Efficiency Measurement and Modeling of a High‐Performance Mg₂(Si,Sn)‐Based Thermoelectric Generator

Abstract

Mg2(Si,Sn) is an attractive material class due to its excellent thermoelectric (TE) properties, its eco‐friendly constituents, its low mass density, and its low price. A lot of research has been done on optimizing its TE properties; however, works on its use in thermoelectric generators (TEG) are scarce. Herein, the first conversion efficiency measurement of a functional, fully Mg2(Si,Sn)‐based TEG, approaching a maximum value of 4% for an applied ΔT = 375 °C, is shown. A maximum power density of 0.9 W cm−2 (related to the cross‐sectional area of the TE legs) at ΔT=375 °C is also reported, which is among the highest performance of silicide‐based modules reported in literature. Efficiency measurements can be tricky due to the uncertainty of heat flow measurement and parasitic heat losses; therefore, assessing the measurement reliability by confronting it to theoretical calculations is necessary. TEG device simulation in a constant property model is used to compare measured data to expected values and a good match is found (<1% deviation for current at maximum power, <4% difference for maximum power output, deviation within measurement uncertainty range for heat flows and efficiency). The significant discrepancy between measurement and calculations of the inner electrical resistance reveals room for improvement. Cracks form due to thermally induced mechanical stress, which dramatically increase the inner electrical resistance. It is shown that by avoiding those cracks, the maximum power output and conversion efficiency of the TEG could be improved by 30%.

High-Performance Functionalized Mg2Si0.9Sn0.1 Thermoelectric Leg Synthesis by a Single-Step Reactive SPS Process

“…Table shows the processing techniques and temperatures used in the literature to fabricate n-type Mg 2 (Si,Sn) with contacts. It has been reported that in n-type Mg 2 (Si,Sn) TE element fabrication, the material synthesis was carried out first and then separate contact fabrication processes were performed separately. ,,,− In addition, the contact layer oxidation and removal process are inevitable due to two different processing sequences, adding additional manufacturing costs for device fabrication.…”

“…In general, the underlying scientific principles for properly selecting contact materials and developing bonding techniques are still limited. Several elemental metallic contacts, such as Cu, ,, Ag, ,, Ni, ,,, and Al, − have been tried to achieve chemically and thermally stable joints on binary magnesium–silicon using different processing techniques. Few reported studies emphasized the use of binary ,, and ternary compounds to overcome the chemical reactivity of elemental metallic contacts.…”

“…Although several efforts have been made on contact making, it was found that the long processing time and high rate of diffusion kinetics lead to higher contact resistance and deterioration of properties of thermoelectric compounds. ,, Recently, aluminum contact fabrication on the Mg 2 (Si,Sn) material reported low contact resistance and high stability . However, the asymmetry in electrical contact resistances due to Al 2 O 3 layer formation is a significant concern in TEG performance. , …”

A Constant Properties Model for the Performance Estimation in Segmented Thermoelectric Generator Elements and Its Experimental Validation Using an n-Type Mg2Si0.3Sn0.7–Bi2Te2.7Se0.3 Segmented Leg

“…24 Mg 2 (Si,Sn)-based TE materials, consisting of earth-abundant and nontoxic chemical elements, are attractive because of their low cost and high TE performance in the medium-temperature (500−800 K) range. Doped n-type Mg 2 (Si,Sn) materials reported zT ≈ 1−1.7 above 723 K. 6,25 The conversion efficiency of Mg 2 (Si,Sn)-based TE devices is reported to be ∼5% 26,27 as compared to the 8−10% for the state-of-the-art mediumtemperature TE devices based on materials such as PbTe, CoSb 3 , and half-Heusler alloys. 28−30 Further enhancement in conversion efficiencies can be achieved by segmenting these materials with low temperature (<500 K) TE materials such as Bi 2 Te 3 .…”

Thermal Stability of Mg2Si0.6Sn0.4 under Oxidation Conditions

“…Thermoelectricity is an elegant way to produce useful energy from waste heat with no production of undesired greenhouse gases. It can be considered as full part of green renewable energy sources, the development of thermoelectric generators (TEGs) for mid-temperature applications being booming these last few years. − Among the available thermoelectric (TE) materials, solid solutions of Mg 2 Si with Mg 2 Sn have attracted considerable attention as useful n -type materials for TEGs working in the mid-temperature range (250–550 °C). − Not only do they exhibit good thermoelectric properties, − but they are also composed of light, non-toxic, inexpensive, and earth-abundant raw elements. , The significant value of 1.5 achieved at 450 °C for the dimensionless thermoelectric figure of merit ZT ( ZT = S 2 T /ρκ, where S is the Seebeck coefficient, κ is the total thermal conductivity, ρ is the electrical resistivity, and T is the absolute temperature) results from the extensive studies performed over the years to improve their thermoelectric properties through different means. − However, their use in TEGs under air remains challenging as they are prone to oxidation and sublimation at temperatures higher than 500 °C, reducing their performance and lifetime. − It becomes even more challenging when the TEGs have to withstand numerous heating/cooling cycles, as it is often the case in actual conditions of use.…”