발간논문

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Solar cell was made from 1 ohm·cm polycrystalline silicon which had been directionally solidified using split, reusable graphite molds coated with Si₃N₄. Silicon n/p homojunction solar cell was fabricated through the following steps; wafer slicing, texturizing etch, P diffusion, anti-reflection coating application, grid formation and contact annealing. The cell area was 9㎠. Under the airmass 1.5 illumination, a short circuit current of 0.232A and an open circuit voltage of 0.582V were obtained with a curve fill factor of 0.666. The maximum energy conversion efficiency was 10.0% under standard terrestrial test conditions (AM 1.5, 100mW/㎠).

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The mullite(3Al₂O₃·2SiO₂) thin films were fabricated by a dipping substrates into coating solution, pulling them up, drying and then heating process. A sol-gel method, which comprises precipitation of aluminum monohydroxide, was used to produce coating solutions. Thickness of the films increased with increasing the viscosity of solution and the number of coating cycle. The adhesion strength of mullite films coated on slide gl-ass was measured with the acoustic emission scratch tester. The critical load of mullite films was measured to be about 14N. Surface hardness could be enhanced up to 2∼3 times that of uncoated substrate by increasing the thickness of mullite films. According to the corrosion test, it turned out that thicker the coating layer became, less was the total corrosion rate. This was attributed to the decrease in porosity.

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The present study has been carried out to investigate the simultaneous behavior of fine ore melting and reduction in a combustion flame during the injection of the mixture of fine iron ore and pulverized coal char with oxygen. Coal char and iron ore(wustite, hematite) particles less than 150 ㎛ in diameter were used for the experiment. The atmospheric temperature of the chamber was set up at 1400℃. The injected weight ratio of Fein ore to in coal char changed from 0.33 to 2.66 and the mole ratio of O/C changed from 0.4 to 1.2. Experimental resuts show that main reduction mechanism of fine wustites is the direct reduction between molten wusitite and unburnt carbon. Similarly for the case of fine hematites, thermal decomposite to magnetite and direct reduction between molten magnetite and solid carbon entrained into the droplet are the main reduction mechanism because melting rate of thermally decomposed hematite is much faster than reduction rate of that by reducing gas. It was observed that reduction degree of fine iron oxide(wustite & hematite) mainly depends on specific combustion heat ratio(㎉/g-ore), the author proposes, which is defined as the ratio of combustion heat of coalchar to the weight of injected fine ore.

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The effects of cusp magnetic field on the fluid flow and mass transfer were simulated by finite difference method in large-scale Czochralski single crystal growth of silicon. The flow intensities were greatly diminished and the distribution of azimutal velocities was similar to the rigid body motions when the cusp magnetic field was applied. The average concentrations and the radial uniformities of oxygen for various crucible and crystal rotation rates at solidification interface were calculated in a cusp magnetic field. The distributions of oxygen in Czochralski grown silicon were critically under the control of melt motions. The average oxygen concentrations are lower and the radial uniformity of the oxygen concentrations at solidification interface became better as the cusp magnetic field was increased. On the basis of the above results, the optimum starting conditions for the operation were numerically calculated.

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Transmission electron microscopy study on the formation mechanism of the as-deposited mixed-phase silicon thin films, deposited on SiO₂ at temperature range 570℃-580℃ by low pressure chemical vapor deposition and annealed at 570℃, has been carried out. Though most part of as-deposited films had an amorphous phase, some crystallites which had a random shape were observed at Si/SiO₂ interfaces. The higher the deposition temperature was, the larger the size of the crystallites was. No remarkable growth of preexisting crystallites was observed in the film deposited at 570℃ for 28 min. and annealed at 570℃ for 1 h. Crystallites which had an elongated elliptical shape were observed in the film annealed at 570℃ for 3 h. The size of the elliptical shaped grains was larger than that of the crystallites grown from preexisting crystallites. From above results, it is concluded that as-deposited mixed-phase silicon films are formed not because films, deposited as an amorphous phase, are crystallized during the deposition process but because films are deposited as a crystalline phase at the initial stage of the deposition process and then deposited as an amorphous phase.

