Sb-doped Mg2Si0 Research Articles

Enhanced thermoelectric performance of Sb-doped Mg2Si0.4Sn0.6via doping, alloying and nanoprecipitation

With the advantages of eco-friendliness, low cost, and low density, Mg2(Si,Sn) solid solutions are promising candidates for thermoelectric applications. In this work, Sb-doped Mg2Si0.4Sn0.6 bulks were prepared with a combined method of solid-state reaction and high pressure synthesis, followed by spark plasma sintering. Our investigations show that Sb doping optimizes the carrier concentration, while Si/Sn alloying effectively suppresses the lattice thermal conductivity and induces a convergence of the two lowest-lying conduction bands. Additionally, numerous coherent Sn-rich nanoprecipitates are formed within micron-sized grains. All these factors contribute synergistically to improving the thermoelectric properties of Mg2Si0.4Sn0.6. The optimal Mg2(Si0.4Sn0.6)0.985Sb0.015 exhibits a power factor higher than 4 000 μW·m−1·K−2 and a lattice thermal conductivity less than 0.8 W·m−1·K−1 at temperatures higher than 600 K, leading to the highest ZT of 1.61 at 823 K. Current work demonstrates an effective approach to enhancing the thermoelectric performance of n-type Mg2X solid solutions through doping, alloying, and microstructure modification.

High-Performance Functionalized Mg2Si0.9Sn0.1 Thermoelectric Leg Synthesis by a Single-Step Reactive SPS Process

Contact electrodes and their adequate joining are the main bottlenecks in realizing low-cost Mg2(Si,Sn) thermoelectric materials for thermoelectric generator applications. The resistance and diffusion chemistry between the thermoelectric elements and contact electrodes play an essential role in achieving high-performance thermoelectric generators for intermediate-temperature applications. A facile single-step spark plasma sintering process was employed to fabricate Sb-doped Mg2Si0.9Sn0.1 and contact joining simultaneously. The single-step-processed thermoelectric element’s power output characteristics with three different contacts (Ni, MnSi, and FeSi2) were investigated by employing a thermoelectric conversion efficiency evaluation system. Our experimental strategy demonstrated that the single-step processing of Mg2(Si0.9Sn0.1)0.95Sb0.05/Ni joining imparts the lowest internal resistance and high-power output characteristics and eliminates the main obstacle of Mg2(Si,Sn) material integration in thermoelectric power generators.

Energy-Efficient Synthesis and Superior Thermoelectric Performance of Sb-doped Mg2Si0.3Sn0.7 Solid Solutions by Rapid Thermal Explosion

Both low cost and high performance play pivotal roles in the commercial application of thermoelectric materials. Herein, a fast, facile and inexpensive thermal explosion (TE) was utilized to prepare eco-friendly Mg2(Si0.3Sn0.7)1-xSbx solid solutions. The phase transformation during the TE process was investigated. The criterion for differentiating TE from conventional solid-state reaction (SSR) was also illustrated, i.e. whether self-propagating high-temperature synthesis (SHS) happens. With Sb doping, carrier density rises and bipolar conduction in high-temperature range is effectively inhibited. Consequently, the dimensionless thermoelectric figure of merit ZTpeak and ZTave are enhanced, which are decent compared with the reported data. Although a comparative analysis of thermoelectric properties suggests that structural homogenization obtained by SSR instead of TE is conducive to high ZT for Mg2Si0.3Sn0.7 materials, the low-cost TE approach can be easily implemented with attributes of high energy efficiency, fast synthesis and large-scale production, thereby quite worth promoting for other thermoelectric materials.

Fast and facile synthesis of Sb-doped Mg2Si0.5Sn0.5 solid solutions with decent thermoelectric performance

Sb-doped Mg2Si0.5Sn0.5 materials were successfully prepared by a fast thermal explosion reaction for the first time. X-ray diffraction (XRD) patterns indicate that this new method easily converts the immiscible composition of 0.5Mg2Si-0.5Mg2Sn into a single-phase Mg2Si0.5Sn0.5 solid solution. With Sb content increasing, the dimensionless thermoelectric figure of merit ZTpeak (peak value) and ZTave (average value) are greatly improved, which are quite comparable to or even better than the values obtained by other methods. More importantly, this facile fabrication technique possesses attractive merits, such as high time- and energy-efficiency, low cost and the potential of mass production. Hence, a significant foundation has been laid for the future synthesis of other Mg-based and functional materials.

