Patent Application
20130180561
The instant disclosure relates to thermoelectric nanocomposites with high thermoelectric performance, methods of manufacturing thereof, and methods of using thereof.
Thermoelectric applications, including both power generation utilizing the Seebeck effect and refrigeration utilizing Peltier effect, have attracted increasing interest worldwide in recent decade. For example, thermoelectric devices are being rapidly developed for waste heat recovery applications, particularly in automobiles, to produce electricity and reduce carbon emissions. The development of efficient thermoelectric devices for both space and terrestrial applications can benefit from availability of compositions with a high thermoelectric figure of merit (zT).
Some embodiments of the disclosure relate to an article of manufacture comprising a matrix and nanoinclusions, wherein the nanoinclusions are uniformly dispersed in the matrix, and wherein the article of manufacture has a thermoelectric figure of merit (zT) of at least 1. The article of manufacture can have a thermoelectric figure of merit (zT) of at least 1.5. The matrix can include, for example, Pb, and the like. The matrix can include at least one composition selected from PbTe, PbSe, and the like. The nanoinclusions can include, for example, Ag, Cu, and the like. The nanoinclusions can include at least one composition selected from Ag2Te, Ag2Se, and the like. At least one dimension of the nanoinclusions is larger than 200 nanometers, or larger than 400 nanometers, or larger than 500 nanometers, or larger than 600 nanometers, or larger than 800 nanometers, or larger than 1 micrometers. The article of manufacture can further include a dopant. The dopant can include at least one composition selected from La and Na, and the like.
Some embodiments of the disclosure relate to a method of manufacturing an article including: heating a first material comprising at least a first element and a second material comprising at least a second element to form a mixture; cooling the mixture to precipitate nanoinclusions comprising the second element; and annealing the mixture. The first element of the first material can include Pb, and the like. The first material can further include at least one composition selected from Te, Se, and the like. The second element of the nanoinclusions can include Ag, Cu, and the like. The nanoinclusions can further include at least one composition selected from Te, Se, and the like. The method can further include repeating the cooling, and/or repeating the annealing. The method can further include doping the article with a dopant. The dopant can include at least one composition selected from La, Na, and the like.
Some embodiments of the instant disclosure are directed to a method of using an article of manufacture in a thermoelectric device, wherein the article of manufacture includes a matrix and nanoinclusions, wherein the nanoinclusions are uniformly dispersed in the matrix, and wherein the article of manufacture has a thermoelectric figure of merit (zT) of at least 1. In some embodiments, the method of using the article of manufacture includes applying a temperature gradient to the article of manufacture; and collecting electrical energy. In some embodiments, the method of using the article of manufacture includes applying electrical energy to the article of manufacture; and transferring heat from a first space at a first operation temperature to a second space at a second operation temperature, wherein the first operation temperature is lower than the second operation temperature.
Thermoelectric (TE) applications have attracted increasing interest worldwide in the last decade as a means to combat the ever growing rate of energy consumption. The two main applications for thermoelectric materials are power generation, which utilizes the Seebeck effect, and solid state cooling, which has its roots in the Peltier effect. Recently, however, power generation has been a prime interest to the automotive industry as a sustainable and emission free waste heat recovery process. Discussion about this can be found at, for example, L. E. Bell, Science (2008), 321, 1457, which is hereby incorporated by reference. The effectiveness of this process is restricted by the overall efficiency of the thermoelectric materials.
A common figure of merit for a thermoelectric material, denoted by z, is defined as z=S2σ/(κE+κL), where S is the Seebeck coefficient, σ is the electrical conductivity, and κE and κL the electronic (or carrier) component and phonon (or lattice) component of the thermal conductivity, respectively. The Seebeck coefficient S for a thermoelectric material is the voltage difference per degree Kelvin. The electrical conductivity a is inverse of the electrical resistivity ρ. The figure of merit z has the units of reciprocal Kelvin. Another figure of merit, which is referred to as thermoelectric figure of merit, can be defined as zT, where T is the absolute temperature in Kelvin, so that zT is a dimensionless quantity.
Materials investigated and optimized over the past 50 years have been conventional, simple semiconductors. Examples include alloys of bismuth telluride, lead telluride, and silicon germanium, with the best of these exhibiting thermoelectric figure of merit zT values of no greater than 1. Recently, this thermoelectric figure of merit zT barrier has been broken, so that thermoelectric figure of merit zT>2 has been achieved in thin film superlattices or quantum well materials with feature sizes of several to tens of nanometers. See, for example, Caylor, J. C., Coonley, K., Stuart, J., Colpitts, T., and Venkatasubramanian, R. Applied Physics Letters (2005), 87, 23105; Venkatasubramanian, R.; Siivola, E.; Colpitts, T.; O'Quinn, B. Nature (2001), 413, 597-602; and Harman, T. C.; Taylor, P. J.; Walsh, M. P.; LaForge, B. E. Science (2003), 297, 2229-2232, each of which is hereby incorporated by reference. The first significant result has been that of Venkatasubramanian (2001) who demonstrated thermoelectric figure of merit zT=2.4 using Bi2Te3—Sb2Te3 quantum well superlattices with 6-nanometer periodicity. Harman and coworkers prepared quantum dot superlattices in the PbTe—PbSeTe system (described as PbSe nanodots embedded in a PbTe matrix) and demonstrated thermoelectric figure of merit zT values of 1.6.
Despite the high thermoelectric figure of merit zT of such thermoelectrics, the performance of devices utilizing superlattice materials has not yet surpassed the performance of bulk Bi2Te3 based devices. This is due to the small size of the thermoelectric elements that currently are achieved from ‘top-down’ fabrication methods, which imply a large, relative contribution of electrical and thermal contact resistances.
Because S, σ (or ρ), and κE have an intimate relationship with the carrier density, strategies of optimizing the carrier density and minimizing the independent parameter κL can be effective route to improve zT. More discussion can be found at, for example, A. F. Ioffe, Semiconductor thermoelements, and Thermoelectric cooling, Infosearch, London, (1957); and G. J. Snyder, E. S. Toberer, Nat Mater (2008), 7, 105, each of which is hereby incorporated by reference.
Two recently proposed mechanisms have shown to enable a significant enhancement of the Seebeck coefficient, and therefore an improvement in zT, by a distortion in the density of states and also carrier pocket engineering. Merely by way of example, due to the complex valence band structure of PbTe, heavily doped p-type PbTe:Na, which does not have resonant states, has been found to have similarly high thermoelectric performance to that of a material that does have resonant states, namely PbTe:Tl. However, this influence on the valence band structure is an electronic effect, indicating the possibility in further enhancing zT for PbTe:Na by reducing the lattice thermal conductivity. See, for example, T. Koga, X. Sun, S. Cronin, M. Dresselhaus, Appl Phys Lett (1998), 73, 2950; O. Rabina, Y. Lin, M. Dresselhaus, Appl Phys Lett (2001), 79, 81; M. S. Dresselhaus, G. Chen, M. Y. Tang, R. G. Yang, H. Lee, D. Z. Wang, Z. F. Ren, J. P. Fleurial, P. Gogna, Adv Mater (2007), 19, 1043; and J. Heremans, V. Jovovic, E. Toberer, A. Saramat, K. Kurosaki, A. Charoenphakdee, G. J. Snyder, Science (2008), 321, 554, each of which is hereby incorporated by reference.
