Page 180 - Carbon Nanotube Fibres and Yarns

Carbon nanotube yarn structures and properties 171

1×10 –4
1 9×10 –5
10 –3 2 8×10 –5 –5
Resisivity (W•m) 10 –4 –5 3 Resisivity (W•m) 7×10 –5 –5 1
6×10
5×10
10
4 4×10 –5 θ 2 3
10 –6 4
1 10 100 10 100
(A) Temperature (K) (B) Temperature (K)
Fig. 7.22 Temperature dependence of resistivity for different types of carbon nanotube
assemblies: (A) MWNT sheet (densified), perpendicular to the pulling direction (1), ran-
domly deposited buckypaper fabricated from the same CVD grown MWNT forest (2),
densified MWNT sheet, parallel to the pulling direction (3), and SWNT (HiPCO) buckypa-
per (4). (B) Three-layer MWNT sheet (1), three-ply yarn (2), one-ply yarn with twist angle
of 23 degrees (3), and one-ply yarn with twist angle of 29 degrees (4) [77]. (Reprinted
with permission from A.E. Aliev, C. Guthy, M. Zhang, S. Fang, A.A. Zakhidov, J.E. Fischer, et al.,
Thermal transport in MWCNT sheets and yarns, Carbon 45 (15) (2007) 2880–2888.)

porosity, which is determined by the densification method used to produce
the yarn. The conductivity of yarns with different levels of porosity should
be compared based on specific conductivity.
If T is the yarn linear density (tex), R is the measured electrical resistance
(Ω) and l is the gauge length (m) used in measurement, the specific conductiv-
3
ity of the yarn in (S/m)/(g/cm ),or S ∙m/tex, can be calculated from Ref. [27]
l
σ = ×10 9 (7.7)
sp
RT
Miao [27] demonstrated the dependence of electrical conductivity on
CNT yarn porosity. The conductivity for twisted yarns followed a linear re-
lationship with the yarn density, as shown in Fig. 7.23A. When the electrical
conductivity is normalized to specific conductivity, a very slow increasing
trend was obtained (Fig. 7.23B). This indicates that the electrical contact
between CNTs in a yarn is not significantly improved by twist insertion.
As shown in Fig. 7.23C and D, when the twist in a twisted yarn is removed
by applying opposite twist, the yarn diameter increased and the electrical
conductivity decreased, but the specific conductivity of the yarn hardly
changed. This is because the yarn diameter increase was almost all caused
by the increased spaces between CNT bundles while the tube-tube contact
within each bundle did not change significantly [6].