Lipeng Hu

Enhancement in thermoelectric performance of bismuth telluride based alloys by multi-scale microstructural effects

Decoupling of interdependent thermoelectric parameters was considered as a crucial strategy to enhance the thermoelectric performance of bulk materials. Here multi-scale microstructural effects have been introduced by a simple hot deformation process to obtain high-performance n-type bismuth telluride based alloys. Highly preferred orientation enables a significant improvement in in-plane electrical conductivity. The donor-like effect (an interaction of antisite defects and vacancies), which can be adjusted by varying hot deformation temperature, was also considered responsible for the remarkable enhancement in power factor. Besides, the in-plane lattice thermal conductivity was greatly reduced by in situ nanostructures and high-density lattice defects generated during the hot deformation process. The present study experimentally demonstrates a successful combination of microscale texture enhancement, atomic scale lattice defects and donor-like effect and recrystallization induced nanostructures as a new approach to improve thermoelectric properties. These effects led to a maximum ZT of 0.95 for the Bi2Te2Se1 sample hot deformed at 823 K, about 80% improvement over that without hot deformation.

The texture related anisotropy of thermoelectric properties in bismuth telluride based polycrystalline alloys

Large size bismuth telluride based polycrystalline alloys have been prepared to investigate the texture-related anisotropy of thermoelectric properties. X-ray diffraction analysis indicated that the n-type alloy was more easily (001) oriented than the p-type one. Both the electrical and thermal conductivities showed strong anisotropy in the textured samples, while the Seebeck coefficient and the ZT were almost isotropic. However, as the electrical and thermal conductivities were measured along different directions, the ZT could be overestimated up to ∼60%. A relationship between the texture degree and the anisotropy of the thermal conductivity has been proposed.

New Insights into Intrinsic Point Defects in V2VI3 Thermoelectric Materials

Defects and defect engineering are at the core of many regimes of material research, including the field of thermoelectric study. The 60‐year history of V2VI3 thermoelectric materials is a prime example of how a class of semiconductor material, considered mature several times, can be rejuvenated by better understanding and manipulation of defects. This review aims to provide a systematic account of the underexplored intrinsic point defects in V2VI3 compounds, with regard to (i) their formation and control, and (ii) their interplay with other types of defects towards higher thermoelectric performance. We herein present a convincing case that intrinsic point defects can be actively controlled by extrinsic doping and also via compositional, mechanical, and thermal control at various stages of material synthesis. An up‐to‐date understanding of intrinsic point defects in V2VI3 compounds is summarized in a (χ, r)‐model and applied to elucidating the donor‐like effect. These new insights not only enable more innovative defect engineering in other thermoelectric materials but also, in a broad context, contribute to rational defect design in advanced functional materials at large.

Hot deformation induced bulk nanostructuring of unidirectionally grown p-type (Bi,Sb)2Te3 thermoelectric materials

Nanostructuring has proved effective in improving the figure of merit in the widely used Bi2Te3 based thermoelectric materials. In this work, a hot deformation induced in situ nanostructuring process is directly applied to the commercial unidirectionally grown p-type Bi0.5Sb1.5Te3 ingots to explore the possibility of commercial application of the “top down” nanostructuring approach, and the thermoelectric properties are investigated over a wide temperature range of 15 K to 520 K. In comparison to the commercial zone melted ingot and the hot pressed sample, it is found that the hot deformed samples exhibit much less texture and significantly reduced lattice thermal conductivity due to in situ formed nanostructures and defects. A high ZT of ∼1.3 is achieved near room temperature, ∼50% improvement compared to that of the zone melted ingot. The hot deformation process thus provides a promising top down approach to prepare high performance Bi2Te3 based thermoelectric materials in a way that is more readily incorporated into the existing procedure of device manufacturing.

Interrelation between atomic switching disorder and thermoelectric properties of ZrNiSn half-Heusler compounds

The interrelation between atomic switching disorder and thermoelectric properties in the half-Heusler alloy ZrNiSn was investigated. ZrNiSn samples were prepared by a time-efficient levitation melting and spark plasma sintering procedure. High-resolution synchrotron radiation powder X-ray diffraction shows that a single phase half-Heusler compound has been obtained. Trace impurities were detected after annealing at 970 K for 5 days. Rietveld refinements were carried out for both unannealed and annealed ZrNiSn samples to study the possible structural disorders. It is found that the generally accepted Zr/Sn antisite defects are not likely to exist. Instead, the refinements revealed interstitial fractional occupancy of Ni on the (½, ½, ½) site, which is normally empty in the half-Heusler phases, but filled in full Heusler materials. The electrical conductivity and Seebeck coefficient from 300 to 900 K of the unannealed and annealed ZrNiSn displayed no obvious distinction, and the room temperature electrical resistivity and absolute Seebeck coefficient of the annealed ZrNiSn even decreased slightly compared to those of the unannealed one, which implies no obvious Zr/Sn disorder transition during the annealing procedure. A slight increase in the thermal conductivity was observed after a long time annealing, possibly due to reduced Ni atomic disorder.

