Conductive Polymer/Carbon Nanotube Structural Materials and Methods for Making Same

The new composite developed by NASA incorporates PGS and CNTs to enhance its thermal conductivity while preserving the mechanical properties of the underlying carbon fiber polymer composite. NASA has also improved the composite manufacturing process to ensure better thermal conductivity not only on the surface, but also through the thickness of the material. This was achieved by adding perforations that enable the additives to spread through the composite. The process for developing this innovative, highly thermally conductive hybrid carbon fiber polymer composite involves several steps. Firstly, a CNT-doped polymer resin is prepared to improve the matrix's thermal conductivity, which is then infused into a carbon fiber fabric. Secondly, PGS is treated to enhance its mechanical interface with the composite. Thirdly, perforation is done on the pyrolytic graphite sheet to improve the thermal conductivity through the thickness of the material by allowing CNT-doped resin to flow and better interlaminar mechanical strength. Finally, the layup of PGS and CNT-CF polymer is optimized. Initial testing of the composite has shown significant increases in thermal conductivity compared to typical carbon fiber composites, with a more than tenfold increase. The composite also has higher thermal conductivity than aluminum alloys, with more than twice the thermal conductivity of the Aluminum 6061 typically used in the aerospace industry. For this new material, NASA has completed a proof-of-concept demonstration and work continues to use the material in a heat exchanger system and further characterize the properties including longevity and radiation impact analysis.

The technology portfolio spans several methods for dispersion and processing of CNTs in polymer resins and composites. CNT/resin systems with high dispersion and long-term stability are provided by three general approaches. One method relies on mechanical dispersion by sonication simultaneous with partial polymerization to increase the resin viscosity to maintain dispersion and enable further polymer processing of the CNT blend into films and other articles. Another approach relies on what is termed donor acceptor bonding, which essentially is a dipole bond created on the CNT/resin interface to maintain dispersion and stability of the CNT/resin blend. This dispersion method also provides advantages in mechanical properties of processed composites due to the interface characteristics. A range of polymer types can be used, including polymethyl methacrylate, polyimide, polyethylene, and others. An additional dry blending approach provides advantages for a variety of thermoplastic and thermoset systems. Use of ball mill mixing achieves effective blending and dispersion of the CNT, even at high loadings. Further processing steps using injection molding or similar melt processing methods have yielded CNT/ polymer composites with a range of useful electronic, optical, and mechanical properties.

NASA's new material is a toughened triaxial braid made from ductile carbon nanotube (CNT) yarn hybridized with carbon fiber, which is ultimately used as reinforcement material to make toughened polymer matrix composites. The CNT yarn component of the reinforcement is solely responsible for adding toughness, while the processes used to optimize the fiber braiding parameters and tensile properties of the carbon fiber-CNT yarn hybrid tow material determine the overall improvement in tensile strength for resin impregnated fiber tows. Bundles of continuous carbon nanotube yarns are combined with a similar format of carbon fiber, yielding an easily scalable process. Advantages of the material include reduced cost by eliminating use of toughening agents, increased ability to conform to highly complex geometries, greater environmental stability compared to aramid fiber reinforcements such as Kevlar, and possibly decreased density. Many hybrid reinforcements exhibit interfacial compatibility issues, which could lead to premature failure via crack propagation at the polymer matrix interface. In contrast, chemical similarities between the CNT yarn and carbon fiber constituents impart NASA's hybrid reinforcement material with excellent interfacial compatibility. Potential applications include aerospace components, composite pressure vessels, wind turbine blades, automotive components, prosthetics, sporting equipment, construction reinforcement material, and other use-cases where strength-to-weight ratio is of utmost importance.

In Glenn's technique, SWCNTs are dispersed in a suitable solvent, such as N-methyl pyrollidinone, and the resulting suspension is saturated with oxygen gas. A singlet oxygen sensitizer is added and the resulting mixture is irradiated under a continuous flow of oxygen for many hours. The resulting oxidized tubes are recovered by filtering the suspension, washing them, and then drying them in a vacuum oven. Singlet oxygen is a highly reactive species and is known to add to a variety of aromatic carbons. Singlet oxygen is prepared by irradiating an oxygen saturated solution with ultraviolet light in the presence of a sensitizer. This method may also be suitable for use in oxidation of multi-wall carbon nanotubes and graphenes. This is an early-stage technology requiring additional development. Glenn welcomes co-development opportunities.

Formation of the inventive polymer composite matrix begins by growing carbon nanotubes directly on a veil substrate. The carbon nanotubes are grown from both sides of a non woven carbon fiber mat. The carbon nanotubes can be single or multi walled and can be grown to predetermined lengths. The veiled substrate is positioned between carbon fiber/ polymer prepreg layers such that the carbon nanotubes protrude into the reinforcement layers. The polymer composite matrix formed following curing of the resin exhibits improved interlaminar strength, fracture toughness and impact resistance. Because of the thinness of the veil layer, electricity can pass from conductive carbon nanotubes on one side of the veil to conductive carbon nanotubes on the other side of the veil. Electricity can also pass between two veils intercalated into the same reinforcement layer when the length of the nanotubes is sufficiently long enough to provide overlap within the reinforcement layers.