Environmental Engineering Reference
In-Depth Information
6.3 Passive Solid-State Thermal Recti
cators
In most publications that relate to passive thermal diode mechanisms the authors
refer to thermal recti
cators. It was as long ago as 1935 when Starr [
67
] published a
paper on thermal and electrical recti
cation using a copper/cuprous oxide combi-
nation and where a recti
ciency of 39 % was achieved. There are a
number of different mechanisms for thermal recti
cation ef
cation, such as the anisotropic
thermal conduction of some materials, a change in the material
'
s thermal conduc-
tivity due to different in
uential mechanisms, memory-shape materials [
68
], the
temperature dependence of thermal conductivity at interfaces [
69
], asymmetrical
nanostructured geometry, nanostructured interfaces, quantum thermal systems,
elastomer liquid crystals, etc. (see Ref. [
70
]). A vast number of different research
activities are being carried out in the
fl
eld of carbon-nanostructured thermal recti-
cation [
71
73
].
Thermal recti
-
cation is a phenomenon in which the thermal transport along a
speci
c axis is dependent on the temperature gradient or the heat
fl
ux. The ther-
mally rectifying
, which apply anisotropy, must exhibit a high thermal
conductivity in one direction and a low thermal conductivity in the other direction.
Similarly, some materials must exhibit a high thermal conductivity in, e.g. the
z plane, while representing heat insulators in the plane x-y. This kind of property
can be, for instance, provided by the application of graphene and it may even be
found on the market.
A review of the different mechanisms and models for thermal recti
“
materials
”
cation can be
found in Roberts and Walker [
74
] and Walker [
75
].
Most publications that refer to the anisotropic behaviour of a material do not
relate to bulk materials. Of these, we should refer to some
ndings from the article
of Kobayashi et al. [
76
], who have experimentally demonstrated thermal recti
-
cation in a device comprising two perovskite cobalt oxides, i.e. LaCoO
3
and
La
0.7
Sr
0.3
CoO
3
, with different thermal conductivities (see Fig.
6.14
for a better
presentation of the problem). The authors reported on a rectifying coefcient of
1.43 for a temperature span of 60 K (the ratio between the two
fl
uxes in different
directions
see Fig.
6.14
).
The dimensions of the two bars resulted in a total length of 12.4 mm. In
Fig.
6.14
the heat
—
q
r
(Wm
−
2
). The materials
A and B are bonded at the centre. The characteristic of the material A is that it
exhibits high thermal conductivity at a low temperature T
C
, and low thermal con-
ductivity at a high temperature T
H
. The material B exhibits
fl
ux is denoted by the heat
fl
ux density
_
the inverse
characteristics.
When materials A and B are in contact with the thermal baths T
C
(heat sink) and
T
H
(heat source), where T
H
is attached to material B, then the total thermal resis-
tance of the bar should be high. The heat
ows through material B to
material A. In contrast, when the conditions for material A are reversed, the total
thermal resistance becomes higher resulting in a heat
fl
ux
q
r1
fl
_
fl
ux
q
r2
that is smaller than
_
q
r1
. The rectifying coef
cient is de
ned by the ratio of
j
q
r1
j
to
j
q
r2
j
.
_
_
_
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