Misplaced Pages

#387612

92-437: YbBiPt (ytterbium-bismuth-platinum; also named YbPtBi) is an intermetallic material which at low temperatures exhibits an extremely high value of specific heat, which is a characteristic of heavy-fermion behavior. YbBiPt has a noncentrosymmetric cubic crystal structure; in particular it belongs to the ternary half-Heusler compounds . YbBiPt was discovered by Zachary Fisk ( Los Alamos National Laboratory ) and coworkers in 1991 in

184-532: A c t o r = σ S 2 [ W / m / K 2 ] {\displaystyle \mathrm {Power~factor} =\sigma S^{2}[W/m/K^{2}]} where S is the Seebeck coefficient , and σ is the electrical conductivity . Although it is often claimed that TE devices with materials with a higher power factor are able to 'generate' more energy (move more heat or extract more energy from that temperature difference) this

276-931: A x {\displaystyle \eta _{\mathrm {max} }} is then calculated from the efficiency of both legs and the electrical and thermal losses from the interconnects and surroundings. Ignoring these losses and temperature dependencies in S , κ and σ , an inexact estimate for Z T {\displaystyle ZT} is given by Z T ¯ = ( S p − S n ) 2 T ¯ [ ( ρ n κ n ) 1 / 2 + ( ρ p κ p ) 1 / 2 ] 2 {\displaystyle Z{\bar {T}}={(S_{p}-S_{n})^{2}{\bar {T}} \over [(\rho _{n}\kappa _{n})^{1/2}+(\rho _{p}\kappa _{p})^{1/2}]^{2}}} where ρ {\displaystyle \rho }

368-463: A crystal (experiencing very little scattering—maintaining electrical conductivity ): this concept is called phonon glass electron crystal. The figure of merit can be improved through the independent adjustment of these properties. The maximum Z T ¯ {\displaystyle Z{\bar {T}}} of a material is given by the material's quality factor where k B {\displaystyle k_{\rm {B}}}

460-466: A polycrystalline Heusler alloy composed of the Ni-Mn-Sn ternary composition space was found to possess a peak compressive strength of about 2000 MPa with plastic deformation up to 5%. However, the addition of Indium to the Ni-Mn-Sn ternary alloy not only increases the porosity of the samples, but it also reduces the compressive strength to 500 MPa. It is unclear from the study what percentage of

552-491: A ZT of 1.5 at 773 K. Later, Snyder et al. (2011) reported ZT~1.4 at 750 K in sodium-doped PbTe, and ZT~1.8 at 850 K in sodium-doped PbTe 1−x Se x alloy. Snyder's group determined that both thallium and sodium alter the electronic structure of the crystal increasing electronic conductivity. They also claim that selenium increases electric conductivity and reduces thermal conductivity. In 2012 another team used lead telluride to convert waste heat to electricity, reaching

644-439: A ZT of 2.2, which they claimed was the highest yet reported. Inorganic clathrates have the general formula A x B y C 46-y (type I) and A x B y C 136-y (type II), where B and C are group III and IV elements, respectively, which form the framework where “guest” A atoms ( alkali or alkaline earth metal ) are encapsulated in two different polyhedra facing each other. The differences between types I and II come from

736-585: A chemical composition of LM 4 X 12 , where L is a rare-earth metal (optional component), M is a transition metal , and X is a metalloid , a group V element or a pnictogen such as phosphorus , antimony , or arsenic . These materials exhibit ZT>1.0 and can potentially be used in multistage thermoelectric devices. Unfilled, these materials contain voids, which can be filled with low-coordination ions (usually rare-earth elements ) to reduce thermal conductivity by producing sources for lattice phonon scattering , without reducing electrical conductivity . It

828-428: A combination of both. The study found higher fracture toughness in samples prepared without a high-energy ball milling step of 2.7 MPa m to 4.1 MPa m , as opposed to samples that were prepared with ball milling of 2.2 MPa m to 3.0 MPa m . Fracture toughness is sensitive to inclusions and existing cracks in the material, so it is as expected dependent on the sample preparation. Half-metallic ferromagnets exhibit

920-433: A conductor when there is both an electric current and a temperature gradient). While all materials have a nonzero thermoelectric effect, in most materials it is too small to be useful. 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 . The most commonly used thermoelectric material

