Thermoelectric materials
However, low-cost materials that have a sufficiently strong thermoelectric effect (and other required properties) are also considered for applications including power generation and refrigeration.
For a single thermoelectric leg the device efficiency can be calculated from the temperature dependent properties S, κ and σ and the heat and electric current flow through the material.
The required carrier concentration is obtained by choosing a nonstoichiometric composition, which is achieved by introducing excess bismuth or tellurium atoms to primary melt or by dopant impurities.
[33][34] The structure of type II materials allows a partial filling of the polyhedra, enabling better tuning of the electrical properties and therefore better control of the doping level.
[43] NASA is developing a Multi-Mission Radioisotope Thermoelectric Generator in which the thermocouples would be made of skutterudite, which can function with a smaller temperature difference than the current tellurium designs.
[44] Homologous oxide compounds (such as those of the form (SrTiO3)n(SrO)m—the Ruddlesden-Popper phase) have layered superlattice structures that make them promising candidates for use in high-temperature thermoelectric devices.
Their ZT values can reach 2.4 for epitaxial SrTiO3 films, and the enhanced thermal stability of such oxides, as compared to conventional high-ZT bismuth compounds, makes them superior high-temperature thermoelectrics.
It was recently found within metamorphic rocks in Slyudyanka, part of the South Baikal region of Russia, and researchers have determined that Sb-doped cuprokalininite (Cu1-xSbxCr2S4) shows promise in renewable technology.
The introduction of antimony enhances the power factor by bringing in extra electrons, which increases the Seebeck coefficient, S, and reduces the magnetic moment (how likely the particles are to align with a magnetic field); it also distorts the crystal structure, which lowers the thermal conductivity, κ. Khan et al. (2017) were able to discover the optimal amount of Sb content (x=0.3) in cuprokalininte in order to develop a device with a ZT value of 0.43.
[55] As to why it was thought to improve the ZT value, the mechanics of cation exchange often bring about crystallographic defects, which cause phonons (simply put, heat particles) to scatter.
[52] As for the metal, copper is an ideal seed particle for any kind of substitution method because of its high mobility and variable oxidation state, for it can balance or complement the charge of more inflexible cations.
Some of the most common conducting polymers investigated for flexible thermoelectrics include poly(3,4-ethylenedioxythiophene) (PEDOT), polyanilines (PANIs), polythiophenes, polyacetylenes, polypyrrole, and polycarbazole.
Organic, air-stable n-type thermoelectrics are often harder to synthesize because of their low electron affinity and likelihood of reacting with oxygen and water in the air.
[65] The addition of rather low amount of graphene or rGO around 1 wt% mainly strengthens the phonon scattering at grain boundaries of all these materials as well as increases the charge carrier concentration and mobility in chalcogenide-, skutterudite- and, particularly, metal oxide-based composites.
One major benefit of this method is that the polymer matrix will generally be highly disordered and random on many different length scales, meaning that the composite material will can have a much lower thermal conductivity.
This problem can be solved by using materials whose transport properties vary along their length thus enabling substantial improvements to the operating efficiency over large temperature differences.
In some nanocrystalline transition metal silicides the power factor is higher than in the corresponding polycrystalline material but the lack of reliable data on thermal conductivity prevents the evaluation of their thermoelectric efficiency.
The thermal conductivity in the cross-plane direction of the lattice is usually very low, but depending on the type of superlattice, the thermoelectric coefficient may increase because of changes to the band structure.
[89] Superlattice structure countermeasures: Counter measures can be taken which practically eliminate the problem of decreased electrical conductivity in a reduced phonon-scattering interface.
[99] However, such single crystalline materials suffer from inability to make useful devices due to their brittleness as well as narrow range of temperatures, where ZT is reported to be high.
[104] In 2020, researchers at Kyung Hee University demonstrated the use of Anderson localization in an n-type semiconductor to improve the thermoelectric properties of a material.
Powder based techniques offer excellent ability to control and maintain desired carrier distribution, particle size, and composition.
Thermoelectric generators have the advantage of no moving parts and do not require any chemical reaction for energy conversion, which make them stand out from other sustainable energy resources such as wind turbine and solar cells; Nevertheless, the mechanical performance of thermoelectric generators may decay over time due to plastic, fatigue and creep deformation as a result of being subjected to complex and time-varying thermomechanical stresses.
In their research, Al-Merbati et al.[115] found that the stress levels around the leg corners of thermoelectric devices were high and generally increased closer to the hot side.
In a separate investigation, Turenne et al.[123] examined the distribution of stress in large freestanding thermoelectric modules and those rigidly fixed between two surfaces for thermal exchange.
They showed that under thermoelectric coupling load, will experience severe joule heat and current density that can be accumulate thermoemechanical stress and miscrostructure evolution.
They also pointed out that the difference in CTE between materials in the flip chip package causes thermal mismatch stress which can later develop the cavities to expand along cathode into cracks.
Also, it is worth noting that they mentioned thermal-electrical coupling can cause electromigration, microcracks and delamination due to temperature and stress concentration that can fail Cu pillar bumps.
Creep deformation is a time-dependent mechanism where strain accumulates as amaterial is subjected to external or internal stressesat a high homologous temperature in excess ofT/Tm= 0.5(whereTmis the melting point in K).
[135][137] Cogeneration power plants use the heat produced during electricity generation for alternative purposes; being this more profitable in industries with high amounts of waste energy.
