Supercomputing Simulates Production with a Promising Wood Derivative
By Jacob Pieper
For decades, the innovation and subsequent production of carbon fiber reinforced polymer, more commonly known as carbon fiber, have been considered revolutionary advancements in the field of materials science.
One major attribute that helps make carbon fiber so appealing as a manufacturing material is its unsurpassed strength-to-weight ratio. Generally speaking, the strength-to-weight ratio for carbon fiber, also known as its specific strength, is twice that of steel and four times that of aluminum.
Interestingly, this ratio may vary slightly depending on the structural composition of the carbon fiber. To create a material that can outrank steel and aluminum in specific strength, the carbon fiber must be manufactured so that its individual layers are positioned in multiple directions, allowing the final product to best handle the stresses it will encounter. This structural alignment will ensure that carbon fiber will reach its maximum utility for any given manufactured part.
The dramatically decreased weight of carbon fiber products has made it ideal for high-end automobile and aerospace manufacturing as well as some sporting goods. Among the other benefits of carbon fiber are its high corrosion resistance and its low thermal expansion compared with steel and aluminum.
Given those positive attributes, carbon fiber seems far-and-away the best material for many of today's industrial and commercial applications. However, carbon fiber production does have some disadvantages.
The first major drawback to carbon fiber production is cost. Today, the carbon fiber being produced ranges from $10 to $20 per pound while one pound of manufacturing steel sells for less than $1.
The second drawback to carbon fiber production, and one that plays a significant role in the high price, is that currently, carbon fiber is produced from polyacrylonitrile (referred to as PAN), a fossil-resource-based polymer.
So by establishing an alternative to PAN, we could decrease the cost and the environmental impact of carbon fiber production, and, as many have claimed, revolutionize materials manufacturing for dozens of real-world applications.
Lignin as a Precursor
One such alternative for carbon fiber production may be achievable by using softwood lignin as a precursor for production instead of PAN.
"Carbon fiber is in high demand because of its light weight and high strength, but the production cost is high," notes researcher Ariana Beste. "If an efficient processing technique for carbon fiber from lignin was developed, carbon fiber production could be made sustainable and cost-effective."
Beste has been working with the Joint Institute for Computational Sciences at the University of Tennessee and Oak Ridge National Laboratory (ORNL) and the Center for Nanophase Materials Sciences at ORNL to help understand the chemical processes that occur during thermal conversion of lignin (part of the fiber production process) in an oxygen environment.
Using Kraken at the National Institute for Computational Sciences (NICS), Beste applied molecular dynamics based on a reactive force field in order to study the thermal decomposition for models of the seven most common linkages in softwood lignin. These linkages are carbon-carbon and ether bridges that connect the individual phenolic units in lignin.
"The reactive force field," says Beste, "allows for chemical bonds to be broken in the simulation, which is essential for the identification of chemical pathways."
In the late 1960s and early 1970s, lignin carbon fiber was commercially available for a brief period of time but was soon out-competed by the more efficiently produced PAN fiber.
Today though, lignin is again being considered as a carbon fiber precursor because it is the primary waste product in biomass production as an energy source. And because of the increased attention being paid to possible alternative energy sources, biomass production has been increasing substantially over the past decade, causing a spike in the amount of lignin waste.
Unfortunately, issues relative to making carbon fiber for commercial production still exist. Among those issues are complications with the fiber spinning process as well as the inferior strength of the completed product.
Overcoming the Obstacles
Currently, those limitations make PAN a more efficient precursor for carbon fiber production. But, with the help of high-performance computing (HPC), we have the ability to simulate the production steps for lignin carbon fiber, making it possible to find solutions to the problems that have previously inhibited widespread lignin carbon fiber production.
"Lignin is a very complicated bio-polymer," says Beste. "Its exact structure is unknown and its composition depends on the origin and isolation technique."
Thus, for lignin to become a substitute for PAN in carbon fiber production, "the production has to be adapted to account for the different mechanical and chemical properties of lignin." This is particularly difficult Beste explains, "because of the lack of understanding of the chemical reactions that occur during thermal treatment."
This thermal treatment is the process by which the fiber undergoes carbonization (in an inert, oxygen-free environment) and oxidation (in an environment where oxygen is present). And while these steps are well understood when PAN is used as a precursor Beste notes, the same cannot be said for a lignin precursor.
Generally speaking, one problem with using lignin is that when the fiber undergoes the thermal treatments, the high heat causes it to melt before the treatment can be completed.
By simulating the oxidation of the seven common softwood lignin linkages, Beste has been able to conclude that four of the seven common linkages she tested are stable enough to allow the lignin fiber to be thermally treated before it is destroyed. The stable linkages include the β-β, 4-O-5, 5-5, and β-5 linkages, which can be seen in the included image (letters B, C, E, and G).
By using the HPC capabilities offered by Kraken, Beste has revealed these four stable linkages and established a foundation on which to continue lignin carbon fiber research.
Her simulations have also paved the way for further research to be done by chemical engineers to improve the efficiency of the processing techniques for lignin carbon fiber production. Ideally, with the help of her research, we will soon see progress toward the sustainable, cost-effective carbon fiber production that Beste anticipates for the future.
Beste's research on this subject was published this year in the Journal of Physical Chemistry.
After the conclusion of her carbon fiber work on the recently decommissioned Kraken, Beste turned her attention to new projects on newer computers. Currently she is investigating ethanol conversion on cerium oxide surfaces using the Cray XC-30 system known as Darter at NICS.
Article posting date: 2 June 2014
About JICS and NICS: The Joint Institute for Computational Sciences (JICS) was established by the University of Tennessee and Oak Ridge National Laboratory (ORNL) to advance scientific discovery and state-of-the-art engineering, and to further knowledge of computational modeling and simulation. JICS realizes its vision by taking full advantage of petascale-and-beyond computers housed at ORNL and by educating a new generation of scientists and engineers well versed in the application of computational modeling and simulation for solving the most challenging scientific and engineering problems. JICS runs the National Institute for Computational Sciences (NICS), which had the distinction of deploying and managing the Kraken supercomputer. NICS is a leading academic supercomputing center and a major partner in the National Science Foundation's eXtreme Science and Engineering Discovery Environment, known as XSEDE. In November 2012, JICS sited the Beacon system, which set a record for power efficiency and captured the number one position on the Green500 list of the most energy-efficient computers.