Preparation and characterization of rubidium-beta-alumina by the gel-to-crystallite conversion method

Preparation of Rb-beta -alumina was realized by the gel-to-crystallite conversion method. Reaction of hydrated aluminum hydroxide gel with RbOH in ethanol medium gave rise to the Rb+-inserted pseudoboehmite precursor under wet chemical conditions. The thermal decomposition of the precursor yielded Rb-beta -alumina. The Rb2O:Al2O3 ratio of monophasic Rb-beta -alumina ranged from 1:10 to 1:22. The extended stability in the compositional range is due to the fact that the conduction planes containing Rb+ and O2- ions can have lower occupancy of Rb+ ions for larger sized alkali ions, permitting the steric separation of the adjoining spinel blocks. High-resolution electron microscopy revealed that the decreasing occupancy of alkali ions in the conduction plane is balanced by changing widths of spinel blocks arising from the shift of tetrahedral Al3+ ions to octahedral sites and an accompanying increase in stacking defects. (C) 2000 Elsevier Science Ltd. All rights reserved

Influence of the preparative conditions on the precursor phases formed during the synthesis of beta-alumina by the wet chemical gel to crystallite conversions

Hydrated alumina gel, Al2O3 . yH(2)O (80<y<120) reacts with sodium hydroxide in water-miscible organic solvents yielding crystallites of either NaxAlOx(OH)(3-x) (0.17<x<0.23), having the gibbsite structure with inserted sodium ions, or NaxAlO1+x(OH)(1-x) (0.17<x<0.23) of boehmite structure. On calcination at elevated temperatures these precursors yield phase-pure beta "-(Na) alumina. A detailed investigation has been undertaken regarding the influence of preparative parameters on the precursor phases formed. It is observed that the non-aqueous solvents destabilise the gel irreversibly in the presence of alkali hydroxide by decreasing the effective ionic pressure within the gel cavities, and at the same time prevent the back diffusion of alkali ions inserted within the precursor lattice. In the presence of strongly desiccative (water attracting) solvents, dehydroxylation of (Na) gibbsite takes place under wet chemical conditions to yield pseudoboehmite. On calcination, both these phases give rise to intermediate alumina which, in turn, converts to beta "-alumina through a continuous solid state proces

Preparation of proton-ββ′′\beta\beta^{\prime\prime}-aluminas by the ion-exchange under hydrothermal conditions and their characterisation

Preparation of proton-$\beta\beta^{\prime\prime}$-aluminas is realised by an ion-exchange process under hydrothermal conditions using dilute acetic acid or sulphuric acid as the medium, and the corresponding alkali \beta\beta-aluminas as the starting materials. The optimum conditions involved in this process is around 425–450 K under the autogenerated pressure below 50 MPa, and a duration of 2–10 h. At higher temperatures and pressures or extended duration, proton-\beta-alumina converts to boehmite, \gamma -AlOOH, in the same medium. This method yields products with a variable range of composition with $[H_2O]:[Al_2O_3]$=1:5 to 1:17. The resulting products convert to \alpha-alumina when heated to temperatures above 1273 K under ambient pressures. High resolution transmission electron microscopy (HRTEM) indicates the variable contents of microsyntactic defects and stacking faults with the changing composition. This study also shows the retention of the \beta-alumina structure during the displacement of alkali by hydroxonium ions under optimum hydrothermal conditions

Preparation of lithium B ''-alumina by the ion-exchange reaction

Preparation of Li-beta"-alumina is realized by the heterogeneous ion-exchange reaction between Na-beta"-alumina and LiCl in the solid state or melt at 800 to 930 K. The utility of Na-beta"-alumina as a template wherein Li+ ions replace Na+ from the conduction planes arises from the high reactivity of the precursor prepared by the novel wet chemical gel-to-crystallite conversion (G-C) technique. High-resolution transmission electron microscopy (HRTEM) revealed the presence of splinter fragments resulting from high density of stacking faults in Li-beta"-alumina. The decrease in intensities of (00l) reflections in powder X-ray diffraction (XRD) patterns on annealing between 873 and 1173 K is due to the intra-diffusion of Li+ from the conduction planes into the spinel blocks. gamma-LiAlO2 and LiAl5O8 are formed when Li-beta"-alumina is heated above 1273 K. Lithium aluminospinel, LiAl5O8, is the only residual phase when heated to 1673 K, which is formed as a result of the evaporation of Li2O

Mechanism of gel-to-crystallite conversion process in the synthesis of nanocrystalline multinary oxides

