Carbonfiber MMCs for Space Applications
The main task of a jet engine is to maintain the structural integrity of parts subjected to combined thermal, mechanical and corrosive stress. The mechanical stress can be significantly reduced in all applications by adapting the CTE to the adjacent materials. Ideally, when there is no CTE mismatch, the stresses are minimized. The mechanical loading of engine walls, which are currently subjected to extreme heating and cooling, results from the combination of applied temperatures, CTE and design. The thermal conductivity would also play an important role by compensating for temperature differences in the chamber wall, but this is not planned. In the currently used reusable launch vehicle propulsion systems, the stresses in the thruster chamber wall result from the high temperature difference between the cooled inner wall and the hot outer wall. The hot outer wall tends to expand, the cooled inner wall tends to contract. The magnitude of the stresses developed is related to the CTE of the material. For a material with an extremely low CTE, no problems are to be expected. It should be noted that the CTE of materials (similar to e.g. the modulus of elasticity) can only be effectively reduced through the use of long fiber reinforcements. Metal, carbon or ceramic continuous fibers have a significantly lower CTE than corresponding highly conductive metallic matrices (Cu, Al, Ni) and depending on the volume fractions used and the type of orientation can proportionally reduce the CTE of the composite material. When using special HM carbon fibers, a drastic increase in thermal conductivity can also be achieved in the directions parallel to the fiber orientation. This is the starting point for the development of metallic, electroformed, 3D-networked carbon fibers that could be used, for example, as radiation-cooled nozzles and combustion chambers for SmallSat launchers. Another application is e.g. as high-temperature-resistant fastening elements in engines. In any case, three-dimensional metal matrix composites of more complicated, almost arbitrary, geometries can be realized through galvanoforming!
Carbon fiber nickel composite for rocket nozzles
Various geometries made of electroformed carbon fiber metal composites
Hotfire test of a carbonfiber-nickel composites (@ TU Wien Space Team)
Fastener made of galvanoformed carbonfiber-nickel, weight-optimized through the use of a lattice structure.
Carbon Nanotube MMCs as heat-sink materials
Incorporating C nanotubes (CNTs) into a metallic matrix is a major challenge in the manufacture of composite materials! Our intention was to use the high intrinsic thermal conductivity of these tiny fibers to improve the overall thermal conductivity of the composite. However, many detailed problems have to be solved for this, starting with the production of stable and homogeneous CNT dispersions, through the individualization of the interwoven CNTs and their homogeneous distribution in the metallic matrix, to a suitable consolidation method that leads to composites of low porosity. We have gained a lot of experience over the years and have now reached a level of experience that allows us to answer this question positively within the framework of the limitation to small volume fractions of CNTs and the use of suitable additions of so called active elements.