Examinations of factors limiting the strain rate available to superplastic deformation lead to the following guide: simultaneously controlling the initial grain size, diffusivity, dynamic grain growth, homogeneity of microstructure and the number of residual defects is essential to heighten the strain rate available to superplastic deformation in oxide ceramics. Along this guide, high-strain-rate superplasticity (HSRS) is attained in materials consisting of tetragonal zirconia, α-alumina and a spinel phase: tensile ductility reached 300-2500% at a strain rate of 0.01-1.0 s-1. Post-deformation microstructure indicates that some secondary phases may suppress cavitation damage and thereby enhance HSRS. The guide is also effective in lowering the limit of deformation temperature for a given strain rate. In monolithic tetragonal zirconia, grain-size refinement combined with doping of aliovalnt cations such as Mg2+, Ti4+ and Al3+ led to HSRS at 1350°C.
Powders of perovskite-type oxides, with the general formula La0.8Sr0.2Co1-xFexO3 (0.0<x<1.0), were synthesized by the amorphous citrate method, and chemical reactions in such a synthesis process were systematically investigated via thermal analysis. A series of decompositions of citric acid derivatives occurred at 575-750 K, and the subsequent oxidation of metal-organic amorphous compounds to perovskite-type oxides occurred at 650-850 K. The apparent activation energies for those processes in La0.8Sr0.2Co0.5Fe0.5O3 (x=0.5) were determined to be 101-105 and 45-49 kJ/mol, respectively, using a non-isothermal technique based on the Kissinger approach. The exothermic peaks of those reactions were temperature-dependent on Fe content (x), and they increased with increasing x, reaching a maximum at x=0.7-0.8, and then decreased. An X-ray diffraction analysis of samples heat-treated at 873 K for 2 h revealed that they have a single-phase perovskite structure for all compositions, with their unit cell volume increasing with Fe content (x). The obtained oxides were nanoparticles with diameters of 10 to 20 nm, which increased with Fe content (x). The amorphous citrate route was found to give excellent starting gel precursors for the low-temperature synthesis of La0.8Sr0.2Co1-xFexO3 nanoparticles.
Oxidation behavior of spark plasma sintered (SPS) magnesium aluminum oxynitride (MgAlON) with different composition was investigated in present work. Results showed that the variation in composition had not obvious effect on the density of MgAlON, but had some effect on excess weight change. Also found that the oxidation behavior of MgAlON was different with oxidation temperature, which was attributed to the transformation from γ-Al2O3 to α-Al2O3. Moreover, the excess weight change was ascribed to the existence of lots of vacancies in γ-Al2O3 and magnesium aluminate spinel (MgAl2O4ss).
Indium nitride (InN) films were grown on the c-face of zinc oxide (ZnO) single crystals by molecular beam epitaxy, and their interfacial structure and crystalline polarity were examined. The c-parameter and the crystallinity of the films strongly depended on the substrate polarity and growth temperature. In particular, the low-temperature-grown films on the oxygen terminated c(−)-face of ZnO (c(−)-ZnO) were single crystalline films with a shorter c-axis than that of bulk InN, while the films grown on the zinc terminated c(+)-face of ZnO (c(+)-ZnO) and the high-temperature-grown films had low crystallinity. Coaxial impact-collision ion scattering spectroscopy (CAICISS) revealed that the low-temperature-grown films on c(−)-ZnO had (0001) In-faces, namely c(+)-polarity (c(+)-InN), and TEM observations suggested the existence of an interfacial layer a few atomic thick in the c(+)-InN/c(−)-ZnO heterostructure. These results on the lattice restriction and the polarity of InN films are discussed in terms of the interfacial structure at the InN/ZnO boundary.
Porous silicon oxycarbide (SiOC) ceramics with porosities ranging from 27% to 88% and a cell density higher than 107 cells/cm3 were made from polysiloxane and polymer microbead blends. The polysiloxane and polymer microbeads were compounded directly using a counter-rotated twin-screw extruder with a filamentary die. The obtained specimens were then transformed into porous SiOC ceramics using two different processes: (1) pyrolysis of the extruded blends and (2) foaming of the blends with gaseous carbon dioxide and subsequent pyrolysis. The pyrolysis process resulted in the production of closed-cell SiOC ceramics, while the combined foaming-pyrolysis process resulted in open-cell SiOC ceramics.
Calcium phosphates with metal ions are expected to show interesting characteristics, such as catalytic activity, as well as high ability to adsorb organic substances. We assumed that the organic substances adsorbed on calcium phosphates could be decomposed by treatment with hydrogen peroxide (H2O2) if the catalytic activity for the decomposition of H2O2 can be introduced to calcium phosphates. We synthesized hydroxyapatite (HAp) containing Ni or Co ions, and octacalcium phosphate (OCP) containing Ni, Co or Mn ions. The HAp containing Co ions and the OCP containing Co or Mn ions showed catalytic activity for the decomposition of H2O2.
