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Spinel Phase (spinel + phase)
Selected AbstractsChemInform Abstract: Effect of Thermal Treatment on the Electronic Conductivity Properties of Cobalt Spinel Phases Synthesized by Electro-Oxidation in Ternary Alkaline Electrolyte (KOH, LiOH, NaOH).CHEMINFORM, Issue 6 2009Myriam Douin Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Novel Multi-functional Mixed-oxide Catalysts for Effective NOx Capture, Decomposition, and Reduction,ADVANCED FUNCTIONAL MATERIALS, Issue 17 2007J. Yu Abstract In this paper, novel multi-functional mixed-oxide catalysts have been rationally designed and developed for the effective abatement of NOx. CaxCo3,,,xAl hydrotalcite-like compounds (where x,=,0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0) are first synthesized by co-precipitation and calcined at 800,°C for 4,h in air to derive the mixed oxides. The resultant mixed oxides are generally of spinel phase, where the CaO phase is segregated when x,,,2.5. It has subsequently been found that the derived oxides are catalytically multi-functional for NOx decomposition, capture, and reduction. For example, the mixed Ca2Co1Al1 -oxide can decompose 55,% NO at 300,°C in 8,% oxygen, completely trap NO for 750,s, and capture 12.88,and 18.06,mg,g,1 NO within 30,and 60,min, respectively. The catalytic activities of the Ca2Co1Al1 -oxide catalyst have been further improved by incorporating La to form a quaternary catalyst Ca2Co1La0.1Al0.9 -oxide. This catalyst significantly enhances the NO decomposition to 75,%, extends the complete trapping time to 1100,s, and captures more NO at 300,°C in 8,% O2 (19.02,mg,g,1 NO within 60,min). The in-situ IR spectra of the catalysts with adsorbed NO indicates that the major nitrogen species formed on the catalysts are various kinds of nitrites and nitrates, which can be readily reduced by H2 within 6,min at 350,°C. Therefore, the excellent catalytic activity of layered double hydroxide (LDH)-based mixed oxides for NO decomposition, storage, and reduction can be achieved by the elegant combination of normal transition metals. [source] Room temperature photoluminescence of the Li2ZnTi3O8 spinel: Experimental and theoretical studyINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2005M. S. C. Câmara Abstract This article describes the characterization of intense photoluminescence observed at room temperature of the Li2ZnTi3O8 spinel phase, obtained by the polymeric precursor method. The evolution of visible photoluminescence is demonstrated by measurement of the photoluminescence signal as a function of the annealing treatment time. The evolution indicates that PL can be attributed to the presence of an inorganic disordered phase. In addition, increased annealing treatment times cause not only a decrease in the total residual content of organic material in the samples, but also an intensified photoluminescence. We discuss the nature of visible photoluminescence at room temperature of the Li2ZnTi3O8 spinel phase in the light of results of both recent experimental and quantum mechanical theoretical studies. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] A comparative in situ Rietveld refinement study: thermal decomposition and transformation of CoAl and CoZnAl layered double hydroxidesJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2008Rune E. Johnsen Rietveld refinement based on in situ X-ray powder diffraction (XRPD) data was combined with thermogravimetric analysis (TGA) and mass spectrometry (MS) to study and compare the phase transformations, thermal stability, microstructural and structural changes of two cobalt-containing nitrate-based layered double hydroxides (LDHs) upon heating in a controlled inert atmosphere of nitrogen. The XRPD data were collected, using synchrotron X-ray radiation, with a time resolution of 107,s, which made it possible to carry out detailed structural studies of the initial layered double hydroxides as well as their decomposition products: spinel for a CoAl,NO3 LDH and spinel/zincite for a CoZnAl,NO3 LDH. Correlating these data with those from the TGA,MS analyses gives us information about the transformation mechanisms. Rietveld refinements of the two spinel phases reveal remarkable differences. The a axis of the spinel formed by decomposition of the CoAl,NO3 LDH increases almost linearly from approximately 598 to 1163,K, mainly due to the dominating thermal expansion, whereas the a axis of the spinel formed by decomposition of the CoZnAl,NO3 shows a more complex temperature dependency. Between approximately 698 and 1073,K, the a axis is almost constant due to pronounced chemical interaction with an additional amorphous phase and the zincite phase, whereas from 1073 up to 1163,K it increases linearly. Calculations, based on the results of the Rietveld refinements, of the size of the octahedral and tetrahedral coordination polyhedra in the spinel show that the octahedra shrink and the tetrahedra expand with increasing temperature. The unusual