•Enthalpies of mixing of UO2 – ThO2 and UO2 – ZrO2 systems have been measured.•Volume mismatch linearly depends on interaction parameter in related systems.•UO2 – ZrO2 is an exception to this ...correlation showing zero heat of mixing.
The enthalpies of formation of cubic urania – thoria (c-ThxU1−xO2+y) and urania – zirconia (c-ZrxU1−xO2, x<0.3) solid solutions at 25°C from end-member binary oxides (c-UO2, and c-ThO2 or m-ZrO2) have been measured by high temperature oxide melt solution calorimetry. The enthalpies of mixing for both systems are zero within experimental error. The interaction parameters for binary solid solutions MO2 – M′O2 (M, M′=U, Th, Ce, Zr, and Hf), fitted by regular and subregular thermodynamic models using both calorimetric and computational data, increase linearly with the corresponding volume mismatch. Cubic UO2 – ZrO2 appears to be an exception to this correlation and shows a zero heat of mixing despite large size mismatch, suggestive of some short-range ordering and/or incipient phase separation to mitigate the strain. The incorporation of ZrO2 into UO2 stabilizes the system and makes it a potential candidate for immobilization and disposal of nuclear waste.
Enthalpies of formation of Co x Zn1–x O solid solutions (both bulk and nanophase materials) at 298 K have been determined using high-temperature oxide melt solution calorimetry in molten sodium ...molybdate (3Na2O·4MoO3) solvent at 973 K. Both the rocksalt and wurtzite phases show an approximately linear dependence of enthalpy of solution on composition, implying a zero heat of mixing in each phase, consistent with negligible lattice parameter changes on substitution of Co2+ for Zn2+. The surface energy of wurtzite Zn0.88Co0.12O solid solution was determined to be 2.33 ± 0.30 J/m2 (anhydrous surface) and 1.65 ± 0.25 J/m2 (hydrous surface), which are very close to values for ZnO. The wurtzite CoO surface energy was estimated to be similar. Here, we argue that, because of the lower surface energies of wurtzite phases than of rocksalt phases, the phase field of the wurtzite solid solution expands to higher CoO content at the nanoscale, suggesting that the reported extended solubility of CoO in ZnO nanoparticles represents thermodynamic stabilization and free energy minimization at the nanoscale. Conversely, the rocksalt Co1–x Zn x O phase shows thermodynamic destabilization, lower zinc content, and easier oxidation (to Co3–x Zn x O4 spinel phase) at the nanoscale than in the bulk.
To provide a complete picture of the energy landscape of Al2O3 at the nanoscale, we directed this study toward understanding the energetics of amorphous alumina (a-Al2O3). 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.
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With the hype of “high entropy” alloys and more recently, “high entropy” ceramics and “high entropy” oxides (HEOs), there has been a great push to investigate and characterize systems ...with 5 or more components. This push has been extremely beneficial for the materials community as it has led to the development of many new systems with targeted applications. However, with our desire to find “new” and “exotic” materials, we have not spent enough time to step back and think deeply about the fundamental thermodynamic constraints that will guide design of future HEOs. Here, we present data-driven discussions with examples that have been collected from the fields of geology and materials science over the past 50 years to highlight critical thermodynamic parameters and principles that can be used for the design of HEOs. The goal of HEOs is to push the limit of the number of components in a single-phase solid solution to achieve unique and tunable properties. True single-phase HEOs are stabilized if the positive entropy of formation more than compensates an unfavorable enthalpy of formation above some critical temperature, making the overall ΔGf negative i.e. the HEO phase is “entropy stabilized”. Under ideal mixing, the number of components in a solid solution does not affect the solubility of an additional component. In real systems, the types of additional components, their structural transformations, and their associated non-ideal interactions influence the solubility limit. Non-ideal interactions can lead to short- or long-range ordering that decreases the overall configurational entropy. Due to the ionic-covalent nature of oxides, this ordering is the norm, not the exception. In the limited cases where mixing is ideal, charge coupled substitutions can work to influence overall configurational entropy contributions due to unique crystallographic sites. Long-range ordering can be minimized by mixing oxide components that have similar charge or are isostructural. Most excitingly, is the realization that surface energies will drastically affect the stability of oxide polymorphs and solubility limits. Thus, nano-materials are an interesting and novel approach that will vastly extend the HEO engineering space. As one can see, there are many avenues for the design and development of “HEOs” that thermodynamics will allow, even though they all may not be driven explicitly by entropy.
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•Comparison of lixiviants for leaching rare earth elements from synthetic phosphogypsum.•Leaching efficiency: H2SO4 > Biolixiviant > Gluconic Acid > H3PO4 for Ce, Nd, Sm, Eu, and ...Yb.•Thermodynamic simulations consistent with experimental results.
Leaching of six individual rare earth (yttrium, cerium, neodymium, samarium, europium, and ytterbium) doped synthetic phosphogypsum samples using a suite of lixiviants was conducted. The lixiviants chosen for this study were phosphoric acid, sulfuric acid, gluconic acid, and a “biolixiviant” consisting of spent medium containing organic acids from the growth of the bacterium Gluconobacter oxydans on glucose. The biolixiviant had a pH of 2.1 and the dominant organic acid was determined to be gluconic acid, present at a concentration of 220 mM. The leaching behaviors of the studied lixiviants were compared and rationalized by thermodynamic simulations. The results suggest that at equivalent molar concentrations of 220 mM the biolixiviant was more efficient at rare earth element (REE) extraction than gluconic acid and phosphoric acid but less efficient than sulfuric acid. Unlike the organic acids, at pH 2.1 the mineral acids failed to extract REE, likely due to different complexation and kinetic effects.
