Significant Enhancement of Thermoelectric Figure of Merit in BiSbTe-Based Composites by Incorporating Carbon Microfiber

Bismuth telluride-based materials are already being commercially developed for thermoelectric (TE) cooling devices and power generators. However, the relatively low efficiency, which is characterized by a TE figure of merit, zT, is the main obstacle to more widespread application. Significant advances in the TE performance have been made through boundary engineering via embedding nanoinclusions or nanoscale grains. Herein, an effective approach to greatly enhance the TE performance of p-type BiSbTe material by incorporating carbon microfibers is reported. A high zT of 1.4 at 375 K and high average zT of 1.25 for temperatures in the range of 300 to 500 K is achieved in the BiSbTe/carbon microfiber (BST/CF) composite materials. Their superior TE performance originates from the low thermal conductivity and the relatively high power factor. A TE unicouple device based on the p-type BST/CF composite material and the commercially available n-type bismuth telluride-based material shows a huge cooling temperature drop in the operating temperature range of 299–375 K, and is greatly superior to the unicouple device made of both commercial p-type and n-type bismuth telluride-based material. The materials demonstrate a high average zT and excellent mechanical properties and are strong candidates for practical applications.     

Enhanced Thermoelectric Performance of p-Type Mg3Sb2 for Reliable and Low-Cost all-Mg3Sb2-Based Thermoelectric Low-Grade Heat Recovery

Bi₂Te₃-based devices have long dominated the commercial market for thermoelectric cooling applications, but their narrow operating temperature range and high cost have limited their possible applications for conversion of low-grade heat into electric power. The recently developed n-type Mg₃Sb₂-based compounds exhibit excellent transport properties across a wide temperature range, have low material costs, and are nontoxic, so it would be possible to substitute the conventional Bi₂Te₃ module with a reliable and low-cost all-Mg₃Sb₂-based thermoelectric device if a good p-type Mg₃Sb₂ material can be obtained to match its n-type counterpart. In this study, by comprehensively regulating the carrier concentration, carrier mobility, and lattice thermal conductivity, the thermoelectric performance of p-type Mg₃Sb₂ is significantly improved through Na and Yb doping in Mg_1.8Zn_1.2Sb₂. Moreover, p- and n-type Mg₃Sb₂ are similar in terms of their coefficients of thermal expansion and their good performance stability, thus allowing the construction of a reliable all-Mg₃Sb₂-based unicouple. The decent conversion efficiency (≈5.5% at the hot-side temperature of 573 K), good performance stability, and low cost of this unicouple effectively promote the practical application of Mg₃Sb₂-based thermoelectric generators for low-grade heat recovery.     

A Resistance Ratio Analysis for CoSb3-Based Thermoelectric Unicouples

In the search for desirable materials for use in thermoelectric generators, CoSb₃-based skutterudites have stimulated much scientific interest due to their high performance capabilities even at high temperatures. In this work, we tested the electrical power-generation characteristics of CoSb₃-based unicouples. We manufactured power-generation unicouples using n-type In_0.25Co_3.95Ni_0.05Sb₁₂ and p-type In_0.25Co_3.0Fe_1.0Sb₁₂ legs. The dimensions of the thermoelectric legs were 10?mm in diameter and 10?mm in height, with Cu sheets and Cu/Mo alloy as the electrode materials. For our unicouples, we evaluated the resistance ratio m?? (=R _o/R), which represents the ratio of the load resistance to the internal resistance of the unicouple. From this analysis of the resistance ratio m??, we obtained a considerable amount of information about the loss factors that caused the difference between the measured power output and the theoretical value. Through these analyses of two types of loss factors, we sought to improve the open-circuit voltage and internal resistance of a unicouple with CoSb₃/Ti/electrode interfaces. In addition, a long-term durability test of the unicouple at high temperature was performed to test the stability of the thermoelectric materials and of the interface between the electrodes and the thermoelectric legs at the same time.     

