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.

Mullite-zirconia composites

Mullite-zirconia composite powders were prepared by the combustion of an aqueous heterogeneous redox mixture consisting of Al(NO3)(3), Zr(NO3)(4)/ZrO(NO3)(2), silica fume and urea/diformyl hydrazine at 500 degrees C. X-ray diffraction data showed that a large amount of tetragonal zirconia existed in the composite powders in spite of high temperature calcination. Milled composite powders showed enhanced densification compared to the unmilled powders and the microstructure of the sintered (1600 degrees C) compacts showed the presence of spherical zirconia grains in intergranular positions along with elongated mullite grains.

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.

Strontium and calcium zirconyl citrates as precursors for the low-temperature synthesis of SrZrO3 and CaZrO3 fine powders

Synthesis and the thermal decomposition behavior of new molecular precursors, strontium, and calcium zirconyl citrates are presented. The pathway to the metazirconate formation has been found to proceed through a multistep process. The precursors yield SrZrO3 and CaZrO3 fine powders at temperatures as low as 650 degrees C. Physico-chemical, spectroscopic, thermoanalytical, and microscopic techniques have enabled the identification of the sequence of events leading to the perovskite formation and proposition of a thermolysis scheme. Retention of the molecular level mixing of the metal ions during the course of the precursor decomposition is supported by these techniques. Prior to the formation of MZrO3 (M = Sr and Ca) an ionic oxycarbonate, M2Zr2O5CO3 (M = SI. and Ca), intermediate is produced by the thermal decomposition of the citrate precursors.

Sliding wear studies in epoxy containing alumina powders

The effect of hard and refractory alumina additions on the mechanical properties of polymer in general and wear behavior in particular is not well studied. In this work, therefore, the changes in wear behavior of epoxy resin due to the additions of alumina powders have been looked into. Using a pin-on-disc set up, dry sliding wear tests were done on both filled (4, 8, & 11 wt. % alumina) and unfilled samples. A sliding velocity of 0.83 m/sec. and a sliding distance of 2 km were employed for the study. Load range used varied from 9.8 N to about 29 N. The experiments point to an increased resistance to wear with an increased presence of filler in the matrix. Further, higher loads result in larger loss of material irrespective of the filler level in the composite.

Preparation and characterization of nanocrystalline powders of bismuth vanadate

The influence of mechanical activation on the formation of Bi2VO5.5 bismuth vanadate (BiV) phase, was investigated by ball-milling a stoichiometric mixture of bismuth oxide and vanadium pentoxide. The structural evolution of the desired BN phase, via an intermediate BiVO4,phase, was investigated using X-ray powder diffraction; (XRD), differential thermal analysis (DTA) and transmission electron microscopy (TEM). Milling for 54h. yielded monophasic gamma-BiV powders with an average crystallite size of 30 nm. The electron paramagnetic resonance (EPR) peaks associated with the V4+ ions are stronger and broader in nanocrystalline (n) BN than in the conventionally prepared microcrystalline (m) BN, suggesting theta significant portion of V5+ has been transformed to V4+ during milling. The optical bandgap of n-BiV was found to be higher than that of m-BiV. High density (97% of the theoretical density), fine-grained (average grain-size of 2 tun) ceramics with uniform grain-size distribution could be fabricated using n-BiV powders. These fine-grained ceramics exhibit improved dielectric, pyre and ferroelectric properties. (C) 1999 Elsevier Science S.A. All rights reserved.

Studies on the variations in compression strengths of graphite powders bearing glass-epoxy composites exposed to humid conditions

The moisture absorption and changes in compression strengths in glass-epoxy (G-E composites without and with discrete quantities of graphite powders introduced into the resin mix prior to its spreading on specific glass fabric (layers) during the lay-up (stacking) sequence forms the subject matter of this report. The results point to higher moisture absorption for graphite bearing specimens. The strengths of graphite-free coupons show a continuous decrease, while the filler bearing ones show an initial rise followed by a drop for larger exposure times. Scanning Fractographic features were examined for an understanding of the process. The observations were explained invoking the effect of matrix plasticizing and the role of interfacial regions.

Preparation of a ZrO2–Al2O3 nanocomposite by high-pressure sintering of spray-pyrolyzed powders

ZrO2–Al2O3 powders were synthesized by spray pyrolysis. These powders were sintered at 1 GPa in the temperature range of 700–1100 °C. The microstructural evolution and densification are reported in this paper. The application of 1 Gpa pressure lowers the crystallization temperature from ∼850 to <700 °C. Similarly, the transformation temperature under 1 GPa pressure for γ → α–Al2O3 reduces from ∼1100 to 700–800 °C range, and that for t → m ZrO2 reduces from ∼1050 to 700–800 °C range. It was possible to obtain highly dense nanocrystalline ZrO2–Al2O3 composite at temperatures as low as 700 °C. The effect of high pressure on nucleation and transformation of phases is discussed.