The present disclosure relates generally to thermoelectric materials, and more particularly to a thermoelectric material including a filled skutterudite crystal structure.
Thermoelectric materials including filled skutterudite crystal structures may be used at least for power generation applications. Such materials generally include a binary skutterudite crystal structure having guest atom(s) introduced into void(s) present in the crystal structure. In an example, the binary skutterudite structure may be a cobalt arsenide material having the general formula CoAs3, a cobalt antimony material having the general formula Co4Sb12, or the like. In some instances, the binary skutterudite structure may include varying amounts of nickel and iron in place of the cobalt.
A thermoelectric material includes a filled skutterudite crystal structure having the formula GyM4X12, where G includes at least i) a rare earth element, ii) an other rare earth element, and iii) an alkaline earth element, where M is selected from cobalt, rhodium, and iridium, and X is selected from antimony, phosphorus, and arsenic. The subscript “y” refers to a crystal structure filling fraction ranging from about 0.001 to about 0.5.
Features and advantages of the present disclosure will become apparent by reference to the following detailed description and drawings, in which like reference numerals correspond to similar, though perhaps not identical, components. For the sake of brevity, reference numerals or features having a previously described function may or may not be described in connection with other drawings in which they appear.
The efficiency of a thermoelectric material is often characterized by a thermoelectric figure of merit, ZT. The figure of merit, ZT, is a dimensionless product and is defined by the formula:
where S, ρ, κ, κL, κe, and T are the Seebeck coefficient (or thermopower), electrical resistivity, total thermal conductivity, lattice thermal conductivity, electronic thermal conductivity, and absolute temperature, respectively. An efficient thermoelectric material generally possesses a combination of a high Seebeck coefficient, a low electrical resistivity, and a low thermal conductivity, and, therefore, may be classified as a material having a suitably high figure of merit, ZT. To drive the figure of merit upwards, the thermoelectric material should be formed in a manner sufficient to i) increase the Seebeck coefficient, ii) decrease the electrical resistivity, and/or iii) decrease the thermal conductivity.
Filled skutterudite structures have been discovered as being a suitable thermoelectric material that exhibits a lower lattice thermal conductivity, and thus a higher figure of merit, ZT. Such a material may include a single element filled skutterudite material such as, e.g., Ba0.24Co4Sb12. This example of the single element filled skutterudite material exhibits a figure of merit, ZT, of about 1.1 at a moderate temperature (e.g., about 850K), as shown in
The inventors of the instant application have discovered that skutterudite thermoelectric materials filled with multiple elements further reduce the lattice thermal conductivity, thereby improving the figure of merit, ZT, beyond what has been achieved with the single element filled structure noted above and, it is believed, for any thermoelectric materials reported thus far. In some examples of the instant disclosure, the skutterudite structure may be filled with at least two elements, one of which is a rare earth element. In other examples, the skutterudite structure may be filled with at least three elements, two of which are rare earth elements. Without being bound to any theory, it is believed that the lattice thermal conductivity of a rare earth element filled skutterudite structure tends to significantly reduce over a wide temperature range, as compared with binary skutterudite structures or skutterudite structures filled with an element other than a rare earth element. This reduced lattice thermal conductivity may be due, at least in part, to the substantially heavy rare earth atoms that rattle inside the interstitial voids of the skutterudite structure, thereby scattering heat-carrying low frequency phonons therein. Phonons having frequencies that are close to the resonance frequencies of the rattling element(s) tend to interact with local modes induced by the rattling element(s) and drive the lattice thermal conductivity down.
It is further believed that the lattice thermal conductivity may also be reduced by introducing guest atoms having different resonance frequencies in the skutterudite structure. As shown in
Accordingly, examples of the multiple element filled skutterudite structure, as disclosed herein, have at least one rare earth element as a guest atom. In many instances, each guest atom is also independently selected to have different phonon resonance frequencies. In an example, the phonon resonance frequencies vary by about 10 cm−1 or more. In another example, the phonon resonance frequencies vary by about 15 cm−1 or more. The examples of the multiple element filled skutterudite thermoelectric material have an average figure of merit, ZT, of at least about 1.4 and, in some cases, even up to about 2.0 at a temperature of about 800K.
The examples of the multiple element filled skutterudite thermoelectric material generally includes a skutterudite body-center-cubic structure (as shown in
The multiple element filled skutterudite material may be formed by inserting the guest atoms, G, interstitially into one or more suitably large voids in the crystal structure of a binary skutterudite compound (shown in
Non-limiting examples of rare earth elements for at least one of the guest atoms G include elements selected from the lanthanide and actinide series of the periodic table of chemical elements. Such elements may include, but are not limited to, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, actinium, thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, and lawrencium.
