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‣ Óxidos do tipo perovskita para reação de redução de no com CO; Perovskite-type oxides to no reduction with CO

TANABE, Eurico Y.; ASSAF, Elisabete M.
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
78.83125%
In this work, the perovskite-type oxides LaNiO3, LaMnO3, La0,7Sr0,3NiO3 and La0,7Sr0,3MnO3 were prepared by co-precipitation and tested in the NO reduction with CO at 400 and 500 ºC for 10 h. The catalysts were characterized by X-ray diffraction, temperature programmed reduction with hydrogen, nitrogen adsorption and chemical analysis. The nonstoichiometric oxygen was quantified by temperature programmed reduction, and the catalytic tests showed that the La0,7Sr0,3MnO3 catalyst presented the higher performance for the reduction reaction of NO with CO. The partial substitution of lanthanum by strontium increased the NO conversion and the N2 yield.

‣ Study of La2-xCaxCuO4 perovskites for the low temperature water gas shift reaction

Maluf, S. S.; Nascente, P. A. P.; Afonso, C. R. M.; Assaf, E. M.
Fonte: ELSEVIER SCIENCE BV; AMSTERDAM Publicador: ELSEVIER SCIENCE BV; AMSTERDAM
Tipo: Artigo de Revista Científica
Português
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The effects of small fractions of calcium (x = 0, 0.05, 0.1, 0.15, and 0.20) on the structure and the catalytic properties of La2-xCaxCuO4 peroviskites have been investigated. The samples have been synthesized using the co-precipitation method. Perovskite-type oxides were characterized by XRD, TPR, XPS, XANES, SEM, and TEM. Catalytic tests for the water gas shift reaction (WGSR) were carried out in a tubular reactor at 290 degrees C. All samples showed a well-defined perovskite structure with surface areas between 6 and 18 m(2) g(-1). The partial substitution of La by Ca enhanced the stability of the perovskites and increased their reduction temperature. All catalysts were actives for WGSR, and the best catalytic performance was obtained for the La1.85Ca0.15CuO4 catalyst, but the samples with 5 and 10% of Ca had the best TOF values for reaction. These results can be associated to promoter effect of calcium, the high surface area, and the reducible species Cu-0 and Cu1+. (C) 2011 Elsevier B.V. All rights reserved.; FAPESP; FAPESP

‣ Stability of LaNiO3 gas diffusion oxygen electrodes

Silva, R. A.; Soares, C. O.; Carvalho, M. D.; Rangel, C. M.; Pereira, M. I. da Silva
Fonte: Springer Publicador: Springer
Tipo: Artigo de Revista Científica
Publicado em //2014 Português
Relevância na Pesquisa
58.215527%
In this work LaNiO3 perovskite-type oxide, prepared by a self-combustion method, was optimized for activity and stability as an anode material for water electrolysis. A full electrochemical study was conducted in order to kinetically characterize electrodes prepared using carbon paper as a base for porous gas-diffusion electrodes in alkaline media, regarding water oxidation and oxygen reduction reactions at room temperature. An electrode stability study was performed by potential cycling and at constant current density, using cyclic voltammetry and electrochemical impedance spectroscopy to check on stability after cycling with complementary scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDS) analysis of fresh and degraded electrodes. Comparison was made using nickel foam as a support for LaNiO3 deposition. Carbon instability in the potential region of interest contrasted with the lower contact resistance between the oxide and support of the Ni foam. Higher metal oxide loadings and dimensional stability were also possible.

‣ Síntese e caracterização de perovskitas LaNi(1-x)Co xO3 como precursores de catalisadores para a conversão do metano a gás de síntese pela reforma com CO2

Lima,Sania Maria de; Assaf,José Mansur
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/04/2007 Português
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LaNiO3 perovskite was modified by partial substitution of nickel by cobalt in order to increase the stability and resistance to carbon deposition during the methane CO2 reforming. The results showed that a suitable combination of precipitation and calcination steps resulted in oxides with the desired structure and with important properties for application in heterogeneous catalysis. The partial substitution of Ni by Co resulted in lower rates of conversion of both the reactants, but the catalyst stability was highly increased. The LaNi0.3Co0.7O3 catalyst, calcined at 800 ºC, was the most active under the reaction conditions.

