Extrusion of alumina fibre using sol-gel precursor

Alumina fibres were prepared by extrusion using boehmite sol as binder. The microstructure and mechanical properties of the fibres were studied. The fibres sintered at 1400 oC have relatively dense microstructure and the highest strength.     

Low-temperature preparation of nanostructured zirconia and YSZ by sol-gel processing

Stable zirconia sol, with a particle size sharply monodispersed at 4 nm, was synthesized via hydrolysis of zirconia tetra-n-butoxide in 2-propanol. Stable composite sols were prepared via an interparticle modification route by introducing yttrium nitrate. Although the nanostructure of the dried gels was insensitive to yttrium addition, the crystallization behaviour was greatly influenced by it. Without yttrium addition the monoclinic zirconia was obtained at 700 K. After interparticle modification with yttrium, the cubic phase of yttria-stabilized zirconia was directly obtained at 726 K. The sintering of both the systems was also enhanced at low temperature. As a result of the low-temperature processing, nanostructured zirconia and YSZ with homogeneous grain sizes of a few tens of nanometres have been successfully prepared.     

Preparation of alumina films by the sol-gel method

This review describes our study on preparation of alumina films by a sol-gel process and their several applications that have been investigated since 1986. Alumina films were prepared from alkoxide or inorganic salt. Both as-prepared alumina films were transparent in ultraviolet, visible and near infrared regions. The alumina from inorganic salt (inorganic alumina) was structureless even after annealed at 300–700°C in air, while the alumina from alkoxide (alkoxide alumina) was in pseudo-boehmite at an annealing temperature lower than 400°C and was in - or -type at 400–700°C. Both alumina films became opaque after annealed at temperatures above 1000°C. The inorganic alumina film annealed at 800°C showed a gas permeability that was influenced by physico-chemical properties of penetrant and alumina. Composite films of alumina and poly(vinyl alcohol) (PVA) were hydrophilic but insoluble in water, and removal of PVA from the composite films by annealing at 600°C led to formation of transparent alumina films. Such properties enabled us to use a counter diffusion method for fabricating -Fe₂O₃-doped alumina films. Alumina films doped with organic dyes such as laser dyes, hole-burning dyes and non-linear optical dyes, which were fabricated by gelation of dye-added alumina sol, exhibited laser emission, hole-spectra and second- or third-harmonic generation properties, respectively. Hydrogenation of alkene was catalyzed by Ni nanoparticles doped alumina films that were prepared by gelation of Ni²⁺ solution-added alumina sol and annealing the Ni²⁺-doped alumina gel in hydrogen gas. Nonlinear optical properties were observed for alumina films doped with CdS, Au and Ag nanoparticles, which were fabricated by gelation of Cd²⁺, HAuCl₄ and AgNO₃ solution-added alumina sols and annealing the Cd²⁺-doped alumina gel in H₂S gas and the Ag⁺- and Au³⁺-doped alumina gels in H₂ gas. Rare earth metal ion-doped alumina films, which were prepared by gelation of rare earth metal ion solution-added alumina sol and annealed the ion-doped alumina gel, exhibited not only normal luminescence but also up-conversion emission, energy transfer type luminescence and long lasting luminescence.     

溶胶-凝胶法制备纳米铝酸锶荧光粉

采用溶胶-凝胶法制备出粒度为20nm、颗粒纳米铝酸锶荧光粉.采用TEM电镜、发射和激发光谱、XRD等方法,研究了铝酸锶荧光粉的结构,发光性能和其组成.结果表明,该荧光粉除了具有较好的纳米粒径,突出表现出优异的发光性能.     

