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Correlation of AE Signal Changes to Rolling Contact Damage Propagation
Nohál, Libor ; Čermák, Jan (referee) ; Hájek, Karel (referee) ; Hutař, Pavel (referee) ; Mazal, Pavel (advisor)
This PhD thesis deals with the experimental study of more precise rolling contact fatigue damage detection using acoustic emission method. A series of experiments was carried out on two representatives bearing steels and the analysis of sensitivity for the presence of contact damage was performed on selected parameters of acoustic emission. The extent of damage was classified into four classes and signal parameters the most characterizing the development of damage were correlated with the extent of damage. It was also verified the influence of lubricants on acoustic emission signals. The results have an impact on the implementation of more precise rolling contact fatigue tests and evaluation of parameters of acoustic emission signal. On the basis of experiments was established methodology for more precise RCF testing method using acoustic emission on test-rig AXMAT II.
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Noise and Transport Analysis of the Niobium Oxide Layers
Sita, Zdeněk ; Hájek, Karel (referee) ; Koktavý, Bohumil (referee) ; Grmela, Lubomír (advisor)
Kondenzátor na bázi oxidu niobu je novým typem pasívní součástky, jehož vývoj byl motivován snahou vyřešit hlavní nedostatky tantalového kondenzátoru – omezený zdroj tantalové suroviny a nebezpečí hoření při průrazu. Chování kondenzátoru na bázi oxidu niobu lze stejně jako u tantalového kondenzátoru popsat prostřednictvím reverzní MIS struktury. Pro studium mechanismu transportu nosičů nábojů v dielektriku Nb2O5 a pro stanovení fyzikálních parametrů, které řídí zbytkový proud, bylo využito měření V-A charakteristik v normálním a reverzním módu při 77 a 300K, dále závislosti kapacity ochuzené vrstvy na napětí a frekvenci, teplotní a časové závislosti zbytkového proudu a spektrální hustoty šumu ve frekvenční a časové doméně při různých napětích. Experimentální data potvrdila platnost navrženého pásového diagramu MIS struktury a poskytla jeho klíčové parametry. Bylo ověřeno, že transport nosičů náboje v NbO kondenzátorech je určen ohmickou, Poole-Frenkelovou a tunelovou složkou v normálním módu, a Schottkyho emisí v reverzním módu. V rozsahu standardních aplikačních napětí dominují v normálním módu Poole-Frenkelova emise a v reverzním módu Schottkyho emise. Při vyšších napětích v normálním módu určuje průrazné napětí kondenzátoru tunelový mechanismus. V reverzním módu rozhoduje o odolnosti vůči tepelnému průrazu kondenzátoru výška bariéry mezi dielektrikem a anodou. Bylo zjištěno, že NbO a Tantalové kondenzátory mají stejný mechanismus vodivosti. Specifika NbO anody se projevují pouze v rozdílných hodnotách parametrů pásového diagramu, nikoliv v principech mechanismu transportu nosičů náboje. To vysvětluje základní rozdíl mezi oběma kondenzátory, který je v kvalitě dielektrické vrstvy na přechodu anody a dielektrika. Nižší potenciálové bariéry a vyšší počet defektů v dielektriku, který je způsoben dalším stabilním oxidem, má za následek vyšší zbytkový proud NbO kondenzátoru. Tento jev však nemá žádný vliv na spolehlivost součástky. Teoretické modely a vybrané testovací metody byly použity k volbě vhodných materiálů anody, ke stanovení vhodných dopantů a k optimalizaci technologie anodické oxidace. Byla nalezena korelace mezi parametry transportu nosičů náboje a spolehlivostí, a na základě experimentálních dat byly navrženy optimalizace výrobního procesu kondenzátorů. Lepší porozumění transportním mechanismům v NbO kondenzátorech umožnilo úplný popis nové součástky na bázi oxidu niobu. Byly zdůrazněny silné a slabé stránky této nové technologie a nalezeny nástroje pro optimalizaci procesů, které umožní vyšší spolehlivost a efektivitu NbO kondenzátorů.
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APPLICATION OF ACOUSTIC EMISSION METHOD FOR MORE DETAILED DIAGNOSTICS OF RADIAL BEARINGS DAMAGE
Hort, Filip ; Pazdera, Luboš (referee) ; Hájek, Karel (referee) ; Samek,, Jiří (referee) ; Mazal, Pavel (advisor)
This thesis studies the possibilities of exploiting the acoustic emission method as a tool for more detailed identification of the radial bearings damage formation. The work includes comparison of different evaluation methods in laboratory tests. Applied couplant between the contact sensor areas and sensing surfaces is also taken into account. The practical part includes the presentation of the experimental station and structural adjustments made. The main part of the work comprises the examples in detail of three types of experimental tests that simulate the possible failure of bearings and their manifestation in the sensing signal. Testing methodology developed with recommended values for setting common AE signal measurement, vibration and temperature is a part of the thesis, too.
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Memcapacitor Emulator
Weiss, Lukáš ; Hájek, Karel (referee) ; Biolek, Dalibor (advisor)
This bacherol's thesis deals with designing a memcapacitor with piecewise linear (PWL) characteristic. The memcapacitor is a part of mem-systems predicted by Leon Chua in 70's and 80's of the last century. A big attention was given to mem-devices and mem-systems in the field of electrical engineering after the memristor's discovery in HP laboratories in 2008. In the first step, the summary of basic information about mem-systems is reported. The second part contains some design solutions of the analogue memcapacitor emulator. The third part implements selected circuit into the PSPICE simulation. Concrete models of modern active integrated circuits are used. The last part contains a circuit solution of the designed model on the PCB.
