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Navrotsky, Alexandra

Publications and source records attributed to Navrotsky, Alexandra.

At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Thermodynamic and structural evolution of mechanically milled and swift heavy ion irradiated Er 2 Ti 2 O 7 pyrochlore

Design and synthesis of thermodynamically metastable yet kinetically achievable materials possessing various desired functional and physical properties have recently drawn tremendous scientific-attention. In addition to conventional heat treatments and wet chemistry approaches, energy deposition into materials can induce unique nonequilibrium phases with distinct structures, chemistry, energetics, and properties. Mechanochemical synthesis and ion beam irradiation are two processing techniques that provide access to phases and states far from equilibrium. By a combination of high temperature oxide melt solution calorimetry, differential scanning calorimetry (DSC), neutron pair distribution function (PDF) analysis, and supplementary powder X-ray diffraction (XRD), the energetics and multiscale structural evolution on annealing of ball milled and swift heavy ion irradiated Er 2 Ti 2 O 7 pyrochlore were investigated. Despite very similar structural modifications of local atomic arrangements and only minor differences in the long range structure, both types of damage yield significant difference in the energetics of the produced material. Here, the energy of destabilization in the milled sample (70.2 ± 8.2 kJ/mol) is much less endothermic than that in the irradiated sample (457.3 ± 8.0 kJ/mol). The DSC profiles, supported by neutron scattering, X-ray diffraction, and solution calorimetry, reveal decoupled annealing events in different temperature ranges, separating crystallization of long range pyrochlore structure from annealing of short range weberite-like domains.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Neutron Spectroscopic and Thermochemical Characterization of Lithium–Aluminum-Layered Double Hydroxide Chloride: Implications for Lithium Recovery

Lithium–aluminum-layered double hydroxide chloride (LDH) has been recently shown to be a promising sorbent material for selective lithium extraction and recovery from geothermal brine. In this research, LDH samples synthesized from two different starting materials (alumina and gibbsite) and with two post-drying conditions (ambient-dried and oven-dried) as well as and Fe-doped LDH are studied using neutron vibrational spectroscopy. The results reveal differences in the ordering of interlayer water molecules. Thermochemical measurements of LDHs show that a higher interlayer ordering of water is directly related to the stability of gibbsite- and alumina-derived LDH. This corroborates with the previous work on increasing the stabilization of LDH via partial substitution of aluminum with iron. These results on the properties of LDH have implications on their lithium extraction abilities and subsequent recovery.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Energetics of porous amorphous low-k SiOCH dielectric films

The trend toward dimension reduction in electronic devices has driven the research and development of low dielectric constant materials to solve resistance capacitance delays and power consumption issues. Amorphous porous hydrogenated silicon oxycarbides form a class of materials that have even lower dielectric constants than other silicon oxycarbides, achieved by introducing organic functional groups as well as porosity to alter the structure and decrease the density of conventional silica. However, these films tend to decompose to SiO 2 during thermal annealing and nano-electronic fabrication processes. This apparent instability leads to the need to investigate how porosity can change the fundamental thermodynamic stability of this class of materials. We used high temperature oxidative molten salt solution calorimetry and cryogenic heat capacity measurements to directly determine enthalpies of formation, heat capacities, and standard entropies of a series of well-characterized porous SiOCH films. Therefore, a full thermodynamic dataset has been obtained for these porous low-k films. All of these samples are either found to be unstable or metastable at or even below 298.15 K with respect to their crystalline counterparts and gaseous products. Thus, there appears little chance of forming thermodynamically stable amorphous porous SiOCH films and their persistence in fabrication and use is controlled by kinetic rather than thermodynamic factors.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Thermochemistry of Surfactant‐Templating of USY Zeolite

Abstract With the aim of understanding the thermochemistry of the introduction of mesoporosity in zeolites by using surfactants, high temperature oxide melt solution calorimetry was used to determine the change in the enthalpy of formation of USY zeolite before and after the introduction of mesoporosity. Our results confirm that this process only slightly destabilizes the zeolite by the additional surface area. However, this can be overcome by the stabilizing effect of the interactions between the surfactant and the zeolite framework.

Linares, Noemi↗
DOE OSTI2019

Adsorption mechanism of alkyl hydroxamic acid onto bastnäsite: Fundamental steps toward rational collector design for rare earth elements

Rare earth element (REE) production is limited in part by inefficient strategies for beneficiation. Hydroxamic acid ligands are promising reagents for the selective flotation of bastnäsite [(Ce,La)FCO 3 ], a major REE ore mineral, but the mechanism and energetics of adsorption are not understood, interfering with the design of new, more efficient reagents. In this work, the adsorption of octyl hydroxamic acid onto bastnäsite was calculated using a combination of experimental and computational methods. In-situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy revealed changes in the hydroxamate functional group vibrational frequencies, corresponding to chelation with cerium cations at the bastnäsite surface. The results indicate a monodentate chemisorption mechanism at low surface loading that changes to bidentate chemisorption at higher concentrations. This interpretation is supported by molecular vibrational frequency shifts calculated using density functional theory (DFT), and orientation of the hydrocarbon chain measured by sum frequency generation (SFG) vibrational spectroscopy. The binding enthalpies of octyl hydroxamic acid interacting with La and Ce-bastnäsite surfaces were measured using isothermal titration calorimetry (ITC) revealing a stronger coordinating ability with bastnäsite than with a common gangue mineral, calcite (CaCO 3 ). Because octyl hydroxamate favors monodentate adsorption at low surface coverages, the relative chelating strength of metal ions could be a poor predictor for selectivity under monolayer adsorption conditions. At higher surface loadings, where the bidentate mode of adsorption is active, selectivity is likely to be limited by increased flotation of gangue ores.

