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.

The first part of this diploma thesis is dealing with conductivity of liquid electrolytes. The second part is about third generation gel electrolytes and thein conductivity. The electrolytes were inorganic salts LiClO4, LiBF4 and NaClO4 in an aprotic propylene carbonate (PC) -used as a solvent. Nanosized Al2O3 and TiO2 particles were, proportionally to the sample's volume, added to the both – blend and gel samples in an amount expressed by weight percentage (wt). Both liquid and gel electrolytes contained following amount on nanosorbent: 3,46% wt, 7,17% wt, 10,0% wt and 12,0% wt. The liquid electrolytes were gelled using commercially available precursor Superacryl (Spofa dental s.r.o.).

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.

Nowadays, nanostructures fixed on solid substrates and colloidal nanoparticles permeate through all areas of human life, in area of sensors and detection as well. This dissertation thesis deals with the fabrication of nanostructures on the surface of planar electrodes via self-ordered nanoporous template of aluminum trioxide. The nanofabrication, as one of many possible techniques, is used to increase the active surface area of electrodes by creating unique surface types with specific properties. These electrodes are very perspective in the applications, such as biomolecules electrochemical detection and measurement. The transformation of aluminum layer into non-conductive nanoporous template in the process of anodic oxidation is a fundamental technique employed to obtain the array of nanostructures in this thesis. The fabrication of high quality nanoporous membranes with narrow pore size distribution on various types of metallic multilayers is one of the key experimental parts in this work. Several problems associated with the production of the thin-film systems, including the dissolving the barrier oxide layer, are discussed and solved. Another part of this work deals with the use of nanoporous membrane as a template for the production of metallic nanostructures via electrochemical metal ions deposition directly into the pores. The obtained nanostructures as nanowires, nanorods or nanodots are characterized by the scanning electron microscopy and energy-dispersive or wavelength X-ray spectroscopy. The electrode surface, modified by gold nanostructures suitable for the detection of biomolecules, has been chosen for the electrochemical measurements, due to the gold biocompatibility. The nanostructured electrodes were characterized by electrochemical impedance spectroscopy and cyclic voltammetry. The effect of nanostructured surface geometrical parameters, including the size of the electrochemically active area, on the results of electrochemical measurements has been observed and compared to flat gold electrodes. Two model biomolecules, namely guanine and glutathione, have been chosen for the study of potential application of these nanostructures in biosensors.

Le but de cette thèse est d’étudier le comportement des asphaltènes dans des condition de hydrotraitement, y compris les propriétés de transport et d’adsorption. La chromatographie d'exclusion stérique inverse (ISEC) ainsi que la spectroscopie d'impédance sont utilisées pour déterminer des paramètres topologiques de solides poreux d’alumine (porosité, taille de pores, tortuosité). Des coefficients de diffusion effectifs de polystyrènes de différentes tailles sont aussi étudiés par chromatographie liquide en conditions non-adsorbantes: les molécules de petites tailles pénètrent plus profondément dans le milieu poreux donc elles prennent plus de temps pour traverser la colonne, tandis que les molécules ayant une taille supérieure à la taille du pore ne sondent que la macroporosité. Avec l'utilisation des méthodes dynamique et «peak parking», il est possible de modéliser le transport des molécules de différentes tailles, et cela aidera à prédire le comportement de molécules d’une taille quelconque. Les colonnes ont été assemblées au laboratoire à partir de poudres et de monolithes d’alumine. Les caractéristiques d'adsorption des asphaltènes modèles sont déterminées et comparées avec une fraction d’asphaltènes extraite d’un brut. Un phénomène de dimérisation ainsi qu’une très forte adsorption sur la surface de l’alumine sont observés avec la molécule modèle. La méthode dynamique a été utilisée avec des colonnes courtes dans des conditions de saturation. Une influence apparente du débit dans l’importance et le mécanisme d’adsorption a pu être constatée. / This work aims to study the complex behaviors of asphaltenes within the hydrotreatment catalytic porous system including transport properties and adsorption. Inverse size-exclusion chromatography (ISEC) and impedance spectroscopy are used to determine the topological characteristics of different alumina porous solids (porosity, pore size, tortuosity). The effective diffusion coefficient of polystyrenes of different sizes was studied via chromatography in non-adsorbing conditions. Elution peaks are used to determine the effect of molecule size on the accessible pore volume and the transport properties therein: molecules of relatively small sizes penetrate further into the porous medium, thus taking more time to navigate the chromatographic setup, while larger molecules traverse much faster, through the macroporosity. The liquid chromatography technique is divided in two different methods. Both methods yield diffusion coefficient values which are modelled, predicting the behavior of molecules of any size. Columns were assembled manually from alumina powders or monoliths. A synthesized asphaltene model molecule was used and its adsorption behavior was determined and compared to an asphaltene fraction recovered from crude oil. The asphaltene model molecule shows a dimerization behavior as well as extremely strong interactions with the alumina surface. Dynamic method was attempted in short alumina columns at saturation conditions and an apparent influence of the flow rate on the extent and mechanics of adsorption was observed.

