Sintering of α-Al2O3-seeded nanocrystalline γ-Al2O3 powders

This paper demonstrates that seeding nanocrystalline transition alumina powders is a viable option for producing high quality, alumina-based ceramics. By using α-Al₂O₃ concentrations of ⩾1.25 wt.% α-Al₂O₃ seed particles (equivalent to 5 ×10¹⁴ seeds/cm³ of γ-Al₂O₃) the sintering temperature is reduced from 1600°C for unseeded γ-Al₂O₃ to 1300–1400°C in dry pressed powders. The scale of the sintered microstructure is related to N_v^−1/3 and thus a 100-nm grain size is obtained. It is apparent that seeding is necessary for producing dense, alumina-based ceramics from nanocrystalline transition alumina powders.     

Controlled synthesis of Bi2O3–YSZ composite powders and their sintering behavior for high-performance electrolytes

Bismuth oxide (Bi₂O₃) is a promising additive to decrease the sintering temperature of yttria-stabilized zirconia (YSZ)-based electrolyte for solid oxide fuel cell application. However, Bi₂O₃ tends to grow into large column bars (>50 µm) in a chemical coprecipitation method, which dramatically limits the mixing uniformity of Bi₂O₃ and YSZ, even much worse than that of mechanical mixing. In this study, the reaction temperature was increased from room temperature to 90°C to increase the number of nucleation during the violate reaction between Bi³⁺ solution and YSZ suspension in NaOH. On this basis, the violence of the reaction was further moderated by adding half of NaOH first, then YSZ powders and the other half of an NaOH solution. The size of Bi₂O₃ was further decreased to sub-micrometer and Bi₂O₃ was homogeneously mixed with YSZ particles, even when its addition amount was as large as 20 mol%. These composite powders effectively promoted the sintering behavior of YSZ. The sintering temperature of YSZ was decreased to 900 and 1000°C with 10 and 5 mol% Bi₂O₃ doping, respectively. Increasing the doping ratio induced severe volatilization of Bi₂O₃ and pore formation. Raising the sintering temperature (no more than 1200°C) enhanced the doping effect of Bi₂O₃ into the YSZ lattice but induced instability in the YSZ crystal structure.     

Synthesis of γ-Al2O3 nanowires through a boehmite precursor route

By regulating the pH values of the reaction solution, the boehmite (γ-AlOOH) nanowires and nanoflakes were successfully synthesized with a simple hydrothermal route using anhydrous AlCl₃, NaOH and NH₃·H₂O as starting materials. Crystalline γ-Al₂O₃ nanowires with diameter of 10–30 nm and length of several hundreds of nanometer have been prepared by thermal decomposition of γ-AlOOH precursor. X-ray diffraction (XRD), transmission electron microscope (TEM), selected area electron diffraction (SAED), and high resolution transmission electron microscope (HRTEM) were used to characterize morphology and structure feathers of the synthesized samples. The pH values of the solution play important roles in the formation of γ-AlOOH nanowires. After calcinated at 500 °C for 2 h, the obtained γ-Al₂O₃ with a linear structure is similar to the γ-AlOOH precursor.     

Efficient α/β-Bi2O3 composite for the sequential photodegradation of two-dyes mixture

A mixture of α/β-Bi₂O₃ and α-Bi₂O₃ powders were obtained by a simple solid state reaction–annealing route at 550 °C. The structure, optical properties and surface area of the commercial α and β-Bi₂O₃ and the synthesized α-phase and α/β-composite were well characterized by X-ray diffraction, diffuse reflectance spectra and N₂ physisorption. The annealed sample at 550 °C showed 20% of β-phase, forming a heterojunction of α/β-Bi₂O₃ whereas annealing at elevated temperature (650 °C) lead to the α-phase. Optical properties showed that the presence of the β-phase is mainly responsible for narrowing the energy band gap. The photocatalytic activity of the commercial α and β-Bi₂O₃ and the synthesized α-phase and α/β-composite were investigated in degradation of single dyes, Indigo Carmine (IC) and Rhodamine-B (RhB) under both UV and visible light-induced photocatalysis. For the best photocatalyst, the photodegradation in a two-dye mixture solution was systematically studied considering the type of dye, the adsorption capacity of the samples and the behavior of dye photodegradation. The photocatalytic performance of α/β-Bi₂O₃ was comparatively much higher than the commercial α and β-Bi₂O₃, indicating that better performance of efficient charge separation and transfer across α/β-Bi₂O₃ composite was obtained. Possible mechanism of the single dye and two-dye mixture degradation was given by using α/β-Bi₂O₃ composite.     

