• Journal of Magnetics
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• v.11 no.1
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• pp.16-19
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• 2006

$2-\mu{m}$ thick GaN epilayer was prepared, and 80 KeV $Co^{-}$ ions with a dose of $3X10^{16}\;cm^{-2}$ were implanted into GaN at $350^{\circ}C$. The implanted samples were post annealed at $700^{\circ}C$. We have investigated the magnetic and structural properties of Co ion-implanted GaN by various measurements. HRXRD results did not show any peaks associated with second phase formation and only the diffraction from the GaN layer and substrate structure were observed. SIMS profiles of Co implanted into GaN before and after annealing at $700^{\circ}C$ have shown a projected range of $\sim390\AA$ with 7.4% concentration and that there is little movement in Co. AFM measurement shows the form of surface craters for $700^{\circ}C$-annealed samples. The magnetization curve and temperature dependence of magnetization taken in zero-field-cooling (ZFC) and field-cooling (FC) conditions showed the features of superparamagnetic system in film. XPS measurement showed the metallic Co 2p core levels spectra for $700^{\circ}C$-annealed samples. From this, it could be explained that magnetic property of our films originated from Co magnetic clusters.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.11
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• pp.2196-2202
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• 1992

The optimum size combination of heat exchangers in a Joule-Thomson(J-T) circuit for small cryogenic systems has been sought analytically, when the circuit is combined with a two-stage Gifford-McMahon(GM) cooler. Full thermodynamic cycle analysis was carried out to predict the performance of the combined refrigeration system. Relevant convective heat transfer coefficients, the computerized properties of helium, and the refrigeration capacity curve of a typical GM cooler have been used in the analysis. The result showed that, by changing the configuration(heat exchanger area ratio) of the system, the performance of the commonly-used GM/J-T refrigerators could be optimized. For the maximum refrigeration performance, the optimum mass flow rate of the refrigerant and the relative size between the heat exchangers have been obtained, when the cooling load was 0.1W at 3.995K with the total heat exchanger area being given.

• Journal of Energy Engineering
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• v.2 no.1
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• pp.95-103
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• 1993

A conceptual thermoelectric heat pump(cooling mode) of small capacity is designed. Its performance is investigated through parametric analysis. COP and cooling capacity decease as the ambient temperature increases with ${\mu}$, J, T$\sub$wi/, fixed. To design a system of fixed capacity comes to calculate ${\mu}$ and J when T$\sub$wi/, and T$\sub$a/ are given. As v is fixed by semi-conductor manufacturers, optimum combination of n and I should be searched for ν. Optimum current could be calculated using ${\mu}$-J curve and optimum value of ${\mu}$. COR$\sub$R/ increases as water flow rate increases and T$\sub$a/ decreases. The effect of heat transfer coefficient at hot(heat releasing) side is more significant than that at cold(heat absorbing) side.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.21 no.2
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• pp.267-275
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• 2012

Austenite stainless steel 304 has properties of high resistance to corrosion and temperature changes. Therefore, this material is widely used in various of industries. However, when the material is subjected to heating and cooling cycles the forming accuracy, for example, the right angle associated with a sharp bend such as corner is lost. This phenomenon is caused by the reversion of the deformation-induced martensite into austenite when the temperature in increased. This result in misfit of a structure or an assembly, and an increase in residual stress. Hence, it is important to understand this process. In this study, to evaluate the mechanical behavior of the deformation-induced martensite and reversed austenite, a scanning acoustic spectroscope including the capability of obtaining both phase and amplitude of the ultrasonic wave (i.e., the complex V(z) curve method) was used. Then, the velocities of the SAW propagating within the specimens made in different conditions were measured. The experimental differences of the SAW velocities obtained in this experiment were ranging from 2,750 m/s to 2,850 m/s, and the theoretical difference was 3.6% under the assumption that the SAW velocity was 2,800 m/s. The error became smaller as the martensite content was increased. Therefore, the SAW velocity may be a probe to estimate the marternsite content.

• Journal of the Korean Vacuum Society
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• v.1 no.1
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• pp.176-182
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• 1992

t-Stress and junction leakage current characteristics of CVD-tungsten have been investigated. Stressversus continuous annealing temperature plot. shows hysteresis curve where the stress level of the cooling curveis higher than that of the heating curve. It is found that the thermal and intrinsic stress of tungsten film depositedby SiH4 reduction is higher than that by Hz reduction.The tungsten film deposited by SiHl reduction is in the tensile stress state below 700"Cnd the stress ofthe film decreses with increasing annealing temperature. The stress state changes into compressive stress atabout 700"Cnd the compressive stress increases rapidly with increasing temperature.Leakage current of the n+/p diode increases rapidly especially in the range of 400-450$^{\circ}$C with increasingdeposition temperature of the CVD-W by SiH4 reduction, which is due to the Si consumption by W encroachment.On the other hand leakage current of the n+/p diode slightly increases with increasing SiH4/WF6 ratio.h increasing SiH4/WF6 ratio.

