Influence of lengths of millimeter-scale single-walled carbon nanotube on electrical and mechanical properties of buckypaper
Nanoscale Research Letters
Influence of lengths of millimeter-scale single-walled carbon nanotube on electrical and mechanical properties of buckypaper
Shunsuke Sakurai
Fuminori Kamada
Don N Futaba
Motoo Yumura
Kenji Hata
The electrical conductivity and mechanical strength of carbon nanotube (CNT) buckypaper comprised of millimeterscale long single-walled CNT (SWCNT) was markedly improved by the use of longer SWCNTs. A series of buckypapers, fabricated from SWCNT forests of varying heights (350, 700, 1,500 μm), showed that both the electrical conductivity (19 to 45 S/cm) and tensile strength (27 to 52 MPa) doubled. These improvements were due to improved transfer of electron and load through a reduced number of junctions for longer SWCNTs. Interestingly, no effects of forest height on the thermal diffusivity of SWCNT buckypapers were observed. Further, these findings provide evidence that the actual SWCNT length in forests is similar to the height.
Carbon nanotube; Buckypaper; Tube length
Background
The effective transfer of phonons, electrons, and load is
known to increase with longer carbon nanotubes (CNTs)
within CNT agglomerates. For example, in the
percolation theory, electron transfer is expected to be achieved
with a lesser number of CNTs by the use of longer CNTs
in accordance with the relation Nc = 5.71 /L2s, where Nc
and Ls are percolation threshold and CNT length,
respectively [
1-4
]. For example, higher electrical
conductivity was observed for transparent conductive films using
network thin films of longer CNTs [
5,6
]. In addition,
Miyata el al. reported a field effect transistor (FET) with
high mobility using long single-walled CNTs (SWCNTs)
[
7
]. Further, in CNT/polymer composites, the beneficial
effect of CNT length on the efficiency of
phonon/electron transport and interfacial load transfer has been
reported [
8-11
]. Such superiority in properties from long
CNTs originates from the fewer CNT junctions, which
interrupt phonon, electron, and load transfer, in a
network structure of CNTs required to span the material.
Although these reports suggest the advantages of long
CNTs on electron, thermal, and mechanical properties
of a CNT assembly, this point has not been explicitly
demonstrated experimentally. In other words, almost all
the above experiments have employed only short CNTs,
on the order of micrometers, with only one exceptional
report by Zhu et al., who reported on the properties of
composite of multiwalled CNTs with thick diameters
(approximately 40 to 70 nm) and bismaleimide (BMI)
[8]. Particularly, there has been no report on the effect of
length on the properties of SWCNTs exceeding 1 mm.
There are three reasons why research on the CNT
length dependence of various properties of CNT
assemblies has been difficult. First, the synthesis of long CNTs
with uniform length in a large quantity is difficult. For
example, Wang et al. reported the synthesis of long
single-wall CNTs with a maximum length of 18.5 cm,
but there were substantial variations in CNT length [
12
].
Cao et al. reported an interesting approach for
lengthtunable CNT growth, but the length did not reach to
millimeter scale [
13
]. Furthermore, several groups
reported the methods for classifying long/short CNTs, but
this was not applied to CNTs that were longer than
10 μm in length [
14-17
]. Secondly, due to the tight
entanglement among CNTs, the dispersion of CNTs
without CNT scission is difficult. Ultrasonic agitation, which
has been typically employed as a dispersion method, is
known to shorten CNTs as it disentangles them [18].
Finally, there is no available method to measure the
lengths of individual CNTs longer than 100 μm. CNTs
with lengths of several micrometers have been evaluated
by atomic force microscopy (AFM) [
8-11,14-17
], but this
method encounters extreme difficultly when obtaining
statistically significant data for long CNTs.
Using water-assisted chemical vapor deposition (CVD),
we reported the synthesis of a vertically aligned SWCNT
array (SWCNT forest) with height exceeding a millimeter
[
19
]. The SWCNT forests possessed several excellent
structural properties, such as long length, high purity,
and high specific surface area. This development opened
up the potential for various new applications of CNTs,
such as high-performance super-capacitors [
20-23
] and
highly durable conductive rubbers [
24,25
]. Subsequently,
many groups reported the growth of long SWCNTs. For
example, Zhong et al. reported the growth of SWCNT
forests reaching 0.5 cm in length [
26
]. Hasegawa et al.
reported growth of SWCNT forests of several millimeters in
length without an etching agent (water) [
27
]. Numerous
studies have also reported the synthesis of multiwalled
CNT forests [
28-30
]. However, the following points
remain unclear at present: the correlations between forest
height and (1) the actual CNT length and (2) the
electrical, thermal, and mechanical properties after formation (...truncated)