A novel alumina catalyst support with high thermal stability derived from silica-modified alumina aerogel

A new alumina catalyst support with high thermal stability was synthesized. The high thermal stability was achieved through the synergetic effect of silica addition and the ultra-low bulk density (aerogel). The amount of silica was varied from 2.5 to 10 wt% and 5 wt% was found to be most effective for suppressing phase transformation; the θ phase remained even after heating at 1400°C for 1 h. The surface areas of the present alumina with 5 wt% silica were 86 and 36 m²/g after heating at 1300 and at 1400°C, respectively. This revised version was published online in November 2006 with corrections to the Cover Date.     

Carbon-silica xerogel and aerogel composites

High surface area carbon-silica xerogels and aerogels were prepared by adding various amounts of activated carbon particles during gelation of tetramethyoxysilane (TMOS). Surface areas and pore structure of the gels were determined by nitrogen and water adsorption and desorption measurements. Carbon increased the surface area of silica gel and the carbon-silica gel composites were found to be hydrophobic. The pore structure of xerogel composites was dominated by carbon whereas that of aerogel composites was apparently controlled by silica component.Also with the Department of Agronomy.     

Morphology and structure of YSZ powders: Comparison between xerogel and aerogel

Yttria-stabilized zirconia (YSZ) fine powders were prepared via sol–gel route in order to shape thermal barrier coatings (TBCs) from these powders. The main objective is to develop new undirectional coatings to allow best thermo-mechanical accommodations compared to conventional processes. To reach this aim, powders have to be able first to be highly dispersed into a sol (non-agglomeration, high specific surface area, etc.) and second to crystallize in the required metastable phase t′. Two routes have been used to dry gels: the conventional way which consists of simple evaporation of the solvent is compared to drying in supercritical conditions. Both YSZ powders after calcination at 950 °C of xerogel (Ex-xero-YSZ powder) and aerogel (Ex-aero-YSZ powder) crystallize in the tetragonal form. N₂ adsorption/desorption analysis of the Ex-xero-YSZ powder indicates an S_w of 2.8 m²/g. For the Ex-aero-YSZ powder, the S_w (26 m²/g) is much higher than of Ex-xero-YSZ, leading to a better sintering capability. This high S_w is correlated to the small crystallite size (26 nm) and the alveolar morphology of Ex-aero-YSZ powders compared to Ex-xero-powder (49 nm). By reducing particles size and increasing the S_w of the powders, supercritical drying appears as a promising way to prepare stable slurries or loaded sols from fine YSZ particles for TBC applications. Indeed, after preparing nanometric powders, they are dispersed into a sol before shaping on superalloys substrates. After thermal treatment at 950 °C for 2 h which corresponds to the working temperature of TBC, the final aim will be to prepare ceramic YSZ coatings.     

镧钡掺杂量对氧化铝孔结构的影响

采用纯γ-氧化铝,制备含镧/钡质量分数为5%、10%、20%、30%的系列改性氧化铝,样品分别在550、950、1050、1200℃条件下进行锻烧,采用比表面积、孔容、孔径、XRD表征老化前后的改性氧化铝孔结构,研究不同镧/钡含量对改性氧化铝孔结构特征的影响.BET和XRD结果表明,镧、钡在高温时产生的大量铝酸盐结构可抑制γ-A2O3相转变,有利于保持γ-Al2O3相结构稳定性和孔结构稳定性,同时较高镧/钡含量也会导致改性氧化铝比表面积降低、孔容、孔径下降,尤其是1200℃高温区表现更为明显,对活性金属分散存在负面影响.综合考虑孔结构和衰减率因素认为,2.5%La2.5%Ba/Al2O3具有最好的表面活性和孔结构稳定性.     

Comparative study of the texture and structure of aerogel and xerogel sulphated zirconia doped with nickel

Aerogels and xerogels based on sulphated zirconia doped with nickel in the same atomic ratios S/Zr?=?0.5 and Ni/Zr?=?0.1 have been synthesized by sol?Cgel method using two drying ways of the solvent in solid preparation steps. Aerogels obtained by drying in supercritical conditions of solvent stabilize zirconia tetragonal phase even at high calcination temperatures and present a developed surface area, whereas xerogels obtained by ordinary solvent drying in an oven are amorphous. TPR and XPS studies revealed that Ni seems to exist in different species in the two kinds of solids and it is more reducible in aerogels.     