Improvement of mechanical properties of cordierite glass-ceramics using modified Egyptian basalt rocks by addition of different amounts of zirconia (ZrO2) was investigated. Crystallization behavior of modified basalt glass was studied by X-ray diffraction (XRD) and scanning electron microscope (SEM). The Vickers hardness of glass-ceramics containing different amounts of ZrO2 and formed at 1050°C for various heat-treatment time was compared. Toughness of the glass-ceramics increased with increasing the amount of transformable ZrO2, which process is probably caused by crack deflection due to the high stress field around transformed ZrO2.
Two kinds of materials, sprayed-on crocidolite and sprayed-on amosite, containing crocidolite and amosite respectively, were treated with aqueous acetic acid solution, the pH of which was adjusted with an ammonium acetate buffer at 5, in order to remove soluble components of cement. The liquids were filtrated with a membrane filter, and the residue collected as crocidolite samples and amosite samples, respectively. The Crocidolite and amosite thus obtained were heated up to 600-1300°C for 1h. Then, power X-ray diffraction (XRD) experiment, scanning electron microscopic (SEM) observation, and thermal analysis (TG/DTA) were carried out for these burned specimens in order to observe the change of the burned materials and melting behaviors together with their thermal properties. In addition, CaCO3 and CaCl2 were mixed with the respective sprayed-on asbestos and sprayed-on crocidolite, and a TG/DTA measurement was conducted on these mixtures. Based on the SEM observation and XRD experiment on the specimens used in the TG/DTA measurements, we tried to decompose the crocidolite and amosite, applying the method of low-temperature decomposition, the applicability of which was previously confirmed in the study on the case of chrysolite. The temperature of the TG/DTA measurement could be raised up to 1000°C, and it became evident that in the cases of specimens where CaCl2 was added, all the asbestos fibers had decomposed, but not in any other specimen. The crocidolite specimen became rounded in shape when it was heated up to 1000°C, and it looked as if it was densified due to burning. CaCO3 and CaCl2 were added to this burned crocidolite, and decomposition of the material after burning was examined. In a DTA thermogram, an endothermic peak was recognized, which corresponds to the formation of a melt of CaCO3-CaO-CaCl2 as summarized in the previous report. Thus it is experimentally verified that burned crocidolite decomposes at high temperatures.
Rapid microwave drying was examined on wet green bodies using nano-sized powders prepared by slip casting and tape casting and was compared with conventional drying techniques. The warpage of dried body is smaller in microwave drying than other conventional drying methods. Clearly, the homogeneous heating of microwave contributed to the uniformity of temperature and water contents in the green body during the drying period. The microwave heating has merits in both rapid drying and uniform structure of dried bodies. It not only reduces the drying periods, but also improves the characteristics of green body. From these results, we believe that microwave drying is useful technique for next generation ceramics production represented by nanotechnology.
Small amounts of inorganic salts like sodium carbonate and calcium acetate were used as impregnants to pulverized limestone, which was then calcined in a powder-particle fluidized bed (PPFB) in nitrogen atmosphere at ambient pressure. Impregnation of 6-μm limestone with 1 mass% sodium carbonate and 1 mass% calcium acetate lowered the decomposition temperature by 30 K, making a 10% increase of calcination conversion at 1073 K. Additive impregnation involving calcium acetate produced macropores of larger than 50 nm in the reactant powders. Calcination with additive impregnation at 1073 K broadened the size of pores of the calcines to larger ones, newly producing 10-100 nm pores. These changes enhanced the ultimate moisture absorptivity by up to 20% as compared to the case without additive impregnation. However, calcination at 1173 K improved only the initial absorption rate.
Solidified materials were prepared by hydrothermal treatment of metakaolinite derived from the thermal dehydroxylation of kaolinite. Hydrothermal solidification was performed under saturated steam pressure at 180°C using powder-compacts consisting of metakaolinite, quartz and slaked lime. The hydrogarnet crystals formed newly after hydrothermal treatment. The bulk density and the bending strength of the hydrothermally solidified materials increased when the powder-compacts was prepared at a higher forming pressure. The mesopore volume and specific surface area of the hydrothermally solidified materials could be controlled by the forming pressure. It was clarified that the increase in the bulk density and the newly formation of fine-sized deposits seem to generate the strength of the hydrothermally solidified materials. And these caused the formation of mesopores.
LiFePO4 has been synthesized using FeC2O4•2H2O, LiOH•H2O and NH4H2PO4 as raw materials by a temperature controllable microwave heating oven. The products were characterized by X-ray diffraction, scanning electron microscope and electrochemical methods. Measurement of dielectric properties of individual raw materials and products at a constant microwave frequency revealed that the starting mixture preheated at 320°C as well as added acetylene black were well microwave absorbers. Single phase olivin-type LiFePO4 with uniform and fine particle sizes was successfully synthesized by microwave heating at 350°C in 15 min. The LiFePO4 cathode materials thus obtained had a higher discharge capacity and better cycle performance than those of the LiFePO4 sample by a conventional solid-state reaction.
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