Environmental Engineering Reference
In-Depth Information
influence of interface scattering, we define two quantities for the
graphene structures: thermal contact resistance
R
c
=
W
δ/σ
,which
equals to the temperature difference between the two thermal
leads per unit heat flux [57], and thermal conductance ratio
σ/σ
0
,
where
W
and
σ
0
are, respectively, the width and the ballistic
thermal conductance of the pristine narrower GNR (i.e.,
n
-ZGNR)
and
0.335 nm is chosen as the layer separation in graphite. The
calculated
R
c
and
δ
=
σ/σ
0
at room temperature, is shown in Figs. 3.15c
and 3.15d, respectively, for the graphene structures with different
width of the narrow part.
As presented in Fig. 3.15d, when
n
(corresponds to the width
of the narrow part) increases,
σ/σ
0
of graphene junctions grows at
small
n
(lessthan10)andbecomes(almost)saturated,whilethatof
QDs exhibits a nearly linear increase. Larger
n
always gives higher
σ/σ
0
for all the three graphene structures, implying that thermal
conduction reduction induced by interface scattering is relatively
weaker as the narrow part gets wider. When the narrow part is
about 2 nm (i.e.,
n
=
10),
σ/σ
0
at 300 K are about 85%, 80%, and
60%forthesingle-interfacejunction,double-interfacejunction,and
QD respectively. To some extent, these predicted values quantify
thermal-transport ability of realistic graphene nanodevices, since
thermalconductanceofthegraphenestructuresvariesslightlywhen
the width of wide part changes. Our results indicate that thermal-
transport ability of single-interface junction is close to that of
double-interface junctions, but obviously better than that of the QD
structure.
As shown in Fig. 3.15c, thermal contact resistance of the
single-interface and double-interface junctions is almost width-
independent, while that of the QD structure gradually decreases
withthewidthofthenarrowpart.Theirregularityappearsat
n
=
2,
presumably caused by predominant edge effects. Thermal contact
resistance of graphene junctions is insensitive to the variance in
ribbon width. Benefitting from this feature, we can extrapolate that
in realistic graphene based nano-junctions, the room temperature
thermal contact resistance between ZGNRs and graphene is around
0.3
×
10
−
9
m
2
K/W. This value is much lower than those between
graphene andsilicon dioxide (5.6
×
10
−
9
-1.2
×
10
−
8
m
2
K/W [58]).
