Thermal stability of Mg2Si0.55Sn0.45 for thermoelectric applications

Abstract

Understanding the thermal stability of the Mg 2 (Si,Sn) system is essential to define their safe temperatures of service. Despite its good thermoelectric performance, Mg 2 (Si,Sn) is subject to a phase separation during thermal cycling due to the miscibility gap, which leads to a degradation of its thermoelectric properties and affects its performance during device operation. Isothermal annealing at 500 C and 750 C were performed with different annealing time to investigate thermal stability of Mg 2 (Si,Sn). During the heat treatment, two phases were formed associated with porosity in the matrix. In addition, thickness of specimen was tracked and a significant expansion was detected. This phenomenon is attributed to the Kirkendall effect. The composition and the structure of the two forming phases were investigated by electron probe microanalysis and X-ray diffraction. Finally, the optimized thermal treatment allowed to stabilize the Mg 2 (Si,Sn) without porosity and the presence of two thermodynamically stabilized phase (Mg 2 Si 0.41 Sn 0.59 and Mg 2 Si 0.58 Sn 0.42) leading to a better reliability of the silicide thermoelectric modules.

Manufacturing and performances of silicide-based thermoelectric modules

“…For the HMS material, the mean resistance value is 8.3 mΩ, with a standard deviation of 2%, while it is 2.1 mΩ with a standard deviation of 13% for the MSS material. The lower resistance value results from the lower intrinsic resistivity (as will be seen later) while the higher standard deviation is due to the presence of some composition inhomogeneities in the material as it has already been shown by Mejri et al [37]. Finally, all the legs issued either from a HMS or a MSS pellet have been qualified.…”

“…The sintered samples are dense, in agreement with the relative density values obtained of 5.1 g/cm 3 (96% of the theorical value) and 2.3 g/cm 3 (99% of the theorical value) for the HMS and MSS materials, respectively. The chemical composition and homogeneity, the crystalline structure and the microstructure as well as the thermomechanical properties of the SPS consolidated pellets of both the HMS and MSS materials used within this work have been already studied [33,37].…”

Interface Engineering Boosting High Power Density and Conversion Efficiency in Mg2Sn0.75Ge0.25‐Based Thermoelectric Devices

“…The distinct diffusion depth for element Sn was only hundreds of nanometers, which was lower than that reported for other TEiMs in the literature. [ 22–27 ] The σ s of Mg 2 Sn 0.75 Ge 0.25 is ≈18.5 MPa, while the Cu 2 MgFe/Mg 2 Sn 0.75 Ge 0.25 interface exhibits a value of up to 15 MPa. This suggests that the diffusion of Sn may have a slight impact on the bonding strength.…”

“…The Mg element was chosen as the priority in the Cu-based TEiM to create an Mg overcompensation environment to enhance stability, considering the susceptibility to Mg deficiency in Mg 2 (Si, Ge, Sn) TEcM. [22][23][24][25][26][27] While a suitable interdiffusion at the TEiM/TEcM interface is necessary for robust interface bonding, [9] if the diffusion is too abrupt, it would result in instability during the service. Given the instability exhibited by the Cu/Mg 2 Sn 0.75 Ge 0.25 interface at elevated temperatures, an element with appreciative diffusion was required, which was also less reactive with Mg 2 (Si, Sn)-based TEcM.…”

“…For example, solid solution [ 15–18 ] and doping [ 16,19–21 ] strategies were employed to optimize PF and ZT values via band convergence and strengthening phonon scattering. Additionally, Mg over‐compensation, [ 22,23 ] heat treatment [ 24 ] and surface protection utilizing BN, [ 25 ] Al 2 O 3 , [ 26 ] or aerogel [ 27 ] have brought further stability to the Mg 2 (Si, Sn)‐based TE materials. Consequently, these TE materials have met the demands for high performance, cost‐effectiveness, and reliability in practical applications.…”