Biomedical Engineering Reference
In-Depth Information
Carbon-based materials display the highest measured thermal
conductivity of any known material at moderate temperatures [77].
In graphite, the thermal conductivity is generally dominated by
phonons, and limited by the small crystallite size within a sample.
Thus, the apparent long-range crystallinity of the CNTs has led to
the speculation that the longitudinal thermal conductivity of the
nanotubes could possibly exceed the in-plane thermal conductivity
of the graphite. The thermal conductivity of the CNTs is dependent
on the temperature and the large phonon mean free paths
(
100 nm) [78]. There seems to be some disagreement into the exact
nature of the thermal conductivity of CNTs, although most agree that
thermal conductivity seems to change depending on temperature, on
electrical current, and vacancy concentration. Hone
~
. [78] found
that the thermal conductivity was temperature dependent, and
was almost a linear relationship. They suggested that the thermal
conductivity was linear in temperature from 7 to 25 K. From 25 to
40 K, the line increases in slope, and it arises monotonically with
temperature to above room temperature. They proposed a model
to explain this behavior, which is summarized by phonon thermal
conductivity tensor:
et al
2
k
C
(9.3)
zz
z
where
k
zz
is the slope of the line on the graph,
C
is the heat
capacity,
is the relaxation time of a
given phonon state, which is approximately 10
u
is the sound velocity, and
t
−11
s. Generally, the
thermal conductivity for a single rope at room temperature could
vary between 1800-6000 Wm
−1
−1
K
. In fact, Berber
et al
. [79] found
−1
−1
a thermal conductivity for SWCNTs ropes of
~
2000 Wm
K
. Also,
Pop
. [80] measured a thermal conductivity for SWCNTs ultrathin
films up to
et al
~
−1
−1
.
The thermoelectric power (TEP) in the CNT mats has been
measured [81]. When a temperature difference,
3500 Wm
K
∆
T
, is applied
between a hot point and a cold point along the length of the CNT
mat, a corresponding thermoelectric voltage,
, was measured
between hot-cold junction, and the thermoelectric power (TEP)
S
∆
V
=
∆
V
/
∆
T
was computed. Adu
et al
. [81] found at room temperature,
µ
a TEP between −45 and −40
V/K in the CNT mat. This indicated
that the mat behaves thermoelectrically as
-type metal, and the
TEP is dominated by the contribution from electrons in metallic
tubes that form percolating conducting pathways through the
sample. Additionally, the TEP is sensitive to oxidation or reduction of
n