1012-509: A greater defect density decreases the lattice thermal conductivity, thereby making a larger figure of merit. In conclusion, Long et al. reported that greater Cu-deficiencies resulted in increases of up to 88% in the ZT value, with a maximum of 0.79. The composition of thermoelectric devices can dramatically vary depending on the temperature of the heat they must harvest; considering the fact that more than eighty percent of industry waste falls within

SECTION 10

#1732787178388

1104-545: A lower elastic, shear , and bulk modulus than in quaternary-, full-, and inverse-Hausler alloys. DFT also predicts a decrease in elastic modulus with temperature in Ni 2 XAl (X=Sc, Ti, V), as well as an increase in stiffness with pressure. The decrease in modulus with respect to temperature is also observed in TiNiSn, ZrNiSn, and HfNiSn, where ZrNiSn has the highest modulus and Hf has the lowest. This phenomenon can be explained by

1196-480: A magnetic moment of around 3.7 Bohr magnetons resides almost solely on the manganese atoms. As these atoms are 4.2 Å apart, the exchange interaction, which aligns the spins, is likely indirect and is mediated through conduction electrons or the aluminium and copper atoms. Electron microscopy studies demonstrated that thermal antiphase boundaries (APBs) form during cooling through the ordering temperatures, as ordered domains nucleate at different centers within

1288-406: A material in thermoelectric systems is determined by the device efficiency . This is determined by the material's electrical conductivity ( σ ), thermal conductivity ( κ ), and Seebeck coefficient (S), which change with temperature ( T ). The maximum efficiency of the energy conversion process (for both power generation and cooling) at a given temperature point in the material is determined by

1380-450: A metallic behavior in one spin channel and an insulating behavior in the other spin channel. The first example of Heusler half-metallic ferromagnets was first investigated by de Groot et al., with the case of NiMnSb. Half-metallicity leads to the full polarization of the conducting electrons. Half metallic ferromagnets are therefore promising for spintronics applications. Thermoelectric materials Thermoelectric materials show

1472-554: A range of 373-575 K, chalcogenides and antimonides are better suited for thermoelectric conversion because they can utilize heat at lower temperatures. Not only is sulfur the cheapest and lightest chalcogenide, current surpluses may be causing threat to the environment since it is a byproduct of oil capture, so sulfur consumption could help mitigate future damage. 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

1564-547: A room-temperature saturation induction of around 8,000 gauss, which exceeds that of the element nickel (around 6100 gauss) but is smaller than that of iron (around 21500 gauss). For early studies see. In 1934, Bradley and Rogers showed that the room-temperature ferromagnetic phase was a fully ordered structure of the L2 1 Strukturbericht type . This has a primitive cubic lattice of copper atoms with alternate cells body-centered by manganese and aluminium . The lattice parameter

1656-504: A temperature-independent figure-of-merit, ZT, between 0.8 and 1.0. Nanostructuring these materials to produce a layered superlattice structure of alternating Bi 2 Te 3 and Sb 2 Te 3 layers produces a device within which there is good electrical conductivity but perpendicular to which thermal conductivity is poor. The result is an enhanced ZT (approximately 2.4 at room temperature for p-type). Note that this high value of ZT has not been independently confirmed due to

1748-402: Is 5.95 Å . The molten alloy has a solidus temperature of about 910 °C. As it is cooled below this temperature, it transforms into disordered, solid, body-centered cubic beta-phase. Below 750 °C, a B2 ordered lattice forms with a primitive cubic copper lattice, which is body-centered by a disordered manganese-aluminium sublattice. Cooling below 610 °C causes further ordering of

1840-472: Is also possible to reduce the thermal conductivity in skutterudite without filling these voids using a special architecture containing nano- and micro-pores. 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. This would mean that an otherwise similar RTG would generate 25% more power at

1932-631: Is approximately given by η m a x = T H − T C T H 1 + Z T ¯ − 1 1 + Z T ¯ + T C T H , {\displaystyle \eta _{\mathrm {max} }={T_{\rm {H}}-T_{\rm {C}} \over T_{\rm {H}}}{{\sqrt {1+Z{\bar {T}}}}-1 \over {\sqrt {1+Z{\bar {T}}}}+{T_{\rm {C}} \over T_{\rm {H}}}},} where T H {\displaystyle T_{\rm {H}}}

SECTION 20

#1732787178388

2024-399: Is based on bismuth telluride ( Bi 2 Te 3 ). Thermoelectric materials are used in thermoelectric systems for cooling or heating in niche applications , and are being studied as a way to regenerate electricity from waste heat . Research in the field is still driven by materials development, primarily in optimizing transport and thermoelectric properties. The usefulness of

