Modeling thermoelectric generator (TEG) performances plays an important role in guiding the design of TEGs to achieve better efficiency. However, a rigorous 1-D TEG modeling performance has not yet been conducted, which prevents reliable prediction of TEG performance. In this work, a detailed 1-D model has been developed to take into account temperature-dependent thermoelectric material properties, heat loss due to radiation and conduction, and Thomson effect. A Lead Telluride (PbTe) TEG was chosen as a sample module and the modeling results agree very well with the experimental results, which proves how powerful the presented detailed 1-D model can be used to predict and validate TEG experimental results. TEG power and efficiency were found to have a respective decrease of 10% and 31% from the simplified model at a temperature gradient of 570 K. While heat loss attributable to conduction and radiation were found to be small, the Thomson effect, which is often neglected, was found to significantly reduce TEG performances. The deep analysis enabled by the new model provides useful guidelines to improve the performance of TEGs. (C) 2017 Elsevier Ltd. All rights reserved.

Modem electric grids that use intermittent renewables require energy storage to maintain reliability. Many potential solutions exist, but latent heat thermal energy storage shows particularly high potential for low cost grid scale energy storage. In this paper, we present the design and initial experimental results for a lab-scale prototype of a novel latent heat thermal storage system. This version of our prototype used 50 kg of aluminum-silicon alloy as a phase change material, and a novel valved thermosyphon concept to control heat flow from a thermal storage tank to thermoelectric generators for dispatchable electricity production. Our results validate the system: the thermal storage system was able to receive heat input, evenly distribute heat to and from the phase change material with small temperature gradients, and controllably dispatch heat to a heat engine for electricity generation on demand. With the basic principle of this technology demonstrated, our next step will be to evaluate and improve system efficiency.

A mathematical model for a thermoelectric generator (TEG) based on constitutive equations has been developed to analyze temperature dependent performance in terms of output power and efficiency. Temperature dependent material properties and thermal losses, which occur as conductive and radiative heat transfer, were considered in the finite element model. Effective material properties were invoked for understanding the influence of temperature dependence of material parameters and related adverse effects on the model TEG. It is shown that analytical equations with effective properties can provide excellent estimation of the performance of a TEG over a broad operating range. The model was simulated, analyzed and validated to examine the effects of different operating conditions and geometry that interact with thermal losses inside the TEG. We believe that this model will further expedite the optimization of TEGs being developed using new material compositions.

In the previous article (1) we have discussed thermoelectric (TE) phenomena. Application of these phenomena in useful devices is limited by the performance of thermoelectric materials. Accordingly, we now discuss characteristics of TE materials, along with delving into the crystal structures, microstructures, and properties of a handful of exemplary materials. The thermoelectric power factor S-2 is introduced as the measure of the electronic performance. In turn, various ways of scattering phonons are related to the structure aspects on one hand and to the performance on the other. Thermal stability of TE materials under repetitive temperature cycling are also discussed-as well as ways of enhancing the thermal stability and thus maintaining also low electric resistivity.

In this work, a comparison between the performance of two- and three-stage cascaded thermoelectric generator (TEG) devices is analyzed based on a prescribed maximum hot side temperature of 973 K, an imposed maximum heat input of 505 W, and a fixed cold side temperature of 473 K. Half-Heusler is used as a thermoelectric (TE) material in the top higher temperature stage and skutterudite as a TE in the bottom lower temperature stage for the two-stage structure. Lead telluride is added in the middle stage to form the three-stage structure. Based on the prescribed constraints, the two-stage cascaded TEG is found to produce a power output of 42 W with an efficiency of 8.3%. The three-stage cascaded TEG produces a power output of 51 W with an efficiency of 10.2%. The three-stage cascaded TEG produces 21% more power than the two-stage does; however, if the system complexity, mechanical robustness, manufacturability, and/or cost of three-stage cascaded TEG outweigh the 21% percent power production increase, the two-stage TEG could be preferable. (C) 2017 Elsevier B.V. All rights reserved.

