As the world races to enhance the efficiency of cars and other vehicles so as to control greenhouse gas emissions and enhance the range of electric vehicles, the hunt is ongoing for lighter materials that are sufficiently robust to be employed in the car’s body.
A circle of carbon fibers that have a diameter of about 10 micrometers. Image Credit: Nicola Ferralis.
Lightweight materials created from carbon fiber, akin to the material used for certain bicycles and tennis rackets, integrate excellent strength coupled with low weight, but these tend to be more expensive to make than analogous structural elements created from aluminum or steel.
Currently, scientists at MIT and elsewhere have formulated a method of creating these lightweight fibers from very inexpensive feedstock: the heavy, gloppy waste substance remaining after petroleum refining, that refineries of late supply for cheaper applications like asphalt, or ultimately treated as waste.
The new carbon fiber is cheap to manufacture and offers benefits over the conventional carbon fiber materials because it can have compressional strength, i.e., it could be employed in load-bearing uses.
The new process has been illustrated recently in the journal Science Advances, in a research article by graduate student Asmita Jana, research scientist Nicola Ferralis, Professor Jeffrey Grossman, and five others at MIT, Oak Ridge National Laboratory in Tennessee, and Western Research Institute in Wyoming.
The study commenced approximately four years ago in response to an appeal from the Department of Energy (DOE), which was looking for methods to create cars that were more efficient and decrease fuel consumption by minimizing their total weight.
If you look at the same model car now, compared to 30 years ago, it’s significantly heavier. The weight of cars has increased more than 15 percent within the same category.
Nicola Ferralis, Research Scientist, MIT
A heavier car necessitates a larger engine, stronger brakes, etc. so the minimizing of the car body’s weight or other parts has a ripple effect that produces extra weight savings.
The DOE is supporting the creation of lightweight structural materials that can match the safety of present-day traditional steel panels but also can be manufactured inexpensively enough to potentially substitute steel altogether in regular vehicles.
Composites created from carbon fibers are not a new concept, but thus far in the automotive world, they have only been employed in a few very costly models. The new study aims to change that by delivering a cost-effective starting material and comparatively simple processing techniques.
Carbon fibers of the quality required for automotive use cost no less than $10 to $12 per pound at present, Ferralis says, and “can be way more,” up to hundreds of dollars a pound for a dedicated application like spacecraft parts. That compares to around 75 cents a pound for steel, or $2 for aluminum, though these prices vary widely, and the materials frequently depend on sources from other countries.
At those costs, he says, manufacturing a pickup truck out of carbon fiber rather than of steel would be approximately twice the cost.
These fibers are usually made from polymers (like polyacrilonitrile) extracted from petroleum but use an expensive transitional step of polymerizing the carbon compounds. The cost of the polymer can be responsible for over 60% of the overall cost of the final fiber, Ferralis explains.
Rather than using a refined and processed petroleum substance, to begin with, the team’s new method uses what is fundamentally the dregs remaining after the refining process, a substance called petroleum pitch.
“It’s what we sometimes call the bottom of the barrel,” Ferralis says.
“Pitch is incredibly messy,” he states. It is a hodgepodge of diverse heavy hydrocarbons, and “that’s actually what makes it beautiful in a way, because there’s so much chemistry that can be exploited. That makes it a fascinating material to start with.”
It is unusable for combustion; although it can burn, it is too dirty a fuel to be unusable, and this is particularly true with stringent environmental protocols.
There’s so much of it. The inherent value of these products is very low, so then it is often landfilled.
Nicola Ferralis, Research Scientist, MIT
A substitute source of pitch, which the researchers also checked out, is coal pitch, a similar material that is a derivative of coking coal, used, for instance, for steel production. That process produces around 80% coke and 20% coal pitch, “which is basically a waste,” he states.
Working in partnership with scientists at Oak Ridge National Laboratory, who had the expertise in engineering carbon fibers under a range of conditions, from lab scale all the way up to pilot-plant scale, the researchers set about discovering ways to estimate the performance in order to guide the selection of conditions for those fabrication trials.
The process that you need to actually make a carbon fiber [from pitch] is actually extremely minimal, both in terms of energy requirements and in terms of actual processing that you need to do.
Nicola Ferralis, Research Scientist, MIT
Jana elucidates that pitch is “made of these heterogeneous set of molecules, where you would expect that if you change the shape or size you would expect the properties to change dramatically,” whereas an industrial material needs to have very stable properties.
By cautiously modeling the ways bonds develop and crosslink between the constituent molecules, Jana could develop a method of estimating how a particular set of processing circumstances would impact the subsequent fiber properties.
“We were able to reproduce the results with such startling accuracy,” she explains, “to the point where companies could take those graphs and be able to predict” characteristics such as density and elastic modulus of the fibers.
The results generated by the study show that by regulating the starting circumstances, carbon fibers could be created that were not only robust in tension, as most similar fibers are, but also robust in compression, meaning they could possibly be used in load-bearing applications. This opens up new possibilities for the practicality of these materials, they state.
DOE’s requirement was for projects to lower the cost of lightweight materials way below $5 a pound, but the MIT team approximates that their technique can achieve better than that, reaching approximately $3 a pound, though they have not yet performed a comprehensive economic analysis.
The new route we’re developing is not just a cost effect. It might open up new applications, and it doesn’t have to be vehicles.
Nicola Ferralis, Research Scientist, MIT
Part of the difficulty of creating the traditional fiber composites is that the fibers have to be made into a cloth and placed in accurate, detailed patterns.
This is because, he explains, “is to compensate for the lack of compressive strength.” It is a matter of engineering to overcome the deficiencies of the material, he says, but with the new method, all that additional complexity would not be required.
The research team comprised Taishan Zhu and Yanming Wang at MIT, Logan Kearney and Amit Naskar at Oak Ridge National Laboratory, and Jeramie Adams at Western Reserve University. The U.S. Department of Energy supported the work.
Journal Reference:
Jana, A., et al. (2022) Atoms to fibers: Identifying novel processing methods in the synthesis of pitch-based carbon fibers. Science Advances. doi.org/10.1126/sciadv.abn1905.
Source: https://mit.edu