β-Zn4Sb3 has one of the highest ZT reported for binary compounds, but its practical applications have been hindered by a reported poor stability. Here we report the fabrication of nearly dense single-phase β-Zn4Sb3 and a study of its thermoelectric transport coefficients across a wide temperature range. Around 425 K we find an abrupt decrease of its thermal conductivity. Past this point, Zn atoms can migrate from crystalline sites to interstitial positions; β-Zn4Sb3 becomes metastable and gradually decomposes into Zn(hcp) and ZnSb. However, above 565 K it recovers its stability; in fact, the damage caused by decomposition can be repaired completely. This is key to its excellent thermoelectric performance at high temperature: the maximum ZT reaches 1.4. Molecular dynamics simulations are used to shed light on the microscopic behavior of the material.

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Zinc antimony stands out among thermoelectrics because of its very low lattice thermal conductivity, close to the amorphous limit. Understanding the physical reason behind such an unusual crystal property is of fundamental interest for the design of new thermoelectric materials. In this work we report the results of atomistic computer simulations on experimentally determined β−Zn 4 Sb 3 structures. We find a remarkably anharmonic behavior of Zn atoms that could be responsible for the low thermal conductivity of Zn 4 Sb 3 : their movement, better explained as diffusive, does not contribute to thermal conduction. Moreover, phonon transport is impeded by a lack of coupling between Zn and Sb atoms in crystalline positions.As one of the best thermoelectric materials at moderate temperature, 1 Zn 4 Sb 3 's remarkable performance (as gauged by its high dimensionless thermoelectric figure of merit, ZT ) is mainly derived from its glass-like lattice thermal conductivity, below 1.0 W m −1 K −1 . Such a low thermal conductivity approaches the theoretical lower bound known as the amorphous limit, introduced by Slack 2 and refined by Cahill and coworkers. 3 . The idea behind this theoretically minimal thermal conductivity is a physically plausible lower bound to phonon mean free paths, 4 such as half a wavelength. Crystalline compounds with thermal conductivities close to 5 or even below 6 this theoretical limit are few and usually display interesting physics.A straight path to the design of efficient thermoelectrics may lie in the extremely poor thermal conduction of Zn 4 Sb 3 . Thus, many attempts at understanding this feature have been made by placing Zn 4 Sb 3 under microscopes. 7-10 For instance, using single-crystal Xray and powder-synchrotron-radiation diffraction, Snyder and coworkers have found that the hexagonal unit cell of Zn 4 Sb 3 containing at least three interstitial Zn atoms follows the rules of valence compounds, and that Zn occupancy at the strongly bonded crystal site reaches only about 90%. 1 This has provided significant insight for explaining the unusual chemical and physical properties of this material. Specifically, interstitial atoms are suspected to be an extremely effective mechanism for reducing thermal conductivity by introducing disorder. 11,12 Despite many attempts, the precise mechanism behind the low lattice thermal conductivity of Zn 4 Sb 3 is still a puzzle, due to the lack of atomic-scale information and the challenges posed by the study of dynamical effects in large systems through ab-initio methods. For instance, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations and density functional theory (DFT) calculations have provided evidence of Zn diffusion and nanovoid formation in β−Zn 4 Sb 3 , 13 but their connection to thermal conduction remains an elusive essential part. More generally, self-diffusion of ions in crystals has long been observed in experiments; 14 its influence on conduction by phonons is also not well understood. Here we performed mole...

The structural stability of thermoelectric materials is a subject of growing importance for their energy harvesting applications. Here we study the microscopic mechanisms governing the structural stability change of zinc antimony at its working temperature, using molecular dynamics combined with experimental measurements of the electrical and thermal conductivity. Our results show that the temperature-dependence of the thermal and electrical transport coefficients is strongly correlated with a structural transition. This is found to be associated with a relaxation process, in which a group of Zn atoms migrated between interstitial sites. This atom migration gradually leads to a stabilizing structural transition of the crystal framework, then results in a more stable crystal structure of β−Zn 4 Sb 3 at high temperature. * Electronic address: jaredlin@163.com (for questions on the experimental part) † Electronic address: zwangzhao@gmail.com 1 arXiv:1611.00894v1 [cond-mat.mtrl-sci]

The current paper reports the results of an experimental investigation into the effect of the circumferential loading on the quality of the billet cross-section and cropping time during a novel bar-cropping process. The new precision bar-cropping process technique uses a continuous circumferential strike on a metal bar with a pre-machined V-shaped notch. According to the relationship between loading mode and crack propagation, several different control curves of strike frequency versus time and stroke versus time were designed and applied in order to analyse how to improve the quality of the cropped billets and decrease the cropping time. The experimental results indicate that the ideal control mode is the combination of a linear decrement in the strike frequency—time curve and a linear increment in the stroke—time curve during the rotary strike cropping, by which higher cross-section quality and shorter cropping time may be achieved. Moreover, a new assessment method is proposed for the purpose of evaluating the cross-section of the cropped billet.

Single-phase Zn4Sb3 and ZnSb-containing samples were prepared by Plasma Activated Sintering. An abrupt decrease of thermal conductivity was found at about 400 K, which is attributed to the microstructure change of Zn4Sb3. Nanoscale inclusions and compositional inhomogeneities were found in Zn4Sb3 sample at 473 K by high-resolution transmission electron microscopy. The phonon scattering is enhanced by increasing grain boundaries and chaotic structure, which reduces the thermal conductivity and increases the thermoelectric performance of Zn4Sb3 at elevated temperature. The Rietveld refinement results show that large ZnSb grains in ZnSb-containing samples will accommodate excess Zn atoms, and then reduce thermoelectric performance.

Dense p-type and n-type SiGe thermoelectric conversion units were fabricated with a double-layer electrode of W/TiB2 or W/MoSi2 by using glass encapsulation hot-isostatic-pressing process. The TiB2 and MoSi2 layers were used to prevent the chemical reaction between the tungsten and SiGe materials. Si3N4 ceramic particles were added into the electrode materials to reduce the mismatch of the thermal expansion between the electrode and the SiGe. Finite element analysis showed that the addition of 40 vol% Si3N4 into the TiB2 layer and 55 vol% Si3N4 into the MoSi2 layer reduced the thermal residual stress to a much lower value than the strength of individual layer. Sintered units had electrical resistivities of (1.5–2.0) × 10−3 Ω cm in the SiGe zone and 10−4 Ω cm in the electrodes. The comparison of the thermoelectric properties of the SiGe sintered with and without electrodes confirmed that the electrodes did not deteriorate the Seebeck coefficient of the SiGe alloys.