992 resultados para Sr^2
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Grain size has marked effects on charge-ordering and other properties of Nd(0.5)A(0.5)MnO(3) (A=Ca or Sr). Thus, the anti-ferromagnetic (AFM) transition in Nd0.5Ca0.5MnO3 is observed distinctly only in samples sintered at 1273 K or higher. The sample with a small grain size (sintered at 1173 K) shows evidence for greater ferromagnetic (FM) interaction at low temperatures, probably due to phase segregation. The FM transition as well as the charge-ordering transition in Nd0.5Sr0.5MnO3 becomes sharper in samples sintered at 1273 K or higher. The sample sintered at 1173 K does not show the AFM-CO transition around 150 K and is FM down to low temperatures; the apparent T-c-T-co gap decreases with the increase in the grain size. The samples sintered at lower temperatures (<1673 K) show evidence for greater segregation of the AFM and FM domains. (C) 2002 Elsevier Science Ltd. All rights reserved.
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We investigate the electronic structure of Ca1-xSrxVO3 using photoemission spectroscopy. Core level spectra establish an electronic phase separation at the surface, leading to a distinctly different surface electronic structure compared to the bulk. Analysis of the photoemission spectra of this system allowed us to separate the surface and bulk contributions. These results help us to understand properties related to two vastly differing energy scales, namely the low-energy scale of thermal excitations ( $\sim\!k_{\rm B}T$) and the high-energy scale related to Coulomb and other electronic interactions.
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We present the magnetic properties of polycrystalline Dy1xSrxMnO3 (0.1 x 0.4) with an orthorhombic (o) crystal structure. The parent compound, o-DyMnO3, undergoes an incommensurate antiferromagnetic ordering of the Mn spins at 39 K, followed by a spiral order at 18 K. A further antiferromagnetic transition at 5 K marks an ordering of the Dy-sublattice. Doping of divalent Sr ions results in diverse magnetization phenomena. The zero-field cooled (ZFC) and field cooled (FC) magnetization curves display the presence of strongly interacting magnetic sublattices. For x = 0.1 and 0.2, a bifurcation between the ZFC and FC magnetization sets in at around 30 and 32 K, respectively. The ZFC magnetization peaks at about 5 K, indicating antiferromagnetic Dy-couplings similar to the case of o-DyMnO3. For x = 0.3, clear signatures of ferrimagnetism and strong anisotropy are found, including negative magnetization. The compound with x = 0.4 behaves as a spin glass, similar to Dy0.5Sr0.5MnO3.
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Homogeneous thin films of Sr(0.6)Ca(0.4)TiO(3) (SCT40) and asymmetric multilayer of SrTiO(3) (STO) and CaTiO(3) (CTO) were fabricated on Pt/Ti/SiO(2)/Si substrates by using pulsed laser deposition technique. The electrical behavior of films was observed within a temperature range of 153 K-373 K. A feeble dielectric peak of SCT40 thin film at 273 K is justified as paraelectric to antiferroelectric phase transition. Moreover, the Curie-Weiss temperature, determined from the epsilon'(T) data above the transition temperature is found to be negative. Using Landau theory, the negative Curie-Weiss temperature is interpreted in terms of an antiferroelectric transition. The asymmetric multilayer exhibits a broad dielectric peak at 273 K. and is attributed to interdiffusion at several interfaces of multilayer. The average dielectric constants for homogeneous Sr(0.6)Ca(0.4)TiO(3) films (similar to 650) and asymmetric multilayered films (similar to 350) at room temperature are recognized as a consequence of grain size effect. Small frequency dispersion in the real part of the dielectric constants and relatively low dielectric losses for both cases ensure high quality of the films applicable for next generation integrated devices. (C) 2011 Elsevier B.V. All rights reserved.