Preparation and self-healing performance of spherical α-Al2O3@MoSi2 particles using the precipitation method

α-Al₂O₃ was coated on the powder surface using the precipitation method to improve the pre-oxidation resistance of molybdenum disilicide in thermal barrier coatings (TBCs). The coating effects of four different aluminum sources (Al(NO₃)₃·9H₂O) to MoSi₂ mass ratios were evaluated by viscosity and micro-morphology. The shell-forming effects on the powder size and post-treatment were thoroughly analyzed to meet the requirements for practical application in TBCs. The results showed that the precipitation method was superior to the sol-gel process in terms of the shell thickness obtained. Optimal shell-forming was generated at 1200 °C in an inert atmosphere. A bonding layer (Al₆Si₂O₁₃) would be formed at the core-shell interface, further preventing oxidation penetration. The self-healing particles MoSi₂@α-Al₂O₃ can effectively seal micro-cracks (width below 300 nm) because of the fluidity of SiO₂ at working temperatures.     

Photocatalytic activity of enlarged microrods of α-Bi2O3 produced using ethylenediamine-solvent

Microrods of α-Bi₂O₃ were prepared by a simple chemical precipitation method, replacing the hydroxyl (OH⁻) ions by the ethylenediamine-solvent as both a precipitating and capping agent; it can also function as a morphological template. The influence of the ethylenediamine solvent concentration on the crystalline structure, optical properties, morphology, and photocatalytic activity was investigated. The prepared samples were characterized by X-ray diffraction, diffuse reflectance spectroscopy, scanning electron microscopy, Fourier transformed IR analysis, thermal analysis and photoluminescence spectroscopy. The adsorption capacity in dark conditions and the photocatalytic activity under UV-light were studied as a function of the photocatalyst load by using indigo carmine dye solution. The highest photoactivity was observed for the non-agglomerated Bi₂O₃ microrods obtained with ethylenediamine-solvent at 40 vol%. The photocatalytic mechanism was discussed as a function of surface oxygen vacancies generated along to the microrod surface caused by the ethylenediamine-solvent.     

Mechanism and microstructural evolution of combustion synthesis of ZrB2–Al2O3 composite powders

Mechanism of combustion synthesis (CS) of ZrB₂–Al₂O₃ composite powders was systematically analyzed by a combustion front quenching method (CFQM). The microstructural evolution during the CS process was investigated by field-emission scanning electron microscopy (FESEM) equipped with energy dispersive X-ray spectrometer (EDS). The combustion temperature and wave velocity were measured by the data acquisition system. Moreover, the phase constituents of the final product were examined by X-ray diffraction (XRD). The thermal behaviors of the stoichiometic powders under the thermal exposure were characterized using differential scanning calorimetry (DSC) and thermogravimetric (TG). The results showed that the combustion reaction started from the melting of the B₂O₃ and Al particles, which was followed by the formation of ZrO₂–B₂O₃–Al solution. The ignition temperature of this system was determined to be around 800 °C. B and Al₂O₃ were then precipitated from the solution. As the CS reaction proceeded, Zr and Al₂O₃ were produced by the reaction between ZrO₂ particles and Al and precipitated from the solution. ZrB₂ could then be formed by the direct reaction between Zr and B. Finally, the ZrB₂–Al₂O₃ composite powders were obtained. Furthermore, a model corresponding to the dissolution–precipitation mechanism was proposed.     

The preparation of core–shell Al2O3/Ni composite powders by electroless plating

Core–shell nanostructured Ni-coated Al₂O₃ composite powders were synthesised by using the electroless plating method. The influence of the chemical components and powder concentration in the Ni coating was investigated by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction techniques. The results show that the concentration of the plating components plays an important role in the formation of core–shell Al₂O₃/Ni composite powders. The nickel content in the composite powders could be effectively controlled by adjusting the nickel chloride content and the concentration of NaH₂PO₂·H₂O in the plating solution. The nanostructure of the crystalline Ni coatings was observed to be very attractive for achieving good bonding between ceramic particles and matrices for composite production.     

