Carbon Nano-Onions and Nanotubes Could Lead to New Advanced Space-Grade Materials

Aerospace Corporation scientists have developed a process for depositing graphitic nano-onion structures in an oxygen (O₂) atmosphere at room temperature by pulsed laser ablation of metal-doped graphite targets and targets of graphite filled organic compounds according to U.S. Patent Application  20100068501.   The company is developing carbon nanotube and nano-onion materials for space applications.  

Gouri Radhakrishnan, Paul M. Adams and Franklin D. Ross produced nano-onion structures in the presence of O2 gas at pressures greater than 0.1 Torr, but not in argon (Ar) atmospheres with comparable pressures. The resulting structures were observed to be 100-200 nm in diameter, significantly larger than nano-onions produced by arc discharge and electron irradiation.

Time-resolved emission spectroscopy was employed to examine differences between evolving plume species under various conditions. The shell structure of these nano-onions makes them good candidates for trapping small molecules. Large 3D cages can serve as an excellent catalyst, again by trapping or adsorbing molecules on surfaces or within the cage. Potential applications for the structures include energy storage, electrochemical applications, e.g., thin-film batteries and microbatteries, and nano-sensors, e.g., gas sensors, protein, and DNA sensors.

TEM analyses show that the materials deposited from ablating a graphite-metal target at 2 Torr O₂ and 2 Torr Ar are very different from each other. Nano-onion structures are observed from a graphite-Ni target with 2 Torr O₂. The formation of such onion-like nanostructures using excimer laser ablation at room temperature has not been reported previously. The production of these nano-onion structures was readily observed in O₂ atmospheres of 0.1 Torr and greater. The TEM images of these structures (FIGS. 3A and 3B) reveal that they are seen as individual onions or clustered onions and have diameters of 100-200 nm. Individual strands of 5-10 nm can also be observed.

High resolution TEM (FIG. 3C) shows continuous lattice fringes in the onion structure, which correspond to graphitic planes. The corresponding selected area electron diffraction (SAED) pattern is shown in FIG. 3D. The lattice spacing measured from structures in 3 different samples is found to be 0.340.+-.0.005 nm, which corresponds to lattice planes in graphite. The three-dimensional nested structure of nano-onions can be seen in the SEM image (FIG. 3E) of a broken onion.

FIG. 3C is a high-resolution TEM image showing lattice fringes, which match the lattice planes in graphite; FIG. 3D is a Selected Area Electron Diffraction (SAED) pattern with inner ring corresponding to the reflection in graphite;  FIG. 3E is a Scanning Electron Microscope (SEM) image showing the internal layer structure of a broken nano-onion; FIG. 4A is a TEM image of material deposited in 2 Torr Ar; FIG. 4B is a SAED pattern corresponding to FIG. 4A. 

 FIG. 5A is a SEM image of material deposited by ablation of graphite-filled phenolic target in 2 Torr O2; and  FIG. 5B is a TEM image corresponding to FIG. 5A showing nano-onions.

The Aerospace Corporation hosted a conference on March 16-17^th to examine the use of carbon nanotubes and nano-onions for space applications. This inaugural interchange meeting, sponsored by the U.S. Air Force Space and Missile Systems Center (SMC), focused on carbon nanotubes materials and their promise for future space platforms with emphasis on technological innovation and device-oriented implementation. The two-day meeting engaged both government and industry communities in reviewing several promising technical approaches of CNT materials to space applications. Representatives from primary DoD contractors, funding agencies and other contributors presented their respective work and discuss their respective vision for CNTs in the field of nano-materials that are relevant to SMC’s mission of researching, developing, and purchasing military space systems.

The Aerospace Corporation has provided independent technical and scientific research, development, and advisory services to national-security space programs since 1960.The greatest challenges that space programs face today are performance, reliability, safety, and cost. Carbon nanotubes (CNTs) would have a substantial effect on space programs, improving each of these factors. Novel design concepts in future aerospace vehicles will depend on new lightweight composites to permit significant reduction of mass and size of components. 

The ability to tailor carbon nanotubes is attractive for these applications because of their extraordinary properties. However, a complete picture of key properties (mechanical, electrical, thermal, and optical) and their role for improved performance in existing materials has yet to emerge. CNT applications in space may include: Spacecraft Attitude Control, Power, Guidance/Navigation Control, Command/Data Handling, Thermal Control, Propulsion, Structures, Payload and Novel Space Applications.  Near earth objects (NEOs)  may even be prevented from colliding with the Earth by using Kevlar and carbon nanotube tether materials. 

The development of new and advanced space-grade materials—and research into their properties and effects on space system components—may lead to lighter, smaller, cheaper, and more capable spacecraft. But before any material can be specified for a space application, it must endure rigorous testing and analysis to determine optimal processing conditions and ensure reliable performance in the hostile space environment. Aerospace research into space materials science will help identify the most promising new formulations and ensure that they can be used to their full potential.   

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