Catalytic synthesis and properties of β-Ga2O3 nanowires grown by metal organic chemical vapor deposition are reported. Au, Ni and Cu catalysts were suitable for the growth of Ga2O3 nanowires under our experimental conditions. The Ga2O3 nanowires grown by using Au, Ni and Cu catalysts showed different growth rates and morphologies in each case.
We found the Ga2O3 nanowires were grown by the Vapor-Solid (VS) process when Ni was used as a catalyst while the Vapor-Liquid-Solid (VLS) was a dominant process in case of Au and Cu catalysts. Also, we found nanowires showed different optical properties depend on catalytic metals. On the other hand, for the cases of Ti, Sn and Ag catalysts, nanowires could not be obtained under the same condition of Au, Cu and Ni catalytic synthesis. We found that these results are related to the different characteristics of each catalyst, such as, melting points and phase diagrams with gallium metal.
K2Ti6O13 whisker have been synthesized by hydrothermal process at low temperature and pressure condition. The average length of the synthesized K2Ti6O13 whisker was about in the range of 300 nm to 1.5 μm. The average diameter of the synthesized K2Ti6O13 whisker was 15 nm to 60 nm. The aspect ratio of the synthesized K2Ti6O13 whisker was below 12.
The average length and diameter of the synthesized K2Ti6O13 whisker can be controlled by reaction temperature and time, KOH molar ratio. K2Ti6O13 whisker was synthesized from more than 210 o C and 4 h at reaction temperature and time. The length of the whisker is increased with increasing reaction temperature. Characterization of the synthesized K2Ti6O13 whisker was carried out using the XRD and FE-SEM.
In this study, spherical monodispersed cerium-doped yttrium aluminum garnet (YAG : Ce 3+ ) phosphor particles were synthesized via homogeneous precipitation method using the mixed solution of yttrium nitrate, cerium nitrate, aluminum nitrate, ammonium aluminum sulfate, and urea as a precipitant. During the process of precursors of monodispersed YAG : Ce 3+ , aluminum ions which form spherical aluminum compounds precipitated first and yttrium compounds precipitated onto the surface of the existing spherical aluminum compounds. Drying process using lyophilization could obtain monodispered spherical YAG : Ce 3+ particles compare to using oven. The thermal calcination process of YAG : Ce 3+ precursors under the temperature of 1200 o C for 6 h was enough to obtain 400~500 nm sized YAG particles with pure YAG phase.
TiO2 films, thickness of 1~30 μm were deposited on glass substrate at room temperature by room temperature granule spray in vacuum. The starting powder was calcinated at 600 o C for 4 h using Al2O3 crucible in the furnace. The particle size of the TiO2, 1.5 μm was measured by a particle size analyzer. The effect of different process parameters such as number of pass, gas flow rate and feeder voltage was studied. As the number of passes increased, the film thickness increased proportionally due to adequate kinetic energy conserved. The effect of three different flow rates (i.e. 15, 25, and 35 LPM) on deposited film was investigated. As gas flow rate increased, the film thickness increased up to 25 LPM and then decreased. Higher feeder voltage with low flow rate of 15 LPM resulted in unsufficient coating thickness due to insufficient kinetic energy. Microstructure of TiO2 films was investigated by scanning electron microscope and high resolution tramission electron microscope.
SiC fiber can be used up to 1800 o C in both inert and air atmosphere without any problems such as melting and oxidation. SiC fibers can be applied to dust filtering processes as a bag filter at a high temperature above 700 o C, which is far beyond the temperature range of currently available industrial bag filter. However the studies for the degradation of SiC fibers were still lacked in the harsh environment of steel industries and thermoelectric power plants. In this study, SiC fibers were reacted with steel dust and thermal power plant dust at a high temperature of 500 o C or higher, and the degraded shape of the fiber surface was observed by SEM. Also the degree of oxygen diffusion on the surface and inside of SiC fiber was analyzed by EDS.
In this study, the effect on the zinc nuclei crystallization caused by changes preprocessing of the zinc crystalline glaze preparation has been studied. The mechanism of the nuclei formation in the crystalline glaze and development of the nuclei by studying the preprocessing step was explained. The preprocessing step was improved by altering mixing process of the materials prior to sintering: number of sieving dispersion process and ultra-sonication prove tests with various duration of sonication. According to the result, the sieving and sonication of the starting materials facilitated the interface reactions of ZnO-SiO2 from 680 o C where low temperature willemite is formulated, and altered Si bonding for the easier bonding between Zn-Si. In other words, solely sieving was enough to accelerate the formation of willemite in low temperature.
When the particles were distributed evenly by sonication, the willemite formation was even more significant.
SrAl2O4 : Eu 2+ ,Dy 3+ phosphorescent phosphors were synthesized by skull melting method. The molar ratio of oxides in the phosphors synthesized by the skull melting technique was SrCO3 : Al(OH)3: Eu2O3: Dy2O3= 1 : 2 : 0.015 : 0.02.
Crystal structure and surface morphology were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. Optical properties of the synthesized SrAl2O4: Eu 2+ ,Dy 3+ were measured by photoluminescence (PL) spectrometer for in-depth study on the excitation, emission and afterglow properties. From the PL measurements, it was found that excitation occurred in the wavelength range from 300 to 420 nm with peak position at 360 nm. The emission spectrum showed a broad curve in the wavelength from 450 to 600 nm with peak position at 530 nm. SrAl2O4: Eu 2+ ,Dy 3+ phosphors exhibited afterglow properties with emission that lasted for a long period.
In this study, the higher temperature thermal property of the fly ash-blast furnace slag system Geopolymer including alumina aggregate was investigated whether that Geopolymer will be or not useful as thermal-resistant construction materials. Under every mixing conditions, the crack on the surface of hardened body was not observed up to 800 o C and it corresponded with fact that level of changes was not significant before and after heating process. Residual compressive strength is most high when mixing Blast-Furnace Slag ratio is 60 wt% until temperature reaches 800 o C. The major hydrates of hardened body of Geopolymer; amorphous halo pattern between 20~35 o (2theta) and mullite (3Al2O3 · 2SiO2) and quartz (SiO2) was found during the experiment. Amorphous halo pattern was a aluminosilicate gel generated by geopolymeric polycondensation and it was found that the halo pattern of aluminosilicate gel was preserved up to 800 o C. The patterns of aluminosilicate gel disappeared from 1,000 o C and crystal phases like gehlenite, calcium silicate, calcium aluminum oxide, microcline was observed with the increase of exposure temperature.
High-purity β-SiC powders for SiC single-crystal growth were synthesized by direct carbonization. The use of high-purity raw materials to improve the quality of a SiC single crystal is important. To grow SiC single crystals by the PVT method, both the particle size and the packing density of the SiC powder are crucial factors that determine the sublimation rate. In this study, we tried to produce high-purity β-SiC powder with large particle sizes and containing low silicon by introducing a milling step during the direct carbonization process. Controlled heating improved the purity of the β-SiC powders to more than 99 % and increased the particle size to as much as ~100 μm. The β-SiC powders were characterized by SEM, XRD, PSA, and chemical analysis to assess their purity. Then, we conducted single-crystal growth experiments, and the grown 4H-SiC crystals showed high structural perfection with a FWHM of about 25-48 arcsec.