1 Conventional Half-Heusler Alloys Advance State -of-the-Art Thermoelectric Properties

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Conventional Half-Heusler Alloys Advance State-of-the-Art

Thermoelectric Properties

Mousumi Mitra1, Allen Benton2, Md Sabbir Akhanda3, Jie Qi1, Mona Zebarjadi3,4, David J.

Singh5, S. Joseph Poon1,4+

1Department of Physics, University of Virginia, Charlottesville, VA 22904

2Department of Physics & Astronomy, Clemson University, Clemson, SC 29631

3Department of Electrical Engineering, University of Virginia, Charlottesville, VA 22904

4Department of Material Science & Eng., University of Virginia, Charlottesville, VA 22904

5Department of Physics & Astronomy, University of Missouri, Columbia, MO 65211

Abstract

Half-Heusler phases have garnered much attention as thermally stable and non-toxic

thermoelectric materials for power conversion in the mid-to-high temperature domain. The most

studied half-Heusler alloys to date utilize the refractory metals Hf, Zr, and Ti as principal

components. These alloys can quite often achieve a moderate dimensionless figure of merit, ZT,

near 1. Recent studies have advanced the thermoelectric performance of half-Heusler alloys by

employing nanostructures and novel compositions to achieve larger ZT, reaching as high as 1.5.

Herein, we report that traditional alloying techniques applied to the conventional HfZr-based half-

Heusler alloys can also lead to exceptional ZT. Specifically, we present the well-studied p-type

Hf0.3Zr0.7CoSn0.3Sb0.7 alloys, previously reported to have a ZT near 0.8, resonantly doped with less

than 1 atomic percent of metallic Al on the Sn/Sb site, touting a remarkable ZT near 1.5 at 980 K.

This is achieved through a significant increase in power factor, by ~65%, and a notable but

appreciably smaller decrease in thermal conductivity, by ~13%, at high temperatures. These

favorable thermoelectric properties are discussed in terms of a local anomaly in the density of

states near the Fermi energy designed to enhance the Seebeck coefficient, as revealed by first-

principles calculations, as well as the emergence of a highly heterogeneous grain structure that can

scatter phonons across different length scales, effectively suppressing the lattice thermal

conductivity. Consequently, the effective mass is significantly enhanced from ~ 7 to 10me within

a single parabolic band model, consistent with the result from first-principles calculations. The

discovery of high ZT in a commonly studied half-Heusler alloy obtained through a conventional

and non-complex approach opens a new path for further discoveries in similar types of alloys.

Furthermore, it is reasonable to believe that the present study will reinvigorate effort in the

exploration of high thermoelectric performance in conventional alloy systems.

+ Correspondence: sjp9x@virginia.edu

Introduction

Thermoelectric materials are essential for energy technology, particularly prospective applications

in waste heat recovery and solar thermal systems. Their importance is that they enable scalable

direct thermal to electrical energy conversion, which is crucial for many applications1-3. The

limiting factor for applications is typically the conversion efficiency, which controls how much

energy can be recovered from a given heat source. The efficiency of a thermoelectric (TE) material

is characterized by the dimensionless figure of merit ZT, ZT=S2σ/(L+e)T, where S is the

temperature-dependent Seebeck coefficient, σ is the electrical conductivity, and L is the lattice

contribution of the thermal conductivity and e is the electronic contribution of thermal

conductivity. Since the early years of thermoelectricity, researchers have found an increasing

number of materials systems that demonstrate promising thermoelectric properties, with ZT~14.

Among thermoelectric (TE) materials, half-Heusler phases (space group

F43m

) have emerged in

recent years as promising materials for large-scale thermoelectric power generation in view of

several favorable material properties5,6. Half-Heusler (HH) alloys exhibit high power factor3, 6-8,

good thermal stability, and practically non-toxicity in comparison with other state-of-the-art

thermoelectric materials. Furthermore, the materials can be produced in large quantities5, 9. The

most studied HH alloys to date belong to the RNiSn and RCoSb types, where R represents

refractory metals Hf, Zr, and Ti. Similar to other thermoelectric materials, there are two basic

approaches to enhancing the ZT of half-Heusler alloys, namely by lowering thermal conductivity

and by raising the power factor. These two TE properties are inter-related and it often poses a

challenge to simultaneously improve them. Nevertheless, advances in materials synthesis and

focused experimental investigation supported by sound physical insight and fundamental

underpinning, have led to high ZT~1.5 in microstructure refined n-type RNiSn based alloys10 and

ZT~1.4-1.5 in novel p-type ZrCoBi11, NbFeSb12, and TaFeSb13 based alloys. Such achievements

would not have been possible without the advances in nanostructuring and composition

exploration and optimization. These advances are built on the deeper understanding of the relevant

physics that inspires such methods as band structure engineering14, 15, hierarchical phonon

scattering16, nanostructure design17 and nano-grain embedment6, 18 contributing to the realization

of high-ZT HH alloys.