‣ Studies on mixed metal oxides solid solutions as heterogeneous catalysts

Arandiyan,H. R.; Parvari,M.
Fonte: Brazilian Society of Chemical Engineering Publicador: Brazilian Society of Chemical Engineering
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/03/2009 Português
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In this work, a series of perovskite-type mixed oxide LaMo xV1-xO3+δ powder catalysts (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0, with 0.5 < δ < 1.5), prepared by the sol-gel process and calcined at 750ºC, provide an attractive and effective alternative means of synthesizing materials with better control of morphology. Structures of resins obtained during the gel formation process by FT-IR spectroscopy and XRD analysis showed that all the LaMo xV1-xO3+δ samples are single phase perovskite-type solid solutions. The surface area (BET) between 2.5 - 5.0 m²/g (x = 0.1 and 1.0 respectively) increases with increasing Mo ratio in the samples. They show high purity, good chemical homogeneity, and lower calcinations temperatures as compared with the solid-state chemistry route. SEM coupled to EDS and thermogravimetric analysis/differential thermal analyses (TGA/DTA) have been carried out in order to evaluate the homogeneity of the catalyst. Finally, the experimental studies show that the calcination temperature and Mo content exhibited a significant influence on catalytic activity. Among the LaMo xV1-xO3+δ samples, LaMo0.7V0.3O4.2 showed the best catalytic activity for the topic reaction and the best activity and stability for ethane reforming at 850ºC under 8 bar.

‣ Synthesis and Characterization of LaNiO3, LaNi(1-x)Fe xO3 andLaNi(1-x)Co xO3 Perovskite Oxides for Catalysis Application

Lima,Sania Maria de; Assaf,José Mansur
Fonte: ABM, ABC, ABPol Publicador: ABM, ABC, ABPol
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/09/2002 Português
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Mixed metal oxides with perovskite-type structure show a great potential to be used in catalysis, electrocatalysis and electronic ceramics. Perovskites oxides catalysts with the composition LaNiO3, LaNi(1-x)Fe xO3 and LaNi(1-x)Co xO3 (x = 0.4 and 0.7) have been synthesized by the precipitation method to be used in the methane reforming to produce hydrogen and synthesis gas. The compounds were characterized by X-ray diffraction, thermogravimetric and differential thermal analysis, inductively coupled plasma atomic emission spectroscopy, surface area measurements, energy dispersive X-ray spectrometry coupled to scanning electron microscopy and temperature programmed reduction. The results showed that a suitable combination of the preparation method with calcination variables (time and temperature) could result in oxides with the desired structure and with important properties at the application point of view in heterogeneous catalysis.

‣ Lanthanoid-free perovskite oxide catalyst for dehydrogenation of ethylbenzene working with redox mechanism