Unless otherwise stated, a nanocomposite and an article of manufacture are used interchangeably in the instant disclosure. In some portions of the disclosure, a nanocomposite or article of manufacture is also referred to as a sample. Exemplary embodiments of nanocomposites or articles of manufacture are illustrated in, for example,
As used herein, nanoinclusions refer to the inclusions in the matrix of a nanocomposite (or article of manufacture) that has a different composition than the matrix. The size of a nanoinclusion can be at a nanometer scale or a micrometer scale. Merely by way of example, a nanoinclusion has at least one dimension that is larger than 1 micrometer.
Unless otherwise stated, carrier density and carrier concentration are used interchangeably in the instant disclosure.
In some embodiments, the numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and so forth, used to describe and claim certain embodiments of the application are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the application are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable.
Strategies to synthesize bulk nanostructured materials for thermoelectrics are inspired by classic metallurgical approaches. Several themes are used in these syntheses of the instant disclosure: (a) solid state partitioning on cooling due to the crossing from a single to two-phase region, (b) formation and subsequent decomposition of a metastable phase, and (c) solidification from the melt. One advantage of these transformations is that the microstructural or nanostructural length scales can be controlled by diffusion, and thus varying processing parameters allows a wide range of microstructures or nanostructures to be obtained. This is described in more detail elsewhere in the instant disclosure. More discussion can be found at, for example, T. Ikeda, L. A. Collins, V. A. Ravi, F. S. Gascoin, S. M. Haile and G. J. Snyder, Chem. Mater., 19, 763-767, (2007); T. Ikeda, S. M. Haile, V. A. Ravi, H. Azizgolshani, F. Gascoin and G. J. Snyder, Acta Materialia, 55, 1227-1239, (2007); T. Ikeda, V. A. Ravi and G. J. Snyder, Acta Materialia, 57, 666-672, (2009); and D. L. Medlin and G. J. Snyder, Current Opinion In Colloid & Interface Science, 14, 226, (2009), each of which is incorporated by reference.
Some embodiments of the instant disclosure are directed to an article of manufacture comprising a matrix and nanoinclusions, wherein the nanoinclusions are uniformly dispersed in the matrix, and wherein the article of manufacture has a thermoelectric figure of merit zT of at least 1. The nanoinclusions can scatter phonons effectively, leading to a low lattice thermal conductivity κL. The article of manufacture can include at least one dopant to optimize the carrier density. The article of manufacture has an improved thermoelectric figure of merit zT.
In some embodiments, the matrix includes at least one composition selected from lead (Pb), selenium (Se), tellurium (Te), antimony (Sb), germanium (Ge), silicon (Si), tin (Sn), bismuth (Bi), arsenic (As), indium (In), thallium (Tl), and the like, or an alloy thereof. In some exemplary embodiments, the matrix includes PbTe, or PbSe. The advantages of PbSe for thermoelectric application include, but not limited to, low cost and tolerance to high temperature. In some exemplary embodiments, the matrix includes PbSexTe1−x, wherein x represents the fraction of PbSe in the alloy of PbTe and PbSe, and can be from (and including) 0 to (and including) 1. In some embodiments, the matrix includes HgCdTe, PbS, InAs, InSb, Cd3As2, Bi2Te3, SnTe, and the like. In some embodiments, the matrix includes a narrow-gap semiconductor. In some embodiments, the matrix has nanoscale features of less than 1 micrometer, or less than 800 nanometers, or less than 600 nanometers, or less than 400 nanometers, or less than 200 nanometers, or less than 100 nanometers, or less than 80 nanometers, or less than 60 nanometers, or less than 40 nanometers, or less than 20 nanometers.
In some embodiments, the nanoinclusions are not isostructural to the matrix. For example, the dimension of the nanoinclusions is larger than the nanoscale features of the matrix so that they enhance the phonon scattering, which can reduce the lattice thermal conductivity κL. In some embodiments, the nanoinclusions have dimensions (along its minor axis if the shape is not isometric) of larger than 20 nanometers, or larger than 40 nanometers, or larger than 50 nanometers, or larger than 60 nanometers, or larger than 80 nanometers, or larger than 100 nanometers, or larger than 120 nanometers, or larger than 150 nanometers, or larger than 180 nanometers, or larger than 200 nanometers, or larger than 250 nanometers, or larger than 300 nanometers, or larger than 400 nanometers, or larger than 500 nanometers. In some embodiments, the nanoinclusions have dimensions (along its major axis if the shape is not isometric) of less than 1 micrometer, or less than 800 nanometers, or less than 600 nanometer, or less than 500 nanometers, or less than 400 nanometers, or less than 300 nanometers, or less than 250 nanometers, or less than 200 nanometers, or less than 150 nanometers, or less than 100 nanometers, or less than 80 nanometers, or less than 60 nanometers, or less than 50 nanometers. In some embodiments, the article includes some smaller nanoinclusions in addition to large nanoinclusions. Merely by way of example, an article includes a matrix with nanoscale features of close to or less than 20 nanometers, large nanoinclusions of 50 nanometers-200 nanometers and small nanoinclusions of less than 50 nanometers. A nanoinclusion can have a shape roughly of a sphere, a rod, a cylinder, an ellipsoid, a plate, and the like. As used herein, “roughly” indicates that the shape of a nanoinclusion may not be perfect. In some embodiments, the nanoinclusions in the matrix have a relatively large scale with at least one dimension that is larger than 200 nanometers, or larger than 400 nanometers, or larger than 500 nanometers, or larger than 600 nanometers, or larger than 800 nanometers. In some embodiments, at least one dimension of the nanoinclusions in the matrix is larger than 1 micrometer. The nanoinclusions with relatively large scale are effective in enhancing phonon scattering, and thereby lowering lattice thermal conductivity κL and improving the thermoelectric performance of the article. In some embodiments, the nanoinclusions are dispersed in the matrix uniformly. In some embodiments, the nanoinclusions are dispersed in the matrix at some other pattern. In some embodiments, the nanoinclusions are dispersed in the matrix randomly. In some embodiments, the average number density of the nanoinclusions in a matrix is from 1 per cubic micrometer to about 200 per cubic micrometer, or from 5 per cubic micrometer to 150 per cubic micrometer, or from 10 per cubic micrometer to 120 per cubic micrometer, or from 20 per cubic micrometer to 100 per cubic micrometer, or from 30 per cubic micrometer to 80 per cubic micrometer, or from 40 per cubic micrometer to 60 per cubic micrometer. In some embodiments, the average number density of the nanoinclusions in a matrix is from 1 per cubic micrometer to about 10 per cubic micrometer, or from 10 per cubic micrometer to about 20 per cubic micrometer, or from 20 per cubic micrometer to about 40 per cubic micrometer, or from 40 per cubic micrometer to about 60 per cubic micrometer, or from 60 per cubic micrometer to about 80 per cubic micrometer, or from 80 per cubic micrometer to about 100 per cubic micrometer, or higher than 100 per cubic micrometer. In some embodiments, the spacing between nanoinclusions is from 10 nanometers to 10 micrometers, or from 50 nanometers to 5 micrometers, or from 100 nanometers to 1 micrometer, or from 150 nanometers to 500 nanometers, or from 200 nanometers to 300 nanometers. In some embodiments, the spacing between nanoinclusions is from 10 nanometers to 50 nanometers, or from 50 nanometers to 100 nanometers, or from 100 nanometers to 200 nanometers, or from 200 nanometers to 400 nanometers, or from 400 nanometers to 600 nanometers, or from 600 nanometers to 800 nanometers, or from 800 nanometers to 1000 nanometers, or larger than 1000 nanometers. In some embodiments, the nanoinclusions (e.g., the size, shape, average number density) do not introduce considerable electronic doping effect to the matrix, and do not significantly affect the carrier density of the matrix. This way, the effect of the nanoinclusions on the improved thermoelectric figure of merit is due primarily to the reduced lattice thermal conductivity κL. The microstructural or nanostructural parameters of the nanoinclusions, including the size, spacing, and the like, can be controlled or adjusted by, for example, adjusting the conditions under which the article is formed. Merely by way of example, annealing time and temperature is proportional to the size growth of the nanoinclusions.