Room-Temperature Mechanical Properties and Slow Crack Growth Behavior of Mg2Si Thermoelectric Materials

Mg₂Si is of interest as a thermoelectric (TE) material in part due to its low materials cost, lack of toxic components, and low mass density. However, harvesting of waste heat subjects TE materials to a range of mechanical and thermal stresses. To understand and model the material??s response to such stresses, the mechanical properties of the TE material must be known. The Mg₂Si specimens included in this study were powder processed and then sintered via pulsed electrical current sintering. The elastic moduli (Young??s modulus, shear modulus, and Poisson??s ratio) were measured using resonant ultrasound spectroscopy, while the hardness and fracture toughness were examined using Vickers indentation. Also, the Vickers indentation crack lengths were measured as a function of time in room air to determine the susceptibility of Mg₂Si to slow crack growth.     

Robust Finite Element Model for the Design of Thermoelectric Modules

Thermoelectric devices for power generation have been receiving increased attention as an emerging sustainable energy technology because of recent advances in thermoelectric materials and the tremendous thermal resources available. Little focus has been given to the effective implementation of thermoelectric materials in power generation modules and efficient module design. With recent exploration into new module configurations, it is imperative that a comprehensive model be developed as a design tool. A new three-dimensional, device-level, multiphysics modeling technique is developed for the purposes of designing and evaluating thermoelectric module configurations. Using the new model, we identify and explore several geometric parameters which are critical to module performance. The impact on device performance of solder, ceramic interface, and electrical contact thickness, as well as the leg spacing, is evaluated for a standard unicouple configuration. Results are compared to the standard one-dimensional constant property models commonly used in thermoelectric module design.     

Design of Apparatus for Ni/Mg2Si and Ni/MnSi1.75 Contact Resistance Determination for Thermoelectric Legs

In recent decades, thermoelectricity has been widely studied as a potential new source of renewable energy. One of the major challenges to improve the efficiency of thermoelectric (TE) devices is to minimize the contact resistance between the active material and the electrodes, since this represents the major loss of charge in a TE module. This article describes the fabrication of an apparatus for TE leg characterization built with commercial and custom-made parts based on the analog one-dimensional transmission-line method. This device permits contact resistance measurements of bulk TE legs. p- and n-type TE materials, Mg₂Si_0.98Bi_0.02 and MnSi_1.75Ge_0.02, respectively, were metallized with nickel foils and used as test materials for contact resistance characterization. Contact resistance values of 0.5 mΩ mm² for Ni/Mg₂Si_0.98Bi_0.02 junctions and 4 mΩ mm² for Ni/MnSi_1.75Ge_0.02 junctions have been measured. Contact resistance measurements are discussed depending on materials processing and the experimental measurement conditions.     

Effect of Synthesis and Sintering Conditions on the Thermoelectric Properties of n-Doped Mg2Si

Magnesium silicide (Mg₂Si)-based alloys are promising candidates for thermoelectric (TE) energy conversion in the middle–high temperature range. The detrimental effect of the presence of MgO on the TE properties of Mg₂Si based materials is widely known. For this reason, the conditions used for synthesis and sintering were optimized to limit oxygen contamination. The effect of Bi doping on the TE performance of dense Mg₂Si materials was also investigated. Synthesis was performed by ball milling in an inert atmosphere starting from commercial Mg₂Si powder and Bi powder. The samples were consolidated, by spark plasma sintering, to a density >95%. The morphology, and the composition and crystal structure of samples were characterized by field-emission scanning electronic microscopy and x-ray diffraction, respectively. Moreover, determination of Seebeck coefficients and measurement of electrical and thermal conductivity were performed for all the samples. Mg₂Si with 0.1 mol% Bi doping had a ZT value of 0.81, indicative of the potential of this method for fabrication of n-type bulk material with good TE performance.     

Modeling and Optimization of Thermoelements by a Combined Analytical and Numerical Method

A combined analytical and numerical process has been developed to model and optimize thermoelements. In this way, the performance of commercial n- and p-type thermoelectric materials can be optimized to deliver the maximum output power and conversion efficiency. The validity of the method is demonstrated using a silicon germanium unicouple.     

