Patent Publication Number: US-2011056531-A1

Title: Method for enhancing the performance of thermoelectric materials by irradiation-processing

Description:
TECHNICAL FIELD 
     The field to which the disclosure relates generally includes thermoelectric material processing and, in particular, to the enhancement of thermoelectric materials by irradiation processing. 
     BACKGROUND 
     Neutron and ion irradiation of materials causes defects that can affect material properties. 
     SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
     A method for enhancing thermoelectric properties in a thermoelectric material may be based on creating a large density of phonon-scattering sites by incorporating nanometer size internal defects in the thermoelectric material by irradiating the material by neutrons or other neutral or charged particles, or electromagnetic radiation (gamma or x-rays). 
     Other exemplary embodiments of the invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while disclosing exemplary embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a schematic drawing of a process for irradiating a thermoelectric material to induce nano-scale defects and additional grain boundaries in accordance with an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following description of the embodiment(s) is merely exemplary (illustrative) in nature and is in no way intended to limit the invention, its application, or uses. 
     The exemplary embodiments, as shown in  FIG. 1 , describe a process of irradiating a thermoelectric material  8  using an irradiation device  16  to form an irradiated thermoelectric material  10  having improved thermoelectric properties. The thermoelectric material  8 , prior to irradiation, may include grain boundaries  12 . In one embodiment, the thermoelectric material  10  after irradiation may include new grain boundaries  13  in addition to grain boundaries  12 . In another embodiment, the irradiated thermoelectric material  10  may also have other beneficial material defects including nanometer length scale (nano-scale) size defects  14 , or features  14 , that may be located at the existing grain boundaries  12 , at the new grain boundaries  13 , and/or in the interior of the grains constituting the irradiated thermoelectric material  10 . 
     The enhancement of performance of the thermoelectric material  10  by irradiation as described above may manifest itself in a variety of engineering advantages when applied to specific devices, but in general may be expected to improve the materials thermoelectric figure of merit (ZT), which itself depends upon other material properties. These other material properties may include the Seebeck coefficient (S), electrical resistivity (ρ) and thermal conductivity (κ), such that ZT=S 2 T/κρ, where T is temperature. 
     Among the potential mechanisms by which radiation may enhance the material&#39;s ZT is a reduction in the material&#39;s thermal conductivity κ, which could be accomplished by the formation of nanometer length scale defects or features  14 , such as those described in  FIG. 1  above. The nature of these defects  14  may include point defects, crystallographic defects (such as the new grain boundaries  13  shown in  FIG. 1 , or lattice mismatching, or twinning, etc.) caused by elastic and inelastic scattering of the irradiation with atoms in the precursor thermoelectric material  8  (i.e. the material irradiated to form material  10 ). 
     Irradiation may lead to direct or immediate creation of the nano-scale defects  14  as described above, or the nano-scale defects  14  could emerge after heat treatment from a heat treatment device  18  and/or through a mechanical treatment device  19 , which may be used in conjunction with the irradiation device  16  as shown in  FIG. 1 . The thermal or mechanical treatment may occur prior, during and/or after the radiation treatment. The nano-scale defects  14  may alternatively emerge as the result of other material processing of larger-scale irradiation-enhanced disorder as is known to those of ordinary skill in the art. 
     In one specific exemplary embodiment, the radiation used to modify the material  8  may be applied internally by incorporating specific isotopes of elements in the precursor alloy or thermoelectric material  8  that naturally undergoes radioactive decay and emits radiation spontaneously. 
     In another specific exemplary embodiment, the radiation used to modify the thermoelectric material  8  may be applied externally by irradiation of the thermoelectric material  8  that then undergoes nuclear reactions between the externally applied radiation and the nuclei, such as by neutron or other particle capture or by gamma ray absorption. 
     In either case (internally applied or externally applied), the excited nuclei subsequently undergo radioactive emissions or nuclear decay, thereby altering short range (crystal lattice) and/or long range (microstructure) material properties, thus yielding an optimized thermoelectric material  10  as illustrated above in  FIG. 1 . 
     Neutron irradiation may offer several conceptual advantages since it is expected to provide maximal penetration of the bulk material  8  (compared to charged particle or electromagnetic irradiation), causing both elastic and inelastic scattering defects  14 , even to the point of amorphization. Some of these defects  14  may be self-healing above a critical temperature, so it is anticipated that for some materials, optimal irradiation conditions may require cryogenic temperatures to freeze in the defects  14  at the necessary densities and distributions, thus yielding metastable structures  10  at the operating temperatures for the applicable thermoelectric device. 
