Rapid Fabrication of Boron Nitride Fine Fibers
Materials and Coatings
Rapid Fabrication of Boron Nitride Fine Fibers (LEW-TOPS-172)
Innovative method offers high yield, low-cost, and safe production
Overview
Hexagonal boron nitride (h-BN) nanofibers have become an area of significant interest to industry given their combination of unique characteristics including excellent thermal conductivity, high electrical resistivity, chemical inertness, light weight, and mechanical strength. Potential applications for h-BN nanofibers include their use as an additive to polymers and other composites to improve material properties, high voltage cable insulation, re-entry shielding, radiation shielding, electric vehicle thermal dissipation, and much more. However, manufacturing of h-BN nanofibers is costly, difficult at scale, and could pose respiratory hazards resulting from their small scale (< 100nm). This has limited the commercial availability of h-BN nanofibers. High-cost BN nanosheets are available, but their use is limited as the form factor is already established and they are difficult to disperse.
h-BN nanofibers could be a critical technology enabling the development of advanced multifunctional materials not just for space systems, but also for a variety of terrestrial applications. So, innovators at NASA’s Glenn Research Center (GRC) developed a polymer derived ceramic (PDC) process utilizing forcespinning technology that enables the rapid, low-cost fabrication of h-BN fibers. Fibers generated using the process have larger diameters (≥ 200nm), mitigating respiratory hazard risk.
The Technology
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.
Benefits
- High yield, low-cost fabrication: NASA’s PDC forcespinning fabrication process offers significantly higher yields relative to alternative methods such as electrospinning, reducing manufacturing cost and enabling production at scale. This could make the commercial production of h-BN nanofibers feasible.
- Excellent material properties: NASA’s h-BN fibers offer the beneficial material properties inherent to h-BN, including light weight and high strength, electrical resistivity, and thermal conductivity.
- Mitigates safety hazards: Because NASA’s process yields yarns with large diameters (≥ 200nm), respiratory hazards associated with their inhalation is mitigated.
- Ease of dispersion: h-BN nanofibers are easier to handle and disperse as a polymer composite additive relative to boron nitride nanosheets with established form factors.
- Enables development of new, multifunctional materials: This low-cost, high yield method to produce h-BN nanofibers is poised to enable the development of new, unique, multifunctional materials with high temperature and voltage performance.
Applications
- Space: Components requiring high temperature and/or voltage capabilities (e.g., re-entry shielding, radiation shielding, lightweight structural components, sensors, electrical component packaging)
- Electronics packaging: Due to enhanced thermal management capabilities and excellent dielectric properties, h-BN nanofibers could be useful when incorporated into materials used in electronics packaging.
- Electric vehicles: Power electronics heat sinks / cooling solutions, battery barrier layers, structural components, thermal management systems, electrical insulation for high-voltage cables
- Structural power: Structural energy storage devices
- Nuclear: Radiation shielding materials
- Composite additives: Improving composite and ceramic composite properties without introducing significant additional weight
- High-voltage cable insulation: Cable insulation for hybrid-electric aircraft, energy infrastructure, etc.
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Similar Results
New Methods in Preparing and Purifying Nanomaterials
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.
Advanced Materials for Electronics Insulation
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.
Conductive Carbon Fiber Polymer Composite
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.
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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.
Conductive Polymer/Carbon Nanotube Structural Materials and Methods for Making Same
Carbon nanotubes (CNTs) show promise for multifunctional materials for a range of applications due to their outstanding combination of mechanical, electrical and thermal properties. However, these promising mechanical properties have not translated well to CNT nanocomposites fabricated by conventional methods due to the weak load transfer between tubes or tube bundles.
In this invention, the carbon nanotube forms such as sheets and yarns were modified by in-situ polymerization with polyaniline, a -conjugated conductive polymer. The resulting CNT nanocomposites were subsequently post-processed to improve mechanical properties by hot pressing and carbonization. A significant improvement of mechanical properties of the polyaniline/carbon nanotube nanocomposites was achieved through a combination of stretching, polymerization, hot pressing, and carbonization.
Creating Low Density Flexible Ablative Materials
The low density flexible ablator can be deployed by mechanical mechanisms or by inflation and is comparable in performance to its rigid counterparts of the same density and composition. Recent testing in excess of 400W/cm2 demonstrated that the TPS char has good structural integrity and retains similar flexibility to the virgin material, there by eliminating potential failure due to fluttering and internal stress buildup as a result of pyrolysis and shrinkage of the system. These flexible ablators can operate at heating regimes where state of the art flexible TPS (non-ablative) will not survive. Flexible ablators enable and improve many missions including (1) hypersonic inflatable aerodynamic decelerators or other deployed concepts delivering large payload to Mars and (2) replacing rigid TPS materials there by reducing design complexity associated with rigid TPS materials resulting in reduced TPS costs.
