Sergey V. Ushakov

Thermochemistry of rare-earth orthophosphates

The enthalpies of formation for the compounds (RE 3+ )PO 4 , (where RE = Sc, Y, La–Nd, Sm–Lu) were determined by oxide-melt solution calorimetry. Calorimetric measurements were performed in a Calvet-type twin microcalorimeter in sodium molybdate (3Na 2 O · 4MoO 3 ) and lead borate (2PbO · 2B 2 O 3 ) solvents at 975 K. The experiments were carried out using both powdered single crystals grown by a flux technique and powders synthesized by precipitation. Formation enthalpies were derived from the drop-solution enthalpies for (RE)PO 4 , RE oxides, and P 2 O 5 . Enthalpies of formation for the (RE)PO 4 compounds with respect to the oxides at 298 K become more negative with increasing RE 3+ ionic radius; i.e., in going from ScPO 4 (−209.8 ± 1.0 kJ/mol), to LuPO 4 (−263.9 ± 1.9 kJ/mol), to LaPO 4 (−321.4 ± 1.6 kJ/mol). From structural considerations, a similar trend is expected for the isostructural RE vanadates and arsenates, as well as for the tetravalent actinide orthosilicates.

Direct measurements of water adsorption enthalpy on hafnia and zirconia

A commercial surface area analyzer and Calvet-type microcalorimeter were combined for measurements of heats of gas–solid interactions, providing enhanced resolution, flexibility, and throughput compared to conventional methods. Integral adsorption enthalpies for half monolayer coverage on HfO2 and ZrO2 surfaces were found to be in the range −130–190 and −110–170kJ per mole of gaseous H2O for differently prepared monoclinic phases and −70 and −90kJ∕mole for tetragonal phases from precipitation. The surface enthalpy for anhydrous tetragonal ZrO2 was derived as 1.23±0.04J∕m2 from water adsorption and high-temperature solution calorimetry data.

Direct calorimetric measurement of enthalpy of adsorption of carbon dioxide on CD-MOF-2, a green metal-organic framework.

The enthalpy of adsorption of CO2 on an environmentally friendly metal-organic framework, CD-MOF-2, has been determined directly for the first time using adsorption calorimetry at 25 °C. This calorimetric methodology provides a much more accurate and model-independent measurement of adsorption enthalpy than that obtained by calculation from the adsorption isotherms, especially for systems showing complex and strongly exothermic adsorption behavior. The differential enthalpy of CO2 adsorption shows enthalpy values in line with chemisorption behavior. At near-zero coverage, an irreversible binding event with an enthalpy of -113.5 kJ/mol CO2 is observed, which is followed by a reversible -65.4 kJ/mol binding event. These enthalpies are assigned to adsorption on more and less reactive hydroxyl groups, respectively. Further, a second plateau shows an enthalpy of -40.1 kJ/mol and is indicative of physisorbed CO2. The calorimetric data confirm the presence of at least two energetically distinct binding sites for chemisorbed CO2 on CD-MOF-2.

Thermochemistry of the alkali rare-earth double phosphates, A3RE(PO4)2

The formation enthalpies for alkali rare-earth compounds of the type K 3 RE(PO 4 ) 2 where RE = Sc, Y, Lu, Er, Ho, Dy, Gd, Nd, or Ce and for A 3 Lu(PO 4 ) 2 compounds with A = K, Rb, or Cs were determined using high-temperature oxide-melt solution calorimetry. Structural phase transitions were observed and characterized using differential scanning calorimetry and high-temperature x-ray diffraction. The formation enthalpy of the K 3 RE(PO 4 ) 2 phases from oxides becomes more exothermic with increasing rare-earth radius for the K 3 RE(PO 4 ) 2 series and with increasing alkali radius for the A 3 Lu(PO 4 ) 2 compounds. The K 3 RE(PO 4 ) 2 phases are stable with respect to anhydrous K 3 PO 4 and REPO 4 . The monoclinic K 3 RE(PO 4 ) 2 compounds undergo a reversible phase transition to a hexagonal (glaserite-type) structure with a phase transition temperature that increases from −99 to 1197 °C with increasing RE ionic radius going from Lu to Ce.

Thermodynamics of formation of coffinite, USiO4

Significance Coffinite, USiO4, is an important alteration mineral of uraninite. Its somewhat unexpected formation and persistence in a large variety of natural and contaminated low-temperature aqueous settings must be governed by its thermodynamic properties, which, at present, are poorly constrained. We report direct calorimetric measurements of the enthalpy of formation of coffinite. The calorimetric data confirm the thermodynamic metastability of coffinite with respect to uraninite plus quartz but show that it can form from silica-rich aqueous solutions in contact with dissolved uranium species in a reducing environment. These constraints on thermodynamic properties support that coffinitization in uranium deposits and spent nuclear fuel occurs through dissolution of UO2 (often forming hexavalent uranium intermediates) followed by reaction with silica-rich fluids. Coffinite, USiO4, is an important U(IV) mineral, but its thermodynamic properties are not well-constrained. In this work, two different coffinite samples were synthesized under hydrothermal conditions and purified from a mixture of products. The enthalpy of formation was obtained by high-temperature oxide melt solution calorimetry. Coffinite is energetically metastable with respect to a mixture of UO2 (uraninite) and SiO2 (quartz) by 25.6 ± 3.9 kJ/mol. Its standard enthalpy of formation from the elements at 25 °C is −1,970.0 ± 4.2 kJ/mol. Decomposition of the two samples was characterized by X-ray diffraction and by thermogravimetry and differential scanning calorimetry coupled with mass spectrometric analysis of evolved gases. Coffinite slowly decomposes to U3O8 and SiO2 starting around 450 °C in air and thus has poor thermal stability in the ambient environment. The energetic metastability explains why coffinite cannot be synthesized directly from uraninite and quartz but can be made by low-temperature precipitation in aqueous and hydrothermal environments. These thermochemical constraints are in accord with observations of the occurrence of coffinite in nature and are relevant to spent nuclear fuel corrosion.

