Strontium and calcium zirconyl citrates as precursors for the low-temperature synthesis of SrZrO3 and CaZrO3 fine powders

Synthesis and the thermal decomposition behavior of new molecular precursors, strontium, and calcium zirconyl citrates are presented. The pathway to the metazirconate formation has been found to proceed through a multistep process. The precursors yield SrZrO3 and CaZrO3 fine powders at temperatures as low as 650 degrees C. Physico-chemical, spectroscopic, thermoanalytical, and microscopic techniques have enabled the identification of the sequence of events leading to the perovskite formation and proposition of a thermolysis scheme. Retention of the molecular level mixing of the metal ions during the course of the precursor decomposition is supported by these techniques. Prior to the formation of MZrO3 (M = Sr and Ca) an ionic oxycarbonate, M2Zr2O5CO3 (M = SI. and Ca), intermediate is produced by the thermal decomposition of the citrate precursors.

Structures of the catalytic site mutants D99A and H48Q and the calcium-loop mutant D49E of phospholipase A(2)

Crystal structures of the active-site mutants D99A and H48Q and the calcium-loop mutant D49E of bovine phospholipase A(2) have been determined at around 1.9 Angstrom resolution. The D99A mutant is isomorphous to the orthorhombic recombinant enzyme, space group P2(1)2(1)2(1), The H48Q and the calcium-loop mutant D49E are isomorphous to the trigonal recombinant enzyme, space group P3(1)21, The two active-site mutants show no major structural perturbations. The structural water is absent in D99A and, therefore, the hydrogen-bonding scheme is changed. In H48Q, the catalytic water is present and hydrogen bonded to Gln48 N, but the second water found in native His48 is absent. In the calcium-loop mutant D49E, the two water molecules forming the pentagonal bipyramid around calcium are absent and only one O atom of the Glu49 carboxylate group is coordinated to calcium, resulting in only four ligands.

Thermodynamic properties of the calcium ferrites CaFe2O4 and Ca2Fe2O5

The chemical potentials of CaO in the two-phase fields Fe2O3 + CaFe2O4 and CaFe2O4 + Ca2Fe2O5 of the pseudobinary system CaO - Fe2O3 have been measured in the temperature range from 975 to 1275 K, relative to pure CaO as the reference state, using solid state galvanic cells incorporating single-crystal CaF2 as the solid electrolyte. The cell was operated under pure oxygen at ambient pressure. The standard Gibbs energies of formation of calcium ferrites, CaFe2O4 and Ca2Fe2O5, were derived from the reversible emfs. The results can be summarized by the following equations:CaO + Fe2O3 --> CaFe2O4;Delta G degrees = - 37,480 + 1.16 T (+/- 250) J/mol 2 CaO + Fe2O3 --> Ca2Fe2O5;Delta G degrees = - 45, 280 - 13.51 T (+/- 275) J/mol These values are compared with thermodynamic data reported in the literature. The results of this study are in excellent agreement with heat capacity data, and in reasonable agreement with earlier measurements of enthalpy and Gibbs energy of formation, but suggest significant revision of enthalpies of formation of calcium ferrites given in current thermodynamic compilations.

Calcium-Decorated Carbyne Networks as Hydrogen Storage Media

Among the carbon allotropes, carbyne chains appear outstandingly accessible for sorption and very light. Hydrogen adsorption on calcium-decorated carbyne chain was studied using ab initio density functional calculations. The estimation of surface area of carbyne gives the value four times larger than that of graphene, which makes carbyne attractive as a storage scaffold medium. Furthermore, calculations show that a Ca-decorated carbyne can adsorb up to 6 H(2) molecules per Ca atom with a binding energy of similar to 0.2 eV, desirable for reversible storage, and the hydrogen storage capacity can exceed similar to 8 wt %. Unlike recently reported transition metal-decorated carbon nanostructures, which suffer from the metal clustering diminishing the storage capacity, the clustering of Ca atoms on carbyne is energetically unfavorable. Thermodynamics of adsorption of H(2) molecules on the Ca atom was also investigated using equilibrium grand partition function.

