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No limit yet for carbon nanotube fibers:
The Rice lab of chemical and biomolecular engineer Matteo Pasquali reported in Carbon it has developed its strongest and most conductive fibers yet, made of long carbon nanotubes through a wet spinning process.
[...] "The goal of this paper is to put forth the record properties of the fibers produced in our lab," Taylor said. "These improvements mean we're now surpassing Kevlar in terms of strength, which for us is a really big achievement. With just another doubling, we would surpass the strongest fibers on the market."
The flexible Rice fibers have a tensile strength of 4.2 gigapascals (GPa), compared to 3.6 GPa for Kevlar fibers. The fibers require long nanotubes with high crystallinity; that is, regular arrays of carbon-atom rings with few defects. The acidic solution used in the Rice process also helps reduce impurities that can interfere with fiber strength and enhances the nanotubes' metallic properties through residual doping, Dewey said.
"The length, or aspect ratio, of the nanotubes is the defining characteristic that drives the properties in our fibers," he said, noting the surface area of the 12-micrometer nanotubes used in Rice fiber facilitates better van der Waals bonds. "It also helps that the collaborators who grow our nanotubes optimize for solution processing by controlling the number of metallic impurities from the catalyst and what we call amorphous carbon impurities."
The researchers said the fibers' conductivity has improved to 10.9 megasiemens (million siemens) per meter. "This is the first time a carbon nanotube fiber has passed the 10 megasiemens threshold, so we've achieved a new order of magnitude for nanotube fibers," Dewey said. Normalized for weight, he said the Rice fibers achieve about 80% of the conductivity of copper.
"But we're surpassing platinum wire, which is a big achievement for us," Taylor said, "and the fiber thermal conductivity is better than any metal and any synthetic fibers, except for pitch graphite fibers."
I wonder how useful the thermal conductivity would be in cooling computer chips?
Journal Reference:
Lauren W. Taylor, Oliver S. Dewey, Robert J. Headrick, et al. Improved Properties, Increased Production, and the Path to Broad Adoption of Carbon Nanotube Fibers, Carbon (DOI: 10.1016/j.carbon.2020.07.058)
(Score: 5, Interesting) by Immerman on Wednesday August 19 2020, @12:16AM
No. Nanotube fibers are *far* weaker than nanotubes themselves. Think of yarn - if you stretch it hard enough to break, it's rarely the individual strands that break, instead the strands pull apart from each other, since each strand is connected to itself via chemical bonds, while it's only connected to its neighbors via much weaker friction (and possibly van-der-Waals forces).
That's dramatically more true with nanotubes, which are held together by carbon-carbon covalent bonds - one of the strongest chemical bonds known, and (I think) the strongest bond known in terms of bond strength per nucleon (a.k.a. strength-to-weight ratio), at least among those elements which can form enough bonds to make a chain. Meanwhile adjacent nanotube fibers share *no* chemical bonds (unless some interesting cross-linking occurs - tubes might partially break and re-join with adjacent ones.
To put it in perspective, graphene (the flattened version of nanotubes) has an intrinsic tensile strength of around 130GPa, while tested nanotubes have demonstrated an applied strength in the 11-63GPa range, compared to the paltry 4.2GPa they've managed here by twisting a bunch of shorter nanotubes together.
As I recall, perfect carbon nanotubes/graphene doesn't have a good enough strength-to-weight ratio to construct a beanstalk-style space elevator to Earth's surface, at least not with an adequate safety margin. It's a bit stronger than theoretically required, but nowhere near the 10x stronger that's a standard engineering safety margin when human lives are on the line (to compensate for imperfections and aging).
And sadly there's good reason to believe that Carbon-Carbon bonds are likely to be the strongest bonds possible, since it's the smallest and lightest element capable of forming four covalent bonds, and bonds generally get weaker the larger the atoms involved, so even some sci-fi unobtantium element we haven't discovered yet is unlikely to be able to form stronger bonds, and certainly not with a higher strength-to-weight ratio since the atoms would have to be more massive than any of the existing elements.
Though, to end on a high note, there is the possibility that a different chemical configuration might be stronger, for mostly geometric reasons. Graphene is a hexagonal grid (like chicken mesh), and so distorts (and weakens) when loaded in any direction. Something that bonded on a rectangular grid, or a straight chain, could potentially be stronger, so long as forces were aligned with the grid.