Panoscopically optimized thermoelectric performance of a half-Heusler/full-Heusler based in situ bulk composite Zr_0.7Hf_0.3Ni_1+xSn: an energy and time efficient way

Abstract

All scale hierarchical architecturing, matrix/inclusion band alignment and intra-matrix electronic structure engineering, the so called panoscopic approach for thermoelectric materials has been demonstrated to be an effective paradigm for optimizing high ZT. To achieve such hierarchically organized microstructures, composition engineering has been considered to be an efficient strategy. In this work, such a panoscopic concept has been extended to demonstrate for the first time in the case of half-Heusler based thermoelectric materials via a composition engineering route. A series of new off-stoichiometric n-type Zr0.7Hf0.3Ni1+xSn (0 ≤x≤ 0.10) HH compositions have been modified to derive HH(1 -x)/full-Heusler (FH)(x) composite with an all scale hierarchically modified microstructure with FH inclusions within the matrix to study the temperature dependent thermoelectric properties. The structural analysis employing XRD, FE-SEM and HR-TEM of these materials reveal a composite of HH and FH, with hierarchically organized microstructures. In such a submicron/nano-composite, the electronic properties are observed to be well optimized yielding a large power factor; α(2)σ (∼30.7 × 10(-4) W m(-1) K(-2) for Zr0.7Hf0.3Ni1.03Sn) and reduced thermal conductivity (∼2.4 W m(-1) K(-1) for Zr0.7Hf0.3Ni1.03Sn) yielding a high ZT∼ 0.96 at 773 K for composition Zr0.7Hf0.3Ni1.03Sn which is ∼250% larger than the normal HH Zr0.7Hf0.3NiSn (ZT∼ 0.27 at 773 K). The enhancement in ZT of these composites has been discussed in terms of primary electron filtering, electron injection and several phonon scattering mechanisms such as alloy scattering, point defect scattering, and grain boundary scattering. The Bergman and Fel model is used to calculate effective thermoelectric parameters of these composites for comparing the experimental results.

Phase Boundary Mapping in ZrNiSn Half-Heusler for Enhanced Thermoelectric Performance

“…Especially, this value is lower than those of the alloyed samples at~13 W m -1 K -1 for Zr 0.9 Ti 0.1 NiSn and~6 W m -1 K -1 for Zr 0.7 Ti 0.3 Ni 1.03 Sn [20,29]. The lowest lattice thermal conductivity is~3.4 W m -1 K -1 at 973 K for ZrNi 1.13 Sn 1.03 , suggesting the effective medium-to-high frequency phonon scattering [16,36,37].…”

“…Since the half-Heusler phase has superior TE properties, more studies are focusing on the boundary of the half-Heusler site. We pointed out the reported experimental solubility limit in ZrNi 1+x Sn, which is around 0:03 ≤ x ≤ 0:05 at 1100-1200 K [20][21][22]. More differently, Romaka et al found this solubility up to x = 0:3 at 1100 K [23].…”

“…With the increasing content of interstitial Ni, the ZT value increased first and decreased when the full-Heusler phase came out. The highest ZT is~0.71 at 973 K for ZrNi 1.11 Sn 1.04 , higher than those of the other Ni selfdoped ZrNi 1+x Sn (see Figure 6(b)) [20,29,35]. In fact, the sample with the full-Heusler phase (e.g., x = 0:13) also has a higher ZT than that of the sample with a low concentration of interstitial Ni (e.g., x = 0:02).…”

“…The Seebeck coefficient (S) and electrical conductivity (σ) were simultaneously measured on a commercial apparatus (ZEM-3, Advance-Riko) from room temperature to 973 K. The temperature-dependent Hall coefficient (R H ) was measured using the van-der-Pauw technique under a reversible magnetic field of 1.5 T. The Hall carrier concentration (n H ) and the Hall mobility (μ H ) were calculated via n H = 1/ðeR H Þ and μ H = R H /ρ, respectively. The thermal conductivity (κ) was calculated using κ = DαC p , where D is the volumetric density determined by the Calculation [20] This work x HH melting point…”

“…So, a nonstoichiometric AB 1+x X is generally designed. However, it is difficult to determine the excess amount of interstitial B atoms since there is a controversial solid solubility limit of B in the vacant position [20][21][22][23][24]. Recently, a convincing study on the solid solubility of Ni in the vacant position of TiNi 1+x Sn half-Heusler has been conducted by using the phase diagram technique [25].…”