P-type Bismuth Antimony Telluride Research Articles

Structural Design Optimization of Micro-Thermoelectric Generator for Wearable Biomedical Devices

Wearable sensors to monitor vital health are becoming increasingly popular both in our daily lives and in medical diagnostics. The human body being a huge source of thermal energy makes it interesting to harvest this energy to power such wearables. Thermoelectric devices are capable of converting the abundantly available body heat into useful electrical energy using the Seebeck effect. However, high thermal resistance between the skin and the device leads to low-temperature gradients (2–10 K), making it difficult to generate useful power by this device. This study focuses on the design optimization of the micro-thermoelectric generator for such low-temperature applications and investigates the role of structural geometries in enhancing the overall power output. Electroplated p-type bismuth antimony telluride (BiSbTe) and n-type copper telluride (CuTe) materials’ properties are used in this study. All the simulations and design optimizations were completed following microfabrication constraints along with realistic temperature gradient scenarios. A series of structural optimizations were performed including the thermoelectric pillar geometries, interconnect contact material layers and fill factor of the overall device. The optimized structural design of the micro-thermoelectric device footprint of 4.5 × 3.5 mm2, with 240 thermoelectric leg pairs, showcased a maximum power output of 0.796 mW and 3.18 mW when subjected to the low-temperature gradient of 5 K and 10 K, respectively. These output power values have high potential to pave the way of realizing future wearable devices.

Effect of Microwave Processing and Glass Inclusions on Thermoelectric Properties of P-Type Bismuth Antimony Telluride Alloys for Wearable Applications

Depending on the application of bismuth telluride thermoelectric materials in cooling, waste heat recovery, or wearable electronics, their material properties, and geometrical dimensions should be designed to optimize their performance. Recently, thermoelectric materials have gained a lot of interest in wearable electronic devices for body heat harvesting and cooling purposes. For efficient wearable electronic devices, thermoelectric materials with optimum properties, i.e., low thermal conductivity, high Seebeck coefficient, and high thermoelectric figure-of-merit (zT) at room temperature, are demanded. In this paper, we investigate the effect of glass inclusion, microwave processing, and annealing on the synthesis of high-performance p-type (BixSb1−x)2Te3 nanocomposites, optimized specially for body heat harvesting and body cooling applications. Our results show that glass inclusion could enhance the room temperature Seebeck coefficient by more than 10% while maintaining zT the same. Moreover, the combination of microwave radiation and post-annealing enables a 25% enhancement of zT at room temperature. A thermoelectric generator wristband, made of the developed materials, generates 300 μW power and 323 mV voltage when connected to the human body. Consequently, MW processing provides a new and effective way of synthesizing p-type (BixSb1−x)2Te3 alloys with optimum transport properties.

High thermoelectric cooling performance of n-type Mg3Bi2-based materials.

Thermoelectric materials have a large Peltier effect, making them attractive for solid-state cooling applications. Bismuth telluride (Bi2Te3)-based alloys have remained the state-of-the-art room-temperature materials for many decades. However, cost partially limited wider use of thermoelectric cooling devices because of the large amounts of expensive tellurium required. We report n-type magnesium bismuthide (Mg3Bi2)-based materials with a peak figure of merit (ZT) of ~0.9 at 350 kelvin, which is comparable to the commercial bismuth telluride selenide (Bi2Te3- x Se x ) but much cheaper. A cooling device made of our material and p-type bismuth antimony telluride (Bi0.5Sb1.5Te3) has produced a large temperature difference of ~91 kelvin at the hot-side temperature of 350 kelvin. n-type Mg3Bi2-based materials are promising for thermoelectric cooling applications.

Thermoelectric Transport Properties of Interface-Controlled p-type Bismuth Antimony Telluride Composites by Reduced Graphene Oxide

We report the thermoelectric transport properties of interface-controlled p-type bismuth antimony telluride (BST) composites using reduced graphene oxide (rGO). The composites were prepared by the spark plasma sintering (SPS) of BST–graphene oxide (GO) hybrid powder, which could induce the in situ reduction of GO into rGO. Compared to the pristine BST, the interface-controlled BST composites exhibited degraded electrical conductivities with similar Seebeck coefficients, consequently resulting in decreased power factors. However, thanks to the suppressed lattice thermal conductivity by the rGO network at the grain boundaries, this disadvantage could be compensated in terms of ZT. Our results will be helpful for understanding thermoelectric transport properties of various graphene-hybrid thermoelectric materials.

Bismuth antimony telluride thin films with unique crystal orientation by two-step method

P-type bismuth antimony telluride thin films have been prepared by flash evaporation and a subsequent two-step synthesis method of homogeneous electron beam (EB) irradiation and thermal annealing. EB irradiation of the thin films was performed in a N2 atmosphere at room temperature and an acceleration voltage of 0.17 MeV. Thermal annealing was performed at 300°C for 1 h under Ar/H2 (95/5%). Thin films prepared using the two-step method exhibit a unique crystal orientation that provides improved electrical conductivity and an improved Seebeck coefficient. With an EB irradiation dose of 0.43 MGy, the power factor of the thin films reached 38.4 μW/(cm·K2). Thermal conductivity was calculated using a phonon transport model and the Wiedemann–Franz law, providing a dimensionless figure of merit (ZT). The ZT was expected to enhance significantly by a two-step method compared with alternate synthesis techniques.

