Search results for "amorphous"
showing 10 items of 790 documents
LOCAL STRUCTURE OF THE CuCu (EDTA) (H2O)2, 2H2O AMORPHOUS COMPLEX : A JOINT MAGNETIC, LAXS, AND MOLECULAR MECHANICS INVESTIGATION
1985
The short range order in the amorphous CuCu (EDTA) (H2O)2, 2H2O is studied by Large Angle X-ray Scattering (LAXS) and molecular mechanics. Three conformations of the complex molecule are shown to be energetically equivalent, two of which are simultaneously present in the amorphous material.
Heterogeneous Dinuclear Rhodium(II) Hydroformylation Catalysts—Performance Evaluation and Silsesquioxane-Based Chemical Modeling
2001
Supported, air stable, and reusable hydroformylation catalysts have been prepared by immobilizing dinuclear rhodium(II) complexes bearing ortho-metalated arylphosphane ligands on amorphous silica and mesoporous MCM-41 supports by phosphane tethers. The oligosilsesquioxane model complex of the catalytic site 1 has been prepared analogously and characterized by single-crystal X-ray diffraction analysis.
First-principles study of nitrogen doping in cubic and amorphous Ge2Sb2Te5
2011
We investigated the structural, electronic and vibrational properties of amorphous and cubic Ge(2)Sb(2)Te(5) doped with N at 4.2 at.% by means of large scale ab initio simulations. Nitrogen can be incorporated in molecular form in both the crystalline and amorphous phases at a moderate energy cost. In contrast, insertion of N in the atomic form is very energetically costly in the crystalline phase, though it is still possible in the amorphous phase. These results support the suggestion that N segregates at the grain boundaries during the crystallization of the amorphous phase, resulting in a reduction in size of the crystalline grains and an increased crystallization temperature.
Selective oxidation of ethane: Developing an orthorhombic phase in Mo–V–X (X=Nb, Sb, Te) mixed oxides
2009
Abstract Mo–V–X (X = Nb, Sb and/or Te) mixed oxides have been prepared by hydrothermal synthesis and heat-treated in N2 at 450 °C or 600 °C for 2 h. The calcination temperature and the presence or absence of Nb determines the nature of crystalline phases in the catalyst. Nb-containing catalysts heat-treated at 450 °C are mostly amorphous solids, while Nb-free catalysts heat-treated at 450 °C and samples treated at 600 °C clearly contain crystalline phases. TPR-H2 experiments show higher H2-consumption on catalysts with amorphous phases. Catalytic results in the oxidative dehydrogenation of ethane indicate that the selective production of the olefin is strongly related to the development of …
Preparation of multi-nanocrystalline transition metal oxide (TiO2–NiTiO3) mesoporous thin films
2005
Ordered mesoporous TiO2–NiTiO3 thin films, with nickel content x < 0.5, were prepared by dip-coating an acidic solution of hydrolysed transition metal chlorides in controlled humidity, followed by a very careful annealing treatment. These latter treatments were studied by in situ SAXS-WAXS experiments involving synchrotron radiation and TEM analysis. They revealed that the walls can be composed of amorphous, single crystalline or multi crystalline nanoparticles depending on x and the conditions applied during the thermal treatment.
Structural Evolution and Vanadium Distribution in the Preparation of V4+-ZrSiO4 Solid Solutions from Gels
2004
Vanadium-containing ZrSiO4-gel precursors with nominal compositions Vx-ZrSiO4 with x= 0.0, 0.002, 0.004, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.1, and 0.2 were prepared using a previously reported procedure and thermally treated over a range of temperature up to the formation of the V4+-ZrSiO4 solid solution. The structural evolution and the V4+ location and its homogenous distribution were followed using powder X-ray diffractometry and electron spin resonance spectroscopy (ESR). Our experiments showed that a tetragonal form of V4+-ZrO2 was the first crystalline phase obtained on heating the gels. On further heating, a phase transformation to the monoclinic form of V4+-ZrO2 took place. Final…
Low-temperature methyl group dynamics of hexamethylbenzene in crystalline and glassy matrices as studied by 2H NMR
1991
Abstract 2 H NMR spectra of hexamethylbenzene (HMB) in protonated crystalline and amorphous matrices at low temperatures are presented. All spectra reveal lineshape changes which can be attributed to methyl group tunnelling. Compared to neat HMB, a drastic increase of the tunnelling frequency is found for all systems. This indicates that the hindering potential originates predominantly from intermolecular forces. We studied the temperature dependence of these spectra and the spin-lattice relaxation in order to exclude a distribution of motional correlation times describing a thermally activated process. In addition, we find a distortion of the methyl tetrahedron.
Glass Transition and Food Technology: A Critical Appraisal
2002
ABSTRACT: Most low water content or frozen food products are partly or fully amorphous. This review will discuss the extent to which it is possible to understand and predict their behavior during processing and storage, on the basis of glass transition temperature values (Tg) and phenomena related to glass transition. Two main conclusions are provisionally proposed. Firstly, glass transition cannot be considered as an absolute threshold for molecular mobility. Transport of water and other small molecules takes place even in the glassy state at a significant rate, resulting in effective exchange of water in multi-domains foods or sensitivity to oxidation of encapsulated materials. Texture pr…
Non-exponential relaxation in disordered materials: Phenomenological correlations and spectrally selective experiments
1998
Abstract In most glass-forming materials external perturbations are relaxed in a non-exponential fashion. It is shown that the degree of non-exponentiality is phenomenologically correlated with the departure from simple thermally activated behavior as measured by the fragility index m. In model glass formers such as the Ge-As-Se ternary alloy, and to some degree for amorphous materials in general, the correlations with these properties are observed also for other characteristic features. These include the specific heat step and the aging kinetics in the glass transformation range. While phenomenological correlations have proven very useful for rationalizing the properties of many glass form…
Effect of Ti(IV) loading on CO oxidation activity of gold on TiO2 doped amorphous silica
2006
Abstract Amorphous SiO 2 , doped with various amount of TiO 2 , was used as support for gold nanoparticles. Gold was deposited through the controlled sol formation technique. The effect of the TiO 2 loading on the structural and surface properties was investigated by transmission electron microscopy, X-ray diffraction, Raman and X-ray photoelectron spectroscopy. The catalytic behavior was tested in the oxidation of CO. A significant enhancement in the activity was observed for the gold supported on mixed oxide with low TiO 2 loading. On the basis of characterization data, the positive effect is assigned to the electronic modification of the Ti(IV) of the newly formed Ti O Si bonds.