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.     

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.     

Nanostructured Thermoelectric Materials and High-Efficiency Power-Generation Modules

For thermoelectric applications, the best materials have high electrical conductivity and thermopower and, simultaneously, low thermal conductivity. Such a combination of properties is usually found in heavily doped semiconductors. Renewed interest in this topic has followed recent theoretical predictions that significant increases in performance are possible for nanostructured materials, and this has been experimentally verified. During exploratory synthetic studies of chalcogenide-based bulk thermoelectric materials it was discovered that several compounds spontaneously formed endotaxially embedded nanostructures. These compounds have some of the best known properties for bulk thermoelectric materials in the 500–800 K temperature range. Here we report our continued efforts to better understand the role of the nanostructures while concurrently furthering the development of these new materials (for example n-type lead–antimony–silver–tellurium, and p-type lead–antimony–silver–tin–tellurium) into thermoelectric power-generation devices.     

Development of Low-Cost Remote-Control Generators Based on BiTe Thermoelectric Modules

This paper presents a new thermogenerator based on moderate-temperature (up to 175°C) BiTe modules available on the open market. Despite this handicap relative to commercial thermogenerators based on high-temperature proprietary-technology PbBi modules (up to 560°C), this new design may become economically competitive due to its innovative thermal sink. Our thermal sink is based on a free-convection water loop built with standard tubing and household hot-water radiators, leading to a more practical, modular design. So, the specific cost of about 55,000 USD/kW obtained for this 120-W prototype is improved to 33,000 USD/kW for a 1-kW unit, which represents about half the price of commercial thermogenerators. Moreover, considering recently launched BiTe modules (that withstand up to 320°C), our proposition could have an even more favorable outlook.     

Fabrication of High-Temperature-Stable Thermoelectric Generator Modules Based on Nanocrystalline Silicon

High-temperature-stable thermoelectric generator modules (TGMs) based on nanocrystalline silicon have been fabricated, characterized by the Harman technique, and measured in a generator test facility at the German Aerospace Center. Starting with highly doped p- and n-type silicon nanoparticles from a scalable gas-phase process, nanocrystalline bulk silicon was obtained using a current-activated sintering technique. Electrochemical plating methods were employed to metalize the nanocrystalline silicon. The specific electrical contact resistance ρ _c of the semiconductor–metal interface was characterized by a transfer length method. Values as low as ρ _c < 1 × 10^?6 Ω cm² were measured. The device figure of merit of a TGM with 64 legs was approximately ZT = 0.13 at 600°C as measured by the Harman technique. Using a generator test facility, the maximum electrical power output of a TGM with 100 legs was measured to be roughly 1 W at hot-side temperature of 600°C and cold-side temperature of 300°C.     

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.     

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 (Bi_0.2Sb_0.8)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 (Bi_0.2Sb_0.8)₂Te₃ alloy.     

Enhancing the Thermoelectric Properties via Modulation of Defects in P-Type MNiSn-Based (M = Hf,Zr, Ti) Half-Heusler Materials

The thermoelectric figure-of-merit (zT) of p-type MNiSn (M = Ti, Zr, or Hf) half-Heusler compounds is lower than their n-type counterparts due to the presence of a donor in-gap state caused by Ni occupying tetrahedral interstitials. While ZrNiSn and TiNiSn, have been extensively studied, HfNiSn remains unexplored. Herein, this study reports an improved thermoelectric property in p-type HfNi_1−xCo_xSn. By doping 5 at% Co at the Ni sites, the Seebeck coefficient becomes reaching a peak value exceeding 200 µV K⁻¹ that breaks the record of previous reports. A maximum power factor of ≈2.2 mW m⁻¹ K⁻² at 973 K is achieved by optimizing the carrier concentration. The enhanced p-type transport is ascribed to the reduced content of Ni defects, supported by first principle calculations and diffraction pattern refinement. Concomitantly, Co doping also softens the lattice and scatters phonons, resulting in a minimum lattice thermal conductivity of ≈1.8 W m⁻¹ K⁻¹. This leads to a peak zT of 0.55 at 973 K is realized, surpassing the best performing p-type MNiSn by 100%. This approach offers a new method to manipulate the intrinsic atomic disorder in half-Heusler materials, facilitating further optimization of their electronic and thermal properties.     

基于镶嵌数据集的大规模遥感影像管理研究

介绍了三种传统影像管理模式的优缺点和存在的问题,同时研究了ESRI公司的镶嵌数据集技术,包括镶嵌数据集的管理模式、动态镶嵌、实时处理和服务共享等特性。通过对实验数据进行分析可知,镶嵌数据集能够高效地管理大规模遥感影像,并能解决传统影像管理中存在的问题。     

Multilayered Ge/SiGe Material in Microfabricated Thermoelectric Modules

Results for low dimensional p-type Ge/SiGe superlattices with Ge quantum wells of 3.43 nm are presented. A range of microfabricated test structures have been developed to characterise the cross-plane electrical and thermal properties of the Ge/SiGe heterostructures. These superlattices were directly grown on 100-mm-diameter silicon wafers by a chemical vapour deposition growth system with rates up to 6 nm/s. Quantum well and quantum mechanical tunnel barriers with dimensions down to \(\sim1\) nm have been designed, grown and tested; they demonstrate a ZT of 0.08 ± 0.011 and power factor of 1.34 ± 0.15 m W m^?1 K^?2 at 300 K. A complete microfabricated module using indium bump-bonding is reported together with preliminary results on unoptimised material and leg dimensions. Routes to optimise the material and modules are discussed.     