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A new series of inorganic-organic hybrid framework compounds, Ln(2)(mu(3)-OH)(C4H4O5)(2)(C4H2O4)]center dot 2H(2)O, (Ln = Ce, Pr and Nd), have been prepared employing a hydrothermal method. Malic acid and fumaric acid form part of the structure. The malate units connect the lanthanide centers forming Ln-O-Ln two-dimensional layers, which are cross-linked by the fumarate units forming the three-dimensional structure. Extra framework water molecules form a dimer and occupy the channels. The water molecules can be reversibly adsorbed. The dehydrated structure did not show any differences in framework structure/ connectivity. The presence of lattice water provides a pathway for proton conductivity. Optical studies suggest an up-conversion behavior involving more than one photon for a neodymium compound.
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SrCrxFe12-xO19 (x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9) hexaferrites were prepared by a microwave-hydrothermal method and subsequently sintered at 950 degrees C for 90 min using the microwave sintering method. The results show that, with increasing Cr3+ content, the lattice parameters changed anisotropically. The average grain sizes of sintered samples were in the range of 280 nm to 660 nm. The saturation magnetization systematically decreased with increasing Cr3+ doping, but the coercivity values increased. The electrical resistivity (log rho) decreased linearly with increasing temperature up to a certain temperature known as the transition temperature (T-c), and T-c decreased with further increase (x>0.5) of the Cr3+ content. This decrease in log rho and the activation energy (E-g) is due to electron hopping and occupancy of doped ions at different lattice sites. We found that the dielectric constant and dielectric loss for all the samples decreased with the Cr3+ content. The structural, magnetic, and electrical properties of Cr3+-doped SrFe12O19 hexaferrites have thus been investigated.
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The electronic structure of the (La0.8Sr0.2)(0.98)Mn1-xCrxO3 model series (x = 0, 0.05, or 0.1) was measured using soft X-ray synchrotron radiation at room and elevated temperature. O K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra showed that low-level chromium substitution of (La, Sr)MnO3 resulted in lowered hybridisation between O 2p orbitals and M 3d and M 4sp valance orbitals. Mn L-3-edge resonant photoemission spectroscopy measurements indicated lowered Mn 3d-O 2p hybridisation with chromium substitution. Deconvolution of O K-edge NEXAFS spectra took into account the effects of exchange and crystal field splitting and included a novel approach whereby the pre-peak region was described using the nominally filled t(2g) up arrow state. 10% chromium substitution resulted in a 0.17 eV lowering in the energy of the t(2g) up arrow state, which appears to provide an explanation for the 0.15 eV rise in activation energy for the oxygen reduction reaction, while decreased overlap between hybrid O 2p-Mn 3d states was in qualitative agreement with lowered electronic conductivity. An orbital-level understanding of the thermodynamically predicted solid oxide fuel cell cathode poisoning mechanism involving low-level chromium substitution on the B-site of (La, Sr)MnO3 is presented. (C) 2015 AIP Publishing LLC.
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Obra atribuda a Quintino Bocaiva, poltico brasileiro, adepto das idias republicanas, membro preeminente da conspirao que deps o ltimo gabinete do imprio. A rendio de Uruguaiana um episdio da Guerra do Paraguai ocorrido em 18/10/1865, no qual a localidade de Uruguaiana ocupada pelo exrcito paraguaio e retomada pouco depois. Inclui transcries de vrios documentos oficiais (despachos, notas, ofcios), do Ministrio de Relaes Exteriores do Paraguai e da Legao Imperial do Brasil, assinados por Jos Berges, Cesar Sauvan Vianna de Lima e Carlos Caneiro de Campos.