2116-680: Is desirable for thermoelectric materials to have high valley degeneracy in a very sharp band structure. Other complex features of the electronic structure are important. These can be partially quantified using an electronic fitness function. Strategies to improve thermoelectric performances include both advanced bulk materials and the use of low-dimensional systems. Such approaches to reduce lattice thermal conductivity fall under three general material types: (1) Alloys : create point defects, vacancies, or rattling structures ( heavy-ion species with large vibrational amplitudes contained within partially filled structural sites) to scatter phonons within

2208-443: Is exact when thermoelectric properties are temperature-independent. 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. In an actual thermoelectric device, two materials are used (typically one n-type and one p-type) with metal interconnects. The maximum efficiency η m

2300-407: Is generally achieved through an inert polymer matrix that is host to thermoelectric filler material. The matrix is generally nonconductive so as to not short current as well as to let the thermoelectric material dominate electrical transport properties. 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

2392-431: Is larger than that of the latter. Conducting polymers are of significant interest for flexible thermoelectric development. They are flexible, lightweight, geometrically versatile, and can be processed at scale, an important component for commercialization. However, the structural disorder of these materials often inhibits the electrical conductivity much more than the thermal conductivity, limiting their use so far. Some of

2484-515: Is limited by the Carnot efficiency T H − T C T H {\displaystyle {\frac {T_{\rm {H}}-T_{\rm {C}}}{T_{\rm {H}}}}} , the first factor in η m a x {\displaystyle \eta _{\mathrm {max} }} , while Z T {\displaystyle ZT} and z T {\displaystyle zT} determines

2576-419: Is necessary to minimize κ phonon and keep the electrical conductivity high. Thus semiconductors should be highly doped. G. A. Slack proposed that in order to optimize the figure of merit, phonons , which are responsible for thermal conductivity must experience the material as a glass (experiencing a high degree of phonon scattering—lowering thermal conductivity ) while electrons must experience it as

2668-637: Is only true for a thermoelectric device with fixed geometry and unlimited heat source and cooling. If the geometry of the device is optimally designed for the specific application, the thermoelectric materials will operate at their peak efficiency which is determined by their z T {\displaystyle zT} not σ S 2 {\displaystyle \sigma S^{2}} . For good efficiency, materials with high electrical conductivity, low thermal conductivity and high Seebeck coefficient are needed. The band structure of semiconductors offers better thermoelectric effects than

2760-420: Is paramount for temperature-sensitive applications (e.g. thermoelectrics ) for which some sub-classes of Heusler compounds are used. However, experimental studies are rarely encountered in literature. In fact, the commercialization of these compounds is limited by the material's ability to undergo intense, repetitive thermal cycling and resist cracking from vibrations. An appropriate measure for crack resistance

2852-414: Is required to produce p-type material which is required for an efficient thermoelectric device. Solid solutions and doped compounds have to be annealed in order to produce homogeneous samples – with the same properties throughout. At 800 K, Mg 2 Si 0.55−x Sn 0.4 Ge 0.05 Bi x has been reported to have a figure of merit about 1.4, the highest ever reported for these compounds. Skutterudites have

YbBiPt - Misplaced Pages Continue

2944-529: Is stabilized by the transition metal X playing a dual role (electron donor as well as acceptor) in the structure. The half-Heusler compounds have distinctive properties and high tunability which makes the class very promising as thermoelectric materials. A study has predicted that there can be as many as 481 stable half-Heusler compounds using high-throughput ab initio calculation combine with machine learning techniques. The particular half-Heusler compounds of interest as thermoelectric materials (space group ) are

3036-501: Is the Boltzmann constant, ℏ {\displaystyle \hbar } is the reduced Planck constant, N v {\displaystyle N_{\rm {v}}} is the number of degenerated valleys for the band, C l {\displaystyle C_{\rm {l}}} is the average longitudinal elastic moduli, m l ∗ {\displaystyle m_{\rm {l}}^{*}}

3128-410: Is the act of intentionally adding an impurity, usually to modify the electrochemical characteristics of the seed material. 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

3220-613: Is the electrical resistivity, and the properties are averaged over the temperature range; the subscripts n and p denote properties related to the n- and p-type semiconducting thermoelectric materials, respectively. Only when n and p elements have the same and temperature independent properties ( S p = − S n {\displaystyle S_{p}=-S_{n}} ) does Z T ¯ = z T ¯ {\displaystyle Z{\bar {T}}=z{\bar {T}}} . Since thermoelectric devices are heat engines, their efficiency

