On the relevance of point defects for the selection of contacting electrodes: Ag as an example for Mg2(Si,Sn)-based thermoelectric generators

Ayachi, Sahar; Deshpande, Radhika; Ponnusamy, Prasanna; Park, Sungjin; Chung, Jaywan; Ryu, Byungki; Müller, Eckhard; Boor, Johannes de

doi:10.1016/j.mtphys.2020.100309

Cited by 20 publications

(43 citation statements)

References 66 publications

(85 reference statements)

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“…There is a difference of about 20 µV/K from the TE material-electrode interface to the center of the n-type material (−110 µV/K at the interface, −90 µV/K in the middle). The appearance of a gradient after contacting is similar to what was reported on contacting with Ag electrodes, in which case the gradient was explained by charge carrier compensation due to Ag diffusion into the TE material [29,39]. This hypothesis was supported by the relatively low formation energies of electrode-related point defects compared to the intrinsic and dopant-induced point defects.…”

Section: Resultssupporting

confidence: 80%

“…Indeed, Li doping is aimed at Mg sites in the Mg2X material; however, it was found that, although Li on Mg In order to verify if the same mechanism can explain our experimental results, hybrid-DFT calculations were performed to study the defect stability of Zn-related defects in Mg 2 Si and Mg 2 Sn. The possible formation of Zn-related point defects is confirmed by comparing the formation energies of the Zn-related point defects to those of intrinsic and extrinsic (Bi and Li) defects from previous papers [39,40]. As the samples of interest presented in this paper are Sn-rich Mg 2 (Si,Sn) solid solutions, only the results of Mg 2 Sn are shown in Figure 5, while the results for Mg 2 Si are shown in Figure S5 in SI.…”

Section: Resultsmentioning

confidence: 69%

“…Contacting an electrode with a Zn layer implies that some Zn could diffuse into the TE material. This can potentially have detrimental effects on the TE properties (and potentially also on the thermal stability of the material) as was already observed with other electrodes such as Ag and Cu [29,32,39]. In order to check for those effects, Seebeck coefficient measurements were performed and reported in Figure 4.…”

Section: Resultsmentioning

confidence: 99%

“…The case of p-type Lidoped Mg 2 Sn under Mg-rich conditions will not be discussed. Indeed, Li doping is aimed at Mg sites in the Mg 2 X material; however, it was found that, although Li on Mg site defects are stable, Li interstitials are even more stable defects [39]. This means that the Li-doped material will always contain a certain proportion of Mg vacancies and Mg-rich conditions will not be obtained.…”

Section: Resultsmentioning

confidence: 99%

“…where E tot [D q ] and E 0 are total energies with and without defects, subscript i indicates the atomic element, µ i is the atomic chemical potential, δn i is the change of number of i-th element in the defective supercell with respect to the pristine one, and E F is the Fermi level of the system. Detailed information can be found in our previous work [39][40][41].…”

Section: Methodsmentioning

confidence: 99%

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

et al. 2021

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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.

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Section: Resultssupporting

confidence: 80%

Section: Resultsmentioning

confidence: 69%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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

et al. 2021

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High‐Performance Thermoelectric Devices Made Faster: Interface Design from First Principles Calculations

Ayachi,

Park,

Ryu

et al. 2023

Advanced Physics Research

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The enormous progress achieved with high‐performance thermoelectric materials has yet to be implemented in high‐performance devices. The bottleneck for this is the material‐specific design of the interface between the thermoelectric material and the electrical connections, particularly identifying suitable contacting electrodes. This has mainly been empirical, slowing down device maturation due to the vast experimental space. To overcome this, an electrode pre‐selection method based on first‐principles electronic structure calculations of charged defect formation energies is established in this work for the first time. Such method allows to predict thermoelectric leg degradation due to impurity diffusion from the electrode into the thermoelectric material and formation of charge carrier traps, causing a majority carrier compensation and performance deterioration. To demonstrate the feasibility of this approach, the charged point defect formation energies of relevant metal electrodes with Mg2(Si,Sn) are calculated. Five hundred ten defect configurations are investigated, and the interplay between intentional doping and electrode‐induced point defects is predicted. These predictions are compared with Seebeck microprobe measurements of local carrier concentrations near the Mg2(Si,Sn)‐electrode interface and a good match is obtained. This confirms the feasibility of electrode screening based on defect formation energy calculations, which narrows down the number of potential electrodes and accelerates device development.

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Instability Mechanism in Thermoelectric Mg₂(Si,Sn) and the Role of Mg Diffusion at Room Temperature

Duparchy,

Deshpande,

Sankhla

et al. 2024

Small Science

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Mg2(Si,Sn) shows great promise as thermoelectric material as it is made from non‐toxic, abundant, and cost‐effective elements offering high performance. This has been emphasized by several thermoelectric generator prototypes, demonstrating technological maturity. However, material stability is paramount for large‐scale applications whereas we reveal here that the thermal stability of n‐type Mg2(Si,Sn) may be limited even at room temperature (RT). Integral thermoelectric properties measurements, locally resolved Seebeck coefficient analysis, scanning electron microscopy/energy‐dispersive X‐ray spectroscopy, and atomic force microscopy are employed to assess changes of n‐type samples stored in ambient atmosphere for years, revealing the evolution of the carrier concentration and transport properties in the material as well as surface degradation. This is caused by the diffusion of loosely bound Mg from the bulk towards the surface and subsequent oxidation, leading to a change of Mg‐based intrinsic defect concentrations, thereby degrading the thermoelectric performance. This microscopic mechanism is backed up by first‐principles calculations, revealing that Mg diffusivity in Mg2(Si,Sn) is high at RT and that diffusion occurs mainly via Mg vacancies. The observed much faster degradation of Sn‐rich Mg2(Si,Sn) can be correlated with the higher density of Mg vacancies in Mg2Sn compared to Mg2Si, as predicted from defect formation energies.

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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

Cited by 20 publications

References 66 publications

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

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

High‐Performance Thermoelectric Devices Made Faster: Interface Design from First Principles Calculations

Instability Mechanism in Thermoelectric Mg₂(Si,Sn) and the Role of Mg Diffusion at Room Temperature

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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

Cited by 20 publications

References 66 publications

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

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

High‐Performance Thermoelectric Devices Made Faster: Interface Design from First Principles Calculations

Instability Mechanism in Thermoelectric Mg2(Si,Sn) and the Role of Mg Diffusion at Room Temperature

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Product

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Instability Mechanism in Thermoelectric Mg₂(Si,Sn) and the Role of Mg Diffusion at Room Temperature