Additionally, non-limiting examples of alkaline earth elements for at least one of the guest atoms G include beryllium, magnesium, calcium, strontium, barium, and radium.
Furthermore, non-limiting examples of alkaline metal elements for at least one of the guest atoms G include lithium, sodium, potassium, rubidium, cesium, and francium.
Yet another example of a multiple filled skutterudite material may generally be identified by the formula AxDyEzM4X12, where A, D, and E are guest atoms G of different chemical natures. Such a thermoelectric material may be referred to as a triple element filled skutterudite material. In this example, A is a rare earth element, D is an alkaline earth element, and E is an alkali metal element, where the subscripts “x,” “y,” and “z” are crystal structure filling fractions of the elements A, D, and E, respectively. In a non-limiting example, “x,” “y,” and “z” each range from about 0.001 to about 0.2. Further, M is a metal selected from cobalt, rhodium, and iridium. In some instances, M may be doped with varying amounts of, e.g., i) nickel, palladium, and platinum, and/or ii) iron, rubidium, and osmium. Also, X is selected from a member of the pnictogen group, such as, e.g., phosphorus, arsenic, and/or antimony. In some instances, X may also be doped with varying amounts of, e.g., i) germanium and tin, and/or ii) selenium and tellurium. Such a triple element filled skutterudite material may also be doped with other n-type or p-type thermoelectric materials for use in a variety of other applications.
Another example of a multiple element filled skutterudite type thermoelectric material is also designated by the formula GyM4X12, where G includes at least i) a rare earth element, ii) another rare earth element, and iii) an alkaline earth element. In this example, M is also a metal selected from cobalt, rhodium, and iridium. Furthermore, X is a member of the pnictogen group, such as antimony, phosphorus, and arsenic. The subscript “y” refers to the crystal structure filling fraction of the guest atoms, which ranges from about 0.01 to about 0.5. In this example, the first rare earth element is different from the second rare earth element. In many cases, the first rare earth element is ytterbium and the other/second rare earth element is selected from a rare earth element other than ytterbium (non-limiting examples of which include lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, lutetium, actinium, thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, and lawrencium). The multiple filled skutterudite thermoelectric material of the instant example may be designated by the formula RxAyBzM4X12, where R is a rare earth element, A is a rare earth element other than R, and B is an alkaline earth element. In a non-limiting example, R is ytterbium and A is a rare earth element other than ytterbium. The subscripts “x,” “y,” and “z” are crystal structure filling fractions of R, A, and B, respectively, where each of “x,” “y,” and “z” ranges from about 0.01 to about 0.2. The elements A and B are also selected so that R, A and B independently have different phonon resonance frequencies. In a non-limiting example, the phonon resonance frequencies of A and B differs by about 15 cm−1. One non-limiting example of such a multiple element filled skutterudite type thermoelectric material has the formula Yb0.07La0.05 Ba0.10Co4Sb12.
Still another example of the multiple element filled skutterudite thermoelectric material is designated by the formula RwAxByCzM4X12, where R is a rare earth element, A is a rare earth element other than R, B is an alkaline earth element, and C is an alkali metal. In a non-limiting example, R is ytterbium and A is a rare earth element other than ytterbium. In the foregoing example of the multiple element filled skutterudite thermoelectric material, M is a metal selected from cobalt, rhodium, and iridium, and X is a member of the pnictogen group such as, e.g., antimony, phosphorus, or arsenic. Additionally, the subscripts “w,” “x,” “y,” and “z” are crystal structure filling fractions of R, A, B, and C, respectively, where such filling fractions range from about 0.01 to about 0.2. An example of such a thermoelectric material includes a binary skutterudite structure having voids filled with ytterbium, lanthanium, barium, and one of sodium or potassium. Again, each of the R, A, B, and C has a different phonon resonance frequency. A non-limiting example of such a multiple element filled skutterudite structure has the formula YbwLaxBayNazCo4Sb12, where the subscripts “w,” “x,” “y,” and “z” ranges from about 0.01 to about 0.2. Another non-limiting example of the multiple element filled skutterudite structure has the formula YbwLaxBayKzCo4Sb12, where the subscripts “w,” “x,” “y,” and “z” ranges from about 0.01 to about 0.2.
