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.     

Processing and microstructure of γ-LiAlO2 ceramics

γ-LiAlO₂ ceramics with different grain sizes were prepared by controlling the sintering process and regulating the size and shape of the precursor powders. It was found that a size gradation of powders promoted the growth of γ-LiAlO₂ grains. Ceramics with an average grain size of 10 μm were prepared from the size-graded powders. It was demonstrated that the shape of the precursor powders greatly affected the grain growth of the ceramics whereas the granulation of the powders restrained the abnormal grain growth. Furthermore nano-sized precursor powders obtained by a sol–gel route made it possible to prepare nano-structured γ-LiAlO₂ ceramics.     

Rheology and chemorheology of aqueous γ-LiAlO2 slurries for gel-casting

The rheological and chemorheological properties of aqueous γ-LiAlO₂ slurries for gel-casting as well as their precursor solutions of methacrylamide and N,N′-methylenebisacrylamide (MAM/MBAM) were studied in the present work. The influences of mean particle size of γ-LiAlO₂ powder, the dispersant content and solid loading, mechanical stirring time on the rheology of γ-LiAlO₂ slurries were investigated systematically. It was demonstrated that the slurries exhibited shear-thinning behaviors with relatively low viscosity which could satisfy the gel-casting process. A rotary rheometer was used to study the chemorheology of the copolymerization of MAM/MBAM solution and the gelation process of γ-LiAlO₂ slurries. The activation energy calculated from the idle time for MAM/MBAM solutions and γ-LiAlO₂ slurries were 27.5 ± 0.4 kJ/mol and 25.3 ± 0.4 kJ/mol, respectively, which revealed the catalytic effect of the γ-LiAlO₂ particles on the copolymerization process.     

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.     

Low-temperature synthesis and microwave dielectric properties of trirutile-structure MgTa2O6 ceramics by aqueous sol–gel process

Trirutile-structure MgTa₂O₆ ceramics were prepared by aqueous sol–gel method and microwave dielectric properties were investigated. Highly reactive nanosized MgTa₂O₆ powders were successfully synthesized at 500 °C in oxygen atmosphere with particle sizes of 20–40 nm. The evolution of phase formation was detected by DTA–TG and XRD. Sintering characteristic and microwave dielectric properties of MgTa₂O₆ ceramics were studied at different temperatures ranging from 1100 to 1300 °C. With the increase of sintering temperature, density, ?_r and Q · f values increased and saturated at 1200 °C with excellent microwave properties of ?_r ～ 30.1, Q · f ～ 57,300 GHz and τ_f ～ 29 ppm/°C. The sintering temperature of MgTa₂O₆ ceramics was significantly reduced by aqueous sol–gel process compared to conventional solid-state method.     

UV/S2O82− process for degrading polyvinyl alcohol in aqueous solutions

This investigation evaluates the effectiveness of the UV/S₂O₈²⁻ process in the degradation of polyvinyl alcohol (PVA) in aqueous solutions without adjusting their pH. The effects of UV wavelength and Na₂S₂O₈ dosage on the efficiency of degradation of PVA were examined. The efficiency of degradation of PVA under UV-254 nm exceeded that under UV-365 nm. A larger Na₂S₂O₈ dosage was associated with a higher efficiency of degradation of PVA under UV-254 nm irradiation. However, an excessive Na₂S₂O₈ dosage inhibited the degradation of PVA by the UV-365 nm/S₂O₈²⁻ process. Both UV-254 nm/S₂O₈²⁻ and UV-365 nm/S₂O₈²⁻ processes exhibited pseudo-first-order kinetics. SO₄⁻ was detected by performing quenching studies using specific alcohols, revealing that SO₄⁻ was found to be the predominant radical in the UV-254 nm/S₂O₈²⁻ process. Additionally, the presence of inorganic anions with various effects inhibited the degradation of PVA by the UV-254 nm/S₂O₈²⁻ process. Complete degradation of PVA (20 mg/L) was achieved within 5 min under UV-254 nm using an Na₂S₂O₈ dosage of 0.12 g/L in the absence of inorganic anions, indicating that UV irradiation to activate S₂O₈²⁻ promotes the degradation of PVA in aqueous solutions without adjusting their pH.     

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.     