3312-455: Is the fixed temperature at the hot junction, T C {\displaystyle T_{\rm {C}}} is the fixed temperature at the surface being cooled, and T ¯ {\displaystyle {\bar {T}}} is the mean of T H {\displaystyle T_{\rm {H}}} and T C {\displaystyle T_{\rm {C}}} . This maximum efficiency equation

3404-477: Is the inertial effective mass, Ξ {\displaystyle \Xi } is the deformation potential coefficient, κ L {\displaystyle \kappa _{\rm {L}}} is the lattice thermal conduction, and T {\displaystyle T} is temperature. The figure of merit, Z T ¯ {\displaystyle Z{\bar {T}}} , depends on doping concentration and temperature of

3496-404: Is the material's toughness , which typically scales inversely with another important mechanical property: the mechanical strength . In this section, we highlight existing experimental and computational studies on the mechanical properties of Heusler alloys. Note that the mechanical properties of such a compositionally-diverse class of materials is expectedly dependent on the chemical composition of

3588-511: Is the process of surrounding a parent crystal with an electrolyte complex, so that the cations (positively charged ions) within the structure can be swapped out for those in solution without affecting the anion sublattice (negatively charged crystal network). What one is left with are crystals that possess a different composition, yet an identical framework. In this way, scientists are granted extreme morphological control and uniformity when generating complicated heterostructures. As to why it

3680-521: The Heusler compounds , more precisely of the so-called half-Heuslers which have XYZ composition. In recent years, there has been a large interest in this material class due to the large variety of physical properties that can be found, and many new Heusler materials have been discovered. Heusler alloy Depending on the field of literature being surveyed, one might encounter the same compound referred to with different chemical formulas. An example of

3772-469: The Wiedemann–Franz law , the higher the electrical conductivity, the higher κ electron becomes. Thus in metals the ratio of thermal to electrical conductivity is about fixed, as the electron part dominates. In semiconductors, the phonon part is important and cannot be neglected. It reduces the efficiency. For good efficiency a low ratio of κ phonon / κ electron is desired. Therefore, it

YbBiPt - Misplaced Pages Continue

3864-410: The load heat energy absorbed at hot junction . {\displaystyle \eta ={{\text{energy provided to the load}} \over {\text{heat energy absorbed at hot junction}}}.} The maximum efficiency of a thermoelectric device is typically described in terms of its device figure of merit Z T {\displaystyle ZT} where the maximum device efficiency

3956-489: 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. Bornite (Cu 5 FeS 4 ) is a sulfide mineral named after an Austrian mineralogist, though it is much more common than the aforementioned cuprokalininite. This metal ore was found to demonstrate an improved thermoelectric performance after undering cation exchange with iron. Cation exchange

4048-481: The thermoelectric effect in a strong or convenient form. The thermoelectric effect refers to phenomena by which either a temperature difference creates an electric potential or an electric current creates a temperature difference. These phenomena are known more specifically as the Seebeck effect (creating a voltage from temperature difference), Peltier effect (driving heat flow with an electric current), and Thomson effect (reversible heating or cooling within

4140-402: The unit cell crystal; (2) Complex crystals : separate the phonon glass from the electron crystal using approaches similar to those for superconductors (the region responsible for electron transport should be an electron crystal of a high-mobility semiconductor, while the phonon glass should ideally house disordered structures and dopants without disrupting the electron crystal, analogous to

4232-453: The Fermi energy so that the average conduction electron energy is close to the Fermi energy, reducing the forces pushing for charge transport. Therefore, semiconductors are ideal thermoelectric materials. In the efficiency equations above, thermal conductivity and electrical conductivity compete. The thermal conductivity κ in crystalline solids has mainly two components: According to

4324-626: The Seebeck coefficient, will increase the quality factor of a material. A large density of states can be created due to a large number of conducting bands ( N v {\displaystyle N_{\rm {v}}} ) or by flat bands giving a high band effective mass ( m b ∗ {\displaystyle m_{\rm {b}}^{*}} ). For isotropic materials m b ∗ = m l ∗ {\displaystyle m_{\rm {b}}^{*}=m_{\rm {l}}^{*}} . Therefore, it