     The source for irradiation (i.e. the irradiation device  16 ) may be selected based on the requirements of radiation type (i.e. neutron, proton, ion, gamma ray, etc.), radiation energy, and radiation flux, which ultimately depend upon the elements used to make the thermoelectric material  8  and the type of radiation induced improvements to the thermoelectric material that are desired, wherein the improvements may include transmutation or otherwise displacing atoms out of their crystal lattice sites. 
     In one exemplary embodiment for neutron irradiation, the irradiation device  16  that may be utilized is a neutron beam. In another exemplary embodiment, the irradiation device  16  may be a particle accelerator. 
     In another exemplary method for irradiation, stable atomic nuclei may be utilized in the precursor thermoelectric material  8 . Next, externally applied non-radioactively-inducing radiation may be applied to the material  8  after and during fabrication, keeping in mind that the starting chemical and isotope composition may need to be specifically altered, selected, or enriched to achieve the benefit. This irradiation may include ions and particles (neutrons, protons, electrons or photons) generated by typical accelerator or reactor technology. In this method, the radioactivity of the thermoelectric material  8  is never enhanced above natural background levels. 
     Furthermore, neutron radiation, both thermal and fast neutrons, can induce elemental transmutation, the radiological activation of a portion of the material&#39;s constituents. The transmuted elements may have a low solubility, or may even be insoluble, in their original crystalline matrix of the thermoelectric material  8 , allowing them to diffuse relatively freely through the host lattice, or diffuse sufficiently under various heat treatment from heat treatment device  18  or mechanical processing from mechanical processing device  19  (for example, mechanical devices applying pressure or subjecting the material to stress), ultimately condensing as nano-scale intragranular inclusions (defects)  14  or grain-boundary structures  12 . Additional defect transformations may occur as the transmuted species reverts to its original elemental species or it adopts a more stable isotopic form of yet another element. Even if the transmuted element remains in the original lattice as a stable isotope, like the nano-scale precipitates of transmuted elements, it represents a point defect  14  and a potential nano-scale inhomogeneity or defect that can lead to enhanced phonon scattering, and thus reduced thermal conductivity or improved thermoelectric power (Seebeck coefficient). 
     Other forms of radiation have their own advantages when it comes to potentially improving the performance of thermoelectric materials via phonon scattering from nano-scale defects  14 . In the case of charged particle beams or ion bombardment from a device  16 , defects  14  can be induced by direct ion implantation into the lattice or into inclusions, and/or the defects  14  can take the form of elongated scattering tracks created by the charged particles that could be tuned to a particular nanometer length scale based on the specific ion and kinetic energy used. In the case of photons, gamma rays, which are a high energy form of electromagnetic radiation, would be most likely to have a substantial impact on the modification and enhancement of thermoelectric materials. Although applying gamma radiation to thermoelectric materials is clearly innovative, for superconducting materials (e.g. Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O 10 ) the critical current density has been observed to improve after gamma-irradiation ( Superconductor Science  &amp;  Technology  19 (1): 151-154 January 2006). For the enhancement of thermoelectric materials, coincident gamma rays and other forms of radiation may be particularly useful. 
     In still another exemplary embodiment, more than one irradiation technology as described above may also be applied, in series or in parallel, to the precursor thermoelectric material  8 . This may also be done in combination with a sequence of thermal and/or mechanical treatments to further enhance the final product, depending upon its ultimate usage. 
     In one embodiment, the materials  8  that may have a relatively high cross section for inelastic scattering. Such exemplary materials  8  may transform during inelastic scattering, as opposed to simply creating isotopes of the same material. Further, such materials  8  may transmutate between atomic species. For example, the irradiation of a Zirconium atom may introduce an additional proton to the nucleus, therein generating a Niobium atom. Further, the irradiated material must not remain radioactive for too long after irradiation such that it is not desirable or available for use in a thermoelectric device. Other thermoelectric precursor materials may include the elements hafnium, vanadium, copper, antimony or tin. 
     One exemplary precursor alloy that may be benefit by irradiation by any of the above methods is ZrNiSn. ZrNiSn has a favorable cross-section for neutron capture. Another precursor alloy is YbAl 3 . Still other precursor alloys are filled-skutterudites. 
     These irradiated materials  10  may find application in any number of uses and devices associated with thermal management. One non-limiting exemplary use is in waste heat recovery systems for automobiles. For example, these materials  10  may be a portion of a thermoelectric device associated with a vehicles exhaust system. Other waste heat recovery systems in which these materials may be used include but are not limited to power plants, fuel cells, or any industrial infrastructure having a large amount of heat. For example, such irradiated thermoelectric material having irradiation induced defect may be used to generate electricity from an energy source such as but not limited to waste heat generate by a vehicle, power plant, fuel cell, or industrial infrastructure. 
     The above description of embodiments of the invention is merely exemplary in nature and, thus, variations thereof are not to be regarded as a departure from the spirit and scope of the invention.