Low-temperature combustion synthesis of nanocrystalline HoFeO3 powders via a sol–gel method using glycin

Single phase nanocrystalline HoFeO₃ powders were successfully synthesized by the sol–gel self-propagation combustion method using glycin (C₂H₅NO₂) as the chelating reagent at a low combustion temperature. Four different mole ratios of glycin to metal nitrate (G/M) were used to prepare HoFeO₃ powders in the experiment. The XRD patterns indicate monophasic HoFeO₃ powders can be formed by further calcination, the SEM micrographs show that the nano-sized grains with distinguishable boundaries had been obtained. The M–H curves show HoFeO₃ powders had the characteristic of antiferromagnetism at 50 K, while the powders had the characteristic of paramagnetism as the ambient temperature reached 100 K or 300 K. The FC/ZFC magnetic measurement results demonstrate that there was a transition from antiferromagnetism to paramagnetism in HoFeO₃ nanopowders as the temperature was increased.     

Green and facile synthesis of cube-like γ-AlOOH and its usage for fabricating Mg stabilized Na-β″-Al2O3 powders

In this work, we describe the discovery of synthesizing γ-AlOOH particles via direct reaction of Al(OH)₃ and H₂O under a green, facile and cost-effective hydrothermal treatment, in which no addition of other additives is required. The as-synthesized γ-AlOOH is in cube-like morphology with micrometres size, which is subsequently used as a precursor to fabricate Mg stabilized Na-β″-Al₂O₃ powder with high β″-Al₂O₃ phase content through solid-state reaction. Time-dependent experiments are carried out to investigate the microstructure and phase evolutions of the γ-AlOOH precursor during the hydrothermal treatment process, and the sintering temperature on the β″-Al₂O₃ phase content and microstructure of the terminal Na-β″-Al₂O₃ powder are also studied. The present approach inspires a new and facile way in the fabrication of γ-AlOOH and Na-β″-Al₂O₃ on a large scale.     

Application of Cr3C2/C composite powders synthesized via molten-salt method in low-carbon MgO–C refractories

High-performance and low-carbon MgO–C refractories are important refractories for smelting ultra-low carbon steel and clean steel. Based on this, Cr₃C₂/C composite powders were synthesized by the molten-salt method, and used as an additive to prepare low-carbon MgO–C refractories under nitrogen atmosphere. The phase, morphology and oxidation kinetics of Cr₃C₂/C composite powders were studied. In addition, the effect of Cr₃C₂/C composite powders on the morphology, mechanical properties, thermal shock resistance, and corrosion resistance of MgO–C refractories was investigated. The results indicated that the Cr₃C₂/C composite powders exhibited superior oxidation resistance than flake graphite. Moreover, the Cr₃C₂/C composite powders were introduced into the MgO–C refractories. Compared with the sample without Cr₃C₂/C composite powders, the introduction of 1 wt% Cr₃C₂/C composite powders significantly improved the thermomechanical properties and corrosion resistance of the material, its CMOR, CCS before and CCS after thermal shock were 9.06 MPa, 50.40 MPa and 32.60 MPa, respectively, and the corrosion index was significantly reduced from 44.6% to 26.5%.     

Improved thermal shock resistance of low-carbon Al2O3–C refractories fabricated with C/MgAl2O4 composite powders

The addition of C/MgAl₂O₄ composite powders can improve the thermal shock resistance of low-carbon Al₂O₃–C refractories attribute to the formation of microcracks in the agglomerated structure, thus consuming more thermal stress and strain energy. Moreover, C/MgAl₂O₄ composite powders additive promote the formation of short fibrous ceramic phases in the refractories, which suggest a bridging role in the interior of the refractories and increase its toughness. Furthermore, the C/MgAl₂O₄ composite powders also result in a remarkable enhancement of the slag corrosion resistance in the refractories.     

In situ synthesis of Al2O3–SiC powders via molten-salt-assisted aluminum/carbothermal reduction method

Al₂O₃–SiC composite powder (ASCP) was successfully synthesized using a novel molten-salt-assisted aluminum/carbothermal reduction (MS-ACTR) method with silica fume, aluminum powder, and carbon black as raw materials; NaCl–KCl was used as the molten salt medium. The effects of the synthesis temperature and salt-reactant ratio on the phase composition and microstructure were investigated. The results showed that the Al₂O₃–SiC content increased with an increase in molten salt temperature, and the salt–reactant ratio in the range of 1.5:1–2.5:1 had an impact on the fabrication of ASCP. The optimum condition for synthesizing ASCP from NaCl–KCl molten salt consisted of maintaining the temperature at 1573 K for 4 h. The chemical reaction thermodynamics and growth mechanism indicate that the molten salt plays an important role in the formation of SiC whiskers by following the vapor-solid growth mode in the MS-ACTR treatment. This study demonstrates that the addition of molten salt as a reaction medium is a promising approach for synthesizing high-melting-point composite powders at low temperatures.     