Ce manuscrit présente une étude des mécanismes de condensation et d'évaporation dans des membranes d'alumine poreuse. Ce matériau poreux possède des pores quasi-cylindriques de taille nanométrique faiblement distribués en diamètres qui, contrairement à beaucoup d'autres milieux poreux, ne sont pas interconnectés. L'alumine poreuse est donc un milieu idéal pour sonder l'influence du confinement sur la condensation et l'évaporation à l'échelle du pore unique.La première partie discute mes résultats dans des membranes disposant de pores droits ouverts aux deux extrémités ou fermés d'un côté. Des mesures d'isothermes de sorption à l'hexane couplées à une étude originale du comportement optique des membranes pour sonder la répartition du liquide dans les pores, indiquent ces derniers ne sont pas parfaitement cylindriques, mais ont une forme conique et possèdent des corrugations marquées. En tenant compte de ces imperfections grâce à des simulations par automate cellulaire, j'ai pu reproduire numériquement les isothermes mesurées. Cet accord montre que la théorie de Saam & Cole décrit bien la condensation et l'évaporation dans un pore unique.La seconde partie met en évidence l'évaporation par cavitation dans les membranes d'alumine poreuse. Un protocole de synthèse spécifique m'a permis d'obtenir des membranes avec des pores en forme d'encrier. Dans ces dernières, j'ai systématiquement observé, optiquement et volumétriquement, une vidange brutale des pores à 0.33 Psat.Celle-ci correspond à la cavitation homogène de l'hexane dans les cavités des encriers, c'est-à-dire à la nucléation thermiquement activée d'une bulle de gaz sphérique dans le liquide sous tension. Il s'agit de la première observation directe d'un tel mécanisme d'évaporation dans des membranes poreuses. / This manuscript presents a study of condensation and evaporation mechanisms in porous alumina membranes. This porous material has almost cylindrical pores of nanometric diameter with a narrow pore size distribution. Unlike many other porous media, their pores are not interconnected. Porous alumina is therefore an ideal material to probe the effects of confinement on condensation and evaporation in a single pore.The first part discusses my results obtained on membranes with straight pores open at both extremities or closed at one side. Sorption isotherms using hexane coupled to an original study of the membranes optical behaviour allow us to probe the liquid distribution within the pores. The results show that our pores are not perfectly cylindrical but funnel shaped and present corrugations. I numerically reproduced the measured isotherms with a cellular automaton, which takes into account the pores defects. This agreement implies that condensation and evaporation in a single pore are properly described by Saam & Cole theory.The second part evidences evaporation by cavitation in porous alumina membranes. A specific synthesis protocol is used to produce membranes with ink-bottle shaped pores. These membranes undergo a systematic and sudden emptying at 0.33 Psat, as measured by the optic and volumetric signals. This is the signature of the homogeneous cavitation of the liquid contained inside the ink-bottles, i.e. the nucleation of a spherical gas bubble in a liquid under tension. This is the first direct observation of such an evaporation process in porous membranes.