In some embodiments, the nanoinclusions introduce electronic doping effect to the matrix such that, in addition to the reduced lattice thermal conductivity κL, the carrier density is improved, and the thermoelectric figure of merit is improved. The nanoinclusions can include, for example, silver (Ag), copper (Cu) and the like, or an alloy thereof. Merely by way of example, the nanoinclusions include an alloy of silver (Ag) and a constituent composition of the matrix, e.g., selenium (Se), tellurium (Te), and the like.
Thermoelectric performance of an article of manufacture for thermoelectric application can be improved by careful control of carrier concentrations through doping. In some embodiments, the article of manufacture is doped with at least one dopant. The article can be doped with an n-type dopant, or a p-type dopant. Effective electron donor dopants (n-type dopants) include, for example, lanthanum (La), thulium (Tm), indium (In), iodine (I), and the like. Effective electron acceptor dopants (p-type dopants) include, for example, sodium (Na), potassium (K), thallium (Tl), and the like. Merely by way of example, thallium (Tl) is a good choice for p-type dopant as it can enhance zT to 1.5 in bulk PbTe by distortion of the electronic density of states. In some embodiments, lanthanum (La)-doping effectively leads to conducting behavior dominated by degenerate charge carriers. The doping concentration can be optimized for different articles including different constituent compositions. In some embodiments, an extrinsic dopant concentration is at least 0.01 at. %, or at least 0.05 at. %, or at least 0.08 at. %, or at least 0.1 at. %, or at least 0.2 at. %, or at least 0.5 at. %, or at least 0.6 at. %, or at least 0.8 at. %, or at least 1 at. %, or at least 1.2 at. %, or at least 1.5 at. %, or at least 1.8 at. %, or at least 2 at. %, or at least 2.2 at. %, or at least 2.5 at. %, or at least 2.8 at. %, or at least 3 at. %. In some embodiments, an extrinsic dopant concentration is lower than 10 at. %, or lower than 8 at. %, or lower than 6 at. %, or lower than 5 at. %, or lower than 4 at. %, or lower than 3 at. %, or lower than 2 at. %, or lower than 1 at. %. In some embodiments, carrier density is at least 1018 per cubic centimeter, or at least 2×1018 per cubic centimeter, or at least 4×1018 per cubic centimeter, or at least 5×1018 per cubic centimeter, or at least 6×1018 per cubic centimeter, or at least 8×1018 per cubic centimeter, or at least 1019 per cubic centimeter, or at least 2×1019 per cubic centimeter, or at least 4×1019 per cubic centimeter, or at least 5×1019 per cubic centimeter, or at least 6×1019 per cubic centimeter, or at least 8×1019 per cubic centimeter, or at least 1020 per cubic centimeter, or at least 2×1020 per cubic centimeter, or at least 4×1020 per cubic centimeter, or at least 5×1020 per cubic centimeter. Merely by way of example, an optimal carrier density is 1019-1020 per cubic centimeter.
The following description regarding some exemplary embodiments of the article of manufacture is for illustration purposes, and is not intended to limit the scope of the disclosure. In some exemplary embodiments, the article of manufacture includes a matrix including PbTe and nanoinclusions including Ag2Te. The matrix has small nanoscale features of less than 20 nanometers. The nanoinclusions are of relatively larger scale. The nanoinclusions are plate-like. Some nanoinclusions have long dimensions of 100-200 nanometers and short dimensions of 50-100 nanometers. Some nanoinclusions have long dimensions of larger than 200 nanometers, some are larger than 1 micrometer. The article of manufacture has improved thermoelectric figure of merit at room temperature and at a temperature higher than room temperature. With the nanoinclusions of Ag2Te in PbTe, no overwhelming electronic doping effects are found: above 400 K the articles show intrinsic semiconductor behavior and the transition temperature from extrinsic to intrinsic conduction is independent of Ag content. The article can be doped to further improve thermoelectric transport with, for example, lanthanum (La), sodium (Na), and the like.
In some embodiments, an article of manufacture as disclosed herein has improved thermoelectric performance. The article of manufacture can have an improved thermoelectric figure or merit, and/or improved theoretically available power generation efficiency (ηmax). This can be due to the band structure complexity and nanostructured effects. As is demonstrated herein in PbTe:Na/Ag2Te, a peak thermoelectric figure of merit zT higher than 1.5 and significant enhancements of average thermoelectric figure of merit zT/thermoelectric efficiency can be realized. Further optimizing the combination of carrier density and nanostructure control can result in an even higher thermoelectric performance. PbTe:Na/Ag2Te and similar PbTe materials are described herein merely for the purpose of illustration. This is not intended to limit the scope of the disclosure. In some embodiments, an article of manufacture including PbSe or a similar composition has improved thermoelectric performance due to the band structure complexity and nanostructured effects; further optimizing the combination of carrier density and nanostructure control results in an even higher thermoelectric performance.
Some embodiments of the instant disclosure are directed to a method of manufacturing an article including: heating a first material including at least a first element and a second material including at least a second element to form a mixture; cooling the mixture to precipitate nanoinclusions including the second element; and annealing the mixture.
In some embodiment, the method of manufacturing an article includes heating the first material including at least a first element and the second material including at least a second element to form a mixture. The heating melts the first material and the second material to form a homogeneous mixture or melt at a first temperature. The first temperature is higher, at a first temperature different, than the higher of the melting temperature of the first material and that of the second material. The first temperature difference can be at least 1 K, or at least 2 K, or at least 5 K, or at least 8 K, or at least 10 K, or at least 12 K, or at least 15 K, or at least 20 K, or at least 25 K, or at least 30 K, or at least 35 K, or at least 40 K, or at least 45 K, or at least 50 K. The heating can be achieved at an essentially constant temperate increase rate. The temperate increase rate can be at least 10 K/hour, or at least 50 K/hour, or at least 80 K/hour, or at least 100 K/hour, or at least 120 K/hour, or at least 150 K/hour, or at least 180 K/hour, or at least 200 K/hour, or at least 220 K/hour, or at least 250 K/hour, or at least 280 K/hour, or at least 300 K/hour, or at least 320 K/hour, or at least 350 K/hour, or at least 380 K/hour, or at least 400 K/hour, or at least 420 K/hour, or at least 450 K/hour, or at least 480 K/hour, or at least 500 K/hour, or at least 520 K/hour, or at least 550 K/hour, or at least 580 K/hour, or at least 600 K/hour, or at least 650 K/hour, or at least 700 K/hour, or at least 750 K/hour, or at least 800 K/hour. The heating can be achieved at a variable temperate increase rate. The essentially constant or variable temperate increase rate can be achieved by controlling, for example, the rate of energy input to the heating process. In some embodiments, the heating is achieved in a closed chamber. In some embodiments, the heating is achieved at or close to the atmospheric pressure. In some embodiments, the heating is achieved under vacuum. Merely by way of example, the chamber pressure is of 10−5 torr or less. In some embodiments, the heating is achieved at a chamber pressure that is higher than the atmospheric pressure. In some embodiments, the heating lasts at least 0.1 hours, or at least 0.5 hours, or at least 1 hour, or at least 1.5 hours, or at least 2 hours, or at least 2.5 hours, or at least 3 hours, or at least 4 hours, or at least 5 hours, or at least 6 hours, or at least 7 hours, or at least 8 hours, or at least 10 hours, or at least 12 hours, or at least 15 hours, or at least 20 hours, or at least 24 hours, or at least 30 hours, or at least 36 hours, or at least 42 hours, or at least 48 hours.