The Fukushima-Daiichi nuclear accident brought together compromised irradiated fuel and large amounts of seawater in a high radiation field. Based on newly acquired thermochemical data for a series ...of uranyl peroxide compounds containing charge-balancing alkali cations, here we show that nanoscale cage clusters containing as many as 60 uranyl ions, bonded through peroxide and hydroxide bridges, are likely to form in solution or as precipitates under such conditions. These species will enhance the corrosion of the damaged fuel and, being thermodynamically stable and kinetically persistent in the absence of peroxide, they can potentially transport uranium over long distances.
Knowledge of thermal expansion and high temperature phase transformations is essential for prediction and interpretation of materials behavior under the extreme conditions of high temperature and ...intense radiation encountered in nuclear reactors. Structure and thermal expansion of Lu2O3 and Yb2O3 were studied in oxygen and argon atmospheres up to their melting temperatures using synchrotron X-ray diffraction on laser heated levitated samples. Both oxides retained the cubic bixbyite C-type structure in oxygen and argon to melting. In contrast to fluorite-type structures, the increase in the unit cell parameter of Yb2O3 and Lu2O3 with temperature is linear within experimental error from room temperature to the melting point, with mean thermal expansion coefficients (8.5 ± 0.6) · 10−6 K−1 and (7.7 ± 0.6) · 10−6 K−1, respectively. There is no indication of a superionic (Bredig) transition in the C-type structure or of a previously suggested Yb2O3 phase transformation to hexagonal phase prior to melting.
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•Lu2O3 and Yb2O3 retain bixbyite-type structure in oxygen and argon to melting.•Thermal expansion is close to linear from room temperature to melting.•There is no indication of Bredig transition or transformation to hexagonal phase.
Previous measurements show that calcium manganese oxide nanoparticles are better water oxidation catalysts than binary manganese oxides (Mn ₃O ₄, Mn ₂O ₃, and MnO ₂). The probable reasons for such ...enhancement involve a combination of factors: The calcium manganese oxide materials have a layered structure with considerable thermodynamic stability and a high surface area, their low surface energy suggests relatively loose binding of H ₂O on the internal and external surfaces, and they possess mixed-valent manganese with internal oxidation enthalpy independent of the Mn ³⁺/Mn ⁴⁺ ratio and much smaller in magnitude than the Mn ₂O ₃-MnO ₂ couple. These factors enhance catalytic ability by providing easy access for solutes and water to active sites and facile electron transfer between manganese in different oxidation states.
Significance Uranium peroxides, metastudtite and studtite, can be formed on exposure of UO ₂ based nuclear fuels to water during geological disposal or as a result of reactor accidents. We report ...detailed structural and thermochemical analysis of the metastudtite decomposition process. The thermodynamic data confirm the irreversible transformation from studtite to metastudtite and show that metastudtite can be a major oxidized corrosion product at the surface of UO ₂ and contribute a significant pathway to dissolution. The prevalence of metastudtite may require additional tailoring of waste forms to minimize this dissolution pathway for uranium.
Metastudtite, (UO ₂)O ₂(H ₂O) ₂, is one of two known natural peroxide minerals, but little is established about its thermodynamic stability. In this work, its standard enthalpy of formation, −1,779.6 ± 1.9 kJ/mol, was obtained by high temperature oxide melt drop solution calorimetry. Decomposition of synthetic metastudtite was characterized by thermogravimetry and differential scanning calorimetry (DSC) with ex situ X-ray diffraction analysis. Four decomposition steps were observed in oxygen atmosphere: water loss around 220 °C associated with an endothermic heat effect accompanied by amorphization; another water loss from 400 °C to 530 °C; oxygen loss from amorphous UO ₃ to crystallize orthorhombic α-UO ₂.₉; and reduction to crystalline U ₃O ₈. This detailed characterization allowed calculation of formation enthalpy from heat effects on decomposition measured by DSC and by transposed temperature drop calorimetry, and both these values agree with that from drop solution calorimetry. The data explain the irreversible transformation from studtite to metastudtite, the conditions under which metastudtite may form, and its significant role in the oxidation, corrosion, and dissolution of nuclear fuel in contact with water.
Bone is a natural nanocomposite. Its mineral component is nanocrystalline calcium phosphate apatite, whose synthetic biomimetic analogs can be prepared by wet chemistry. The initially formed ...crystals, whether biological or synthetic, exhibit very peculiar physicochemical features. In particular, they are nanocrystalline, nonstoichiometric, and hydrated. The surface of the nanocrystals is covered by a non-apatitic hydrated layer containing mobile ions, which may explain their exceptional surface reactivity. For their precipitation in vivo or in vitro, for their evolution in solution, for the 3D organization of the nanocrystals, and for their consolidation to obtain bulk ceramic materials, water appears to be a central component that has not received much attention. In this mini-review, we explore these key roles of water on the basis of physicochemical and thermodynamic data obtained by complementary tools including FTIR, XRD, ion titrations, oxide melt solution calorimetry, and cryo-FEG-SEM. We also report new data obtained by DSC, aiming to explore the types of water molecules associated with the nanocrystals. These data support the existence of two main types of water molecules associated with the nanocrystals, with different characteristics and probably different roles and functions. These findings improve our understanding of the behavior of bioinspired apatite-based systems for biomedicine and also of biomineralization processes taking place in vivo, at present and in the geologic past. This paper is thus intended to give an overview of the specificities of apatite nanocrystals and their close relationship with water.
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