Special Heat Transfer Mechanisms: Active and Passive Thermal Diodes - Magnetocaloric Energy Conversion: From Theory to Applications - page 228

Wang et al. reported [ 81 ] on the important role of interface collisions on thermal
recti cation. Schmotz et al. [ 82 ] demonstrated a thermal diode mechanism based on

a standard silicon processing technology using the recti

cation of phonon transport.
They developed a thermal diode that consists of an array of differently shaped holes
milled into a thin silicon membrane. The authors reported on a recti cation ratio for

the heat ux of 1.7 at a temperature of 150 K. Based on their experiences the

authors suggested that the thermal diode should serve as a building block in full
analogy to electrical circuits, such as memory, gates and transistors. The heat

fl

ux in
this particular device is carried mainly by phonons, while leaving the electronic
degree of freedom completely unaffected [ 82 ].
To generalize the effect we refer to the work of Roberts and Walker [ 74 ], who
divided the thermal recti

fl

cation mechanisms into:

bulk mechanisms,

￿

molecular — nanoscale mechanisms.

￿

6.3.1 Bulk Mechanisms

6.3.1.1 Metal/Insulator Coupling

The heat ux is transported mainly by electrons in the metal and phonons in the

insulator. The two materials transport heat with different carriers. Therefore,
the transfer between the electrons and phonons occurs [ 74 ]. In the vicinity of the
interface in the metal, the electrons scatter with the phonons. These are then
transmitted into the insulator or re

fl

ected at the interface into the metal. The elec-
tron-phonon scattering and the phonon transmission lead to an effective contact
resistance [ 74 ].

fl

6.3.1.2 Thermal Strain/Warping at Interfaces

The interface is composed of materials with different temperature-dependent
properties. If the temperature on each side of the interface varies, this will result in
different effective contact areas.

6.3.1.3 Thermal Potential Barrier at Interfaces

Electronic effects at interfaces can be observed in both the metal-oxide interfaces
and the metal metal interfaces. According to Rogers [ 83 ] the differences in the

work function produced a thermally rectifying effect. The work function is
the energy required to remove an electron from the highest

-

lled level of a solid. In
the case of a junction of two metals with different work functions, the electrons will