Tailoring the chemical bonding of GeTe-based alloys by MgB2 alloying to simultaneously enhance their mechanical and thermoelectric performance

Self Cite

High‐performance thermoelectric (TE) devices require not only a high figure of merit (ZT) but also mechanical strength and thermal stability. Here, a simultaneous enhancement of ZT as well as mechanical properties is obtained in GeTe‐based alloys by adding boron. This material is then assembled with n‐type CoSb3 skutterudite into TE modules. The improved ZT values result from the increase in charge carrier mobility due to the reduced interfacial barrier height. A peak ZT of 2.2 at 773 K can be achieved in Ge0.84Pb0.1Sb0.06TeB0.07, which shows a negligible change in the coefficient of thermal expansion upon the phase transition from the rhombohedral to the cubic phase, ensuring good thermal stability of the device. Resulting from the boron‐induced grain refinement and dispersion strengthening, the average compressive strength and Vickers hardness of Ge0.9Sb0.1TeB0.07 can be enhanced to ≈227 MPa and ≈202 Hv, respectively. The improved mechanical properties facilitate the assembly of devices and lower the interfacial contact resistance. As a synergy of increased ZT and mechanical strength, a high output power density of ≈1.76 W cm−2 at ΔT = 425 K and an energy conversion efficiency of 7.4% at ΔT = 477 K can be achieved in the TE modules.

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Boron Strengthened GeTe‐Based Alloys for Robust Thermoelectric Devices with High Output Power Density

Bai

et al. 2021

Self Cite

“…Among the several types of second phases that have been incorporated into TE materials to enhance their zT value, silicon carbide nanoparticles have attracted significant interest. Many semiconductors, such as SiC, [ 131–142 ] MgB 2 , [ 143,144 ] TiN, [ 145,146 ] B, [ 147–149 ] and others, [ 150–152 ] have been confirmed to improve both the mechanical and thermoelectric performance of TE materials.…”

Section: Discontinuous Interface Modificationmentioning

confidence: 99%

“…Parallelly, subtle changes in the CTE near the phase transition temperature of GeTe (680 K) have been observed for MgB 2 ‐doped Ge 0.9 Sb 0.1 Te samples, a phenomenon which can be attributed to the simultaneous action of Sb and Mg alloys on tuning the chemical bonding and lattice structures, as well as the local distortions generated around the Mg sites. [ 144 ]…”

Section: Discontinuous Interface Modificationmentioning

confidence: 99%

Current State‐of‐the‐Art in the Interface/Surface Modification of Thermoelectric Materials

Lehmann

Bahrami

et al. 2021

Thermoelectric (TE) materials are prominent candidates for energy converting applications due to their excellent performance and reliability. Extensive efforts for improving their efficiency in single‐/multi‐phase composites comprising nano/micro‐scale second phases are being made. The artificial decoration of second phases into the thermoelectric matrix in multi‐phase composites, which is distinguished from the second‐phase precipitation occurring during the thermally equilibrated synthesis of TE materials, can effectively enhance their performance. Theoretically, the interfacial manipulation of phase boundaries can be extended to a wide range of materials. High interface densities decrease thermal conductivity when nano/micro‐scale grain boundaries are obtained and certain electronic structure modifications may increase the power factor of TE materials. Based on the distribution of second phases on the interface boundaries, the strategies can be divided into discontinuous and continuous interfacial modifications. The discontinuous interfacial modifications section in this review discusses five parts chosen according to their dispersion forms, including metals, oxides, semiconductors, carbonic compounds, and MXenes. Alternatively, gas‐ and solution‐phase process techniques are adopted for realizing continuous surface changes, like the core–shell structure. This review offers a detailed analysis of the current state‐of‐the‐art in the field, while identifying possibilities and obstacles for improving the performance of TE materials.

“…[35][36][37] Figure 1a compares the n h -dependent μ of the Cu-alloyed and Cufree GeTe. [33,34,36,38,39] Particularly, Xie et al [36] reported that Cu 2 Te-alloyed Ge 1−y Sb y Te has a room-temperature μ of ≈40 cm 2 V −1 s −1 and correspondingly a high zT of >2.0 in (Ge 0.92 Sb 0.08 Te) 0.97 (Cu 2 Te) 0.03 . Li et al [39] observed that (GeTe) 0.92 (CuSbTe 2 ) 0.08 has a high μ of >40 cm 2 V −1 s −1 in Cu(Sb, Bi)Te 2 -alloyed GeTe due to the similar electronegativity between Cu and Ge which weakens the alloy scattering of carriers.…”

Section: Introductionmentioning

confidence: 99%

High Carrier Mobility and High Figure of Merit in the CuBiSe₂ Alloyed GeTe

Yin

Liu

et al. 2021

Dimensionless figure-of-merit (zT) is the most typical descriptor to evaluate thermoelectric materials, which is defined as zT = S 2 σT/κ, where S, σ, T and κ are the Seebeck coefficient, the electrical conductivity, the absolute temperature, and the thermal conductivity, respectively. [4,5] κ includes the lattice thermal conductivity (κ l ) and the electrical thermal conductivity (κ e ). [6,7] To achieve high energy conversion efficiency, high zT values with high power factor (S 2 σ) and low κ are required. According to extensive investigations, S 2 σ can be increased by quantum confinement, [8,9] modulation doping, [10] band convergence, [5,11] and resonant state engineering. [12,13] Whereas, κ (mainly κ l ) is mainly reduced by introducing a variety of lattice defects as phonon scattering centers such as point defects, [14,15] dislocations, [16,17] stacking faults, [18,19] nano precipitates, [20,21] and hierarchical architecture structures. [22,23] GeTe is a promising mid-temperature material with a phase transition from the rhombohedral phase (R-GeTe) to the cubic phase (C-GeTe) at ≈700 K. However, the intrinsic high carrier concentration (n h for p-type GeTe, ≈10 21 cm −3 due to massive Ge vacancies) limits its S 2 σ and thus zT (≈0.75 at ≈740 K). [24,25] To According to theMott's relation, the figure-of-merit of a thermoelectric material depends on the charge carrier concentration and carrier mobility. This explains the observation that low thermoelectric properties of GeTe-based materials suffer from the degraded carrier mobility, on account of the fluctuation of electronegativity and ionicity of various elements. Here, highperformance CuBiSe 2 alloyed GeTe with high carrier mobility due to the small electronegativity difference between Cu and Ge atoms and the weak ionicity of CuTe and BiTe bonds, is developed. Density functional theory calculations indicate that CuBiSe 2 alloying increases the formation energy of Ge vacancies and correspondingly reduces the amount of Ge vacancies, leading to an optimized carrier concentration and a high power factor of ≈37.4 µW cm −1 K −2 at 723 K. Moreover, CuBiSe 2 alloying induces dense point defects and triggers ubiquitous lattice distortions, leading to a reduced lattice thermal conductivity of 0.39 W m −1 K −1 at 723 K. These synergistic effects result in an optimization of the carrier mobility, the carrier concentration, and the lattice thermal conductivity, which favors an enhanced peak figure-of-merit of ≈2.2 at 723 K in (GeTe) 0.94 (CuBiSe 2 ) 0.06 . This study provides guidance for the screening of GeTe-based thermoelectric materials with high carrier mobility.