Absorption plasticization of alumina сeramic fiber

The dependence of tensile strength and elongation at failure strain on the change in oiling-emulsion concentration was studied. A correlation between the tensile strength and elongation values at failure strain with the change in the surface activity and specific electrical conductivity of the oiling emulsion was observed with a change in the oiling-emulsion concentration from 2 to 20%. This indicates that electro-physical processes play an important role in the adsorption mechanism: free electrical charges on the fiber surface under mechanical loading.     

Synthesis and characterization of sol–gel alumina nanofibers

Abstract Alumina nanofibers of high aspect ratio with surface area of >300 m² g⁻¹ has been prepared successfully in bulk quantities by the sol–gel method. The synthesis parameters including the binary water–alcohol solvent system to aluminium isopropoxide ratio, pH, type of solvent and aging temperature affect the uniformity and formation of nanofibers. It is proposed that alumina nanofibers were formed by the curling of the nanosheets upon condensation after the hydrolysis. The phase evolution of alumina nanofibers from pseudoboehmite to α phase has been shown by XRD and FTIR. ²⁷Al NMR investigations show that the Al atoms are six and four coordinated. The morphology of the alumina nanofibers does not change much as the calcination temperature was increased. In addition, the average pore size increases and the BET surface area decreases as a function of calcination temperature. The thermal behavior of alumina nanofibers was investigated by TGA. Graphical Abstract      

Sol–gel derived mesoporous and microporous alumina membranes

Stable polymeric and colloidal boehmite sols were prepared by sol–gel process through controlled hydrolysis/condensation reactions. The particle sizes of the colloidal sols were in the 12–25 nm range depending on the process parameters and about 2 nm for polymeric sols. The presence of a significant increase in the microporosity content of the heat treated polymeric membranes relative to the mesoporous colloidal membranes might make the design of thermally stable microporous alumina membranes with controlled pore structures possible. The phase structure evolution in the 600–800 °C range had shown that the crystallization of the gamma alumina in the amorphous matrix starts at about 800 °C. This indicated that the pore structure stability may be enhanced through processing up to this relatively high temperature in polymeric alumina derived unsupported membranes. The permeance values of the two and three layered colloidal alumina membranes were observed to be independent of pressure which implies that the dominant gas transport mechanism is Knudsen diffusion in these structures. This was also supported by the 2.8 nm BJH pore sizes of the colloidal membranes. The Knudsen diffusion equation derived permeances of the polymeric alumina membranes with thicknesses of about 300 nm were determined to be very close to the experimentally determined permeance values.     

Rapid solidification of cryolite and cryolite–alumina melts

Abstract  

Rapid solidification processing (with a cooling rate in the interval 10⁵–10⁶ K s⁻¹) was used to prepare deeply undercooled cryolite–alumina melts. These samples were analyzed by XRD, infrared, and Raman spectroscopy. Besides cryolite, the amorphous phase and a low amount of ι-Al₂O₃ were detected. Annealing of the quenched sample revealed the transformation of metastable amorphous phases into different products depending on the annealing conditions. The results obtained showed that all of the elements (Na, Al, O, and F) are probably present in the amorphous parts of the quenched samples.     

High surface area sol–gel alumina–titania nanocatalyst

Alumina–titania mixed oxide nanocatalysts with molar ratios = 1:0.5, 1:1, 1:2, 1:5 have been synthesized by adopting a hybrid sol–gel route using boehmite sol as the precursor for alumina and titanium isopropoxide as the precursor for titania. The thermal properties, XRD phase analysis, specific surface area, adsorption isotherms and pore size details along with temperature programmed desorption of ammonia are presented. A specific surface area as high as 291 m²/g is observed for 1:5 Al₂O₃/TiO₂ composition calcined at 400 °C, but the same composition when calcined at 1,000 °C, resulted in a surface area of 4 m²/g, while 1:0.5 composition shows a specific surface area of 41 m²/g at 1,000 °C. Temperature programmed desorption (of ammonia) results show more acidic nature for the titania rich mixed oxide compositions. Transmission electron microscopy of low and high titania content samples calcined at 400 °C, shows homogeneous distribution of phases in the nano range. In the mixed oxide, the particle size ranges between 10–20 nm depending on titania content. The detailed porosity data analysis contributes very much in designing alumina–titania mixed oxide nanocatalysts.     

