Thermal Conductivity and Dimensionless Figure of Merit of Thermoelectric Rhodium Oxides Measured by a Modified Harman Method

We have developed a modified Harman method to extract the thermoelectric signal using a squared AC current in the presence of Joule heating, and have measured the thermal conductivity and dimensionless figure of merit of single crystals of the layered rhodium oxide Bi_0.78Sr_0.4RhO_3+d and the pseudo-one-dimensional rhodium oxide Ba_1.2Rh₈O₁₆. We find that these rhodium oxides exhibit a small thermal conductivity of 30 mW/cm K at 200 K and rather large ZT of 0.02 below 200 K. We believe that this method will be a powerful tool for thermal conductivity measurements in sub-millimeter-sized crystals.     

Optimal Electrical Conductivity for Maximum Thermoelectric Figure of Merit

A frequently employed approach for determination of the maximum thermoelectric figure of merit of a material involves a calculation of its maximum electrical power factor and the corresponding thermal conductivity. In this study, we show that the thermoelectric figure of merit determined using this approach is likely to be limited by the Lorenz factor. The maximum thermoelectric figure of merit is achieved at a different electrical conductivity. A simple way of estimating the optimal electrical conductivity for obtaining the maximum thermoelectric figure of merit is presented.     

无量纲优值对热电制冷系统性能影响分析

建立了无量纲稳态系统热力学模型。并用该模型分析了优值系数对系统性能的影响,优值系数是热电制冷器性能的内在制约,散热和温度条件则是热电制冷性能的外在制约,无量纲优值体现了二者对系统的影响。热电制冷系统的特殊优势再度受到人们关注,但在热电材料优值系数受到限制的现实条件下,热电制冷系统在能效上是难以与压缩式制冷空调系统比较的。     

Electrostatic Control of the Thermoelectric Figure of Merit in Ion-Gated Nanotransistors

Semiconductor nanostructures have raised much hope for the implementation of high-performance thermoelectric generators. Indeed, they are expected to make available reduced thermal conductivity without a heavy trade-off on electrical conductivity, a key requirement to optimize the thermoelectric figure of merit. Here, a novel nanodevice architecture is presented in which ionic liquids are employed as thermally-insulating gate dielectrics. These devices allow the field-effect control of electrical transport in suspended semiconducting nanowires in which thermal conductivity can be simultaneously measured using an all-electrical setup. The resulting experimental data on electrical and thermal transport properties taken on individual nanodevices can be combined to extract ZT, guide device optimization and dynamical tuning of the thermoelectric properties.     

Use of the Harman Technique for Figure of Merit Measurements of Cascade Thermoelectric Converters

The diagnostic capabilities of the high-speed Harman technique were investigated for measurements of the figure of merit Z in cascade thermoelectric converters with nonuniform legs (inhomogeneous or of an irregular shape). It was shown that, for multistage modules (two stages and more) and for any module with inhomogeneous legs (segmented or graded), the application of the standard Harman technique may lead to incorrect results. Some modifications of the Harman technique are proposed that make it suitable for testing of one- and multistage modules with homogeneous and nonuniform legs.     

Thermoelectric Figure of Merit and Quantum Mobility of Holes in Copper-Doped Antimony-Telluride Single Crystals

Semiconductors - The thermoelectric properties of Sb2 – xCuxTe3 single crystals (0 ≤ x ≤ 0.10) synthesized by the Bridgman method are studied in the temperature...     

Thermoelectric Figure of Merit Enhancement in Bi2Te3-Coated Bi Composites

This study examines the thermoelectric behavior of composites containing hydrothermally processed tellurium-coated bismuth particles of various sizes. Since only a very thin layer of Bi₂Te₃ forms on the particle surface, the high-pressure compacted composite is still dominated by bismuth as the main ingredient (??96% Bi). Thermoelectric figure of merit ZT values are derived from measurements of thermal conductivity, electrical resistivity, and Seebeck coefficient. As expected, a ZT value almost three times higher than that of bismuth is found. This enhancement appears to be caused mainly by lowered thermal conductivity due to the significant number of grain boundaries, short phonon mean free path in the coating layers, and lattice mismatch.     

Evaluation of Temperature-Dependent Effective Material Properties and Performance of a Thermoelectric Module

We devised a novel method to evaluate the temperature-dependent effective properties of a thermoelectric module (TEM): Seebeck coefficient (S _m), internal electrical resistance (R _m), and thermal conductance (K _m). After calculation, the effective properties of the module are converted to the average material properties of a p–n thermoelectric pillar pair inside the module: Seebeck coefficient (S _TE), electrical resistivity (ρ _TE), and thermal conductivity (k _TE). For a commercial thermoelectric module (Altec 1091) chosen to verify the novel method, the measured S _TE has a maximum value at bath temperature of 110°C; ρ _TE shows a positive linear trend dependent on the bath temperature, and k _TE increases slightly with increasing bath temperature. The results show the method to have satisfactory measurement performance in terms of practicability and reliability; the data for tests near 23°C agree with published values.     