Observation of specific optical phonon modes dominating Li ion diffusion in γ-LiAlO2 ceramic

In γ-LiAlO₂ ceramic, Li ion diffusion plays a key role both in tritium release as a solid tritium breeder material in nuclear fusion and in phase stability as a matrix material in molten carbonate fuel cells (MCFC). Yet fundamental understanding of the diffusive process in γ-LiAlO₂ is still missing, especially considering the interaction of the lattice dynamics and Li ion motion. Herein, we demonstrated that two specific optical phonon modes are coupled with Li ion diffusion in γ-LiAlO₂, by investigating the temperature-dependent lattice dynamics via Raman scattering experiments, as well as first-principles calculations and neutron diffraction experiments (ND). The high-temperature ND experiments showed that γ-LiAlO₂ undergoes partial Li ion disordering in Li sublattice at high temperature. Notably, the lattice dynamics studies demonstrated that B₁ mode at 226 cm^?1 and A₁ mode at 263 cm^?1 are responsible for Li ion motion through the oscillation of Li–O in <010> and <001> direction, respectively. Our results further suggest that the behavior of tritium release and phase stability in γ-LiAlO₂ at elevated temperature may be controlled by tuning the two specific phonon modes through photon/electron-phonon coupling.     

Piezoelectric porous α-quartz membrane by aqueous gel-casting with enhanced antifouling and mechanical properties

Porous α-quartz ceramics with improved mechanical strength were fabricated by using silica sol as a bonding agent in an aqueous gel-casting process. α-quartz dispersion was prepared with a solid load of 75 wt% and a viscosity of 1.66 Pa·s. α-Al₂O₃ particles were added to promote dispersion of the bonding agent. The optimum amounts of amorphous silica and α-Al₂O₃ were 10 % and 8 % of the mass of α-quartz powder, respectively. The thermal treatment of α-quartz green bodies was optimized for mechanical and permeation properties. Electrical fields up to 1 kV/mm were applied to the green bodies to accomplish α-quartz grains orientation and hence piezoelectric response. This caused an increase of piezo-acoustic response by 58 % at an excitation frequency of 200 kHz. In-situ mitigation of fouling was obtained by applying an alternating voltage of 60 V, during TiO₂ suspension filtration, which resulted in an increase of the stationary flux by 23 %.     

Synthesis,characterization, and microwave dielectric properties of ZnTiTa2O8 ceramics with ixiolite structure obtained through the aqueous sol–gel process

Nano-sized ZnTiTa₂O₈ powders with ixiolite structure, with particle sizes ranging from 10 nm to 30 nm, were synthesized by thermal decomposition at 950 °C. The precursors were obtained by aqueous sol–gel and the compacted and sintered ceramics with nearly full density were obtained through subsequent heat treatment. The microstructure and electrical performance were characterized by field emission scanning electron microscopy, x-ray diffraction, and microwave dielectric measurements. All the samples prepared in the range 950–1150 °C exhibit single ixiolite phase and relative density between ~87% and ~94%. The variation of permittivity and Q·ƒ value agreed with that of the relative density. Pure ZnTiTa₂O₈ ceramic sintered at 1050 °C for 4 h exhibited good microwave dielectric properties with a permittivity of 35.7, Q·ƒ value of 57,550 GHz, and the temperature coefficient of resonant frequency of about −24.7 ppm/°C. The relatively low sintering temperature and excellent dielectric properties in the microwave range would make these ceramics promising for applications in electronics.     

The influence of additives on the grinding of γ-alumina for the production of high-alumina ceramics

Reactivity of alumina can be increased by decreasing its ultimate crystal size. Dry ball milling, with some organic grinding aids, minimised mill packing and promoted fine grinding of γ-alumina. The effects of oleic acid, stearic acid, aeropromotor 765 and naphthenic acid on the size distribution and specific surface of the ground product were investigated. Aeropromotor 765 and naphthenic acid were both found to be effective in grinding, but their combined effect is more pronounced after firing. Ground products obtained by dry ball milling had satisfactory bulk density and cold crushing strength. The effect of these reagents on the shrinkage of alumina fired bodies had been shown. Distinct optima corresponding to the relative effect of these reagents on the firing characteristics of alumina bodies were correlated with their effectiveness in grinding. At the optimum conditions, a ground product pressed at 720 kg/cm² fired at 1550°C for 30 minutes, had a bulk density of 1.94 g/cm³, a cold crushing strength of 296.6 kg/cm² and 8.95% shrinkage.     