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A rea estudada est inserida na Faixa Ribeira, Segmento Central da Provncia Mantiqueira (Almeida et al., 1973, 1977, 1981), que representa um cinturo de dobramentos e empurres gerado no Neo-proterozico/Cambriano, durante a Orognese Brasiliana, na borda sul/sudeste do Crton do So Francisco (Almeida, 1971, 1977; Cordani et al., 1967, 1973; Cordani & Brito Neves, 1982; Teixeira & Figueiredo, 1991). Neste contexto, o Complexo Quirino o embasamento retrabalhado do Terreno Paraba do Sul (Heilbron et al., 2004). O Complexo Quirino formado por extensos corpos de ortognaisses foliados a homogneos, leuco a mesocrticos, de granulometria mdia grossa, composicionalmente variando entre granitides tonalticos/granodiorticos a granticos, e apresentando enclaves de rochas ultramficas, mficas e clcio-silicticas (ricas em tremolita). Os ortognaisses tonalticos/granodiorticos apresentam porfiroblastos de plagioclsio e a hornblenda como mfico principal, contrastando com os de composio grantica que apresentam porfiroblastos de K-feldspato e biotita predominante. Como acessrios aparecem zirco, titanita, apatita e epidoto. Tambm esto associados a estes ortognaisses, granitides neoproterozicos que formam corpos individualizados ou lentes anatticas no conjunto paleoproterozico. Estes so compostos predominantemente por biotita gnaisse e hornblenda-biotita gnaisse. A anlise litogeoqumicas dos ortognaisses do Complexo Quirino demonstrou a existncia de duas sries magmticas distintas. A primeira pertencente srie clcio-alcalina de alto-K apresenta uma composio mais expandida grantica-adameltica/granodioritica/tonaltica e correlacionvel aos bt-ortognaisses e alguns hb-bt-ortognaisses. Os ortognaisses da srie mdio-K apresentam composio predominantemente tonaltica, sendo correlacionveis maioria dos hornblenda-biotita gnaisses. Enclaves lenticulares de metapiroxenticos e anfibolticos ocorrem em muitos afloramentos. Tambm ocorrem granitides neoproterozicos de composio granticas a quartzo-monzonticas O estudo isotpico de Sm-Nd e Sr demonstrou que os ortognaisses da srie clcio-alcalina de alto-K e aqueles da srie clcio-alcalina de mdio-K possuem idades modelo TDM variando entre paleoproterozicas a arqueanas, consistentes com dados U-Pb em zirco publicados na literatura. A srie clcio-alcalina de alto-K mais antiga (2308 9,2 Ma a 2185 8 Ma) do que a srie calcio-alcalina de mdio-K (2169 3 a 2136 14 Ma) e a existncia de zirces herdados com idades mnimas de 2846 Ma e 2981 Ma para srie de mdio-K e 3388 16 para srie de alto-K. Os granitides brasilianos possuem idades de cristalizao neoproterozica correlacionada a Orognese Brasiliana (602 a 627 Ma) (Viana, 2008; Valladares et al., 2002)./Com base nos dados de Sr e Sm-Nd foi possvel caracterizar 4 grupos distintos. Os grupos 1 e 2 so formados por rochas de idade paleoproterozica (2,1 a 2,3 Ga) com idades modelo TDM variando de 2,9 e 3,4 Ga, εNd entre -8,1 e -5,8 e 87Sr/86Sr(t) = 0,694707 (Grupo 1) e TDM variando de 2,5 a 2,7 Ga, εNd entre -5,8 e -3,1 e 87Sr/86Sr(t) = 0,680824 (Grupo 2), formados no paleoproterozico com contribuio de uma crosta arqueana. O grupo 3 formado por rochas juvenis de idade paleoproterozica, com idades de cristalizao variando entre 2,0 e 2,2 Ga e com idades modelo TDM variando de 2,1 a 2,2 Ga e εNd entre + 1,5 e + 1,2. O grupo 4 formado durante o neoproterozico (645 Ma) por rochas possivelmente de idade paleoproterozico com idades modelo TDM igual a 1,7 Ga e εNd igual a -8,3.