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Composite coatings composed of ceramic powder and adhesive were studied to develop a method for repair or maintenance of facilities under particle erosion conditions. Wear resistant ceramic powders such as Al₂O₄, SiC, and WC were mixed with epoxy or urethane base adhesive. The resulting composite materials were coated on SS41 steel plate and tested to assess a particle erosion properties. Test results indicate that both the shape and size of the ceramic filler influence the erosion rate of the composite coatings. Epoxy based coatings are snowing better erosion resistance than that of the urethane based coatings. Erosion resistance of the epoxy/ceramic composite coatings is superior to that of the commercial metal paste which is used for anti-wear repair. Finally, it is careful to rank the erosion resistance of the adhesive based composite coatings.

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In this study, tensile tests were carried out at room temperature to evaluate the influence of δ precipitates on serration behavior of 2090 Al-Li alloy by using reversion heat treatment for the first time. The heat treatment conditions include solution heat treatment, aging and reversion heat treatment. Three different initial strain rates. 2×10^(-3)/sec, 2×10^(-4)/sec and 5×10^(-5)/sec, were used as experimental variables. For all heat treatment conditions, the serration amplitude increased, but the critical strain for the onset of serration decreased with decreasing strain rate. The aged specimen showed the highest serration amplitude and the lowest critical strain for the onset of serration at a given strain rate. The reversion heat treatment after aging decreased the serration amplitude and increased the critical strain for the onset of serration. The results indicate that serration behavior of 2090 Al-Li alloy is affected by work softening due to shearing of δ precipitates as well as by Portevin-Le Chatelier effect caused by the solute atoms.

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MoSi₂ alloys were prepared by mechanical alloying of Mo and Si powders. The microstructures of powders were characterized by XRD and SEM after different milling times. The formation procedures of MoSi₂ and phase transformations with temperature were investigated through the DSC and the RTA experiments. The powders milled for 6 hours showed the structure of fine mixture of Mo and Si. The tetragonal MoSi₂ phase and the hexagonal MoSi₂ phase appeared gradually in the powders milled for longer time. After 24 hours of milling, the average grain sizes of Mo and tetragonal MoSi₂ showed the nanocrystaliine structure. The DSC results showed exothermic peak at about 650℃ for the powders which were milled for 24 hours. The results showed that the unreacted fine mixture of Mo and Si transformed to the MoSi₂ phase by fast interfacial reaction which were in good agreement with the RTA results. The activation energy of this reaction was obtained from the Kissinger analysis and found to be 2.3 eV.

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In order to investigate the effect of Ce on the thermal stability of Al Ti alloy, Al- 8wt.%(Ti+Ce) powders with different Ti to Ce atomic ratio were mechanically alloyed in an attritor. The steady state was obtained after 16 hours milling for all the alloys. The mechanically alloyed powders were vacuum hot pressed and isothermally aged for 100 hours at 400 550℃. The coarsening of A1- Ti intermetallic compounds was retarded by addition of Ce and the thermal stability was improved. The lattice parameter variations of Al-Ti-Ce intermetallic compounds were measured, and the addition of Ce increased the overall lattice mismatch between the intermetallic compound and Al matrix, and so the interfacial energy. Therefore, when considered the solubility and diffusivity of Ce in Al, it was considered that the low solid solubility of Ce in Al matrix was the major factor to the increased thermal stability of Al-Ti alloy.

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A thermodynamic description of the Fe-Cr-Ni-Mo-Mn-Si-C-N system (basic stainless steel system) has been obtained by the use of thermodynamic models for the Gibbs energy of individual phases. Thermodynamic calculations enabled evaluation and prediction of phase equilibria and phase stabilities in alloy steels. The α/γ equilibria in duplex stainless steels, solubilities of M_(23)C_6 in austenitic steels stainless and liquidus and solidus temperatures in various types of steels were calculated and compared with experimentally reported data. It was concluded that the present thermodynamic description can be used in predicting phase equilibria in stainless steels systems where no experimental data is available and can give fundamental information for allay design of new steels. Some technical approach methods were presented in order to relate thermodynamic calculations with phase transformations which are governed by kinetics as well as thermodynamics.

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