thermal behaviour of the octahedra is discussed and attributed to the low formation temperature of the cobalt aluminium spinel phase. Finally, the intensity of a low-angle scattering (LAS) signal observed in the XRPD patterns was correlated with the decomposition of the LDH, and determination of the specific surface areas gave the temperature-dependent BET surface areas. [source] Formation and Densification Behavior of MgAl2O4 Spinel: The Influence of Processing ParametersJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2008Ibram Ganesh Different types of dense stoichiometric and nonstoichiometric magnesium aluminate (MgAl2O4) spinel (MAS) ceramics were prepared following a conventional double-stage firing process using different commercially available alumina and magnesia raw materials. Stoichiometric, magnesia-rich, and alumina-rich spinels were sintered at 1500°,1800°C for 1,2.5 h. The influence of the different processing parameters (average particle size, degree of spinel phase, green density, mass of the powder compact, sintering temperature, holding time at the peak temperature, and starting composition) on the densification behavior of MAS was assessed by measuring the bulk density, apparent porosity, and water absorption capacity, and microstructural observations. Most of the MAS compositions tested exhibited excellent sintering properties. [source] Effect of Controlling Parameters on the Reaction Sequences of Formation of Nitrogen-Containing Magnesium Aluminate Spinel from MgO, Al2O3, and AlNJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2004Siddhartha Bandyopadhyay The reaction sequences of the formation of nitrogen-containing magnesium aluminate spinel from MgO, Al2O3, and AlN were investigated as a function of temperature through dilatometric study and as a function of time through isothermal heat treatments. The natures of reactions are described through the appearance of phases in conjunction with densification behavior and the change in lattice parameter of the spinel phase. Although the dilatometric study provides the detail insights of the formation sequence, the isothermal runs reveal new information about the differential rate of reactivity of the reacting species that suggested a tentative controlling mechanism. Through the initial formation of magnesium aluminate, oxygen-rich solid solution (MgAlON) forms, which ultimately reacts with the rest of AlN to reach its nominal composition. Nitrogen diffusion through MgAlON lattice seems to be rate controlling. [source] Single-Source Sol-Gel Synthesis of Nanocrystalline ZnAl2O4: Structural and Optical PropertiesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2001Sanjay Mathur Nanometer-sized zinc aluminate (ZnAl2O4) particles were synthesized from heterometal alkoxides, [ZnAl2(OR)8], possessing an ideal cation stoichiometry for the ZnAl2O4 spinel. ZnAl2O4 is formed at 400°C, which is the lowest temperature reported for the formation of monophasic ZnAl2O4. 27Al magic-angle spinning nuclear magnetic resonance spectroscopy revealed that ZnAl2O4 possesses an inverse structure at <900°C, while the normal spinel phase is observed at higher temperatures. The homogeneity of the in-depth composition and Zn:Al stoichiometry (1:2) was confirmed by electron spectroscopy for chemical analysis. Evaluation of the valence-band spectra of ZnAl2O4 and ZnS suggested that the hybridization of O 2p and Zn 3d orbitals is responsible for lowering the bandgap in the latter. The average crystallite size showed an exponential relationship to the calcination temperature (X-ray diffractometry and transmission electron microscopy data). The optical spectra of different spinel powders (average particle sizes, 20,250 nm) showed that the absorption edge exhibits a blue shift as particle size decreases. [source] Cathodoluminescent characteristics of Sm-doped ZnAl2O4 nanostructured powdersPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2005E. Martinez-Sanchez Abstract Cathodoluminescent (CL) powders of zinc aluminate activated with samarium ions have been prepared by a co-precipitation chemical process. Different doping concentrations in the start mixture and sintering temperatures for the precipitated powder were studied. It was observed that the crystalline characteristics of the powders depend upon the sintering temperature. For temperatures higher than 450 °C, the powders presented the cubic spinel phase with a lattice parameter value of 8.0756 Å, and grain sizes from 50 to 100 nm with semispherical shape. The CL emission spectra showed broad bands localized at 564, 602, 644, and 703 nm associated with 4G5/2 , 6H5/2, 4G5/2 , 6H7/2, 4G5/2 , 6H9/2, and 4G5/2 , 6H11/2 transitions in the Sm3+ ions, respectively. Concentration quenching of the cathodoluminescence occurred at about 4.26 at% of the activator (Sm) concentration inside the sintered powders. