MIT Researchers found out that a phenol-formaldehyde polymer changed into a glassy carbon material in a process quite like baking to reach its best combination of low density and high strength at 1,000 °C (1,832 °F).
Presently, they have confirmed that, they can accomplish a similar glassy transformation, but at a more industrially-reachable temperature of 800 °C by incorporating a small fraction of carbon nanotubes to this material.
As the preliminary polymeric hydrocarbon, known as a phenol-formaldehyde polymeric resin, is heated from 600 °C, the size of its crystallites increases until it attains a plateau at 1,000 °C. Postdoc Itai Y. Stein says scientific literature confirms that this plateau holds until well over 2,000 °C. The incorporation of 1% by volume of aligned carbon nanotubes to the starting material allows it to attain the plateau crystallite size at a temperature 200 °C lower.
What we’re showing is that by adding carbon nanotubes, we reach this plateau region earlier.
Itai Y. Stein, Postdoctoral Student, MIT
The findings were published in the August 22nd online issue of the Journal of Materials Science. The co-authors were Stein, former Materials Processing Center-Center for Materials Science and Engineering (MPC-CMSE) Summer Scholars Ashley L. Kaiser (2016) and Alexander J. Constable (2015), Postdoc Luiz Acauan, and the Senior Author, Professor of Aeronautics and Astronautics Brian L. Wardle. Kaiser is currently a Graduate Student in Wardle’s lab.
This work has the interesting finding that nanostructures assist in fabricating [and] manufacturing the glassy carbon composites. Early lessons with nano-materials broadly showed that nanostructures impede manufacturing, however, we are finding a theme across several research areas that when controlled, the nanostructures can be utilized to enhance manufacturing, sometime significantly. While the nanostructures — here, aligned carbon nanotubes — are valuable as reinforcement to the glassy carbon, they can also be utilized to improve the manufacturability. Ashley and Itai are taking this work even further to test the limits.
Brian L. Wardle, Professor of Aeronautics and Astronautics, MIT
Crystallite size is closely linked to hardness, which is a measure of mechanical performance such as toughness and strength. It is one of the most vital properties of the glassy carbon material.
“If you look at the hardness normalized by the density, we previously found that the first point in the plateau region is the best point, because there the glassy carbon material is the least dense and hardest,” Stein says.
The main finding of the previous paper was that more disorder in the organization of carbon crystallites caused lower density and greater hardness in the glassy carbon material, which was achieved by baking a phenol-formaldehyde polymer without the presence of any oxygen. The transformed material is also referred to as pyrolytic carbon or PyC.
Although the polymer changes into a graphite-like material, it never attains the more highly ordered structure of graphite. This variance is established by X-ray diffraction (XRD) analysis of samples baked with and without, carbon nanotubes and compared to a standard indicator for graphite known as the Bernal stacking order. The type of disorder in crystallites in this case is called turbostratic stacking, where the planes that contain the crystallites are arbitrarily rotated with regard to one another because of holes (or vacancies) and curvature. XRD studies performed at the Center for Materials Science and Engineering’s shared experimental facilities also confirmed the crystallite size evolution with respect to baking temperature.
To envisage this disorder compared to the flawless hexagonal structure of graphene or repeating layered structure of graphite, Stein recommends imagining a stack of flat square pieces of paper. The papers easily stack into a flawless square with minimal space between each sheet. But if each piece of paper is pulled out, crumpled and then casually flattened out again, it would be exasperating trying to reorder the sheets into a perfect stack.
Similar disorder happens in the molecular structure of the glassy carbon, as the precursor phenol-formaldehyde polymer starts with a complex mix of carbon-rich compounds and the baking temperature is not sufficiently high to break down all of them into simpler carbon structures. Raman spectroscopy results verified the occurrence of these defects in the carbon structure. Another method, Fourier Transform Infrared Spectroscopy, established the existence of hydrogen and oxygen groups within the crystallites.
“It originates from the polymeric precursor that we use, the phenol-formaldehyde, and they’re just stuck; they can’t leave,” Stein explains.
