In-situ observations using high-temperature transmission electron microscopy (TEM) is a promising technique for obtaining new findings and developing phenomenological theory for ceramic materials at high temperatures. By observing the changes in hydrothermal BaTiO₃ due to heating, we clarify the vanishing mechanism of internal pores and the mechanism of BaCO₃ phase generation/vanishing at high temperatures. Through observations of the grain growth of BaTiO₃ prepared by radio-frequency plasma chemical vapor deposition, we calculate the grain-boundary diffusion coefficient, which is an important parameter controlling the sinterability of ceramics. Observations of the shrinkage and disappearance of Ba₅Nb₄O₁₅, as well as simulations, indicate that high energy and low diffusivity at the grain boundary make grains vanish while maintaining a truncated spherical shape. Finally, observations of the structural changes in layered perovskite BaLn₂Mn₂O₇ (Ln = Gd, Pr, Eu) reveal a first-order phase transition that has not been previously reported.

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Grain boundaries, interfaces and surfaces are very attractive areas from the viewpoint of lattice discontinuity. Doped elements and point defects often segregate there, which largely changes bulk properties such as microstructure, mechanical strength, electrical conductivity. To understand the details of the dopant effects, it is necessary to use nano-scale analysis techniques based on transmission electron microscopy with energy dispersive X-ray spectroscopy and electron energy-loss spectroscopy with nano/sub-nano electron probe. This review contains some nano-scale analysis studies performed in our research group for the grain size control of BaTiO₃ polycrystals, electrical properties across single grain boundaries of n-type BaTiO₃, SrTiO₃ and ZnO, interface structural change of WC–Co based cemented carbides and the preparation of novel dislocation nano-wires.
Grain boundaries in BaTiO₃ sinters were revealed to exhibit grain boundary faceting by doping a very small amount of excess TiO₂. The formation of the facets due to extra Ti–O₂ bonding is closely related to abnormal grain growth, which is often observed in TiO₂-excess BaTiO₃ sinters. A similar faceting feature is also observed in VC-doped WC–Co cemented carbides. Doped VC strongly segregates to WC/Co interfaces to form micro facets. The formation of the micro facets resulted in WC grain size reduction. In ZnO boundaries, doped Pr was revealed to segregate at the grain boundary. The segregated Pr ions are situated in specific atomic columns. Further, electron energy-loss spectroscopy has revealed that the valence of the segregated Pr ions is 3+, which means that they are not acceptors but donors to ZnO. The improvement of varistic effect observed in Pr-doped ZnO is closely related to Zn vacancies enhanced by Pr doping. Meanwhile, dislocations are a kind of one-dimensional lattice defect including extra half planes. By pipe-diffusion of dopants at dislocation cores, we can successfully develop conducting nano-wires in insulating sapphire crystals.

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A brief overview of siloxane-based low-density aerogels and aerogels-like xerogels is presented. Aerogels are highly porous solids composed of inorganic oxides, metals, cross-linked polymers and carbons, and are known to possess a number of excellent physical properties such as high visible-light transparency with low refractive index, low dielectric properties, and extremely-low thermal conductivity. Aerogels are therefore regarded as a promising candidate for applications such as superinsulators; however, a mass production and applications of aerogels have been significantly discouraged due to the lack of mechanical properties since the first invention in 1931. This review introduces the substantial effort to improve the mechanical properties of aerogels with particularly highlighting our recent findings on elastic organic–inorganic hybrid aerogel monoliths obtained from methyltrimethoxysilane (MTMS) using the controlled sol–gel chemistry.

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Nano-sized Ag–Pd alloy and Ag–Pd–glass composite powders were prepared by flame spray pyrolysis. The Ag–Pd molar ratio was fixed at 70–30. The mean sizes of the Ag–Pd and Ag–Pd–glass powders were 29 and 25 nm, respectively. The Ag–Pd alloy powder began to be oxidized rapidly at approximately 365°C and oxidation of alloy powders was finished at 420°C. On the other hand, oxidation of the Ag–Pd–glass composite powders gradually occurred around 400°C and oxidation of the powders was finished at 530°C. The specific resistances of the conducting films formed from the nano-sized Ag–Pd–glass composite powders were 76, 59, 35 µΩ·cm at firing temperatures of 550, 600 and 800°C, respectively.

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The imaginary component, ξ_D31β of the nonlinear piezoelectric coefficient of the third-higher term, ξ_D31, which quantitatively expresses the magnitude of the nonlinearity, was measured in Pb(Mn_1/3Sb_2/3)O₃–PbZrO₃–PbTiO₃ (PMS–PZ–PT) system ceramics. The ξ_D31β was determined from the phase delay of the third-higher harmonic voltage generated in the samples during constant-current driving. The dependence of ξ_D31β on the composition was similar to that of ξ_D31. Using the ξ_D31β, two theoretical equations for the current-jumping phenomenon and the change of mechanical quality factor in high-power driving are constructed. The calculation curves simulated by those theoretical equations agreed with the experimental values measured about the two phenomena. It is revealed that the nonlinear loss component during high-power vibration is quantitatively estimated by introducing the ξ_D31β.

