Abstract:
A thermoelectric material having enhanced Seebeck coefficient is characterized by a microstructure comprising nanoscale Pb inclusions dispersed in matrix substantially composed of PbTe. The excess Pb is obtained either by adding Pb in an amount greater than the stoichiometric amount needed to form PbTe, or by adding an additive effective to getter Te so as to produce the desired excess. The method is generally applicable to enhance thermoelectric properties of compounds of Pb, Sn or Ge, and Te, Se, or S.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/579,454, filed Jun. 14, 2005, which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD OF INVENTION  
       [0002]     This invention relates to thermoelectric materials, and more particularly thermoelectric materials composed of PbTe or the like and containing nanoscale inclusions effective to enhance the Seebeck coefficient.  
       BACKGROUND OF INVENTION  
       [0003]     Thermoelectric materials are known that convert thermal energy directly into electrical energy or vice versa. The efficiency of a thermoelectric material is characterized by “The Thermoelectric Figure of Merit,” which is defined as the square of its Seebeck coefficient times its electrical conductivity divided by its thermal conductivity. The Seebeck coefficient is a measure of the “thermoelectric pumping power”, i.e. the amount of heat that a material can pump per unit of electrical current. The electrical conductivity is a measure of electrical losses in a material, and the thermal conductivity is a measure of heat that is lost as it flows back against the heat pumped by a material.  
         [0004]     A relatively high-efficiency thermoelectric material has been proposed that includes PbSeTe/PbTe quantum dot superlattice (QDSL) structures. The combination of a relatively larger power factor with low lattice thermal conductivity provides a significant increase in the thermoelectric figure of merit for these QDSL structures compared to their bulk alloys. The proposed QDSL structures are prepared using molecular beam epitaxy or MBE. U.S. patent application Ser. No. 10/808,180, filed Mar. 24, 2004, incorporated herein by reference, describes a methodology for producing bulk PbTe-based structures having useful thermoelectric properties. The PbTe structures are prepared by grinding PbTe material to form a powder, and pressing and sintering the powder. The resulting sintered microstructure was found to contain nanometer-sized grains that enhanced the thermopower, which was attributed to the selective scattering of electrons depending on their energy.  
         [0005]     While the aforementioned thermoelectric material exhibits an enhanced Seebeck coefficient, there exists a need for still further improvement in the thermoelectric performance of materials produced by bulk processes. Moreover, it is desired to provide a method for manufacturing thermoelectric materials that does not require powder methodology.  
       SUMMARY OF THE INVENTION  
       [0006]     In accordance with this invention, a thermoelectric material has a microstructure characterized by a matrix and inclusions dispersed in the matrix. The matrix is substantially composed of a MA compound, wherein M is selected from the group consisting of lead, Pb; tin, Sn; and germanium, Ge; and A is selected from the group consisting of tellurium, Te; selenium, Se; and sulfur, S. A preferred compound is lead telluride, PbTe. The inclusions are composed of M in an amount in excess of the quantity required for forming the matrix. The excess M is obtained either by adding M in an amount greater than the stoichiometric amount needed to form MA, or by adding an additive effective to getter A so as to produce the desired M excess. The microstructure includes inclusions having maximum dimension less than 100 nanometers, referred to herein as nanoscale inclusions. Preferably, at least 50 percent of the inclusions are between 10 and 35 nanometers. The nanoscale inclusions are present in an amount up to about 33 percent by volume, preferably between 3 and 20 percent. It is found that the formation of nanoscale inclusions significantly improves the Seebeck coefficient of the material.  
         [0007]     In one aspect of this invention, a method is provided for forming a thermoelectric material composed of MA compound, wherein M is selected from the group consisting of Pb, Sn and Ge, and A is selected from the group consisting of Te, Se and S. The method comprises forming a melt of M and A wherein M is present in an amount greater than a stoichiometric amount for forming the MA compound. The melt is solidified to produce a thermoelectric material having a microstructure characterized by nanoscale inclusions dispersed in a matrix substantially composed of MA compound. The nanoscale inclusions are composed of elemental M, have a maximum dimension less than 100 nanometers, and are present in an amount up to about 33 percent by volume.  
