Google Search

A5: Nanoscale and low dimensional effects

Precipitates in Ge-Sb-Te and Sn-Bi-Se materials as nanostructured thermoelectrics

Felix Fahrnbauer1, Stefan Schwarzmüller2, Philipp Urban1, Nadja Giesbrecht2, Christina Fraunhofer2, Gerald Wagner1, G. Jeffrey Snyder3 and Oliver Oeckler1
1 Leipzig University, GERMANY2 LMU Munich, GERMANY3 California Institute of Technology, UNITED STATES

Micro- and nanocscale heterostructures receive increasing attention as highly efficient thermoelectric materials. Exsolution leading to nano-precipitates is an intriguing way towards enhanced phonon scattering without significant loss of electrical conductivity.[1] Metastable Ge-Sb-Te (GST) materials exhibit a pronounced intrinsic nanostructure due to short-range defect ordering upon quenching.[2] In these compounds, one Ge atom can be substituted by two Cu atoms to a certain extent. Higher Cu contents in Cu2Ge11Sb2Te15 lead to rocksalt-type ~Ge11Sb2Te14 with agglomerated precipitates of Cu2Te. The thermoelectric figure of merit (ZT) of 1.5 at 500 °C is slightly higher than that of pure GST despite a lower electrical conductivity. Quenching GST melts with additional CoSb3 yields heterogeneous nanocomposites with skutterudite-type precipitates. Therein, GeTe partially replaces Sb in CoSb3, thus distorting the characteristic Sb4 units and changing the electronic structure.[3] For metastable (GeTe)19Sb2Te3, the precipitation of CoSb2(GeTe)0.5 improves the ZT value by an order of magnitude. Additional partial substitution of Sb by As further increases the electrical conductivity up to 80 % without a significant change of the Seebeck coefficient. This substitution also makes the system more robust concerning thermal treatment as the heating and cooling curves of the electrical properties converge despite an irreversible phase transition of the GST material at ~250 °C.

(SnSe)nBi2Se3 (~2.5 < n < 4) crystallizes analog to GST in a rocksalt-type structure and is interesting as an intermediate system between layered tellurides and chain-like sulfides. Upon quenching, a partial cation ordering and structure distortion in combination with the exsolution of SnSe leads to a heterostructure with a low thermal conductivity.

 

[1] S.N. Guin, D.S. Negi, R. Datta, K. Biswas, J. Mater. Chem. A 2014, 2, 4324.

[2] T. Rosenthal, M.N. Schneider, C. Stiewe, M. Döblinger, O. Oeckler, Chem. Mater. 2011, 23, 4349.

[3] X. Su, H. Li, Q. Guo, X. Tang, Q. Zhang, C. Uher, J. Electron. Mater. 2011, 40, 1286.