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Li2CoTi3O8 pigment is a cobalt-based blue pigment and is highly safe as it is actually used for cosmetics such as an eye shadow and a lipstick. Despite its excellent features, available reports on Li2CoTi3O8 blue pigment are quite limited. In this study, we demonstrate that the colors of CoO-lean compositions, cyanic blue, are substantially same as that of stoichiometric Li2CoTi3O8, which contributes further reduction of Co resources. In addition, we also demonstrate that green color pigments can be synthesized just by increasing the CoO compositions. Li2CO3, CoO and TiO2 (anatase) powders were used as starting materials. The mixed powders were heat-treated at 1100 °C in air for 2 h to obtain Li2CoTi3O8 powders, and they were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and CIE1976-L*a *b* color coordinates measurement. Quantitative color measurement revealed that 10% reduction of CoO from stoichiometric Li2CoTi3O8 is possible without much affecting the human eye feeling. Green colors appeared for the CoO-rich compositions, and the dichroism of the Li2CoTi3O8 pigment was attributable to the valence change of Co ions from divalent (cyanic) to trivalent (greenish).

Co3O4-based materials attract a lot of attention particularly for electrochemical applications. Here, we report a unique Co3O4 hierarchical micro- and nanostructures (HMNS), viz., micrometer-sized mesoporous granules assembled with Co3O4 facetted nanocrystals. CoCl2·6H2O and urea (Co: urea = 2:15 in mol fraction) were dissolved in a water/ethanol mixed solution and hydrothermally heated at 160 °C for 0.5 h, 4 h and 6 h. The three precursors were calcined at 350 °C for 3 h in air atmosphere. The product via hydrothermal treatment for 4 h was composed of 'soft' cube-like granules, which actually was the assembly of well-facetted Co3O4 microcrystals with pore-size distribution of ∼10–100 nm. The other product via hydrothermal treatment for 6 h was composed of 'hard' cube-like granules, which actually was the assembly of fine and equiaxed Co3O4 nanocrystals with narrow pore-size of distribution of ∼10 nm. Macro- and mesoporous Co3O4 powders with controlled pore-size distribution were successfully synthesized via facile hydrothermal and post-thermal treatments. The 'soft' cube-like Co3O4 granules (Co3O4 HMNS) exhibited much higher electrochemical performance than a commercial Co3O4 powder with the particle size of ∼1–2 μm.

The synthesis of pure brookite is generally much more difficult than that of pure anatase. The hydrothermal conversion, recently developed by Kozawa et al., is a facile method to synthesize brookite TiO2 from inverse spinel-type Mg2TiO4 under a mild hydrothermal condition e.g. in 1 M HCl solution at 100 °C. However, slight rutile TiO2 is usually co-existed under the reported conditions. The aim of this study is to prepare a high-purity brookite TiO2 powder by the hydrothermal conversion from Mg2TiO4. We investigated the conditions of Mg2TiO4 preparation and hydrothermal conversion, and it was found that the most important factor for the high-purity brookite synthesis, i.e. decreasing the co-existing rutile TiO2, was to prepare the Mg2TiO4 precursor without MgTiO3. Using an MgO-rich composition and adding a second calcination step for the Mg2TiO4 preparation yielded a high-purity (99.3 wt%) brookite powder with a surface area of 27.7 m2/g. Under the current experimental conditions, addition of a surfactant or an alcohol for the hydrothermal treatment was not apparently effective for the high-purity brookite synthesis.

Since the thermal-expansion anisotropy of pseudobrookite-type MgTi2O5 is not as prominent as isomorphic Al2TiO5, MgTi2O5 is thermally more stable than Al2TiO5. To realize the future structural applications of porous MgTi2O5, influence of impurities on the phase and microstructure stabilities should be studied. Here, impurity effects on phase and microstructure stabilities of reactively-sintered porous MgTi2O5 have been studied. As model impurities, Li2CO3 and amorphous SiO2 have been selected. Li2CO3 or SiO2 powder (0.1–4.0 wt%) was added to MgCO3 (basic) and TiO2 anatase powders with a molar ratio of 1:2. Uniaxially-pressed mixed powders were sintered at 1100 °C in air for 2 h to obtain porous MgTi2O5 bars. Appropriate Li2CO3 and SiO2 doping (~0.5 wt%) promoted uniform grain growth. However, with the excess amount of the impurities (> 2 wt%), the coefficient of thermal expansion of Li2CO3- and SiO2-doped MgTi2O5 ceramics decreased obviously above 900 °C and 950 °C due to the secondary sintering. To keep the fine-grained porous microstructure of Li2CO3- and SiO2-doped samples with preventing the secondary sintering, impurity levels should be preferably limited less than ~1.0 wt%.

Porous CaAl12O19 ceramics attract much attention toward many potential applications, such as high-temperature filters, catalyst supports, and lightweight structural materials at high temperatures. In this study, we have focused on the reactive sintering and microstructure development of homogeneously bulk porous CaAl12O19. Boehmite AlOOH powders with different particle size [0.08 µm (C01) and 0.7 µm (C06)] were used as Al2O3 sources. CaCO3 and boehmite powders were wet-ball milled, and the green compacts were sintered in air at 520 °C, 750 °C, 1100 °C, 1200 °C, 1300 °C, 1400 °C, 1500 °C (3 h) and 1600 °C (3 h), in order to analyze the microstructure development. At 1500 °C (3 h), microstructure became macroscopically homogeneous. The porosities of the C01 and C06 samples sintered at 1500 °C were calculated to be 51% and 56%, respectively. Stacking of the plate-like grains and their card-house structures are observed. Such card-house structures stabilize the open-porous structure even at 1600 °C. Just by changing the size of starting boehmite AlOOH powders, the size of the CaAl12O19 plate-like particles was altered with keeping similar porosity.

