Jennifer Lewis (Harvard University)
26 June 2014. Engineers at Harvard University developed an ink made of epoxy resins and carbon fibers, which when arrayed into a cellular pattern with three-dimensional printing, can produce a strong lightweight composite with the properties of balsa wood. Materials scientist and engineering professor Jennifer Lewis and postdoctoral researcher Brett Compton, now at Oak Ridge National Laboratory, describe the new material online, published last week by the journal Advanced Materials (paid subscription required).
Balsa wood has long been known for its high strength in proportion to its weight, especially to model airplane builders. These same strength-to-weight properties make balsa a desirable material for wind turbine blades for generating electric power. But 95 percent of the world’s supply of balsa comes from one country, Ecuador, making the price and availability of this important material dependent on actions of that country’s government, a situation history has shown can lead to undesirable consequences.
“Balsa wood has a cellular architecture that minimizes its weight,” says Lewis in a university statement, “since most of the space is empty and only the cell walls carry the load. It therefore has a high specific stiffness and strength.” The researchers adapted this design concept for their new material, starting with an ink based on epoxy resins, enhanced with nanoscale clay particles and the chemical dimethyl methylphosphonate, a plasticizer and stabilizer, to add viscosity.
Lewis and Compton then added carbon fibers and tiny silicon carbide filaments to this mix. Adding these fibers gave the researchers the capability to add strength and toughness to the material, by controlling the orientation of the fibers through the ink nozzle of the 3-D printer. The team printed the ink through a 3-D printer to form hexagonal cells in a honeycomb pattern, then building the cells into the new material.
Tests of the material show it has the stiffness of wood, which is 10 times stiffer than commercial 3-D polymers, but just as strong. The researchers believe this kind of material can help reduce the weight of automobiles, increasing their fuel efficiency, as well as substituting for balsa in wind turbines.
The ability to adjust the composition of fillers and orientation of fibers in the ink also opens up ways for designing new materials with 3-D printers. “As we gain additional levels of control in filler alignment and learn how to better integrate that orientation into component design, we can further optimize component design and improve materials efficiency,” says Compton. “Eventually, we will be able to use 3-D printing technology to change the degree of fiber filler alignment and local composition on the fly.”
The following video demonstrates the material taking shape with 3-D printing.
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