Preparation of γ-LiAlO2 green bodies through the gel-casting process

An aqueous gel-casting process using methacrylamide (MAM) as monomer and N,N′-methylenebisacrylamide (MBAM) as crosslinker was developed for γ-LiAlO₂ ceramics. The precursor γ-LiAlO₂ powder with mean particle size of 3.4 μm was synthesized via solid-state reaction process. The zeta potential of the γ-LiAlO₂ powders in deionized water and viscosity of the slurries with different solid loadings were measured. It was found that the solid loading of the γ-LiAlO₂ slurry that met the requirement of gel-casting could reach 56 vol.%. The effects of pH values and the kind of initiator on the copolymerization of MAM/MBAM were investigated. The green bodies with the binder content of 5.3 wt% exhibited a compressive strength of 12.6 MPa and a yield point as high as 6.4 MPa. SEM results revealed homogeneous and compact morphology of the green bodies. In addition, the compressive strength of the sintered γ-LiAlO₂ bodies were studied simultaneously.     

Rheological behavior of aqueous polymer-plasticized γ-LiAlO2 pastes for plastic forming

The rheological properties of aqueous methylcellulose (MC) plasticized γ-LiAlO₂ pastes with high solid loading were investigated.     

An aqueous gel-casting process for γ-LiAlO2 ceramics

Methyl cellulose (MC) was used as the gelation agent in the gel-casting of γ-LiAlO₂ because it could form strong gels during heating. The thermal gelation behavior of the MC solution was studied based on the measurement of its apparent viscosity as a function of temperature. The effects of MC solution concentration and solid loading on the rheological properties of the γ-LiAlO₂ slurries were studied systematically. It was found that all the slurries exhibited a shear-thinning behavior, which was considered to be favorable for the casting process. The compressive strength and relative density of the dried γ-LiAlO₂ green bodies were measured, and the microstructure of the green and sintered bodies was investigated.     

The hydrolysis process of γ-AlON powder and preparation of highly transparent ceramics with aqueous gel-casting

The untreated γ-AlON powders were proposed to prepare highly transparent γ-AlON ceramics by aqueous gel-casting technique and two-step sintering process. The related mechanism of hydrolysis of γ-AlON powders was discussed. The surface of the synthesized γ-AlON powders is covered with a layer of aluminum hydroxides compound. As the period of hydrolysis extends, the surface layer dissolved, and the boehmite and bayerite gradually formed, which is characterized by various techniques including XRD, FT-IR, XPS and TEM. The low viscosity slurry with high solids loading (53 vol%) was obtained by γ-AlON powders without any surface modification treatments as a result of the slow hydrolysis process. In addition, compared with the dry pressed sample, the sample (4.12 mm in thickness) prepared by aqueous gel-casting exhibits superior in-line transmittance of 81–83% in the visible region, which could be ascribed to the more homogenous and denser microstructure of the green body. Finally, the Weibull modulus and characteristic strength of γ-AlON transparent ceramic were determined to be 4.85 and 295 MPa, respectively.     

Investigation of the effect of ZrO2 and ZrO2/Al2O3 additions on the hot-pressing and properties of equimolecular mixtures of α- and β-Si3N4

In this work, hot-pressing of equimolecular mixtures of α- and β-Si₃N₄ was performed with addition of different amounts of sintering additives selected in the ZrO₂–Al₂O₃ system. Phase composition and microstructure of the hot-pressed samples was investigated. Densification behavior, mechanical and thermal properties were studied and explained based on the microstructure and phase composition. The optimum mixture from the ZrO₂–Al₂O₃ system for hot-pressing of silicon nitride to give high density materials was determined. Near fully dense silicon nitride materials were obtained only with the additions of zirconia and alumina. The liquid phase formed in the zirconia and alumina mixtures is important for effective hot-pressing. Based on these results, we conclude that pure zirconia is not an effective sintering additive. Selected mechanical and thermal properties of these materials are also presented. Hot-pressed Si₃N₄ ceramics, using mixtures from of ZrO₂/Al₂O₃ as additives, gave fracture toughness, K_IC, in the range of 3.7–6.2 MPa m^1/2 and Vicker hardness values in the range of 6–12 GPa. These properties compare well with currently available high performance silicon nitride ceramics. We also report on interesting thermal expansion behavior of these materials including negative thermal expansion coefficients for a few compositions.     