4416-651: The air. These materials often have a figure of merit that is still too low for commercial applications (~0.42 in PEDOT:PSS ) due to the poor electrical conductivity. Hybrid composite thermoelectrics involve blending the previously discussed electrically conducting organic materials or other composite materials with other conductive materials in an effort to improve transport properties. The conductive materials that are most commonly added include carbon nanotubes and graphene due to their conductivities and mechanical properties. It has been shown that carbon nanotubes can increase

4508-531: The alloys themselves, and therefore trends in mechanical properties are difficult to identify without a case-by-case study. The elastic modulus values of half-Heusler alloys range from 83 to 207 GPa, whereas the bulk modulus spans a tighter range from 100 GPa in HfNiSn to 130 GPa in TiCoSb. A collection of various density functional theory (DFT) calculations show that half-Heusler compounds are predicted to have

4600-436: The average toughness of Ti 1−x (Zr, Hf) x NiSn ranges from 1.86 MPa m to 2.16 MPa m , increasing with Zr/Hf content. The preparation of samples may affect the measured fracture toughness however, as elaborated by O’Connor et al. In their study, samples of Ti 0.5 Hf 0.5 Co 0.5 Ir 0.5 Sb 1−x Sn x were prepared using three different methods: a high-temperature solid state reaction , high-energy ball milling , and

4692-431: The band structure of metals. The Fermi energy is below the conduction band causing the state density to be asymmetric around the Fermi energy. Therefore, the average electron energy of the conduction band is higher than the Fermi energy, making the system conducive for charge motion into a lower energy state. By contrast, the Fermi energy lies in the conduction band in metals. This makes the state density symmetric about

SECTION 50

#1732787178388

4784-526: The beginning of a mission and at least 50% more after seventeen years. NASA hopes to use the design on the next New Frontiers mission. Homologous oxide compounds (such as those of the form ( SrTiO 3 ) n (SrO) m —the Ruddlesden-Popper phase ) have layered superlattice structures that make them promising candidates for use in high-temperature thermoelectric devices. These materials exhibit low thermal conductivity perpendicular to

4876-520: The best thermoelectric materials around 1000 °C and are therefore used in some radioisotope thermoelectric generators (RTG) (notably the MHW-RTG and GPHS-RTG ) and some other high^temperature applications, such as waste heat recovery . Usability of silicon-germanium alloys is limited by their high price and moderate ZT values (~0.7); however, ZT can be increased to 1–2 in SiGe nanostructures owing to

4968-474: The charge of more inflexible cations. Therefore, either the cuprokalininite or bornite minerals could prove ideal thermoelectric components. Half-Heusler (HH) alloys have a great potential for high-temperature power generation applications. Examples of these alloys include NbFeSb, NbCoSn and VFeSb. They have a cubic MgAgAs-type structure formed by three interpenetrating face-centered-cubic (fcc) lattices. The ability to substitute any of these three sublattices opens

5060-441: The charge reservoir in high-T c superconductors ); (3) Multiphase nanocomposites : scatter phonons at the interfaces of nanostructured materials, be they mixed composites or thin film superlattices . Materials under consideration for thermoelectric device applications include: Materials such as Bi 2 Te 3 and Bi 2 Se 3 comprise some of the best performing room temperature thermoelectrics with

5152-693: The complicated demands on the growth of such superlattices and device fabrication; however the material ZT values are consistent with the performance of hot-spot coolers made out of these materials and validated at Intel Labs. Bismuth telluride and its solid solutions are good thermoelectric materials at room temperature and therefore suitable for refrigeration applications around 300 K. The Czochralski method has been used to grow single crystalline bismuth telluride compounds. These compounds are usually obtained with directional solidification from melt or powder metallurgy processes. Materials produced with these methods have lower efficiency than single crystalline ones due to

5244-444: The composite material will can have a much lower thermal conductivity. The general procedure to synthesize these materials involves a solvent to dissolve the polymer and dispersion of the thermoelectric material throughout the mixture. Bulk Si exhibits a low ZT of ~0.01 because of its high thermal conductivity. However, ZT can be as high as 0.6 in silicon nanowires , which retain the high electrical conductivity of doped Si, but reduce

5336-526: The context of material research devoted to correlated electron systems such as heavy-fermion metals and Kondo insulators . Then the material was studied in detail due to its unconventional properties at very low temperatures (below 1 K). YbBiPt crystallizes in the MgAgAs structure , which is also known as the half-Heusler structure. YbBiPt exhibits metallic behavior, e.g. continuously decreasing electrical resistivity upon cooling. The temperature dependence of