Porous zirconium titanate ceramics synthesized by sol–gel process

Zirconium titanate, ZrTiO₄, was synthesized by sol–gel method from zirconium butoxide and titanium isopropoxide. Amorphous ZrTiO₄ powder was ground, calcined at 500 °C, and milled to homogenize size distribution of the powder. Milled powder was pressed into tablets and sintered at 900–1400 °C for 8 h. Differential scanning calorimetry and dilatometric studies indicated crystallization of ZrTiO₄ at 600–700 °C. Raman spectroscopy and X-ray diffraction analysis confirmed presence of crystallized ZrTiO₄ already at 900 °C, and crystallite size was determined by Scherrer equation. Scanning electron microscopy showed that ZrTiO₄ grains begin to sinter at higher temperatures, starting from 1200 °C, while preserving high porosity up to 1300 °C as confirmed by dilatometry and mercury intrusion porosimetry.     

Improved γ-alumina support based pseudo-boehmite shaped by micro-extrusion process for oxygen carrier support application

γ-Alumina extrudates for chemical-looping combustion in fluidized bed reactors were shaped by varying acetic acid concentrations between 0.07 and 3.76 M. Influence of pseudo-boehmite peptization on structural properties, microstructure, chemical phases and attrition resistance was determined. With addition of acetic acid, the d₉₀ of boehmite agglomerates after 1 h kneading decreased from 134 to 40 μm at pH 4. Due to this, the extrusion diameter was reduced from 1500 to 200 μm, as well as median pore radii (from 30.1 to 5.3 nm). Porosity was about 70%. Addition of more than 1.87 M acid lead to a slight increase in mesopore sizes caused by some pore blocking caused by the formation of aluminium acetate salts. A small micropore surface was determined with t-layer model from Harkins and Jura. Higher attrition resistance was observed for samples peptized with lower acid concentration because of the closer contact between particles after decomposition.     

新型衬底材料γ-LiAlO2热稳定性研究

研究了温度梯度法生长的γ-LiAlO2晶体在1100℃不同气氛处理的热稳定性。借助扫描电子显微镜和X射线衍射分析发现：1100℃／10h真空和空气处理后的样品表面分别形成了均匀微孔状和岛状LiAl5O8多晶;而富Li气氛处理后样品表面仍然光滑且仍为LiAlO2。用Leitz大视场显微镜观察重新抛光后样品内部宏观缺陷,发现空气和真空处理后样品缺陷增多,分别为岛状和条纹状缺陷;而富Li气氛处理后样品中缺陷明显减少。对比不同气氛处理的4块样品吸收光谱,确认了真空和空气处理产生的196nm吸收峰可能是Li空位吸收峰。     

Adsorption of γ-picoline onto acid-activated bentonite from aqueous solution

The adsorption of γ-picoline onto acid-activated bentonite was investigated. The adsorption reached a maximum at pH = 7. Pseudo-first-order, pseudo-second-order, the Elovich equation, and intraparticle diffusion models were used to analyze the kinetic data obtained at different concentrations. The pseudo-second-order model was best applicable to describe the adsorption of γ-picoline on acid-activated bentonite. The Langmuir model provided good correlation with the experimental data. The adsorption of γ-picoline was endothermic with ∆H° = 29 J/mol.     

An improved adsorption–catalytic process for removing volatile organic compounds from exhaust gases

New methods are developed for conducting adsorption–catalytic processes to remove volatile organic compounds (VOCs) from exhaust gases at industrial enterprises. New flowsheets are proposed for these processes, in particular a system with localized heating of a part of the catalyst bed to initiate the combustion of adsorbed VOCs, and a system separating a full adsorption–catalytic bed into parallel sections with nonsimultaneous regeneration. Studies combine pilot-scale experiments and mathematical modeling. The flowsheet, in which the initiating heater is located directly in the catalytic adsorbent bed considerably reduces (by at least two orders of magnitude) the energy expenditures on regeneration, both in terms of specific energy consumption for purifying a unit volume of exhaust gases and in terms of the power required for the heater. Separating the bed into several sections allows a severalfold reduction in the maximum concentrations of pollutants and the gas temperature at the outlet of the adsorption–catalytic system during its operation. The proposed methods are characterized by high efficiency of gas purification and low energy consumption, so they can be widely used in protecting the atmosphere against VOC emissions.     

