The influence of talc addition on the pyroplastic deformation of alumina-strengthened porcelain was investigated in this study. Along with the lower content of feldspar, the addition of talc contributed to the crystallization of the cordierite during the firing process, and the resulting cordierite crystals formed complicated filler structures. The porcelain samples made with the addition of talc showed an unprecedented pyroplastic deformation characteristic with almost no changes in the pyroplastic deformation index value during further firing at over 100°C after the water absorption reached almost zero. This improvement of pyroplastic deformation was thought to synchronize with the crystallization of cordierite.
With rapid progress of high performance computers and algorithm, we are now capable to obtain physical properties and thermodynamical quantities of substances through first principles calculations with the accuracy comparable to experiments. Here, the author makes a short review of our works related to the use of first principles calculations for the study of engineering ceramics. It includes 1) chemical bonding of SiAlONs and solute arrangements, 2) phonon calculations and related properties, 3) theoretical lattice thermal conductivity, 4) cluster-expansion method to study solid solution and order/disorder phenomena, and 5) materials informatics approach for discovery of low thermal conductivity materials.
In this paper, the crystallization of the glass with Y2Si2O7-mullite eutectic composition was examined. A dendrite Y2Si2O7 crystalline grows from the surface of the glass bulk at 1302 K. A competitive growth of Y2Si2O7 phase and mullite phase occurs at 1430 K. The activation energy for the competitive growth can be estimated to 329 kJ/mol. The activation energy for the crystallization of hypoeutectic and hypereutectic composition increases with increasing the amount of the primary phase.
Microstructural evolution of Si3N4 ceramics and thermal/mechanical properties were investigated with respect to the initial α-to-β ratios of the starting powders. The fast growth of elongated β-grains was observed in the α powder, while the growth of relatively equiaxed grains occurred in the β powder. In the cases where α/β mixed raw powder was used, exaggerated bimodal microstructural evolution occurred as a result of the abnormal grain growth. These microstructural variations of the Si3N4 had a significant influence on the mechanical and thermal properties, such as strength, fracture toughness and thermal conductivity. The bimodal microstructure, which consisted of large elongated grains surrounded by fine matrix grains, resulted in both high flexural strength and high fracture toughness. Controlling the microstructural evolution, resulted in a high thermal conductivity of 86 W/mK, a high flexural strength of 912 MPa, and a high fracture toughness of 7.88 MPa.m1/2 from α/β mixed raw powder. The achievement of both high thermal and mechanical properties of Si3N4 ceramics was highly essential for thermal management applications.
Improvement of thermal and mechanical properties of carbon fiber reinforced thermoplastic (CFRTP) was studied by incorporating exfoliated hexagonal boron nitride (h–BN) with high aspect ratio into matrix resin composed of polypropylene (PP). The thermal diffusion and propagation of model CFRTP composite composed of a single carbon fiber and matrix was investigated by microwave (MW) irradiation. As result, the thermal propagation of composite with 2.5 vol.% exfoliated h–BN was even similar to that of composite with 5.0 vol.% raw h–BN. Furthermore, the thermal degradation of the composite was suppressed by the exfoliated h–BN at low filler content. Moreover, three–point bending test showed that the specific strength and the specific rigidity of CFRTP composite composed of 2.5 vol.% exfoliated h–BN/PP were increased by 22 and 37%, respectively, as compared to matrix with only PP. The mechanical property of CFRTP with 2.5 vol.% exfoliated h–BN/PP was higher than that of CFRP with 5.0 vol.% raw h–BN/PP. Furthermore, the number of failure cycles of CFRTP composite composed of exfoliated h–BN was increased by one orders as compared to raw h–BN one. Thus, the addition of exfoliated h–BN into matrix resin had a significant influence on the thermal and mechanical properties of CFRTP.
The electrical conductivities of Sr2CoRO6 (R=Mo, Nb) under different oxygen partial pressures and their crystal structures were investigated. Their conducting mechanisms were proposed. X-ray photoelectron spectra revealed that the dominated oxidation state of Co2+ was in Sr2CoMoO6 but Co3+ in Sr2CoNbO6. The crystal lattice structures of Sr2CoRO6 were identified by X-ray diffraction and their parameters were calculated by using Rietveld method. The refined crystal structures of Sr2CoRO6 have been used to construct their own conducting channel model. And the electrical conductivities were measured by using an electrochemical workstation with four-terminals method through controlling the oxygen partial pressure of the testing environment. The results show that the electrical conductivity of Sr2CoMoO6 is 0.36 S·cm−1, much lower than that of Sr2CoNbO6 (7.81 S·cm−1) in air at 973 K, which is assigned to the difference of their conducting channels. And the electrical conductivity of Sr2CoMoO6 increased linearly with the decrease in PO2, reaching 2.05 S·cm−1 when PO2 = 10−21 atm at 973 K, while Sr2CoNbO6 presented an opposite trend. It was supposed that the different valences of cobalt in Sr2CoMoO6 and Sr2CoNbO6 lead to the different electron conducting channels and different changing trends of electrical conductivities with the decrease in oxygen partial pressure.