5428-659: The crystal lattice and are often out of step with each other where they meet. The anti-phase domains grow as the alloy is annealed. There are two types of APBs corresponding to the B2 and L2 1 types of ordering. APBs also form between dislocations if the alloy is deformed. At the APB the manganese atoms will be closer than in the bulk of the alloy and, for non-stoichiometric alloys with an excess of copper (e.g. Cu 2.2 MnAl 0.8 ), an antiferromagnetic layer forms on every thermal APB. These antiferromagnetic layers completely supersede

5520-424: The door for wide variety of compounds to be synthesized. Various atomic substitutions are employed to reduce the thermal conductivity and enhance the electrical conductivity. Previously, ZT could not peak more than 0.5 for p-type and 0.8 for n-type HH compound. However, in the past few years, researchers were able to achieve ZT≈1 for both n-type and p-type. Nano-sized grains is one of the approaches used to lower

5612-523: The fact that the elastic modulus decreases with increasing interatomic separation : as temperature increases, the atomic vibrations also increase, resulting in a larger equilibrium interatomic separation. The mechanical strength is also rarely studied in Heusler compounds. One study has shown that, in off-stoichiometric Ni 2 MnIn, the material reaches a peak strength of 475 MPa at 773 K, which drastically reduces to below 200 MPa at 973 K. In another study,

SECTION 60

#1732787178388

5704-448: The high thermal conductivity, which is intrinsic to highly symmetric HH structure, has made HH thermoelectric generally less efficient than other classes of TE materials. Many studies have focused on improving HH thermoelectric by reducing the lattice thermal conductivity and zT > 1 has been repeatedly recorded. The magnetism of the early full-Heusler compound Cu 2 MnAl varies considerably with heat treatment and composition. It has

5796-423: The layers while maintaining good electronic conductivity within the layers. Their ZT values can reach 2.4 for epitaxial SrTiO 3 films, and the enhanced thermal stability of such oxides, as compared to conventional high-ZT bismuth compounds, makes them superior high-temperature thermoelectrics. Interest in oxides as thermoelectric materials was reawakened in 1997 when a relatively high thermoelectric power

5888-465: The left-most element on the periodic table comes first, uses the Zintl interpretation of semiconducting compounds where the chemical formula XY 2 Z is written in order of increasing electronegativity. In well-known compounds such as Fe 2 VAl which were historically thought of as metallic (semi-metallic) but were more recently shown to be small-gap semiconductors one might find both styles being used. In

5980-500: The manganese and aluminium sub-lattice to the L2 1 form. In non-stoichiometric alloys, the temperatures of ordering decrease, and the range of anealing temperatures, where the alloy does not form microprecipitates, becomes smaller than for the stoichiometric material. Oxley found a value of 357 °C for the Curie temperature , below which the compound becomes ferromagnetic. Neutron diffraction and other techniques have shown that

6072-481: The material of interest. The material quality factor B {\displaystyle B} is useful because it allows for an intrinsic comparison of possible efficiency between different materials. This relation shows that improving the electronic component N v m l ∗ Ξ 2 {\displaystyle {\frac {N_{\rm {v}}}{m_{\rm {l}}^{*}\Xi ^{2}}}} , which primarily affects

6164-423: The maximum reversibility of the thermodynamic process globally and locally, respectively. Regardless, the coefficient of performance of current commercial thermoelectric refrigerators ranges from 0.3 to 0.6, one-sixth the value of traditional vapor-compression refrigerators. Often the thermoelectric power factor is reported for a thermoelectric material, given by P o w e r   f

6256-507: The most common conducting polymers investigated for flexible thermoelectrics include poly(3,4-ethylenedioxythiophene) (PEDOT), polyanilines (PANIs), polythiophenes, polyacetylenes, polypyrrole, and polycarbazole. P-type PEDOT:PSS (polystyrene sulfonate) and PEDOT-Tos (Tosylate) have been some of the most encouraging materials investigated. 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

6348-576: The most common difference is X 2 YZ versus XY 2 Z, where the labels of the two transition metals X and Y in the compound are swapped. The traditional convention X 2 YZ arises from the interpretation of Heuslers as intermetallics and is used predominantly in literature studying magnetic applications of Heuslers compounds. The XY 2 Z convention on the other hand is used mostly in thermoelectric materials and transparent conducting applications literature where semiconducting Heuslers (most half-Heuslers are semiconductors) are used. This convention, in which

