Binary iron sulfides are of major interest and importance in environmental, geological, and planetary science. Oxidative high-temperature oxide melt drop-solution calorimetry in molten sodium ...molybdate (...*...) solvent at 975 K was applied to determine the energetics of formation of the binary iron monosulfide solid solution (pyrrhotite) ..., 0 ≤ x ≤ 1). The enthalpies of formation from elements are consistent with earlier data in the literature, available for a few compositions. Within the experimental errors, the enthalpies of formation of the solid solution from the end-members ... and FeS (..., kJ/mol) at 25 ...C equal to zero. Under the assumption of random distribution of Fe vacancies, the Gibbs free energies of mixing of ... (0 ≤ x ≤ 1) are estimated. Our data support the two-sublattice model proposed by Waldner and Pelton (2005). (ProQuest: ... denotes formulae/symbols omitted.)
The thermodynamic stability of nanocrystalline SnO2–TiO2 solid solutions was studied experimentally. Microcalorimetry of water adsorption revealed a systematic decrease in the surface energy with ...increasing Ti4+ content in the SnO2‐rich compositions, consistent with previous reports of Ti4+ segregation on the surface. The surface energy change was accompanied by an increase in the magnitude of the heat of water adsorption, also indicating a modification of the SnO2 surface by Ti4+. Supporting the water adsorption data, calculations using high‐temperature oxide melt solution calorimetry data also suggest a decrease in the interface energies. A thermodynamic analysis showed that the observed surface energy decrease is responsible for an increase in the stability of solid solutions in the nanophase regime. Although a miscibility gap is expected in this system from bulk phase diagrams, the surface energy contribution modifies the bulk trend and promotes extensive solid solutions when the surface area is above a critical value dependent on the surface energy and the bulk enthalpy of mixing.
Nanocrystalline SnxTi1-xO2 rutile solid solutions are important materials for gas sensors and catalysts. Although thermodynamic data are available for nanocrystalline SnO2 and TiO2 and for bulk ...(coarse-grained) SnxTi1-xO2 solid solutions, there is a lack of experimental thermochemical data on the energetics of SnxTi1-xO2 nanoparticles. Sn0.586Ti0.414O2 rutile solid solution nanoparticles were synthesised. The surface energies of anhydrous and hydrated nanoparticles were measured by combining high-temperature oxide melt solution calorimetry molten 2PbO.B2O3 at 800 C and water adsorption calorimetry. The surface energy of the anhydrous surface was 2.02 plus or minus 0.03 J/m2, and that of the hydrated surface was 1.68 plus or minus 0.03 J/m2, which are between the previously reported surface energies of rutile SnO2 and TiO2. The integral heat of water adsorption was -80 kJ/mol, with a chemisorbed maximum coverage of about 6 H2O/nm2, which are also between those for TiO2 and SnO2 reported previously. The strongly positive (unfavourable) energetics of mixing in SnO2-TiO2 bulk solid solutions are predicted to change little at the nanoscale, and the extensive solid solution seen in the nanophase system prepared near room temperature reflects kinetic hindrance to exsolution of an initially homogeneous precipitate rather than thermodynamic stability.
Abstract
Understanding composition‐structure‐property relationships of high‐alumina nuclear waste glasses are important for vitrification of nuclear waste at the Hanford Site. Two series of glasses ...were designed, one with varying Al:Si ratios and the other with (Al + Na):Si ratios based on the international simple glass (ISG, a simplified nuclear waste model glass), with Al
2
O
3
ranging from 0 to 23 mol% (0 to 32 wt%). The glasses were synthesized and characterized using electron probe microanalysis, X‐ray photoelectron spectroscopy, small angle X‐ray scattering, high‐temperature oxide melt solution calorimetry, and infrared spectroscopy. Glasses were crystal free, and the lowest Na
2
O and Al
2
O
3
glass formed an immiscible glass phase. Evolution of various properties—glass‐transition temperature, percentage of 4‐coordinated B, enthalpy of glass formation—and infrared spectroscopy results indicate that structural effects differ based on the glass series.
•Cr3C2 is a useful transient liquid phase sintering agent for carbide-based ceramics.•Low atomic mass cations (e.g., Cr or Ti) can rapidly diffuse through oxide scales.•Minor second phase additions ...can promote limited liquid formation for pore filling.•Oxycarbide interlayer may comprise nanocrystalline graphite in amorphous oxide.
Hafnium carbide-based solid solution ceramics with yttria additions were consolidated via hot pressing utilizing chromium carbide as a transient liquid sintering agent. Selected samples were oxidized at temperatures exceeding 2200°C under high velocity airflow to determine their potential for service in ultra-high temperature environments. Cross-sectional examination revealed a complex multi-layer scale comprising a highly porous exterior region, a dense multi-phase interior region, and an oxygen- and carbon-containing interlayer consisting of nanocrystalline graphite in an amorphous or nanocrystalline oxide matrix. Outward diffusion of mobile cations is speculated to play an important role in establishing the observed scale morphology.
Akaganeite, β-FeOOH, is a commonly occurring ferric mineral in the environment and is a sorbent, ion exchanger, and catalyst. It is often fine-grained (nanophase) and frequently contains excess ...water. Its enthalpy of formation was studied by solution calorimetry in aqueous HCl. The enthalpy of water adsorption was studied by a new calorimetric technique combining a Calvet microcalorimeter and an automated gas dosing system, used for surface adsorption measurements. Akaganeite samples with surface areas of 30−280 m2/g were used. Sample characterization was performed by X-ray powder diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, Brunauer−Emmett−Teller method, scanning electron microscopy, and transmission electron microscopy. Surface enthalpy and enthalpy of water adsorption are reported for the first time. By adsorbing water, akaganeite decreases its effective surface enthalpy from 0.44 J/m2 to 0.34 J/m2. The enthalpy of formation of akaganeite can vary by 10−12 kJ/mol as a function of the surface area. The standard enthalpy of formation of akaganeite with zero surface area was refined and is −554.7 ± 1.9 kJ/mol. Thus, the standard enthalpy of formation and surface enthalpy of akaganeite are between those of goethite and lepidocrocite. The more metastable the polymorph, the lower its surface energy.
Low-temperature heat capacities of a series of low dielectric constant amorphous films with different compositions were measured from 1.8 to 300 K using a Quantum Design Physical Property Measurement ...System (PPMS). By using piece wise functions to fit the heat capacities, the characteristic Debye temperatures ΘD and the standard molar entropies are determined. The standard molar entropies of these materials range from 8.8 J·K−1·mol−1 to 17.5 J·K−1·mol−1. Together with the formation enthalpies obtained by high temperature oxidative solution calorimetry in molten sodium molybdate solvent, the corresponding Gibbs free energies from elements and crystalline constituents (and gaseous products as required) are obtained. The Gibbs free energy terms of these materials are dominated by the enthalpy term rather than the entropy. These samples are thermodynamically stable at room temperature with respect to elements and the samples with oxygen incorporated are generally thermodynamically more stable than the others. However, compared to crystalline binary counterparts and gases, some of these materials possess either positive or close-to-zero Gibbs free energies of formation, indicating that they are thermodynamically metastable; while, for the rest, which are stable at ambient conditions, elevation of temperature will eventually lead to decomposition.
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