Distribution Coefficient Of Boron Research Articles

Thermodynamics of Boron Removal from Silicon Using CaO-MgO-Al2O3-SiO2 Slags

Slag refining is one of few metallurgical methods for removal of boron from silicon. It is important to know the thermodynamic properties of boron in slags to understand the refining process. The relation of the distribution coefficient of boron to the activity of silica, partial pressure of oxygen, and capacity of slags for boron oxide was investigated. The link between these parameters explains why the distribution coefficient of boron does not change much with changing slag composition. In addition, the thermodynamic properties of dilute boron oxide in CaO-MgO-Al2O3-SiO2 slags was determined. The ratio of the activity coefficient of boron oxide and silica was found to be the most important parameter for understanding changes in the distribution coefficient of boron for different slags. Finally, the relation between the activity coefficient of boron oxide and slag structure was investigated. It was found that the structure can explain how the distribution coefficient of boron changes depending on slag composition.

Equilibrium boron distribution between Fe–C–Si–Al melt and boron-bearing slag

HSC 6.1 Chemistry software (Outokumpu) and a simplex–lattice experiment design are employed in thermodynamic modeling of the equilibrium boron distribution between steel containing 0.2% C, 0.35% Si, and 0.028% Al (wt % are used throughout) and CaO–SiO2–Al2)3–8% MgO–4% B2O3 slag over a broad range of chemical composition at 1550 and 1600°C. For each temperature, mathematical models (in the form of a reduced third-order polynomial) are obtained for the equilibrium boron distribution between the slag and the molten metal as a function of the slag composition. The results of simulation are presented as graphs of the composition and equilibrium distribution of boron. The slag basicity has considerable influence on the distribution coefficient of boron. For example, increase in slag basicity from 5 to 8 at 1550°C decreases the boron distribution coefficient from 160 to 120 and hence increases the boron content in the metal from 0.021% when L B = 159 to 0.026% when L B = 121. In other words, increase in slag basicity favorably affects the reduction of boron. Within the given range of chemical composition, the positive influence of the slag basicity on the reduction of boron may be explained in terms of the phase composition of the slag and the thermodynamics of boron reduction. Increase in metal temperature impairs the reduction of boron. With increase in temperature to 1600°C, the equilibrium distribution coefficient of boron increases by 10, on average. On the diagrams, we see regions of slag composition with 53–58% CaO, 8.5–10.5% SiO2, and 20–27% Al2O3 corresponding to boron distribution coefficients of 140–170 at 1550 and 1600°C. Within those regions, when the initial slag contains 4% B2O3, we may expect boron concentrations in the metal of 0.020% when L B = 168 and 0.023% when L B = 139.

Boron removal from metallurgical-grade silicon by CaO–SiO2 slag refining

Boron removal from metallurgical-grade silicon (MG-Si) using CaO–SiO2 slag was studied by employing a medium-frequency electromagnetic induction furnace. The relationship between the optical basicity (Λ) of the CaO–SiO2 slag and the distribution coefficient of boron (LB) was investigated. Consequently, the local minimum and maximum LB values of 0.72 and 1.58 are obtained when Λ = 0.56 and Λ = 0.71, respectively. The boron content in MG-Si decreases gradually with refinement time increasing, down to a minimum value of 4.73 × 10−6. The controlling step in the removal of boron from MG-Si is not the chemical reaction at the interface of the slag and silicon. Instead, the controlling step is a diffusion mass transfer, in which boron impurities diffuse from molten silicon to the interface of the slag and silicon, or B2O3 formed by the chemical reaction diffuses from the slag–silicon interface to molten slag.

Thermodynamics of boron distribution in solvent refining of silicon using ferrosilicon alloys

Distribution coefficient of boron between purified solid silicon and iron–silicon melt at infinite dilution of boron was evaluated to investigate the effectiveness of boron removal from silicon by solvent refining using iron. The estimated values for distribution coefficient of boron at infinite dilution are: 0.49±0.01 (1583K), 0.41±0.03 (1533K) and 0.33±0.04 (1483K). The distribution coefficient increases with temperature showing less boron removal at higher temperatures. The estimated values for self interaction parameter of boron are as follow: −96±12 (1583K), −111±28 (1533K) and −159±45 (1483K). Activity coefficient of boron in solid silicon was obtained and related to temperature as lnγBinsolidSi0=(16317±282)1T-(7.06±0.18).