Effect of yttrium and praseodymium on properties of Ce0.75Zr0.25O2 solid solution for CH4–CO2 reforming

Ce_0.75Zr_0.25O₂ solid solutions doped with Y³⁺ or Pr⁴⁺/Pr³⁺ were prepared by the co-precipitation method, and their physicochemical properties were characterized by means of N₂ adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, FT-Raman, and H₂ temperature-programmed reduction and thermogravimetric analysis. Their performance in CH₄–CO₂ reforming was also tested in an atmospheric fixed-bed reactor. Ce_0.75Zr_0.25O₂ and Y³⁺ or Pr⁴⁺/Pr³⁺ doped Ce_0.75Zr_0.25O₂ solid solutions are of CaF₂ structure, and the thermal stability of Ce_0.75Zr_0.25O₂ is enhanced by doping Y³⁺ or Pr⁴⁺/Pr³⁺. Comparing with Ce_0.75Zr_0.25O₂, the migration of bulk lattice oxygen species become easier and the content of surface oxygen species is higher in the doped Ce_0.75Zr_0.25O₂, which is due to either oxygen vacancies or/and structural distortion resulted from the doping. The activity of the solid solutions in CH₄–CO₂ reforming is closely related to the surface oxygen species. Y³⁺ or Pr⁴⁺/Pr³⁺ doped Ce_0.75Zr_0.25O₂, especially the former, show higher activity than Ce_0.75Zr_0.25O₂, and Y³⁺ doped Ce_0.75Zr_0.25O₂ possesses better stability. All of the catalysts have good coke resistance. The catalyst deactivation is mainly due to the catalyst sintering.     

Synthesis and characterization of MgAl2O4–ZrO2 composites

Different types of dense 5–97% ZrO₂–MgAl₂O₄ composites have been prepared using a MgAl₂O₄ spinel obtained by calcining a stoichiometric mixture of aluminium tri-hydroxide and caustic MgO at 1300 °C for 1 h, and a commercial yttria partially stabilized zirconia (YPSZ) powder as starting raw materials by sintering at various temperatures ranging from 1500 to 1650 °C for 2 h. The characteristics of the MgAl₂O₄ spinel, the YPSZ powder and the various sintered products were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), BET surface area, particle size analysis, Archimedes principle, and Vickers indentation method. Characterization results revealed that the YPSZ addition increases the sintering ability, fracture toughness and hardness of MgAl₂O₄ spinel, whereas, the MgAl₂O₄ spinel hampered the sintering ability of YPSZ when sintered at elevated temperatures. A 20-wt.% YPSZ was found to be sufficient to increase the hardness and fracture toughness of MgAl₂O₄ spinel from 406 to 1314 Hv and 2.5 to 3.45 MPa m^1/2, respectively, when sintered at 1600 °C for 2 h.     

Effects of γ-radiation versus H2O2 on carbon steel corrosion

The effect of ionizing radiation on steel corrosion is an important materials issue in nuclear reactors. In the presence of ionizing radiation water decomposes into both oxidizing and reducing species (e.g., OH, H₂O₂, O₂⁻) whose net interactions with steels are not fully understood. The effect of radiation on the corrosion kinetics of carbon steel has been studied at pH 10.6 and room temperature, using electrochemical and chemical speciation analyses. The present study investigates the effect of γ-radiation on carbon steel corrosion and compares it with that of chemically added H₂O₂, which is considered to be the key radiolytically produced oxidant at room temperature. Various oxide films were pre-grown potentiostatically on carbon steel electrodes, and then exposed to either γ-radiation at a dose rate of ∼6.8 kGy h⁻¹ or to H₂O₂ in a concentration range of 10⁻⁶ to 10⁻² M. The corrosion kinetics were studied by monitoring the corrosion potential (E_CORR), and periodically performing linear polarization (LP) and electrochemical impedance spectroscopy (EIS) measurements.     

