Jafar Safarian

Vacuum Refining of Molten Silicon

Metallurgical fundamentals for vacuum refining of molten silicon and the behavior of different impurities in this process are studied. A novel mass transfer model for the removal of volatile impurities from silicon in vacuum induction refining is developed. The boundary conditions for vacuum refining system—the equilibrium partial pressures of the dissolved elements and their actual partial pressures under vacuum—are determined through thermodynamic and kinetic approaches. It is indicated that the vacuum removal kinetics of the impurities is different, and it is controlled by one, two, or all the three subsequent reaction mechanisms—mass transfer in a melt boundary layer, chemical evaporation on the melt surface, and mass transfer in the gas phase. Vacuum refining experimental results of this study and literature data are used to study the model validation. The model provides reliable results and shows correlation with the experimental data for many volatile elements. Kinetics of phosphorus removal, which is an important impurity in the production of solar grade silicon, is properly predicted by the model, and it is observed that phosphorus elimination from silicon is significantly increased with increasing process temperature.

Thermodynamic and Kinetic Behavior of B and Na Through the Contact of B-Doped Silicon with Na2O-SiO2 Slags

Boron (B) is the most problematic impurity to be removed in the processes applied for the production of solar grade silicon. Boron removal from liquid silicon by sodium-silicate slags is experimentally studied and it is indicated that B can be rapidly removed within short reaction times. The B removal rate is higher at higher temperatures and higher Na2O concentrations in the slag. Based on the experimental results and thermodynamic calculations, it is proposed that B removal from silicon phase takes place through its oxidation at the slag/Si interfacial area by Na2O and that the oxidized B is further gasified from the slag through the formation of sodium metaborate (Na2B2O4) at the slag/gas interfacial area. The overall rate of B removal is mainly controlled by these two chemical reactions. However, it is further proposed that the B removal rate from silicon depends on the mass transport of Na in the system. Sodium is transferred from slag to the molten silicon through the silicothermic reduction of Na2O at the slag/Si interface and it simultaneously evaporates at the Si/gas interfacial area. This causes a Na concentration rise in silicon and its further decline after reaching a maximum. A major part of the Na loss from the slag is due to its carbothermic reduction and formation of Na gas.

Vacuum Evaporation of Pure Metals

Theories on the evaporation of pure substances are reviewed and applied to study vacuum evaporation of pure metals. It is shown that there is good agreement between different theories for weak evaporation, whereas there are differences under intensive evaporation conditions. For weak evaporation, the evaporation coefficient in Hertz-Knudsen equation is 1.66. Vapor velocity as a function of the pressure is calculated applying several theories. If a condensing surface is less than one collision length from the evaporating surface, the Hertz-Knudsen equation applies. For a case where the condensing surface is not close to the evaporating surface, a pressure criterion for intensive evaporation is introduced, called the effective vacuum pressure, peff. It is a fraction of the vapor pressure of the pure metal. The vacuum evaporation rate should not be affected by pressure changes below peff, so that in lower pressures below peff, the evaporation flux is constant and equal to a fraction of the maximum evaporation flux given by Hertz-Knudsen equation as 0.844

Kinetics and Mechanism of Phosphorus Removal from Silicon in Vacuum Induction Refining

\dot{n}_{\hbox{Max} }

Removal of SiC particles from solar grade silicon melts by imposition of high frequency magnetic field

. Experimental data on the evaporation of liquid and solid metals are included.

Mechanisms and Kinetics of Boron Removal from Silicon by Humidified Hydrogen

Natural Science Foundation of Fujian Province, China [2007J0012]; Key Technological Program of Fujian Province, China [2007HZ0005-2]; Norwegian Research Council [BASIC-10341702]

Microscopic Study of Carbon Surfaces Interacting with High Carbon Ferromanganese Slag

Abstract Vacuum induction refining is a process that can be applied to remove phosphorus from molten silicon for the production of solar grade silicon. Pure silicon was doped by phosphorus to make molten silicon containing around 17 ppmw phosphorus. The kinetics of phosphorus removal from this silicon was studied at 0.5 Pa through the application of vacuum induction refining. It was observed that vacuum removal of phosphorus occurs through a first-order reaction. The rate constants of phosphorus evaporation were determined as 2.28 × 10-6 m/s and 4.93 × 10-6 m/s at 1500 °C and 1600 °C, respectively. Moreover, an apparent activation energy 213.1 kJ/mol for phosphorus evaporation from molten silicon was calculated. It was found that mass transfer of phosphorus in the melt is not rate limiting in the inductively stirred silicon melt. The vacuum removal of phosphorus is mix-controlled by chemical reaction and gas phase mass transfer. Under medium vacuum conditions, the mass transfer in the gas phase is more rate-limiting than the chemical reaction at higher refining temperatures.

SILICON PURIFICATION THROUGH MAGNESIUM ADDITION AND ACID LEACHING

The thermodynamic activities in the silicon binary melts with Al, Ca, Mg, Fe, Ti, Zn, Cu, Ag, Au, Sn, Pb, Bi, Sb, Ga, In, Pt, Ni, Mn and Rh are studied. The silicon activities along the liquidus are calculated through a quasi-regular solution model using the recently determined liquidus constants for the silicon binary systems. The silicon activities at its melting point are calculated considering regular solution approximation. The activities of the other melt component at the silicon melting point are also calculated through the graphical integration of the Gibbs–Duhem equation for the activity coefficient, which are further utilized to determine the corresponding activities along the liquidus. The calculated activities are presented graphically, and it is indicated that the results are consistent with the reported activity data in the literature. The activities in the dilute solutions are also calculated graphically. Moreover, the activities of particular dilute solute elements in silicon are calculated through a simple formula, which is a function of the liquidus constants.