Synthesis and hierarchical pore structure of biomorphic manganese oxide derived from woods

Biomorphic manganese oxides on two different wood templates (fir and paulownia) were fabricated by infiltration with nitrate and subsequently calcination. X-ray diffraction (XRD) test and microscopy observation (FESEM and TEM) were employed to characterize the phase and structure of biomorphic manganese oxides. The pore structure of the resulting products was studied through mercury intrusion and nitrogen adsorption measurement. Infrared (IR) adsorption properties were investigated by Fourier Transmission Infrared (FTIR). The final oxide products contain hierarchical pore structure from μm to nm scale, and also show unique pore size and distribution with hierarchy on nanoscale derived from the specific wood template, the changeable connectivity of nanoscale pore channel controlled by calcinations temperature. The rise of temperature leads to diminishing of average pore diameter and volume but more uniform pore size distribution and higher degree of pore connectivity. The overall collapse in IR adsorption spectra of biomorphic manganese oxides with the rise of calcination temperature is related to the effect of nanoscale pore structure especially the increasing of pore connectivity.     

Surface area and pore structure of talc-magnesite

The effect of thermal treatment on talc-magnesite in the presence of air in the temperature range 500–1000°, was studied by nitrogen adsorption and followed by t.g.a., d.t.a. and X-ray analysis. From the nitrogen adsorption isotherms, the specific surface area as well as pore-size distribution was estimated. The surface area increased with temperature, to reach a maximum at 600°. On immersion of the calcination products in water, magnesium hydroxide was formed. Decomposition of the hydration products in air at 450° resulted in solids of relatively higher surface areas, whereas decomposition in vacuo at the same temperature produced more active solids. Surface area of the products produced in vacuo decreased only slightly with the primary decomposition temperature, and they are characterised by sharp pore-size distribution curves. Surface areas of the products produced in air were dependent on the primary calcination temperature, although they showed pore-size spectra similar to those of the other products. Products obtained by direct decomposition in air showed wider pore spectra that shifted towards higher pore radii with rise of temperature.     

表面活性剂对活性氧化铝孔结构的影响

研究了十二烷基苯磺酸钠和C12脂肪醇聚氧乙烯(3)醚对活性氧化铝孔结构的影响。实验结果表明,十二烷基苯磺酸钠和C12脂肪醇聚氧乙烯(3)醚能够使氧化铝平均孔径从11.98 nm分别移至6.32 nm和9.42 nm,比表面从193 m2/g分别增至241 m2/g和230 m2/g。这两种表面活性剂之所以能显著影响氧化铝孔分布,主要是因为,以氧原子为极性头的表面活性剂,不仅能够进入拟薄水铝石粒子间隙,而且能够插入拟薄水铝石层间,其氧原子与拟薄水铝石层表面的羟基以及水分子,通过氢键形成一种新的复合氧化铝。在焙烧的过程中,表面活性剂能够减小拟薄水铝石层表面和微孔内壁的应力,避免了层间的坍塌和毛细孔的收缩,从而影响氧化铝孔分布,且极性头小的表面活性剂影响效果优于极性头大的表面活性剂。     

Surface area and pore structure of ceric oxide

Adsorption-desorption isotherms of nitrogen at ?196°, and of methanol and cyclohexane vapours at 35°, were determined on a sample of microcrystalline hydrated ceria and its products of dehydration formed in air at temperatures up to 1000°. The nitrogen adsorption isotherms were analysed by the de Boer t-method. In one particular sample, the V_a-t plot gave a straight line passing through the origin, which continued until near the saturation pressure. This sample is known to be porous, and its pore size distribution curve gave a most probable hydraulic radius of 35 ǎ. In analysing the pore structure, the corrected modelless method was used, and there is an indication that the pores are most likely to be cylindrical in shape. Sintering seems to take place through material transport from wider pores to fill narrower ones, where the total number of pores is reduced. Adsorption of larger molecules than nitrogen, namely methanol and cyclohexane, gave surface area values which are slightly lower than those calculated from nitrogen adsorption. It seems that slight persorption effects can take place in minor fractions of micropores present. However, the remainder of pores present in all samples investigated are accessible to the three adsorbate molecules.     

Surface area and pore structure of thorium dioxide

Water vapour isotherms at 35° were measured on two samples of thorium dioxide, one crystalline and the other an amorphous gel. Specific surface areas and pore size distributions were estimated and compared with those obtained from nitrogen adsorption at ?196°. The effect of thermal treatment on both samples was also investigated at 35–500°. It was shown that the adsorption of water vapour on thoria gel is purely physical in nature, and that the gel surface is characterised by the existence of pores of very particular sizes, namely of hydraulic radii 4–6 ǎ. On the other hand, water seems to interact chemically with the surface of the crystalline sample. In this case the specific surface area could not be evaluated from the adsorption branch of the isotherm, but was successfully evaluated through the application of the ‘corrected modelless’ method to the desorption branch of the water isotherms. This gave the surface area of the total pore system which was found to be in good agreement with the B.E.T.-nitrogen area.     

