Encapsulated Thermoelectric Modules for Advanced Thermoelectric Systems

An encapsulated thermoelectric (TE) module consists of a vacuum-tight stainless-steel container in which an SiGe or BiTe TE module is encapsulated. This construction enables maximum performance and durability because: the thermal expansion mismatch between the hot and cold sides of the container can be accommodated by a sliding sheet in the container; the TE module inside is always kept in a vacuum environment, therefore no oxidation can occur; and the pressure difference between the inside and outside of the container reduces thermal contact resistance inside the container. Our encapsulated SiGe module features higher operating temperature—up to 650°C for both hot and cold sides. Other high-temperature modules and conventional BiTe modules, including both-sides and one-side skeleton types, have been encapsulated. Several variants of the encapsulated module are available. Encapsulated thermoelectric modules with integrated coolers contain cooling panels through which water can pass. If the module hot side is heated by a radiating heat source (radiation coupling) or convection of a hot gas or fluid (convection coupling), no pressing force on the module is necessary. It therefore features minimum contact resistance with the cooling duct, because no pressure is applied, maximum TE power, and minimum installation cost. Another, larger, variant is a quadruple flexible container in which four modules (each of maximum size 40 mm × 40 mm) are encapsulated. These encapsulated modules were used in a powder metallurgy furnace and were in use for more than 3000 h. Application to cryogenic temperatures simulating the liquid nitrogen gas vaporizer has been also attempted.     

Numerical Simulation of Performance and Thermomechanical Behavior of Thermoelectric Modules with Segmented Bismuth-Telluride-Based Legs

The approach of using segmented legs to build thermoelectric (TE) modules can enhance the performance of TE generators. This approach is based on the selection of materials for different segments that are optimized in terms of their TE properties with respect to the temperature range to which they are exposed during module operation. For this purpose, by carefully controlling the chemical composition of ternary and quaternary bismuth-telluride-based alloys, we have optimized the figure of merit ZT of p-type and n-type alloys implemented by a powder technology approach. The alloys were prepared by mechanical alloying followed by hot extrusion, and their mechanical and TE properties were fully characterized as a function of temperature, which gave us a solid database for simulation of modules containing these materials. Finite-element numerical simulation was applied to evaluate the impact of TE materials properties on the level of mechanical stresses generated by thermal gradients in modules made of segmented legs. Keeping the same total length of two-segment p- and n-type legs, the relative length of each segment was varied to obtain an 8% relative increase of generated electrical power compared with homogeneous legs of the same total length. Under these conditions, the presence of solder interface between the two segments and between the segments and the copper conductors of the module concentrates plastic strain, leading to a significant reduction of the stress level in the TE materials compared with that resulting from using nonsegmented legs. Leg segmentation not only leads to improved TE performance but could also significantly modify the maximum values and distribution of thermomechanical stresses in the modules, depending on how it is realized. The study presents how this numerical simulation tool can be used to optimize the design of segmented modules.     

Improvement in the Durability and Heat Conduction of uni-leg Thermoelectric Modules Using n-type Mg2Si Legs

We have fabricated several kinds of uni-leg thermoelectric (TE) modules using Sb-doped n-type Mg₂Si. In order to evaluate the influence of the structure of the modules on their durability with respect to heat-cycling, modules of two different types were evaluated. One was a conventional-structured module, in which the upper and lower surfaces of the legs were each fixed to a ceramic substrate. The other was a ‘half skeleton’ module, in which the ‘cold-side’ substrate was removed and a thermal-conductive sheet was used instead of a ceramic plate for the cold-side insulator. From the result of this evaluation, it was confirmed that, although some variation in the output power was observed for the ‘half-skeleton’ module, the power variation was markedly less than for the conventional-structured module. Additionally, to improve the output power of the module, we replaced the Al₂O₃ substrate with Si₃N₄, which has a higher thermal conductivity than the Al₂O₃ substrate. The observed output power of a module (25 mm × 24 mm × 8.3 mm) fabricated using the Si₃N₄ substrate was 1,293 mW at ΔT = 500 K. The output value of the module using the Si₃N₄ plate was improved by 29 % compared with the output value of the module using the Al₂O₃ substrate. Moreover, based on the structures of these modules, a 36 mm × 41 mm × 8.3 mm module was fabricated. The expected value of the output power of the module was 1.9 W at ΔT = 500 K.     

