Hybrid-functional and quasi-particle calculations of band structures of Mg2Si, Mg2Ge, and Mg2Sn

We perform hybrid functional and quasi-particle band structure calculations with spin-orbit interaction to investigate the band structures of Mg2Si, Mg2Ge, and Mg2Sn. For all Mg2X materials, where X = Si, Ge, and Sn, the characteristics of band edge states, i.e., band and valley degeneracies, and orbital characters, are found to be conserved, independent of the computational schemes such as density functional generalized gradient approximation, hybrid functionals, or quasi-particle calculations. However, the magnitude of the calculated band gap varies significantly with the computational schemes. Within density-functional calculations, the one-particle band gaps of Mg2Si, Mg2Ge, and Mg2Sn are 0.191, 0.090, and -0.346 eV, respectively, and thus severely underestimated compared to the experimental gaps, due to the band gap error in the density functional theory and the significant relativistic effect on the low-energy band structures. By employing hybrid-functional calculations with a 35% fraction of the exact Hartree-Fock exchange energy (HSE-35%), we overcame the negative band gap issue in Mg2Sn. Finally, in quasi-particle calculations on top of the HSE-35% Hamiltonians, we obtained band gaps of 0.835, 0.759, and 0.244 eV for Mg2Si, Mg2Ge, and Mg2Sn, respectively, consistent with the experimental band gaps of 0.77, 0.74, and 0.36 eV, respectively.Comment: 23 pages, including 84 references, 5 tables, 3 figure

Impact of the Dopant Species on the Thermomechanical Material Properties of Thermoelectric Mg2Si0.3Sn0.7

Thermoelectric generators are an excellent option for waste heat reuse. Materials for such devices have seen their thermoelectric properties improving constantly. The functioning of a generator, however, does not only depend on thermoelectric properties. Thermal and mechanical properties play a decisive role in the feasibility of any thermoelectric generator. To shed light on the properties exhibited by thermoelectric materials, we present the temperature dependent characterization of Young’s modulus and coefficient of thermal expansion for Mg(2)Si(0.3)Sn(0.7). Comparing undoped to Bi-doped n-type and Li-doped p-type material, we investigated the influence of doping in the relevant temperature regime and found the influences to be minor, proving similar properties for n- and p-type. We found a Young’s modulus of 84 GPa for the p-type and 83 GPa for the n-type, similar to that of the undoped compound with 85 GPa. The thermal expansion coefficients of undoped, as well as n- and p-type were equally similar with values ranging from 16.5 to 17.5 × 10(−6) 1/K. A phase analysis was performed to further compare the two materials, finding a similar phase distribution and microstructure. Finally, using the gathered data, estimations on the possible thermally induced stresses under a temperature difference are provided to evaluate the relevance of knowing temperature dependent thermal and mechanical properties

In-situ Sample Preparation of Oxidizing and Contaminating Samples for High Quality EDS and WDS Quantification Using FIB-SEM

Energy dispersive and wavelength dispersive X-ray spectroscopy (EDS and WDS) are very important tools in materials research to obtain information about the chemical composition of a sample. A planar and clean surface within a homogeneous material is essential to achieve a proper quantitative analysis with those methods. Often, surfaces tend to contaminate or oxidize very fast under atmospheric conditions. Usually samples cannot be transferred to the microscope without exposure to these conditions. Electron microscopes themselves provide a high vacuum free of contamination. Surfaces prepared with a focused ion beam (FIB) are smooth and sufficiently free of contamination, but are not perpendicular to the electron beam. In this work, an in situ preparation procedure was developed to improve the accuracy of quantitative analytical results using a FIB-SEM equipped with EDS and WDS. For this, the geometric obstacles had to be bypassed to achieve a FIB-prepared surface free of contamination or oxidization, perpendicular to the electron beam and suitable for the analysis

Upscaled Synthesis of n‐and p‐Type Thermoelectric Skutterudite Single Legs by Gas Atomization and Current‐Assisted Sintering

