Atomic Disorders Induced by Silver and Magnesium Ion Migrations Favor High Thermoelectric Performance in α‐MgAgSb‐Based Materials

Abstract

wileyonlinelibrary.com =, where S , σ , κ , and T are the Seebeck coeffi cient, the electrical conductivity, the thermal conductivity, and the absolute temperature, respectively. Good thermoelectric materials possess the feature of "phonon glass, electron crystal." [ 1 ] The state-of-the-art strategies for improving ZT s for existing materials can generally be divided into two categories: one is to reduce the lattice thermal conductivity through nanostructuring or hierarchical architecturing; [2][3][4] the other is to enhance the electrical transport properties by resonant doping [ 5 ] or heavy band convergence. [ 6,7 ] Recently, Thermoelectric devices can directly convert thermal energy to electricity or vice versa with the effi ciency being determined by the materials' dimensionless fi gure of merit ( ZT ). Since the revival of interests in the last decades, substantial achievements have been reached in search of high-performance thermoelectric materials, especially in the high temperature regime. In the near-room-temperature regime, MgAgSb-based materials are recently obtained with ZT ≈ 0.9 at 300 K and ≈1.4 at 525 K, as well as a record high energy conversion effi ciency of 8.5%. However, the underlying mechanism responsible for the performance in this family of materials has been poorly understood. Here, based on structure refi nements, scanning transmission electron microscopy (STEM), NMR experiments, and density function theory (DFT) calculations, unique silver and magnesium ion migrations in α-MgAg 0.97 Sb 0.99 are disclosed. It is revealed that the local atomic disorders induced by concurrent ion migrations are the major origin of the low thermal conductivity and play an important role in the good ZT in MgAgSb-based materials.

Mg‐Ion Inversion in MgO@MgO−Al2O3 Oxides: The Origin of Basic Sites

“…The local environment of Mg speciesi ns ynthetic a-MgAgSb-based materials has been investigated to ascertain the presence of octa-and tetra-coordinate Mg species. [32] Motivatedb yt he results of this previouss tudy,w ec on-ducted as olid-state 25 Mg NMR analysisf or the present x MgOÀ Al 2 O 3 samples. The obtained spectra are shown in Figure 1d, together with that of pure MgO as ar eference.…”

“…Thus, Mg inversion increases with Al IV increase as Mg/Al molar ratio in x MgO−Al 2 O 3 decreases from 3 to 0.5 (Table ). The local environment of Mg species in synthetic α‐MgAgSb‐based materials has been investigated to ascertain the presence of octa‐ and tetra‐coordinate Mg species . Motivated by the results of this previous study, we conducted a solid‐state 25 Mg NMR analysis for the present x MgO−Al 2 O 3 samples.…”

Hierarchical Chemical Bonds Contributing to the Intrinsically Low Thermal Conductivity in α‐MgAgSb Thermoelectric Materials

“…The above experimental and theoretical joint efforts have demonstrated that the low κ l of α‐MgAgSb can be attributed to the unique lattice dynamics—the coexistence of global and local weak chemical bonds. Recently, Li et al argued the ion mutual migrations in α‐MgAgSb as the primary reason for its low κ l , based on ab initio molecular dynamics (AIMD) calculations and experiments . In order to rationalize this, we did the AIMD calculation for α‐MgAgSb at 300 K, using a much larger supercell (192 atoms) and a total of 30 000 time steps (60 ps).…”

Manipulation of Phonon Transport in Thermoelectrics

“…Considering the propagation of a sound wave in gas, liquid, or solid, its velocity gets higher as the chemical bonds become stronger. This leads to an expectation of a low group velocity for a low lattice thermal conductivity (Equation ), in materials with either global or local soft bonds. This can be directly seen from the phonon dispersion (Figure b), where the slope of k ‐vector versus phonon frequency characterizes the phonon group velocity ( v g ) and can further be quantified as the square root of the force constants ( F , f ) over the atomic mass ( M , m ) .…”