High Pressure Behavior of Chromium and Yttrium Molybdate (Cr2Mo3O12, Y2Mo3O12).

The high pressure behavior of negative thermal expansion materials continues to be of interest, as their potential use in controlled thermal expansion composites can be affected by irreversible pressure-induced phase transitions. To date, it is not possible to predict the high pressure behavior of these compounds, necessitating measurements on each composition. In this work, high pressure synchrotron powder X-ray diffraction studies of Cr2Mo3O12 and Y2Mo3O12 were conducted in a diamond anvil cell. Chromium molybdate, which adopts the monoclinic P21/a structure under ambient conditions, was found to not undergo any crystalline-crystalline transitions up to 8.9 GPa. The orthorhombic ambient pressure polymorph of yttrium molybdate was found to undergo a phase transition to the monoclinic P21/a scandium tungstate structure below 0.13 GPa. This structure is frequently observed for related materials at low temperatures, but has never been reported for Y2Mo3O12. No additional changes in this material were observed up to 4.9 GPa. The fact that the monoclinic polymorphs of these materials do not undergo phase transitions within the studied pressure range makes them unique among A2M3O12 materials, as most isostructural compositions undergo at least one phase transition to crystalline high pressure phases.

Low Temperature Synthesis and Characterization of AlScMo3O12.

Recent interest in low and negative thermal expansion materials has led to significant research on compounds that exhibit this property, much of which has targeted the A2M3O12 family (A = trivalent cation, M = Mo, W). The expansion and phase transition behavior in this family can be tuned through the choice of the metals incorporated into the structure. An undesired phase transition to a monoclinic structure with large positive expansion can be suppressed in some solid solutions by substituting the A-site by a mixture of two cations. One such material, AlScMo3O12, was successfully synthesized using non-hydrolytic sol-gel chemistry. Depending on the reaction conditions, phase separation into Al2Mo3O12 and Sc2Mo3O12 or single-phase AlScMo3O12 could be obtained. Optimized conditions for the reproducible synthesis of stoichiometric, homogeneous AlScMo3O12 were established. High resolution synchrotron diffraction experiments were carried out to confirm whether samples were homogeneous and to estimate the Al:Sc ratio through Rietveld refinement and Vegard's law. Single-phase samples were found to adopt the orthorhombic Sc2W3O12 structure at 100 to 460 K. In contrast to all previously-reported A2M3O12 compositions, AlScMo3O12 exhibited positive thermal expansion along all unit cell axes instead of contraction along one or two axes, with expansion coefficients (200-460 K) of αa = 1.7 × 10-6 K-1, αb = 6.2 × 10-6 K-1, αc = 2.9 × 10-6 K-1 and αV = 10.8 × 10-6 K-1, respectively.

Two Decades of Negative Thermal Expansion Research: Where Do We Stand?

Negative thermal expansion (NTE) materials have become a rapidly growing area of research over the past two decades. The initial discovery of materials displaying NTE over a large temperature range, combined with elucidation of the mechanism behind this unusual property, was followed by predictions that these materials will find use in various applications through controlled thermal expansion composites. While some patents have been filed and devices built, a number of obstacles have prevented the widespread implementation of NTE materials to date. This paper reviews NTE materials that contract due to transverse atomic vibrations, their potential for use in controlled thermal expansion composites, and known problems that could interfere with such applications.

Autohydration of nanosized cubic zirconium tungstate.

Nanoparticulate zirconium tungstate prepared through hydrothermal methods was found to autohydrate under ambient conditions. This results in positive thermal expansion, limiting its usefulness for controlled thermal expansion composites. TEM and BET studies provided strong evidence that the cause of autohydration is a result of structural defects present in the nanoparticles, while kinetics are governed by surface area, suggesting that processing methods can be used to minimize or overcome this problem.

Facile synthesis of troilite.