Transparent polycrystalline aluminum oxide is a promising optical material, particularly in applications that require ballistic protection. However, the rhombohedral crystal structure of alumina limits its transparency due to birefringent scattering. One method of reducing birefringent scattering is to align the particles along the same crystallographic direction, minimizing the refractive index mismatch. This dissertation explores the use of high aspect-ratio platelet-morphology alumina powder in order to process a crystallographically aligned polycrystalline alumina part, with improved optical properties. The optimal hot-pressing parameters of non-pre-aligned platelet alumina were explored, showing that a low pre-load pressure (0MPa), a high maximum temperature (1800°C), a low maximum pressure (10MPa), and a long isothermal hold time (>5hrs) yields dense, transparent parts. These parameters resulted in samples with a high in-line transmission (>65%) despite a large grain size (>60μm). This is due to a high degree of crystallographic orientation, which minimizes the refractive index mismatch between grains, reducing birefringent scattering. Pre-alignment resulted in a further increase in crystallographic orientation, indicating that the pre-alignment procedure effectively aligns the platelets along the same crystallographic orientation. However, pre-alignment resulted in a minimal improvement in optical properties due to the pre-aligned platelets decreasing the densification. Mechanical properties were characterized, resulting in a flexure stress and Vickers hardness of approximately 175MPa and 17GPa, respectively. These low mechanical properties are due to the large grain size. The Vickers hardness was also characterized along different alignment/hot-pressing directions, showing that the hardness matches that of sapphire along corresponding crystallographic directions. Modifications to the Rayleigh-Gans-Debye model were made, accounting for crystallographic orientation. The modified model more closely matches the experimental optical data, illustrating the importance of accounting for crystallographic alignment. This dissertation emphasizes the importance of characterizing optical losses in transparent ceramics and how they relate to the microstructure, as well as the significance of crystallographic alignment in a birefringent transparent ceramic like alumina.

<div> <div> <div> <p>Transparent alumina is a candidate material for ballistic applications where visible or infrared wavelength transmission is required. However, the transparency of polycrystalline alumina can be limited due to the rhombohedral crystal structure being inherently birefringent. Birefringence causes light scattering at grain boundaries and is detrimental to the transparency. It has been shown experimentally that the application of a high magnetic field during processing can lead to crystallographic alignment and the reduction of birefringent light scattering. This alignment method is effective but is limited in terms of scalability. This research addresses these limitations through the use of simple and cost-effective shear and elongational forming processes such as uniaxial warm pressing and direct ink writing (DIW) for the improvement of final sintered transparency. To further support the improvement of these processes as alternatives and to evaluate the possibility of using powder ratios to improve the alignment, this research will also investigate the sintering behavior during hot-pressing of equiaxed and platelet powders. </p> <p>Platelet ceramic-filled thermoplastic blends were developed and formed into sheets through uniaxial warm pressing. The solids loading (30 – 40 vol.%) and platelet diameter (1.2 and 11μm) were varied to compare effects on viscosity, percent reduction, and final alignment. All ceramic- filled thermoplastic polymer blends exhibited pseudoplastic behavior. Crystallographic alignment of green body samples was quantified by the orientation parameter (r) and grain misalignment angle (full width at half maximum, FWHM) obtained from rocking curve analysis. Blends with 11μm diameter platelets displayed a higher temperature sensitivity constant, better flow properties, and higher alignment compared to blends with 1.2μm diameter platelets. Optimal samples produced with blends containing 30 vol.% of 11μm diameter platelets demonstrated an alignment of r = 0.251 +/- 0.017; FWHM = 11.16° +/- 1.16°. A sample with optimal alignment was hot-pressed to transparency and obtained an in-line transmission of 70.0% at 645nm. The final alignment of this pre-aligned hot-pressed sample (r = 0.254 +/- 0.008; FWHM = 11.38° +/- 0.54°) improved when compared to a non-pre-aligned sample (r = 0.283 +/- 0.005; FWHM = 13.40° +/- 0.38°).</p><p>Additionally, the use of direct ink writing, an additive manufacturing technique, as a viable alignment process for producing transparent alumina was investigated. Highly loaded (> 54 vol.%) equiaxed alumina suspensions were developed with platelet additions ranging from 0-20vol.% of the total solids loading. An increase in the amount of platelet powders from 5-20vol.% increased the dynamic yield stress from 104Pa to 169Pa and decreased in the equilibrium storage modulus from 17,036Pa to 13,816Pa. It was found that the DIW process significantly increased the alignment in one orientation when compared to samples cast from the same suspensions and this behavior may be connected to the rheological properties. Lastly, an optical analysis showed that sample developed with 5vol.% platelet suspensions had higher in-line transmission values across the visible spectrum when compared to samples developed with 20vol.% suspensions. A sample cast from a 5vol.% platelet suspensions had the lowest grain alignment but possessed an in-line transmission of 42.8% at 645nm, which was the highest of the samples produced in this study. An optical loss analysis showed, that this sample has the lowest backwards scattering losses due to residual porosity and this result was supported by the density data. It is suggested that the alignment of the DIW samples is more complex and a more advanced texture analysis will need to be conducted to properly characterize the grain alignment.</p><p>Lastly, the densification behavior of equiaxed and platelet powder ratios with no intentional pre-alignment was investigated. An initial sintering investigation identified the optimum maximum pressure selected for the hot-pressing process as 20MPa. Under the selected hot- pressing parameters, the effects of 0, 25, 50, 75, and 100wt.% equiaxed powder additions on the sintering behavior, optical properties, and grain alignment was investigated. The data showed that an increase in the amount of equiaxed powders decreased the initial powder compact displacements rate. Additionally, an increase in the wt.% equiaxed powders from 0wt% to 75wt% decreases the in-line transmission from 70.9% to 40.2%, respectively at 645nm. Lastly, an increase in the wt.% equiaxed powders from 0wt% to 75wt decreased the alignment from (r = 0.321 +/- 0.005; FWHM = 16.26° +/- 0.40°) to (r = 0.509 +/- 0.022; FWHM = 34.63° +/- 2.61°), respectively.</p></div></div></div>

