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Ever since the birth of thermoelectrics, it has been well known that semiconductors (materials with a relative small bandgap) give the best thermoelectric performance. From quantum mechanics, it is also well known that low dimension quantum confinement leads to changes in the band alignment of a material. Thus, a semimetallic material can be made semiconducting by using low dimensionality quantum confinement effects. BiSb alloys have been of particular interest for thermoelectric application in the temperature range of 70K to 100K. In bulk form, BiSb alloys can either be semimetal or semiconductor, depending on the alloy composition. Moreover, semimetallic BiSb alloys can be made semiconducting by using the low dimensionality quantum confinement concept. With these two previous concepts in mind, it is valuable to further explore the dependence of the band alignment for different alloy concentrations and different confinement conditions for BiSb alloys.
Following the study of the effect of the Sb concentration and of the wire diameter on the semimetallic or semiconducting phase of BiSb alloy nanowires, we now examine the corresponding effect of the Sb concentration and the film thickness on the properties of BiSb alloy films. A band structure phase diagram is calculated, giving the details on the dependence of the relative band edge position on the film thickness and the Sb concentration. This phase diagram gives a first hand guideline for choosing the film thickness and the Sb concentration to better improve the thermoelectric performance of BiSb alloy films.
The evolution of modulated reactants designed to form (Bi2Te3)5(TiTe2)5(SnTe)5(TiTe2)5 and (Bi2Te3)5(SnTe)5 are compared and contrasted. The modulated reactant designed to form (Bi2Te3)5(SnTe)5 interdiffused at 220°C forming SnBi2Te4 rather than the desired superlattice structure. The second sample was designed to have TiTe2 as a potential diffusion barrier to prevent the formation of SnBi2Te4. This second sample remained layered after annealing at 220°C. SnTe crystals are observed in the high angle diffraction pattern after this annealing, but there is evidence for the beginning of SnBi2Te4 formation. Annealing this sample at 300°C results in the formation of SnBi2Te4. The interdiffusion of Sn and Bi appears to occur before the formation of the desired TiTe2 structure.
A series of CexCo4Sb12 samples were synthesized, with x = 0 to 2. The crystallinity of the samples was studied using quantitative Rietveld analysis, and revealed that the samples did not crystallize completely. These data correlated with changes in the lattice parameter, DSC peak temperature, and DSC peak area. The crystallinity was confirmed using electron back-scatter diffraction (EBSD), which showed crystallites with a background of an amorphous matrix. This unusual morphology may improve thermoelectric properties by decreasing thermal conductivity.
The transient Harman technique is used to characterize the cross-plane ZT of InGaAs/InGaAlAs superlattice structures with embedded ErAs nanoparticles in the well layers. ErAs nanoparticles have proven to substantially reduce the thermal conductivity while slightly increasing the electrical conductivity of bulk InGaAs. The InGaAs/InGaAlAs superlattice structure was designed to have a barrier height of approximately 200meV. Although ErAs nanoparticles provide free carriers inside the semiconductor matrix, additional doping with Si increased the Fermi energy to just below the barrier height. The bipolar transient Harman technique was used to measure device ZT of samples with different superlattice thicknesses in order to extract the intrinsic cross-plane ZT of the superlattice by eliminating the effects of device Joule heating and parasitics. High-speed packaging is used to reduce signal ringing due to electrical impedance mismatch and achieve a short time resolution of roughly 100ns in transient Seebeck voltage measurement. The measured intrinsic cross-plane ZT of the superlattice structure is 0.13 at room temperature. This value agrees with calculations based on the Boltzmann transport equation and direct measurements of specific film properties. Theoretical calculations predict cross-plane ZT of the superlattice to be greater than 1 at temperatures greater than 700K.
Effect of Ga addition on the thermoelectric properties of Ba-Ge type-III clathrate has been investigated as a function of Ga content and temperature. The substitution of Ga atom for Ge atom leads to the decrease of carrier (electron) concentration. Electrical conduction is of n-type for all clathrate compounds investigated and the values of electrical resistivity and Seebeck coefficient increase with the increase in the Ga content and in temperature. Both electronic and lattice thermal conductivity decrease with the increase in the Ga content because of the decreased carrier concentration and the increased extent of the rattling motion of Ba atoms encapsulated in open-dodecahedron, respectively. A very high ZT value of 1.25 is obtained at 670 °C for Ba24Ga15Ge85.
