A research team including scientists from the National Institute of Standards and Technology (NIST) has now succeeded in developing an advanced image gathering and processing technique for mapping the nanoscale structure of carbon nanotubes within a composite material in three dimensions.
The NIST group surveyed carbon nanocomposites with four different concentrations of nanotubes: a) 0.5 percent, b) 2.5 percent, c) 4 percent, d) 7 percent. They found that as the concentration of carbon nanotubes rises, they come into contact and intersect. More intersections improve the material's electrical and thermal conductivity, and the physical contact causes them to conform to one another, which straightens them and improves the material's strength. (Credit: Natarajan/NIST)
Researchers from the Massachusetts Institute of Technology and the University of Maryland also contributed to the study. The overall properties of carbon nanotubes largely depend on the manner in which they are arranged and distributed in a material. The data will enable researchers who study composite materials to design and test realistic computer models of materials with varied electrical, thermal and mechanical features.
Carbon fiber composites are known for their light weight and high strength. Carbon nanotube (CNT) composites or nanocomposites with smaller and more number of carbon filaments not only exhibit high strength, but are also good conductors of heat and electricity.
Alex Liddle from NIST, an author of this research paper stated that while researchers were able to determine the bulk properties of a nanocomposite, they did not know precisely why different composite formulations exhibited different properties.
Figuring out why these materials have the properties they do requires a detailed, quantitative understanding of their complex 3-D structure. We need to know not only the concentration of nanotubes but also their shape and position, and relate that to the properties of the material.
It is a challenge to view the arrangement of carbon nanotubes in composites as they are enclosed by an epoxy resin, which also contains carbon atoms. Even with advanced probes, the contrast is very low for software image processors to differentiate them easily.
It is truly tedious to mark thousands of carbon nanotubes in an image. Therefore, Bharath Natarajan from NIST developed an image processing algorithm that can differentiate CNTs from an epoxy resin.
The full potential of a CNT in terms of its strength and its electrical and thermal conductivity is realized when it is straight and stretched out however when CNTs are suspended in an epoxy resin, they spread out, bundle and twist into different shapes. Our analysis revealed that the benefits of CNTs increases in a non-linear fashion as their concentration increases. As the concentration raises, the CNTs come into contact, increasing the number of intersections, which increases their electrical and thermal conductivity, and the physical contact causes them to conform to one another, which straightens them, increasing the material's strength.
The fact that improving the CNT concentration enhances its properties is not strange but now researchers realize how this influences the properties of materials and why previous models of the performance of nanocomposite materials never matched their actual performance.
We've really only seen the tip of the iceberg with respect to this class of material. There are all sort of ways other researchers might slice and dice the data to model and eventually manufacture optimal materials for thermal management, mechanical reinforcement, energy storage, drug transport and other uses.
A part of this research was conducted at NIST's Center for Nanoscale Science and Technology (CNST), a national user facility available to researchers from industry, academia and government.