The thermal conductivity of SiC ceramics sintered with BeO exhibited a higher value of 270W (mK)-1 at room temperature than that of the other non-oxide ceramics. We focused on the microstructure formed by addition of a small amount of BeO. TEM images revealed new microstructural data for understanding the thermal conductivity of this SiC ceramics. BeO addition in the SiC sintering formed a liquid phase, and densification of SiC ceramics was improved. A kind of very thin intergranular film existed as an amorphous state on the surfaces of almost all of the grains. During the liquid-phase sintering process, impurities of Fe, Ni, Al, V, Ti, and O were also precipitated at the triple-grain junction from the liquid phase.
Carbon materials were fully covered with a molten silicate glass in order to improve their steam oxidation resistance and thermal cyclic fatigue. The wettability of the carbon to molten glass was modified by soaking in a Si-N precursor solvent followed by thermal decomposition. When glass coating was performed under higher N2 partial pressure, the molten glass completely sealed the carbon substrate and infiltrated into the interior without the production of cristobalite due to the oxidation of the pyrolyzed species. However, coating under a lower N2 partial pressure, such as an Ar atmosphere, led to the formation of cristobalite at the interface between the carbon substrate and silicate glass. In this case, the glass covered only the surface of the substrate and many pores formed at the interface. The pores formed as a result of the production of large amounts of CO from oxidation of the carbon substrate. The structural changes of the interface, resulting from differences in the N2 partial pressure during sealing with glass, are in good agreement with thermodynamic considerations. The coating of molten glass on carbon substrates under higher N2 partial pressure significantly improved the thermal cyclic fatigue of the carbon substrates in addition to the steam oxidation resistance.
Porous hydroxyapatite (HAp) bodies were prepared by the simple process. Porous fibers consisting of polyacrylonitrile (PAN) and HAp were fabricated by extrusion of mixtures of PAN and HAp in dimethylformamide (DMF) solution. Fibers obtained by this extrusion method were chopped and molded. Subsequently, these compactions from chopped fibers consisting of HAp and PAN were sintered at various temperatures in air. The sintered HAp bodies had high porosity and a number of pores with those diameter some hundreds μm. These porous HAp bodies can be used as a scaffold in the body.
The aluminum nitride powder synthesized by combustion synthesis (SHS) process usually includes course particles because of its high reaction temperature. These powders are not suitable for sintering, and strong pulverization is necessary. The reduction of the primary particle size of AlN powder synthesized by SHS was studied by adding reaction controlling agents. The results showed that some additives cause a remarkable decrease in its reaction temperature and increase in specific surface area of the product powders. Pulverization and sintering of these products are then carried out and the characteristics of the sintered body were evaluated. The average particle sizes, after 30 minutes pulverization by planetary mill, were 1.3μm for the AlN synthesized with 3mass% NH4F as a reaction controlling agent, and 1.7μm without NH4F. Sintering of these powders was carried out by using spark plasma sintering (SPS) at 1823K and obtained relative densities were 98.7% and 91.9%, respectively.
Recently, Bio-cast ceramics are paid attention because of preparing ceramics with microstructure characteristic of a plant, reducing cost and solving exhaustible resource problems. The ceramics derived from a plant are porous and are recognized as filter, catalyst carrier and others for the application. For bamboo, in particular, the growth is much faster in plants and the growth and development area extends in the world. So in this study, porous SiC ceramics derived from the bamboo charcoal as a source of carbon was produced. Bamboo charcoal was reacted with silicon by melting impregnation technique at 1450°C for 0.1 ∼ 5h under a flow of Ar gas (100ml/min). XRD analysis showed that as heating time increased, more SiC was synthesized, but a small amount of free silicon remained.
