Understanding the electron and phonon transport characteristics is crucial for designing and developing high performance thermoelectric materials. Weak scattering effects on charge carriers, characterized by deformation potential and alloy scattering potential, are favorable for thermoelectric solid solutions to enable high carrier mobility and thereby promising thermoelectric performance. Mg2(Si,Sn) solid solutions have attracted much attention due to their low cost and environmental compatibility. Usually, their high thermoelectric performance with ZT ∼ 1 is ascribed to the band convergence and reduced lattice thermal conductivity caused by alloying. In this work, both a low deformation potential Ξ = 13 eV and a low alloy scattering potential U = 0.7 eV are found for the thermoelectric alloys by characterizing and modeling of thermoelectric transport properties. The band convergence is also verified by the increased density‐of‐states effective mass. It is proposed that, in addition to band convergence and reduced lattice thermal conductivity, the low deformation potential and alloy scattering potential are additional intrinsic features that contribute to the high thermoelectric performance of the solid solutions.

3776 www.MaterialsViews.com wileyonlinelibrary.cominter-convert heat and electricity. [ 1,2 ] The conversion effi ciency largely depends on the fi gure of merit of the material, ZT = α 2 σT / κ , where α is the Seebeck coeffi cient, σ the electrical conductivity, κ the thermal conductivity (including the lattice thermal conductivity κ L and the electronic thermal conductivity κ e ), and T the absolute temperature. Among the ZTgoverning physical properties, the power factor ( PF = α 2 σ ) and the lattice thermal conductivity κ L are relatively less interdependent, leading to two basic strategies to improving ZT . One is to enhance the PF by tuning the carrier concentration, [ 2 ] or increasing the effective mass via band convergence [3][4][5] and resonant levels. [ 6,7 ] The other is to reduce κ L via enhancing phonon scattering in solid solutions and/ or at the grain boundaries, [ 7,8 ] for example, in PbTe(PbSe), [ 9 ] Bi 2 Te 3 [ 10,11 ] and fi lled skutterudites. [ 12 ] In the same spirit, the material parameter β = ( m */ m e ) 3/2 μ / κ L has been used as a criterion for high-ZT materials, [ 13 ] where m * is the carrier effective mass, and μ the carrier mobility in cm 2 V s −1 and κ L in W cm −1 K −1 .Notably, Mg 2 X (X = Si, Ge, and Sn)-based materials exhibit large β values, [ 14 ] promising mid-temperature TE performance, [ 3,15,16 ] and also they are made of non-toxic and naturally abundant constituent elements. To date, the best TE performance with ZT ∼1 has been attained in Mg 2 (Si,Sn) solid solutions, in line with enhancing the PF (i.e., the fi rst strategy) via implementing low carrier scattering potential, [ 17 ] band convergence, [ 18 ] and group-VA elements doping. [ 3,15 ] However, the experimentally minimum κ L of Mg 2 (Si,Sn) (≈1.5 W m −1 K −1 ) is substantially higher than the state-of-the-art TE materials, such as Bi 2 Te 3 (<1.0 W m −1 K −1 ) [ 10 ] and PbTe (≈1.0 Wm −1 K −1 ). [ 4 ] Hence the promise of Mg 2 (Si,Sn)-based materials as an emerging class of green mid-temperature TE materials hinges on how to further reduce the lattice thermal conductivity without degrading the PF much.The optimal operation temperature range of Mg 2 (Si,Sn) is in the mid-temperature range, where the characteristic wavelength of heating-carrying phonons, λ , is on the order of its lattice constant, a , according to the relation λ ≈ ( θ D / T ) a , where θ D is A point defect chemistry approach to improving thermoelectric (TE) properties is introduced, and its effectiveness in the emerging mid-temperature TE material Mg 2 (Si,Sn) is demonstrated. The TE properties of Mg 2 (Si,Sn) are enhanced via the synergistical implementation of three types of point defects, that is, Sb dopants, Mg vacancies, and Mg interstitials in Mg 2 Si 0.4 Sn 0.6-x Sb x with high Sb content ( x > 0.1), and it is found that i) Sb doping at low ratios tunes the carrier concentration while it facilitates the formation of Mg vacancies at high doping ratios ( x > 0.1). Mg vacancies act as acceptors and phonon scatters; ii) the concentration...

While adding extra Mg is intended to compensate for the Mg loss during the synthesis, it often leads to Mg interstitials in Mg 2 (Si,Sn) materials and profoundly affects their thermoelectric properties. Herein we studied the electrical conductivity, Seebeck coefficient, and thermal conductivity of Mg 2(1+x) Si 0.38 Sn 0.6 Sb 0.02 (0.05 # x # 0.12) as a function of Mg excess between 300 K and 730 K. The presence of interstitial Mg was corroborated by X-ray powder diffraction, X-ray photoelectron spectroscopy, Hall coefficient measurement, and compositional analysis. The electrical properties have been analyzed in the framework of a single parabolic band model to gain more insight on the roles of Mg interstitials. We found that increasing Mg excess content increased the carrier concentration, electronic effective mass, and electrical conductivity, while it decreased the Seebeck coefficient and led to a non-monotonic change in the lattice thermal conductivity. As a result, a maximum ZT $ 0.85 was attained at 700 K for Mg excess x ¼ 0.1, a $60% enhancement compared to that of the sample x ¼ 0.05. The Mg interstitials thus provide an extra tuning parameter in optimizing the thermoelectric properties of Mg 2 (Si,Sn)-based materials.

The development and application of second generation high temperature superconducting (2G-HTS) tapes have attracted much attention in China recently. Progress in upscaling high performance 2G-HTS tape production at Shanghai Superconductor Technology (SST) is reported in this paper. With ion beam assisted deposition, biaxially textured buffer layers with a configuration of CeO 2 /LaMnO 3 /MgO/Y 2 O 3 /Al 2 O 3 /C-276 have successfully been fabricated. In-plane and out-ofplane texture degrees of CeO 2 films achieve 2°-4°and 2°, respectively. A multi-plume multi-turn pulsed laser deposition (PLD) system combined with the so-called 'radiation assisted conductive heater' has been proposed and further developed for REBCO layer deposition. Our effort was focused on minimizing the temperature variations in the deposition region by modifying the heating shield that assists the conductive heater of the drum-like cylinder. A tape travelling speed of 100-180 m h −1 can be achieved with a steady temperature profile when passing through the deposition zone, which is very beneficial for the growth of the REBCO layer. Taking advantage of the liquid phase growth mode, several compositions of superconducting films with a thickness in the range of 1-2.5 μm have been grown with high growth rates of over 40 nm s −1 . Furthermore, the microstructures and superconducting performance were investigated in detail. Based on these studies, superconducting tapes with piece lengths of up to 500 meters have been developed. High I c values at 77 K, self-field (over 520 A cm −1 width) or at low temperature, high magnetic field conditions (over 560 A/4 mm width at 4.2 K, 10 T, perpendicular field) have been achieved. Lamination and jointing techniques have also been developed by SST for power and magnet applications.

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