Czochralski (CZ) growth process is one of the most important techniques for growing high quality sapphire single crystal for LED application. In this study, the inductively-heated CZ growth processes for the sapphire crystal of 300 mm length have been analyzed numerically using finite element method. The hot zone structures were modified with the crucible geometry change and the additional insulation layer installed above the crucible. The results show that the solid-liquid interface height decreased from about 80 mm at initial stage to 40 mm after mid-stage due to achieve the growth speed balance. Also the optimal input power of the modified system was similar with the original one due to the compensation effects of the crucible geometry and additional insulation. The crystal temperature grown by the modified CZ grower was increased about 10 K than the original one. Therefore the sapphire crystal of 300 mm height was grown successfully.
In this study, a c-axis displacement and an internal stress of the sapphire crystal of 300 mm length have been analyzed numerically and the crystal length having no sub-grain defects have been predicted. The hot zone structures were modified with the crucible geometry change and the additional insulation layer installed above the crucible. The simulation results show that the c-axis displacement difference between the original hot zone and others originated from the sub-grain defect formations in the sapphire ingot. When the crystal grown by CZ (Czochralski) grower using the modified hot zone,the crystal length having no sub-grain defects was increased about 57 mm maximum than the original one. When the simulation results compared with the experimental one, the predicted crystal length having no sub-grain defects were well corresponded with the experiment one in c-axis wafer of the 300 mm sapphire ingot. Therefore the sapphire crystal of 250 mm length having no sub-grain defects was successfully grown by CZ process.
Recently, it has been interested much that AlN (Aluminum Nitride) crystals can be applied to UV LEDs and high power devices as like GaN and SiC crystals. The reports about commercial grade of AlN wafers in the world have been absent, however several results for growth of large size of AlN single crystals have been reported from abroad. In this report, the result of AlN single crystals of a diameter of about 8 mm grown are reported. Optical microscopic characterization was applied to observe the form of the crystals and the crystal quality was evaluated by FWHM measurement by DCXRD rocking curve analysis.
A stoichiometric mixture of evaporating materials for MgGa2Se4 single crystal thin films was prepared from horizontal electric furnace. To obtain the single crystal thin films, MgGa2Se4 mixed crystal was deposited on thoroughly etched semi-insulating GaAs(100) substrate by the Hot Wall Epitaxy (HWE) system. The source and substrate temperatures were 610oC and 400oC, respectively. The crystalline structure of the single crystal thin films was investigated by double crystal X-ray diffraction (DCXD). The temperature dependence of the energy band gap of the MgGa2Se4 obtained from the absorption spectra was well described by the Varshni’s relation, Eg(T) = 2.34 eV − (8.81 × 10−4 eV/K)T2/(T + 251 K). The crystal field and the spin-orbit splitting energies for the valence band of the MgGa2Se4 have been estimated to be 190.6 meV and 118.8 meV, respectively, by means of the photocurrent spectra and the Hopfield quasicubic model. These results indicate that the splitting of the Δso definitely exists in the Γ5 states of the valence band of the MgGa2Se4/GaAs epilayer. The three photocurrent peaks observed at 10 K are ascribed to the A1-, B1-exciton for n = 1 and C27-exciton peaks for n = 27.
Surface chemical modification via air and hydrogen heat treatment was found to relieve the aggregation of nanodiamond (ND) seed particles and lead to a significantly enhanced nucleation density for ultrananocrystalline diamond (UNCD) film growth. After heat treatment in air and hydrogen, modification of surface functionalities and increase in the zeta potential were observed. Mean size of the ND aggregates was also dramatically reduced from ~2 μm to ~55 nm. Si surface seeded with ND particles heat-treated at 600oC in hydrogen produced a much higher nucleation density of ~2.7 × 1011 cm−2 compared to untreated ND seeds.
Carbon coils could be synthesized using C2H2/H2 as source gases and SF6 as an incorporated additive gas under the thermal chemical vapor deposition system. A 304-type stainless steel was used as a substrate with nickel powders as the catalyst. The surface of the substrate was pretreated using a sand paper or a mechanical drill to enhance the production yield of the carbon coils. The characteristics of the deposited carbon nanomaterials on the substrates were investigated according to the surface state on the stainless steel substrate. The protrusion induced by the grooves on the substrate surface could enhance the formation of the carbon nanomaterials having the coils geometries. The cause for the enhancement of the carbon coils formation by the grooves was suggested and discussed with the surface energies for the interaction between as-growing carbon elements. Finally, we could obtain the massive production yield of the carbon coils by the surface pretreatment using SiC sand papers on the several tens grooved stainless steel substrate.