In some embodiments, the first material includes a first element that forms a matrix of an article of manufacture. The first material can include more constituent compositions of the matrix. The matrix includes at least one composition selected from lead (Pb), selenium (Se), tellurium (Te), antimony (Sb), germanium (Ge), silicon (Si), tin (Sn), bismuth (Bi), arsenic (As), indium (In), thallium (Tl), and the like, or an alloy thereof. In some exemplary embodiments, the matrix includes PbTe, or PbSe. In some exemplary embodiments, the matrix includes PbSexTe1−x, wherein x represents the fraction of PbSe in the alloy of PbTe and PbSe, and can be from (and including) 0 to (and including) 1.
In some embodiments, the second material includes a second element that forms nanoinclusions of an article of manufacture. Merely by way of example, the second element includes silver (Ag) or copper (Cu). The second material can include more constituent compositions of the matrix or the nanoinclusions of an article of manufacture.
In some embodiments, the method of manufacturing an article includes cooling the mixture to precipitate nanoinclusions including the second element. The cooling is performed by contacting a coolant directly or indirectly with the mixture so that the mixture is at a second temperature. By cooling, the second element of the second material precipitates from the matrix to form nanoinclusions. The nanoinclusions include at least the second element. The nanoinclusions can further include other constituent compositions of the article of manufacture. Merely by way of example, the nanoinclusions include an alloy of the second element. As used herein, “indirectly” means that the coolant and the mixture are separated from each other by a partition, e.g., the wall of a container holding the mixture. The coolant can be at least one medium selected from a liquid (e.g., oil, water, and the like), and a gas (air, an inert gas, and the like). Merely by way of example, the cooling is achieved by cold water quenching. The second temperature is lower, at a second temperature difference, than the melting temperature of at least one of the first material and the second material. The second temperature difference can be at least 1 K, or at least 2 K, or at least 5 K, or at least 8 K, or at least 10 K, or at least 12 K, or at least 15 K, or at least 20 K, or at least 25 K, or at least 30 K, or at least 35 K, or at least 40 K, or at least 45 K, or at least 50 K, or at least 80 K, or at least 100 K, or at least 150 K, or at least 200 K, or at least 250 K, or at least 300 K, or at least 350 K, or at least 400 K, or at least 450 K, or at least 500 K, or at least 550 K, or at least 600 K. The cooling can be achieved at an essentially constant temperate decrease rate. The temperate decrease rate can be at least 10 K/hour, or at least 50 K/hour, or at least 80 K/hour, or at least 100 K/hour, or at least 120 K/hour, or at least 150 K/hour, or at least 180 K/hour, or at least 200 K/hour, or at least 220 K/hour, or at least 250 K/hour, or at least 280 K/hour, or at least 300 K/hour, or at least 320 K/hour, or at least 350 K/hour, or at least 380 K/hour, or at least 400 K/hour, or at least 420 K/hour, or at least 450 K/hour, or at least 480 K/hour, or at least 500 K/hour, or at least 520 K/hour, or at least 550 K/hour, or at least 580 K/hour, or at least 600 K/hour, or at least 650 K/hour, or at least 700 K/hour, or at least 750 K/hour, or at least 800 K/hour. The cooling can be achieved at a variable temperate decrease rate. The essentially constant or variable temperate decrease rate can be controlled by, for example, the flow rate of the coolant.
In some embodiments, the method of manufacturing an article includes annealing the mixture. The mixture is annealed at a third temperature. The third temperature is lower, at a third temperature difference, than the lower of the melting temperature of the first material and that of the second material. The third temperature difference can be at least 1 K, or at least 2 K, or at least 5 K, or at least 8 K, or at least 10 K, or at least 12 K, or at least 15 K, or at least 20 K, or at least 25 K, or at least 30 K, or at least 35 K, or at least 40 K, or at least 45 K, or at least 50 K, or at least 80 K, or at least 100 K, or at least 150 K, or at least 200 K, or at least 250 K, or at least 300 K, or at least 350 K, or at least 400 K, or at least 450 K, or at least 500 K, or at least 550 K, or at least 600 K. In some embodiments, the annealing lasts at least 0.1 hours, or at least 0.5 hours, or at least 1 hour, or at least 1.5 hours, or at least 2 hours, or at least 2.5 hours, or at least 3 hours, or at least 4 hours, or at least 5 hours, or at least 6 hours, or at least 7 hours, or at least 8 hours, or at least 10 hours, or at least 12 hours, or at least 15 hours, or at least 20 hours, or at least 24 hours, or at least 30 hours, or at least 36 hours, or at least 42 hours, or at least 48 hours, or at least 54 hours, or at least 60 hours, or at least 66 hours, or at least 72 hours, or at least 78 hours, or at least 84 hours, or at least 90 hours, or at least 96 hours.
Operation conditions including, for example, the temperate decrease rate (or cooling rate), annealing time and temperature, and the like, or a combination thereof, can effect the microstructure or nanostructure of the article including the microstructure or nanostructure of the matrix and/or of the nanoinclusions. Merely by way of example, annealing time and temperature is proportional to the size growth of the nanoinclusions. In some embodiments, the annealing time and temperature are chosen to achieve desired the microstructure or nanostructure of the article including the microstructure or nanostructure of the matrix and/or of the nanoinclusions. In some embodiments, the annealing are repeated to further improve or adjust the microstructure or nanostructure of the article (e.g., by improving or adjusting the microstructural or nanostructural parameters of the nanoinclusions), at the same condition as the previous annealing process, or at a different condition.
In some embodiments, the method of manufacturing an article includes further cooling and/or further annealing. The cooling can be repeated at least once, at the same condition as the previous cooling process, or at a different condition. The annealing can be repeated, at the same condition as the previous annealing process, or at a different condition.
In some embodiments, the method of manufacturing an article includes doping the article with a dopant. Effective electron donor dopants (n-type dopants) include, for example, lanthanum (La), thulium (Tm), indium (In), iodine (I), and the like. Effective electron acceptor dopants (p-type dopants) include, for example, sodium (Na), potassium (K), thallium (Tl), and the like. The doping can be performed after the heating. In some embodiment, the doping are performed before the cooling. In some embodiment, the doping are performed before the annealing. In some embodiment, the doping are performed after the annealing.
An article manufactured according to the method described herein has improved thermoelectric performance, e.g., improved thermoelectric figure of merit. This can be due to the complexity of the valence band structure and nanostructure effects, as well as optimized combination of carrier density and nanostructure. The article of manufacture can have a thermoelectric figure of merit of 1 or higher. The article of manufacture can have an improved theoretically available power generation efficiency (ηmax).
A person of ordinary skill in the art, reading the instant disclosure, would know how to arrange the order and conditions for the cooling, annealing and doping processes to manufacture the described article of manufacture.
Some embodiments of the instant disclosure are directed to a method of using an article of manufacture in a thermoelectric device, wherein the article of manufacture includes a matrix and nanoinclusions, wherein the nanoinclusions are uniformly dispersed in the matrix, and wherein the article of manufacture has a thermoelectric figure of merit zT of at least 1.
In some embodiments, the method of using the article of manufacture includes applying a temperature gradient to the article of manufacture; and collecting electrical energy. In some embodiments, the method of using the article of manufacture includes applying electrical energy to the article of manufacture; and transferring heat from a first space at a first operation temperature to a second space at a second operation temperature, wherein the first operation temperature is lower than the second operation temperature.
