Aero-Astro Magazine Highlight

Nano and the next-gen materials

By Brian Wardle

While much work has been done to bring the nanoscale properties of carbon nanotubes to bear on macro-scale engineering materials and structures, little improvement in the performance of macro-scale structures had been gained despite nearly a decade’s worth of research.

Brian Wardle in his lab with an experimental arrangement for synthesizing carbon nanotubes. (William Litant photograph)

Recently, I was visiting an aircraft production line. I was suspended on a platform that allowed me to view the unfinished interior of a primary commercial transport structure made from advanced composites. The scientific and technical innovations to arrive at such a structure are at least 30 years in the making and represent efforts from the entire global advanced composites community.

After I reviewed technical aspects with the engineers leading the visit, I was both energized and left wondering how best to use this experience in the learning environments for my students.

I’ve been working in advanced aerospace structures, primarily in advanced composites, since my graduate work at MIT in the mid ’90s. Since joining the Aero-Astro faculty, I have turned my group’s attention to opportunities afforded by nano- and micro-structures, with a recent large effort in the areas of nanoscience and nanotechnology. While much work has been done to bring the nanoscale properties of carbon nanotubes to bear on macro-scale engineering materials and structures, little improvement in the performance of macro-scale structures had been gained, despite nearly a decade’s worth of research. In 2003, my group started work on a new vision for using aligned carbon nanotubes (CNTs — perfect, hollow cylinders of carbon with exceptional physical properties) to enhance existing advanced composites. Advanced composites composed of aligned micron-diameter carbon fibers and polymers are prevalent in many aerospace applications. Rather than mixing nanometer-diameter CNTs into polymers in a random orientation, I envisioned working with CNTs as a second fiber to improve strength properties as well as provide multifunctionality. I conceived several hybrid architectures building upon existing advanced composite architectures and processing, using the aligned CNTs to attain new levels of performance.

We discovered that working with aligned CNTs not only makes sense for property tailoring and maximization, but it also allows for a new way of solving the difficult manufacturing challenges of dispersing and organizing CNTs into advanced polymers. (One does not want to start with 50 billion CNTs per square centimeter in a bag and then begin organizing them to make meter-scale structures!) Rather than mixing and/or organizing the CNTs, we grow aligned CNTs where they will have the desired engineering effect (e.g., in situ growth on woven fibers) and then impregnate with the polymer. My group has 15 people working on nano-engineered composite topics ranging from fabrication and characterization of standard mechanical samples, to probing nanoscale thermal and electrical transport properties in collaboration with groups in MIT Materials Science and Mechanical Engineering. We are performing both basic and applied research to address the physics, materials, processing, manufacturing, and mechanics challenges to realize these nano-engineered composites.

Fuzzy-fiber reinforced polymers are the nano-engineered version of carbon fiber reinforced plastics. FFRPs enhance existing advanced fiber and polymer composites with aligned carbon nanotubes, forming a three-dimensional nano-reinforced composite (left). Aligned-CNTs are grown in situ on the surface of woven advanced fiber cloth as a first step in the fabrication (right).

Nanostitched composites are existing aligned-fiber advanced composites reinforced at the weak ply interface with aligned CNTs. The processes developed for the nanostitch are compatible with existing composite processing routes.

My group has built a significant presence in what we have termed “nano-engineered composites.” This idea brings together the promise of nanotechnology with practical and achievable approaches for implementing nanostructures in macrostructures, to achieve new levels of true multifunctional structural performance. We have recently demonstrated significant macroscopic (laminate-level) structural and electrical engineering property improvements. In mid-2007, I formed an aerospace industry consortium — Nano-Engineered Composite aerospace Structures — to build on a platform research. We will be working closely with nine industry partners for the next three years. Our hope is that we will enable the next-generation(s) of materials for aerospace and other demanding applications.

Macro-scale engineering property improvements provide a positive context in which to explore fundamental materials and physics questions regarding nanoscale interactions, and particularly how those aggregate to yield engineering-relevant macroscopic properties. An example is what happens when polymers crosslink in the presence of nanostructures, particularly when the nanostructures are closely spaced together. Consider that in some of the structures we have made, the CNT spacing is about 20 nm, which is on the order of the characteristic length of polymer chains. Many polymer scientists believe, and there is some evidence for this, that the polymers change character near the nanostructures and that, in fact, a secondary “interphase” polymer is created. We still don’t know if this is true, but one of my group’s new contributions is a novel experimental platform for fabricating articles to test this hypothesis. Helping to answer these fundamental scientific questions is both rewarding and enjoyable. However, as an engineer, I am still driven by practical considerations, such as how to optimally place the CNTs for improved structural performance.

Brian Wardle is the MIT Aeronatics and Astronautics Draper Assistant Professor, He is pursuing research in nano-engineered advanced composites, power MEMS devices (fuel cells and vibrational energy harvesters), advanced composite systems durability and damage resistance/tolerance, and structural health monitoring technologies. He received a B.S. in Aerospace Engineering from Penn State University in 1992 and completed S.M. and Ph.D. work at MIT Aero-Astro in 1995 and 1998, respectively. He directs MIT’s Nano-Engineered Composite aerospace STructures Consortium and is a principal member of the Technology Laboratory for Advanced Materials and Structures. He may be reached at
wardle@mit.edu