Huge amount of ash is generated from industrial wastes. In order to recycle this, the production of glass powder from ash is very useful because the use of glass powder as filler for concrete improves its properties. The purpose of this study is to clarify the formation mechanism of glass powder from powder ash and to obtain an optimum condition for its production. In this study, three kinds of ashes (sewage sludge ash, paper sludge ash, and coal ash) were used for the production of glass powder. Glass fraction and the ratio of spherical particles for the glass powder were measured. The crystalline phases in the glass powder was identified, and melting behavior of the ashes was investigated. It was found that melting process is very important to produce spherical glass powder and that the basicity defined as the ratio CaO/SiO2 is a good measure to control the composition of the ashes for the glass powder production.
Yttria-stabilized zirconia powder was synthesized by a hydrolysis method using zirconium oxychloride and yttrium chloride as a starting materials. The effect of PEG addition was investigated on the hydrolysis of zirconium oxychloride and characteristics of the obtained yttria-stabilized zirconia powder. By the addition of PEG, the initial hydrolysis of zirconium oxychloride was promoted and the well-dispersed hydrous zirconia powder was obtained. In this case, however, a slight amount of un-reacted zirconium oxychlodride remained even after 115hrs. On the other hand, for the sample without PEG, hydrolysis of starting material was nearly completed after 115hrs but large aggregates of hydrous zirconia were formed. From the results of pH titration, it was found that difference of surface properties of yttria-stabilized zirconia powder prepared with PEG between before and after milling in water was smaller than that of the powder obtained without PEG. Furthermore, the yttria-stabilized zirconia powder containing PEG showed higher content of tetragonal phase than the powder without PEG. These results indicated that zirconium hydroxide as seed particles for hydrolysis reaction was dispersed well by the action of PEG and as a consequence highly dispersion of hydrous zirconia and homogeneous mixing of yttrium hydroxide were achieved.
Self-assembled quantum dot formation in heteroepitaxial growth attracts much attention of many researchers. Although a large number of experimental studies are made on the quantum dot, it is also very important to establish the numerical method which can examine the shape, size and arrangement of the quantum dots growing on the substrate surface. In this study, phase-field model, which can simulate the growth process of the quantum dot in which the thin film with planer surface transforms to the periodic islands, is developed. The driving force of the surface morphology change is elastic strain energy generated by the lattice mismatch of substrate and thin film. Two dimensional simulations are performed by using finite difference method for phase-field and finite element method for stress field. The time evolutions of the surface morphology, stress distribution and free energy are evaluated. As a result, it is clarified that the surface of thin film changes to the island morphology so as to reduce the elastic strain energy while the surface energy increases. Phase-field model developed here, furthermore, is applied to the dislocation network, substrate steps and multilayer structure, which are utilized in order to achieve uniform size and distribution of the quantum dots arrangement. The preferential island formations on top of dislocation, step and embedded inclusion are observed and the island size becomes more uniform.
The martensitic transformation of Ni-Ti shape memory alloy was investigated by X-ray diffraction analysis aided X-ray diffraction pattern simulation. Using the wire shape samples the X-ray diffraction patterns were measured at every 10°C from 0°C to 80°C. The software that simulated the X-ray diffraction patterns using by the data of already known lattice constants was developed on the basis of the Bragg's law and the intensity laws. First we confirmed that the X-ray pattern measured by arranging wire samples and attaching the sample holder coincided that of the powder method. Next the X-ray pattern at austenite phase (80°C) was compared with the simulated pattern of B2 crystal structure by the program and the state was identified as B2 structure. On the other hand at martensite phase (0°C) B2 and B19' structures were mixed in the crystal. Although the shape memory effect was expressed at this environmental temperature, it is considered that not only B19' structure but B2 structure is also related to the effect of shape memory effect.
Electromagnetic acoustic resonance (EMAR) is a contactless resonant method with an electromagnetic acoustic transducer (EMAT). This method is free from extra energy losses, resulting in the measurement of intrinsic ultrasonic attenuation in solids. In this study, the EMAR was applied to detect the creep damage of an Austenite Stainless Steel, SUS316L. The material was exposed to the temperature of 973K at various stresses. We measured ultrasonic attenuation for 1-8-MHz frequency range as the creep advanced. The attenuation coefficient exhibits much larger sensitivity to the damage accumulation than the velocity. In a short interval, between 60% and 70% of whole life, attenuation experiences a large peak, being independent of the stress. This novel phenomenon is interpreted as resulting from microstructure changes, especially, dislocation mobility. This is supported by TEM observations for dislocation structure. This technique has a potential to assess the damage advance and to predict the creep life of metals.
