简介概要

Vacuum distillation refining of metallurgical grade silicon (Ⅰ)——Thermodynamics on removal of phosphorus frommetallurgical grade silicon

来源期刊:中国有色金属学报(英文版)2007年增刊第1期(Part ⅡB)

论文作者:魏奎先 马文会 戴永年 杨斌 刘大春 汪镜福

文章页码:1022 - 1025

Key words:metallurgical grade silicon; solar grade silicon; refining; vacuum distillation; thermodynamics; phosphorus removal

Abstract: The removal of impurity phosphorus from metallurgical grade silicon is one of the major problems on purification of metallurgical grade silicon for solar grade silicon preparation. The thermodynamics on vacuum refining process of the metallurgical grade silicon was studied via separation coefficient of impurity phosphorus in the metallurgical grade silicon and vapor-liquid equilibrium composition diagram of Si-P binary alloy at different temperatures. The behaviors of impurity phosphorus in the vacuum distillation process were examined. The results show that the vacuum distillation should be taken to obtain silicon with less than 10-7 P, and the impurity phosphorus is volatilized easily by vacuum distillation in thermodynamics. Phosphorus is distilled from the molten silicon and concentrated in vapor phase.

基金信息:the National Natural Science Foundation of China
Sustentation Project of Science and Technology of China
the Doctorial Programs Foundation of Ministry of Education of China



详情信息展示

Vacuum distillation refining of metallurgical grade silicon (Ⅰ)——Thermodynamics on removal of phosphorus from metallurgical grade silicon

WEI Kui-xian(魏奎先)1, 2, MA Wen-hui(马文会)1, 2, DAI Yong-nian(戴永年)1, 2,

YANG Bin(杨 斌)1, 2, LIU Da-chun(刘大春)1, 2, WANG Jing-fu(汪镜福)1, 2

1. National Engineering Laboratory for Vacuum Metallurgy, Faculty of Materials and Metallurgical Engineering, Kunming University of Science and Technology, Kunming 650093, China;

2. Key Laboratory of Vacuum Metallurgy of Non-ferrous Metals of Yunnan Province, Kunming 650093, China

Received 15 July 2007; accepted 10 September 2007

Abstract: The removal of impurity phosphorus from metallurgical grade silicon is one of the major problems on purification of metallurgical grade silicon for solar grade silicon preparation. The thermodynamics on vacuum refining process of the metallurgical grade silicon was studied via separation coefficient of impurity phosphorus in the metallurgical grade silicon and vapor-liquid equilibrium composition diagram of Si-P binary alloy at different temperatures. The behaviors of impurity phosphorus in the vacuum distillation process were examined. The results show that the vacuum distillation should be taken to obtain silicon with less than    10-7 P, and the impurity phosphorus is volatilized easily by vacuum distillation in thermodynamics. Phosphorus is distilled from the molten silicon and concentrated in vapor phase.

Key words: metallurgical grade silicon; solar grade silicon; refining; vacuum distillation; thermodynamics; phosphorus removal

1 Introduction

Silicon has been widely used in the photovoltaic industry and semiconductor industry. Metallurgical grade silicon (MG-Si) is obtained from the carbothermic reduction of silica, containing some impurities such as Fe, Al, Ca and P, which affect the physiochemical properties of silicon materials[1]. In order to meet the requirement of solar grade silicon (SOG-Si), we should try to remove the impurities from metallurgical grade silicon.

Phosphorus is a typical impurity in silicon, and the required maximum limit for phosphorus content is 0.1×10-6 for SOG-Si[2]. Unfortunately, the segregation coefficient of phosphorus in silicon is large (0.35[3]), so it is difficult to remove phosphorus by zone or unidirectional solidification refining. The MG-Si can be refined by vacuum distillation[4-5], electron beam melting[1,6-8], solidification refining[9], and acid leach-ing treatment with addition of calcium[10], to meet the necessity of SOG-Si. In this study, the aim is to investigate systemically the thermodynamic performance for MG-Si refining via vacuum distillation to provide a theoretical base for SOG-Si production.

2 Relationship between vapor pressure of Si, P and temperature

Based on the different properties of elements contained when vaporizing and condensing, crude metal can be separated from impurities by vacuum distillation refining. The difference in vapor pressure of each metal at different temperatures is the basic principle of crude metal vacuum distillation. Relationship between the saturated pressure and temperature is shown in Eqn.(1), and the coefficients A, B, C and D for different substance are listed in Table 1.

Table 1 Values of coefficients of A, B, C and D for different substance[11]

lg p* =AT -1+BlgT+CT+D                       (1)

According to Eqn.(1) and Table 1, the relationship between saturated vapor pressure (p*, Pa) of phosphorus in the MG-Si and temperature (T, K) is obtained as

                      (2)

         (3)

According to Eqns.(2) and (3), the saturated pressure of liquid of different substances can be worked out at a given temperature. Fig.1 presents the relationship between lg p* and T for phosphorus and silicon. As shown in Fig.1, phosphorus has a high saturated pressure with a normally low volatile rate, and silicon has low saturated pressure, which means it is difficult for silicon to volatile into gas phase from raw material at atmospheric pressure. However, under vacuum condition, the volatile rate will increase for both phosphorus and silicon. At the same temperature, the saturated pressure of phosphorus is much higher than that of silicon. At   1 732 K, silicon begins to melt and phosphorus sublime. The saturated pressure of phosphorus exceeds 108 Pa, which means that it would be distilled out of the silicon. As a result, theoretically, phosphorus and silicon can be separated by using vacuum distillation.

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