Low-temperature synthetic pathways can result in crystallization of metastable materials. These methods have been widely explored for the preparation of metal oxides. Adaptation of nonhydrolytic sol-gel chemistry to non-oxide systems offers an elegant route to transition-metal sulfides. The method can be exploited for the facile and reproducible synthesis of iron sulfide crystallizing in the troilite structure. This phase is only found in meteorites and planets and has previously been obtained by high-temperature or high-energy ball-milling methods. "Nonhydrolytic" sol-gel processing results in direct crystallization of troilite with no need for further calcination.

The compositional and physicochemical homogeneity of male femoral cortex increases after the sixth decade.

The temporal and spatial fluctuations in the dynamics of secondary osteonal remodeling impart heterogeneity to the compositional quality of bone. Bone mineral density (BMD) fails to reflect this heterogeneity as being a single score, and thus it cannot resolve the overlap between healthy individuals and those who experience fractures. Such information on tissue heterogeneity is lacking in the literature. In the current study, specimens were prepared from mid-diaphyseal portions of human femora (N=16, age range 52-85 years old) and grouped based on the anatomical location (anterior, lateral, medial and posterior quadrants). Raman microscopy was used to obtain multiple measurements from each specimen which allowed the construction of histograms of mineralization, crystallinity and carbonation. The coefficient of variation (COV) and skewness were extracted from histograms as measures of heterogeneity. Results demonstrated that average mineralization of the medial quadrant and the data pooled over quadrants significantly increased with age. The mean carbonation increased within the observed age range for the pooled data. The variations of values about the mean became tighter for mineralization, crystallinity and type-B carbonation with age, indicating an overall reduction in compositional heterogeneity of aging femoral cortex. Skewness values indicated that the distributions of histograms were not Gaussian. We conclude that age-related changes in mean tissue composition are confounded with changes in the variation of tissue make-up about the mean. Future studies will establish as to whether compositional heterogeneity correlates with the mechanical strength of bone.

An addition to the oxoacid family: H2B12(OH)12.

The acid H(2)B(12)(OH)(12) can be isolated as a crystalline solid by protonation of the hydroxylated borane anion, B(12)(OH)(12)(2)(-). This acidic compound has low solubility in water, conducts protons in the solid state, and has thermal stability to a temperature of 400 degrees C. The conductivity mechanism is a Grotthuss mechanism with a low activation enthalpy (9-13 kcal/mol). This new acid represents an addition to the class of oxoacids, of which sulfuric and phosphoric acid are the most prominent examples.

Electrocatalytic activity of ordered intermetallic phases for fuel cell applications.

The electrocatalytic activities of a wide range of ordered intermetallic phases toward a variety of potential fuels have been studied, and results have been compared to those of a pure polycrystalline platinum (Pt(pc)) electrode. A significant number of the ordered intermetallic phases exhibited enhanced electrocatalytic activity when compared to that of Pt, in terms of both oxidation onset potential and current density. The PtBi, PtIn, and PtPb ordered intermetallic phases appeared to be the most promising electrocatalysts tested thus far for fuel cell applications. PtPb, in particular, showed an onset potential that was 100 mV less positive and a peak current density approximately 40 times higher than those observed for Pt in the case of methanol oxidation. The ability to control the geometric and electronic structures of the electrocatalytic material by using ordered intermetallic phases has been shown to be a promising direction of inquiry in the search for superior electrocatalysts for fuel cell applications.

Electrocatalytic oxidation of formic acid at an ordered intermetallic PtBi surface.

The electrocatalytic oxidation of formic acid at a PtBi ordered intermetallic electrode surface has been investigated using cyclic voltammetry, rotating disk electrode (RDE) voltammetry and differential electrochemical mass spectrometry (DEMS). The results are compared to those at a polycrystalline platinum electrode surface. The PtBi electrode exhibits superior properties when compared to polycrystalline platinum in terms of oxidation onset potential, current density, and a much diminished poisoning effect by CO. Using the RDE technique, a value of 1.4 x 10(-4) cm s-1 was obtained for the heterogeneous charge transfer rate constant. The PtBi surface did not appear to be poisoned when exposed to a CO saturated solution for periods exceeding 0.5 h. The results for PtBi are discussed within the framework of the dual-path mechanism for the electrocatalytic oxidation of formic acid, which involves formation of a reactive intermediate and a poisoning pathway.