Metal-ceramic composites in fusion reactor environments
Name: Sam Humphry-Baker
Mentor: Dr Luc Vandeperre
Sponsor: Imperial College Research Fellowship
Powder-processed composites based on the carbides and borides of tungsten are used extensively in manufacturing tools and mining applications, however they have been recently identified as promising materials for nuclear fusion reactors1. In both applications, these materials are exposed to very high temperatures, mechanical stresses and oxidation, while in fusion devices irradiation damage poses an additional challenge.
This project is focussed on tailoring these materials to enhance their performance in extreme environments. Our strategy is based on developing enhanced metal-ceramic composites, whereby ceramic particles are combined with a small amount of ductile metallic alloy to improve the overall mechanical properties and manufacturability. The defining microstructural feature of these composites is their high density of metal-ceramic interfaces. These interfaces control heat transport, vacancy diffusion, and dislocation motion – and are therefore key to understanding their interesting properties. By engineering the density and chemistry of these interfaces using advanced processing techniques, we will design materials that can be employed in more demanding conditions.
The work can be divided into several complimentary work streams. First, materials will be processed at Imperial, using advanced powder consolidation equipment such as the vacuum hot-press. Sintering studies will be aided by the state-of-the-art thermal analysis facilities within CASC. Once synthesised, materials will be studied under oxidation2 and mechanical stresses at Imperial – predominantly using the thermogravimeter and temperature creep tester. Complimenting this work, irradiation studies will be conducted at external facilities and brought back to college for post-mortem characterisation. The work benefits from on-going collaboration with Tokamak Energy Ltd and their supporting of a PhD studentship within the Nuclear-CDT.
1. Modelling the power deposition into a spherical tokamak fusion power plant. C.G. Windsor, J.G. Morgan, P.F. Buxton, A.E. Costley, G.D.W. Smith, A. Sykes Nucl. Fusion. 57 , 36001, 2016.
2. Oxidation resistant tungsten carbide hardmetals. S.A. Humphry-Baker, K. Peng, W.E. Lee, Int. J. Refract. Met. Hard Mater. 66, p: 135–143, 2017.
PhD Student: Ben Currie
Whilst tungsten monocarbide (WC) has been used extensively in the tooling industry within hard metals, use of monolithic WC has remained minimal. This has led research to be focused on the hard metals themselves, whilst the properties of monolithic WC have remained relatively unstudied. Recently there has been renewed interest in the research of monolithic WC due to its promise as a neutron shielding material in compact spherical tokamaks. This application will expose the shielding material to an environment of extremely high temperatures and neutron fluxes. Whilst monolithic WC is of interest due to its high chemical and thermal stability, low activation properties, high attenuation of both gamma and neutrons as well as high hardness and sputtering resistance, its high temperature mechanical properties are still not fully understood and its mechanical properties after irradiation are yet to be studied at all.
This project therefore aims to use techniques such as compressive creep testing, compressive yield testing, mutual indentation and nanoindentation in conjunction with ion implantation to study the effects of the fusion environment on monolithic WC, expanding on the literature and assessing its suitability as a neutron shielding material.
PhD Student: James Davidson
Development of smaller fusion reactors presents several challenges to the design, primarily the proximity of the fusion plasma to the magnets that confine it. Unlike in standard reactor designs, there is only a small space in which the shielding materials need to be placed, therefore highly effective shielding materials are required. Currently, most conventional shielding materials are unable to provide adequate neutron shielding efficiency to the toroidal magnets, leading to increased degradation and heat deposition, reducing lifetime and performance.
Simulations work has shown that ceramic compounds in the tungsten-boron system are able to provide high levels of neutron shielding. This is due to the high scattering cross section of the tungsten and the high neutron capture cross section of boron.
While the shielding properties of the W-B system are ideal, the mechanical properties make manufacturing an issue due to their brittle nature leading to high hardness and low toughness. In order to improve these properties, composites manufactured with mixtures of pure tungsten and tungsten borides will be investigated in this project. One aspect of this work will be modifying the W-WB composite using several thermomechanical processing techniques to improve the toughness. The focus of this work will be to see how these thermomechanical processing techniques affect the microstructure and to relate these changes to the mechanical properties of the W-WB composite.