Enhanced Thermoelectric Properties of Bi_0.4Sb_1.6Te₃ Through the Decoration of SnO₂ Nanoparticles

Thermoelectric energy harvesting has emerged as a promising solution to mitigate global climate change by enabling the conversion of waste heat into electricity. The efficiency of thermoelectric devices relies heavily on the performance of thermoelectric materials, evaluated through the thermoelectric figure of merit (ZT). ZT is defined as ZT = S²σT/κ, where S, σ, T, and κ represent the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. To achieve high ZT values, thermoelectric materials must exhibit high σ and S values while minimizing κ, particularly κ_eleand κ_lat. However, independently controlling these parameters is challenging, making simultaneous optimization of these thermoelectric properties difficult.<br/>Numerous studies over the past few decades have focused on enhancing the power factor (σS²) by manipulating carrier density or conducting band engineering. Additionally, reducing κ_lat through nanostructuring or maximizing phonon scattering has been explored to achieve low thermal conductivity. Among various thermoeletric materials, Bi_0.4Sb_1.6Te₃ (BST) stands out as a material capable of converting temperature differences into electricity. Consequently, BST-based alloys have been investigated to improve thermoelectric performance by incorporating other materials as nanoparticles, aiming to increase the power factor and decrease thermal conductivity. However, the trade-off relationship between electrical and thermal properties presents challenges in simultaneous control.<br/>To overcome this trade-off, we propose a novel approach called the metallization of surface reduction nanoparticle method. This method enables independent control of electrical and thermal properties, offering enhanced stability in the interface between the base material and the surface. SnO₂, a typical n-type semiconducting oxide, exhibits unconventional behavior due to disruptions in the stoichiometric ratio of tin and oxygen caused by interstitial Sn atoms or O vacancies. The reduction reaction from the surface allows the existing base material (SnO₂) to form crystals without surface amorphousness, facilitating sequential arragement (SnO₂ - SnO_x - Sn rich) from SnO₂ (in the core) to metallic Sn (on the surface).<br/>Based on this principle, we fabricate n-type SnO₂-decorated BST alloys using a combination of microwave irradiation, ball milling, and spark plasma sintering processes. These processes prevent the synthesis of SnO₂ nanoparticles with other elements from BST, enabling the decoration of pure SnO₂ nanoparticles onto the surface of the BST matrix (BST_SnO₂). We confirm the presence of approximately 100nm-sized SnO₂ nanoparticles as a secondary phase on the BST matrix, exhibiting excellent dispersion. By decorating the surface with SnO₂ nanoparticles, the average thermoelectric figure of merit (ZT) improves from 0.78 to 1.24 at 400K. The observed enhancement in thermoelectric performance can be attributed to two primary factors. Firstly, in terms of electrical properties, the power factor improves due to a slight reduction in electrical conductivity and a significant increase in the Seebeck coefficient. Secondly, in terms of thermal properties, the total thermal conductivity experiences a slight reduction due to the reduced electrcial thermal conductivity and the suppression of bipolar thermal conductivity.