Low-temperature performance of Al4B2O9 nanowires

In this paper we report on the synthetic investigation of single-crystalline aluminum borate (Al₄B₂O₉) nanowires in large scale by a direct calcination of a precursor powder made of Na₂B₄O₇·10H₂O and Al (NO₃)₃·9H₂O at a low temperature of 850 °C. The nanowires, with the diameter of 20-40 nm and the length up to several micrometers, possess smooth surfaces and uniform sizes along the entire wire. The growth mechanism of the nanowires is attributed to a solid-liquid-solid process, which controls the nanowire morphology.     

Subsolidus phase equilibria in the Al2O3-CeO2-PbO and Al2O3-CeO2-RuO2 systems

The subsolidus phase equilibria in air for the Al₂O₃-CeO₂-PbO and Al₂O₃-CeO₂-RuO₂ systems were studied with the aim of obtaining information on possible interactions between a CeO₂-based solid electrolyte in solid-oxide fuel cells (SOFCs) and other oxides. No ternary compound was found in either of the systems. The tie line in the Al₂O₃-PbO-CeO₂ system is between Al₂Pb₂O₅ and the CeO₂.     

Subsolidus phase equilibria in the RuO2-Bi2O3-ZrO2 system

Subsolidus equilibria in air in the RuO₂-Bi₂O₃-ZrO₂ system were studied with the aim of obtaining information on possible interactions between a Bi₂Ru₂O₇-based cathode and a ZrO₂-based solid electrolyte in solid-oxide fuel cells (SOFCs). No ternary compound was found in the system. The tie lines are between Bi₂Ru₂O₇ and ZrO₂, and between Bi₂Ru₂O₇ and gamma-Bi₂O₃—the ZrO₂ stabilised Bi₂O₃ phase, stable at temperatures over 710 °C.     

Modification and optimization of nano-crystalline Al2O3 combustion synthesis using Taguchi L16 array

Nanocrystalline Al₂O₃ powders have been synthesized by combustion method using 8 new fuels. The effectiveness of important factors on the production of nanopowders was investigated and optimized using Taguchi L₁₆ array design. The products were characterized by XRD, BET, TGA, EDX, FESEM, and TEM analyses. Results demonstrated that the alumina nanoparticles had crystallite sizes between 8.31 nm and 13.54 nm. The optimized sample had the specific surface area of 72 m²/g and crystallite size of 7.25 nm. The synthesis of γ-alumina was modified in order to achieve higher specific surface area (122.63 m²/g). A nano-network of alumina powders woven by alumina nano-fibers has been fabricated successfully by modified combustion synthesis. The length and diameter of fibers were about 160 nm and 10 nm respectively.     

Structural properties of Al2O3-La2O3 binary oxides prepared by sol-gel

The structural properties of La₂O₃ and Al₂O₃-La₂O₃ binary oxides prepared by sol-gel were studied by XRD, HRTEM and UV-vis. The binary oxides with high lanthana contents show an amorphous structure after calcination at 650 °C. At calcination temperatures higher than 1000 °C there is a phase transformation from the amorphous state to the crystalline LaAlO₃ with a perovskite structure. The structure of La₂O₃ is consistent with the hexagonal system; however, some crystalline microdomains with a monoclinic structure were detected by HRTEM. Islands of La₂O₃ and LaAl₁₁O₁₈ phases were detected at high lanthana concentration in the binary oxide. The modification in the coordination shell of the Al³⁺ cations due to the interaction with La³⁺ cations confirms the formation of phases with a perovskite structure and the presence of islands of the LaAl₁₁O₁₈ phase.     

Nanorod alumina-supported Ni-Zr-Fe/Al2O3 catalysts for hydrogen production in auto-thermal reforming of ethanol

Nanorod alumina-supported Ni-Zr-Fe/Al₂O₃ catalysts were prepared by co-impregnation, characterized by TEM, TPR, XRD, XPS, and TPD-pyridine, and tested in auto-thermal reforming of ethanol. The characterization results indicate that, with iron and zirconia promotion, the Ni_xFe_1−xAl₂O₄ mixture spinel forms, the valence of the surface Ni species is modified, and the acidity decreases. As a result, during a 30-h test over the Ni-Zr-Fe/Al₂O₃ catalyst, sintering is restrained, and the selectivity to hydrogen remains around 85.79% without obvious loss, while the un-promoted Ni/Al₂O₃ shows poor stability and selectivity.     

