Combustion synthesis of oxide materials for nuclear waste immobilization

Oxide materials like perovskite, zirconolite, hollandite, pyrochlore, NASICON and sphene which are used for nuclear waste immobilization have been prepared by a solution combustion process. The process involves the combustion of stoichiometric amount of corresponding metal nitrates and carbohydrazide/tetraformyl trisazine/diformyl hydrazide at 450 degrees C. The combustion products have been characterized using powder X-ray diffraction, infrared spectroscopy, and Si-29 MAS-NMR. The fine particle nature of the combustion derived powders has been studied using density, particle size, BET surface area measurements and scanning electron microscopy. Sintering of combustion derived powder yields 85-95% dense ceramics in the temperature range 1000 degrees-1300 degrees C.

Controlled combustion synthesis and properties of fine-particle NASICON materials

Fine-particle NASICON materials, Na1+xZr2P3-xSixO12 (where x = 0.0, 0.5, 1.0, 1.5, 2.0 and 2.5), have been prepared by controlled combustion of an aqueous solution containing stoicthiometric amounts of sodium nitrate, zirconyl nitrate, ammonium perchlorate, diammonium hydrogen phosphate, fumed silica and carbonohydrazide. Formation of NASICON has been confirmed by powder XRD, Si-29 NMR and IR spectroscopy. These NASICON powders are fine (average agglomerate size 5-12 mum) with a surface area varying from 8 to 30 m2 g-1. NASICON powders pelletized and sintered at 1100-1200-degrees-C for 5 h achieved 90-95% theoretical density and show fine-grain microstructure. The coefficient of thermal expansion of sintered NASICON compact was measured up to 500-degrees-C and changes f rom -3.4 x 10(-6) to 4.1 x 10(-6) K-1. The conductivity of Sintered Na3Zr2PSi2O12 compact at 300-degrees-C is 0.236 OMEGA-1 cm-1.

Photocatalytic Inactivation of Escherichia coli with LbL Fabricated Immobilized TiO2 Thin Films

Photocatalysis using semiconductor catalyst such as TiO2, in presence of UV light, is a promising technique for the inactivation of various microorganisms present in water. In the current study, the photocatalytic inactivation of Escherichia coli bacteria was studied with commercial Degussa Aeroxide TiO2 P25 (Aeroxide) and combustion synthesized TiO2 (CS TiO2) catalysts immobilized on glass slides in presence of UV irradiation. Thin films of the catalyst and polyelectrolytes, poly(allyl amine hydrochloride) and poly(styrene sulfonate sodium salt), were deposited on glass slides by layer by layer (LbL) deposition method and characterized by SEM and AFM imaging. The effect of various parameters, namely, catalyst concentration, surface area and number of bilayers, on inactivation was studied. Maximum inactivation of 8-log reduction in the viable count was observed with 1.227 mg/cm(2) of catalyst loaded slides. With this loading, complete inactivation was observed within 90 min and 75 min of irradiation, for Aeroxide and CS TiO2, respectively. Further increase in the catalyst concentration or increasing number of bilayers had no significant effect on inactivation. The effect of surface area on the inactivation was studied by increasing the number of slides and the inactivation was observed to increase with increasing surface area. It was also observed that the immobilized catalyst slides can be used for several cycles leading to an economic process. The study shows potential application of TiO2, for the inactivation of bacteria, in its fixed form by a simple immobilization technique.

Sonochemical reaction engineering

Ultrasound has been widely used by chemists to enhance yields as well as rates of homogeneous as well as heterogeneous chemical reactions. The effect of ultrasound on the course of chemical reactions is mediated through cavitation bubbles it generates. High temperatures and pressures are attained inside the cavitating bubbles when they collapse. The extreme conditions so generated lead to the formation of reactive intermediates, e.g., free radiacls, inside the bubbles, which cause chemical reactions to occur when they enter the surrounding liquid. This is the mechanism through which ultrasound influences the path of homogeneous reactions. The cavitation bubbles collapse asymmetrically in the vicinity of solids, e.g., catalyst particles. Asymmetric collapse lead to formation of high speed microjets. The microjets can enhance transport rates, the increase surface area through pitting as well as particle fragmentation through collisions. Both can alter the rates of heterogeneous reaction rates. It however appears that these effects do not exhaust the scope of the influence of ultrasound on heterogeneous reactions. Modelling and quantitative prediction of the effect of ultrasound on chemical reactions is however at a stage of infancy as the phenomena are complex. Only a few examples of modelling exist in literature. Apart from this, reactor design and scaleup pose significant problems. Thus sonochemical reaction engineering offers large scope for research and development efforts.

