• 한국분말재료학회지
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• 제6권1호
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• pp.75-80
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• 1999

The formation of $n-TiO_2$ powder by oxidation of Ti-ethoxied vapor in a flat flame burner reactor maintained under 20 torr has been studied. The produced powder were characterized in terms of crystal structure, chemical composition by XRD and TEM. The results showed that the powder consisted of loose agglomerated anatase and rutile particles and their size were about 10 nm and 20 nm, respectively. In the course of synthesis, changes of the flame color were happened to each condition during heating up the bubbler. The flame color transition phenomena reveled that a critical precursor delivery rate was needed for the powder formation (obtainable powder yield). The critical precursor delivery rate was estimated by a simple function of the bubbler temperature and the carrier gas flow rate. The critical precursor delivery rate was reviewed as an important variable of the nanopowder synthesis.

• 한국산업융합학회 논문집
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• 제5권3호
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• pp.165-171
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• 2002

The thermosyphon has been used as a heat transmission device in the heat recovery of low level energy and cooling for heat generating equipments. Many studies on the working fluids and wicks have been reported to improve the heat transfer efficiency of the thermosyphon. A low temperature heat pipe with acetone is chosen in the present study to compare the heat transfer characteristics due to pouring amount of working fluid, magnitude of power supplied and tilt angles. The thermosyphon made ⵁ$15.88{\times}0.8t{\times}600mm$ of copper, evaporation section 200mm, insulation section 25mm, condensation 375mm. Heat transfer rate of the thermosyphon increase as magnitude of power supplied increase and observe dry out phenomenon at 5~10% of pouring amount of working fluid. So thermosyphon at the 150kJ/s judged to need 12% or more. Heat transfer rate of the thermosyphon have nothing to do with tilt angles. Dry out phenomenon of the thermo syphon makes it possible that a low temperature thermosyphon may be used to control temperature and heat transfer of a system when the critical quantity of a working fluid is supplied in the thermosyphon.

• 한국세라믹학회지
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• 제48권6호
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• pp.641-647
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• 2011

The present study aims to identify the effect of sintering atmosphere [$O_2$, 75$N_2$-25 $H_2$ (mol%) and $H_2$] on microstructural evolution at the relatively low sintering temperature of 1040$^{\circ}C$. Samples sintered in $O_2$ showed a bimodal microstructure consisting of fine matrix grains and large abnormal grains. Sintering in 75 $N_2$ - 25 $H_2$ (mol %) and $H_2$ caused the extent of abnormal grain growth to increase. These changes in grain growth behaviour are explained by the effect of the change in step free energy with sintering atmosphere on the critical driving force necessary for rapid grain growth. The results show the possibility of fabricating $(K_{0.5}Na_{0.5})NbO_3$ at low temperature with various microstructures via proper control of sintering atmosphere.

• Journal of Magnetics
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• 제4권1호
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• pp.26-32
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• 1999

The HDDR process can be used as an effective means of processing of the coercive Nd-Fe-B-type or the Sm2Fe17Nx materials. The HDDR (hydrogenation, disproportionation, desorption, recombination) processed materials are feartured with a very fine microstructure. The thermomagnetic characteristics of the Nd-Fe-B-type or the Sm2Fe17Nx materials with fine microstructure due to the HDDR process were investigated. It has been found that the fine-microstructured hard magnetic materials showed an unusual TMA (Thermomagnetic analysis) tracting featured with a low and constant magnetization at lower temperature range and a peak just below their Curie temperatures when a low external field is applied. This thermomagnetic characteristic was immediate particularly in the TMA with a low applied field. This thermomagnetic characteristic was interpreted in terms of the competition between two counteracting effects; the decrease in magnetication due to the thermal agitation at an elevated temperature and the increase in magnetization resulting from the rotation of magnetization of the fine grains comparable to a critical single domain size due to the decreased magnetocrystalline anisotropy at an elevated temperature.

• ETRI Journal
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• 제27권3호
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• pp.312-318
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• 2005

Mid-span spectral inversion (MSSI) has to utilize high optical pump power, for its operation principle is based on a nonlinear optical wavelength conversion. In this paper, a low pump-power operation of MSSI-based chromatic dispersion compensation (CDC) has been achieved successfully, for the first time to our knowledge, by introducing a noise pre-reduction scheme in cascaded wavelength conversions with periodically poled $LiNbO_3$ waveguides at a relatively low operation temperature. As preliminary studies, phase-matching properties and operation-temperature dependence of the wavelength converter (WC) were characterized. The WC pumped at 1549 nm exhibited a wide conversion bandwidth of 59 nm covering the entire C-band and a conversion efficiency of -23.6 dB at 11 dBm pump power. CDC experiments were implemented with 2.5 and 10 Gb/s transmission systems over 100 km single-mode fiber. Although it is well-known that the signal distortion due to chromatic dispersion is not critical at a 2.5 Gb/s transmission, the clear recovery of eye patterns was identified. At 10 Gb/s transmission experiments, eye patterns were retrieved distinctly from seriously distorted ones, and notable improvements in bit-error rates were acquired at a low pump power of 14 dBm.

