National Repository of Grey Literature 52 records found  1 - 10nextend  jump to record: Search took 0.00 seconds. 
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.
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.
Nuclear magnetic resonance in magnetic systems
Křišťan, Petr ; Štěpánková, Helena (advisor) ; Mašláň, Miroslav (referee) ; Schneeweiss, Oldřich (referee)
Title: NMR in magnetic systems Author: Petr Křišťan Department: Department of Low Temperature Physics Supervisor: prof. RNDr. Helena Štěpánková, CSc. Abstract: Magnetically ordered iron oxides, namely magnetite, maghemite, hexagonal ferrite M or system Fe-B, with nano or submicron dimensions of particles are the main subject of study of the presented thesis. The materials were investigated mainly by 57 Fe nuclear magnetic resonance (NMR). The results were thoroughly analyzed and compared with the results obtained by other methods (Mössbauer spectroscopy, ZFC/FC magnetic measurement, X-ray diffraction or TEM). In case of the maghemite nanoparticles a regular distribution of vacancies in octahedral positions was verified by the help of NMR in external magnetic fields and at various temperatures. The experi- mental results were also compared with ab-initio calculations. In thin layers of barium M type hexaferrite, effects of reduced particle size on 57 Fe NMR spectra were observed. The NMR methods were also successfully applied to investigation of system FeMoCuB of amorphous and nanocrystalline ribbons, where 57 Fe NMR was able to resolve formation of different phases in depen- dence on the process of preparation. Due to different NMR excitation condi- tions of signal from strontium M type hexaferrite and maghemite,...
Nanoscale phases on surface Fe-6Si magnetic sheets
Švábenská, Eva ; Roupcová, Pavla ; Král, Lubomír ; Bulín, Tomáš ; Vondráček, Martin ; Foldyna, Josef ; Čechal, J. ; Schneeweiss, Oldřich
Analysis of the nanoscale phases which appear on the surface Fe-6Si samples were carried out after the various treatments – grinding and etching, annealing, and water jet abrasion. The basic information on structure, chemical and phase composition was obtained by X-Ray Powder Diffraction (XRD), Scanning Electron Microscopy with EDX, Glow Discharge Optical Emission Spectrometry (GDOES), Mössbauer Spectroscopy and X-ray Photoelectron Spectrometry (XPS). The results show high stability of the surface phase composition after the mechanical and heat treatments. Results obtained from the surface analysis in micrometer depth (XRD, EDX, GDOES) do not show any changes after the different treatments. Iron oxides were detected in XPS and conversion electron Mössbauer spectra (CEMS) which analyze the surface composition in a nanometer scale. In addition to, fine changes in atomic ordering on the surface can be observed after mechanical and heat treatments in the CEMS spectra.
Investigation of vacancies in Fe-Al alloys
Lukáč, František ; Čížek, Jakub (advisor) ; Král, Robert (referee) ; Schneeweiss, Oldřich (referee)
Title: Investigation of vacancies in Fe-Al alloys Author: RNDr. František Lukáč Department: Department of Low Temperature Physics Supervisor: doc. Mgr. Jakub Čížek, Ph.D., Department of Low Temperature Physics, Faculty of Mathematics and Physics Abstract: Fe-Al alloys exhibit relatively high vacancy concentrations, which significantly influence the mechanical properties of these alloys. Positron annihi- lation spectroscopy was employed for investigations of vacancies in this work and the vacancy concentrations in Fe-Al alloys were determined by positron lifetime spectroscopy and measurements of positron diffusion length of implanted mono- genergetic positrons. The correlation of hardness with the vacancy concentration was characterized by determination of the vacancy hardening coefficient in Fe- Al alloys. The thermal evolutions of hardness and the vacancy concentration were compared during the annealing of the Fe-Al alloys with various composi- tions. Coincidence Doppler broadening of annihilation peak was employed for determination of the chemical composition of quenched-in vacancies in Fe-Al al- loys. Increased concentration of Al atoms in surrounding vacancies in Fe-Al alloys was related to the attractive interaction between the Al atoms and the vacancy. Quantum mechanical ab initio calculations were...
Superconductivity and electronic properties of y-U alloys and their hydrides
Tkach, Ilya ; Havela, Ladislav (advisor) ; Mihálik, Marián (referee) ; Schneeweiss, Oldřich (referee)
Title: Superconductivity and electronic properties of γ-U alloys and their hydrides. Author: Ilya Tkach 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: Low-temperature electronic properties for U-Mo and U-Zr splats stabilized in γ-U were investigated. Magnetic measurements revealed Pauli paramagnetic behavior with temperature independent susceptibility for U-Mo alloys. U-Mo and U- Zr splats become superconducting at low temperatures with Tc up to 2.1 K and critical field 5-6 T. The superconductivity of -U can be considered as a bulk effect and can be described by the BCS theory, while -U superconductivity is not a real bulk effect. U-Mo and U-Zr alloys absorb hydrogen at high pressures (p ≥ 4.5 bar) and form hydrides with stoichiometry analogous to UH3. The hydrides with Mo have an amorphous structure based on β-UH3 phase, while hydrides with Zr have a crystalline structure of the -UH3 type. (UH3)1-xMox hydrides are ferromagnetic with enhanced TC up to 202 K and magnetic moments 1.1 μB/U in comparison with pure β-UH3 (175 K; 0.87 μB/U)....
Local structure of hexagonal ferrites studied by NMR
Kouřil, Karel ; Štěpánková, Helena (advisor) ; Pištora, Jaromír (referee) ; Schneeweiss, Oldřich (referee)
Title: Local structure of hexagonal ferrites studied by NMR Author: Karel Kouřil Department/Institute: Department of Low Temperature Physics Supervisor of the doctoral thesis: prof. RNDr. Helena Štěpánková, CSc. Abstract: Hexagonal ferrites of M, W, X and Y structure types were studied by means of NMR, electronic structure calculations and magnetoelectric experi- ments. Presented results deal with cation distribution, localization of ferrous ions, interpretation of NMR spectra of studied materials and effects of size reduction. In oriented layers M type strontium ferrite and submicron particles of M type barium ferrite pronounced effects of reduced size were observed on 57 Fe NMR spectra. Performance of magnetoelectric barium-strontium Y type hexaferrites with divalent zinc cations improved upon thermal treatment of samples while distribution of zinc was not significantly altered. In Sc sub- stituted BaM, Sc content was found to be uniform throughout TSSG grown crystal. In LaSrM systems electron localization in 2a sublattice was observed, co-substitution with La+Zn, La+Co and La+Cu was found to lead to partial charge compensation. In strontium ferrites of type W and X localization of ferrous ions in octahedral sites in SS block pair was observed. Keywords: NMR, hexagonal ferrites, structure,...
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...

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