International transport property-measurement round-robins have been conducted by the thermoelectric annex under the International Energy Agency (IEA) Implementing Agreement on Advanced Materials for Transportation (AMT). Two previous round-robins used commercially available bismuth telluride as the test material, with the objectives of understanding measurement issues and developing standard testing procedures. This round-robin extended the measurement temperature range to 773 K. It was designed to meet the increasing demands for reliable transport data for thermoelectric materials used for power-generation applications. Eleven laboratories from six IEA-AMT member countries participated in the study. A half-Heusler ( n -type) material prepared by GMZ Energy was selected for the round-robin. The measured transport properties had a narrower distribution of uncertainty than previous round-robin results. The study intentionally included multiple testing methods and instrument types. Over the full temperature range, the measurement discrepancies for the figure of merit, ZT , in this round-robin were ±11.5 to ±16.4% from the averages.

The accurate determination of the thermoelectric properties of a material becomes increasingly difficult as the temperature rises. However, it is the properties at elevated temperatures that are important if thermoelectric generator efficiency is to be improved. It is shown that the dimensionless figure of merit, ZT, might be expected to rise with temperature for a given material provided that minority carrier conduction can be avoided. It is, of course, also necessary that the material should remain stable over the whole operating range. We show that the prediction of high temperature properties in the extrinsic region is possible if the temperature dependence of carrier mobility and lattice thermal conductivity are known. Also, we show how the undesirable effects arising from mixed or intrinsic conduction can be calculated from the energy gap and the relative mobilities of the electrons and the positive holes. The processes involved are discussed in general terms and are illustrated for different systems. These comprise the bismuth telluride alloys, silicon-germanium alloys, magnesium-silicon-tin and higher manganese silicide.

Potential usefulness of thermoelectric (TE), effects is hard to overestimate-while at the present time the uses of those effects are limited to niche applications. Possibly because of this situation, coverage of TE effects, TE materials and TE devices in instruction in Materials Science and Engineering is largely perfunctory and limited to discussions of thermocouples. We begin a series of review articles to remedy this situation. In the present article we discuss the Seebeck effect, the Peltier effect, and the role of these effects in semiconductor materials and in the electronics industry. The Seebeck effect allows creation of a voltage on the basis of the temperature difference. The twin Peltier effect allows cooling or heating when two materials are connected to an electric current. Potential consequences of these phenomena are staggering. If a dramatic improvement in thermoelectric cooling device efficiency were achieved, the resulting elimination of liquid coolants in refrigerators would stop one contribution to both global warming and destruction of the Earth's ozone layer.

The structure of Bi2Te3 (Seebeck coefficient Standard Reference Material (SRM™ 3451)) and the related phase Sb2Te3 have been characterized as a function of temperature using the neutron powder diffraction (NPD) and the extended X-ray absorption fine structure (EXAFS) techniques. The neutron structural studies were carried out from 20 K to 300 K for Bi2Te3 and from 10 K to 298 K for Sb2Te3. The EXAFS technique for studying the local structure of the two compounds was conducted from 19 K to 298 K. Bi2Te3 and Sb2Te3 are isostructural, with a space group of R3¯m. The structure consists of repeated quintuple layers of atoms, Te2-M-Te1-M-Te2 (where M = Bi or Sb) stacking along the c-axis of the unit cell. EXAFS was used to examine the bond distances and static and thermal disorders for the first three shells of Bi2Te3 and Sb2Te3 as a function of temperature. The temperature dependencies of thermal disorders were analyzed using the Debye and Einstein models for lattice vibrations. The Debye and Einstein temperatures for the first two shells of Bi2Te3 are similar to those of Sb2Te3 within the uncertainty in the data. However, the Debye and Einstein temperatures for the third shell of Bi-Bi are significantly lower than those of the third shell of Sb-Sb. The Einstein temperature for the third shell is consistent with a soft phonon mode in both Bi2Te3 and Sb2Te3. The lower Einstein temperature of Bi-Bi relative to Sb-Sb is consistent with the lower value of thermal conductivity of Bi2Te3 relative to Sb2Te3.

