Mechanochemical Synthesis, Accelerated Aging, and Thermodynamic Stability of the Organic Mineral Paceite and Its Cadmium Analogue
Not Available
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Engineering topics
Navrotsky, Alexandra
Publications and source records attributed to Navrotsky, Alexandra.
Compositional control of radionuclide retention in hollandite-based ceramic waste forms for Cs-immobilization
Hollandite materials, as a class of crystalline nuclear waste forms, are promising candidates for the immobilization of radioactive elements, such as Cs, Ba, as well as a variety of lanthanide and transition-metal fission products. In this study, three Ga-doped titanate hollandite-type phases, Ba 1.33 Ga 2.67 Ti 5.33 O 16 , Ba 0.667 Cs 0.667 Ga 2 Ti 6 O 16 , and Cs 1.33 Ga 1.33 Ti 6.67 O 16 , were synthesized using a solid-state reaction route. All synthesized phases adopted a single phase tetragonal structure, as determined by powder X-ray diffraction (XRD), and elemental analysis confirmed the measured stoichiometries were close to targeted compositions. Extended X-ray absorption fine structure spectroscopy (EXAFS) was used to determine the local structural features for the framework of octahedrally coordinated cations. EXAFS data indicated that Cs 1.33 Ga 1.33 Ti 6.67 O 16 possessed the most disordered local structure centered around the Ga dopant. The enthalpies of formation of all three hollandite phases measured using high-temperature oxide melt solution calorimetry were found to be negative, indicating enthalpies of formation of these hollandites from oxides are thermodynamically stable with respect to their constituent oxides. Furthermore, the formation enthalpies were more negative and hence more favorable with increased Cs content. Finally, aqueous leaching tests revealed that high Cs content hollandite phases exhibited greater Cs retention as compared to low Cs content hollandite. While preliminary in nature, this work draws attention to the links between the capacity for radionuclide retention, atomistic level structural features and bulk thermodynamic properties of materials.
nuclear (including radiation effects), materials a↗
Bio- and mineral acid leaching of rare earth elements from synthetic phosphogypsum
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.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Thermodynamics and Stability of Rhabdophanes, Hydrated Rare Earth Phosphates REPO4 · n H2O
Not Available
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Phase transformations in oxides above 2000°C: experimental technique development
Here, the oxidation of boride and carbide-based ultra-high-temperature ceramics is the primary limiting factor for their use as aerodynamic surfaces. Understanding the behaviour of the oxides that can result from oxidation of metal borides and carbides at very high temperatures is essential to optimise and tailor the performance of these materials; yet experimental thermodynamic and structural data for refractory oxides above 2000°C are mostly absent. The following techniques that can be applied to fill this gap are discussed: (i) commercial ultra-high-temperature differential thermal analysis for investigation of phase transformations and melting in inert environments to 2500°C, (ii) a combination of laser heating with a splittable nozzle aerodynamic levitator for splat quenching and drop calorimetry from temperatures limited only by sample evaporation, (iii) synchrotron X-ray and neutron diffraction on laser-heated aerodynamically levitated oxide samples for in situ observation of phase transformations in variable atmospheres, refinement of high-temperature structures and thermal expansion. Recent experimental findings include anomalous thermal expansion of the defect fluorite phase of YSZ, thermodynamics of pyrochlore–fluorite transformation from high-temperature structure refinements, and measurement of thermal expansion to the melting temperatures and fusion enthalpies of Zr, Hf, La, Yb and Lu oxides. These methods provide temperatures, enthalpies and volume change for phase transformations above 2000°C, which are required for thermodynamic assessments and calculation of phase diagrams of multicomponent systems.
