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Stanovení koeficientů tepelné vodivosti modifikovaného dřeva buku
Stárek, Šimon
The bachelor thesis is dedicated to the influence of selected wood modifications on its thermal properties. For the experimental part of the work, European beech wood (Fagus sylvatica L.) was used, which was modified by acetylation (using acetic anhydride) and thermal modification at temperatures of 200 and 220 ˚C. Values of thermal conductivity coefficient, specific heat capacity, and thermal diffusivity were measured using the Thermophysical Tester RTB device. The obtained data were statistically evaluated and compared with available literature. Compared to control samples, a decrease in thermal conductivity of approximately 3 % was observed in samples thermally modified at 200 ˚C. Samples thermally modified at 220 ˚C showed much significant decrease in thermal conductivity, that being over 15 %, due to a more pronounced decrease in the density of the modified wood. Despite the fact that the density of acetylated samples increased, very similar results were achieved. This decrease in thermal conductivity was mainly caused by the different thermal properties of the chemical used for impregnation process. Modified wood with these thermal properties can find applications in the construction industry, where the priority is to achieve the best possible energy efficiency of components made of wood.
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Determination of crystal field parameters in selected tetragonal compounds
Zubáč, Jan ; Javorský, Pavel (advisor) ; Prchal, Jiří (referee)
The thesis deals with a crystal electric field and its effect on magnetic properties of the in- termetalic tetragonal rare-earth compounds. Using the self-flux method we have prepared single crystals of two R2TIn8 type compounds, namely Nd2RhIn8 and Nd2IrIn8. These compounds or- der antiferromagnetically in low-temperature region. We have verified a proper structure through EDX analysis, X-ray powder diffraction and Laue diffraction method. Calculations of the crystal field parametres based on experimental magnetic susceptibilities data were performed. Apart from Nd2RhIn8, we have computed crystal field parameters for isostructural Nd2CoIn8 and Dy2CoIn8 compounds and cerium compound CeCuAl3. These compounds are studied simultaneously at our department.
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Magnetic excitations in cerium compounds
Vlášková, Kristina ; Javorský, Pavel (advisor) ; Prchal, Jiří (referee)
Title: Magnetic excitations in cerium compounds Author: Kristina Vlášková Department: Department of Condensed Matter Physic Supervisor: doc. Mgr Pavel Javorský, Dr., Department of Condensed Matter Physics Abstract: Thanks to presence of only one electron in 4f shell of cerium ion, Ce based compounds reveal wide range of exceptional magnetic properties like valence fluctuations, magnetic ordering or spin glass behavior. In present thesis we mainly focus on tetragonal CeCuxAl4-x system and its magnetic behavior. Former results of inelastic neutron experiments opened discussion about energy levels of cerium ions in CeCuAl3. Except two crystal field excitations also one additional energy level was present, phenomenon of vibron quasi-bound state was proposed to explain observed energy structure. Measurement and analysis of specific heat and neutron inelastic scattering on compounds with various Cu content should help to understand phenomenon of vibron state and also its development with Cu/Al concentration. Another two types of cerium compounds will be investigated in means of specific heat and resistivity measurement, polycrystalline sample of CePt4Al and single crystal of Ce3Ru4Al12. Preparation, characterization and measurement of low temperature properties are subjects of this work. Results will be discussed...
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Magnetic properties of Ce compounds studied by specific heat
Čermák, Petr ; Javorský, Pavel (advisor)
Materials containing the 4f (rare earth) or 5f (actinides) exhibit a large variety of interesting physical properties. The Ce-based compounds have a special place among the rare-earth compounds. The Ce atom contains only a single f-electron that is responsible for the magnetic behavior. The 4f states in compounds with the heavy rare earths have a well localized character, whereas many Ce-based compounds are on the borderline between the localized and itinerant behavior. These compounds show large variety of the magnetic ground states what is a result of the competition between the long-range order of the RKKY type and the screening of the localized moments by conduction electrons. We observe nonmagnetic states with a mixed valence (between Ce3+ and Ce4+), metallic systems with a long-range order of the Ce moments (ferromagnetic, antiferromagnetic or more complex structures). To analyze the electronic properties, the heat capacity data, and namely their low-temperature part, play an indispensable role. This diploma work comprise the sample preparation of selected cerium compounds, their phase characteristics and the heat capacity measurements at low temperatures (0.4 - 300 K). The main part is focused on the data analysis and comparison with theoretical models.