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<p>The anisotropy of 1.3 - 2.3 MeV protons in interplanetary space has been measured using the Caltech Electron/Isotope Spectrometer aboard IMP-7 for 317 6-hour periods from 72/273 to 74/2. Periods dominated by prompt solar particle events are not included. The convective and diffusive anisotropies are determined from the observed anisotropy using concurrent solar wind speed measurements and observed energy spectra. The diffusive flow of particles is found to be typically toward the sun, indicating a positive radial gradient in the particle density. This anisotropy is inconsistent with previously proposed sources of low-energy proton increases seen at 1 AU which involve continual solar acceleration.</p> <p>The typical properties of this new component of low-energy cosmic rays have been determine d for this period which is near solar minimum. The particles have a median intensity of 0.06 protons/ cm^(2)-sec-sr-MeV and a mean spectral index of -3.15.The amplitude of the diffusive anisotropy is approximately proportional to the solar wind speed. The rate at which particles are diffusing toward the sun is larger than the rate at which the solar wind is convecting the particles away from the sun. The 20 to 1 proton to alpha ratio typical of this new component has been reported by Mewaldt, et al. (1975b).</p> <p>A propagation model with _(rr) assumed independent of radius and energy is used to show that the anisotropy could be due to increases similar to those found by McDonald, et al. (1975) at ~3 AU. The interplanetary Fermi-acceleration model proposed by Fisk (1976) to explain the increases seen near 3 AU is not consistent with the ~12 per cent diffusive anisotropy found.</p> <p> The dependence of the diffusive anisotropy on various parameters is shown. A strong dependence of the direction of the diffusive anisotropy on the concurrently measured magnetic field direction is found, indicating a _ less than _ to be typical for this large data set.</p>
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<p>The cross sections for the two antiproton-proton annihilation-in-flight modes,</p> <p>p + p <sup>+</sup> + <sup>-</sup></p> <p>p + p k<sup>+</sup> + k<sup>-</sup></p> <p>were measured for fifteen laboratory antiproton beam momenta ranging from 0.72 to 2.62 GeV/c. No magnets were used to determine the charges in the final state. As a result, the angular distributions were obtained in the form [d/d (<sub>C.M.</sub>) + d/d ( <sub>C.M.</sub>)] for 45 <sub>C.M.</sub> 135. </p> <p>A hodoscope-counter system was used to discriminate against events with final states having more than two particles and antiproton-proton elastic scattering events. One spark chamber was used to record the track of each of the two charged final particles. A total of about 40,000 pictures were taken. The events were analyzed by measuring the laboratory angle of the track in each chamber. The value of the square of the mass of the final particles was calculated for each event assuming the reaction</p> <p>p + p a pair of particles with equal masses.</p> <p>About 20,000 events were found to be either annihilation into <sup></sup>-pair or k <sup></sup>-pair events. The two different charged meson pair modes were also distinctly separated.</p> <p>The average differential cross section of p + p <sup>+</sup> + <sup>-</sup> varied from ~ 25 b/sr at antiproton beam momentum 0.72 GeV/c (total energy in center-of-mass system, s = 2.0 GeV) to ~ 2 b/sr at beam momentum 2.62 GeV/c (s = 2.64 GeV). The most striking feature in the angular distribution was a peak at <sub>C.M.</sub> = 90 (cos <sub>C.M.</sub> = 0) which increased with s and reached a maximum at s ~ 2.1 GeV (beam momentum ~ 1.1 GeV/c). Then it diminished and seemed to disappear completely at s ~ 2.5 GeV (beam momentum ~ 2.13 GeV/c). A valley in the angular distribution occurred at cos <sub>C.M.</sub> 0.4. The differential cross section then increased as cos <sub>C.M.</sub> approached 1.</p> <p>The average differential cross section for p + p k<sup>+</sup> + k<sup>-</sup> was about one third of that of the <sup></sup>-pair mode throughout the energy range of this experiment. At the lower energies, the angular distribution, unlike that of the <sup></sup>-pair mode, was quite isotropic. However, a peak at <sub>C.M.</sub> = 90 seemed to develop at s ~ 2.37 GeV (antiproton beam momentum ~ 1.82 GeV/c). No observable change was seen at that energy in the <sup></sup>-pair cross section.</p> <p>The possible connection of these features with the observed meson resonances at 2.2 GeV and 2.38 GeV, and its implications, were discussed. </p>
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Er3+ doped multicomponent fluoride based glass was prepared. These precursor fluoride glass samples were then heated using different schedules. Crystalline phase particles were successfully precipitated in the multicomponent fluoride glass samples after heat treatment. The influence of heat treatment on the spectroscopic properties of Er3+ in multicomponent fluoride based glass samples were discussed. Small changes of the Judd-Ofelt parameters Omega(i) (i = 2,4,6) were found in multicomponent fluoride glass samples before and after heat treatment compared to oxyfluoride telluride glass. Preparation conditions used to produce transparent multicomponent fluoride glass ceramics doped with rare-earth ions are discussed. (c) 2007 Elsevier B.V. All rights reserved.
