Key Engineering Materials Vol. 617

Abstract: In this study, SiC/SiO₂ core-shell nanowires (SiCNWs) were fabricated by thermal evaporation method without any catalyst, using pre-oxidized silicon powder and methane (CH₄) gas as precursors. The reaction temperature was 1340°C in an inert atmosphere. The SiCNWs produced by this process had a single crystal β-SiC core ranging from 20 to 80 nm in diameter and low-crystalline SiO₂ shell about 10-20 nm thick, and up to 1 mm long. The exhaust gas from the production system was analyzed by gas chromatography and the growth activity of SiCNWs was captured by digital camera during a soaking period. From the results, CO gas was detected only when SiCNWs were growing and was not found when methane gas was stopped. It was clarified that CO gas was produced as a by-product during the formation of SiCNWs. The formation mechanism of SiCNWs synthesized by thermal evaporation method was suggested to be oxide-assisted growth mechanism.

Abstract: We studied an adsorption method as a technique of collection, and a vitrification method of iodine toward safekeeping. In this study, iodine-adsorptions on magnesium compounds were evaluated, and fixation of iodine into glass was tried. As a result, it was revealed that 1 g of magnesium oxide with high surface area successfully adsorbed 2.23 g of iodine. Iodine-adsorbed magnesium compound was tried to be vitrified with MgO-P₂O₅ glass.

Abstract: Thin carbon nanotubes (CNTs) were heated at 2800°C to heighten the crystallinity of the CNTs. The microstructures and fracture toughness of the thin CNTs and the heat-treated thin CNTs/alumina composites were investigated. Using such thin CNTs was effective for obtaining finer microstructures of the composites but the heat-treatment made the uniform dispersion of the CNTs difficult. The composites containing the heat-treated thin CNTs showed higher fracture toughness than those containing the thin CNTs and the thick CNTs used in previous study when their content was the same.

Abstract: The influence of Cu₂O addition on crystallization process and microstructure of the transparent mica glass-ceramics was investigated. Cu⁺ ions which were added as Cu₂O were oxidized to Cu²⁺ ions during preparation of parent glasses and the Cu²⁺ ions were reduced to Cu⁺ ions again during heating of the parent glasses. By heating the parent glasses to which 10-20 mol% Cu₂O was added at 650-700°C, not only nanosized micas but also Cu₂O nanoparticles separeted. Their separation temperatures were lowered as the additive amount of Cu₂O increased. Micas separated at 750°C should contain Cu⁺ and Cu²⁺ ions in the structure. Metallic Cu was observed only in the specimen to which 1 mol% Cu₂O was added.

Abstract: C-and BN-interphases on SiC fibers for unidirectional SiC_f/SiC composites were formed by EPD process, and their microstructure and mechanical properties were investigated. Whereas the C-SiC_f/SiC composites showed a pseudo-ductile fracture behavior with large amount of fiber pullout, the BN-SiC_f/SiC composites fractured in a brittle manner without fiber pullout in spite of sufficient thickness of BN interphase. It is inferred from the results of EDS that sintering additives would react with h-BN-interphase, and the interphase did not act effectively for toughening the SiC_f/SiC composites.

Abstract: Low-temperature synthesis of spinel (MgAl₂O₄) was investigated by capsule hot isostatic press (HIP) using hydroxides as starting materials; Mg(OH)₂, MgO, Al(OH)₃, g-AlOOH, g-Al₂O₃. MgAl₂O₄ has been conventionally prepared above 1400 °C, whereas the mixtures containing the hydroxides reacted at 400-500 °C for 1 h under 200 MPa of isostatic pressure to form MgAl₂O₄. The hydroxides showed high reactivity to form MgAl₂O₄ with high crystallinity. However, the products more or less contained Mg(OH)₂. The single phase of MgAl₂O₄ was obtained by adding 20 mass% of water to the starting mixtures of Mg(OH)₂ - Al(OH)₃, Mg(OH)₂ - g-Al₂O₃ and MgO - Al(OH)₃. The average grain size of the MgAl₂O₄ was 70 – 100 nm. It was concluded that the transformation of AlOOH or g-Al₂O₃ was promoted by a small amount of water and that Mg(OH)₂ rapidly reacted with the active intermediate compound between AlOOH or g-Al₂O₃ and a-Al₂O₃.