Disorder or complexity? Understanding a nanoscale template structure on alumina

One strategy in creating functional nanostructures is templating where active nanoparticles are arranged on a regular nanoscale array of anchor sites on an inert substrate. An extraordinarily well ordered substrate with a 4.2 nm template periodicity is an alumina (aluminum oxide) film grown on a Ni3Al(111) metallic alloy support. Templating on the alumina film is facilitated by a dot and a network superstructure that can readily be prepared but has not yet been understood at the atomic scale. By imaging the alumina surface with dynamic scanning force microscopy (SFM) operated in the noncontact mode (NC-AFM), we reveal that the main structural element of the oxide film is a lattice of hexagons with a 0.29 nm side length that is pinned to the 0.51 nm periodicity of the substrate. The surface unit cell is defined by distinguished sites forming the dot structure. Pinning the oxide film to the substrate furthermore results in a honeycomb-like topographic modulation referred to as the network structure. These findings demonstrate how long range order is generated by the superposition of complex structures that locally exhibit apparent atomic disorder.     

Ni–Mo2C supported on alumina as a substitute for Ni–Mo reduced catalysts supported on alumina material for dry reforming of methane

In this study, alumina-supported NiMo catalysts were carburized to obtain alumina-supported nickel–molybdenum carbides as potential catalysts for dry reforming of methane. The typical carbide was compared with a low carburized material (in 5% H₂/CH₄) and a reduced NiMo catalyst. It was shown that the passivated alumina-supported NiMo catalysts by carbon lead to higher reactivity, selectivity, and stability for dry methane reforming reaction.     

Solvent-free synthesis of alumina supported cobalt catalysts for Fischer–Tropsch synthesis

A novel mechano-synthesis method has been elaborated in this work for the design of efficient cobaltbased Fischer–Tropsch catalysts. The process aims to reduce the total number of steps involved in the synthesis of solid catalysts and thus to avoid relevant toxic solutions generated during the catalyst preparation. The mechano-synthesis of the Co/Al_2O_3 catalyst was processed in a low-energy vibratory micro mill and high energy planetary ball mill. Porous spherical γ-aluminas(1860 μm and 71 μm mean particle diameter) were used in this work as host compounds. Co_3O_4(3 μm mean particle diameter) has provided guest particles for mechano-synthesis. The catalysts were characterized by textural(surface area, porosity and particle size) and structural analyses(X-ray diffraction, TPR, SEM-EDX and microprobe). The microprobe images show deposition of Co_3O_4 on the surface of the alumina and indicated no Co_3O_4 diffusion inside the alumina pores. SEM-EDX mapping illustrated that cobalt coating tended to occur on surface of rounded shape of cracked alumina fragments. After milling, the crystallite size of Co_3O_4 decreased to 15 nm from 30 to 50 nm. The TPR profiles indicated very low concentrations of inactive cobalt aluminate mixed compounds which are usually produced during the catalyst preparation by impregnation.In Fischer–Tropsch synthesis, the catalysts prepared using mechano-synthesis methods showed catalytic performance comparable to the catalysts prepared by impregnation.     

Growth of porous anodic alumina films in hot phosphate–glycerol electrolyte

Growth of porous anodic alumina films has been examined at 10 V in hot phosphate-containing glycerol electrolyte containing 0.1 to 0.57 mass% water. The growth rate of the films is highly dependent upon the water content of the electrolyte, reducing markedly at a water content of 0.1 mass%, an opposite trend to that found previously for the formation of porous films on titanium and niobium. Chemical dissolution of the anodic alumina is also suppressed in electrolyte of low water content. GDOES depth profiles revealed that an increased water content of the electrolyte promoted the incorporation of phosphorus species into the films, although chemical dissolution reduced the amounts of phosphorus in the outer regions. Carbon species also appeared to be present in films, particularly at lower water content. Using a niobium oxide outer layer to suppress chemical dissolution resulted in films that were about 1.2 times the thickness of the consumed aluminium for an electrolyte containing 0.25 mass% water. The expansion suggests a possible contribution of field-assisted flow of film material in the growth of the porous anodic film.     

