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Phase Mixture (phase + mixture)

Distribution by Scientific Domains

Chemistry	50%
Physics and Astronomy	33%

Selected Abstracts

Minerals as Model Compounds for Cu/ZnO Catalyst Precursors: Structural and Thermal Properties and IR Spectra of Mineral and Synthetic (Zincian) Malachite, Rosasite and Aurichalcite and a Catalyst Precursor Mixture

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 10 2009
Malte Behrens
Abstract The Cu/ZnO system is a model for Cu/ZnO/Al2O3 catalysts, which are employed industrially for the synthesis of methanol. These catalysts are usually prepared from mixed basic carbonate precursors. A complex phase mixture, with constituents structurally related to the minerals rosasite andaurichalcite, is present at the industrially applied composition (Cu/Zn , 70:30). Using minerals and phase-pure synthetic samples as references, a comprehensive characterisation of such a phase mixture, including the determination of the individual compositions of the different phases, has been attempted by complementary analytical laboratory techniques (XRD, TGA, IR). The results are critically discussed in light of the complexity of the system. A thermally very stable carbonate species , well-known for mixed synthetic systems , is also detected for the mineral reference samples. Significant amounts of amorphous phases are found to be present in the synthetic zincian malachite sample but not in synthetic aurichalcite or the catalyst precursor. A simplified explanation for the shift of the characteristic 20 reflection of the malachite structure as a function of Zn incorporation based on the varying average Jahn,Teller distortion of the MO6 octahedra is proposed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

Seismic singularities at upper-mantle phase transitions: a site percolation model

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2004
Felix J. Herrmann
SUMMARY Mineralogical phase transitions are usually invoked to account for the sharpness of globally observed upper-mantle seismic discontinuities. We propose a percolation-based model for the elastic properties of the phase mixture in the coexistence regions associated with these transitions. The major consequence of the model is that the elastic moduli (but not the density) display a singularity at the percolation threshold of the high-pressure phase. This model not only explains the sharp but continuous change in seismic velocities across the phase transition, but also predicts its abruptness and scale invariance, which are characterized by a non-integral scale exponent. Using the receiver-function approach and new, powerful signal-processing techniques, we quantitatively determine the singularity exponent from recordings of converted seismic waves at two Australian stations (CAN and WRAB). Using the estimated values, we construct velocity,depth profiles across the singularities and verify that the calculated converted waveforms match the observations under CAN. Finally, we point out a series of additional predictions that may provide new insights into the physics and fine structure of the upper-mantle transition zone. [source]

Defects and structure of µc-SiOx:H deposited by PECVD

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3-4 2010
Lihong Xiao
Abstract Electronic transport and paramagnetic defects detected by Electron Spin Resonance (ESR) in both intrinsic and -type silicon oxide prepared by PECVD were investigated. The structure and alloy composition of the material were varied all the way from microcrystalline silicon (µc-Si:H) to amorphous silicon oxide (a-SiOX:H). The transition-phase-mixture material is called "microcrystalline silicon oxide" (µc-SiOX:H). In undoped samples we find a strong reduction of the dark conductivity from 10 -3to 10 -12 S/cm and an increase of the spin density from1017 to 3×1019 cm -3 as the crystallinity decreases from 80% to 0%. The variation of the dark conductivity in phosphorous doped samples was even higher from 101 to 10 -12 S/cm. ESR spectra of intrinsic material consist of a single featureless line with g-values in the range of 2.0043,2.005 depending on the structure and alloying. The spectra of -type material exhibit a broader range of g-values of 1.998,2.0043 due to strong variations of the Fermi level over the entire crystallinity range. The results are discussed within the frame of current understanding of µc-SiOX:H as a phase mixture of µc-Si:H crystallites embedded in a-SiOX:H matrix (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Prediction of spinodal wavelength in continuously cooled metallic liquid

ANNALEN DER PHYSIK, Issue 1 2009
C. Borchers
Abstract The spinodal decomposition of a deeply undercooled metallic liquid Zr41.2 Ti13.8Cu12.5Ni10Be22.5 between 800 and 700 K is analysed in the framework of the theory of Cahn and Hilliard for continuous cooling, and the wavelength with maximum amplification is predicted, using as input parameters thermodynamic values gained in experiments. Electron microscopical studies show the microstructure of glass forming alloys Zr41.2Ti13.8Cu12.5Ni10Be22.5. The as-cast material exhibits a two phase mixture of amorphous regions with different compositions. Evidence for spinodal decomposition is given, and the computed maximum-amplitude wavelength corresponds well with the one found in the experiments. [source]

X-Cell: a novel indexing algorithm for routine tasks and difficult cases

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2003
Marcus A. Neumann
X-Cell is a novel indexing algorithm that makes explicit use of systematic absences to search for possible indexing solutions from cells with low numbers of calculated reflections to cells with high numbers of reflections. Space groups with the same pattern of systematic absences are grouped together in powder extinction classes, and for a given peak number range an independent search is carried out in each powder extinction class. The method has the advantage that the correct cell is likely to be found before the rapid increase of possible solutions slows down the search significantly. A successive dichotomy approach is used to establish a complete list of all possible indexing solutions. The dichotomy procedure is combined with a search for the zero-point shift of the diffraction pattern, and impurity peaks can be dealt with by allowing for a user-defined portion of unindexed reflections. To rank indexing solutions with varying numbers of unindexed reflections, a new figure of merit is introduced that takes into account the highest level of agreement typically obtained for completely incorrect unit cells. The indexing of long and flat unit cells is facilitated by the possibility to search for rows or zones in reciprocal space first and then to use the lattice parameters of the dominant row or zone in the unit-cell search. The main advantages of X-Cell are robustness and completeness, as demonstrated by a validation study on a variety of compounds. The dominant phase of phase mixtures can be indexed in the presence of up to 50% of impurity peaks if high-quality synchrotron data are available. [source]