High efficiency Mg2(Si,Sn)-based thermoelectric materials: scale-up synthesis, functional homogeneity, and thermal stability

Elastic mechanical thermodynamic and thermoelectric properties of pristine and titanium doped Mg2Si: a density functional theory study

Herein, we report a systematic investigation of the effect of Titanium doping on the structural, elastic, mechanical, thermodynamic, and thermoelectric (TE) dynamics of Mg2Si Compounds using first-principle investigation. The present study has been carried out using the full potential linearized augmented plane wave method as implemented inWien2kcode undermBJexchange potentials. The investigations revealed that Mg2-xTixSi compounds have structural stability with cubic phase (Fm-3msymmetry) and possess degenerate semiconducting nature. The analysis of elastic constants revealed mechanical stability of the investigated compounds following Born criteria. Thermodynamic investigations have been carried out in the temperature range of 100-1500 K at zero pressure and the quantities like heat capacity, Debye temperature, Grüneisen constant, and thermal expansion coefficient have been critically analyzed. Lastly, the TE performance of Mg2-xTixSi compounds has been predicted by estimating the thermopower (S2σ) and TE figure of merit (zT) in the temperature range of 300-1500 K. The predicted value ofzTmaxfor Mg2-xTixSi compound is 0.67 at 800 K forx= 0.25 titanium content, suggesting materials promising application for TE energy harvesting and mechanical devices.

Low-Cost Magnesium-Based Thermoelectric Materials: Progress, Challenges, and Enhancements

Magnesium-based thermoelectric (TE) materials have attracted considerable interest due to their high ZT values, coupled with their low cost, widespread availability, nontoxicity, and low density. In this review, we provide a succinct overview of the advances and strategies pertaining to the development of Mg-based materials aimed at enhancing their performance. Following this, we delve into the major challenges posed by the severe working conditions, such as high temperature and thermal cycling, which adversely impact the behavior and long-term stability of the TE modules. Challenges include issues like the lack of mechanical strength, chemical instability, and unreliable contact. Subsequently, we focus on the key methodologies aimed at addressing these challenges to facilitate the broader application of the TE modules. These include boosting the mechanical strength, especially the toughness, through grain refining and additions of second phases. Furthermore, strategies targeted at enhancing the chemical stability through coatings and modifying the microstructure, as well as improving the contact design and materials, are discussed. In the end, we highlight the perspectives for boosting the practical applications of Mg-based TE materials in the future.

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 Mg2Si0.3Sn0.7. 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.

Overcoming Asymmetric Contact Resistances in Al-Contacted Mg2(Si,Sn) Thermoelectric Legs

Thermoelectric generators are a reliable and environmentally friendly source of electrical energy. A crucial step for their development is the maximization of their efficiency. The efficiency of a TEG is inversely related to its electrical contact resistance, which it is therefore essential to minimize. In this paper, we investigate the contacting of an Al electrode on Mg2(Si,Sn) thermoelectric material and find that samples can show highly asymmetric electrical contact resistivities on both sides of a leg (e.g., 10 µΩ·cm2 and 200 µΩ·cm2). Differential contacting experiments allow one to identify the oxide layer on the Al foil as well as the dicing of the pellets into legs are identified as the main origins of this behavior. In order to avoid any oxidation of the foil, a thin layer of Zn is sputtered after etching the Al surface; this method proves itself effective in keeping the contact resistivities of both interfaces equally low (<10 µΩ·cm2) after dicing. A slight gradient is observed in the n-type leg's Seebeck coefficient after the contacting with the Zn-coated electrode and the role of Zn in this change is confirmed by comparing the experimental results to hybrid-density functional calculations of Zn point defects.

Developing Contacting Solutions for Mg2Si1-xSnx-Based Thermoelectric Generators: Cu and Ni45Cu55 as Potential Contacting Electrodes

Magnesium silicides can be used for thermoelectric energy conversion as high values of figure of merit zT were obtained for n-type (1.4 at 500 °C) and p-type (0.55 at 350 °C) materials. This, however, needs to be complemented by low resistive and stable contacting to ensure long-term thermogenerator operation and minimize losses. In this study, we selected Cu and Ni₄₅Cu₅₅ as contacting electrodes for their high electrical conductivity, similar coefficient of thermal expansion (CTE), and good adhesion to Mg₂(Si,Sn). Both electrodes were joined to Mg₂Si_0.3Sn_0.7 pellets by hot pressing in a current-assisted press. Microstructural changes near the interface were analyzed using SEM/EDX analysis, and the specific electrical contact resistance r_c was estimated using a traveling potential probe combined with local Seebeck scanning. Good contacting was observed with both electrode materials. Results show low r_c with Cu, suitable for applications, for both n- and p-type silicides (<10 μΩ·cm²), with the occurrence of wide, highly conductive diffusion regions. Ni₄₅Cu₅₅ joining also showed relatively low r_c values (∼30 μΩ·cm²) for n- and p-type but had a less inhomogeneous reaction layer. We also performed annealing experiments with Cu-joined samples at 450 °C for 1 week to investigate the evolution of the contact regions under working temperatures. r_c values increased (up to ∼100 μΩ·cm²) for annealed n-type samples but remained low (<10 μΩ·cm²) for p-type. Therefore, Cu is a good contacting solution for p-type Mg₂(Si,Sn) and a potential one for n-type if the diffusion causing contact property degradation can be prevented.