Renewable and Sustainable Energy Reviews 112 (2019) 158–169 Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser Strategies for engineering phonon transport in Heusler thermoelectric T compounds ∗ ∗∗ ∗∗∗ Sadeq Hooshmand Zaferania,c, , Reza Ghomashchia,b, , Daryoosh Vashaeec,d, a School of Mechanical Engineering, University of Adelaide, SA, 5005, Australia b ARC Research Hub for Graphene Enabled Industry Transformation, University of Adelaide, Adelaide, SA, 5005, Australia c Department of Electrical and Computer Engineering, North Carolina State University, NC, 27606, United States d Department of Materials Science and Engineering, North Carolina State University, NC, 27606, United States ARTICLE INFO ABSTRACT Keywords: Thermoelectric generators, which can convert waste heat directly into electricity, are promising candidates for Thermoelectricity capturing low-grade heat and enhancing the efciency of the heat engines. This would lead to decreasing the Heusler alloys fossil fuel usage and greenhouse gas emission. Many Heusler compounds have been studied for thermoelectric Thermal conductivity application due to their desired characteristics such as sizeable thermoelectric power factor, non-toxicity, and Phonon scattering high stability over a wide temperature range. The primary restriction for Heusler thermoelectric materials has Waste heat recovery been their high lattice thermal conductivity, which reduces their thermoelectric fgure of merit. Several stra- tegies have been carried out to ameliorate this restriction by engineering the phonon transport properties. This article discusses several approaches such as bulk nanostructuring, the creation of point defects and vacancies, impurity doping, and multiphase engineering of the material structure for reducing the thermal conductivity of the Heusler compounds. The efectiveness of each of these methods depends on temperature; hence, the working temperature must be taken into account when designing the material structure and the composition to achieve the optimum performance for practical applications. 1. Introduction for energy production but also decreases waste heat emissions [8–11]. For thermoelectric materials, the dimensionless fgure of merit (zT) Climate change as a result of global warming may be traced back to is a measure for evaluating the efciency of energy conversion and is human activities, industrial processes, and greenhouse gas emission defned as: [1,2]. In this feld, the application of renewable energy and the prac- S2 tical use of waste heat are very appealing felds of study [3–6]. Ap- zT = T e+ l (1) parently, the trend is encouraging as based on the report of “interna- tional energy agency, world energy outlook 2017″, there has been a where S is the Seebeck coefcient (VK−1), σ is the electrical con- −1 −1 rapid deployment of clean energy technologies and an indirect falling ductivity (Ω m ), T is temperature (K), κe and κl are the electronic costs [7]. One of the available techniques towards improving energy and the lattice thermal conductivities (Wm−1K−1), respectively efciency is to endeavor to convert the wasted thermal energy back into [12,13]. With the advances in new thermoelectric materials, the ther- more useful energy such as electricity. Thermoelectric generators moelectric technology is growing to fnd new market space for power (TEGs), capable of converting waste heat directly into electricity, have generation, cooling, or detection and imaging applications [14–17]. already attracted worldwide attention as evidenced by the expansion of Consequently, new types of thermoelectric devices and fabrication research activities on thermoelectric materials and manufacturing of TE techniques are also being developed [18,19]. High-performance ther- modules and systems. Waste heat recovery is an environmentally moelectric materials have been promoted following new designs such as friendly method which not only reduces the consumption of fossil fuels the reduction of the thermal conductivity by using larger phonon ∗ Corresponding author. School of Mechanical Engineering, University of Adelaide, SA, 5005, Australia. ∗∗ Corresponding author. School of Mechanical Engineering, University of Adelaide, SA, 5005, Australia. ∗∗∗ Corresponding author. Department of Electrical and Computer Engineering, North Carolina State University, NC, 27606, United States. E-mail addresses: [email protected] (S. Hooshmand Zaferani), [email protected] (R. Ghomashchi), [email protected] (D. Vashaee). https://doi.org/10.1016/j.rser.2019.05.051 Received 23 September 2018; Received in revised form 3 May 2019; Accepted 22 May 2019 Available online 30 May 2019 1364-0321/ © 2019 Elsevier Ltd. All rights reserved. S. Hooshmand