Synthesis and dissolution behavior of β-TCP and HA/β-TCP composite powders

Calcium phosphate powders, β-TCP and biphasic HA/β-TCP, were synthesized by calcining the powders obtained from the co-precipitation method using Ca(NO₃)₂·4H₂O and (NH₄)₂HPO₄. The effects of the initial Ca/P ratio and pH of the solution on the phase evolution and in vitro dissolution behavior of the powders in a Ringer's solution were investigated. The Ca/P ratio of the resulting powders was strongly dependent on the pH of the solution and weakly dependent on the initial Ca/P ratio. Single phase TCP powder was obtained at pH=7.4 and the initial Ca/P ratio had a little effect on the resulting Ca/P ratio. Biphasic composite powders were prepared at pH=8.0 and the Ca/P ratio of resulting powder was controllable by adjusting the initial Ca/P ratio. TCP powder showed the highest dissolution rate in the Ringer's solution and biphasic composite powder exhibited an intermediate dissolution behavior between that of HA and TCP.     

Synthesis and characterization of α/β-Bi2O3 with enhanced photocatalytic activity for 17α-ethynylestradiol

The α/β-Bi₂O₃ photocatalyst was successfully synthesized by a novel solvothermal-calcination method. The physical and chemical properties of as-prepared samples were characterized based on XRD, XPS, SEM, TEM, EDS, BET, UV–vis DRS and PL techniques. The synthesized α/β-Bi₂O₃ photocatalyst exhibited enhanced photocatalytic activity for 17α-ethinylestradiol (EE2), and 96.9% of EE2 was degraded after only 24 min of visible-light irradiation using α/β-Bi₂O₃ as photocatalyst. The reaction rate constant over α/β-Bi₂O₃ photocatalyst was 1.42, 2.23, 9.22 and 54.1 times higher than pure β-Bi₂O₃, α-Bi₂O₃+β-Bi₂O₃, α-Bi₂O₃ and P25 respectively. Effect of catalyst dosage and pH value was investigated. The possible photocatalytic mechanism has been discussed on the basis of the theoretical calculation and the experimental results. α/β-Bi₂O₃ was a fairly stable and efficient photocatalyst under the studied experimental conditions, proving that the α/β-Bi₂O₃ photocatalyst was a promising photocatalyst for the practical application.     

Synthesis and process modeling of mesoporous γ-Al2O3 granules using the biopolymer chitosan via experimental design

In this study, biopolymer chitosan is presented as a template for synthesizing and shaping the mesoporous γ-Al₂O₃ macrospheres. This porous γ-Al₂O₃ granule has a high surface area (310 m²/g), high pore volume (.6148 cm³/g), and pore diameter between 2 and 10 nm. The full factorial design based on a mathematical model was implemented to study the acid concentration, chitosan amount, ammonia concentration, and aging time affecting the responses (Brunauer–Emmett–Teller surface area and pore volume). Predicted responses were found to be in satisfactory agreement with experimental values (R² = .9580 and .9109, respectively). The adequacy of the model was examined by analyzing the residual distribution plots and Pareto graph. X-ray diffraction, scanning electron microscopy (SEM), thermogravimetric analysis, and N₂ adsorption/desorption techniques are employed to characterize the structure of the prepared γ-alumina sample.     

Construction of a novel Ca2Bi2O5/α-Bi2O3 semiconductor heterojunction for enhanced visible photocatalytic application

Bi³⁺-containing compounds have been intensively investigated for their potential application as photocatalysts for degrading pollutants and splitting water. In this work, a Ca₂Bi₂O₅/α-Bi₂O₃ heterojunction photocatalyst was successfully prepared via the facile sol–gel method. The excess of the initial Bi raw material can result in the Ca₂Bi₂O₅/α-Bi₂O₃ heterojunction of the final products. The as-synthesized nanoparticles were investigated via X-ray diffraction, transmission electron microscopy, scanning electron microscopy, energy-dispersive spectrometry, UV–Vis optical absorption, and X-ray photoelectron spectroscopy. The band energy of the Ca₂Bi₂O₅ substrate semiconductor was 2.49 eV and characterized with a direct transition nature. The photocatalytic effect on the photodegradation of Rhodamine B solutions was evaluated. Ca₂Bi₂O₅/α-Bi₂O₃ heterojunctions showed improved photocatalytic abilities compared with single Ca₂Bi₂O₅ and α-Bi₂O₃ under viable light irradiation. The mechanism was discussed in terms of the microstructure, luminescence intensities, and decay curves (lifetimes). The photo-produced electrons and holes can be adequately separated in Ca₂Bi₂O₅/α-Bi₂O₃ heterojunctions ensuring its photocatalytic activities. The present results can serve as reference for investigating the optical properties of Bi semiconductors.     

