We present the behavior
of multiple solid solutions within ternary
(Ba
x
Ca1–x
)B6 and (Ba
x
Sr1–x
)B6 compounds and demonstrate that nanodomain
formation is preferred over uniform solid solutions under certain
processing conditions. Instead of the expected single solid solution
of M1 and/or M2 atoms within the MB6 phase, we note separation into nanodomain regions rich in either
M1 or M2. This phase separation has been observed
from detailed analyses of the shapes of the peaks in X-ray diffraction
data, where peak splitting and asymmetry are the result of multiple
solid solutions with lattice parameters differing by up to 1.4%. High-resolution
transmission electron microscopy confirms the presence of these nanodomains,
which are about 2–3 nm in size, and reveals varying degrees
of lattice misalignment. We also present X-ray diffraction analysis
of (Ba
x
Ca1–x
)B6 powders calcined from 1273 to 1973 K and document
the enhancement in sample homogeneity as the separated phases merge
into a uniform solid solution. As subsequent calcinations at lower
temperatures do not result in a re-separation of phases, the nanodomains
are deemed metastable. The greatest degree of phase separation is
observed in the (Ba
x
Ca1–x
)B6 system, which corresponds to the largest
difference in cation radii (0.161 vs. 0.134 nm for Ba2+ and Ca2+, respectively). Analysis of the chemical reactions
that occur during synthesis suggests that the decomposition of the
metal precursors (nitrates and carbonates) to metal oxides may cause
selective MB6 phase formation in mixed-cation hexaborides.
We demonstrate the synthesis of high-surface-area, low-density refractory aerogels. The monolithic hafnium boride (HfB 2 ) and zirconium boride (ZrB 2 ) aerogels are prepared via borothermal reduction of precursor hafnia and zirconia aerogels, respectively, consisting of a fine mixture of boron nanoparticles and the metal oxide. This precursor boron−metal oxide (B−MO 2 ) composite aerogel was synthesized by modifying the pure ethanol solvent typically used in the epoxide-initiated sol−gel synthesis of metal oxide aerogels with an ethanolic boron nanoparticle suspension. After reduction, precursor aerogels are converted to metal boride aerogels containing primary particles in the sub-100 nm regime. The relative densities of the HfB 2 and ZrB 2 aerogels are 3 and 7%, respectively, and could be tailored by simply changing the density of the precursor aerogels via modifying the reagent concentrations or the drying conditions. Thermal conductivities of the ZrB 2 monoliths ranged from 0.18 to 0.33 W/(m K). The surface areas of the HfB 2 and ZrB 2 aerogels were 10 and 19 m 2 /g, respectively. Successful reduction of the aerogels to the diboride phase was confirmed by X-ray diffraction.
We present the effect of pulsed direct current on metal ion diffusion in CaB-SrB diffusion couples, showing that the diffusivity of Ca and Sr across the diffusion couple interface is higher toward the positive electrode when subjected to a current flow of 2.2 kA at a temperature of 2007 K. We attribute this enhanced mobility to the movement of negatively charged metal vacancies toward the positive electrode in the system. Energy-dispersive spectroscopy is used to map the concentration of Ca and Sr in the region near the interface, and diffusion profiles are fitted with error functions. The concentration curves display concentration-dependent Boltzmann-Matano diffusivity. Total dopant values (Q) have been used to numerically compare the differences between Ca diffusion in SrB and Sr diffusion in CaB. We determine an enhancement of 3.8× for Ca into SrB versus an enhancement of 1.8× for Sr into CaB. No new phases are formed at the interface between CaB and SrB, since hexaboride compounds readily form solid solutions. The results elucidate the role of pulsed direct current on the diffusion of metal ions in hexaboride compounds.
We present the mechanisms of formation of mesoporous scandia-stabilized zirconia using a surfactant-assisted process and the effects of solvent and thermal treatments on the resulting particle size of the powders. We determined that cleaning the powders with water resulted in better formation of a mesoporous structure because higher amounts of surfactant were preserved on the powders after washing. Nonetheless, this resulted in agglomerate sizes that were larger. The water-washed powders had particle sizes of >5 μm in the as-synthesized state. Calcination at 450 and 600 °C reduced the particle size to ∼1-2 and 0.5 μm, respectively. Cleaning with ethanol resulted in a mesoporous morphology that was less well-defined compared to the water-washed powders, but the agglomerate size was smaller and had an average size of ∼250 nm that did not vary with calcination temperature. Our analysis showed that surfactant-assisted formation of mesoporous structures can be a compromise between achieving a stable mesoporous architecture and material purity. We contend that removal of the surfactant in many mesoporous materials presented in the literature is not completely achieved, and the presence of these organics has to be considered during subsequent processing of the powders and/or for their use in industrial applications. The issue of material purity in mesoporous materials is one that has not been fully explored. In addition, knowledge of the particle (agglomerate) size is essential for powder handling during a variety of manufacturing techniques. Thus, the use of dynamic light scattering or any other technique that can elucidate particle size is essential if a full characterization of the powders is needed for achieving postprocessing effectiveness.
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