Special Heat Transfer Mechanisms: Active and Passive Thermal Diodes - Magnetocaloric Energy Conversion: From Theory to Applications - page 210

Fig. 6.2 Examples of the operating characteristics of a thermal diode

of different thermal diode characteristics. A particular thermal diode can possess
several such mechanisms. Unidirectional fl ow relates to thermal diodes, which

allow a heat ux in only one direction, i.e. laterally or vertically with respect to the

heated or cooled element. Another principle relates to thermal diodes that allow a
bidirectional heat

fl

ux. In certain cases some mechanisms may provide a concen-
trated and some a dispersed heat

fl

ux (a similar situation relates to the application of
so-called nanostructured metamaterials that apply transformation thermodynamics).
The anisotropic heat

fl

ux may relate to different structures, material properties, as
well as mechanisms. The heat valve is a typical thermal diode mechanism that acts
as the thermal switch (insulator

fl

conductor).
Important Note: The alternation of the magnetic

-

eld in magnetic refrigeration
can also serve as the driving source for different kinds of thermal diode mecha-
nisms. This fact must not be neglected in the design of future magnetocaloric
devices.

6.2 Active Solid-State Thermal Diodes

This kind of thermal diode requires an external energy source for its operation,
which can be provided by a magnetic eld, electric current, applied voltage or

electric eld, pressure, mechanical stress, etc.

6.2.1 Thermoelectrics

Thermoelectrics can be applied as an effective thermal switch or heat-valve
mechanism in magnetic refrigeration [ 4 6 ]. Thermoelectric modules are used for

electricity generation or for cooling and heat pumping. They work on two different
principles. If the purpose is electricity generation, then the phenomenon called the

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