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We have developed novel bioresorbable β-tricalcium phosphate (β-TCP) cements on the basis of chelate-setting mechanism of inositol phosphate (IP6). The starting cement powders (IP6/β-TCP powders) were prepared by surface-modifying β-TCP particles with IP6. The cement specimen was fabricated by mixing the IP6/β-TCP powder in pure water at desired powder/liquid ratios, and examined the effects of powder properties of the IP6/β-TCP powder on the mechanical strength (compressive strength) of the cement specimens. We focused on the crystalline phase, particle size, specific surface area (SSA), and crystallite size among powder properties. The crystalline phase of resulting cement specimen was β-TCP single phase or mixture of β-TCP and calcium-deficient apatite (CDAp). The β-TCP cement with compressive strength of 13 MPa was fabricated from the finely-ground β-TCP powders prepared by ball-milling commercially-available β-TCP powder for 4 h using zirconia beads with 10 mm in diameter. Meanwhile, the β-TCP/CDAp biphasic cement had maximum compressive strength of about 23 MPa among the examined cement specimens, it was fabricated from the ball-milled commercially-available β-TCP powder for 3 h using zirconia beads with 10 mm in diameter, and then for 3 h using zirconia beads with 2 mm in diameter. In order to make the determining factors of the compressive strength clear, we examined the relationship between powder properties (particle size, SSA and crystallite size) and compressive strength. The strength of the IP6/β-TCP cement was not dependent on the particle size of the IP6/β-TCP powder; meanwhile, the strength was enhanced with increasing SSA and decreasing crystallite size. Thus, the IP6/β-TCP powder with higher SSA and smaller crystallite size may be useful in the fabrication of chelate-setting β-TCP cement with enhanced mechanical properties.

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The effects of nickel oxide (Ni₂O₃) additives on electrical characteristics and microstructural phases of ZnO varistors with low residual voltage ratio are studied. The breakdown electrical fields increase remarkably with the amount of the doped nickel oxide increased. The minimum of the residual voltage ratios of the samples reaches 1.39. While the amount of the doped nickel oxide is more than 2 mol %, a new phase of chrombismite Bi₁₆CrO₂₇ is observed from X-ray diffraction (XRD) and scanning electron microscopy (SEM). The δ-Bi₂O₃ phase decreases and the BiO_2−x phase increases correspondingly. In addition, the capacitance versus voltage (C–V) characteristics of the samples are measured and analyzed.

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Polycarbosilane (PCS) or silicon carbide (SiC) fillers were used as fillers in fabricating partially interconnected, open-cell porous SiC ceramics by carbothermal reduction of polysiloxane-derived SiOC and subsequent sintering process. The effects of filler type (PCS or SiC), filler content (0–50 mass %) and sintering temperature (1800–1950°C) on microstructure, porosity, and strength of the polysiloxane-derived porous SiC ceramics were investigated. The spherical pore morphology was retained with SiC filler addition, while irregular cells were dominant with PCS filler addition after sintering at low temperatures. The pore morphology became mostly irregular while the grain morphology changed from a mixture of small, equi-axed grains and large faceted grains to the large faceted grains with increase in sintering temperature from 1800 to 1950°C. Porosity decreased and strength increased with increase in sintering temperature irrespective of the filler source and content. PCS filler addition continuously increased the porosity and decreased the strength, whereas the porosity decreased and strength increased beyond 30 mass % SiC filler addition in the starting composition. Less strength was recorded for the ceramics prepared with PCS fillers. It was possible to adjust the porosity from 34 to 59% and flexural strength from 16 to 96 MPa by controlling the filler type, filler content, and sintering temperature.

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Pure ZnS and CdS were synthesized under hydrothermal conditions. The reactions of Zn or ZnO with S at 240°C yielded pure ZnS. Similar reactions at lower temperatures gave plate crystals of an unknown phase along with ZnS and ZnO. Pure CdS was obtained by the reaction between CdO and S at 240°C while the reaction of metal Cd and S gave a mixture. The as-prepared CdS crystals had rod shapes.

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Oriented PbTiO₃ ceramics were obtained by slip casting under a high magnetic field, and the orientation factor was 90%. Such highly oriented perovskite ceramics by slip casting under a high magnetic field have never been reported as far as we know. It was thought that dispersibility and crystallinity of the powder were important for orientation by slip casting under a high magnetic field. In this study, we controlled the dispersibility and crystallinity of powder by calcination temperature. As a result, the sample obtained from the powder calcined at 1100°C was oriented, but the one obtained from the powder calcined at 900°C was not. The powder calcined at 900°C is more aggregated than that at 1100°C and the crystallinity of the powder calcined at 1100°C is higher than that of the powder calcined at 900°C. It became clear that highly oriented PbTiO₃ ceramics can be fabricated from the powder with high dispersibility and high crystallinity by slip casting under a high magnetic field.

