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Tracking Resistance Of
Various Materials In Vacuum
Šedivý, Matúš
Many vacuum instruments used in industry and scientific research apply high voltage components. Vacuum performs as an insulator with theoretically infinite breakdown voltage. Surface of dielectrics between two electrodes inside a vacuum chamber is the weakest link for a possible electrical breakdown (tracking). This paper focuses on examination of a vacuum pressure influence to the surface breakdown voltage of a various materials. Results were compared and led to determination of an appropriate materials for a design of internal connectors in electron microscopes produced by the company Thermo Fisher Scientific.
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Influence of low-temperature plasma on enhaced ceramic processing
Rolek, Ondřej ; Kachlík, Martin (referee) ; Pouchlý, Václav (advisor)
This masters’s thesis contains literary research about basic shaping methods of ceramic materials concerning the slip casting method. In addition, stabilization of ceramic slips, drying of ceramic green bodies and their subsequent sintering are described. This thesis also includes a description of plasma technology, including DCBD (Diffuse Coplanar Barrier Discharge) technology. In experimental part of this thesis the Al2O3 powder was treated with DCBD technology. From the prepared powder, the green body was prepared by the slip casting method, which was subsequently sintered by one-step and two-step sintering. The thesis describes the methods of preparation of ceramic samples and their evaluation. Evaluated experiments have shown that plasma technology has a positive effect on the treatment of ceramic powder Al2O3. Alumina ceramic samples of high relative density ( 99%) and low grain size (
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Advanced ceramic layered materials with heterogeneous microstructure for ballistic applications
Mařák, Vojtěch ; Kachlík, Martin (referee) ; Drdlík, Daniel (advisor)
This bachelor thesis deals with a preparation technology of layered ceramic materials (laminates) with heterogeneous microstructure containing Al2O3 as a matrix with addition of SiC. For the preparation of these materials, dry and wet shaping of ceramic powder with subsequent SPS (Spark Plasma Sintering) was chosen. Due to the goals of the bachelor thesis, the dry shaping proved to be less suitable due to the difficult preparation of the layered structure and the significant wear of the graphitic dies used in the SPS technology. However, the information obtained about mechanical and physical properties has led to the design of the ceramic suspension used in wet shaping (slip-casting). As optimal design 5 vol % of SiC in Al2O3 was chosen. Via this way, ceramic laminates with a sharp layer interface were prepared and the graphitic die wear was minimized. Prepared ceramic samples reached relative density up to 99 % and hardness up to 20.7 GPa. The obtained knowledge has shown the need to optimize the technology of suspension preparation to create ceramic material suitable for ballistic applications.
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Investigation of high voltage influence on different materials in high and low vacuum
Šedivý, Matúš ; Šandera, Josef (referee) ; Hejátková, Edita (advisor)
The beginning of this thesis contains an overview of properties of the insulators, and description of insulators that were used for in depth research of surface breakdown in vacuum. Furthermore, this work focuses on mechanisms of an electric breakdown initiation at the interface of the solid insulator and surrounding low pressure gas. Multiple methods for measurements of dielectric strength are examined. The experimental part describes the measurements performed in the vacuum chamber. The results of these measurements are then analysed. In conclusion, used insulators are compared and suggestions for further work are given.
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ELECTRON BEAM REMELTING OF PLASMA SPRAYED ALUMINA COATINGS
Matějíček, Jiří ; Veverka, J. ; Čížek, J. ; Kouřil, J.