• 유체기계공업학회:학술대회논문집
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• 2004.12a
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• pp.83-88
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• 2004

In this study, the flow and thermal performance of the heat sink and fan-sink were experimentally studied to predict the operating condition of the fan-sink. The experiments of the flow and thermal resistance of the heat sink with various inlet blockage, which were occurred by the shapes of the axial fans, were conducted for the proof of the effects of the inlet blockages. The greater the inlet blockage of the heat sink, the higher the pressure drop and lower the thermal resistance of the heat sink will be. The operating point of the fan-sink was predicted by the pressure drop curve with the inlet blockage, which was corresponded to the selected fan and the fan performance curve, and verified by the performance test of the fan-sink. The predicted operating point of the fan-sink had good agreement with the result of the performance test of the fan-sink within $0.7\%$ of the volume flow rates. Measured thermal resistance of the fan-sink was equivalent to that of the heat sink with the same inlet blockage of the fan-sink. It was shown that the heat transfer characteristics of the heat sink were influenced by the flow interaction between the selected fan and the heat sink. To improve the thermal resistance of the heat sink, it is necessary to consider appropriate flow patterns of the fan outlet entering into the heat sink.

• Journal of the Korean Ceramic Society
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• v.35 no.6
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• pp.640-646
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• 1998

Nucleation and crystallzation of a quaternary glass system for lead titanate glass-ceramics were in-vestigated using DTA(differential thermal analysis ) with variation of nucleation temperature and crystal growth time. Glass samples containing 60mol% of PbO-{{{{ { TiO}_{2 } }} were prepared from melts by the conventional normal cooling method in a cylindrical brass mould. The glass sample was nucleated between 40$0^{\circ}C$ and 50$0^{\circ}C$ for a given time and showed the maximum nucleation rate at 46$0^{\circ}C$ The DTA crystallization peak temperature decreased with increasing nucleating time and decreasing heating rate during DTA runs which indicated an increase of the number of nuclei produced in the system.

• The Journal of the Acoustical Society of Korea
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• v.36 no.6
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• pp.371-377
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• 2017

The performance of an air conditioner is closely related to the cooling performance of a split-type outdoor unit so that, in most of the relevant preceding studies, the independent performance of an axial fan in an outdoor unit has been studied. However, there is a lack of research on the effects of other components in an outdoor units was rarely investigated. Therefore, in this paper, the effects of the fan orifice among other parts on the flow performance of the outdoor unit was numerically investigated. A virtual fan tester consisting of 18 million grids was developed for highly resolved flow simulation. The unsteady RANS (Reynolds-averaged Navier-Stokes) equations are numerically solved by using finite-volume CFD (Computational Fluid Dynamics) techniques. In order to verify the validity of the numerical methods, the predicted P-Q curve of the cooling fan in a full outdoor unit is compared with the measured one. Optimization of orifice shape was carried out for maximum flow performance of the outdoor unit using the validated numerical method.

• The Journal of the Acoustical Society of Korea
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• v.37 no.6
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• pp.396-403
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• 2018

In this paper, a high-performance and low-noise centrifugal fan is developed for cooling automotive seats which provide a driver with pleasant driving environment. First, the flow characteristics of the existing fan unit was analyzed using a fan performance tester and CFD (Computational Fluid Dynamics) simulations. The analysis of the predicted flow field indicated vortex flow near the tip of fan hub and stagnation flow on the top of fan hub. Two design points are devised to reduce the vortex flow and the stagnation flow observed in the existing fan unit. First, the cut-off clearance which is the minimum distance between the fan blade and the fan housing is increased to reduce the vortex strength and, as a result, to reduce the overall sound pressure level. Second, the hub shape is more modified to eliminate the stagnation flow. The validity of proposed design is confirmed through the numerical analysis. Finally, a prototype is manufactured with a basis on the numerical analysis result and its improved flow and noise performances are confirmed through the P-Q curves measured by using the Fan Tester and the SPL (Sound Pressure Level) levels measured in the anechoic chamber.

• Journal of Magnetics
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• v.11 no.1
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• pp.36-42
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• 2006

In this report, we demonstrate a comprehensive analysis of the effects of Au addition on the microstructure and magnetic properties of $Fe_{73.5}Si_{13.5}B_{9}Nb_{3}Au_1$ Finemet-type alloy. It was found that the as-quenched alloys were the amorphous state and turned into nanocrystalline state under heat treatments. The DSC analysis indicates that the sharply exothermal peak corresponding to the crystallization of the $\alpha-Fe(Si)$ was observed at $547-579^{\circ}C$ depending on the heating rates, which is little higher than that of original Finemet (542-$570{^{\circ}C}$, respectively). Besides, the thermomagnetic result confirmed that the full substitution of Cu by Au with the single phase structure in the M(T) curve along cooling cycle. Ultrasoft magnetic properties of the nanocrystallized samples were significantly enhanced by the proper annealing such as the increase of permeability and the decrease of the coercivity. The optimum annealing condition was found at the annealing temperature of $540^{\circ}C$ and the increase of the annealing time up to 90 min.