Controlled precipitation of W2B4 platelets and of β-WB nanolaminates for the in situ reinforcement of ternary TiB2-W2B4-CrB2 ceramics

Mechanical reinforcement of hard ultrahigh-temperature materials often requires complex processing techniques involving the mixing of platelet-shaped particles into the matrix - ultimately leading to inhomogeneities that compromise strength and fracture toughness. Thus, this paper investigates in situ reinforcement of a TiB₂ ceramic matrix by controlled precipitation of W₂B₄ platelets and β-WB nanolaminates. The results showed successful precipitation of: (i) W₂B₄ platelets according to the phase diagram, (ii) epitaxial β-WB lamellas, and (iii) spinodal, i.e. fully coherent W-rich lamellas which change to either W₂B₄ platelets or β-WB during ageing. The study of phase amount and microstructure as a function of homogenization temperature and annealing temperature/time regimes allows one to control the volume fraction, size as well as aspect ratio of precipitates. This enables the control of fracture toughness affected by crack deflection, particle pull-out, crack bridging and crack branching. Composition and milling treatment most affect microstructure. Successful precipitation is possible at 1650 °C.     

Densification and characterization of SiO2B2O3CaOMgO glass/Al2O3 composites for LTCC application

A glass/ceramic composite using lead-free low melting glass (SiO₂B₂O₃CaOMgO glass) with Al₂O₃ fillers was investigated. X-ray diffraction analysis revealed that the anorthite and cordierite phase appeared in the sintered composites. The dilatometric analysis showed that the onset of shrinkage took place at ∼624 °C for all the samples and the onset temperature was independent on the content of glass. The low melting glass significantly promoted densification of the composites and lowered the sintering temperature to ∼875 °C. The addition of 50 wt% glass sintered at 875 °C showed ε_r of 7.3, tan δ of 1.15×10⁻³, TEC of 5.41 ppm/°C, thermal conductivity of 3.56 W/m °C, and flexural strength of 184 MPa. The results showed that the SiO₂B₂O₃CaOMgO glass/Al₂O₃ composites were strong potential candidates for low temperature cofired ceramic substrate applications.     

Spinelisation and properties of Al2O3–MgAl2O4–C refractory: Effect of MgO and Al2O3 reactants

The effect of particle size of MgO and Al₂O₃ on the spinel formation associated with permanent linear change on reheating (PLCR) and microstructure of Al₂O₃–MgAl₂O₄–C refractory is investigated as a function of heating cycle at 1600 °C with 2 h holding at each cycle. It was found that rate of spinel formation and associated volume expansion is very much dependent on the reactivity and particle size of the reactant. When the reactants are very fine and reactive there is considerable amount of spinel formation, whereas coarser reactants with lower reactivity show negligible formation of spinel phase and associated expansion. Magnesia and alumina with moderate reactivity develops optimum PLCR of the refractory. It continuously increases with the number of heating cycles. The SEM photomicrographs show that in Al₂O₃–MgAl₂O₄–C refractory the spinel phase is formed in between the calcined bauxite grain and the EDX analysis indicates that the spinel phase formed is stoichiometric in nature.     

