Title: Materials for fusion applications and their interaction with tokamak plasma Author: Veronika Klevarová Department: Department of Physics of Materials Supervisor: doc. RNDr. Miloš Janeček, CSc., Department of Physics of Materials Abstract: Tungsten represents a perspective option in the context of fusion devices first-wall materials. In the first part of this work, set of tungsten samples with variable grain size was prepared by spark plasma sintering. Specimens were exposed to steady state deuterium plasma beam and high energy heat pulses, simulating thus the normal operation in the tokamak. As a consequence of the exposure, samples surfaces were roughened, as-prepared grains were recovered and in some cases cracks were formed. Moreover, post-irradiation analysis of the damaged samples revealed activation of in-grain slip systems within the loaded surfaces. Threshold grain diameter for this mechanism was determined to be between 5.5 - 6.6 μm at the particular loading conditions. However, damaged features showed to depend more on the fabrication parameters than on the grain diameter. Synergistic effects of simultaneous loading were proven to be important since those reduced the heat propagation within the volume of the tested samples. In the second part of this thesis, introduction to plasma-surface...

Bulk commercially pure titanium was prepared by powder metallurgy, namely by cryogenic milling and spark plasma sintering, with aim to produce ultra-fine grained material with enhanced strength. The microstructure of milled powders was investigated in detail by a novel method called transmission EBSD, which allowed the first direct observation of texture within the powder particles. This texture is similar to rolling texture, because of the similar nature of the defor- mation during milling. Microstructure observations revealed grains with the size under 100 nm. The influence of sintering parameters on material properties were studied by scan- ning electron microscopy including EBSD, X-ray diffraction and by microhardness measurements. The trade-off relationship between porosity and grain size was identified, fully dense material with ultra-fine grained microstructure could not be produced. Increased oxygen content was identified as a main strengthening factor, while porosity has significant deteriorating effect on mechanical properties. The texture of powder was retained in the bulk material. The possibility of stabilizing the microstructure by mechanical alloying of Ti with yttrium oxide nanoparticles was investigated with mixed results. The stabiliza- tion was successful, but several issues...

Ionic conductivity of metal oxides is critical for the function of a broad range of different components, such as electrolytes in solid oxide fuel cells and alloys designed for high temperature applications. In both cases the ionic conductivity can be studied by in situ impedance spectroscopy, which is also able to reveal information on the dielectric properties of the metal oxides, and in some cases the influence of their microstructure. The focus of this thesis is on impedance spectroscopy measurements of α-alumina in the temperature range 400-1000 °C. This metal oxide has found extensive use as the protective scale on heat resistant alloys. Some unpublished work on oxygen ion conductivity of yttria-stabilized zirconia is also included. The low electrical conductivity of α-alumina can be a source for errors and misinterpretations during impedance spectroscopy measurements. A major disturbance originates from leakage currents that appear in the experimental setup. These leakage currents are due to conduction through the gas phase around the sample, conduction on the sample surface, or poor insulation in the sample holder. It was shown that below 700 °C, conduction on the sample surface could severely distort the measurement. The magnitude of the distortions appeared to be sensitive to the type of electrodes used. The use of a so-called guard electrode was shown to effectively block the surface conduction in the measurements. Conductivity of metal oxides is known to be dependent on their microstructure. Generally it is believed that ionic conductivity is favoured along grain boundaries and dislocations. The influence of microstructure on conductivity was studied for α-alumina in the temperature range 400-1000 °C. The conductivity of a series of highly pure and dense samples with narrow grain size distributions was measured by impedance spectroscopy. It appeared that the activation energy for conduction increased with decreasing grain size. Results based purely on impendence spectroscopy have some inherently weaknesses. For instance no information on the nature of the charge carrier can be found. Therefore the charge transport in single crystalline α-alumina was simulated by the molecular dynamics method. The results from the simulation were then compared to results from impedance measurements on single crystalline α-alumina. From the simulation it turned out that diffusion of aluminium ions had lower activation energy than diffusion of oxygen. The activation energy of oxygen was close to the measured activation energy, and the mobility of oxygen was higher than for aluminium. Therefore the dominating charge carrier was suggested to be oxygen ions.

Oxid uraničitý (UO2) je v současnosti nejčastěji používaným materiálem jakožto palivo v komerčních jaderných reaktorech. Největší nevýhodou UO2 je jeho velmi nízká tepelná vodivost, a protože se při štěpení UO2 v jaderném reaktoru vytváří velké množství tepla, vzniká v UO2 peletě velký teplotní gradient. Tento teplotní gradient způsobuje vznik velkého tepelného napětí uvnitř pelety, což následně vede k tvorbě trhlin. Tyto trhliny napomáhají k šíření štěpných plynů při vysoké míře vyhoření paliva. Tvorba trhlin a zvýšený vývin štěpného plynu posléze vede ke značnému snížení odolnosti jaderného paliva. Tato práce se zabývá problematikou zvyšování tepelné vodivosti jaderného paliva na modelu materiálu (CeO2). V této práci jsou studovány podobnosti chování CeO2 a UO2 při konvenčním slinováním a při „spark plasma sintering.“ Způsob jak zvýšit tepelnou vodivost použitý v této práci je včlenění vysoce tepelně vodivého materiálu, karbidu křemíku (SiC), do struktury CeO2 pelet. Od karbidu křemíku je očekáváno, že zvýší tok tepla z jádra pelety, a tím zvýší tepelnou vodivost CeO2. V této práci je také porovnávána podobnost chování SiC v CeO2 matrici s chováním SiC v UO2, které bylo popsáno v literatuře.

