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Abstract  

Kinetics of the oxidation of magnetite (Fe3O4) to hematite (a-Fe2O3) are studied in air using simultaneous TG/DSC. The mechanism is complex and the differences between the kinetic conclusions and Arrhenius parameters based on either TG or DSC are discussed. As in our previous work on CaCO3 [1], the determination of a satisfactory baseline for the DSC results adds considerable uncertainty to those kinetic results. Consequently the calculations based on the TG data are considered superior. Solid state reactivity varies from one source of material to another and the results are compared for two different commercial samples of magnetite, both presumably prepared by wet chemical methods. These materials are much more reactive than the material studied previously [2], which had been coarsened and refined at high temperatures. In that earlier study, the metastable spinel, g-Fe2O3, was formed as an intermediate in the oxidation to the final stable form, a-Fe2O3. The exothermic reaction of the gamma to alpha form of the product during the oxidation process destroys the direct comparison between the TG and DSC results, since the former only detects the change in mass of the sample and not the crystallographic transformation. The TG results, however, represent the true oxidation process without superposition of the structural aspects.

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Journal of Thermal Analysis and Calorimetry
Authors: M. Jiménez de Haro, L. Pérez Maqueda, E. Stepkowska, J. Ma Martínez, and J. Pérez-Rodríguez

Abstract  

Grinding and contact with water or salt solution increased the specific surface (ssa) but lowered the first dehydration effect (escaping up to 150C) and increased the second dehydration effect (150 to 500C). The dehydroxylation was moved to lower temperatures and was only ΔM(500-1100C)=3.70.3 % as compared to 5.5% in the parent vermiculite (V). Except ΔM(20-150C), the mass losses measured at the remaining T ranges, were consistent in the ground samples, thus the grinding for 2 min caused the homogenization of the crystal structure of vermiculite [ΔM(150-500C)=7.60.7%]. DTA curves after grinding and cation exchange indicate an important exothermal peak at 795-870C, its temperature depending on exchangeable cation. It indicates the formation of high temperature phases (enstatite, forsterite, spinel). The lowest temperature of the peak (795C) was observed in V-gr-Li, here lithium silicate was formed. The highest peak temperature (870C) was found in V-gr-K, where almost only forsterite developed. These exothermal peaks were very weak in unground V with various exchangeable cations.

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Lithium cobaltate LiCoO 2 and Li-Mn oxides nanopowders are synthesized by plasma chemical method in RF-IC plasma flow. Plasma forming gases are air or nitrogen, additionally air is used as oxidizing and quenching gas. As-synthesized and annealed in air nanopowders are investigated by powder XRD, BET, DTA/TGA, SEM, TEM methods and wet chemical analysis; lattice parameters are estimated by the programme SCANIX. Evaporation of initial powder mixtures Li 2 CO 3 +Co with Li/Co molar ratio 1:1 or Li 2 CO 3 +(Mn 2 O 3 +MnO 2 ) with various Li/Mn molar ratios, subsequent quenching and condensation of products results in obtaining of nanopowders with SSA of 13-23 m 2 /g and average particle size of 67-95 nm. After synthesis nanopowders contain admixtures of lithium nitrate hydrate LiNO 3 ·nH 2 O by-phase because of plasma flow composition and air environment. After heat-treating at 600-1000°C pure LiCoO 2 , LiMn 2 O 4 spinel, Li 2 MnO 3 , LiMn 2 O 4 +Li 2 MnO 3 are formed with good crystallinity and average particle size of 70-240 nm. Prepared nanopowders can be applied as cathode materials for Li-ion batteries with appropriate electrochemical properties for high discharge rate.

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Journal of Thermal Analysis and Calorimetry
Authors: D. Bhosale, N. Choudhari, S. Sawant, V. Patil, P. Kulkarni, and V. Kelkar

Abstract  

Homogeneous solid solution oxalates of Fe2+, Cu2+, Mg2+ and Zn2+ metals were prepared by co-precipitation from respective metal acetate solutions with oxalic acid solution. The thermogravimetric (TG) analysis of co-precipitated oxalate complexes with general formula MgxCu(0.50-x)Zn0.50Fe2(C2O4)3nH2O (x=0.00, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50) were carried out by manual method in static air atmosphere. The total mass loss % and stepwise mass loss % values are in good agreement with theoretically calculated mass loss % values. The thermal decomposition of oxalate complexes occur at relatively lower temperatures (561 to 698 K). The lowering of decomposition temperatures may be attributed to earlier initiation of Fe2+ oxalate in oxalate complexes. At temperatures between 598–698 K the thermal decomposition of Cu-Mg-Zn-Fe solid solution oxalate complexes leads to formation of ferrites of spinel structure. After tampering at 873 and 1273 K, homogeneous ferrites arise, which is revealed from XRD studies.

