Materials for space structures

A range of applications of nanomaterials lies in the construction of spacecrafts and space structures due their improved mechanical characteristics (higher firmness and stability and concurrently a lower density) compared with conventional materials. Nanomaterials could in particular contribute to the reduction of the lift-off masses of spacecrafts leading to substantial cost savings and also ensure safer and more flexible space missions. In the context of space structures, different material classes should be taken into consideration.

5.2.1.1 Nanoparticle reinforced polymers

The mechanical properties of polymers can be improved by dispersion of nanoparticles into the polymer matrix. As suitable nanoparticles e.g. silicates (in particular montmorillonite clay), POSS (Polyhedral Oligomeric

Silesquioxanes) or also carbon nanotubes (CNT, see section 5.2.1.2) are

. Nanoparticle for impro-

considered. As polymer matrices for example epoxide, nylon, polyphe- ved mechanical proper-

nole or polyimide can be used. The reinforcement effect of the nanopar- ties of polymers ticles is usually based on chemical connections, whereby a network between nanoparticles and the polymer matrix is formed. Nanoparticle reinforced polymers can be formed and extruded like conventional polymers but possess however thermal and mechanical properties, which lie between those of organic polymers and inorganic ceramics. Due to its high mechanical firmness and resistance against heat and radiation, na-noparticle reinforced polymers have application potentials for various components in space, among other things as housings of solid-propellant rockets, as heat protection material in rocket nozzles, electrical isolations or fire protection applications. The development of nanoparticle reinfor-

ced polymers is promoted by NASA e.g. in the frame of the SBIR programme. First tests for the space qualification of nanoparticle reinforced polymers have already been accomplished by NASA at the exterior of the ISS. Also within the range of aviation, nanoparticle reinforced polymers are investigated intensively at present as lightweight structure materials for airplane bodies (Chen 2001, Rice 2001).

5.2.1.2 Carbon Nanotubes

Illustration 10: Vision of a space elevator based on ultra-strong CNT materials

(Source:

Carbon nanotubes (CNT) with diameters of 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 ap-prox. 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. A substantial part of the nanotechnology programme of NASA is based on the development and application of CNT based materials, sensors and electronics (see NASA 2000). 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.

If it should succeed in the future to manufacture favourable 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 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 illustration 10).

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 approx. 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, also problems concerning the transfer of the molecular properties to macroscopic materials are still unsolved, e.g the dispersion of CNT in composite matrices or spinning of CNT to macroscopic fibers. 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. Such investigations are accomplished at present by NASA and also in Germany for example by the technical university Hamburg-Harburg (Gojny et al. 2002). Only recently scientists of 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 325 MPa and are therefore six times stronger than conventional polymers (Mamedov et al. 2002). However this can not compete with the mechanical properties of conventional carbon fiber reinforced polymers, which exhibit tensile strengths over 2 GPa, 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.

Another approach for synthesis of CNT materials is the spinning of CNT to macroscopic fibers. The spinability 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 (Ericson et al. 2002). Recently Chinese researchers of the Tsinghua university succeded in the production of a 200 |im thick yarn from carbon nanotu-bes by dragging a bundle of CNT grown on a silicon substrate up to 30 cm length similarly to spinning silk (Jiang et al. 2002). 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).

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 electrical 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 molecular electronics, as described in more detail in the following sections.

CNT polymer composites with application potential in space structures

Research efforts for the production of conti-nous CNT fibers

Electric conducting CNT composites for electrostatic isolations

5.2.1.3 Metal-Matrix-Composites

By reinforcement of metals with ceramic fibers, in particular silicium carbide, but also alumium oxide or aluminum nitride, their thermo-mechanical properties can be improved. Such metal matrix composites (MMC), e.g. SiC in aluminum alloys or TiN in Ti/Al alloys, possess due to their high heat resistance, firmness, thermal conductivity, controllable

Nanostructuring improves thermomecha- thermal expansion and low density, a high potential for aerospace appli-

nical properties of cations and are examined at present regarding the replacement of magne-

MMC materials sium and aluminum in various structures of spacecrafts and aeroplanes.