Plasma sprayed alumina coatings find numerous applications in various fields, where they enhance the properties of the base material. Examples include thermal barriers, wear resistance, electrical insulation, and diffusion and corrosion barriers. A typical structure of plasma sprayed coatings, containing a multitude of voids and imperfectly bonded interfaces, gives them unique properties - particularly low thermal conductivity, high strain tolerance, etc. However, for certain applications such as permeation barriers or wear resistance, these voids may be detrimental.\nThis paper reports on the first experiments with remelting of plasma sprayed alumina coatings by electron beam technology, with the purpose of densifying the coatings and thereby eliminating the voids. Throughout the study, several parameters of the e-beam device were varied - beam current, traverse velocity and number of passes. The treated coatings were observed by light and electron microscopy and the thickness, structure and surface morphology of the remelted layer were determined and correlated with the process parameters. Based on the first series of experiments, the e-beam settings leading to dense and smooth remelted layer of sufficient thickness were obtained. In this layer, a change of phase composition and a marked increase in hardness were observed.\n
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Study of Sintering of Nanoceramic Materials
Dobšák, Petr ; Hanykýř, Vladimír (referee) ; Havlica, Jaromír (referee) ; Šída, Vladimír (referee) ; Cihlář, Jaroslav (advisor)
The topic of the Ph.D. thesis was focused on the process of sintering alumina and zirconia ceramic materials with the aim to compare kinetics of sintering sub-micro and nanoparticle systems. Zirconia ceramic powders stabilized by different amount of yttria addition in the concentration range of 0 – 8 mol% were used. The different crystal structure (secured by yttria stabilization) of zirconia, as found, did not play statistically proven role in the process of zirconia sintering. The possible influence was covered by other major factors as particle size and green body structure, which does affect sintering in general. According to the Herrings law, the formula predicting sintering temperature of materials with different particle size was defined. The predicted sintering temperatures were in good correlation with the experimental data for zirconia ceramic materials prepared from both, coarser submicrometer, and also nanometer powders. In case of alumina ceramics the predicted and experimentally observed sintering temperature values did not match very well. Mainly the nanoparticle alumina materials real sintering temperature values were markedly higher than predicted. The reason was, as shown in the work, strong agglomeration of the powders and strong irregularities of particle shape. The major role of green body microstructure in the sintering process was confirmed. The final density of ceramic materials was growing in spite of sintering temperature, which was decreasing together with pore - particle size ratio (materials with similar particle size were compared). Sintering temperature was increasing together with growing size of pores trapped in the green body structure. Clear message received from the above mentioned results was the importance of elimination of stable pores with high coordination number out off the green body microstructure during shaping ceramic green parts. Same sintering kinetics model was successfully applied on the sintering process of submicro- and also nanometer zirconia ceramics. Activation energy of nanometer zirconia was notably lower in comparison to submicrometer material. For the sintering of nanoparticle zirconia was typical so called “zero stage” of sintering, clearly visible on kinetic curves. It was found out, that processes running in zirconia “green” material during zero stage of sintering are heat activated and their activation energy was determined. Pores of submicrometer zirconia were growing in an open porosity stage of sintering just a slightly (1.3 times) compared to the nanoparticle zirconia, where the growth was much higher (5.5 times of the initial pore diameter). This difference was most probably caused by preferential sintering of agglomerates within the green bodies and by particle rearrangement processes which appears in the zero stage of sintering of nanoparticular ceramics. The technology of preparation of bulk dense ytria stabilized zirconia nanomaterial with high relative density of 99.6 % t.d. and average grain size 65nm was developed within the thesis research.
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Transparent polycrystalline ceramics for ballistic and high-temperature applications
Malík, Petr ; Pouchlý, Václav (referee) ; Trunec, Martin (advisor)
Alumina based transparent ceramic bodies were prepared within this diploma thesis by the gel casting process followed by presintering to closed porosity stage with subsequent hot isostatic pressing. The problematic drying of large bodies was solved by means of careful temperature and relative humidity control and by osmotic drying. Almost perfectly dense ceramics were achieved thanks to colloidal processing of TM-DAR and TM-UF ceramic powders providing a final structure with grain size of 381 nm, resp. 302 nm. High real in-line transmission of 72,3 % was achieved at wavelength of 632,8 nm and thickness of 0,8 mm. Among mechanical properties high values of hardness HV10 = 22,5 GPa, fracture toughness KIC = 3,99 MPam0,5, Young’s modulus E = 400 GPa a bending strength o = 494 MPa were achieved.
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Study of electric properties of nanocomposites
Libra, Miroslav ; Rozsívalová, Zdenka (referee) ; Polsterová, Helena (advisor)
The present master´s thesis deals with the electrical properties of nanocomposite materials. Samples for the experiment are made epoxi resin and oxides TiO2 and Al2O3 as nanofillers in different percent performace. The samples nanocomposites are measured temperature dependence of the resistivity inside, dissipation factor and relative permittivity. It discusses the effect of the filler on the resulting electrical properties of the polymer.
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