Watanabe, Ryo; Ikushima, Maiko; Mukawa, Kei; Sumomozawa, Fumitaka; Ogo, Shuhei; Sekine, Yasushi
Fonte: Frontiers Media S.A. Publicador: Frontiers Media S.A.
Tipo: Artigo de Revista Científica
Publicado em 23/10/2013 Português
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For the development of highly active and robust catalysts for dehydrogenation of ethylbenzene (EBDH) to produce styrene; an important monomer for polystyrene production, perovskite-type oxides were applied to the reaction. Controlling the mobility of lattice oxygen by changing the structure of Ba1 − xSrxFeyMn1 − yO3 − δ (0 ≤ x ≤ 1, 0.2 ≤ y ≤ 0.8), perovskite catalyst showed higher activity and stability on EBDH. The optimized Ba/Sr and Fe/Mn molar ratios were 0.4/0.6 and 0.6/0.4, respectively. Comparison of the dehydrogenation activity of Ba0.4Sr0.6Fe0.6Mn0.4O3 − δ catalyst with that of an industrial potassium promoted iron (Fe–K) catalyst revealed that the Ba0.4Sr0.6Fe0.6Mn0.4O3 − δ catalyst showed higher initial activity than the industrial Fe–K oxide catalyst. Additionally, the Ba0.4Sr0.6Fe0.6Mn0.4O3 − δ catalyst showed high activity and stability under severe conditions, even at temperatures as low as 783 K, or at the low steam/EB ratio of 2, while, the Fe–K catalyst showed low activity in such conditions. Comparing reduction profiles of the Ba0.4Sr0.6Fe0.6Mn0.4O3 − δ and the Fe–K catalysts in a H2O/H2 atmosphere, reduction was suppressed by the presence of H2O over the Ba0.4Sr0.6Fe0.6Mn0.4O3 − δ catalyst while the Fe–K catalyst was reduced. In other words...

‣ Caracterização dos eletrocatalisadores LaMnO3, LaFeO3, LaFe0,2Mn0,8O3 E La0,5Fe0,5MnO3 preparados por autocombustão assistida por microondas para cátodos de células a combustível do tipo SOFC

Costa, Romário Cezar Pereira da
Fonte: Universidade Federal do Rio Grande do Norte; BR; UFRN; Programa de Pós-Graduação em Química; Físico-Química; Química Publicador: Universidade Federal do Rio Grande do Norte; BR; UFRN; Programa de Pós-Graduação em Química; Físico-Química; Química
Tipo: Dissertação Formato: application/pdf
Português
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Materials consisting of perovskite-type oxides (ABO3) have been developed in this work for applications in fuel cell cathodes of solid oxide type (SOFC). These ceramic materials are widely studied for this type of application because they have excellent electrical properties, conductivity and electrocatalytic. The oxides LaMnO3, LaFeO3, LaFe0.2Mn0.8O3 e La0.5Fe0.5MnO3 were synthesized by the method of microwave assisted combustion and after sintering at 800°C in order to obtain the desired phases. The powders were characterized by thermogravimetry (TG), X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM) and voltammetric analysis (cyclic voltammetry and polarization curves). The results obtained by XRF technique showed that the microwave synthesis method was effective in obtaining doping oxides with values near stoichiometric. In general, powders were obtained with particle size less than 0.5 μm, having a porous structure and uniform particle size distribution. The particles showed spherical form, irregular and crowded of varying sizes, according to the analysis of SEM. The behavior of the oxides opposite the thermal stability was monitored by thermogravimetric curves (TG), which showed low weight loss values for all samples...

‣ Propriedade fotoluminescente do Ca1-(x+y)YbxTmyTiO3 obtido pelo método hidrotérmico assistido por micro-ondas

Pereira, Sandra de Cássia
Fonte: Universidade Federal de Goiás; Brasil; UFG; Programa de Pós-graduação em Química (RC); Regional de Catalão (RC) Publicador: Universidade Federal de Goiás; Brasil; UFG; Programa de Pós-graduação em Química (RC); Regional de Catalão (RC)
Tipo: Dissertação Formato: application/pdf
Português
Relevância na Pesquisa
48.44539%
In recent years, the perovskite type oxides doped with rare-earth ions has attracted great interest in the research, especially in the field of photoluminescence (PL). This property has always attracted the interest of the scientific community, especially since their discovery room temperature. The present work aims to study the PL property CaTiO3 (CT) doped with rare-earth ions Yb (III) and Tm (III). Samples of CT pure and doped with rare-earth ions, Ca1-(x+y)YbxTmyTiO3 (x and y = 0, 0,01, 0,0075, 0,005, ou 0,0025 ), were obtained by co-precipitation method and processing using hydrothermal system assisted by microwave , which was used to be a relatively new, promising little used method of processing. The samples were characterized by different techniques, the X-ray diffraction confirmed to obtain a single phase material of orthorhombic perovskite type structure, confirming that the hydrothermal synthesis method is effective in the samples of interest in shorter periods of time. Ultraviolet-visible absorption spectroscopy, allowed calculation of the "gap" optic, confirming the presence of intermediate levels between the valence band and the conduction in the doped samples. The characterization by SEM and EDX confirmed, respectively...

