Achieving High Thermoelectric Performance in Rare-Earth Element-Free CaMg ₂ Bi ₂ with High Carrier Mobility and Ultralow Lattice Thermal Conductivity

Abstract

CaMg2Bi2-based compounds, a kind of the representative compounds of Zintl phases, have uniquely inherent layered structure and hence are considered to be potential thermoelectric materials. Generally, alloying is a traditional and effective way to reduce the lattice thermal conductivity through the mass and strain field fluctuation between host and guest atoms. The cation sites have very few contributions to the band structure around the fermi level; thus, cation substitution may have negligible influence on the electric transport properties. What is more, widespread application of thermoelectric materials not only desires high ZT value but also calls for low-cost and environmentally benign constituent elements. Here, Ba substitution on cation site achieves a sharp reduction in lattice thermal conductivity through enhanced point defects scattering without the obvious sacrifice of high carrier mobility, and thus improves thermoelectric properties. Then, by combining further enhanced phonon scattering caused by isoelectronic substitution of Zn on the Mg site, an extraordinarily low lattice thermal conductivity of 0.51 W m-1 K-1 at 873 K is achieved in (Ca0.75Ba0.25)0.995Na0.005Mg1.95Zn0.05Bi1.98 alloy, approaching the amorphous limit. Such maintenance of high mobility and realization of ultralow lattice thermal conductivity synergistically result in broadly improvement of the quality factor β. Finally, a maximum ZT of 1.25 at 873 K and the corresponding ZTave up to 0.85 from 300 K to 873 K have been obtained for the same composition, meanwhile possessing temperature independent compatibility factor. To our knowledge, the current ZTave exceeds all the reported values in AMg2Bi2-based compounds so far. Furthermore, the low-cost and environment-friendly characteristic plus excellent thermoelectric performance also make the present Zintl phase CaMg2Bi2 more competitive in practical application.

Electronic Orbital Alignment and Hierarchical Phonon Scattering Enabling High Thermoelectric Performance p-Type Mg 3 Sb 2 Zintl Compounds

“…Recently, Zintl phase compounds have been extensively investigated as potential mid-temperature TE materials [ 19 – 21 ]. Typical Zintl phases consist of electropositive cations and electronegative polyanions [ 22 , 23 ].…”

High Thermoelectric Performance of CaMg2Bi2 Enabled by Dynamic Doping and Orbital Alignment

“…Moreover, the Yb element has an excellent advantage over other divalent elements in improving the mobility due to the characteristic of Yb's 4f state. [ 11,13 ] According to the calculated results of band structure shown in Figure 7d, Yb/Ba co‐doping realizes the orbital alignment, namely Δ E = 0 eV, and then increases the valley degeneracy N v in Ca 0.5 Yb 0.25 Ba 0.25 Mg 2 Bi 1.98 sample. The experimental results indeed confirm that the effective mass is increased from 0.5 to 0.7 m e after doping, as shown in Figure 7e.…”

“…e) The Seebeck coefficient versus n H in different works. [ 11,13,14,16 ] The solid line represents the restructured SPB model predication with different effective mass. The n H of CaMg 2 Bi 1.98 , Ca 0.5 Yb 0.25 Ba 0.25 Mg 2 Bi 1.98 , (Ca 0.5 Yb 0.25 Ba 0.25 ) 0.995 Na 0.005 Mg 2 Bi 1.98 samples are 0.25 × 10 19 cm –3 , 0.92 × 10 19 cm –3 , and 2.84 × 10 19 cm –3 , respectively.…”

“…f) Comparison of temperature‐dependent PF of different works. [ 10–13,16 ] (Ca 0.75 Ba 0.25 ) 0.995 Na 0.005 Mg 1.95 Zn 0.05 Bi 1.98 , [ 13 ] Ca 0.5 Yb 0.5 Mg 2 Bi 2 , [ 16 ] Ca 0.6 Eu 0.2 Yb 0.2 Mg 2 Bi 2 , [ 11 ] Ca 0.685 Ag 0.015 Ba 0.3 Mg 2 Bi 2 , [ 12a ] Ca 0.995 Li 0.005 Mg 1.9 Zn 0.1 Bi 1.98 , [ 12b ] Ca 0.995 Na 0.005 Mg 21 Bi 1.98 . [ 10 ]…”

“…The high ZT value of 1.3 at 873 K has been reported in Ca 0.6 Yb 0.2 Eu 0.2 Mg 2 Bi 2 and (Ca 0.75 Ba 0.25 ) 0.995 Na 0.005 Mg 1.95 Zn 0.05 Bi 1.98 samples. [11,13] Despite these witnessed advancements, there is an essential topic that has been neglected for a long time: Since the Bi impurity is inevitably formed due to the volatilization of cation Mg during the preparation process, whether in single crystal or polycrystalline AMg 2 Bi 2 -based materials, [8,9,12,14] how does the Bi second phase evolve with temperature and how does it affect the thermoelectric properties?…”