Highlights

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Structural transition of Ba₆Ge₂₅ was studied by positron annihilation and XRD.

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XRD measurements suggest rearrangement of lattice structure at 200–250 K.

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Positron annihilation results indicate change of electron density during transition.

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The structure transition also affects thermal, electric and magnetic properties.

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Movement of Ba atoms might be responsible for the structure transition.

Abstract

It has been demonstrated that InSb nanoinclusions, which are formed in situ, can simultaneously improve all three individual thermoelectric properties of the n-type half-Heusler compound (Ti,Zr,Hf)(Co,Ni)Sb (Xie WJ, He J, Zhu S, Su XL, Wang SY, Holgate T, et al. Acta Mater 2010;58:4795). In the present work, the same approach is adopted to the p-type half-Heusler compound Ti(Co,Fe)Sb. The results of resistivity, Seebeck coefficient, thermal conductivity and Hall coefficient measurements indicate that the combined high-mobility electron injection, low energy electron filtering and boundary scattering, again, lead to a simultaneous improvement in all three individual thermoelectric properties: enhanced Seebeck coefficient and electrical conductivity as well as reduced lattice thermal conductivity. A figure of merit of ZT ≈ 0.33 was attained at 900 K for the sample containing 1.0 at.% InSb nanoinclusions, a ∼450% improvement over the nanoinclusion-free sample. This represents a rare case that the same nanostructuring approach works successfully for both p-type and n-type thermoelectric materials of the same class, hence pointing to a promising materials design route for higher-performance half-Heusler materials in the future and hopefully will realize similar improvement in thermoelectric devices based on such half-Heusler alloys.

Abstract

FeSb₂Te, a ternary derivative of binary CoSb₃, displays anomalous electrical and thermal transport properties because of considerable modifications in the band structure induced by Fe and significant mixed valence state (namely Fe²⁺ and Fe³⁺) scattering of phonons. The substitution of Te for Sb generates more holes without notably affecting the band structure, while markedly improving the electrical conductivity and retaining a high Seebeck coefficient due to the enhanced density of states, thereby leading to dramatically increased power factors. Furthermore, the heat carrying phonons are strongly scattered with increasing x value because of the formation of solid solutions between two end members: □FeSb₂Te and □FeSb₃ (where □ can be viewed as a vacancy). Consequently, high thermoelectric figures of merit were achieved in the FeSb_2+xTe_1−x compounds, with the largest ZT value reaching ∼0.65 for the sample with x = 0.2. This is the highest value among all p-type unfilled skutterudites and is comparable with some filled compositions. Prospects for further improving the performance of p-type FeSb₂Te-based skutterudites are discussed.

Abstract

Since the vibration modes of the pnicogen rings in CoSb₃-based skutterudites fall within the range of frequencies of heat-carrying phonons, disruption of the rings by doping should have a strong influence on heat transport in this material. To test the premise, single-phase double-doped CoSb_2.75Ge_0.25−xTe_x (x = 0.125–0.20) compounds were synthesized by combining melt spinning with a spark plasma sintering method. Following the melt-spinning process, the side of the ribbons contacting the copper drum is featureless and reflects its amorphous nature while the free surface of the ribbons is composed of 30–80 nm grains. After spark plasma processing the average grain size of the bulk samples is about 200 nm. High-resolution transmission electron microscopy images show an in situ nanostructure consisting of circular, 15 nm diameter dots of Te- and Ge-enriched skutterudite phase embedded in the skutterudite matrix. Transport properties were measured from 2 to 800 K as a function of Te and Ge content on the pnicogen (Sb) rings and the results were correlated with the structural data. Double-doping on pnicogen rings with Ge and Te, and using melt-spinning processing, results in binary skutterudite compounds that possess an impressive figure of merit of ZT ∼ 1.1 at 750 K.

Abstract

β-Zn₄Sb₃ compounds doped with minute amounts of Cd were synthesized by the MS-SPS technique, which involves melt spinning (MS) followed by spark plasma sintering (SPS), and the microstructures, thermoelectric and thermodynamic properties were systematically characterized. The non-equilibrium MS-SPS technique generates multi-scale nanostructures in the MS-prepared ribbon-shape samples and the resulting compacted bulk materials. These unique multiple nanostructures result in substantial reductions in lattice thermal conductivities, particularly for samples with a large number of ZnSb nanodots with sizes of 10–30 nm. Meanwhile, Cd-doping remarkably improves the electrical properties of the (Zn_1−xCd_x)₄Sb₃ compounds by a slight decrease in electrical conductivity and an apparent enhancement of the Seebeck coefficient. Therefore, the dimensionless figure of merits are significantly improved and the maximum value reaches ∼1.30 for the (Zn_0.99Cd_0.01)₄Sb₃ sample at 700 K, representing ∼13% and ∼23% improvements compared with the undoped MS-SPS sample and the 1% Cd-doped melting ingot, respectively. In particular, this value shows no degradation after 10 heat cycles from 300 to 700 K or 30 h annealing at 680 K in vacuum, whereas the ZT of neat sample decreases by ∼20% to a relatively low value of ∼1.0 after 30 h annealing. The enhanced thermal stability of ZT along with the suppressing effect on the low-temperature α–β phase transition clearly indicates a large improvement in thermodynamic stability as a result of minute Cd-doping. All the above-mentioned benefits make the minute Cd-doped β-Zn₄Sb₃ compound prepared by the MS-SPS technique a promising candidate for mid-range temperature thermoelectric power generation applications.

Abstract

We report an induction-melting spark-plasma-sintering synthesis process of the nanocomposite material composed of (TiZrHf)(CoNi)Sb coarse grains and in situ formed InSb nanoinclusions that occur primarily on the grain boundaries. We were able to qualitatively control the amount of InSb nanoinclusions by varying the In and Sb contents in the starting materials. The effects of the nanoinclusion formation and the matrix–nanoinclusion boundaries on the thermoelectric properties have been studied and correlated. In particular, the nanoinclusion-induced electron injection and electron filtering mechanisms helped to simultaneously decrease the resistivity, enhance the Seebeck coefficient and reduce the thermal conductivity of the nanocomposite. A figure of merit of ZT ∼ 0.5 was attained at 820 K for the sample containing 1 at.% InSb nanoinclusions, which is a 160% improvement over the sample containing no nanoinclusions. The experimental results are discussed in the context of the effective medium model formerly proposed by Bergman and Fel.