Preceramic polymer routes, which produce ceramic products called polymer-derived ceramics (PDCs) from molecular precursors, have been developed as a powerful tool to synthesize reduced ceramics in different morphologies (fibers, coatings and monoliths) for over half a century. In recent years, pore control of porous ceramic monoliths has attracted attention for improving their functionalities, which has driven the preceramic polymer routes to be integrated with other synthetic techniques for porous materials such as templating, foaming, emulsion and phase separation. This article briefly overviews porous PDC monoliths with a special emphasis on those derived from porous preceramic polymer gels which are prepared via the sol–gel process accompanied by spinodal decomposition. Ceramization behaviors of Si- and Ti-based preceramic polymers are highlighted in terms of crystal transition and variation of pore properties in different length scales.
Heat- and corrosion-resistant catalytic materials are essential in the field of environmental protection and energy production. In this article, recent progress in material research in this field is reviewed based on publications from the author’s research group. In an automotive three-way catalyst (TWC), thermal deactivation by sintering of platinum group metal (PGM) nanoparticles can be suppressed by controlling the interfacial bonding to the surface of the support, which provides an anchoring effect. A similar concept is useful in solar thermochemical cycles to produce clean fuels, which are conducted in a high-temperature and corrosive environment containing sulfuric acid vapor. A further challenge in both applications is the replacement of PGM catalysts by economically viable catalysts. Thermally stable multicomponent transition metal oxides were proposed as a possible candidate for PGM-free TWC. A positive synergy between the different functionalities of metal elements results in high catalytic performance. Molten phases of metal vanadates, which are used for solar thermochemical cycles of sulfur, are another example of PGM-free catalysts. These examples highlight the critical roles of each metal element and their combination for obtaining synergy, which are required to further understand the ways to simultaneously achieve catalytic activity and thermal/corrosion stability.
Although Pb2+ substitution for Bi3+ increases the hole concentration, p, in Pb-substituted Bi-based superconductors, the Pb substitution effect has yet to be studied in detail. Herein the Pb substitution effect on the electronic properties of Bi2201, 2212, and 2223 superconductors is investigated to elucidate the role of Pb on the correlation between p and formal valence, v. Consistent with Mott-Hubbard theory, p increases as v increases in the Pb-free Bi2201 and Pb-free Bi2212 phases. In contrast, p decreases with increasing v for the Pb-substituted Bi2201 phase, which contradicts Mott-Hubbard theory but can be explained by the increase in Pb4+. For the Pb-substituted Bi2212 phase, p increases with increasing v from 2.0 to 2.2, and then remains constant at p = 0.10–0.20 from v 2.2 to 2.63. When v increases from 2.0 to 2.2, the hole is doped at a Cu site, which is the same behavior as that of the Pb-free Bi2212 phase. As v increases from 2.2 to 2.63, the hole is doped in a Pb site and not a Cu one. For the Pb-substituted Bi2223 phase, p decreases with increasing v, and a mixed-valence state between Pb2+ and Pb4+ may coexist. As the v increases, the Pb valence increases and the Cu valence decreases. These results indicate two factors influence p. One is correlated with the substitution of Sr2+ for La3+ or Y3+ for Ca2+, which obeys Mott-Hubbard theory. The other is correlated with the substitution of Pb2+/4+ for Bi3+, which does not obey Mott-Hubbard theory. The results strongly suggest that Pb substitution for Bi does not necessarily dope a hole but decreases the hole concentration.
The magnetic and electronic properties of CeOInS2 and their influence on phase transition were analyzed in this study. High-temperature XRD measurements of CeOInS2 revealed that orthorhombic CeOInS2 transformed into tetragonal CeOInS2 at a high temperature of 636 K. The transport properties of CeOInS2 showed semiconducting behavior, with a larger temperature dependence of electronic resistivity in the tetragonal phase compared to that in the orthorhombic phase. Unlike structurally similar Ce(O,F)BiS2 superconductors that show long-range magnetic ordering, CeOInS2 neither exhibited superconductive transition nor long-range magnetic ordering at temperatures between 2 and 300 K.
The electrical conductivity of porous and high-purity β-Ga2O3 ceramics was measured as a function of oxygen partial pressure, po2, at 700 to 900 °C. In the high po2 range, the conductivity was proportional to about −1/4th power of po2, while in the lower po2 range less than about 10−5 atm its exponent was −0.1 to −0.13. This suggests that different types of defects were formed at high and low po2. The point defect of doubly ionized interstitial gallium ion causes the exponent of −1/4, the same value of the experiment in high po2. When po2 was changed, the electrical conductivity first changed sharply, followed by a slow change, suggesting some migrations and/or formation of crystalline defects, and so on. In the Ga2O3 samples, many dislocations, with density of about 1012 cm−2, were observed. As the source of carriers, line defects, such as dislocations, should be considered as well as point defects.
The dissolution behavior of calcium aluminosilicate (CAS) glasses with various CaO and Al2O3 concentrations was investigated at 180 °C in NaOH aqueous solutions with an initial pH of 13.2–14.0. A CaO–Na2O–Al2O3–SiO2–H2O (C–N–A–S–H) layer was formed on the immersed glass surface due to the dissolution and penetration of the glass and solution constituents. An excellent alkali durability was achieved by the minimization of Al2O3 addition and a [CaO]/[SiO2] molar ratio of 1.10–1.20 for the glass composition design. The results of the infrared spectroscopy indicated that the C–N–A–S–H layer contained H2O and CO32− species from the ambient atmosphere and consisted of the network structure of SiO4–AlO4 tetrahedra. The hydrothermal reactions between the CAS glasses and NaOH solution promoted the formation of the C–N–A–S–H layer and precipitation of Ca(OH)2 with an immersion time. Stable Al-substituted tobermorite and hillebrandite phases were finally formed on the immersed glass surfaces due to the hydrothermal reactions.
Prestress in alumina-strengthened porcelain, caused by the mismatch in thermal shrinkage between the porcelain matrix and alumina particles during porcelain cooling, is estimated using Raman spectroscopy by monitoring the frequency shift of Raman band at 417 cm−1, which is assigned to the A1g vibrational mode of α-alumina. This spectroscopic estimation supports the validity of prestress which is expected to accumulate during porcelain cooling beginning at ∼850 °C.