The Judgment Of The All-melting-moment During Using Electron Beam To Purify Poly-crystalline Silicon

ince 2009,under several beneficial policies such as “Golden sun project”,the subsidyof solar grid-connected power generation,the installed capacity of PV industry in our country increase from less 100 MW to 70 GW even more until 2016,it has increased by 700 times and was the number one of the world for many years.Every year,there is a need of over 200 thousand tons poly-crystalline silicon in our country.The poly-crystalline silicon’s use ratio in ingot casting is about 65% for one time,the rest of it need to be recycled by purification or acid pickling.The electron beam melting is a efficient and environmental way to recycle poly-crystalline silicon because it can remove the transpirable impurities such as P,Al for the characteristics of high vacuum and high melting speed and it can also remove the metal impurities by controlled directional solidification.In industrial production of electron beam melting,we can only judge the smelting time by inspection window because it is hard to know the state of silicon melt by thermometry directly,so the lack of unified standard of judgment lead to the difference of smelting time in different production batch.Under this situation,there are several problems such as the volatility of product’s quality,poor efficiency of energy consumption because the removal of transpirable impurities is sensitive to smelting time,which has influenced the standard industrial production.So it is important to get an unified standard of judgment in all-melting-moment to control the smelting time and promote the industrialization of using electron beam to recycle poly-crystalline silicon.In this paper,we try to measure the melt temperature indirectly and use the change of cooling water temperature difference which can reflect the change of melt temperature to analyze the state of melt and we judge the all-melting-moment through a great deal of experimental analysis.Then we verify the feasibility and evaluate the effectiveness of this method by detection of product’s electrical property.The specific research results of this paper as follow:（1）During the process of fusion and in the case of electron beam gun being shut during smelting,when the change of cooling water temperature difference is rising from a moment as exponential function below:y=A₁·exp（-x/B₁）+A₂·exp（-x/B₂） This moment is when silicon being all melting.The values of A₁、B₁、A₂、B₂ are different because production environment is different,such as the quality of silicon and the inflow cooling water temperature.（2）The removal quality of impurity is sensitive to smelting time,a slight difference in smelting time will lead to the change of p-n type when the smelting time make donor impurity and acceptor impurity nearly compensate each other.（3）Using this method to calculate the smelting time again and combing it with the logical curve of resistivity changing by smelting time,the resistivity distribution of product,we can draw a conclusion that the method of judging all-melting-moment by the change of cooling water temperature difference is effectual.