1,720,992 research outputs found
Thermal expansion of C3S and Mg-doped alite
No full textThe lattice thermal expansion coefficients were measured using in situ high temperature x-ray powder diffraction on triclinic tricalcium silicate (C3S) in the temperature range 25-970 degrees C and on monoclinic alite of composition Ca(2.879)Mg(0.112)Al(0.0439)Si(0.971)O5 in the temperature range 30-1150 degrees C. Full profile refinements were performed using the Rietveld method. The structural model of Mumme [1] was adopted for the alite crystal structure, implying a space group Cm and cell constants a=12.18, b=7.04, c=9.25 Angstrom, beta=116.11 degrees at 25 degrees C. All three lattice parameters show similar temperature behaviour and relative thermal expansion coefficients. There is a clear change in slope at about 900 degrees C, which is assumed to be related to the phase change between two monoclinic forms of alite. Refinements of C3S were conducted using the structural model of Golovastikov et al. [2] implying a space group and cell constants a=11.67, b=14.28, c=13.72 Angstrom, alpha=105.3, beta=94.2, gamma=90.0 degrees at 25 degrees C. The C3S lattice parameter b shows a different trend in temperature with respect to a and c. The same behaviour is reproduced by the relative thermal expansion coefficient alpha. Three clear changes in slope are found and regarded as phase transition points between triclinic modifications of C3S. Integrated intensity analysis for selected peaks, together with TG/DTA in the temperature range 25-1150 degrees C, were also used in order to preliminarly characterize the phase transitions
High temperature in situ Rietveld study of Fe,Mg partitioning in olivine
No full textA natural olivine sample from a mantle peridotite .xenolith has been studied by in situ
high-temperature powder diffractìon, The structure has been successfully refined from powder
data at three temperatures (25,600, and 800 °C) using the Rietveld method. The study shows
that the full-profìle technique is well suited for the structure artalysis of high-temperature
powder diffraction data. The results indicate that, in this temperature range, tbere is no
signifìcant ordering of the Fe,Mg cations in the two crystallographically independent octahedrai
sites. This has implications for the thermodynamic modeling of olivine at upper mantle
condìtions, The present experiments allowed measurement of the lattice thermal expansion of
olivine in the temperature range 25-800 •C, and assessment of the temperature dependence of
the isotropic atomic displacement parameters
Phase-composition and Mechanism of Formation of Ba-beta-alumina-type Systems For Catalytic Combustion Prepared By Precipitation
No full textA novel preparation method is proposed for Ba-beta-Al2O3-type systems to be used in high-temperature catalytic combustion, consisting of precipitation in aqueous medium. Differential thermal analysis-thermogravimetry, X-ray diffraction, Fourier transform-infrared, TEM and surface area data are presented for Ba-Al-0 samples with Al/Ba atomic ratios in the range 14-9 calcined at different temperatures up to 1670 K. The final materials consist of a barium-poor Ba-beta(I)-Al2O3 phase together with 1 % alpha-Al2O3 in the case of Al/Ba = 1 4 and of a barium-rich Ba-beta(II)-Al2O3 Phase together with 2% BaAl2O4 in the case of Al/Ba = 9. A monophasic sample with a Ba-beta-Al2O3 structure is obtained in the case of Al/Ba = 12; this phase is constituted by the simultaneous presence of both beta(I) and beta(II) structure types. Two routes operate above 1370 K in the formation of the Ba-beta-Al2O3 phases involving solid-state reactions between gamma-Al2O3 and BaAl2O4, and gamma-Al2O3 and dispersed barium compounds. Based on the analogies between the structures of gamma-Al2O3 and of the Ba-beta-Al2O3 phases it is suggested that the formation of the latter occurs via diffusion of barium ions within oxygen close-packed planes of the gamma-Al2O3-type spinel structur
The Crystal-structure of Ba-beta-alumina Materials For High-temperature Catalytic Combustion
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
On the Morphological Properties of Tungsta-titania De-noxing Catalysts
No full textTungsta-titania samples with different W loadings up to 15 wt. % and calcined at different temperatures have been prepared and characterized by surface area measurements, mercury porosimetry, x-ray diffraction, microstructural analysis, and laser Raman spectroscopy. It has been found that W inhibits the initial sintering of TiO2 (anatase) and the anatase --> rutile transformation. The morphological and structural properties of the samples (surface area, porosity, and phase composition) have been related to the microscopic properties of the materials such as crystallite dimensions and defects concentration. A model for the sintering of TiO2 is discussed. This model is based on the diffusion of surface hydroxyls, formed upon adsorption of water on surface oxygen vacancies. A role for W is proposed in terms of stabilization of both material defects and surface hydroxyls
Synthesis of Alcohols From Carbon Oxides and Hydrogen .18. Preparation Chemistry, Phase-transformations and Catalytic Behavior of Unpromoted Mn-Cr-O Systems In the Synthesis of Alcohols From Carbon-monoxide and Hydrogen
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