U. T. Dallas-led Research Team Produces Strong, Transparent Carbon Nanotube Sheets

Carbon nanotubes are like minute bits of string, and untoldtrillions of these invisible strings must be assembled to make usefulmacroscopic articles that can exploit the phenomenal mechanical andelectronic properties of the individual nanotubes. In the Aug. 19 issueof the prestigious journal Science, scientists from the NanoTechInstitute at UTD and a collaborator, Dr. Ken Atkinson from CommonwealthScientific and Industrial Research Organization (CSIRO), a nationallaboratory in Australia, report such assembly of nanotubes into sheetsat commercially useable rates.

Starting from chemically grown, self-assembled structures inwhich nanotubes are aligned like trees in a forest, the sheets areproduced at up to seven meters per minute by the coordinated rotationof a trillion nanotubes per minute for every centimeter of sheet width.By comparison, the production rate for commercial wool spinning is 20meters per minute. Unlike previous sheet fabrication methods usingdispersions of nanotubes in liquids, which are quite slow, thedry-state process developed by the UTD-CSIRO team can use theultra-long nanotubes needed for optimization of properties.

Strength normalized to weight is important for manyapplications, especially in space and aerospace, and this property ofthe nanotube sheets already exceeds that of the strongest steel sheetsand the Mylar and Kapton sheets used for ultralight air vehicles andproposed for solar sails for space applications, according to theresearchers. The nanotube sheets can be made so thin that a squarekilometer of solar sail would weigh only 30 kilograms. While sheetsnormally have much lower strength than fibers or yarns, the strength ofthe nanotube sheets in the nanotube alignment direction alreadyapproaches the highest reported values for polymer-free nanotube yarns.

The nanotube sheets combine high transparency with highelectronic conductivity, are highly flexible and provide giantgravimetric surface areas, which has enabled the team to demonstratetheir use as electrodes for bright organic light emitting diodes fordisplays and as solar cells for light harvesting. Electrodes that canbe reversibly deformed over 100 percent without losing electricalconductivity are needed for high stroke artificial muscles, and the Science article describes a simple method that makes this possible for the nanotube sheets.

The use of the nanotube sheets as planar incandescent sourcesof highly polarized infrared and visible radiation is also reported inthe Sciencearticle. Since the nanotube sheets strongly absorb microwave radiation,which causes localized heating, the scientists were able to utilize akitchen microwave oven to weld together plexiglas plates to make awindow. Neither the electrical conductivity of the nanotube sheets northeir transparency was affected by the welding process -- whichsuggests a novel way to imbed these sheets as transparent heatingelements and antennas for car windows. The nanotube sheets generatesurprisingly low electronic noise and have an exceptionally lowdependence of electronic conductivity on temperature. That suggeststheir possible application as high-quality sensors - which is a veryactive area of nanotube research.

"Rarely is a processing advance so elegantly simple that rapidcommercialization seems possible, and rarely does such an advance soquickly enable diverse application demonstrations," said the article'scorresponding author, Dr. Ray H. Baughman, Robert A. Welch Professor ofChemistry and director of the UTD NanoTech Institute. "Synergisticaspects of our nanotube sheet and twisted yarn fabrication technologieslikely will help accelerate the commercialization of both technologies,and UTD and CSIRO are working together with companies and governmentlaboratories to bring both technologies to the marketplace."

The breakthroughs resulted from the diverse expertise of thearticle's co-authors. Dr. Mei Zhang and Dr. Shaoli Fang, NanoTechInstitute research scientists, first demonstrated the nanotube sheetfabrication process, and this result was translated into diverseapplications by the entire team. The other team members include Dr.Anvar Zakhidov, associate director of the NanoTech Institute;Christopher Williams, Zakhidov's graduate student from the UTD PhysicsDepartment; Dr. Sergey Lee and Dr. Ali Aliev, research scientists atNanoTech Institute, in addition to Atkinson and Baughman.

The applications possibilities seem even much broader than thepresent demonstrations, Baughman said. For example, researchers fromthe Regenerative Neurobiology Division at Texas Scottish Rite Hospitalfor Children, Dr. Mario Romero, Director, and Dr. Pedro Galvan-Garcia,Senior Researcher Associate, and Dr. Larry Cauller, associate professorin UTD's neuroscience program, have initial evidence suggesting thathealthy cells grow on these sheets - so they might eventually beapplied as scaffolds for tissue growth.

Baughman said that numerous other applications possibilitiesexist and are being explored at UTD, including structural compositesthat are strong and tough; supercapacitors, batteries, fuel cells andthermal-energy-harvesting cells exploiting giant-surface-area nanotubesheet electrodes; light sources, displays, and X-ray sources that usethe nanotube sheets as high-intensity sources of field-emittedelectrons; and heat pipes for electronic equipment that exploit thehigh thermal conductivity of nanotubes. Multifunctional applicationslike nanotube sheets that simultaneously store energy and providestructural reinforcement for a side panel of an electrically poweredvehicle also are promising, he said.

UTD researchers began collaborating with their counterparts atCSIRO last year. In November 2004, the organizations achieved abreakthrough by downsizing to the nanoscale methods used to spin wooland other fibers to produce futuristic yarns made from carbonnanotubes.

The latest research was funded by the Defense AdvancedResearch Projects Agency, an agency of the United States Department ofDefense, the U.S. Air Force Office of Scientific Research, the TexasAdvanced Technology Program, the Robert A. Welch Foundation and theStrategic Partnership for Research in Nanotechnology.