The impact of contact resistance on thermoelectric (TE) device performance grows more significant as devices are scaled down. To improve and understand the effects of contact resistance on bulk TE device performance, a reliable experimental measurement method is needed. There are many popular methods to extract contact resistance, but they are only well suited for measuring metal contacts on thin films and do not necessarily translate to measuring contact resistance on bulk TE materials. The authors present a measurement technique that precisely measures contact resistance on bulk TE materials by making and testing stacks of bulk, metal-coated TE wafers using TE industry-standard processes. An equation that uses the Z of the stacked device to extract the contact resistance is used to reduce the sensitivity to resistivity variations of the TE material. Another advantage of this technique is that it exploits realistic TE device manufacturing techniques and results in an almost device-like structure. The lowest contact resistivity measured was 1.1 × 10−6 Ω cm2 and 1.3 × 10−6 Ω cm2 for n- and p-type materials, respectively using a newly developed process at 300 K. The uncertainty in the contact resistivity values for each sample was 10% to 20%, which is quite good for measurements in the 10−6 Ω cm2 range.

As thermoelectric (TE) element length decreases, the impact of contact resistance on TE device performance grows more significant. In fact, for a TE device containing 100-μm tall Bi2Te3TE elements, the figure of merit ratio (ZTDevice/ZTMaterial) drops from 0.9 to 0.5 as the contact resistivity increases from 5 × 10−07 to 5 × 10−06 Ω-cm2. To understand the effects of contact resistance on bulk TE device performance, a reliable experimental measurement method is needed. There are many popular methods to extract contact resistance such as Transmission Line Measurements (TLM) and Kelvin Cross Bridge Resistor method (KCBR), but they are only well-suited for measuring metal contacts on thin films and do not necessarily translate to measuring contact resistance on bulk thermoelectric materials. The authors present a new measurement technique that precisely measures contact resistance (on the order of 5 × 10−07 Ω-cm2) on bulk thermoelectric materials by processing stacks of bulk, metal-coated TE wafers using TE industry standard processes. One advantage of this technique is that it exploits realistic TE device manufacturing techniques and results in an almost device-like structure, therefore representing a realistic value for electrical contact resistance in a bulk TE device. Contact resistance measurements for metal contacts to n- and p-type Bi2Te3 alloys are presented and an estimate of the accuracy of the measurements is discussed.

Recent research and development of high-temperature thermoelectric materials has demonstrated great potential for converting automobile exhaust heat directly into electricity. Thermoelectrics based on classic bismuth telluride have also started to impact the automotive industry by enhancing air-conditioning efficiency and integrated cabin climate control. In addition to engineering challenges of making reliable and efficient devices to withstand thermal and mechanical cycling, the remaining issues in thermoelectric power generation and refrigeration are mostly materials related. The dimensionless figure of merit, ZT, still needs to be improved from the current value of 1.0 to 1.5 to above 2.0 to be competitive with other alternative technologies. In the meantime, the thermoelectric community could greatly benefit from the development of international test standards, improved test methods, and better characterization tools. Internationally, thermoelectrics have been recognized by many countries as a key component for improving energy efficiency. The International Energy Agency (IEA) group under the Implementing Agreement for Advanced Materials for Transportation (AMT) identified thermoelectric materials as an important area in 2009. This paper is part I of the international round-robin testing of transport properties of bulk thermoelectrics. The main foci in part I are the measurement of two electronic transport properties: Seebeck coefficient and electrical resistivity.

Thermionic energy conversion is a process that allows direct conversion of heat into electrical energy without mechanically moving components. In a thermionic converter electrons from the emitter traverse a small gap, are collected by a counter-electrode, the collector, and a self generated voltage develops across the gap. We have prepared prepared an ultra-nanocrystalline diamond (UNCD) based thermionic electron emitter that exhibits a low effective work function of typically 1.4 eV. This was attributed in part to reduced band bending and to the negative electron affinity (NEA) surface. A thermionic energy converter comprised of 2 diamond electrodes were positioned to establish a 25 micron gap and the emitter which was operated at temperatures up to 700 Celsius with a self generated open circuit voltage of 0.35 V. The reduced power output of the device was in part attributed to space charge effects and diamond film resistivity. Utilizing surface ionization effects at the emitter by introducing atomic hydrogen into the converter gap resulted in significant power output increase. With atomic hydrogen in the gap, the converter was operated up to 750 Celsius indicative of efficient surface ionization for charge transfer as well as a stable NEA diamond surface.