Calcium ruthenates: Determination of Gibbs energies of formation using electrochemical cells

Metallic Ru has been found to coexist separately with CaO, RuO2, and the interoxide phases, Ca2RuO4, Ca3Ru2O7, and CaRuO3, present along the pseudobinary system CaO-RuO2. The standard Gibbs energies of formation (Df((ox))G(o)) of the three calcium ruthenates from their component oxides have been measured in the temperature range 925-1350 K using solid-state cells with yttria-stabilized zirconia as the electrolyte and Ru+RuO2 as the reference electrode. The standard Gibbs energies of formation (Deltaf((ox))G(o)) of the compounds can be represented by Ca2RuO4:Deltaf((ox))G(o)/J mol(-1)=-38,340-6.611 T (+/-120), Ca3Ru2O7 : Df((ox))G(o)/J mol(-1)=-75,910-11.26 T (+/-180), and CaRuO3 : Deltaf((ox))G(o)/J mol(-1)=-35,480-3.844 T(+/-70). The data for Ca2RuO4 corresponds to the stoichiometric composition, which has an orthorhombic structure, space group Pbca, with short c axis ("S'' form). The structural features of the ternary oxides responsible for their mild entropy stabilization are discussed. A three-dimensional oxygen potential diagram for the system Ca-Ru-O is developed as a function of composition and temperature from the results obtained. Using the Neumann-Kopp rule to estimate the heat capacity of the ternary oxides relative to their constituent binary oxides, the standard enthalpies of formation of the three calcium ruthenates from the elements and their standard entropies at 298.15 K are evaluated. (C) 2003 The Electrochemical Society.

Efficient syntheses of benzothiazepines as antagonists for the mitochondrial sodium-calcium exchanger: Potential therapeutics for type II diabetes

Type II diabetes mellitus is a chronic metabolic disorder that can lead to serious cardiovascular, renal, neurologic, and retinal complications. While several drugs are currently prescribed to treat type II diabetes, their efficacy is limited by mechanism-related side effects (weight gain, hypoglycemia, gastrointestinal distress), inadequate efficacy for use as monotherapy, and the development of tolerance to the agents. Consequently, combination therapies are frequently employed to effectively regulate blood glucose levels. We have focused on the mitochondrial sodium-calcium exchanger (mNCE) as a novel target for diabetes drug discovery. We have proposed that inhibition of the mNCE can be used to regulate calcium flux across the mitochondrial membrane, thereby enhancing mitochondrial oxidative metabolism, which in turn enhances glucose-stimulated insulin secretion (GSIS) in the pancreatic beta-cell. In this paper, we report the facile synthesis of benzothiazepines and derivatives by S-alkylation using 2-aminobenzhydrols. The syntheses of other bicyclic analogues based on benzothiazepine, benzothiazecine, benzodiazecine, and benzodiazepine templates are also described. These compounds have been evaluated for their inhibition of mNCE activity, and the results from the structure-activity relationship (SAR) studies are discussed.

Physical chemistry of the reduction of calcium oxide with aluminium in vacuum

The physical chemistry of "aluminothermic" reduction of calcium oxide in vacuum is analyzed. Basic thermodynamic data required for the analysis have been generated by a variety of experiments. These include activity measurements in liquid AI-Ca alloys and determination of the Gibbs energies of formation of calcium aluminates. These data have been correlated with phase relations in the Ca-AI-0 system at 1373 K. The various stages of reduction, the end products and the corresponding equilibrium partial pressures of calcium have been established from thermodynamic considerations. In principle, the recovery of calcium can be improved by reducing the pressure in the reactor. However,, the cost of a high vacuum system and the enhanced time for reduction needed to achieve higher yields makes such a practice uneconomic. Aluminum contamination of calcium also increases at low pressures. The best compromise is to carry the reduction up to the stage where 3CaO-Al,O, is formed as the product. This corresponds to an equilibrium calcium partial pressure of 31.3 Pa at 1373 K and 91.6 Pa at 1460 K. Calcium can be extracted at this pressure using mechanical pumps in approximately 8 to 15 hr, depending on the size and the fill ratio of the retort and porosity of the charge briquettes.

Production of niobium powder by electronically mediated reaction (EMR) using calcium as a reductant

Explored in this study is an electronically mediated reaction (EMR) route for the production of niobium powder using calcium as a reductant for niobium oxide (Nb2O5). Feed material, Nb2O5, and reductant calcium alloy containing aluminum and nickel were charged into electronically isolated locations in a molten salt (e.g. CaCl2) at 1173 K. The current flow through an external path between the feed and reductant locations was monitored. A current approximately 0.4 A was measured during the reaction in the external circuit connecting cathode and anode location. Niobium powder with low aluminum and nickel content was obtained although liquid Ca–Al–Ni alloy was used as the reductant. This clearly demonstrates that niobium metal powder can be produced by an electronically mediated reaction (EMR), without direct physical contact between feed (Nb2O5) and reductant (calcium). Mechanism of calciothermic reduction of Nb2O5 in the molten salt is discussed using an isothermal chemical potential diagram.

