Microstructural investigation and performance evaluation of slip-cast alumina supports

In this work we successfully obtained slip-cast alumina supports with tubular shape. It was investigated the influence of both the starting powder particle size and heat treatment program on the pore structure and water permeability of the prepared materials. This study is supported by a series of experimental tests, including Archimedes method, mercury intrusion porosimetry, scanning electron microscopy, and cold crushing tests. We observed that the heat treatment temperature exhibited a more significant effect on the porosity than the sintering time. It was noticed that, in a general way, the higher the sintering temperature, the smaller the porosity and the larger the apparent density of the prepared materials. In addition, the raise of the sintering temperature from 1100 to 1500 °C increased the cold crushing strength of the tested samples. This behavior was not observed for F2, which seems to be related to the poor sinterability of the starting alumina powder used in its preparation. Among the samples prepared in this work, F2 showed the highest water permeability, followed by F3 and F1. The water permeation behavior of these materials is discussed on the basis of their mean pore size, porosity, genus per unit volume, and pore network tortuosity.     

Preparation of Al2O3 and MgAl2O4-based samples with tailored macroporous structures

In this work we successfully obtained freeze-cast alumina (Al₂O₃) and magnesium aluminate spinel (MgAl₂O₄) samples. Camphene was used as the freezing vehicle in this study. The specimens prepared herein were examined by Archimedes tests, scanning electron microscopy, and X-ray powder diffraction. Cold crushing tests were also carried out at room temperature. It was observed that the pore structure of Al₂O₃ samples can be tailored by changing the solid loading and freezing rate; the higher the solid loading and freezing rate, the finer the pore structure of the freeze-cast sample. MgAl₂O₄-based specimens were fabricated by keeping the solid loading in the starting slurry at 30 vol% and using liquid nitrogen as the cooling agent. The material obtained from a 60 Al₂O₃?40 MgO slurry showed a spinel amount of about 90%, an expressive total porosity (63 ± 3%), and a significant cold crushing strength (58 ± 6 MPa). In addition, this material exhibited the finest pore structure among the composition studied herein, showing a mean pore size of about 4 µm.     

Gas permeation and thermomechanical properties for macroporous alumina focused on necking size at grain boundaries

The gas permeation and thermomechanical properties of macroporous alumina used as a support substrate for microporous ceramic permselective membranes were investigated. The porosity, pore size, and apparent necking size between grains of macroporous alumina were systematically varied, and the relationships between the porous microstructure and material properties were examined. The grain necking size at alumina grain boundaries was evaluated by microstructural observations. The nitrogen gas permeance of the porous alumina increased with increasing pore size. All the measured thermal and mechanical properties decreased with increasing porosity. The properties of porous alumina samples with extensive grain necking showed higher values even in samples with the largest pore size. The high thermal conductivity of porous alumina with extensive grain necking was due to the low interfacial thermal resistance at grain boundaries. Porous alumina with extensive grain necking had high thermal shock strength due to the higher thermal conductivity. It was demonstrated that a porous structure combining high gas permeability and excellent fracture resistance could be successfully achieved.     

Freeze-cast alumina pore networks: Effects of processing parameters in steady-state solidification regimes of aqueous slurries

Aqueous alumina slurries with varying solids loading and particle size were freeze cast under seven freezing conditions to investigate the influence of these on pore network characteristics including pore size and geometric specific surface area. Slurry temperatures were recorded in situ to determine freezing front position and velocity during solidification, which were then analyzed via regression and modeled using solidification theory. Classic mathematical models for the time dependence of freezing front position and velocity were found to hold for freeze-cast slurries. Building on these, a one-phase Stefan problem was used to describe freezing kinetics. Models for freezing front velocity were combined with solidification theory to obtain predictions of microstructural feature size from freezing kinetics. Results showed that, while there may be a dependence on solids loading, the combination of mathematical modeling of solidification and classic solidification theory is applicable to freeze-cast ceramics and accurately describes pore network characteristics from processing parameters.     

