Page 151 - Carbon Nanotube Fibres and Yarns

144 Carbon Nanotube Fibers and Yarns

the strong van der Waals force, CNTs tend to form bundles during growth
and processing.
Undensified CNT webs drawn from a CNT forest (Chapter 2) have an
extremely high porosity, estimated to be in the vicinity of 99.97% [27]. The
porosity of a twisted CNT yarn decreases with the degree of twist inserted
into the yarn, as shown in Fig. 7.5. A highly twisted CNT yarn can have a
packing fraction as high as 0.6 (a porosity of 0.4) [14]. This is similar to the
fiber packing fraction of a highly densified textile yarns [28].
If the twist in a twisted yarn is removed, the yarn diameter can increase
substantially, as shown in Fig. 7.6. The change of yarn porosity is believed to
be mainly due to the widening of the voids between CNT bundles while
voids within a CNT bundle do not change so significantly [6].

1.4 1
Forest 1 0.9 Forest 1
1.2 1 Forest 2 0.8 Forest 2
Forest 3
Bulk density (g/cm 3 ) 0.8 Porosity 0.6
Forest 3
0.7
0.5
0.6
0.4
0.4
0.2
0.2 0.3 φ =0.238
0.1
0 0
50
40
30
20
50
40
30
20
(A) 0 10 Surface twist angle (degrees) 60 70 (B) 0 10 Surface twist angle (degrees) 60 70
5000 t/m 15000 t/m

(C) (D)
Fig. 7.5 CNT yarn bulk density and porosity. (A) Relationship between twist angle
and yarn bulk density, (B) relationship between twist angle and yarn porosity [14],
and (C, D) SEM images of FIB sections milled through CNT yarns with two levels
of twist [25]. (Panels (A and B) reprinted with permission from M. Miao, J. McDonnell,
L. Vuckovic, S.C. Hawkins, Poisson’s ratio and porosity of carbon nanotube dry-spun
yarns, Carbon 48 (10) (2010) 2802–2811.; Panels (C and D) reprinted with permission
from K. Sears, C. Skourtis, K. Atkinson, N. Finn, W. Humphries, Focused ion beam milling of
carbon nanotube yarns to study the relationship between structure and strength, Carbon
48 (15) (2010) 4450–4456.)