Effect of pore structure on gas permeability constants of porous alumina

Porous alumina was fabricated using different particle size, sintering temperature, and particle size and content of poly (methyl-methacrylate) (PMMA) as pore former. The Forchheimer equation was used to investigate the relationship between porosity and average pore size, and obtain the permeability constants k₁ and k₂ (the viscous effect and the inertial effect, respectively). Compared to Darcy's law, the Forchheimer equation established a more realistic and reliable relationship between fluid pressure and fluid velocity. k₁ and k₂ were found to be more sensitive to the average pore size than to the porosity of alumina. Moreover, reliable relationships were confirmed between the average pore size and k₁, k₂, and their ratio (k₁/k₂).     

The influence of preparation methods on the pore structure of alumina

Structural characterization of eleven kinds of aluminas prepared by different methods was carried out by measurement of the adsorption of nitrogen, X-ray diffraction and scanning electron microscopy. Aluminas prepared using aqueous ammonia as precipitation reagent exhibited a single pore size distribution, the maximum being at a radius of about 20Å, whereas aluminas prepared with urea exhibited a twin peaked pore size distribution, with maxima at radii of about 20Åand 50Å. The pore structures of aluminas prepared from aluminium nitrate by pyrolysis and from sodium aluminate with carbon dioxide were similar to the former, while aluminas prepared by pyrolyses of aluminium isopropoxide and of aluminium trichloride had a pore structure similar to the latter.Based on scanning electron microscopy and nitrogen adsorption-desorption hysteresis loop observations, the crystallite structures of aluminas and pore structures were discussed in connection with the preparation methods employed.     

Surface area and pore structure of stannic oxide gel from organic vapour adsorption

Surface area measurements were performed on stannic oxide gel and its thermal dehydration products obtained both in vacuo and in presence of air in the temperature range 100–600°C by benzene and carbon tetrachloride adsorption at 35°C. The presence or absence of air is found to impose changes in the surface area and consequently vary the pore structure of the solid material. Pore size analyses for both micro- and mesopores are performed. The low temperature samples are characterised by the presence of micropores only. At 350°C widening of pores commences and at higher temperatures the results indicate the existence of pores of “submicro” dimensions together with the mesopores. These changes in pore structure go in parallel with the gradual development of the crystalline cassiterite structure.     

Effect of alumina fiber content on pore structure and properties of porous ceramics

Porous Al₂O₃ ceramics with different contents of alumina fibers were prepared by gel-casting process. The effects of Al₂O₃ fiber content on pore size distribution, porosity, compressive strength, and load-displacement behavior of the ceramic materials were investigated. Initial results showed that with the increase of Al₂O₃ fiber content, the pore size and porosity of the material is increased, and the compressive strength is decreased. However, upon increasing the fiber content from 50 wt% to 67 wt%, the performance of the samples changed greatly. The compressive strength of the material increased, while the porosity remained unchanged, the pore size increased greatly, and the shape of the load displacement curve changed. It showed that when the fiber content increased from 50 wt% to 67 wt%, the loading body in the fiber-reinforced porous ceramics changed from particles to fibers.     

Granule rearrangement and pore structure of a spray-dried alumina compact observed by X-ray tomography

This study examined the compaction behavior of spray-dried Al₂O₃ granules with special emphasis on granule rearrangement and the resulting pore structure under different loads. A numerical simulation based on the finite element method was performed to estimate the density gradient in cylindrical compacts with different aspect ratios. The simulation results were compared with the experimental observations in terms of the density gradient, microstructural change, and pore distribution. A non-uniform pressure distribution in the compact resulted in a density gradient, where the top circumference showed the highest density, decreasing toward the center and bottom. The three-dimensional pore structures at different positions of the compact were visualized by X-ray micro-computed tomography, while two-dimensional images were also obtained using a liquid immersion technique for comparison. Good agreement between the simulation and experimental results was found, showing a density difference of ≤ 2 %.     

