Enhancing Thermoelectric Performance of TiNiSn Half-Heusler Compounds via Modulation Doping

Abstract

MNiSn (M = Ti, Zr, and Hf) half-Heusler (HH) compounds are widely studied n-type thermoelectric (TE) materials for power generation. Most studies focus on Zr- and Hf-based compounds due to their high thermoelectric performance. However, these kinds of compositions are not cost-effective. Herein, the least expensive alloy in this half-Heusler family-TiNiSn-is investigated. Modulation doping of half-metallic MnNiSb in the TiNiSn system is realized by using spark plasma sintering. It is found that MnNiSb dissolves into the TiNiSn matrix and forms a heavily doped Ti1-xMnxNiSn1-xSbx phase, which leads to largely enhanced carrier concentration and also slight increase of carrier mobility. As a result, the electrical conductivity and power factor of the modulation doped compounds are greatly improved. A maximum power factor of 45 X 10(-4) W K-2 m(-1) is obtained at 750 K for the modulation doping system (TiNiSn)(1-x) + (MnNiSb)(x) with x = 0.05, which is one of the highest reported values in literature for TiNiSn systems. Furthermore, the lattice thermal conductivity is also suppressed due to the enhanced phonon scattering. Beneficial from the improved power factor and suppressed lattice thermal conductivity, a peak zT of 0.63 is obtained at 823 K for x = 0.05, which is an similar to 70% increase compared to the peak zT of TiNiSn. These results highlight the potential application of inexpensive TiNiSn-based TE materials and the effectiveness of modulation doping in enhancing the TE performance of HH compounds

Manipulation of Ni Interstitials for Realizing Large Power Factor in TiNiSn‐Based Materials

“…Data of TiNi 1+ z Sn, TiNi 1.06 Sn 1− z Sb z , TiNiCu z Sn, and (TiNiSn) 1− z + (MnNiSb) are from refs. .…”

“…To determine the origin of the superior PF, the Hall mobility (μ H ) of Ti 1− y Ta y Ni 0.92 Sn (labeled Ni‐deficient samples) is plotted versus carrier concentration ( n H ) in Figure d, including the data of Ni‐sufficient samples, i.e., Ti 1− y Ta y NiSn (this work, temperature‐dependent thermoelectric properties are shown in Figure S4 in the Supporting Information), TiNi 1+ z Sn, TiNi 1.06 Sn 1− z Sb z , TiNiCu z Sn, and (TiNiSn) 1− z + (MnNiSb) z . It is worth noting that the μ H of the Ni‐deficient samples shows an increase of 34–251% in comparison to the Ni‐sufficient samples.…”

“…Additionally, strategies for PF improvement by engineering intrinsic defects have been applied to various thermoelectric materials, especially p‐type NbFeSb with an ultrahigh PF . Consequently, benefiting from the successful manipulation of the Ni interstitials, decent average PF (PF avg ) has been achieved in Ni‐deficient samples, showing 38–95% improvement compared with that of Ni‐sufficient samples (Figure f) …”

“…Correspondingly, a record‐high power factor (with the peak value ≈100 µW cm −1 K −2 obtained in Ti‐doped NbFeSb‐based materials) highlights the superior electrical performance of the half‐Heusler materials . Among this class of materials, TiNiSn‐based compounds have generated much interest, although their relatively low carrier mobility (below 20 cm 2 V −1 s −1 ) and inferior power factor (≈30 µW cm −1 K −2 ) have impeded their development for high‐performance . Since their counterpart MNiSn (M = Zr, Hf) compounds commonly possess large power factor values (≈50 µW cm −1 K −2 ) resulting from higher carrier mobility (over 30 cm 2 V −1 s −1 ), it is reasonable that the power factor of TiNiSn‐based compounds can be improved via carrier mobility enhancement.…”

The Effects of Excess Co on the Phase Composition and Thermoelectric Properties of Half-Heusler NbCoSb

“…To improve phonon transport properties, various approaches have been used to enhance phonon scattering and decrease κ L , such as introducing secondary phase [ 5 ], nanoscale crystalline grain [ 6 ] and point defect [ 7 ]. To enhance electron transport properties, a series of band structure engineering approaches have been developed to tune the power factor ( S 2 σ ) by optimizing carrier concentration [ 8 ] or energy resonant doping [ 9 ].…”