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.     

Thermal Cycling Effects on the Thermoelectric Properties of n-Type In,Ce-Based Skutterudite Compounds

n-Type In-filled CoSb₃ is a known skutterudite compound that has shown promising thermoelectric (TE) properties resulting in high dimensionless figure of merit values at elevated temperatures. Use in various waste heat recovery applications will require survival and operation after exposure to harsh thermal cycling environments. This research focused on uncovering the thermal cycling effects on TE properties of n-type In_0.2Co₄Sb₁₂ and In_0.2Ce_0.15Co₄Sb₁₂ skutterudite compositions as well as quantifying their temperature-dependent structural properties (elastic modulus, shear modulus, and Poisson??s ratio). It was observed that the Seebeck coefficient and resistivity increased only slightly in the double-filled In,Ce skutterudite materials upon thermal cycling. In the In-filled skutterudites the Seebeck coefficient remained approximately the same on thermal cycling, while the electrical resistivity increased significantly after thermal cycling. Results also show that the thermal conductivity marginally decreases in the case of In-filled skutterudites, whereas the reduction is more pronounced in In,Ce-based skutterudite compounds. The possible reason for this kind of reduction can be attributed to grain pinning effects due to formation of nanoinclusions. High-temperature structural property measurements (i.e., Young??s modulus and shear modulus) are also reported. The results show that these structural properties decrease slowly as temperature increases and that the compounds are structurally stable after numerous (up to 200) thermal cycles.     

Thermoelectric Module Design Under a Given Thermal Input: Theory and Example

Established thermoelectric theory enables direct calculation of the power output and conversion efficiency if the temperature difference across a module is given. However, in some applications such as those using a radioisotope or solar radiation as a heat source, the thermal input remains constant while the temperature difference varies with the geometry of the thermoelectric module. In this paper, a theoretical framework for thermoelectric module design under a given thermal input is presented. It provides a convenient approach for module geometry optimization. The usefulness of the theory is demonstrated through a design study, in which an appropriate thermoelement length for a solar thermoelectric system is determined by considering conflicting requirements for a longer length to obtain a greater temperature difference and for a shorter length to produce a larger power output.     

Efficiency Study of a Commercial Thermoelectric Power Generator (TEG) Under Thermal Cycling

Thermoelectric generators (TEGs) make use of the Seebeck effect in semiconductors for the direct conversion of heat to electrical energy. The possible use of a device consisting of numerous TEG modules for waste heat recovery from an internal combustion (IC) engine could considerably help worldwide efforts towards energy saving. However, commercially available TEGs operate at temperatures much lower than the actual operating temperature range in the exhaust pipe of an automobile, which could cause structural failure of the thermoelectric elements. Furthermore, continuous thermal cycling could lead to reduced efficiency and lifetime of the TEG. In this work we investigate the long-term performance and stability of a commercially available TEG under temperature and power cycling. The module was subjected to sequential hot-side heating (at 200°C) and cooling for long times (3000 h) in order to measure changes in the TEG’s performance. A reduction in Seebeck coefficient and an increase in resistivity were observed. Alternating-current (AC) impedance measurements and scanning electron microscope (SEM) observations were performed on the module, and results are presented and discussed.     

Enhancement of Thermoelectric Figure of Merit for Bi0.5Sb1.5Te3 by Metal Nanoparticle Decoration

Introducing nanoinclusions in thermoelectric (TE) materials is expected to lower the lattice thermal conductivity by intensifying the phonon scattering effect, thus enhancing their TE figure of merit ZT. We report a novel method of fabricating Bi_0.5Sb_1.5Te₃ nanocomposite with nanoscale metal particles by using metal acetate precursor, which is low cost and facile to scale up for mass production. Ag and Cu particles of ??40?nm were successfully near-monodispersed at grain boundaries of Bi_0.5Sb_1.5Te₃ matrix. The well-dispersed metal nanoparticles reduce the lattice thermal conductivity extensively, while enhancing the power factor. Consequently, ZT was enhanced by more than 25% near room temperature and by more than 300% at 520?K compared with a Bi_0.5Sb_1.5Te₃ reference sample. The peak ZT of 1.35 was achieved at 400?K for 0.1?wt.% Cu-decorated Bi_0.5Sb_1.5Te₃.     