This paper studies the crack propagation direction of type 304 steel tube specimen in reversed torsion tests. Strain controlled torsion low cycle fatigue tests were carried out for type 304 steel tube specimens and the number of cycles to failure was obtained. The crack directions were also observed by intermitting the tests. Main cracks propagated in the maximum shear strain directions at high strain ranges but in the principal strain directions at low strain ranges. The transition of the main crack direction from the maximum shear strain direction to the principal strain direction by lowering strain range was discussed based on the observations of micro crack directions. The main crack is the crack that directly leaded to the failure of specimen and the micro crack is the crack whose length is less than 100μm. Most micro cracks initiated in the maximum shear strain directions and they branched in the principal strain directions as they grew. The critical length of branching was related to the strain range. The critical crack length at high strain ranges was longer than that at low strain ranges, which corresponded well with the main crack propagation directions. The aspect ratio of cracks was discussed in relation to the main crack directions and the energy release was discussed regarding the aspect ratio.
Fatigue crack propagation tests were performed in thin-walled tubular specimens with a hole made of a stainless steel (SUS 316NG) under cyclic torsion with and without superposed static and cyclic axial loading. The propagation path of mode I fatigue cracks followed the direction perpendicular to the maximum of the total range of the normal stress including the compressive component of the stress. The propagation rate was faster than the uniaxial data when compared at the same range of the stress intensity factor. Even when compared at the same value of the effective stress intensity range, the crack propagation rate was faster under combined loading than under uniaxial loading because of excessive plasticity ahead of the fatigue crack. The J-integral range determined from the relation between load and displacement was proved to be an appropriate parameter for crack propagation with excessive plasticity under combined loading.
Among many other uses, rubber is used in tubes and wire covers. Rubber exhibits considerable changes in mechanical properties over time and in temperature. To confirm the reliability of these products, we must evaluate how they degrade in humidity, sunlight, and under thermal loading. We therefore studied the thermal degradation of chlorosulfonated polyethylene rubber (CSM) and ethylene propylene diene methylene linkage (EPDM). We first measured thermal reduction of elongation and of fatigue strength of CSM and EPDM. To evaluate the degrading of these properties, we used Arrhenius's equation to confirm the relationship between temperature and exposure time. Using these evaluation methods we could predict thermal reduction of fatigue strength by taking into account the temperature and exposure time. The acceleration in thermal degradation of elongation and that of fatigue strength agreed well quantitatively. We next measured the viscoelastic behavior of virgin CSM and EPDM material and of thermally degraded material. We evaluated the thermal degradation of mechanical loss tangent by using Arrhenius's equation to confirm the relationship between the temperature and the time. The acceleration in thermal degradation of a mechanical loss tangent was in good agreement with the acceleration in thermal degradation of elongation and of fatigue strength.
This study aims at clarifying the electrochemical compatibility between carbon steel in chloride-contaminated base concrete and stainless steel in repair concrete. For this purpose, patch repair to RC members deteriorated by chloride-induced corrosion of rebar was simulated. The results can be summarized as follows; The half-cell potential of stainless steel is less affected than that of carbon steel by chloride in concrete. In the repaired specimens, the type of steel used in repair does not affect the relationship between the chloride content and potential or polarization of carbon steel in a base RC member. Anodic polarization curve of stainless steel differs from that of carbon steel, especially in the positive potential range, which, however, can be ignored in actual applications in concrete. Therefore, the use of stainless steel does not accelerate re-deterioration of RC member by electrochemical incompatibility, and is effective for the repair of RC member deteriorated by chloride-induced corrosion.