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Li-doped nanosized TiO2 powder with enhanced photocalatylic acivity under sunlight irradiationAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 10 2010Soraa Bouattour Abstract This work reports on the synthesis of Li-doped TiO2 nanoparticles using the sol,gel process and solid-state sintering, and investigates their potential use as a photocatalyst for degradation under sunlight excitation of different organic model compounds in aqueous solution. The structure of the nanocrystals was examined by X-ray diffraction, UV-vis ground state diffuse reflectance absorption spectra and X-ray photoelectron emission spectroscopy. Results showed that samples prepared by sol,gel process and calcined at 400 °C are composed of a mixture of anatase and rutile phases, in contrast to the one prepared by solid-state sintering, which exhibits an anatase phase with Li being involved in a spinel phase. The photocatalytic degradation of aqueous solutions of different aromatic compounds was successfully achieved under sunlight excitation in presence of Li-doped TiO2 prepared via sol,gel process. It was shown that the calcination temperature and the preparation mode greatly affect the photocatalytic efficiency. Copyright © 2010 John Wiley & Sons, Ltd. [source] A comparative in situ Rietveld refinement study: thermal decomposition and transformation of CoAl and CoZnAl layered double hydroxidesJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2008Rune E. Johnsen Rietveld refinement based on in situ X-ray powder diffraction (XRPD) data was combined with thermogravimetric analysis (TGA) and mass spectrometry (MS) to study and compare the phase transformations, thermal stability, microstructural and structural changes of two cobalt-containing nitrate-based layered double hydroxides (LDHs) upon heating in a controlled inert atmosphere of nitrogen. The XRPD data were collected, using synchrotron X-ray radiation, with a time resolution of 107,s, which made it possible to carry out detailed structural studies of the initial layered double hydroxides as well as their decomposition products: spinel for a CoAl,NO3 LDH and spinel/zincite for a CoZnAl,NO3 LDH. Correlating these data with those from the TGA,MS analyses gives us information about the transformation mechanisms. Rietveld refinements of the two spinel phases reveal remarkable differences. The a axis of the spinel formed by decomposition of the CoAl,NO3 LDH increases almost linearly from approximately 598 to 1163,K, mainly due to the dominating thermal expansion, whereas the a axis of the spinel formed by decomposition of the CoZnAl,NO3 shows a more complex temperature dependency. Between approximately 698 and 1073,K, the a axis is almost constant due to pronounced chemical interaction with an additional amorphous phase and the zincite phase, whereas from 1073 up to 1163,K it increases linearly. Calculations, based on the results of the Rietveld refinements, of the size of the octahedral and tetrahedral coordination polyhedra in the spinel show that the octahedra shrink and the tetrahedra expand with increasing temperature. The unusual thermal behaviour of the octahedra is discussed and attributed to the low formation temperature of the cobalt aluminium spinel phase. Finally, the intensity of a low-angle scattering (LAS) signal observed in the XRPD patterns was correlated with the decomposition of the LDH, and determination of the specific surface areas gave the temperature-dependent BET surface areas. [source] Structural Evolvement of Heating Treatment of Mg/Al-LDH and Preparation of Mineral Mesoporous MaterialsACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 2 2006CHEN Tianhu Abstract, Although hydrotalcite, or layered double hydroxides (LDHs), is not a common mineral, it is an important material that can be easily synthesized in laboratory. In this study, structural evolvement and BET surface area changes of heat treated Mg/Al-LDH is evaluated by XRD, TEM and N2 -BET analyses. The results indicate that the magnesium-aluminum LDH with carbonate as interlayer anion, periclase-like oxides was formed at temperatures of 400,800°C. Meanwhile, 2,3 nanometer mesoporous were formed during decomposition of LDH. However, the heat treated samples still preserve the morphology of the original LDH plates. Periclase-like formed from LDH heat treatment may re-hydrolyze and recover the structure of LDH. However, crystallinity of the recovered LDH is lower than that of the original LDH. This heat treatment will result in formation of (Mg, Al)-oxide nano-crystals and nanopores among the nano-crystals. When heating temperature exceeds 1000, the periclase-like (Mg, Al)-oxide is transformed into a composite with periclase (MgO) and spinel phases. The periclase can be re-hydrolyzed and dissolved in HCI solution. After acid treatment, the sample with a high surface area is composed of spinel nano-crystals and nanopores among them. Our results will provide a new and economic way to synthesize mesoporous materials through pathways of phase transformation of precursor materials with different composition. [source] | |||||||||||||