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2009, Vol.19, No.3

  • 1.

    Theoretical gravity studies on roles of convection in crystal growth of Hg2Cl2-Xe by physical vapor transport under normal and high gravity environments

    Geug-Tae Kim | Moo Hyun Kwon | 2009, 19(3) | pp.107~115 | number of Cited : 4
    Abstract PDF
    Particular interest in the role of convection in vapor crystal growth has arisen since some single crystals under high gravity acceleration of 10g0 appear considerably larger than those under normal gravity acceleration (1g0). For both ΔT = 60 K and 90 K, the mass flux increases by a factor of 3 with increasing the gravity acceleration from 1g0 up to 10g0. On the other hand, for ΔT = 30 K, the flux is increased by a factor of 1.36 for the range of 1g0 ≤ g ≤ 10g0. The maximum growth rates for 1g0, 4g0, 10g0 appear approximately in the neighborhood of y = 0.5, and the growth rates shows asymmetrical patterns, which indicate the occurrence of either one single or more than one convective cell. The maximum growth rate for 10g0 is nearly greater than that for 1g0 by a factor of 2.0 at PB = 20 Torr. For three different gravity levels of 1g0, 4g0 and 10g0, the maximum growth rates are greater than the minimum rates by a factor of nearly 3.0, based on PB = 20 Torr. The mass flux increases with increasing the gravity acceleration, for 1g0 ≤gy≤ 10g0, and decreases with increasing the partial pressure of component B, xenon (Xe), PB. The |U|max is directly proportional to the gravity acceleration for 20 Torr ≤ PB≤ 300 Torr. As the partial pressure of PB (Torr) decreases from 300 Torr to 20 Torr, the slopes of the |U|maxs versus the gravity accelerations increase from 0.1 sec to 0.17 sec. The mass flux of Hg2Cl2 is exponentially decayed with increasing the partial pressure of component B, PB (Torr) from 20 Torr up to 300 Torr .
  • 2.

    Effects of total pressure and gravity level on the physical vapor transport of Hg2Cl2-Cl2 system

    Jeong-Gil Choi | Moo Hyun Kwon | Geug-Tae Kim | 2009, 19(3) | pp.116~124 | number of Cited : 3
    Abstract PDF
    Our computational studies for the physical vapor transport crystal growth of Hg2Cl2-Cl2 system evidence suggests that the PVT growth process exhibits the diffusion-dominated behaviors for aspect ratios more than and equal to 10, which would provide purely diffusive transport conditions adequate to microgravity environments less than 10−3g0. Also, the regimes of high temperature difference based on the fixed source temperature of 380 oC, where ΔT is relatively large enough for the crystal growth of mercurous chloride, the transport rates do not keep increasing with ΔT but tend to some constant value of 2.12 mole cm−2s−1. For the aspect ratios of 5, 10, and 20, the transport rate is directly proportional to the total pressure of the system under consideration. For Ar = 5, the rate is increased by a factor of 2.3 with increasing the total pressure from 403 Torr to 935 Torr, i.e., by a factor of 2.3. For both Ar = 10 and 20, the rate is increased by a factor of 1.25 with increasing the total pressure from 403 Torr to 935 Torr.
  • 3.

    Preparation and characterization of niobium carbide crystallites

    Jeong-Gil Choi | 2009, 19(3) | pp.125~129 | number of Cited : 3
    Abstract PDF
    The preparation and characterization of niobium carbide crystallites were investigated in this study, and in particular, the effect of preparation conditions were studied on the synthesis of niobium carbides crystallites. For this purpose, various characterization techniques including x-ray diffraction, BET surface area, and oxygen uptake measurements were employed to characterize the synthesized niobium carbide crystallites. The niobium carbide crystallites were prepared using niobium oxide and methane gas or methane-hydrogen mixture. Using x-ray diffraction a lattice parameter of 4.45 Å and a crystallite size ranging from 52 Å to 580 Å was found. BET surface areas ranged from 3.2 m2/g to 16.6 m2/g and oxygen uptake values varied from 0.5 μmol/g to 6.1 μmol/g. It was observed that niobium carbide crystallites were active for ammonia decomposition reaction. While the BET surface area increased with increasing the oxygen uptake, the conversion of ammonia decomposition reaction decreased. These results indicated that the ammonia decomposition over these materials was considered to be structure-sensitive.
  • 4.