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0008]     This invention will be further described with reference to the accompanying drawings in which:  
         [0009]      FIG. 1  is a graph showing Seebeck coefficient as a function of electron mobility for PbTe materials in accordance with this invention;  
         [0010]      FIG. 2  is a photomicrograph obtained using a scanning electron microscope and showing PbTe materials in accordance with this invention;  
         [0011]      FIG. 3  is an x-ray diffraction pattern for a PbTe material in accordance with this invention;  
         [0012]      FIG. 4  is a graph showing resistivity and Hall effect as a function of temperature for PbTe materials in accordance with this invention; and  
         [0013]      FIG. 5  is a graph showing Seebeck coefficient and Nernst coefficient as a function of temperature for PbTe materials in accordance with this invention. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0014]     In accordance with a preferred embodiment of this invention, thermoelectric material having enhanced Seebeck coefficient was formed from lead telluride, PbTe, formulations containing lead in an amount greater than the stoichiometric quantities. Preferably, excess lead was obtained by addition of a tellurium getter, such as silver, Ag. The getter addition results in excess lead over stoichiometric amounts and thereby produces nanoscale lead inclusions dispersed in a PbTe matrix. As used herein, nanoscale includions refer to inclusions charaterized by a maximum dimension less than 100 nanometers. Preferably, at least 50% by volume of the inclusions have a maximum dimenison between about 10 and 35 nanometers. It is found that thermoelectric material haivng the nanoscale inclusions exhibits a significant increase in Seebeck coefficient as compared to stoichiometric PbTe material.  
         [0015]     By way of a preferred example, a sample was prepared by adding a minor amount of Ag to a stoichiometric PbTe formulation. Pb shot, Te chips and Ag powder were added in the following proportions, based upon atomic percent, 49.5% Pb, 49.5% Te, and 1% Ag. The metals were heated above the meltng points of all consituents in an open crucible in an inert atmosphere and then quickly cooled. The resulting material is enriched with Pb realtive to stiochiometric proportions, due to a reaction between Ag and Te to form Ag 2 Te. The presence of elemental Pb was confirmed by an x-ray diffraction pattern. Also, the material was observed to exhibit superconductivity below 7° K. The Ag-containing material was charachertized by n-type conductivity and an electron concentration of 8×10 19  cm −3 . To reduce the electron concentration, an amount of 3 grams of Ag-containing material was mixed with 10 grams of PbTe chips commercially available from Alfa Corporation under catalog # 88285. The mixture was heated at 1000° C. overnight under a vacuum of &lt;1.0E-5 Torr inside of a quartz ampoule. The ampoule was slowly cooled at a rate of 0.1° C. per minute to 900° C., to solidify the semiconductor material. Thereafter further cooled to ambient temperature. The resulting material is referred to as sample  78 .  
         [0016]      FIG. 1  is a graph showing Seebeck coefficient, in microvolts per degree Kelvin, as a function of electron concentration per cubic centimeter, for PbTe materials at ambient temperature. The solid line indicates the Seebeck coefficient for conventional PbTe formed by traditional crystal growth techniques. Hall measurements were carried out on sample  78  to determine carrier concentration and Seebeck coefficient. The results are shown in  FIG. 1  for sample  78  and demonstrate a substantial increase in Seebeck coefficient. For purposes of comparison, a sample  73  having a composition similar to the above material containing 1 percent silver was prepared and cooled rapidly to prevent formation of nanoscale Pb inclusions, and showed a Seebeck coefficient similar to conventional material.  
         [0017]     To further illustrate the dependence of the Seebeck coefficient in samples containing nanoscale inclusions, additional samples  116 ,  118 B; and  118 N were prepared by the process described for sample  78 , but having varied electron conductivity. It will be understood that minor variations in tellurium vacancies during processing result in significant variations in electrical conductivity. Seebeck coefficients are shown in  FIG. 1  and are increased relative to comparable material formed without Ag addition or Pb inclusions.  
