WO2006016907A2

WO2006016907A2 - Photoactuators based on carbon nanotube-nafion composites

Info

Publication number: WO2006016907A2
Application number: PCT/US2005/013606
Authority: WO
Inventors: Igor A. Levitsky; Peter T. Kanelos; William B. Euler
Original assignee: The Board Of Governors For Higher Education, State Of Rhode Island And Providence Plantations
Priority date: 2004-04-21
Filing date: 2005-04-21
Publication date: 2006-02-16
Also published as: WO2006016907A3

Abstract

A photo-mechanical actuating material is provided that is composed of an organic composite and a method of forming such a structure.

Description

PHOTOACTUATORS BASED ON CARBON NANOTUBE-NAFION COMPOSITES

PRIORITY INFORMATION

This application claims priority to U.S. Provisional Application No. 60/564,199 filed on April 21, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to direct conversion of light energy to mechanical energy at an unprecedented magnitude.

2. Description of the Prior Art

Most actuators have been electromechanical, in both micro- and nano-scale technologies. There have been a few semiconductor photoactuators. However, the magnitude of actuation is more than 1000 times smaller than reported here. Pure carbon nanotubes fibers also exhibit photoactuation but of a smaller magnitude.

Photomechanical actuators could replace electromechanical actuators in micro- and non-scale technologies resulting in smaller micro-machines due to the lack of wiring needed to provide physical displacement.

SUMMARY OF THE INVENTION

A photo-mechanical actuator based on a carbon nanotubes/Nafion bi-layer. A key feature of the invention is the direct conversion of light energy to mechanical energy at an unprecedented magnitude.

An object of the present invention is to provide a photo-mechanical actuating material composed of an organic composite and a method of forming such a structure.

It is a further object of the invention to provide a layer of Nation which includes carbon nanotubes.

DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention will become more apparent as the description proceeds with reference to the accompanying drawings, wherein:

FIGS. 1 is a schematic of a photoactuator; FIG. 2 is a schematic showing a bi-layer composite; FIG. 3 is a graph of photo-mechanical displacement;

FIGS. 4 A and 4B are photographs of cantilever displacement, 4 A being prior to light stimulation and 4B with photo stimulation; and

FIG. 5 is a graph of the linearity of photo-mechanical actuation.

DETAILED DESCRIPTION OF THE INVENTION

Generally, the invention includes a photoactuator based on a carbon nanotube-Nafion composite as shown at 10 in Figure 1.

As shown in Figure 2, the photoactuator consists of bilayer strip 12. One layer is made of nafion 14, an ionomeric, perfluorinated polymer, that has a thickness of 25 to 200 microns. The second layer 16 is composed of single walled carbon nanotubes with a thickness of 1 to 20 microns. When the bilayer composite

12 is exposed to light, it has been shown to exhibit mechanical actuation as a result of exposure to various wavelengths of light the strip and bends in the direction of the Nafion. The bending direction is independent of the direction of the light source.

The layer of Nafion 14 includes single- walled carbon nanotubes and the ionomeric polymer Nafion. The extent of actuation is wavelength dependent and parallels the absorption of the semiconducting single walled carbon nanotubes. The maximum deflection of the actuator tip depends upon the light intensity and is approximately linear in light intensity. At the maximum light levels available in the NIR, the strains reaches 0.3-0.4% .

The mechanism of actuation appears to be based upon a photoconductivity effect at the nafion-nanotube interface. The photocurrent was measured in a sample where the actuation was restricted. The actuation is caused by the electrostatic forces. The difference in this actuator is that the electric current is generated by light rather than by an external electric foiled. There is a direct conversion of light energy to mechanical energy at an unprecedented magnitude.

Carbon nanotubes are highly conjugated network of carbon atoms. They have extremely high surface area and unique conducting and electrical properties. They have superior physical properties (e.g. high Young's modulus and work density). Single-walled carbon nanotubes have been shown to exhibit actuation via the stretching and contracting of the conjugated carbon network. Single walled nanotubes (SWNTs) exhibit electronic/physical properties such as band gap width, actuating efficiency, and expansion coefficient are dependent on their diameter, chirality, and bundle structure. The nanotubes used for this research had typical diameters of 0.8-1.1 nm, assorted chiralities and formed bundles of 50-100 nanotubes. The NIR absorption spectra shows peaks characteristic of the valence, conducting transition for semiconducting nanotubes. A semiconducting transition occurs between 1000-1600 nm.

