Thermoelectric materials with grain boundary: Strategies to parameters

<p indent="0mm">Thermoelectric conversion technology is a novel clean energy technology that directly converts heat into electricity utilizing the thermoelectric effect of materials. The optimization of thermoelectric performance in materials has garnered widespread attention and research, where the carrier and phonon transports directly determine the final figure of merit (<italic>ZT</italic>). Grain boundaries, the most common microstructures or defects in polycrystalline materials, significantly impact the transport behaviors of carriers (electrons or holes) and phonons. On the one hand, grain boundaries are the locus of various defects, which adversely affect carrier transport and thereby inhibit electrical performance. The most intuitive effect is carrier scattering at grain boundaries, which encompasses grain boundary scattering and barrier scattering. Furthermore, the trap effect and depletion region effect at grain boundaries also impede carrier transport. On the other hand, grain boundaries can also act as intense scattering centers for phonons, hindering heat transport. Phonons can be scattered at grain boundaries, altering their direction and energy of propagation, leading to changes in phonon transport paths. Consequently, these altered phonon transport behaviors enhance the thermal resistance and diminish the thermal conductivity of the material. Additionally, the irregular arrangement of atoms at grain boundaries may give rise to localized phonon states, creating an additional barrier to phonon transport. Therefore, understanding and controlling electron-phonon transport behaviors at grain boundaries is crucial for enhancing the thermoelectric performance of polycrystalline materials. This review paper focuses on the impact of microstructures on the behaviors of carriers and phonons at grain boundaries, and systematically summarizes effective strategies for regulating thermoelectric transport parameters based on grain boundary engineering. Firstly, we delve into effective mechanisms and propose strategies to optimize electrical transport by analyzing carrier behaviors at grain boundaries. These include dynamical carrier concentration regulation, carrier mobility optimization (through conducting paths, high dielectric constant phases, low-angle grain boundaries, and phases aligned with the matrix band structure), and Seebeck coefficient enhancement via energy filtering. Secondly, we investigate effective scattering mechanisms to hinder thermal transport by examining phonon behaviors at grain boundaries, and put forward strategies to reduce lattice thermal conductivity, such as increasing grain boundary density (via grain refinement and nano-second phases). Furthermore, we explore the mechanisms for decoupling electron-phonon transport through a comprehensive analysis of their behaviors at grain boundaries, and develop methods to synergistically enhance thermoelectric transport properties. This includes optimizing grain size and introducing coherent or semi-coherent interfaces. Finally, we underscore the pivotal role of grain boundaries in optimizing carrier and phonon transport. Moreover, we suggest that multi-scale theoretical simulations and <italic>in-situ</italic> experimental characterization of grain boundary microstructures will contribute significantly to a deeper understanding of transport mechanisms. The exploration and development of novel strategies and methods for grain boundary structure design will more effectively decouple carrier and phonon transport, thereby establishing the relationship between microstructures, grain boundary regulation, and thermoelectric properties. This review provides an essential theoretical and experimental foundation for the future design and optimization of thermoelectric materials.