Impurity Band Effects in Thermoelectric Materials

The possibility of improvement of the power factor through introduction of impurity bands is considered. The contributions to the Seebeck coefficient and electrical conductivity from carriers in an impurity band are expressed in terms of the Fermi energy and the position of the impurity levels relative to the edge of the main band. Surprisingly, it is found that the presence of an impurity band generally brings little benefit, and in fact, when it lies near the edge of the main band, the power factor may be significantly reduced. Our findings are consistent with the observation that the figure of merit of PbTe is improved by the presence of deep-lying thallium states. They also indicate that, for Bi₂Te₃, compensation for nonstoichiometry by counterdoping may not yield the highest power factor.     

Dopant Distributions in PbTe-Based Thermoelectric Materials

Atom-probe tomography (APT) is utilized to characterize the dopant distribution in two thermoelectric materials systems: (1) PbTe-2?mol.%SrTe-1?mol.%Na₂Te, and (2) codoped PbTe-1.25?mol.%K-1.4?mol.%Na. We observe the presence of Na-rich precipitates of a few nanometers in diameter in both systems. Both concentration frequency distribution analyses and partial radial distribution functions are employed to analyze the tendency for dopant clustering detected by APT. In the codoped sample, K accumulates significantly in Na-rich precipitates, while in the Sr-containing sample, Sr is homogeneously distributed. High-resolution transmission electron microscopy also reveals the presence of precipitates having platelet morphology, which are oriented parallel to the {001} planes.     

High-Efficiency Thermoelectric Module Based on High-Performance Bi0.42Sb1.58Te3 Materials

Bismuth-telluride-based alloy is the sole thermoelectric candidate for commercial thermoelectric application in low-grade waste heat harvest near room temperature, but the sharp drop of thermoelectric properties at higher temperature and weak mechanical strength in zone-melted material are the main obstacles to its wide development for power generation. Herein, an effective approach is reported to improve the thermoelectric performance of p-type Bi_0.42Sb_1.58Te₃ hot-pressed sample by incorporating Ag₅SbSe₄. A peak ZT of 1.40 at 375 K and a high average ZT of 1.25 between 300 and 500 K are achieved. Such outstanding thermoelectric performance originates from the synergistic effects of improved density-of-states effective mass, reduced bipolar thermal conductivity by the boosted carrier concentration, and suppressed lattice thermal conductivity by the induced phonon scattering centers including substitute point defects, dislocations, stress–strain clusters, and grain boundaries. Comprised of the p-type Bi_0.42Sb_1.58Te₃ + 0.10 wt% Ag₅SbSe₄ and zone-melted n-type Bi₂Te_2.7Se_0.3, the thermoelectric module exhibits a high conversion efficiency of 6.5% at a temperature gradient of 200 K, indicating promising applications for low-grade heat harvest near room temperature.     

Variations of Thermoelectric and Mechanical Properties of Large Lead Telluride Samples Produced by a Short-Term Sintering Method

Although lead telluride is a widely used thermoelectric (TE) material for generator applications in the intermediate temperature range, its mechanical properties have not been fully understood yet. Especially sintered PbTe samples have hardly been investigated with regard to their mechanical properties and homogeneity in general. The aim of the present study is to comprehend the TE and mechanical properties of large PbTe samples produced by a short-term sintering process. The potential and Seebeck microprobe technique was used to reveal the spatial distribution of the Seebeck coefficient on the samples’ surfaces at room temperature. Microhardness measurements were performed with a Vickers indenter. The results demonstrate that, despite a homogeneous density, the functional and mechanical properties vary along the samples, which is attributed to varying local parameters during sintering.     

A Novel Optimization Method for the Electric Topology of Thermoelectric Modules Used in an Automobile Exhaust Thermoelectric Generator

Based on Bi₂Te₃ thermoelectric modules, a kind of automobile exhaust thermoelectric generator (AETEG) with a single-column cold-source structure was designed. To enhance its net power and efficiency, the output performance of all the thermoelectric modules was tested with a temperature monitoring unit and voltage monitoring unit, and modeled using a back-propagation (BP) neural network based on various hot-source temperatures, cold-source temperatures, load currents, and contact pressures according to the temperature distribution of the designed heat exchanger and cooling system. Then, their electric topology (series or parallel hybrid) was optimized using a genetic algorithm to achieve the maximum peak power of the AETEG. From the experimental results, compared with when all the thermoelectric modules were connected only in series or parallel at random, it is concluded that the AETEG performance is evidently affected by the electric topology of all the single thermoelectric modules. The optimized AETEG output power is greatly superior to the other two investigated designs, validating the proposed optimized electric topology as both feasible and practical.     

Effects of Microstructural Evolution on the Thermoelectric Properties of Spark-Plasma-Sintered Ti0.3Zr0.35Hf0.35NiSn Half-Heusler Compound

The MNiSn (M = Ti, Zr, Hf) half-Heusler semiconducting compounds are widely investigated due to their good potential for thermoelectric (TE) power generation applications. In the current work, the evolution of the transport and structural properties of the Ti_0.3Zr_0.35Hf_0.35NiSn compound upon various thermal treatments was studied. The nominal composition was arc melted, ball milled, and spark plasma sintered (SPS). Following SPS, large Hf-rich domains were found by scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS). Subsequently, the samples were subjected to homogenization treatments at 1163 K for 480 h and 610 h under argon atmosphere. Following these thermal treatments, the relative amount of the Hf-rich domains was reduced and they became smaller in size, with increasing thermal treatment duration. Nevertheless, no uniphased structure was reached. The dissolution of the Hf-rich domains in the half-Heusler matrix resulted in increase of both the Seebeck coefficient and electrical resistivity values and a decrease of the carrier concentration, attributed to elimination of these metallic domains. Resulting from the high atomic disorder caused by substitution at the M site, low thermal conductivity values of ~3.8 W m^?1 K^?1 were obtained leading to high ZT values of up to 0.82 following SPS.