NbFeSb Half-Heusler Alloy Thermoelectric Transport Achieves High Power Factor with Complete Scattering Process Consideration

Thermoelectric materials, which convert heat into electricity and vice versa, offer a promising route to waste heat recovery and solid-state refrigeration, but achieving high efficiency remains a significant challenge. Bhawna Sahni, Yao Zhao, and Zhen Li, working with colleagues at various institutions, investigate the electronic and thermoelectric transport properties of NbFeSb, a high-performing half-Heusler alloy, to understand the factors limiting its performance. The team develops a highly efficient computational method, based on the Boltzmann Transport equation, to model how electrons move through the material and are scattered by different obstacles, including vibrations and impurities. Their results reveal that scattering from polar phonons and ionized impurities strongly influences the material’s ability to conduct electricity, while the impact of other scattering mechanisms is comparatively weaker, particularly at lower temperatures, offering crucial insights for optimising half-Heusler and other polar thermoelectric materials. This advancement not only explains the observed power-factor performance of NbFeSb, but also provides a broadly applicable and computationally affordable method for designing improved thermoelectric materials in the future.

Researchers employed a sophisticated computational method incorporating the Boltzmann Transport equation, meticulously accounting for how electrons interact with various factors within the material, including acoustic phonons, non-polar optical phonons, both within and between energy valleys, polar optical phonons, and ionized impurities, providing a comprehensive understanding of electron behavior.

First-Principles Prediction of Thermoelectric Performance

This research focuses on predicting and improving the performance of thermoelectric materials, substances capable of converting heat directly into electricity and vice versa. The goal is to identify and design materials with higher thermoelectric efficiency, measured by the figure of merit, which is crucial for practical applications. The team utilizes first-principles calculations, based on Density Functional Theory, combined with the Boltzmann Transport Equation to accurately model electron behavior and consider complex interactions within the material. The calculations involve detailed analysis of electron scattering processes, crucial for understanding how efficiently electrons move through the material, including interactions with vibrations in the crystal lattice (phonons), defects or impurities, and the random arrangement of atoms in alloys.

Advanced computational techniques, including the use of Wannier functions, streamline these calculations and improve accuracy. The team explores various materials, including half-Heusler alloys, skutterudites, oxides, and two-dimensional materials, seeking to identify promising candidates for high-performance thermoelectric applications. The research highlights the importance of manipulating the material’s band structure to optimize thermoelectric properties. Reducing thermal conductivity through alloying, nanostructuring, and defect engineering is also a key focus. Accurate calculations of electron and phonon scattering rates are essential for predicting thermoelectric properties.

NbFeSb Electronic Structure and Transport Properties

Scientists have achieved a detailed understanding of electronic and thermoelectric transport properties in the half-Heusler alloy NbFeSb, a material known for its potential in energy conversion. The research team employed a highly efficient computational method to investigate how electrons move through the material and are scattered by various factors, providing insights into optimizing its thermoelectric performance. Calculations reveal a band gap of 0. 51 eV, consistent with previous reports, and demonstrate that conduction bands are primarily influenced by niobium d-orbitals, while valence bands are dominated by iron d-orbitals, followed by niobium and antimony contributions.

The study meticulously distinguished between different scattering mechanisms that impede electron flow, including interactions with acoustic phonons, non-polar phonons, polar phonons, and ionized impurities. Results demonstrate that polar phonon and ionized impurity scattering are approximately twice as strong in determining electrical conductivity and power factor for p-type carriers at 300 K, and five times stronger for n-type carriers. This strength diminishes with increasing temperature, highlighting the temperature-dependent nature of these scattering processes. The peak power factor for p-type NbFeSb was measured at 11.

45mW/mK², while the n-type material achieved 5. 92mW/mK² at a significantly lower carrier density. Calculated conductivity values were found to be around three times higher than experimental measurements, while the Seebeck coefficient was somewhat lower, suggesting the presence of defects in experimental samples that limit conductivity. However, computational power factor values showed reasonable agreement with existing p-type experimental data, being approximately two times higher. This work establishes a hierarchy of scattering processes based on their strengths, providing a deep understanding of internal mechanisms and enabling broader application of these findings to other half-Heusler materials and beyond.

NbFeSb Thermoelectric Transport and Scattering Mechanisms

This research presents a detailed investigation into the electronic and thermoelectric transport properties of the half-Heusler alloy, NbFeSb, a material known for its high thermoelectric performance. Scientists employed a computational method based on the Boltzmann transport equation to model how electrons move through the material, carefully accounting for various scattering processes that impede their flow, including interactions with acoustic and non-polar phonons, polar phonons, and ionized impurities. The results demonstrate good agreement between the calculated power-factor values and existing experimental data, establishing an upper limit for performance in this material and providing insights into the relative strengths of different scattering mechanisms. The study reveals that both polar phonon and ionized impurity scattering significantly influence transport properties, while the impact of non-polar phonon scattering is comparatively weaker, particularly for electrons at lower temperatures.

This understanding extends beyond NbFeSb, offering valuable knowledge applicable to other half-Heusler and polar thermoelectric materials generally. Importantly, the developed computational approach is material agnostic and substantially reduces the computational cost compared to fully ab initio methods, while maintaining a high level of accuracy. While the calculations align reasonably well with measured p-type data, the team acknowledges a discrepancy, with computational results approximately twice as high, suggesting areas for further refinement. Future work could focus on improving the accuracy of calculations for n-type performance, for which fewer reports currently exist.