         [0018]     Sample  116  was examined by scanning electron microscope. As shown in  FIG. 2 , the SEM image reveals light colored precipitates dispersed in a gray matrix. In general, the precipitates had maximum dimensions less than 100 nanometers. It is pointed out that the dark gray craters are believed to be thermal etch pits formed during preparation of the sample for SEM imaging.  
         [0019]      FIG. 3  is an x-ray diffraction pattern of sample  116 , showing the both PbTe and elemental Pb. Peaks indicated by arrows  10  are attributed to the elemental Pb. Sample  116  was also observed to be superconducting below 7° K.  
         [0020]     In a further example of this invention, a sample of PbTe material was prepared by compacting and sintering a powder. The powder was prepared by pulverizing sample  78  using a motar and pestle to form coarse particles, and then ball milling the coarse particles to form a fine powder using a ball mill with n-heptane and zirconia balls. This powder was then isostatically pressed into two pellets, and the resulting pellets placed in a quartz ampoule. The pellets were subjected to a H 2  thermal cleaning process to remove oxygen, and the ampoule was subsequently partially backfilled with an H 2  atmosphere. The material were then sintered for 161 hours at 347° C. The resulting material is shown as samples  79  and  79   b  in  FIG. 1  and exhibited dramatically enhanced Seebeck coefficient.  
         [0021]      FIG. 4  shows electrical resistivity and Hall coefficient as a function of temperature for sample  118 .  FIG. 5  shows Seebeck coefficient and Nernst coefficient as a function temperature, also for sample  118 . It will be appreciated that the Nernst effect allows direct determination of the scattering parameter. From these values, electron mobility, density, effective mass, and scattering paramter may be calculated, see Yu. I. Ravich, B. A. Efimova, and I. A. Smirnov, Semiconducting Lead Chalcogenides (Plenum, N.Y., 1970). The resulting scattering parameter as a function of temperature, assuming a parabolic band structure, is shown in  FIG. 5 .  
         [0022]     While not wishing to be limtied to any particular theory, it is believed that, in the above examples, lead in amounts greater than stoichiometric forms precipitates or inclusion of dimensions less than 100 nm. When an electrical current is conducted through the material, the inclusions scatter electrons, thus altering the scattering parameter (the exponent of the energy dependence of the electron scattering time) and increasing the Seebeck coefficient.  
         [0023]     In the examples, the thermoelectric material was based upon PbTe compound. This invention may be employed to enhance other suitable thermoelectric compounds. In general, suitable thermoelectric compounds include compounds characterized by the formula MA, wherein M is a metal selected from Pb, Sn and Ge, and A is selected from Te, Se and S.  
         [0024]     In accordance with this invention, nanoscale inclusions are formed by an excess of M over stoichiometric amounts. As used herein, stoichiometric amounts refers to a theoretical amounts for forming MA, e.g., PbTe. Preferably, the excess M is between about 3 and 6 percent. In the examples, excess Pb was obtained by addition of a getter. As used herein, getter refers to a additive having a greater affinity for A than M. A preferred getter is silver. Alternately, excess M may be obtained by additions of M in amounts greater than stoichiometric. In general, it is believed that excess M sufficient to precipitate inclusions in an amount greater than about 3 percent by volume results in significant improvement in Seebeck coefficient. However, large additions, e.g., 33 percent, may result in formation of a continuous phase that would porduce an electrical short through the material. A preferred volume of inclusions is between about 5 and 20 percent.  
         [0025]     In the described examples, the inclusions were formed by slowly cooling the metal over an extended time to allow diffusion of the excess M and formation of the PbTe matrix. Alternately, the melt may be rapidly cooled and thereafter annealed for a time and at a temperature sufficient to diffuse the excess M to form the desired nanocscale inclusions.  
         [0026]     While this invention has been described in terms of the preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.