Surface Tensions of Fe–(30–40Mol%) Si–C Alloys at 1523–1773K

Materials Transactions, Vol. 54, No. 10 (2013) pp. 1968 to 1974 ©2013 The Japan Institute of Metals and Materials Surface Tensions of Fe(3040 mol%)SiC Alloys at 15231723 K Takeshi Yoshikawa Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505, Japan To analyze the Marangoni effect on fluid flow in an FeSi solution during solution growth of SiC, knowledge of the temperature coefficient of surface tension of the solution is essential. In this investigation, the surface tensions of Fe(3040 mol%) Si alloys and alloys with added carbon were measured at 15231723 K by the maximum bubble pressure method. Surface tensions and temperature coefficients were precisely determined. The measured surface tensions were also compared with estimations based on the modified Butler’s model, and the effect of carbon on the surface tension of FeSiC alloys was assessed. [doi:10.2320/matertrans.M2013211] (Received June 4, 2013; Accepted July 23, 2013; Published September 6, 2013) Keywords: ironsilicon alloy, surface tension, Marangoni effect, surface active effect, modified Butler’s model 1. Introduction heating.9) A remarkable SiC grown ridge on the seed crystal was observed at the edge of the area that was in contact with Silicon carbide (SiC) is a wide band gap semiconductor, the cylindrical liquid bridge. However, the growth was and is a promising base material for high temperature insignificant in interior regions. Because the growth of the electronics and high-voltage power devices because of its low ridge became larger with larger temperature gradients, the on-resistance and high electrical breakdown field. Physical Marangoni effect might affect the convection around the vapor transport (PVT), based on the sublimation of SiC meniscus of the liquid bridge. powder, is the standard method to produce SiC single To analyze the Marangoni effect on the fluid flow of the crystals. However, this process requires temperatures above solution during the solution growth process, it is essential to 2200 K, resulting in high costs and an inadequate quality of know the temperature and composition dependence of the the grown crystals. surface tension of the FeSiC alloy. The surface tension of Recently, solution growth has been identified as a more binary FeSi1014) and FeC13,1519) alloys has been inves- promising process for producing high quality SiC crystals tigated by many researchers. However, studies on the surface with a smaller dislocation density because the solidliquid tension of FeSiC alloys have been limited and the reported interface is close to the equilibrium state during growth. data are controversial. Belton20) used the sessile drop method Earlier work using solution growth with Si-based solvents, to measure the surface tension of FeSiC alloys with such as SiGe or SiAl solvents, clarified the epitaxial varying Si content (28 mol%) and with a constant C content growth,1,2) but growth rates did not exceed 10 µm/h. The of 12 mol% at 1723 K. He found that increasing the Si growth kinetics of solution growth of SiC was thus regarded content remarkably decreased the surface tension. He to be far slower than conventional sublimation processes in suggested that the decrease was caused by the surface-active which the growth rate is generally larger than 500 µm/h. effect of the associative adsorption of SiC. Kawai et al.13) However, the growth rate was improved by selecting suitable used the sessile drop method to measure the surface tension solvents for carbon solubility, and a growth rate of over of an Fe(110 mol%)Si(13 mol%) C alloy at 1673 100 µm/h was achieved at temperatures above 2000 K using 1923 K, and found its maximum value to be 2 mol% Si at SiTi3,4) and SiCr5,6) solvents by the top-seed growth constant C content. They also found a positive temperature method. dependence for the surface tension of the Fe50 mol% Si The author’s group has attempted to achieve rapid solution alloy, which is anomalous for the endothermic mixing alloy. growth at lower temperature using an FeSi solvent. We have If Belton’s suggestion that the remarkable change in the investigated the phase relation of the FeSiC system, as well surface tension is attributed to SiC adsorption is valid, solutal as the solubility of SiC in the liquid phase, to clarify an Marangoni effect besides thermal effect can affect the fluid optimal solvent composition for rapid SiC growth.7) Fe(35 flow in SiC solution growth conditions. 