• 한국전기전자재료학회논문지
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• 제22권7호
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• pp.606-611
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• 2009

AC loss is one of the important factors for commercialization of a high temperature superconducting (HTS) cable from an economic point of view. But AC loss characteristics of the HTS-cable are not elucidated completely because of its complex structure. As an earlier stage of analyzing the AC loss in the 22.9 kV/50 MVA, 100m HTS-cable system of Korea Electric Power Corporation (KEPCO) which is now in collaboration with us, a two-dimensional (2D) numerical model, which takes into account the nonlinear conductivity properties of a high temperature superconductor, has been developed. In order to examine our 2D model, we have prepared several single-layer cable samples whose AC losses are sufficiently reliable due to their simple structure. The AC losses of the samples were experimentally investigated and then compared with our 2D model. The results show that the numerically calculated AC losses are not in good agreement with the measured ones for the cylindrical cable and deca-cable samples with low critical current density. However, the numerically calculated and measured AC losses are relatively in good agreement for the deca-cable and hex-cable samples with high critical current density, although the difference between these two loss data in the deca-cable sample tends to increase in the low current region.

• 한국표면공학회지
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• 제31권6호
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• pp.345-358
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• 1998

The evolution of surface roughness and morphology in epitaxial Si and $Si_{0.7}Ge{0.3}$ alloys grown by UHV opm-beam sputter deposition onto nominally-singular, [100]-, and [110]-mi-scut Si(001) was investigated by stomic force microscopy and trasmission electron microscopy. The evolution of surface roughness of epitaxial Si films grown at $300^{\circ}C$ is inconsistent with conventional scaling and hyperscaling laws for kineti roughening. Unstable growth leading to the formation of mounds separated by a well-defined length scale is observed on all substrates. Contraty to previous high-temperature growth results, the presence of steps during deposition at $300^{\circ}C$ increases the tendency toward unstable growth resulting in a much earlier development of mound structures and larger surface roughnesses on vicival substrates. Strain-induced surface roughening was found to dominate in $Si_{0.7}Ge{0.3}$ alloys grown on singular Si(001) substrates at $T_S\ge450^{\circ}C$ where the coherent islands are prererentially bounded along <100> directions and eshibt {105} facetting. Increasing the film thickness above critical values for strain relaxation leads to island coalescence and surface smoothening. At very low growth temperatures ($T_s\le 250^{\circ}C$), film surfaces roughen kinetically, due to limited adatom diffusiviry, but at far lower rates than in the higher-temperature strain-induced regime. There is an intermediate growth temperature range, however, over which alloy film surfaces remain extremely smooth even at thicknesses near critical values for strain relaxation.

• 한국초전도ㆍ저온공학회논문지
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• 제16권4호
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• pp.7-16
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• 2014

Rare-earth barium copper oxide (REBCO) coated conductor (CC) tapes have already been commercialized but still possess some issues in terms of manufacturing cost, anisotropic in-field performance, $I_c$ response to mechanical loads such as delamination, homogeneity of current transport property, and production length. Development on improving its performance properties to meet the needs in practical device applications is underway and simplification of the tape's architecture and manufacturing process are also being considered to enhance the performance-cost ratio. As compared to low temperature superconductors (LTS), high temperature superconductor (HTS) REBCO CC tapes provide a much wider range of operating temperature and a higher critical current density at 4.2 K making it more attractive in magnet and coil applications. The superior properties of the REBCO CC tapes under magnetic field have led to the development of superconducting magnets capable of producing field way above 23.5 T. In order to achieve its optimum performance, the electromechanical properties under different deformation modes and magnetic field should be evaluated for practical device design. This paper gives an overview of the effects of mechanical stress/strain on $I_c$ in HTS CC tapes due to uniaxial tension, bending deformation, transverse load, and including the electrical performance of a CC tape joint which were performed by our group at ANU in the last decade.

• Progress in Superconductivity
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• 제6권1호
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• pp.13-18
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• 2004

High-.ate in-situ$ YBa_2$Cu$Cu_3$$O_{7-x}$ (YBCO) film growth was demonstrated by means of the electron beam co-evaporation. Even though our oxygen pressure is low, ∼$5 ${\times}$10^{-5}$ Torr, we can synthesize as-grown superconducting YBCO films at a deposition rate of around 10 nm/s. Relatively high temperatures of around 90$0^{\circ}C$ was necessary in this process so far, and it suggests that this temperature at a given oxygen activity allows a Ba-Cu-O liquid formation along with an YBCO epitaxy. Local critical current density shows a clear correlation with local resistivity. Homogeneous transport properties with a large critical current density ($4 ∼ 5 MA/\textrm{cm}^2$ at 77K, 0T) are observed in top faulted region while it is found that the bottom part carries little supercurrent with a large local resistivity. Therefore, it is possible that thickness dependence of critical current density is closely related with a topological variation of good superconducting paths and/or grains in the film bodies. The information derived from it may be useful in the characterization and optimization of superconducting films for electrical power and other applications.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2004년도 제22회 춘계학술대회논문집
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• pp.774-778
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• 2004

The atomization process of a circular $SF_{6}$ liquid jet issued into an otherwise quiescent, high-pressure $N_2$ gas was observed to explore the breakup mechanism of liquid ligaments involved in turbulent atomization. Both liquid and gas temperatures were fixed at a room temperature but the gas pressure was elevated to more than twice the critical pressure of $SF_{6}$. Therefore, the liquid surface was in a thermodynamic state close to a critical mixing condition with suppressed vaporization. Since the surface tension and the surface gas density approach zero and the surface liquid density, respectively, phenomena equivalent to those which would appear when a very high speed laminar flow of water were injected into the atmospheric-pressure air can be observed by issuing $SF_{6}$ liquid at low speeds in micro-gravity environment which avoid disturbances due to gravity forces. The instability ob near-critical mixing surface jet was quantitatively characterized using a newly developed device, which could issue a very small amount of $SF_{6}$ liquid at small constant velocity into a very high-pressure $N_2$ gas.