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Solid solutions in the MgO-Al2O3-Cr2O3 system

Effects of polymorphism, temperature and pressure

Journal of Thermal Analysis and Calorimetry
Authors: A. Turkin and V. Drebushchak

Abstract  

Coexisting solid solutions with spinel and corundum structure were synthesized at 1773 K and two pressures, 1 bar and 25 kbar. Samples were analyzed by electron microprobe analysis and X-ray powder diffraction. Pressure and temperature were shown to affect the properties of the solid solutions in different ways. Pressure governs the composition of the defect spinel Mg1−xAl2O4, and temperature changes the cation distribution between coexisting phases. This allows one to separate the effects of cation exchange and magnetic contribution to the heat capacity in thermodynamic modeling. The defect spinel itself can form only because γ-Al2O3 exists, polymorph with spinel structure. Thermodynamic considerations argue in favor of eskolaite-spinel assemblages prevailing over corundum-picrochromite ones at very high temperatures deep in the Earth.

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Abstract  

Samples of the MgO–Al2O3–SiO2 ternary system, constituted by 28.5 mol% ofMgO, 28.5 mol% of Al2O3 and 43mol%of SiO2, were activated in a roll mill and calcined at different temperatures. The influence of the grinding time, the used SiO2 precursor and activation medium, furthermore the mass ratio between the powdered sample and zirconia cylinders was investigated on the reactivity of the MgO–Al2O3–SiO2 ternary system. FTIR spectra and the X-ray powder diffraction patterns indicates the formation of Mg(OH)2 at 393 K, of forsterite (MgSi2O5) and enstatite (MgSiO3) at 1223 K and of spinel (MgAl2O4) between 1223 and 1523 K in some samples. The presence of cordierite (Mg2Al2Si5O18) was observed at 1523 K, a reaction pathway concerning its formation was proposed.

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The thermal behaviour of hydroxide mixtures aimed at Mg-Al spinel preparation is reported. The mixtures of hydroxides were prepared by precipitation reaction from Mg and Al nitrate solutions, stoichiometric to the spinel formation. Hydroxide mixtures with different phase distributions were investigated, prepared by varying the precipitation procedure. The results were related to the thermal behaviour of mechanical mixtures of separately precipitated hydroxides. The spinel formation was identified performing XRD analysis on powder samples heated at different temperatures. The coprecipitated mixtures are completely decomposed to spinel at 400‡C. The presence of the Mg-Al mixed hydroxide phase in the mixture is of primary concern to get spinel at low temperatures.

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Journal of Radioanalytical and Nuclear Chemistry
Authors: N. Nava, J. Jacobs, A. García, J. Rosink, M. Valenzuela, L. Van Ijzendoorn, and H. Brongersma

Abstract  

ZnxMg1-xFe2O4 mixed oxide spinels (x=0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) were prepared by conventional solid state reaction method. Bulk and surface characterization of these ferrites were carried out by different techniques. The ratios Zn/Fe and Mg/Fe determined by PIXE and AAS were nearly as expected from the synthesis mixture, i.e. following the substitution model. XRD shows only a spinel structure (fcc) with increasing lattice parameter as x increases from 0 to 1. The bulk ratios of tetrahedral to octahedral sites occupied by Fe cations measured by NGR are in good agreement with the theoretical bulk stoichiometry. LEIS results indicate a Mg substitution by Fe cations at the surface. Only octahedral sites are proposed at the surface. The transition from inverse to normal spinel was follow through NGR-spectra occurring at x=0.4.

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Abstract  

The electrical conductivity of pure and Mn-doped MgAl2O4 spinel has been studied in nitrogen atmosphere as a function of temperature. The conduction process is explained by the motion of cation vacancies in each of -irradiated and unirradiated samples. The effect of -irradiation on the conductivity and activation energy has been discussed.

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Abstract  

The products of dickite heated in air at 1000 to 1300°C were studied using curve-fitting of transmission and photoacoustic infrared and micro-Raman spectra. The spectra were compared with those of mullite, Al-spinel, corundum, cristobalite, amorphous silica and meta-dickite. Bands that characterize crystalline phases appeared at 1100°C and became stronger with increasing temperature. Mullite, Al-spinel, corundum and amorphous silica were identified by their characteristic bands. The characteristic IR bands of cristobalite overlap those of mullite and amorphous silica, and its presence was therefore established from intensity ratios of the appropriate bands. The research clearly demonstrated the advantage of using curve-fitting for the identification of high temperature phases in the study of the thermal treatment of kaolin-like minerals by infrared and Raman spectroscopy. This technique seems to be a useful method for materials analysis in the ceramic industry.

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Products of hydrothermal treatment of the initial amorphous system MnxFe2−2x(OH)6−4x for 0≤x1 in 0.1x intervals, and products of their further thermal treatment, were examined by chemical analysis, X-ray, IR, and DTA techniques supported by magnetic measurements. After hydrothermal growth for lowx, hematite and goethite phases occurred. Although the goethite phase was still identifiable atx=0.6, formation of a solid solution with the isostructural groutite was not found. The ferrimagnetic spinel phase, which resists heating up to 400‡C, was present at 0.5≤x≤0.9. At higher temperatures, it transformed into the rhombohedral hematite type phase or into the cubic bixbyite phase. AtT≥900‡C, a ferrimagnetic spinel structure reappeared up tox=0.8. For x=0.9, the low- and high-temperature forms of the hausmannite phase occurred, forx= 1 passing from one form into another through Mn5O8 and partritgeite.

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Abstract  

The electrical conductivity of pure and Co-doped MgAl2O4 spinels was measured at 700–1100 K. The results of both pure and doped spinels were found to be fitted with an exponential expression with two different activation-energy parameters, in two ranges of temperature. The effect of -irradiation on the electrical conductivity of pure and doped spinels was also studied. The kinetic parameters of the isothermal annealing of the induced irradiation damages in the spinels were determined. All the results were found to be consistent with the ionic conduction which was suggested to be due to the motion of the cation vacancy.

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Abstract  

Heat capacity of stoichiometric homogeneous spinel MgFe2O4 was measured from 5 to 305 K and thermodynamic functions were derived for temperatures up to 725 K using our previous high-temperature experimental data for the same sample. Anomaly in C p was found at very low temperatures. Experimental data below 20 K contain large (up to 25% near 5 K) error arising from the difference in the thermal history between the experimental series. Magnetic contribution to the low-temperature heat capacity was tested, and the linear function was found to fit experimental data better than the three-halves power derived from the spin-wave theory.

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Abstract  

Hydrolysis of ZnO in acidic pH provides a facile route for synthesis of layered double hydroxides (LDH) bearing Zn2+ with Cr3+ and Al3+ without having carbonate ion in the inter-layer position. The thermal decomposition of the prepared LDH show that with the increase of Cr3+ content in the system there is an increase in the thermal stability of the compounds. In case of Zn-Cr LDH there is a non-mass loss transition around 420°C. Segregation of parent LDH structure to a bivalent oxide and a spinel takes place in both Zn-Cr and Zn-Al LDH only at temperatures above 550°C.

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