Nafion is an ion-exchange polymer which has a poly(tetrafluoroethylene) backbone. It contains mobile cations (M+) that are ionically bound to the negatively charged functional groups (X-). They are hygroscopic, which means it absorbs moisture from the atmosphere.

Nafion is thought to have a polar /non-polar microphase separation. The presence/absence of the hydrophilic clusters contributes to polymer chain contraction/extension.

SAMPLE PREPARATION

As shown in Figure 2, a bi-layer composite 12 was fabricated by airbrushing a chloroform/single- walled carbon nanotubes suspension onto a heated 16 (approximately 50 ⁰C) Nafion substrate 14. Cantilevers (lmm x 13mm) were cut from the bi-layer composite to explore displacement due to photostimulation.

A graphite/Nafion bi-layer was fabricated as a control group of the photo¬ mechanical actuation experiments. For the detection of a photocurrent through the system, a cantilever was coated with a thin, conducting layer of gold (approximately 200nm thick) onto which electrical contacts 20, 22 were attached as shown in Figure 1. The semi-transparent layer of gold 30 was sputtered onto the Nafion surface opposite the SWNT layer to measure photo-induced current.

In order to provide evidence for a cooperative mechanism between SWNTs and Nafion, photo-mechanical displacement was measured against a Graphite/Nafion composite. Figure 3 shows a graph of the results. With the photo-mechanical displacement of SWNT/Nafion on top and the Graphite/Nafion on the bottom.

Photo-mechanical actuation of the SWNT/Nafion cantilever was captured using a CCD camera mounted to an optical microscope and interfaced with a computer monitor. Figure 4 A illustrates a cantilever displacement via photoactuation. And 4B is a cantilever prior to light stimulation with maximum displacement with photostimulation after 30 seconds.

The relationship between the wavelength of light and photo-mechanical actuation (spectral dependence) was measured. In order to measure the spectral dependence a linear relationship between intensity and displacement must be shown.

Figure 5 shows the linearity of photo-mechanical actuation at lOOOnm (graph 24) and 1300 nm (graph 26).

The spectral dependence of the photo-mechanical actuation was compared to the absorption spectrum of the SWNTs. A strong correlation between the photo- response and SWNT absorption was seen for the range in which semiconducting SWNTs are found lOOOnm- 1600nm.

Thus, there was a successful fabrication of photo-mechanical actuator from an organic composite material. The lack of photo-mechanical actuation in the graphite/nafion control group points to a cooperative mechanism between the SWNTs and Nafion. Experiments show photo-response to be of a greater magnitude when light travels through the Nafion layer first, this leads us to conclude the mechanics is an interfacial phenomenon. Quantum mechanical excitation of the SWNTs coupled with chain configuration within the Nafion may be two contributing factors to the photo-mechanical displacement.

The foregoing description has been limited to a few embodiments of the invention. It will be apparent, however, that variations and modifications can be made to the invention, with the attainment of some or all of the advantages. Therefore, it is the object of the claims to cover all such variations and modifications as come within the true spirit and scope of the invention. What is now claimed is:

Claims

CLAIMS 1. A photo-mechanical actuator, said actuator comprising a layer of nafion on a substrate. 2. The actuator of claim 1, wherein the layer of nafion includes carbon nanotubes. 3. The actuator of claim 1, wherein the substrate is plastic. 4. The actuator of claim 1, wherein the layer of nafion is formed by airbrushing a chloroform/single-walled carbon nanotube suspension onto a heated substrate. 5. The actuator of claim 4, wherein the substrate comprises a nafion substrate. 6. A method of forming a mechanical actuator comprising: providing a substrate; and forming an actuator comprising a layer of nafion on said substrate. 7. The method of claim 6, wherein the layer of nafion includes carbon nanotubes. 8. The method of claim 6, wherein the substrate is plastic. 9. The method of claim 6, wherein the layer of nafion is formed by airbrushing a chloroform/single-walled carbon nanotube suspension onto a heated substrate. 10. The method of claim 9, wherein the substrate comprises a nafion substrate.