CoSb3‐based Skutterudites are among the best materials for thermoelectric generator (TEG) applications in the intermediate temperature range up to 500 °C. Synthesis of these materials is usually performed on a laboratory scale in materials research. In order to be suitable for an industrial low cost production of TEG technologies capable of delivering large amounts of thermoelectric (TE) materials are needed. A process mastering this challenge is gas atomization, which has been adapted to the requirements of TE materials, in particular CoSb3‐based Skutterudites .It is found that despite rapid solidification taking place in the atomization process the produced powder material contains only traces of the target Skutterudite phase. Microstructure investigation shows a very fine dispersion on the micrometer scale of CoSb, CoSb2, and Sb phases in the atomized particles, making diffusion paths for the formation of the Skutterudite phase short. This allows the use of short‐term heat treatment to achieve almost single phase material of high functional homogeneity. Different thermal post‐treatments are evaluated leading to a content of >98% of the Skutterudite phase in large ingots. Doping and filling by varying the starting composition is applied to tune the materials to n‐ and p‐type conduction, respectively, and led to an increase of their thermoelectric figure of merit ZT up to values of 0.9 and 0.72 for n‐ and p‐type material, respectively. Gas atomization can be used in combination with current‐assisted hot pressing to produce p‐ and n‐type Skutterudite single legs in large quantities in comparably short time. The results on ZT shown in the present work are measured on still not optimized materials and prove the possibility of Skutterudites fabrication by gas atomization with moderate ZT values

Native point defects and low pp-doping efficiency in Mg2(Si,Sn)Mg_2 (Si,Sn) solid solutions: A hybrid-density functional study

We perform hybrid-density functional calculations to investigate the charged defect formation energy of native point defects in $Mg_2 Si$, $Mg_2 Sn$, and their solid solutions. The band gap correction by hybrid-density functional is found to be critical to determine the charged defect density in these materials. For $Mg_2 Si$, $Mg$ interstitials are dominant and provide unintentional $n$-type conductivity. Additionally, as the $Mg$ vacancies can dominate in $Mg$-poor $Mg_2 Sn$, $p$-type conductivity is possible for $Mg_2 Sn$. However, the existence of low formation energy defects such as $Mg_{Sn}^{1+}$ and $I_{Mg}^{2+}$ in $Mg_2 Sn$ and their diffusion can cause severe charge compensation of hole carriers resulting in low $p$-type doping efficiency and thermal degradation. Our results indicate that, in addition to the extrinsic doping strategy, alloying of $Mg_2 Si$ with $Mg_2 Sn$ under $Mg$-poor conditions would be necessary to enhance the $p$-type conductivity with less charge compensation.Comment: Main: 17 pages (including title, abstract, main, references, figure captions. 4 figures). This manuscript is accepted for publication in JALCOM. The article will be published as Gold Open Acces

Layered germanium tin antimony tellurides: element distribution, nanostructures and thermoelectric properties

In the system Ge-Sn-Sb-Te, there is a complete solid solution series between GeSb2Te4 and SnSb2Te4. As Sn2Sb2Te5 does not exist, Sn can only partially replace Ge in Ge2Sb2Te5; samples with 75% or more Sn are not homogeneous. The joint refinement of high-resolution synchrotron data measured at the K-absorption edges of Sn, Sb and Te combined with data measured at off-edge wavelengths unambiguously yields the element distribution in 21R-Ge0.6Sn0.4Sb2Te4 and 9P-Ge1.3Sn0.7Sb2Te5. In both cases, Sb predominantly concentrates on the position near the van der Waals gaps between distorted rocksalt-type slabs whereas Ge prefers the position in the middle of the slabs. No significant antisite disorder is present. Comparable trends can be found in related compounds; they are due to the single-side coordination of the Te atoms at the van der Waals gap, which can be compensated more effectively by Sb3+ due to its higher charge in comparison to Ge2+. The structure model of 21R-Ge0.6Sn0.4Sb2Te4 was confirmed by high-resolution electron microscopy and electron diffraction. In contrast, electron diffraction patterns of 9P-Ge1.3Sn0.7Sb2Te5 reveal a significant extent of stacking disorder as evidenced by diffuse streaks along the stacking direction. The Seebeck coefficient is unaffected by the Sn substitution but the thermal conductivity drops by a factor of 2 which results in a thermoelectric figure of merit ZT = similar to 0.25 at 450 degrees C for both Ge0.6Sn0.4Sb2Te4 and Ge1.3Sn0.7Sb2Te5, which is higher than similar to 0.20 for unsubstituted stable layered Ge-Sb-Te compounds