A study on the thermoelectric properties of layered cobaltates is presented, based on the dynamic mean field theory for strongly correlated electron systems. Electron correlation results in a crossover from coherent quasi-particle excitation at low temperature to incoherent excitation at high temperatures in cobaltates. With an extremely narrow quasi-particle bandwidth (hωc ∼ 50 meV), the thermal destruction of Fermi-liquid occurs at the moderate crossover temperature TM (∼ 200 K), and suggests a new scaling for thermoelectric power S of cobaltates (S ∼ kT/hωc ∼ T/TM) at low temperatures. At high temperatures, the dominating incoherent excitation leads to a weak temperature dependent S, and electric resistivity ρ approaches the Mott-limit ha/e2 ∼ a few mΩ·cm for cobaltates, where a is a lattice constant.
We report the fabrication and characterization of thin film power generators composed 400 p- and n-type ErAs:InGaAs/InGaAlAs superlattice thermoelectric elements. The thermoelectric elements incorporate erbium arsenide metallic nanoparticles into the semiconductor superlattice structure to provide charge carriers and create scattering centers for phonons. 10 µm p- and n-type InGaAs/InGaAlAs superlattices with embedded ErAs nano-particles were grown on InP substrates using molecular beam epitaxy. Thermal conductivity values were measured using the 3ω method and cross-plane Seebeck coefficients were determined using Seebeck device test patterns. 400 element ErAs:InGaAs/InGaAlAs thin film power generators were fabricated from superlattice elements 10 µm thick and 200 µm × 200 µm in area. The output power was 4.7 milliwatts for an external electrical load resistor of 150 Ω at about 80 K temperature difference drop across the generator. We discuss the limitations to the generator's performance and provide suggestions for further improvement.
We present a comparative study of the microstructure of Ca3Co4O9 single crystals and c-axis oriented Ca3Co4O9 thin films grown on glass substrates. Though both crystals and films have similar values of Seekbeck coefficient and electric resistivity at room temperature, their microstructures are rather different. Extensive high resolution transmission electron microscopy (TEM) studies reveal that the films grown on glass substrates have abundant stacking faults, which is in contrast to the perfect crystalline structure found in the single crystal sample. The c-axis lattice constants derived from the x-ray diffraction (XRD) and TEM measurements for the single crystal sample and the thin film are virtually the same, suggesting that the thin film on the glass substrate was not strained.
Polycrystalline-sintered samples of thallium based substances, (Tl2Te)100−x(Sb2Te3)x (x= 0, 1, 5, 10), were prepared by melting Tl2Te and Sb2Te3 ingots followed by annealing in sealed quartz ampoules. The thermoelectric properties were measured from room temperature to around 600 K. The values of the Seebeck coefficient of all samples are positive, indicating a p-type conduction characteristic. The maximum value of the power factor is 6.53×10−4 Wm−1K−2 at 591 K obtained for x= 10 (Tl9SbTe6), which is about one order lower than those of state-of-the-art thermoelectric materials. All samples indicate an extremely low thermal conductivity, for example that of Tl2Te is approximately 0.35 Wm−1K−1 from room temperature to around 600 K. Although the electrical performance of the samples is not so good, the ZT value is relatively high due to the extremely low thermal conductivity. The maximum ZT value is 0.42 at 591 K obtained for Tl9SbTe6.
Research into thermoelectric materials has recently undergone a push into lower dimensional materials in the hopes that quantum confinement effects will enhance the performance of these structures. It has already been demonstrated that 2D superlattice materials show enhanced properties. More recently, materials known to have good thermoelectric properties, such as Bi2Te3 or PbTe, have been grown in low dimensional morphologies. We investigate synthesis techniques for growing low dimensional structures of Bi-Te materials with the aim of incorporating them into a composite material alongside bulk Bi2Te3.
Polycrystalline-sintered samples of Tl2GeTe3, Tl4SnTe3, and Tl4PbTe3 were prepared by a solid-state reaction. Their thermoelectric properties were evaluated at temperatures ranging from room temperature to ca. 700 K by using the measured electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ). Despite their poor electrical properties, the dimensionless figure of merit ZT of all the compounds was relatively high, i.e., 0.74 at 673 K for Tl4SnTe3, 0.71 at 673 K for Tl4PbTe3, 0.29 at 473 K for Tl2GeTe3, due to the very low lattice thermal conductivity of the compounds.