包覆型Al_2O_3粉体制备低温烧成多孔陶瓷膜支撑体

采用22μmα-Al2O3为骨料,0.5μmα-Al2O3为烧结助剂,一是采用简单机械混合得到上述2种氧化铝的混合粉体,二是通过粉体表面修饰的方法,将0.5μmα-Al2O3包覆在22μmα-Al2O3表面得到包覆型氧化铝粉体。采用上述2种原料,通过干压成型法制备出片状多孔支撑体,考察了不同烧成温度下2种粉体路线制备出的多孔支撑体的弯曲强度、平均孔径、孔隙率和纯水通量。结果表明:在获得相同支撑体性能的前提下,以包覆型氧化铝粉体为原料制备出的支撑体的烧成温度大大低于采用简单机械混合后粉体制备支撑体的。在1550℃的烧成温度下,包覆型氧化铝粉体制备的支撑体的机械强度为34.2MPa,孔隙率为34%,平均孔径为2.34μm,纯水通量为205m3/(m2·h·MPa)。     

ADSORPTION OF H_2PtCl_6 ON ALUMINA

The adsorption kinetics and equilibrium of H_2PtCl_6 on various types of alumina were studied,and IRSand DRS were used to characterize these supports before and after impregnating with H_2PtCl_6 solution.Thedata indicated that the uptake of H_2PtCl_6 on supports strongly depends on the Lewis acidity of alumina andthat the adsorption rate is controlled by pore diffusion for supports of high adsorption capacity.Relationshipbetween uptake and acidity is interpreted in terms of active hydroxyl groups on alumina.     

Design and preparation of high permeability porous mullite support for membranes by in-situ reaction

The natural mineral kaolin combined with alumina additives Al(OH)₃,α-Al₂O₃ and AlF₃ was used to prepare porous mullite ceramic membrane supports using an in-situ reaction. The effects of composition and sintering temperature on the sintering behavior, pore structure, permeability and microstructure of the resulting porous mullite supports were extensively investigated. The experimental results showed that excess SiO₂ in kaolin can be consumed by adding alumina precursors, which resulted in a stiff skeleton of interlinked needle-like mullite crystals in-situ during the sintering. The needle-like mullite crystals touched each other and formed a short network, which acted as a porous skeletal network structure. This network resulted in a highly permeable porous structure. The resulting support is suitable for the preparation of asymmetric ceramic membranes. The densification and pore structure of the support can be effectively adjusted by control of the quantity of alumina precursors in the composition and the sintering temperature. Sintering the subject mullite compositions at 1500 °C for two hours resulted in support structures with an average porosity of 45.9%, an average pore size of 1.3 µm and a penetrating porosity of 35.9%.     

Influence of oxidation temperature on the gas permeation and separation properties in a microporous carbon membrane

Of thermosetting polymers, polyphenylene oxide (PPO) is considered as one of the promising alternative polymeric precursors for carbon membrane preparation. In this study, the PPO derived carbon membranes were prepared by carbonization and followed by air-oxidation as post-treatment method to modify the membrane pore structures. The characterization of the pore properties showed that air-oxidation enlarged the pore structure for the postoxidized carbon materials. The permeation results for the post-oxidized carbon membranes showed that the extent of the permeation modification was strongly dependent on the oxidation temperature. In the binary mixture gas systems, the permeation performance of the adsorbing gas species increased due to the surface diffusion mechanism. It is considered in the oxidation effect on the permeation modification that the post-oxidation of the carbon membranes increased gas permeation and separation properties.     

