A Correlative Study of Interfacial Segregation in a Cu-Doped TiNiSn Thermoelectric half-Heusler Alloy

The performance of thermoelectric materials depends on both their atomicscale chemistry and the nature of microstructural details such as grain boundaries and inclusions. Here, the elemental distribution throughout a TiNiCu0.1Sn thermoelectric material has been examined in a correlative study deploying atom-probe tomography (APT) and electron microscopies and spectroscopies. Elemental mapping and electron diffraction reveal two distinct types of grain boundary that are either topologically rough and meandering in profile or more regular and geometric. Transmission electron microscopy studies indicate that the Cu dopant segregates at both grain boundary types, attributed to extrusion from the bulk during hot-pressing. The geometric boundaries are found to have a degree of crystallographic coherence between neighboring grains; the rough boundaries are decorated with oxide impurity precipitates. APT was used to study the three-dimensional character of rough grain boundaries and reveals that Cu is present as discrete, elongated nanoprecipitates cosegregating alongside larger substoichiometric titanium oxide precipitates. Away from the grain boundary, the alloy microstructure is relatively homogeneous, and the atomprobe results suggest a statistical and uniform distribution of Cu with no evidence for segregation within grains. The extrusion suggests a solubility limit for Cu in the bulk material, with the potential to influence carrier and phonon transport properties across grain boundaries. These results underline the importance of fully understanding localized variations in chemistry that influence the functionality of materials, particularly at grain boundaries.

Automated summary

The performance of thermoelectric materials is influenced by both atomic-scale chemistry and microstructural details such as grain boundaries. This study used atom-probe tomography (APT) and electron microscopies and spectroscopies to examine the elemental distribution in a TiNiCu0.1Sn thermoelectric material. The research identified two types of grain boundaries, one rough and meandering and the other regular and geometric. Cu dopant was found to segregate at both types of grain boundaries, with the rough boundaries also decorated with oxide impurity precipitates. APT revealed that Cu exists as discrete, elongated nanoprecipitates cosegregating with larger substoichiometric titanium oxide precipitates in the rough grain boundaries. These findings highlight the importance of understanding localized chemistry variations, particularly at grain boundaries, in influencing material functionality.