Probing anisotropy of Seebeck coefficient and enhanced thermoelectric performance of Mg2Si0.35Sn0.65 single crystal

A single crystal of Sb-doped Mg2Si0.35Sn0.65 has been successfully prepared using high temperature-gradient directional solidification. The composition distribution of the crystal is uniform, and the preferred growth direction of the crystal is [111] orientation. The Seebeck coefficient (S) of the crystal is anisotropic in different orientations, and the electrical and thermal conductivity are isotropic properties. The S value in the [111] orientation can reach −265 μVK−1, which is much larger than the reported results. Also, the power factor in the [111] orientation has a value of 5.56 mWm−1K−2, which leads to a maximum ZT of 1.3 at 730 K. Results of Hall testing and first principles calculations agree well with the experiment results.

In situ nanostructure design leading to a high figure of merit in an eco-friendly stable Mg2Si0.30Sn0.70 solid solution

The relationship between the temperature and the composition as well as the microstructure of a Sb-doped Mg2Si0.30Sn0.70 solid solution was systematically studied according to the Mg2Si–Mg2Sn pseudo-binary phase diagram.

Effect of Annealing on Microstructure and Thermoelectric Properties of Sb-Doped Mg2Si0.5Sn0.5 Solid Solution

A 0.33 mol.% Sb-doped Mg2Si0.5Sn0.5 solid solution was synthesized by combining a liquid–solid reaction and hot-pressing process. The effect of annealing (1068 K, 250 h) on microstructure and thermoelectric properties of the solid solution was studied by x-ray diffraction (XRD), scanning electron microscopy, electron probe microanalysis, transmission electron microscopy, and thermoelectric measurements. The successful synthesis of the solid solution with an antifluorite structure was confirmed by XRD. The as-prepared sample contained Si, Sn, and MgO inclusions tens of nanometers in size. After annealing, Si and Sn inclusions disappeared, while the MgO nanoparticles remained almost unchanged; the charge carrier concentration and electrical conductivity decreased and the lattice thermal conductivity increased. As a result, the thermoelectric figure of merit ZT ∼ 0.34 at 394 K for the as-prepared sample deteriorated to ∼0.24 at 388 K after the annealing. The results suggest the presence of a high density of point defects, such as Mg interstitials in the as-prepared sample. The density of these Mg interstitials was reduced by the annealing, thereby affecting the charge carrier concentration and electrical conductivity. The increase in the lattice thermal conductivity upon annealing is attributed to the disappearance of point defects, grain boundaries (grain growth) and Si and Sn inclusions, which all act as phonon scattering centers. Thus, point defects and nanoinclusions might be important for optimizing the thermoelectric properties of a material. This work provides new insights into the effect of annealing on the microstructure and its relationship with the thermoelectric properties of Sb-doped Mg2Si0.5Sn0.5 solid solutions. It also provides hints for developing Mg2Si0.5Sn0.5-based materials with superior thermoelectric properties.

B23-O-16Correlation of thermoelectric properties to microstructure of an annealed Sb-doped Mg2Si0.5Sn0.5solid solution with TEM

