Ultralow Thermal Conductivity and High-Temperature Thermoelectric Performance in n-Type K_2.5Bi_8.5Se₁₄

Abstract

We studied the narrow band-gap (0.55 eV) semiconductor K 2.5 Bi 8.5 Se 14 as a potential thermoelectric material for power generation. Samples of polycrystalline K 2.5 Bi 8.5 Se 14 prepared by spark plasma sintering exhibit exceptionally low lattice thermal conductivities (κ lat ) of 0.57−0.33 W m −1 K −1 in the temperature range of 300−873 K. The physical origin of such low κ lat in K 2.5 Bi 8.5 Se 14 is related to the strong anharmonicity and low phonon velocity caused by its complex low symmetry, large unit cell crystal structure, and mixed occupancy of Bi and K atoms in the lattice. High-resolution scanning transmission electron microscopy studies and microanalysis indicate that the K 2.5 Bi 8.5 Se 14 sample is a single phase without intergrowth of the structurally related K 2 Bi 8 Se 13 phase. The undoped material exhibits an n-type character and a figure-of-merit (ZT) value of 0.67 at 873 K. Electronic band structure calculations indicate that K 2.5 Bi 8.5 Se 14 is an indirect band-gap semiconductor with multiple conduction bands close to the Fermi level. Phonon dispersion calculations suggest that K 2.5 Bi 8.5 Se 14 has low phonon velocities and large Gruneisen parameters that can account for the observed ultralow κ lat . The degree of n-type doping can be controlled by introducing Se deficiencies in the structure, providing a simple route to increase the ZT to ∼1 at 873 K.

Entropy-Induced Multivalley Band Structures Improve Thermoelectric Performance in p-Cu7P(SxSe1–x)6 Argyrodites

“…In the past decades, the large efforts of the thermoelectric community are devoted to the increase of the power factor through the optimization of carrier concentrations, 8 band engineering, 9 resonance scattering, 10 and advanced electronic structure engineering. 11,12 The successful reduction of the lattice thermal conductivity was attained using grain boundary engineering, 13,14 nanostructuring, 15,16 point defect engineering, 17,18 lattice anharmonicity, 19,20 and lattice softening. 21−23 Except for enhancing the ZT of the existing thermoelectrics, numerous efforts were also devoted to exploring the novel TE materials within the "Phonon Liquid Electron Crystals" (PLEC) concept.…”

“…Therefore, the simple tuning of one parameter leads to compensation in two others. In the past decades, the large efforts of the thermoelectric community are devoted to the increase of the power factor through the optimization of carrier concentrations, band engineering, resonance scattering, and advanced electronic structure engineering. , The successful reduction of the lattice thermal conductivity was attained using grain boundary engineering, , nanostructuring, , point defect engineering, , lattice anharmonicity, , and lattice softening. − …”

High Figure of Merit in Gallium-Doped Nanostructured n-Type PbTe-xGeTe with Midgap States

“…For example, σ and κ ele are directly related by the Wiedemann–Franz relation, and S and σ have opposite trends with respect to the carrier concentration ( n ). The strategies toward high ZT aim at either increasing PF or decreasing κ lat with minimal loss of S . − Moreover, the efficiency (η) of the thermoelectric device is directly dependent on the average figure of merit, ZT avg , as η = [( T H – T C )/ T H ] [(1 + ZT avg ) 1/2 – 1]/[(1 + ZT avg ) 1/2 + T C / T H ], where T H is the hot-side temperature and T C is the cold-side temperature of the device. The ZT avg is calculated by the following relationship, .…”

Phonon scattering mechanism in thermoelectric materials revised via resonant x-ray dynamical diffraction

“…Understanding a mechanism for impeding thermal conductivity, while keeping a high electric conductivity, is key to design better thermoelectric materials. [1][2][3][4][5][6][7][8][9][10] Filled skutterudites are a family of materials displaying good potential for thermoelectric applications. [11][12][13][14][15][16][17][18] This is usually attributed to an enhancement of phonon scattering, which lowers the thermal conductivity, by localized vibrational modes denoted rattling modes.…”