Abstract
The way a material is synthesized and processed has an immense effect on its microstructure, which in turn has a big impact on its transport properties. Here we compare the thermoelectric (TE) properties of n- and p-type Bi2-xSbxTe3 (x: 0 and 1.5) materials synthesized through two different routes, specifically mechanochemical alloying (MA) -as a solid-state synthesis route, and microwave-assisted (MW) polyol synthesis -as a solution synthesis route. Reaction time is significantly reduced in the MW synthesis, leading to a significantly lower energy consumption (i.e. higher energy efficiency) per batch as compared to the MA route. The resultant materials are compared for their crystallinity, phase purity, morphology, and microstructure. Spark plasma sintering was used to prepare pellets and the resultant consolidates were evaluated for their transport properties. TE properties and microstructure of the specimens were investigated in relation to processing conditions and composition. MA samples formed fused structures (from 200 nm to several micrometers in size) composed of smaller particles. MW synthesized materials exhibit hexagonal platelet morphology, high crystallinity and phase purity. They also show lower thermal conductivity, leading to a higher resultant TE figure-of-merit -ZT. TE properties of Bi2-xSbxTe3 samples were studied on sintered cylindrical pellet samples, where the highest ZT values achieved were 1.04 (@ 440 K) for MW-Bi2Te3 and 0.76 (@ 523 K) for MW-Bi0.5Sb1.5Te3 samples, while MA-Bi2Te3 and MA-Bi0.5Sb1.5Te3 samples showed maximum ZT values of 0.74 (@ 460 K) and 0.27 (@ 300 K), respectively as n- and p-type TE materials. The observed trend is much higher ZT values for MW samples, ascribed to their higher degree of texturing and nanostructured grains reducing the thermal conductivity, thus achieving a better overall performance, verifying the prospect to enhance ZT using MW-assisted solution synthesis approach.
Highlights
Thermoelectric (TE) materials are capable of directly interconverting heat energy and electrical power
The residual strain remaining in the powder after mechanical milling leads to a parallel shift, while the nonuniform strain will lead to broadening of the diffraction peaks (Schaffer and McCormick, 1992)
Strain and the crystallite size were calculated for all samples using the Williamson–Hall approach (Mote et al, 2012; Khachotourian et al, 2016; Lim and Quah, 2020), details of which and the results obtained are summarized in Supplementary Table S1
Summary
Thermoelectric (TE) materials are capable of directly interconverting heat energy and electrical power. An ideal TE material is anticipated to have a high power factor (S2σ) accompanied with a lower thermal conductivity (κtot). Bismuth telluride (Bi2Te3) and its alloys (Bi2−xSbx2SeyTe3−y) are presently the most promising bulk TE materials, in the ambient temperature region, due to their high TE figure of merit, ZT (Boyer and Cissé, 1992). The first one is the particularities of the electronic structure, which may cause a decrease in the electrical conductivity but at the same time lead to a substantial increase in thermopower; as a result, the power factor (S2σ) of nano-sized TEs can be higher than that of their bulk counterparts. The second phenomenon is the presence of a large number of interfaces (grain boundaries), which may effectively scatter phonons (κlat) while exerting a minimal effect on the transport of charge carriers (σ), mainly due to the very small size of constituent nano-sized building blocks (Shevelkov, 2008)
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