A new p-type high entropy semiconductor AgMnGeSbTe 4 with a band gap of ≈0.28 eV is reported as a promising thermoelectric material. AgMnGeSbTe 4 crystallizes in the rock-salt NaCl structure with cations Ag, Mn, Ge, and Sb randomly disordered over the Na site. Thus, a strong lattice distortion forms from the large difference in the atomic radii of Ag, Mn, Ge, and Sb, resulting in a low lattice thermal conductivity of 0.54 W m −1 K −1 at 600 K. In addition, the AgMnGeSbTe 4 exhibits a degenerate semiconductor behavior and a large average power factor of 10.36 µW cm −1 K −2 in the temperature range of 400-773 K. As a consequence, the AgMnGeSbTe 4 has a peak figure of merit (ZT) of 1.05 at 773 K and a desirable average ZT value of 0.84 in the temperature range of 400-773 K. Moreover, the thermoelectric performance of AgMnGeSbTe 4 can be further enhanced by precipitating of Ag 8 GeTe 6 , which acts as extra scatting centers for holes with low energy and phonons with medium wavelength. The simultaneous optimization in power factor and lattice thermal conductivity yields a peak ZT of 1.27 at 773 K and an average ZT of 0.92 (400-773 K) in AgMnGeSbTe 4 -1 mol% Ag 8 GeTe 6 .
A nanostructure has a significant role in enhancing the power factor and preventing the heat propagation for thermoelectric materials. Herein, we propose a unique segregated and percolated (SP) microphaseseparated structure to enhance the thermoelectric performance of SnTe. The SP structure is composed of insoluble SnTe and AgCuTe, in which AgCuTe with ultralow lattice thermal conductivity undergoes a solid-phase welding during a spark plasma sintering process and forms continuous percolated layers at the interface of isolated SnTe. The SP structure achieved a simultaneous scattering for low energy holes due to the energy offset of the valence band maximum between SnTe and AgCuTe and for phonons due to the noncoherent interfaces between SnTe and AgCuTe, resulting in a high Seebeck coefficient of ∼219.4 μV/K and a low lattice thermal conductivity of ∼1.1 W m −1 K −1 at 800 K for (SnTe) 0.55 (AgCuTe) 0.45 . The thermoelectric performance was further enhanced by means of the cosubstitution of In and Mn for Sn in the SnTe lattice, inducing resonance levels and extra phonon scattering. As a result, the SP structure combined with In/Mn codoping enable us to achieve a low lattice thermal conductivity of 0.47 W m −1 K −1 , a peak ZT of ∼1.45 at 800 K, and a high average ZT of ∼0.73 (400−800 K) for (Sn 0.98 In 0.01 Mn 0.01 Te) 0.75 (AgCuTe) 0.25 .
An increasing number of studies have reported producing
composite
structures by combining thermoelectric and functional materials. However,
combining energy filtering and ferroelectric polarization to enhance
the dimensionless figure of merit thermoelectric ZT remains elusive. Here we report a composite that contains nanostructured
BaTiO3 embedded in a Bi0.5Sb1.5Te3 matrix. We show that ferroelectric BaTiO3 particles
are evenly composited with Bi0.5Sb1.5Te3 grains reducing the concentration of free charge carriers
with increasing BaTiO3 content. Additionally, as a result
of the energy-filtering effect and ferroelectric polarization, the
Seebeck coefficient was improved by ∼10% with a ∼10%
improvement in power factors. The BaTiO3 phase can effectively
scatters phonons reducing lattice thermal conductivity κl (0.5 W m–1 K–1) and increasing ZT to 1.31 at 363 K in Bi0.5Sb1.5Te3 composites with 2 vol % BaTiO3 content giving
an improvement of ∼25% over pure Bi0.5Sb1.5Te3. Our work indicates that the introduction of ferroelectric
nanoparticles is an effective method for optimizing the ZT of Bi0.5Sb1.5Te3-based thermoelectric
materials.
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