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    ABSTRACT: The diffusion of n-alkanes ranging from length n-C8 to n-C20 in the zeolite silicalite is studied using classical molecular dynamics simulations. Different simulations were performed using a united-atom hydrocarbon model with a rigid zeolite framework, an all-atom hydrocarbon model with a rigid zeolite framework, and an all-atom hydrocarbon model with a flexible zeolite framework, all at 300 K. The latter two models have never previously been used to simulate longer alkanes in silicalite. Diffusion coefficients measured using a rigid zeolite framework exhibited a periodic dependence on chain length in the [010] direction in line with the previously observed phenomenon of resonant diffusion, regardless of the hydrocarbon model used. Explanations are considered in terms of the location of low energy traps within the silicalite structure, presenting a diffusion barrier. A monotonic dependence on diffusivity with chain length was observed however, on using an all-atom hydrocarbon model and a flexible framework, which was attributed to the occurrence of pore 'breathing' assisting diffusion. It was also noted that the calculated diffusion coefficients were up to an order of magnitude lower, and experimental diffusion coefficients are in much closer agreement when the latter model is used.
    Physical Chemistry Chemical Physics 10/2013; 15(43). DOI:10.1039/c3cp52653d
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    ABSTRACT: We investigated the high-P,T annealing and mechanical properties of nanocomposite materials with a highly nitrided bulk composition close to Ti 3 N 4 . Amorphous solids were precipitated from solution by ammonolysis of metal dialkylamide precursors followed by heating at 400–700 °C in flowing NH 3 to produce reddish-brown amorphous/nanocrystalline materials. The precursors were then densified at 2 GPa and 200–700 °C to form monolithic ceramics. There was no evidence for N 2 loss during the high-P,T treatment. Micro-and nanoindentation experiments indicate hardness values between 4–20 GPa for loads ranging between 0.005–3 N. Young's modulus values were measured to lie in the range 200–650 GPa. Palmqvist cracks determined from microindentation experiments indicate fracture toughness values between 2–4 MPa·m 1/2 similar to Si 3 N 4 , SiC and Al 2 O 3 . Significant variations in the hardness may be associated OPEN ACCESS Materials 2011, 4 1748 with the distribution of amorphous/crystalline regions and the very fine grained nature (~3 nm grain sizes) of the crystalline component in these materials.
    Materials 12/2011; 4(10):1747-17623390. DOI:10.3390/ma4101747
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    ABSTRACT: We studied the low-frequency Raman and X-ray scattering behavior of amorphous silicon (a-Si) at high pressure throughout the range where the density-driven polyamorphic transformation between the low-density amorphous (LDA) semiconductor and a novel metallic high-density amorphous (HDA) polyamorph occurs. The experimental data were analyzed with the aid of molecular dynamics (MD) simulations using the Stillinger-Weber potential. The heat capacity of a-Si obtained from the low pressure Raman data exhibits non Debye-like behavior, but the effect is small, and our data support the conclusion that no boson peak is present. The high-pressure Raman data show the presence of a distinct low frequency band for the HDA polyamorph in agreement with ab initio MD simulations. Spatially resolved synchrotron X-ray diffraction was used to study the high pressure behavior of the a-Si sample throughout the LDA-HDA transition range without interference by crystallization events. The X-ray data were analyzed using an iterative refinement strategy to extract real-space structural information. The appearance of the first diffraction peak (FDP) in the scattering function S(Q) is discussed in terms of the void structure determined from Voronoi analysis of the MD simulation data.
    The Journal of Physical Chemistry B 08/2011; 115(48):14246-55. DOI:10.1021/jp205090s
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    ABSTRACT: Indium sesquioxide is widely used as a transparent conducting oxide in modern optoelectronic devices; the rising cost of indium has generated interest in the nanoscale properties of In(2)O(3), and questions arise as to the nature of its physicochemical properties below the bulk regime. We report the stable and metastable stoichiometric clusters of (In(2)O(3))(n), where n = 1-10, as predicted from an evolutionary search within the classical interatomic potential and quantum density functional energy landscapes. In contrast to the paradigm set by ZnO, which favours high symmetry bubble-like structures, the In(2)O(3) nanoclusters are found to tend towards dense, low symmetry structures approaching the bulk system at remarkably small molecular masses. Electronic characterisation is performed at the hybrid density functional and many-body GW levels to obtain accurate predictions of the spectroscopic properties, with mean values of the ionisation potentials and electron affinities calculated as 7.7 and 1.7 eV, respectively.
