Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 13 de 13
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
1.
Inorg Chem ; 2020 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-32806009

RESUMO

The potential of the perovskite system Nd1-xSrxCoO3-δ (x = 1/3 and 2/3) as cathode material for solid oxide fuel cells (SOFCs) has been investigated via detailed structural, electrical, and electrochemical characterization. The average structure of x = 1/3 is orthorhombic with a complex microstructure consisting of intergrown, adjacent, perpendicularly oriented domains. This orthorhombic symmetry remains throughout the temperature range 373-1073 K, as observed by neutron powder diffraction. A higher Sr content of x = 2/3 leads to stabilization of the cubic perovskite with a homogeneous microstructure and with a higher oxygen vacancy content and cobalt oxidation state than the orthorhombic phase at SOFC operation temperature. Both materials are p-type electronic conductors with high total conductivities of 690 and 1675 S·cm-1 at 473 K in air for x = 1/3 and 2/3, respectively. Under working conditions, both compounds exhibit similar electronic conductivities, since x = 2/3 loses more oxygen on heating than x = 1/3, associated with a greater loss of p-type charger carriers. However, composite cathodes prepared with Nd1/3Sr2/3CoO3-δ and Ce0.8Gd0.2O2-δ present lower ASR values (0.10 Ω·cm2 at 973 K in air) than composites prepared with Nd2/3Sr1/3CoO3-δ and Ce0.8Gd0.2O2-δ (0.34 Ω·cm2). The high activity for the oxygen electrochemical reaction at intermediate temperatures is likely attributable to a large disordered oxygen-vacancy concentration, resulting in a very promising SOFC cathode for real devices.

2.
Dalton Trans ; 48(36): 13651-13661, 2019 Sep 17.
Artigo em Inglês | MEDLINE | ID: mdl-31465070

RESUMO

The partial substitution of up to 5% Nd+3 by Ca+2 results in the oxide Nd1.90Ca0.10MgTiO5.94 that presents some remarkable structural features with a noticeable influence on its properties. In this oxide with a monoclinic perovskite-like structure and an octahedral tilting scheme (a-a-b+), both A- and B-ions are arranged in a rock-salt like manner, representing therefore the first example of a type of perovskite theoretically predicted. Besides this unprecedented arrangement of A- and B-ions, the oxygen vacancies created through doping with acceptor ions are trapped by association with the acceptor defects and hence the mobility of these vacancies is strongly limited. The oxygen conductivity of the substituted material is lower and the activation energy for oxygen motion is higher than those of the parent oxide, in which the concentration of anion vacancies is only due to intrinsic defects.

3.
Dalton Trans ; 44(16): 7643-53, 2015 Apr 28.
Artigo em Inglês | MEDLINE | ID: mdl-25811695

RESUMO

A solid solution forms for Sr3NdNb(3-x)Ti(x)O(12-δ) with approximate limits 0 ≤ x ≤ 0.06. The system crystallizes with a 12R-type hexagonal perovskite structure in the space group R3, as determined by neutron diffraction and selected area electron diffraction. The electrical properties of the end members have been investigated by impedance spectroscopy in the temperature range 550-800 °C under various gas atmospheres and as a function of oxygen and water-vapour partial pressure. Proton transport dominates under wet oxidising conditions in the temperature range 550-700 °C, as confirmed by the H(+)/D(+) isotope effect. Acceptor doping considerably enhances proton conductivity with a value of 3.3 × 10(-6) S cm(-1) for the bulk response of x = 0.06 at 700 °C in moistened air. The presence of a -» slope for both doped and undoped samples in the range 10(-19) ≤ pO2 ≤ 10(-8) atm at 900 °C indicates n-type transport under reducing conditions following the extrinsic model attributable to acceptor centres. The conductivity is essentially independent of pO2 at 600 °C under dry oxidising conditions, consistent with oxide-ion transport; a positive power-law dependence at higher temperature indicates extrinsic behaviour and a significant electron-hole contribution. The dielectric constant at RT of nominally stoichiometric Sr3NdNb3O12 is εr ∼ 37, with a moderately high quality factor of Q × f ∼ 16,400 GHz at fr ∼ 6.4 GHz. The temperature coefficient of resonant frequency of x = 0 is τf ∼ 12 ppm °C(-1), which lowers to -3 ppm °C(-1) for the Ti-doped phase x = 0.06.


