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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: 2, Informative) by khallow on Wednesday August 19 2020, @02:45AM
The other repliers nailed the problems. I'll note that it also depends on where you want that space tether. There are other uses that can be done with present day fibers such as a skyhook [wikipedia.org] in Earth orbit or a lunar space elevator [wikipedia.org] from the Moon to the Earth-Moon L1 point (which is between the Earth and Moon, about 90% of the way straight towards the Moon).
I don't know how strong a Martian tether needs to be, but it might need exotic materials in order to have an adequate safety margin.