In this manuscript, distinct problems concerning carbon-bonded materials processing and characterization were analyzed, from bulk to cellular samples. The main motivation was to address central topics that would enhance the comprehension of the material’s behavior, as well as trigger targeted improvements. Some of the topics this thesis covers are: Non-linear Young’s modulus behavior of carbon-bonded alumina at high temperatures; Influence of the processing route on the cold crushing strength of carbon-bonded alumina foam filters; Geometric characterization of ceramic foam filters as a tool to understand processing parameters; Use of advanced techniques such as computer tomography and finite element modelling to correlate processing parameters and mechanical behavior. In most of the analyses, non-standard computational strategies were adopted. In those cases, algorithms were written to facilitate the evaluations, or even enable it in the first place. All the algorithms’ concepts are described in this thesis and their codes are available in the Appendices. The current work was carried out within the framework of the Collaborative Research Center 920 (CRC 920) “Multifunctional filters for metal melt filtration - a contribution to zero defect materials” at the Technische Universität Bergakademie Freiberg funded by the German Research Foundation (DFG).

Um einen dauerhaften Einsatz Al2O3-basierter Feuerfestwerkstoffe in Hochtemperaturanlagen zur Synthesegaserzeugung zu ermöglichen, erfolgte die Entwicklung neuer Materialien mit verbesserter Thermoschock- und Korrosionsbeständigkeit gegenüber flüssiger Kohleschlacke. In einem industrienahen Spinell-Alumina-Verbundwerkstoff lässt sich die Mikro- und Porenstruktur durch Zugabe von 6 Gew.-% eines Spinell-reichen Zements optimieren, wodurch die Infiltration und zugleich die Korrosion durch saure Braunkohleasche erheblich reduziert werden konnte. Die Zugabe von 2,5 Gew.-% TiO2 zu einer Al2O3-Gießmasse führt ebenfalls zur Verbesserung der Thermoschockbeständigkeit und verhindert während des Tiegeltests die weitere Auflösung der Al2O3-Matrix, indem sich auf dem Werkstoff durch Reaktion mit MgO der Schlacke eine dichte in situ Spinell-Schutzschicht ausbildet. So erfolgte die Entwicklung von Materialien mit hervorragenden thermomechanischen Eigenschaften und ausreichender Korrosionsbeständigkeit, welche als umweltfreundliche und kostengünstige Alternative zu den derzeit eingesetzten Cr2O3-reichen Werkstoffen angesehen werden können.