Assuntos
Compostos de Cálcio/química , Óxidos/química , Titânio/química , Cristalografia por Raios X , Espectroscopia Dielétrica , Condutividade Elétrica , Microscopia Eletrônica de Transmissão , Conformação Molecular , Neodímio/química , Nióbio/química , Estrôncio/química , Temperatura
4.
Dalton Trans ; 44(8): 3801-10, 2015 Feb 28.
Artigo em Inglês | MEDLINE | ID: mdl-25609145

RESUMO

We report the rich magnetic behaviour of Sr2CoNb1-xTixO6 (0 ≤ x ≤ 0.5) oxides as a result of their complex microstructure. Although these oxides show an average simple-cubic perovskite structure, they present a flexible microstructure due to short-range ordering between Co/Ti and Nb cations in the perovskite B-sites. The microstructure consists of double-cubic perovskite domains grown in a simple-cubic perovskite matrix. The size and number of the double-cubic perovskite domains decrease as the Ti content increases. As a result of aliovalent substitution of Nb(5+) by Ti(4+) in the parent Sr2CoNbO6 mixed-valence Co(3+)/Co(4+) oxides are obtained. A spin glass-like state has been observed at low temperatures for all the series, with freezing temperatures increasing with the Ti-content in the range 22 to 33 K. Furthermore, the x = 0.3 and x = 0.5 samples show non-interacting superparamagnetic particle-like dynamics associated with relatively high amounts of Co(4+), with "blocking temperatures" of 13 and ∼16 K, respectively. The complex magnetic behaviour of the title oxides seems to be connected with the clustering of magnetic Co(3+) and the distribution of Co(4+) as a result of the microstructure.

5.
Microsc Microanal ; 20(3): 687-91, 2014 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-24576358

RESUMO

A solid solution of Nd 1-x Sr x CoO 3-δ (with x=0, 1/3, 2/3, and 1) has been prepared and characterized by a combination of X-ray diffraction, electron microscopy, and electron energy-loss spectroscopy (EELS). The structural characterization indicates that Nd-doped materials present an orthorhombic symmetry with a=√2xa p, b=√2xa p, and c=2xa p (a p refers to lattice parameter of simple cubic perovskite), while SrCoO2.5 has an orthorhombic symmetry with a=√2xa p, b=4xa p, and c=√2xa p. EELS analysis revealed that Co are in 3+ oxidation states but in different spin configurations.

6.
Inorg Chem ; 49(18): 8578-82, 2010 Sep 20.
Artigo em Inglês | MEDLINE | ID: mdl-20731409

RESUMO

A new ternary manganese vanadate, NaMnVO(4), was synthesized by solid state reaction route, and its crystal structure and magnetic properties were characterized by X-ray diffraction, magnetic susceptibility and specific heat measurements, and by density functional calculations. NaMnVO(4) crystallizes in the maricite-type structure with space group Pnma, a = 9.563(1) A, b = 6.882(1) A, c = 5.316(1) A, and Z = 4. NaMnVO(4) contains MnO(4) chains made up of edge-sharing MnO(6) octahedra, and these chains are interlinked by VO(4) tetrahedra. The magnetic susceptibility has a broad maximum at T(max) = 24 K and follows the Curie-Weiss behavior above 70 K with θ = -62 K. NaMnVO(4) undergoes a three-dimensional antiferromagnetic ordering at T(N) = 11.8 K. The spin exchanges of NaMnVO(4) are dominated by the intrachain antiferromagnetic exchange, and the interchain spin exchanges are spin-frustrated. The most probable magnetic structure of the ordered magnetic state below T(N) was predicted on the basis of the extracted spin exchanges.

7.
Inorg Chem ; 46(13): 5390-7, 2007 Jun 25.
Artigo em Inglês | MEDLINE | ID: mdl-17539630

RESUMO

The structural characterization of NaNbWO(6), prepared by the ceramic route, has been performed. Electron diffraction has shown the presence of two related phases in a 1:1 ratio, whose lattice parameters correspond to those of the well-known tetragonal tungsten bronzes (TTB) and those of a monoclinically distorted phase. In addition to basic unit cells, the morphology of the two phases has been found to be similar, but they present a slight difference in the W/Nb ratio. (1)H and (23)Na magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectra of NaNbWO(6) and its proton-exchanged derivatives have been interpreted on the basis of the ideal TTB structure. The average structure and the morphology remain unchanged in Na(1-x)H(x)NbWO(6) derivatives. (1)H and (23)Na MAS-NMR spectroscopies have been used to monitor changes produced during exchange processes. It has been shown that the exchange of Na ions is mainly produced, but not exclusively, at tetragonal channels. However, a large amount of Na ions at the pentagonal channels do not exchange with protons, suggesting that these ions are needed to stabilize the TTB-like structure. A tentative distribution of sodium ions in the most-exchanged oxide, deduced from NMR results, approximately (Na(0.46))(p)(Na(0.08))(s)H(0.46)NbWO(6), has been proposed. NMR spectra of Na(1-x)H(x)NbWO(6) indicate that two different OH groups are formed upon exchanging. The study of samples hydrated with D(2)O allowed us to conclude that deuterons of adsorbed water exchange with protons of the two OH groups. The proton-deuteron exchange is slow at room temperature but is strongly enhanced at 90 degrees C. This observation relates to the proton conductivity displayed by exchanged products under a humid atmosphere.