Encapsulated Thermoelectric Modules and Compliant Pads for Advanced Thermoelectric Systems

The described encapsulated thermoelectric (TE) module consists of a vacuum-tight stainless-steel container with dimensions of 55 mm × 50 mm × 11 mm in which a SiGe or BiTe TE module is placed. This construction enables maximum performance and durability because: (1) the thermal expansion mismatch between the hot and cold sides of the container can be accommodated by a sliding sheet that is present in the container; (2) the TE module inside is always kept in a vacuum environment so that oxidation cannot occur; and (3) the difference in pressure between the inside and outside of the container reduces the thermal contact resistance inside the container. The design also includes a compliant pad made of porous material that is infiltrated with braze filler material. If heated to a temperature above the melting temperature of the braze filler, the thermal gap conductance of the interface is enhanced due to the strong affinity of the braze filler to an adjacent member. It is possible for this interface to slide as long as the braze filler is in the liquid state. This design strategy provides high-flux, direct conduction paths to the heat source and heat sink as well as a large temperature gradient across the TE module itself (1.7 times that in the case of the conventional design strategy); therefore, the module can provide a power output that is three times greater than that of a conventional module.     

Properties of p- and n-Type PbTe Microwires for Thermoelectric Devices

In thermopower measurements, microwires fabricated from as-purchased bulk PbTe exhibits p-type behavior between room temperature and ～600 K. At higher temperatures, it undergoes majority carrier inversion and exhibits n-type behavior. We report on the preparation and properties of potassium oxide and Zn-doped PbTe microwires, which exhibit stable p- and n-type behavior, respectively, between room temperature and 725 K. Thermoelectric figures of merit (ZT) are reported for device components prepared from bundles of such p- and n-type microwires in a glass matrix.     

Miniature Thermoelectric Modules Developed for Cycling Applications

Semiconductors - In the paper the test data on new cycle-resistant thermoelectric modules are presented and discussed. These modules can be applied in medical equipment for polymerase chain...     

Test System for Thermoelectric Modules and Materials

We present a design for a complex measuring device that enables its user to assess the parameters of power-generating thermoelectric modules (TEMs) (or bulk thermoelectric materials) under a wide range of temperatures (T _cold = 25°C to 90°C, T _hot < 450°C) and mechanical loading (P = 0 N to 10⁴ N). The proposed instrument is able to monitor the temperature and electrical output of the TEM, the actual heat flow through the module, and its mechanical load, which can be varied during the measurement. Key components of our testing setup are (i) a measuring chamber where the TEM/material is compressed between thermally shielded heating blocks equipped with a mechanical loading system and water-cooled copper-based cooler, (ii) an electrical load system, (iii) a type K thermocouple array connected to a data acquisition computer, and (iv) a thermostatic water-based cooling system with electronically controlled flow rate and temperature of cooling water. Our testing setup represents a useful tool able to assess, e.g., the thermoelectric parameters of newly developed TEMs and materials or to evaluate the thermoelectric parameters of commercially available modules and materials for comparison with values declared by the manufacturer.     

Thermoelectric Properties of Two-Phase PbTe with Indium Inclusions

The thermoelectric figure of merit (zT) can be increased by introduction of additional interfaces in the bulk to reduce the thermal conductivity. In this work, PbTe with a dispersed indium (In) phase was synthesized by a matrix encapsulation technique for different In concentrations. x-Ray diffraction analysis showed single-phase PbTe with In secondary phase. Rietveld analysis did not show In substitution at either the Pb or Te site, and this was further confirmed by room-temperature Raman data. Low-magnification (~1500×) scanning electron microscopy images showed micrometer-sized In dispersed throughout the PbTe matrix, while at high magnification (150,000×) an agglomeration of PbTe particles in the hot-pressed samples could be seen. The electrical resistivity (ρ) and Seebeck coefficient (S) were measured from 300 K to 723 K. Negative Seebeck values showed all the samples to be n-type. A systematic increase in resistivity and higher Seebeck coefficient values with increasing In content indicated the role of PbTe-In interfaces in the scattering of electrons. This was further confirmed by the thermal conductivity (κ), measured from 423 K to 723 K, where a greater reduction in the electronic as compared with the lattice contribution was found for In-added samples. It was found that, despite the high lattice mismatch at the PbTe-In interface, phonons were not scattered as effectively as electrons. The highest zT obtained was 0.78 at 723 K for the sample with the lowest In content.     

