Optimization of the figure of merit in ${\mathrm{Bi}}_{100\text{\ensuremath{-}}x}{\mathrm{Sb}}_{x}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ nanocomposites

Optimization of the figure of merit in ${Bi}_{100 − x} {Sb}_{x} / {Al}_{2} O_{3}$ nanocomposites

Hyungyu Jin and Joseph P. Heremans

Phys. Rev. Materials 2, 115401 – Published 26 November 2018

Abstract

Bismuth-antimony ( $B i_{100 − x} S b_{x}$ ) alloys have the highest thermoelectric figure of merit of all $n$ -type thermoelectric materials below 200 K. They are the only Te-free thermoelectric alternatives to the tetradymite materials for applications at and below room temperature. Single-crystal $B i_{100 − x} S b_{x}$ alloys show the maximum figure of merit $z T \sim 0.5$ at 200 K along the trigonal axis crystallographic direction, but the cost associated with single-crystal growth and the tendency of single crystals to cleave preclude their use. Mechanically robust polycrystalline $B i_{100 − x} S b_{x}$ / $A l_{2} O_{3}$ nanocomposites are shown here to be able to reach competitive $z T$ values. Two compositions are investigated, $B i_{82} S b_{18}$ and $B i_{88} S b_{12}$ . Thermal and electrical transport properties confirm significant reduction of lattice thermal conductivity in the nanocomposite samples, but the concurrent loss of electrical conductivity leads to an unfavorable net effect on $z T$ . In contrast, a large increase in thermopower is observed in the $B i_{82} S b_{18}$ / $A l_{2} O_{3}$ nanocomposite system, which is attributed to a better optimized doping level. Accordingly, the $z T$ of a $B i_{82} S b_{18}$ / $A l_{2} O_{3}$ nanocomposite sample is shown to reach $z T \sim 0.4$ at 240 K, which rivals that of single crystals. Near room temperature, the $z T$ of the nanocomposite sample is improved by $\sim 60 %$ over that of the single-crystalline sample. Galvano- and thermomagnetic analysis suggests a strong effect of carrier concentration on the $z T$ of ${Bi}_{100 − x} {Sb}_{x} / {Al}_{2} O_{3}$ nanocomposite samples.

Received 17 May 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.2.115401

Physics Subject Headings (PhySH)

Lattice thermal conductivity Thermomagnetic effects Thermopower

Composite materials Solid solutions Thermoelectrics

Interdisciplinary PhysicsCondensed Matter, Materials & Applied Physics

Authors & Affiliations

Hyungyu Jin^1,* and Joseph P. Heremans²

¹Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, South Korea
²Department of Mechanical and Aerospace Engineering, Department of Materials Science and Engineering, and Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA

^*hgjin@postech.ac.kr

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Vol. 2, Iss. 11 — November 2018

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Figure 1
Calculated thermoelectric figure of merit ( $z T$ ) of $B i_{100 − x} S b_{x}$ alloys at 300 K from the data on the undoped single-crystalline samples in Ref. [5]. The result for the polycrystal (“average”) was obtained by taking weighted averages of the data along the three crystallographic directions. Symbols indicate calculated data points and lines are added to guide the eye.
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Figure 2
Scanning electron microscopy (SEM) pictures for $B i_{82} S b_{18}$ / $A l_{2} O_{3}$ nanocomposite samples with (a), (b) 5 vol % and (c), (d) 20 vol % $A l_{2} O_{3}$ nanoparticles. Scale bar is shown on the bottom-right corner of each picture.
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Figure 3
Temperature dependence of thermoelectric properties of $B i_{82} S b_{18}$ alloy samples. (a) Thermal conductivity ( $κ$ ); (b) electrical resistivity $(ρ)$ ; (c) Seebeck coefficient ( $S$ ); (d) figure of merit ( $z T$ ). The data for the single-crystalline sample were calculated by taking a weighted average of the data from Ref. [5] along the three crystallographic directions. Inset in (a) shows the magnetic field dependence of $κ$ normalized by the zero-field value at $T = 100 K$ . Figure legend is included in (c). Symbols are experimental data points and lines are added to guide the eye.
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Figure 4
Temperature dependence of (a) carrier concentration ( $n$ ), (b) Hall mobility $(μ_{H})$ , and (c) Nernst coefficient ( $N$ ) of $B i_{82} S b_{18}$ alloy samples. Figure legend is included in (a). (d) Dependence of $(3 / 2 + λ) m^{*}$ on $A l_{2} O_{3}$ concentration at $T = 100$ and 140 K, where $λ$ is the scattering parameter and $m^{*}$ is the density-of-state effective mass of electrons. Symbols are experimental data points and lines are added to guide the eye.
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Figure 5
Temperature dependence of thermoelectric properties of $B i_{88} S b_{12}$ alloy samples. (a) Thermal conductivity ( $κ$ ); (b) electrical resistivity $(ρ)$ ; (c) Seebeck coefficient ( $S$ ); (d) figure of merit ( $z T$ ). The data for the single-crystalline sample were calculated by taking a weighted average of the data from Ref. [5] along the three crystallographic directions. Figure legend is included in (c). Symbols are experimental data points and lines are added to guide the eye.
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Figure 6
Temperature dependence of (a) carrier concentration ( $n$ ), (b) Hall mobility $(μ_{H})$ , and (c) Nernst coefficient ( $N$ ) of $B i_{88} S b_{12}$ alloy samples. Figure legend is included in (a). (d) Dependence of $(3 / 2 + λ) m^{*}$ on $A l_{2} O_{3}$ concentration at $T = 100$ and 140 K, where λ is the scattering parameter and $m^{*}$ is the density-of-state effective mass of electrons. Symbols are experimental data points and lines are added to guide the eye.
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Physical Review Materials

Optimization of the figure of merit in Bi100−xSbx/Al2O3 nanocomposites

Hyungyu Jin and Joseph P. Heremans

Phys. Rev. Materials 2, 115401 – Published 26 November 2018

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Optimization of the figure of merit in ${Bi}_{100 − x} {Sb}_{x} / {Al}_{2} O_{3}$ nanocomposites