6440-465: The normal magnetic domain structure and stay with the APBs if they are grown by annealing the alloy. This significantly modifies the magnetic properties of the non-stoichiometric alloy relative to the stoichiometric alloy which has a normal domain structure. Presumably this phenomenon is related to the fact that pure manganese is an antiferromagnet although it is not clear why the effect is not observed in

6532-543: The number and size of voids present in their unit cells . Transport properties depend on the framework's properties, but tuning is possible by changing the “guest” atoms. The most direct approach to synthesize and optimize the thermoelectric properties of semiconducting type I clathrates is substitutional doping, where some framework atoms are replaced with dopant atoms. In addition, powder metallurgical and crystal growth techniques have been used in clathrate synthesis. The structural and chemical properties of clathrates enable

6624-651: The optimization of their transport properties as a function of stoichiometry . 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. Partially filled variants can be synthesized as semiconducting or even insulating. Blake et al. have predicted ZT~0.5 at room temperature and ZT~1.7 at 800 K for optimized compositions. Kuznetsov et al. measured electrical resistance and Seebeck coefficient for three different type I clathrates above room temperature and by estimating high temperature thermal conductivity from

6716-406: The porosity increase from the indium addition reduces the strength. Note that this is opposite to the outcome expected from solid solution strengthening , where adding indium to the ternary system slows dislocation movement through dislocation-solute interaction and subsequently increases the material's strength. The fracture toughness can also be tuned with composition modifications. For example,

6808-672: The present article semiconducting compounds might sometimes be mentioned in the XY 2 Z style. Although traditionally thought to form at compositions XYZ and X 2 YZ, studies published after 2015 have discovered and reliably predicted Heusler compounds with atypical compositions such as XY 0.8 Z and X 1.5 YZ. Besides these ternary compositions, quaternary Heusler compositions called the double Half-Heusler X 2 YY'Z 2 (e.g. Ti 2 FeNiSb 2 ) and triple Half-Heusler X 2 X'Y 3 Z 3 (for e.g. Mg 2 VNi 3 Sb 3 ) have also been discovered. These "off-stoichiometric" (that is, differing from

6900-732: The published low temperature data they obtained ZT~0.7 at 700 K for Ba 8 Ga 16 Ge 30 and ZT~0.87 at 870 K for Ba 8 Ga 16 Si 30 . Mg 2 B (B =Si, Ge, Sn) compounds and their solid solutions are good thermoelectric materials and their ZT values are comparable with those of established materials. The appropriate production methods are based on direct co-melting, but mechanical alloying has also been used. During synthesis, magnesium losses due to evaporation and segregation of components (especially for Mg 2 Sn) need to be taken into account. Directed crystallization methods can produce single crystals of Mg 2 Si , but they intrinsically have n-type conductivity, and doping, e.g. with Sn, Ga, Ag or Li,

6992-467: The random orientation of crystal grains, but their mechanical properties are superior and the sensitivity to structural defects and impurities is lower due to high optimal carrier concentration. 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. Some possible dopants are halogens and group IV and V atoms. Due to

7084-529: The reported figure of merit in either respective manuscript. Cuprokalininite (CuCr 2 S 4 ) is a copper-dominant analogue of the mineral joegoldsteinite . 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 (Cu 1-x Sb x Cr 2 S 4 ) shows promise in renewable technology. Doping

7176-437: The same atoms will not be positioned on top of each other, impeding phonon conductivity perpendicular to the layers. Recently, oxide thermoelectrics have gained a lot of attention so that the range of promising phases increased drastically. Novel members of this family include ZnO, MnO 2 , and NbO 2 . All variables mentioned are included in the equation for the dimensionless figure of merit , zT , which can be seen at

7268-715: The semiconducting ternary compounds with a general formula XYZ where X is a more electropositive transition metal (such as Ti or Zr), Y is a less electropositive transition metal (such Ni or Co), and Z is heavy main group element (such as Sn or Sb). This flexible range of element selection allows many different combinations to form a half-Heusler phase and enables a diverse range of material properties. Half-Heusler thermoelectric materials have distinct advantages over many other thermoelectric materials; low toxicity, inexpensive element, robust mechanical properties, and high thermal stability make half-Heusler thermoelectrics an excellent option for mid-high temperature application. However,