掺杂PbTe晶体的热电性质研究

介绍了利用区熔法生长掺杂热电变换材料ＰｂＴｅ晶体,并对生长样品的杂质分布、热电特性进行了测量和分析。     

Low-Temperature Synthesis and Thermoelectric Properties of n-Type PbTe

n-Type PbTe compounds were synthesized at temperatures as low as 430°C. After synthesis, the materials were ground, cold pressed, and sintered at 600°C. The effect of this low-temperature synthesis on the structural features and thermoelectric properties of as-prepared and PbI₂-doped materials was investigated for the first time. The Seebeck coefficient, and electrical and thermal conductivity were measured in the temperature range 2 K ≤ T ≤  610 K. The results show that all materials exhibit n-type conduction and the thermoelectric properties are improved by doping. ZT values reach 0.5 at 610 K, and the discrepancies with the literature are discussed.     

Thermoelectric Properties of Nanocrystalline PbTe Synthesized by Mechanical Alloying

In this work, nanocrystalline lead telluride powder was synthesized from high-purity elements by mechanical alloying by means of a planetary ball-milling procedure. The milling medium was tungsten carbide, and the diameter of the balls was varied in order to investigate the effect on the structural features of the material. Phase transformations and crystallite evolution during ball-milling were followed by powder x-ray diffraction (PXRD). The broadened PXRD peaks were analyzed with Voigt functions, revealing small crystalline size and stress introduced during the mechanical alloying process. Transmission electron microscopy (TEM) studies confirmed the material’s nanostructure, as well as the effect of ball diameter on the size of the crystals. Thermoelectric properties are discussed in terms of the Seebeck coefficient and the nominal carrier concentration, as determined by Hall-effect measurements. The enhancement of the Seebeck coefficient is reported to be higher compared with other PbTe-based nanocomposites.     

Development of Skutterudite Thermoelectric Materials and Modules

Multifilling with La, Ba, Ga, and Ti in p-type skutterudite and Yb, Ca, Al, Ga, and In in n-type skutterudite remarkably reduces their thermal conductivity, resulting in enhancement of their dimensionless figure of merit ZT to ZT?=?0.75 for p-type (La,Ba,Ga,Ti)₁(Fe,Co)₄Sb₁₂ and ZT?=?1.0 for n-type (Yb,Ca,Al,Ga,In)_0.7(Co,Fe)₄Sb₁₂. A thermoelectric module technology suitable for these skutterudites including diffusion barrier and electrode materials has been established. The diffusion barrier materials allow the electrode to coexist stably with the p/n skutterudites in the module??s working temperature range of room temperature to 600°C. Under conditions of hot/cold-side temperatures of 600°C/50°C, a skutterudite module with size of 50?mm?×?50?mm?×?7.6?mm exhibited generation performance of 32?W power output and 8% thermoelectric conversion efficiency.     

Symmetry Analysis of Thermoelectric Energy Converters with Inhomogeneous Legs

Symmetry analysis has been applied to thermoelectric energy converters [thermoelectric generators (TEG), coolers (TEC), and heaters (TEH)] with inhomogeneous legs. The features of the crystallographic symmetry of thermoelectric materials and the symmetry of legs, thermocouples, and modules are studied. The effect of symmetry on the figure of merit Z of thermoelectric energy converters is considered. A general rule for proper placement of legs in thermoelectric converters is developed. A modified tetratomic classification for thermoelectric energy converters with inhomogeneous legs (TEG^a, TEG^b, TEC, and TEH) is proposed. An increase in Z for thermoelectric energy converters with inhomogeneous legs is due to the bulk thermoelectric effect. An increase in Z gives the reduction of irreversible processes in the modules (Joule heating and thermal conductivity), accompanying breaking of the symmetry of the legs. It is found that violations of the symmetry requirements can lead to significant energy losses in converters.     

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.     

Procedure and Equipment for Measuring Parameters of Thermoelectric Generator Modules

Equipment for measuring parameters of thermoelectric generator modules in the temperature range from 30°C to 600°C, and in the expanded range to 800°C, has been developed. To determine module efficiency, the equipment employs a direct heat flux measurement using heat meters. Steps to minimize measurement errors were taken in the development. This equipment can be used for studies on the development of thermoelectric modules, as well as for their quality control during commercial production. Consumers will find this equipment useful for testing generator modules prior to mounting them into final products.     

