Thermoelectric Properties of Metals and Semiconductors

Abstract

Thermoelements are usually made from crystalline solids. They are not commonly single crystals, but their crystalline nature can be observed under a microscope. Sometimes, the transport properties vary with crystalline orientation but, until we deal with specific examples, we shall suppose that the properties are uniform in all directions.The transport of electric charge is due to quasi-free electrons in the solid. The solids of interest to us are metals and semiconductors. In such materials, the electrons carry not only the charge but also the thermal energy. In other words, there is an electronic component of the thermal conductivity. As we shall see later, heat can also be carried by the thermal vibrations of the atoms in a crystal but, for the moment, we confine ourselves to the electronic effects.The idea of conduction by electrons was proposed by Drude and Lorentz using the principles of classical physics. The classical free electron theory predicted that the specific heat should be much larger for a metal than for an electrical insulator but, in reality, there is very little difference. This discrepancy disappeared when Sommerfeld [1] took account of the newly developed quantum theory but neither the classical nor the quantum mechanical-free electron theories were able to explain why some solids are metallic conductors and others are insulators. It was only when notice was taken of the interaction of the electrons with the periodic potential that exists in a crystal lattice that further progress could be made. It was shown that, through this interaction, the energy of the electrons must lie in discrete bands that are separated by forbidden regions or energy gaps. Interestingly, Sommerfeld's theory can still be applied to the current-carrying electrons, if they are assigned an effective mass rather than the mass of a free electron.According to quantum theory, the probability that an electron state of energy, E, will be occupied is given by the Fermi distribution function f 0 .E/ D

Inorganic thermoelectric materials: A review

“…Although the PF of NaCo 2 O 4 is not that sufficient, as required for TE applications, nonetheless, it opened the door to explore oxide‐based materials as feasible TE materials. The study on oxide‐based materials has been growing rapidly owing to their attractive features such as high chemical and thermal stability, earth's abundance, relatively inexpensive, nontoxicity, chemically inert, and robust properties that make them suitable for the TE power generation, particularly for high‐temperature applications …”

“…It has two prominent features such as its melting point is very high ~1300 K and it has magnificent stability at high temperatures, which is why this material has been used in RTGs by NASA, for the Mars mission in 2013, and other high temperature applications. Nevertheless, the main challenges associated with this material are its high κ lat due to the low mass of SiGe, and their strong chemical bonds, which further minimize the ZT . Thus, the optimization of SiGe alloys is mainly based on the reduction of its κ lat .…”

“…Since the discovery of the thermoelectric effect, by Thomas Johann Seebeck in the year 1821, numerous materials have been explored, to find the most suitable for the TE power generation, starting from inorganic (inorganic material includes both metals and semiconductors) to organic materials. Although the beginning of the research on TE materials was ponderous, nevertheless, since the mid‐20th century, the basic science of TE effect was clearly understood and the classic inorganic TE materials, such as Bi 2 Te 3 , PbTe, and SiGe alloys, were acknowledged as magnificent TE materials which offered ZT values close to 1 . Since then, the field of thermoelectrics has quickly expanded, especially due to the prodigious development of novel techniques such as using low‐dimensional structure, nanocomposites, band engineering, and carrier filtering …”

“…Although the beginning of the research on TE materials was ponderous, nevertheless, since the mid-20th century, the basic science of TE effect was clearly understood and the classic inorganic TE materials, such as Bi 2 Te 3 , PbTe, and SiGe alloys, were acknowledged as magnificent TE materials which offered ZT values close to 1. 22,23 Since then, the field of thermoelectrics has quickly expanded, especially due to the prodigious development of novel techniques such as using low-dimensional structure, 24,25 nanocomposites, 23,26 band engineering, [27][28][29][30] and carrier filtering. 31 Recently, a number of reviews on TE materials have been published that, in summary, mainly focused on bulk TE materials, 32 both inorganic and organic TE materials, 33 progress and emerging TE materials, 13,34,35 design of advanced TE materials, 36 and high efficiency TE materials.…”

Hybrid Plasmonics for Energy Harvesting and Sensing of Radiation and Heat

“…Hence, it is established an electron-density gradient and a corresponding electric potential across the hot and cold ends of the conductor. [108,109] The Seebeck effect is the working principle of thermoelectric generators (TEGs), which are solid-state energy harvesting devices that can convert static temperature gradients to electricity without any moving parts. Figure 3-1a illustrates the structure of a typical TEG, consisting of a pair of p-type and n-type thermoelectric legs.…”