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

1. A nanofiber yarn comprising a plurality of nanofibers twisted into a yarn along an alignment axis, wherein

(a) the nanofibers of the plurality of nanofibers have a ratio of nanofiber length to nanofiber circumference of at least 50;

(b) the yarn has a helix angle measured relative to the alignment axis of from 5° to 30°; and

(c) the yarn has tensile strength of at least 280 MPa.

2. The nanofiber yarn of claim 1 , wherein the yarn is an unplied yarn.

3. The nanofiber yarn of claim 1 , wherein the yarn has a diameter of less than 20 microns.

4. The nanofiber yarn of claim 1 , wherein the yarn has a helix angle of 20° and a tensile strength between 320 MPa and 350 MPa.

5. The nanofiber yarn of claim 1 , wherein the nanofibers of the plurality of nanofibers have a length to thickness ratio of at least 1000.

6. The nanofiber yarn of claim 1 , wherein at least 20% of the nanofibers migrate from near surface of yarn to an interior and then back to surface in distance less than 50% of yarn length.

7. The nanofiber yarn of claim 1 , wherein the yarn has a density-normalized tensile strength of between 310 MPa/g cm −3 and 575 MPa/g cm −3 .

8. The nanofiber yarn of claim 7 , wherein the yarn includes a first straight section without a knot, a second straight section without a knot, and an overhand knot between the first straight section and the second straight section.

9. The nanofiber yarn of claim 8 , wherein the overhand knot has a density greater than the density of the first straight section and the second straight section.

10. The nanofiber yarn of claim 8 , wherein the overhand knot has a tensile strength that is from 20% to equal to a tensile strength of the first and the second straight sections.

11. The nanofiber yarn of claim 8 , wherein the overhand knot has an electrical conductivity greater than an electrical conductivity of the first and the second straight sections.

12. The nanofiber yarn of claim 1 , wherein the yarn has a diameter of 5 microns and a linear density of 10 μg/m.

13. The nanofiber yarn of claim 1 , wherein the nanofibers of the plurality of nanofibers are multiwalled nanofibers.

14. The nanofiber yarn of claim 1 , wherein the nanofibers of the plurality of nanofibers are carbon nanofibers.

15. The nanofiber yarn of claim 1 , wherein a nanofiber of the plurality of nanofibers have a width of less than 100 nm.

16. The nanofiber yarn of claim 1 , wherein the yarn has a diameter of 1 micron or less.

17. A nanofiber fabric comprising:

(a) a first sheet of multiwalled nanotubes, wherein the multiwalled nanotubes of the first sheet are aligned in a first direction; and

(b) a second sheet of multiwalled nanotubes on the first sheet of multiwalled nanotubes, wherein

(i) the multiwalled nanotubes of the second sheet are aligned in the first direction,

(ii) the first sheet and the second sheet are aligned so that the nanotubes of the first sheet and the second sheet are both aligned in the first direction.

18. The nanofiber fabric of claim 17 , wherein the first sheet and the second sheet are free of binder.

19. The nanofiber fabric of claim 17 , wherein at least one of the first sheet and the second sheet individually have a density-normalized tensile strength of at least 120 MPa/g cm −3 .

20. The nanofiber fabric of claim 19 , further comprising at least sixteen additional sheets of multiwalled nanotube sheets having nanotubes aligned in the first direction are disposed on at least one of the first sheet and the second sheet, thus forming a stack.

21. The nanofiber fabric of claim 20 , wherein a density-normalized tensile strength of the stack is at least 465 MPa/g cm −3 .