36 MATERIALS SCIENCE↗
Lithium aluminum–layered double hydroxide chlorides ( LDH ): Formation enthalpies and energetics for lithium ion capture
Layered aluminum double hydroxide chloride sorbents, LiCl∙Al 2 (OH)6. n H 2 O, Li-LDH, have shown promising application in selective Li extraction from geothermal brines. Maintaining LiCl uptake capacity and retaining a long cycle life are critical to widespread application of sorbent materials. To elucidate the energetics of Li capture, enthalpies of LDH with different Li content have been measured by acid solution calorimetry. The formation enthalpies generally become less exothermic as the Li content increases, which indicates that Li intercalation destabilizes the structure, and the enthalpies seem to approach a limit after the Li content x = 2Li/Al exceeds 1. To improve stability, metal doping of the aluminum LDH structure with iron was performed. Introduction of a metal with greater electron density but a similar ionic radius was postulated to improve the stability of the LDH crystal structure. Here, the calorimetric results from Fe-doped LDH samples corroborate this as they are more exothermic than LDH-lacking Fe. This suggests that Fe doping is an effective way to stabilize the LDH phase.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Combined computational and experimental investigation of high temperature thermodynamics and structure of cubic ZrO 2 and HfO 2
Structure and thermodynamics of pure cubic ZrO 2 and HfO 2 were studied computationally and experimentally from their tetragonal to cubic transition temperatures (2311 and 2530 degrees C) to their melting points (2710 and 2800 degrees C). Computations were performed using automated ab initio molecular dynamics techniques. High temperature synchrotron X-ray diffraction on laser heated aerodynamically levitated samples provided experimental data on volume change during tetragonal-to-cubic phase transformation (0.55 +/- 0.09% for ZrO 2 and 0.87 +/- 0.08% for HfO 2 ), density and thermal expansion. Fusion enthalpies were measured using drop and catch calorimetry on laser heated levitated samples as 55 +/- 7 kJ/mol for ZrO 2 and 61 +/- 10 kJ/mol for HfO 2 , compared with 54 +/- 2 and 52 +/- 2 kJ/mol from computation. Volumetric thermal expansion for cubic ZrO 2 and HfO 2 are similar and reach (4 +/- 1).10 -5 /K from experiment and (5 +/- 1).10 -5 /K from computation. Finally, an agreement with experiment renders confidence in values obtained exclusively from computation: namely heat capacity of cubic HfO 2 and ZrO 2 , volume change on melting, and thermal expansion of the liquid to 3127 degrees C. Computed oxygen diffusion coefficients indicate that above 2400 degrees C pure ZrO 2 is an excellent oxygen conductor, perhaps even better than YSZ.
36 MATERIALS SCIENCE↗
Surface energy of fayalite and its effect on Fe-Si-O oxygen buffers and the olivine-spinel transition
The surface energy (hydrated surfaces) of fayalite ($α$-Fe 2 SiO 4 ) was determined to be 2.47 ± 0.25 J/m 2 using high-temperature oxide melt solution calorimetry. This is larger than the surface energy of magnetite (Fe 3 O 4 ), but lower than that of forsterite ($α$-Mg 2 SiO 4 ). The changes in the positions of the quartz-fayalite-magnetite (QFM) and quartz-iron-fayalite (QIF) buffers with particle size reduction were calculated. QFM is lowered in $f_{O_2}$ by 3–7 log units as a function of temperature for 30 nm particles while QIF is raised by 1–2 log units. The estimated surface energy difference between olivine and spinel polymorphs decreases the pressure of the olivine-spinel transition in Fe 2 SiO 4 by about 1 GPa.