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A new laser crystal Nd3+:(La, Sr)(Al, Ta)O-3 (abbreviated as Nd3+:LSAT) has been grown by the conventional Czochralski method. The absorption and luminescence spectra of trivalent neodymium in Nd3+:LSAT crystal were measured at room temperature. The value of absorption and emission cross-section was calculated. The Judd-Ofelt analysis was applied to the crystal to get the phenomenological parameters (Omega(i), i = 2,4,6), the line strengths, the radiative transition rates, the branching ratios and the radiative lifetime. (C) 2006 Elsevier B.V. All rights reserved.
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Os granitoides do Domnio Cambuci, na regio limtrofe entre os estados do Rio de Janeiro e Esprito Santo, foram separados em quatro principais grupos: (1) Complexo Serra da Bolvia (CSB) - Ortogranulitos e Ortognaisses Heterogneos; Ortognaisse Cinza Foliado; e charnockitos da Regio de Monte Verde (2) Leucogranitos/leucocharnockitos gnaissificados da Sute So Joo do Paraso (SSJP) (3) Granito Cinza Foliado (4) Leucogranito isotrpico. O CSB caracterizado pelo magmatismo de carter calcioalcalino do tipo I, oriundo em ambiente de arco vulcnico (Sute Monte Verde) e retrabalhamento crustal (ortogranulitos leucocrticos). O Ortogranulito esverdeado fino, considerado no presente estudo como rocha do embasamento para o Terreno Oriental, cristalizada durante o paleoproterozoico - Riaciano (2184,3 21 Ma) e recristalizada durante o evento metamrfico Brasiliano no neoproterozoico - Edicariano (607,2 1,5 Ma), cuja idade TDM de 2936 Ma. O Ortogranulito leucocrtico mdio cristalizou-se no neoproterozoico Edicariano (entre 592 e 609 Ma) e idade TDM ca. 2100 Ma, ao qual apresenta registro de herana no paleoproterozoico. A Sute Monte Verde caracteriza-se por um magmatismo calcioalcalino e a Sute Crrego Fortaleza, por um magmatismo calcioalcalino de alto K, ambas com assinatura de arco magmtico. Registram dois pulsos magmticos, em no Neoproterozoico - Edicarano: um em 592 2 Ma, idade do charnoenderbito, com idade TDM 1797 Ma, e outro em 571,2 1,8 Ma (injeo de um charnockitoide). Para todas as rochas do CSB so registradas feies protomilonticas, milonticas e localmente ultramilonticas. Os dados geoqumicos indicam que os granitoides da SSJP so da srie calcioalcalina de alto K, gerados no Neoproterozoico (idades que variam desde 610,3 4,7 Ma at, 592,2 1,3 Ma. As idades TDM revelam valores discrepantes para duas amostras: 1918 Ma e 2415 Ma, sugerindo que tenham sido geradas de diferentes fontes. O Granito Cinza Foliado da Srie Shoshontica, metaluminoso do tipo I e, de ambincia tectnica de granitos intraplaca. Entretanto, poderiam ter sido fomados em ambiente de arco cordilheirano, havendo contaminao de outras fontes crustais. Fato este pode ser confirmado pelas as idades TDM calculadas ≈ 1429 1446 Ma. O Leucogranito isotrpico ocorre em forma de diques de direo NW, possui textura macia e inequigranular. Dados geoqumicos revelam que so granitoides metaluminosos do tipo I da srie shoshontica, e, de acordo com a ambincia tectnica, so granitos intraplaca. O Leucogranito Isotrpico representa o magmatismo ps-colisional ao qual ocorreu entre 80 a 90 Ma de anos aps o trmino do evento colisional na regio central da Faixa Ribeira. O Leucogranito Issotrpico cristalizou-se no cambriano (512,3 3,3 Ma e 508,6 2,2 Ma) e com idades TDM ca. 1900
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