The effect of annealing (1068 K, 250 h) on microstructure and thermoelectric properties of a compound, a 0.33 mol% Sb-doped Mg2Si0.5Sn0.5 solid solution, was studied with TEM, thermoelectric measurements, XRD, SEM, and EPMA (electron probe microanalysis). The solid solution was synthesized by combining a liquid-solid reaction and hot-pressing process. The as-prepared sample contained Si, Sn and MgO inclusions tens of nm in size. After annealing, Si and Sn inclusions disappeared, while the MgO nanoparticles remained almost unchanged; the charge carrier concentration and electrical conductivity decreased and the lattice thermal conductivity increased. As a result, the thermoelectric figure of merit ZT ~ 0.34 at 394 K for the as-prepared sample deteriorated to ~ 0.24 at 388 K after the annealing. The results suggest the presence of a high density of point defects, such as Mg interstitials in the as-prepared sample. The density of these Mg interstitials was reduced by the annealing, thereby affecting the charge carrier concentration and electrical conductivity. The increase in the lattice thermal conductivity upon annealing is attributed to the disappearance of point defects, grain boundaries (grain growth) and Si and Sn inclusions, which all act as phonon scattering centers. Thus point defects and nanoinclusions might be important for optimizing the thermoelectric properties of a material. This work provides new insights into the effect of annealing on the microstructure and its relationship with the thermoelectric properties of Sb-doped Mg2Si0.5Sn0.5 solid solutions. It also provides hints for developing Mg2Si0.5Sn0.5-based materials with superior thermoelectric properties. B23-O-15 doi:10.1093/jmicro/dfv174

Suppressing the bipolar contribution to the thermoelectric properties of Mg2Si0.4Sn0.6 by Ge substitution

The optimized thermoelectric figure of merit (ZT) of Mg2Si0.4Sn0.6 peaks at about 750 K because its relatively narrow band gap results in pronounced bipolar transport at higher temperatures. To suppress the bipolar transport, we have conducted a combined experimental and theoretical investigation of Ge-substitution effects on the band structures and thermoelectric properties of Sb-doped Mg2Si0.4Sn0.6−yGey (y = 0, 0.1, and 0.2) synthesized by solid state reaction. The measured transport properties of these compositions can be interpreted by a triple-parabolic-band model based on first-principle calculation of band structures. The results show that the bipolar conduction in the temperature range up to 800 K was effectively suppressed by Ge substitution that widens the band gap. As a side effect, Ge substitution induces the separation of two otherwise converged conduction bands in Mg2Si0.4Sn0.6, leading to reduced thermoelectric performance at low temperatures. The result of these two competing effects is that Ge-substituted samples achieve the maximum power factor similar to that of Mg2Si0.4Sn0.6, but with an increased peak temperature. In addition, Ge substitution reduces the bipolar thermal conductivity while maintaining the low lattice thermal conductivity of Mg2Si0.4Sn0.6.

Thermoelectric properties of Sb-doped Mg2(Si0.95Ge0.05) synthesized by spark plasma sintering

Abstract Magnesium silicide (Mg 2 Si) has recently attracted much interest as an n -type thermoelectric (TE) material for converting waste heat into electric power. The objective of this work was to reveal a mechanism to increase the thermoelectric properties of Mg 2 Si by Sb-doping and Ge-doping. Generally, Mg 2 Si is synthesized by all-molten method. However, this synthesis method has some problems, for example, mass defects of Mg for Mg 2 Si composition caused because the melting point of Mg 2 Si is very close to the boiling point of Mg. In this study, we tried to synthesize high purity Mg 2 Si from Mg and Si alloyed with a dopant (Sb, Ge) by spark plasma sintering (SPS). The Ge-doped samples had a higher ZT value than the ZT value of the Sb-doped sample of the same concentration without Ge because the thermal conductivity of the former was lower. The maximum ZT value of Sb0.23 at%-doped Mg 2 (Si 0.995 Ge 0.05 ) was 0.74 at 756 K.

Thermal stability of Mg2Si0.3Sn0.7 under different heat treatment conditions

Thermal stability of Sb-doped Mg2Si0.3Sn0.7 was studied by changing the heat treatment conditions (atmosphere, coating, temperature, and time).

Synergetic effect of Zn substitution on the electron and phonon transport in Mg2Si0.5Sn0.5-based thermoelectric materials.

