Simultaneous improvement in electrical and thermal properties of interface-engineered BiSbTe nanostructured thermoelectric materials

Highlights

• BiSbTe thermoelectric materials with Te interfacial layers to realize energy filtering and phonon scattering.

• Thermoelectric properties were systematically studied in thin films and bulk pellets.

• ZT values were enhanced well above 1.3 ± 0.14.

Abstract

Over the past decade, nanostructuring has become the core of thermoelectric (TE) material research because it creates numerous internal interfaces that provide an effective way to tune the electrical and/or thermal properties of TE materials. Herein, we report a synthesis of interface-engineered BiSbTe nanostructured TE materials by introducing chemically synthesized molecular Te_n²⁻ polyanions into BiSbTe particles, from which BiSbTe nanostructured materials with high-density Te interfacial layers are prepared in thin films and sintered pellets. These Te layers form the contact potential well at the BiSbTe-Te junction to realize energy dependent carrier scattering and scatter phonons effectively, thus resulting in simultaneous improvement in the electrical and thermal properties to increase the ZT value well above 1.3 ± 0.14 that is increased by 40% compared to bulk BiSbTe. The findings of current study can open up new chemical design spaces for interface-engineered electronic and TE materials.

Introduction

Thermoelectric (TE) effects that enable direct conversion from electrical to thermal energy and vice versa have been of great interest due to their potential for sustainable energy harvesting from various waste heat sources and environmentally benign solid-state cooling [1], [2], [3]. The conversion efficiency of TE materials is evaluated by the dimensionless figure of merit, ZT=(S²σT/κ), where S, σ, T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity, respectively. Recently, significant efforts have been made to introduce various-typed nanostructures such as nanoinclusions [4], [5], [6], [7], [8], [9], [10], [11] and nanograins [12], [13], [14], [15], [16], [17], [18] into TE materials, which generated numerous interfaces and grain boundaries. They actively scatter phonons resulting in the reduction of the thermal conductivity, significantly increasing ZT values. Recent most reports to show enhanced ZT values by nanostructuring have focused on this effect, giving rise to challenges in improvement of electrical properties.

Interface engineering of nanostructured TE materials can provide the additional degree of control over the electrical as well as thermal transport. For example, interfaces with well-designed electronic structures can realize the energy dependent carrier scattering to enhance the Seebeck coefficient by obstructing the flow of low energy carriers across interfaces [19], [20], [21], [22], [23], [24], [25]. However, systematic control over their composition, structure, and related electronic structures is still an underexplored area because selective modifications of interfaces alone without the detriment to the primary properties of host grains are very challenging. For example, the consolidation of nanoscale TE particles through typical high temperature processes can cause unwanted structural integration and compositional blend of interfaces into the bulk matrix [26], [27], [28], [29], [30].

Selective introduction of new molecular or nanoscale additives with appropriate electronic structure into between TE grains can provide an effective way to form designed interfaces well-distributed in TE materials. Son, et al. reported the use of “Bi nanocrystal glue” as an additive to TE microscale particles for the formation of doped or de-doped interfacial layers in BiSbTe grains. However, Bi nanocrystals were easily reacted with and thermally integrated into host BiSbTe [27], which remains challenges in thermal stability and functionality of additives.

Here, we report a synthesis of Bi_xSb_2-xTe₃ nanostructured TE materials with homogeneously-distributed Te interfacial layers and systematically studied their electrical and thermal transport in thin films and sintered pellets. Bi_xSb_2-xTe₃ provides a good model system to study the effects of new interfaces on electrical and thermal properties because Bi_xSb_2-xTe₃ show the highest ZT near room temperature and its structure- and composition-dependent TE properties are well understood [31]. Furthermore, Te has an excellent electronic structure as a second phase to make interfaces with Bi_xSb_2-xTe₃ because it is known to have the appropriate work function (∼4.95 eV) to form the potential well for scattering low-energy hole carriers at the metal-semiconductor junction of Te-Bi_xSb_2-xTe₃ [32]. As well, a Te phase would not cause any structural and compositional changes in Bi_xSb_2-xTe₃ host grains during heat treatment due to its limited solubility in Bi_xSb_2-xTe₃ [33].

Recently, Kim et al. reported that the formation of dense dislocation arrays at grain boundaries were formed during liquid-phase sintering of melt spun BiSbTe with excess Te, which dramatically reduced the thermal conductivity and enhanced the ZT value up to 1.86 [33]. Although excess Te phase in this material has a beneficial effect on controlling the formation of defects, the effect of Te itself at grain boundaries on TE properties of BiSbTe was not systematically studied due to the difficulty in the controlled synthesis of BiSbTe which have selectively formed Te at interfaces. In this work, synthesized molecular Te_n²⁻ polyanions were used as an additive for Bi_xSb_2-xTe₃ and clear liquid solution of Te_n²⁻ polyanion was homogeneously impregnated into Bi_xSb_2-xTe₃ TE particles due to the fluidic property, which led to the formation of Te interfacial layers between Bi_xSb_2-xTe₃ grains via heat treatment (Fig. 1). One should note that these samples with Te interfacial layers are totally different from the material reported by Kim et al. which has dislocation arrays at grain boundaries rather than a pure Te phase [33]. The resulting materials of Bi_xSb_2-xTe₃ with Te interfacial layers exhibited both enhanced electrical conductivity and Seebeck coefficient as well as reduced thermal conductivities simultaneously by the realization of energy dependent carrier scattering and phonon scattering effects, as a consequence, and the ZT values increased by ∼40%, compared with Bi_xSb_2-xTe₃ materials without Te interfacial layers.