溶胶-凝胶法制备纳米CeO2晶体

CeO2是一种稀土类研究较为广泛且多用途的功能材料.本实验以六水硝酸铈和聚乙二醇为原料,采用溶胶法制备纳米CeO2晶体.主要研究了反应温度、焙烧温度对粉体性质的影响.结果表明,聚乙二醇溶胶法可制得纳米CeO2.在65℃下溶胶制备的CeO2粉体较均匀,分散性也较好,随着焙烧温度升高,CeO2保持立方晶型不变,粒径增大,结晶更完善.800℃焙烧所得的CeO2晶的形貌趋于球形,直径在20～50nm.     

A simple semi sol-gel method for preparation of alumina monoliths with hierarchical pore structures

A simple semi sol-gel method for preparation of stable alumina monoliths is presented. The approach is based on chemical binding of boehmite by hydrolysis products of aluminium nitrate. The crystalline phase of the monoliths depends on the calcination temperature, and the size and shape of the alumina crystallites were determined by transmission electron microscopy and modelling of X-ray diffraction patterns. The monoliths have bimodal pore size distributions with mesopores ranging from 3 to 20 nm and macropores ranging from 10 to 40 μm. Incipient wetness impregnation resulted in gold particles, ranging from 4 to 20 nm, supported on the monoliths. The size of the gold particles depended largely on the crystalline phase of the support, but also on the amount of gold precursor. The catalytic activity of the functionalised materials in liquid-phase oxidation of glucose was higher in continuously stirred batch reactor tests compared to continuous flow fix bed tests. In both cases the activity was improved as the size of gold particles decreased.     

Effect of aluminium alkoxide with donor-functionalized group on texture and morphology of the alumina prepared by sol-gel processing

A new aluminium alkoxide, Al(OCH₂CH₂OCH₂CH₂OCH₃)₃, with donor-functionalized group was prepared and characterized by elemental analysis, mass spectrometry, and ¹H, ¹³C and ²⁷Al NMR spectroscopy. The prepared aluminium alkoxide was used as a precursor for the synthesis of alumina by the sol-gel processing. The physiochemical properties of alumina that were obtained from hydrolysis of Al(OCH₂CH₂OCH₂CH₂OCH₃)₃ were investigated and compared with that of alumina synthesized from aluminium 2-butoxide. Utilizing Al(OCH₂CH₂OCH₂CH₂OCH₃)₃ as a precursor in sol-gel processing resulted in formation of alumina with significantly lower surface area, smaller pore volume, and pore size in comparison with alumina obtained from aluminium 2-butoxide. A novel morphology was observed for the alumina prepared from aluminium alkoxide with donor-functionalized group.     

溶胶-凝胶法制备CeO2防紫外膜

用溶胶-凝胶法在普通玻璃基片上成功制备了CeO2防紫外薄膜。采用交叉镀膜的方法,将TiO2膜层作为缓冲层,成功制备了不同膜层配比的防紫外膜。实验表明,不同膜层配比,紫外线透过率及可见光透过率均不同,在一定范围内,随着CeO2溶胶浓度增大,紫外吸收能力增大,溶胶的陈化时间对薄膜紫外屏蔽性能影响不大。     

Non-stoichiometric NiZn ferrite by sol-gel processing

Non-stoichiometric Ni_0.5Zn_0.5Fe_2−xO_4−3/2x ferrites over a wide range of x = 0∼0.8 are synthesized by a sol-gel processing. Phase evolution, crystal structure and crystallite size of spinel ferrites are dependent on annealing temperature and the amount of Fe deficiency. The crystallite size of spinel increases with annealing temperature and grows faster in stoichiometric ferrites than that of non-stoichiometric. Fe deficiency results in the partial reduction of spinel ferrite to zincite ZnO. XRD indicates that the crystallization temperature of ZnO is increased to about 700 °C. Zincite reduces the number of ferrite crystallites and disfavors the growth of spinel ferrites. The lattice parameters decrease with Fe deficiency and are insensitive to the variation in composition in the samples annealed at lower temperature due to the segregation of ZnO and lattice expansion in the ultrafine crystallites.     