The Researchers' former paper revealed that the existence of these more complex carbon compounds in the material reinforces it by leading to 3D connections that are tough to break. The new research reveals that the carbon nanotubes do not have any effect on these hydrogen or oxygen substructures in the material.
Stein says that, for the present study, the goal was to investigate what happens when carbon nanotubes are incorporated and the baking temperature is raised; specifically, what impact, if any, the nanotubes make on crystallite growth. They discovered that the nanotubes impact the crystallite formation process on the meso-scale, which is measured in tens of nanometers, while everything else stays untouched. Significantly, only the crystallite size is influenced by the incorporation of the carbon nanotubes.
“We were surprised to see no change in the graphitic nature of our polymer as it is being baked in the presence of carbon nanotubes," Stein says. "Nonetheless, that is a very interesting finding because we can reduce the processing temperature without affecting the structure of the resulting glassy carbon. Since the properties of the glassy carbon depend on its structure, this finding could allow an industrial process of this technology to realize significant energy savings.”
Faster structural evolution
“The carbon nanotubes allow the composite’s structure to evolve faster at the meso-scale, so it reaches its final state at a lower processing temperature,” Kaiser adds. “These nanotubes also decrease the overall weight of the material. This way, we are able to produce our composite at a lower temperature while decreasing its density and maintaining its excellent properties.”
Stein observes that in the previous work the Researchers also demonstrated that raising processing temperature over 1,000 °C caused the material to be weaker.
“So we are essentially reducing the temperature you need to go to reach the best properties,” Stein says of the new report. “If you look at the hardness normalized by the density, this [800 ° C] is the best point, because this is where the glassy carbon is expected to be the least dense and hardest.”
Stein states that the lower processing temperature may also render these phenolic materials more compatible with metals whose melting points are lower than 1,000 °C, which in turn may be beneficial for 3D printing.
“The application we specifically thought of using this in is meta-materials,” he says. “If you can use nanotubes to reduce the temperature you bake at, if you want to convert it to carbon, just pure carbon, then that could make it more accessible. That 200 °C is a big difference for many processes.”
In the latest findings, the MIT team experimented on a material with only 1% carbon nanotubes by volume. Subsequently, they plan to study the influence of increasing the proportion of carbon nanotubes to 20% by volume. “We just want to see if the nanotubes make it stronger,” Stein says. They will also study the effect on thickness and size of the crystallites from the integrated carbon nanotubes.
A whole range of structural composites would benefit from this study, particularly next-generation ultra-lightweight nano-structures.
Piran R. Kidambi, Assistant Professor of Chemical and Biomolecular Engineering, Vanderbilt University
“The study found that aligned carbon nanotube-glassy carbon matrix nanocomposites at the meso-scale evolved much faster with a plateau in crystallite sizes (an important quality metric) at a temperature up to 200 °C lower compared to having a pure glassy carbon matrix,” Kidambi says. “Lower temperatures are good news for manufacturing to minimize heating costs in processing, and recent models tell us that slender crystallites are desirable since they increase glassy carbon hardness. Hence a combination of a plateau in crystallite sizes and lower temperatures is very interesting from a manufacturing perspective. This is high-quality research that uses fundamental insights to inform and guide manufacturing/synthesis routes for superior composites.”
Summer Scholar work
Kaiser’s research work as a 2016 MPC-CMSE Summer Scholar comprises of the bulk of the paper’s experimental results, excluding the Raman spectroscopy results. “It is a very robust and focused contribution,” Stein says.
“I was thrilled to be involved in this research when I was a Summer Scholar,” Kaiser says. “Now, being able to come back to MIT as a graduate student, rejoin the Wardle group, and publish this work is very exciting. I’m eager to continue working on composites as I pursue my PhD here in materials science and engineering.”
This research received support from the Department of Defense, National Science Foundation MRSEC Program, and the MIT Materials Processing Center. Airbus, Embraer, Lockheed Martin, Saab AB, ANSYS, Hexcel, Saertex, and TohoTenax also provided partial support through MIT's Nano-Engineered Composite Aerospace Structures Consortium. Stein was supported partly by a National Defense Science and Engineering Graduate Fellowship.