Materials and Coatings
See for yourself how NASA materials and coatings can solve real-world problems.
Niobium Titanium Nitride Thin Film Coating
The Niobium Titanium Nitride (NbTiN) Thin Film Coating can optically couple light to a bolometric detector, which is suspended on an ultra-thin dielectric membrane. The coating can also filter out low frequency spectral components, which would increase the photon-limited noise of the detector. NbTiN thin film coatings are fabricated on dielectric substrates using a specialized reactive sputtering co-deposition process. Two different sputtering sources are used, in which one source contains a niobium sputtering target and the other contains a titanium sputtering target. The niobium and titanium are deposited in a nitrogen-rich environment. NbTiN coating can be used by depositing it on one side of an ultra-thin silicon membrane and have a well-defined optical impedance requirement for a specific application. NbTiN coating can be deposited on non-silicon membranes as well. The NbTiN coating have low intrinsic stress, which makes it mechanically compatible with integration on ultra-thin dielectric membranes. The coating possesses the optical impedance required for a high optical efficiency absorption. Furthermore, the coating has a very low superconducting transition temperature, which enables it to filter out radiation at certain frequencies. The NbTiN coating is especially useful for ultrasensitive cryogenic bolometric detector applications. The NbTiN coating can be fabricated in a reproducible manner, while simultaneously not complicating the fabrication process of detector architectures.
Resin Transfer Molding (RTM) 370 Resin for High-Temperature Applications
RTM370 imide resin was developed to address the limitations of conventional imide resins, which are generated from commercially available symmetrical biphenyl dianhydride and oxydianiline (ODA). These resins form symmetrical dianhydride or diamine compounds that result in a substance with much higher viscosity than is viable for RTM, RFI, and VARTM. RTM370 harnesses the unique properties of asymmetric biphenyl dianhydride (a-BPDA) used in combination with a kinked ODA and a 4-(Phenylethynyl) phthalic anhydride endcap to form a mixture that can be melted without the use of solvents, and achieve the desired low-melt viscosity. RTM370 displays a high softening temperature (Tg = 370°C) and can be melted at 260-280°C. It can then be injected into fiber preforms under pressure (200 psi) or through a vacuum (VARTM) to form composites with excellent toughness. The resin can also be made into powder prepregs by melting the resin powders so that they fuse onto fibers. Recently, carbon fiber filled RTM370 imide resins have been fabricated into composites by laser sintering. This exciting advancement in additive manufacturing represents a new frontier for high-temperature composites. Not only are RTM370 composites lightweight, durable, and impact-resistant, they also possess outstanding abrasion resistance and significant thermo-oxidative stability (as demonstrated in long-term isothermal aging at 288°C for 1,000 hours). In summary, this groundbreaking approach yields a vastly superior resin for fabricating high-quality composites with improved performance, durability, and adaptability. RTM370's unique, solvent-free melt process is simpler, more environmentally friendly, and more cost-effective than competing systems, lending it broad appeal for a variety of Earth-based applications.
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.
Polyamides are polymers that are similar to polyimides (another polymer that has been developed for use in aerogels). However, because the amide link is a single chain while the imide link is a ring structure, polyamide aerogels can be made less stiff than polyimides, even though a similar fabrication process is used. The precursor materials can be made from any combination of diamine and diacid chloride. Furthermore, NASA Glenn researchers have found methods for using combinations of diamines and disecondary amines to produce polyamide aerogels with tunable glass transition temperatures, for greater control of features such as flexibility or water-resistance. In the first step of the fabrication process, an oligomeric solution is produced that is stable and can be prepared and stored indefinitely as stock solutions prior to cross-linking. This unique feature allows for the preparation and transport of tailor-made polyamide solutions, which can later be turned into gels via the addition of a small amount of cross-linker. When the cross-linking agent is added, the solution can be cast in a variety of forms such as thin films and monoliths. To remove the solvent, one or more solvent exchanges can be performed, and then the gel is subjected to supercritical drying to form a polyamide aerogel. NASA Glenn's polyamide aerogels can be fully integrated with the fabrication techniques and products of polyimide aerogel fabrication, so hybrid materials which have the properties of both classes are easily prepared. As the first aerogels to be composed of cross-linked polyamides, these materials combine flexibility and transparency in a way that sets them apart from all other polymeric aerogels.
Adaptive Thermal Management System
Efficient thermal management has long been an issue in both commercial systems and in the extreme environments of space. In space exploration and habitation, significant challenges are experienced in providing fluid support systems such as cryogenic storage, life support, and habitats; or thermal control systems for launch vehicle protection, environmental heat management, or electronic instruments. Furthermore, these systems operate in dynamic, transient modes and often under extremes of temperature or pressure. The current technical requirements associated with the thermal management of these systems result in control issues as well as significant life-cycle costs. To combat these issues, the Adaptive Thermal Management System (ATMS) was developed to help provide the capability for tanks, structural walls, or composite substrate materials to switch functionality (conductive or insulative) depending on environmental conditions or applied stimuli. As a result, the ATMS provides the ability to adapt between both heating and cooling modes within a single system. For example, shape memory alloy (SMA) elements are used to actuate at certain design temperatures to create a conductive bridge between two metal plates allowing broad-area heat rejection from the hotter surface. Upon cooling to the lower design set-point, the SMA elements return to their original shapes, thereby breaking the conductive path and returning the system to its overall insulative state. This technology has the potential to be applied to any system that would have the need for a self-regulating thermal management system that allows for heat transfer from one side to another.
