Design, calibration, and operation of a system for drop‐and‐catch (DnC) calorimetry on oxides from temperature above 1500°C are described. This system allows the measurement of heat contents and ...heats of fusion by drop calorimetry of small (100 mg or less) samples held by containerless levitation at high temperature and dropped in a calorimeter at room temperature. The spheroids, 2‐3 mm in diameter, prepared by laser melting of powders, are aerodynamically levitated in a splittable nozzle levitator and laser heated to the desired temperature monitored by radiation thermometry. The sample is dropped by splitting the nozzle and caught by splittable water‐cooled calorimetric plates at 25°C, which provide complete enclosure of the sample to avoid heat loss by radiation. The drop time is ~0.1 seconds, calorimeter equilibration time after the drop is ~15 minute. DnC experiments are automated with software‐controlled laser power and programmable delay between splitting the nozzle and catching the sample. The fusion enthalpy of Al2O3 measured by DnC calorimeter, 120 ± 10 kJ/mol, agrees well with previously reported values. The system can be used for measurements of fusion enthalpies of refractory oxides amenable to laser heating as well as for splat quenching of oxide melts.
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.
Silicon oxycarbides synthesized through a conventional polymeric route show characteristic nanodomains that consist of sp2 hybridized carbon, tetrahedrally coordinated SiO4, and tetrahedrally ...coordinated silicon with carbon substitution for oxygen, called “mixed bonds.” Here we synthesize two preceramic polymers possessing both phenyl substituents as unique organic groups. In one precursor, the phenyl group is directly bonded to silicon, resulting in a SiOC polymer-derived ceramic (PDC) with mixed bonding. In the other precursor, the phenyl group is bonded to the silicon through Si-O-C bridges, which results in a SiOC PDC without mixed bonding. Radial breathing-like mode bands in the Raman spectra reveal that SiOC PDCs contain carbon nanoscrolls with spiral-like rolled-up geometry and open edges at the ends of their structure. Calorimetric measurements of the heat of dissolution in a molten salt solvent show that the SiOC PDCs with mixed bonding have negative enthalpies of formation with respect to crystalline components (silicon carbide, cristobalite, and graphite) and are more thermodynamically stable than those without. The heats of formation from crystalline SiO2, SiC, and C of SiOC PDCs without mixed bonding are close to zero and depend on the pyrolysis temperature. Solid state MAS NMR confirms the presence or absence of mixed bonding and further shows that, without mixed bonding, terminal hydroxyls are bound to some of the Si-O tetrahedra. This study indicates that mixed bonding, along with additional factors, such as the presence of terminal hydroxyl groups, contributes to the thermodynamic stability of SiOC PDCs.
New worlds, new chemistry, new ceramics Navrotsky, Alexandra; Householder, Megan
International journal of ceramic engineering & science,
November 2021, 2021-11-00, 20211101, 2021-11-01, Volume:
3, Issue:
6
Journal Article
Peer reviewed
Open access
Space missions have documented the chemical diversity and complexity of planets in our solar system. Astronomers have found thousands of exoplanets orbiting other stars in our galaxy. Almost every ...star in our galaxy has one or more orbiting planets and they appear to be much more diverse than the few sampled in our solar system. This diversity implies immense chemical, ceramic, geological, and mineralogical complexity. Constrained by the limited data, understanding this range of planets poses a classic inverse problem in materials science, namely, to constrain the composition, phase relations, and evolution of a planet based on a few remote observations of its properties. The purpose of this paper is to provide a general view of the Earth and of planets in our solar system and beyond, to introduce fundamental concepts of planetary structure and chemistry, and to identify outstanding questions and opportunities from the point of view of chemistry, materials science, and ceramics. This paper is composed of two parts. First, an introduction is followed by a discussion of the structure, geophysics, and phase equilibria in the Earth, to orient the non‐geologic reader to the basic phenomena and concepts. This is followed by a loose transcription of a lecture given by Navrotsky at the (virtual) 2020 American Ceramic Society meeting. This talk stressed the evolving nature of planetary science, especially inspired by the discovery of myriads of exoplanets, highlighted the Materials of the Universe Center at Arizona State University, and briefly described and compared planets in our solar system.
