Unravelling the interplay between defects, dopant species, and thermoelectric properties in p-type Mg2X (X=Si, Ge, Sn)
Final Report Abstract
Thermoelectricity deals with the direct and reversible conversion of heat into electrical energy and vice versa. Thermoelectric devices can use electrical power for heat pumping and cooling and are therefore used for temperature regulation, e.g., in car seats, semiconductor lasers, and the biochemical industry. On the other hand, they can also make use of waste heat and convert it into usable electrical energy. Heat-to-electricity conversion with a high efficiency requires materials with 휎푆 2 good thermoelectric properties, i.e., with a large thermoelectric figure of merit 푧푇 = 휅 푇, where 휎 is the electrical conductivity, 푆 the Seebeck coefficient, 휅 the thermal conductivity, and 푇 the absolute temperature. The development of efficient materials with high 푧푇 for this temperature regime has therefore been a focus of the thermoelectric materials research in the last decades, resulting in attractive thermoelectric properties for several material classes including Mg2Si-based solid solutions. The constituent elements of Mg2Si are among the most abundant in the earth’s crust, giving this class a crucial advantage in view of potential large-scale applications and economical attractiveness. Moreover, magnesium silicide based solid solutions have the lowest low mass density 휌 among all relevant thermoelectric materials, making them especially suitable for airborne or mobile applications. This project dealt with the understanding and further development of (p-type) Mg2X (X=Si,Ge,Sn), whose application of p-Mg2X in TE devices is partially hindered by the lower performance of the ptype compared to the n-type. The central hypothesis of this project was, that intrinsic (Mg-related) and/or extrinsic (dopant related) point defects are responsible for that. It was indeed confirmed that the performance is limited in particular for the Si-rich solid solutions by a too low carrier concentration and the underlying mechanism was unveiled. However, it was also shown that the superior properties of Sn-rich solid solutions (compared to Si-rich) is more due to the electronic band structure than point defects. In summary, based on controlled synthesis, TE property characterization, microstructural analysis and TE transport modelling it was possible to: 1. Establish a link between material composition and defect concentrations; this included the study of different dopant as well as Mg content and the Si:Ge:Sn ratio. 2. Establish a relationship between synthesis parameters and defect concentrations combining TE transport and defect chemistry (for n-type material). 3. Quantify the effect of defects on thermoelectric properties like carrier concentration and mobility. 4. Develop models that capture defect effects, both for isoelectronic defects (Si:Sn) and those induced by dopants or variation in Mg content. Experimental and modelling results allow for an optimization of p-Mg2X and a rationale adjustment of e.g. carrier concentration and composition in dependence of different application scenarios. Synthesis and characterization of p-type Mg2Ge lead to surprisingly good TE performance, probably caused by a temperature dependent electronic band structure, the fundamental reason for which remains to be uncovered. The second surprise was the relevance of point defects not only for material optimization, but also for contact development, opening up a full new field for further research.
Publications
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"Systematic analysis of the interplay between synthesis route, microstructure, and thermoelectric performance in p-type Mg2Si0.2Sn0.8". Materials Today Physics 2019, 100133
Kamila, H.; Goyal, G. K.; Sankhla, A.; Ponnusamy, P.; Mueller, E.; Dasgupta, T.; de Boor, J.
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"Density of States Effective Mass for p-Type Mg2Si-Mg2Sn Solid Solutions: Comparison between Experiments and First Principles Calculations". The Journal of Physical Chemistry C 2020
de Boor, J.; Berche, A.; Jund, P.
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"On the applicability of the single parabolic band model to advanced thermoelectric materials with complex band structures". Journal of Materiomics 2021, 7 (3), 603-611
de Boor, J.
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"On the relevance of point defects for the selection of contacting electrodes: Ag as an example for Mg2(Si,Sn)-based thermoelectric generators". Materials Today Physics 2021, 16, 100309
Ayachi, S.; Deshpande, R.; Ponnusamy, P.; Park, S.; Chung, J.; Park, S.; Ryu, B.; Müller, E.; de Boor, J.
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"On the role of Mg content in Mg2(Si,Sn): Assessing its impact on electronic transport and estimating the phase width by in situ characterization and modelling". Materials Today Physics 2021, 21, 100471
Sankhla, A.; Kamila, H.; Naithani, H.; Mueller, E.; de Boor, J.
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"Understanding the dopability of p-type Mg2(Si,Sn) by relating hybrid-density functional calculation results to experimental data". Journal of Physics: Energy 2022
Kamila, H.; Ryu, B.; Ayachi, S.; Sankhla, A.; Müller, E.; de Boor, J.
