As the world strives to improve the efficiency of cars and other vehicles to reduce greenhouse gas emissions and improve the range of electric vehicles, the search for materials that are ever lighter and strong enough for use in car bodies is underway.
Lightweight carbon fiber materials, similar to the material used for some tennis racquets and bicycles, combine exceptional strength with low weight, but have been more expensive to produce than comparable steel or aluminum structural elements. Now, researchers at MIT and elsewhere have found a way to make these lightweight fibers from an ultra-cheap raw material: the heavy, viscous waste left over from petroleum refining, the material refineries supply today. today for low value applications such as asphalt, or possibly treat as waste.
Not only is the new carbon fiber cheap to manufacture, but it offers advantages over traditional carbon fiber materials as it can have resistance to compression, which means it could be used for load-bearing applications. . The new process is described in the log Scientists progressin an article by graduate student Asmita Jana, research scientist Nicola Ferralis, Professor Jeffrey Grossman and five others from MIT, Wyoming’s Western Research Institute and Tennessee’s Oak Ridge National Laboratory.
The research began about four years ago in response to a request from the Department of Energy, which was looking for ways to make cars more efficient and reduce fuel consumption by reducing their overall weight. “If you look at the same car model today, compared to 30 years ago, it is significantly heavier,” says Ferralis. “The weight of cars has increased by more than 15% in the same category.”
A heavier car requires a bigger engine, more powerful brakes, etc., so reducing the weight of bodywork or other components has a ripple effect that produces further weight savings. The DOE is pushing for the development of lightweight structural materials that match the safety of today’s conventional steel panels, but can also be manufactured at a low enough price to potentially completely replace steel in vehicles standard.
Composites made from carbon fibers are not a new idea, but so far in the automotive world they have only been used in a few very expensive models. The new research aims to reverse the trend by providing a low-cost starting material and relatively simple processing methods.
Carbon fiber of the grade required for automotive use currently costs at least $10 to $12 a pound, according to Ferralis, and “can cost significantly more,” up to hundreds of dollars a pound for specialty applications such as spacecraft components. This compares to around 75 cents per pound for steel, or $2 for aluminum, although these prices fluctuate widely and materials are often dependent on foreign sources. At those prices, he says, making a pickup out of carbon fiber instead of steel would roughly double the cost.
These fibers are generally made from polymers (such as polyacrilonitrile) derived from petroleum, but using an expensive intermediate step of polymerizing carbon compounds. The cost of the polymer can represent more than 60% of the total cost of the final fiber, says Ferralis. Instead of using a refined, processed petroleum product to start with, the team’s new approach uses what is essentially the dregs left over after the refining process, a material known as petroleum pitch. “It’s what we sometimes call bottom of the barrel,” says Ferralis.
“The pitch is incredibly messy,” he says. It’s a hodgepodge of heavy hydrocarbons mixed together, and “that’s actually what makes it beautiful in a way, because there’s so much chemistry that can be harnessed. It makes for a fascinating material for to start.”
It is useless for combustion – although it can burn, it is too dirty a fuel to be practical, and this is especially true with tightening environmental regulations. “There are so many,” he says, “the intrinsic value of these products is very low, so they often get buried.” Another source of pitch, which the team also tested, is coal pitch, a similar material that is a by-product of coking coal, used for example in steel production. This process produces about 80% coke and 20% coal pitch, “which is basically waste,” he says.
Working in conjunction with researchers at Oak Ridge National Laboratory, who had the expertise in fabricating carbon fibers under a variety of conditions, from lab scale to pilot plant scale, the team s endeavored to find ways to predict performance in order to guide the choice of conditions for these fabrication experiments.
“The process you need to make a carbon fiber [from pitch] is actually extremely minimal, both in terms of energy requirements and in terms of the actual processing you need to perform,” says Ferralis.
Jana explains that pitch is “made up of these heterogeneous sets of molecules, where you would expect that if you change the shape or the size, you would expect the properties to change drastically”, whereas a industrial material must have very consistent properties.
By carefully modeling how bonds form and crosslink between constituent molecules, Jana was able to develop a way to predict how a given set of processing conditions would affect the properties of the resulting fiber. “We were able to reproduce the results with surprising accuracy,” she says, “to the point that companies could take these graphs and be able to predict” characteristics like fiber density and modulus of elasticity.
The work produced results showing that by adjusting the starting conditions, it was possible to make carbon fibers that were not only tensile-resistant, like most of these fibers, but also compressive-resistant, meaning that ‘they could potentially be used in carrier applications. This opens up entirely new possibilities for the utility of these materials, they say.
The DOE’s call was for projects to bring the cost of lightweight materials below $5 a pound, but the MIT team believes their method can do better than that, hitting something like $3 a pound, although they have not yet done a detailed economic analysis. To analyse.
“The new route we are developing is not just a cost effect,” says Ferralis. “It could open up new applications, and it doesn’t have to be vehicles.” Part of the complication of making conventional fiber composites is that the fibers must be formed into a fabric and laid out in precise, detailed patterns. The reason is, he says, “to compensate for the lack of compressive strength.” It’s a matter of engineering to overcome the shortcomings of the material, he says, but with the new process, all that extra complexity wouldn’t be necessary.
The research team included Taishan Zhu and Yanming Wang from MIT, Jeramie Adams from Western Reserve University, and Logan Kearney and Amit Naskar from Oak Ridge National Laboratory.
Researchers find a new way to start the carbon fiber manufacturing process
Asmita Jana et al, Atoms to fibers: identification of new processing methods in pitch-based carbon fiber synthesis, Scientists progress (2022). DOI: 10.1126/sciadv.abn1905. www.science.org/doi/10.1126/sciadv.abn1905
Provided by Massachusetts Institute of Technology
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