발간논문

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Solar-grade polycrystalline silicon was directionally solidified by the Bridgman method using split, reusable graphite molds coated with Si₃N₄ powder. The impurity content, grain size and dislocation etch-pit density(EPD) of the polycrystalline silicon wafers which affect the conversion efficiency of the solar cell were investigated and compared with the reference values reported for high efficiency silicon solar cell. According to the X-ray diffraction analysis, preferred orientation was <111>. The resistivity decreases from 3 to 2Ω·㎝, while the carrier concentration increases from 4.0 to 6.0×10^(15)㎝^(-3) with increasing growth rate and fraction solidified. The presence of B which can be acting as a carrier, is thought to be the main factor causing such a decrease in resistivity.

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Oxide layers of an Fe-36wt%Ni Invar alloy produced by heating the specimen in air in the temperature range between 560℃ and 700℃ were investigated. According to XRD and AES analysis the oxide layers were composed of thin Fe₂O₃ on the surface, thick Fe_(3-x)Ni_xO₄(0≤x≤1) in the middle and thin NiO near the phase boundary. A thermodynamic evaluation has revealed that the Fe_(3-x)Ni_xO₄ layer is formed by the reaction of FeO with Ni which was enriched to a concentration above 70 at% through preferred oxidation of Fe at the initial stage of oxidation. NiO can be formed, on the other hand, by the reaction between Fe₂NiO₄ and Ni when the concentration of Ni reached over 90 at%. From the results of the XRD and AES analysis as well as the thermodynamic evaluation, the oxidation process of an Fe-36wt%Ni Invar alloy is proposed.

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Indentation fractures were obtained using a vickers diamond indentor on seven cemented carbide grades. Three different loads were applied using a Rockwell hardness tester. Indentation parameters, diagonals of Vickers indentation marks, and the length of the surface cracks, were measured. The specimens for the indentation tests were made of short bar specimens which had been tested. The measured indentation parameters along with short bar K_(lc) values were used for evaluating three equations derived by different researchers.

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Functionally gradient W/Mo material was developed by using a laser surface-alloying method. Tungsten was first coated on a molybdenum substrate and then CO₂ laser beam was irradiated in order to make W/Mo surface-alloying layer. By a successive W coating and laser irradiation treatment, it was possible to develop functionally gradient W/Mo material. The thermal property of this W/Mo material was measured by a laser flash method and then compared with the theoretical values.

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Casting conditions for producing unidirectionally solidified Al-1wt%Si alloy by mean of horizontal continous casting process adopting the heated mold have been studied. It has been shown that the casting speed R to obtain the alloy rod with mirror surface increases as the distance between mold and cooler decreases. To obtain the unidirectionally solidified rod with mirror surface the liquid/solid interface position has to be located between 2 and 7㎜ toward casting direction from the mold exit. Both the size d and the interphase spacings S of eutectic Si phase formed in the unidirectionally solidified rod, decrease as the casting speed increases and the distance between mold and cooler decreases. The relationship between the interphase spacing S and casting speed R can be described as S²R=const. in accordance with many other researcher`s results.

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In this paper, we described the microstructure evolution of Zr-2.5wt% Nb alloy during hot rolling in the α+β phase field after β quenching. The α′and retained β phases produced after β quenching transformed into α′and equilibrium a phases during subsequent hot rolling. The elongated αbecame shorter by penetration of β into a through high angel α/α boundaries as the number of rolling passes increased. Defects introduced during hot rolling process helped β transforming into α′with lower activation energy, and part of β transformed into α′by burst transformation during cooling after hoc rolling.

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