Space missions have documented the chemical diversity and complexity of planets and moons in our own solar system. Astronomers have found thousands of exoplanets orbiting other stars in our galaxy, whose diversity implies immense chemical, ceramic, geological, and mineralogical complexity. The purpose of this paper is to provide an overview of the Earth and of planets and moons in our solar system and beyond, to introduce fundamental concepts of planetary structure and chemistry, and to identify outstanding questions and opportunities from the point of view of chemistry, materials science, and ceramics.
A general overview of the trends in structural and thermodynamic properties that have been identified within the hydrated normal rare earth carbonates and the rare earth hydroxycarbonates is ...presented. Based upon available literature, we demonstrate the trends in crystallographic unit cell parameters, thermal stability, aqueous solubility, and thermochemical properties. These trends can be attributed to both the unique chemistry and strong similarity of the rare earth elements. There are also inconsistent trends that signal research needs to better understand the structure–energy relationships of the rare earth carbonates.
The surface and interface enthalpies of CeO2 were studied by high temperature oxide melt solution calorimetry combined with water adsorption calorimetry. The surface enthalpies of the hydrated and ...anhydrous surfaces are 0.86 ± 0.02 and 1.16 ± 0.02 J/m2, respectively. The water adsorption enthalpies are similar for nano and bulk ceria; for the nano ceria the integral adsorption enthalpy for chemisorbed water relative to water vapor is −59.82 ± 0.74 kJ/mol with coverage of 8.79 ± 0.39 H2O/nm2 and for the bulk it is −61.69 ± 1.26 kJ/mol with coverage of 8.15 ± 0.66 H2O/nm2. The interfacial enthalpy is 0.81 ± 0.14 J/m2. The obtained energies are in good agreement with reported data from atomistic simulations and less direct experimental determinations.
Over 60 years of nuclear activities have resulted in a global legacy of radioactive wastes, with uranium considered a key radionuclide in both disposal and contaminated land scenarios. With the ...understanding that U has been incorporated into a range of iron (oxyhydr)oxides, these minerals may be considered a secondary barrier to the migration of radionuclides in the environment. However, the long-term stability of U-incorporated iron (oxyhydr)oxides is largely unknown, with the end-fate of incorporated species potentially impacted by biogeochemical processes. In particular, studies show that significant electron transfer may occur between stable iron (oxyhydr)oxides such as goethite and adsorbed Fe(II). These interactions can also induce varying degrees of iron (oxyhydr)oxide recrystallization (<4% to >90%). Here, the fate of U(VI)-incorporated goethite during exposure to Fe(II) was investigated using geochemical analysis and X-ray absorption spectroscopy (XAS). Analysis of XAS spectra revealed that incorporated U(VI) was reduced to U(V) as the reaction with Fe(II) progressed, with minimal recrystallization (approximately 2%) of the goethite phase. These results therefore indicate that U may remain incorporated within goethite as U(V) even under iron-reducing conditions. This develops the concept of iron (oxyhydr)oxides acting as a secondary barrier to radionuclide migration in the environment.
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•Rare earths are used in applications involving multicomponent oxide materials.•This paper summarizes thermodynamic properties of these diverse oxide materials.•Ionic size is a major ...factor in determining the systematics of phase stability
Rare earth elements (RE) are incorporated into a large variety of complex oxide phases to provide tailored mechanical, electrical, optical, and magnetic properties. Thermodynamics control phase stability, materials compatibility in use, corrosion, and transformation. This review presents, in one compilation, the thermodynamic properties of a large number of such materials and discusses systematic trends in energetics and the factors controlling stability.
Abstract
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 °C) to their ...melting points (2710 and 2800 °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. 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 °C. Computed oxygen diffusion coefficients indicate that above 2400 °C pure ZrO
2
is an excellent oxygen conductor, perhaps even better than YSZ.
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