Preparation of CaTiO3 fine powders by the hydrothermal method

Fine powders consisting of 0.1–0.5 μm size crystallites of CaTiO3 are prepared at 150–200°C by the hydrothermal method starting from hydrated titania gel and reactive calcium oxide suspended as an aqueous slurry in an autoclave. The resulting high-purity CaTiO3 is characterised by TEM, X-ray powder diffraction, chemical analyses and sintering characteristics. The hydrothermally prepared CaTiO3 powders are sinterable to high-density ceramics below 1400°C. The dc conductivity behaviour of the chemically reduced ceramics is presented.

Hydrothermal preparation of Ba(Ti,Zr)O3 fine powders

Fine powders consisting of aggregated submicron crystallites of Ba(Ti,Zr)O3 in the complete range of Ti/Zr ratios are prepared at 85–130°C by hydrothermal method, starting from TiO2 + ZrO2 · xH2O mixed gel and Ba(OH)2 solution. The products obtained below 110°C incorporate considerable amounts of H2O and OH− within the lattice. As-prepared BaTiO3 is cubic and converts to tetragonal phase after the heat treatment at 1200°C, accompanied by the loss of residual hydroxyl ions. TEM investgations of the growth features show a transformation of the gel to the crystallite. Ba2+ ions entering the gel produce chemical changes within the gel, followed by dehydration, resulting in a cubic perovskite phase irrespective of Ti/Zr. The sintering properties of these powders to fine-grained, high density ceramics and their dielectric properties are presented.

Hydrothermal synthesis of Ba(Ti, Sn)O3 fine powders and dielectric properties of the corresponding ceramics

Fine powders of submicron-sized crystallites of BaTiO3 were prepared at 85–130°C by the hydrothermal method, starting from TiO2.ξH2O gel and Ba(OH)2 solution. The products obtained below 110°C incorporated considerable amounts of H2O and OH− in the lattice. As-prepared BaTiO3 is cubic and converts to the tetragonal phase after heat treatment at 1200°C, accompanied by the loss of residual OH− ions. Hydrothermal reaction of SnO2.ξH2O gel with Ba(OH)2 at 150–260°C gives rise to the hydrated phase, BaSn(OH)6.3H2O, due to the amphoteric nature of SnO2.ξH2O which stabilises Sn(OH)62− anions in basic media. On heating in air or releasing the pressure in situ at 260°C, BaSn(OH)6.3H2O converts to BaSnO3 through an intermediate, BaSnO(OH)4. Solid solutions of Ba(Ti,Sn)O3 are directly formed from (TiO2 + SnO2)..ξH2O gel up to 35 mol% SnO2. At higher Sn contents, the hydrothermal products are mixtures of BaSn(OH)6.3H2O and BaTiO3, which on annealing at 1000°C result in monophasic Ba(Ti,Sn)O3. The sintering characteristics and the dielectric properties of the ceramics prepared out of these fine powders are presented. The dielectric properties of fine-grained Ba(Ti,Sn)O3 ceramics are explained on the basis of the prevailing diffuse phase transition behaviour.

Precipitation of rutile and anatase (Tio12) fine powders and their conversion to MTiO3 (M = Ba, Sr, Ca) by the hydrothermal method

Fine powders of TiO2 (rutile) with high degree of crystallinity are formed from aqueous titanium oxychloride solution under hydrothermal conditions at 160–230°C and 15–100 kg/cm2 for 1–2 hours. The anatase phase is produced from the same medium when sulfate ion impurity is present, with Image . Both these fine powders are converted to BaTiO3, SrTiO3 or CaTiO3 when suspended in Ba(OH)2 or Sr(OH)2 solution or in an aqueous slurry of carbonate-free CaO with Image , at 180–280°C and 12–65 kg/cm2 for 4–8 hours. The resulting fine powders contain monocrystallites of the perovskite phase with 0.1–1.5 μm particle size.

Sacrificial photolysis of water on TiO2 fine powders prepared by the hydrothermal method

The often discussed role of surface hydroxylation of TiO2 particles as an essential characterestics for their photocatalytic activity can be verified by preparing TiO2 powders by hydrothermal method since hydroxylated surface layers will be better retained on these particles formed in superheated water. Thus, fine powders of TiO2 (rutile) with high degree of crystallinity are formed from titanium oxychloride in the mixed solvent of water and 2-propanol at 160–230°C and 20–120 atm. The anatase phase is produced from the same medium when sulfate ion impurity is present, with Image . TiO2 powders are washed free of anions and 2-propanol by ultrafiltration and are Pt mounted by a photochemical method. Aqueous suspensions of both forms of TiO2 neither as such nor after Pt-loading, do not produce H2 on band gap illumination whereas, H2 is generated in presence of hole scavengers such as EDTA, TEOA, sulfite or hypophosphite. The effects of hole scavenger concentration, Pt : TiO2 ratio, particulate suspension density and the nature of hole scavengers on H2 production are presented. Platinised rutile powders are equally active as anatase in sacrificial systems.