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The aim of this study was to understand the processing – structure – property relationships in spark plasma sintered (SPS) boron carbide (B4C) and B4C-titanium diboride (TiB2) ceramic composites. SPS allowed for consolidation of both B4C and B4C-TiB2 composites without sintering additives, residual phases, e.g., graphite, and excessive grain growth due to long sintering times. A selection of composite compositions in 20% TiB2 feedstock powder increments from 0% to 100%, was sintered at 1900°C for 25 minutes hold time. A homogeneous B4C-TiB2 composite microstructure was determined with excellent distribution of TiB2 phase, while achieving ~99.5% theoretical density. An optimum B4C-23 vol.% TiB2 composite composition with low density of ~3.0 g/cm3 was determined that exhibited ~30-35% increase in hardness, fracture toughness, and flexural bend strength compared to commercial armor-grade B4C. This is a result of a) no residual graphitic carbon in the composites, b) interfacial microcrack toughening due to thermal expansion coefficient differences placing the B4C matrix in compression and TiB2 phase in tension, and c) TiB2 phase aids in crack deflection thereby increasing the amount of intergranular fracture. Collectively, the addition of TiB2 serves as a strengthening and toughening agent, and SPS shows promise for the manufacture of hybrid ceramic composites.

Sintering is an important processing step for obtaining the necessary mechanical stability and rigidity of ceramic bulk materials. Both mass and heat transfer are essential in the sintering process. The importance of radiation heat transfer is significantly enhanced at high temperatures according to the well-known Stefan-Boltzmann’s law. In this thesis, we modified the pressure-less spark plasma sintering set-up to generate intense thermal radiation, aiming at rapid consolidation of ceramic bulk materials. This approach was named as “Sintering by Intense Thermal Radiation (SITR)” as only thermal radiation contributed. Firstly, the heat and mass transfer mechanisms during the SITR process were studied by choosing zirconia ceramics as references. The results revealed that the multiple scattering and absorption of radiation by the materials contributed to the heat diffusion. The observed enhanced densification and grain growth can be explained by a multiple ordered coalescence of zirconia nanocrystals using high heating rates. Secondly, the temperature distribution during the SITR process was investigated by both numerical simulation and experimental verifications. It showed that the radiator geometry, sample geometry and radiating area were influencing factors. Besides, the change of material and geometry of the radiators resulted in an asymmetric temperature distribution that favored the formation of SiC foams. The foams had gradient structures with different open porosity levels and pore sizes and size distributions. Finally, ceramic bulk materials were successfully fabricated by the SITR method within minutes. These materials included dense and strong ZrO2 ceramics, Si3N4 foams decorated with one-dimensional nanostructures, and nasal cavity-like SiC-Si3N4 foams with hierarchical heterogeneities. Sufficient densification or formed strong necks were used for tailoring these unique microstructures. The SITR approach is well applicable for fast manufacture of ceramic bulk materials because it is clean and requires low energy consumption and properties can be controlled and tuned by selective heating, heating speed or temperature distribution. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Manuscript.</p><p> </p>

