(321c) Phase Transformation of Silicon Nanowires As a Route for the Mass Production of Mg2Si Nanowires

Solid state thermoelectric modules can convert unused waste heat energy directly into electricity without emitting any additional greenhouse gases. The performance of thermoelectric materials that serve as building blocks in the fabrication of thermoelectric modules can be gauged by examining their figure of merit, zT,which is given by zT = S² σT / (κ_e + κ_l). Here, S is the Seebeck coefficient of the material, σ is the electrical conductivity of the material, and κ_e and κ_l are the thermal conductivities of the material from electronic and lattice contributions, respectively. Enhancing the zT values of materials beyond the current state-of-the-art requires materials that exhibit lower thermal conductivities and higher electrical conductivities. Theoretical predictions indicate that accomplishing this task, while navigating the constraints imposed by the Wiedemann-Franz law, is possible by selectively lowering the κ_l of materials. Selective reduction of the κ_l of materials is possible by phonon confinement and phonon scattering when they are synthesized in ultra-thin, single-crystalline nanowire format. One of the chemical compositions of materials useful for the fabrication of thermoelectrics is magnesium silicide (Mg₂Si). Previous studies have indicated that bulk and nanocrystalline Mg₂Si is a promising thermoelectric material in the 500 to 800 K temperature range. It is also non-toxic, comprised of earth-abundant elements, and inexpensive. However, it is not yet clear whether Mg₂Si in nanowire format performs better than Mg₂Si in bulk or other nanocrystalline formats. In this context, there is a need to develop a method for the mass production of Mg₂Si nanowire powders and measure their thermoelectric performance.

 A possible route for the mass production of Mg₂Si nanowire powders is the phase transformation of pre-synthesized silicon nanowires. To accomplish this, it is imperative to understand whether the phase transformation of silicon nanowires leads to the formation of Mg₂Si nanowires. Secondly, it is important to deduce the conditions under which the transformation of single-crystalline silicon nanowires leads to the formation of single-crystalline Mg₂Si nanowires. Hence, the aim of this presentation is to discuss phase transformation of silicon nanowires in detail. The silicon nanowires necessary for the phase transformation studies have been obtained by electroless etching of wafers, and the phase transformation of so-obtained silicon nanowires into Mg₂Si was accomplished by supplying magnesium vapor over their surfaces at elevated temperatures. This process resulted in the formation of polycrystalline Mg₂Si nanowires. The nucleation and growth steps involved in this process, along with the modification made to the experiments to attain single-crystalline silicon nanowire to single-crystalline Mg₂Si nanowire transformation, will be discussed in this presentation.