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Aluminum hydroxide is an eye-catching and extensively researched adsorbent for fluoride removal and its defluoridation performance is closely related to the preparation method and crystalline phase. In this research, the defluoridation performances of aluminum hydroxides with different crystalline phases are compared and evaluated in terms of fluoride removal capacity, sensitivity to pH values and residual Al contents after defluoridation. It is found that the defluoridation performance of different aluminum hydroxides follows the order of boehmite > bayerite > gibbsite. The fluoride adsorption on aluminum hydroxides follows the pseudo-second-order kinetic model and Langmuir isotherm model, and the maximum defluoridation capacities of boehmite, bayerite and gibbsite are 42.08, 2.97 and 2.74 mg m−2, respectively. The pH values and FTIR analyses reveal that the ligand exchange between fluoride and surface hydroxyl groups is the fluoride removal mechanism. Different aluminum hydroxides have different surface hydroxyl group densities, which results in the different defluoridation capacities. This work provides a new idea to prepare aluminum hydroxide with outstanding defluoridation performance. HIGHLIGHTS The defluoridation performance of boehmite, bayerite and gibbsite is compared.; Boehmite exhibits better defluoridation performance than bayerite and gibbsite.; Fluoride removal mechanism was revealed through pH value and FTIR analyses.; Adsorption capacity of aluminum hydroxide depends on surface hydroxyl group density.;

Ultrasound can accelerate and change the reaction process and is widely used in the field of hydrogen production and storage. In this study, ultrasound (US) and AlOOH suspension (AH) are used to promote hydrogen production from Al hydrolysis. The results indicate that both US and AH greatly shorten the induction time and enhance the hydrogen production rate and yield. The promoting effect of US and AH on Al hydrolysis originates from the acoustic cavitation effect and catalytic effect, respectively. When AH is used in combination with US, Al hydrolysis has the best hydrogen production performance and the hydrogen yield can reach 96.6 % within 1.2 h, because there is a synergistic effect on Al hydrolysis between AH and US. Mechanism analyses reveal that the micro-jets and local high temperature environment arising from acoustic cavitation improve the catalytic activity of AlOOH, while the suspended AlOOH particles enhance the cavitation effect of US. This work provides a novel and feasible method to promote hydrogen production from Al hydrolysis.

Abstract Hydrogen generation via Al‐water reaction promoted by special catalysts has great potential, which can provide hydrogen source for portable fuel cells. In this work, we use wet chemical reduction method to prepare a high‐activity Fe/AlOOH composite with better stability and demonstrate its usage as a catalyst for Al‐water reaction. It is found that Fe/AlOOH composite not only significantly promotes the Al‐water reaction but also shortens the induction time. The catalytic activity of Fe/AlOOH directly correlated with its active area, which, in turn, is closely related to the AlOOH:Fe molar ratio. In addition, increasing the weight content of Fe/AlOOH, reaction temperature, and the Al:water ratio improve the hydrogen generation performance of Al‐water reaction. Mechanism analyses show that water dissociation on AlOOH surface and galvanic microinteractions between Fe and Al promotes the hydration of surface passivating oxide film of Al particle, which improves the Al‐water reaction dynamics. The present work provides a new way to produce hydrogen from Al‐water reaction for portable devices.

Superconducting MgB2–MgO composite with about 75% mole concentration of MgO was synthesized in situ by a single-replacement reaction. The resistance versus temperature curve shows that the composite has a high superconducting transition temperature to 38.0 K and metallic transport behavior with low resistivity and highly residual resistance ratio comparable to a pure MgB2 sample. These characteristics are explained by using the statistical percolation model and a conductivity expression with temperature for the metal–insulator MgB2 composite is given in this paper. The results indicate that MgB2 superconductor can tolerance a high content of insulating contamination and this metal–insulator MgB2–MgO composite can be utilized for the superconducting fault current limiter. [ABSTRACT FROM AUTHOR]

Superconducting Mg[B.sub.2]-MgO composite with 75% mole concentration of MgO is prepared in situ by a single-replacement reaction. The results have shown that Mg[B.sub.2] superconductor can tolerate a high content of insulating contamination and the metal-insulator Mg[B.sub.2]-MgO composite is used for the superconducting fault current limiter.