The novel low temperature synthesis of nanocrystalline MgAl2O4 spinel using “gel” precursors

Nanocrystalline MgA1₂O₄ was synthesized by pyrolysis of the homogeneous spinel precursor, Mg(C₄H₅O₂)₂·(H₂O):Al₂(OH)₃(C₃H₃O₂)₃·(H₂O)₂:NH₄(C₃H₃O₂)·(H₂O). The spinel precursor gel is composed of a mixture of magnesium and aluminum acrylate salts dissolved in an ammonium acrylate gel. The reaction of acrylic acid with magnesium hydroxide in water produced magnesium methacrylate, and a similar reaction of basic aluminum acetate produced the aluminum salt of acrylic acid. The reaction of acrylic acid with ammonium hydroxide produced the ammonium acrylate gel. Ammonium acrylate gel was found to absorb many times its own weight of water. Although similar properties have been identified in PMMA and in the polymer of sodium acrylate, only the ammonium acrylate gel exists as an unpolymerized monomer. The spinel precursor gel is a pre-ceramic material that yields an amorphous oxide phase at 425°C that begins to crystallize to the MgAl₂O₄ spinel at 600°C and is fully crysta1line at 1030°C with a uniform particle size of 7-100 nm. The spinel precursor gel and the nanocrystalline MgA1₂O₄ spinel were characterized by XRD and TEM.     

Mechanochemical synthesis of NaNbO3

The mechanochemical synthesis of NaNbO₃ is studied. It is shown that NaNbO₃ can be prepared by milling the constituents, i.e. Na₂CO₃ and Nb₂O₅ in the planetary mill. After 40 h of mechanochemical treatment NaNbO₃ nanoparticles in the range of 10-20 nm are obtained. Furthermore, the high-energy milling leads to the mechanochemically-triggered carbonate decomposition, which has been observed for a few cases in the open literature.     

Novel solution phase metathetic pathway for the synthesis of MnV2O6·H2O

A novel solution-based approach for the metathetic synthesis of hydrated MnV₂O₆ driven by microwave energy has been successfully demonstrated. The metathetic pathway of synthesizing this technologically important material is confirmed by the presence of high lattice energy by-product NaCl as per the reaction.

     

Thermal shock resistance of the porous Al2O3/ZrO2 ceramics prepared by gelcasting

Porous Al₂O₃/ZrO₂ ceramics with porosity varying from 6% to 50% were fabricated by gelcasting using polystyrene (PS) as pore-forming agent. The effects of sintering temperature on porosity, strength as well as pore size were investigated. The flexural strength of these porous ceramics at room temperature significantly decreases as the porosity increases. Thermal shock resistance of these ceramics was improved by increasing the porosity. Both the critical difference temperature (ΔT_c) and residual strength of high porosity ceramics were higher than those of low porosity ceramics. These improvements can be attributed to the pores in the specimens which relax the thermal shock stress and arrest the propagation of microcracks effectively, which is confirmed by XRD analysis of specimens which encountered different thermal shock temperature difference.     

Reaction chemistry during self-propagating high-temperature synthesis (SHS) of H3BO3-ZrO2-Mg system

In the present investigation, the ZrB₂ powder is produced by SHS of mixture containing H₃BO_3, ZrO₂ and Mg. The thermal analysis and XRD study reveal the reaction mechanism of ZrB₂ formation by SHS process. Synthesis of H₃BO₃-Mg system results in formation of Mg₃(BO₃)₂ and MgB₄ phases, whereas ZrO₂ is partially reduced to Zr₃O and Zr during synthesis of ZrO₂-Mg system. The reaction between elemental Zr and MgB₄ results in ZrB₂ formation. The particle size of ZrB₂ is found to decrease with the addition of SHS diluent.     