Description of gel-to-crystallite conversion process is made towards the synthesis of nanocrystalline titanates and aluminates. Thermodynamic and kinetic factors governing the conversion of a gel to meta-stable and stable nanocrystalline products(s) are discussed. Correlations between these factors and the preparative conditions employed for the syntheses of titanates and aluminates are arrived at

Extended stability ranges of \beta-aluminas with increasing ionic radii of metal alkali ions

We propose a new technique for constructing code-generator generators, which combines the advantages of the Graham-Glanville parsing technique and the bottom-up tree parsing approach. Machine descriptions are similar to Yacc specifications. The construction effectively generates a pushdown automaton as the matching device. This device is able to handle ambiguous grammars, and can be used to generate locally optimal code without the use of heuristics. Cost computations are performed at preprocessing time. The class of regular tree grammars augmented with costs that can be handled by our system properly includes those that can be handled by bottom-up systems based on finite-state tree parsing automata. Parsing time is linear in the size of the subject tree. We have tested the system on specifications for some systems and report table sizes

Synthesis of Beta Alumina from Aluminum Hydroxide Precursor Formed by Gel to Crystallite Conversions

A novel method is reported for the preparation of phase-pure $\beta"$-(Na) alumina by the gel to crystallite conversion technique. Coarse gels of hydrated alumina, $Al_20_3.YH_20(8O<Y<120)$, were reacted with sodium hydroxide solution in ethanol, yielding crystalline $Na_x,Al0_x,(OH)_{3-x}(0.17 < x < 0.23)$ having the gibbsite structure with inserted sodium ions. Thermal decomposition of this precursor above 573 K yielded the intermediate $Na_x,AlO_{1+x},(OH)_{1-x}$, with the boehmite structure. At higher temperatures this intermediate gave rise to \gamma-alumina, $Na_xA1_20_{3-x}(0H)_{3x}$ and $NaAl_{7/3}O_4$ with the defect spinel structure. It is proposed that because of the cubic close packing of oxygen ions present in the intermediate alumina, formation of $\beta"$- alumina takes place above 1273 K as a continuous solid-state process with minimal rearrangement of the close packed oxide ions. The $\beta"$-alumina phase thus formed is free of intergrowths of the $_\beta$-phase, as revealed by HREM and is stable up to near-melting conditions

Low-temperature zirconia oxygen gauges based on RuO<SUB>2</SUB> electrode

A low temperature oxygen gauge based on zirconia electrolyte has been developed. It makes use of RuO2 as electrode material in place of platinum in conventional gauges. The low interfacial impedance of the RuO2 electrode makes it possible to keep the cell resistance below 106 &#969; even at low temperatures. Nernst's law tests indicate that this cell can give theoretical outputs down to 498 K campared to 923 K for gauges with platinum electrodes. Faraday's law tests confirm its good performance over a wide range of oxygen concentrations. High electronic conductivity, single oxide phase, slight non-stoichiometry and good adherence are responsible for the good performance of RuO2 as electrode. An activation energy of 90.95 kJ/mole observed for the interface shows that the vacancy movement in the electrolyte is the rate controlling step. The evaporation of RuO2 as RuO4 gives rise to flow dependent output. This can be overcame by operating the cells at low temperatures though at the cost of speedy response. The low operating tempratures lead to a compact gauge with good stability

Novel approach for the bulk synthesis of nanocrystalline yttria doped thoria powders via polymeric precursor routes

Three different polymeric precursor routes viz., (i) amorphous citrate process, (ii) Pechini process and (iii) polyethylene glycol assisted process were compared for synthesising nanocrystalline powders of 7.5 mol% yttria doped thoria (YDT). In each of the processes, parameters such as metal-to-fuel ratio or composition of fuel were varied and the effects were analysed. TG/DTA studies were conducted to identify the ignition temperatures of the precursors. Also, a novel experimental procedure with controlled combustion was devised for the preparation of powders based on the thermal analysis data. All the processes result in phase pure and nanocrystalline powders. The average crystallite size of the powders ranged between 9 and 18 nm. The powder samples were analysed for their carbon content and studied for their sinterability. Densities as high as 99% th.d. could be achieved by sintering the compacts of powders obtained from (i) amorphous citrate process with CA/M ratios 2.0 and 3.0, (ii) Pechini processes (independent of fuel composition) and (iii) PEG-assisted process using PEG 4000 at a relatively low temperature of 1500 degreesC for 2 h. (C) 200