Basics of thermometric

1. Thoughts of
Thermoelectric Material
Subhra Sourav Jana

2.  60% of the energy in industry is wasted in the form of heat. But we know that
gradually the amount of fossil fuel depletes. At the same time continuous use of fossil
fuel keeps on increasing the risk of global warming.
 One of the purpose of thermoelectric material is to utilize that waste heat in such crisis
of energy.
 Thermoelectric material is used in conversion of heat to electricity
 Seebeck effect was first observed in 1821 by Baltic German physicist Thomas Johann
Seebeck.
 The most important parameter scientists are concerned about is figure of merit (ZT).
High ZT increases the efficiency.
 Till now research is going on and we have got ZT between 2-3.
 ZT of 4-10 can assist the material to achieve efficiency 15%-20%.
Introduction

3. Figure of merit, ZT=
𝑆2 𝞂 T
𝑘
To improve the ZT 𝞂 𝑆 But 𝑘
 𝑆2 𝞂 is called Power factor of the material.
 Point to be noted that S is a material property. That depends on the electronic structure of
the material, its band structure.
 K =kl +ke This implies that thermal conductivity is sum of phononic and
electronic contribution. Hence , The massage we want to convey is that we can not
increase the 𝞂 as much as possible.
 Phonon is a energy packet of lattice vibration. Heating can increase the internal bond
energy between the atoms and that increases the lattice vibration.
Important parameter to increase the efficiency of
Thermoelectric material is to increase the ZT
Discussion of Different parameter

4.  Mott equation says that Seebeck Coef (S)
𝑆 =
8𝜋2
𝑘 𝐵
2
3𝑒ℎ2
𝑚∗ 𝑇
𝜋
3𝑛
2/3
 Doping in the system can increase
the carrier concentration.
 Carrier concentration should be in
the range of 1019 that means to be
semi metallic or degenerate
semiconductor.
 Higher concentration than that is
not effective as it increases the
thermal conductivity by electronic
contribution.
 Increase in doping concentration
can increase the carrier
concentration (n) hence the S falls
down.
 But we may argue that sometimes increase in same can increase the DOS or
scattering of electron and that enhance the effective mass of electron (m*). But
effect of carrier concentration is dominant over m*. That is why S drops with n
Continued…

5.  Interesting phenomena is we can not increase the power factor aimlessly even is we
neglect the electronic contribution in thermal. The reason is given below.
 If we want to increase the S then the conductive channel should be as far as it
possible from the chemical potential of the system. That can be changed by doping.
Or in the other way higher density of states which will conduct the carrier should
be as as it possible from the fermi energy.
 But we have to remember at the same time that the conductivity function gradually
decreases after few of KBTs. (KB= Boltzman constant, T= Absolute Temperature ).
So the summery is PF has its own limitation.
 As it is mentioned thermal conductivity has two contribution electronic and lattice.
We can not do anything with electronic part but lattice part (kl) can be engineered
by nanostructured grain, alloying, complex lattice structure, nano inclusions,
composites.
 Nano structured grain can reduce the thermal conductivity as the grain boundary
helps to scatter the phonons. Because grain boundary act as defect structure.
 Alloying with different atoms can be helpful. Atoms with different atomic mass can
act as phonon scatterer again.
 Interfaces are the best place to scatter the phonon. Need to remember that it can
also affect the electrons. But there should not be not much energy difference in
the path of electron. Or else electron scattering will be dominant and that will
decreases the electrical conductivity.
Continued…

6. The End
Thank you