Reduced Thermal Conductivity in Nanostructured AgSbTe2 Thermoelectric Material, Obtained by Arc-Melting

Abstract

AgSbTe2 intermetallic compound is a promising thermoelectric material. It has also been described as necessary to obtain LAST and TAGS alloys, some of the best performing thermoelectrics of the last decades. Due to the random location of Ag and Sb atoms in the crystal structure, the electronic structure is highly influenced by the atomic ordering of these atoms and makes the accurate determination of the Ag/Sb occupancy of paramount importance. We report on the synthesis of polycrystalline AgSbTe2 by arc-melting, yielding nanostructured dense pellets. SEM images show a conspicuous layered nanostructuration, with a layer thickness of 25–30 nm. Neutron powder diffraction data show that AgSbTe2 crystalizes in the cubic Pm-3m space group, with a slight deficiency of Te, probably due to volatilization during the arc-melting process. The transport properties show some anomalies at ~600 K, which can be related to the onset temperature for atomic ordering. The average thermoelectric figure of merit remains around ~0.6 from ~550 up to ~680 K.

Optimizing Thermoelectric Properties through Compositional Engineering in Ag-Deficient AgSbTe2 Synthesized by Arc Melting

“…Although in AgSbTe 2 a bipolar contribution is evident as there is initially an increment of the Seebeck coefficient, followed by a plateau up to 700 K followed by a sharp reduction, this behavior is not visible in the measured temperature range for the nonstoichiometric samples in this study. Maximum values of the Seebeck coefficients of 157 and 209 μV K –1 were measured for Ag 0.7 Sb 1.12 Te 2 and Ag 0.7 Sb 1.12 Te 1.95 Se 0.05 , respectively, both at 750 K. These values are notably lower than the Seebeck coefficient of 340 V K –1 at 540 K reported for the arc-melted AgSbTe 2 reference . This decrease might be associated with an increase in the carrier concentration, which is consistent with the significant variation in electrical resistivity, reduced by 1 order of magnitude through compositional engineering in Ag-deficient samples (Figure b).…”

“…It is noteworthy that the typical Ag 2 Te impurity peaks are not present in either composition studied here, as confirmed by differential scanning calorimetry (DSC) analysis (see Supporting Information, section SI1). In our previous work on arc-melted AgSbTe 2 , the presence of Ag 2 Te was almost negligible in the XRD pattern, but still detectable . The low amount of impurities has been reported as a consequence of quenching the melted material in the synthesis process, which thwarts the formation of higher melting point impurities .…”

“…Thermoelectric Properties. The evolution of the electrical transport properties of Ag 0.7 Sb 1.12 Te 2 and Ag 0.7 Sb 1.12 Te 1.95 Se 0.05 with temperature is shown in Figure 6 and compared with previously reported 38 similarly synthesized stoichiometric AgSbTe 2 . A monotonic increase in the Seebeck coefficient is observed as the temperature increases (Figure 6a) in both Ag-deficient samples.…”

“…Despite the induced vacancies in the crystal structure and the reduced covalent radius of Sb (1.39 Å) vs Ag (1.45 Å), 39 the lattice parameter of nominal Ag 0.7 Sb 1.12 Te 2 increases with respect to stoichiometric AgSbTe 2 (6.0788 Å). 38 This expansion of the unit cell agrees with a reduced bonding strength, with bond length increasing from d(Ag/Sb−Te) = 3.03939 Å to d(Ag/Sb−Te) = 3.04459(4) Å, which can be a consequence of increased Sb Ag substitution and filling of antibonding states as previously described. 40 The reduced bond order can lead to reduced lattice thermal conductivity, altered bandwidth, and effective mass.…”

“…Maximum values of the Seebeck coefficients of 157 and 209 μV K −1 were measured for Ag 0.7 Sb 1.12 Te 2 and Ag 0.7 Sb 1.12 Te 1.95 Se 0.05 , respectively, both at 750 K. These values are notably lower than the Seebeck coefficient of 340 V K −1 at 540 K reported for the arcmelted AgSbTe 2 reference. 38 This decrease might be associated with an increase in the carrier concentration, which is consistent with the significant variation in electrical resistivity, reduced by 1 order of magnitude through compositional engineering in Ag-deficient samples (Figure 6b). The electrical properties of pristine AgSbTe 2 are mainly hindered by its high electrical resistivity because of deficient carrier concentration, 8 while using this vacancy-engineering strategy might produce an enhancement of the number of charge carriers, similar to what has been reported for other aliovalent doped samples.…”