Phase Stability of Mixed-Cation Alkaline-Earth Hexaborides

Abstract

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.

Mechanical Properties of an Fe‐Based SAM2×5‐630 Metallic Glass Matrix Composite with Tungsten Particle Additions

“…Additional details on the SPS process can be found elsewhere. [21][22][23][24][25] Phase development/devitrification of the samples was characterized by X-ray diffraction (Ultima IV, Rigaku, Japan) using CuKα radiation, a step size of 0.02 degrees 2θ, a count time of 1 s, and scanning from 20 to 80 degrees 2θ. Densities of the samples were evaluated by the Archimedes method using a Mettler Toledo Density Kit (Mettler Toledo, Columbus, OH).…”

Nanodomains and local structure in ternary alkaline-earth hexaborides

“…Additionally, the PDF results suggest that each composition appears phase uniform over length scales of less than 1 nm; however, over length scales greater than 1 nm, the barium-containing compositions appear to contain coherent nanodomains. This is supported by transmission electron microscopy imaging and earlier powder X-ray diffraction (PXRD) studies of these materials (Cahill, Alberga et al, 2017).…”

“…Both are annealed under argon at NIST to reduce intrinsic defect concentrations. The three ternary alkaline-earth hexaborides (MB 6 , M = Ca 0.5 Sr 0.5 , Ca 0.5 Ba 0.5 and Sr 0.5 Ba 0.5 ) were identical to those previously reported by Cahill and co-workers (Cahill, Alberga et al, 2017), prepared using combustion synthesis techniques (Kanakala et al, 2010(Kanakala et al, , 2011Kelly et al, 2010). Precursors were heated to 773 K with carbohydrazide as a fuel source.…”

“…These parameters were fixed during analysis of the specimens. Least-squares refinements of the experimental PDF data were carried out with the software PDFgui (Farrow et al, 2007), with models initialized using the parameters of the prior Rietveld analysis (Cahill, Alberga et al, 2017) and symmetry constrained by the standard Pm3m space-group model for these materials. For each fit, a lattice parameter, the boron z position and the isotropic displacement parameters were refined.…”

Suppressing η‐phase development in steel‐cemented tungsten carbide: A spark plasma sintering study

“…The Fe‐cemented composite had a relative density of 99.7% and properties comparable to the Co‐cemented composite. Achieving fast heating rates and short sintering cycles is also possible with spark plasma sintering (SPS), which has been demonstrated for sintering oxides, nonoxides,, and metals …”