‣ Produção de hidrogênio a partir da fração aquosa do bio-óleo utilizando óxidos mistos a base de lantânio e níquel

Resende, Karen Abreu
Fonte: Universidade Federal de Uberlândia Publicador: Universidade Federal de Uberlândia
Tipo: Dissertação
Português
Relevância na Pesquisa
48.5279%
A condensação dos gases oriundos da pirólise da biomassa gera um composto líquido chamado bio-óleo, que pode ser dividido em duas frações: uma aquosa e uma não aquosa. A fração aquosa não possui um alto valor agregado, sendo normalmente descartada. Porém, essa mistura apresenta uma imensa quantidade de compostos orgânicos, o que faz com que ela seja uma fonte renovável em potencial de hidrogênio. Devido à diversidade de compostos orgânicos encontrados nessa fração aquosa, é comum a utilização de compostos modelos nos estudos envolvendo o tema. Entre os compostos modelos estudados, o mais freqüente é o ácido acético, pois, geralmente, ele é um dos que se encontra em maior quantidade na mistura. Dos processos reacionais de reforma aplicados para a produção do hidrogênio a partir da fração aquosa do bio-óleo, a reforma a vapor é o mais citado na literatura, mas a reforma oxidativa também é utilizada. Sendo assim, o objetivo dessa dissertação foi estudar a produção de hjidrogênio a partir da fração aquosa do bio-óleo. Inicialmente estudou-se o comportamento termodinâmico da reação de reforma a vapor da fração aquosa do bio-óleo, com o objetivo de determinar a melhor temperatura e pressão para realizar essa reação. Nesse estudo foram utilizados dois compostos modelo para representar essa mistura: ácido acético e acetol. Além disso...

‣ Study on the properties of piezoelectric materials and manganese-based oxide perovskites /

Hezareh, Talayeh.
Fonte: Brock University Publicador: Brock University
Tipo: Electronic Thesis or Dissertation
Português
Relevância na Pesquisa
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Perovskite type piezoelectric and manganese oxide materials have gained a lot of attention in the field of device engineering. Lead zirconium titananium oxide (PbZri.iTiiOa or PZT) is a piezoelectric material widely used as sensors and actuators. Miniaturization of PZTbased devices will not only perfect many existing products, but also opens doors to new applications. Lanthanum manganese oxides Lai-iAiMnOa (A-divalent alkaline earth such as Sr, Ca or Ba) have been intensively studied for their colossal magnetoresistance (CMR) properties that make them applicable in memory cells, magnetic and pressure sensors. In this study, we fabricate PZT and LSMO(LCMO) heterostructures on SrTiOa substrates and investigate their temperature dependency of resistivity and magnetization as a function of the thickness of LSMO(LCMO) layer. The microstructure of the samples is analysed through TEM. In another set of samples, we study the effect of application of an electric field across the PZT layer that acts as an external pressure on the manganite layer. This verifies the correlation of lattice distortion with transport and magnetic properties of the CMR materials.

‣ Reforma de biogás para produção de hidrogênio usando catalisadores tipo perovskitas a base de lantânio e níquel, dopados com cério

