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Síntese de manganita dopada com estrôncio e cobaltita dopada com antimônio e caracterização microestrutural

Tarragó, Diego Pereira
Fonte: Universidade Federal do Rio Grande do Sul Publicador: Universidade Federal do Rio Grande do Sul
Tipo: Dissertação Formato: application/pdf
POR
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A grande variedade de propriedades dos óxidos com estrutura perovskita permite sua aplicação em diversas áreas da tecnologia. Em especial, as propriedades eletroquímicas e a estabilidade térmica de alguns destes compostos faz destes materiais os mais usados em cátodos de dispositivos para geração de energia como as células a combustível de óxido sólido (SOFC). As SOFC são dispositivos que podem atingir altas eficiências na conversão de energia elétrica e são passíveis de utilização na geração de energia sustentável e distribuída. Para estas células, as perovskitas compostas de manganita de lantânio dopada com estrôncio (LSM) preenche a maioria dos requisitos para aplicação como cátodos da SOFC. Nas células a combustível de óxido sólido de temperatura intermediária (IT-SOFC) há a possibilidade de construir a célula com filmes finos dos materiais cerâmicos suportados em um interconector metálico. No entanto, para os cátodos das IT-SOFC as manganitas não possuem bom desempenho eletroquímico devido à menor temperatura de operação do dispositivo, sendo necessário o desenvolvimento de condutores mistos para aumentar os sítios reativos para reação de redução do O2. Assim, o desenvolvimento de cátodos utilizando a perovskita cobaltita de estrôncio dopada com antimônio (SCS) tem sido proposto para esta aplicação. Portanto...

Effect of the sintering temperature on the properties of nanocrystalline Ca1−xSmxMnO3 (0 ≤ x ≤ 0.4) powders

Lopes, M E; Melo Jorge, M. E.; Sério, S.; Nunes, M. R.; Matos, I.
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
POR
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Nanocrystalline Ca1−xSmxMnO3 (0 ≤ x ≤ 0.4) manganites were prepared by a soft chemical method (Pechini method) followed by auto-combustion and sintering in air at 1073 or 1473 K. Single-phase powders with general composition Ca1−xSmxMnO3 were obtained after 18 h annealing. The particle and grain sizes of the substituted Sm-manganites did not exhibit variation with samarium content, but increase with increasing the sintering temperature. All manganites show two active IR vibrational modes near 400 and 600 cm−1 characteristic of the BO6 octahedron vibrations. For the samples sintered at Ts = 1473 K, the partial substitution of calcium by samarium in the CaMnO3 phase induces a marked decrease in the electrical resistivity, in the temperature range of 300–900 K, and at the same time a metal-to-insulator transition occurs; for Ts = 1073 K all the samples present semiconductor behaviour. With the increase of the annealing temperature the grain size increases and a metal–semiconductor transition appears. The results can be ascribed to the Mn4+/Mn3+ ratio and particle grain size. The effects of particle size on the electrical properties can be attributed to the domain status, changes in the Mn–O–Mn bond angle and Mn–O bond length.

Synthesis and electrical properties of nanocrystalline Ca1−xEuxMnO3±δ

Lopes, ME; Silveira, C; Nunes, M R; Melo Jorge, M E
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
POR
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46.95%
Nanopowders of Ca1−xEuxMnO3 (0.1≤x≤0.4) manganites were synthesized as a single phase using the auto gel-combustion method. The citrate method shows to be simple and appropriate to obtain single phases avoiding segregation or contamination. The Ca1−xEuxMnO3 system has been synthesized at 800 °C during 18 h, against the conventional method of mixing oxides used to obtain these materials at higher temperatures of synthesis. The formation reaction was monitored by X-ray diffraction (XRD) analysis and an infrared absorption technique (FTIR). The polycrystalline powders are characterised by nanometric particle size, ∼48 nm as determined from X-ray line broadening analysis using the Scherrer equation. Morphological analysis of the powders, using the scanning electron microscope (SEM), revealed that all phases are homogeneous and the europium-substituted samples exhibit a significant decrease in the grain size when compared with the undoped samples. The structure refinement by using the Rietveld method indicates that the partial calcium substitution by europium (for x≥0.3) modifies the orthorhombic structure of the CaMnO3 perovskite towards a monoclinic phase. All manganites show two active IR vibrational modes around 400 and 600 cm−1. The high temperature dependence of electrical resistivity (between 25 and 600 °C) allows us to conclude that all the samples exhibit a semiconductor behaviour and the europium causes a decrease in the electrical resistivity by more than one order of magnitude. The results can be well attributed to the Mn4+/Mn3+ ratio.