As it has been reported, the strength of MMC could be increased up to 25 % through nanostructuring and beyond that, superplasticity and a better resistance against material fatigue can be obtained in comparison to conventional MMC.26 Nanostructured ceramic fibers can be manufactured for example by modified flame synthesis on a several kg per day scale.27 Different research activities can be noticed in the frame of the SBIR-programme of NASA.

5.2.1.4 Nanocrystalline metals and alloys

The thermomechanical characteristics of metals and alloys can also be improved by controlling the nano-/microstructure of the materials. Melting points and sintering temperatures can be reduced up to 30 %, if the material is made of nanopowders. Another advantage is the easy formabi-lity of the materials through superplasticity. In a SBIR project of NASA, nano-crystalline aluminum alloys were developed for space applications by the company DWA Aluminum Composites in co-operation with different US-American aerospace companies. Such materials are investigated as alternatives for titanium in components of liquid rocket engines (e.g. lines and turbopumps), since they are lighter and less susceptible to embrittlement by hydrogen.

5.2.1.5 Nanostructured ceramics/ceramic nanopowders

Within ceramics a special focus lies on the production of controlled mic-ro/nano-structured grain sizes. An objective is the improvement of ther-momechanical properties, fracture toughness and formability ("super-plasticity") of this brittle material class. In addition, the sintering temperatures and the consolidation time of ceramic materials can be reduced by applying nanopowders, which saves not only money but also allows new manufacturing techniques like coprocessing of ceramics and metals.28 Ceramic nanopowders meanwhile can be manufactured with high chemical purity and adjustable powder grain size. Both gas or liquid phase processes are used for the production of ceramic nanopowders, for

26 personal communication 30.08.2001, Dr. J.C. Whithers, MER Corporation

27 see http://www.argonide.com/alumina_fibers.html

28 „Rapid Densification of Ceramic Monoliths and Composites" press release of NASA-MSFC, October 1998

non-oxidic powders (e.g. Si3N4, SiC, TiCN) preferentially gas phase processes and for oxidic powders (e.g. Al2O3, SiO2) also sol gel procedures.

For space application, nanostructured ceramic composites will play a role in particular as thermal and oxidative protection for fiber-reinforced construction materials (e.g. coating of carbon fiber materials with boron nitride, see section 5.2.2). Further application could arise in sensor tech- High-strength transpa-nology, optoelectronics and for space structures. An interesting develop- rent corundum ceramics ment is the production of high-strength transparent bulk ceramics. The Fraunhofer institute IKTS for example has developed a procedure for manufacturing sub |im structured corundum ceramics (Al2O3), which possess high firmness (600 - 900 MPa), scratch resistance and transparency (Krell 2002). A controlled grain growth during the sintering process makes it possible to avoid porosity to a large extent, which guarantees a dense texture and thus a high firmness. Applications in space may be seen within the range of transparent exterior surfaces and skins of spacecrafts or sensor windows.

A further relevant topic are nanostructured gradient materials, in which the gradient can be adjusted both regarding thermomechanical or chemical properties. These materials could be used for example in the production of photonic structures in optical data communication or in the pro- Electroph°retic depositi-, „ . . , , . , on of nanoparticles for duction of micromechanical and microelectronic components with a high

^ ° the near-net-shaping of degree of miniaturization. Problematic however is the shaping and com- complex components pacting of nanoparticles to components. So most of conventional shaping techniques for ceramics cannot be applied economically with nanopartic-

les, since the ceramic fragment formation depends usually on the particle size and thus long process times must be taken into account. Solutions are offered here e.g. through the formation of nanoscale ceramic particles by means of electrophoretic deposition (EPD). The EPD process, in which particles are moved through a dispersion medium by an electrical field with a size independent speed and are deposited on a ceramic green body, allows a near net shaping of complex components.

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