Surface and pore structure of deoiled acid-and heat-treated spent bleaching clays

Samples of spent bleaching clay were deoiled by hexane, methanol, hexane-methanol, and supercritical CO₂ extractions. The deoiled clays were regenerated by acid and heat treatments. Nitrogen adsorption isotherms for these samples are type IV with hysteresis loops corresponding to type H3, indicating slit-shaped pores. Used deoiled and dried samples have smaller surface areas and pore volumes than unused clay. The surface areas and pore volumes increased after heat treatment. Acidified heat-treated deoiled samples have smaller surface areas and greater pore volumes than unused clay, except for the methanol-deoiled sample. Thus, heat and acid treatments removed substances adsorbed in the pores that were not removed by solvents or CO₂ extraction. This was confirmed from the ratios of the cumulative surface area/BET surface area, as well as analysis of the pore size distributions, which indicated an increase in mesopores with radii of between 25 and 100 Å. The t-plots showed that smaller pores with sizes between 7 and 25 Å, present originally in the unused clays, were closed by heat treatment. These pores were absent in the deoiled and the heat-treated samples except for the heat-treated sample that was deoiled by hexane followed by methanol. Smaller pores, accompanied by an increase in surface area, were also observed for all deoiled samples after acid and heat treatments.     

Organic/inorganic double-precursor cross-linked alumina aerogel with high specific surface area and high-temperature resistance

Alumina aerogel has drawn a great research interest in the aerogel community owing to its great high-temperature heat resistance. The typical preparation can be divided into two approaches depending on the type of precursors. However, the alumina aerogel derived from organic aluminum alkoxides suffers from poor monolithic integrity, whereas the one from inorganic aluminum salts presents unsatisfied thermal stability. In this work, we develop a novel organic/inorganic double-precursor cross-linking method to prepare alumina aerogels. Aluminum chloride hexahydrate is used to provide an acid condition for the hydrolysis and condensation reaction of aluminum sec-butoxide (ASB), and serves as a reactant to co-condensate with the hydrolyzed ASB to form an interpenetrating chain structure. The resulting alumina aerogel exhibits a high specific surface area (SSA) of 667 m²/g and good high-temperature thermal insulation performance. Moreover, it can still retain SSA of 224 and 105 m²/g after heating at 1000 and 1300°C, respectively.     

Surface and pore structure modification of cellulose fibers through cellulase treatment

The surface and pore structure of cellulose fibers have a significant impact on the properties and performance in applications. Cellulase enzymatic hydrolysis of cellulose fibers can result in changes to the surface and pore structure, thus providing a useful tool for fiber modification. This research characterizes these changes using various test methods such as fiber dimension, water retention value (WRV), hard‐to‐remove (HR) water content, freezing and nonfreezing bound water content, polymer adsorption, and crystallinity index. For a high‐dosage cellulase treatment (600 U/g dry solid), the fiber length was significantly decreased and the fibers were “cut” in the cross direction, not in the axial direction. The swelling capacities as measured by the WRV and HR water content increased for the high‐dosage treatment. Three independent measurements (nonfreezing bound water, polymer adsorption, and crystallinity index) are in good agreement with the statement that the amorphous regions of cellulose fibers are a more readily available substrate relative to crystalline regions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3833–3839, 2007     

大孔容活性氧化铝的生产

活性氧化铝在多个工业领域有重要用途。孔容是产品的重要技术指标之一。如何生产大孔容活性氧化铝,已成为研究、生产的重要课题。报道了以氢氧化铝为原料,应用快脱法,生产高质量活性氧化铝的工业生产结果,其产品具有大孔容、低松装密度、高强度、大比表面积的特点。多年的工业实践证明,快脱法是生产大孔容活性氧化铝的有效途径。实验研究了生产过程中固相的物相组成。实验已证明,拟薄水铝石可作为生产活性氧化铝的扩孔剂,在快脱法作业程序中,掺加适量的大孔容拟薄水铝石,可使球状氧化铝的孔容提高到0.76 mL/g。     

Tailored graded pore structure in zirconia toughened alumina ceramics using double-side cooling freeze casting

Ceramics with graded and continuously aligned open pores were investigated using a double-side cooling freeze casting setup. The ceramic preforms with tailored lamellae spacing (wavelength), wall thickness and graded pore structure were used to infiltrate with a second phase for the fabrication of graded interpenetrating phase composites. The effects of solid content, temperature setting and gradient, cooling rate and the introduction of electrophoretic deposition (EPD) on the freezing velocity of the ceramic suspension were analysed. On the bottom of the ceramic specimen, a dense layer was formed and tailored with the use of EPD. The ceramic was characterised by a graded open pore structure with wavelengths up to 115 μm and interconnected microstructure. The effect of solid content on the degree of supercooling and the effect of temperature gradient on the average freezing velocity were investigated. The addition of EPD before freeze casting affected significantly the microstructure, the wavelength decreased and the wavelength gradient became smaller compared to simple freeze casting.