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Local Order (local + order)

Distribution by Scientific Domains

Polymers and Materials Science	50%

Selected Abstracts

Molecular Dynamics Simulations of Amorphous Si,C,N Ceramics: Composition Dependence of the Atomic Structure

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2003
Nicoletta Resta
We have performed classical molecular dynamics simulations of amorphous Si,C,N materials. The dependence of the local order and of the microstructure on the chemical composition was investigated. Our simulations show that for a stoichiometric nitrogen/silicon ratio equal to or higher than 4/3, the amorphous ceramic separates into different amorphous domains, namely C-rich, SiN-rich, and SiC-rich phases. Below this ratio, the material is composed of mixed structures, homogeneously spread within the material. For a very particular composition range, we found that carbon atoms crystallize into monoatomic graphitic layers surrounding the SiN-rich domains. [source]

Annealing effects on crystallized Al-doped a-Ge:H thin films

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2005
F. Fajardo
Abstract Annealing effects on crystallized aluminum doped hydrogenated amorphous germanium (a-Ge:H) thin films have been studied by Raman spectroscopy, as a function of Al concentration ([Al/Ge] ,10,6,10,2 range). All the as-deposited Al-doped a-Ge:H samples with [Al/Ge] < 1.3 at% possess a Raman spectra that includes the signal given by the amorphous phase and a shoulder at around 300 cm,1, that evolves with increasing impurity content, to a well defined peak, corresponding to the contribution of the TO vibration of crystallized Ge. The scattering intensity of samples having 1.4 , [Al/Ge] , ,1.8 at% is dominated by the c-Ge TO vibration. Aluminum concentrations above [Al/Ge] > ,1.8 at% do not indicate crystallization of a-Ge:H films to levels detectable by Raman spectroscopy. An abrupt crystallization was observed to occur after annealing at Ta , 500 °C for all the range of the Al-doped samples, including the intrinsic one. These results and the comparison with data of the local order and coordination of Ga atoms into the a-Ge:H network, suggest that Al-induced crystallization originates from fourfold coordinated aluminum atoms that act as crystallization seeds. These seeds have only small influence on the abrupt crystallization of Al doped a-Ge:H films due to annealing treatments. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Amorphous blends of poly(zwitterions) and zwitterionomers of the ammonioalkoxydicyanoethenolate type with some alkali metal salts

POLYMERS FOR ADVANCED TECHNOLOGIES, Issue 10 2001
Monique Galin
Abstract Poly(zwitterions) and zwitterionomers of the ammonioethoxydicyanoethenolate type (functional dipolar unit R3N+,(CH2)2,O,CO,C,,(CN)2, µ,=,25.9 D) show the very specific property of solvation of some alkali metal salts to yield amorphous blends. For homopolymers in the (meth)acrylic series, solvation is observed up to a ratio r,=,[salt]/[zwitterion] of 1 for LiClO4 and NaSCN and of 0.5 for NaCF3SO3: it results in a significant plasticization (increasing order LiClO4,<,NaSCN,<,NaCF3SO3) and in the development in some cases of a poorly defined (lamellar?) local order, as evidenced by the presence of a single broad peak in the small-angle x-ray scattering (SAXS) patterns (Bragg distances of about 15,20 Å). For the amorphous blend of a biphasic poly(tetramethyleneoxide) segmented zwitterionomer and NaCF3SO3 (r,=,0.5), selective solvation of the salt in the hard zwitterionic domains induces a transition from a lamellar structure (zwitterionic sublayer of about 9,Å thickness) to an hexagonal packing of ionic-zwitterionic cylinders (radius of about 15 Å). Ionic conductivity, measured in a narrow range of temperature just above the glass transition temperature, is characterized for most systems by an activation energy of about 1,1.8,eV; the drastic decrease of the conductivity by a factor of 103, when going from the homopolymer to the zwitterionomer blends, is typical of the inhibition of the ionic percolation process by the lack of connectivity of the ionic-zwitterionic domains. Copyright © 2001 John Wiley & Sons, Ltd. [source]

Simultaneous fitting of X-ray and neutron diffuse scattering data

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 1 2007
D. J. Goossens
Conventional crystallographic refinement uses the Bragg-peak intensities and gives the single-site average crystal structure. Information about short-range order and local order is contained in the diffuse scattering that is distributed throughout reciprocal space. Models of the short-range order in materials can now be automatically refined. The complementarity of X-ray and neutron diffraction data, and the value of simultaneously refining a structural model against both types of data, has long been known. This paper presents the first refinement of a short-range-order model against comprehensive X-ray and neutron diffuse scattering data simultaneously. The sample is the organic molecular crystal benzil, C14H10O2 (for neutron work H is replaced by D). The technique gives new insights into local order in crystalline materials, including the dynamic correlation structure indicative of the dynamics of molecules in the crystalline state, and successfully overcomes limitations of using only the X-ray data set. [source]