A partial list of technologies and subject areas of possible interest include:
Batteries and Supercapacitors
A number of nano materials could enhance energy storage densities in batteries and supercapacitors. Nanotechnology could reduce the charge leakage between electrodes and dielectrics in batteries and supercapacitors, thereby reducing long-term charge reduction from unwanted currents. Successful implementation of silicon nanotubes or other nanomaterials could reduce the dielectric-electrode reaction and dramatically improve battery storage life. Increasing battery life would have important ramifications for space vehicles and for energy management in a number of terrestrial applications including plug-in hybrid vehicles, fuel cell-battery hybrid designs, among other opportunities.
Composite Materials
Carbon nanotube based composites offer an interesting combination of mechanical, electrical, thermal, and optical properties, potentially useful for a number of clean energy applications such as flywheels, fuel cell bipolar plates, and structural materials. These technologies are also of great interest in for NASA space missions.
Nanotubes have been dispersed in range of matrices including polymers, resins, and pitch materials to produce high performance engineering materials including films, fibers, and bulk composites. Although the initial data suggests that these composites offer enhanced strength, stiffness, and electrical conductivity, weak nanotube-polymer bonding at the interface and non-uniform distribution of nanotubes in the polymer matrix remain problems. Modifying single-walled carbon nanotubes with functionalized groups allows formation of strong covalent bonds with the polymer matrix - and has been reported with some success.
In the case of electrically conductive composites requiring mechanical strength as well, blended polymers have been used. The carbon nanotube filler preferentially segregates in a single phase allowing for the formation of a 3-D triple continuous structure capable for electric conduction. The second phase is a continuous neat polymer phase and provides mechanical strength.
Energy Efficiency
A variety of nanotechnologies and materials have been identified that reduce energy losses in a wide range of settings through friction and viscosity management, industrial coatings, surface passivation, and similar applications. The achievement of energy savings through efficiency would help mitigate over sizing electricity generation and storage technologies required to meet the peak loads and longer-term energy needs of space vehicles. Successful development of nanotechnology based energy efficiency would find substantial market opportunity in terrestrial applications also.
Environmental Toxicity
Safety assessment of engineered nanomaterials has not yet been fully evaluated. Preliminary studies with fullerenes, carbon nanotubes and quantum dots are mixed as to their toxicity assessment. In the case of fullerenes, studies have shown toxicity in human dermal fibroblasts, rat hepatocytes, and juvenile bass. Preliminary research has reported that the toxicity may be due to the leaking of the solvent used to purify the bucky balls. This leakage was not detected using traditional assays. However, it was recently reported in a gene expression microarray experiment that samples from zebrafish treated with fullerenes resulted in a gene expression signature showing toxicity. The authors concluded that this toxicity is due to a metabolite of the solvent used in purification not the fullerene molecules themselves.
Additional health study of nanomaterials is warranted. Toxicogenomics, a relatively new discipline within the field of toxicology that evaluates compound-cellular interactions using high-throughput screening technologies, is a prime candidate for the study. In particular, the application of genomics (using gene expression microarrays containing a full complement of an organism's genes) and proteomics (using two-dimensional gel electrophoresis annotated by mass spectrometry to profile all the proteins within an organism), would be especially interesting.
While these preliminary studies give an indication that some toxicity may be present, no comprehensive study has been performed to date. A study based on the use of appropriate primary human cells will allow for very controlled treatment conditions. Checking for any deleterious effects from nanomaterials using genomic and proteomic methods can assess compound-cell interactions at the global cellular level by investigating the signal from all the genes or proteins in a tissue or organism.
Fuel cells
Low temperature PEM fuel cell commercialization is inhibited due to ongoing issues with performance and cost. Carbon nanotubes have at least two applications within the PEM structure that could dramatically impact both performance and cost. Carbon nanotubes are being investigated for use as a catalyst support structure in the Membrane Electrode Assembly (MEA) of fuel cells. Successful introduction of nanotubes could reduce the amount of platinum catalyst needed for long-term functionality of the device. Because platinum is a major cost driver for PEM fuel cells, reduced platinum use could enhance the commercial prospects for these devices in terrestrial applications.
Additionally, nanotube catalyst support could be an enabling technology that allows the use of alternate membrane materials that can be used at higher temperatures. The current generation polymer electrolyte membranes operate at around 80°C. With use of nanomaterials, membranes based on metal-oxanes can be developed allowing operation at much higher temperature (~ 200°C or higher). By pushing PEM fuel cell operating temperatures to over 150°C, the ability of weakly bound impurity atoms to block catalyst sites could be reduced, the effect of which could be longer fuel cell lifetimes.
Carbon nanotubes have also been proposed for use in bipolar plates of PEM fuel cells. Important characteristics of these plates include high electrical and thermal conductivity, high strength, low mass, ability to withstand acidic environments, and low-cost manufacturing. A number of materials are commonly used as bipolar plate materials including stainless steel and graphite. The use of carbon nanotubes in an electrically conductive thermoplast that could be injection molded could result in a compelling mix of mechanical and electrical properties that result in high performance and low cost.
Hydrogen Storage
The ability to efficiently store high volumes of hydrogen is known to be a key enabling technology for vehicle based fuel cell engines. In fact, the U.S. Department of Energy has created a program to research hydrogen storage technologies. Called the ‘Grand Challenge," the program is evaluating a number of hydrogen storage techniques involving high pressure gases, liquid hydrogen, and metal hydride technologies. Single wall nanotubes are another technology of interest. Although initial studies on SWNT have not been favorable, the problem appears to be related to an inability to store and remove hydrogen atoms from the interior of the nanotubes. A number of researchers are investigating the use of metallic intercalation to maintain needed tube diameter, much like the stair step structure binding the double helix in DNA molecules. Successfully modifying common SWNT to enhance hydrogen storage capability could provide substantial benefit for NASA, while potentially advancing fuel cell vehicle prospects.
Quantum Wire
Highly conductive quantum wire is produced from metallic-type single wall carbon nanotubes, sometimes referred to as "armchair" nanotubes. The production of large quantities of metallic-type tubes has proved difficult and is the subject of numerous research projects exploring processing alternatives, separations techniques, and yield improvement. While HARC has previously described a program with Rice University to further explore a seeded growth and separation technique, the development of bulk metallic nanotube production technique using pulsed laser vaporization (PLV) is also of interest.
The PLV technique as employed at NASA Johnson Space Center produces pure SWNTs by using two Nd:YAG (Neodymium: Yttrium-Aluminum-Garnet) pulsed lasers that impinge on a composite graphite and metal catalyst target. The process has produced as much as 0.3-0.4 grams per hour, with the exact yield dependent on the amount and type of catalysts, laser power and wavelength, temperature, pressure and type of buffer gas, and geometry of the fluid flow near the carbon target. Metallic-type SWNT produced by the PLV technique could be used in a number of ways to further enhance and accelerate the production of quantum wire at Rice University's Carbon Nanotechnology Laboratory or elsewhere.
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