Four different-sized pure Al powders were used to react with distilled water at 55°C in a low vacuum, and their effect on the reaction dynamics and hydrogen-generation rate was investigated. It was found that the Al particle sizes not only affect the hydrogen-generation rate but also affect the induction time for the beginning of the reaction. The induction time strongly depends on the Al particle sizes, which obviously increased with the Al particle sizes. The hydrogen-generation rate increased with the decreasing Al particle sizes, because the small-sized Al powder has a high surface area. Mechanism analyses revealed that the long induction time for the large Al particles probably originates from a long time for their H diffusion to reach a saturation concentration in the bulk Al metal. The reaction rate of Al with water could be qualitatively explained by a shrinking core model. [ABSTRACT FROM AUTHOR]

Unlike traditional batteries, small fuel cells have a high energy density and can work uninterruptedly, being better energy suppliers for portable devices. Such devices require an economically viable fuel. Recent findings showed that metal Al particle surfaces could be modified by fine ceramic oxide grains through a ceramic processing procedure, and the modified Al powder could continuously react with pure water and generate hydrogen under ambient conditions. The reaction of Al with water produces as much as 3.7–4.8 wt% hydrogen, and the reaction byproducts are chemically neutral. Metal Al is cheap and hydrogen generation from the reaction between surface-modified Al particles and water is a simple process. These features make this new process a cost-efficient way of generating hydrogen for small fuel cells in comparison with other portable hydrogen-generation materials and technologies. In this paper, the state of the art of portable hydrogen-generation materials is surveyed, and the ceramic oxide-modified Al–hydrogen technology and its potential are highlighted. [ABSTRACT FROM AUTHOR]

Long crack R-curve of a porous Si3N4 with aligned fibrous grains was investigated, using a chevron-notched beam technique. A crack was constrained to propagate normal to the grain alignment. The crack growth resistance of aligned porous SigN4 was much larger compared with that of dense Si3N4 ceramics. Microstructure observations showed that pullouts of fibrous grains in aligned porous Si3N4 markedly increased during crack propagation relative to those of dense Si3N4, due to the existence of pores. The efficient grain pullouts in porous Si3N4 increased the bridging stress at the crack wake. [ABSTRACT FROM AUTHOR]

Al[sub 2]O[sub 3]-5 vol% SiC particle composites doped with 800 ppm rare-earth impurities (Y[sup 3+], Nd[sup 3+], and La[sup 3+]) were fabricated by hot-pressing at a temperature of 1550°C. Doping of rare earth impurities in Al[sub 2]O[sub 3]-SiC composites led to a fracture mode change from transgranular in dopant-free composites to intergranular in rare-earth-doped composites. The fracture mode change obviously increased the crack deflection so that the fracture toughness of rare-earth-doped composites was higher than that of the composites without dopants, especially for the Nd[sup 3+]- and La[sup 3+]-doped composites. It was found that the fracture-mode change originated from a weak grainboundary bonding caused by co-segregation of the rare-earth dopants and Si[sup 4+] ions dissolved from the SiC particle surfaces. [ABSTRACT FROM AUTHOR]

Examines the synthesis of porous silicon nitride using freeze-drying process. Sublimation of ice during freeze drying for generation of pores; Obtainment of porous silicon nitride with high porosity; Importance of vapor/solid phase reaction to formation of mechanism of fibrous grains.

Examines the effects of agglomeration on mechanical properties of porous zirconia fabricated by partial sintering. Use of unidirectional pressures to prepare green compacts; Types of oxide ceramics; Improvement on the uniformity of microstructures and overall mechanical properties.

Presents a model analysis of multiple cracking in ceramic and superplastic laminates based on bending theory. Two basic processes that occur after a ceramic layer fractures; Relaxation process of the residual stress in the ceramic layer; Shear process of superplastic flow; Agreement of theoretical predications with experimental results.

