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Plazmochemická příprava olovnato-wolframové bronzi
Brožek, Vlastimil ; Janča, J. ; Eliáš, M.
Tungsten bronzes are materials based on tungsten oxide that exhibit different colours, excellent chemical resistivity, metallic appearance and good conductivity. The doped tungsten bronzes may by used as anodic catalyst in fuel cells. The tungsten bronzes are prepared by reduction of tungstates. The reduction process is known for very long time, but the correlation between their stoichiometry, conductivity and colour are not yet completely understood. The published results about colour of products are very often misleading. In this paper, we study chemical reactions between lead-tungstate and hydrogen plasma. The reduction was performed in an inductively coupled r.f. discharge. The r.f. generator operated at the frequency of 27 MHz with the power output of 1 kW. The PbWO4 was heated by the discharge to the temperature of 900°C. The resulting samples were analysed by SEM, XRD and three point L*a*b* method. The XRD spectra of our plasmachemical product show diffraction patterns of metal tungsten and lead-tungstate bronze with a cubic phase similar to Pb0,261WO3 (P4/mbm). The diffraction peaks of original lead-tungstate bronze are higher for the sample reduced only 60 min than for 120 min. To obtain data for process kinetics observation, further experiments were performed with ground lead-tungstate powder at two size fractions. Analytical balance, after determining the W and Pb content, showed the bronze composition to be most likely close to Pb 1-X = 0,225.
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Perpectives of cyech lithium sources exploitation
Brožek, Vlastimil ; Dufek, V.
In the mineral reserve balance of Czech republic there is filed more than 53 Mt of Sn-W ores with 0,208 - 0,286 % Li content, but unfortunately this low content did not lately allow to take advantage of their exploitation in competition with cheap processing e.g. of German company CHEMETALL. In sixtieth years after promising start of lithium carbonate production from zinnwaldite in Lachema Kaznějov the researchers of Institute of Chemical Technology Prague prepared metallic lithium by classical electrolytic process from chloride melt. The metal was destined first for lithium hydride an lithium aluminium hydride production. The establishing of such production failed from different reasons. The predominant part of domestic lithium sources is constituted as ferrous micas. Therefore the possibility to obtain concentrates, which would be acceptable from economic point of view reveals in magnetic separation, especially by the use of highly intensive superconducting magnets. They can deliver concentrates with about five time higher lithium content.
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Behaviour of tungsten carbide during plasma deposition
Brožek, Vlastimil ; Flemr, V. ; Matějíček, Jiří ; Neufuss, Karel
Analysis of the spheroidized tungsten carbide powder prepared by plasma spraying showed marked changes in chemical and phase composition, especially in dependence on particle size. In sub-micrometric or nanometric particles, the bonded carbon content is reduced to nearly zero and increases with particle diameter. Nanometric particles consist of almost pure tungsten. this permits to draw some conclusions about the tungsten carbide decomposition and carbon transport in molten carbide under extreme conditions of plasma melting and deposition onto solid substrates.
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