Device-Level Optimization of n-Type Mg3(Sb,Bi)2-Based Thermoelectric Modules toward Applications: A Perspective

In recent decades, improvements in thermoelectric material performance have made it more practical to generate electricity from waste heat and to use solid-state devices for refrigeration. However, despite the development of successful strategies to enhance the figure-of-merit zT, optimizing devices for large-scale applications remains challenging. High zT values do not guarantee excellent device performance, and maintaining high zT over a wide temperature range is difficult. Thus, device-level structural optimization is crucial for maximizing overall energy conversion efficiency. Proper interfacial and structure design strategies, including contact layer selection, multi-stage optimization, and size matching for the n- and p-type thermoelectric legs, are necessary for advancing device performance. Additionally, thermal stability issues, device assembly techniques, mechanical properties, and manufacturing costs are crucial considerations for large-scale applications. To achieve actual applications, the thermoelectric community must look beyond simply aiming for high zT values. This article focuses on modules based on n-type Mg₃(Sb, Bi)₂, one of the most promising commercially available thermoelectric materials, and discusses the influence of various parameters on the modules and on the corresponding device-level optimization strategies.     

Topology Optimization of Segmented Thermoelectric Generators

The thermoelectric (TE) power output, \(f_P\), and conversion efficiency, \(f_{\eta }\), for segmented thermoelectric generators (TEGs) have been optimized by spatially distributing two TE materials (BiSbTe and Skutterudite) using a numerical gradient-based topology optimization approach. The material properties are temperature-dependent, and the segmented TEGs are designed for various heat transfer rates at the hot and cold reservoirs. The topology-optimized design solutions are characterized by spike-shaped features which enable the designs to operate in an intermediate state between the material phases. Important design parameters, such as the device dimensions, objective functions and heat transfer rates, are identified, investigated and discussed. Comparing the topology optimization approach with the classical segmentation approach, the performance improvements of \(f_P\) and \(f_{\eta }\) design problems depend on the heat transfer rates at the hot and the cold reservoirs, the objective function and the device dimensions. The largest performance improvements for the problems investigated are \(\approx \) 6%.     

Stress Analysis and Output Power Measurement of an n-Mg2Si Thermoelectric Power Generator with an Unconventional Structure

We examine the mechanical stability of an unconventional Mg₂Si thermoelectric generator (TEG) structure. In this structure, the angle θ between the thermoelectric (TE) chips and the heat sink is less than 90°. We examined the tolerance to an external force of various Mg₂Si TEG structures using a finite-element method (FEM) with the ANSYS code. The output power of the TEGs was also measured. First, for the FEM analysis, the mechanical properties of sintered Mg₂Si TE chips, such as the bending strength and Young’s modulus, were measured. Then, two-dimensional (2D) TEG models with various values of θ (90°, 75°, 60°, 45°, 30°, 15°, and 0°) were constructed in ANSYS. The x and y axes were defined as being in the horizontal and vertical directions of the substrate, respectively. In the analysis, the maximum tensile stress in the chip when a constant load was applied to the TEG model in the x direction was determined. Based on the analytical results, an appropriate structure was selected and a module fabricated. For the TEG fabrication, eight TE chips, each with dimensions of 3 mm × 3 mm × 10 mm and consisting of Sb-doped n-Mg₂Si prepared by a plasma-activated sintering process, were assembled such that two chips were connected in parallel, and four pairs of these were connected in series on a footprint of 46 mm × 12 mm. The measured power generation characteristics and temperature distribution with temperature differences between 873 K and 373 K are discussed.     

Development of an Mg2Si Unileg Thermoelectric Module Using Durable Sb-Doped Mg2Si Legs

Mg₂Si unileg structure thermoelectric (TE) modules, which are composed only of n-type Mg₂Si legs, were fabricated using Sb-doped Mg₂Si. The Mg₂Si TE legs used in our module were fabricated by a plasma-activated sintering method using material produced from molten commercial doped polycrystalline Mg₂Si, and, at the same time, nickel electrodes were formed on the Mg₂Si using a monobloc plasma-activated sintering technique. The source material used for our legs has a ZT value of 0.77 at 862 K. The TE modules, which have dimensions of 21 mm × 30 mm × 16 mm, were composed of ten legs that were connected in series electrically using nickel terminals, and the dimensions of a single leg were 4.0 mm  × 4.0 mm × 10 mm. From evaluations of the measured output characteristics of the modules, it appeared that the electrical resistance of the wiring that is used to connect each leg considerably affects the power output of the unileg module. Thus, we attempted to reduce the wiring resistance of the module and fabricated a module using copper terminals. The observed values of the open-circuit voltage and output power of the Sb-doped Mg₂Si unileg module were 496 mV and 1211 mW at ΔT = 531 K (hot side: 873 K; cool side: 342 K).     