Thermodynamic properties of calcium titanates: CaTiO3, Ca4Ti3O10, and Ca3Ti2O7

The chemical potentials of CaO in two-phase fields (TiO2 + CaTiO3), (CaTiO3 + Ca4Ti3O10), and (Ca4Ti3O10 + Ca3Ti2O7) of the pseudo-binary system (CaO + TiO2) have been measured in the temperature range (900 to 1250) K, relative to pure CaO as the reference state, using solid-state galvanic cells incorporating single crystal CaF2 as the solid electrolyte. The cells were operated under pure oxygen at ambient pressure. The standard Gibbs free energies of formation of calcium titanates, CaTiO3, Ca4Ti3O10, and Ca3Ti2O7, from their component binary oxides were derived from the reversible e.m.f.s. The results can be summarised by the following equations: CaO(solid) + TiO2(solid) → CaTiO3(solid), ΔG° ± 85/(J · mol−1) = −80,140 − 6.302(T/K); 4CaO(solid) + 3TiO2(solid) → Ca4Ti3O10(solid), ΔG° ± 275/(J · mol−1) = −243,473 − 25.758(T/K); 3CaO(solid) + 2TiO2(solid) → Ca3Ti2O7(solid), ΔG° ± 185/(J · mol−1) = −164,217 − 16.838(T/K). The reference state for solid TiO2 is the rutile form. The results of this study are in good agreement with thermodynamic data for CaTiO3 reported in the literature. For Ca4Ti3O10 Gibbs free energy of formation obtained in this study differs significantly from that reported by Taylor and Schmalzried at T = 873 K. For Ca3Ti2O7 experimental measurements are not available in the literature for direct comparison with the results obtained in this study. Nevertheless, the standard entropy for Ca3Ti2O7 at T = 298.15 K estimated from the results of this study using the Neumann–Koop rule is in fair agreement with the value obtained from low-temperature heat capacity measurements.

Colloidal calcium nanoparticles: digestive ripening in the presence of a capping agent and coalescence of particles under an electron beam

The nanochemistry of calcium remains unexplored, which is largely due to the inaccessibility of calcium nanoparticles in an easy to handle form by conventional methods of synthesis as well as its highly reactive and pyrophoric nature. The synthesis of colloidal Ca nanoparticles by the solvated metal atom dispersion (SMAD) method is described. The as-prepared Ca-THF nanoparticles, which are polydisperse, undergo digestive ripening in the presence of a capping agent, hexadecyl amine (HDA) to afford highly monodisperse colloids consisting of 2-3 nm sized Ca-HDA nanoparticles. These are quite stable towards precipitation for long periods of time, thereby providing access to the study of the nanochemistry of Ca. Particles synthesized in this manner were characterized by UV-visible spectroscopy, high resolution electron microscopy, and powder X-ray diffraction methods. Under an electron beam, two adjacent Ca nanoparticles undergo coalescence to form a larger particle.

Selective biodissolution of calcium and iron from bauxite in the presence of Bacillus polymyxa

A bacterium Bacillus polymyxa was found to be capable of selective removal of calcium and iron from bauxite. The bioleached residue was found to be enriched in its alumina content with insignificant amounts of iron and calcium as impurities. The developed bio- process was found to be capable of producing a bauxite product which meets the specifica- tions as a raw material for the manufacture of alumina based ceramics and refractories. The role of bacterial cells and metabolic products in the selective dissolution of calcium (present as calcite) and iron (present as hematite and goethite) from bauxite was assessed and possi- ble mechanisms illustrated. The effect of different parameters such as sucrose concentra- tion, pH, pulp density and time on selective biodissolution was studied. It was observed that periodic decantation and replenishment of the leach medium was beneficial in improving the dissolution kinetics. Calcium removal involves chelation with bacterial exopolysaccha- tides and acidolysis by organic acid generation. Hematite could be solubilized through a reductive dissolution mechanism.