발간논문

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Ultrafine particles of Fe₃O₄ were prepared by means of AOT/water/cyclohexane microemulsion system. Reactants added to the microemulsion system were FeCl₃ FeCl₂ and NH₄OH solutions whose amounts of addition were controlled properly in terms of burets. From the results of this study, the available condition for the preparation of Fe₃O₄ could be described as follows : In the preparing the microemulsion system, the volume ratio of cyclohexane to water was 3.0 and the mole ratio of water to AOT was 7.0, respectively. The concentration of FeCl₃ and FeCl₂ solutions was 1.4M and that of NH₄OH was 15M. The temperature of the microemulsion system was maintained at 30℃, which was stirred continuously with the rate of 1000∼1200rpm. The available amounts of the reactants added to the microemulsion system were obtained and the combinations of the reactants suitable for the preparation of Fe₃O₄, were determined at the triangular graph within experimental conditions of this study.

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Specimens of carbon steel coated with chlorinated rubber primer have been immersed in 5% and 0.05% NaCl solutions and in a 0.35% (NH₄)₂SO₄+0.05% NaCl solution. Their electrochemical impedances at their corrosion potentials have been measured as a function of immersion time. Measured impedance data have been analyzed according to an equivalent circuit representing impedances across the organic film and the interface between the film and substrate. The film and substrate have been deteriorated more at the higher salt concentration. The corrosion at the interface has been controlled by the anodic dissolution of steel substrate and accelerated with time.

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In this study, A. C. Impedance spectra of various surface treated sheets-EG(electrogalvanized sheet), Cr(chromated sheet) and AF(anti-finger printed sheet) were measured at cathodic polarization potential in the solution of 0.2% NaCl. In order to evaluate corrosion resistance, Rp values measured through A. C. impedance spectra at the different cathodic polarization (O.C.P., oxygen reduction and hydrogen evolution potential), were compared to the result of salt spray test. Rp values measured at the dissolved oxygen reduction polarization was reliable and consistant with the results of SST. As impedance spectra was analyzed through several equivalent circuit models with transfer resistance and diffusion impedance, corrosion resistance could be predicted in a short time.

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The vacuum refining of nickel base Rene´ 80 was studied by using a 2㎏ laboratory vacuum induction melting apparatus. It was possible to reduce the sulfur content of superalloy Rene´ 80 to below 2 ppm by strong induction stirring of melt and a use of calcia lined crucible. At initial carbon contents below 0.04%, the degree of decarburization after first refining and second refining were 77% and 57%, respectively. At initial carbon contents above 0.04%, the degree of decarburization decreased with increasing initial carbon content.

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Grain growth was studied on the straight rolled molybdenum sheet doped with CaO and MgO and compared with that of pure molybdenum sheet. Texture change was also investigated during annealing. Optical, scanning and transmission electron micrographs were observed to link the grain growth with the change of recrystallization texture. The rolling texture consisted of one strong component, {100}$lt;011$gt; and two weak components, {211}$lt;011$gt; plus {111}$lt;011$gt; in doped molybdenum and one strong component, {100}$lt;011$gt; and one weak component, {211}$lt;011$gt; in pure molybdenum. When annealing was done up to 1978℃, the recrystallization texture of pure molybdenum sustained a rolling texture, {100}$lt;011$gt;. However, that of doped molybdenum only retained a rolling texture, {100}$lt;011$gt; at a temperature below 1950℃ and changed into near (211)[011] single crystal over this temperature. Upon annealing at this critical temperature, 1950℃, a new large grain was nucleated rapidly from a rolling plane by Nielsen-type coalescence mechanism and subsequently secondary recrystallization occurred at the expense of small matrix grains. Its orientation was far from approximately 30˚about $lt;100$gt;.

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Due to the great hardness, hard nitrides have been extensively used in cutting tools and wear-resistant parts. The nitride also shows excellent high-temperature strength and good corrosion resistance. The purpose of this research is to develop the technology for the synthesis of titanium nitride by the reaction of titanium metal and nitrogen. The titanium nitride(TiN_x) can be manufactured by the several methods. In the present work, bulk TiN_x has been synthesized by the reaction of titanium and nitrogen followed by homogenizing annealing. The homogeneous TiN_(x≥1.0) can be obtained at 1000℃ with the lattice parameter and specific gravity of 0.424㎚ and 5.39g/㎤, respectively similar to that of pure grade TiN.

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Solidification behaviors of silicon thin film in zone melting recrystallization with halogen lamps have been studied in terms of the substrate preheating temperature. Computer simulations were carried out to analyze the temperature profiles and interface instabilities in liquid and solid phases near the solidifying interface. It is expected from the computer simulations that when the preheating temperature is relatively low, the non-planar interface should be developed at the solidifying front and defect density should be low. On the other hand, when the preheating temperature is relatively high, the planar interface and high defect density are expected. At low preheating temperature, below 1000℃, regular defects corresponding to uniform cells were observed. Above 1000℃ Y-shaped irregular defects corresponding to planar interface were observed.

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Sputter deposited cobalt-based alloy thin films have structural heterogeneities consisted of two phase structures those are composed of cobalt-rich particles and oxide-rich boundaries. Size of the particles increases with increasing incident angle of sputtered atoms. The films sputter deposited with negative substrate bias show so-called `high temperature` structures with large and relaxed particle shapes, mainly due to the less effect of oxygen. Size of the particles decreases with increasing substrate temperature below 300℃ during sputter deposition. Pure Co thin films show decreasing particle size by polymorphic transformation from hcp and fcc mixed structure to fcc structure. CoB alloy thin films deposited without substrate bias show also decreasing particle size by increasing number of nucleation sites based on selective oxidation of boron. On the other hand, CoB alloy films deposited with negative substrate bias do not show any changes of particle size keeping columnar structures. However, at substrate temperature above 300℃, particle growth is dominant resulting in increasing particle size. Post depostion annealing of all cobalt-based thin films used in this experiment causes particle growth, high temperature structures, and two phase structures with structural and compositional heterogeneities.

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Sputter deposited CoB and CoZrNb thin films have structural and compositional heterogeneities consisted of cobalt-rich particles and oxide-rich boundaries. The origin of the heterogeneities are considered as the oxygen incorporation into thin films during sputter deposition as shown in TEM morphological differences between CoB alloy films and pure Co films, between CoZrNb alloy films deposited without and with substrate bias, and between Co-based films before and after postdeposition annealing. The compositional heterogeneities are verified directly and indirectly by several experimental methods. One indirect method is the observation of different contrast of particles and boundaries in TEM micrographs. Measurement of the Curie temperature in CoB films is the other indirect method for observation of compositional heterogeneities. EDS in STEM is adopted as a direct method in CoZrNb thin films deposited with substrate bias and annealed after deposition, because the heterogeneous structure is enlarged enough to use electron beam in STEM on these films. Measurement of magnetostriction of magnetic films is also suggested as one of the methods observing the compositional heterogeneities in CoZrNb thin films.

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We have recrystallized p-Si thin film using periodically-arrayed seed windows and grid patterns. With continuous seeding effects from periodically-arrayed seed windows, nearly single crystalline films with few defects could be recrystallized. The recrystallized film had same crystallographic orientation with single crystalline substrate. Thermal profile at solidification front could be modified periodically by buried p-Si grid patterns in capping oxide. As a result, defect trails having same spacing with grid patterns could be localized. Defects were localized with grid spacing up to 80㎛, but above 80㎛ defects became irregular due to unstable solidifying front. At relatively low incident beam intensity, films having low defect density could be obtained. As the beam intensity increased the defects became irregular.

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