Porous MgAl2O4 filters were prepared by reactive sintering of a MgCO3 (basic) powder and 4-type Al2O3 source powders with different particle size and phases (α-Al2O3, fine and coarse boehmite AlOOH, and γ-Al2O3). The mixed powder compacts were reactively sintered in air at 1200, 1400 or 1600 °C for 2 h to obtain MgAl2O4. Single-phase porous MgAl2O4 was successfully obtained by the reactive sintering at 1400 and 1600 °C. From the SEM observation, the initial particle sizes of Al2O3 sources strongly affected the final microstructure of porous MgAl2O4, rather than the phases of Al2O3 sources. The porous MgAl2O4 filter made from fine boehmite source exhibited good performance to remove submicron-sized colloidal particles (simulating bacteria) from a suspension. Possibility of the thermal recovery of porous MgAl2O4 filter was demonstrated.

Volume-shrinkage of a sample in reactive sintering generally tends to be larger than that in conventional sintering. New techniques to suppress the volume shrinkage are eagerly needed for actual manufacturing. Recently, we have reported that reactively sintered porous MgTi2O5 from hydromagnesite and TiO2 rutile showed less volume shrinkage than that from hydromagnesite and TiO2 anatase. The result demonstrated that the compositional control of starting polymorphs can be a potential technique to optimize the volume shrinkage. In this paper, in order to evolve the reactive sintering technique, volume-changes during reactive sintering were dynamically monitored by thermomechanical analysis (TMA). The dimensional change behavior measured by TMA was linked up with the reaction behavior clarified by high-temperature X-ray diffraction (HT-XRD). In dilatometry curves, transient volume expansions were observed and they were well-explained by the formation and crystal growth of intermediate MgTiO3 and objective MgTi2O5 particles.

Reactive sintering is an environmentally-friendly processing for porous ceramics. If zero volume-shrinkage in the reactive sintering is realized, the reactive sintering will be more widely applied. Here, we report a new reactive sintering technique to realize near-zero volume-shrinkage (NZVS) in the MgO–TiO 2 system. Porous MgTi 2 O 5 pellets have been reactively sintered from LiF-doped hydromagnesite/TiO 2 mixed powders with changing TiO 2 anatase/rutile compositions. By using the anatase and rutile mixtures as TiO 2 source, volume-shrinkage during the sintering was well-controlled. In particular, the NZVS is realized for the sintering temperatures at 1000–1100 °C. To clarify the mechanism of the NZVS phenomenon, MgTi 2 O 5 particle formation behavior from the identical starting powders without pelletizing was investigated in detail. From the microstructural observation for MgTi 2 O 5 powders, TiO 2 rutile prevented the crystal growth of MgTi 2 O 5 particles.

Mesoporous titania nanocrystals were obtained through the surfactant-assisted templating method (SATM). The synthesis was carried out in the presence of two series of micellar surfactant systems, namely (C n H 2 n +1 ) 2 NCH 3 and (C n H 2 n +1 )NH 2 with n = 8–14. The sol–gel processed nanocrystalline titania were characterized by small angle X-ray scattering (SAXS), transmission electron microscopy (TEM), X-ray diffraction (XRD), N 2 adsorption–desorption measurements. The morphologies of titania were affected by the formation conditions, e.g., carbon chain length of hydrophilic tails of alkylamine surfactants, and calcination conditions. SAXS experiments indicated that the structure of titania sol under SATM was explained by the mass fractal model. The synthesized powders exhibited anatase-type mesoporous structure with surface area of up to 215 m 2 g −1 .

Mesoporous high surface area and high crystallinity MO 2 powders (TiO 2 , CeO 2 , ZrO 2 , and HfO 2 ) were synthesized by a modified sol–gel method using laurylamine hydrochloride, metal alkoxide and acetylacetone. The prepared MO 2 powders, characterized by XRD, nitrogen adsorption isotherm, SEM, TEM, and SAED, had crystalline size of about 5–15 nm, specific surface area of 44–80 m 2 /g, and a narrow pore size distribution with average pore diameter of about 3–6 nm. This synthesis method provides a new simple route to fabricate nanostructured materials under mild conditions.

The anisotropic thermal expansion behavior of CaAl 4 O 7 was studied by molecular dynamics (MD) simulation in the temperature range from 298 to 700 K. The monoclinic crystal structure (space group C2/c) has been successfully reproduced by using a simple 2-body potential. The principal distortions ( λ 1 , λ 2 , λ 3 ) and the coefficients of linear thermal expansion ( α 1 , α 2 , α 3 ) along with the principal axes have been calculated from the temperature dependence of the unit-cell parameters, a , b , c , and β . The distortion λ 3 was much smaller than the other two distortions for all the temperature range, which was partially consistent with the experimental work performed by Fukuda et al. The anisotropic thermal expansion behavior can be explained by the crystallographic geometry, i.e. directional difference in the packing density.