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Kumar et al. Energy Mater. 2025, 5, 500109 https://dx.doi.org/10.20517/energymater.2025.22 Page 3 of 17

Recent advances in TE performance using the HEA concept have been introduced, including the n-type
[21]
PbSe-based high-entropy material (Pb Sb 0.012 Sn Se Te S , ZT = 1.8 at 900 K) , p-type PbSe-based
0.25 0.25
0.5
0.89
0.1
high-entropy material (Pb Na 0.025 Cd Se S Te , ZT = 2.0 at 900 K) , and p-type GeTe based
[22]
0.5 0.25
0.935
0.04
0.25
high-entropy material (Ge Ag Sb Pb Bi Te, ZT = 2.7 at 750 K) On the other hand, the HEA
[23]
0.13
0.11
0.01
0.12
0.61
concept has not yet been successful in improving the TE performance of bismuth tellurides for
[17]
room-temperature applications .
Here we investigated the anisotropic thermoelectric properties of the composites consisting of p-type
bismuth antimony telluride (BST: Te-excess Bi Sb Te ) and heavy metallic HEA nanoparticles (HEA:
0.4
3
1.6
TaNb HfZrTi). Since the Te-excess effectively enhances the TE performance of the p-type bismuth
2
antimony telluride [9,11] , the composition of the Bi Sb Te with additional Te was used for the BST matrix.
3
1.6
0.4
The TaNb HfZrTi is one of the representative HEAs with a body-centered cubic (BCC) phase and high
2
mechanical strength . To minimize a chemical doping effect, which can decrease the ZT value by deviating
[24]
from the optimized carrier concentration, a mixed powder with the BST and HEA was used in the hot-press
sintering process. The carrier concentrations, Seebeck coefficients (S), and Fermi energies (E ) of
F
theBST+HEA (x = 0, 0.1, 0.5, 1.0 vol%) samples are not significantly affected by the addition of HEA. On
x
the other hand, the significantly enhanced electrical conductivity σ and reduced κ are observed in the x =
L
0.1 vol% sample for the parallel (Pa) direction to the hot-press direction. The enhanced Pa-direction σ and
the decrease of κ of theBST+HEA samples are mainly affected by the enlarged carrier mean free paths λ e
x
L
and decrease of phonon mean free path λ , which is a realization of PGEC . The increased λ can be
[15]
ph
e
explained by the improved grain connectivity due to the HEA nanoparticles for the Pa-direction of the BST
grains. Grain connectivity, which refers to the interconnection between grains, has a significant impact on
transport characteristics [25-29] . The increased λ in theBST+HEA samples further supports that the HEA
x
ph
nanoparticles can improve the grain connectivity of the BST grains. As a result, the ZT values of
theBST+HEA samples are enhanced from 1.09 (x = 0) to 1.33 (x = 0.1 vol%) at 350 K in the Pa-direction
x
due to the improved σ and reduced κ These results clearly show that incorporating HEA nanoparticles is a
L.
promising strategy for improving the performance of the anisotropic thermoelectric materials. Additionally,
controlling of the grain connectivity could be a good concept to improve the transport characteristics in
[30]
various materials such as superconductors , batteries [31,32] , solar cells [33,34] , and semiconductors [35,36] .
EXPERIMENTAL
Materials
The polycrystalline Te-excess BST samples were synthesized using the conventional melting method. The
stoichiometric amounts of Bi (99.999%), Sb (99.999%), and Te (99.999%) were sealed in evacuated quartz
[9]
tubes under high vacuum with the excess tellurium (~7 W%) . The elements in the quartz ampule were
melted at 923 K for 24 h and then water-quenched. The obtained ingots were pulverized into the powder
using an agate mortar and pestle under an argon atmosphere. The TaNb HfZrTi HEA powders were
2
synthesized by the planetary ball milling method using powder elements. The stoichiometric ratios of Ta
(99.98%), Nb (99.8%), Hf (99.6%), Zr (99.5%), and Ti (99.5%) powders were loaded into a stainless-steel jar
with stainless-steel balls under an argon atmosphere. The ball milling process was carried out at a rotation
[24]
speed of 400 rpm for 12 h .
The obtained BST and HEA (0, 0.1, 0.5 and 1.0 vol%) powders were mixed using a physical multimixer in
vibration mode for 24 h. The mixed powders were loaded into a graphite die with an inner diameter of
15 mm and sintered using the hot-press method at 763 K under uniaxial pressure of 50 MPa for 60 min. The
relative densities of the sintered samples were above 95% compared to the calculated densities.