Merely by way of example, thermoelectric modules including the article of manufacture disclosed herein are used to harness waste heat from automotive exhaust (500 K-800 K) to produce electricity and reduce CO2 emissions. The efficiency of such thermoelectric generators is determined by the temperature difference, yielding the Carnot limit, and the material efficiency.
The following examples are for illustrative purposes only and are not intended to limit the scope of the disclosure or its various embodiments in any way.
The following examples are included to demonstrate embodiments disclosed herein. It is appreciated by those of skill in the art that the methodology and compositions disclosed in the examples which follow represent methodology discovered by the inventors to function well in the practice of the disclosure, and thus can be considered to constitute particular modes for its practice. However, those of skill in the art can, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the disclosure.
The pseudo-binary phase diagram of PbTe-Ag2Te (see
Similar behavior in the PbTe-Sb2Te3 phase diagram can be harnessed to yield Widmanstätten precipitates of Sb2Te3 in a matrix of PbTe.
Pure elements were used for the preparation of (PbTe)50−z(Ag2Te)2z/3 nanocomposites with z=0, 1, 2, 3 and 4 (correspond to the Pb-contents of 50 mol. %, 49 mol. %, 48 mol. %, 47 mol. % and 46 mol. % as shown in the original PbTe-Ag2Te phase diagram, respectively), which were labeled as Ag1.3 for x=1.3 mol. %, Ag2.7 for x=2.7 mol. %, Ag4.1 for x=4.1 mol. % and Ag5.5 for x=5.5 mol. % in the following discussion (see Table 1 for corresponding formula of (PbTe)1−x(Ag2Te)x with 0≦x≦5.5 mol. %). The purities for the starting Pb (chunk), Ag (shot with size of 2 millimeters) and Te (chunk) were 99.999% or higher (all from Alfa Aesar). The mixture of the elements was loaded into a quartz ampoule followed by sealing under vacuum with a chamber pressure of 10−5 ton or less. The ampoule was subsequently heated to 1273 K (point 1 in
Thermoelectric performance of an article of manufacture for thermoelectric application can be improved by careful control of carrier concentrations through doping. The results on (PbTe)1−x(Ag2Te), nanocomposites indicate that this system is suitable for studying and optimizing thermoelectric transport in PbTe nanocomposites. First, with the addition of Ag2Te in PbTe, no overwhelming electronic doping effects were found: above 400 K the samples showed intrinsic semiconductor behavior and the transition temperature from extrinsic to intrinsic conduction is independent of Ag content. The concentration of Ag2Te resulting in nano-precipitates (nanoinclusions) can be adjusted independently from the dopant. Pb/Sb, Pb and the nominally charge balanced NaSb(Bi)Te2 and AgSbTe2 can cause significant electronic doping in PbTe. See, for example, J. P. Heremans, C. M. Thrush and D. T. Morelli, Phys. Rev. B, 70, 115334, (2004); J. P. Heremans, C. M. Thrush and D. T. Morelli, J. Appl. Phys., 98, 063703, (2005); J. R. Sootsman, H. Kong, C. Uher and J. J. D. W. P. H. T. C. G. Kanatzidis, Angew. Chem. Int. Ed., 47, 8618, (2008); M. Zhou, J. Li and T. Kita, J. Am. Chem. Soc., 130, 4527, (2008); and A. Gueguen, P. F. P. Poudeu, C. Li, S. Moses, C. Uher, J. He, V. Dravid and K. M., Chem. Mater., 21, 1683, (2009), each of which is hereby incorporated by reference. PbS(SnTe) composites with PbTe can also be intrinsic, or can be extrinsically doped with PbI2. See, for example, J. Androulakis, C. Lin, H. Kong, C. Uher, C. Wu, T. Hogan, B. A. Cook, T. Caillat, K. M. Paraskevopoulos and M. G. Kanatzidis, J. Am. Chem. Soc., 129, 9780, (2007), which is hereby incorporated by reference. Second, by having minimal electronic contribution to the thermal conductivity, the thermal conductivity reduction due to nanoparticles (nanoinclusions) in PbTe was demonstrated: the lattice contribution to the thermal conductivity was effectively reduced in the whole temperature range (see
Both p- and n-type optimized PbTe thermoelectric materials can be doped with an extrinsic dopant concentration corresponding to an optimal carrier concentration of 1019-1020 per cubic centimeter. See, for example, I. B. Cadoff and E. Miller, Thermoelectric Materials and Devices. Reinhold Publishing Corporation, New York: Reinhold, 1960, which is hereby incorporated by reference. Effective electron donor dopants in PbTe include La, Tm, In and I, whereas electron acceptors include Na, K, and Tl. More discussion can be found at, for example, I. B. Cadoff and E. Miller, Thermoelectric Materials and Devices. Reinhold Publishing Corporation, New York: Reinhold, 1960; D. L. Partin, J. Appl. Phys., 57, 1997, (1985); B. A. Akimov, E. N. Korobeinikova, L. I. Ryabova, and M. E. Tamm, Sov. Phys. Semicond., 25, 208, (1991); Y. Gelbstein, Z. Dashevsky and M. P. Dariel, Physica B, 363, 196, (2005); and J. P. Heremans, V. Jovovic, E. S. Toberer, A. Saramat, K. Kurosaki, A. Charoenphakdee, S. Yamanaka and G. J. Snyder, Science, 321, 554, (2008), each of which is hereby incorporated by reference. Tl can be a good choice for p-type dopant as it can enhance zT to ˜1.5 in bulk PbTe by distortion of the electronic density of states. See, for example, J. P. Heremans, V. Jovovic, E. S. Toberer, A. Saramat, K. Kurosaki, A. Charoenphakdee, S. Yamanaka and G. J. Snyder, Science, 321, 554, (2008), which is hereby incorporated by reference.
One challenge for controlling the doping in PbTe in the presence of Ag2Te is the possibility that substitutional Ag at Pb sites or Ag interstitials can compensate other donors. For example, Sb is an n-type dopant when it substitutes Pb in PbTe, but in the presence of Ag2Te, Sb readily forms compensated AgSbTe2, which is present as nanoparticles (nanoinclusions) or dissolves into PbTe. Either mechanism reduces the doping effectiveness of Sb.
The phase purity, homogeneity, and microstructure were examined by x-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) equipped with an energy dispersive spectrometer (EDS). A JEOL 2010F TEM operated at 200 kV was used for the transmission electron microscopy, electron diffraction, and energy dispersive x-ray spectroscopy (EDS, Oxford Inc.) studies. To prepare specimens for TEM, material was mechanically thinned then dimpled in a Gatan 656 Dimple Grinder. Final thinning was conducted using low energy Ar ion milling (Fischione 1010) at cryogenic temperatures. Atom probe tomography was utilized to analyze the compositional homogeneity in the matrix using a LEAP® (Imago Scientific Instruments). This analysis used needle-like specimens with tip diameters of less than 100 nanometers, which was achieved by processing in a FEI Nova600 Dual beam FIB equipped with an “Xtreme Access” micro-manipulator as previously described in, for example, G. B. Thompson, H. L. Fraser and M. K. Miller, Ultramicroscopy, 100, 25, (2004), which is hereby incorporated by reference.
