Nanograined Thermoelectrics with Bi2Te3-based Compounds Fabricated at Low Temperatures

By Bismay Prakash RoutReviewed by Susha Cheriyedath, M.Sc.Feb 3 2022

In a recent study published in the journal ACS Applied Energy Materials, researchers fabricated p-type Bi_0.5Sb_1.5Te₃ thermoelectric compounds at a relatively low sintering temperature of 130 ℃ with a high dimensionless figure of merit (ZT) value of 0.56 at 450 K using liquid-phase-assisted sintering (i.e. cold-sintering).

Study: Cold-Sintered Bi₂Te₃‑Based Materials for Engineering Nanograined Thermoelectrics. Image Credit: Gorodenkoff/Shutterstock.com

This ZT value was comparable to commercial Bi_0.5Sb_1.5Te₃ ingots with a ZT value of 0.8-1.0, which are produced using hot consolidation processes at a temperature of 400-500 ℃. The axial compaction and liquid evaporation during cold sintering facilitated grain reorientation, reduced grain size, improved mass transfer speed, promoted twin boundaries formation, and resulted in a very low lattice thermal conductivity of 0.57 W.m⁻¹.K⁻¹ at 107 ℃ due to phonon scattering.

Bi₂Te₃-Based Thermoelectrics Compounds

Thermoelectric materials like Bi₂Te₃-based ceramics materials convert heat into electrical energy and vice versa via the Peltier effect and Seebeck effect. The higher the ZT value over a wider range of temperatures, the higher the conversion efficiency.

They are commonly prepared using energy-intensive zone melting (ZM) processes such as hot working, hot pressing, and spark plasma powder sintering at a temperature of 400-500 ℃. However, high-temperature fabrication results in higher manufacturing costs, poor mechanical properties, highly preferential grain orientation, low output, and high manufacturing defects. Meanwhile, cold-sintering alleviates such issues and facilitates low lattice thermal conductivity.

The cold liquid-phase sintering mechanism of ceramics has two stages. Firstly, the streamlined edges of the wetted powder particles are eliminated during the grain rearrangement process, which promotes a smooth and faster grain rearrangement process under adequate pressure and temperature. Secondly, the capillary forces generated in the void space between ceramic particles gradually decrease the porosity during grain surface dissolution-precipitation in the liquid phase, which densifies the entire mass of the ceramic.

About the Study

In this study, researchers fabricated p-type Bi_0.5Sb_1.5Te₃ thermoelectric ceramic at a relatively low sintering temperature. First, all three pure metal powders of Bi, Sb, and Te were mixed in a composition ratio of 1:3:6 and vacuum-sealed in quartz tubes followed by heating in a muffle furnace to melt at a temperature of 800 ℃ for 6 hours. The prepared ingots were cooled in an argon environment to prevent any surface oxidation.

Subsequently, the ingots were milled and the obtained powders were sieved to get Bi_0.5Sb_1.5Te₃ powders of size below 30 μm. After that, the powders were mixed with deionized water and compressed under a pressure of 240 MPa at five low sintering temperatures of 110, 120, 130, 140, and 150 °C, respectively, for 20 min to obtain Bi_0.5Sb_1.5Te₃ pellet samples.  

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For characterization, XRD was used to analyze crystal structure and phase composition, FE-SEM for fracture morphology and grain microstructure, EDS for element composition, laser flash method for thermal diffusivity, and the Archimedes’ method for sample density measurement.

Observations

All samples had identical diffraction peaks with a density above 97% of theoretical density. Deionized water eliminated sharp particle edges, which reduced the particle size and increased the specific surface area. Higher specific surface area resulted in higher surface energy that resulted in higher mass transport and negligible preferential orientation, which is good for industrial standards owing to the homogeneous properties of the sintered samples.

Samples sintered below 120 °C exhibited a more porous structure due to incomplete densification, whereas the samples sintered above 130 °C indicated less number of pores due to enhanced atomic diffusion. Moreover, the low sintering temperature resulted in less grain growth, and the small grain sizes (<30 μm) led to enhanced phonon scattering, thus, decreasing the lattice thermal conductivity of the samples. Also, the generated twin boundaries with a 60° misorientation angle assisted in phonon scattering.

For samples sintered at 120 °C and above, the electrical conductivity decreased with increasing temperature, whereas samples sintered at 110 °C and below indicated a temperature-activated abnormal electrical transport. The sample sintered at 130 °C demonstrated the highest conductivity value of 404.67 S.cm^-1 at room temperature.

Conclusions

In summary, the researchers of this study incorporated low-temperature liquid-phase sintering (cold sintering) to fabricate p-type Bi_0.5Sb_1.5Te₃ thermoelectric ceramics.

The prepared samples, especially those sintered at 130 °C demonstrated fewer pores, high density, negligible preferential grain orientation, highest electrical conductivity, and very low lattice thermal conductivity due to phonon scattering at twin boundaries. More importantly, the ZT value of the prepared Bi_0.5Sb_1.5Te₃ samples was comparable to that of commercial ones.

Hence, this cold sintering method is a promising energy-efficient and low-cost manufacturing method for high density and highly conductive Bi_0.5Sb_1.5Te₃ thermoelectric ceramics.

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