58 GEOSCIENCES↗
DOE OSTI2019

Enthalpies of formation and phase stability relations of USi, U 3 Si 5 and U 3 Si 2

U–Si intermetallic compounds have drawn great attention due to their potential application as nuclear fuels. However, the thermodynamic properties and phase equilibria of this binary system from ambient to high temperature conditions are not fully understood. Via high temperature oxidative drop calorimetry and detailed characterization of the initial and final phases, we have experimentally determined the standard enthalpies of formation of USi and U 3 Si 5.07 at 298 K to be –43.2 ± 6.2 and –43.8 ± 9.0 kJ/mol·atom, respectively. The energetics of the tetragonal USi (t-USi, space group I 4/ mmm ) phase has also been calculated with Density Functional Theory (DFT) for the first time. Combining the obtained formation enthalpies with the heat capacities measured previously, we assessed the thermodynamic stability of t-USi relative to a phase assemblage of two other U–Si phases, U 3 Si 5.07 and U 3 Si 2 , from ambient temperature to 1200 K. The tetragonal USi is thermodynamically more stable than U 3 Si 5.07 + U 3 Si 2 , which supports previously published phase diagram (H. Okamoto and T. Massalski, 1990 [1]): specifically, at least one stable USi phase exists when the U content is 50 at.%. In conclusion, further thermodynamic and phase equilibrium studies are needed for a more comprehensive understanding of the U–Si system across broader compositional and temperature ranges.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2019

Heat capacity and thermodynamic functions of crystalline forms of the metal-organic framework zinc 2-methylimidazolate, Zn(MeIm) 2

Zeolitic imidazolate frameworks (ZIFs) are comprised of metal atoms connected with imidazole-like linkers, and these frameworks have potential for applications in molecular sieving, gas sequestration, and catalysis. Furthermore, these materials form true polymorphs with the same chemical composition but different topologies. In this work, we present the results of low temperature heat capacity and inelastic neutron scattering studies of the sodalite (SOD) and diamondoid ( dia ) topologies of the popular zinc 2-methylimidazolate framework, Zn(MeIm) 2 . Molar heat capacities from 1.8 K to 300 K are presented, along with theoretical fits and the values of C p,m °, Δ 0 T S m °, Δ 0 T H m °, and Φ m ° calculated from those fits. The Gibbs energy of the transformation from SOD to dia is -(4.6 ± 2.2) kJ, and this transformation is primarily enthalpically driven. The results of this study are compared with previous measurements on the zinc 2-ethylimidazolate framework, Zn(EtIm) 2 . Inelastic neutron scattering measurements confirm the presence of low energy modes and suggest that the higher heat capacity of SOD at low temperatures is due to the dynamics of the methyl groups on the methylimidazolate linkers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Sample seal-and-drop device and methodology for high temperature oxide melt solution calorimetric measurements of PuO 2

Thermodynamic properties of refractory materials, such as standard enthalpy of formation, heat content, and enthalpy of reaction, can be measured by high temperature calorimetry. In such experiments, a small sample pellet is dropped from room temperature into a calorimeter operating at high temperature (often 700 °C) with or without a molten salt solvent present in an inert crucible in the calorimeter chamber. However, for hazardous (radioactive, toxic, etc.) and/or air-sensitive (hygroscopic, sensitive to oxygen, pyrophoric, etc.) samples, it is necessary to utilize a sealed device to encapsulate and isolate the samples, crucibles, and solvent under a controlled atmosphere in order to prevent the materials from reactions and/or protect the personnel from hazardous exposure during the calorimetric experiments. We have developed a sample seal-and-drop device (calorimetric dropper) that can be readily installed onto the dropping tube of a calorimeter such as the Setaram AlexSYS Calvet-type high temperature calorimeter to fulfill two functions: (i) load hazardous or air-sensitive samples in an air-tight, sealed container and (ii) drop the samples into the calorimeter chamber using an “off-then-on” mechanism. As a case study, we used the calorimetric dropper for measurements of the enthalpy of drop solution of PuO 2 in molten sodium molybdate (3Na 2 O · 4MoO 3 ) solvent at 700 °C. The obtained enthalpy of –52.21 ± 3.68 kJ/mol is consistent with the energetic systematics of other actinide oxides (UO 2 , ThO 2 , and NpO 2 ). In conclusion, this capability has thus laid the foundation for thermodynamic studies of other Pu-bearing phases in the future.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Functionality in metal–organic framework minerals: proton conductivity, stability and potential for polymorphism

Rare metal–organic framework (MOF) minerals stepanovite and zhemchuzhnikovite can exhibit properties comparable to known oxalate MOF proton conductors, including high proton conductivity over a range of relative humidities at 25 °C, and retention of the framework structure upon thermal dehydration. They also have high thermodynamic stability, with a pronounced stabilizing effect of substituting aluminium for iron, illustrating a simple design to access stable, highly proton-conductive MOFs without using complex organic ligands.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