Xavier, Thiago Padovani
Fonte: Universidade Federal de Uberlândia Publicador: Universidade Federal de Uberlândia
Tipo: Dissertação
Português
Relevância na Pesquisa
48.83125%
No presente trabalho, foi avaliado o comportamento de óxidos do tipo perovskitas La1-xCexNiO3 (x = 0; 0,05 e 0,10), atuando como catalisadores, frente às reações de reforma seca do metano (RSM) e reforma de biogás (RB). Os catalisadores foram sintetizados pelo método sol-gel (ou citrato) e pelo método de combustão com uréia. Posteriormente, foram avaliados estruturalmente e quanto ao seu desempenho catalítico. Os precursores catalíticos foram caracterizados por difração de raios X, medidas de área específica BET, redução a temperatura programada e análise termogravimétrica. Todos os catalisadores apresentaram baixos valores de área específica (< 10 m2 g-1). A fase principal LaNiO3, presente em todas as amostras calcinadas, foi convertida em La2O3, Ni0 e La(OH)3 após redução, sendo observada a fase CeO2 nas amostras dopadas com cério. A presença da fase La(OH)3 foi mais evidente nas amostras sintetizadas pelo método de combustão. Os tamanhos médios de cristalito, referentes à fase principal LaNiO3, se mantiveram entre 13 – 17 nm. A amostra La0,90Ce0,10NiO3 sintetizada pelo método de combustão apresentou o menor tamanho médio de cristalito enquanto que as amostras La0,95Ce0,05NiO3 e La0,90Ce0,10NiO3 sintetizadas pelo método sol-gel tem o maior valor. O método sol-gel produziu tamanhos médios de cristalito do NiO (20 – 25 nm) menores do que os produzidos pelo método de combustão (31 – 38 nm). Porém...

‣ Lanthanum based high surface area perovskite-type oxide and application in CO and propane combustion

Silva,P. R. N.; Soares,A. B.
Fonte: Fundação Editora da Universidade Estadual Paulista Júlio de Mesquita Filho - UNESP Publicador: Fundação Editora da Universidade Estadual Paulista Júlio de Mesquita Filho - UNESP
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2009 Português
Relevância na Pesquisa
59.5283%
The perovskite-type oxides using transition metals present a promising potential as catalysts in total oxidation reaction. The present work investigates the effect of synthesis by oxidant co-precipitation on the catalytic activity of perovskite-type oxides LaBO3 (B= Co, Ni, Mn) in total oxidation of propane and CO. The perovskite-type oxides were characterized by means of X-ray diffraction, nitrogen adsorption (BET method), thermo gravimetric and differential thermal analysis (ATG-DTA) and X-ray photoelectron spectroscopy (XPS). Through a method involving the oxidant co-precipitation it's possible to obtain catalysts with different BET surface areas, of 33-44 m²/g, according the salts of metal used. The characterization results proved that catalysts have a perovskite phase as well as lanthanum oxide, except LaMnO3, that presents a cationic vacancies and generation for known oxygen excess. The results of catalytic test showed that all oxides have a specific catalytic activity for total oxidation of CO and propane even though the temperatures for total conversion change for each transition metal and substance to be oxidized.

‣ The LaNiO3 perovskite-type oxides for the production of hydrogen through steam reforming of ethanol. The effect of the preparation method

Noronha, Fábio Bellot; Silva, Adriana Maria da; Costa, Lídia Oazem de Oliveira da; Mattos, Lisiane Veiga; Lima, Sonia M. de; Reis, Andressa Andrade Alves da Silva
Fonte: Instituto Nacional de Tecnologia Publicador: Instituto Nacional de Tecnologia
Tipo: apresentação
Português
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69.201807%
Escócia; The perovskite-type oxide with general formula ABO3 is a promising material as catalyst for the ethanol conversion reactions to hydrogen. These mixed oxides are able to produce highly dispersed metallic particles upon reduction, which inhibits the formation of carbon [1]. Recently, we have studied the performance of LaNiO3 perovskite-type oxide precursor as a catalyst for both steam reforming (SR) and oxidative steam reforming (OSR) of ethanol [2]. Catalyst deactivation took place depending on the reaction conditions and it was attributed to the deposition of carbon on the surface of the catalyst. The aim of this work was to study the effect of the preparation method of LaNiO3 perovskite-type oxide precursor on catalyst stability during SR and OSR of ethanol.

