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Davis, CA, United States

Hayun S.,Peter A Rock Thermochemistry Laboratory | Tran T.,Peter A Rock Thermochemistry Laboratory | Ushakov S.V.,Peter A Rock Thermochemistry Laboratory | Thron A.M.,University of California at Davis | And 3 more authors.
Journal of Physical Chemistry C | Year: 2011

Measuring the surface energy of highly hygroscopic materials has remained a thorny problem for many years, mainly because obtaining an anhydrous surface state and maintaining this condition during the surface energy assessment has been considered an impractical task. In this work, we developed synthetic and calorimetric approaches that overcome these difficulties and applied them to measure the surface energy of anhydrous nanocrystalline magnesium oxide. Anhydrous MgO with specific surface area of ∼300 m2 g -1 was synthesized by laser ablation in a controlled oxygen partial pressure environment. High resolution transmission electron microscopy and X-ray diffraction showed cubic nanoparticles with sizes ranging from 5 to 10 nm (as controlled by the partial pressure) and with the periclase crystal structure. The surface energy of the anhydrous state was assessed using high temperature oxide melt drop solution calorimetry and differential scanning calorimetry; the surface energies were 1.2 ± 0.1 and 1.3 ± 0.1 J m-2, respectively. These values are slightly higher than from previously reported experiments and are consistent with a less hydrated surface. © 2011 American Chemical Society. Source

Tavakoli A.H.,Peter A Rock Thermochemistry Laboratory | Maram P.S.,Peter A Rock Thermochemistry Laboratory | Widgeon S.J.,Peter A Rock Thermochemistry Laboratory | Widgeon S.J.,University of California at Davis | And 7 more authors.
Journal of Physical Chemistry C | Year: 2013

To provide a complete picture of the energy landscape of Al 2O3 at the nanoscale, we directed this study toward understanding the energetics of amorphous alumina (a-Al2O 3). a-Al2O3 nanoparticles were obtained by condensation from gas phase generated through laser evaporation of α-Al2O3 targets in pure oxygen at25 Pa. As-deposited nanopowders were heat-treated at different temperatures up to 600 C to provide powders with surface areas of 670-340 m2/g. The structure of the samples was characterized by powder X-ray diffraction, transmission electron microscopy, and solid-state nuclear magnetic resonance spectroscopy. The results indicate that the microstructure consists of aggregated 3-5 nm nanoparticles that remain amorphous to temperatures as high as 600 C. The structure consists of a network of AlO4, AlO5, and AlO6 polyhedra, with AlO5 being the most abundant species. The presence of water molecules on the surfaces was confirmed by mass spectrometry of the gases evolved on heating the samples under vacuum. A combination of BET surface-area measurements, water adsorption calorimetry, and high-temperature oxide melt solution calorimetry was employed for thermodynamic analysis. By linear fit of the measured excess enthalpy of the nanoparticles as a function of surface area, the surface energy of a-Al2O3 was determined to be 0.97 ± 0.04 J/m2. We conclude that the lower surface energy of a-Al2O3 compared with crystalline polymorphs γ- and α-Al2O3 makes this phase the most energetically stable phase at surface areas greater than 370 m2/g. © 2013 American Chemical Society. Source

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