Zaferani, et al. Renewable and Sustainable Energy Reviews 112 (2019) 158–169 scattering via nanostructuring [20,21] and nano-inclusions [22–24] as As depicted in Fig. 2.a, in the acoustic branches, including long- well as the increase of the power factor by designing complex structures itudinal (LA) and transverse (TA) modes, the group velocities of all the [25–29], creating of resonant energy levels close to the band edges acoustic branches, Vgj = dωj/dq (j = LA, TA), are higher than that of [30], using low dimensional structures [31–33], and via carrier fltering the longitudinal optical (LO) and transverse optical (TO) modes with [34–36]. New materials based on bulk heterostructures, or nano- their velocity being nearly zero at q ≈ 0. The transverse mode makes composites, have especially taken considerations because of their sim- either a doubly degenerate or two separate branches. In each classif- plicity of manufacture and similarity with the current type of the cation, the transverse modes have lower group velocity than the long- thermoelectric devices. Even though nanostructuring techniques have itudinal ones due to the higher frequency of the longitudinal modes at a turned out to be gainful in numerous material frameworks because of specifc wave vector. In other words, the longitudinal branches gen- the inherent spectral discrepancies of the electron and phonon transport erally carry more energies in comparison to the transverse branches parameters [37], these strategies are also linked to the degradation of with the same momentum. the charge carrier mobility or have less efciency at high working In a solid, if A represents the number of atoms in a unit cell, the temperatures. Therefore, in a few materials, they have resulted in a outcome will be 3A phonon branches, in which, three branches are signifcant decrease in the power factor prompting little or no en- acoustic, and the rest are optical. Also, Table 1 shows the classifcation hancement of the fgure-of-merit [20,38,39]. of acoustic and optical branches as longitudinal and transverse with In this regard, Heusler alloys have a simple lattice structure and as their counts [61]. such phonon characteristics compared to those of the caged structures Moreover, both the acoustic and optical components of phonon like Skutterudites [40–42] and Clathrates [43–45]. Therefore, they energy propagate transversally and longitudinally within the crystalline generally have higher thermal conductivity while also a more sub- solids as schematically illustrated in Fig. 2.b. In respect to the appli- stantial thermoelectric power factor. Therefore, methods for reducing cation temperature range, the transverse acoustic (TA) (Fig. 2b) bran- the thermal conduction in Heusler based alloys are highly desired. ches have low frequency and are characteristics of low temperature Concerning material selection, it is desired to fnd compounds con- while the longitudinal acoustic (LA) (Fig. 2b) types have higher fre- taining nontoxic and eco-friendly elements with no or little rare earth quencies and are dominant at high temperature. components, which are sustainable in harsh condition and at high It is worth noting that a higher number of atoms (A) in a unit cell temperature In this feld, the Heusler compounds have attracted much can be a recipe for obtaining lower thermal conductivity. Because, attention due to their particular characteristics as non-toxic materials when the number of optical components of the phonon energy in- with magnetic or semiconducting behavior and also remarkable stabi- creases, the absorbed energy can be transferred less by the acoustic lity at various temperature intervals [46,47]. branches. In this case, more energy is attributed to the optical branches, These compounds are divided into full- and half-Heusler types which have a negligible contribution to heat transfer ascribed to their which are based on ternary intermetallic materials [48–53] with the low group velocity. Moreover, due to the limited phonon frequency stoichiometric composition of X2YZ and XYZ, respectively [12,54]. The (∼10 THz), by increasing the optical branches, the acoustic phonon full-Heusler alloys (X2YZ) have an L21 structure with fcc lattice unit frequency declines as the acoustic branches transfer only a partial cells containing four atoms as X at (1/4,1/4,1/4) and (3/4,3/4,3/4), Y amount of energy in the crystal. This fact infuences the lattice thermal at (1/2,1/2,1/2) and Z at (0,0,0) in Wyckof coordinates, and their conductivity mostly at high temperature in which the high-frequency corresponding space group is Fm-3m (No. 225) [12,54]. Also, the half- phonons are the majority of heat carriers [61]. Heusler alloys