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Introduction The synthesis of metal chromites with spinel structures is a subject that acquired keen researcher interest from early times because of their technological applications [ 1 ]. In the last decade, the synthesis of

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In this paper the influence of alkaline-earth admixtures on the synthesis of lanthanum chromites of La1–xMxCrO3 (x=0; 0.3; M=Ca, Sr, Ca+Sr) type was studied. The formation mechanism as well as the phase composition evolution, under non-isothermal and isothermal conditions, were investigated by thermal analysis and X-ray diffraction. The structure of the compounds and of the solid solutions formed depends on the solute type by means of the structural distortion induced. The crystallinity of the chromites obtained is obviously influenced by both the temperature and the thermal treatment plateau.

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Acceptor doped lanthanum chromites are potential interconnect materials to be used in high temperature Solid Oxide Fuel Cells (SOFC). However, instability of these materials when exposed to low oxygen partial pressure causes a volume expansion that can be detrimental to the SOFC performance. The stability of La0.8Sr0.2Cr0.97V0.03O3 is determined as function ofpO2 and temperature by isothermal thermogravimetry and dilatometry. The experimental data are analysed using a simple model for the defect chemistry. The relation between expansion behaviour and change in defect chemistry is discussed using a simple structural model.

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Mössbauer spectroscopy was applied to natural chromite samples from ophiolite complexes in the Philippines. Chemical and structural characterization of the chromite samples was also carried out using X-ray absorption fine structure (XAFS), X-ray diffraction (XRD) and X-ray fluorescence spectrometry (XRF). The Mössbauer spectra of the samples consisted of quadrupole doublets ascribable to Fe3+ in octahedral site, Fe3+ in tetrahedral site, and Fe2+ in tetrahedral site. The relative percentage of Fe3+ and Fe2+ ions suggested that these Philippine samples were formed under relatively high oxygen fugacity.

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Journal of Thermal Analysis and Calorimetry
Authors: Érika Marinho, A. Souza, Danniely de Melo, Iêda Santos, Dulce Melo and Walquíria da Silva

Abstract  

Because of their electrical, magnetic and catalytic properties rare earth and transition metal mixed oxides are important compounds. Lanthanum chromites have been extensively used as solid oxide fuel cell (SOFC) interconnect materials. In this work, lanthanum chromites partially substituted by alkaline earth metals were synthesized by the urea combustion process. TG and DSC techniques were used to evaluate the presence of the organic material in the powder after reaction on the hot plate. The powders were calcinated at 900C and characterized by XRD and SEM. The results show that the particles have nanometric dimensions and the perovskite structure was formed.

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Calcium chromite, CaCr2O4, was prepared and its purity and stoichiometry were ascertained by chemical analysis and X-ray diffraction methods. The thermal diffusivity, specific heat capacity and thermal conductivity of calcium chromite were measured by Laser Flash method using an Ulvac-Sinku Riko TC-3000 series instrument in the temperature range of 298 to 1100 K. The heat capacity data were utilised to calculate the thermodynamic parametersenthalpy increments, entropy increments and free energy increments—in the above temperature range.

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The catalytic effect of two different groups of chromites on the non-isothermal decomposition of KClO4was studied. TG and DSC curves of the thermal behaviour obtained for KClO4-alkaline earth chromites (Series 1) mixtures indicate the formation of K2Cr2O7, through a solid-solid interaction, before accelerating the decomposition stage of KClO4. Such an accelerating effect becomes more pronounced in the case of admixing KClO4 with some transition metal chromites (Series 2). The results were discussed taking into consideration the electronic configuration of cations and the electrical properties of the chromite catalysts. The presence of coordinatively active cations, able to form surface complex with the oxygen of the perchlorate anion, was proved to be necessary for obtaining an active catalyst. The kinetic parameters and models describing the catalyzed thermal decomposition process of KClO4 were evaluated by using a computer program that allows the analysis using five different methods. It was found that the adopted kinetic model for pure KClO4 and that mixed with catalysts from Series 1 is one-dimensional movement of phase boundary. On the other hand, random nucleation mechanism was achieved in the presence of catalysts from Series 2. Finally, a tentative reaction mechanism consistent with the obtained results was suggested.

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Introduction Transition metal and rare earth oxides have been known to exhibit interesting magnetic, electrical, and catalytic properties bringing tremendous potentialities to these materials. Lanthanum chromites [ 1 ], for

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Terrestrial zircon and apatite, as examples of uranium rich refractory minerals, and chromite, also highly refractory but uranium poor, were neutron-irradiated for their133Xe release studies. Uranium determination by133Xe was found not to be successful in these refractory samples, due to incomplete fissiogenic-xenon release at 1600°C as revealed by stepwise heating experiment. The high temperature release peaks appeared at ≾1500°C in these terrestrial minerals studied, but at 1000∼1200°C in reported meteoritic whitlockite and chromite.

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