Lead-Antimony-Silver-Tellurium (L-A-S-T) materials, synthesized at Michigan State University, show promising thermoelectric properties at high temperatures for use in power generation applications. Recent scaled-up quantities of L-A-S-T show a ZT=1.4 at 700 K approaching the figure of merit for samples made in small quantities. These materials are of great interest for power generation applications with hot side temperatures in the range of 600-800 K. Developing these materials into working devices requires minimization of the thermal and electrical parasitic contact resistances, so various fabrication methods are under investigation. To examine each method, a new measurement system has been developed to characterize these devices under various load and temperature gradients. An introduction to the system will be presented, as well as results for devices made of the L-A-S-T materials.
We have studied the thermoelectric properties of thallium compounds as novel thermoelectric materials. Especially, we focus on the Ag-Tl-Te ternary system, in which we found that Ag9TlTe5 exhibits an excellent thermoelectric figure of merit (ZT= 1.23) because of its extremely low thermal conductivity (around 0.22 Wm−1K−1). In this paper, we studied the thermal conductivity of four kinds of ternary silver thallium tellurides: AgTl3Te2, AgTlTe, Ag8Tl2Te5 and Ag9TlTe5, for which we found room temperature values of 0.39, 0.26, 0.14 and 0.21 Wm−1K−1, respectively. In order to understand the extremely low thermal conductivity, we performed an ultrasonic pulse echo measurement and evaluated some thermophysical properties.
A brief overview of the research activities at the Thermionic Energy Conversion (TEC) Center is given. The goal is to achieve direct thermal to electric energy conversion with >20% efficiency and >1W/cm2 power density at a hot side temperature of 300–650C. Thermionic emission in both vacuum and solid-state devices is investigated. In the case of solid-state devices, hot electron filtering using heterostructure barriers is used to increase the thermoelectric power factor. In order to study electron transport above the barriers and lateral momentum conservation in thermionic emission process, the current-voltage characteristic of ballistic transistor structures is investigated. Embedded ErAs nanoparticles and metal/semiconductor multilayers are used to reduce the lattice thermal conductivity. Cross-plane thermoelectric properties and the effective ZT of the thin film are analyzed using the transient Harman technique. Integrated circuit fabrication techniques are used to transfer the n- and p-type thin films on AlN substrates and make power generation modules with hundreds of thin film elements. For vacuum devices, nitrogen-doped diamond and carbon nanotubes are studied for emitters. Sb-doped highly oriented diamond and low electron affinity AlGaN are investigated for collectors. Work functions below 1.6eV and vacuum thermionic power generation at temperatures below 700C have been demonstrated.
We determine the plasmon energies of the skutterudites CoP3, CoAs3 and CoSb3 by electron energy loss spectroscopy, and compare with calculated values from the Drude model and density functional theory (DFT). For these compounds, whose doped versions have potential applications as thermoelectric materials, there is a relatively large discrepancy between experiment and theory based on the Drude model as well as the DFT-calculations. We also study the transitions from occupied to unoccupied states near the Fermi-level that show up at energies lower than the plasmon energies. The features observed are in general agreement with the DFT-calculations.
The die-casting growth process combined with an advanced version of the Bridgman method was employed for manufacturing the multicrystalline bulk crystal of Si1−xGex. This process provides a form of phase transformation which is completely different from that predicted by the Si-Ge phase diagram. By combining this growth with subsequent heat treatment of the precipitated sample, the variation in the germanium content obtained was within ± 4 % for Si0.65Ge0.35 sample with a carrier concentration in the mid-1018 cm−3. The power factor obtained exhibited a quite flat characteristic over the temperature range of room temperature to 800 K. However, there was a drop in the Seebeck coefficient at about 800 K, which corresponded to a rise in the electrical conductivity. The value of the thermal conductivity was about 0.04 W/cmK at temperatures ranging from 600 to 900 K. The maximum value of the figure of merit obtained for the grown Si0.65Ge0.35 sample was 0.19 at 773 K.
