National Repository of Grey Literature 58 records found  1 - 10nextend  jump to record: Search took 0.00 seconds. 
Electronic properties and structure of selected rare earth and uranium compounds: influence of impurities
Pospíšil, Jiří ; Sechovský, Vladimír (advisor) ; Schneeweiss, Oldřich (referee) ; Sedmidubský, David (referee)
Title: Electronic properties and structure of selected rare earth and uranium compounds; influence of impurities Author: Jiří Pospíšil Abstract: This thesis is devoted to studies of influence of impurities and/or chemical varieties on magnetic and superconducting properties of selected metallic materials containing rare earth and uranium. The research generally consisted of the defined preparation of studied materials, detailed composition and structure characterisation, experiments focussed on determination of mutually related magnetic, transport and thermal properties followed by data analysis of relations between material composition/quality and material properties. The technology phase was carried out in two ways - 1) preparation of the best pure materials as possible and 2) controlled doping of pure materials. Three main study cases have been chosen: SmPd2Al3 - the representative of the complex magnetism of the Sm3+ ion, the so far unexplored superconductor YPd2Al3 were selected as the RE materials candidates and the controversial ferromagnetic superconductor UCoGe. The key ingredient of the initial technology phase was the refining the commercially available best element metals to the laboratory best achievable purity grade by the unique Solid State Electrotransport (SSE) method. SSE has been also...
Structural defects in II-VI semiconductors
Šedivý, Lukáš ; Belas, Eduard (advisor) ; Hulicius, Eduard (referee) ; Schneeweiss, Oldřich (referee)
Title: Structural defects in II-VI semiconductors Author: Lukáš Šedivý Department: Institute of Physics of Charles University Supervisor: Doc. Ing. Eduard Belas, CSc., Institute of Physics, Faculty of Mathematics and Physics, Charles University Abstract: The single crystalline CdTe doped by chlorine is an excellent material for man- ufacturing x-ray and gamma-ray room-temperature semiconductor detectors thanks to the large linear attenuation coefficient, the possibility to make it high-resistive at the room temperature, and good electron mobility and the lifetime. This thesis aimed to examine the effect of annealing in well-defined ambient component pressure, Cd or Te, on the crys- tal's defect structure. The first experimental is devoted to eliminating the second phase defects - inclusions - present in CdTe : Cl, which significantly decay the crystal quality and detection performance. The following experimental parts are focused on the detailed inves- tigation of the point defect structure of CdTe : Cl. The annealing interval bisection method for reaching high resistivity material is introduced. The equilibrium defect structure is in- vestigated using the in-situ high-temperature Hall effect measurements. The results are interpreted through an advanced model of the defect structure considering also dissocia-...
Exchange bias in metamagnetic heterostructures
Zadorozhnii, Oleksii ; Schneeweiss, Oldřich (referee) ; Uhlíř, Vojtěch (advisor)
Výměnná anizotropie je zajímavý fyzikální jev vznikající na rozhraní antiferomagnetických (AF) a feromagnetických (FM) materiálů, který již je široce používán v elektronickém průmyslu a magnetickém záznamu. Přestože byl tento jev dlouhou dobu intenzivně studován, jeho přesný mechanizmus zatím nebyl uspokojivě vysvětlen. V této práci je představen přehled studií dokumentujících výměnnou anizotropii v tenkých dvojvrstvách, včetně experimentálních výsledků a teoretických modelů. Experimentální úkoly této diplomové práce zahrnovaly jak výrobu, tak měření různých modelových systémů vykazujících výměnnou anizotropii. Dvojvrstva Fe/FeRh, kde vrstva FeRh prochází fázovou přeměnou z AF fáze na FM fázi při 360 K, poskytuje možnost nastavení parametrů výměnné anizotropie. Dále byly zkoumány účinky výměnné anizotropie a tvarové anizotropie v mikrostrukturách Fe/FeRh. Konečně, přítomnost výměnné anizotropie byla zkoumána mezi FM a AF fází koexistujícími během fázové přeměny v nanodrátech FeRh. Vzorky byly vyrobeny pomocí magnetronového naprašování a elektronové litografie. Všechny prezentované systémy byly analyzovány pomocí magnetooptické Kerrovy mikroskopie. Výměnná anizotropie byla úspěšně nalezena v systému Fe/FeRh, přičemž její velikost byla téměř identická co do rozsahu i orientace s výsledky v literatuře, přestože námi vyrobená dvojvrstva měla horší kvalitu FM-AF rozhraní. Bylo také prokázáno, že v tomto systému existuje tzv. tréninkový efekt (Training effect), což je výrazným důkazem existence výměnné anizotropie. U nanodrátů bylo změřena významná výměnná anizotropie mezi koexistujícími fázemi FM a AF během fázové přeměny.
U in metastable systems: structure, magnetism, superconductivity
Buturlim, Volodymyr ; Havela, Ladislav (advisor) ; Heczko, Oleg (referee) ; Schneeweiss, Oldřich (referee)
Title: U in metastable systems: structure, magnetism, and superconductivity Author: Volodymyr Buturlim Department / Institute: Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University Supervisor of the doctoral thesis: Doc. RNDr. Ladislav Havela, CSc., Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Prague, The Czech Republic Abstract: This thesis presents studies of U-Nb and U-Ti alloys and hydrides synthesized via different technological paths. Investigation of the microstructure of the alloys with different concentrations of Ti (Nb) allowed to find the optimum setting for stabilization of the metastable bcc U allotrope. Ultra-fast cooling accompanied by alloying leads to retention of materials with high degree of atomic disorder apparent from the studies of their transport properties. The alloys exhibit a weakly paramagnetic ground state and low-temperature superconductivity, the critical temperature of which has only moderate variations with Ti (Nb) concentrations and which has very high upper critical fields. Interaction with hydrogen allows to stabilize two distinctive forms of hydride: β-UH3 and UTi2Hx. β-UH3 alloyed by Ti (Nb) orders ferromagnetically with transition temperatures exceeding 170 K, weakly influenced...