Effect of B2O3-Bi2O3-SiO2-ZnO glass on the sintering and microwave dielectric properties of 0.83ZnAl2O4-0.17TiO2

The 0.83ZnAl₂O₄-0.17TiO₂ (ZAT) ceramics were synthesized by solid state ceramic route. The effect of 27B₂O₃-35Bi₂O₃-6SiO₂-32ZnO (BBSZ) glass on the microwave dielectric properties of ZAT was investigated. The crystal structure and the microstructure of the ceramic-glass composites were studied by X-ray diffraction and scanning electron microscopic techniques. The low frequency dielectric loss was measured at 1 MHz. The dielectric properties of the sintered samples were measured in the microwave frequency range by the resonance method. Addition of 0.2 wt% of BBSZ improved the dielectric properties with quality factor (Q_u × f) > 120,000 GHz, temperature coefficient of resonant frequency (τ_f) = −7.3 ppm/°C and dielectric constant (?_r) = 11.7. Addition of 10 wt% of BBSZ lowered the sintering temperature to about 950 °C with Q_u × f > 10,000 GHz, ?_r = 10 and τ_f = −23 ppm/°C. The reactivity of 10 wt% BBSZ added ZAT with silver was also studied. The results show that ZAT doped with suitable amount of BBSZ glass is a possible material for low-temperature co-fired ceramic (LTCC) application.     

Characterization of nanosized Al2(WO4)3

Nanosized aluminum tungstate Al₂(WO₄)₃ was prepared by co-precipitation reaction between Na₂WO₄ and Al(NO₃)₃ aqueous solutions. The powder size and shape, as well as size distribution are estimated after different conditions of powder preparation. The purity of the final product was investigated by XRD and DTA analyses, using the single crystal powder as reference. Between the specimen and the reference no difference was detected. The crystal structure of Al₂(WO₄)₃ nanosized powder was confirmed by TEM (SAED, HRTEM). In additional, TEM locality allows to detect some W₅O₁₄ impurities, which are not visible by conventional X-ray powder diffraction and thermal analyses.     

Fluorite-like phases in the BaF2-BiF3-Bi2O3 system—synthesis, conductivity and defect clustering

A fluorite-like solid solution Ba_1 − xBi_xO_zF_{2 + x − 2z} on the basis of cubic BaF₂ was synthesised in the BaF₂-Bi₂O₃-BiF₃ system and the homogeneity range at 873 K was determined. The samples were studied by X-ray powder diffraction and electron diffraction, and their transport properties were measured by the complex impedance method at 300-623 K. Tendencies of variation of lattice parameters and transport properties were determined. These tendencies are discussed on the basis of a defect clustering hypothesis. Thermal treatment at 573 K of the solid solution, quenched from 873 K results in the formation of a new ordered tetragonal fluorite-like phase with lattice parameters a = 9.5355(4) Å, c = 18.151(1) Å.     

On structure and mechanical properties of ultrasonically cast Al-2% Al2O3 nanocomposite

An investigation on the structure of an ultrasonically cast nanocomposite of Al with 2 wt.% nano-sized Al₂O₃ (average size ∼10 nm) dispersoids showed that the nanocomposite was consisting of nearly continuous nano-alumina dispersed zones (NDZs) in the vicinity of the grain boundaries encapsulating Al₂O₃ depleted zones (ADZs). The mechanical properties were investigated by nanoindentation and tensile tests. The nano-sized dispersoids caused a marginal increase in the elastic modulus, and a significant increase in the hardness (∼92%), and tensile strength (∼48%). Subsequent cold rolling to achieve a reduction ratio of 2 resulted in an appreciable increase in the hardness due to change in morphology of the microstructure. Estimation of the strength on the basis of inter-particle spacing, which was measured by transmission electron microscopy, could not be accounted for on the basis of Orowan mechanism, and therefore, strengthening mechanisms like local climb and/or cross slip might have a role in this room temperature (0.32T_M) deformation process.     