High-Performance Polyimide Powder Coatings
Powder coatings are used throughout industry to coat a myriad of metallic objects. This method of coating has gained popularity because it conserves materials and eliminates volatile organic compounds. Resins traditionally chosen for powder coatings have low melting points that enable them to melt and flow into a smooth coating before being cured to a durable surface. High-performance resins, such as Teflon, nylon, and polyimide, have not been found suitable for use in powder coatings because of their high melting points. However, KSC's newly developed polyamic acid resins with low melting points can be used in a powder coating. These polyamic acid resins, when sprayed onto metal surfaces, can be cured in conventional powder coating ovens to deliver high-performance polyimide powder coatings. The polyimide powder coatings offer superior heat and electrical stability as well as superior chemical resistance over other types of powder coatings.
Silicon Oxide Coated Aluminized Polyimide Film Radiator Coating
The SiOx Coated Aluminized Polyimide Film Radiator Coating uses all the exposed surfaces on the six sides of a CubeSat as radiators. All the internal components are thermally coupled to the radiators. Waste heat from the internal components is transferred by conduction to the radiators through its aluminum structure or frame. SiOx thin film coated aluminized polyimide film is used as the radiator coating. Its total thickness is approximately 0.05 mm, which is predominately the polyimide film thickness. Polyimide film is known commercially as Kapton. The coating is bonded to the CubeSat exterior by using an acrylic transfer adhesive. SiOx Coated Aluminized Polyimide Film Radiator Coatings absorptance and emittance can be tailored to meet the component thermal requirements by altering the SiOx thickness. Since the SiOx is a thin film, altering its thickness has no significant effect on the total thickness of the radiator coating. An indium tin oxide (ITO) thin film can be added to make the coating conductive, if needed, without affecting the absorptance or emittance. This coating, with or without ITO, can be used for various CubeSat applications. By tailoring the absorptance and emittance of this coating, external MLI blankets and active heater control are eliminated. The thermal connection between heat generating components and the battery eliminates the need for a battery heater.
Automated transfer of large-area defect-free graphene using a fluid transfer system
The innovation is a series of inter-connected fluid reservoirs. The first reservoir comprises an etching agent that dissolves the growth substrate of a graphene sample. Subsequent reservoirs contain deionized water designed to wash off the etching agent. A graphene sample comprising a polymeric top coat, graphene layer, and growth substrate is floated in the first etchant reservoir. When the growth substrate has dissolved in the etchant fluid, the level of that reservoir is raised with additional etchant fluid. The rising etchant fluid level causes the etchant to flow into the next reservoir, creating a gentle current. The graphene sample floats along the current and is subsequently transferred into the next deionized water reservoir. The etchant is washed off in the deionized water. Once all the etchant is washed off, an application substrate is placed at the bottom of the deionized water reservoir. When the deionized water is drained, the graphene sample is mated with the application substrate via Van Der Waals forces. This innovation democratizes the the production of graphene, allowing it to be processed reliably and easily in house. This system can be utilized as a post-processing system for graphene production providing graphene substrates while keeping sensitive research and development safely in-house.
Advanced Protective Coatings for Graphite-Based Nuclear Propulsion Fuel Elements
To protect the graphite (Gr)-based substrates in a nuclear systems fuel elements from hot hydrogen attacks, earlier researchers developed a method to deposit (via chemical vapor deposition) a protective niobium carbide (NbC) or zirconium carbide (ZrC) coating in the inner cooling channels of the fuel elements through which the hydrogen propellant flows. Unfortunately, the significant difference in the coefficients of thermal expansion (CTE) between the Gr-based substrate and the ZrC coating leads to debonding at intermediate temperatures, thereby exposing the substrate to hot hydrogen attack despite the NbC or ZrC coating. Innovators at Glenn have proposed a solution to this problem by introducing additional layers of compliant metallic coatings to accommodate the differences in CTE between the ZrC and the Gr-substrate,thereby potentially increasing life and durability. In this configuration,the innermost layer is composed of molybdenum carbide (Mo2C), and additional outer layers are made of molybdenum (Mo) and niobium (Nb) layers. The MoC acts as a diffusion barrier to minimize the diffusion of carbon into the refractory metal layers and the diffusion of Mo or Nb into the Gr-based substrate. The Mo layer is deposited on top of the Mo2C layer. A Nb layer is deposited on the Mo layer with the ZrC forming the outside layer of the coating. A thin Mo layer on the ZrC helps to seal the cracks on the ZrC and acts a diffusion barrier to hydrogen diffusion into the coating.The Mo and Nb layers are compliant so that differences in the thermal expansion of ZrC and other layers can be accommodated without significant debonding or cracking. They also act as additional diffusion barriers to hydrogen diffusion towards the Gr-substrate. Overall, Glenn's pioneering use of layered coatings for these components will potentially increase the durability and performance of nuclear propulsion rockets.
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