Byung- Moon

Numerical and Experimental Study of the Electromagnetic Continuous Casting of Silicon

For the preparation of multi-crystalline Si ingot, an EMC (Electromagnetic Continuous Casting) technique using a segmented Cu cold crucible under a high frequency alternating magnetic field was utilized. Numerical and experimental works were systematically carried out in order to optimize the effect of Joule heating and electromagnetic pressure. A Si melt of 5.5 cm height was able to be kept non-contact with the crucible wall when the induction coil current of 970 A was applied. A segmented graphite crucible, which was attached at the upper part of the cold crucible, was introduced to enhance significantly the heating efficiency of Si melt keeping non-contact condition.

Optimization of Heat Treatment Parameters of Mo-Free High-Cr Cast Iron Mill Balls

A Mo-free high-Cr cast iron with superior impact-wear resistance was tried to develop for mill balls by alloy design and subsequent heat treatment. The Cr/C ratio was varied up to 7.7 in order to minimize and eventually eliminate Mo addition, still ensuring hardenability. For the proper balance between abrasion resistance and toughness, the combined effects of each heat treatment parameters on the mechanical properties were systematically investigated using Taguchi method and response surface method (RSM), well recognized as powerful tools of the designs of experiments (DOE). It was found that hardness and impact energy were mainly affected by tempering and destabilization conditions, respectively. It is noted that high impact energy of 5.2 J/cm2 was obtained without deteriorating hardness (54 HRc) during the verification experiments under the heat treatment condition optimized by the DOE analyses.

The effects of heat treatment condition and Si distribution on order-disorder transition in high Si steels

The decomposition and formation of ordered phases in steels with 5 to 6.5 %Si were investigated by a combined analysis of TEM and electrical resistivity measurements in order to determine the optimal heat treatment conditions for removal of ordered phases. TEM diffraction pattern study revealed that B2 ordered phase in 6.5 %Si steel was sufficiently dissolved by heat treatment at 850 °C for 1 h, and then rapidly re-formed during cooling. The critical cooling rate, above which the suppression of B2 phase formation was possible, increased rapidly with Si content higher than 5.6%. DO3 phase appeared only in the case of as-cast samples containing Si content as high as 6.5%. The measurement of electrical resistivity change during the heat treatment showed that the atomic movement was substantially spurred above 800 °C, resulting in an active order-disorder transition. The removal of solidification segregation is necessary to reduce the amount of B2 ordered phase in the as-cast or hot-rolled state, to lower the heat treatment temperature for dissolution of B2 ordered phase, and to shorten the heat treatment time.

The Effect of Crucible Configuration on Joule and Pinch Effects in EMCC Process of Silicon

For the development of an economical production technology of multi-crystalline silicon, an EMCC (Electro Magnetic Continuous Casting) process using a segmented Cu cold crucible was practiced. In order to enhance significantly the heating efficiency of silicon melt keeping non-contact condition during continuous melting and casting processes, the effects of Joule heating and electromagnetic pressure in molten silicon were optimized by systematically varying crucible design and configuration. Throughout the present investigation, multi-crystalline silicon ingot was successfully produced at the casting speed of above 1.5 mm/min under a non-contact condition.

Tribological Behavior and Surface Chemistry Characteristics of Quasicrystal Coatings

The aim of this study was to investigate the effects of air plasma spraying process parameters on the tribological behavior and surface characteristics of quasicrystal coatings. For this purpose, the response surface method (RSM), one of the designs of experiment (DOE) was utilized to systematically investigate the combined effects of each process parameters. The RSM analysis revealed that the superior tribological performance of quasicrystal coatings was obtained by the optimized condition of specific micro-hardness and porosity level. It was specially quoted that the relatively low hardness of quasicrystal coatings caused the improvement of abrasive wear resistance during the current investigation.

Cold Workability and Magnetic Properties of 6% Si Steel

In order to develop an economical production method of high Si steel sheet, 6%Si (by weight percent, unless specified otherwise) steel was prepared by a combined process of conventional casting and hot- and cold-rolling. Tension and nano-hardness tests and TEM analysis were carried out to examine the effects of ordered phases, Si-content, and testing temperature on cold workability. By optimizing the successive processes of casting, hot-rolling, heat treatment, and cold-rolling, 0.5 mm thick 6%Si steel sheet was successfully produced without crack formation. As Si content increased from 3% to 6%, core loss (W10/50) of the 0.5 mm thick Si steel sheet decreased from 1.36 W/kg to 0.89 W/kg.

Boron removal from UMG-Si by hybrid melting utilizing steam plasma torch and EMCM

The specific purification process was developed to remove impurity element boron from UMG-Si (Upgraded Metallurgical Grade Silicon) by the steam plasma and EMCM (Electro Magnetic Continuous Melting). Some plasma reactive gases lead to volatilization of boron on the surface of liquid silicon because of the lower evaporation pressure than that of silicon. Boron is removed as volatile compounds such as BxHzOy , B(OH)x, BxOy. The experiment was carried out in various surrounding gas conditions of plasma. Injecting gas (hydrogen and steam) with argon which is carrier gas of steam plasma torch was best for the refinement. The removal ratio of boron impurity was increased with increasing the flow ratio of steam. The final Boron concentration was decreased to 0.17 ppmw from 2.9 ppmw of initial content.

Binder-Free Pellet Fabrication of Ultra-Fine Poly-Si Powder for Economical Production of Solar Grade Si Ingot

The consolidation process of ultra fine Si powders, generated as by-product during the decomposition process of silane gases, was systematically investigated for use as economical solar-grade feedstock. Si powder compacts were tried to fabricate by a consolidation process without a binding agent and then their density ratio and strength were evaluated. The Si powders in as-received state were not pure enough to be used alone as solar grade feedstock material. After the adequate chemical treatments, a sufficiently high purity above solar-grade was able to be achieved.

Effects of Atomizing Gas on Microstructural Characteristics and Mechanical Properties of Spray Formed High Si Steels

In order to investigate the feasibility of spray forming process for production of high Si steel sheets, 4.8~6.4%Si steels were prepared by spray forming process using N2 and Ar gases and their cold workability was compared with that of conventional ingot cast samples. Atomizing gas affected significant effect on deposit features, microstructural characteristics and mechanical properties of the spray formed high Si steels.

Compaction and Cold Crucible Induction Melting of Fine Poly Silicon Powders for Economical Production of Polycrystalline Silicon Ingot

The consolidation and casting processes of fine silicon powders, by-product of high purity silicon rods making process in the current method, were systematically investigated for use as economical solar-grade feedstock. Morphology, size, and contamination type of the fine silicon powders were inspected by combined analysis of SEM, particle size analyzer, and FT-IR. Silicon powder compacts were tried to fabricate by a consolidation process without a binding agent and then their density ratio and strength were evaluated. Finally, the electrical resistivity of the specimens prepared by an electromagnetic casting method was examined for purity assessment.