Direct Energy, 2018a

8 THERMOELECTRICS 185
thermocouples
heater
heat
sink
Figure 8.3: Components used to measure thermalconductivity.
values are listed in Table 8.4, found in Section 8.3. Understanding thermal
conductivity is complicated because a number of mechanisms are responsible
for the conduction of heat. Heat may be transported by phonons,
photons, electrons, or other mechanisms, and each mechanism depends on
temperature and the properties of the material. Good thermoelectric devices
have sma lthermalconductivity. Often metals have large thermal
conductivity and insulators have small thermal conductivity.
The apparatus to measure thermalconductivity consists of a heater, a
heat sink, and a number of thermocouples [110] [112]. To measure thermal
conductivity experimentally, start with a bar of material with a known
cross sectionalarea A. Heat one end of the bar with respect to the other,
wait for a steady thermalstate, and measure the temperature at each end
of the bar. Next, calculate the temperature gradient dT
dx
in units m K along
the length of the bar. Also measure the rate that heat is supplied to the
bar, dQ
dt , in units J s . By denition, thermalconductivity is the ratio
κ =
(power dissipated in heater)(distance between thermocouples)
(cross sectionalarea)(change in temp)
[109, p. 49]. The thermal conductivity can be calculated from
(8.33)
( )
− dQ
dt
κ =
A ( ) . (8.34)
dT
dx
This technique works well for low to moderate temperatures and materials
with high thermalconductivity. Other methods exist to measure thermal
conductivity and are advantageous for dierent temperature or conductivity
ranges.
Another way to understand thermalconductivity is to think of it as
the product of the amount of heat transported by some particle times the
velocity of that particle. This viewpoint applies whether or not actual
particles are involved in the heat transport. More specically, thermal