Search

Polymer derived ceramics (PDCs) refers to ceramic materials formed through the pyrolysis of a pre-ceramic polymer. The use of the PDC process enables the fabrication of complex, lightweight, mechanically robust shapes that are too difficult to machine otherwise. The PDC process also allows for granular control over the chemistry, resulting in better fiber homogeneity and allowing for application-specific tailoring. NASA’s PDC process to rapidly fabricate multifunctional h-BN nanofibers entails the following steps. First a liquid-based polymer precursor solution containing boron and nitrogen is made. Next, the precursor undergoes a forcespinning process, which causes the solvent to evaporate, leaving behind only polymeric nanofiber preforms. These preforms are then cured via UV exposure or other means to link the polymer chains to one another. Finally, the crosslinked polymers are heat treated under specific conditions to convert the polymer fibers into ceramics. This NASA innovation offers the ability to make low-cost, layered h-BN fiber mats or weaved fabrics of flexible h-BN from spun yarns at scale. The size of the fibers (> 200 nm) makes them easier to handle and disperse relative to nanotubes or nanosheets and mitigates respiratory hazards. The process offers high yields relative to alternative fabrication processes such as electrospinning. The resulting h-BN nanofibers have a broad range of potential applications and are poised to enable the development of new, multifunctional materials.

Sometimes called white graphite, affordable and plentiful hBN possesses the same kind of layered molecular structure as graphite. In graphite, this structure has allowed next-generation nanomaterials like carbon nanotubes and graphene to be produced. With hBN, however, the process of converting the substance into boron nitride nanotubes (BNNT) has been too difficult to yield commercial quantities. Glenn innovators have created several new methods that could enable greater adoption of this unique nanomaterial. In the initial stage, the starter reactant is mixed with a selected set of chemicals (a metal chloride, for example) and an activation agent (such as sodium fluoride). This mixture causes hBN to become less resistant to intercalation. The intercalated product can then be exfoliated by heating the material in air, and giving the material a final rinse with a liquid-phase ferric chloride salt to dissolve any embedded impurities without damaging its internal structure. These efficiently exfoliated nanomaterials can be used to form advanced composite materials (e.g., layered with aluminum oxide to form hBN/alumina ceramic composites). Nanomaterials fabricated from hBN can also take advantage of the material's unique combination of being an electrical insulator with high thermal conductivity for applications ranging from microelectronics to energy harvesting. Glenn's innovations have enabled a significantly improved matrix composite material with the potential to make a significant impact on the commercial materials market.

Many researchers have attempted to use polymer-ceramic composites to improve the thermal and dielectric performance of polymer insulation for high voltage, high temperature electronics. However, using composite materials has been challenging due to manufacturing issues like incomplete mixing, inhomogeneous properties, and void formation. Here, NASA has developed a method of preparing and extruding polymer-ceramic composites that results in high-quality, flexible composite ribbons. To achieve this, pellets of a thermoplastic (e.g., polyphenylsulfone or PPSU) are coated with an additive then mixed with particles of a ceramic (e.g., boron nitride or BN) as shown in the image below. After mixing the coated polymer with the ceramic particles, the blended material was processed into ribbons or films by twin-screw extrusion. The resulting ribbons are highly flexible, well-mixed, and void free, enabled by the coated additive and by using a particle mixture of micronized BN and nanoparticles of hexagonal BN (hBN). The polymer-ceramic composite showed tunable dielectric and thermal properties depending on the exact processing method and composite makeup. Compared to the base polymer material, the composite ribbons showed comparable or improved dielectric properties and enhanced thermal conductivity, allowing the composite to be used as electrical insulation in high-power, high-temperature conditions. The related patent is now available to license. Please note that NASA does not manufacturer products itself for commercial sale.