발간논문

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In this study wettability between Al₂O₃ and Al to the variation of SiO₂ concentration in Al₂O₄ was investigated. Alumina specimens containing from 0 Lo 20wt%SiO₂ were prepared by using the coventional ceramic processing techniques. Wettability was measured by using the dip coverage method and was compared with wettability measured by using the sessile drop method. Also the distributions of Al and Si element around the interface were investigated. As the results of these investigation, wetting process in the dip coverage method was shorter than that of the sessile drop method, and the equation of reaction rate for wetting process between alumina and aluminium was almost agreed with the following equation. Qn[ 1/(1- α)]=K_0(t-τ²) Both the started time and the completed time of wetting became short with SiO₂ concentration and this tendency bacame severe with higher melt temperature. The Si content around the interface did not change.

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In order to investigate the interface phenomena, namely wettability, interface reaction, interfacial bonding, between Al₂O₃, and various Al alloys, wettability with the addition of alloying elements was measured by the dip coverage method, and the distribution of alloying elements and interfacial compounds at the interface were examined by using SEM-EDAX, Auger electron spectroscope and X-ray diffractometer. The interfacial bonding strength was also estimated by the indentation method. Wettability of Al was considerably improved by addition of Mg, Cu, Ni or Si. Among the four elements, Mg was the most effective. Mg was segregated at the interface of AC8A/Al₂O₃-20%SiO₂;the segregated Mg formed MgAl₂O₄, an interfacial compound. In pure Al/Al₂O₃ the inierface bonding strength was not affected by the concentration of SiO₂ in A1₂O₃ but in AC8A/Al₂O₃ the interface bonding strength increased with SiO₂ concentration and was 6.05 kgf/㎟ at 20%SiO₂.

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A study was conducted to investigate the effect, of cold rolling and electron beam welding on microstructures and mechanical properties in C250 and T250 maraging steels. Microstructures of cold rolled specimen performed intermediate solution treatment(850℃, 1hr)were finer than those of directly cold rolled specimen due to recrystallization during solution treatment. Even though the hardness distribution trends of both C250 and T250 cold rolled maraging steels after electron beam welding were nearly the same, the restoration trends of hardness on welded zone after aging treatment were different were different between those two maraging steels. The segregation of Ni, Mo, Ti elements on welded zone in T250 maraging steel was severer than that in C250 maraging steel, which resulted in poor hardness restoration in T250 maraging steel.

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The microstructure and surface morphology of Cu thin films have been investigated and effects of microstructure and surface morphology on the patterning of microcircuits by wet chemical etching have been studied. Using 3 target DC magnetron sputtering unit, 2-5㎛ thick Cu thin films are deposited on the water-cooled substrates which are polycrystalline alumina, single crystal sapphire, and slide glass. 50㎚ thick Cr film is used as an adhesion layer between Cu and substrates. As the sputtering pressure increases, the microstructure is more open, and pores or microcracks in the column boundaries are frequently observed, which are in good agreement with the structure zone model proposed by Thornton. The morphology of Cu films deposited onto sapphire and slide glass substrates are similar at the same sputtering pressure, but the microstructure deposited on coarser polycrystalline alumina substrates contains more cracks and open boundaries, which are attributed to self-shadowing effcets during film growth. The micropatterns of 25㎛ in line width are successfully obtained by photolithography and wet chemical method regardless of sputtering conditions. Line morphology becomes smoother in the film formed at lower sputtering pressure. This effect is related to the open boundaries in the columnar structure. Line patterns are not much affected by grain or column size of the film.

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Thermoelectric properties of Bi₂Te₃-base thick films were studied in order to obtain the optimum manufacturing conditions;binder contents, sintering temperature and time, and amounts of dopants. Blank screen printing and subsequent sintering process were proved to be suitably; for the fabrication of thick films, if the fine powders(◎2㎛)were mixed with 18% of the binder;propylene glycol 3wt% and ethyl alcohol 15wt%. The thick films of intrinsic Bi₂Te₂ were found to have n-type property and the figure of merit of 4.25×10^(-4)K^(-1) when doped with 4 at.% Se and sintered at 450℃ for 4 hrs. The figure of merit of sintered p-type Bi_(0.5)Sb_(1.5)Te₃ and n-type Bi₂Te₂Se_(0.3) with 10 at% Sb increased as the sintering temperature became higher;the former showed maximum values of 5.96×10^(-4)K^(-1) when sintered at 550℃ for 4 hrs, and the latter could be reached to 4.62×10^(-4)K^(-1) when sintered at. 525℃ for 4 hrs.

