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Now showing items 113 - 128 of 6688

  • First-Principles Characterization of Amorphous Phases of MB(12)H(12), M = Mg, Ca

    Kulkarni, Anant D.   Wang, Lin-Lin   Johnson, Duane D.   Sholl, David S.   Johnson, J. Karl  

    Complex borohydrides have been studied as candidate materials for vehicular hydrogen storage. In particular, M(BH(4))(2), where M = Mg or Ca, has received considerable attention because of its high gravimetric capacity for H(2). Experimental observations indicate an initial decomposition reaction: 6M(BH(4))(2) -> 5MH(2) + MB(12)H(12) + 13H(2), where the MB(12)H(12) species are amorphous. We use first-principles density functional theory to study the structural and energetic properties of MB(12)H(12). The purpose of these calculations is to investigate why these materials form amorphous structures. We have identified two possible reasons for MB(12)H(12) adopting apparently amorphous rather than crystalline structures. First, our calculations reveal a large number of structurally distinct polymorphs having near-degenerate energies. We simulate an X-ray diffraction (XRD) pattern for each polymorph and compute the Boltzmann-averaged XRD patterns for MgB(12)H(12), and CaB(12)H(12). These average patterns appear to be amorphous, having very broad peaks. Therefore, we predict that the amorphous MB(12)H(12) materials observed experimentally may be an intimate mixture of a very large number of structurally distinct crystallites. Second, our first-principles molecular dynamics calculations indicate that cations are highly mobile near room temperature; analysis of cation radial distribution functions gives liquid-like rather than solid-like features that could also result in an experimental observation of an amorphous XRD pattern near room temperature.
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  • Luminescence of Pb(2+) in MAl(2)B(2)O(7) (M = Ca, Sr)

    Pekgozlu, Ilhan   Tascioglu, Suelin   Menger, Ayhan  

    Pb(2+) doped SrAl(2)B(2)O(7) was prepared by solution combustion synthesis. The synthesized material was determined by powder XRD. The photoluminescence properties of the synthesized phosphor were investigated at room temperature using a spectrofluorometer. The emission peak of Pb(2+) doped SrAl(2)B(2)O(7) was observed at 420 nm upon excitation at 277 nm. The Stokes shift of SrAl(2)B(2)O(7):Pb(2+) was calculated to be 12 292 cm(-1). The luminescence behavior of Pb(2+) in both SrAl(2)B(2)O(7) and CaAl(2)B(2)O(7) was discussed.
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  • The Non-Centrosymmetric Borate Oxides, MBi2B2O7 (M: Ca, Sr).

    J. Barbier; L. M. D. Cranswick  

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  • The Non-Centrosymmetric Borate Oxides, MBi2B2O7 (M: Ca, Sr).

    J. Barbier   L. M. D. Cranswick  

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  • Sb NQR in filled skutterudites MFe4Sb12 (M=Na, Ca, La)

    A.A. Gippius   E.N. Morozova   K.S. Okhotnikov   E.A. Alkaev   A.V. Shevelkov   M. Baenitz   A. Leithe-Jasper   H. Rosner   R. Viennois   Yu. Grin   F. Steglich  

    Temperature-dependent nuclear quadrupole resonance (NQR) investigations on the 121,123Sb nuclei in filled skutterudites MFe 4Sb 12 (M=Na, Ca, La) are reported. These new compounds show a nice interplay of ferromagnetic and antiferromagnetic correlations best seen in the relaxation rate. Substructures observed in the 121,123Sb NQR spectra give strong evidence for static (or dynamic) disorder on the filler site. Therefore, we performed ab-initio linearized augmented plane wave (LAPW) calculations for the Sb NQR substructure frequencies for various La and Ca displacement along different direction and compared the obtained values with the experimental data. [All rights reserved Elsevier]
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  • Nitrous Oxide Decomposition Over MCO(3)-Co(3)O(4) (M = Ca, Sr, Ba) Catalysts

    Abu-Zied, B. M.   Soliman, S. A.  

    In this paper, nitrous oxide decomposition over a series of MCO(3)-Co(3)O(4) (M = Ca, Sr, Ba) catalysts having M/Co ratios of 0.1-0.4 has been studied. The various catalysts were characterized using thermal (TGA, DTA), XRD, IR and N(2) sorption techniques. N(2)O decomposition activity was found to be dependent on the type of the alkaline earth cation, the M/Co ratio, cobalt oxide crystallites sizes, and the calcination temperature.
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  • High thermoelectric figure of merit in nanostructured p-type PbTe–MTe (M = Ca, Ba)

    Biswas, Kanishka   He, Jiaqing   Wang, Guoyu   Lo, Shih-Han   Uher, Ctirad   Dravid, Vinayak P.   Kanatzidis, Mercouri G.  

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  • Spectral analysis of Eu3+:B2O3–Al2O3–MF2 (M = Zn, Ca, Pb) glasses

    P. Abdul Azeem   M. Kalidasan   K. Rama Gopal   R.R. Reddy  

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  • Thermoelectric Properties of CuAl1?xMxO2 (M=Zn, Ca)

    Kurotori, Takuya   Sugihara, Sunao  

    Substitution was performed with Ca 2+ and Zn 2+ in the Al 3+ site of CuAlO 2. We investigated the influence of substitution on thermoelectric properties. Power factors were the highest near 850 K for the samples of CuAl 0.999Zn 0.001O 2 (6.8times10 -5 Wm -1K -2). CuAl 0.999Ca 0.001 O 2 (7.4times10 -5 Wm -1K -2 ). CuAlO 2 (6.6times10 -5 Wm -1K -2)
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  • Theoretical investigations of the chemical bonding in MM'O-2 clusters (M,M' =3D Be,Mg,Ca)

    Ponec, Robert   Cooper, David L.  

