Thermoelectric performance of n-type Mg2Ge

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.

Influence of the Mn5Ge3/Ge ohmic-contact interface on the Seebeck coefficient of the Mn5Ge3/Ge bilayer

Thermoelectricity is a well-known effect that can be used to convert heat energy into electrical energy. However, the yield of this conversion is still low compared to current photovoltaic technology. It is limited by the intrinsic properties of materials, leading to intensive materials science investigations for the design of efficient thermoelectric (TE) materials. Interface engineering was shown to be a valuable solution for improving materials' TE properties, supporting the development of multiphase TE materials. In particular, interfaces have been suggested to promote the increase of the Seebeck coefficient of materials without significantly impacting their electrical conductivity through the so-called energy filtering effect. This work aims at determining experimentally the effect of a metal/semiconductor interface exhibiting an ohmic character on the effective Seebeck coefficient of multiphase materials, focusing on the n-type Mn5Ge3/p-type Ge interface. This interface is shown not to contribute to carrier transport, but to contribute to carrier concentration filtering due to carrier injection or recombination. The Seebeck coefficient of the bi-phase material is shown to be dependent on the direction carriers are crossing the interface. The interface effect mainly results from a modification of charge carrier concentrations in the semiconductor.

Magnetic moment impact on spin-dependent Seebeck coefficient of ferromagnetic thin films

Magnetic materials may be engineered to produce thermoelectric materials using spin-related effects. However, clear understanding of localized magnetic moments (µ_I), free carriers, and Seebeck coefficient (S) interrelations is mandatory for efficient material design. In this work, we investigate µ_I influence on the spin-dependent S of model ferromagnetic thin films, allowing µ_I thermal fluctuations, ordering, and density variation influence to be independently investigated. µ_I influence on free carrier polarization is found to be of highest importance on S: efficient coupling of free carrier spin and localized magnetic moment promotes the increase of S, while spin-dependent relaxation time difference between the two spin-dependent conduction channels leads to S decrease. Our observations support new routes for thermoelectric material design based on spin-related effects in ferromagnetic materials.

Non-Rigid Band Structure in Mg2Ge for Improved Thermoelectric Performance

Magnesium silicide and its solid solutions are among the most attractive materials for thermoelectric generators in the temperature range of 500-800 K. However, while n-type Mg2(Si,Ge,Sn) materials show excellent thermoelectric performance, the corresponding p-type solid solutions are still inferior, mainly due to less favorable properties of the valence bands compared to the conduction bands. Here, Li doped Mg2Ge with a thermoelectric figure of merit zT of 0.5 at 700 K is reported, which is four times higher than that of p-type Mg2Si and double than that of p-type Mg2Sn. The reason for the excellent properties is an unusual temperature dependence of Seebeck coefficient and electrical conductivity compared to a standard highly doped semiconductor. The properties cannot be captured assuming a rigid band structure but well reproduced assuming two parabolic valence bands with a strong temperature dependent interband separation. According to the analysis, the difference in energy between the two bands decrease with temperature, leading to a band convergence at around 650 K and finally to an inversion of the band positions. The finding of a combination of a light and a heavy band that are non-rigid with temperature can pave the way for further optimization of p-type Mg2(Si,Ge,Sn).

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

Considering the need for large quantities of high efficiency thermoelectric materials for industrial applications, a scalable synthesis method for high performance magnesium silicide based materials is proposed.

Effect of the Fabrication Technique on the Thermoelectric Performance of Mg-Based Compounds-A Case Study of n-Type Mg2Ge

High performance, low cost, and low toxicity have been the main characteristics associated with magnesium-based thermoelectric materials. Nevertheless, the high volatility of magnesium creates challenges in the synthesis of these materials. In this work, n-type Mg₂Ge is synthesized using a solid-state technique, fully characterized, and compared with Mg₂Ge fabricated through different processes. We have found that Bi is an ineffective dopant in Mg₂Ge and precipitates into Mg₂Bi₃. Regardless of the technique used, the loss of Mg by evaporation and formation of precipitates in Bi-doped samples resulted in a low charge carrier concentration and, consequently, a low power factor. The precipitates significantly reduced the lattice thermal conductivity, however, leading to a figure-of-merit, zT, of 0.4 at 725 K, improving the previously reported figure-of-merit, zT, of 0.2 for Sb-doped Mg₂Ge. This work highlights the impact of the fabrication technique on the thermoelectric performance of Mg-based compounds.