Evidence of nanostructuring and reduced thermal conductivity in n-type Sb-alloyed SnSe thermoelectric polycrystals

Abstract

SnSe has been recently reported as an attractive thermoelectric material, with an extraordinarily high, positive, Seebeck coefficient. Here, we describe the synthesis, structural, microscopic, and thermoelectric characterization of Sn1–xSbxSe intermetallic alloys prepared by a straightforward arc-melting technique. Sb-doped tin selenide was synthesized as strongly nanostructured polycrystalline pellets. Neutron diffraction studies reveal that Sb is placed at the Sn sublattice in the crystal structure, showing concentrations as high as 30%, and generates a significant number of Sn vacancies, while the increase of the interlayer distances favors the nanostructuration. The material is nanostructured both out-of-plane in nanometer-scale layers and in-plane by ∼5 nm undulations of these layers. This nanostructuring, along with an increased amount of Sn vacancies, accounts for a reduction of the thermal conductivity, which is highly desirable for thermoelectric materials. The phonon mean free path is estimated to be on the order of 2 nm from low temperature, thermal conductivity, and specific heat, in accordance with the nanostructuration observed by high-resolution transmission electron microscopy. The thermal conductivity of SnSe is characterized by three independent techniques to assure a room temperature value of Sn0.8Sb0.2Se of κ ∼ 0.6 W/m K. The freshly prepared Sb-doped compounds exhibit an abrupt change in the type of charge carriers, leading to large, negative Seebeck coefficients, although the arc-melt synthesized pellets remain too resistive for thermoelectric applications. Cold-pressed pellets evolve to be p-type at room temperature, but reproducibly turn n-type around 500 K, with increased electrical conductivity and maximum observed figure of merit, ZT ∼ 0.3 at 908 K.

High‐Performance Thermoelectric SnSe: Aqueous Synthesis, Innovations, and Challenges

“…A summary of ZTs for SnSe‐based thermoelectric materials. a) The timeline for state‐of‐the‐art SnSe bulks thermoelectric materials,11–124,169–182 the performance achieved by solution route are circled by yellow. b) Temperature‐dependent ZT and c) corresponding peak and average ZT values for polycrystalline SnSe through different fabrication techniques 13,16,22,46,58,62,95,99,101.…”

High thermoelectric performance of rapidly microwave-synthesized Sn1−δS

“…The Sn deficiency can induce a so-called broad defect-band above the valence band, at least for the sister compound SnSe, according to ab initio calculations. 67 This would explain the reduced band-gap, the higher values of n H and m eff and the enlarged Seebeck coefficient.…”

“…66 This is a clear indication of the effect of Sn vacancies on the electronic properties of the microwave-synthesized sample. 67 The temperature dependence of the electrical resistivity is shown in Fig. 7b.…”

SnSe:Kx intermetallic thermoelectric polycrystals prepared by arc-melting

“…Applied pressure transforms SnSe into a semimetal [69]. SnSe can be doped with a wide variety of elements, including Ge [70][71][72], Sb [73], or alloyed with SnTe [8] and SnS [74].…”

“…Arc-melting produces highly nanostructured SnSe efficiently [3] and is a versatile synthesis procedure for various dopings [70,75]. This synthesis produces material with elemental ratios or doping different from the nominal amount, as either dopant, tin or selenide is easily lost to the arc [73,76]. The polycrystalline n-type SnSe pellets produced by arcmelting can reach high zT up to 1.8 [77].…”