    Motivated by the known stability of the somewhat unusual Be2O2 rhombus, which features a short Be-Be distance but no direct metal-metal bonding, we investigate the nature of the bonding interactions in the analogous clusters MM'O-2 (M, M' =3D Be, Mg, Ca). CCSD/cc-pVTZ and CCSD(T)/cc-pVQZ calculations, amongst others, are used to determine optimized geometries and the dissociation energies for splitting the MM'O-2 clusters into metal oxide monomers. The primary tools used to investigate the chemical bonding are the analysis of domain-averaged Fermi holes, including the generation of localized natural orbitals, and the calculation of appropriate two- and three-center bond indices. Insights emerging from these various analyses concur with earlier studies of M2O2 rhombic clusters in that direct metal-metal bonding was not observed in the MM'O-2 rings whereas weak three-center (3c) bonding was detected in the MOM' moieties. In general terms, these mixed MM'O-2 clusters exhibit features that are intermediate between those of M2O2 and M'O-2(2), and the differences between the M and M' atoms appear to have little impact on the overall degree of 3c MOM' bonding.
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  • MgM (M = Al and Ca) oxides as basic catalysts in transesterification processes

    Albuquerque, Monica C. G.   Santamaria-Gonzalez, Jose   Merida-Robles, Josefa M.   Moreno-Tost, Ramon   Rodriguez-Castellon, Enrique   Jimenez-Lopez, Antonio   Azevedo, Diana C. S.   Cavalcante, Celio L., Jr.   Maireles-Torres, Pedro  

    This work investigates the use of MgAl and MgCa oxides as catalysts for transesterification processes, in order to evaluate their potential as heterogeneous catalysts for biodiesel production. The catalysts were characterized by XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), SEM (Scanning electron microscopy), CO2-TPD and N-2 adsorption. MgCa oxides were more active than MgAl in the transesterification of ethyl butyrate with methanol at 60 degrees C and atmospheric pressure. The highest activity was found for a bulk Mg:Ca molar ratio of 3.8, with a conversion of 45%, whereas MgO was inactive. Moreover, lixiviation of the active phase was not observed, thus excluding the contribution of the homogeneous catalysis to the studied transesterification process. in the methanolysis of sunflower oil, the activity follows the same tendency observed in the transesterification of ethyl butyrate with methanol. A FAME (fatty acid methyl esters) yield of 92% has been reached with a methanol:oil molar ratio of 12, a reaction temperature of 333 K and a 2.5 wt.% of the Mg:Ca(3.8) catalyst. (C) 2008 Elsevier B.V. All rights reserved.
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  • Photoluminescence properties of M2MgSi2O7: Re2+(M=Ba, Sr, Ca)

    Dawei He   Yanning Shi   Dan Zhou   Tao Hou  

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  • Approximants in the Ag-In-M and Au-Sn-M (M = Ca or Rare Earth Metals) Systems

    Morita, Yoshiki   Tsai, An Pang  

    Formation of 1/1 Yb-Cd type approximants has been verified in the Ag-In-RE (RE: rare earth metals) system for RE = Tm, Tb, Er, Dy, Gd, Pr, Cc, or Eu. The compositions of the 1/1 approximants determined by wavelength dispersive X-ray analysis (WDX), are in the range of 42-48 at. % Ag, 38-42 at. % In, and 14-15 at. % REs. By employing the empirical rule of e/a similar to 2.0, where e/a is the valence electron concentration, a number of approximants are newly found in the Au-Sn-RE systems for RE = Sin, Pr, Cc, Dy, Tb, Gd, or Eu, where the 1/1 approximants are formed in the range of 61-64 at. % Au, 20-23 at. % Sri, and 14-15 at. % REs. The 2/1 approximants only form in Ag(43)In(43)Eu(14) and Au(60)Sn(25)M(15) for M = Eu, Yb, or Ca, where the atomic size for Eu, Yb, and Ca is larger than that for the other REs and they are all divalent. Basically, the 2/1 and 1/1 approximants share the same e/a as the quasicrystals found in the In-Ag-M, Au-Sn-M, and Cd-M groups. It is likely that the phase selection among the quasicrystalline and the 1/1 and 2/1 approximant phases is dominated by the effective atomic size factor, R(r,e). [DOI: 10.1143/JJAP.47.7975]
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  • Theoretical investigations of the chemical bonding in MM′O2 clusters (M, M′?=?Be, Mg, Ca)

    Ponec Robert   Cooper David L.  

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  • Thermoelectric Property of Ca(Mn1-x,Mx)O3 (M=Ta, Mo, Nb and Zn)

    Horiguchi, Ken-ichi; Teduka, Yasuo; Sugihara, Sunao  

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  • SbNQR in filled skutterudites MFe4Sb12 (M = Na, Ca, La) RID D-1139-2010

    Gippius, AA   Morozova, EN   Okhotnikov, KS   Alkaev, EA   Shevelkov, AV   Baenitz, M   Leithe-Jasper, A   Rosner, H   Viennois, R   Grin, Y   Steglich, F  

    Temperature-dependent nuclear quadrupole resonance (NQR) investigations on the Sb-121.123 nuclei in filled skutterudites MFe4Sb12 (M = Na, Ca, La) are reported. These new compounds show a nice interplay of ferromagnetic and antiferrornagnetic correlations best seen in the relaxation rate. Substructures observed in the Sb-121.123 NQR spectra give strong evidence for static (or dynamic) disorder on the filler site. Therefore, we performed ab-initio linearized augmented plane wave (LAPW) calculations for the Sb NQR Substructure frequencies for various La and Ca displacement along different direction and compared the obtained values with the experimental data. (c) 2006 Elsevier B.V. All rights reserved.
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