Module Geometry and Contact Resistance of Thermoelectric Generators Analyzed by Multiphysics Simulation

Thermoelectric device performance is determined by not only the properties of the thermoelectric material but also the geometrical design and thermal matching of the materials. Leg length and contact quality strongly influence thermoelectric generator efficiency. Experimental results for contact properties are compared with the latest performance measurements on modules manufactured from Bi₂Te₃ compounds. Module performance is related to the obtained contact resistance and thermoelectric material properties. The different influences are studied using thermoelectric multiphysics finite-element modeling of examples where, in addition to the thermoelectric field equations, further effects such as convection and radiation as well as the temperature dependency of the material properties are taken into account. Extensive thermoelectric device modeling is used to understand the experimental findings with respect to contact properties and geometry.     

Fabrication of Highly (0?0?l)-Textured Sb2Te3 Film and Corresponding Thermoelectric Device with Enhanced Performance

An approach for fabrication of highly (0?0?l)-textured Sb₂Te₃ thin film with layered structure by the magnetron sputtering method is reported. The composition, microstructure, and thermoelectric properties of the thin films have been characterized and measured by x-ray diffraction, scanning electron microscopy with energy-dispersive x-ray spectroscopy, and a thermoelectric (TE) measurement system, respectively. The results show that well-oriented (0?0?l) Sb₂Te₃ thin film with layered structure is beneficial for improvement of thermoelectric properties, being a promising choice for planar TE devices. The power generation and cooling performance of a layered p-Sb₂Te₃ film device are superior to those of the ordinary thin-film device. For a typical parallel device with 38 layered Sb₂Te₃ film elements, the output voltage, maximum power, and corresponding power density are up to 10.3?mV, 11.1?μW, and 73?mW/cm², respectively, for a temperature difference of 76?K. The device can produce a 6.1?K maximum temperature difference at current of 45?mA. The results prove that enhanced microdevice performance can be realized by integrating (0?0?l)-oriented Sb₂Te₃ thin films with a layered architecture.     

Design and Fabrication of Segmented GeTe/(Bi,Sb)2Te3 Thermoelectric Module with Enhanced Conversion Efficiency

GeTe and (Bi,Sb)₂Te₃ are two representative thermoelectric (TE) materials showing maximum performance at middle and low temperature, respectively. In order to achieve higher performance over the whole temperature range, their segmented one-leg TE modules are designed and fabricated by one-step spark plasma sintering (SPS). To search for contact and connect layers, the diffusion behavior of Fe, Ni, Cu, and Ti metal layers in GeTe is studied systematically. The results show that Ti with a similar linear expansivity (10.80 × 10⁻⁶ K⁻¹) to GeTe, has low contact resistance (3 µΩ cm²) and thin diffusion layer (0.4 µm), and thus is an effective metallization layer for GeTe. The geometric structure of the GeTe/(Bi,Sb)₂Te₃ segmented one-leg TE module and the ratio of GeTe to (Bi,Sb)₂Te₃ are determined by finite element simulation method. When the GeTe height ratio is 0.66, its theoretical maximum conversion efficiency (η_max) can reach 15.9% without considering the thermal radiation and thermal/electrical contact resistance. The fabricated GeTe/(Bi,Sb)₂Te₃ segmented one-leg TE module showed a η_max up to 9.5% with a power density ≈ 7.45 mW mm⁻², which are relatively high but lower than theoretical predictions, indicating that developing segmented TE modules is an effective approach to enhance TE conversion efficiency.     

Analysis of the Microstructure of Mg2Si Thermoelectric Devices

High-performance Mg₂Si thermoelectric devices have been obtained by spark plasma sintering of high-purity, pre-synthesized, all-molten Mg₂Si powder. We studied the effects of source powder particle size on thermoelectric performance. To improve the performance, further investigation of the microstructure of the devices is needed. In this work we studied the microstructure of grain boundaries and interfaces between electrodes and Mg₂Si sintered bodies to increase understanding of Mg₂Si thermoelectric devices.     