The several examples of the filled skutterudite thermoelectric material disclosed hereinabove may be used to make a variety of thermoelectric devices, an example of which is shown in
To further illustrate example(s) of the present disclosure, various examples are given herein. It is to be understood that these are provided for illustrative purposes and are not to be construed as limiting the scope of the disclosed example(s).
Data for several known thermoelectric materials were retrieved from literature to determine the materials' respective figure of merit, ZT, values. Such materials include single-filled skutterudite structures (Ba0.3CO3.95Ni0.05Sb12 and La0.9CoFe3Sb12), and alloys including Bi2Te3, PbTe, and SiGe. The thermoelectric figure of merit, ZT, for temperatures ranging from about 0K to about 1400K for these thermoelectric materials are shown in
Further, a sample of a multiple-filled skutterudite thermoelectric material was prepared and tested to determine its figure of merit, ZT, value. This sample was a multiple-filled skutterudite structure having the chemical formula Ba0.08Yb0.09Co4Sb12. The sample was prepared according to the method described in L. D. Chen, et al., J. Appl. Phys. 90, 1864 (2001), which is herein incorporated by reference in its entirety.
Thermal and electrical transport properties of the prepared sample were measured at temperatures ranging from about 0K to about 900K. For example, thermal diffusivity measurements were made using an Anter Flashline™ FL5000 laser flash system equipped with a six-sample carousel and an aluminum block furnace. The sample was formed into discs that were about 12.6 mm in diameter and about 1 mm in thickness for use in the Anter Flashline™ FL5000 laser flash system. Also, data related to the heating and cooling properties of the same were measured using a Netzsch Pegasus® 404 C high temperature differential scanning calorimeter (DSC). The heating and cooling data were then used to calculate the specific heat (Cρ) of the sample following an ASTM standard procedure, such as ASTM Standard E1269, “Standard Test Method for Determining Specific Heat Capacity by Differential Scanning Calorimetry,” ASTM International, West Conshohocken, Pa., 2005, which is herein incorporated by reference in its entirety.
The thermal conductivity (κ) of the sample was calculated using the equation κ=α×D×Cρ, where α is the thermal diffusivity, D is the mass density, and Cρ is the specific heat. The thermal resistivity (ρ) and the Seebeck coefficient (S) of the sample were then measured using an ULVAC ZEM-3 system. The sample was cut into 2 mm×2 mm×11 mm parallelepipeds in order to use the ULVAC ZEM-3 system.
The figure of merit, ZT, was calculated using the equation
where S is the Seebeck coefficient, T is the temperature, ρ is the thermal resistivity, and κ is the thermal conductivity. The figure of merit, ZT, over the temperature range of 0K to 1400K of the double-filled skutterudite type thermoelectric material (Ba0.08Yb0.09Co4Sb12) is shown in
As shown in
Data for several known thermoelectric materials were retrieved from literature to determine the materials' respective figure of merit, ZT, values. Such materials include single-filled skutterudite structures such as Ba0.24Co4Sb12 and Yb0.12Co4Sb12, and alloys such as Bi2Te3, PbTe, and SiGe. The thermoelectric figure of merit, ZT, for temperatures ranging from about 0K to about 1400K for these thermoelectric materials are shown in
An example of a double element filled skutterudite material (Ba0.08Yb0.09Co4Sb12), and two examples of a triple element filled skutterudite material (Ba0.08Yb0.04La0.05Co4Sb12 and Ba0.01Yb0.07La0.05Co4Sb12) were prepared according to the same preparation method as the double element filled thermoelectric material prepared above for Example 1.
The thermoelectric figure of merit, ZT, for temperatures ranging from about 0K to about 1400K for the prepared materials were calculated according to the same procedure described above for Example 1 and were plotted on the graph depicted in
While several embodiments have been described in detail, it will be apparent to those skilled in the art that the disclosed embodiments may be modified. Therefore, the foregoing description is to be considered exemplary rather than limiting.
The instant application is a continuation-in-part of co-pending U.S. application Ser. No. 12/396,875 filed Mar. 3, 2009, which claims the benefit of U.S. Provisional Application Ser. No. 61/036,715 filed Mar. 14, 2008, the contents of which are incorporated herein by reference.
This invention was made in the course of research and/or development supported by the U.S. Department of Energy, under Government Contract No. DE-FC26-04NT42278. The U.S. government has certain rights in the invention.
Number | Date | Country | |
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61036715 | Mar 2008 | US |
Number | Date | Country | |
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Parent | 12396875 | Mar 2009 | US |
Child | 12434299 | US |