Atom probe tomography of a Cu-doped TiNiSn thermoelectric material : nanoscale structure and optimization of analysis conditions

Funding: The Oxford Atom Probe facility is funded by EPSRC (EP/M022803/1) and the Glasgow plasma focused ion beam system was funded by EPSRC grant EP/P001483/1. Thermoelectric materials were developed under joint EPSRC grants EP/N017218/1 and EP/N01717X/1.Cu-doping and crystallographic site occupations within the half-Heusler (HH) TiNiSn, a promising thermoelectric material, have been examined by atom probe tomography. In particular, this investigation aims to better understand the influence of atom probe analysis conditions on the measured chemical composition. Under a voltage-pulsing mode, atomic planes are clearly resolved and suggest an arrangement of elements in-line with the expected HH (F-43m space group) crystal structure. The Cu dopant is also distributed uniformly throughout the bulk material. For operation under laser-pulsed modes, the returned composition is highly dependent on the selected laser energy, with high energies resulting in the measurement of excessively high absolute Ti counts at the expense of Sn and in particular Ni. High laser energies also appear to be correlated with the detection of a high fraction of partial hits, indicating nonideal evaporation behavior. The possible mechanisms for these trends are discussed, along with suggestions for optimal analysis conditions for these and similar thermoelectric materials.PostprintPeer reviewe

Substitution Versus Full-Heusler Segregation in TiCoSb

Half-Heuslers (HHs) are promising thermoelectric materials with great compositional flexibility. Here, we extend work on the p-type doping of TiCoSb using abundant elements. Ti0.7V0.3Co0.85Fe0.15Sb0.7Sn0.3 samples with nominal 17.85 p-type electron count were investigated. Samples prepared using powder metallurgy have negative Seebeck values, S ≤ −120 µV K−1, while arc-melted compositions are compensated semiconductors with S = −45 to +30 µV K−1. The difference in thermoelectric response is caused by variations in the degree of segregation of V(Co0.6Fe0.4)2Sn full-Heusler and Sn phases, which selectively absorb V, Fe, and Sn. The segregated microstructure leads to reduced lattice thermal conductivities, κlat = 4.5−7 W m−1 K−1 near room temperature. The largest power factor, S2/ρ = 0.4 mW m−1 K−2 and ZT = 0.06, is observed for the n-type samples at 800 K. This works extends knowledge regarding suitable p-type dopants for TiCoSb

Electronic scattering in half-Heusler thermoelectrics from resistivity data

Funding: The EPSRC (EP/N01717X/1) and Leverhulme Trust (RPG-2020-177) are acknowledged for funding. The STFC is acknowledged for access to the Materials Characterisation Laboratory at the Rutherford Appleton Laboratory.A key part of optimising thermoelectric materials is understanding the electronic scattering mechanism. For half-Heusler (HH) thermoelectrics, the dominant mechanisms are acoustic phonon scattering in pure systems and alloy scattering in highly alloyed systems. In this report, the significance of the residual resistivity ρ0 is highlighted. Large ρ0 values can lead to misidentification of the dominant scattering mechanism when only high-temperature ρ(T) data is available. A straightforward approach to analyse ρ(T) is proposed and applied to a range of HH systems. This reveals large levels of structural disorder in XIVNiSn, whilst XVFeSb has the strongest coupling with acoustic phonons. The electronic scattering mechanism depends sensitively on composition, with acoustic (ρ ~ T1.5), metallic (~T1) and alloy (~T0.5) scattering observed within the main HH families. With the aid of velocity of sound, band mass and carrier concentration data, the deformation potential can be obtained, enabling quantification of the interaction between phonons and carriers, from fits to resistivity data. This work provides a route for the analysis of experimental ρ(T) data that can be applied to a range of thermoelectric materials.Publisher PDFPeer reviewe

Compositions and thermoelectric properties of XNiSn (X = Ti, Zr, Hf) half-Heusler alloys

Neutron powder diffraction has been used to investigate the experimental compositions of single and multiphase half-Heusler samples.</p