Chemistry Reference
In-Depth Information
9.3.2.3
ELECTRICAL PROPERTIES
Because of the regularity and exceptional electronic structure of graphene,
the structure of a nanotube affects its electrical properties strongly. It has
been concluded that for a given
(n, m)
nanotube, while
n=m
, the nanotube
is metallic; if
n−m
is a multiple of 3, then the nanotube is semiconduct-
ing with a very small band gap, if not, the nanotube is a moderate semi-
conductor. However, some exceptions are in this rule, because electrical
properties can be strongly affected by curvature in small diameter carbon
nanotubes. In theory, metallic nanotubes can transmit an electric current
density of 4 × 109A/cm
2
, which is more than 1,000 times larger than those
of metals such as copper, while electro migration lead to limitation of cur-
rent densities for copper interconnects.
Because of nanoscale cross-section in carbon nanotubes, electrons
spread only along the tube's axis and electron transfer includes quantum
effects. As a result, carbon nanotubes are commonly referred to as one-di-
mensional conductors. The maximum electrical transmission of a SWNT
is
2G
0
, where
G
0
= 2e
2
/h
is the transmission of a single ballistic quantum
channel.
9.3.2.4
THERMAL PROPERTIES
It is expected that nanotubes act as very good thermal conductors, but they
are good insulators laterally to the tube axis. Measurements indicate that
SWNTs have a room temperature thermal conductivity along its axis of
about 3500 W·m
−1
·K
−1
; higher than that for copper (385 W·m
−1
·K
−1
). Also,
SWNT has a room temperature thermal conductivity across its axis (in the
radial direction) of around 1.52 W·m
−1
·K
−1
, which is nearly as thermally
conductive as soil. The temperature constancy of carbon nanotubes is ex-
pected to be up to 2800 °C in vacuum and about 750 °C in air.
9.3.2.5 DEFECTS
As with any material, the essence of a crystallographic defect affects the
material properties. Defects can happen in the form of atomic vacancies.
High levels of such defects can drop the tensile strength up to 85%. A
main example is the Stone Wales defect, which makes a pentagon and
