Siam Physics Congress 2017

Thermoelectric Half Heusler phases to harvest waste heat

25 May 2017, 09:50

25m

Ballroom 1

Invited Speaker Condensed Matter Physics A6: Condensed Matter and Accelerators

Speaker

Prof. Peter Rogl (1Christian Doppler Laboratory for Thermoelectricity at the Institute of Materials Chemistry and Research, University of Vienna)

Description

Thermoelectric materials have the ability to convert a heat flow into an electrical charge carrier flow (energy flow). TiNiSn-based thermoelectric half-Heusler phases (noncentrosymmetric, cubic MgAgAs-type) have proven to be excellent thermoelectrics. The large-scale production and particularly nanostructuring of materials by preferably system-inherent phases need a profound knowledge not only of isothermal phase relations, temperature dependent solubilities but also of the solidification behavior.
The present paper covers a detailed experimental investigation of the constitution of the [Ti,Zr]-Ni-Sn systems including a liquidus projection and a Scheil diagram, as well as a CALPHAD calculation of the entire constitution diagrams. For the binary systems TiNiSn-ZrNiSn and TiNiSn-HfNiSn, thermodynamic spinodal/binodal curves have been determined from solubility data and DFT calculations. With the relevant elastic moduli also the corresponding critical points of coherent spinodal demixing were calculated.
From a series of compounds Ti${}_{\text{1-x-y}}$Zr${}_{\text{x}}$Hf${}_{\text{y}}$NiSn and respective Sn/Sb substituted alloys the thermoelectric behavior was characterized in the temperature range from 4.2 to 875 K. Due to a particularly low thermal conductivity at a high Seebeck coefficient, Hf-containing n-type alloy reached a figure of merit ZT = 1.5 at 850 K whereas for Hf-free, n-type Ti${}_{\text{0.5}}$Zr${}_{\text{0.5}}$NiSn${}_{\text{0.98}}$Sb${}_{\text{0.02}}$ a ZT ~ 1.2 at 850 K was found yielding an thermoelectric leg-efficiency of $\eta$ ~ 11$$. These excellent TE data on spinodally demixed alloys prove that Hf-free and therefore cheap half-Heusler alloys can be promising materials for a large scale production route. The experimental data (backed by SEM/TEM analyses) are compared with DFT calculations and discussed.