Investigation of Cubic PbS/AgSbS 2 System for Thermoelectric Applications
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Investigation of Cubic PbS/AgSbS2 System for Thermoelectric Applications Iliya Todorov1, Duck-Young Chung1, and Mercouri Kanatzidis1,2 1 Materials Science Division, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL, 60439 2 Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL, 60208 ABSTRACT Investigation of a family of bulk cubic compounds with general formula AgPbmMS2+m (M = Sb, Bi) is reported. These PbS-based cubic structured quaternary systems combine a set of desirable features for efficient ZT thermoelectric materials for solar thermal power generation. AgPbmMS2+m (M = Sb, Bi) possess an average NaCl structure (Fm-3m symmetry), high melting point (991 - 1095 oC for M = Sb; 865 - 1091 ûC for M = Bi), relatively wide energy gap (0.7 > Eg (eV) > 0.39 for both Sb and Bi). The systematic variation of lattice parameters, energy gap and melting point is reported. Preliminary charge transport properties are reported along with variable temperature thermal conductivity data of selected members. INTRODUCTION As importance of thermoelectric materials as a source of energy production and conversion has been rising, significantly enhanced figures of merit ZT = (σS2/κ)T were reported during the past decade. For instance, nanostructured thin film superlattices of Bi2Te3 and Sb2Te3 grown from chemical vapor deposition have ZT ~ 2.4 at room temperature.[1] PbSe/PbTe quantum dot superlattices grown by molecular beam epitaxy is reported to exhibit ZT ~ 2.5-3 at 550 K [2, 3]. These outstanding ZT’s of superlattices primarily originate from the depressed lattice thermal conductivity of the systems. For bulk materials a ZT values of ~1.7 was reported for select members of cubic family, AgPbmSbTem+2 (specifically LAST-18 (m = 18): LAST stands for Lead Antimony Silver Tellurium), at 700 K.[4, 5] Although appropriately doped members of this family exhibit high ZT values, their operating temperature is not high enough for solar thermal power generation applications because they require materials stable at 1000 K. In an effort to develop bulk thermoelectric materials for efficient solar thermal power generation, we begun to assess the family AgPbmMS2+m (M = Sb, Bi) because of its expected considerable thermal stability at high temperature. Moreover, the S system is composed of readily available and abundant elements, and also they are much less investigated as thermoelectric materials compared to PbTe-base materials. We initiated systematic investigations of the fundamental physical properties of a whole family of quaternary cubic system, AgPbmMS2+m (M = Sb, Bi). We report here the structural features examined by powder and single crystal X-ray diffraction and high resolution TEM, thermal analysis, band gap measurements as well as preliminary thermoelectric property (electrical conductivity, thermopower, and thermal conductivity) measurements for a large number of members in this system in the undoped state.
EXPERIMENT The starting materials, PbS and AgMS2 (M = Sb, Bi), were prepar
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