Pushing the limit - the ExtreMat project

Materials World magazine
1 Oct 2008

The production industry is moving towards a high-tech, knowledge-based economy. Key drivers are new materials and a processing rate that allows the operational windows of existing systems to widen or enables the creation of new products and systems by performing multiple functions.

ExtreMat partners are working to generate new commercial materials, multi-material components and crosscutting processing technologies for use in fields such as nuclear fusion, space and aerospace. It is a joint research project of 37 industrial and scientific partners from 12 countries, co-funded by the European Commission under the Sixth Framework Programme for Research and Technological Development.

Research on materials for extreme environments focuses on four aspects – self-passivating protection, heat sink and radiation resistance materials, and ExtreMat compounds.

Protective materials

The main goals for new protection materials are the ability to withstand temperatures up to 2,000°C, as well as heat flux pulses and cool-down shocks up to 1,000K/s. The materials must offer a self-passivating effect when exposed to physicochemically aggressive media such as hydrogen or oxygen, and offer passive stability by surface regeneration. Both carbon- and silicon carbide-(SiC) based materials are being explored for different applications.

Doped carbon-based materials are aimed at fusion first wall applications. By metal doping, resistance to chemical erosion by hydrogen and thermal shock is achieved. A strong mitigation of particle emission from heat-loaded surfaces is a further benefit. The results obtained with titanium-(Ti) doped graphites demonstrate that the chemical erosion by hydrogen can be reduced by factors of five to 10, depending on the temperature.

Reduced particle emission has been proven in high heat flux tests with surface heat loads of 2.4GW/m2 during four microsecond pulses (see images below). This mainly results from the higher thermal conductivity of Ti-doped graphite material, which is a consequence of the catalytic effect of the titanium carbide particles on the formation of graphite during thermal treatment at over 2,700°C during manufacture. A joining technology for Ti-doped graphite to copper heat sinks by brazing is under development.

Meanwhile, SiC-based multilayer ceramics that are manufactured by tape casting and subsequent sintering are being developed for thermal protection of the nose and wing leading edges of space vehicles. At these positions, oxidation resistance at temperatures as high as 1,900°C is required. Integrating porous SiC layers in a multilayer architecture lowers thermal conductivity and provides a mechanism for crack deviation at the interfaces that positively affect toughness (see image, right). Thermal loads up to 380MW/m2 during four microsecond pulses are successfully sustained. The multilayered SiC also survived 100 laboratory space re-entry simulations in terms of thermal cycle, temperature gradients, pressure and composition of the gaseous atmosphere without suffering significant degradation.

The self-passivating behaviour of the material against oxidation has been demonstrated by exposure to air at 1,600°C over 30 hours – a thin silicon dioxide protective layer formed on the surface. For thermal protection of turbine engine components, chemical stability against water vapour attack is also required. These multilayer ceramics represent an economically attractive alternative to conventional chemical vapour deposition (CVD) coated ceramic matrix composites.

Heat sinks

Further development in many technical fields is hindered by the increasing problem of thermal management. Overheating causes more than 50% of failures in electronic devices and temperature is also a limiting factor in construction of heat exchangers, fusion reactors, brake systems and aircraft engine parts.

Dissipation of up to 20MW/m2 will only be possible with heat sink materials that have a thermal conductivity of ~600W/(mK). They must withstand large temperature changes without disintegration and deterioration of properties, and have the capability to reduce complex thermo-mechanical stresses after bonding to, supporting or protecting structures by tailoring their thermal expansion in the range of four to nine ppm/K.

From over 40 promising material systems, two main groups have been selected – copper alloys reinforced with fibres or (nano)particles for heat sinks in reactors and engines where performance at temperatures up to 1,000°C is crucial, and metal matrix composites based on highly conductive phases such as diamond, highly graphitised carbon fibres and flakes, and carbon nanotubes for use in advanced electronics.

Several advanced technologies, including gas pressure infiltration of melts, squeeze casting, isostatic pressing, rapid sintering, physical vapour deposition and plasma coating, have been applied. Development of composites has been supported by tailoring the interfaces between constituents at nanoscopic levels to stabilise them in a range of working temperatures without degrading thermal conductivity.