Sinteractive anodic powders improve densification and electrochemical properties of BaZr0.8Y0.2O3-δ electrolyte films for anode-supported solid oxide fuel cells

The difficult sintering of BaZr0.8Y0.2O 3-δ (BZY20) powders makes the fabrication of anode-supported BZY20 electrolyte films complex. Dense BZY20 membranes were successfully fabricated on anode substrates made of sinteractive NiO-BZY20 powders, prepared by a combustion method. With respect to traditional anode substrates made of powders prepared by mechanical mixing, the anode substrates made of the wet-chemically synthesized composite NiO-BZY20 powders significantly promoted the densification of BZY20 membranes: dense BZY20 films were obtained after co-pressing and co-firing at 1300 °C, a much lower temperature than those usually needed for densifying BZY20 membranes. Improved electrochemical performance was also observed: the supported BZY20 films maintained a high proton conductivity, up to 5.4 × 10-3 S cm-1 at 700 °C. Moreover, an anode-supported fuel cell with a 30 m thick BZY20 electrolyte film fabricated at 1400 °C on the anode made of the wet-chemically synthesized NiO-BZY20 powder showed a peak power density of 172 mW cm-2 at 700 °C, using La0.6Sr0.4Co 0.2Fe0.8O3-δ-BaZr0.7Y 0.2Pr0.1O3-δ as the cathode material, with a remarkable performance for proton-conducting solid oxide fuel cell (SOFC) applications.

BaZr0.8Y0.2O3--NiO composite anodic powders for proton-conducting SOFCs prepared by a combustion method

BaZr0.8Y0.2O3- (BZY)-NiO composite powders with different BZY-NiO weight ratios were prepared by a combustion method as anodes for proton-conducting solid oxide fuel cells (SOFCs). After heating to 1100C for 6 h, the composite powders were made of a well-dispersed mixture of two phases, BZY and NiO. Chemical stability tests showed that the BZY-NiO anodic powders had good stability against CO2, whereas comparative tests under the same conditions showed degradation for BaCe0.7Zr 0.1Y0.2O3--NiO, which is at present the most used anode material for proton-conducting SOFCs. Area specific resistance (ASR) measurements for BZY-NiO anodes showed that their electrochemical performance depended on the BZY-NiO weight ratio. The best performance was obtained for the anode containing 50 wt BZY and 50 wt NiO, which showed the smallest ASR values in the whole testing temperature range (0.37 cm2 at 600C). The 50 wt BZY and 50 wt NiO anode prepared by combustion also showed superior performance than that of the BZY-NiO anode conventionally made by a mechanical mixing route, as well as that of Pt.

Sacrificial photolysis of water on Ti1−xSnxO2 ultrafine powders

Ultrafine powders of extra pure Ti1−xSnxO2, where o < x < 1, prepared by the hydrothermal method are pale yellow in color. They show photocatalytic activity after platinization, in the visible light (420–550 nm) for H2-production from aqueous solutions containing sacrificial donors such as hypophosphite. The spectral sensitization is shown to be due to peroxotitanium species in the rutile-type structure. Peroxide ion, O22−, arises from the dimerization of O−, the hole centres, produced during the disproportionative decomposition of residual hydroxyls: OH− = O− + H. Higher OH contents in TixSnxO2 is due to the amphoteric chemistry of oxocompounds of tin.

Characterization of barium titanate fine powders formed from hydrothermal crystallization

A detailed evaluation of size, shape and microstrains of BaTiO3 crystallites produced by hydrothermal crystallization at 90 – 180 °C and 0.1 – 1.2 MPa, from amorphous TiO2· xH2O (3 < × < 8) gel and aqueous Ba(OH)2 is presented, using X-ray line-broadening and TEM studies. Whereas the concentration of Ba(OH)2 and the acceptor impurities affect the crystallite shape, the stoichimetry with respect to Ba/Ti, donor as well as acceptor impurities, and the temperature of crystallization influence the microstrains. It is shown that strains in the crystallites are related to the point defects in the lattice. Compensation of the residually present hydroxyl ions in the oxygen sublattice by cation vacancies results in strains leading to metastable presence of the cubic phase at room temperature. Studies on the diffuse phase transition behaviour of these submicron powders show that the stable tetragonal phase is produced only on annealing at high temperatures where the mobility of cations vacancies are larger. Heat-treatment reduces anisotropy and strain in undoped samples, whereas annealing is less effective in doped materials. Comparison of the crystillite size by TEM showed better agreement with the Warren—Averbach method.