For bulk thermoelectrics, improvement of the figure of merit ZT to above 2 from the current values of 1.0 to 1.5 would enhance their competitiveness with alternative technologies. In recent years, the most significant improvements in ZT have mainly been due to successful reduction of thermal conductivity. However, thermal conductivity is difficult to measure directly at high temperatures. Combined measurements of thermal diffusivity, specific heat, and mass density are a widely used alternative to direct measurement of thermal conductivity. In this work, thermal conductivity is shown to be the factor in the calculation of ZT with the greatest measurement uncertainty. The International Energy Agency (IEA) group, under the implementing agreement for Advanced Materials for Transportation (AMT), has conducted two international round-robins since 2009. This paper, part II of our report on the international round-robin testing of transport properties of bulk bismuth telluride, focuses on thermal diffusivity, specific heat, and thermal conductivity measurements.

Materials with the half-Heusler structure possess interesting electrical and magnetic properties, including potential for thermoelectric applications. MgAgSb is compositionally and structurally related to many half-Heusler materials but has not been extensively studied. This work presents the high-temperature x-ray diffraction analysis of MgAgSb between 27 and 420 degrees C, complemented with thermoelectric property measurements. MgAgSb is found to exist in three different crystal structures in this temperature region, taking the half-Heusler structure at high temperatures, a Cu2Sb-related structure at intermediate temperatures, and a previously unreported tetragonal structure at room temperature. All three structures are related by a distorted Mg-Sb rocksalt-type sublattice, differing primarily in the Ag location among the available tetrahedral sites. Transition temperatures between the three phases correlate well with discontinuities in the Seebeck coefficient and electrical conductivity; the best performance occurs with the novel room temperature phase. For application of MgAgSb as a thermoelectric material, it may be desirable to develop methods to stabilize the room temperature phase at higher temperatures.

Due to the rich reserves of the raw materials, along with their low cost and nontoxic nature, Mg2Si1−xSnx-based compounds have generated intense attention from the international thermoelectric community for their application in thermoelectric power generation within the intermediate temperature range. In this work, we have adopted a two-step solid state reaction followed by a spark plasma sintering process to prepare a series of Sb-doped Mg2.16(Si0.4Sn0.6)1−ySby (0 ≤ y ≤ 0.055) solid solutions. We discuss the influence of Sb doping and the microstructure on the thermoelectric properties. Our results confirm that Sb acts as an effective n-type dopant and we estimate the maximum amount of Sb the Mg2Si0.4Sn0.6 structure can accommodate to be ∼2.3% by XRD, DSC and EPMA analyses. The electron transport properties and low-temperature electronic heat capacity measurements reveal that both the light conduction band and the heavy conduction band contribute to the transport in n-type Mg2Si0.4Sn0.6 solid solutions. The highest density-of-states effective mass and power factor were observed for Mg2.16(Si0.4Sn0.6)0.985Sb0.015 with an electron concentration of n ≈ 1.67 × 1020 cm−3, which is likely to be due to the Fermi level positioned within ∼2kBT of both the heavy and light conduction bands providing contributions from both bands. In addition, doping with Sb does not seem to affect the lattice thermal conductivity above room temperature. TEM analysis indicates the presence of Sn-rich precipitates with the size of several tens of nanometers dispersed in the Mg2Si0.4Sn0.6 matrix. Such a nanophase may enhance the boundary scattering of phonons and contribute to a low lattice thermal conductivity. Owing to the above characteristics of the band structure and the microstructure, the Mg2.16(Si0.4Sn0.6)0.985Sb0.015 solid solution with n = 1.67 × 1020 cm−3 possessed excellent thermoelectric properties and achieved a high ZT value of 1.3 at 740 K. Further reductions in the lattice thermal conductivity could be achieved via optimization of the nanophase inclusions, leading to a further enhancement of the figure of merit for Mg2Si0.4Sn0.6-based solid solutions.