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Ultrapure metallic materials
Prachařová, Marcela ; Kalbáčová Vejpravová, Jana (advisor)
Title: Ultrapure metallic materials Author: Bc. Marcela Janatová Department: Department of Condensed Matter Physics Supervisor: RNDr. Jana Poltierová Vejpravová, Ph.D. Supervisor's e-mail address: jana@mag.mff.cuni.cz Abstract This thesis deals with crystal structure and magnetic properties of RCr2Si2 (R = Pr, Tb, Er) and RCr2Si2C (R = La, Ce, Pr, Nd) compounds. All prepared samples were characterized by powder X-ray diffraction and investigated by specific-heat, magne- tization and AC-susceptibility measurements in the temperature range 2-900 K and in magnetic fields up to 9 T. The results suggest non-trivial magnetic ordering of the Cr sublattice in all studied compounds, accompanied with ferromagnetic ordering of the Pr and Nd magnetic moments below the Curie temperature of 30 and 21 K in PrCr2Si2C and NdCr2Si2C, respectively, and possible metamagnetism in CeCr2Si2C. For RCr2Si2C compounds, the experimental evidence of Cr magnetism is corrobora- ted by results of extended first-principles calculations based on the density functional theory. Keywords: RCr2Si2 compounds; RCr2Si2C compounds; Magnetic ordering; Heat ca- pacity; Magnetization 1
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Identification of Thermal Conductivity and Thermal Capacity of Building Materials by the "Hot Wire Method"
Průša, David ; Čermák,, Jan (referee) ; Šťastník, Stanislav (advisor)
This aim of task deals with study of heat dissipation mechanisms and the description of physical phenomena, which is accompanied by non-stationary measurement of thermal characteristics by the method "hot-wire method". In particular, we observe the coefficient of thermal conductivity and its dependence on various variables such as the temperature of the measured sample, its moisture state, the volume of the sample and its porosity. The above mentioned findings are used for the invention of the measuring device of a nonstationary gauge, which is based on regular heating and is dedicated to measuring the thermal conductivity coefficient and the heat capacity by the "hot-wire method" method. In the last part of the thesis is verified functionality of the proposed measuring device, the suitability of the created algorithm for the processing of the measured data and the evaluation of the results was verified. The reproducibility of the measurements was verified and the measured results were compared with the measurement methods, which are commonly used. the influence of humidity on the coefficient of thermal conductivity.
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Identification of Thermal Conductivity and Thermal Capacity of Building Materials by the "Hot Wire Method"
Průša, David ; TROJAN,, Karel (referee) ; Šťastník, Stanislav (advisor)
This aim of task deals with study of heat dissipation mechanisms and the description of physical phenomena, which is accompanied by non-stationary measurement of thermal characteristics by the method "hot-wire method". In particular, we observe the coefficient of thermal conductivity and its dependence on various variables such as the temperature of the measured sample, its moisture state, the volume of the sample and its porosity. The above mentioned findings are used for the invention of the measuring device of a nonstationary gauge, which is based on regular heating and is dedicated to measuring the thermal conductivity coefficient and the heat capacity by the "hot-wire method" method. In the last part of the thesis is verified functionality of the proposed measuring device, the suitability of the created algorithm for the processing of the measured data and the evaluation of the results was verified. The reproducibility of the measurements was verified and the measured results were compared with the measurement methods, which are commonly used. the influence of humidity on the coefficient of thermal conductivity.
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Anomalous heat response in nonequilibrium stochastic models
Miřátský, Václav ; Netočný, Karel (advisor) ; Ryabov, Artem (referee)
The bachelor thesis deals with heat capacity generalization to systems in non-equilibrium steady states. The non-equilibrium state is kept by contact with many baths and by action of non-potential forces. Evolution of system is understood to be a Markovian process described in the first part of this thesis. After that the concept of reversible heat is presented and heat capacity matrix is introduced. Following chapter deals with the usage of heat capacity matrix for analysation of two-level model in contact with two baths. At the end the built formalism is used for describtion of transport of fermions through a quantum dot. 1
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