Process-Structure-Property Relationships for Laser Additive Manufacturing of Thermoelectric Materials for Low and High Temperature Applications

Thermoelectric materials enable direct, solid-state conversion of heat to electricity and vice versa, so they offer functional power generation and heat pumping capabilities. Additive manufacturing offers the potential to structure thermoelectric materials and devices at multiple length scales, thus improving both intrinsic properties, overall system performance, and application integration. We report on experimental and computational investigation of process-structure-property relationships for laser powder bed fusion of thermoelectric materials including bismuth telluride (for applications near 100°C) and silicon germanium (for applications near 1000°C). Strategies for non-spherical powders were developed, and the process parameters that lead to conduction mode melting were determined. The process has been demonstrated on multiple tools, both custom setups and commercial tools. The structure of both single melt tracks and bulk parts were characterized at nano-, micro-, and meso-scales for varying process parameters such as laser power, scan speed, and powder layer thickness. Finite element modeling of laser energy deposition and heat diffusion within the powder bed resulted in simulations of spatial and temporal temperature gradients during laser processing of thermoelectric material. These simulations were compared to in situ, long wavelength infrared sensor data collected during processing experiments. The ratio of temperature gradient and solidification rate was used to predict the transition between equiaxed and columnar grains and compare the prediction to the experimentally observed grain morphology. A kinetic Monte Carlo simulation was used to simulate the formation of grain structure during solidification. Thermoelectric properties (Seebeck coefficient, electrical conductivity, and thermal conductivity) were measured on fabricated bulk samples, and structure and property characterization results indicate a link between the process-dependent nanostructure and the Seebeck coefficient, including a transition between n- and p-type behavior and graded Seebeck coefficient. These results indicate functional grading of thermoelectric properties during the additive manufacturing process may be possible. Finally, the power generation performance impact of unique device shapes (e.g., layered, hourglass, and hollow shapes) enabled by additive manufacturing was modeled for high and low temperature and heat flux boundary conditions, and selected promising shapes were fabricated. This work provides knowledge about how rapid melting and solidification of thermoelectric materials during laser additive manufacturing impacts the material structure and properties. The results show how variations in process parameters can be leveraged to enable functional grading of thermoelectric properties within the material by manipulating the material at the nano and micro-scales. The additive manufacturing technique enables fabrication of unique shapes at the mesoscale that show improvements for system-level performance of thermoelectric devices.