LiF–CaH2 alumina electrolytes for intermediate temperature fuel cell applications

Hydrofluoride-based electrolytes with proton conduction have been successfully used in intermediate temperature fuel cell applications. Among the various hydrofluoride electrolytes, LiF–CaH₂ and its composite with Al₂O₃, i.e., LiF–CaH₂–Al₂O₃, are the most promising candidates which show more advantages than the other hydrofluorides. In this communication, we put our emphasis on the LiF–CaH₂–Al₂O₃ electrolytes and their applications for intermediate temperature fuel cells. Furthermore, new fuel cell processes from hydride ions, H⁻, and the electrochemical behaviour of LiF–CaH₂–Al₂O₃ electrolytes and fuel cells are discussed in more detail.     

One-pot synthesis of α-aminophosphonates catalyzed by antimony trichloride adsorbed on alumina

SbCl₃ adsorbed on Al₂O₃ is found to be an efficient and recyclable catalyst in promoting three-component coupling reactions of aldehydes (aromatic and aliphatic), amines (aryl amines, aliphatic amines and esters of S-α-amino acids) and dialkylphosphites to afford the corresponding α-aminophosphonates in high yields. The ethyl ester of S-phenylalanine was observed to yield the corresponding α-aminophosphonate with S,S-diastereoisomer formed in dominance over the S,R-diastereoisomer.     

Optical absorption in solution processed thin films of calcia–alumina binary compounds

UV–visible optical spectra were obtained at room temperature in air for solution-processed thin films of 12CaO·7Al₂O₃(C12A7) on the MgO <100> single crystal substrates after the post-deposition heat treatment at 1,100 °C. Two absorption peaks were observed at 5.3 and 6.1 eV and their presence was attributed to the complex electronic structure of C12A7 and its different energy levels due to the extra-framework species and the framework itself. The peak at 5.3 eV is believed to be associated with O²⁻-Cage Conduction Band type transition whereas the Framework Valence Band to Framework Conduction Band transition is responsible for the peak at 6.1 eV. The nature of electronic transition was found to be direct from the analysis due to Tauc’s law.     

β-Cyclodextrin for design of alumina supported cobalt catalysts efficient in Fischer-Tropsch synthesis

Addition of β-cyclodextrin during catalyst preparation strongly affects the structure and catalytic performance of alumina supported cobalt catalysts for Fischer-Tropsch synthesis. Impregnation of the support with solutions containing β-cyclodextrin leads to higher metal dispersion and spectacularly enhances both reaction rate and heavy hydrocarbons productivity in Fischer-Tropsch synthesis.     

A novel low temperature synthesis technique of sol–gel derived nanocrystalline alumina abrasive

The effect of ball milling process, co-doped seed and two step sintering technique on the properties of sol–gel derived alumina abrasive sintered at low temperature was investigated. The results showed that ball milling time with 10 h can be effective in enhancing the activity of the precursor and the microstructural uniformity of sintered alumina abrasive. A small amount of Al₂O₃–(NH₄)₃AlF₆ co-doped seed addition had potential synergistic effects for reducing α-Al₂O₃ phase transformation temperature and improving the mechanical property of alumina abrasive. A remarkable suppression of grain growth was achieved by controlling sintering temperature with two-step sintering method. Therefore, by using ball milling process, co-doping α-Al₂O₃–(NH₄)₃AlF₆ seed and two-step sintering technique, the sol–gel derived uniform nanocrystalline alumina abrasive is easily achieved at low temperature. Nanocrystalline alumina abrasive prepared at these conditions exhibited excellent mechanical properties and wear resistance compared to fused corundum abrasive and those sol–gel derived corundum abrasive with conventional sintering methods.     