40 mol%) Si alloy was suitable as a solvent because its SiC In previous studies, Fe(3040 mol%) Si alloys were used solubility was over 100 times greater than that of Si-based as solvent alloys for the solution growth of SiC.8,9) In the alloys or molten silicon. The solution growth was carried out present work, the surface tension of these alloys and those at 16231723 K by a temperature difference method using the with added carbon were measured at 15231723 K by liquid bridge of the FeSi solvent under induction heating.8) maximum bubble pressure method. The estimation was also Because homo-epitaxial growth of 4H- and 6H-SiC was conducted based on Butler’s model.2123) The effect of achieved at a rate of 100200 µm/h, the FeSi solvent was temperature and composition change on the surface tension determined as effective for solution growth. To evaluate the was discussed. diffusion control growth, the solution growth was conducted at 1623 K by the temperature difference method in conditions 2. Alloy Composition designed to suppress natural convection under resistance Fe(3040 mol%) Si alloy was selected as a target alloy +Corresponding author, E-mail: [email protected] composition. The author’s group7) investigated the phase Surface Tensions of Fe(3040 mol%)SiC Alloys at 15231723 K 1969 Table 1 Thermodynamic parameters describing the excess properties of the liquid phase for each binary in the FeSiC system. Ex Excess Gibbs energy; ¦G = XFeXSiLFeSi,liq + XFeXCLFeC,liq + XSiXCLSiC,liq ¹1 Li-j,liq/J·mol Reference 2 3 LFeSi,liq = ¹151128 + 29.125T + (¹33882 ¹ 2.5015T)(XFe ¹ XSi) + (33954 ¹ 11.256T)(XFe ¹ XSi) + (21289 ¹ 0.8650T)(XFe ¹ XSi) 24) 2 LFeC,liq = ¹124320 + 28.500T + (¹19300)(XFe ¹ XC) + (49260 ¹ 19.000T)(XFe ¹ XC) 25) LSiC,liq = 8700 7) 3.5 3.0 Fe-30mol%Si 2.5 (graphite satd.) Fe-37.5mol%Si 2.0 (SiC satd.) Mol% C 1.5 Fe-35mol%Si 1.0 (SiC satd.) 0.5 Fe-40mol%Si (SiC satd.) 0 1473 1573 1673 1773 Temperature, T / K Fig. 1 Estimated solubility of carbon in liquid FeSi alloys saturated with graphite or SiC. relationship of the FeSiC system at 15231723 K. Graphite or SiC was determined to be in equilibrium with the Fe Si- Fig. 2 Experimental apparatus for the maximum bubble pressure method. based liquid at Si contents smaller or larger than 35 mol%, respectively. The liquid composition determined by experimental data was found to agree with estimations by The furnace was then heated under a H2 gas flow (5 sccm). thermodynamic calculation using the thermodynamic proper- To eliminate the oxygen contamination into the sample, 24) 25) ties of the liquid phase in the binary FeSi, FeC and high purity H2 gas (dew point <¹203 K) was employed and SiC7) systems. passed through silica gel and magnesium perchlorate, sponge For the surface tension measurements of FeSiC alloys titanium heated at 1073 K to remove H2O and oxygen before in the present work, the liquid alloy was saturated with supply to the sample. graphite (Fe30 mol% Si) or SiC (Fe35, 37.5, 40 mol% Si). Measurements were performed after maintaining the target The equilibrium carbon concentrations of the alloys were temperature for more than 0.5 h for FeSi alloys and 1 h for estimated for the respective alloys by thermodynamic FeSiC alloys. First, the position of the melt surface was 7) analysis as described in previous work using the thermo- determined from the pressure change in the Al2O3 capillary dynamic properties listed in Table 1. The calculations as it was lowered at a rate of 0.01 mm/s into the surface. were conducted using a thermodynamic software package The immersion depth of the capillary tip was then controlled (FactSage 6.3). to within 212 mm, and the maximum bubble pressure Figure 1 shows the calculated result of the temperature was measured. The alumina capillary was prepared from dependence of carbon content of the FeSi alloys with an alumina pipe (O.D. 2.0 mm, I.D. 1.5 mm; Degussit) by varying Si content. The carbon content is largest for the polishing and finishing its tip to a sharp edge (3045°) with Fe30 mol% Si alloy, determined as 3.5 mol% at 1723 K. emery paper (#2000). The diameter of the tip was determined by an optical microscope, and the measurement error was 3. Experimental within 10 µm. The motion of the capillary was controlled with an electric actuator (ELS-4; Oriental Motor Co., Ltd.). Surface tension measurements of FeSi alloys and FeSi The pressure in the capillary was measured using a differ- C alloys were carried out using the SiC electric resistance ential pressure transmitters (FKCS22V5; Fuji Electric furnace (Fig. 2). A total of 3040 grams of electric iron Systems Co., Ltd.) calibrated with a water manometer.

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