Search

Neutron and x‐ray diffraction measurements have been performed on CaO–MgO–Al2O3–SiO2 (CMAS) glasses doped with NiO–FeXO at room temperature, along with x‐ray measurements on aerodynamically levitated liquids at ≥2000 K. The disordered structures have been modeled using empirical potential structure refinement to investigate the relation between the aluminosilicate network and the modifying cations. The SiO4 and AlO4 tetrahedra are found to have wider Si–O and Al–O bond distance distributions in the glass, and the first Ca–On coordination shell is highly distorted, redistributing different populations of long and short bonds between the liquid and the glass. The addition of Fe and Ni at low aluminosilicate content increases the number of free oxygens not bonded to AlO4 or SiO4. Mg–O and Fe–O are both found to be predominantly fourfold and fivefold in the liquid and glassy states. Despite these low coordination numbers, their bond angle distributions indicate that they are predominantly in nontetrahedral‐type geometries, with ferrous and ferric iron possessing similar coordination environments. The Ca–O and Mg–O average coordination numbers and enthalpies of solution are consistent with their higher reactivity within relatively acidic aluminosilicate melts. [ABSTRACT FROM AUTHOR]

Ytterbium‐doped lanthanum titanate glasses were prepared by levitation melting for the detailed characterization of the Yb3+${\rm Yb}^{3+}$ spectroscopic properties in the rare‐earth titanate glass host. Low‐temperature fluorescence spectroscopy reveals distinct site‐selectivity in both static and lifetime fluorescence measurements suggesting an absence of clustering as well as significant variation of local ytterbium environments. Typical site‐selectivity behavior of a shrinking Stark manifold with lower excitation energy is observed. At 77 K, both the mean emission frequency and the fluorescence lifetime initially increase as the excitation energy decreases from about 11100 to 10750 cm−1${\rm cm}^{-1}$ and then slightly decrease at lower excitation energy. Temperature‐dependent lifetime measurements between 77 and 420 K show a decreasing lifetime with increasing temperature and are well described by a two‐level thermal activation model. The temperature‐dependent fluorescence spectroscopy coupled with a room temperature white light absorption measurement allow the determination of the Stark energy levels of Yb3+${\rm Yb}^{3+}$ in lanthanum titanate glass as well as the calculation of the laser cross‐sections. [ABSTRACT FROM AUTHOR]

Rare earth and barium titanates are useful as ferroelectric, dielectric, and optical materials. Measurements of their thermophysical properties in the liquid state can help guide melt processing technologies for their manufacture and advance understanding of fragile liquids' behavior and glass formation. Here, we report the density, thermal expansion, viscosity, and surface tension of molten BaTi2O5, BaTi4O9, and 83TiO2-17RE2O3 (RE = La or Nd). Measurements were made using electrostatic levitation and droplet oscillation techniques in microgravity, which provide access to quiescent liquid droplets and deep supercooling of 510–815 K below the equilibrium melting points. Densities were measured over 900–2400 K. Viscosities were similar for all four compositions, increasing from ∼10 mPa s near 2100 K to ∼30 mPa s near 1750 K. Surface tensions were 450–490 dyn cm−1 for the rare earth titanates and 383–395 dyn cm−1 for the barium titanates; surface tensions of all compositions had small or negligible temperature dependence over 1700–2200 K. For solids recovered after melt quenching, x-ray microtomography revealed the fracture mechanics in crystalline products and minimal internal porosity in glass products, likely arising from entrapped gas bubbles. Internal microstructures were generally similar for products processed either in microgravity or in a terrestrial aerodynamic levitator. [ABSTRACT FROM AUTHOR]

Understanding and controlling liquid–liquid phase separation in aluminosilicates is crucial for optimizing glass properties. However, the metastable nature of aluminosilicates' phase separation has made it difficult to study experimentally, and uncertainty persists regarding the compositional and temperature extents of the miscibility gap. Here, we present new experimental evidence that suggests a consolute temperature between 1440 and 1590°C and endmember compositions of 7 and 62 mol.% Al2O3 for the phase‐separated glasses. Using containerless melt processing, deeply supercooled liquids over the 0–60 mol.% Al2O3 range are probed with in situ small‐ and wide‐angle X‐ray scattering, which simultaneously reveals changes in nanoscale density heterogeneity and atomic structure. Correlations between phase separation and atomic coordination environments are compared for liquids and glasses. Pair distribution function analysis shows mean O–(Si + Al) coordination increases with Al2O3 content and decreases with temperature. [ABSTRACT FROM AUTHOR]

Several high-temperature energy conversion and storage technologies rely on redox cycling of Fe-based materials, including storage materials in solid-oxide Fe-air batteries and oxygen carriers in chemical-looping combustion. The materials' macroporosity necessary for gas flow is, however, irreversibly diminished during redox cycling due to (i) large volume changes during the redox transformations, (ii) foam sintering at elevated operating temperature (550-900 °C), and (iii) formation and growth of Kirkendall microporosity. To address these challenges, we use directional freeze-casting to create highly porous, lamellar, Fe-composite foams containing uniformly distributed sintering inhibitor (SI) particles-either Y 2 O 3 -stabilized ZrO 2 (YSZ), CeO 2 , or ZrO 2 -at 0, 5, 10, or 15% of the solid volume. We characterize these foams before, during, and after redox cycling (Fe/FeO/Fe 3 O 4 , via H 2 O and H 2 ) at 800 °C using operando synchrotron X-ray microtomography, metallography, and scanning electron microscopy. Shrinkage of the foam volume and formation of a gas-blocking shell surrounding the foam are reduced as the SI fraction increases. Volumetric shrinkage after the first five redox cycles is decreased from 66% (for pure-Fe foams) to 45% (for all Fe-composites containing 5 vol % SI). Foams containing 15 vol % YSZ show no volumetric shrinkage after five cycles, although, after 20 cycles, they have shrunk 53%. Post-cycling analysis reveals segregation of the SI particles to the cores of individual lamellae, surrounded by thick layers of sintered Fe on the lamellae surfaces. This segregation occurs due to Fe diffusion through FeO to the lamellae surfaces during oxidation, leaving behind the SI particles, which are then pushed into clusters by FeO/Fe 3 O 4 contraction during reduction. The SI is thus rendered ineffective, which explains why foam densification is delayed (compared with pure-Fe foams), rather than fully prevented, after repeated cycling.