quartz-iron-fayalite (QIF) buffers↗
Thermochemical Measurements of Alkali Cation Association to Hexatantalate
Ion association is an important process in aqueous dissolution, precipitation, and crystallization of ionic inorganic, organic, and biological materials. Polyoxometalates (POMs) are good model compounds for understanding the complex relationships between lattice energy, ion-pairing in solution, and salt solubility. Here we perform calorimetric measurements to elucidate trends in cluster stability, lattice energy, and ion-pairing behavior studies of simple hexatantalate salts in neat water, parent hydroxide solutions, and molybdate melts, extending previous studies on the isostructural hexaniobates. High temperature calorimetry of alkali salts of hexatantalate reveals that the enthalpies of formation from oxides of the K, Rb, and Cs salts are more similar to each other than they are for their niobate analogues and that the tantalate cluster is energetically less stable than hexaniobate. Aqueous dissolution calorimetry reveals that the cesium salt of hexatantalate has a similar concentration dependence on its dissolution enthalpy to that of hexaniobate. However, unlike rubidium hexaniobate, rubidium hexatantalate also exhibits increased concentration dependence, indicating that hextantalate can undergo increased ion-pairing with alkali salts other than cesium, despite the dilute environments studied. Dissolution enthalpies of POM salts in the parent alkali hydroxides shows that protonation of clusters stabilizes lattices even more than the strongly associating heavy alkali cations do. Additionally, neither weak nor strong lattice ion associations necessarily correlates with respectively high or low aqueous solubility. These studies illuminate the importance of considering ion-pairing among the interrelated processes in the aqueous dissolution of ionic salts that can be extended to serving as a model of cation association to metal oxide surfaces.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Thermodynamics of radiation induced amorphization and thermal annealing of Dy 2 Sn 2 O 7 pyrochlore
Thermodynamics and annealing behavior of swift heavy ion amorphized Dy 2 Sn 2 O 7 pyrochlore were studied. Its amorphization enthalpy, defined as the total energetic difference between the irradiation amorphized and undamaged Dy 2 Sn 2 O 7 states, was determined to be 283.6 ± 6.5 kJ/mol by high temperature oxide melt drop solution calorimetry. It has been an enigma that stannate and some other pyrochlores do not follow the general r A /r B -radiation resistance relation seen in most pyrochlore systems. In this paper, we use the amorphization enthalpy, which reflects all the complex chemical and structural characteristics, as a more effective parameter to correlate the radiation damage resistance of pyrochlores with their compositions. It successfully explains the superior radiation damage resistance of the stannate pyrochlores compared with titanate pyrochlores. Differential scanning calorimetry (DSC) reveals a strong exothermic event starting at 978 K, which is attributed to long-range recrystallization based on X-ray diffraction (XRD) analysis, similar to the effect previously observed in Dy 2 Ti 2 O 7 . A second pronounced heat event beginning at ~1148 K, which results from local structural rearrangement, is clearly decoupled from the first event for irradiated Dy 2 Sn 2 O 7 . Both the heat releases measured by DSC on heating to 1023 and 1473 K, and the excess enthalpies of the annealed samples indicate that the recovery to the original, ordered state was not fully achieved up to even 1473 K, despite XRD showing the apparent restoration of crystalline pyrochlore structure. The remaining metastability may be attributed to local disorder in the form of weberite-like short-range domains in the recrystallized material. Intriguingly, the second event for different pyrochlores generally starts at similar temperatures while the onset of the long range recrystallization is compositionally dependent. Finally, the amorphization and thermal annealing behavior observed in irradiated Dy 2 Sn 2 O 7 may provide insights into the general mechanisms of radiation damage and recovery of pyrochlores relevant to their nuclear applications.
36 MATERIALS SCIENCE↗
Energetics of bulk lutetium‐doped Ce 1−x Lu x O 2−x/2 compounds
Abstract The dissolution enthalpy , Δ H DS , and the formation enthalpy, Δ H f,ox , of bulk lutetium‐doped cerium oxide (LuDC) were studied at 701°C in molten sodium molybdate. For the composition range of Ce 1− X Lu X O 2− X /2 , studied 0 ≤ X ≤ 0.3, the Δ H DS decreases linearly and smoothly with lutetium content according to Δ H DS , kJ/mol = 73.5(1.0)−165.1(5.5)· x ). The enthalpy of formation, Δ H f,ox , becomes more exothermic linearly with lutetium content. No anomaly in Δ H f,ox is observed at low Lu 2 O 3 concentration as reported previously for several other rare‐earth‐doped ceria systems, suggesting possible differences in clustering and microstructure, which may also be related to difference in processing conditions.
Yavo, Nimrod↗
Thermodynamic evidence of flexibility in H 2 O and CO 2 absorption of transition metal ion exchanged zeolite LTA
Absorption thermodynamics on the framework flexibility of TMI-exchanged zeolite LTA driven by water/CO 2 molecules.
Chemistry↗