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GaN deposition on a tungsten substrate
Pikna, Štěpán ; Piastek, Jakub (referee) ; Čalkovský, Vojtěch (advisor)
This bachelor thesis is focused on deposition of GaN nanocrystals on the etched tungsten tips. Motivation was to prepare these GaN structures on the Schottky cathode made by company ThermoFisher Scientific and measure its field emission. In the theoretical part of the thesis GaN and tungsten field emission properties are introduced. The experimental part begins with tungsten tip etching optimalization, where the right values for best tips are temperature 20 °C, depth of the tip 2,5 mm and solution NaOH used. Further the gallium structures were prepared on these tips using molecular beam epitaxy (MBE). The right temperature to prepare GaN nanocrystals was determined as 200 °C. The deposition of gallium was set to 2 hours and following nitridation was 3 hours. Finally, the field emission from GaN prepared on copper foil with graphene was measured and compared with other experiments.
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Automation of dust charging experiment
Pechal, Radim ; Pavlů, Jiří (advisor) ; Žilavý, Peter (referee)
Title: Automation of dust charging experiment Author: Radim Pechal Department: Department of Surface and Plasma Science Supervisor: RNDr. Jiří Pavlů, PhD. Supervisor's e-mail address: Jiri.Pavlu@mff.cuni.cz Abstract: Since the dust grains are expected to cause problems during operation of tokamaks, there is a developing research of these devices which raises interest in dust grain charging processes research as well. Clarification of the dust grain charging processes may help with their solutions. This thesis deals with an issue of designing a part of a new experimental setup for dust grain research built at the Department of Surface and Plasma Science, Charles University in Prague. The base of this experimental setup is original linear quadrupole trap enabling dust grain trapping and its charging by electron, ion, and UV beams. In the thesis, a method of stabilization of one part of grain oscillation frequency detection is described. The thesis is focused on a development of a control unit and high voltage power supply for an image intensifier. To measure charging processes tungsten dust grains were used. This material will be probably used in construction of ITER tomakak. Electron equilibrium characteristics and other parameters were measured. Two samples of tungsten dust grains were used for...
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Tungsten-steel and tungsten-chromium composites prepared by RF plasma spraying
Matějíček, Jiří ; Klečka, Jakub ; Čížek, Jan ; Veverka, Jakub ; Vilémová, Monika ; Chráska, Tomáš ; Ganesh, V.
For future fusion reactors, materials able to withstand harsh environments are needed. In particular, this concerns the plasma-facing components which are foreseen to consist of tungsten based plasma-facing armor and structural and cooling part made of steel. Currently, joining of these materials presents a significant challenge. The stress concentration at their interface, which arises due to thermal exposure and the difference in thermal and mechanical properties, can be reduced by composite/graded interlayers. Plasma spraying is among the prospective technologies for their preparation. In this work, tungsten-steel and tungsten-chromium composites were prepared by radio-frequency inductively coupled plasma (RF-ICP) spraying. Initial optimization of the spraying process for pure materials as well as their mixtures was carried out. Basic characterization of the layers for their structure, porosity and composition is presented.
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RF-ICP deposition of tungsten coatings
Zlatník, Rostislav ; Houdková, Šárka (referee) ; Čížek, Jan (advisor)
The main object of this work is the technology of radio frequency controlled inductively coupled plasma, abbreviated RF-ICP. The specific goal of the work was the application of surface layers of tungsten and analysis of the influence of changes in process conditions on the resulting microstructure and chemical composition of deposited coatings. The monitored conditions were torch power, choice of carrier gas, powder feedrate.
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Preparation Of Cold Field Emission Cathodes Withultra Sharp Tips
Jurčík, Michal
This article deals with preparation of very sharp cold field emission cathodes made from polycrystalline tungsten wire which is widely used by other researchers and manufacturers. Common manufacturing process of electrochemical etching in NaOH solution is improved by separation of electrolyte surfaces between anode and cathode. This method prevents hydrogen bubbles, formed on cathode, to fine down the electrolyte surface near anode that cause shockwaves which affect the progress of etching. Separation of surfaces leads to sharper tips of the final cathode which is the main goal of my research.
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On tungsten spraying using inductively coupled plasma system - First results
Klečka, Jakub ; Mušálek, Radek ; Vilémová, Monika ; Lukáč, František
Thanks to its unique properties such as high melting point and density, tungsten and tungsten-based alloys are commonly used in a wide range of applications. Among others, these materials are promising candidates for the plasma facing components in the future fusion reactors. One of considered ways of production of these parts is plasma spraying. There are however several limitations for conventional gas stabilized torches, mainly in plasma enthalpy (i.e. the ability to efficiently melt tungsten particles in considerable feed rates) and susceptibility of tungsten to oxidation (which complicates spraying in oxidizing open-air atmosphere). The radio frequency inductively coupled plasma torch (RF-ICP) is a unique system which can potentially overcome both these problems and can be used for efficient tungsten spraying. The tungsten powder, which can be finer than the one used for the conventional systems, is fed axially into the hot plasma core, both factors lead to a more efficient melting of the particles. The deposition is performed in a chamber with controlled atmosphere of inert gas or decreased pressure, the oxidation is therefore suppressed. In this first study carried out with the newly commissioned RF-ICP system TekSpray 15 (Tekna), samples of tungsten coatings on graphite substrates were prepared. The X-ray diffraction and SEM images of the free surfaces and cross-sections were obtained, documenting high purity of the deposits and appropriate flattening of the splats leading to a dense coating microstructure. The effect of substrate preheating on the microstructure, porosity and hardness was also studied.
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Automated Electrochemical Etching Prototype for Cold Field Emission Cathodes
Jurčík, Michal
This article deals with a setup for automated cold cathode etching, its difficulties and benefits. As the method is well know there are no articles which describes this procedure in detail at least for tungsten wires from which cathodes are made. To provide stable and repeatable process of cathodes manufacturing, the experimental setup was proposed with full hardware control with aim to assure etching circuit disconnection in order of hundreds of nanoseconds.
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Powder feeding analysis in RF-ICP
Zlatník, Rostislav ; Veverka, Jakub (referee) ; Čížek, Jan (advisor)
The work is focused on the problematics of inductively coupled plasma, which is used for deposition of layers. We focused on two goals in the work. The first one focused on powder feeder, which is working on the vibration principle and is used in an apparatus that is using inductively coupled plasma. The aim is to determine its properties, dependence on different input parameters and working conditions. Is important to verify the suitability of its use for this kind of application. The assumption was to find the ideal configuration of the feeder for the given powder. After an overall assessment of the feeder properties, it was recognized as inappropriate for this application. The second goal is to spray tungsten layers with three different tungsten powders. Layers made with an inductively coupled plasma technology delivers promising results, but this issue needs to be intensively addressed in the future.
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