Synthesis and characterization of barium bis(citrato) oxozirconate(iv) tetrahydrate--A new molecular precursor for fine particle BaZrO3

Barium metazirconate (BaZrO3) fine powder has been produced by thermally decomposing a molecular precursor, barium bis(citrato)oxozirconate(IV) tetrahydrate at about 700-degrees-C. The precursor, Ba[ZrO(C6H6O7)2] . 4H2O (BZO) has been synthesized and characterized by employing a combination of spectroscopic and thermoanalytical techniques. The precursor undergoes thermal decomposition in three major stages: (i) dehydration to give an anhydrous barium zirconyl citrate, (ii) decomposition of the anhydrous citrate in a multistep process to form an ionic oxycarbonate intermediate, Ba2Zr2O5CO3, and (iii) decomposition of the oxycarbonate to produce BaZrO3 fine powder. The particle size of the resultant BaZrO3 is about 0.2 mum, and the surface area is found to be 4.0 m2 g-1.

Synthesis and properties of tialite, beta-AL2TIO5

Tialite, beta-Al2TiO5, a low expansion material, has been synthesised by the combustion of corresponding metal nitrates and carbohydrazide (CH) or urea redox mixtures at 500-degrees-C. As prepared powders contained tialite, rutile, and corundum in the mole ratios of 50:25:25 (CH) and 20:40:40 (urea). The combustion derived powders, when calcined 30 min at 1300-degrees-C, gave single phase beta-tialite having a surface area of 20-25 m2 g-1 and a particle size of 0.79-1.03 mum.

Rapid Synthesis of Ultrahigh Adsorption Capacity Zirconia by a Solution Combustion Technique

Tetragonal ZrO2 was synthesized by the solution combustion technique using glycine as the fuel. The compound was characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, and BET surface area analysis. The ability of this compound to adsorb dyes was investigated, and the compound had a higher adsorption capacity than commercially activated carbon. Infrared spectroscopic observations were used to determine the various interactions and the groups responsible for the adsorption activity of the compound. The effects of the initial concentration of the dye, temperature, adsorbent concentration, and pH of the solution were studied. The kinetics of adsorption was described as a first-order process, and the relative magnitudes of internal and external mass transfer processes were determined. The equilibrium adsorption was also determined and modeled by a composite Langmuir-Freundlich isotherm.

Combustion synthesis and properties of the NASICON family of materials

The fine-particle NASICON family of materials, MZr2P3O12(where M = Na, K, ½Ca and ¼Zr) and NbZrP3O12, have been prepared by the combustion of aqueous heterogeneous mixtures of stoichiometric amounts of metal nitrate, zirconyl nitrate, niobium phosphate, diammonium hydrogen phosphate, ammonium perchlorate and carbohydrazide (CH) at 400 °C. The formation of NASICON materials was confirmed by powder X-ray diffraction (XRD), IR, solid-state (31P) NMR spectroscopy and thermal expansion coefficient measurements. The combustion-synthesized NASICON powders have an average agglomerate size of 9�13 µm with a specific surface area varying from 8 to 28 m2 g�1. The powders pelletized and sintered in the range 1100�1200 °C for 5 h achieved 95�97% theoretical density and showed fine-grain microstructure. The coefficient of thermal expansion of a sintered compact was measured up to 500 °C and ranged from �1.5 × 10�6°C�1 to 1.0 × 10�6°C�1 depending on the composition.

Combustion synthesis of yttria

Fine-particle, sinter-active yttria has been prepared by combustion of a redox compound, Y(N2H3COO)3·3H2O and mixtures of Y(N2H3COO)3·3H2O�NH4NO3 or NH4ClO4 as well as yttrium nitrate and hydrazine-based fuels. The fineparticle nature of the combustion-derived yttria has been investigated using powder density, particle size and BET surface area measurements. The uniaxially, cold-pressed fine-particle yttria when sintered at 1450�1500 °C achieved 98% theoretical density and showed a fine-grain (1�2 µm) microstructure.

Reactivity Of Silica From Rice Husk With Lime

The paper correlates the reactivity of rice husk ash with its physicochemical properties such as crystallinity, surface area, microstructure, particle size distribution, porosity and solubility. These properties, in tum, are dependent on the time-temperature conditions under which the ash is prepared. It is found that the reactivity of the ash cannot be quantified by any one of these parameters alone, though they all indicate it qualitatively. Therefore, a method for quantifying this property was developed, by which the Reactivity Index is obtained. There is only a gradual change in the reactivity index of RHA with ashing temperature, as in many other properties, like surface area, porosity and total volume of gas absorbed by unit mass of the silica ash. This reactive index is found to be useful in determining the optimum ash/lime ratios required to give the best performance for RHA-lime composites.

