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Synthesis, processing and properties of high-temperature ceramic oxygen conductors
Alemayehu, Adam
Ceramic oxygen conductors are important materials used as solid electrolytes in solid oxide fuel cells and oxygen sensors. Among the many electrolyte materials, cerium dioxide is attracting a lot of attention. This is mainly due to ceria's lower operating temperature than the widely used yttria-stabilized zirconia. Moreover, the doping of cerium dioxide with other lanthanide ions is easily carried out and the final material is stable due to the similar properties of different lanthanides. The most common doping element for cerium dioxide is gadolinium. In this thesis, a study of the precipitation of cerium and gadolinium with oxalic acid was studied focusing especially on the homogeneity of the final oxalate precipitate. The thermal decomposition of the oxalates into oxides and the homogeneity of the final oxide materials were studied. Additionally, large crystals of lanthanide oxalates were obtained by homogeneous precipitation using oxamic acid. The consolidation of the prepared oxides via ultrafast high- temperature sintering was compared with the conventional furnace sintering and the effect of electric fields on grain growth of CeO2 nanocrystalls was described. Key words: ceramic materials, oxygen conductors, oxalates, cerium dioxide, sintering, field- assisted sintering
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Synthesis, processing and properties of high-temperature ceramic oxygen conductors
Alemayehu, Adam ; Tyrpekl, Václav (advisor) ; Jankovský, Ondřej (referee) ; Grasso, Salvatore (referee)
Ceramic oxygen conductors are important materials used as solid electrolytes in solid oxide fuel cells and oxygen sensors. Among the many electrolyte materials, cerium dioxide is attracting a lot of attention. This is mainly due to ceria's lower operating temperature than the widely used yttria-stabilized zirconia. Moreover, the doping of cerium dioxide with other lanthanide ions is easily carried out and the final material is stable due to the similar properties of different lanthanides. The most common doping element for cerium dioxide is gadolinium. In this thesis, a study of the precipitation of cerium and gadolinium with oxalic acid was studied focusing especially on the homogeneity of the final oxalate precipitate. The thermal decomposition of the oxalates into oxides and the homogeneity of the final oxide materials were studied. Additionally, large crystals of lanthanide oxalates were obtained by homogeneous precipitation using oxamic acid. The consolidation of the prepared oxides via ultrafast high- temperature sintering was compared with the conventional furnace sintering and the effect of electric fields on grain growth of CeO2 nanocrystalls was described. Key words: ceramic materials, oxygen conductors, oxalates, cerium dioxide, sintering, field- assisted sintering
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Preparation and characterization of bi-phasic magnetic nanoparticles
Alemayehu, Adam ; Tyrpekl, Václav (advisor) ; Brázda, Petr (referee)
Title: Preparation and characterization of bi-phasic magnetic nanoparticles Author: Bc. Adam Alemayehu Department: Department of Innorganic Chemistry Supervisors: doc. RNDr. Daniel Nižňanský, Ph.D. RNDr. Václav Tyrpekl, Ph.D. Consultant: M.Sc. Dominika Zákutná Abstract The work is focusing on the magnetic properties of ferrimagnetic spinel ferrites before and after the introduction of a shell on a core particle with different magnetic properties. The core nanoparticles were prepared by hydrothermal decomposition of oleate precursors. The introduction of shell was carried out by hydrothermal treatment of the already prepared core particles. The phase composition, spinel structure and the sizes of the prepared samples were investigated by powder X-ray diffraction and by transmission electron microscopy. Formation of the shell on top of the core particle was determined indirectly, using Mössbauer spectroscopy at room temperature. The interparticle interactions and the particle shape was studied by small angle X-ray scattering. The magnetic properties of the prepared samples were measured on a SQUID magnetometer. Key words: magnetic nanoparticles, core, shell, spinel structure, powder X-ray diffraction, Mössbauer spectroscopy, magnetic measurements, TEM
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Size evolution of spiral magnetic structure in cobalt chromite nanoparticles
Alemayehu, Adam ; Nižňanský, Daniel (advisor) ; Klicpera, Milan (referee)
Název práce: ývoj spirální magnetické chromitových nanočásticích Katedra anorganické chemie, PřF UK Vedoucí bakalářské práce: RNDr. Daniel Nižňanský, Ph. mail vedoucího: Magnetické nanomateriály hrají důležitou roli v posledních desetiletích díky zajímavým aplikacím . Multiferroické materiály jsou materiály, u nichž existují dva typy jevů, magnetické a feroelektrické uspořádání. To znamená, že se může indukovat magnetizac ým polem nebo elektrická magnetickým polem. chromitý patří do druhé skupiny feroických látek, nazývaných multiferroika typu II. Tyto eroické materiály jsou relativně nové. V případě, že v těchto materiálech existuje feroelektrika, musí být v těchto materiálech přítomen zvláštní typ magnetického uspořádání. chromitý ) krystalizuje ve struktuře kubického spinelu, kde jsou umístěny v tetraedrických a okta ický polohách zhlucích dochází k ferrimagnetickému uspořádání CoCr při systém dále prochází magnetickým fázovým přechodem uspořádání do řady magnetických spirál s dlouhým dosahem při přechodové teplotě 26 K Současně byla pozorovaná dielektrická anomálie pod spirálním magnetickým přechodem této práci je prezentován vývoj spirálového uspořádání se změnou velikosti částic. íčová slova: prášková röntgenová difrakce, difuzní rozptyl neutronů, hydrotermální metoda
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