Preparation of silica–alumina composite membranes for hydrogen separation by multi-step pore modifications

In the present paper, a silica–alumina composite membrane for hydrogen separation was prepared within an α-alumina support by the multi-step pore modification. The α-alumina support has an asymmetric structure composed of a thin dense skin layer and a thick coarse layer and the average pore size of its skin layer is 80 nm. The composite membrane layer was formed in the vicinity of the interphase between the two layers of the support by two consecutive steps; namely, in situ silica sol–gel reaction and soaking and vapor deposition. In order to enhance the hydrogen selectivity, palladium (Pd) particles were impregnated in the final step utilizing Pd-acetate as a Pd precursor. Although both silica and Pd induced the surface diffusion, Pd was more effective for selective hydrogen adsorption than silica. This multi-step method produced a porous membrane with moderate hydrogen selectivity and satisfactory hydrogen permeance at high temperature and at high transmembrane pressure. The separation factor of hydrogen relative to nitrogen was maintained at about 10 even when the transmembrane pressure was as high as 110 kPa, and the hydrogen permeance was still much higher than that of non-porous polymeric membranes. In addition, the microstructural distributions of Si and Pd within the intermediate membrane layer were examined by a scanning electron microscopy (SEM) and an energy dispersive X-ray analysis (EDX)     

The non-isothermal kinetics of mullite formation in mechanically activated kaolinite–alumina ceramic system

The non-isothermal kinetics of mullite formation from both non-activated and mechanically activated kaolinite?+?alumina ceramic system have been studied by differential thermal analysis (DTA). The mixture of kaolinite and alumina was activated mechanically in a planetary mill, while amorphization in the kaolinite and alumina structure was studied by X-ray diffraction analysis. The activation energies depending on the conversion for mullite formation have been calculated from the DTA curves by using the non-isothermal method of Coats and Redfern at heating rates of 5, 10, 15, and 20?°C?min^?1. The mechanical activation of the kaolinite and alumina mixture resulted in the decrease in activation energy values for mullite formation.     

Processing and properties of sintered submicron IR transparent alumina derived through sol–gel method

For the first time, sintered alumina with high transparency in mid infrared region, composed of submicron grains, has been fabricated using sol–gel processing. Commercially available boehmite powder was used to prepare the stable sol. The sol was mixed with appropriate amount of sintering aids and alumina seeds. The sol was further gelled, dried, and heat treated at 1000?°C for producing alumina powder. The powder was further shaped into pellets by compaction and sintered at temperatures between 1200 and 1400?°C in air. Sintered samples were further pressed hot isostatically to produce sintered submicron transparent alumina. The synthesized powder was characterized for its morphology and phase. The sintered and hot isostatically pressed samples were characterized for their physical, mechanical, and optical properties. The present method produced transparent alumina with transparency upto 87% in mid-wave infrared region. These transparency values were at par with the transparency of single crystal sapphire in the mid-wave infrared region and the hardness values were even superior than sapphire.

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Cobalt supported on CNTs-covered γ-and nano-structured alumina catalysts utilized for wax selective Fischer-Tropsch synthesis

Cobalt supported on carbon nanotubes(CNTs)-covered alumina has been recently developed and successfully utilized as a catalyst in Fischer-Tropsch synthesis(FTS).Problems associated with shaping of Co/CNTs into extrudates or pellets as well as catalyst attrition rendered these materials unfavorable for industrial applications.In this investigation regularγ-and nano-structured(N-S)alumina as well as CNTs-covered regularγ-and N-S-alumina supports were impregnated by cobalt nitrate solution to make new cobalt-based catalysts which were also promoted by Ru.The catalysts were characterized and tested in a micro reactor to evaluate their applicability in FTS.γ-Al2O3 was prepared by calcination of bohemite and N-S-Al2O3 was prepared by sol-gel method using aluminum chloride as starting material.Catalyst evaluations indicated that N-S-Al2O3 was superior to regularγ-Al2O3 and that CNTs-covered alumina supports were favored over non-covered ones in terms of activity and heavy hydrocarbon selectivity.These were justified by porosimetric characteristics of the catalysts and existence of CNTs points of view. CNTs-covered catalysts also showed higher wax selectivity and better resistance to deactivation.Furthermore,TPR analysis indicated that the cobalt aluminate phase,which is responsible for the permanent deactivation of alumina supported Co-based catalysts,did not form on alumina supported Co-based catalysts covered with CNTs due to weaker interactions between cobalt and alumina.