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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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Plazmové nástřiky ve zvýšeném gravitačním poli
Brožek, Vlastimil
Porosity – and thus density – of plasma sprayed materials can be influenced by kinetic parameters of the particles flying in the plasma jet. These parameters can be improved by increased gravity field. In an analogy to centrifugal casting, we have verified the possibility of plasma deposition on substrates placed on the inside perimeter of a centrifuge. A WSP® plasma generator with a 100 kW power was used. The calculated gravity acceleration was 307 ms-2, corresponding to a 30 MPa pressure on the substrate. Alternative method of ceramics densification, HIPping at 150 MPa, reduces porosity to below 5%. In our case, porosity of an alumina deposit was reduced from 8.74% to 7.82%.
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Interakce vodíkového plazmatu s ferity
Hofman, R. ; Vacík, M. ; Brožek, Vlastimil ; Eliáš, M. ; Janča, J.
Ferrites are a well-known ferroelectric materials based on metal oxides with different oxidation levels. They found thanks its advisable properties a broad field of exercise, especially in electronics. In our work we made an effort to amplify this field of exercise. An intention of our work was explaining how the ferrites behave in strongly reducing atmosphere realized in hydrogen plasma. The emphasis was laid on readiness and selectivity of reduction. And further we are interested in morphology and granulometry of obtained products and last but not least in mechanical properties, especially of obtained ceramics composite with metal matrix ( MMC
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Plazmochemická redukce a karbidace ilmenitu
Janča, J. ; Eliáš, M. ; Brožek, Vlastimil
Plasma spraying is a thermal spraying method applied mostly to produce coatings and functionally graded materials. Plasma spraying of ilmenite free-standing parts and coatings on metallic substrates has been mastered at IPP Praha. This report concerns compact ilmenite parts undergoing a reduction in hydrogen plasma, inductively and capacitive generated. In accordance with theoretical predictions, metallic iron was selectively reduced from ilmenite, while any form of hydrogen present in the plasma did not reduce titania. After the reduction, the products did not change their dimensions and shape, apart from a minute contraction (whose extent is now being measured). The thermal expansion coefficient of ilmenite is 9,98.10-6 K-1 in the 100-750°C range; for our products, it is slightly lower. The goal of further research is to find conditions to obtain anatase structure of the titania in the formed composite.
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Impregnace keramických membrán anatasem
Vacík, M. ; Hofman, R. ; Mastný, L. ; Šrank, Z. ; Brožek, Vlastimil
Catalytic properties of anatase have been known for a certain time but its practical usage has been hindered by technological limits of preparation of either compact massive intermediate products of various shapes which are necessary as a component of catalytic reactors or nanosized anatase powder. Our aim was to prepare ceramic membrane which would contain catalytic form of titanium oxide - i.e. anatase in its entire volume or on its surface only. Ceramic membranes of any shape (mostly tabular or piped) may be prepared by hot spattering technology. If plasma torch is used we call this process plasma spraying. The advantage of plasma spraying is in chance to provide semi-finished membranes with high dimension accuracy with defined radius of voids and/or as functionally graded materials, i.e. with changing concentration of some ceramic, metallic or combined components. The pores developed in membranes can be readily filled up with substances that allow modification of its properties. In this area, the possibility to impregnate membrane ceramic intermediates with other functional components, e.g. photocatalytic forms of titanium oxide, arises as well.
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