Higher thermoelectric performance of Zintl phases (Eu _0.5 Yb _0.5 ) _1−x Ca _x Mg ₂ Bi ₂ by band engineering and strain fluctuation

Abstract

Complex Zintl phases, especially antimony (Sb)-based YbZn 0.4 Cd 1.6 Sb 2 with figure-of-merit (ZT) of ∼1.2 at 700 K, are good candidates as thermoelectric materials because of their intrinsic "electron-crystal, phonon-glass" nature. Here, we report the rarely studied p-type bismuth (Bi)-based Zintl phases (Ca,Yb,Eu)Mg 2 Bi 2 with a record thermoelectric performance. Phase-pure EuMg 2 Bi 2 is successfully prepared with suppressed bipolar effect to reach ZT ∼ 1. Further partial substitution of Eu by Ca and Yb enhanced ZT to ∼1.3 for Eu 0.2 Yb 0.2 Ca 0.6 Mg 2 Bi 2 at 873 K. Density-functional theory (DFT) simulation indicates the alloying has no effect on the valence band, but does affect the conduction band. Such band engineering results in good p-type thermoelectric properties with high carrier mobility. Using transmission electron microscopy, various types of strains are observed and are believed to be due to atomic mass and size fluctuations. Point defects, strain, dislocations, and nanostructures jointly contribute to phonon scattering, confirmed by the semiclassical theoretical calculations based on a modified Debye-Callaway model of lattice thermal conductivity. This work indicates Bi-based (Ca, Yb,Eu)Mg 2 Bi 2 is better than the Sb-based Zintl phases. technology that converts heat into electricity, is currently used in subsea and spacecraft (1), and is foreseen to play an important role in the power industry and automobiles (2-5). Widespread applications are currently limited as a result of the low efficiency of TE materials (6). TE efficiency depends on the Carnot term as well as the TE figure-of-merit, ZT, defined as ZT = (S 2 σ/κ)T, where S, σ, κ, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. S 2 σ is known as the power factor (PF) (7). Even though PF can be enhanced by electronic structure engineering, and κ can be reduced by an increase in phonon scattering, it is very difficult to independently increase PF and simultaneously decrease κ because they are oppositely related to carrier concentration and effective mass.As an alternative to evaluating the maximum ZT, a dimensionless material parameter B at particular temperature has proven to be useful (8-10).where m*, m 0 , μ, κ Lat , and T are the carrier effective mass, free electron mass, carrier mobility, lattice thermal conductivity, and absolute temperature, respectively. Therefore, heavy effective mass, high carrier mobility, and low lattice thermal conductivity are highly desirable for good TE performance. Practically, some strategies and concepts have been proposed to achieve this goal, e.g., band convergence and resonant states for heavier effective mass (11-13), band alignment and weak electron-phonon and alloy scattering to achieve high carrier mobility (14-16), and alloying or nanostructuring to enhance phonon scattering (17)(18)(19) (22,23), and A y Mo 3 Sb 7-x Te x (24). In particular, Zintl phases AB 2 Sb 2 (A = Ca, Yb, Eu, Sr; B = Zn, Mn, Cd, Mg) (25-29) crystalli...

Insights into the design of thermoelectric Mg3Sb2 and its analogs by combining theory and experiment

“…106,107 However, it should be noted that SOC also plays an important role in decreasing energy gaps of Bi-based compounds. 105 29 which provides a solid confirmation of the zero-Δ rule. Though simple and powerful, the zero-Δ rule only has resulted in the discovery of a few potential TE candidates.…”

“…1). Experimentally, strategies such as doping, [45][46][47]49,50 alloying, 15,16,18,19,21,23,25,29,35,36 and controlling vacancy concentrations 27 have been used to optimize electrical transport properties and to reduce thermal conductivity through point defect scattering. As nearly all these compounds are persistently p-type, it is a remarkable breakthrough that the low-cost Te-doped Mg 3 Sb 2 -based compounds were recently discovered to show exceptional n-type TE properties at low and intermediate temperatures ( Fig.…”

Growth and transport properties of Mg3X2 (X = Sb, Bi) single crystals

“…Recently, the surprising discovery of high-performance in n-type Mg3(Sb,Bi)2-based Zintl compounds has ignited intensive research activity on this system, [20,21] especially since almost all the other known Zintl compounds display only good p-type TE properties due to their intrinsic point defects. [22,23] The Zintl phase Mg3Sb2, which crystalizes in a hexagonal layered structure composed of Mg 2+ and (Mg2Sb2) 2layers, was discovered for the first time in 1933. [24] Since the beginning of this century, research attention has been paid to this compound but initial studies only focused on its p-type TE properties.…”

Recent Advances in Thermoelectric Power Generation Technology

“…The modern thin film thermoelectric technology targets the heat flux source to provide thermal control. [37][38][39][40][41]. It is also noteworthy point that band gap range of Mg based thermoelectric materials are close band gaps of semiconductor materials as shown in Table 2.…”