992 resultados para Sr^2
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Three new oxides Sm2SrCo2O7, Sm2BaCo2O7 and Gd2SrCo2O7 have been synthesized successfully by solid state reaction mathod. The X-Ray diffraction spectra show that they are all isostructural with Sr3Ti2O7, and Ln(2)SrCo(2)O(7)(Ln=Sm,Gd) crystallized in tetragonal system, Sm2BaCo2O7 in orthrhombic system. The Co-O bonds in CoO2 planes of Ln(2)SrCo(2)O(7) are shorter than those of LnSrCoO(4)(Ln=Sm, Gd), and so their delectrons are more delocalized and their electrical resistivities are smaller. The electrical resistivities versus temperature in the range 300 similar to 1100K showed that the five brides show the characters of weakly localized systems. In the lower temperature range, the magnetic behaviors of Gd2SrCo2O7 and GdSrCoO4 fit Curie-Weiss law well, and the magnetic exchange reaction in CoO2 sublattices of Gd2SrCo2O7 is ferromagnetic, but that of GdSrCoO4 is antiferromagnetic. The other three oxides with Sm3+ showed complex magnetic behaviors which is perhaps related with the complexity of Sm3+.
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Three new oxides Ln(2)MCo(2)O(7) (Ln = Sm, Gd; M = Sr, Ba) have been synthesized in solid state reaction method. The powder X-ray diffraction spectra show that they are all isostructural with Sr3Ti2O7. The electrical resistivities in the temperature range 300-1100 K show that they are all semiconductors, and a transition to metals is observed at 1053, 1053, and 573 K for Sm2SrCo2O7, Gd2SrCo2O7, and Sm2BaCo2O7, respectively. The magnetic suspectivities of Gd2SrCo2O7 in the temperature range 300-673 K fit the Curie-Weiss law well. A plateau is observed in the curves of Sm(2)MCo(2)O(7) (M = Sr, Ba) which is attributed to the configuration state change of Co(III) from low spin to high spin. (C) 1995 Academic Press, Inc.
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:Sm_2SrCo_2O_7,Gd_2SrCO_2O_7Sm_2BaCo_2O_7.Sr_3Ti_2O_7,Sm_2BaCo_2O_7,.LnSrCoO_4,Ln_2SrCo_2O_7(Ln=Sm,Gd)CoO_2Co-O,,.3001100K,,. 3001100K,,GdSrCoO_4Gd_2SrCo_2O_7Curie-Weiss,CoO_2,;Sm~(3+),Sm~(3+).
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The phosphors MMgF(4)(M = Ca, Sr, Ba) doped with samarium ions are synthesized in different atmospheres using solid phase reaction at high temperature. Samarium has been first stabilized in the divalent state in SrMgF4 and BaMgF4 matrices. Effects of matrices on the valent state of samarium ions are briefly discussed.
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<p>Epitaxial (001)-oriented 0.7Pb(Mg<sub>0.33</sub>Nb<sub>0.67</sub>)O<sub>3</sub>-0.3PbTiO<sub>3</sub> (PMN-PT) thin films were deposited by pulsed laser deposition on vicinal SrTiO<sub>3</sub> (001) substrates using La<sub>0.7</sub>Sr<sub>0.3</sub>MnO<sub>3</sub> as bottom electrode. Detailed microstructural investigations of these films were carried out using X-ray diffraction (XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM). Polarization-field hysteresis curves were measured at room temperature. Spontaneous polarization P s , remnant polarization P r and coercive voltage V c were found to be 25 C/cm<sup>2</sup>, 15 C/cm<sup>2</sup> and 0.81 V, respectively. Field dependent dielectric constant measurements exhibited butterfly shaped curves, indicating the true ferroelectric nature of these films at room temperature. The dielectric constant and the dielectric loss at 100 kHz were found to be 238 and 0.14, respectively. The local piezoelectric properties of PMN-PT films were investigated by piezoelectric force microscopy and were found to exhibit a local piezoelectric coefficient of 7.8 pm/V.</p>
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<p>The ability to directly utilize hydrocarbons and other renewable liquid fuels is one of the most important issues affecting the large scale deployment of solid oxide fuel cells (SOFCs). Herein we designed La<sub>0.2</sub>Sr<sub>0.7</sub>TiO<sub>3</sub>-Ni/YSZ functional gradient anode (FGA) supported SOFCs, prepared with a co-tape casting method and sintered using the field assisted sintering technique (FAST). Through SEM observations, it was confirmed that the FGA structure was achieved and well maintained after the FAST process. Distortion and delamination which usually results after conventional sintering was successfully avoided. The La<sub>0.2</sub>Sr<sub>0.7</sub>TiO<sub>3</sub>-Ni/YSZ FGA supported SOFCs showed a maximum power density of 600mWcm<sup>-2</sup>at 750C, and was stable for 70h in CH<sub>4</sub>. No carbon deposition was detected using Raman spectroscopy. These results confirm the potential coke resistance of La<sub>0.2</sub>Sr<sub>0.7</sub>TiO<sub>3</sub>-Ni/YSZ FGA supported SOFCs.</p>
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<p>The electrochemical performance of one-dimensional porous La<sub>0.5</sub>Sr<sub>0.5</sub>CoO<sub>2.91</sub> nanotubes as a cathode catalyst for rechargeable nonaqueous lithium-oxygen (Li-O<sub>2</sub>) batteries is reported here for the first time. In this study, one-dimensional porous La<sub>0.5</sub>Sr<sub>0.5</sub>CoO<sub>2.91</sub> nanotubes were prepared by a simple and efficient electrospinning technique. These materials displayed an initial discharge capacity of 7205 mAh g<sup>-1</sup> with a plateau at around 2.66 V at a current density of 100 mA g<sup>-1</sup>. It was found that the La<sub>0.5</sub>Sr<sub>0.5</sub>CoO<sub>2.91</sub> nanotubes promoted both oxygen reduction and oxygen evolution reactions in alkaline media and a nonaqueous electrolyte, thereby improving the energy and coulombic efficiency of the Li-O<sub>2</sub> batteries. The cyclability was maintained for 85 cycles without any sharp decay under a limited discharge depth of 1000 mAh g<sup>-1</sup>, suggesting that such a bifunctional electrocatalyst is a promising candidate for the oxygen electrode in Li-O<sub>2</sub> batteries.</p>
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