Fabrication and application of nanofiber ribbons and sheets and twisted and non-twisted nanofiber yarns

A nanofiber yarn that includes a plurality of nanofibers twisted into a yarn along an alignment axis. The nanofibers of the plurality of nanofibers have a ratio of nanofiber length to nanofiber circumference of at least 50. The yarn has a helix angle measure drelative to the alignment axis of from 5? to 30?. The yarn has tensile strength of at least 280 MPa. A nanofiber fabric that includesa first sheet of multiwalled nanotubes and a second sheet of multiwalled nanotubes on the first sheet of multiwalled nanotubes. The multiwalled nanotubes of the first sheet are aligned in a first direction. The multiwalled nanotubes of the second sheet are aligned in the first direction. The first sheet and the second sheet are aligned so that the multuwalled nanotubes of the first sheet and the second sheet are both aligned in the first direction.