In this study, we report the synthesis and characteristics of gallium oxide (Ga2O3) nanoparticles prepared by the polymerized complex method. Ga2O3 nanoparticles were synthesized using Ga(NO3)3, ethylene glycol, and citric acid as the starting materials at a low temperature of 500~800oC. The temperature of the weight reduction by the loss of organic precursor was revealed using TG-DTA analysis. The crystal structural change of Ga2O3 nanoparticles by the annealing process was investigated by XRD analysis. The morphologies and the size distributions of Ga2O3 nanoparticles were analyzed using SEM.
Silicon carbide (SiC) has recently drawn an enormous industrial interest because of its useful mechanical properties such as thermal resistance, abrasion resistance and thermal conductivity at high temperature. Especially, high purity SiC is applicable to the fields of power semiconductor and lighting emitting diode (LED). In this work, high purity carbon powders as raw material for high purity SiC were prepared by a RF induction thermal plasma. Dodecane (C12H26)as hydrocarbon liquid precursor has been utilized for synthesis of high purity carbon powders. It is found that the filtercollected carbon powders showed smaller particle size (10~20 nm) and low crystallinity compared to the reactor-collected carbon powders. The purities of reactor-collected and filter-collected carbon powders were 99.9997 % (5N7) and 99.9993 %(5N3), respectively. In addition, the impurities of carbon powders synthesized by RF induction thermal plasma were mainly originated from the surrounding environment.
Fabrication of nanocomposite material for the Fe2O3-Al system by mechanical alloying (MA) has been investigated at room temperature. It is found that α-Fe/Al2O3 nanocomposite powders in which Al2O3 is dispersed in α-Fe matrix are obtained by mechanical alloying of Fe2O3 with Al for 5 hours. The change in magnetization and coercivity also reflects the details of the solid state reduction process of hematite by pure metal of Al during mechanical alloying.
Densification of the MA powders was performed in a spark plasma sintering (SPS) machine using graphite dies at 1000oC and 1100oC under 60 MPa. Shrinkage change after SPS of MA'ed sample for 5 hrs was significant above 700oC and gradually increased with increasing temperature up to 1100oC. X-ray diffraction result shows that the average grain size of α-Fe in α-Fe/Al2O3 nanocomposite sintered at 1100oC is in the range of 180 nm. It can be also seen that the coercivity (Hc) of SPS sample sintered at 1000oC is still high value of 88 Oe, suggesting that the grain growth of magnetic α-Fe phase during SPS process tend to be suppressed.
The effects of CeO2 on catalytic activity of CeO2-TiO2 for the selective catalytic reduction (SCR) of NOx were investigated in terms of structural, morphological, and physico-chemical analyseis. CeO2-TiO2 catalysts were synthesized with three different additions, 10, 20, and 30 wt% of CeO2, by the sol-gel method. The XRD peaks of all specimens were assigned to a TiO2 phase (anatase) and the peaks became broader with the addition of CeO2 because it was dispersed as an amorphous phase on the surface of TiO2 particles. The specific surface area of TiO2 increased with the addition of CeO2from 60.6306 m2/g to 116.2791 m2/g due to suppression of TiO2 grain growth by CeO2. The 30 wt% CeO2-TiO2 catalyst,having the strongest catalytic acid sites (BrΦnsted and Lewis), showed the highest NOx conversion efficiency of 98 % at 300oC among the specimens. It was considered that CeO2 contributes to the improvement of the NOx conversion of CeO2-TiO2 catalyst by increasing specific surface area and catalytic acid sites.
Phosphate has been removed in waste water by chemically synthesized graphene oxide. Removing efficiency of phosphate was investigated using phosphate dispersion aqueous solution, and 70 % of phosphate was removed in phosphate dispersion solution by chemically synthesized graphene oxide solution. Removing efficiency of phosphate was increased from 70 % to 80 % with assistant of iron nano-particle in chemically synthesized graphene oxide solution. Phosphate removing capacity was up to 89.37 mg/g at initial phosphate concentration of 100 mg/l and temperature of 303 K. The Freundlich was applied to describe the equilibrium isotherms and the isotherm constants were determined.