Investigates the microstructure and mechanical properties of porous alumina ceramics fabricated by aluminum hydroxide (Al(OH)[sub 3]) decomposition. Determination of the fracture strength of porous ceramics; Interrelation between pore structures and Al(OH)[sub 3] particles; Increase in fracture toughness in specimens with lower porosity.

Examines the fabrication of high-surface-area alumina ceramics by Al(OH)[sub 3] decomposition. Types of starting powders used; Effect of Al(OH)[sub 3] addition on Al[sub 2]O[sub 3] surface area; Factors attributing to the production of teta-Al[sub 2]O[sub 3].

Previous works indicated that Al particle surfaces could be modified by fine γ-Al2O3 grains, which can be used as a hydrogen-generation material, where the fine γ-Al2O3 grains were produced by the decomposition of the Al(OH)3 phase in the mixture. In this work, commercially available γ-Al2O3 powder was directly used as the modification agent and mixed with Al powder and heat treated at an elevated temperature. It was found that the modified Al powder produced by directly using γ-Al2O3 has an obvious shorter time for complete hydrogen generation than that using Al(OH)3-produced γ-Al2O3. Microstructure analyses revealed that directly using γ-Al2O3 powder has a better coverage of fine γ-Al2O3 grains on Al particle surfaces than the Al(OH)3-produced γ-Al2O3. This implies that a uniform distribution of modification agents on Al particle surfaces is an important factor for the Al–water reaction dynamics. [ABSTRACT FROM AUTHOR]

The effect of temperature on the reaction of γ-Al2O3-modified Al powders with distilled water was investigated. It was found that by increasing the temperature up to 40°C, the hydrogen generation speed can be enhanced one to two orders of magnitude relative to that at room temperature (18°C). X-ray analyses and transmission electron microscopy observations revealed that the reaction by-product at 40°C is bayerite (Al(OH)3), which is chemically neutral. The present results imply that slightly increasing the temperature is an effective way to get the target hydrogen generation speed. [ABSTRACT FROM AUTHOR]

Metal Al particle surfaces were modified by fine γ-Al2O3 grains using a mixture of Al and Al(OH)3 powders, which was pressureless sintered at a temperature of 600°C in vacuum. All the constituents on original Al particle surfaces were transformed into γ-Al2O3 phase after sintering. No dense passive oxide layers on Al particle surfaces were formed even after the modified Al particles were exposed to oxidation environment. The modified Al particles could continuously react with pure water and generate hydrogen at room temperature. The present finding implies that metal Al could become hydrogen-generation material by surface modification. [ABSTRACT FROM AUTHOR]

In this work, 800 ppm of Zr4+ dopants were added to AI2O3-5 vol% SiC particle composite. Zr4+ doping led to a weak AI203 grain-boundary bonding so that the fracture mode changed from transgranular in undoped composite to intergranular in Zr4+-doped composite. The fracture mode change increased the fracture toughness of the composite. Transmission electron microscopy and energy-dispersive spectroscopy examinations revealed that the weak grain-boundary bonding in the doped composite was caused by the segregation of Zr4+ and Si4+ ions at the AI2O3 grain boundary. [ABSTRACT FROM AUTHOR]

Porous silicon carbide (SIC) ceramics were fabricated by an oxidation-bonding process in which the powder compacts are heated in air so that SiC particles are bonded to each other by oxidation-derived SiO[sub 2] glass. Because of the crystallization of amorphous SiO[sub 2] glass into cristobalite during sintering, the fracture strength of oxidation-bonded SiC ceramics can be retained to a relatively high level at elevated temperatures. It has been shown that the mechanical strength is strongly affected by particle size. When 0.6 µm SiC powders were used, a high strength of 185 MPa was achieved at a porosity of ∼31%. Moreover, oxidation-bonded SiC ceramics were observed to exhibit an excellent oxidation resistance. [ABSTRACT FROM AUTHOR]