Microstructure and permeability of porous YSZ ceramics fabricated by freeze casting of oil-in-water suspension

Porous yttria-stabilized zirconia (YSZ) ceramics are fabricated through freeze casting of oil-in-water suspension followed by sintering at 1250−1550 °C. The pore structure, compressive strength and permeability of porous YSZ ceramics are tailored via altering the emulsion content and sintering temperature. The samples obtained using higher emulsion content or at lower sintering temperature show larger Darcian and non-Darcian constants due to their higher open porosity and larger pore size. Furthermore, the investigation on individual contributions of viscous and inertial resistances on the total pressure drop during permeation process indicates that the viscous resistance increases but the inertial resistance decreases with increasing the emulsion content or decreasing the sintering temperature for samples. Porous YSZ ceramics obtained in this work with a k₁ range of 3.14 × 10⁻¹³–1.12 × 10⁻¹² m² are appropriate for applications in filters and membrane supports.     

Three-dimensional alumina nanotemplate

Cylindrically and pentagonally shaped three-dimensional (3-D) alumina nanotemplates were fabricated by electrochemical anodization of high purity aluminum. Crack-free 3-D nanotemplates were achieved by anodizing various aluminum pre-formed structures through their concave surfaces. A critical parameter for obtaining crack-free 3-D nanotemplates was manipulating the stress at the aluminum/aluminum-oxide interface, because tensile stress at this interface can cause radial cracks. Two different cathode configurations were used to produce 3-D nanotemplates: inner and outer cathode configuration. The outer cathode configuration allowed fabrication of 3-D alumina templates with smaller diameters (<500 μm) and the inner cathode configuration allowed fabrication of 3-D alumina templates with larger diameters (>500 μm). A larger pore density was observed at the inner surface of 3-D template than at the outer surface due to the curvature of the template. The pore diameter of alumina nanotemplate was strongly depended on the applied current density. No defect was observed independent of anodizing solutions and applied current density. This cost-effective and manufacturable method can be utilized for preparation of hierarchical nanostructures.     

无机铝源合成有序介孔氧化铝的影响因素研究

有序介孔氧化铝具有较大的比表面积、均匀且窄的孔径分布、有序的孔结构等特点,在多相催化反应及吸附分离过程中具有十分重要的应用价值,研究其合成及应用具有重要意义.以廉价的无机铝盐为铝源,结合模板剂,利用溶胶-凝胶法,通过改变模板剂的种类、老化时间和温度等影响因素,制备有序介孔氧化铝.采用多种测试技术对其结构进行表征,探讨不...     

Freeze-cast composite scaffolds prepared from sol-gel derived 58S bioactive glass and polycaprolactone

This work deals with the preparation of freeze-cast scaffolds using sol-gel derived 58S bioactive glass and a hypoeutectic naphthalene-camphor mixture as the starting powder and freezing vehicle, respectively. After the freeze-casting step, samples were air sintered at 1250?°C for 2?h, which led to the crystallization of 58S. The obtained scaffolds were subsequently infiltrated with poly(ε-caprolactone) (PCL), a biodegradable polymer with potential application for bone tissue repair. The prepared materials were examined by helium pycnometry, laser granulometry, scanning electron microscopy (SEM), Archimedes tests, X-ray microtomography (micro-CT), Fourier transform infrared spectroscopy (FTIR), N₂ adsorption, X-ray diffraction (XRD), and uniaxial compression tests. Samples cytotoxicity was evaluated by (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) tetrazolium reduction (MTT) and LIVE/DEAD assays. Their biocompatibility was also examined after soaking in a simulated body fluid (SBF) solution at 37?°C for up to 14 days. It was observed that the infiltration of PCL into the 58S scaffolds greatly increased their mechanical stability. Moreover, it was shown that these composites displayed a high cell viability (above 70%), which reveals that they did not interfere in the production of osteoblast cells. A hydroxyapatite coating was observed on the samples surface upon soaking in SBF, reinforcing that they are biocompatible materials. As far as we know, this is the first time that freeze-cast scaffolds were obtained using sol-gel derived 58S particles and a naphthalene-camphor mixture. Besides, as the infiltration of PCL into freeze-cast bioactive glass scaffolds improved their mechanical stability without impairing their bioactivity, this is a promising approach to prepare samples for load-bearing applications in bone tissue engineering.     

