Based upon the analysis of materials cycling and processing on the earth, a thermodynamic concept for energetical and environmental problems has been proposed. It concludes that solution processing using aqueous solutions should be the most important processing even for advanced materials. According to this concept, energetical and environmental features of soft solution processing (SSP) are discussed in general, using also some particular examples, such as BaTiO3. Applications of the SSP are shown with special emphasis on hydrothermal and/or electrochemical synthesis of thin films and integration issues. Soft solution processing allows one to fabricate in aqueous solutions shaped/sized/oriented ceramics in only one step, without excess energies for firing/sintering or melting and without expensive equipment, providing an environmentally friendly route for the preparation of advanced ceramic materials.

The problems encountered to tailor simultaneously various specific chemical or physical properties are discussed. Selected polymeric precursors used in association with fine powders allow the control of the nano/microstructure of composites and hence the preparation of functional (FGM) and hierarchical reinforced (HRC) composites, making it possible to combine several kinds of fibers, interphases, and matrices in the same composite (hot microwave absorbent), to control the fiber/matrix interface (long life times composites), to achieve net-shape sintering of 3D composite matrices, and to prepare thick films of metal-ceramic composites with tailored microwave absorption (radar stealthiness).

La2CaCu2O6 (n = 2) was obtained at temperatures as low as 800 °C by firing the coprecipitates from a mixed aqueous solution of La, Ca, and Cu acetates which were titrated with tartaric acid. The firing temperature was much lower than that in solid state reactions which are usually conducted above 1000 °C. Both La2Ca2Cu3O8 (n = 3) and La2Ca3Cu4O10 (n = 4) were obtained at 700 °C in a similar preparation method. Low temperature firing was very important for these preparations. A porous sintered body of La2CaCu2O6 with a relative density of about 15% was also obtained using this preparation method. This porous sintered body could be effectively oxygen-doped by electrochemical oxidation showing the presence of a new superconducting phase with Tc around 30 K.

SiC particles coated uniformly with Al ions (0.25 mass% Al2O3) in an aluminum nitrate solution were consolidated by filtration through a gypsum mold. Hot-pressing in vacuum gave dense SiC (above 99% relative density) in the temperature range of 1900–1950 °C under a pressure of 39 MPa. The microstructures of dense SiC consisted of 2–5 μm grains of low aspect ratios (below 2). The fracture toughness and flexural strength of SiC increased gradually as the hot-pressing temperature became higher and were 4.3 Mpa m0.5 and 350 MPa, respectively, with hot-pressing at 1950 °C. Crack propagation in SiC shifted from intergrain to intragrain with increasing hot-pressing temperature.

A successive hydrogenation system was constructed using a two-compartment cell separated by a Pd sheet. The hydrogenation rate changed greatly, depending on the kind of substrates used. For the purpose of improving the hydrogenation rate, the surface of a Pd sheet was modified with highly active catalysts such as Pd black, Pt, and Au by using active hydrogen passing through the Pd sheet as a reducing agent. As a result, the hydrogenation rate of unsaturated organic compounds such as 4-methylstyrene was markedly increased by the surface modification with these catalysts due to the increase in reaction zone and appearance of new active sites.

Crystallization of amorphous titania prepared by hydrolysis of ethoxide was accelerated even by a small amount of water in the vapor phase. The existence of water promoted the change of localized structure of the amorphous titania to anatase structure, which resulted in acceleration of anatase nucleation. The anatase crystals grew in steam by solid-state epitaxial growth, but stopped growing in a short time. The growth of anatase crystals under hydrothermal conditions could be divided into the following two stages: the first stage with fast growth rate by the solid-state epitaxial growth and the second stage with slow growth rate by the dissolution and deposition process.

Three types of antimonic acid (Sb2O5 · nH2O) films, (111)-oriented and nonoriented polycrystalline films of cubic Sb2O5 · nH2O and amorphous Sb2O5 · nH2O films, were prepared on quartz glass substrates with a spin-coating technique using aqueous slurries, which were formed by reacting H2O2 with metallic Sb powder or Sb(O-n-C3H7)3. The morphologies and structures of the films were observed, and the electrical conductivity of the films was determined at 20 °C in the relative humidity range 9–100%. The electrical conductivity of the (111)-oriented film, which increases from 1.69 × 10−5 to 2.89 × 10−3 S cm−1 as the relative humidity changes from 11 to 85%, is approximately one order of magnitude larger than those of the nonoriented and amorphous films.