Distribution of Boron Between Silicon and CaO-MgO-Al2O3-SiO2 Slags

Abstract Production of solar-grade silicon through metallurgical refining methods can be less expensive than current production methods both in terms of energy use and capital cost. Slag refining is a potential metallurgical refining method for removal of boron from silicon. The distribution of boron between silicon and binary CaO-SiO2 and MgO-SiO2 slags, and between silicon and ternary CaO-MgO-SiO2 and CaO-Al2O3-SiO2 slags has been investigated in this work at 1873 K (1600 °C). The distribution coefficient of boron has been found to be between 2 and 2.5 in the binary CaO-SiO2 and MgO-SiO2 systems. It has also been found to be in the same range across the entire CaO-MgO-SiO2 system. In these systems the distribution coefficient is unaffected by slag composition. The distribution coefficient has been found to decrease with increasing Al2O3 content in the slag in the ternary CaO-Al2O3-SiO2 system.

Boron removal in purifying metallurgical grade silicon by CaO–SiO2 slag refining

Boron removal from metallurgical grade silicon (MG-Si) using a calcium silicate slag was studied. The results show that it is impossible basically to remove boron using a pure SiO2 refining. The oxidizing ability of CaO–SiO2 slag for boron removal was characterized by establishing the thermodynamic relationship between the distribution coefficient of boron (LB) and the activities of SiO2 and CaO. The experimental results show that the distribution coefficient and the removal efficiency of boron are greatly improved with the increase of CaO proportion in the slag. The maximal value of LB reaches 1.57 with a slag composition of 60%CaO-40%SiO2 (mass fraction). The boron content in the refined silicon is reduced from 18×10−6 to 1.8×10−6 using slag refining at 1600 °C for 3 h with a CaO–SiO2/MG-Si ratio of 2.5, and the removal efficiency of boron reaches 90%.

Removing boron from metallurgical grade silicon by a high basic slag refining technique

A new purification method of removing boron from metallurgical grade silicon (MG-Si) using a high baisicity slag was developed in this paper. The typical impurities Al, Ca, Ti, B, P etc in MG-Si can be removed by the binary calcium sillicate slag and it is especially efficient for removing impurity Boron. It was found that the maximal distribution coefficient of boron between calcium sillicate slag and silicon reaches to 1.57 when the mass ratio of CaO/SiO2 was 1.5 and the composition was 60%CaO-40%SiO2. It showed that the oxidizability of calcium sillicate slag was affected and restricted by the basicity and the mass ratio of acid oxide SiO slag according to the thermodynamic relationship. The boron concentration in MG-Si can be reduced from 18x10-6 to 4.5x10-6 and 1.4x10-6, respectively, when using the ternary slags 40.5%CaO-49.5%SiO2-10%Li2O and 32CaO-38%SiO2-30%Li2O.

Distribution coefficient of boron in Si crystal ingots grown in cusp-magnetic Czochralski process

Silicon single crystals are grown by the Czochralski method with various growing conditions. Effective segregation coefficient of boron is found to depend on the magnetic field in cusp-magnetic Cz method. Effects of zero-Gauss plane (ZGP), ZGP shape and magnetic intensity (MI) on the dopant concentration and its distribution in the crystal are experimentally investigated. The shape of ZGP is not only flat but also parabolic due to the magnetic ratio (MR), which is the ratio of the lower to upper electric-current densities in the configurations of the cusp-magnetic field. Equilibrium distribution coefficient of boron calculated by BPS model is 0.698. With the crystal rotation ( w) of 16 rpm and the crucible rotation of −0.5 rpm, the effective distribution coefficient ( k e) is 0.728 in zero magnetic intensity and increases up to 0.8093 in the parabolic ZGP shape. Although the magnetic strength near the crystal–melt interface decreases with increasing MR, it increases in the bulk melt, and hence k e increases. Flow stability in the bulk melt influences k e. At the magnetic field and growing conditions, k e increases with increasing initial charge size of the silicon melt. There is no significant influence of ZGP on the radial distribution of the boron concentration. Simulation results of melt flow in the presence of a parabolic ZGP are outlined, and the segregation results in the experiments are compared with published experimental data.