Current high-ZT p-type half-Heusler alloys are based on novel compositions that exploit the

physics of heavy hole band12 and high band degeneracy as well as soft phonons11, 13. For the more

conventional p-type half-Heusler alloys, some of us18 leveraged nano phases of ZrO2 throughout

the grain boundaries of p-type Hf0.3Zr0.7CoSn0.3Sb0.7 to reduce the thermal conductivity, yielding

a maximum ZT of ~0.8 at 900 K. The latter p-type alloy has had little advancement until Chen and

Ren8 designed optimal composition schemes in HfZrCoSnSb, showing that a ZT~1 can be

achieved. On the other hand, the advancements mentioned have also inspired new ways of thinking

about conventional half-Heusler thermoelectric materials. In their earlier work, Simonson et al.19

reported direct evidence of enhanced near band edge density of states in lightly doped n-type half-

Heusler alloys by exploiting the concept of resonant doping, giving rise to an enhanced Seebeck

coefficient. The finding may provide an additional opportunity for improving the performance of

p-type half-Heusler alloys. In this article, we report significant improvements in the thermoelectric

properties of a previously reported p-type Hf0.3Zr0.7CoSn0.3Sb0.7 alloy upon doping with minute

amounts of metallic Al electronically, very dissimilar to semimetallic Sb. Nonetheless, we find

that Al atoms do enter the lattice on the Sb/Sn site and, crucially, this results in a remarkable

increase in ZT from 0.8 to 1.5. This increase can be ascribed to the emergence of a sharp peak in

the density of states of a similar p-type half-Heusler alloy ZrCo(Sn0.3Sb0.7)1-xAlx, yielding

“resonant states” near the band edge. The present findings could pave the way for refocusing

traditional doping techniques to yield state-of-the-art thermoelectric performance in common

thermoelectric materials.

Experimental Procedures

Elements Hf, Zr, Co, Sn, Sb, and Al were weighted according to the nominal composition

Hf0.3Zr0.7Co(Sn0.3Sb0.7)1-xAlx (x=0, 0.005, 0.01, 0.015, 0.02). Following prior work20, a small

amount (~5%) of excess Sb was added to compensate for the evaporation of Sb during arc melting.

Since the preferential doping site of Al in Hf0.3Zr0.7Co(Sn0.3Sb0.7) is not known a priori, additional

alloys were synthesized to investigate the dopability of the (Hf,Zr) sublattice and Co sublattice.

The elements were loaded into an arc furnace in preparation for melting under the Argon

atmosphere. Because of the low concentration of Al as a dopant element, Al was pre-melted with

small amounts of Hf and Zr to ensure homogeneous dissolution of Al in the final product. The

ingot was re-melted twice to improve the overall homogeneity. Afterward, the ingots were

pulverized into fine 10-30 µm size powders, followed by consolidation using Spark Plasma

Sintering (Thermal Technologies® SPS 10-4) under an axial pressure of 50 MPa at 1073 K for 10

minutes and then at 1423 K for 5 minutes in vacuum. Details of the sample preparation were

reported in previous publications with appropriate modifications.21, 22

The crystal structure was investigated via X-ray diffraction (XRD) technique using Cu-Kα X-rays

(1468.7 eV) on a PANalytical Empyrean Diffractometer at a 3°/min scan rate. Microstructures and

elemental composition profiles of the samples were analyzed via scanning electron microscopy

(SEM) using backscattered electron (BSE) imaging on an FEI Quanta 650 operating at an

accelerating voltage of 15 keV, a spot size of 4 nm, and a working distance of approximately 10

mm. The microstructures of the half-Heusler alloys were characterized by electron backscatter

diffraction (EBSD) using a Helios UC G4 Dual Beam FIB-SEM. The samples were mounted in

nonconductive epoxy. Before EBSD, the surfaces of the samples were mechanically polished with

SiC abrasive papers of grit sizes 600, 1200, 2500, and 4000. The surface treatment was continued

by polishing with diamond polishing suspensions of 0.25 micrometer, followed by 0.05-

micrometer colloidal silica suspension. The (EBSD) images were evaluated to plot the grain size

distribution curves using the Crystal Imaging software attached to the instrument.