A obtenção da sílica vítrea pelo processo de sinterização por plasma pulsado (SPS), a partir de matérias-primas de diferentes estruturas (cristalina e amorfa) e diferentes granulometrias, foi investigada. Análises de difração de raios X, transmitância óptica, microscopia óptica e eletrônica de varredura, e medições de densidade foram realizadas nas amostras sinterizadas. Sílicas vítreas transparentes foram fabricadas a partir de pós de quartzo atingindo-se temperaturas finais entre 1450 e 1600°C, enquanto que nanopó e pó amorfo de sílica formaram consolidados transparentes com temperaturas máximas próximas de 1200° C. Taxas de aquecimento entre 40 e 150°C/min. foram utilizadas nas sinterizações, com tempos de processo menores que 40 minutos. As maiores taxas de aquecimento exigiram uma maior temperatura final para a fusão completa do material cristalino e causaram, de forma indireta, a presença de aglomerações de micro-bolhas nas amostras obtidas com o nanopó. Um gradiente radial de temperatura (decrescente do centro para as bordas) foi observado nos consolidados fabricados com os pós cristalinos, facilitando o processo de fechamento da porosidade aberta. A presença ou ausência de material não fundido e de bolhas nas amostras foram analisadas por difratogramas de raios X, microscopia óptica e medidas de densidade. A análise da transmitância indicou uma quantidade praticamente nula de grupos OH nos compactos sinterizados com os pós cristalinos e em torno de 20 ppm no caso dos sólidos fabricados com matéria-prima sol-gel. Amostras de nanopós de sílica dopada com titânia (~6% em massa) foram processadas com temperaturas finais de 1200 e 1400°C apresentando coloração azulada e negra, respectivamente, e aumento dos clusters de titânia para a temperatura mais elevada. A sinterização a uma temperatura em torno 1200°C mantida por apenas 4 minutos resultou na conversão completa da fase anatase para a fase rutilo da titânia. / The production of vitreous silica by the spark plasma sintering (SPS) process, starting from raw materials of different structures (crystalline and amorphous) and granulometry were investigated. Analysis of X-ray diffraction, optical transmittance, optical and scanning electron microscopy, and density measurements were performed on the sintered compacts. Transparent vitreous silica was fabricated from quartz powder at final temperatures ranging from 1450 and 1600°C while silica nanopowder and silica powder formed transparent compacts at temperatures around 1200°C. Heating rates between 40 and 150° C/min. were used in processes with durations smaller than 40 minutes. Higher heating rates demanded higher final temperatures to complete the fusion process and caused, indirectly, the formation of micro-bubbles agglomerations in the samples produced from the nanopowder. A radial gradient of temperature (decreasing from the center to the border) was observed at the compacts fabricated with the crystalline powders favoring the closure of the open porosity. The presence or the absence of non-fused material and bubbles in the samples was analyzed by X-ray diffraction, optical microscopy and density measurements. The transmittance analysis indicated an almost zero quantity of OH groups in the compacts sintered from crystalline powders and about 20 ppm in the solids fabricated from the sol-gel raw material. Nanopowder samples of silica titania (~6 wt % of titania) were processed with final temperatures of 1200°C and 1400°C presenting blue and black coloration, respectively, and an increase of the titania clusters for the highest temperature. The sintering at temperatures near 1200°C with a holding time of just 4 minutes caused the complete anatase-rutile conversion in titania.

Cerâmicas piezelétricas possibilitam posicionamento e sensoriamento de precisão ou captação de energia mecânica valendo-se do efeito piezelétrico, capaz de converter energia mecânica em elétrica ou o contrário. Para aprimorar ou estender as aplicações dessas cerâmicas, mecanismos flexíveis podem ser acoplados a elas, formando um Dispositivo Piezelétrico Flextensional (DPF). No projeto desse tipo de estrutura, o conceito de Material com Gradação Funcional (MGF) é interessante, já que esses materiais apresentam variações graduais de suas propriedades efetivas, permitindo a alternância entre um material mais flexível e um mais rígido de acordo com a intensidade de deslocamento desejada em cada região da estrutura. Assim, neste trabalho, implementa-se o Método de Otimização Topológica (MOT) no projeto de estruturas gradadas com o intuito de identificar as vantagens e desvantagens da utilização do conceito de MGF em DPF. Esse método combina algoritmos de otimização e o Métodos dos Elementos Finitos (MEF) para distribuir material dentro de um domínio fixo através de um modelo de material, que no presente caso é o de Material Isotrópico Sólido com Penalização (MISP) adaptado a MGF. Na fabricação desses dispositivos otimizados, utiliza-se a Sinterização por Jato de Plasma (SJP) para a obtenção de tarugos gradados que são submetidos a processos de eletro-erosão e de corte a laser. Por fim, para a verificação dos resultados numéricos, utiliza-se um vibrômetro para aferir os deslocamentos dos protótipos de atuadores fabricados. / Piezoelectric devices enable precision positioning and sensing or mechanical energy harvesting based on the piezoelectric effect. In flextensional piezoelectric devices, flexible coupling structures are attached to ceramics to improve or extend the application possibilities. On the design of this kind of structure, the concept of Functionally Graded Materials (FGM) can be interesting, since it allows gradual variations of its effective properties along some direction by mixing two or more materials. Thus, in order to identify the advantages and disadvantages of using FGM, graded flexible coupling structures that maximize the performance of piezoelectric devices are obtained by implementing the Topology Optimization Method (TOM). This method combines optimization algorithms and the Finite Element Method (FEM) to distribute material inside a fixed domain. In this work, the formulation is based on the Solid Isotropic Material with Penalization (SIMP) material model adapted for the FGM concept, which can represent continuous change in material properties along the domain. Resulting optimal graded topologies of coupling structures are presented and compared with homogeneous structures. Finally, graded devices are manufactured through Spark Plasma Sintering (SPS) technique in order to be characterized, validating numerical results. The numerical results demonstrate the TOM efficacy in designing functionally graded piezoelectric devices and show, by its implementation, significant gains in graded mechanisms performance when compared with analogous homogeneous. Furthermore, the feasibility of proposed manufacturing process is confirmed, allowing the fabrication of prototypes with expected behavior.