Low-temperaturesynthesisofnanocrystalline Ca1 xHoxMnO3 d (0pxp0.3) powders

Lopes, M E; Isasi, P H; Nunes, M R; Melo Jorge, M E
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
ENG
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Nanocrystalline Ca1 xHoxMnO3 d (0

Properties of Ca1−xHoxMnO3 perovskite-type electrodes

Ferreira, B M; Melo Jorge, M E; Lopes, M E; Nunes, M R; Silva Pereira, M I
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
26.27%
In this work, Ca1−xHoxMnO3 (x = 0, 0.1 and 0.2) perovskite oxide pelleted electrodes were prepared from the respective powders obtained by the citrate route method at 1173 K. The electrodes exhibit particle size that decreases with the holmium content in the oxide. All the samples reveal semiconductor behaviour and the presence of holmium induces a marked decrease in the electrical resistivity. The results can be well attributed to the changes in the Mn4+/Mn3+ ratio. Electrodes were characterized by cyclic voltammetry and chronopotentiometry. Cyclic voltammetric studies indicate a similar behaviour of the electrodes, irrespective of their composition. Two pairs of peaks were identified and associated, one to the Mn4+/Mn3+ redox couple and the other to the Mn7+/Mn4+ and Mn6+/Mn4+ redox couples. The voltammetric data provide evidence that the electrodes roughness factor increases with the introduction of Ho-ions in the oxide structure, what is consistent with the crystallite size obtained by X-ray diffraction (XRD) and the morphology observed by scanning electron microscopy (SEM). The Ho substituted electrodes present higher current density when compared with CaMnO3 electrodes what can be attributed both to higher electrical conductivity and smaller particle size. The chronopotentiometric studies have shown that the discharge occurs by different mechanisms for the oxide electrodes with and without Ho.

Experimental observation of charge ordering in nanocrystalline Pr$_{0.65}$Ca$_{0.35}$MnO$_{3}$

Biswas, Anis; Das, I.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.51%
Observation of charge ordering in single crystalline and bulk polycrystalline systems of various rare-earth based manganites is well documented. However, there is hardly any manifestation of the same when the grain size is reduced to nanoscale. We have observed charge ordering in case of nanocrystalline Pr$_{0.65}$Ca$_{0.35}$MnO$_{3}$ of average particle size 40 nm. This phenomenon is attributed to the primary role played by the martensitic character of the charge order transition in the material.; Comment: 4 pages including 4 figures

Magnetocaloric properties of nanocrystalline La$_{0.125}$Ca$_{0.875}$MnO$_{3}$

Biswas, Anis; Samanta, Tapas; Banerjee, S.; Das, I.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.63%
Some recent experimental studies show the invisibility of antiferromagnetic transition in the cases of manganites when their particle size is reduced to nanometer scale. In complete contrast to these cases, we have observed the signature of antiferromagnetic transition in the magnetocaloric properties of nanocrystalline La$_{0.125}$Ca$_{0.875}$MnO$_{3}$ of average particle size 70 and 60 nm similar to its polycrystalline bulk form. The system exhibit inverse magnetocaloric effect in its polycrystalline and nanocrystalline form. An extra ferromagnetic phase is stabilized at low temperature for the sample with particle size $\sim 60$ nm.; Comment: 3 Figures

Magnetocaloric effect in nano- and polycrystalline manganite $La_{0.7}Ca_{0.3}MnO_3$

Pekala, M.; Drozd, V.; Fagnard, J. F.; Vanderbemden, Ph.; Ausloos, M.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 29/10/2007
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$La_{0.7}Ca_{0.3}MnO_3$ samples were prepared in nano- and polycrystalline forms by sol-gel and solid state reaction methods, respectively, and structurally characterized by synchrotron X-ray diffraction. The magnetic properties determined by ac susceptibility and dc magnetization measurements are discussed. The magnetocaloric effect in this nanocrystalline manganite is spread over a broader temperature interval than in the polycrystalline case. The relative cooling power of the poly- and nanocrystalline manganites is used to evaluate a possible application for magnetic cooling below room temperature.; Comment: 6 pages, 5 (double) figures, 1 table, 16 references; submitted to Appl. Phys. A

Neutron scattering study of magnetic phase separation in nanocrystalline La$_{5/8}$Ca$_{3/8}$MnO$_3$