‣ Evaluation of the performance of Ni/La2O3 catalyst prepared from LaNiO3 perovskite-type oxides for the production of hydrogen through steam reforming and oxidative steam reforming of ethanol

Noronha, Fábio Bellot; Mattos, Lisiane Veiga; Lima, Sonia M. de; Silva, Adriana Maria da; Assaf, José M; Jacobs, Gary; Davis, Burtron H; Costa, Lídia Oazem de Oliveira da
Fonte: Instituto Nacional de Tecnologia Publicador: Instituto Nacional de Tecnologia
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
88.91375%
This paper studies the performance of LaNiO3 perovskite-type oxide precursor as a catalyst for both steam reforming and oxidative steam reforming of ethanol. According to results of temperatureprogrammed desorption of adsorbed ethanol and by carrying out diffuse reflectance infrared Fourier transform spectroscopy analyses of ethanol steam reforming, ethanol decomposes to dehydrogenated species like acetaldehyde and acetyl, which at moderate temperatures, convert to acetate by the addition of hydroxyl groups.

‣ Magnetic and Orbital States and Their Phase Transition of the Perovskite-Type Ti Oxides: Strong Coupling Approach

Mochizuki, Masahito; Imada, Masatoshi
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 12/02/2001 Português
Relevância na Pesquisa
48.68389%
The properties and mechanism of the magnetic phase transition of the perovskite-type Ti oxides, which is driven by the Ti-O-Ti bond angle distortion, are studied theoretically by using the effective spin and pseudospin Hamiltonian with strong Coulomb repulsion. It is shown that the A-type antiferromagnetic (AFM(A)) to ferromagnetic (FM) phase transition occurs as the Ti-O-Ti bond angle is decreased. Through this phase transition, the orbital state changes only little whereas the spin-exchange coupling along the c-axis is expected to change from positive to negative nearly continuously and approaches zero at the phase boundary. The resultant strong two-dimensionality in the spin coupling causes rapid suppression of the critical temperature, as observed experimentally. It may induce large quantum fluctuations in this region.; Comment: 13 pages, 15 figures

‣ A high-mobility two-dimensional electron gas at the heteroepitaxial spinel/perovskite complex oxide interface of {\gamma}-Al2O3/SrTiO3

Chen, Y. Z.; Bovet, N.; Trier, F.; Christensen, D. V.; Qu, F. M.; Andersen, N. H.; Kasama, T.; Zhang, W.; Giraud, R.; Dufouleur, J.; Jespersen, T. S.; Sun, J. R.; Smith, A.; Nygård, J.; Lu, L.; Büchner, B.; Shen, B. G.; Linderoth, S.; Pryds, N.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 01/04/2013 Português
Relevância na Pesquisa
49.339844%
The discovery of two-dimensional electron gases (2DEGs) at the heterointerface between two insulating perovskite-type oxides, such as LaAlO3 and SrTiO3, provides opportunities for a new generation of all-oxide electronic and photonic devices. However, significant improvement of the interfacial electron mobility beyond the current value of approximately 1,000 cm2V-1s-1 (at low temperatures), remains a key challenge for fundamental as well as applied research of complex oxides. Here, we present a new type of 2DEG created at the heterointerface between SrTiO3 and a spinel {\gamma}-Al2O3 epitaxial film with excellent quality and compatible oxygen ions sublattices. This spinel/perovskite oxide heterointerface exhibits electron mobilities more than one order of magnitude higher than those of perovskite/perovskite oxide interfaces, and demonstrates unambiguous two-dimensional conduction character as revealed by the observation of quantum magnetoresistance oscillations. Furthermore, we find that the spinel/perovskite 2DEG results from interface-stabilized oxygen vacancies and is confined within a layer of 0.9 nm in proximity to the heterointerface. Our findings pave the way for studies of mesoscopic physics with complex oxides and design of high-mobility all-oxide electronic devices.; Comment: 25pages...

