In Situ Evolution of Secondary Metallic Phases in Off-Stoichiometric ZrNiSn for Enhanced Thermoelectric Performance

Abstract

The full-Heusler (FH) inclusions in the half-Heusler (HH) matrix is a well-studied approach to reduce the lattice thermal conductivity of ZrNiSn HH alloy. However, excess Ni in ZrNiSn may lead to the in situ formation of FH and/or HH alloys with interstitial Ni defects. The excess Ni develops intermediate electronic states in the band gap of ZrNiSn and also generates defects to scatter phonons, thus providing additional control to tailor electronic and phonon transport properties synergistically. In this work, we present the implication of isoelectronic Ge-doping and excess Ni on the thermoelectric transport of ZrNiSn. The synthesized ZrNi1.04Sn1–x Ge x (x = 0–0.04) samples were prepared by arc-melting and spark plasma sintering, and were extensively probed for microstructural analysis. The in situ evolution of minor secondary phases, i.e., FH, Ni–Sn, and Sn–Zr, primarily observed post sintering resulted in simultaneous optimization of the electrical power factor and lattice thermal conductivity. A ZT of ∼1.06 at ∼873 K was attained, which is among the highest for Hf-free ZrNiSn-based HH alloys. Additionally, ab initio calculations based on density functional theory (DFT) were performed to provide comparative insights into experimentally measured properties and understand underlying physics. Further, mechanical properties were experimentally extracted to determine the usability of synthesized alloys for device fabrication.

Relevance of Solidification Kinetics for Enhanced Thermoelectric Performance in Al-Doped Higher Manganese Silicides

“…In this context, waste heat utilization employing thermoelectrics (TE), a heritage space technology for power generation, appears very promising and relies on the ability to attain low-cost materials with a high TE figure of merit ( zT ). , As a dimensionless parameter, zT is commonly used for determining the cumulative efficacy of thermal and electrical transport properties in materials for TE conversion and is expressed as italiczT = ( S 2 σ κ ) T , where S , σ, and κ represent the Seebeck coefficient, electrical conductivity, and thermal conductivity at the absolute temperature ( T ), respectively . High zT is desirable for efficient TE conversion, and is observed largely in expensive chalcogenide-based semiconductors constituting toxic and/or costly materials such as Bi 2 Te 3, − PbTe, − GeTe, , and SnSe. − Thus, in recent years, many inexpensive and nontoxic options, primarily in half Heuslers − and silicides − based alloys, are actively explored for TE applications.…”

A mechanistic view of defect engineered VFeSb half-Heusler alloys

“…Moreover, the presence of low melting constituent elements such as antimony and tin may cause non-homogeneity in the stoichiometry of HH alloys, as they vaporize during melting and remain in a segregated impurity phase post melting, thus making the synthesis of HH alloys highly susceptible to impurity phases. 40–46…”

“…Moreover, the presence of low melting constituent elements such as antimony and tin may cause non-homogeneity in the stoichiometry of HH alloys, as they vaporize during melting and remain in a segregated impurity phase post melting, thus making the synthesis of HH alloys highly susceptible to impurity phases. [40][41][42][43][44][45][46] Among the pre-existing HH alloys, structurally ordered cubic VFeSb alloys are a prospective n-type candidate for near-room temperature TE applications owing to their remarkably highpower factor near room temperature. [47][48][49][50][51] Interestingly, VFeSb alloys are homologous compounds of NbFeSb HH alloys for which significant progress has been reported lately using microstructural engineering and substitution.…”

Realization of Band Convergence in p-Type TiCoSb Half-Heusler Alloys Significantly Enhances the Thermoelectric Performance

“…216). The face-centered cubic (fcc) lattice can be viewed as three interpenetrated sublattices of equal unit cell size. , This family offers a wide range of chemical compositions, even though compounds with 18 valence-electron count (VEC) are the most promising for thermoelectric applications due to their narrow band gap and sharp slope in the density of states near the Fermi level. , Among HH compounds, XNiSn and XCoSb (X = Ti, Zr, Hf) have been widely explored, notably leading to n-type compositions (with X = Zr) with high thermoelectric performance. − ,− In comparison, p-type analogues based on TiCoSb exhibit lower TE performance despite various approaches employed such as doping, ,− nanostructuring, and nanocomposite …”