Thermal stability of mixed-cation alpha-sialon ceramics

A series of alpha-sialon (alpha') compositions containing mixed stabilising cations were prepared, by introducing additional CaO to a basic Sm alpha-sialon compositions. The thermal stability of these Sm-Ca-containing alpha-sialon phases was investigated using XRD, SEM and EDXS techniques. It was found that the addition of calcium into the Sm alpha-sialon systems greatly improved the stability of the alpha-sialon phases. Calcium was found to be incorporated into the alpha-sialon structure, coexistent with the samarium, and partitioning of the calcium and samarium was observed between the alpha' phase and grain boundary phases. This indicates a technique which may be used to improve the thermal stability of the alpha' phase while maintaining good refractory phases at the gialon grain boundaries. (C) 2003 Elsevier Science B.V. All rights reserved.

Oxidation of toluene in waste gas streams using mesoporous Ti-hexagonal mesoporous silica

Various mesoporous catalysts with titanium loadings between 0.5 and 4 Ti wt. % and surface areas between 600 and 1,600 m(2)/g were synthesized using the molecular designed dispersion technique. These catalysts were tested using toluene oxidation in a fixed bed reactor at temperatures between 300 and 550degreesC. The reaction products were found to be CO2 and CO with selectivity towards CO2 above 80% for all catalysts. The catalytic activity of the catalysts increases with titanium loading. The total conversion at 550degreesC was not affected by the textural porosity, but increased textural porosity did significantly reduce the ignition temperature by up to 50degreesC. The Thiele modulus was calculated to be much less than one for all these materials indicating that the reaction rate is not diffusion, limited.

The microstructure of mechanically alloyed MoSi2 with Ni and Al additions

The effect of Ni and Al additions on grain boundary silica in mechanically alloyed and hot isostatically pressed (HTPed) MoSi2 was investigated. Mechanical alloying Mo and Si in the absence of Al produced finely dispersed silica within a fine grained structure. Mechanically alloyed and HIPed Mo and Si with Ni and Al partially transformed the silica to crystalline oxide phases, including Al2O3. An improvement in high temperature properties is not expected due to the retention of a grain boundary silica film. Rapid grain growth resulted during HIPing, possibly due to the formation of a Ni/Fe/Al liquid phase.

Molecular Sieve Silica Membranes

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An electron microscopic study of nickel sulfide inclusions in toughened glass

It has been known since the early sixties that nickel sulfide inclusions cause spontaneous fracture of toughened (thermally tempered) glass, but despite the considerable amount of work done on this problem in the last four decades, failures still occur in the field with regularity. In this study we have classified (by viewing through a 60x optical microscope) inclusions into two groups, which are classic and atypical nickel sulfides. The classics look like the nickel sulfide inclusions found at the initiation-of-fracture of windows that have broken spontaneously. We have compared the structure and composition of the atypical inclusions with the structure and composition of the classics. All of the classic and atypical nickel sulfide inclusions studied in this work were found to have a composition in the range of Ni52S48 to Ni48S52. Inclusions on the nickel rich side of stoichiometric NiS were found to be two-phase assemblies, and inclusions on the sulphur rich side of NiS were single phase. It had been proposed that the atypicals were passive, and of a different composition to the classics. However, we found that the difference between passive and dangerous nickel sulfide inclusions was not a difference in composition but rather a difference in the type of material in the internal pore space. The passive's had carbon char in their internal pore space, whereas the pore space of dangerous inclusions contained Na2O. The presence of Na2O and carbon char with the inclusions indicates that the formation of the inclusions results from a reaction of a nickel-rich phase with sodium sulphate and carbon. (C) 2001 Kluwer Academic Publishers.

A nanometre-scale non-periodic structural variation in high temperature superconducting ceramics and the implications for properties

Non-periodic structural variation has been found in the high T-c cuprates, YBa2Cu3O7-x and Hg0.67Pb0.33Ba2Ca2Cu3O8+delta, by image analysis of high resolution transmission electron microscope (HRTEM) images. We use two methods for analysis of the HRTEM images. The first method is a means for measuring the bending of lattice fringes at twin planes. The second method is a low-pass filter technique which enhances information contained by diffuse-scattered electrons and reveals what appears to be an interference effect between domains of differing lattice parameter in the top and bottom of the thin foil. We believe that these methods of image analysis could be usefully applied to the many thousands of HRTEM images that have been collected by other workers in the high temperature superconductor field. This work provides direct structural evidence for phase separation in high T-c cuprates, and gives support to recent stripes models that have been proposed to explain various angle resolved photoelectron spectroscopy and nuclear magnetic resonance data. We believe that the structural variation is a response to an opening of an electronic solubility gap where holes are not uniformly distributed in the material but are confined to metallic stripes. Optimum doping may occur as a consequence of the diffuse boundaries between stripes which arise from spinodal decomposition. Theoretical ideas about the high T-c cuprates which treat the cuprates as homogeneous may need to be modified in order to take account of this type of structural variation.

