Jong-Kwon Lee

Growth Kinetics of MoSi2 Coating Formed by a Pack Siliconizing Process

Growth kinetics of MoSi 2 coating formed by pack siliconizing process of Mo substrate was investigated on the basis of the dependence of its growth rate on Si content in the pack powders. Pack siliconizing was carried out using (1-70) wt % Si-5 wt % NaF-bal SiC packs at 1100°C in a hydrogen atmosphere. The chemical vapor deposition (CVD) of Si on Mo substrate was also performed at 1100°C using SiCl 4 -H 2 gas mixture to determine the maximum growth rate of MoSi 2 coating obtainable in the bulk Si/mo diffusion couple. The growth kinetics of the MoSi 2 coating formed by the pack siliconizing process obeyed a parabolic rate law irrespective of compositions of pack powders. The growth rates of MoSi 2 coating increased with increasing Si content in the pack powders up to 40 wt % Si but attained a constant value in the pack powders with over 40 wt % Si, which was equal to that obtained by the CVD process. The rate-limiting step for growth of MoSi 2 coating formed by the pack siliconizing process was subject to theoretical considerations. Three models such as a gas-phase diffusion controlled model, a solid-state diffusion controlled model, and a dynamic equilibrium model of gas-phase diffusion and solid-state diffusion were evaluated. The theoretically predicted results based on the dynamic equilibrium model and the experimentally obtained results were found to be in good agreement.

Formation of MoSi2–SiC composite coatings by chemical vapor deposition of Si on the surface of Mo2C layer formed by carburizing of Mo substrate

Abstract The formation process of MoSi 2 /β-SiC composite coating by chemical vapor deposition of Si on the Mo 2 C layer formed in advance on a Mo substrate by a carburizing process was investigated using optical microscopy, field-emission scanning electron microscopy cross-sectional transmission electron microscopy and X-ray diffraction. The composite coating was composed of equiaxed MoSi 2 grains with average size of 300 nm and the β-SiC particles with average size of 92 nm, which were mostly located at the grain boundaries of MoSi 2 . The morphology of β-SiC particles was oblate-spheroidal shape and volume percentage was approximately 19.3%. The number of cracks in the composite coating was smaller than that for the monolithic MoSi 2 coating by the reduced mismatch of thermal expansion coefficient with the Mo substrate. The formation mechanism of MoSi 2 /β-SiC composite coating was suggested on the basis of microstructure observation.

Effect of Cl/H input ratio on the growth rate of MoSi2 coatings formed by chemical vapor deposition of Si on Mo substrates from SiCl4–H2 precursor gases

Abstract Under chemical vapor deposition (CVD) conditions of Si on Mo substrate limited by mass transport of reactant gas species through a gas boundary layer, the effect of the Cl/H input ratio on the growth rate of MoSi 2 coating at 1200 °C was investigated using a horizontal hot-wall reactor and SiCl 4 –H 2 gas mixtures. The growth kinetic of the MoSi 2 coating obeyed a parabolic rate law irrespective of Cl/H input ratios. The growth rate of MoSi 2 coating initially increased with increasing Cl/H input ratio until the maximum growth rate was reached, and then decreased inversely proportional to the Cl/H input ratio at higher Cl/H input ratio. This suggests that there is a limit to the increase in growth rate for MoSi 2 coating due to the etching effect of Si with respect to the input ratio of Cl/H. The etching effect of the Cl/H input ratio on the growth rate of the MoSi 2 coating was explained by thermodynamic calculations based on the variation of silicon activity on the surface of the MoSi 2 coating. The mass balance between the Si flux supplied by the mass transport step and that consumed by solid-state diffusion to form MoSi 2 coating was considered to be responsible for the activity of silicon.

