPa,0.2 MPa,0.3 MPa,0.4 MPa,0.6 MPa,0.8 MPa respectively.

Fig.7.The Diagram of KPΘfor Reaction(3)versus temperature under different pressures1-6 note 0.1 MPa,0.2 MPa,0.3 MPa,0.4 MPa,0.6 MPa,0.8 MPa respectively.

SiCl2generation ratio decreases with increasing feed molar ratio of gas phase zinc toas shown in Figs.8 and 9.It indicates that the higher feed molar ratio of gas phase zinc to SiCl4can restrict the side reactions,which is accord to the results reported by Uesawa[20].Therefore,the higher feed molar ratio of gas phase zinc to SiCl4has a positive effect on the gas phase zinc reduction process.SiCl2generation ratio decreases slightly under the range of feed molar ratio of gas phase zinc to SiCl4of 1 to 10 at 1200 K,and decreases quickly at 1400 K as shown in Fig.8.Thus,the feed molar ratio of gas phase zinc to SiCl4has greater effect on the SiCl2generation ratio at higher temperature.The change of SiCl2generation ratio is very small under the range of feed molar ratio of gas phase zinc to SiCl4from 1 to 10 at 0.1 MPa,and begins to be higher when pressure increases as shown in Fig.9.Therefore,the feed molar ratio of gas phase zinc to SiCl4has a bigger effect on the SiCl2generation ratio at higher pressure as temperature does.

When the temperate and pressure are 1200 K and 0.2 MPa,respectively,which are optimum obtained above,SiCl2generation ratio decreases when the feed molar ratio of zinc to SiCl4is lower than 4,and keeps as a constant when the feed molar ratio of zinc to SiCl4is higher than 4 as shown in Fig.9(a).Therefore,increasing feed molar ratio of zinc to SiCl4can restrict the side reaction only when the feed molar ratio of gas phase zinc to SiCl4is lower than 4.Increasing feed molar ratio can cause the deeper degree of the main Reaction(1)when the feed molar ratio of zinc to SiCl4is higher than 4.However,the influence is very small as reported by Hou[17].Therefore,although the excess of zinc is important in the zinc reduction process from the above analysis,the feed mole ratioshould not be excessively large.Otherwise,the following considerations arise.(1)The content of Zn may reach saturated concentration.Under the condition,the state of zinc is liquid or/and solid which cannot react with SiCl4.Thus,the part of zinc must be purified again for cyclic utilization.Then,the cost and energy consumption in the process must rise greatly;and(2)the gaseous zinc has strong reducibility.So,the technical indexes for the related equipment are higher and the service time must be smaller when the concentration of gaseous zinc is higher.As a result,the cost must rise in gas phase zinc reduction process.Therefore,the optimum feed molar ratio of zinc to SiCl4should be about 4 in actual production.

From the above analysis,the conditions of 1200 K,0.2 MPa and feed molar ratio of Zn to SiCl4of 4 are preferred,as shown in Table 2.Under these conditions,the SiCl2generation ratio is very low,which shows that the side reactions can be restricted and the silicon yield ratio should increase.The theoretical silicon yield ratio is 90.3%when the reactions reach equilibrium under the above conditions.However,the silicon yield is about 67% in actual production.It is obvious that the equilibrium silicon yield ratio is much higher than that in actual production.There may be the following reasons:(1)the conditions,which are temperature,pressure and feed ratio of gas phase zinc to SiCl4,cannot be a

Fig.8.SiCl2generation ratio with nZn/nSiCl4at 1200 K(a),1300 K(b),1400 K(c).

Fig.9.SiCl2generation ratio with nZn/nSiCl4under 0.1 MPa(a),0.3 MPa(b)and 0.6 MPa(c).

Table 2 Suitable operational conditions and ideal operational conditions determined by authors and their comparison with actual operation conditions[13]

From the above analysis,the conditions of 1200 K,0.2 MPa and feed molar ratio of Zn to SiCl4of 4 are preferred,as shown in Table 2.Under these conditions,the SiCl2generation ratio is very low,which shows that the side reactions can be restricted and the silicon yield ratio should increase.The theoretical silicon yield ratio is 90.3%when the reactions reach equilibrium under the above conditions.However,the silicon yield is about 67% in actual production.It is obvious that the equilibrium silicon yield ratio is much higher than that in actual production.There may be the following reasons:(1)the conditions,which are temperature,pressure and feed ratio of gas phase zinc to SiCl4,cannot be a constant in the actual production,especially for temperature.The temperature affects the silicon yield heavily.The side-reaction occurs more easily when the temperature is higher which causes the lower polycrystalline silicon yield;and(2)the polycrystalline silicon loss is the other reason caused the lower polycrystalline silicon yield.There are two aspects for polycrystalline silicon loss.One is 14%of the polycrystalline silicon,which cannot up to solar-grade silicon,deposits on the wall of the fluidized bed reactor(FBR)[14].The other one is the silicon fine,which is the product of heterogeneous reactions in the gas phase, flows into the next cooling system with the off-gas.Therefore,the silicon yield ratio can be increased by optimizing the operational conditions such as temperature,pressure and feed molar ratio of SiCl4and Zn,and improving the production equipment.Thus,the further research on the subject is required and is left for a future study.

The present study provides useful results for the industrial production of polycrystalline silicon by gas phase zinc reduction process.The interesting results can provide a useful theory to optimize the conditions to increase the polycrystalline silicon ratio.However,the reported polycrystalline yield are lower,indicating that there is some kinetic limitation and the reactor may not have been fully optimized.Hence,there is a strong need to study reactor models for each of these reactors in detail and examine how the process can be improved further.This is the chance and challenge for the researchers interested in the gas phase zinc reduction process.

4.Conclusions

(1)SiCl2generation ratio,which stands for the degree of side reactions,increases with increasing temperature when pressure and feed molar ratio of gas phase zinc to SiCl4are kept as a constant.Thus,the temperature must be low in the gas phase zinc reduction process.The temperature can be controlled at about 1200 K,which is slightly higher than the boiling temperature of zinc(1184 K),in practical production since zinc must been maintained solely gaseous state.

(2)The higher pressure can restrict the SiCl2generation ratio,which indicates that the polycrystalline silicon yield ratio increases with increasing pressure.However SiCl2generation ratio keeps as a constant when the pressure is higher than 0.2 MPa at the optimum temperature of 1200 K.Therefore,the optimum operation pressure should be about 0.2 MPa in actual production.

(3)The excess zinc can restrict SiCl2generation ratio,indicating that the excess zinc has a positive influence on the production ratio of silicon.However,the feed molar ratioshould notof 4 is be overly large.Therefore,the molar ratio preferred.

(4)The conditions of 1200 K,0.2 MPa andof 4 are suitable in zinc reduction process.Under these conditions,SiCl2generation ratio is very low,which indicates that the side reactions can be restricted.