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We present a procedure for fabricating transparent silica glass that involves the sintering of green bodies prepared from an inorganic–organic nanocomposite. The nanocomposite was prepared from fumed silica and poly(vinyl alcohol) (PVA). We investigated the SiO₂–PVA nanocomposite by Fourier transform infrared spectrometry (FT-IR) measurement, transmission electron microscopy and electron energy-loss spectroscopy, and the results revealed homogeneously dispersed SiO₂ nanoparticles and PVA. We examined the relationship between the pH and ζ potential of the SiO₂ suspension and the formability of the SiO₂–PVA nanocomposite. Formability of the SiO₂–PVA nanocomposite was dependent on the pH of the SiO₂ suspension, and a monolithic SiO₂–PVA nanocomposite without cracks was obtained using a SiO₂ suspension at around the isoelectric point (pI). By sintering the SiO₂–PVA nanocomposite in air at 1100°C, monolithic transparent silica glass was obtained with no cracks. The glass is highly transparent from the ultraviolet to the visible region.

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Electrically conductive carbon nanotube (CNT)-dispersed ceramics with high strength were fabricated by varying the CNT quantity and the firing temperature. We investigated the effect of these factors on density, electrical conductivity, bending strength, and microstructure of the developed ceramics. The relative density of the CNT dispersed Si₃N₄ ceramics was higher than 90% except for the sample containing less than 1 wt % of CNTs and fired at a temperature of 1800°C. It was confirmed that CNTs exist in the samples with a higher density. The pullout length of CNTs on the fracture surface in samples fired at higher temperatures was shorter because of the degradation of the CNTs. By TEM observation, CNTs were found to exist in the grain boundary in the Si₃N₄ ceramics, and their diameter was found to be almost the same as that of raw CNTs. Electrical conductivity appeared in the samples by adding CNTs more than 1 wt % though the sample by adding 0.5 wt % CNTs was insulator. The electrical conductivities of the samples increased with an increase in the firing temperature, which was explained by the grain growth of β-Si₃N₄. The bending strength of the samples with 1 wt % of CNTs was as high as that of samples without CNTs. Thus, CNT-dispersed Si₃N₄ ceramics having both electrical conductivity and a higher bending strength were obtained by controlling the CNT quantity and the firing temperature.

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We investigated the fabrication conditions of a solid electrolyte Ta₂O₅ thin film suitable for an electrochromic (EC) all-solid-state switchable mirror glass to reduce material and processing costs. The film was deposited by reactive dc magnetron sputtering in a mixture gas of argon and oxygen. The electrochemical properties of the film on a WO₃/ITO/glass substrate and the optical switching properties of the EC switchable mirror device were investigated, considering the thickness of the Ta₂O₅ thin film. All the films on WO₃/ITO/glass substrates in this work exhibited similar current density, as measured by cyclic voltammetry. The electrochemical properties of the film with thickness reduced by 75% were almost the same as those of the conventional film. However, the device constructed with the thinner film displayed poor optical switching properties. Switching from the reflective state to the transparent state upon application of a voltage, the device with a 100-nm-thick film required a switching time that was 3 times longer than that of the conventional device constructed with a 400-nm-thick film. These results indicate that suitable fabrication conditions for the solid electrolyte Ta₂O₅ thin film of EC all-solid-state switchable mirror glass can potentially be found that reduce consumption of materials and shorten processing time.

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A new method of synthesis of single phase ZrP₂O₇ was studied using hydrothermal reaction of amorphous ZrO(OH)₂ with H₃PO₄ and subsequent air calcination. The reaction steps to ZrP₂O₇ were analyzed by means of XRD, TG/DTA and FT-IR. The hydrothermal reaction at 200°C for 18 h yielded well-crystallized tetraphosphate, Zr₃(PO₄)₄, loaded with unreacted H₃PO₄. Upon heating above 500°C, the mixture caused the stoichiometric reaction to form ZrP₂O₇, but impurities such as ZrO₂ and Zr₂P₂O₉ were not formed. TEM observation confirmed that ZrP₂O₇ after calcination at 900°C consisted of crystallites in size of several ten nanometers.

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La_1.8Sr_1.2Mn₂O₇, La_0.5Sr_1.5MnO₄ and La_0.8Sr_0.2MnO₃ were prepared and their particulate matter oxidation was investigated by thermogravimetry method. Of the three perovskite oxides, La_1.8Sr_1.2Mn₂O₇ gave a higher oxidation activity than the other two oxides at a practical level of temperature. The properties of these oxides were also investigated by X-ray photoelectron spectroscopy. The Mn 2p core-level spectra of the La_1.8Sr_1.2Mn₂O₇ only shifted towards low energies. These suggested that an electron transfer from Mn–O planes to adsorbed oxygen in the La_1.8Sr_1.2Mn₂O₇ was effective for improving the particulate matter oxidation activity.

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