An electrochemical study of H2O2 decomposition on single-phase γ-FeOOH films

The electrochemical reduction and oxidation kinetics of hydrogen peroxide on γ-FeOOH films chemically deposited on indium tin oxide substrates were studied over the pH range of 9.2-12.6 and the H₂O₂ concentration range of 10⁻⁴ to 10⁻² mol dm⁻³. The Tafel slopes for H₂O₂ reduction and oxidation obtained from polarization measurements are 106 ± 4 and 93 ± 15 mV dec⁻¹, respectively, independent of pH and the concentration of H₂O₂. Both the reduction and oxidation of H₂O₂ on γ-FeOOH have a first-order dependence on the concentration of molecular H₂O₂. However, for the pH dependence, the reduction has an inverse first-order dependence, whereas the oxidation has a first-order dependence, on the concentration of OH⁻. For both cases the electroactive species is the molecular H₂O₂, not its base form, HO₂⁻. Based on these observations, reaction kinetic mechanisms are proposed which involve adsorbed radical intermediates; HOOH⁻ and HO for the reduction, and HO₂H⁺, HO₂, and O₂⁻ for the oxidation. These intermediates are assumed to be in linear adsorption equilibria with OH⁻ and H⁺ in the bulk aqueous phase, respectively, giving the observed pH dependences. The rate-determining step is the reduction or oxidation of the adsorbed H₂O₂ to the corresponding intermediates, a reaction step which involves the use of Fe^III/Fe^II sites in the γ-FeOOH surface as an electron donor-acceptor relay. The rate constant for the H₂O₂ decomposition on γ-FeOOH determined from the oxidation and reduction of Tafel lines is very low, indicating that the γ-FeOOH surface is a very poor catalyst for H₂O₂ decomposition.     

Influences of La and Er on structure and properties of Bi2O3–B2O3 glass

Bi₂O₃·2B₂O₃ glasses doped with La₂O₃ and Er₂O₃ were prepared by the melting-quenching method with AR-grade oxides. IR analysis was used to investigate the glass network structure. The characteristic temperatures including the glass transition temperature (T_g), crystallization temperature (T_p), and melting temperature (T_m) were estimated by DSC. The coefficient of thermal expansion (α), mass density (D), and Vickers hardness (H_v) were also measured. The results show that the basic network structure of Bi₂O₃·2B₂O₃ glasses doped with rare-earth oxides consists of chains composed of [BO₃], [BO₄], and [BiO₆] units. La₂O₃ and Er₂O₃ act as network modifiers. As the doping concentrations of the rare-earth oxides were increased, T_g increased and α decreased, indicating that a more rigid glass was obtained. Er₂O₃ reduces the melting temperature and prevents glass crystallization. La₂O₃ contributes to the improvement of the microhardness of Bi₂O₃·2B₂O₃ glass.     

Hydrogen-bond mediated and concentrate-dependent NaHCO3 crystal morphology in NaHCO3–Na2CO3 aqueous solution: Experiments and computer simulations

Adding Na₂CO₃ to the NaHCO3cooling crystallizer, using the common ion effect to promote crystallization and improve product morphology, is a new process recently proposed in the literature. However, the mechanism of the impact of Na₂CO₃ on the crystal morphology is still indeterminate. In this work, the crystallization of NaHCO₃ in water and Na₂CO₃ –NaHCO₃ aqueous solution was investigated by experiments and ...     

Nanofluids Containing γ-Fe2O3 Nanoparticles and Their Heat Transfer Enhancements

Homogeneous and stable magnetic nanofluids containing γ-Fe₂O₃ nanoparticles were prepared using a two-step method, and their thermal transport properties were investigated. Thermal conductivities of the nanofluids were measured to be higher than that of base fluid, and the enhanced values increase with the volume fraction of the nanoparticles. Viscosity measurements showed that the nanofluids demonstrated Newtonian behavior and the viscosity of the nanofluids depended strongly on the tested temperatures and the nanoparticles loadings. Convective heat transfer coefficients tested in a laminar flow showed that the coefficients increased with the augment of Reynolds number and the volume fraction.     