7360-538: The small bandgap (0.16 eV) Bi 2 Te 3 is partially degenerate and the corresponding Fermi-level should be close to the conduction band minimum at room temperature. The size of the band-gap means that Bi 2 Te 3 has high intrinsic carrier concentration. Therefore, minority carrier conduction cannot be neglected for small stoichiometric deviations. Use of telluride compounds is limited by the toxicity and rarity of tellurium. Heremans et al. (2008) demonstrated that thallium -doped lead telluride alloy (PbTe) achieves

7452-428: The specific heat shows an anomaly at 0.4K and linear behavior at yet lower temperatures with the enormous Sommerfeld coefficient (which describes the linear-in-temperature contribution to the specific heat caused by metallic electrons) of 8J/(mol Yb K), which indicates an effective mass of the charge carriers that is extremely large even for heavy-fermion standards. The crystal structure of YbBiPt makes it an example of

7544-419: The stoichiometric alloy. Similar effects occur at APBs in the ferromagnetic alloy MnAl at its stoichiometric composition. Some Heusler compounds also exhibit properties of materials known as ferromagnetic shape-memory alloys . These are generally composed of nickel, manganese and gallium and can change their length by up to 10% in a magnetic field. Understanding the mechanical properties of Heusler compounds

7636-488: The tensile strength of the polymer composite they are blended with. However, they can also reduce the flexibility. Furthermore, future study into the orientation and alignment of these added materials will allow for improved performance. The percolation threshold of CNT’s is often especially low, well below 10%, due to their high aspect ratio. A low percolation threshold is desirable for both cost and flexibility purposes. Reduced graphene oxide (rGO) as graphene-related material

7728-416: The thermal conductivity due to elevated scattering of phonons on their extensive surfaces and low cross-section. Combining Si and Ge also allows to retain a high electrical conductivity of both components and reduce the thermal conductivity. The reduction originates from additional scattering due to very different lattice (phonon) properties of Si and Ge. As a result, Silicon-germanium alloys are currently

7820-400: The thermal conductivity via grain boundaries- assisted phonon scattering. Other approach was to utilize the principles of nanocomposites, by which certain combination of metals were favored on others due to the atomic size difference. For instance, Hf and Ti is more effective than Hf and Zr, when reduction of thermal conductivity is of concern, since the atomic size difference between the former

7912-472: The thermoelectric materials figure of merit z T {\displaystyle zT} , given by z T = σ S 2 T κ . {\displaystyle zT={\sigma S^{2}T \over \kappa }.} The efficiency of a thermoelectric device for electricity generation is given by η {\displaystyle \eta } , defined as η = energy provided to

8004-422: The top of this page. The goal of any thermoelectric experiment is to make the power factor, S σ , larger while maintaining a small thermal conductivity . This is because electricity is produced through a temperature gradient, so materials that can equilibrate heat very quickly are not useful. The two compounds detailed below were found to exhibit high-performing thermoelectric properties, which can be evidenced by

8096-574: The well-known XYZ and X 2 YZ compositions) Heuslers are mostly semiconductors in the low temperature T  = 0 K limit. The stable compositions and corresponding electrical properties for these compounds can be quite sensitive to temperature and their order-disorder transition temperatures often occur below room-temperatures. Large amounts of defects at the atomic scale in off-stoichiometric Heuslers helps them achieve very low thermal conductivities and make them favorable for thermoelectric applications. The X 1.5 YZ semiconducting composition

8188-436: Was also used to enhance figure of merit of thermoelectric materials. 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. However, significant growth of ZT after addition of graphene or rGO

8280-457: Was observed mainly for composites based on thermoelectric materials with low initial ZT. When thermoelectric material is already nanostructured and possesses high electrical conductivity, such an addition does not enhance ZT significantly. Thus, graphene or rGO-additive works mainly as an optimizer of the intrinsic performance of thermoelectric materials. Hybrid thermoelectric composites also refer to polymer-inorganic thermoelectric composites. This

8372-421: Was reported for NaCo 2 O 4 . In addition to their thermal stability, other advantages of oxides are their low toxicity and high oxidation resistance. Simultaneously controlling both the electric and phonon systems may require nanostructured materials. Layered Ca 3 Co 4 O 9 exhibited ZT values of 1.4–2.7 at 900 K. If the layers in a given material have the same stoichiometry, they will be stacked so that

8464-466: 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. According to the Debye-Callaway formalism, a model used to determine the lattice thermal conductivity, κ L , the highly anharmonic behavior due to phonon scattering results in a large thermal resistance. Therefore,

#387612