Synthesis and properties of rare earth doped lamp phosphors

Red, blue and green emitting lamp phosphors such as EU(3+) doped Y2O3 (red phosphor), EU(2+) doped Ba0.64Al12O18.64, BaMgAl10O17 and BaMg2Al16O27 (blue phosphors) and Ce0.67Tb0.33MgAl11O19 and Eu2+, Mn2+ doped BaMgAl10O17 (green phosphors) have been prepared by the combustion of the corresponding metal nitrates (oxidizer) and oxalyl dihydrazide/urea/carbohydrazide (fuel) mixtures at 400 degrees-500 degrees C within 5 min. The formation of these phosphors has been confirmed by their characteristic powder X-ray diffraction patterns and fluorescence spectra. The phosphors showed characteristic emission bands at 611 nm (red emission), 430-450 nm (blue emission) and 515-540 nm (green emission). The fine-particle nature of the combustion derived phosphors has been investigated using powder density, particle size and BET surface area measurements.

Combustion synthesis and properties of nanostructured ceria-zirconia solid solutions

Nanostructured ceria-zirconia solid solutions (Ce1 − xZrxO2, X = 0 to 0.9) have been synthesized by a single step solution combustion process using cerous nitrate, zirconyl nitrate and oxalyl dihydrazide (ODH) / carbohydrazide (CH). The as-synthesized powders show extensive XRD line broadening and the crystallite sizes calculated from the XRD line broadening are in the nanometer range (6–11 nm). The combustion derived ceria zirconia solid solutions have high surface area in the range of 36–120 m2/g. Calcination of Ce1 −xZrxO2 at 1350 °C showed three distinct solid solution regions: single phase cubic (x ≤ 0.2), biphasic cubic-tetragonal (0.2 < x Image .8) and tetragonal (x > 0.8). When x ≥ 0.9, the metastable tetragonal phase formed transforms to monoclinic phase on cooling after calcination above 1100 °C. The homogeneity of Ce1 − xZrxO2 has been confirmed by EDAX analysis. The Temperature Programmed Reduction (TPR) measurement of Ce0.5Zr0.5O2 was carried out with H2 and the TPR profile showed two water formation peaks corresponding to the utilization of surface and bulk oxygen.

Evaluation of strain-rate effects in transitional round jets using direct numerical simulation

The coherent flame model uses the strain rate to predict reaction rate per unit flame surface area and some procedure that solves for the dynamics of flame surfaces to predict species distributions. The strainrate formula for the reaction rate is obtained from the analytical solution for a flame in a laminar, plane stagnation point flow. Here, the formula's effectiveness is examined by comparisons with data from a direct numerical simulation (DNS) of a round jetlike flow that undergoes transition to turbulence. Significant differences due to general flow features can be understood qualitatively: Model predictions are good in the braids between vortex rings, which are present in the near field of round jets, as the strain rate is extensional and reaction surfaces are isolated. In several other regions, the strain rate is compressive or flame surfaces are folded close together. There, the predictions are poor as the local flow no longer resembles the model flow. Quantitative comparisons showed some discrepancies. A modified, consistent application of the strain-rate solution did not show significant changes in the prediction of mean reaction rate distributions.

Amorphous, Monoclinic, and Tetragonal Porous Zirconia Through a Controlled Self-Sustained Combustion Route

Porous, large surface area, metastable zirconias, are of importance to catalytic, electrochemical, biological, and thermal insulation applications. Combustion synthesis is a very commonly used method for producing such zirconias. However, its rapid nature makes control difficult. A simple modification has been made to traditional solution combustion synthesis to address this problem. It involves the addition of starch to yield a starting mixture with a ``dough-like'' consistency. Just 5 wt% starch is seen to significantly alter the combustion characteristics of the ``dough.'' In particular, it helps to achieve better control over reaction zone temperature that is significantly lower than the one calculated by the adiabatic approximation typically used in self-propagating high-temperature synthesis. The effect of such control is demonstrated by the ability to tune dough composition to yield zirconias with different phase compositions from the relatively elusive ``amorphous'' to monoclinic (> 30 nm grain size) and tetragonal pure zirconia (< 30 nm grain size). The nature of this amorphous phase has been investigated using infrared spectroscopy. Starch content also helps tailor porosity in the final product. Zirconias with an average pore size of about 50 mu m and specific surface area as large as 110 m2/g have been obtained.

Mesoporous MnO2 synthesized by using a tri-block copolymer for electrochemical supercapacitor studies

Mesoporous MnO2 is prepared from KMnO4 by using a tri-block copolymer, namely, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) as a reducing as well as a structure-directing agent. The as synthesized MnO2 samples are poorly crystalline with mesoporosity having pore diameter between 8 and 40 nm. BET surface area as high as 273 m(2) g(-1) is obtained. By heating, the poorly crystalline MnO2 turns into a well crystalline form at 400 degrees C with nanorod morphology. However, the surface area decreases for the heated samples. Samples of MnO2 prepared by varying the ratio of KMnO4 and the copolymer, and also the heated samples are subjected to electrochemical characterization for supercapacitor studies. High specific capacitance values on mass basis are obtained for the as prepared mesoporous MnO2 samples. (C) 2011 Elsevier Inc. All rights reserved.