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Memristor with PWL-type characteristic
Holas, Jiří ; Hájek, Karel (referee) ; Biolek, Dalibor (advisor)
Bachelor’s thesis deals with an analog emulation of memristor memory with PWL-type characteristic which is designed for modern digital memories. The first part of the document summarizes the basic memristor characteristics and rules of their proper modeling. The next parts present the circuit implementation and computer simulation. The printed circuit board design and prototype measurement are included.
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The Noise Spectroscopy of Radiation Detectors Based on the CdTe
Zajaček, Jiří ; Štourač, Ladislav (referee) ; Hájek, Karel (referee) ; Grmela, Lubomír (advisor)
The main object of this work is noise spectroscopy of CdTe radiation detectors (-rays and X–rays) and CdTe samples. The study of stochastic phenomenon and tracing redundant low-frequency noise in semiconductor materials require long-term measurements in time domain and evaluate suitable power spectral densities (PSD) with logarithmic divided frequency axes. We have used the means of time-frequency analysis derived from the discrete wavelet transform (DWT) and we have designed the effective algorithm for PSD estimation, which is comparable with an original analog method. CdTe single crystal with Au contacts we can imagine as a series connection of two Schottky diodes with a resistor between them. The bulk resistance at constant temperature and other constant parameters changes due to the carrier concentration changing only. The p-type CdTe sample shows metal behavior with every temperature changes. Semiconductor properties of the sample begin to dominate just after some period of time. This behavior is caused by the hole mobility changing. The voltage noise spectral density of 1/f noise depends on the quantity of free carriers in the sample. All the studied samples have very high value of low frequency noise, much higher than it should have been according to Hooge’s formula. The excess value of low frequency noise is caused by the low carrier concentration within the depleted region.
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Improving Extraction of Information From Executable Files
Hájek, Karel ; Kolář, Dušan (referee) ; Zobal, Lukáš (advisor)
This thesis deals with extension of an open-source decompiler project called RetDec maintained by the Avast company. The goal is to develop an extension of data extraction from executable files for malware analysis improvement. The thesis proposes several possible improvements on data extraction in the RetDec project. The most useful of these suggested enhancements are then selected and implemented. The selected enhancements involve calculating a hash of symbol names in Linux executable files and a more extensive analysis of Authenticode format, a Microsoft technology for digital signing of executable files for Windows operating systems. The thesis implements the selected additional data extractions in the RetDec project and tests them on real-world malware samples.
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Transport of Electric Charge in Tantalum Capacitor
Pelčák, Jaromír ; Koktavý, Bohumil (referee) ; Hájek, Karel (referee) ; Grmela, Lubomír (advisor)
The task of the thesis was studding of tantalum capacitors with solid electrolytes properties. Ta – Ta2O5 – MnO2 capacitor by its construction represents MIS structure, where tantalum anode has metal conductivity and MnO2 cathode is semiconductor. Isolation layer consists of tantalum pentoxide Ta2O5 with relative permitivity r = 27. Dielectric thickness is typically in range from 30 to 150nm. The capacitor charge is not only stored and accumulated on electrodes but also in localised states (oxide vacancies) in isolation layer. The capacitor connected in normal mode represents MIS structure polarized in reveres direction when the applied voltage higher potential barrier between semiconductor - MnO2 cathode and isolation of Ta2O5. The transport of charge carriers via isolation layer is determined by Poole-Frenkel mechanisms and tunnelling. Poole-Frenkel mechanism of charge transport is dominant in low intensity of electric field. Tunnelling determines current at higher electric field intensity. During low intensity of electric field ohmic component is also presented which is determined by volume of resistance of impurities in isolation layer due to donor states of oxygen vacancies. Based on the modelling of measured VA characteristics is possible to estimate determine dielectric thickness of Ta2O5 and determine share of Poole-Frenkelov and tunnel current and charge transportation. The thesis is described charge transport and charge concentration on tantalum capacitor in low frequency area and analysis of capacitor behaviour at frequency band. The first impulse for the thesis was an effort to create equivalent circuit diagram of tantalum capacitor in respect of its physical and electrical behaviour. There is an opportunity to study and determine electric charge transport and its accumulation based on the equivalent circuit diagram structure. There is also a chance to define and trace potential barriers and charge distribution in the capacitor structure based on an measurement and carried out experiments. This methodology and analysis consists of electrical characteristic determination to create physical model of the capacitor describing it function, properties and behaviour.
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Diagnostics and therapy of spiritual emergency - orienting study
Janečková, Barbora ; Bahbouh, Radvan (advisor) ; Břicháček, Václav (referee) ; Hájek, Karel (referee)
The doctoral dissertation contains of a definition of spiritual emergency, differential diagnosis, description of kinds of spiritual emergency. It looks on the therapeutical possibilities in working with these states. Part of the dissertation is analysis of semistructured interviews with clients and therapists who work with these kinds of problems. In the analysis of the interviews with therapists I center on their experience with the dynamics of spiritual emergence, its manifestation and possible existence of similar traits of clients with this diagnosis. Important parts of the interviews are the most effective ways of working with these clients. I was also interested in their own perception on how spiritual emergence occurs and on the possible change in the quality of life of the client after this episode. In the analysis of the interviews with the clients I question when spiritual emergence occurred, how it started, how long did it last and what were the reactions from outside. Important part is centered on the interventions which the client experienced and their efficiency. Case studies demonstrate typical traits described at spiritual emergency, as quite dramatic beginning, complete recovery and also an important change in value system.
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