Dhital, Chetan; de la Cruz, Clarina; Opeil, C.; Treat, A.; Wang, K. F.; Liu, J. -M.; Ren, Z. F.; Wilson, Stephen D.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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We demonstrate that magnetic phase separation and competing spin order in the colossal magnetoresistive (CMR) manganites can be directly explored via tuning strain in bulk samples of nanocrystalline La$_{1-x}$Ca$_x$MnO$_3$. Our results show that strain can be reversibly frozen into the lattice in order to stabilize coexisting antiferromagnetic domains within the nominally ferromagnetic metallic state of La$_{5/8}$Ca$_{3/8}$MnO$_3$. The measurement of tunable phase separation via magnetic neutron powder diffraction presents a direct route of exploring the correlated spin properties of phase separated charge/magnetic order in highly strained CMR materials and opens a potential avenue for realizing intergrain spin tunnel junction networks with enhanced CMR behavior in a chemically homogeneous material.; Comment: 6 pages, 4 figures. New figure and text added to manuscript

Nanosized Sodium-Doped Lanthanum Manganites: Role of the Synthetic Route on their Physical Properties

Malavasi, Lorenzo; Mozzati, Maria C.; Polizzi, Stefano; Azzoni, Carlo B.; Flor, Giorgio
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 17/10/2003
Relevância na Pesquisa
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In this paper we present the results of the synthesis and characterisation of nanocrystalline La1-xNaxMnO3+delta samples. Two synthetic routes were employed: polyacrylamide-based sol-gel and propellant synthesis. Pure, single phase materials were obtained with grain size around 35 nm for the sol-gel samples and around 55 nm for the propellant ones, which moreover present a more broaden grain size distribution. For both series a superparamagnetic behaviour was evidenced by means of magnetisation and EPR measurements with peculiar features ascribable to the different grain sizes and morphology. Preliminary magnetoresistivity measurements show enhanced low-field (< 1 T) magnetoresistance values which suggest an interesting applicative use of these manganites.; Comment: 31 Pages 10 Figures to appear in Chem. Mater

Direct experimental evidence of multiferroicity in a nanocrystalline Zener polaron ordered manganite

Shukla, Vinay Kumar; Mukhopadhyay, Soumik; Das, Kalipada; Sarma, A.; Das, I.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 24/12/2014
Relevância na Pesquisa
26.51%
We discuss the particle size driven tunability of the coexistence of ferromagnetism and ferroelectricity in Pr0.67Ca0.33MnO3(PCMO) with the help of x ray diffraction, magnetization, impedance spectroscopy, and remanent polarization measurements. The remanent polarization measurements using the positive up negative down method clearly prove the existence of ferroelectricity in PCMO with phase separation between Zener polaron(ZP) ordered P21nm and disordered Pbnm structures. We also find that the ferroelectric response is enhanced in nanocrystalline samples so long as ZP ordering is not destroyed while the long-range antiferromagnetic ordering at low temperature in bulk system is replaced by ferromagnetic correlations in nanoparticles. The conclusion that by reducing the crystallite size it might be possible to make ferromagnetism and ferroelectricity coexist near room temperature should be generally applicable to all ZP ordered manganites.; Comment: 10 pages, 14 figures

Strain heterogeneity and magnetoelastic behavior of nanocrystalline half-doped La, Ca manganite, La_0.5Ca_0.5MnO_3

Pagliari, L.; Dapiaggi, M.; Maglia, F.; Sarkar, T.; Raychaudhuri, A. K.; Chatterji, T.; Carpenter, M. A.
Fonte: IOP Publishing Publicador: IOP Publishing
Tipo: Article; published version
EN
Relevância na Pesquisa
26.51%
This is the final version. It was first published by IOP Science at http://iopscience.iop.org/0953-8984/26/43/435303/.; Elastic and anelastic properties of La_0.5Ca_0.5MnO_3 determined by Resonant Ultrasound Spectroscopy in the frequency range ~100 ? 1200 kHz have been used to evaluate the role of grain size in determining the competition between ferromagnetism and Jahn-Teller/charge order of manganites which show colossal magneto resistance. At crystallite sizes of ~75 and ~135 nm the dominant feature is softening of the shear modulus as the charge order transition point, Tco (~225 K), is approached from above and below, matching the form of softening seen previously in samples with ?bulk? properties. This is consistent with a bilinear dominant strain/order parameter coupling, which occurs between the tetragonal shear strain and the Jahn-Teller (?^+_3) order parameter. At crystallite sizes of ~34 and ~42 nm the charge ordered phase is suppressed but there is still softening of the shear modulus, with a minimum near T_co. This indicates that some degree of pseudoproper ferroelastic behaviour is retained. The primary cause of the suppression of the charge ordered structure in nanocrystalline samples is therefore considered to be due to suppression of macroscopic strain...