Powder synthesis and characterization of nanocrystalline CeO2 via the combustion processes

Nanocrystalline CeO₂ powders were synthesized by the combustion reactions using citric acid and glycol as fuels and nitrate as an oxidant. The adiabatic flame temperatures in the auto ignition processes of the precursors were calculated theoretically. XRD measurements indicated that the powders produced in the combustion processes were cubic fluorite CeO₂ phase. The size and morphology of the particles and extent of agglomeration in the powders were studied using transmission electron microscopy (TEM) and the particle size analyzer respectively. Blue shifts of the absorption peak of the as-prepared powders were observed.     

Structure study of Bi4Ti3O12 produced via mechanochemically assisted synthesis

Nanosized bismuth titanate was prepared via high-energy ball milling process through mechanically assisted synthesis directly from their oxide mixture of Bi₂O₃ and TiO₂. Only Bi₄Ti₃O₁₂ phase was formed after 3 h of milling time. The excess of 3 wt% Bi₂O₃ added in the initial mixture before milling does not improve significantly the formation of Bi₄Ti₃O₁₂ phase comparing to stoichiometric mixture. The formed phase was amorphized independently of the milling time. The Rietveld analysis was adopted to determine the crystal structure symmetry, amount of amorphous phase, crystallite size and microstrains. With increasing the milling time from 3 to 12 h, the particle size of formed Bi₄Ti₃O₁₂ did not reduced significantly. That was confirmed by SEM and TEM analysis. The particle size was less than 20 nm and show strong tendency to agglomeration. The electron diffraction pattern indicates that Bi₄Ti₃O₁₂ crystalline powder is embedded in an amorphous phase of bismuth titanate. Phase composition and atom ratio in BIT ceramics were determined by X-ray diffraction and EDS analysis.     

Growth of nano-particles of Al2O3, AlN and iron oxide with different crystalline phases in a thermal plasma reactor

The paper presents the experimental results showing that the crystalline phase of the nano-particles, synthesized in a DC transferred arc thermal plasma reactor, critically depend on the operating pressure in the reaction zone. The paper reports about the changes in crystalline phases of three different compounds namely: aluminium oxide (Al₂O₃), aluminium nitride (AlN) and iron oxide (Fe_xO_y) synthesized at 760 Torr and 500 Torr of operating pressures. The major outcome of the present work is that the phases having higher defect densities are more probable to form at the sub-atmospheric operating pressures. The variations in the crystalline structures are discussed on the basis of the change in the temperature during the nucleation process, prevailing at the boundary of the plasma, on account of the ambient pressures. The as-synthesized nano-particles were examined by X-ray diffraction analysis and transmission electron microscopy. In addition, the confirmatory analysis of the crystalline phases of iron oxides was carried out with the help of Mössbauer spectroscopy.     

A mild solvothermal route to α-Fe2O3 nanoparticles

A mild solvothermal route has been developed to synthesize α-Fe₂O₃ nanoparticles using Fe(NO₃)₃ as a starting material. The results from XRD and TEM indicate the α-Fe₂O₃ powders possess a rhombohedrally centered hexagonal structure, and the size of particles from alcohothermal method at 160 °C is about 50-100 nm.     

One-pot synthesis of CuFeSe2 cuboid nanoparticles

Ternary semiconducting CuFeSe₂ nanocrystals of a particular shape and size were successfully synthesized using a cost-effective and simple one-pot chemical route. X-ray powder diffraction and field emission scanning electron microscopy results indicated that the as-synthesized CuFeSe₂ comprised cuboid nanoparticles with dimensions of 50–150 nm as well as a tetragonal phase. Elemental analysis yielded an atomic ratio of Cu:Fe:Se of 1:1.06:2.17. The synthesis temperature and the solvent octadecylamine were significant in determining the structural phases and morphologies of the final products. The optimal condition for synthesizing the tetragonal CuFeSe₂ phase with cuboid nanoparticles was a reaction temperature of 200 °C for 1 h in octadecylamine solvent. A possible mechanism of the formation of ternary CuFeSe₂ nanoparticles with controllable shapes is discussed.