Carbon nanotubes have exceptional electrical and mechanical properties that make them useful in many applications. Arrays of vertically aligned carbon nanotubes can be synthesized, but little is known about the mechanical properties of such arrays. In a recent study, published in Nanotechnology, these properties have been characterized and an application has been proposed.
Parallel plate design
Vertically aligned carbon nanotube arrays with the desired geometry were used as-grown on metal electrodes patterned on a silicon chip (see image). When a voltage was applied between the arrays they were electrostatically actuated and attracted towards each other. The separation between the arrays was studied in situ during actuation. The arrays of carbon nanotubes behaved as solid cohesive units, whose separation could be reproducibly changed by more than 30% by applying appropriate actuation voltages.
Electrostatic simulations of similar devices were performed. The results were compared with the experimental observations to extract the mechanical properties of the arrays. Although individual nanotubes have a tensile strength of tens to hundreds of GPa, thus stronger than steel, the arrays were found to have an effective Young's modulus of only ca 4 MPa, thus lower than rubber. This is believed to be due to nanotubes being able to slip and slide against each other in the low-density arrays.
During actuation, the separation between the arrays of carbon nanotubes was changed. Because the arrays form a simple parallel plate capacitor, the value of the capacitance could be controlled by the actuation voltage. The capacitance was determined from radio-frequency measurements and a capacitance change of more than 20% was found.
The very low value of the effective Young's modulus for carbon nanotube arrays allows relatively large structures to be actuated with rather low voltages. The good agreement between model calculations and the experimental measurements should make it possible to design device geometries that provide specific operating characteristics and take advantage of the special properties of carbon nanotube arrays.