Piezocatalytic performance of Fe2O3−Bi2MoO6 catalyst for dye degradation

A Fe₂O₃−Bi₂MoO₆ heterojunction was synthesized via a hydrothermal method. Scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray, powder X-ray diffraction, Fourier transform infrared spectroscopy and ultra-violet−visible near-infrared spectrometry were performed to measure the structures, morphologies and optical properties of the as-prepared samples. The various factors that affected the piezocatalytic property of composite catalyst were studied. The highest rhodamine B degradation rate of 96.6% was attained on the 3% Fe₂O₃−Bi₂MoO₆ composite catalyst under 60 min of ultrasonic vibration. The good piezocatalytic activity was ascribed to the formation of a hierarchical flower-shaped microsphere structure and the heterostructure between Fe₂O₃ and Bi₂MoO₆, which effectively separated the ultrasound-induced electron–hole pairs and suppressed their recombination. Furthermore, a potential piezoelectric catalytic dye degradation mechanism of the Fe₂O₃−Bi₂MoO₆ catalyst was proposed based on the band potential and quenching effect of radical scavengers. The results demonstrated the potential of using Fe₂O₃−Bi₂MoO₆ nanocomposites in piezocatalytic applications.     

Modifications of CeO2–ZrO2 solid solutions by nickel and sulfate as catalysts for NO reduction with ammonia in excess O2

Nickel and sulfate were impregnated on CeO₂–ZrO₂ to improve the activity and selectivity of catalyst for NO abatement with ammonia. The performance of catalyst is related to the types of surface acid sites. Lewis acid sites, of which the strength is increased by modification of nickel, are considered as the essential active sites for low-temperature NH₃–SCR reaction. The introduction of Brønsted acid sites by sulfate modification weakens the strong oxidation of ammonia but enhances the ammonia adsorption capacity of catalyst. Therefore, the high-temperature activity of catalysts is also improved.     

Experiments and transient finite element simulation of γ-Y2Si2O7/B2O3-Al2O3-SiO2 glass coating on porous Si3N4 substrate under thermal shock

A dense γ-Y₂Si₂O₇/B₂O₃-Al₂O₃-SiO₂ glass coating was fabricated by slurry spraying method on porous Si₃N₄ ceramic for water resistance. Thermal shock failure was recognized as one of the key failure modes for porous Si₃N₄ radome materials. In this paper, thermal shock resistance of the coated porous Si₃N₄ ceramics were investigated through rapid quenching thermal shock experiments and transient finite element analysis. Thermal shock resistance of the coating was tested at 700 °C, 800 °C, 900 °C and 1000 °C. Results showed that the cracks initiated within the coating after thermal shock from 800 °C to room temperature, thus leading to the reduction of the water resistance. Based on the finite element simulation results, thermal shock failure tended to occur in the coating layer with increasing temperature gradient, and the critical thermal shock failure temperature was measured as 872.24 °C. The results obtained from finite element analysis agree well with that from the thermal shock tests, indicating accuracy and feasibility of this numerical simulation method. Effects of thermo-physical properties for the coating material on its thermal shock resistance were also discussed. Thermal expansion coefficient of the coating material played a more decisive role in decreasing the tangent tensile stress.     

The role of CeO2–ZrO2 as support in the ZnO–ZnCr2O4 catalysts for autothermal reforming of methanol

Autothermal reforming of methanol for hydrogen production was investigated over ZnO–ZnCr₂O₄ supported on a series of metal oxides (Al₂O₃, CeO₂, ZrO₂ and CeO₂–ZrO₂). CeO₂–ZrO₂ mixed oxides with Ce /Zr molar ratio of 4/1 was found to be the optimal support which showed significant effect on the catalytic activity and selectivity. The ZnO–ZnCr₂O₄/CeO₂–ZrO₂ and ZnO–ZnCr₂O₄ catalysts were characterized by XRD, TEM, H₂-TPR and XPS. The results show that CeO₂–ZrO₂ mixed oxides have significant effect on the catalytic performance and the supported catalyst shows more uniform temperature distribution in the catalyst bed which was mainly due to its reasonable redox properties.     

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.