Four compositions of (PbTe)1−x(Ag2Te), were considered here (x=1.3 mol. %, 2.7 mol. %, 4.1 mol. %, 5.5 mol. %), all of which had compositions (Table 1) greater than the solubility limit for Ag2Te at the annealing temperature (770 K). After melting (step 1 in
The high temperature structure of alpha-Ag2Te (α-Ag2Te) contains a face-centered-cubic-like arrangement of Te with Ag cations distributed among a variety of interstitial sites. At 415 K, a slight structural distortion occurs and the β (beta) phase with a monoclinic cell is formed. The lattice mismatch between the Ag2Te (both modifications) and PbTe crystal structures is close to 2%, suggesting these precipitates (i.e. nanoinclusions) can be semi-coherent and oriented with respect to the matrix. Electron diffraction (
To develop robust models for the thermal conductivity, the microstructural geometry was examined. SEM and TEM images indicated that the majority of the precipitates in samples with more than 1.3 mol. % Ag2Te were plate-like with long dimensions of 100-200 nanometers and short dimensions of 50-100 nanometers. A small fraction of the population observed have at least one dimension as large as 1 micrometer. In the Ag5.5 sample, the average number density of the short plate-like precipitates is 55±30 per cubic micrometer based on TEM observations for which an area density was measured and the thickness of the TEM foil was approximated by measuring distinct features at various tilt angles in the TEM. This gave an inter-precipitate (inter-nanoinclusion) distance of 250 nanometers. The solid-state precipitation mechanism was supported by the observation that only a few precipitates (nanoinclusions) formed in the quickly quenched ingot of the Ag5.5 sample before the final anneal at 773 K.
(PbTe)1−x(Ag2Te)x samples discussed in Example 2 and Example 3 were further examined for the electronic and thermal transport properties.
Thermal conductivity measurements, shown in
The (PbTe)1−x(Ag2Te)x samples all showed reduced phonon (lattice) thermal conductivity κL compared to a typical doped PbTe thermoelectric material. This effect can be attributed both to alloy scattering in the PbTe solid solution matrix and to boundary scattering from the nano-precipitates (nanoinclusions). The Ag1.3 sample, which had a low concentration of nanoparticles, has reduced lattice thermal conductivity that largely agrees with that predicted by the Debye-Callaway model (
The x=1.3 mol. % data (Ag1.3) agreed well with the alloy model because only very few Ag2Te particles (nanoinclusions) were present, allowing this composition to be treated as a single-phase alloy. However, for greater Ag content, the thermal conductivity continued to drop, which were simply explained with the expected alloy effect. In the pseudo-binary (PbTe)1−x(Ag2Te)x system, compositions of x>1 mol. % falls in the two-phase coexistence region for temperatures lower than 750 K (
The experimental lattice thermal conductivity in the Ag2Te—PbTe nanocomposites was clearly lower than that expected by a composite of alloys, and even below that expected if the interfaces had infinite Kapitza resistance, indicating that the thermal conductivity of the PbTe alloy matrix was reduced by at least one additional mechanism. This reduced κL arose both from the alloy scattering in the PbTe matrix and the scattering of long mean-free path phonons at the interfaces of the precipitates. Most model calculations and experimental studies have investigated extremely fine (<20 nm) nano-sized particles distributed in a matrix phase to reduce the lattice thermal conductivity. See, for example, Y. K. Koh, C. J. Vineis, S. D. Calawa, M. P. Walsh and D. G. Cahill, Appl. Phys. Lett., 94, 153101, (2009); W. Kim, J. Zide, A. Gossard, D. Klenov and S. Stemmer, Phys. Rev. Lett., 96, 045901, (2006); G. H. Zhu, H. Lee, Y. C. Lan, X. W. Wang, G. Joshi, D. Z. Wang, J. Yang, D. Vashaee, H. Guilbert, A. Pillitteri, M. S. Dresselhaus, G. Chen and Z. F. Ren, Phys. Rev. Lett., 102, 196803, (2009); K. F. Hsu, S. Loo, F. Guo, W. Chen, J. S. Dyck, C. Uher, T. Hogan, E. K. Polychroniadis and M. G. Kanatzidis, Science, 303, 818, (2004); B. Poudel, Q. Hao, Y. Ma, Y. Lan, A. Minnich, B. Yu, X. Yan, D. Wang, A. Muto, D. Vashaee, X. Chen, J. Liu, M. S. Dresselhaus, G. Chen and Z. Ren, Science, 320, 634, (2008); D. M. Rowe, Thermoelectrics Handbook: Macro To Nano. CRC/Taylor & Francis, Boca Raton, 2006; and M. S. Dresselhaus, G. Chen, Z. F. Ren, G. Dresselhaus, A. Henry and J.-P. Fleurial, JOM, Journal of the Minerals, Metals and Materials Society, 61, 86, (2009), each of which is hereby incorporated by reference. The traditional room temperature estimate of the phonon mean free path is only 3 nanometers in PbTe, leading to this emphasis on small nanoparticles, and nanoparticle spacing of the order 10 nanometers. However, a more detailed analysis shows that 50% and 10% of the heat in PbTe at room temperature is carried by the phonons having mean free paths greater than 42 nanometers and 860 nanometers respectively. Indeed, a ˜20% reduction in phonon thermal conductivity can be expected in PbTe by scattering those phonons with mean free paths greater than ˜200 nanometers (the inter-precipitate distances are ˜250 nanometers in the samples here). The total thermal conductivity was reduced in the entire measured temperature range as shown in
The experimental thermal conductivity was also very close to that expected for a supersaturated solid solution of (PbTe)1−x(Ag2Te)x using the Debye-Callaway model described above (not shown in
La was added as an n-type dopant. More description about La dopant can be found at, for example, K. Ahn, C. Li, C. Uher and M. G. Kanatzidis, Chem. Mater., 21, 1361, (2009), which is hereby incorporated by reference. The La-doped series with composition of (PbLazTe1+z)0.945(Ag2Te)0.055 were identified as La1 for z=1%, La2 for z=2%, La3 for z=3% and La4 for z=4% (see Table 1 for corresponding formula of (Pb1−yLayTe)1−x(Ag2Te)x). The dashed arrow in
Electrical resistivity and Hall effect were measured by the Van der Pauw technique. A reversible magnetic field of 2 T was used for the Hall effect measurement in the temperature range of 300-675 K. The Hall carrier density was obtained by nH=1/eRH, where RH is the Hall coefficient and e is the electron charge. Seebeck coefficient was obtained by measuring the thermal power under a temperature gradient of 10 K using chromel-niobium thermocouples. Consistent results were obtained both from measurements utilizing constant temperature gradient and measurements at constant average temperature with more than 50 varying temperature differences between +5 K and −5 K. Thermal conductivity was obtained by the measurement of thermal diffusivity using a laser flash method (Netzsch LFA 457). All of the measurements were carried out under vacuum in the temperature range of 300 K-775 K. Heat capacity Cp was estimated using the method of Dulong-Petit with a value of 0.15 J/g·K, close to the experimental value from 150 K to 270 K. More description can be found at, for example, D. H. Parkinson and J. E. Quarrington, Proc. Phys. Soc., 67, 569, (1954), which is hereby incorporated by reference. The actual Cp value may be 10% higher at 775 K (and corresponding thermoelectric figure of merit zT 10% lower) as reported elsewhere at, for example, M. Zhou, J. Li and T. Kita, J. Am. Chem. Soc., 130, 4527, (2008), which is hereby incorporated by reference. The thermal conductivity κ was then calculated from the experimental density, heat capacity, and the thermal diffusivity. Measurement reproducibility was confirmed by the consistency of the heating and cooling thermal cycles on the same sample.
Repeated measurements were performed on re-synthesized thermoelectric composition (article of manufacture) (La0.028Pb0.972Te)0.947(Ag2Te)0.053 (identified as La3), and the samples with the same composition resulted in a thermoelectric figure of merit zT ranging from 1.5 to 1.7 at 775 K, which can be due to variations of the carrier density. Seebeck coefficient and resistivity measurements on this composition were also confirmed in the temperature range of 300 K-650 K, by using an ULVAC ZEM-3 system. The combined experimental uncertainty for the determination of thermoelectric figure of merit zT was considered to be close to 20%.
