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Quantitative scanning calorimetry on microgram-sized samples opens a broad, new range of opportunities for studying the thermodynamic properties of quantity-limited materials, including those produced under extreme conditions or found as rare accessory minerals in nature. We calibrated the Mettler Toledo Flash DSC 2+ calorimeter to obtain quantitative heat capacities in the range 200–350 °C, using samples weighing between 2 and 11.5 μg. Our technique is applied to a new set of oxide materials to which it has never been used before, without the need for melting, glass transitions, or phase transformations. Heat capacity data were obtained for silica in the highmore » pressure stishovite (rutile) structure, dense post-stishovite glass, standard fused quartz, and for TiO2 rutile. These heat capacities agree within 5%–15% with the literature values reported for rutile, stishovite, and fused SiO2 glass. The heat capacity of post-stishovite glass, made by heating stishovite to 1000 °C, is a newly reported value. After accurate calibrations, measured heat capacities were then used to calculate masses for samples in the microgram range, a substantial improvement over measurement in conventional microbalances, which have uncertainties approaching 50%–100% for such small samples. Since the typical uncertainty of heat capacities measured on 10–100 mg samples in conventional differential scanning calorimetry is typically 7% (1%–5% with careful work), flash differential scanning calorimetry, using samples a factor of 1000 smaller, increases the uncertainty of heat capacity measurements by a factor of <3, opening the door for meaningful measurements on ultra-small, high-pressure samples and other quantity-limited materials.« less

A group of multi-component oxides based on BaZrO₃ have been prepared using a solid-state reaction method and examined in terms of their water uptake and thermodynamics of formation. Depending on the type and amount of acceptor substitution, the synthesized compounds exhibit various proton defect concentrations, reaching up to 0.2 mol/mol for a compound containing 10 different elements in the B-sublattice, where 50% of them are acceptors. Further, for the most promising materials, van’t Hoff plots were created and the enthalpies and entropies of hydration were calculated. At higher temperatures, these parameters do not differ from the values for the referencemore » yttrium doped barium zirconate. However, at lower temperatures they are more negative, indicating a more exothermic process of proton incorporation.« less

Surface energetics of zinc sulfide nanoparticles determines their structure, properties, and occurrence. Using a combination of experimental techniques, we investigated the thermodynamics of the two polymorphs, sphalerite and wurtzite at bulk and nanoscale to understand their occurrence. Calorimetric measurements confirmed that wurtzite has a lower surface energy than sphalerite, which causes a reversal in phase stability at the nanoscale, with wurtzite energetically stable for particle size below 10 nm. Taking these surface energies into account, a simple model of the thermodynamics of the sphalerite - wurtzite transformation as a function of particle size and temperature can explain the occurrence ofmore » the zinc sulfide polymorphs in environments as diverse as ore bodies and planetary atmospheres.« less

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 andmore » 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.« less

An alternative option for treating anion-enriched reprocessed nuclear waste streams is to immobilize technetium-99 (⁹⁹Tc, β = 293.7 keV, t_1/2 = 2.1 × 10⁵ years) and other anions in micro- and mesoporous materials. Here we determine the thermodynamic stability of anion bearing sodalites, Na₈Al₆Si₆O₂₄X₂ (X = SO₄, ReO₄, Cl, I), to improve our understanding of the driving forces that control framework assembly using high temperature oxide melt solution calorimetry. Raman and FTIR spectroscopy illustrate a strong dependence for vibrational features on anion size and enabled the development of a linear model that predicted the vibrational features for numerous anion bearingmore » sodalites to within ±20 cm^–1 (i.e., OH, F, Br, ClO₄, NO₃, and MnO₄). The largest negative enthalpy of formation from elements and the lack of structural water demonstrate that the perrhenate sodalite (Na₈Al₆Si₆O₂₄[ReO₄]₂), a chemical analogue for pertechnetate sodalite (Na₈Al₆Si₆O₂₄[TcO₄]₂), is more thermodynamically stable than all other anion bearing sodalites evaluated. The enthalpies of the reaction between nepheline and the sodium salt, which provides the guest anion species, was negative only for the ReO₄ and NO₃ bearing sodalites. Overall, we report for the first time the enthalpy of the ion exchange reactions for different anion bearing sodalites relative to the perrhenate sodalite, which is a key step in gaining the ability to tune sodalite material properties and structure during treatment and the immobilization of ⁹⁹Tc in the presence of competing anions.« less

The defect fluorite yttrium niobate Y₃NbO₇ and pyrochlore yttrium titanate Y₂Ti₂O₇ solid solutions have been synthesized via a solid state synthesis route. The resulting stoichiometry of the oxides is Y_2+xTi_2-2xNb_xO₇, where x = 0 to x = 1. All of the samples were single-phase; however, for those with a predominant fluorite phase, a small amount of additional pyrochlore phase was detected. The volume of the solid solution unit cells linearly increases with increase in yttrium niobate content. The water uptake increases with (x) and the protonic defect concentration reaches almost 4.5 × 10^-3 mol mol^-1 at 300 °C. The calculatedmore » enthalpy of formation from oxides suggests strong stability for all of the compositions, with the values of enthalpy ranging from -84.6 to -114.3 kJ mol^-1. The total conductivity does not have a visible dependence on Y₃NbO₇ content. For each compound, the total conductivity is higher in wet air. Interestingly, for samples where x < 0.5, the ratio of conductivity in hydrogen to air increases with increasing temperature, while for x > 0.5, the trend is the opposite.« less