New Methods in Preparing and Purifying Nanomaterials

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.

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.

Commercial soft magnetic cores used in power electronics are limited by core loss and decreased ferromagnetism at high temperatures. Extending functional performance to high temperatures allows for increased power density in electric systems with fixed power output and elevated operating temperature. The innovators at Glenn developed a unique composition and process to improve the temperature capability of the material. Nanocomposite soft magnetic materials are typically comprised of a combination of raw materials including iron, silicon, niobium, boron, and copper. Instead of niobium, NASA's material utilizes small cobalt and tantalum additions. The raw materials are combined to form an amorphous precursor through melt spinning. NASA&#39s innovation with the fabrication lies in the thermal annealing step, which nucleates and crystallizes the precursor to form the composite structure of the material. By adjusting the temperature and magnetic field of the thermal annealing step, Glenn's process results in good coupling between the crystalline and amorphous matrix phases. Innovators at Glenn demonstrated the temperature robustness using small test cores of their material and are investigating additional quality attributes compared to other well-known soft magnetic materials (see two Figures below).

Aimed at structural applications up to 2700°F, NASA's patented technologies start with two types of high-strength SiC fibers that significantly enhance the thermo-structural performance of the commercially available boron-doped and sintered small-diameter “Sylramic” SiC fiber. These enhancement processes can be done on single fibers, multi-fiber tows, or component-shaped architectural preforms without any loss in fiber strength. The processes not only enhance every fiber in the preforms and relieve their weaving stresses, but also allow the preforms to be made into more shapes. Environmental resistance is also enhanced during processing by the production of a protective in-situ grown boron-nitride (iBN) coating on the fibers. Thus the two types of converted fibers are called “Sylramic-iBN” and “Super Sylramic-iBN”. For high CMC toughness, two separate chemical vapor infiltration (CVI) steps are used, one to apply a boron nitride coating on the fibers of the preform and the other to form the SiC-based matrix. The preforms are then heat treated not only to densify and shrink the CVI BN coating away from the SiC matrix (outside debonding), but also to increase its creep resistance, temperature capability, and thermal conductivity. One crucial advantage in this suite of technologies lies in its unprecedented customizability. The SiC/SiC CMC can be tailored to specific conditions by down-selecting the optimum fiber, fiber coating, fiber architecture, and matrix materials and processes. In any formulation, though, the NASA-processed SiC fibers display high tensile strength and the best creep-rupture resistance of any commercial SiC fiber, with strength retention to over 2700°F.

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.