Synthesis of γ-Ga2O3-Al2O3 solid solutions by spray pyrolysis method

Synthesis of the γ-Ga₂O₃-Al₂O₃ solid solutions by spray pyrolysis was examined. Spherical particles were obtained using an aqueous solution of Al(NO₃)₃ and Ga(NO₃)₃ with HNO₃. For Ga-rich composition, γ-phase solid solutions were directly crystallized by the spray pyrolysis. For Al-rich composition, spray pyrolysis gave amorphous products unless a sufficient thermal energy was supplied during the spray pyrolysis. Subsequent calcination of the amorphous products gave γ-Ga₂O₃-Al₂O₃ solid solutions. However, physical properties of the solid solutions were affected by the spray pyrolysis conditions.     

Synthesis of ZrB2/SiC composite powders by single-step solution process from organic–inorganic hybrid precursor

A precursor of a zirconium diboride/silicon carbide (ZrB₂/SiC) composite was synthesised via an organic–inorganic hybrid derived from gum karaya, tetraethyl orthosilicate, boric acid and zirconyl chloride starting materials. Fourier transform infrared spectroscopy of the as-synthesised dried hybrid revealed the formation of Si–O, Zr–O–C and B–O–B. X-ray diffraction revealed that the powder consists of only ZrB₂ and β-SiC. Scanning electron microscopy and TEM of the composite powders showed that SiC and ZrB₂ occurred in intimately mixed aggregates of spheroidal submicron sized particles for low (3M) boric acid concentration, while at high (5M) boric acid concentration, the two phases are larger with the ZrB₂ adopting a blocky, angular morphology (～10–30?μm long by 5?μm wide and thick), while the SiC remains spheroidal with ～1?μm diameter particles in 10–20?μm diameter aggregates. Thermogravimetry–differential thermal analysis with the help of X-ray diffraction analysis revealed that the formation temperature was low at 1275°C for ZrB₂ and 1350°C for the SiC with 40?wt-% yield.     

Attenuation properties of epoxy-Ta2O5 and epoxy-Ta2O5-Bi2O3 composites at γ-ray energies 59.54 and 662 keV

Epoxy resin filled with suitable high Z elements can be a potential shield for X-rays and γ-rays. In this work, we present the γ-ray attenuation properties of epoxy composites filled with (0–30 wt%) Tantalum pentoxide (Ta₂O₅) and Ta₂O₅-Bi₂O_3, which were prepared by open mold cast technique. X-ray diffraction patterns showed crystalline peaks of Ta₂O₅ and bismuth oxide (Bi₂O₃) in the prepared epoxy-Ta₂O₅ and epoxy-Ta₂O₅-Bi₂O₃ composites. Homogeneity of the samples at higher filler wt% was revealed by SEM images. Mechanical characterization showed the enhanced mechanical strength of epoxy-Ta₂O₅-Bi₂O₃ composites compared to epoxy-Ta₂O₅. Higher storage modulus and glass transition temperature of the epoxy-Ta₂O₅-Bi₂O₃ composites showed enhanced stiffness and thermal stability when compared to neat and epoxy-Ta₂O₅. Decrease in the value of tan(δ) at higher content of filler loadings indicated the good adhesion between filler and matrix. Mass attenuation coefficients of epoxy-Ta₂O₅ (30 wt%) composites at γ-ray energies 59.54 and 662 keV were found to be 0.876 cm² g^–1 and 0.084 cm² g^–1, while that of epoxy-Ta₂O₅-Bi₂O₃ (30 wt% Bi₂O₃) composite were 1.271 cm² g^–1 and 0.088 cm² g^–1, respectively. The epoxy-5% Ta₂O₅-30% Bi₂O₃ composites with higher μ/ρ value and tensile strength may be a potential γ-ray shield in various radiation environments.