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Pleasanton, CA, United States

Ross N.L.,Virginia Polytechnic Institute and State University | Spencer E.C.,Virginia Polytechnic Institute and State University | Levchenko A.A.,Setaram Inc. | Kolesnikov A.I.,Oak Ridge National Laboratory | And 7 more authors.
Materials Research Society Symposium Proceedings

We present low-temperature inelastic neutron scattering spectra collected on two metal oxide nanoparticle systems, isostructural TiO 2 rutile and SnO 2 cassiterite, between 0-550 meV. Data were collected on samples with varying levels of water coverage, and in the case of SnO 2, particles of different sizes. This study provides a comprehensive understanding of the structure and dynamics of the water confined on the surface of these particles. The translational movement of water confined on the surface of these nanoparticles is suppressed relative to that in ice-lh and water molecules on the surface of rutile nanoparticles are more strongly restrained that molecules residing on the surface of cassiterite nanoparticles. The INS spectra also indicate that the hydrogen bond network within the hydration layers on rutile is more perturbed than for water on cassiterite. This result is indicative of stronger water-surface interactions between water on the rutile nanoparticles than for water confined on the surface of cassiterite nanoparticles. These differences are consistent with the recently reported differences in the surface energy of these two nanoparticle systems. © 2011 Materials Research Society. Source

Levchenko A.,Setaram Inc.
11AIChE - 2011 AIChE Annual Meeting, Conference Proceedings

The new hydrogen storage materials discovered in the last few years present a number of challenges to the research community from a viewpoint of materials characterization. These include both evaluation of the performance in real-world applications of hydrogen storage materials and understanding the underlying fundamental mechanisms controlling their properties. This presentation gives an overview of state-of-the-art characterization techniques for hydrogen storage materials. It covers simultaneous TPD-gas sorption/mass-spectrometry measurements, volumetric PCT isotherm measurements on thin-film and extremely small metal-hydride samples, as well as direct van't Hoff and in-situ calorimetric measurements of enthalpy of formation of hydrides. Challenges in performing measurements on particular types of materials or under demanding conditions are discussed. Source

Trofymluk O.,University of California at Davis | Levchenko A.A.,Setaram Inc. | Navrotsky A.,University of California at Davis
Microporous and Mesoporous Materials

The thermodynamics of mesoporous silicas (MCM-41, MCM-48, SBA-15, and SBA-16) were studied by solution calorimetry at 323 K in 25% aqueous HF. The enthalpies of formation were determined for calcined mesoporous silica (MS) and organic structure-directing agent (SDA) occluded samples (SDA: n-hexadeciltrimethylammonium bromide or CTAB, Pluronic P123, and Pluronic F127). The following are the measured interaction enthalpies between the MS and SDA: MCM-41/CTAB, -6.1 kJ/mol SiO2; MCM-48/CTAB, -12.3 kJ/mol SiO 2; SBA-15/P123, -19.7 kJ/mol SiO2; SBA-16/F127, -19.9 kJ/mol SiO2. Per unit surface area, these interactions are -0.08, -0.15, -0.43, and -0.40 J/m2, respectively. Though these SDA-framework interaction energies are still small in magnitude, they are somewhat more exothermic than those in silica zeolite formation, reflecting the greater metastability of the MS materials and the role of the long chain SDA in stabilizing and space-filling the large pores. The cubic MS (SBA) show stronger SDA interactions than the hexagonal (MCM). The interaction energies confirm a complex landscape of many competing structures of similar energy; with the role of SDA kinetic in selecting a specific structure rather than energetic in strongly stabilizing a given state, as has already been noted for zeolites. The enthalpies of the calcined MS relative to quartz determined by HF solution calorimetry in this study are in excellent agreement with those determined previously by high temperature oxide melt solution calorimetry. Source

Levchenko A.A.,Setaram Inc. | Kolesnikov A.I.,University of California at Davis | Trofymluk O.,Oak Ridge National Laboratory | Navrotsky A.,University of California at Davis

Bundles of (10,10) single-wall carbon nanotubes (SWCNTs) have been studied by high-temperature oxidation calorimetry and inelastic neutron scattering to obtain standard formation enthalpies and entropies at 298 K. SWCNTs are found to be only moderately less stable than graphite, and are significantly more stable than their fullerene counterparts. They are 7 kJ mol -1 metastable in terms of enthalpy relative to graphite, and just 5 kJ mol -1 less stable than diamond. Despite striking differences in vibrational dynamics of carbon atoms in SWCNTs and graphite, their thermodynamic properties at room and higher temperatures are dominated by the same set of high energy vibrations, reflected in very similar vibrational entropies. However, the energetics of SWCNTs are governed by the diameter-dependent enthalpic contributions, but not the specifics of phonon density of states. © 2010 Elsevier Ltd. All rights reserved. Source

Levchenko A.A.,University of California at Davis | Levchenko A.A.,Setaram Inc. | Jain P.,University of California at Davis | Trofymluk O.,University of California at Davis | And 3 more authors.
Journal of Physical Chemistry B

The dynamics of glass-forming liquids under nanoconfinement is key to understanding a variety of phenomena in nature and modern technology. We report a 13C NMR spectroscopic study that directly demonstrates that a-relaxation in bulk glycerol involves an isotropic rotational jump of the constituent molecules. The activation energy of this motion is ∼78 kJ mol-1 in the bulk, which abruptly changes to a low value of ∼27.5 kJ mol-1, characteristic of β-processes, upon confinement of glycerol into ∼2 nm pores in mesoporous silica. This observation implies that the molecular dynamics associated with structural relaxation near glass transition are inherently different in supercooled glycerol in the bulk and under extreme nanoconfinement. © 2010 American Chemical Society. Source

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