Taylor S. Wood and Dr. Richard Vanfleet, Physics and Astronomy
Abstract
Carbon nanotubes have an unusually high strength-to-weight ratio and thus present an exciting material for use in reinforcing the structural integrity of microstructures. However, despite their desirable properties, carbon nanotubes have proved difficult to incorporate in materials as strengthening elements. Our group has developed a method for patterning and infiltrating, or filling, carbon nanotube forests to make structures. Infiltration proceeds by flowing an ethylene/argon mixture across a sample at a temperature of 900 °C, thus depositing amorphous carbon and locking the spaces in the nanotubes together. Using cantilever structures made of this carbon nanotube/carbon composite material, we have begun to measure key mechanical properties of this composite material. We are able to determine the maximum applied force that a carbon-infiltrated microstructure can withstand and the Young’s modulus and yield stress of the composite material.
Introduction
Microdevices’ tiny size makes them very advantageous in a variety of applications. We use them in many everyday electronic devices without even knowing it. However, although they can be very functional, without strong mechanical properties, these devices will not withstand the wear of their use. One thing that considerably affects the overall robustness and strength of these microdevices is the material out of which it is made. Because of this, carbon nanotubes have proven to be an effective material for use in microfabrication. In this study, we used carbon nanotubes to make microstructures and studied their mechanical properties for eventual use in microdevices.
Experimental Methods
Microfabrication began by spinning a 1 μm thick layer of AZ 3312 photoresist, a photosensitive material. Using a mask, we then subjected the photoresist to a high-intensity ultra-violet light, thus exposing a specific pattern. We then develop the photoresist to remove the exposed area— in our case a cantilever pattern (see Figure 1). A thin film of iron is then applied over the exposed area as a catalyst for carbon nanotube growth. In our study, we deposited iron at both 4 nm and 7 nm thicknesses. We then created three-dimensional carbon nanotube structures in the pattern of our cantilever mask through chemical vapor deposition. Carbon nanotubes were grown in a tube furnace to a temperature of 750 °C in a gaseous mixture of hydrogen and ethylene (see Figure 2). The sample was then heated to a temperature of 900 °C and subjected to a mixture of ethylene and argon for 30 minutes or 2 hours (see Figure 3).
After fabrication, we then removed the structures from the substrate and tested them. Each cantilever was force-tested using an Instron Materials Testing apparatus. Each cantilever was slowly depressed as the Instron measured the force resistance per millimeter. From this value, we were able to calculate values for the Young’s modulus (E), or elastic behavior of a material, and yield stress (S), or point of plastic deformation, by the following equations:
where F represents the applied resistive force, d represents the distance the beam is deflected before breaking, w is the width of each beam (300 μm), l is the length of each beam (2 mm), and h is the height of each beam (approximately 200 μm). We then compared the results to values of other known materials.
Data and Results
Surprisingly, samples infiltrated with carbon for 30 minutes and samples infiltrated with carbon for 2 hours showed little variation in the Young’s modulus and yield stress (see Table 1). Preliminary results, however, did show that the iron thickness had some effect on the Young’s modulus and yield stress. In the cases of both mechanical properties, we found an increased value in the 7 nm iron samples as opposed to the 4 nm samples (see Table 2).
Conclusions and Future Research
Through this study we were able to characterize the Young’s modulus and yield stress, two mechanical properties important in the design and fabrication of microdevices. Our final results showed that the composite carbon nanotube/carbon material had a Young’s modulus between 2 and 7 GPa and a yield stress between 5 and 21 MPa. Although important and interesting, these numbers were significantly lower than our expected values and less than that of many other materials, such as aluminum. We theorize that this has primarily been due to poor carbon infiltration. In the future we hope to adjust the carbon infiltration parameters so that our samples will be more completely filled and thus stronger. Additionally, we have recently found evidence that before breaking, each cantilever beam puts a strain on the outside border of the overall structure. We hope to repeat these experiments, taking into account this phenomenon.