MICROSTRUCTURE AND COMPOSITION OF FINE PARTICLES RELEASED BY CAR BRAKING
Švábenská, Eva ; Roupcová, Pavla ; Pizúrová, Naděžda ; Schneeweiss, Oldřich
Vehicular traffic is connected with large volume of fine particles released during brake processes of cars. Our research is focused on the phase, structure and chemical analysis of the fine particles taken from some car brake parts by their services. The information on structure and phase composition was obtained by X-Ray Powder Diffraction, Mossbauer Spectroscopy, scanning electron microscopy with EDX and transmission electron microscopy. The results of the wear debris analysis are compared with original brake materials components. Most of recognized particles are based mainly on iron oxides. Wear brake particles are discussed in the relation to the potential risk to the environment and human health.
CHANGES IN ELECTRICAL STEEL SHEETS INDUCED BY HEAT TREATMENT
Bulín, Tomáš ; Švábenská, Eva ; Hapla, Miroslav ; Ondrušek, C. ; Schneeweiss, Oldřich
Surface coating of electrical steel sheets is very important due to minimizing eddy current losses in electrical machines. The quality of coating can differ according to the composition. The coating layer has limited heat stability and after crossing of the guaranteed temperature it can be significantly degraded and its insulation properties may change importantly. The aim of this study is to describe changes in parameters of magnetic behavior of the non-oriented Si electrical steel sheet due to degradation of the coating by annealing in air. The parameters were acquired from the measuring of magnetic hysteresis loops on two types of samples with different excitation. The basic information on structure was obtained by optical microscopy. Acquired data are compared between samples with the damaged coating layer by annealing and the original coating layer. The results are discussed from the point of view of the possibility of using sheets with damaged insulation in electrical machines.
MICROSTRUCTURE AND COMPOSITION OF FINE PARTICLES RELEASED BY CAR BRAKING
Švábenská, Eva ; Roupcová, Pavla ; Pizúrová, Naděžda ; Schneeweiss, Oldřich
Vehicular traffic is connected with large volume of fine particles released during brake processes of cars. Our research is focused on the phase, structure and chemical analysis of the fine particles taken from some car brake parts by their services. The information on structure and phase composition was obtained by X-Ray Powder Diffraction, Mossbauer Spectroscopy, scanning electron microscopy with EDX and transmission electron microscopy. The results of the wear debris analysis are compared with original brake materials components. Most of recognized particles are based mainly on iron oxides. Wear brake particles are discussed in the relation to the potential risk to the environment and human health.
CHANGES IN ELECTRICAL STEEL SHEETS INDUCED BY HEAT TREATMENT
Bulín, Tomáš ; Švábenská, Eva ; Hapla, Miroslav ; Ondrušek, C. ; Schneeweiss, Oldřich
Surface coating of electrical steel sheets is very important due to minimizing eddy current losses in electrical machines. The quality of coating can differ according to the composition. The coating layer has limited heat stability and after crossing of the guaranteed temperature it can be significantly degraded and its insulation properties may change importantly. The aim of this study is to describe changes in parameters of magnetic behavior of the non-oriented Si electrical steel sheet due to degradation of the coating by annealing in air. The parameters were acquired from the measuring of magnetic hysteresis loops on two types of samples with different excitation. The basic information on structure was obtained by optical microscopy. Acquired data are compared between samples with the damaged coating layer by annealing and the original coating layer. The results are discussed from the point of view of the possibility of using sheets with damaged insulation in electrical machines.
Hyperfine interactions and electronic structure of magnetite
Řezníček, Richard ; Štěpánková, Helena (advisor) ; Máca, František (referee) ; Schneeweiss, Oldřich (referee)
The present work focuses on analyses of the experimental 57 Fe nuclear magnetic resonance data and the results of ab initio calculations in order to achieve deeper insight into the hyperfine and electronic structure of magnetite both below and above the Verwey phase transition. Parameters of hyperfine interactions and electronic structure data of the Cc phase of magnetite were extracted from the results of ab initio calculations based on the recently reported crystal structure. Published experimental dependences of the 57 Fe nuclear magnetic resonance frequencies on the external magnetic field direction were quantitatively reanalyzed, yielding hyperfine anisotropy data, which were compared to the analogous hyperfine field parameters from the ab initio calculations. The findings were interpreted in the context of up-to-date charge ordering models for magnetite. Attention was also paid to the impact of various cationic substitutions and defects on the electronic structure and hyperfine interactions in magnetite manifested in the 57 Fe nuclear magnetic resonance spectra: below the Verwey transition, signal intensity shifts in the spectra were analyzed; above the Verwey transition, the mean field model was adapted for an analysis of temperature dependence of satellite signal frequencies, while the anomalous...
Properties of iron nanoparticles sealed in protective media
Schneeweiss, Oldřich ; Hapla, Miroslav ; Zbořil, R. ; Mašláň, M.
Iron nanocrystalline powder was prepared by the reduction of ferrihydride. The samples were sealed in Ar in a plastic bag to avoid any contact with air in the form of (i) the as-prepared powder, (ii) the powder mixed with a vacuum grease, and (iii) the powder after a very slow oxidation at room temperature which formed an oxidic shell. Their phase compositions were investigated by XRD and Mossbauer spectroscopy. These data were completed by magnetic measurements. The intense amorphous-like component was observed in the Mossbauer spectrum of the as-prepared powder. The origin of this component is discussed.

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