Epitaxially grown LiNbO3 thin films were prepared on sapphire(012) (R-plane) substrates by a dipping-pyrolysis process using two kinds of lithium sources, i.e., Li-2-ethylhexanoate and Li-trifluoroacetate, which are pyrolyzed in air to Li2CO3 and LiF, respectively. Crystallization temperature and alignment of LiNbO3 films on sapphire(012) were found to depend greatly on the Li sources. Dominantly [100]-oriented epitaxial LiNbO3 films were crystallized at around 600 °C using 2-ethylhexanoate, whereas dominantly [012]-oriented epitaxial LiNbO3 films resulted at a temperature as low as 400 °C using trifluoroacetate.

Polycrystalline films of cobalt oxides (Co3O4 and CoOOH) with and without Sm were fabricated on platinum substrates by electrochemical oxidation in an autoclave. The cobalt oxides were directly crystallized at temperatures higher than about 100 °C during electrolysis. The composition of the cobalt oxides strongly depended on the electrode potential; i.e., Co3O4 and CoOOH were formed in non-noble and relatively noble potential regions, respectively. When Sm was incorporated in the CoOOH by the electrolysis in solutions containing Sm3+, only the c lattice constant was increased due to an expansion in the layer structure.

Fine-grained monoclinic zirconia ceramic was made from well-dispersed zirconia sol prepared by the hydrolysis of zirconium chloride oxide octahydrate. Dechlorinated and concentrated zirconia sol was consolidated by pressure filtration. The relative green density of the compact was improved by the following cold isostatic pressing treatment at 400 MPa. The compact was densified by pressureless sintering to >98% of theoretical density in air at 1100 °C, which is lower than that of monoclinic to tetragonal transformation of pure zirconia. The average grain size of the sintered monoclinic zirconia ceramics was 92 nm.

Titania gels in liquid media crystallized from amorphous to anatase at temperatures as low as 120 °C, while the onset of crystallization in air was 370 °C. The crystallization rate in water was much faster than in methanol and n-hexane, and the crystallite size of anatase prepared in water was larger than of those prepared in organic solvents. The crystallized powders were capable of efficient hydrogen evolution following band gap irradiation in the presence of a sacrificial hole acceptor such as methanol. The photocatalytic activity of powder crystallized in methanol was superior to those prepared in water and air.

Hydroxy double salts (HDS's) comprise a class of layered materials that are similar to layered double hydroxides (LDH's) and show a comparable intracrystalline reactivity. Their anion exchange reactions occur with preincorporated anions in the hydroxide layer and with anions bound as gegenions of the positively charged layers, respectively. In this study, nitrate and acetate anions of HDS's were exchanged with anionic monoand di-carboxylic acids, and we confirmed that interlayer spacing of HDS's increased depending on the size of mono- and di-carboxylic acids. Moreover, we have prepared photofunctional materials by exchange reaction with azobenzene-p-carboxylic acid and 4–4′-azobenzenedicarboxylic acid.

A layered perovskite type oxide, K2La2Ti3O10, was prepared by a gel technique using the polymerized complex (PC) method. A single phase K2La2Ti3O10 was obtained by adding twice the potassium required for stoichiometry. The Ni–K2La2Ti3O10 catalyst prepared by the PC method exhibited a higher photocatalytic activity for decomposition of H2O into H2 and O2 than that prepared by a conventional solid state reaction.

