More Like this

1. Micro- and Nano-structures Formed in Silicon Germanium Undergoing Laser Melting for Additive Manufacturing

   https://doi.org/10.1007/s11837-024-06941-4  

   Welch, Ryan; Şişik, Bengisu; LeBlanc, Saniya (November 2024, JOM)

   Abstract Thermoelectric materials offer a unique solution for active cooling or conversion of heat to electricity within a thermal protection system due to their solid-state nature. Yet, the integration of thermoelectrics into thermal protection systems is hindered by conventional manufacturing processes, which limit the material’s shape. Laser additive manufacturing can enable freeform shapes that allow integration of thermoelectrics into systems that are favorable for thermoelectric energy conversion. Through modeling and experimentation, this work presents single melt line processing and structures of silicon germanium, a high-temperature thermoelectric material, for laser powder bed fusion. Experiments consisted of single melt lines with an Nd-YAG laser and 50-µm spot size on Si₅₀Ge₅₀and Si₈₀Ge₂₀powder compacts. We found that laser processing of silicon germanium alloys causes oxidation and processing defects that are resolved through rescanning strategies. Rapid cooling results in a microstructure with silicon-rich grains and germanium entrapped near grain boundaries for Si₈₀Ge₂₀and dendritic structures in Si₅₀Ge₅₀which are linked to the degree of undercooling during solidification. Laser-processed silicon germanium contains crystalline defects, nanoscale precipitates, and an average grain size of 24 µm. This work informs laser additive manufacturing of silicon germanium parts and uncovers process-structure relationships of laser-processed silicon germanium alloys. 

   more » « less
2. Laser additive manufacturing of powdered bismuth telluride

   https://doi.org/10.1557/jmr.2018.390  

   Zhang, Haidong; Hobbis, Dean; Nolas, George S.; LeBlanc, Saniya (December 2018, Journal of Materials Research)

   Traditional manufacturing methods restrict the expansion of thermoelectric technology. Here, we demonstrate a new manufacturing approach for thermoelectric materials. Selective laser melting, an additive manufacturing technique, is performed on loose thermoelectric powders for the first time. Layer-by-layer construction is realized with bismuth telluride, Bi 2 Te 3 , and an 88% relative density was achieved. Scanning electron microscopy results suggest good fusion between each layer although multiple pores exist within the melted region. X-ray diffraction results confirm that the Bi 2 Te 3 crystal structure is preserved after laser melting. Temperature-dependent absolute Seebeck coefficient, electrical conductivity, specific heat, thermal diffusivity, thermal conductivity, and dimensionless thermoelectric figure of merit ZT are characterized up to 500 °C, and the bulk thermoelectric material produced by this technique has comparable thermoelectric and electrical properties to those fabricated from traditional methods. The method shown here may be applicable to other thermoelectric materials and offers a novel manufacturing approach for thermoelectric devices. 

   more » « less
3. Nano‐ and Micro‐Structures Formed during Laser Processing of Selenium Doped Bismuth Telluride

   https://doi.org/10.1002/admi.202100185  

   Welch, Ryan; Hobbis, Dean; Birnbaum, Andrew J.; Nolas, George; LeBlanc, Saniya (July 2021, Advanced Materials Interfaces)

   Abstract Laser processing of thermoelectric materials provides an avenue to influence the nano‐ and micro‐structure of the material and enable additive manufacturing processes that facilitate freeform device shapes, a capability that is lacking in thermoelectric materials processing. This paper describes the multiscale structures formed in selenium‐doped bismuth telluride, an n‐type thermoelectric material, from laser‐induced rapid melting and solidification. Macroscale samples are fabricated in a layer‐by‐layer technique using laser powder bed fusion (also known as selective laser melting). Laser processing results in highly textured columnar grains oriented in the build direction, nanoscale inclusions, and a shift in the primary charge carriers. Sparse oxide inclusions and tellurium segregation shift the material to p‐type behavior with a Seebeck coefficient that peaks at 143 µV K^–1at 95 °C. With an average relative density of 74%, fabricated parts have multiscale porosity and microscale cracking that likely resulted from low powder layer packing density and processing parameters near the transition threshold between conduction and keyhole mode processing. These results provide insights regarding the pathways for influencing carrier transport in thermoelectric materials via laser melting‐induced nanoscale structuring and the laser processing parameters required to achieve effective powder consolidation and hierarchical structuring in thermoelectric parts. 

   more » « less
4. Origins of twin boundaries in additive manufactured stainless steels

   https://doi.org/10.1016/j.actamat.2024.120035  

   Nie, Y; Chang, YT; Charpagne, MA (May 2024, Acta Materialia)

   316L and 304L stainless steels and a compositional gradient of both are fabricated using the same processing parameters via laser directed energy deposition additive manufacturing. In those alloys, the increase in chromium-to-nickel ratio is accompanied with grain refinement and formation of a high density of twin boundaries, i.e. sigma3 boundaries. By means of electron microscopy, crystallographic and thermodynamic calculations, we demonstrate that two mechanisms arising from the ferrite-to-austenite solidification mode are at the origin of twin boundary formation and grain refinement: 1) inter-variant boundaries emerging from the encounter of pairs of austenite grains formed from a common ferrite orientation with Kurdjumov-Sachs orientation relationship; 2) icosahedral short-range-ordering-induced (ISRO) nucleation of twin-related grains directly from the solidifying liquid. These findings define new routes to achieve grain boundary engineering in a single step in FeCrNi alloys, by tailoring the solidification pathway during the AM process, enabling the design of functionally graded materials with site-specific properties. 

   more » « less
5. Numerical Analysis of the Melt Pool Kinetics in Selective Laser Melting Based Additive Manufacturing of Thermoelectric Powders

   https://doi.org/10.1115/MSEC2024-124375  

   Suresh, Jagannath; Goyal, Gagan K; Wang, Haozheng; Zuo, Lei (June 2024, American Society of Mechanical Engineers)

   Abstract Thermoelectric generators convert heat energy to electricity and can be used for waste heat recovery, enabling sustainable development. Selective Laser Melting (SLM) based additive manufacturing process is a scalable and flexible method that has shown promising results in manufacturing high zT Bi2Te3 material and is possible to be extended to other material classes such as Mg2Si. The physical phenomena of melting and solidification were investigated for SLM-based manufacturing of thermoelectric (Mg2Si) powders through comprehensive numerical models developed in MATLAB. In this study, Computational Fluid Dynamics (CFD)-based techniques were employed to solve conservation equations, enabling a detailed understanding of temperature evolution within the molten pool. This approach was critical for optimizing processing parameters in our investigation, which were also used for printing the Mg2Si powders using SLM. Additionally, a phase field-based model was developed to simulate the directional solidification of the Mg2Si in MATLAB. Microstructural parameters were studied to correlate the effects of processing parameters to the microstructure of Mg2Si. 

   more » « less