Development of an integrated process for the production of high-purity γ-aminobutyric acid from fermentation broth

γ-Aminobutyric acid (GABA), a natural non-protein amino acid, plays an irreplaceable role in regulating the life activities of organisms. Nowadays, the separation and purification of food-grade GABA from fermentation broth is still a great challenge. This research utilized monosodium glutamate as a substrate for the production of high-purity GABA via an integrated process incorporating fermentation, purification, and crystallization. Firstly, 147 g·L^-1 GABA with a yield of 99.8% was achieved through fed-batch fermentation by Lactobacillus brevis CE701. Secondly, three integrated purification methods by ethanol precipitation were compared, and crude GABA with a purity of 89.85% was obtained by the optimized method. Thirdly, GABA crystals with a purity of 98.69% and a yield of 60% were further obtained through a designed crystallization process. Furthermore, the GABA industrial production process model was established by Superproper Designer V10 software, and material balance and economic analysis were carried out. Ethanol used in the process was recovered with a recovery of 98.79% through Aspen simulated extractive distillation. Then the fixed investment (equipment purchase and installation costs) for an annual production of 80 t GABA will be about 833000 USD; the total annual production cost (raw material cost and utility cost) will be about 641000 USD. The annual sale of GABA may be at the range of 2400000-4000000 USD and the payback period will be about 1-2 year. This integrated process provides a potential way for the industrial-scale production of food-grade GABA.     

H2/CO2 separation characteristics of γ-Al2O3 membrane by aging stage in sol-gel process

γ-AlO(OH) sol solution was prepared by aluminum isopropoxide as an initial material. γ-Al₂O₃ membrane on the α-Al₂O₃ support was continuously made through coating and thermal treatment from the α-AlO(OH) sol. Previous work [Yoo et al., 1997] has shown that the aging stage in the sol preparation process mainly affects the characterization of γ-Al₂O₃ particles as well as γ-AlO(OH) sol. Based on this fact, the γ-Al₂O₃ membrane was prepared with two aging conditions in the present study. The separation characteristics experiment of the H₂/CO₂ mixture was performed on these membranes. As a result of the study, the mechanism of gas transport on the two membranes was proved as Knudsen diffusion. The ideal separation factor was reached at the value of the calculated Knudsen separation factor; however, permeability increased and selectivity decreased selectivity according to the aging.     

ZnO–Ag ceramics for ethanol sensors

The structure and electrical properties of the ZnO–Ag (0.001–3 wt%) ceramics in air and in air with the ethanol addition are studied. It was found that electrical parameters are sensitive to the concentration of ethanol in air due to a decrease in the barrier height at the grain boundaries. Silver addition causes a decrease in the density of material due to the formation of Ag inclusions. This can be the main reason of higher sensitivity to ethanol observed in ZnO–Ag ceramics in comparison with other ZnO–based ceramics.     

Environmental-barrier coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass: Part I,YAlO3 and γ-Y2Si2O7

The high-temperature (1500?°C) interactions of two promising dense, polycrystalline EBC ceramics, YAlO₃ (YAP) and γ-Y₂Si₂O₇, with a calcia-magnesia-aluminosilicate (CMAS) glass have been explored as part of a model study. Despite the fact that the optical basicities of both the EBC ceramics and the CMAS are similar, they both react with the CMAS. In the case of the Si-free YAlO₃, the reaction zone is small and it comprises three regions of reaction-crystallization products, including Y-Ca-Si apatite solid-solution (ss) and Y₃Al₅O₁₂ (YAG)(ss). In contrast, only Y-Ca-Si apatite(ss) forms in the case of Si-containing γ-Y₂Si₂O₇, and the reaction zone is an order-of-magnitude thicker. These CMAS interactions are analyzed in detail, and are found to be strikingly different than those observed in Y-free EBC ceramics (β-Yb₂Si₂O₇ and β-Sc₂Si₂O₇) in the accompanying Part II paper. This is attributed to the presence of the Y in the YAlO₃ and γ-Y₂Si₂O₇ EBC ceramics.