    Physical Chemistry Chemical Physics 08/2010; 12(30):8446-53. DOI:10.1039/c0cp00056f
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    ChemPhysChem 08/2010; 11(11):2341-4. DOI:10.1002/cphc.201000306
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    ABSTRACT: We review recent developments and applications of computational modelling techniques in the field of materials for energy technologies including hydrogen production and storage, energy storage and conversion, and light absorption and emission. In addition, we present new work on an Sn2TiO4 photocatalyst containing an Sn(II) lone pair, new interatomic potential models for SrTiO3 and GaN, an exploration of defects in the kesterite/stannite-structured solar cell absorber Cu2ZnSnS4, and report details of the incorporation of hydrogen into Ag2O and Cu2O. Special attention is paid to the modelling of nanostructured systems, including ceria (CeO2, mixed Ce(x)O(y) and Ce2O3) and group 13 sesquioxides. We consider applications based on both interatomic potential and electronic structure methodologies; and we illustrate the increasingly quantitative and predictive nature of modelling in this field.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 07/2010; 368(1923):3379-456. DOI:10.1098/rsta.2010.0111
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    ABSTRACT: We examine oxygen incorporation in alpha-Al2O3 using electronic structure techniques. We demonstrate that the ground-state configuration is a peroxide split interstitial, which is more than 2eV lower in energy than the oxide closed-shell interstitial species in alumina, which proves to be only a transition state. Our results have general implications for the nature of oxygen interstitials in close-packed oxides.
    Chemical Physics Letters 05/2010; 492(1):44-48. DOI:10.1016/j.cplett.2010.04.029
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    ABSTRACT: We have investigated the origin of antiferromagnetism of CoO in the rocksalt structure using spin-polarized density functional theory calculations. We find that in the rocksalt structure, the superexchange interaction between the occupied and unoccupied e(g) states plays the dominant role, which leads to an antiferromagnetic ground state, but the system also has a strong direct exchange interaction between the partially occupied minority spin t(2g) states that leads to the unusual situation that the ferromagnetic phase is more stable than most antiferromagnetic configurations. (C) 2010 American Institute of Physics. [doi:10.1063/1.3402772]
    Applied Physics Letters 04/2010; 96(16). DOI:10.1063/1.3402772
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    ABSTRACT: We examined the solid-state water-soluble amorphous precursors that are formed by partial thermal decomposition of Al(NO3)3·9H2O (aluminum nitrate nonahydrate: ANN) using Raman and FTIR and solid-state magic-angle spinning NMR spectroscopy. We also studied the species formed in the aqueous alumosols formed by dissolution of the pre-ceramic precursors using 27Al NMR spectroscopy. Species identified in the alumosols included the Al3+(H2O)6 monomer, the [AlO4Al12(OH)24(H2O)12]7+(Al13) Keggin ion, and the Al30 polycation, [Al30O8(OH)56(H2O)24]18+, as well as various other oligomers or nanoparticles containing IV-, V- and VI-coordinated Al3+ ions.
    Journal of Non-Crystalline Solids 08/2009; DOI:10.1016/j.jnoncrysol.2008.11.044
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    ABSTRACT: Since the pioneering work of Bridgman it has been known that pressure affects the glass transition of polymers and liquid state viscosities. Usually the Tg and viscosity both increase as a function of pressure as expected from ‘free volume’ theories. However, H2O provided a notable exception in that the viscosity passes through a minimum at low temperature. It was thought that this might be linked to the anomalous thermal expansion behavior. However further research on geologically important aluminosilicate liquids revealed that they could show anomalous viscosity decreases with increasing pressure and this behavior is given a structural interpretation as five-fold coordinated Si4+ and Al3+ species are formed. Also the existence of polyamorphism or density-driven liquid–liquid phase transitions in certain systems can lead to anomalies in the Tg or ηvs. P relations. This may be the case for H2O, for example. Current research is focusing on investigating structural changes in liquids and glasses at high pressure as the rich variety of behavior is becoming recognized. Both experimental studies and computer simulations are important as the underlying phenomonology is linked to changes in the glass or liquid structure as a function of densification.
    Journal of Non-Crystalline Solids 05/2009; DOI:10.1016/j.jnoncrysol.2009.01.036
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