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[en] The transformations induced in two clays differing in Ca content, by firing under reduced conditions up to 10800C, were studied by X-ray diffraction, scanning electron microscopy, and magnetization measurements. In the calcareous clay, gehlenite forms at lower temperatures (9000C) and, in addition, wollastonite forms at higher temperatures (10800C). Ferric iron persists even under strongly reducing conditions and its presence is attributed to trapping in gehlenite. Extensive vitrification is observed in the noncalcareous clays. Ferrous iron, produced by dissociation of iron oxides, is partly dissolved into the vitreous matrix and partly incorporated into the spinel mineral hercynite. The key role of Ca in controlling the above transformations was verified by studying the clays after removal or addition of calcite. The interaction of Ca with the clay constituents and its progressive attack on the quartz grains forming wollastonite zones was observed with the electron microscope. The bulk magnetic properties of the samples depended principally on the amount of iron oxides present, which in turn were strongly affected by firing temperature and type of clay. Metallic iron was detected in strongly reducing atmospheres
[en] Single-phase polycrystalline powder samples of the double perovskite oxide LaSrMnCoO6 were synthesized by the Pechini (citrate-gel) technique. The structural, magnetic and electrical properties of the obtained powders were investigated by X-ray diffraction, electron microscopy, dc magnetization, ac susceptibility and dc resistivity measurements. The crystal structure of the new compound was found to be cubic of space group Fm3-barm at room temperature. Below 225 K, the samples exhibit ferrimagnetic behavior with a spin-glass-like character. Resistivity measurements indicate semiconducting behavior with two different conductivity mechanisms: thermally activated behavior below 190 K and variable range hopping above 190 K
[en] Atomic positional disorder of a single-phase natural carbonate fluorapatite (francolite) is revealed from analysis of the atomic displacement parameters (ADPs) refined from neutron powder diffraction data as a function of temperature and carbonate content. The ADPs of the francolite show a strong disturbance at the P, O3, and F sites. When it is heat treated to partially or completely remove the carbonate, the ADPs as well as the other structural parameters resemble those of a fluorapatite (Harding pegmatite) that was measured under the same conditions. The various structural changes are consistent with a substitution mechanism whereby the planar carbonate group replaces a phosphate group and lies on the mirror plane of the apatite structure. (c) 2000 Materials Research Society
[en] The removal of metal ions (Pb, Cd, Zn, and Cu) from spiked aqueous solutions using four lignite samples (TH2, TH7, MT2, and MT8) of different quality, from different areas in Greece, was investigated. Cation exchange capacity, humic and fulvic acid content, and the BET specific surface area of the samples were determined, proximate and ultimate analyses were conducted and the mineralogy of their low temperature ash was studied. Equilibrium and kinetic studies were performed in batch conditions. Competitive adsorption of the four elements examined was also investigated. It was observed that the four lignite samples were considerably effective in removing Pb, Cd, Zn, and Cu ions from aqueous solutions, with the sample MT2 being the most effective. Among the elements, Pb appeared to have the strongest affinity based on a mass uptake by lignite samples. The same behaviour was observed during the competitive adsorption experiments. Kinetic experiments proved that, in all cases, equilibrium was achieved within 45 min. Sorption isotherm studies were conducted by varying the initial concentration of the elements. MATLAB software was used to fit experimental data to Langmuir and Freundlich equations. The data were better fitted to the Langmuir equation. Attempt was made to correlate the adsorption behaviour of the lignite samples with the mineralogy of their low temperature ashes and their content in humic and fulvic acids.
[en] An X-ray Diffractometer for Powder Crystallography is described along with experimental results and future plans. This is an intermediate instrument toward a long linear array system. Three channels of a silicon microstrip detector, are the detecting elements in the present instrument. Each detector channel is followed by a VLSI readout chain, which consists of a charge preamplifier with pulse shaping circuitry, a discriminator, and a 16-bit counter. Control and data acquisition is performed with a custom made PC readout card. A motorized goniometer scans the angle range of interest. Calibration of the system is done with reference samples and data which are captured with a one-channel conventional NaI detector. (Copyright (c) 1998 Elsevier Science B.V., Amsterdam. All rights reserved.)