[Paper Review] High thermoelectric figure of merit in p-type Half-Heuslers by intrinsic phase separation

Elisabeth Rausch, Siham Ouardi+4 more|arXiv (Cornell University)|Feb 11, 2015

Advanced Thermoelectric Materials and Devices3 citations

TL;DR

This study achieves a record ZT of 1.15 in p-type Ti₀.₂₅Hf₀.₇₅CoSb₀.₈₅Sn₀.₁₅ Half-Heusler by leveraging intrinsic micrometer-scale phase separation, which enhances phonon scattering and reduces lattice thermal conductivity. The approach avoids energy-intensive nanostructuring, offering a scalable alternative for high-performance thermoelectrics.

ABSTRACT

Improvements in the thermoelectric properties of Half-Heusler materials have been achieved by means of a micrometer-scale phase separation that increases the phonon scattering and reduces the lattice thermal conductivity. A detailed study of the p-type Half-Heusler compounds Ti(1-x)Hf(x)CoSb0.85Sn0.15 using high-resolution synchrotron powder X-ray diffraction and element mapping electron microscopy evidences the outstanding thermoelectric properties of this system. A combination of intrinsic phase separation and adjustment of the carrier concentration via Sn substitution is used to realize a record thermoelectric figure of merit for p-type Half-Heusler compounds of ZT around 1.15 at 710C in Ti0.25Hf0.75CoSb0.85Sn0.15. The phase separation approach can form a significant alternative to nanostructuring processing time, energy consumption and increasing the thermoelectric efficiency.

Motivation & Objective

• To improve the thermoelectric performance of p-type Half-Heusler materials beyond current nanostructuring methods.
• To investigate the role of intrinsic phase separation in reducing lattice thermal conductivity.
• To optimize carrier concentration via Sn substitution on the Sb site for enhanced power factor.
• To demonstrate a scalable, low-energy alternative to ball-milling and rapid consolidation for high-ZT materials.
• To correlate microstructural features with macroscopic thermoelectric properties in Ti1-xHfxCoSb0.85Sn0.15 system.

Proposed method

• Synthesized Ti1-xHfxCoSb0.85Sn0.15 ingots via arc melting and multi-step annealing to ensure homogeneity.
• Performed high-resolution synchrotron X-ray powder diffraction (XPD) to precisely determine crystal structures and phase compositions.
• Used backscattered electron imaging and energy-dispersive X-ray spectroscopy (EDX) to map elemental distribution and identify phase-separated regions.
• Applied PB-Phi(Rho-z) matrix correction for semi-quantitative EDX composition analysis with 25 kV acceleration voltage.
• Measured Seebeck coefficient, electrical conductivity, thermal diffusivity, and specific heat to calculate ZT using κ = Cpαρ.
• Conducted thermal cycling tests to verify stability and reproducibility of thermoelectric properties.

Experimental results

Research questions

• RQ1 Can intrinsic phase separation in p-type Half-Heusler materials significantly reduce lattice thermal conductivity?
• RQ2 How does Sn substitution on the Sb site affect carrier concentration and power factor in Ti1-xHfxCoSb0.85Sn0.15?
• RQ3 To what extent does phase separation enhance ZT compared to conventional nanostructuring approaches?
• RQ4 Is the phase-separated microstructure stable under thermal cycling conditions relevant to device operation?
• RQ5 Can the ZT performance of p-type Half-Heuslers match or exceed that of state-of-the-art n-type systems?

Key findings

• A record ZT of 1.15 was achieved at 710 °C in Ti₀.₂₅Hf₀.₇₅CoSb₀.₈₅Sn₀.₁₅, representing a 15% improvement over previous p-type Half-Heusler materials.
• Intrinsic micrometer-scale phase separation into Hf-rich and Ti-rich Half-Heusler phases was confirmed by high-resolution XPD and EDX mapping.
• The second phase exhibited a slight Co excess and was identified as a Heusler-type phase (L2₁ structure), likely forming coherent intermetallic precipitates.
• Phase separation led to enhanced phonon scattering, reducing lattice thermal conductivity without compromising electrical transport.
• The ZT value of 1.15 matches the performance of state-of-the-art n-type Ti₀.₅Zr₀.₂₅Hf₀.₂₅NiSn₀.₉₉₈Sb₀.₀₀₂ (ZT=1.2 at 560 °C), enabling complementary use in full thermoelectric modules.
• Thermoelectric properties remained stable over multiple heating and cooling cycles, confirming reproducibility and long-term stability.