Following the idea that there exists an optimal bandwidth for maximizing the thermoelectric figure of merit (ZT), we conduct detailed calculations in this paper to search for the optimal ZT in Bi2Te3/Sb2Te3 quantum dot (QD) nanocomposites (NCs) with Bi2Te3 QDs uniformly embedded in Sb2Te3 matrix where electron minibands are formed. The two-channel transport model, which considers both the miniband transport by the quantum-confined carriers and the background transport by the bulklike carriers, is used for electrical transport, while the lattice thermal conductivity is modeled using the modified effective medium approximation. Simultaneous decrease of the lattice thermal conductivity and the Lorenz number leads to an enhanced ZT in QD NCs when the Seebeck coefficient is not dramatically decreased. The optimal structural parameters that result in optimal electronic structure for maximizing ZT are found, with the consideration of realistic carrier scattering physics including phonon bottleneck effect. The optimal QD size is found to be similar to 6 nm, and the optimal interdot distance depends on the QD size and the doping concentration. For a given QD size, the maximum ZT is determined by the minimum of Lorenz number, which occurs when the quantum-confined carrier transport overwhelms the bulklike carrier transport.

There is an interest in higher-ZT thermoelectric materials for high-watt-density cooling of electronics. Reducing thermal conductivity through increased phonon scattering in nanomaterials has been shown to be effective and is being investigated by many groups. Solution-based synthesis is a method for making thermoelectric nanomaterials that can provide particle sizes <20 nm and can be scaled to production quantities of materials. We are exploring an approach that proceeds through formation of an “ink” that contains inorganic colloidal nanocrystals of thermoelectric materials. This approach has the advantage that, by adjustments within the basic synthesis process, the size, shape, and composition of the nanocrystals can be tightly controlled to study changes in the transport properties. Currently we are making materials from inks that contain Bi2Te3 nanocrystals with Sb2Te3 ligands, suspended in a solvent. Powders formed by curing the inks are made into solid pellets by hot pressing, and the pellets are used for characterization and transport property measurements. The best result from our thermoelectric property measurements is ZT = 0.9 with power factor of 27 μW/cm K2, which to our knowledge is the best value for solution-based synthesis.

We have successfully developed a Seebeck coefficient Standard Reference Material (SRM™), Bi2Te3, that is essential for interlaboratory data comparison and for instrument calibration. Certification measurements were performed using a differential steady-state technique on 10 samples (15 measurements) randomly selected from a batch of 390 bars. The certified Seebeck coefficient values are provided from 10 to 390 K, and they are further supported by transient measurements. The availability of this SRM will validate measurement results, leading to a better understanding of the structure/property relationships and underlying physics of potential high-efficiency thermoelectric materials.

Mg 2 (Si 0.3 Sn 0.7 ) 1− y Sb y (0 ≤ y ≤ 0.04) solid solutions were prepared by a two-step solid-state reaction method combined with the spark plasma sintering technique. Investigations indicate that the Sb doping amount has a significant impact on the thermoelectric properties of Mg 2 (Si 0.3 Sn 0.7 ) 1− y Sb y compounds. As the Sb fraction y increases, the electron concentration and electrical conductivity of Mg 2 (Si 0.3 Sn 0.7 ) 1− y Sb y first increase and then decrease, and both reach their highest value at y = 0.025. The sample with y = 0.025, possessing the highest electrical conductivity and one of the higher Seebeck coefficient values among all the samples, has the highest power factor, being 3.45 mW m −1 K −2 to 3.69 mW m −1 K −2 in the temperature range of 300 K to 660 K. Meanwhile, Sb doping can significantly reduce the lattice thermal conductivity ( κ ph ) of Mg 2 (Si 0.3 Sn 0.7 ) 1− y Sb y due to increased point defect scattering, and κ ph for Sb-doped samples is 10% to 20% lower than that of the nondoped sample for 300 K < T < 400 K. Mg 2 (Si 0.3 Sn 0.7 ) 0.975 Sb 0.025 possesses the highest power factor and one of the lower κ ph values among all the samples, and reaches the highest ZT value: 1.0 at 640 K.