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Journal of Thermal Analysis and Calorimetry
Authors: M. Tachibana, T. Tojo, H. Kawaji, T. Atake, N. Morita, H. Ikuta, Y. Uchimoto, and M. Wakihara

Abstract  

Heat capacity of spinel LiCr1/6Mn11/6O4-d (d=0, 0.0184)was measured between 5 and 300 K. Both compounds showed no anomaly in the measured temperature range, especially around the room temperature where a structural phase transition is reported for the parent compound LiMn2O4. The non-stoichiometric compound LiCr1/6Mn11/6O3.9816 has greater heat capacity than that of the stoichiometric LiCr1/6Mn11/6O4. Molecular dynamics study on the vibrational property of LiMn2O4-d revealed that the lattice defects in the non-stoichiometric compound increase the low frequency phonons compared with the stoichiometric compound. It should be related to the greater heat capacity of the non-stoichiometric compound LiCr1/6Mn11/6O3.9816.

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Abstract  

The thermal stability of the solid solutions of Fe2Mo1–xTixO4 for x=0.0 to 1.0 in air, had been investigated in the temperature range 303–1173 K using differential thermal analysis and thermogravimetry (DTA and TG). The products obtained by heating the sample in air, at different temperatures, have been characterized by X-ray diffraction and IR-studies. The results show that all the ferrite samples undergo surface oxidation during initial heating. On heating to 823 K, the samples undergo oxidation of the octahedral site cations only and forma cation deficient spinel phase. On further heating in air, the ferrites undergo complete oxidation.

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Abstract  

Spinel (MgAl2O4) was synthesized mechanochemically (MC) by grinding MgO with γ-Al2O3 up to 10 hours. Examination of the MC product by neutron diffraction and infrared spectroscopy showed that it has a higher degree of inversion than its thermally produced counterpart—47% as against 10% respectively. X-ray studies showed that MgAl2O4 crystallites grow equidimensionally at a much higher rate than in the case of α-Al2O3. The higher degree of inversion and higher formation rate when γ-Al2O3 is used, is attributed to similarities in oxygen framework of MgO, Al2O3 and spinel and to the higher retention of the cations coordination number. The equidimensional growth is attributed to the presence of multiple soft modes, the {111} planes.

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The thermal behaviour of hydroxide mixtures, precursors to the synthesis of MgFe2O4 spinel powders, was investigated.

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Abstract  

Chromium doped spinels LiCrYMn2−YO4 (0.2≤Y≤0.8) has been synthesized by the sucrose-aided combustion procedure. The thermal behaviour, phase homogeneity and structural characteristics of the samples were studied by thermal analysis, coupled mass spectrometry, and room-and high-temperature X-ray diffraction methods. It was found that the ‘as prepared’ samples contained residual organic impurities undetectable for X-ray diffraction, that burn out completely at 400°C. Samples treated between 400 and 750°C are single phase spinels, whose crystallites size increase from 10 to 50 nm on increasing the temperature. Cr-doping enhances the thermal stability of the spinels, which augments on increasing the Cr content Y. The enhanced thermal stability of the spinels has been accounted for based on the high excess stabilization energy of Cr3+ in octahedral ligand field.

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The thermochemical reactivity of the spinel-type quaternary metal oxide Cu1−xZnxAl2O4 has been investigated for different Cu:Zn ratios. In oxygen or inert gas atmospheres no considerable reduction is observed. In molecular hydrogen metal selective reduction of the Cu is found at relatively high temperature. The solid reduction product is made up of sintered, poorly dispersed metallic copper on a Zn-Al-O metal oxide support, a potential catalyst for the methanol synthesis. Owing to the measured high reduction temperature leading to the mentioned sintering of the metallic copper, the activity of this system cannot be high.

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Reaction Kinetics, Mechanisms and Catalysis
Authors: Souhila Boumaza, Aline Auroux, Simona Bennici, Amel Boudjemaa, Mohamed Trari, Aissa Bouguelia, and Rabah Bouarab

Abstract  

Cu-based mixed oxides were prepared by a co-precipitation method and characterized by XRD analysis, BET and H2-TPR. For all catalytic systems, it was found by XRD analysis that the spinel phase is the majority phase. Catalysts were tested for the production of hydrogen via the water gas shift reaction between 150 and 250 °C. It was demonstrated by TPR that Cu–Zn–Al and Cu–Al were easily reduced. This may be responsible for relatively high catalytic activity in comparison with the results obtained by Cu–Co and Zn–Al catalysts prepared by the same method. The oxide systems can be ranked as follows: Cu–Zn–Al > Cu–Al > Cu–Mn, Cu–Cr > Cu–Fe ≫ Zn–Al > Cu–Co at 250 °C.

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Abstract  

The electrical conductivity and IR-spectra of pure and Cu-doped Fe3O4 spinels were measured at 300–1000 K. Two breaks in the conductivity-temperature curves have been observed for all investigated pure and doped samples. One of these two breaks were found near the Curie point of the investigated spinel. The electrical conduction in -irradiated and non-irradiated pure and Cu-doped Fe3O4 occurred by a hopping mechanism due to a fast electron exchange between Fe2+ and Fe3+-ions present on octahedral sites. The Seebeck-voltage of the irradiated and non-irradiated pure and Cu-doped samples has been measured. The effect of -irradiation on the conductivity values //, activation energy and type of defects was discussed.

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Abstract  

Thermogravimetric analysis was used in order to study the reduction in air of submicronic powders of Co3−xMnxO4 spinels, with 0 ≤ x ≤ 1. For x = 0 (i.e. Co3O4), cation reduction occurred in a single step. It involved the CoIII ions at the octahedral sites, which were reduced to Co2+ on producing CoO. For 0 < x ≤ 1, the reduction occurred in two stages at increasing temperature with increasing amounts of manganese. The first step corresponded to the reduction of octahedral CoIII ions and the second was attributed to the reduction of octahedral Mn4+ ions to Mn3+. From the individual weight losses and the electrical neutrality of the lattice, the CoIII and Mn4+ ion concentrations were calculated. The distribution of cobalt and manganese ions present on each crystallographic site of the spinel was determined. In contrast to most previous studies that took into account either CoIII and Mn3+ or Co2+, CoIII and Mn4+ only, our thermal analysis study showed that Co2+/CoIII and Mn3+/Mn4+ pairs occupy the octahedral sites. These results were used to explain the resistivity measurements carried out on dense ceramics prepared from our powders sintered at low temperature (700–750 °C) in a Spark Plasma Sintering apparatus.

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A series of samples of the system Ni0.65Zn0.35CuxFe2−xO4 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) are prepared by the usual ceramic technique. X-ray analysis shows that they are cubic spinel (single phase). The lattice parameter, theoretical density (D x), bulk density (D) and the porosity (P) are measured for the samples. The vacancy concentration of oxygen is an important parameter in the sintering process of spinel ferrites. The decrease in the population of Fe3+ ion in the octahedral sites with the introduction of Cu2+ results in the decrease of lattice parameter. The DTA tracing shows a strong exothermic peak at 90°C.

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