REPOSITORY > RESULTS

Abstract

Fusion technology has a great potential to safely provide an inexhaustible quantity of electricity without producing greenhouse gases and minimal hazardous waste compared to conventional energy sources, such as nuclear power plants. For materials intended for fusion plasma-facing applications, the essential properties are structural stability at elevated temperature, adequate thermal conductivity, strength and ductility, thermal shock and thermal fatigue resistance, and stability under neutron exposure. For such applications, tungsten and tungsten-based materials are a reasonable choice due to their advantageous combination of physical properties.
This thesis introduces three research topics with one common denominator – microstructure analysis of tungsten and tungsten-based materials developed by the Slovenian research group within the European Fusion Programme. The first research topic relates to my first-author scientific article entitled “The role of tungsten phases formation during tungsten metal powder consolidation by FAST: Implications for high-temperature application”. In the article, my co-authors and I have explored and utilised the research methods appropriate for the microstructure analysis of tungsten consolidated by field-assisted sintering technology (FAST). In the study, in-situ secondary phases formation after consolidation was analysed and identified as tungsten dioxide (WO2). The presence of oxide inclusions motivated us to explore the role of tungsten carbide (WC) as an oxygen binder for removing the oxide impurities from the tungsten matrix, as described in my second first-author scientific article entitled “Tungsten carbide as a deoxidation agent for plasma-facing tungsten-based materials”. To completely remove the oxygen impurities and obtain a pure tungsten body, we have to introduce a minimum of 5.8 - 8.8 vol % WC as a carbon source to the initial mixture. Determined from stoichiometry calculations, the oxygen is removed in the form of carbon monoxide and carbon dioxide.
A surplus amount of WC will lead to the in-situ formation of a thermally stable ε-W2C as a secondary phase in the tungsten matrix. The presence of ε-W2C phase can influence the performance of plasma-facing materials under irradiation, which my co-authors and I experimentally explored in my third first-author article entitled “Non-uniform He bubble formation in W/W2C composite: Experimental and ab-initio study”, exploring the influence of helium on the microstructure of the multi-phase material, i.e., W/W2C composite described in my previous articles. The experimental observations of helium bubble formation in the W/W2C composite were complemented by first-principles-based density functional theory (DFT) calculations to establish a fundamental understanding of helium clustering, migration and dissolution in tungsten metal and tungsten carbide, W2C. The study deduces that helium will be preferentially trapped by large structural defects, which in the end, severely affect the material integrity for plasma-facing applications.