Increasing Seebeck Coefficients and Thermoelectric Performance of Sn/Sb/Te and Ge/Sb/Te Materials by Cd Doping

Abstract

Ge and Sn in (GeTe)nSb2Te3 (GST) and (SnTe)nSb2Te3 compounds, respectively, are partially substituted by Cd. These layered compounds (n = 1) consist of NaCl‐type slabs that are separated by van der Waals gaps. Resonant X‐ray diffraction yields different Cd‐atom distributions in Cd0.2Ge0.8Sb2Te4 and Cd0.2Sn0.8Sb2Te4, which indicate how van der Waals gaps in metastable cubic GeTe‐rich or SnTe‐rich compounds (n ≥ 7) are surrounded. The latter exhibit promising thermoelectric properties with figures of merit ZT ≈ 0.8 at 450 °C. Alloying with Sb2Te3 increases the ZT value of SnTe by a factor of up to two because of an increased Seebeck coefficient and a reduced thermal conductivity; the hole concentration is significantly reduced. A further improvement in ZT by ≈20% is possible by Cd‐doping, most likely due to a more covalent bonding character, yielding a ZT of ≈1.1 at 450 °C for Cd1.2Sn10.8Sb2Te15. In order to understand the electronic transport properties of these p‐type semiconductors, the single parabolic band model is applied. By contrast, Cd doping in GST materials reduces the Hall mobility and therefore decreases the power factor compared to undoped GST materials.

Electronic origin of the high thermoelectric performance of GeTe among the p-type group IV monotellurides

“…The resistivity and Hall coefficient were measured using the Van der Pauw technique under a reversible magnetic field of 1.5 T. The Seebeck coefficient was obtained from the slope of the thermopower vs temperature gradient from 0 to 5 K. 68 The thermal diffusivity (D) was measured by a laser flash technique (model: LFA-457, Netzsch, Selb, Germany). A Dulong-Petit limit of the heat capacity (C p ) is used, which is consistent with the measurements 40 for GeTe alloys and with those in many recent reports 47,69,70 The thermal conductivity is determined via κ = dC p D, where d is Table 1 Calculated band parameters for GeTe, PbTe and SnTe Thermoelectric performance of p-type GeTe J Li et al the density. All of the transport property measurements were carried out from 300 to 800 K, and the reproducibility is confirmed by multiple measurements on the same sample and in multiple samples with similar carrier concentrations.…”

Hall‐effect Measurements and Transport Properties of Heterostructures in the Model System NiTe2‐Sn12Sb2Te15

“…[25] Cations and vacancies are disordered in the NaCl-type structure of Sn 12 Sb 2 Te 15 . [8] Comparison of PXRD patterns before and after thermal treatment during the LFA measurements show neither shifts of the reflections of the NaCl-type matrix phase nor any changes concerning the side phase NiTe 2 ( Figure 1). These observations are confirmed by the results of Rietveld refinements, which yield similar lattice parameters of both phases in all samples.…”

“…This is enabled in a targeted way by means of single parabolic band (SPB) modeling [6,7] based on Halleffect data as shown for related single-phase materials such as Cd-substituted (GeTe) n Sb 2 Te 3 or (SnTe) n Sb 2 Te 3 with n = 1, 7 or 12. [8] The thermoelectric performance of SnTe can be enhanced both by increasing and by decreasing the charge carrier concentration n H by means of doping, because there are contributions from both a light and a heavy valence band. [9] In this unusual case, a two-band model can be applied to model experimental data of S and zT as a function of n H , in this case using a SPB model for the heavy and a non-parabolic Kane band model for the light band.…”

“…[18] For a better comparison of the changes with respect to the pristine matrix material, the composition Sn 12 Sb 2 Te 15 was chosen as it has been described extensively in the literature. [4,8] Experimental Section Synthesis: Bulk samples were prepared by fusing the elements Ni (Alfa, 99.999 %), Sn (Alfa Aesar, 99.999 %), Sb (Smart Elements, 99.999 %) and Te (Alfa Aesar, 99.999 %) in silica glass ampules under dry argon atmosphere at 900-950°C for 1 d, followed by quenching in air with subsequent annealing at 500-550°C for 3-4 d. The furnace was switched off afterwards to slowly cool the samples down to room temperature.…”

Cobalt germanide precipitates indirectly improve the properties of thermoelectric germanium antimony tellurides

“…In the GST class of materials, many elements are suitable for substitution, e.g. Mn, 41 Sn, 42 Cd, 43 Li, 44,45 Ag, 46 or Cr 47 replacing Ge; As 48 or In 49,50 replacing Sb, and Se 35 replacing Te. These substitutions change the electronic structure and charge carrier density and thus influence the thermoelectric performance.…”