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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.
