Nanotube research accelerated greatly following the independent discoveries by Bethune at IBM and Iijima at NEC of single-walled carbon nanotubes and methods to specifically produce them by adding transition-metal catalysts to the carbon in an arc discharge.
It has been very difficult to make larger amounts of carbon nanotubes and to make them longer. It has been even more difficult to combine lots of carbon nanotubes and make the combined material close to the strength of individual carbon nanotubes.
In 2008, it was found individual CNT shells have strengths of up to ≈100 gigapascals (15,000,000 psi). Although the strength of individual CNT shells is extremely high, weak shear interactions between adjacent shells and tubes lead to significant reduction in the effective strength of multi-walled carbon nanotubes and carbon nanotube bundles down to only a few GPa.