Thermoelectric and Mechanical Properties of Novel Hot-Extruded PbTe n-Type Material

Lead telluride-based materials demonstrate the highest thermoelectric performance in the temperature range from 200°C to 400°C, and they are of interest for numerous waste heat recovery applications. Unfortunately, these conventionally grown materials are usually very brittle, which results in significant material loss during module manufacturing and a decrease in module reliability when subjected to continuous vibrations common for automotive applications. We present a hot extrusion process developed for the first time for PbTe which yields polycrystalline materials with strong mechanical properties combined with high thermoelectric performance. n-Type lead telluride was extruded from conventionally synthesized and powdered material at temperatures in the range of 450°C to 520°C depending on material stoichiometry. The extruded rods were of cylindrical shape with 2.54?cm diameter and lengths up to 40?cm. Young??s modulus measured using mechanical spectroscopy varied from 59?GPa to 51?GPa for temperatures in the range of 20°C to 300°C. Slicing and dicing of extruded rods to obtain cubical samples with 2?mm side demonstrated no difficulties, illustrating the material homogeneity and its potential for manufacturing module legs. The microstructure of the material was studied by scanning electron microscopy. Doping with antimony iodide during the milling process controls the conduction electron concentration in the range from 1?×?10¹⁹?cm^?3 to 6?×?10¹⁹?cm^?3. For optimized doping of 0.08?wt.% SbI₃, the maximum thermoelectric figure of merit (ZT) reaches a value of 0.99 at 380°C, as measured by the Harman method. The combination of high thermoelectric performance and improved fracture toughness makes this novel hot-extruded polycrystalline PbTe material highly competitive for many applications.     

Design of Bulk Thermoelectric Modules for Integrated Circuit Thermal Management

Various parameters affecting the performance of bulk thermoelectric (TE) modules used for integrated circuit (IC) thermal management are studied. An effective circuit model is developed that takes into account various ideal and nonideal effects in the module. It is shown that there is an optimum module thickness and an optimum operating current which depend on the overall heat dissipation and on the external thermal resistances. Optimized TE modules with ZT~0.8, will have a cross section over leg length ratio of 0.037m, can increase the chip operation power by 15% in comparison with the case without a TE cooler while maintaining the chip temperature below 100degC. This is for a package thermal resistance of 0.2K/W. Prospects for TE material with higher ZT values and the effect of contact resistance on the power dissipation density are also discussed. The results presented in this paper can be used in applications other than in the IC thermal management when external thermal resistances dominate the performance of TE modules     

The Effects of Thermoelectric Film Thickness on Performance of In-Plane Thermoelectric Modules

We fabricated in-plane thermoelectric modules (4?mm?×?4?mm) on a 4-??m-thick substrate using a vacuum deposition process through a shadow mask. In this study, a thermoelectric p?Cn pair was established using multilayered films of p and n thermoelectric thin films and an insulator film with a hole at the center. The output power was 58?nW at 443?K using the multilayered microgenerator. We discuss the effects of device thickness on the efficiency of the microgenerator to increase the output electric power. We evaluated the output power of the in-plane thermoelectric generator with a substrate using a one-dimensional heat conduction model, and it was found to depend on the thickness of the thermoelectric film. If the thermoelectric film is very thin, the power factor is more important than the nondimensional figure of merit, ZT. Metal thin films with high power factor are more efficient than semiconductors with low power factors even though their thermal conductivities are high. When the thermoelectric thin film is thick, ZT should be higher for larger output power of the device.     

A Power-Generation Test for Oxide-Based Thermoelectric Modules Using p-Type Ca3Co4O9 and n-Type Ca0.9Nd0.1MnO3 Legs

Metal oxides are considered to be promising thermoelectric (TE) materials, especially for high-temperature power-generation applications, because they have many advantages such as low price, light weight, thermal stability, nontoxicity, and high oxidation resistance. For these reasons, oxide-based TE modules were fabricated using p-type pure Ca₃Co₄O₉ and n-type Ca_0.9Nd_0.1MnO₃ legs for power generation at temperatures in excess of 1000?K. This study involved the use of Ag sheets with a Ag paste as electrode materials and alumina plates as a substrate for the modules. The p-type pure Ca₃Co₄O₉ legs were manufactured by spark plasma sintering, and the n-type Ca_0.9Nd_0.1MnO₃ legs were sintered by a conventional process at atmospheric pressure. From a unicouple, a power density as high as 93.2?mW/cm² under a temperature condition of ??T?=?727?K (T _hot?=?1175?K) was obtained. This high power density is believed to be a result of the modified contact of the electrode (notch process) and the optimized material properties (the SPS process and a dopant effect) along with the high ??T obtained in this study (reduced thermal losses because of good packing of thermal insulation). Areas of concern for future research include the following: (1) the measured open-circuit voltage from the present unicouples was only 94.3% of the theoretical voltage, and (2) the internal resistance value was as high as 490% of the theoretical resistance.