High performance n-type bismuth telluride based alloys for mid-temperature power generation

Currently more than 60% of primary energy used in industry or life is lost as waste heat in the temperature range of 400–900 K, and much attention is paid to mid-temperature thermoelectric (TE) power generation. Here we combine several strategies, i.e. alloying, doping and hot deformation, to improve the TE performance of n-type bismuth telluride based TE alloys for mid-temperature power generation. Se alloying was adopted to widen the band gap and suppress intrinsic conduction at elevated temperatures. When Se atoms completely substitute the Te(2) atoms, the crystal structure of Bi2Te3 based alloys tends to be more ordered, resulting in the maximum value of the band gap. And the induced alloying scattering significantly reduces the lattice thermal conductivity. Then SbI3 donor doping was used to increase the electron concentration to further suppress the detrimental effects of bipolar conduction. Finally we applied repetitive hot deformations to further improve the figure of merit zT and a peak zT of ∼1.1 was obtained at about 600 K in the 0.1 at% SbI3–Bi2Te1.9Se1.1 alloy, which was hot-deformed three times. The results demonstrated the great potential of the alloy for application in mid-temperature TE power generation.

Enhancing Thermoelectric Performance of n-Type Hot Deformed Bismuth-Telluride-Based Solid Solutions by Nonstoichiometry-Mediated Intrinsic Point Defects

Bismuth-telluride-based solid solutions are the unique thermoelectric (TE) materials near room temperature. Various approaches have been applied to enhance the thermoelectric performance, and much progress has been made in their p-type materials. However, for the n-type counterparts, little breakthrough has been obtained. We herein report on enhancing thermoelectric performance of n-type bismuth-telluride-based alloys by nonstoichiometry to mediate the point defects, combined with one-time hot deformation. The improved power factor of 3.3 × 10-3 W m-1 K-2 and reduced lattice thermal conductivity contribute to a high figure-of-merit, zT, of 1.2 at 450 K for n-type Bi2Te2.3Se0.69 alloys with Se deficiency. The high zT is comparable to that of Bi2Te2.3Se0.7 hot deformed three times, which is a practically complicated process. The results demonstrate that nonstoichiometry can be an effective and simple strategy in mediating intrinsic point defects and enhancing the thermoelectric performance of bismuth-telluride-based alloys.

Hot deformation induced defects and performance enhancement in FeSb2 thermoelectric materials

The effect of hot deformation induced defects and texture on thermoelectric properties of FeSb2 bulk crystals has been investigated. The transport properties of the samples along both parallel and perpendicular direction of pressing were measured from 3 K to 300 K. The results showed that thermal conductivity of the deformed samples was significantly reduced. After twice deformation, the thermal conductivity of the sample along the perpendicular direction of pressing was decreased to 4 W/mK, which was only one third of that before deformation. Transmission electron microscopy observation revealed the presence of high density of lattice defects in the deformed samples. The lattice thermal conductivity was analyzed using the Debye-Callaway approximation, and the results showed that the deformation induced lattice imperfections play an important role in enhancing phonon scattering. In addition, both the electrical resistivity and Seebeck coefficient exhibited a weak anisotropy in the deformed samples. The fi...

Dry calibration of electromagnetic flowmeters based on numerical models combining multiple physical phenomena (multiphysics)

This paper presents a method for dry calibration of an electromagnetic flowmeter (EMF). This method, which determines the voltage induced in the EMF as conductive liquid flows through a magnetic field, numerically solves a coupled set of multiphysical equations with measured boundary conditions for the magnetic, electric, and flow fields in the measuring pipe of the flowmeter. Specifically, this paper details the formulation of dry calibration and an efficient algorithm (that adaptively minimizes the number of measurements and requires only the normal component of the magnetic flux density as boundary conditions on the pipe surface to reconstruct the magnetic field involved) for computing the sensitivity of EMF. Along with an in-depth discussion on factors that could significantly affect the final precision of a dry calibrated EMF, the effects of flow disturbance on measuring errors have been experimentally studied by installing a baffle at the inflow port of the EMF. Results of the dry calibration on an ...

Magnetic field estimation in measurement dead domain for dry calibration of electromagnetic flowmeter

Advances in computing technology enable dry calibration of large-diameter electromagnetic (EM) flowmeters at low cost, which has been recognized as an effective alternative to traditional flow rigs. Dry calibration requiring no actual liquid in the measuring pipe utilizes the magnetic field distribution reconstructed from measured boundary conditions to determine the sensitivity of the EM flowmeter. However, because sensors have finite sizes, and the fact that inner linings of the measuring pipe deform due to mechanical stresses, a measurement dead domain (MDD) exists between the measured boundary surface and the pipe wall. As the MDD is often close to the magnetic exciting unit, neglecting it results in significant errors in dry calibration. This paper offers a practical method combining iterative optimization and reconstruction to estimate the magnetic field in the MDD from the field data on the measured boundary surface. The method has been validated on an off-the-shelf industrial EM flowmeter by comparing the estimated field in the MDD with experimental measurements. It has been demonstrated that accurately accounting for the immeasurable field in the MDD eliminates more than two-thirds of the dry calibration errors. The estimation method illustrated here can also be extended to measure other physical fields which obey similar governing equations.