Abstract: The microstructures and mechanical properties of arc-melting processed Mo3Si-Mo5Si3 eutectic have been investigated. The Vickers hardness of Mo3Si-Mo5Si3 eutectic alloy at room temperature is on the order of 1350Hv. The fracture toughness value of the alloy at room temperature is 1.39MPam1/2 measured by Single edge-notched beam specimen technique and 1.61MPam1/2 measured by Indentation technique. The compressive strengths at 1300 oC and 1400 oC under a strain rate of 10-4s-1 are about 550MPa and 300MPa respectively.
Abstract: SiC nanorod was grown by APCVD using TMS. Grown SiC nanorods had a 10 ~ 60nm diameters and lengths of several micrometers. Nanorod.s diameters and lengths were different by kind of catalysts. Nanorod.s growth scheme was divided by two regions with diameter.s variations. At first region, nanorod was grown by VLS mechanism, but at the second region, nanorod growth was made by VS reaction. These differences were made from limitations of growth rate and deactivation effects. Growth temperature, time and flow rates of source gases were affected nanorod.s diameters and its lengths. And the kind of catalysts and coating methods were affected growth direction and microstructures too.
Abstract: Bilayer with inner layer of ZrO2-3mol% Y2O3 (TZ-3Y) and outer layer of zirconiaalumina (AKP-30) composite is fabricated using gel-casting for the inner layer and dip coating for the outer layer in aqueous system. The processing additives for gel-casting, such as dispersant, monomer, dimmer, and initiator, are adjusted and optimized. From which the solid loading of each starting material is determined. The bilayer structure can be in-situ formed by dip coating, which are verified by the sintered microstructure of the bilayer investigated.
Abstract: It was pointed out that one of the causes of recent failure to launch rocket was due to the fracture of nozzle throat insert made of graphite materials. The relationship between mechanical properties and microfracture process in graphite was not enough analyzed. To ensure the reliability of such aerospace equipment, we considered the necessity of assurance by non-destructive evaluation, evaluation of mechanical property for graphite material and design based on fracture probability. In this study, four-point bending test and fracture toughness test were used to evaluate mechanical properties. Mean strength, Weibull parameters, and R-curve for crack propagation were estimated. AE measurement during tests was performed in order to obtain location and stress of microfracture. AE results were analyzed by stochastic process theory. The result of AE demonstrates that microfracture process during bending test is divided into three stages. AE behavior in fracture toughness test was also closely related to crack propagation.
Abstract: Formation energies of various defects in Al2O3 and SiO2 are calculated by using the plane-wave pseudopotential method. Also, the formation energies of Schottky defects and Frenkel defects are evaluated on the basis of these calculations. It is shown that formation energies of these defects are higher in SiO2 than in Al2O3. In other words, less defects are formed in SiO2 than in Al2O3. It is also found that the principal defect is the cation Frenkel defect in Al2O3 but the anion Frenkel defect in SiO2. These results agree with the experimental results that Al ions diffuse preferably in Al2O3 but oxygen ions diffuse in SiO2 at high temperatures.
Abstract: Ceramic functional coating is frequently applied to the structure material of turbine blades, and it is very important to investigate its properties during thermal shock, thermal cycle and thermo-mechanical fatigue. In this study, we measured acoustic emission (AE) caused by microfracture of the coatings to investigate the fracture mechanism during thermo-mechanical test of coatings. We have developed the method to detect AE signal using laser interferometer, and it was demonstrated that it was very powerful tool for non-contact measurement of AE. This technique was applied to the thermal cycle test of coatings, and microfracture mode was analyzed by simulation of wave propagation using finite element method (FEM). The change in energy release rate of interfacial cracks in the specimen due to heating and cooling was also calculated by FEM and correlation between propagation of interfacial crack and AE behavior was evaluated.
Abstract: Spherical fine powders of tungsten oxide were prepared by emulsion evaporation method. The characteristics of the powders prepared were examined by means of TGA, XRD, SEM and image analyzer. The emulsions were prepared by vigorous mixing of aqueous phase containing tungsten ion and the organic phase containing kerosene, surfactant and paraffin oil. Precursors were made by evaporating the emulsion in the kerosene bath at 160 °C and then calcined at 650 °C. The average particle size of the tungsten oxide powders was 0.6 ㎛ and their shapes were spherical. As the HLB value of the surfactant increased and the concentration of tungsten ions decreased, the mean particle size of tungsten oxide powders decreased whereas agglomeration size increased
Abstract: A development of synthetic process via rare earth complex precursors was applied to a formation of rare earth manganites which can not be easily formed by conventional (normal) process. Several compounds of rare earth manganites with various structures were successfully synthesized by this method and their crystal structures were identified by powder X-ray diffractometry. Both of the orthorhombic and rhombohedral LaMnO3 phases were prepared. LnMn2 O5 phase was more easily prepared by this method than by another method. The solid solution of (Sr,La)2
MnO4 phase for various compositions were tried to prepare a single phase, especially for a composition region Sr/La ratio < 1. since this phase is normally formed for only region of Sr/La ratio .. 1 by the solid state reaction method.
Abstract: A low-temperature co-fired composition with sintering temperature of <900
°C was developed using rare earth derived borate glass (La2O3-B2O3-TiO2) and a conventional BNT (BaONd2O3-TiO2) ceramic. The sintering behavior, phase evaluation, sintered morphology, and microwave dielectric properties were investigated. It was found that increasing the sintering temperature from 750 to 850°C leads to increases in shrinkage, densities and microwave dielectric properties (≈ 20 for εr and >8000 for Q*f0). However, after 850 °C , the values of these properties decreased. It is suggested therefore that this new composite has good potential as a new candidate
for low temperature co-fired ceramic (LTCC) materials.
Abstract: The chemical and electrical characteristics of the grain boundary of the polycrystalline semiconducting SrTiO3 ceramics, which were synthesized by hot-press sintering Na-coated semiconducting SrTiO3 powders, were investigated in terms of process parameters such as sintering temperature. Diffusion kinetic parameters such as diffusion coefficients and activation energy of diffusing ions at particular heat-treatment conditions were obtained by fitting experimental data with computer-simulated results as well as electrical features near the grain boundary of the ceramics.