8.
Ultramicroscopy ; 107(6-7): 445-52, 2007.
Artigo em Inglês | MEDLINE | ID: mdl-17254714

RESUMO

Two complex perovskite-related structures were solved by ab initio from precession electron diffraction intensities. Structure models were firstly derived from HREM images and than have been confirmed independently using two and three-dimensional sets of precession intensities. Patterson techniques prove to be effective for ab initio structure resolution, specially in case of projections with no overlapping atoms. Quality of precession intensity data may be suitable enough to resolve unknown heavy oxide structures.

9.
Chemistry ; 13(3): 910-5, 2007.
Artigo em Inglês | MEDLINE | ID: mdl-17048285

RESUMO

Two new oxides of the Ruddlesden-Popper series have been isolated and structurally characterized in the Sr-Co-Ta-O system. X-ray and electron diffraction and high-resolution electron microscopy show that polycrystalline Sr(3)CoTaO(7) constitutes the n=2 member of a new Sr(n+1)(CoTa)(n)O(3n+1) homologous series, the essential feature of which is the existence of two connected Co/Ta octahedral layers, separated by Sr atoms. Sr(2)CoTaO(6), the n=infinity member of the series, shows a particular short-range ordering of Co and Ta at the octahedral sites leading, as shown by high-resolution electron microscopy, to the disordered intergrowth of simple and double perovskite type domains. Strategies to stabilize new oxides of this series are discussed.

10.
Chem Commun (Camb) ; (12): 1356-7, 2004 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-15179462

RESUMO

The pore topology of ITQ-15 zeolite consists of an ultra-large 14-ring channel that is intersected perpendicularly by a 12-ring pore; acid sites have been introduced in its framework and this unique structure shows advantages over unidirectional ultralarge pore zeolites for diffusing and reacting large molecules.

11.
J Electron Microsc (Tokyo) ; 52(1): 41-7, 2003.
Artigo em Inglês | MEDLINE | ID: mdl-12741487

RESUMO

The synthesis and microstructural characterization, by means of selected-area electron diffraction and high-resolution electron microscopy, of Ba7Rh6O18 is reported. This material, isostructural to Ba7Co6O18, is formed by rows of one trigonal prism and five face-sharing octahedra running parallel to the c-axis of a trigonal unit cell with parameters a = 1.004(5) and c = 3.165(7) nm. The substitution of Sr by Ba is accommodated by means of twin formation due to a rearrangement of the rows of polyhedra.

12.
Chemistry ; 8(21): 4973-9, 2002 Nov 04.
Artigo em Inglês | MEDLINE | ID: mdl-12487134

RESUMO

The (alpha =3, beta =2) member of the (A3ABO6) (A3B3O9) homologous series has been stabilised in the Sr-Rh-O system for a [Sr10(Sr0.5Rh1.5)TP(Rh6)Oh]O24 composition. The structural characterisation has been performed by powder X-ray and electron diffraction measurements and high-resolution electron microscopy. In this structure, three face-sharing [RhO6] octahedra linked by one [Rh/SrO6] trigonal prism comprise the infinite one-dimensional chain that runs parallel to the c axis of a trigonal unit cell (Pc1), with parameters a=9.6411(1) and c=21.2440(4) A.

13.
Chemistry ; 8(24): 5694-700, 2002 Dec 16.
Artigo em Inglês | MEDLINE | ID: mdl-12693051

RESUMO

A new perovskite-related oxide with the LaBaCuCoO5.2 composition has been stabilised. Its structure can be described as formed by the recurrent intergrowth of two alternating blocks of YBaCuFeO5 (2ac, i.e., two-fold perovskite superlattice) and YBa2Fe3O8 (3ac) structural types. From the starting material LaBaCuCoO5.2-delta (delta = 0), the rigorous control of the oxygen content has allowed the stabilisation of three new five fold perovskite-related superstructures with the compositions delta = 0.4, 0.8 and 1.1, which can also be described as recurrent intergrowths of two blocks showing 2ac and 3ac periodicity. The reduction process takes place through the 3ac periodic blocks, when 0 < delta < 0.8. Further oxygen decrease seems to involve the 2ac periodic blocks made up of pyramidal layers, giving rise to infinite layer units. The stability limit for these fivefold superstructures is delta = 1.1. In agreement with this the as-synthesised materials constitute an example of topotactic reaction, since their basic structure is kept through the reduction process.

SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA