Journal of Powder Materials

Theorerical Review of Grain Boundary Faceting: J Korean Powder Metall Inst. 1999;6(3):197-208.

Oxidation Behavior of the Simulated Supent Fuel at 400-700°C: J Korean Powder Metall Inst. 1999;6(3):209-214.

Abstract PDF: The oxidation behavior of the simulated spent fuel of burn up 33 MWD/kgU was investigated to predict that of the spent fuel in the temperature ranges of 400 to 700°C and was compared with those of UO_2. The forms of uranium oxides after the oxidation were conformed by XRD analyses. The oxidation rate at each the temperature and the activation energy were obtained. After complete oxidation, the simulated spent fuel was converted to U_3O_8 and pulverized to powder due to the density difference between the simulated spent fuel and uranium oxides. The activation energies were 85.35 and 30.77kJ/mol in the temperature ranges of 400≤T(°C)≤500 and 500≤T(°C)≤700, respectively.

Effects of Elemental Powder Particle Size and Shape on the Synthesis of Ni Silicides by Mechanical Alloying: J Korean Powder Metall Inst. 1999;6(3):215-223.

Abstract PDF: The synthesis of Ni_5Si_2,;Ni_2Si and NiSi has been investigated by mechanical alloying (MA) of Ni-27.9at%Si, Ni-33.3at%Si and Ni-50.0at%Si powder mixtures. As-received and premilled elemental powders were subjected to MA. The as-received Ni powder was spherical and the mean particle size 48.8mum, whereas the premilled Ni powder was flaky and the mean particle diameter and thickness were found to be 125 and 5mum, respectively. The mean surface area of the premilled Mi powder particle was 3.5 times as large as that of the as-received Ni powder particle. The as-received Si powder was was 10.0mum. Self-propagating high-temperature synthesis (SHS) reaction, followed by a slow reaction (a solid state diffusion), was observed to produce each Ni silicide during MA of the as-received elemental powders. In other word , the reactants and product coexisted for a long period of MA of time. Only SHS reaction was, however, observed to produce each Ni silicide during MA of the premilled elemental powders, indicating that each Ni sillicide formed rather abruptly at a short period of MA time. The mechanisms and reaction rates for the formation of the Ni silicides appeared to be influenced by the elemental powder particle size and shape as well as the heat of formation of the products (Ni_5Si_2longrightarrow-43.1kJ/mol.at., Ni_2Sirightarrow-47.6kJ/mol.at.).

Characterization of Plasma Sprayed Cr_2O_3 Coatings: J Korean Powder Metall Inst. 1999;6(3):224-230.

Abstract PDF: Oxide powders of Cr_2O_3 ;and;Cr_2O_3+3%TiO_2 were prepared by spray drying, plasma densification and fused+crushed processes. The oxide coating layers were made by plasma spray and characterized by microstructure, hardness, adhesion strength and relative density. The optimum spray distance for the high quality coatings by spraying dried powders was found to be 9cm. A small amount addition of TiO_2;in;Cr_2O_3 powder significantly improved coating characteristics by lowering the melting point of powders. The hardness and relative density of coating layers of the plasma densified powders were comparable to fused and crushed powders, however, the adhesion strength was much higher in the former case.

Microstrucual Characterization of Vacuum Gas Gas Atomized AZ31+1%MM Alloy Powders: J Korean Powder Metall Inst. 1999;6(3):231-237.

Abstract PDF: In this study, the characteristics of gas atomized Mg-3wt%Al-1wt%Zn-1wt%MM alloy powders under vacuum condition were investigated. In spite of the low fluidity and easy oxidation of the molten magnesium, the spherical powders could be successfully produced by using a modified three pieces nozzle attached to the gas atomization unit. It was found that most of the solidified powders less than 50mum in diameter were single crystal and the solidified structure showed a typical dendritic morphology due to supercooling prior to nucleation. The secondary dendrite arm spacing decreased as the size of powders decreased. The Mg-Al-Ce intermetallic compounds with chemically stable phase were found in the interdendritic regions of alpha-Mg. It is considered that formation of the chemically stable phase may possibly affect to improve the corrosion resistance. Therefore, it is expected that the materials formed of these Mg-Al-Zn-MM alloy powders shows better mechanical properties and corrosion resistance due to the structural refinement.

Powder Characteristics of n-TiO_2 Powder Synthesized by Chemical Vapor Synthesis: J Korean Powder Metall Inst. 1999;6(3):238-245.

Abstract PDF: The preparation of n-TiO_2 powder by the Chemical Vapor Synthesis process (CVS) was studied using the liquid metal organic precursor (TTIP). The residence time and the collection methods were considered as main processing variables through the experiments. The CVS equipment consisted of a micropump and a flashvaporizer, a tube furnace and a tubular collection device. The synthesis was performed at 1000°C with various sets of collection zone. The residence time and the total system pressure were controlled in the range of 3~20 ms and 10 mbar, respectively. Nitrogen adsorption, X-ray diffraction and electron microscopy were used to determine particle size, specific surface area and crystallographic structure. The grain size of the as-prepared n-TiO_2 powder was in the range of 2~8 nm for all synthesis parameters and the powder exhibited only little agglomeration. The relationship between particle characteristics and the processing variables is reviewed based on simple growth model.

Growth of Elongated Grains in \alpha'-Sialon Ceramics: J Korean Powder Metall Inst. 1999;6(3):246-250.

Abstract PDF: The effects of the beta-Si_3N_4 starting particle size and alpha/beta phase transformation during sintering process on the microstructure evolution of Yttrium alpha-Sialon ceramics were investigated. As-received beta-Si_3N_4 powder (mean particle size: 0.54mum) and classified beta-Si_3N_4 powder(mean particle size: 0.26mum) were used as starting powders. With decreasing the starting particle size, the growth of elongated grains was enhanced, which resulted in the whisker -like microstructure with elongated grains. These results were discussed in relation to the two-dimensional nucleation and growth theory. In the specimen heat treated at 1600°C for 10h before sintering at 1950°Cfor 1h under 40atm(2-step sintering), the grain size was smaller than of the 1-step sintering at 1950°C for 1h. However, bimodal microstructure evolution were not not remarkable in both sample, which is ascribed to the alpha-phase contents existing in beta-Si_3N_4 starting powder.

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