Melting and solidification of bismuth antimony telluride under a high magnetic field: A new route to high thermoelectric performance

Imposing an intensity variable high static magnetic field during a traditional melting-solidification (MS) method has been used as a new method to prepare p-type bismuth antimony telluride thermoelectric materials in this work. On this basis, we present a systematic study of the nucleation, crystal orientation, microstructure, electrical and thermal transport properties of the obtained alloy ingots solidified under different magnetic field intensities. A c-axis alignment of bismuth antimony telluride in the direction perpendicular to the magnetic field, formation of BSTII nanorods, and a simultaneous optimization of the electrical and thermal transport properties have been observed. Consequently, an enhanced ZTmax=1.71 at 323K has been achieved in a polycrystalline Bi0.5Sb1.5Te3 sample solidified under a 2T magnetic field.

P-Type Bismuth Telluride-Based Composite Thermoelectric Materials Produced by Mechanical Alloying and Hot Extrusion

We produced six different composites of p-type bismuth antimony telluride alloys and studied their structure and thermoelectric properties. The components of the composites were obtained in powder form by mechanical alloying. Mixed powders of two different compositions were consolidated by hot extrusion to obtain each bulk composite. The minimum grain size of bulk composites as revealed by scanning electron microscopy shows a 50% reduction compared with the conventional (Bi0.2Sb0.8)2Te3. X-ray diffraction (XRD) analysis only shows peak broadening with no clear indication of separate phases, and indicates a systematic decrease of crystallite size in the composite materials. Scattering mechanisms of charge carriers were evaluated by Hall-effect measurements. The thermoelectric properties were investigated via the Harman method from 300 K up to 460 K. The composites show no significant degradation of the power factor and high peak ZT values ranging from 0.86 to 1.04. The thermal conductivity of the composites slightly increases with respect to the conventional alloy. This unexpected behavior can be attributed to two factors: (1) the composites do not yet contain a significant number of grains whose sizes are sufficiently small to increase phonon scattering, and (2) each of the combined components of the composites corresponds to a phase with thermal conductivity higher than the minimum value corresponding to the (Bi0.2Sb0.8)2Te3 alloy.

Experimental evidence of enhancement of thermoelectric properties in tellurium nanoparticle-embedded bismuth antimony telluride

Abstract

Conductive scanning probe microscopy of nanostructured Bi2Te3

In order to explain the unique thermoelectric properties of bulk nanocomposite p-type bismuth antimony telluride, its structural and electrical properties are investigated using transmission electron microscopy (TEM) and atomic force microscopy with a conductive probe (C-AFM). The material is observed to contain both nano- and micro-sized grains with sizes varying from 10 nm to 3 µm. This unique structure promotes phonon scattering, thereby decreasing the thermal conductivity to below 1 W mK(-1) at room temperature. Moreover, the C-AFM data show that the electrical conductivity of nanosized grains is higher than the bulk value and reaches 1600 S cm(-1). This results in a moderate increment of the overall electrical conductivity, thereby increasing the figure of merit (ZT) up to 1.4 at 100 °C. In addition to demonstrating a powerful scanning probe microscopy (SPM) based investigation technique that requires minimal sample preparation, our findings contribute towards better understanding of the enhancement of thermoelectric properties of nanocomposite thermoelectric materials.

Synthesis and evaluation of lead telluride/bismuth antimony telluride nanocomposites for thermoelectric applications

Heterogeneous nanocomposites of p-type bismuth antimony telluride (Bi 2− x Sb x Te 3) with lead telluride (PbTe) nanoinclusions have been prepared by an incipient wetness impregnation approach. The Seebeck coefficient, electrical resistivity, thermal conductivity and Hall coefficient were measured from 80 to 380 K in order to investigate the influence of PbTe nanoparticles on the thermoelectric performance of nanocomposites. The Seebeck coefficients and electrical resistivities of nanocomposites decrease with increasing PbTe nanoparticle concentration due to an increased hole concentration. The lattice thermal conductivity decreases with the addition of PbTe nanoparticles but the total thermal conductivity increases due to the increased electronic thermal conductivity. We conclude that the presence of nanosized PbTe in the bulk Bi 2− x Sb x Te 3 matrix results in a collateral doping effect, which dominates transport properties. This study underscores the need for immiscible systems to achieve the decreased thermal transport properties possible from nanostructuring without compromising the electronic properties.

Improved Thermoelectric Performance of p-Type Bismuth Antimony Telluride Bulk Alloys Prepared by Hot Forging

The thermoelectric (TE) performance of Bi0.5Sb1.5Te3 polycrystalline alloys has been improved by a simple hot-forging process. No obvious texture was observed in the x-ray diffraction (XRD) patterns of the hot-forged samples. Transport property measurements indicated that the hot-forged samples possessed extremely low thermal conductivities. A maximum ZT value of ∼1.1 at room temperature was obtained for the sample forged under 30 MPa pressure, being almost 50% more than that of the initial unforged alloy. High-resolution transmission electron microscopy (HRTEM) observations suggested that the high density of lattice defects of the hot-forged samples could be responsible for the extremely low thermal conductivities.

Structure Study of Bulk Nanograined Thermoelectric Bismuth Antimony Telluride

The microstructures of bulk nanograined p-type bismuth antimony telluride with a thermoelectric dimensionless figure-of-merit ZT = 1.4 are investigated using transmission electron microscopy. It is found that the bulk material contains both nano- and microsized grains. Between the nanograins, bismuth-rich interface regions with a 4 nm thickness were detected. In addition, nanoprecipitates as well as other defects are also found to be embedded in the nanograins. The high ZT is attributed to the slight increase in the electrical conductivity, and to the large decrease of the thermal conductivity.