Exploring Resonance Levels and Nanostructuring in the PbTe−CdTe System and Enhancement of the Thermoelectric Figure of Merit

Abstract

We explored the effect of Cd substitution on the thermoelectric properties of PbTe in an effort to test a theoretical hypothesis that Cd atoms on Pb sites of the rock salt lattice can increase the Seebeck coefficient via the formation of a resonance level in the density of states near the Fermi energy. We find that the solubility of Cd is less than previously reported, and CdTe precipitation occurs to create nanostructuring, which strongly suppresses the lattice thermal conductivity. We present detailed characterization including structural and spectroscopic data, transmission electron microscopy, and thermoelectric transport properties of samples of PbTe-x% CdTe-0.055% PbI(2) (x = 1, 3, 5, 7, 10), PbTe-1% CdTe-y% PbI(2) (y = 0.03, 0.045, 0.055, 0.08, 0.1, 0.2), PbTe-5% CdTe-y% PbI(2) (y = 0.01, 0.03, 0.055, 0.08), and PbTe-1% CdTe-z% Sb (z = 0.3, 0.5, 1, 1.5, 2, 3, 4, 5, 6). All samples follow the Pisarenko relationship, and no enhancement of the Seebeck coefficient was observed that could be attributed to a resonance level or a distortion in the density of states. A maximum ZT of approximately 1.2 at approximately 720 K was achieved for the PbTe-1% CdTe-0.055% PbI(2) sample arising from a high power factor of approximately 17 microW/(cm K(2)) and a very low lattice thermal conductivity of approximately 0.5 W/(m K) at approximately 720 K.

Evolution of Pb1-xCdxTe Solid Solution Structure at High Temperatures

“…This value matches perfectly the result given recently in Ref. [11]. The remaining Cd is still present in investigated sample but not as a constituent of the crystal phase (solid solution) but in the form of nanoclusters or small liquid droplets, which cannot be detected in the X-ray diffraction experiment (one has to mention that for bulk Cd the melting temperature is equal to T = 594 K but for Cd in the form of nanoclusters this temperature should be significantly smaller).…”

“…These results did not confirm the solubility limit known from the literature and suggested the necessity of some correction of the relevant phase diagram. Very recently, our findings were confirmed in part by the value of the solubility limit x < 0.01 [11], which was much lower than that suggested previously.…”

Thermoelectric alloys between PbSe and PbS with effective thermal conductivity reduction and high figure of merit

“…30). High zT was also found by different studies in PbTe-MTe 31-34 (M: Ca, Mg, Sr, Ba) and PbTe-CdTe, 35,36 where they were shown to have nano-scale features that are also suggested as important for good thermoelectric properties. For the less studied chalcogenides, high zT was also reported for systems such as PbSe-PbS, 37 PbSe-MSe [38][39][40] (M: Ca, Sr, Ba), and PbS-XS 41,42 (X: Ca, Sr, Zn, Cd), where similar nanostructures were also observed.…”

“…These are behaviors of solid solutions with statistically random atomic distribution. Recently many lead chalcogenides [31][32][33]35,36,39,[48][49][50][51] (including PbSe with PbS addition < 16%) are found with nanostructures on the <10 nm scale using TEM. These features on the nanoscale are not necessarily in conict with our SEM observation at larger scales.…”

Thermoelectric properties of Nb-doped SrTiO3 ceramics enhanced by potassium titanate nanowires addition

“…Very recently, we prepared Nb-doped SrTiO 3 composites by adding nanostructured yttria stabilized zirconia (YSZ) 8) and mesoporous silica (MS) 9) by the pressure-less sintering method. These additions could effectively reduce thermal conductivity by enhancing phonon scattering at grain boundaries and increase the electrical conductivity by promoting densification, and then enhanced the ZT value, which strongly triggered our interest to investigate other effective nanosized additions to further enhance the thermoelectric properties of Nbdoped SrTiO 3 .…”