The microstructure, defect structure and thermoelectric properties of binary and some ternary Re silicide have been investigated as a new class if thermoelectric material. Binary Re silicide is identified to contain many Si vacancies, which are arranged in an ordered manner in the underlying tetragonal C11b structure so that the silicide is formulated to be ReSi1.75 with a monoclinic unit cell and contains four differently oriented domains accompanied by the twinned microstructure. The density and arrangement of Si vacancies can be controlled by ternary alloying. When the number of valence electrons of a ternary element is smaller than that for Re, the density of Si vacancies decreases with ternary additions, whereas the density of Si vacancies increases with ternary additions when the number of valence electrons of a ternary element is larger than that for Re. For both cases, the variation of the density of Si vacancies upon ternary alloying is accompanied by the introduction of the so-called shear structure.Binary ReSi1.75 exhibits nice thermoelectric properties as exemplified by the high value of dimensionless figure of merit (ZT) of 0.70 at 800 °C when measured along , although the ZT value along  is just moderately high. The ZT value is further increased to 0.8 with a small amount (2% substitution for Re) of Mo addition, by which an incommensurate microstructure is formed as result of extensive shear operation on the nano-scale.
Thermoelectric technology capable of solid state electric power generation and cooling has been has been known for almost 180 years. Only in the past 50 years has this technology found its way out of the laboratory and into niche military, space, and commercial products. Most of the profitable commercial products have made their appearance in the past decade. Many of you are working hard to advance the state of the art in thermoelectric materials, and I am sure that you do not want to wait another 180 years, 50 years, or even another decade to commercialize your results. There are many potential ways to commercialize this technology, but the area that I think represents the biggest market is the automotive industry. There are over 17 million automobiles sold in the US each year and over 60 million worldwide. With the possible exception of the electric power industry, I know of no other market segment that is even close to the potential of the automotive industry for using a high volume of thermoelectric materials. Every vehicle produced has an electrical system supplied by a one to two kilowatt generator with increasing power demand as electrical features are added. A high percentage of vehicles have air conditioning systems with 3 to 5 kilowatts of cooling. Sufficiently advanced thermoelectric materials can be the heart of systems that supplement or replace the mechanical or electro-mechanical devices performing these functions today. This paper addresses the boundary conditions for the function, quantity, and value needed to commercialize thermoelectric technology. Timing to introduce subsystems with this technology is also addressed. Thermoelectric technology has to compete with the existing technologies and other emerging technologies to be successfully commercialized. While it seems out of reach today, there is even the potential that sufficiently advanced thermoelectric materials and device construction could one day replace the internal combustion engine and even rival fuel cells in energy conversion efficiency.
Radioisotope Thermoelectric Generators (RTGs) have proved to be reliable, long-lived sources of electrical power that have enabled the conduct of a number of important NASA deep space missions since 1961. Past RTGs have used two types of thermoelectric materials: PbTe/TAGS and SiGe. In an effort to further improve both the thermoelectric efficiency and specific power of the next generation of RTGs, JPL is investigating a number of potential high-efficiency, high-temperature thermoelectric materials that could operate at a hot-side temperature of up to 1275 K. Among the materials being studied are the refractory CeyRu4−xIrxSb12 filled skutterudite compounds. We have synthesized polycrystalline samples for x ≤ 1.5 by a powder metallurgy technique. Dense samples have been hot-pressed from the pre-reacted powders and characterized by a variety of techniques including electron probe microanalysis, differential thermal analysis and thermogravimetic analysis. Seebeck coefficient, electrical resistivity, Hall coefficient, and thermal conductivity measurements have been conducted on the samples from room temperature to 1275 K. Results show that the samples are phase stable up to 1275 K. The results of the transport property measurements are presented and discussed.
We report structural, electrical, and thermopower properties of epitaxial and topotaxial NaxCoO2 thin films on (0001) sapphire substrate. Topotaxial NaxCoO2 films were prepared by converting an epitaxial Co3O4 film to NaxCoO2 by annealing in Na vapor and epitaxial NaxCoO2 films were obtained by pulsed laser deposition. All the films are c-axis oriented. For topotaxial films, annealing in different Na vapor pressures resulted in films with different Na concentrations, which showed distinct transport properties. For directly deposited epitaxial films by pulsed laser deposition, deposition parameters are found to control the Na concentration and hence the film properties. The largest thermoelectric power of the samples made by different methods is found to be similar in the range of 70-100 μV/K at room temperature