Production of alumina fibre through jute fibre substrate

Alumina fibre has been produced using jute fibre as substrate material at temperatures lower than 1600 C in a reducing atmosphere. Processed jute fibre was chemically pretreated by saturation with Al₂Cl₆ · 12H₂O, coked and then pyrolysed to obtain alumina fibre. Chemical pretreatment conditions have been determined by following weight loss measurements of the jute fibre at 0.1 to 0.6 N solutions of NaOH, KOH, NH₄OH, Na₂CO₃, K₂CO₃, HCl and acetic acid. The effect of heat treatment on the jute fibre and jute fibre + aluminium salt has been studied from 150 to 1600 C. Trace elements present (Fe₂O₃, SiO₂, K₂O, Na₂O, CaO, MgO, ZnO, MnO, V₂O₅, P₂O₅, CuO) on heat-treated products have been determined by atomic absorption spectrometry. Optical and scanning electron micrographs of representative samples showing growth mechanism are presented. The effect of copper, nickel and platinum catalysts and fluxing agents such as K₂O and Na₂O in fibre formation has also been examined. Particle size and surface area analyses of intermediate and final products have been carried out. Changes in 2 values are plotted for various products from X-ray diffraction studies. It is conceived that the porous surface of cellulosic fibrils in the jute fibre adsorbs the AlCl₃ molecules which decompose to oxide and are gradually shaped to the fibrous form during the course of thermal treatment in a reducing atmosphere and due to the high surface area.     

Rare-earth doped zirconia fibres by sol-gel processing

Polycrystalline zirconia fibres, doped with 2–8 mol% of oxides of trivalent lanthanum, praseodymium, neodymium, samarium, gadolinium, and dysprosium (in decreasing cation size), were prepared by spinning of acetate-derived sols and baking the gel fibres thus obtained at 900–1300 °C for 1 h. The larger sized dopants lanthanum, praseodymium and neodymium (Group A) gave rise to tetragonal zirconia, with or without cubic zirconia, at 900 °C which converted partly or fully to monoclinic zirconia, in certain cases accompanied by a cubic zirconate phase at higher temperatures. The smaller sized dopants samarium, gadolinium and dysprosium (Group B) generated only tetragonal or cubic, or both polymorphs of zirconia, depending on the cation type, concentration and temperature. This stabilization of higher symmetry polymorphs with Group B dopants was associated with relatively large crystallite size (especially when calcined at 1300 °C). The maximum tensile strength values of usable fibres calcined at 1300 °C were found to decrease with increasing size in dopant dysprosium > gadolinium > samarium > neodymium, praseodymium, lanthanum=0). Although all the dopant cations were larger in size than Zr⁴⁺ (in the same oxygen coordination), the relative closeness in size of Group B cations with Zr⁴⁺ was considered to be the reason behind the obtained differences in properties.     

Adhesion of chromium metallization on alumina surfaces prepared by sol-gel techniques

The adhesion of a sputter-deposited Cr metallization layer to alumina films prepared by a solution-gelation method has been investigated using a pull test. Alumina films with a range of thicknesses (1 to 6 m) were prepared by dipping commercially available polycrystalline alumina substrates into hydrolysed aluminium butoxide sols and fired for 1 h at 500, 900, or 1200 C. Monolithic, crack-free films resulted both from pure alumina and Ti-doped alumina sols. The adhesion strength was dependent on the thickness of the alumina films, as well as on the temperature of the heat treatment. Failure occurred in part between the alumina film and the substrate and in part between the alumina film and the chromium layer. For alumina films fired at 500 C, the adhesion strengths of 1 m thick films were greater than those measured for 3 and 6 m thick films because of the formation of greater mechanical bonds between alumina films and the substrate. The adhesion strength of the chromium layers was greatly improved by firing at 1200 C. This increase in adhesion strength was attributed to an increase in the surface roughness of these specimens, which occurred due to crystallization of the sol layer. The adhesion strengths of films doped with Ti was not significantly different from those of the undoped films.