The temperature dependence of electrical resistivity and Seebeck coefficient were measured with

a ZEM 3 (ULVAC Riko, Japan) electrical properties measurement system. Thermal diffusivity

(D) was measured via the Laser Flash technique (LFA 457 MicroFlash System). Heat capacity,

Cp, was measured via Differential Scanning Calorimetry (Netzsch DSC 404C), and material

density (ρ) was measured using the Archimedes method. Thermal conductivity was calculated

using the relation κ = DρCν. The lattice component to the thermal conductivity was estimated by

applying the Weidemann-Franz law to the electrical part of the total thermal conductivity equation,

κL = κ - κe. The Wiedemann-Franz law represents a standard model of heat conduction through

current carriers that gives κe = LσT, where L is the Lorenz number, and σ is the electrical

conductivity. Room-temperature mobility (µ) and carrier concentration (n) was obtained via the

Hall Effect using the electrical transport option (ETO) of Versalab. Samples used for the

measurement were polished to a thickness of approximately 0.3 mm. A magnetic field (B) was

applied perpendicular to the supplied electrical current (I). In this configuration, the resistance

developed across the transverse leads (R⊥) is measured while the magnetic field (B) was varied

from 0.1 T to -0.1 T at constant I. The slope obtained from the R⊥ vs. B plot was used to estimate

the Hall coefficient (RH) using the formula RH=R

⊥

d/B. Hall carrier concentration was calculated

as n=1/eRH, and the Hall mobility was calculated as μ = RHσ.

Results

Materials Structure

The phase purity of the samples was confirmed through room temperature powder XRD. As can

be seen in Figure 1, the peaks correspond well to the established pattern18. Phase analyses of the

samples were implemented via a search-match technique on experimentally acquired peak files

using the PDF4+ database and High-score plus software. We have added the standard Bragg peaks

of the material in Figure 1. The nature of Bragg’s sharp peaks indicates the material is highly

crystalline. All the indexed peaks could be ascribed to the crystal structure of the half-Heusler

ZrCoSb phase (JCPDS 54-0448). All the samples appear to be single-phase with the same

MgAgAs-type crystallographic structure without any impurity phase within the detection limit of

XRD.

Additional XRD measurements were performed on samples doped on the Hf/Zr site and the Co

site via stoichiometric deficit on those sites. The consensus is that Al causes secondary phases and

phase separation immediately. The impurity phases Al16Co7 and Al1.4Co0.6 have appeared in the

samples, marked as star and triangle signs, which again confirms from the SEM image as discussed

in the subsequent section of the supplementary materials. Several X-ray diffraction patterns can

be seen in the Supplementary Materials (Figure S1).

Figure 1: Room Temperature powder X-ray diffraction patterns of Hf0.3Zr0.7Co(Sn0.3Sb0.7)1-xAlx

where x = 0 (blue), 0.01 (green), 0.015 (yellow), 0.02 (purple).

Since the doping ratio of Al is so tiny, any secondary phases that form would be below the

resolution limit of XRD. EDS was used to obtain the compositional profile of the sample at the

microscopic level. Figure 2a shows the SEM image of a specific region in the x = 0.015 sample.

EDS line scan through the x = 0.015 sample shows practically no spatial variability among the

elements (Figure 2b). In Figure 2c, we see general homogeneity in the elemental mapping of the

x = 0.015 samples between the elements. Based on the uniform density of bright orange dots shown

in the EDS mapping, Al atoms are distributed homogeneously in the sample. Results for the x = 0

(undoped), and x = 0.01 samples are shown in the supplementary materials (Figure S2). Both the

undoped and x=0.01 samples also exhibit composition homogeneity. However, the x = 0.02 sample

shows clear phase separation undetected by XRD. This finding indicates that no more than ~0.7

at.% Al can be doped into the Sn/Sb sublattice.

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1ConventionalHalf-HeuslerAlloysAdvanceState-of-the-ArtThermoelectricPropertiesMousumiMitra1,AllenBenton2,MdSabbirAkhanda3,JieQi1,MonaZebarjadi3,4,DavidJ.Singh5,S.JosephPoon1,4+1DepartmentofPhysics,UniversityofVirginia,Charlottesville,VA229042DepartmentofPhysics&Astronomy,ClemsonUniversity,Clemson,SC...

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