Separation of mercury (II), lead (II), and silver (I) from aqueous solution using an aminopolycarboxylate silica

A quartz crystal microbalance modified by the attachment of silica particles derivatized with the aminopolycarboxylate ligand N-[(3-trimethoxysilyl)propyl]ethylenediamine-N,N',N'-triacetic acid has been employed to assess conditions under which mercury (II), lead (II), and silver (I) nitrates may be separated in aqueous solution. The separation protocol, which involved removal of Hg(II), as [HgI4](2-), and Pb(II) with H+ was successfully applied to a batchwise separation of the 3 metal ions.

Multi-fluorescent silica colloids for encoding large combinatorial libraries

Large chemical libraries can be synthesized on solid-support beads by the combinatorial split-and-mix method. A major challenge associated with this type of library synthesis is distinguishing between the beads and their attached compounds. A new method of encoding these solid-support beads, 'colloidal bar-coding', involves attaching fluorescent silica colloids ('reporters') to the beads as they pass through the compound synthesis, thereby creating a fluorescent bar code on each bead. In order to obtain sufficient reporter varieties to bar code extremely large libraries, many of the reporters must contain multiple fluorescent dyes. We describe here the synthesis and spectroscopic analysis of various mono- and multi-fluorescent silica particles for this purpose. It was found that by increasing the amount of a single dye introduced into the particle reaction mixture, mono- fluorescent silica particles of increasing intensities could be prepared. This increase was highly reproducible and was observed for six different fluorescent dyes. Multi-fluorescent silica particles containing up to six fluorescent dyes were also prepared. The resultant emission intensity of each dye in the multi-fluorescent particles was found to be dependent upon a number of factors; the hydrolysis rate of each silane-dye conjugate, the magnitude of the inherent emission intensity of each dye within the silica matrix, and energy transfer effects between dyes. We show that by varying the relative concentration of each silane-dye conjugate in the synthesis of multi-fluorescent particles, it is possible to change and optimize the resultant emission intensity of each dye to enable viewing in a fluorescence detection instrument.

Hydrophobicity of Templated Silica Xerogels for Molecular Sieving Applications

Silica xerogels were prepared by a sol-gel process catalyzed by acid with tetraethylorthosilicate, and using an organic covalent ligand template (methyltriethoxysilane) or a noncovalent template C6 surfactant (triethylhexylammonium bromide). The influence of hydrotreatment on the structure of templated xerogels is examined in terms of surface area, micropore volume, average pore size, and pore size distribution, and compared against a blank xerogel (nontemplated). The role of surface functional groups was evaluated using Si-29 nuclear magnetic resonance. The structural integrity of the xerogel was maintained to a large extent in samples that had a high contribution of Q(4) species (siloxane groups). Xerogel matrix densification occurred when there was a large concentration of Q(3) and Q(2) species (silanol groups), which also were responsible for increased hydrophilicity. The templated xerogels resulted in up to a 25% concentration of methyl functional groups (T-3 and T-2 species), leading to hydrophobic xerogels. The best results in terms of structural integrity and hydrophobicity were obtained with templated xerogels prepared with the C6 surfactant. The results in this study suggest that surfactant-enhanced condensation reactions lead to structures with a high contribution of Q(4) groups, which are not susceptible to water attack, but are strong enough to oppose matrix densification during rehydration.

Characterisation of templated xerogels for molecular sieve application

This paper presents the results of the characterisation of templated silica xerogels as precursor material for molecular sieve silica membranes for gas separation. The template agent integrated in the xerogel matrix is a methyl ligand covalently bended to the siloxane network in the form of methyltriethoxysilane (MTES). Several surface and microstructural characterisation techniques such as TGA, FTIR, NMR, and nitrogen adsorption have been employed to obtain information on the reaction mechanisms involved in the sol-gel processing of such molecular sieves. The characterisation results show the effects of processing parameters such as heat treatment temperature, and the concentration of the covalently bonded template on the development of the pore structure. It was found that calcination temperature significantly enhanced the condensation reactions thus resulted in more Si-O-Si groups being formed. This was also confirmed with the data of FTIR characterisation showing enhanced silicon bands at higher heat treatment temperatures. As a result of the promoted densification and shrinkable pore network the micropore volume also reduced with increasing methyl ligand molar ratio. However, the mean pore diameter does not change significantly with calcination temperature. While the contribution of the templates towards controlling pore size is less precise, increasing the methyl ligand molar ratio results in the broadening of the pore size distribution and lower pore volume. Higher template concentration induces the collapse of the xerogel matrix due to capillary stress promoting dense xerogels with low pore volume (C) 2001 Elsevier Science B.V. All rights reserved.

Glass transition in Australian honeys

Differential scanning calorimetry (DSC) was used to study the glass transition in 10 Australian honeys by scanning at 10 degrees Cmin(-1) from -130 to 50 degreesC after annealing at -50 degreesC. The honeys had moisture contents 14.9 to 18.0%, seven were from Eucalyptus species. The glass transition temperatures (T-g) ranged from -46 degrees to -38 degreesC and were significantly (p