Soft Magnetic Nanocomposite for High-Temperature Applications

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.

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.

This technology was developed to address the limitations of traditional, single-fed linear alternators, which require permanent magnets, adhesive bonding organics, and heavy iron laminations for flux control. They experience eddy-current losses and require electromagnetic interference protection. Furthermore, they have a limited operational temperature range (only up to 250°C), which typically declines to below 200°C as the adhesive bonding organics outgas and degrade over time. Consequently, they are limited to approximately 93% efficiency at ambient temperatures. Glenn's novel linear alternator addresses all of the limitations of its predecessors and engenders a number of desirable new qualities - notably the ability to reduce eddy-current losses by 25% and operate at 99% efficiency at temperatures up to 950°C. It features a concentric, additively manufactured monolithic copper plunger and stator. The stator is a stationary single copper Halbach array, whereas the plunger is a moving electromagnetic copper Halbach array. A direct current is delivered through the conductive piston flexure support, which also provides reactive power for resonance. It creates a fixed magnetic field similar to that of a permanent magnet, but the magnetic field is channeled inward by the Halbach mover, doubling its strength. By utilizing standard double-fed induction control methods, the reactive power can be transferred and adjusted between both coils. This maximizes system efficiency and minimizes weight. This innovative technology will enable a new class of vastly superior linear alternators with the ability to operate at extreme temperatures with increased performance and efficiency. This is an early-stage technology requiring additional development. Glenn welcomes co-development opportunities.

Solid state power conversion devices, such as thermoelectrics, depend upon temperature gradients for their operation. For example, aeronautic gas turbine engines maintain the necessary temperature gradients throughout their systems due to the enthalpic processes of combustion, which offers the possibility of generating electrical power for use in primary and secondary electrical systems in the aircraft. However, until now thermoelectric materials have not been able to withstand the combination of high temperatures and oxidative environments present in gas turbine engines. Glenn's innovation overcomes these limitations by using a doped oxide pyrochlore (crystal compound) semiconductor as the thermoelectric material. The material has a low thermal conductivity, which allows it to maintain a thermal gradient and sufficient electrical conductivity to produce an electromotive force. The pyrochlore allows the thermoelectric material to be present within a gas turbine engine, converting heat directly into electricity and functioning at high temperatures without oxidizing in air. Glenn's innovative thermoelectric material permits the benefits of solid-state power conversion devices to improve fuel efficiencies for a broader range of applications than has ever been possible. This innovation is in the early stages of development, and Glenn welcomes opportunities for co-development.

SiC fiber tows and preform materials are commonly used as reinforcements in CMCs, to make parts for use in harsh, high-temperature environments such as aircraft engines. These materials are desirable for numerous high-temperature applications because of their very low weight and outstanding thermo-chemical inertness. However, the multiple-step process using electric furnaces to produce these materials have numerous drawbacks: they are very expensive ($10,000 to $25,000 per spool); they involve high temperatures (greater than 2000°C); they require high power (more than 700 watts); and they produce much wasted material. Glenn innovators have discovered an efficient way to improve the quality and strength of SiC fiber tows using a unique microwave-furnace design that induces molecular heating. Glenn's innovation relies on microwave sintering to convert a polymer to ceramic fibers/tows/yarns, or to manipulate commercially available SiC fibers to increase strength and improve other qualities. Not only can higher quality tows be produced, but also - for the first time - old, damaged, or otherwise unusable fibers can be improved and recycled, thereby saving significant costs by increasing yield. Even entire engine components can be placed in the furnace and restored. The desired results can be achieved in minutes rather than the usual hours or even days. Glenn's low-temperature microwave process provides greater control with less power, while also eliminating plasma generation and minimizing arcing events. Because this method also facilitates the shaping of the SiC fiber after initial processing, fabricating preforms with 2D or 3D architectures becomes simpler. Glenn's creative processing method makes producing SiC tows and preforms much less expensive, opening them up for increased use in a broad range of applications.