Abstract: A CO₂ blowing method was used to obtain a highly concentrated magnesium phosphate solution (47 times more concentrated than conventional solutions). In the present study, we exposed the solution to microwave heating. Fine and uniform spherical particles were obtained by microwave heating at a heating rate of 2030oC·min^-1. Conversely, the products formed plate-like particles after a retention time of 5 min with a low heating rate. The spherical particles had a relatively uniform size of 0.51.0 μm and were composed of magnesium phosphate octahydrate, while the plate-like particles were composed of magnesium phosphate pentahydrate. Spherical particles could not be obtained by heating at a gradual heating rate using a hot plate; instead, large and non-uniform plate-like particles were formed. From this result, it is evident that microwave heating enables the production of unusual shaped particles that are not accessible using ordinary heating methods.

Abstract: The effect of superheated steam on the decomposition of CaCO₃ and MnCO₃ and on the solid state reactions of ZnCO₃-FeOOH and MnCO₃-CaCO₃ was investigated. A newly developed apparatus for the experiments under 1 atm of pure water vapor was used. CaCO₃ decomposed at 800 oC in the superheated steam to form the single phase of CaO. On the other hand, the decomposition was uncompleted in air. CaCO₃ transformed into CaO via Ca (OH)₂ in superheated steam. During the transformation of carbonate into hydroxide, the crystal lattice is temporarily disordered to make it active, leading lower decomposition temperature of CaCO₃. MnCO₃ decomposed to form γ-Mn₂O₃ at above 1000 oC in air, whereas γ-Mn₂O₃ was obtained at 800 oC in the superheated steam. The solid state reaction in the steam was suppressed for the mixture of ZnCO₃ and FeOOH. This seemed to be due to the large difference in decomposition temperature between ZnCO₃ and FeOOH. MnCO₃ reacted with CaCO₃ to form CaMn₂O₄ at 800 oC in the superheated steam. However, a higher temperature of 1000 oC was required to cause the reaction in air. The low-temperature transformation of MnCO₃ and CaCO₃ in the superheated steam would affect the reaction. It was concluded that the reactivity of carbonate in super heated steam was promoted by the Hedvall effect, which was caused by the formation of intermediate phase such as hydroxide.

Abstract: Nanosize Mg²⁺, Zn²⁺, Sr²⁺, and Na⁺ mono- and di-substituted calcium hydroxyapatites were synthesized using an aqueous precipitation route under controlled conditions. The chemical composition and the micro-structural features of the polycrystalline samples were characterized by chemical analysis, thermal analysis (TG/DTA) combined with MS for evolved gas analysis, FTIR and XRD spectroscopy as well as SEM. The correlations between chemical composition and the crystal morphology were elucidated.

Abstract: Transition Edge Sensor (TES) is an energy dispersive X-ray detecting system with high energy resolution. The energy resolution of this system depends on the steepness of superconducting transition curve from normal to superconducting state, heat capacitance and the operating temperature. The TES is based on the dilution refrigerator cooled by about 100 mK. The energy resolution is calculated about 1-2 eV for the detector with maximum detecting energy as 10 eV. The energy resolution also depends on the superconducting current flowing through the TES device because the superconducting current is affected by the temperature stability of the refrigerator. The fluctuation of the superconducting current means the fluctuation of the X-ray spectrum peak center. We have developed the sensitivity correction system to stabilize the peak center of the X-ray spectrum. The peak center of X-ray spectrum correlates with heater power to keep the base temperature of TES device at a constant temperature. The peak center of X-ray spectrum is calibrated by monitoring the heater power at constant time interval using the correlation curve between the peak center of X-ray spectrum and heater power.