    Luminescence properties of a new Tb3+ ion activated long persistent phosphor

    박병석 | Choi Jong Keon | 2009, 19(3) | pp.130~134 | number of Cited : 0
    Abstract PDF
    A new long persistent phosphors of CaZrO3 was synthesized at high temperature with weak reduction atmosphere by a traditional solid state reaction method. Photoluminescence spectra analysis showed that the CaZrO3 doped with Tb3+ emitted green-yellow emission caused by the energy level transition from the 5D3 and 5D4 to 7F1~7F6. The main emission spectra of 542 nm peak by the 5D4→7F5 transition was revealed through synthesizing at high temperature in N2 gas atmosphere. The afterglow emission spectra of CaZrO3 : Tb3+ long persistent phosphores arise at 546 nm peak of narrow range. After the 254 nm ultraviolet light excitation source was switched off, the green-yellow long persistent phosphor can be observed which could last for 8 h in the limit of light perception of dark-adapted human eyes (0.32 mcd/m2).
  • 5.

    Cr6+ leaching property of cement using high performance lignin

    Hyun Park | Kim Kyung Nam | 2009, 19(3) | pp.135~143 | number of Cited : 0
    Abstract PDF
    To study possibility of Cr 6+ solid solution and efficiency of admixture, leaching property of Portland cement mortar was investigated by using KSLT (Korea Standard Leaching Test), TCLP (Toxicity Characteristic Leaching Procedure), Soacking procedure, as adding excess Cr 6+ . As a result, admixture of lignin type showed high compressive strength of mortar but no changing leaching property.
  • 6.

    Preparation and photocatalytic activity of ACF/TiO2 composites by using titanium n-butoxide and acid modified activated carbon fiber

    Oh Won Chun | Ko Weon Bae | 권호정 and 2other persons | 2009, 19(3) | pp.144~151 | number of Cited : 0
    Abstract PDF
    Photocatalytic degradation of methylene blue (MB) in aqueous solution was investigated using TiO2 coated on various acid modified activated carbon fiber (ACF). The ACFs/TiO2 composites were prepared from titanium n-butoxide (TNB) as titanium precursor and various acid modified ACFs. The prepared samples are heat treated at 973 K. Then the ACF/TiO2 composites were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX). Moreover, photocatalytic degradation of MB by the ACF/TiO2 composites was determined under UV irradiation. The results shows that the photocatalytic activity of ACF/TiO2 composites (AT1~AT4) prepared with TNB and various acid modified ACF was much better than that of ACF/TiO2 composite (AT) prepared with TNB and nonacid modified ACF, and the effects improved with order of sample AT3 > AT4 > AT1 > AT2.
  • 7.

    Comparison of flux and natural sapphire after heat-treatment

    김기인 | 서진교 | Jong-Wan Park and 1other persons | 2009, 19(3) | pp.152~158 | number of Cited : 3
    Abstract PDF
    Various fabrication methods have been used to synthesize sapphire which has qualities of jewelry well beyond the industrial class. Among them, the flux sapphire of Chatham Company which has as high value as jewelry was selected in order to compare natural and synthetic sapphire. First, the WD-XRF (Wavelength dispersive x-ray fluorescence spectrometer) was used to analyze the chemical composition of natural and synthetic sapphire. Although natural sapphire had very diverse chemical compositions, flux sapphire had small quantities of Mo, Pt and Pb elements in addition to the similar chemical ingredients to natural one. Pt is decisive proof of flux sapphire. Next, by investigating spectroscopic characteristics using UV-VIS Spectrophotometer after heat treatment at high temperatures of 1300 o C and 1500 o C, the variation of 690 nm absorbance related to Cr 3+ was detected in the natural sapphire while those of the 690 nm absorbance (related to Cr 3+ ) as well as absorbance of 376 nm and 388 nm (Fe 3+ ) were seen in the flux sapphire. It was found that the difference in the absorbance variation of flux sapphire is greater than that of natural sapphire after heat treatment. The chemical composition and spectrum analysis were utilized to compare the natural sapphire and the flux synthetic sapphire.