La is an n-type dopant in PbTe. More discussion about La can be found at, for example, G. T. Alekseeva, M. V. Vedernikov, E. A. Gurieva, P. P. Konstantinov, L. V. Prokofeva and Y. I. Ravich, Semiconductors, 32, 716, (1998); and K. Ahn, C. Li, C. Uher and M. G. Kanatzidis, Chem. Mater., 21, 1361, (2009), each of which is incorporated herein by reference. La is less likely to be compensated by Ag as there are no known Ag—La—Te compounds. Some charge compensation was observed La-doped PbTe alloyed with Ag metal (Pb1−xLaxTe—Ag): electron concentration in Pb0.99La0.01Te dropped from 5×1019 per cubic centimeter to 1-2×1019 per cubic centimeter when 5 at.% to 10 at.% of Ag was added. See., for example, K. Ahn, C. Li, C. Uher and M. G. Kanatzidis, Chem. Mater., 21, 1361, (2009), which is hereby incorporated by reference. It has been concluded in the Ahn reference that Pb1−xLaxTe—Ag crystallized in a NaCl-type structure without noticeable impurity phase formation, that the lattice parameter continuously decreased with increasing Ag content, and that a zT of close to 1.2 was obtained at 720 K in the (Pb1−xLaxTe—Ag) where no evidence of nanoparticles (nanoinclusions) was shown and no evidence of Ag2Te was seen.
The results of the instant disclosure showed that La was an effective n-type dopant in LaxPb1−xTe—Ag2Te, while retaining the nanocomposite structure. TEM observations on the La-doped sample La3 (
La-doped (PbTe)0.945(Ag2Te)0.055 composites (articles of manufacture) exhibited n-type heavily doped semiconducting behavior in the entire temperature range measured (
A maximum in electrical resistivity was also found at T close to 400 K in the La-doped series. This feature exhibited a hysteresis between heating up and cooling down measurements, and strengthens as the Ag2Te concentration was increased. This may result from the beta (β)→alpha (α) monoclinic-to-cubic phase transition of Ag2Te, in which an obvious resistivity enhancement and other electrical transport abnormalities have been reported in the literature. See, for example, F. F. Aliev, Semiconductors, 37, 1057, (2003), which is hereby incorporated by reference. However, the effect of this transition on lattice thermal conductivity and Seebeck coefficient was very weak (
The well established Pisarenko relation of Seebeck vs. carrier density assuming an acoustic phonon scattering mechanism in bulk PbTe (see, for example, Y. I. Ravich, B. A. Efimova and I. A. Smirnov, Semiconducting Lead Chalcogenides. Plenum, New York, 1970, which is hereby incorporated by reference) gave a good description of the experimental data for La-doped samples (the inset in
With the measured thermal diffusivity, density and Dulong-Petit heat capacity (Table. 1), the total thermal conductivity κ (sum of lattice thermal conductivity κL and electronic thermal conductivity κE) was determined as shown in
Comparisons of lattice thermal conductivity κL for the large nanoparticle (nanoinclusion) La2 sample with small nanoparticle (nanoinclusion) PbTe-based nanocomposites of the optimized thermoelectric compositions reported in the literature (
High thermoelectric figure of merit zT at T>650 K in La doped PbTe-Ag2Te nanocomposites with an appropriate density of relatively large (100 nanometers -200 nanometers) nanoparticles (nanoinclusions) originated from exceptionally low lattice thermal conductivity κ not found in many related systems with small (<20 nanometers) nanoparticles (nanoinclusions). Undoped PbTe-Ag2Te nanocomposites demonstrated this effect without the complication of the electronic component κE to the thermal conductivity. With the combination of independent carrier concentration optimization by La-doping, the thermoelectric figure of merit zT in La- doped PbTe-Ag2Te nanocomposites reached as high as 1.6 at 775 K. This and similar synthetic approaches, based on both considerations of the equilibrium phase diagram and precipitation kinetics as well as careful control of the dopant chemistry, can be broadly applicable to other thermoelectric material systems.
P-type PbTe/Ag2Te nanocomposites were obtained by Na-doping (PbTe:Na/Ag2Te). PbTe/Ag2Te nanocomposites with a composition of (PbTe)0.945(Ag2Te)0.055 were pre-synthesized as described above and elsewhere (for example, Y. Pei, J. Lensch-Falk, E. S. Toberer, D. L. Medlin, G. J. Snyder, Adv Funct Mater 2011, 21, 241, which is hereby incorporated by reference) and then used as starting materials for making PbTe:Na/Ag2Te together with appropriate amounts of Na and Te metals. The nominal concentration of Na (normalized to Pb) is 0˜3 at % and the final samples for this study were synthesized with the same method as described above, including sealing, melting, quenching, annealing and hot pressing. Phase components were checked using X-ray diffraction and scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS).
Hot pressed disk-shape samples with relative density of 98% or higher were used for the measurements. Details on measuring the transport properties are described above and elsewhere. See, for example, Y. Pei, A. LaLonde, S. Iwanaga, G. J. Snyder, Energ Environ Sci (2011), DOI: 10.1039/c0ee00456a; and Y. Pei, J. Lensch-Falk, E. S. Toberer, D. L. Medlin, G. J. Snyder, Adv Funct Mater 2011, 21, 241, each of which is hereby incorporated by reference. The heat capacity Cp in kB per atom was calculated to be 3.07+4.7×10−4×(T/K−300)), which can be accurate for lead chalcogenides. See, for example, R. Blachnik, R. Igel, Z Naturforsch B 1974, B 29, 625; M. Zhou, J. F. Li, T. Kita, J Am Chem Soc (2008), 130, 4527; and Y. Pei, A. LaLonde, S. Iwanaga, G. J. Snyder, Energ Environ Sci 2011, DOI: 10.1039/c0ee00456a, each of which is hereby incorporated by reference. The thermal conductivity for most of the recently reported high zT PbTe materials was determined using a heat capacity of or close to the Dulong-Petit approximation by +/−5%. See, for example, J. Heremans, V. Jovovic, E. Toberer, A. Saramat, K. Kurosaki, A. Charoenphakdee, G. J. Snyder, Science (2008), 321, 554; P. F. P. Poudeu, A. Gueguen, C. I. Wu, T. Hogan, M. G. Kanatzidis, Chem Mater (2010), 22, 1046; and Y. Pei, J. Lensch-Falk, E. S. Toberer, D. L. Medlin, G. J. Snyder, Adv Funct Mater (2011), 21, 241, each of which is hereby incorporated by reference. It should be noted that this method determines a value close to 10% higher than the Dulong-Petit law (3 kB per atom) at T>700 K. The uncertainty for each measurement of Seebeck coefficient S, and thermal conductivity κ is close to 5%, resulting in a combined error close to 20% in thermoelectric figure of merit zT-determination.
Annealing the high temperature oversaturated solid solution phase of PbTe/Ag2Te in the low temperature, two-phase region created homogenously distributed Ag2Te nanoinclusions in a PbTe matrix.
