B Removal by Zr Addition in Electromagnetic Solidification Refinement of Si with Si-Al Melt

Abstract

The metallurgical treatment producing solar-grade Si (SoG-Si) from metallurgical-grade Si (MG-Si) is a promising method for the manufacturing of low-cost Si solar cells. Among the impurity elements in MG-Si, B is difficult to remove through solidification refining and vacuum melting because of its large segregation coefficient [0.8 at 1687 K (1414°C)] [1] and low vapor pressure. [2] Although processes such as oxidation through H 2 O-added plasma melting [3,4] and slag treatment [5,6] are currently employed for B removal, there is a strong demand to further reduce the cost of the refining process for manufacturing SoG-Si.Recently, a solvent refining process using Al-Si melt combined with electromagnetic solidification has shown outstanding results in purifying Si more effectively and economically. [7,8] Some attempts have also been made to improve B removal based on this solvent refining process, such as using Si-Al-Sn (10-30 mol pct) [9] and Si-Al-Zn (<40 mol pct) [10] melts as the refining solvents, but adding large amounts of Sn or Zn possibly does not make this process a low-cost manufacturing process for SoG-Si. If the B removal process can be improved by adding small amounts of additives, it will become more practical. It is notable that a small amount of Ti has been used as an additive to enhance B removal because B and Ti can form a thermodynamically stable compound, TiB 2 . [11] The B content of refined Si was reduced from 170 to 1.1 ppma by adding 933 ppma Ti.In this study, we used Zr as a new additive to improve B removal, considering that Zr has a stronger affinity for B than Ti, [12] and the segregation coefficient of Zr [1.6 9 10 À8 , 1687 K (1414°C)] [13] is much smaller than that of Ti [2 9 10 À6 , 1687 K (1414°C)], [1] which implies that the added Zr is potentially easier to be removed than the added Ti in solidification refining. Therefore, B removal by Zr addition in electromagnetic solidification refining of Si with Si-Al melt was investigated.An induction furnace (20 kHz) was employed in this study to carry out the electromagnetic solidification refining experiments. A schematic of the apparatus is shown in Figure 1. In order to avoid oxidation of the samples, the air in the quartz chamber (50 mm O.D., 44 mm I.D., 900 mm length) was first evacuated by a vacuum pump (ultimate pressure <6 Pa). The chamber was then filled with Ar gas (99.99 pct) at a flow rate of 100 ml/min. The surface temperature of the Si-Al melt was monitored by a dual-wavelength infrared pyrometer through a prism. The perpendicular position of the sample, together with the quartz chamber, can be controlled using a stepping motor.Ten grams of bulk Si (99.9999 pct) and Al shot (99.999 pct) together with differing amounts of Si-1 wt pct Zr and Si-1 wt pct B alloys, prepared as described below, were placed in a high-purity dense graphite crucible (25 mm O.D., 17 mm I.D., 75 mm length). The bottom of the crucible was placed level with the lower end of the induction coils, as shown in Figure 1. The Si-1 wt pct B and Si-1 wt pct Zr...

Effects of small amounts of transition metals on boron removal during electromagnetic solidification purification of silicon with Al–Si solvent

“…Yoshikawa et al [10] reported a small amount of Ti (<933 ppma) could strengthen B removal in the Al-Si solventpurification process. Our previous study also confirmed that a small amount of Zr (<1057 ppma) was significantly responsible for strengthen B removal [11].…”

Removal of B from Si by Hf addition during Al–Si solvent refining process

“…The B content of refined Si was reduced from 170 to 1.1 ppma by adding 933 ppma Ti, and the added Ti could be simultaneously reduced from 933 to 1.6 ppma. In a previous study [16], we added a small amount of Zr (<1057 ppma) to improve B removal in the electromagnetic solidification refining process.…”

Effects of Slag Treatment Conditions on Boron Removal from Metallurgical Silicon by United Gas-Slag Refining Technology

“…Wu et al [12] used 8 L/min compressed oxygen to purify metallurgical silicon by bottom blowing at 1873 K. The boron content was reduced from 35 ppmw to 18 ppmw, and the removal rate of Al and Ca impurities exceeded 90%, but the experiments also revealed problems such as the severe oxidation of the silicon substrate and large silicon loss. Solvent re ning [13] and plasma re ning [14] are two highly effective methods for the removal of boron impurities in industrial silicon, but their complicated re ning process and high energy consumption hinder their application. Blowing re ning [15,16] is a simple and inexpensive method for the removal of impurities such as Al, Ca, Ti, and B.…”