Quantum Dot Thermoelectric Conversion
The direct conversion of temperature differences into electricity, or thermoelectric conversion, is attracting attention as a source of renewable energy.
This technology operates on the principle of the Seebeck effect, by which charge carriers move from the high-temperature side to the low-temperature side of a substance with different temperatures at its ends, resulting in a voltage difference between those ends.
The materials used for thermoelectric conversion are mainly semiconductors. The electrons in n-type semiconductors and the holes in p-type semiconductors function as charge carriers for electrical conduction. The greatest advantage of this simple principle is the ease of generating electricity through waste heat, a byproduct that is otherwise difficult to use. As there is no drive component in the thermoelectric conversion element, the method offers greater quietness, durability, and reliability than other power generation methods, as well as ease of scaling down and the absence of maintenance requirements. With these advantages, it is already finding use in nuclear batteries for unmanned planetary probes, power generation using radiant heat from slabs in steelworks, wristwatches that use body temperature as a heat source, and other applications; power generation using automobile exhaust gas and solar heat are also at the experimental stage. Applications remain limited to this specialized range of uses, however, as efficiency is currently 8–16% lower than other power generation engines, and the discovery and development of more efficient materials is needed. The theoretical efficiency of a thermoelectric conversion material is expressed by the following formula, with TH the temperature of the high-temperature part and TL the temperature of the low-temperature part.
ZT here refers to a dimensionless performance metric determined by the physical properties of the material, and is expressed as follows.
Here, a is the Seebeck coefficient, k is the thermal conductivity, r is the electrical conductivity, and T is the absolute temperature.
ZT is an important value in performance evaluation since it is able to represent the efficiency of thermoelectric conversion more simply than Carnot efficiency does. ZT between 1.5 and 2 in automobile exhaust usage and ZT between 0.5 and 1 in micro-power generation are considered targets for practical use. It can be seen from the formula above that obtaining high ZT requires the use of material that has high electrical conductivity, a high Seebeck coefficient, or low thermal conductivity. These values are specific to certain materials; many substances have been proposed as materials for thermoelectric conversion. Since the discovery of the Seebeck effect in metals by Thomas Johann Seebeck in 1821, investigation has focused on thermoelectric effects in metalloids and simple metals such as bismuth (Bi) and antimony (Sb). Research subsequently expanded to address intermetallic compound semiconductors using substances such as bismuth telluride (Bi2Te3) and silicon–germanium (SiGe) 12 alloy, which are still under study as thermoelectric-conversion materials. Bi2Te3 compounds are thermoelectric conversion materials that feature relatively high ZT in the low-temperature range, from room temperature to about 450 K. Conversely, SiGe has the advantage of high stability at high temperature ranges and low environmental burden. In subsequent research, the substance known as phonon glass electron crystal (PGEC), which combines glass-like low thermal conductivity with high electron mobility by which electrons act as if in a crystal, was proposed as suited to thermoelectric conversion materials. Another proposed new concept controls lattice thermal conductivity through the rattling motion of guest atoms in the cages of a cage-like structural material. Research under this concept is focusing on skutterudite compounds, half-Heusler compounds, clathrate compounds, oxides, and other new thermoelectric conversion materials with ZT higher than those of earlier materials. At the same time, the theory that ZT increases in low-dimensional systems has spurred active research into nanolevel structural control, a method made possible by recent technological advances. There have also been reports demonstrating a maximum ZT of 6.9 in a two-dimensional quantum well and a maximum ZT of 14 in nanowire when Bi2Te3 is used. This significant improvement in ZT is thought to be due to an increase in density of electronic states due to quantum confinement effects, as well as a decrease in thermal conductivity due to the limiting of phonon mean free path.
The concept of phonons is commonly used in discussions of thermal conduction in nanostructured thermoelectric materials. Phonons are quantized lattice vibrations and are the primary transporters of heat in semiconductors. As an example, Fig. 1 shows a schematic diagram of phonon transport in a nanostructured crystal when phonons move along the mean free path in a crystal of length L. If L is sufficiently large relative to L, then the phonons diffuse before reaching the interface. Conversely, if L is sufficiently small relative to L, then the phonons reach the interface without diffusing. In the latter case, phonon transport is greatly affected by the interface, and thermal conduction capacity is limited, thereby reducing thermal conductivity. Through this principle, nanostructuring shortens the representative length L and interfaces increase, reducing thermal conductivity. At the same time, thinking of electrical conduction as the movement of electrons allows discussion of electrical conductivity in the same manner as thermal conductivity. As interfaces increase when nanostructuring is used to reduce thermal conductivity, electrons scatter and electrical conductivity decreases. However, as the electron mean free path is far smaller than the phonon mean free path, nanostructuring has less of an effect on electrons than it has on phonons, and electrical conductivity decreases less than thermal conductivity decreases. Accordingly, it is thought that fabricating nanostructures of appropriate size can reduce thermal conductivity and increase ZT while maintaining electrical conductivity. Working from these theories, many studies have begun to use structural control at the nanolevel to increase ZT. Examples include quantum wells, superlattice structures, and nanowires.。