Combustion Synthesis of Metal Chromite Powders

Fine-particle metal chromites (MCr2O4, where M = Mg, Ca, Mn, Fe, Co, Ni, Cu, and Zn) have been prepared by the combustion of aqueous solutions containing the respective metal nitrate, chromium(III) nitrate, and urea in stoichiometric amounts. The mixtures, when rapidly heated to 350°C, ignite and yield voluminous chromites with surface areas ranging from 5 to 25 m2/g. MgCr2O4, sintered in air at 1500°C for 5 h, has a density of 4.0 g/cm3.

Wet chemical syntheses of ultrafine multicomponent ceramic powders through gel to crystallite conversion

Coarse (BOn/2)-O-n+/xH(2)O (10

A simple method for the preparation of plasma-sprayable powders based on ZrO2

Plasma-sprayable powders of calcia, magnesia and yttria-stabilized zirconia have been prepared by using polyvinyl alcohol binders. The powders have been characterized for sprayability by spray coating on steer plates previously coated with an NiAl bond coat. The suitability of these coatings for thermal barrier applications have been examined. Thermal barrier and related properties, along with phase stability and mechanical properties, have been found to be good. Failure of the thermal barrier coating has been observed to occur at the interface between the bond coat and the substrate, due to the formation of a pile-up layer consisting of Fe-Zr-Al-O compound.

Photocatalysis on Fine Powders of Perovskite Oxides

Fine powders of semiconductor oxides have been widely used as photocatalysts for many reactions. Among the various photocatalytic reactions, water splitting has been given much importance, since it is a promising chemical route for solar energy conversion. Perovskite oxides, in particular SrTiO, have been commonly used as photocatalysts because some of them can decompose H,O into H, and 0, without an external bias potential (1). In turn, this is because the conduction band (CB) edges of some of the perovskite oxides are more negative than the H+/H, energy level. Since the catalytic activity is related to the surface properties of the solids, fine powders rather than single crystals are used. Photocatalysis on fine powers can be conveniently discussed in three parts, viz. preparation, characterization and their catalytic activity. Presently, photo-decomposition of water using SrTiO, fine powders is discussed in greater detail, although other photocatalytic reactions on various perovskite oxides are also briefly dealt with.

Reactivities of aluminium and aluminium-magnesium alloy powders in polymeric composites

Polymeric compositions containing Al-Mg alloys show higher reactivities, in comparison with similar compositions containing aluminium. This is observed irrespective of the amount of oxidizer, type of oxidizer used, type of polymeric binder, and over a range of the particle sizes of the metal additive. This is evident from the higher calorimetric values obtained for compositions containing the alloy, in comparison to samples containing aluminium. Analysis of the combustion residue shows the increase in calorimetric value to be due to the greater extent of oxidation of the alloy. The interaction between the polymeric binder and the alloy was studied by coating the metal particles with the polymer by a coacervation technique. On ageing in the presence of ammonium perchlorate, cracking of the polymer coating on the alloy was noticed. This was deduced from differential thermal analysis experiments, and confirmed by scanning electron microscopic observations. The increase in stiffness of the coating, leading to cracking, has been traced to the cross-linking of the polymer by magnesium.

Phase evolution and densification of spray pyrolysed ZrO2-Al2O3 powders

Devitrification of spray pyrolysed, amorphous ZrO2-Al2O3 solid solution produces nanocrystalline microstructures (grain sizes 10-20 nm). In this study, spray pyrolysed amorphous ZrO2-40 mol% Al2O3 powder displayed good sinterability during decomposition, after spraying, of the nitrate precursors up to 1023K. Hot pressing of fully pyrolysed, pre-sintered (more than 70% dense) pellets at 923K and 750 MPa produced an amorphous pellet with less than 2% porosity. The results indicate the possibility of producing dense, amorphous pellets that can be heat treated further to produce nanocrystalline microstructures conducive for superplasticity.