VPS is a thermal spray material deposition process that can provide high deposition rates and rapid quenching for thermoelectric materials. Beside its promising industrial scale production, thermal spray can directly spray thermoelectric materials on different kinds of shapes, including cylinders, spheres, and curved surfaces. Magnesium silicide (Mg2Si) samples have been synthesized by VPS, which has been shown to be an effective way to reduce the thermal conductivity of material compared with conventional hot press method. Since Mg2Si is extremely sensitive to oxygen and temperature in more traditional thermal spray techniques, the VPS approach using low power has maintained a close representation of the feedstock powder composition.
In this work, the reduced thermal conductivity observed in the Mg2Si material is most likely due to reduced heat conduction by pores and cracks in the VPS samples. Carrier concentration represents a tradeoff between electrical conductivity and Seebeck coefficient. The Seebeck coefficient increases with a decrease in carrier concentration if the phase remains the same, however, the electrical conductivity increases as carrier concentration increases (the mobility does not change significantly). Low-temperature electrical measurements also suggest a 3D variable hopping effect for both hot press and VPS samples. VPS samples achieved ZT values of almost 30% of those hot-pressed samples using the same starting powder. The work suggests that vacuum thermal spray can be a viable means to directly fabricate thermoelectric materials for high-volume applications.
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