Energetics of metastudtite and implications for nuclear waste alteration

Significance Uranium peroxides, metastudtite and studtite, can be formed on exposure of UO2 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 UO2 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, (UO2)O2(H2O)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 UO3 to crystallize orthorhombic α-UO2.9; and reduction to crystalline U3O8. 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.

Fluorite-pyrochlore transformation in Eu2Zr2O7—direct calorimetric measurement of phase transition, formation and surface enthalpies

The energetics of the order-disorder phase transformation in the binary oxide system, Eu2O3–ZrO2, is studied by powder X-ray diffraction and high temperature drop solution calorimetry. The nanocrystalline defect fluorite phase of Eu2Zr2O7 is synthesized on crystallization of an amorphous precursor from aqueous precipitation. The defect fluorite transforms to an ordered pyrochlore above 1200 °C. Aerodynamic levitation combined with laser heating is used to prepare coarse defect fluorite, which is otherwise impossible by conventional synthesis techniques. Formation enthalpies from oxides are −62.4 ± 2.6 and −24.6 ± 3.7 kJ mol−1 for the pyrochlore and defect fluorite phase, respectively. The transformation enthalpy from pyrochlore to defect flourite in the coarse sample is 37.8 ± 3.1 kJ mol−1 at 25 °C. The enthalpy of water vapor adsorption on the surface of the nanocrystalline defect fluorite Eu2Zr2O7 is −75 ± 2.5 kJ mol−1 H2O for coverage of 9.5 ± 0.8 H2O/nm2. The calculated surface enthalpies for the anhydrous and hydrous surfaces of defect fluorite Eu2Zr2O7 are 1.47 ± 0.13 and 1.01 ± 0.15 J m−2, respectively.

Hafnia: Energetics of thin films and nanoparticles

Crystallization energetics of amorphous hafnia powders and thin films on platinum substrates was studied by differential scanning calorimetry and time-resolved high temperature x-ray diffraction. For initially amorphous 25 and 20 nm films from atomic layer deposition, crystallization enthalpy decreases from −38 to −32 kJ/mol, and crystallization temperature increases from 388 to 417 °C as thickness decreases. Enthalpy of water vapor adsorption on the surface of monoclinic hafnia was measured for both bulk powder and nanoparticles and was found to vary from −110 to −130 kJ/mol for coverage of ∼5 H2O/nm2. The enthalpies of monoclinic hafnia with various surface areas, prepared by crystallization and annealing of an amorphous hafnia precursor, were measured by high temperature oxide melt solution calorimetry. Under the previously used assumption that the interfacial enthalpy is 20% of the surface enthalpy, the surface enthalpy was calculated from experimental data as 2.8±0.1 J/m2 for the hydrated surface and...

Energetics of CO2 and H2O Adsorption on Zinc Oxide

Adsorption of H2O and CO2 on zinc oxide surfaces was studied by gas adsorption calorimetry on nanocrystalline samples prepared by laser evaporation in oxygen to minimize surface impurities and degassed at 450 °C. Differential enthalpies of H2O and CO2 chemisorption are in the range -150 ±10 kJ/mol and -110 ±10 kJ/mol up to a coverage of 2 molecules per nm(2). Integral enthalpy of chemisorption for H2O is -96.8 ±2.5 kJ/mol at 5.6 H2O/nm(2) when enthalpy of water condensation is reached, and for CO2 is -96.6 ±2.5 kJ/mol at 2.6 CO2/nm(2) when adsorption ceases. These values are consistent with those reported for ZnO prepared by other methods after similar degas conditions. The similar energetics suggests possible competition of CO2 and H2O for binding to ZnO surfaces. Exposure of bulk and nanocrystalline ZnO with preadsorbed CO2 to water vapor results in partial displacement of CO2 by H2O. In contrast, temperature-programmed desorption (TPD) indicates that a small fraction of CO2 is retained on ZnO surfaces up to 800 °C, under conditions where all H2O is desorbed, with adsorption energies near -200 kJ/mol. Although molecular mechanisms of adsorption were not studied, the thermodynamic data are consistent with dissociative adsorption of H2O at low coverage and with several different modes of CO2 binding.

Thermodynamics of Oxide Systems Relevant to Alternative Gate Dielectrics

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