Similar to PbTe:Na (PbTe doped with Na), Na was found to be an effective p-type dopant in PbTe/Ag2Te as indicated by the Hall coefficient (RH) and Seebeck coefficient S measurements. The doping solubility of Na in PbTe/Ag2Te was found to be much smaller than in pure PbTe. The measured room temperature Hall density (pH=1/eRH, e is the electron charge) in PbTe:Na/Ag2Te was much smaller than that in PbTe:Na. The nominal concentrations of Na were comparable in PbTe:Na/Ag2Te and previous PbTe:Na. The measured hall density pH in PbTe:Na/Ag2Te did not exceed 4×1019 per cubic centimeter, while room temperature Hall density pH can be as high as 14×1019 per cubic centimeter (according to, for example, Y. Pei, A. LaLonde, S. Iwanaga, G. J. Snyder, Energ Environ Sci 2011, DOI: 10.1039/c0ee00456a, which is hereby incorporated by reference). The most heavily doped samples with room temperature Hall density pH of 2.5, 3.1 and 3.7×1019 per cubic centimeter were used for following discussions and marked as 2 .5e19, 3.1e19 and 3.7e19, respectively.
The room temperature Seebeck coefficient S versus Hall density pH (Pisarenko plot, solid curve in
The Seebeck coefficient S was flattening at high doping levels (Hall density PH>3×1019 per cubic centimeter). This can be due to the complex valence structure as shown in the inset of
Rather than a general Seebeck coefficient S being proportional to absolute temperature T, Seebeck coefficient S increased significantly (
It is reasonable to observe a reduced mobility of PbTe:Na/Ag2Te as compared with PbTe:Na, due to both the enhanced scattering of carriers at the phase boundaries and the enhanced point defect scattering in the matrix phase. More discussion can be found at, for example, F. Wald, Journal of the less-common metals (1967), 13, 579; and Y. Pei, J. Lensch-Falk, E. S. Toberer, D. L. Medlin, G. J. Snyder, Adv Funct Mater (2011), 21, 241, each of which is hereby incorporated by reference. Shown in the inset of
As can be seen in
The observed reduction in thermal conductivity κ can be, at least, partly due to the reduced electrical conductivity a and thus a reduced electronic component κE to the thermal conductivity κ as well. It is difficult to accurately estimate the κE via the Wiedemann-Franz law (κE=LσT, where L, σ and T are the Lorenz number, electrical conductivity, and absolute temperature, respectively) because of the difficulty in determining the Lorenz number L in p-PbTe due to the complex valence band structure and non-parabolicity of these bands. More discussion about these issues can be found at, for example, I. A. Smirnov, Y. I. Ravich, Sov Phys Semicond+ (1967), 1, 739; I. A. Smirnov, M. N. Vinogradova, N. V. Kolomoets, L. M. Sysoeva, Soviet Physics Solid State, Ussr (1968), 9, 2074; and Y. I. Ravich, B. A. Efimova, I. A. Smirnov, Semiconducting lead chalcogenides, Plenum Press, New York (1970), each of which is hereby incorporated by reference. For simplicity, an estimation of Lorenz number L was made using a single parabolic band (SPB) model assuming an acoustic phonon scattering mechanism, which resulted in a Lorenz number L with a deviation of less than 10% when compared to a more rigorous single non-parabolic band and multiple band model calculation. More discussion regarding the models can be found at, for example, I. A. Smirnov, M. N. Vinogradova, N. V. Kolomoets, L. M. Sysoeva, Soviet Physics Solid State, Ussr (1968), 9, 2074; C. M. Bhandari, D. M. Rowe, in CRC handbook of thermoelectrics, (Ed: D. M. Rowe), CRC Press, Boca Raton, Fla. (1995), 43; S. Ahmad, S. D. Mahanti, Phys Rev B (2010), 81, 165203; and Y. Pei, J. Lensch-Falk, E. S. Toberer, D. L. Medlin, G. J. Snyder, Adv Funct Mater (2011), 21, 241, each of which is hereby incorporated by reference.
By subtracting the electronic component κE from the total thermal conductivity κ, the obtained lattice component κL was calculated and given in
As compared with La-doping discussed above (using the same estimation of heat capacity Cp), Na-doping in PbTe/Ag2Te nanocomposites included the electronic effect of complex band structure for better electronic transport properties, therefore resulted in a significant enhancement of the thermoelectric figure of merit zT, particularly at low temperatures (
Most importantly, the current effort of combining both complex band structure and nanostructures enabled a thermoelectric figure of merit zT higher than 1.5 at T>650 K. Moreover, as shown in
Band structure complexity and nanostructured effects are simultaneously considered as an effective approach for improving thermoelectric performance. As is demonstrated in PbTe:Na/Ag2Te, a peak thermoelectric figure of merit zT higher than 1.5 and significant enhancements of average thermoelectric figure of merit zT/thermoelectric efficiency were realized. Further optimizing the combination of carrier density and nanostructure control can result in an even higher thermoelectric performance in similar PbTe materials.
As will be appreciated by one skilled in the art the elements and structures disclosed here could be used in many combinations and appreciate that these form part of the current invention.
The various methods and techniques described above provide a number of ways to carry out the application. Of course, it is to be understood that not necessarily all objectives or advantages described can be achieved in accordance with any particular embodiment described herein. Thus, for example, those skilled in the art will recognize that the methods can be performed in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objectives or advantages as taught or suggested herein. A variety of alternatives are mentioned herein. It is to be understood that some preferred embodiments specifically include one, another, or several features, while others specifically exclude one, another, or several features, while still others mitigate a particular feature by inclusion of one, another, or several advantageous features.
Furthermore, the skilled artisan will recognize the applicability of various features from different embodiments. Similarly, the various elements, features and steps discussed above, as well as other known equivalents for each such element, feature or step, can be employed in various combinations by one of ordinary skill in this art to perform methods in accordance with the principles described herein. Among the various elements, features, and steps some will be specifically included and others specifically excluded in diverse embodiments.
Although the application has been disclosed in the context of certain embodiments and examples, it will be understood by those skilled in the art that the embodiments of the application extend beyond the specifically disclosed embodiments to other alternative embodiments and/or uses and modifications and equivalents thereof.
In some embodiments, the terms “a” and “an” and “the” and similar references used in the context of describing a particular embodiment of the application (especially in the context of certain of the following claims) can be construed to cover both the singular and the plural. The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (for example, “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the application and does not pose a limitation on the scope of the application otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the application.
Preferred embodiments of this application are described herein, including the best mode known to the inventors for carrying out the application. Variations on those preferred embodiments will become apparent to those of ordinary skill in the art upon reading the foregoing description. It is contemplated that skilled artisans can employ such variations as appropriate, and the application can be practiced otherwise than specifically described herein. Accordingly, many embodiments of this application include all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the application unless otherwise indicated herein or otherwise clearly contradicted by context.
All patents, patent applications, publications of patent applications, and other material, such as articles, books, specifications, publications, documents, things, and/or the like, referenced herein are hereby incorporated herein by this reference in their entirety for all purposes, excepting any prosecution file history associated with same, any of same that is inconsistent with or in conflict with the present document, or any of same that may have a limiting affect as to the broadest scope of the claims now or later associated with the present document. By way of example, should there be any inconsistency or conflict between the description, definition, and/or the use of a term associated with any of the incorporated material and that associated with the present document, the description, definition, and/or the use of the term in the present document shall prevail.
In closing, it is to be understood that the embodiments of the application disclosed herein are illustrative of the principles of the embodiments of the application. Other modifications that can be employed can be within the scope of the application. Thus, by way of example, but not of limitation, alternative configurations of the embodiments of the application can be utilized in accordance with the teachings herein. Accordingly, embodiments of the present application are not limited to that precisely as shown and described.
This application claims priority under 35 U.S.C. §119(e) from U.S. Provisional Applications Ser. No. 61/299,830 filed Jan. 29, 2010, which is hereby incorporated by reference in its entirety.
The U.S. Government has certain rights in this invention pursuant to Grant No. W911NF-08-C-0058 awarded by the Army.
| Number | Date | Country | |
|---|---|---|---|
| 61299830 | Jan 2010 | US |