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Comparative thermogravimetric and heat-flux DSC investigations of phase formations by heating of sinteractive powders, which were prepared by thermal decomposition of a NiCO3∶ MnCO3=1∶2 mixture and thermal decomposition of oxalate mixed crystals NiMn2(C2O4)3.6H2O, show the metastability of the defect spinel from the oxalate precursor and its high reactivity.

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Abstract  

The purified bentonite parent clay, fraction ≤; 2 mm of montmorillonite type, has been pillared by various polyhydroxy cations, Al, AlFe and AlCu, using conventional pillaring methods. The thermal behavior of PILCs was investigated by combination of X-ray diffraction (XRD), thermal analysis (DTA, TG) and low temperature N2 adsorption/desorption (LTNA). Thermal stability of Al-, AlFe- and AlCu-PILC samples was estimated after isothermal pretreatment in static air on the temperatures 300, 500, 600 and 900C. Crucial structural changes were not registered up to 600C, but the fine changes in interlayer surrounding and porous/microporous structure being obvious at lower temperatures, depending on the nature of the second pillaring ion. AlFe-PILC showed higher thermal stability of the texture, the AlCu-PILC having lower values and lower thermal stability concerning both overall texture and micropore surface and volume. Poorer thermal stability of AlCu-PILC sample at higher temperatures was confirmed, the presence of Cu in the system contributing to complete destruction of aluminum silicate structure, by 'extracting' aluminum in stabile spinel form.

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The high-temperature (950–1500°) changes in synthetic montmorillonite of relatively simple chemical composition, studied by X-ray diffraction analysis, infrared spectroscopy and electron microscopy, are described. It was found that this montmorillonite belongs to the Wyoming type and the high-temperature phases involve cristobalite, mullite, anorthite and spinel. Only mullite crystallized from this sample on heating for two hours at 1500°.

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Abstract  

The CO2 absorption properties and the microstructure of (Ba,Ca)(Fe,Mg)O3 - have been studied by TGA, XRD, and Mössbauer spectrometry. Paramagnetic doublets of FeIV and FeIII appeared in the Mössbauer spectra of cubic (Ba0.5Ca0.5)(Fe0.5Mg0.5)O3 - heated in CO2 up to 600 °C, and a pair of sextets of tetrahedral FeIII (Hin = 43 T) and octahedral FeIII (Hin = 51 T) were produced above 800 °C, and an additional sextet characteristic of FeIII in a spinel structure (Hin = 48 T) was observed at 1000 °C. On the other hand, a pair of sextets of tetrahedral and octahedral FeIII of the orthorhombic (Ca0.95Ba0.05)(Fe0.5Mg0.5)O3 - showed hardly any change after absorption of CO2. It is concluded that only a small portion of Mg entered the orthorhombic phase of (Ca0.95Ba0.05)(Fe,Mg)O3 - and Mg preferred the octahedral B site of the perovskite lattice. The excess Mg formed separate CaO-MgO mixed oxide, and the primary mechanism of CO2-trapping is the formation of CaMg(CO3)2.

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Abstract  

Perovskite oxides of composition (Sr1-xCax)(Fe0.5Co0.5)O3- were investigated for CO2 absorption properties and were proved to be useful as materials for CO2 absorption in the temperature range from 550 to 850 °C. The absorption rate of CO2 increased with Ca doping. The mechanical treatment of perovskite oxides for several minutes, especially for the oxides containing a small amount of Ca, was found to be effective for activating the oxides for CO2 absorption and for reducing the starting temperature of CO2 absorption by about 80 °C. However, it was not less effective to treat the oxides for a long time. The site distortion due to Sr and Ca ions at site A and the mixed valence states at site B were confirmed to be effective for CO2 absorption at high temperatures. During the absorption of CO2, a spinel compound was formed according to the following reaction: 2(Sr,Ca)(Fe,Co)O2.5 + CO2 (Sr,Ca)CO3 + (Sr,Ca)(Fe,Co)2O4.

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Introduction The spinel ferrites, MeFe 2 O 4 (where Me is a metallic element or a group of metallic elements), are from decades in the centre of many researches all over the world due to the different and interesting

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