Elucidating the ball-milling-induced crystallization mechanism of amorphous NbCo1.1Sn via atomic-scale compositional analysis

Abstract

NbCoSn-based half-Heusler is a promising thermoelectric material candidate due to its outstanding advantages. The recently introduced nanostructure tailoring via an amorphous precursor has greatly reduced thermal conductivity. Although the nanocrystallization of amorphous NbCo1.1Sn has been observed during ball milling, its mechanism remains unclear even though it affects the microstructure of the milled powder, which is important for optimizing the final microstructure after sintering. In this study, the crystallization mechanism of the amorphous matrix induced by ball milling was investigated and the phase transformation sequence of amorphous NbCo1.1Sn-NbCo2Sn full-Heusler was verified. In the ball milling process, the introduction of oxygen and the formation of niobium oxide were observed. Through the oxygen content comparison experiment and atomicscale compositional research, i.e., atom probe tomography, it was proven that the introduced oxygen concentration determines whether the amorphous matrix crystallizes into NbCo2Sn full-Heusler or simply transforms into a Co-rich amorphous region. Furthermore, a ball milling-induced crystallization mechanism based on atomic diffusion and free volume theories in amorphous materials was suggested. In this mechanism, the free volumes become localized and some exceed the volume of the near constituent atom. Then, atomic diffusion occurs with different diffusivities depending on the atomic radius. In the Nb-depleted region originated by oxidation, the more dominant Co diffusion than Sn induces the Co-saturated region and relative Sn-depleted region. If this region reaches the chemical composition of NbCo2Sn, it transforms into NbCo2Sn full-Heusler via polymorphous crystallization through the ordering reaction in order to reach thermodynamical stability. The suggested crystallization mechanism deepens the understanding of the nature of the ball-milled feedstock and provides insight for optimizing the process to develop an ideal microstructure for realizing reduced thermal conductivity.