Most materials shrink laterally like a rubber band when stretched, so their Poisson's ratios are positive. Likewise, most materials contract in all directions when hydrostatically compressed and decrease density when stretched, so they have positive linear compressibilities. We found that the in-plane Poisson's ratio of carbon nanotube sheets (buckypaper) can be tuned from positive to negative by mixing single-walled and multiwalled nanotubes. Density-normalized sheet toughness, strength, and modulus were substantially increased by this mixing. A simple model predicts the sign and magnitude of Poisson's ratio for buckypaper from the relative ease of nanofiber bending and stretch, and explains why the Poisson's ratios of ordinary writing paper are positive and much larger. Theory also explains why the negative in-plane Poisson's ratio is associated with a large positive Poisson's ratio for the sheet thickness, and predicts that hydrostatic compression can produce biaxial sheet expansion. This tunability of Poisson's ratio can be exploited in the design of sheet-derived composites, artificial muscles, gaskets, and chemical and mechanical sensors.

Typical TEM images of CNTs extracted from an undeformed pillar. In situ videos: (included)PillarCompression_25xspeed.mpg PillarCompression_25xspeed.avi A pillar compression performed at a strain rate of 0.001 s -1 , then sped up to 25× the original rate for easier viewing.PillarCompression_25xspeed_data.mpg PillarCompression_25xspeed_data.aviStress-strain data gathered simultaneously with the pillar compression shown in 'PillarCompression_25xspeed.mpg'. Both videos may be played side-by-side to correlate the buckling events and stress-strain humps.

No abstract

Artificial muscles and electric motors found in autonomous robots and prosthetic limbs are typically battery-powered, which severely restricts the duration of their performance and can necessitate long inactivity during battery recharge. To help solve these problems, we demonstrated two types of artificial muscles that convert the chemical energy of high-energy-density fuels to mechanical energy. The first type stores electrical charge and uses changes in stored charge for mechanical actuation. In contrast with electrically powered electrochemical muscles, only half of the actuator cycle is electrochemical. The second type of fuel-powered muscle provides a demonstrated actuator stroke and power density comparable to those of natural skeletal muscle and generated stresses that are over a hundred times higher.

A series of rich premixed flames are used to realize a post-flame gas mixture for optimum carbon nanotube (CNT) growth using inexpensive hydrocarbon fuels. The mixture of CO, CO 2 , H 2 , and H 2 O is varied through use of hydrocarbon fuels with different H/C ratio in flames with different fuel/air ratios. Both SEM and HRTEM imaging are used to correlate the nanotube morphology and internal structure to the reaction gas composition. The variations observed are understood in light of the gas composition and the interaction of the reactive components with both the deposited Co catalyst particles and supporting metal substrate. Further comparisons between flames producing the same CO or H 2 concentrations identify the roles of these gases in CNT synthesis. Optimal flame synthesis conditions, defined upon a H 2 and CO concentration map, are gauged on the basis of CNT length, relative surface density, and level of graphitic structure.

No abstract

A simple model is developed to predict the complex mechanical properties of carbon nanotube sheets (buckypaper) [Hall et al., Science 320 504 (2008)]. Fabricated using a similar method to that deployed for making writing paper, these buckypapers can have in-plane Poisson's ratios changed from positive to negative, becoming auxetic, as multiwalled carbon nanotubes are increasingly mixed with single-walled carbon nanotubes. Essential structural features of the buckypapers are incorporated into the model: isotropic in-plane mechanical properties, nanotubes preferentially oriented in the sheet plane, and freedom to undergo stress-induced elongation by both angle and length changes. The expressions derived for the Poisson's ratios enabled quantitative prediction of both observed properties and remarkable new properties obtainable by structural modification.

No abstract