Silicon carbide, tungsten and carbon fibre-reinforced copper matrix composites have been produced and submitted to neutron irradiation testing to evaluate feasibility for fusion applications. Regarding materials for electronics, thermal conductivities from 600-700W/(mK) have already been achieved with diamond and pitch-based carbon fibre-reinforced metals. The technology for continuous manufacturing of copper-SiC-fibre monolayers at the industrial scale, as well as industrially applicable techniques for gas pressure infiltration of particle/fibrous preforms, were developed to support future commercialisation.

In the industrialisation phase, various combinations of heat sink and protection materials will be prepared to demonstrate and test mutual joinability and performance of joints under extreme loading.

Radiation resistance

Radiation resistant materials are essential for advanced nuclear applications with prote-ctive, heat removal and structural functions. The materials should maintain structural stability under intense irradiation at the lowest possible activation – metallic materials at neutron doses up to 150dpa (displacements per atom) and operation temperatures of up to 700°C and non-metallic materials at neutron doses up to 10dpa and operation temperatures up to 1,000°C.

Materials under development are oxide dispersion strengthened (ODS) steels and tungsten-based alloys, as well as carbon- and SiC-based materials. Research aims to investigate fracture behaviour and radiation effects, identify testing parameters for pre- and post-irradiation testing, select the most promising material compositions and manufacturing routes, produce raw materials and final test specimen, characterise microstructure, physical and mechanical properties, as well as plan and implement of the neutron irradiation tests.

Reduced activation, radiation-resistant ODS ferritic steels have been developed through small ingots with the composition Fe-(12-14)Cr-1.2W- 0.3Ti-0.3%Y2O3 (in weight per cent). The ingots were prepared by mechanical alloying and hot isostatic pressing (HIPping). Their densities are 99.3-99.5% of the theoretical density and the ductile-to-brittle transition temperature of 12Cr ODS ferritic steels is close to 0°C. All ingots contain a homogenous distribution of oxide particles below five nanometres in size, and the numerous internal interfaces originating from these particles are expected to act as sinks for irradiation-induced defects. Such materials will be used as structural components in fission and fusion reactors at temperatures around 800°C.

Within the ExtreMat neutron irradiation programme, about 1,000 specimens have been prepared for the irradiation test, comprising materials newly developed in subprojects. The samples are being exposed to individually defined temperatures and accumulated neutron doses. Testing of materials under neutron irradiation comprises a huge effort to design the irradiation rig inside the reactor, and achieve pre-irradiation characterisation.

ExtreMat compounds

Emphasis is also placed on finding new joining processes, interlayers, coatings and integrated diffusion barriers for the new materials to improve wetting with molten brazing alloys, as well as to reduce residual stress from bonding, prevent embrittlement by suppression of interfacial material diffusion at high temperatures and inhibit hydrogen permeation. These techniques can be used in the multi-material compound formation of ceramic interlayers in metallic materials, as well as for joining materials developed within other ExtreMat subprojects. Such technologies include –

• Hydrogen diffusion barriers against tritium diffusion in nuclear fusion. Under the specific conditions of metal-cooled fusion reactors, erbium oxide deposited by CVD on tungsten and steels acts as an alternative to conventional hydrogen barriers.

• Surface tailoring to provide compatible joining of ceramic matrix composites to metallic materials such as SiC that are bonded to copper-based heat sinks. Atomic deposition of thin interface films, followed by sophisticated characterisation such as Rutherford back-scattering and sessile drop technique for in-vacuum wetting angle measurements at high temperatures, open enhanced operation regimes (temperature, and mechanical loads) for compound materials.

• Improved matching of the coefficient of thermal expansion (CTE) of different constituents of composites used in heat sink applications by applying interlayers, tungsten and molybdenum. These are deposited by thermal spray or CVD, or implemented as solid sheets, and are the main elements used for interlayers with intermediate CTE. As a highlight, tungsten-copper layers with tailored CTE and an almost four-times increase of thermal diffusivity can be produced by a combination of plasma spraying and HIPping.

In addition to the materials and technologies development results, ExtreMat partners have formed new collaborations and created a trend in materials research. Within the European materials community, ‘materials for extreme environments’ is a recognised phrase. This is reflected in a specific topic at the bi-annual EUROMAT conference, as well as in the international attendance of 150 participants in an ExtreMat-organised conference in June 2008.

Further information