High‐Performance Flexible Thermoelectric Devices Based on All‐Inorganic Hybrid Films for Harvesting Low‐Grade Heat

The fabrication of a flexible thermoelectric (TE) device that contains flexible, all‐inorganic hybrid thin films (p‐type single‐wall carbon nanotubes (SWCNTs)/Sb₂Te₃ and n‐type reduced graphene oxide (RGO)/Bi₂Te₃) is reported. The optimized power factors of the p‐type and n‐type hybrid thin films at ambient temperature are about 55 and 108 µW m^?1 K^?2, respectively. The high performance of these films that are fabricated through the combination of vacuum filtration and annealing can be attributed to their planar orientation and network structure. In addition, a TE device, with 10 couples of legs, shows an output power of 23.6 µW at a temperature gradient of 70 K. A prototype of an integrated photovoltaic‐TE (PV‐TE) device demonstrates the ability to harvest low‐grade “waste” thermal energy from the human body and solar irradiation. The flexible TE and PV‐TE device have great potential in wearable energy harvesting and management.     

Direct evidence for effect of molecular orientation on thermoelectric performance of organic polymer materials by infrared dichroism

A direct evidence by infrared dichroism is reported for the first time for the effect of molecular orientation on thermoelectric (TE) performance of organic polymer materials. The preferred orientation was induced by mechanical uni-axial stretching of the films of neat polyaniline (PANI) and its nanocomposites with reduced graphene oxide (rGO) or multi-walled carbon nanotube (MWCNT). Five characteristic bands of Fourier transform infrared (FTIR) spectra were chosen, and quantitative investigations were carried out using the dichroic ratios measured by polarized FTIR spectra. The influences of draw ratio and content of inorganic carbon nanoparticles were taken into account. The results show that the TE performance (including anisotropic TE function) can be conveniently tuned by polymer molecular orientation induced by mechanical stretching, which shed light on the understanding of molecular mechanism towards structure-TE performance relationship, and will speed up the applications of organic polymer TE materials.     

Thermoelectric Performance Enhancement in BiSbTe Alloy by Microstructure Modulation via Cyclic Spark Plasma Sintering with Liquid Phase

The widespread application of thermoelectric (TE) technology demands high-performance materials, which has stimulated unceasing efforts devoted to the performance enhancement of Bi₂Te₃-based commercialized thermoelectric materials. This study highlights the importance of the synthesis process for high-performance achievement and demonstrates that the enhancement of the thermoelectric performance of (Bi,Sb)₂Te₃ can be achieved by applying cyclic spark plasma sintering to Bi_xSb_2–xTe₃-Te above its eutectic temperature. This facile process results in a unique microstructure characterized by the growth of grains and plentiful nanostructures. The enlarged grains lead to high charge carrier mobility that boosts the power factor. The abundant dislocations originating from the plastic deformation during cyclic liquid phase sintering and the pinning effect by the Sb-rich nano-precipitates result in low lattice thermal conductivity. Therefore, a high ZT value of over 1.46 is achieved, which is 50% higher than conventionally spark-plasma-sintered (Bi,Sb)₂Te₃. The proposed cyclic spark plasma liquid phase sintering process for TE performance enhancement is validated by the representative (Bi,Sb)₂Te₃ thermoelectric alloy and is applicable for other telluride-based materials.     

Synthesis and Characterization of Al-Doped Mg2Si Thermoelectric Materials

Magnesium silicide (Mg₂Si)-based alloys are promising candidates for thermoelectric (TE) energy conversion for the middle to high range of temperature. These materials are very attractive for TE research because of the abundance of their constituent elements in the Earth’s crust. Mg₂Si could replace lead-based TE materials, due to its low cost, nontoxicity, and low density. In this work, the role of aluminum doping (Mg₂Si:Al = 1:x for x = 0.005, 0.01, 0.02, and 0.04 molar ratio) in dense Mg₂Si materials was investigated. The synthesis process was performed by planetary milling under inert atmosphere starting from commercial Mg₂Si pieces and Al powder. After ball milling, the samples were sintered by means of spark plasma sintering to density >95%. The morphology, composition, and crystal structure of the samples were characterized by field-emission scanning electron microscopy, energy-dispersive spectroscopy, and x-ray diffraction analyses. Moreover, Seebeck coefficient analyses, as well as electrical and thermal conductivity measurements were performed for all samples up to 600°C. The resultant estimated ZT values are comparable to those reported in the literature for these materials. In particular, the maximum ZT achieved was 0.50 for the x = 0.01 Al-doped sample at 600°C.