In the early days of nanotech commercialization, it was popular to add nanomaterials to composites-- like carbon nanotubes (CNTs) in bicycles— in order for them to be “stronger than steel”. But that was mostly just marketing copy. The CNTs merely replaced the resins that had previously filled out the material matrix. It was pretty unclear whether they actually imparted any of the capabilities that nanomaterials possessed, like strength and flexibility.
The issue has come to be known as the “Valley of Death” in composite material design. Essentially, materials scientists were finding that the extraordinary properties of nanowires were disappearing—the properties of an entire composite were limited by the properties of other materials found in the material’s matrix.
A couple of years back some joint research between industry and academia attempted to address this shortcoming and take advantage of nanomaterials' superior strength. It looks like it's finally happened.Researchers at the University of Western Australia have found a way for composite materials to actually match the strength and flexibility of the nanowires that have been placed inside them.
"In a normal metal matrix-nanowire composite, when we pull the composite to a very high stress, the nanowires will experience a large elastic deformation of several percent,” explains Yinong Liu, a professor at the University of Western Australia, in a press release. “That is okay for the nanowires, but the normal metals that form the matrix cannot. They can stretch elastically to no more than 1 per cent. Beyond that, the matrix deforms plastically.”
In research published in the journal Science (“A Transforming Metal Nanocomposite with Large Elastic Strain, Low Modulus, and High Strength”), Liu and his colleagues discovered that the shape-memory alloy nickel-titanium (NiTi) had enough elasticity to be used with nanowires in a material matrix without losing the superior functionality of the nanowires.
"NiTi is a shape memory alloy, a fancy name but not totally new,” says Liu in the press release. “It is no stronger than other common metals but it has one special property that is its martensitic transformation. The transformation can produce a deformation compatible to the elastic deformation of the nanowires without plastic damage to the structure of the composite. This effectively gives the nanowires a chance to do their job, that is, to bear the high load and to be super strong. With this we have crossed the ‘Valley of Death'!”
The resulting composite has proved to be twice as strong as high-strength steels and it enjoys elastic strain limits that are 5 to 10 times greater than the best spring steels currently available.
One obvious application is in medical implants. More intriguingly, the material's high elastic strain levels could enable breakthroughs in electronics, optoelectronics, piezoelectrics, piezomagnetics, photocatalytics, and chemical sensing properties. Even better (from my perspective), it might lead to bike frames that actually benefit from the nanomaterial used in them.
Image: Shijie Hao/China University of Petroleum