Special Heat Transfer Mechanisms: Active and Passive Thermal Diodes - Magnetocaloric Energy Conversion: From Theory to Applications - page 227

Fig. 6.14 Schematic gure of two bonded materials with different, temperature-dependent thermal
conductivities and the variation of the thermal conductivity (above, for a material such as LaCoO 3
and below for a material such as La 0.7 Sr 0.3 CoO 3 ), which is required for the thermal recti cation

Kurabayashi [ 77 ] published an article on the anisotropic thermal properties of
solid polymers. He also reviewed the existing theories and experimental methods
for analysing the polymer orientation. The basis for the anisotropic thermal con-
ductivity of oriented polymers is in the difference between the thermal energy
transport mechanisms in the parallel and perpendicular directions with respect to
molecules. We should therefore not neglect the possibility of applying polymers.
Moreover, the recent developments in polymer science show [ 78 ] that such mate-
rials may be serious candidates for future thermal conductors.
Most of the phenomena described in publications on thermal recti cation by

anisotropic thermal conductivity relate to the nanoscale, and most of the work
performed has been based on theoretical investigations. Because of the large
number of publications on anisotropic thermal conductivity we refer below only to
some of them.
Li et al. [ 79 ] proposed the coupling of two nonlinear one-dimensional lattices.
They theoretically analysed mechanisms that enable heat ux through a system in

one direction and where the system acts as an insulator when the temperature
gradient is reversed. They also discussed the potential experimental realization of
such nanoscale systems.
In 2006, Chang et al. [ 80 ] demonstrated nanoscale thermal recti

fl

cation with the
use of high thermal conductivity carbon and boron nitride nanotubes. These were
deposited externally and non-homogeneously using heavy molecules [ 80 ]of
C 9 H 16 Pt. The resulting nanoscale system yielded asymmetric thermal conductance
with higher heat ux in the direction of decreasing mass density. The authors

predicted that solitons may be responsible for the observed phenomenon. They also
reported on a recti

fl

cation of 2 % at room temperature; however, higher values of
7 % were also measured.