Carbon nanotubes (CNT), with diameters of a few nanometers, as fullerene derivatives represent pure carbon compounds and occur in different modifications, e.g. single walled (SWCNT) or multi-walled (MWCNT). CNT possess unusual mechanical characteristics (on molecular level approximately 50 times stronger than steel and outstanding thermal and electrical conductivity). Due to their special properties, CNT possess numerous application potentials in space, among other things, within the ranges of space structures, thermal control devices, sensor technology, electronics and biomedicine.
How NASA is Using Carbon Nanotubes
A substantial part of the nanotechnology programme of NASA is based on the development and application of CNT based materials, sensors and electronics. In particular, the huge potential for mass savings in space structures makes CNT very interesting for space applications. A further advantage of CNT composites is that the changes of the mechanical properties of the material can be indicated through changes of the electrical resistance and so possible damages could in principle be easily detected by simply monitoring the electric conductance of the material.
Cheaper and Improved Carbon Nanotubes Could Lead to Visionary Inventions
If it should succeed in the future to manufacture favourably-priced CNT with defined characteristics on industrial scale and to transfer the outstanding molecular properties into macroscopic materials, not only improved conventional spacecraft will be possible, but also the type of space applications which sound very visionary at present. Conceivable, for example, is a space elevator, consisting of a self-supporting CNT rope, which is connected from earth to a geostationary object in space (see figure 1).
Figure 1. Vision of a space elevator based on ultra-strong carbon nanotube materials.
The High Price of Carbon Nanotubes is Slowing Progress
At present, however, technical applications of CNT based materials for structures are still far away. This is, on the one hand, due to the very high price, particularly for SWCNT, which amounts to approximately 500$ per gram, depending on the purity and quality of the product. The high price is due to the fact that CNT can be produced so far only on a laboratory scale, with quantities up to 100g per day through different gas-phase processes (flame synthesis, catalytic CVD, electrical arc discharge, laser ablation, etc), and require a complex cleaning procedure. On the other hand, problems concerning the transfer of the molecular properties to macroscopic materials are also still unsolved, e.g the dispersion of CNT in composite matrices or spinning of CNT to macroscopic fibers.
Problems with Producing Carbon Nanotube Composites
A problem with the production of CNT composites, e.g. reinforced polymers, is the alignment and the adhesion of the CNT in the matrix. CNT tend here to agglomerate, so that the loading rate with CNT is limited to a little weight percentage. A solution could be the chemical modification of the CNT and the chemical binding to the polymer-matrix.
Benefits of Synthesising Carbon Nanotubes with Polymer Composites
Such investigations are accomplished at present by NASA and also in Germany, for example, by the technical university Hamburg-Harburg. Only recently, scientists at the University of Oklahoma and the University of Erlangen-Nuernberg succeeded in the synthesis of SWCNT/polymer composites, with a sandwich structure containing about 50% weight percentage of SWCNT. These composites exhibit a tensile strength of up to 325MPa, and are therefore six times stronger than conventional polymers. However, this cannot compete with the mechanical properties of conventional carbon fiber reinforced polymers, which exhibit tensile strengths over 2GPa, and further technological breakthroughs should be made to exploit the potential of CNT for the production of ultralight, high-strength hybrid materials, which could be used for various structure applications in space.
Spinning Carbon Nanotubes to Macroscopic Fibers and Dragging CNT Bundles on to Silicon Substrates
Another approach for synthesis of CNT materials is the spinning of CNT to macroscopic fibers. The spinnability of CNT, however, is limited by the bad solubility in organic solvents. By dispersion of SWCNT in strong acids, however, fibers with a mostly uniform alignment and promising mechanical and electrical properties have already been achieved. Recently, Chinese researchers at Tsinghua University succeeded in the production of a 200 micron thick yarn from carbon nanotubes, by dragging a bundle of CNT grown on a silicon substrate up to 30cm length, similarly to spinning silk. If it should be possible in the future to weave such CNT fibers into macroscopic objects, numerous applications will arise also in space, e.g. in materials for electromagnetic radiation shielding or protection against mechanical impacts for space stations or astronaut suits).
Expected Timescales for Carbon Nanotube Space Applications
While applications of CNT materials for structure applications are to be expected rather in a long-term time horizon, due to their high price and problems with the scalability of production processes, other applications of CNT, such as fillers for electrically conductive polymer composites, e.g. for antistatic insulating materials, could be realized earlier. Such materials are developed, among other things, in the context of a SBIR project of NASA by the US-American companies Triton-Systems and Foster-Miller. In addition, a multiplicity of further space relevant applications of CNT is conceivable, for example, in the sensor technology or in molecular electronics.