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Removal of impurities such as antimony, arsenic, tin, lead and bismuth from molten blister copper by cryolite flux has been investigated in order to develope a new pyrometallurgical copper refining process. A 300g of molten blister copper in which Cu₂O was dissolved as an oxygen source was treated with cryolite. Experiments were carried out in argon atmosphere at 1200℃. At added oxygen of 0.5wt. %, antimony, tin and arsenic contents in molten blister copper decreased rapidly from 0.12wt.%, and 0.113wt.% to 6.0×10^(-3) wt.%, 1.4×10^(-3) wt.% and 1.0×10^(-3) wt.%, respectively. The cryolite flux, however, had only a moderate effect on the removal of lead whereas virtually no effect on the removal of bismuth. A brief thermodynamic analysis was carried out to estimate the solubility limits of impurity oxides in molten blister copper and the experimental results were well explained by the thermodynamic analysis.

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A mathematical model which describes the rate phenomena in particulate flash reaction system has been developed and examined with experimental results. The developed model faithfully reproduces the kinetic data and fluid profiles. The temperature history of reacting particles which can be hardly measured by experiment is also calculated and analyzed. From this study it is found that the heating of particle is mainly due to the convective heat transfer between particle and surrounding gas, after particle reaches about 700K, strong exothermicity of reaction leads the sharp increase of particle temperature. Fragmentation of particle seems to lead acceleration of desulfurization which causes the discrepancy of model prediction for large particles.

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The effect of heat treatment on the microstructures of Ti-48at%Al alloys was investigated. TiAl intermetallic compounds were melted by the plasma arc melting system. The effect of the solidification rate on the cast microstructures was also studied by using three types of mold, i. e., steel, graphite and ceramic. It was found that dendrite arm spacing and thickness of the lamellar structure decreased, and Al segregation at interdendritic r increased as the solidification rate in the molds was increased. The specimen cast in a ceramic mold showed the most rapid rate of recrystallization. In lamellar structures, the recrystallization does not occur from the interface of γ and α₂ phase, but rather from the growth of the new laths, maintaining their crystallographic relationship. In the compressive test at room temperature, the difference of the yield strength that occurred from the various mold decreased with the increase of heat treatment time.

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The effect of organic liquids (methanol+toluene, methanol+acetone and methanol+ ethylacetate) with the carburizing medium and nitrogen with a carrier gas on the internal oxidation of commercial carburized steels during carburizing in the atmosphere of cracked methanol gas were investigated. After water quenching, the outmost layer of carburized steel appeared uniform distribution of internal oxide at the grain boundaries and grain interiors. Below outmost layer the oxides formed at the grain boundaries. With increasing oxygen concentration in orgainc liquid and decreasing nitrogen addition to the carburizing atmosphere, the depth of internal oxide layer became thicker. It has also been found that the depth of internal oxidation increase with the silicon and manganes contents. It. was concluded that the control of the internal oxidation in drip- feed carbrizlng atmosphere is possible through lowering the CO gas content of the carburizing aimosphere with the addition of N₂ gas or by using the organic liquid having a superior cracking behavior (with smaller oyxgen radials) as a carburizing medium.

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Two kinds of specimens with different microstructure were prepared by changing casting variables in vacuum investment casting. The effect of microstructural difference on the creep behavior, creep crack initiation and creep crack propagation was studied in Ni base superalloy RENE 80. The specimen having fine grained and uniformly distributed carbide exhibited superior creep properties to the large grained specimen tested at 760℃. However above 870℃, two kinds of specimens showed similar creep behavior. At higher temperature creep, crack initiation was principally caused by the reaction between the grain boundary exposed at the surface and the test atmosphere rather than the internal microstructural changes. The crack initiated at the surface propagated to the internal area and this kind of crack propagation leads to fracture of material.

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