Microstructure and permeability of porous Si3N4 supports prepared via SHS

Porous Si₃N₄ ceramics with tailored pore structures were fabricated via self-propagating high temperature synthesis (SHS) using Polymethylmethacrylate (PMMA) as pore forming agent. The pore structures, mechanical properties and permeation performance of porous Si₃N₄ ceramics were investigated by altering the particle sizes and amount of PMMA. With the increasing content of PMMA, the flexural strength of samples decreased from 102.5 MPa to 9.4 MPa. The tortuosity which showed irregular variation affected gas permeability directly. The samples with 20 wt% content of PMMA exhibited the maximum Darcian and non-Darcian constants with the smallest tortuosity. Moreover, the comparison of permeability coefficients with other ceramics via different pore forming methods in literature was presented. The specimens exhibited great permeability due to the large pore sizes created by the elongated and coarsened β-Si₃N₄ grains during the SHS process, providing a low-cost and environmentally friendly method for preparing high permeability porous Si₃N₄ supports.     

Processing and characterization of sol–gel titania membranes

In this work we present a structural characterization of sol–gel titania membranes obtained in both supported and unsupported forms. We used two commercial grade alumina supports obtained from Whatman and Rojan Advanced Ceramics. The unsupported membranes were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), differential scanning calorimetry (DSC), nitrogen sorption, and X-ray powder diffraction (XRD). Morphological studies were performed in both supported and unsupported membranes using a field emission scanning electron microscope (FE-SEM). In order to evaluate the performance of the supported membranes, single-gas permeation experiments were carried out at room temperature with nitrogen, helium, and carbon dioxide. We concluded from nitrogen sorption experiments that increasing the membrane heat treatment temperature leads to samples with lower specific surface areas and greater pore sizes. Close packed titania particles of uniform size were observed in SEM micrographs of unsupported membranes. The SEM analyses also revealed the presence of titania coatings on supported membranes. Some of the obtained membranes showed a separation capacity for He/CO₂ and He/N₂ larger than that expected for the Knudsen mechanism in the investigated pressure range. However, a good part of the analyzed samples showed an improvement of their separation capacity with increasing the feed pressure.     

Processing,structural characterization and performance of alumina supports used in ceramic membranes

Although there are various commercially available methods for the separation and capture of gas species such as CO₂ and H₂, they are energy expensive and in some cases environmentally unfriendly. The membrane separation process presents advantages such as its relative simplicity, ease of use, low energy consumption, and application in the separation of both liquid and gas mixtures. For these reasons, the membrane technology has achieved in the last years a great commercial and strategic importance. In this work we present a structural characterization of an alumina support, before and after the deposition of a titania film on its surface. The obtained asymmetric membranes are intended to be used in gas separation processes. The alumina supports and titania coatings were prepared by dry-pressing and sol–gel process, respectively. The processed samples were characterized by X-ray diffraction (XRD), nitrogen sorption, scanning electron microscopy (SEM), and X-ray microtomography (μ-CT). In order to evaluate the membranes performance, single-gas permeation experiments were performed at room temperature with nitrogen, helium, and carbon dioxide. We observed that although the alumina supports obtained in this work have not been submitted to any surface finishing procedure, the obtained membranes have potential application in gas separation processes. We observed that increasing the pressure feed leads to improving their separation capacity.     

Chlorophyll adsorption by alumina-pillared acid-activated clays

Clays, acid-activated to an optimum level, have been pillared with alumina to give semi-crystalline expanded materials with surface acidities, pore volumes and average pore diameters generally higher than those of the corresponding pillared materials derived from a clay matrix not previously acid-activated. The chlorophyll adsorption capacity of the pillared acid-activated materials is significantly greater than that of pillared, nonactivated clays. The procedures used in the preparation of these pillared acid-activated clays (i.e., temperature of pillaring, method of drying and calcination temperature) have a significant influence on chlorophyll adsorption capacity because they influence both the physical and the chemical properties of the final pillared material. This variation provides a useful means of relating the various properties of the pillared materials to the chlorophyll adsorption capacity. As a result, a correlation has been demonstrated between adsorption capacity and a combination of pore volume and number of strong acid sites (of strength pK_a < −1.5) present in the pillared material. Optimal adsorbents were obtained from freeze-dried samples prepared by exchange at 20°C and calcination in air at 500°C.     