Formation of crack-free MoSi2/α-Si3N4 composite coating on Mo substrate by ammonia nitridation of Mo5Si3 layer followed by chemical vapor deposition of Si

Abstract The formation of crack-free MoSi2/α-Si3N4 composite coating on a Mo substrate by ammonia nitridation followed by chemical vapor deposition (CVD) of Si on the Mo5Si3 layer at 1100 °C which was already formed on a Mo substrate was investigated using optical microscopy, field-emission scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (XTEM) and X-ray diffraction (XRD). This study demonstrated that the crack-free MoSi2/α-Si3N4 composite coating with some pores could be produced on a Mo substrate. Two composite layers (γ-Mo2N/α-Si3N4 and Mo3Si/α-Si3N4) with different microstructure were formed by ammonia nitridation of Mo5Si3 layer, and the subsequent CVD of Si resulted in the formation of three composite layers (MoSi2/oblate-spheroidal type α-Si3N4, γ-Mo2N/rod type α-Si3N4 and Mo3Si/α-Si3N4). The volume percentage of α-Si3N4 phase in the MoSi2/α-Si3N4 composite coating ranged from 30 to 33%, suggesting that the thermal expansion coefficient of composite coating was close to that of Mo substrate.

Kinetics of chemical vapor deposition of Si on Ni substrates from a SiCl4–H2 gas precursor mixture

Abstract The kinetics of chemical vapor deposition (CVD) of Si on the Ni substrate at deposition temperatures between 850 and 1050 °C using hot-wall reactor and SiCl 4 –H 2 gas mixture was investigated. The deposition rate of Si on the Ni substrate obeyed a linear rate law. The activation energy for Si deposition was approximately 117 kJ/mol at low temperatures, indicating that the rate-limiting step for deposition of Si was the chemical reaction step to deposit Si on the surface of substrate. At high temperatures above 975 °C, activation energy was approximately 29 kJ/mol, indicating that that was the mass transport step of reactant gas species through a gas boundary layer from the main gas stream to the surface of Ni substrate. Under CVD conditions of Si limited by the mass transport step, there was a limit in the increase of the Cl/H input ratio to increase the deposition rate of Si on Ni substrate due to etching effect of Si.

Formation process and microstructural evolution of Ni-silicide layers grown by chemical vapor deposition of Si on Ni substrates

Abstract The formation process and microstructural evolution of Ni-silicide layers formed by chemical vapour deposition (CVD) of Si on Ni substrate at deposition temperatures between 900 and 1050 °C using horizontal hot-wall reactor and SiCl 4 –H 2 gas mixtures was investigated. The Ni-silicide layers grew sequentially in a series of γ-Ni 5 Si 2 , δ-Ni 2 Si and θ-Ni 2 Si layers with increasing deposition time. After an incubation time necessary for nucleation of each Ni-silicide phase, the growth kinetics of each layer obeyed a parabolic rate law and was controlled by solid-state diffusion of Ni leading to void formation at the interface of the Ni-silicide layers and the Ni substrate. The growth rates of γ-Ni 5 Si 2 and δ-Ni 2 Si layers were faster at the early deposition stage than those at the later deposition stage. The formation process and microstructures of each Ni-silicide layer was influenced by the effect of deposition parameters especially on the difference between the Si flux supplied by CVD process and the Ni flux diffusing through each Ni-silicide layer.

Formation and Interface Analysis of Ti ∕ Ni ∕ Ti ∕ Au Ohmic Contacts on n -Type 6H–SiC

We present the results of TiNiTiAu multilayer ohmic contacts on n -type 6H-SiC and their interface analysis. The as-deposited contacts show rectifying behavior and, with the increase in annealing temperature, they gradually transform to high-quality ohmic contacts exhibiting linear current-voltage characteristics. The interface evolution was analyzed through glancing angle X-ray diffraction, Auger electron spectroscopy, and atomic force microscopy. The Ti Si2 and Ni2 Si formed at the interfaces during the low-temperature annealing initiate the conversion from Schottky to ohmic behavior, while the increased Ni2 Si formation at high-temperature annealing makes the perfect ohmic contacts. The results were interpreted through the thermodynamic reaction mechanisms. © 2007 The Electrochemical Society.