Unveiling the phonon scattering mechanisms in half-Heusler thermoelectric compounds

Abstract

Charge-compensation-induced Frenkel defects were proven as the major reason for phonon scattering of p-type half-Heusler ZrCoSb.

High‐Pressure‐Sintering‐Induced Microstructural Engineering for an Ultimate Phonon Scattering of Thermoelectric Half‐Heusler Compounds

“…Following our previous work, these 4d lattice sites are occupied by Co atoms due to their lowest formation energy. [ 18 ] We thus verify a coexistence of half‐Heusler (HH) and intermediate‐Heusler (IH) phases, but not a full‐Heusler (FH) phase. This behavior is different from the Ni‐based compounds such as ZrNiSn where HH, IH, FH phases were observed simultaneously.…”

“…Theoretically, the respective values of α are −1 and −0.5 if phonons are mainly scattered by the intrinsic phonon–phonon interaction or point defects. [ 18 ] Besides, the boundary‐dominant phonon scattering is usually observed at cryogenic temperatures with the theoretical value of α being either 3 or 2 depending on the assumption whether the scattering rate is phonon‐frequency sensitive. [ 51 ] In some special cases such as nanowires, positive values of α are obtained at above 300 K, [ 52 ] thus manifesting the role of boundary scattering in increasing α (i.e., decreasing the |α|).…”

“…On the other hand, these compounds usually possess a large κ L that hinders their zT improvement. [ 17 ] For example, the κ L of pristine ZrCoSb is ≈21 W m −1 K −1 at 300 K, [ 18 ] much larger than that of nanostructured Bi 2 Te 3 (≈0.9 W m −1 K −1 ) [ 19 ] or nanostructured MgAgSb (≈0.6 W m −1 K −1 ). [ 20 ] Therefore, the efforts to improve the half‐Heusler compounds have mainly been focused on reducing the κ L during the past decades, where the most effective approach is to introduce 0D point defects.…”

“…[ 34–36 ] Recently, we reported that point‐defect scattering has been the major phonon scattering mechanism of half‐Heusler compounds other than the intrinsic phonon–phonon interaction, whereas scattering from other defects is less pronounced. [ 18 ] The insufficient contributions from the higher‐dimension defects (such as 2D grain boundaries) greatly attenuated the scattering intensity of phonons, especially at lower frequencies that carry the most heat.…”

“…Microscopic studies are subsequently applied to clarify the origins of the κ L reduction, which evidence a structural intricacy including a remarkable grain refinement to the sub‐100 nm level (2D defects) and a shared occupation of 4c and 4d lattice sites (0D defects). By applying the phonon transport model with a BvK–Debye dispersion, [ 18 ] we uncover a synergistic phonon scattering by these defects, thus showing the potential of high‐pressure sintering in engineering the microstructures to modify the phonon transport and improve the thermoelectric performance.…”