The reactions of rare earth (RE) acetates with iron acetylacetonate in 1,4-butanediol at 300 °C (glycothermal reaction) yielded two novel phases depending on the ionic size of the RE element: one was obtained for Er-Lu and the other for Tb and Dy. The former phase was hexagonal REFeO3, while the latter phase has not been identified. The reaction of Y or Ho acetate yielded the mixture of these two phases. When the reactions were carried out in the presence of seed crystals of yttrium aluminum garnet (Y3Al5O12), these phases were not formed but RE iron garnet (RE3Fe5O12) grew on the seed, which suggests that spontaneous nucleation of RE iron garnet does not occur, but crystal growth proceeds easily under the glycothermal conditions. Hydrothermal reaction of the same starting materials yielded a mixture of Fe2O3 and an amorphous RE phase.

A new porous material was prepared from a layered compound, K4Nb6O17, through the exfoliation of its layers. A composite of the niobate sheets and MgO particles were obtained by precipitating the exfoliated two-dimensional niobate sheets with MgO fine particles. Porous niobium oxide was obtained by removal of the MgO particles from the composite after thermal treatment. It had a large surface area and showed higher photocatalytic activity than the original H+/K4Nb6O17 for H2 evolution from various aqueous alcohol solutions.

Solution processing methods have been used to prepare a solid electrolyte, copper-doped bismuth vanadate, and several different lithium transition metal oxide cathode materials. Dense thin films of the bismuth vanadate were prepared by pyrolysis of metal organic precursors deposited on various oxide substrates. A high degree of crystal orientation was obtained using single crystal MgO substrates. The Pechini process was used to prepare powders of the different materials and a variety of results were obtained. The bismuth vanadate exhibited a second phase of BiVO4 while LiNiO2 and the LiCoxNi1−xO2 solid solution require further efforts at obtaining the proper phase and stoichiometry. The LiCoO2 system formed readily and exhibited good electrochemical performance.

BaTiO3, SrTiO3, and BaxSr1−x TiO3 thin films, as well as multilayers in the SrTiO3-BaTiO3 system, have been prepared on Ti substrates in newly constructed flow-system equipment by the hydrothermal-electrochemical method. The synthesis parameters (temperature of 120–200 °C, flow rate of 1–50 cm3/min) allow fabrication of dense, single-phase films with different morphology by controlling nucleation and/or growth rates. The flow system enables also an easy fabrication of SrTiO3/BaTiO3 and BaTiO3/SrTiO3 multilayers with variable chemical composition and microstructure across the film thickness. The multilayers can be prepared in only one experiment by simply changing the kind of flowing solution and/or adjusting the processing conditions.

Pure tetravalent nickel in γ-type cobalt substituted nickel oxyhydroxide, Ni0.70Co0.30O2K0.30(H2O)0.42, could be obtained by the “chimie douce” reaction. The presence of tetravalent nickel is confirmed by comparing the Ni K-edge XANES spectrum of the sample with those of reference compounds having various nickel valency and similar layer structure. The Co K-edge XANES spectrum indicates that the trivalent cobalt remains unchanged regardless of the nickel valency. The structural modification during chimie douce reaction observed from XRD patterns and the result of iodometric titration are consistent with the Ni and Co K-edge XANES data.

A novel wet process to synthesize metal oxide thin films has been developed. The process is called the Liquid-Phase Deposition (LPD) method. In this method, metal oxide or hydroxide thin films are formed on the substrate through the ligand-exchanging (hydrolysis) equilibrium reaction of metal-fluoro complex species and the F− consumption reaction of a F− scavenger. The LPD method is a unique soft solution process, and is performed by very simple procedures. In this paper, we develop a method of preparing composite oxide thin films, Pt-dispersed titanium oxide, and iron-nickel binary oxide thin films.

Aluminosilicate sodalite Na8[SiAlO4]6Cl2 single crystals are synthesized by hydrothermal processing at 873–973 K and 100–150 MPa in noble metal capsules to avoid contamination. The starting material is aluminosilicate gel, and spontaneous nucleation followed by its growth takes place. The largest size of the single crystals obtained is 1 mm across. Longer aging and heating result in larger single crystals. It is also found that the aluminum source in the synthesis gel and the element used in the noble metal capsules influence the single crystal growth. Judging from x-ray diffraction (XRD) (powder and single-crystal), optical microscopy, and thermogravimetry and differential thermal analysis (TG-DTA), the sodalite single crystals grown have good quality compared with the conventional powder.