‣ Magnetic Phase Transition of the Perovskite-type Ti Oxides

Mochizuki, Masahito; Imada, Masatoshi
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 24/04/2000 Português
Relevância na Pesquisa
48.68389%
Properties and mechanism of the magnetic phase transition of the perovskite-type Ti oxides, which is driven by the Ti-O-Ti bond angle distortion, are studied theoretically by using the effective spin and pseudo-spin Hamiltonian with strong Coulomb repulsion. It is shown that the A-type antiferromagnetic(AFM(A)) to ferromagnetic(FM) phase transition occurs as the Ti-O-Ti bond angle is decreased. Through this phase transition, the orbital state is hardly changed so that the spin-exchange coupling along the c-axis changes nearly continuously from positive to negative and takes approximately zero at the phase boundary. The resultant strong two-dimensionality in the spin coupling causes a rapid suppression of the critical temperature as is observed experimentally.; Comment: 9 pages, 5 figures

‣ Spin and Orbital States and Their Phase Transitions of the Perovskite-Type Ti Oxides: Weak coupling Approach

Mochizuki, Masahito
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 14/02/2002 Português
Relevância na Pesquisa
48.8392%
The magnetic phase diagram of the perovskite-type Ti oxides as a function of the GdFeO3-type distortion is examined by using the Hartree-Fock analysis of a multiband d-p Hamiltonian from a viewpoint of competitions of the spin-orbit interaction, the Jahn-Teller (JT) level-splitting and spin-orbital superexchange interactions. Near the antiferromagnetic (AFM)-to-ferromagnetic (FM) phase boundary, A-type AFM [AFM(A)] and FM states accompanied by a certain type of orbital ordering are lowered in energy at large JT distortion, which is in agreement with the previous strong coupling study. With increasing the GdFeO3-type distortion, their phase transition occurs. Through this magnetic phase transition, the orbital state hardly changes, which induces nearly continuous change in the spin coupling along the c-axis from negative to positive. The resultant strong two-dimensionality in the spin coupling near the phase boundary is attributed to the strong suppression of T_N and T_C, which is experimentally observed. On the other hand, at small GdFeO3-type without JT distortions, which correspond to LaTiO3, the most stable solution is not G-type AFM [AFM(G)] but FM. Although the spin-orbit interaction has been considered to be relevant at the small or no JT distortion of LaTiO3 in the literature...

‣ Towards stable bifunctional oxygen electrodes and corrosion resistant gas diffusion layers for regenerative fuel cells

Silva, R. A.; Soares, C. O.; Carvalho, M. D.; Jorge, M. E. Melo; Gomes, A.; Pereira, M. I. da Silva; Rangel, C. M.
Fonte: Laboratório Nacional de Energia e Geologia Publicador: Laboratório Nacional de Energia e Geologia
Tipo: Conferência ou Objeto de Conferência
Publicado em //2013 Português
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58.601387%
ABSTRACT: Regenerative fuel cells (RFCs) can provide very high energy storage at minimal weight in a dual mode system, by combining an electrolyzer and a fuel cell. Although RFCs are an appealing technology their development is still at an early stage. One key issue is the search for highly active electrocatalysts for both oxygen reduction and water oxidation. Presently, platinum is the best electrocatalyst for the oxygen reduction reaction (ORR), but has a poor oxygen evolution (OER) performance while metal oxides catalyze the OER but not the ORR. Yet, the search for the development of bi-functional oxygen electrodes is also associated to structurally stable gas diffusion layers - they must be capable of withstanding high potentials when cells are operated in the electrolyzer mode and in addition, mass transport limitations when used as a cathode in fuel cell mode. A novel approach is used in this work to tackle the issue, focussing on the development of stable gas diffusion electrodes for the oxygen reactions, having as a base high surface area LaNiO3. Previous work by the authors has optimised the synthesis of the mentioned perovskite-type oxide, prepared by a self-combustion method. The high electrochemical surface area and low porosity of the oxide has been indicated by electrochemical impedance spectroscopy (EIS) and BET measurements. A full characterization has been the subject of recent publications [1...