arXiv subjects

Nirma Kumari

Publications and source records attributed to Nirma Kumari.

3 recordsLinked to original sources

arXiv2026

High Thermoelectric Performance via Stacking-Controlled Symmetry Breaking in Layered XZnBi (X = Rb, Cs) Zintl Materials

High thermoelectric efficiency requires high Seebeck coefficient, high electrical conductivity, and low thermal conductivity. However, strategies that suppress thermal conductivity often simultaneously degrade electrical conductivity, making effective electrical-thermal decoupling highly challenging. Here, we show that atomic-layer stacking order change in XZnBi (X = Rb, Cs) provides an efficient route to achieve such decoupling. Even though electronic transport coefficients and relaxation times remain largely insensitive to stacking order due to preserved Fermi-surface topology, the lattice thermal conductivity exhibits a strong stacking dependence, with AB stacking significantly suppressing it below 1 Wm$^{-1}$K$^{-1}$ at temperatures above 300 K. The stacking transition from AA to AB breaks structural symmetries. It increases the three-phonon phase space and available scattering channel, substantially suppressing phonon transport by about 50$\%$ in both materials. As a result, the AB-stacked phases yield high ZT values of 1.96 (1.69) in n-type CsZnBi (RbZnBi) at 900 K, which is about 40$\%$ (30$\%$) higher than AA stacking. These findings establish the XZnBi family as promising thermoelectric candidates and highlight stacking-order controlled phonon transport as a robust strategy for advancing thermoelectric material design.

cond-mat.mtrl-sci↗

arXiv2025

Reference compositions for bismuth telluride thermoelectric materials for low-temperature power generation

Thermoelectric (TE) technology enables direct heat-to-electricity conversion and is gaining attention as a clean, fuel-saving, and carbon-neutral solution for industrial, automotive, and marine applications. Despite nearly a century of research, apart from successes in deep-space power sources and solid-state cooling modules, the industrialization and commercialization of TE power generation remain limited. Since the new millennium, nanostructured bulk materials have accelerated the discovery of new TE systems. However, due to limited access to high-temperature heat sources, energy harvesting still relies almost exclusively on BiTe-based alloys, which are the only system operating stably near room temperature. Although many BiTe-based compositions have been proposed, concerns over reproducibility, reliability, and lifetime continue to hinder industrial adoption. Here, we aim to develop reference BiTe-based thermoelectric materials through data-driven analysis of Starrydata2, the world's largest thermoelectric database. We identify Bi0.46Sb1.54Te3 and Bi2Te2.7Se0.3 as the most frequently studied ternary compositions. These were synthesized using hot pressing and spark-plasma sintering. Thermoelectric properties were evaluated with respect to the processing method and measurement direction. The results align closely with the median of reported data, confirming the representativeness of the selected compositions. We propose these as reference BiTe materials, accompanied by transparent data and validated benchmarks. Their use can support the standardization of TE legs and modules while accelerating performance evaluation and industrial integration. We further estimated the performance of a thermoelectric module made from the reference composition, which gives the power output of over 2.51 W and an efficiency of 3.58% at a temperature difference of 120 K.

cond-mat.mtrl-sci↗

arXiv2024

Wiedemann-Franz Law and Thermoelectric Inequalities: Effective ZT and Single-leg Efficiency Overestimation

We derive a thermoelectric inequality in thermoelectric conversion between the material figure of merit (ZT) and the module effective ZT using the Constant Seebeck-coefficient Approximation combining with the Wiedemann-Franz law. In a P-N leg-pair module, the effective ZT lies between the individual ZT values of the P- and N-legs. In a single-leg module, however, the effective ZT is less than approximately one-third of the leg's ZT. This reduction results from the need for an external wire to complete the circuit, introducing additional thermal and electrical losses. Multi-dimensional numerical analysis shows that, although structural optimization can mitigate these losses, the system efficiency remains limited to below half of the ideal single-leg material efficiency. Our findings explain the single-leg efficiency overestimation and highlight the importance of optimizing the P-N leg-pair module structure. They also underscore the need for thermoelectric leg-compatibility, particularly with respect to Seebeck coefficients.

physics.app-ph↗