Purification Of Pot Material Of Monocrystalline Silicon By Electron Beam Melting

As the basic material in photovoltaic industry,the requirement for silicon increases yearby year with the rapid development of photovoltaic industry.During solar-grade silicon production process,a large amount of scrap silicon exists due to impurity enrichment in some areas of silicon ingot or being cut in the form of silicon powder.Pot material of monocrystalline silicon is the residual silicon at the bottom of crucible containing a large amount of impurities during the preparation of monocrystalline silicon by CZ method,which occupies a considerable proportion in scrap silicon.In recent years,there are about one thousand tons pot material of monocrystalline silicon in China.The regeneration of this part material is significant for the recycling of solar grade silicon.Pot material of monocrystalline silicon contains a large number of metal impurities,and may contains many impurities from group ?A or ?A such as P,Al and As.Volatile impurities,such as P,Al,As,etc.,can be removed easily by evaporation during electron beam melting;meanwhile,metal impurities can be removed during silicon directional solidification induced by electron beam.Therefore,electron beam melting is considered as a suitable method for the regeneration of pot material of monocrystalline silicon.In this paper,the feasibility of purification of this material by electron beam melting was investigate,and the relationship between electrical properties and impurity content was discussed.On this basis,the influence of electron beam melting parameters on purification efficiency was analyzed.The conclusions are as follows:(1)Impurities such as P,Fe,Al,Ca,Ti,Ga,Mg,Zn,etc.can be effectively removed from pot material of monocrystalline silicon by electron beam melting.The removal efficiencies are 77.42%,94.82%,92.63%,87.86%,84.33%,99.56%,90.29%,81.97%respectively.The obtained silicon ingot meets the production requirements.(2)The resistivity of the obtained silicon ingot is mainly determined by impurity contents of group ?A and ?A such as B,P and Al.The results show that,if the ingot is N-type with a high resistivity(?5 ?·cm),the impurity content of group ?A and ?A can meet the requirements for solar cells.The minority carrier lifetime of the ingot is inversely proportional to the total atomic concentration of metal impurities per volume,so the distribution of metal impurities can be reflected directly by the distribution of lifetime.Therefore,the content of metal impurities in the obtained silicon ingot can be evaluated based on the test results of resistivity and lifetime.(3)The results show that the cutting line for the removal of high impurity area can be determined through the distribution of lifetime.The transverse average lifetime of 0.84 ?s is considered as the standard for cutting.The high impurity area can be removed completely and the loss rate of silicon ingot is lowest.(4)During purification of pot material of monocrystalline silicon using electron beam melting,the yield of silicon ingot can be increased from 76.29% to 83.98% by extending refining time,but the metal impurity removal efficiency can not be improved obviously.The removal efficiency can also not be improved by reducing the cooling intensity during solidification process,and the yield is only 71.84%.The removal efficiency can be improved obviously by extending beam current reduction time,and the yield increases to 84.75%.If the cooling intensity is reduced and beam current reduction time is extended at the same time,the removal efficiency of the bottom part can be improved but upper part not,and the yield is79.67%.(5)The results show that the removal efficiency and the yield can be optimized by extending solidification time.Therefore,the optimum process parameters for purification of pot material of monocrystalline silicon by electron beam melting are: refining time 20 min,cooling water flow of solidification crucible 500 L/min,and beam current reduction time 134 min.