Mg3Sb2-based Zintl compound: a non-toxic, inexpensive and abundant thermoelectric material for power generation

Abstract

The deployment of thermoelectric materials for deriving an enhanced figure of merit (ZT) for power generation in inexpensive, non-toxic and relatively abundant bulk homogeneous solid relies on the extent of achieving the ''phonon-glass electron crystal'' (PGEC) characteristics. Here, a proof of principal has been established experimentally in the present work for a Zintl compound of Mg 3 Sb 2 and its derivative of isoelectronically Bi doped Bi; Mg 3 Sb 22x Bi x (0 ¡ x ¡ 0.4) alloys in Mg 3 Sb 2 . Single phase p-type Mg 3 Sb 2 compounds, with Mg and Sb powders as starting materials, have been prepared directly by spark plasma sintering (SPS) in a one step process. The structural refinements of this hexagonal Zintl compound by X-ray diffraction analysis (XRD) and high resolution transmission electron microscopy (HRTEM) investigation reveal that they are single phase devoid of any oxides or Sb precipitates. Transport measurements indicate low thermoelectric figure of merit (ZT = 0.26 at 750 K) for Mg 3 Sb 2 . However, an optimum doping of 0.2 at% with iso-electronic Bi ions at the Sb site enhances the ZT to 0.6 at 750 K, which is comparable with the present day industrial materials such as Bi based tellurides and selenides which are toxic. We note that the system becomes metal with carrier density exceeding 15 6 10 20 /cm 3 for x ¢0.25. The substantial increase in ZT in Mg 3 Sb 22x Bi x (0 ¡ x ¡ 0.4) owes to a partial decoupling of the electronic and phonon subsystem, as expected for a Zintl phase compound. While the reduction in thermal conductivity in Mg 3 Sb 22x Bi x (0 ¡ x ¡ 0.4) accounts to mass fluctuations and grain boundary scattering, the enhancement in the electronic power-factor is attributed to the presence of heavy and light bands in its valence band structure. The latter has been confirmed by means of both X-ray photo electron spectroscopy studies and first-principles density functional based calculations. These measurements established that a high figure of merit can be achieved in this class of materials with appropriate doping. Further, relative abundance of the material ingredients combined with its one step synthesis leads to a cost effective production and less toxicity makes the material an environmentally benign system for thermoelectric power generation.

Graphene boosts thermoelectric performance of a Zintl phase compound

“…8 Here, we have employed the XPS to compare the electronic structure of bare Mg 3 Sb and Mg 3 Sb 2 /GNS nanocomposites. Fig.…”

“…The elemental analysis of the samples was performed using energy dispersive spectroscopy (EDS) attached to the FE-SEM. The detail of TEM specimen preparation of Mg 3 Sb 2 has been described elsewhere 8 and same procedure has been followed for present nanocomposites.…”

“…It has a fairly large effective mass, high thermo power, low thermal conductivity but unfortunately a poor electrical conductivity which is largely the reason for the low thermoelectric performance. In our earlier report, 8 we demonstrated the increase in ZT in Mg 3 Sb 2Àx Bi x (0 # x # 0.4) alloys by isoelectronic substitution of Bi 3À on Sb 3À site. The enhancement in ZT was attributed to the increased Seebeck coefficient and reduced thermal conductivity.…”

“…as well as a low value of k. A large value for the power factor of a material requires it to possess a large effective mass (m*) and high carrier mobility m. 2 Several strategies such as doping, [3][4][5][6] solid solution alloying, [7][8][9][10] and nanostructuring/nanocomposite, [11][12][13][14][15][16] have been utilized to modify the structure, hence improving the thermoelectric properties. Such microstructural modications lead to several important mechanisms e.g.…”

“…Despite the large increase in the Seebeck coefficient and signicant reduction in the thermal conductivity, there was no signicant improvement in the electrical conductivity resulting to only moderate improvement in ZT of these materials. 8 In this work, we have adopted nanocomposite approach by incorporating graphene nanosheet (GNS), in Mg 3 Sb 2 (with high m* and low k) for further improving the ZT of this potential thermoelectric material. Combination of higher m and lower k would be important to attain superior thermoelectric performance.…”