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Microstructural Stability of Weld Joints of Austenitic and Ferritic Steels
Šohaj, Pavel ; Stránský, Karel (referee) ; Sopoušek, Jiří (referee) ; Foret, Rudolf (advisor)
This doctoral thesis summarizes the theoretical and experimental knowledge in the field of dissimilar weld joint of progressive austenitic and ferritic creep-resistant steels. The following materials were selected for the presented study: 316Ti stabilized austenitic stainless steel, martensitic 9-12 %Cr steel P92 and ferritic ODS steel MA 956. The main attention was focused on the long-term microstructural stability during high temperature exposure of heterogeneous joints of the austenite / ferrite type. The literature analysis critically evaluates the current state of knowledge in the field of microstructural stability of advanced creep-resistant steels weld joints. The practical experimental part was carried out in two directions. On the basis of the chemical composition phase equilibrium calculations were performed for each steel using the ThermoCalc software, giving the basic concepts about the dependence of the phase composition and the chemical composition of phases on temperature. In parallel with these calculations the laboratory joints 316Ti/P92 made by resistance welding and the MA 956/316Ti electron beam weld joints were prepared, analyzed in as-weld state and further annealed at different temperature conditions. Exposed joints were subjected to microstructure and phase analysis. The stability of the weld interface was mainly observed. Attention was also focused on the agreement between the calculation and experimental data in comparison with data published in the literature. Based on the calculations, experimental results and published data the suitability of the combination of materials is discussed in the thesis and reasoning about the behavior of studied weld joints during long-term high temperature exposure was formulated. Based on the results the expected degree of microstructural stability of 316Ti/P92 joint was confirmed, while the joints MA 956/316Ti were found to be unstable.
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THE APPLICATION OF NANOMATERIALS FOR LEAD FREE SOLDERS DEVELOPMENT
Pešina, Zbyněk ; Pinkas, Jiří (referee) ; Spousta, Jiří (referee) ; Sopoušek, Jiří (advisor)
The present dissertation is motivated by the search for alternatives of lead-free soldering by nanoparticles of metals and their alloys. The research focuses on the possibility of replacing lead-free solders by nanoparticles. This issue is currently being addressed by the use of lead-free solders but their properties are not entirely equivalent to properties of lead-tin based alloys. The theoretical part of the dissertation first summarizes up-to date knowledge on the development of lead-free alloys currently used for soldering in the electronics. The work compares these lead-free solder candidates with previously used Pb-Sn alloys. The second section of the theoretical part is devoted to nanotechnology that offers possible solutions of problems associated with the use of lead-free solders. The text contains a description of the properties of nanocrystalline materials in comparison with those of compact alloys having the same chemical composition. The possibility of preparation of nanoparticles and potential problems associated with small particle sizes are also presented. Introduction of the experimental part focuses on the preparation of nanoparticles of pure metals and alloys by chemical and physical ways as well as on an instrumentation for characterisation and analysis. Attention is focused on the silver in nanoparticle form that exhibits the low temperature sintering effect, which is thermally activated by decomposition of oxide envelope covering the Ag nanoparticles. This factor is critical for low-temperature sintering and thus also for possible future applications. The thermal effects of the low sintering process were studied by methods of thermal analysis. The preparation of the Cu / Ag nano / Cu joints was carried out in-situ in inert atmosphere and under the action of atmospheric oxygen. In both cases varying conditions of thermal treatment were used. The cross sections of the prepared joints were then used for the metallographic analysis of the local mechanical properties of the resulting silver layer, for the chemical composition evaluation of the resulting layers of the joint, and for the microstructure study. Strength characteristics are represented by testing shear strength of individual joints.
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Differential Equations in Chemistry
Bartlová, Hana ; Sopoušek, Jiří (referee) ; Franců, Jan (advisor)
The work deals with study of chemical reaction kinetics. The work is divided into three parts. In the first part basic concepts are summarized for better understanding of the subject. The second part deals with derivation of the reaction models and numerical experiments. The last part is focused on the oscillating reactions.
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Characterization of silver nanoparticles
Župková, S. ; Sopoušek, J. ; Šídlo, M. ; Buršík, Jiří
Understanding and characterization of colloidal nanoparticles is very important for their uses in the research and technology. Colloidal silver solutions have many interesting properties such as surface plasmon resonance. Plasmons are optical effects observed for silver, gold, and copper and the other nanoparticles. The value of the wavelength of this effect depends on the particle size and shape as well as on the properties of solvent or absorbent presented. As sample there were used nanoparticles of silver, which were dispersed in aqueous solution. The fluorescence, size and zeta-potential of two samples each with different date of preparation were measured. The colloidal solutions were investigated by means of fluorescence spectroscopy, dynamic light scattering, electron microscopy, and electrophoretic light scattering for zetapotential evaluation.
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Analytical electron microscopy of lead-free nanopowder solders
Buršík, Jiří ; Sopoušek, J. ; Zálešák, Jakub ; Buršíková, V.
During the last decade, the EU legislative regulations enforced lead-free solders and hence initiated an extensive search for the best replacement of lead-containing solders. Parallel to new binary and ternary bulk solders, metal nanoparticles are also considered as potential candidates for solder materials. It is known that physical, electric and thermodynamic properties of nanoobjects are significantly different from those of the bulk materials. The oxidation, high reactivity of the surfaces and aggregation are frequent problems of nanotechnology applications. The nanoparticles of pure metals and alloys exhibit the depression of the melting point compared to bulk material, hence they are able to aggregate and to form firm interlayer joints at low temperatures. Exploiting this effect can save energy, work and materials.
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Electron microscopy of nanoparticles for lead-free soldering prepared by wet chemical synthesis
Buršík, Jiří ; Škoda, D. ; Vykoukal, V. ; Sopoušek, J.
The nanoparticles of pure metals and alloys exhibit the depression of the melting point compared to bulk material, hence they are able to aggregate and to from firm interlayer joints at low temperatures. Exploiting this effect in soldering industry can save energy, work and materials. Using emulsions with nanopowders might be the solution of demanding task of replacing classical Sn-Pb solders by their lead-free substitutes. In this work, Ag- and Sn-based nanopowders were prepared as potential low-toxic constituents of novel solders by a chemical wet synthesis from chemicals of high purity. Various ways of preparation and further storage of the product were examined. Resulting nanoparticles (their size distribution, morphology and tendency for clustering) were characterization by means of scanning and transmission electron microscopy.
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