발간논문

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Ca^(2+)-ion conductors based on calcium hexaluminate with MgO have been synthesized by direct solid-solid reaction and the phase compositions near Al₂O₃-rich region of the Al₂Z₃-CaO-MgO system at 1700℃ were examined by x-ray diffraction analysis. It has been found that MgO·Al₂O₃ spinel dissolves into the CaO·6Al₂O₃ magnetoplumbite structure to form a distorted magnetoplumbite structure. The electrical conductivity of this single phase was higher than the other multi-phases. The electrical conductivity of the phase having the composition of CaO:Al₂O₃:MgO = 1:6.6:0.4 in mole ratio was about 1×10^(-2) ohm^(-1)㎝^(-1) at 1400℃ which is high enough to use as a good solid electrolyte for high temperature sensor applications.

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A new state equation of gas derived from liquid theory has been applied to calculate critical constants of metal gases. The parameters a and b of new equation were obtained by using the values of heat of vaporization and collision diameter. The equilibrium volumes of gas and liquid can be calculated from the equation of state. No adjustable parameters, which exhibit different values for different substances, are involved in the calculation. The calculated critical constants of alkali metal gases are then compared with the observed values. The calculated constants of various metal gases which do not have experimental values were compared with the results of Grosse showing a good agreement with each other.

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Magnetic and magnetoresistance properties as functions of Co composition and annealing temperatures were investigated in the Cu_(1-x)Co_x (x=0.2∼0.4) alloy films deposited by thermal co-evaporation. The MR ratio of 12.5% at room temperature which was the largest among the reported values was obtained in the 25 at.%Co-Cu granular alloy film aged at 500℃ for 140 minutes. At higher measuring temperature and limited external field, the maximum MR ratio occurs for larger Co size. In the low Co concentration($lt;25at.%) alloy films, they behaved mainly as paramagnetic in the as-deposited state at room temperature and above that composition, it showed the ferromagnetic character. The enhanced MR ratio up to annealing temperature of 500℃ was due mainly to the reduction of the Cu matrix resistivity, which resulted from the Co precipitation and Cu matrix grain growth. The MR ratio and the magnetization decreased by raising the annealing temperature from 500℃ to higher temperature. These were thought to be due to re-solution of precipitated Co back into the Cu matrix at higher temperature. The Co content of the diffused interface layer decreased and a result of that, the interface spin-dependent scattering power decreased.

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High-temperature thermomechanical fatigue (TMF) failure of a 9 vol% tungsten-fiber-reinforced copper matrix composite was characterized. It was found that deformation of the copper matrix controls failure of the composite under both in-phase and out-of-phase TMF conditions. Cracks were initiated inside the copper matrix and propagated through cavity nucleation and coalescence processes. Final failure of the composite ensued from the failure of fibers. Matrix damage of the out-of-phase TMF tested specimen appeared more severe than that of the in-phase TMF One-dimensional elastic-inelastic analysis showed that the matrix of the out-of-phase TMF specimen experienced more inelastic tensile and compressive deformation, while under the in-phase condition, the matrix experienced all tensile deformation. The analysis explained the Large matrix damage under the out-of-phase TMF condition and the shorter fatigue life of the in-phase tested specimen.

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Correlations of the critical fracture strain ε_f and the fracture toughness J_(lc) with the microroughness measured on the fracture surfaces have been investigated for HP9-4-20 steel and ASTM A710 steel with different strength level. The critical fracture strain ε_f was proportional to the local microroughness (1/3)·ln(M²/3f_p) measured on the fracture surfaces. Fracture toughness J_(lc) was in good agreement with (σ_o/3)·ln(M²/3f_p)·l_o and could be estimated using the microroughness parameter M, the characteristic distance l_o, and the effective yield stress σ_o.

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The effects of post-homogenization cooling methods, viz. air cooling and furnace cooling, on the tensile properties of a strip cast 8xxx series Al alloy have been investigated in this study. Furnace cooling, compared to air cooling, resulted in faster softening behavior during annealing at the temperature range below 300℃, although little difference was observed above 300℃. This difference in softening behavior at lower temperature range was attributed to the difference in recrystallization temperatures in the two specimens. Furnace cooled specimen contained more dynamically recovered structure at cold rolled state than the air cooled specimen. Possible reasons for this difference in cold worked structures and recrystallization temperatures depending on the cooling methods were discussed in terms of solute and precipitate distribution.

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The torsion tests in the range of 790∼1100℃, 3.7×10^(-1)∼5.0×10^0/sec were performed to study the high temperature continuous deformation behavior of Nb-microalloyed structural steels and carbon steel. The flow curves exhibited the characteristic of dynamic recrystallization(DRX). The dependence of the flow stress on temperature(T) and stain rate(ε·) was expressed by power law, ε˙ = A(σ^n′)_pexp(-Q/RT). The dynamically recrystallized grain related to the value of the Zener-Hollomon parameter Z. Under the Z value of 10^(13) order it was found that the grain size ranged from 20㎛ in Nb microalloyed steel to 30㎛ in carbon steel.

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TiC_xN_(1-x) coatings were deposited on high speed steel and Si substrates by PECVD with CH₄/N₂ inlet fraction of 0.3, 0.4, 0.5 and 0.6. The chemical composition, surface and fracture cross-section morphology, preferred orientation and microhardness of the coatings were analyzed. It has been found that the compostiion of TiC_xN(1-x) coatings was changed from TiC_(0.54)N_(0.46) to TiC_(0.60)N_(0.40) with an increas of CH₄/N₂ inlet fraction. When CH₄/N₂ inlet fraction was small, TiC_xN_(1-x) coating showed the columnar grains with a domed top. The faceted columnar grains, however, began to develop with the increase of CH₄/N₂ inlet fraction. The grain size increased as well. At CH₄/N₂=0.3, the coatings had (200) preferred orientation, while no preferred orientation was observed at CH₄/N₂=0.6. It is suggested that these results are caused by enhanced adatom mobility and surface diffusion with CH₄/N₂ inlet fraction.

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Microstructures and mechanical properties of Mo and Co alloy coating layers made by plasma spray coating method have been examined by measn of optical microscopy, X-ray diffraction, scanning electron microscopy, microvickers hardness tests, surface roughness tests and wear resistance tests. The splat is relatively flat with the boundary in Mo layers, whereas wavy with interfaces at some angle to the surfaces in Co alloy layers. The Mo coating layers were shown to be crystalline state but the Co alloy layers to be amourphous state in X-ray diffraction profiles. The surface roughness, hardness, wear resistance of the Co alloy layers were superior to those of the Mo layers. The wear mechanism in plasma coating layers was splat delamination in which the wavy splat is much harder to be delaminated than the falt splat.

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