On the Significance of the Thermoelectric Figure of Merit <Emphasis Type="Italic">Z</Emphasis>

When the electrical conductivity, σ, thermal conductivity, λ, and thermopower, S, of a material are all assumed to be constant over the temperature range of interest, then the well-known thermoelectric (TE) figure of merit, Z = σS ²/λ, arises as part of the derivation of conversion efficiency in a TE generator. However, there are an infinite number of parameter sets (σ, λ, S) that yield any given Z. So, are they truly equivalent? This paper reviews the historical basis for Z as a metric for TE quality and discusses results of simulations on three systems having different parameter sets but the same Z. The three systems exhibit different power generation capabilities, illustrating that Z is not sufficient to specify the likely performance of a TE material in a system. Instead, a systems analysis is required that incorporates, at a minimum, source and sink temperatures and thermal resistances.     

Analysis of Ce‐ and Yb‐Doped TAGS‐85 Materials with Enhanced Thermoelectric Figure of Merit

Doping of TAGS‐85 with 1 at% Ce or Yb forms a dilute magnetic semiconductor system with non‐interacting localized magnetic moments that obey the Curie law. X‐ray diffraction patterns and slight broadening in ¹²⁵Te NMR, attributed to paramagnetic effects, suggest that Ce and Yb atoms are incorporated into the lattice. ¹²⁵Te NMR spin‐lattice relaxation and Hall effect show similar hole concentrations of ≈10²¹ cm^?3. At 700 K, the electric conductivity of the Ce‐ and Yb‐doped samples is similar to that of neat TAGS‐85, while the thermal conductivity and the Seebeck coefficient are larger by 6% and 16%, respectively. Possible mechanisms responsible for the observed increase in thermopower may include i) formation of resonance states near the Fermi level and ii) carrier scattering by lattice distortions and/or by paramagnetic ions. Due to the increase in the Seebeck coefficient up to 205 μV K^?1, the thermoelectric power factor of Ce‐ and Yb‐doped samples reaches 36 μW cm^?1 K^?2, which is larger than that measured for neat TAGS‐85, 27 μW cm^?1 K^?2. The increase in the Seebeck coefficient overcomes the increase in the thermal conductivity, resulting in a total increase of the figure of merit by ≈25% at 700 K compared to that observed for neat TAGS‐85.     

Development of a Measurement System for the Figure of Merit in the High-Temperature Region

New equipment has been developed for evaluating the figure of merit, ZT, on the basis of the Harman method in the temperature range between room temperature and 650 K. In this temperature range, the sample holder in the vacuum chamber has a different construction as compared with the sample holder constructed for the temperature range below room temperature. Several issues that need to be considered, such as compensation for the thermal radiation effect, suppression of heat leakage from the lead wires, and the setup method for the lead wires on the sample, are examined in the considered temperature region. Evaluations of ZT are successfully made for typical thermoelectric materials, (Bi,Sb)₂Te₃ and CeFe₃CoSb₁₂. We then demonstrate that the influence of thermal radiation between the high- and low-temperature edges of the sample induced by the Peltier effect on the estimated value of ZT is negligible at around 600 K. Furthermore, the change in the thermoelectric properties due to repetition of the thermal cycle is studied, and a typical hysteresis behavior is observed in the considered thermoelectric materials. It is revealed that heating the sample to a high temperature causes a change in its thermoelectric properties, which one must take into account for practical applications of thermoelectric materials.     

Mechanical Properties of Anisotropic Conductive Adhesive Film Under Hygrothermal Aging and Thermal Cycling

Mechanical properties of anisotropic conductive adhesive film (ACF) were investigated experimentally under various environmental conditions. The temperature sweep test was conducted to investigate the effects of temperature on dynamical mechanical properties of the ACF. The ACF exhibited transitions to the glass state, viscoelastic state, and rubber state with increasing temperature, and its glass-transition temperature (T _g) was determined to be 149°C. The creep-recovery behaviors of the ACF were investigated, and it was found that the initial strains, instantaneous strains, and creep or recovery rates increased with increasing temperature. No obvious creep phenomenon was observed at low temperatures (≤0°C). The creep strain and creep rates at any time decreased with increasing hygrothermal aging time. The uniaxial tensile behaviors of the ACF were also investigated under hygrothermal aging and thermal cycling. The results show that the Young’s modulus and tensile strength of the ACF decrease with increasing hygrothermal aging time; however, they increase at first and then decrease with increasing thermal cycling time. T _g decreases slightly for the ACF after hygrothermal aging; however, it increases after thermal cycling.     

Effects of Underfill Materials and Thermal Cycling on Mechanical Reliability of Chip Scale Package

The thermomechanical reliability of chip-scale packages (CSPs) with various underfills was evaluated by measure the electrical resistance under thermal shock and four-point bending fatigue tests. The underfill containing cycloaliphatic-type epoxy resin had lower resistance than without cycloaliphatic-type epoxy resin under thermomechanical fatigue test because the cycloaliphatic-type epoxy resin was able to mechanically relax more than the other types. The lifetimes of the CSPs under thermomechanical fatigues were strongly dependent on the properties of the underfill.