High-temperature electrochemical properties of Cs-substituted CaWO4 and BaWO4 have been investigated. When cesium ions are partly substituted for calcium site of Scheelite-type structured CaWO4 to form oxide ion vacancies as Ca1−xCsxWO4−x/2, oxide ion conduction appears at elevated temperatures. This defect structure for the oxide ion conduction is in contrast to the PbWO4-based oxide ion conductor with oxide ion interstitials, Pb1−xLaxWO4+x/2. The measured electric conductivity of Ca1−xCsxWO4−x/2 (x = 0.15) is high as 10−2 Scm−1 at 750°C with the oxide ion transport number relatively close to unity. Essentially the similar results are obtained for the barium analogous system Ba1−xCsxWO4−x/2, although the conductivity is still lower than CaWO4-based system.
Ca1−x/2Sr1−x/2SiO4:xEu2+ (x = 0.1–0.9) phosphors were synthesized using propylene glycol-modified silane (PGMS), and their luminescence properties were studied as a function of Eu concentration. The Ca1−x/2Sr1−x/2SiO4:xEu2+ phosphors showed strong orange-red or red emissions. The wavelength at the maximum emission intensity shifted from 610 to 639 nm as Eu concentration increased from x = 0.1 to x = 0.7. The origin of the red emission was discussed in terms of Eu2+ substitution at smaller Ca (n) sites.
B-type carbonate apatite (CO3Ap) has recently emerged as an attractive bone scaffold material because it exhibits significantly higher osteoconductivity than its hydroxyapatite analogue. Highly soluble acidic calcium phosphate salts, such as calcium hydrogen phosphate dehydrate (DCPD) and monocalcium phosphate monophosphate (MCPM), are typically used as bone cement and easily produce large calcium phosphate crystals. However, few studies have focused on these salts as precursors despite extensive investigations on the production of CO3Ap. In this study, DCPD and MCPM were converted into CO3Ap by hydrothermal treatment in the presence of at least 0.5 mol/L NaHCO3, which acted as a neutralizer and CO32− releaser. The obtained material maintained precursor morphologies and was therefore regarded as a pseudomorph of DCPD and MCPM mesocrystal structures. This synthetic approach is expected to facilitate the fabrication of shape-controlled CO3Ap compacts from DCPD and MCPM.
In this paper, mineral and sintering properties of high alumina-containing clay (Yamase clay) derived from a weathered granite stone were investigated, and compared with those of low alumina-containing clay derived from a weathered sandstone (Hobashira clay) for the ancient Karatsu ware. Content of Al2O3 in Yamase clay was 30.8–33.1 mass %, and showed higher than that of Hobashira clay (13.1–15.8 mass %). Yamase and Hobashira clay had a mineral composition of kaolinite (64.3), α-quartz (10.6), muscovite (11.9), albite (10.3), and microcline (2.9 mass %), and kaolinite (12.9), α-quartz (52.9), muscovite (23.4), albite (6.9), and microcline (3.9 mass %), respectively, by the Rietveld analysis. Bulk densities of Yamase clay heated at 1200, 1300, and 1400°C were 1.99, 2.15, and 2.35 g/cm3, and Yamase clay had a higher refractoriness than Hobashira clay. After heating Yamase clay at 1300–1400°C, the heated body was composed of fine needle-like mullite (49.8) and glass (50.2 mass %) without the bloating of the body.
Bulk polycrystalline MgTi2O5 has low coefficients of thermal expansion (CTE), high thermal shock resistance and high temperature stability. However, polycrystalline MgTi2O5 generally includes extensive internal microcracks and hence it has poor mechanical properties. In this study, dense MgTi2O5 samples with fewer microcracks were prepared by spark plasma sintering (SPS) of commercially available MgCO3 (basic) and TiO2 (anatase) powders at 1000–1200°C for 20 min under 20–80 MPa in vacuum. The samples sintered above 1100°C were composed of MgTi2O5 phase with trace intermediate MgTiO3. High relative density and flexural strength, 99.9% and 341.5 MPa, were obtained for the samples sintered at 1200°C under 80 MPa. This strength value is, to the best of our knowledge, the highest value among pseudobrookite-type ceramics. The bulk CTE values of the samples were almost identical for all samples, ∼10 × 10−6 K−1, which confirms the fewness of microcracks.
Gallium nitride (GaN) powder was prepared from β-gallium oxide (β-Ga2O3) powder using polymeric carbon nitride (PCN) as a nitridation reagent and characterized by powder X-ray diffraction, 71Ga magic-angle spinning nuclear magnetic resonance spectroscopy, Raman spectroscopy, and scanning electron microscopy. β-Ga2O3 was completely nitridated to GaN at the temperature above 800°C. The change in the morphology after the nitridation can be explained by two successive reactions, i.e., the reduction of Ga2O3 to gaseous Ga2O and its subsequent nitridation to GaN, with the rate of the former reaction being slower than that of the latter.