Boron segregation control in silicon crystal ingots grown in Czochralski process

Boron-doped silicon single crystals of 207 mm diameter with various growing conditions are grown from a large amount of the melt in the cusp-magnetic Czochralski method, and the effects of growing parameters on dopant concentrations in the crystals are experimentally investigated. Equilibrium distribution coefficient of boron calculated by BPS model is 0.716. With the crystal rotation ( ω ) of 13 rpm and the crucible rotation of - 0.5 rpm , the effective distribution coefficient ( k e ) is 0.751 in zero magnetic strength and increases up to 0.78 in the magnetic strength of 640 G. For ω ⩾ 7 rpm , there is no significant influence of ω on k e . With ω ⩽ 3 rpm , k e is almost unity. The experimental results are compared with theory.

The influence of boron localized vibrations on the equilibrium distribution of boron between the surface and bulk of a silicon crystal

The distribution coefficient of boron in the system of silicon crystal + boron surface phase is calculated in a wide temperature range. A realistic model of the lattice vibrations of silicon is used. The effect of boron localized vibrations is included in the calculation. It is shown that the boron localized vibrations significantly decrease the distribution coefficient and impede the penetration of the boron atoms into the interior of the silicon crystal.

Redistribution of boron during thermal oxidation of silicon studied by SIMS using Ar+ bombardment and the MISR method. Part I: Methodology

AbstractQuantitative depth profiling of boron through SiO2/Si interfaces, when profiling with Ar+, is complicated by the matrix effects. The presence of oxygen affects the sensitivity of boron by three orders of magnitude and accurate determination of the distribution of the distribution coefficient of boron at the interface is only possible when the matrix effects can be elminated or taken into account in the quantification. By using matrix‐sensitive correlation factors, a sensitivity curve of boron at different primary ion current densitites was established. This relation shows a good long‐term reproducibility. Moreover, a physical interpretation of the shape of the sensitivity curve of boron is given by comparison with pure Si and SiO2. Tentative explanations for the behaviour of different SixOyz+ species in conditions of varying primary ion current density and oxygen backfill are given. The influence of the oxygen backfill pressure on the sputter rate of silicon was investigated and problems arising in the profilation of boron in SiO2/Si interfaces were addressed. Practical applications of the described methodology are discussed in Part II of this series.

Thermodynamic study of the solubility process of boron in silicon carbide, grown from the vapour phase

The thermodynamics of the solubility process of the acceptor impurity boron in silicon carbide is investigated. The thermodynamic analysis of equilibrium in the Si-C-B system has been carried out and the temperature dependences of the partial pressures of interacting components are determined for the temperature interval (1800–3000) K. The calculation of the Fermi level in p-SiC(B) is carried out. The heat of solution and the distribution coefficient of boron in silicon carbide are determined on the basis of the thermodynamic analysis of the Si-C-B system, the experimental temperature dependence of boron solubility and the calculation of the Fermi level and Fermi-Dirac distribution in p-SiC(B).

The Distribution Coefficient of Boron in Germanium

The equilibrium distribution coefficient of boron in germanium was found to be 12.2. The variation with growth rate was interpreted in terms of a theory by Burton, Prim, and Slichter. As the growth rate was increased the width of the diffusion dominated layer initially increased followed by a later decrease. This was due to a change from a thermally produced melt flow pattern to a hydrodynamically produced pattern. The results show that a slow growth rate and a high rotation rate should be used to obtain equilibrium distribution coefficients.

Distribution Coefficient of Boron in Germanium

The effective distribution coefficient of boron in germanium has been measured as a function of crystal growth rate. Its behavior is shown to agree with that predicted by the theory of Burton et al. for a solute whose equilibrium distribution coefficient is greater than unity. The coefficient decreases rapidly with increasing growth rate from its equilibrium value 17.4 to 2.2 at a growth rate of 10—2 cm/sec.