Mg2Si1-xSnx alloys are a prospective material for thermoelectric generators at moderate temperatures. The thermoelectric properties of Mg2Si0.5Sn0.5-based thermoelectric materials with only Zn substitution or Zn/Sb co-doping were investigated. Isoelectronic Zn substitution did not affect the carrier concentration, but improved the carrier mobility. Zn atoms incorporated into a Sb-doped Mg2Si0.5Sn0.5 matrix simultaneously boosted the power factor and suppressed the lattice thermal conductivity, leading to an enhancement of the thermoelectric figure of merit ZT of the resulting bulk materials. The interplay between the electron and phonon transport of Mg2Si0.5Sn0.49Sb0.01 substituted with Zn at Mg sites results in an enhancement of the ZT by 25% at ∼730 K, from ZT≈ 0.8 in Mg2Si0.5Sn0.49Sb0.01 to ZT≈ 1.0 in Mg1.98Zn0.02Si0.5Sn0.49Sb0.01. Solid solutions in the Mg2Si-Mg2Sn system appear to be more promising for thermoelectric applications.

High thermoelectric figure of merit of Mg2Si0.55Sn0.4Ge0.05 materials doped with Bi and Sb

Thermoelectric properties of new Bi- and Sb-doped Mg2Si0.55Sn0.4Ge0.05 compounds prepared by powder methods were studied in the temperature range 300–823 K. The materials exhibited compositional inhomogeneites consisting of Sn-rich and Sn-poor areas. Doping with Bi or Sb had a very strong influence on the thermoelectric properties. A high figure of merit was obtained, with a value ∼1.4 for Bi members and ∼1.2 for Sb members at high temperatures. These values are the highest reported on this system.

The Influence of Grain Boundary Scattering on Thermoelectric Properties of Mg2Si and Mg2Si0.8Sn0.2

The temperature dependences of the Seebeck coefficient, and electrical and thermal conductivities of bulk hot-pressed Sb-doped n-type Mg2Si and Mg2Si0.8Sn0.2 samples were measured in the temperature range from 300 K to 850 K together with the Hall coefficients at room temperature. The features of the complex band structure and scattering mechanisms were analyzed based on experimental data within the relaxation-time approximation. Based on the obtained model parameters, the possibility of improvement of the thermoelectric figure of merit due to nanostructuring and grain boundary scattering was theoretically analyzed for both Mg2Si and the solid solution.

Roles of interstitial Mg in improving thermoelectric properties of Sb-doped Mg2Si0.4Sn0.6 solid solutions

While adding extra Mg is intended to compensate for the Mg loss during the synthesis, it often leads to Mg interstitials in Mg2(Si,Sn) materials and profoundly affects their thermoelectric properties. Herein we studied the electrical conductivity, Seebeck coefficient, and thermal conductivity of Mg2(1 + x)Si0.38Sn0.6Sb0.02 (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 = 0.1, a ∼60% enhancement compared to that of the sample x = 0.05. The Mg interstitials thus provide an extra tuning parameter in optimizing the thermoelectric properties of Mg2(Si,Sn)-based materials.

Low-temperature solid state reaction synthesis and thermoelectric properties of high-performance and low-cost Sb-doped Mg2Si0.6Sn0.4

Mg2Si1−xSnx compounds are a type of low-price, environment-friendly medium temperature thermoelectric materials with very important prospects for practical application, and the exploration of high performance Mg2Si1−xSnx compounds is currently attracting worldwide interest. In this study, Sb-doped Mg2Si0.6Sn0.4 compounds were prepared through a two-step, low-temperature solid state reaction method combined with the spark plasma sintering technique for rapid densification. The influence of Sb doping amount on the thermoelectric properties of Mg2Si0.6−ySn0.4Sby (0 ⩽ y ⩽ 0.015) compounds was investigated. The solid solubility limit of Sb in Mg2Si0.6Sn0.4 compounds was estimated around y = 0.0125. As y increased, the electrical conductivity of Mg2Si0.6−ySn0.4Sby (0 ⩽ y ⩽ 0.0125) compounds increased considerably, while the absolute value of the Seebeck coefficient and the lattice thermal conductivity decreased. The sample with y = 0.0125 had the highest ZT, reaching 1.11 at 860 K, and the samples with 0.005 ⩽ y ⩽ 0.015 all attained ZTmax > 0.95. The adopted synthesis process also showed very good repeatability and regularity in obtaining thermoelectric properties, together with the capability of precise composition control of Mg2Si0.6−ySn0.4Sby, making it promising for the practical application of Mg2Si based thermoelectric materials.