Microstructure and gas permeation performance of porous silicon nitride ceramics with unidirectionally aligned channels

Porous Si₃N₄ ceramics with unidirectionally aligned channels were prepared via freezing ceramics suspension with distinct solid contents under different freezing temperatures. The samples obtained using lower solid content in ceramic suspension at higher freezing temperature exhibit larger Darcian and non-Darcian constants due to their higher open porosity, larger pore size and lower tortuosity. Moreover the investigation on individual contributions of viscous energy losses and inertial energy losses on the total pressure drop during permeation process indicates that with decreasing the solid content or the freezing temperature the viscous energy losses increase but the inertial energy losses decrease for samples owing to the differences in their pore structures. It is worth mentioning that porous Si₃N₄ ceramics with unidirectionally aligned channels exhibit larger Darcian and non-Darcian constants than those with similar pore size distributions and open porosity owing to their lower tortuosity, thus rendering them appropriate for filters and membrane supports.     

Effect of freeze-drying treatment on the optical properties of SPS-sintered alumina

This study looks at the influence of alumina powder processing on the preparation of transparent alumina by Spark Plasma Sintering (SPS). Zeta potential measurements were carried out on alumina suspensions in order to determine the best dispersion conditions. Stable slurries were submitted to a spray freeze drying process and their sintering behavior was compared with the corresponding non spray freeze dried powders. Transparent alumina samples were successfully prepared from alumina powders by Spark Plasma Sintering. An optical model considering pore and grain size distributions has been developed to obtain information about porosity in dense materials. It was found that the final density and, accordingly, the optical properties were improved when spray freeze dried starting powder was used.     

Al2O3@TiO2复合生物载体的制备及其BSA吸附特性

通过钛酸盐和氧化钛溶胶两种制备途径在氧化铝（Al₂O₃）支撑体外包覆介孔氧化钛（TiO₂）层,分析了这两种途径制备的氧化钛表面结构与性质的差异及其对于BSA蛋白吸附性能的影响。以粒径分别为0.37、10.4和21.8 mm的三种氧化铝支撑体作为基材,用XRD、Raman分析了两种路径制备的氧化钛晶型均为锐钛矿相;N₂吸脱附分析其比表面积均在（100±10）m²·g^-1,两者具有相似的介孔结构。FESEM分析发现钛酸钾溶胶制备的氧化钛的形貌受到支撑体孔径的影响。对于在牛血清蛋白（BSA）吸附实验中,钛酸钾溶胶法包覆的20 mm粒径复合载体（SP20@K-TiO₂）吸附量为22.18 mg·g^-1,高于相同条件下氧化钛溶胶制备的Al₂O₃@TiO₂复合载体。SP20@K-TiO₂ 中TiO₂吸附量为150.88 mg·g^-1,相比于TiO₂粉体,复合载体中TiO₂对BSA蛋白吸附的性能得到提升。     

成孔剂的量对多孔氧化铝支撑体孔结构的影响

以氧化铝为骨料,淀粉及其它有机粘结剂为成孔剂制备出管状多孔氧化铝支撑体。系统地考察了成孔剂对多孔氧化铝支撑体孔结构的影响。研究表明:当支撑体中成孔剂的质量分数 <10%时,支撑体的孔隙率稳定在35%左右。当成孔剂的质量分数在 10% ~25%之间时,支撑体的孔隙率随成孔剂量的增大而显著增加。当成孔剂的质量分数>25%时,在保证支撑体完整性的前提下,支撑体的孔隙率随成孔剂量的增大变化不大,稳定在 45%左右。成孔剂质量分数的增加会使支撑体的平均孔径增大,孔径分布变宽,平均孔径与最可几孔径的差值增大。