Synthesis of thermoelectric magnesium-silicide pastes for 3D printing, electrospinning and low-pressure spray

In this work, eco-friendly magnesium-silicide (Mg₂Si) semiconducting (n-type) thermoelectric pastes for building components concerning energy-harvesting devices through 3D printing, spray and electrospinning were synthetized and tested for the first time. The Mg₂Si fine powders were obtained through the combination of ball milling and thermal annealing under Ar atmosphere. While the latter process was crucial for obtaining the desired Mg₂Si phase, the ball milling was indispensable for homogenizing and reducing the grain size of the powders. The synthetized Mg₂Si powders exhibited a large Seebeck coefficient of ~ 487 µV/K and were blended with a polymeric solution in different mass ratios to adjust the paste viscosity to the different requirements of 3D printing, electrospinning and low-pressure spray. The materials produced in every single stage of the paste synthesis were characterized by a variety of techniques that unequivocally prove their viability for producing thermoelectric parts and components. These can certainly trigger further research and development in green thermoelectric generators (TEGs) capable of adopting any form or shape with enhanced thermoelectric properties. These green TEGs are meant to compete with common toxic materials such as Bi₂Te₃, PbTe and CoSb that have Seebeck coefficients in the range of ~ 290-700 μV/K, similar to that of the produced Mg₂Si powders and lower than that of 3D printed bulk Mg₂Si pieces, measured to be ~ 4866 μV/K. Also, their measured thermal conductivities proved to be significantly lower (~ 0.2 W/mK) than that reported for Mg₂Si (≥ 4 W/mK). However, it is herein demonstrated that such thermoelectric properties are not stable over time. Pressureless sintering proved to be indispensable, but difficultly achievable by long thermal annealing (even above 32 h) in inert atmosphere at 400 °C, at least for bulk Mg₂Si pieces constituted by a mean grain size of 2-3 μm. Hence, for overcoming this sintering challenge and become the silicide's extrusion viable in the production of bulk thermoelectric parts, alternative pressureless sintering methods will have to be further explored.