High-temperature Thermoelectric Devices Research Articles

Effect of Percolation on Structural and Electrical Properties of MIC Processed SiGe/Al Multilayers

The effect of metal induced crystallization (MIC) is widely used in the production of electronic devices by forming large grained polycrystalline Si from amorphous Si in contact with Al. This effect can also be utilized in conjunction with silicon-germanium (SiGe) alloys and thus provides means of a possible low cost production of future high temperature thermoelectric devices. In this work, sputter deposited multilayer systems of Si80Ge20/Al thin films have been investigated. The effect of MIC is used to crystallize the initially amorphous SiGe while simultaneously doping it with Al. As metallic phases would be detrimental to the thermoelectric performance, special interest is directed to the Al layers and their dissociation during the annealing treatment. A percolation limit regarding the thickness and continuity of the Al layers was found, but no detrimental side effects with respect to the MIC process could be detected.

Synthesis of Lanthanum Telluride Thin Films By Electrodeposition from Ionic Liquids

Thermoelectric devices allow for conversion between electrical and thermal energy and have a great deal of potential as both cooling devices and alternative energy sources. Currently they are limited by their low efficiency, and so are only used in very specialized applications. Some potential materials for use in high temperature thermoelectric devices include rare earth telluride compounds. One such material is bulk lanthanum telluride, La3-xTe4, 0 ≤ x ≤ 1/3, which was found to have a ZT greater than unity at temperatures near 1000 C [1]. Lanthanum is already commonly used in many industrial applications, including as lanthanum hydride in the cathode of nickel metal hydride batteries [2]. Nanostructured materials have been shown to have a higher thermoelectric efficiency relative to bulk materials due to the enhancement of phonon scattering through quantum confinement [3]. This has generated a great deal of interest in low-dimensional and nanostructured thermoelectric devices. Since lanthanum telluride is usually made by ball milling, the creation of complex nanostructures or thin films with traditional methods is difficult. We have developed a technique to electrochemically deposit lanthanum telluride thin films from an ionic liquid at low temperatures (approximately 70 C). The films were deposited on noble metal substrates from 1-ethyl-3-methylimidazolium bromide ionic liquid based electrolytes containing to 0.05 M to 0.25 M lanthanum nitrate and 0.025M (HTeO2)+. The ratios of lanthanum to tellurium in the deposited films were determined by relative concentrations of the anions and cations in the electrolyte. The technique used for the deposition of these thin films could ultimately be used in the synthesis of nanostructured lanthanum telluride. This electrochemical method promises to be simpler, less expensive, and more efficient than current techniques. 1. A. May, J-P. Fleurial and G. J. Snyder, Phys. Rev. B, 78, 125205 (2008). 2. T. Sakai et al., J. Less-Common Met., 161, 193-202 (1990). 3. L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B, 47, 12727 (1993).

Enhancement of thermoelectric efficiency in vapor deposited Sb2Te3 and Sb1.8In0.2Te3 crystals

Pure and indium doped antimony telluride (Sb2Te3) crystals find applications in high performance room temperature thermoelectric devices. Owing to the meagre physical properties exhibited on the cleavage faces of melt grown samples, an attempt was made to explore the thermoelectric parameters of p‐type crystals grown by the physical vapor deposition (PVD) method. The crystal structure of the grown platelets (9 mm× 8 mm× 2 mm) was identified as rhombohedral by x‐ray powder diffraction method. The energy dispersive analysis confirmed the elemental composition of the crystals. The electron microscopic and scanning probe image studies revealed that the crystals were grown by layer growth mechanism with low surface roughness. At room temperature (300 K), the values of Seebeck coefficient S (⊥ c) and power factor were observed to be higher for Sb1.8In0.2Te3 crystals (155 μVK−1, 2.669 × 10−3 W/mK2) than those of pure ones. Upon doping, the thermal conductivity κ (⊥ c) was decreased by 37.14% and thus thermoelectric efficiency was improved. The increased figure of merit, Z = 1.23 × 10−3 K−1 for vapour grown Sb1.8In0.2Te3 platelets indicates that it could be used as a potential thermoelectric candidate.

High Temperature Thermoelectric Device Concept Using Large Area PN Junctions

A new high temperature thermoelectric device concept using large area nanostructured silicon p-type and n-type (PN) junctions is presented. In contrast to conventional thermoelectric generators, where the n-type and p-type semiconductors are connected electrically in series and thermally in parallel, we experimentally demonstrate a device concept in which a large area PN junction made from highly doped densified silicon nanoparticles is subject to a temperature gradient parallel to the PN interface. In the proposed device concept, the electrical contacts are made at the cold side eliminating the hot side substrate and difficulties that go along with high temperature electrical contacts. This concept allows temperature gradients greater than 300 K to be experimentally applied with hot side temperatures larger than 800 K. Electronic properties of the PN junctions and power output characterizations are presented. A fundamental working principle is discussed using a particle network model with temperature and electric fields as variables, and which considers electrical conductivity and thermal conductivity according to Fourier’s law, as well as Peltier and Seebeck effects.

Structural and Thermoelectric Properties of Zn-Doped Cuprous Aluminate Delafossite

The polycrystalline Zn-doped CuAlO2 ceramics were obtained by solid state reaction method. The mixture of high purity grade of CuO, Al2O3 and Zn (CH3COO)22H2O powders was ground and then pressed by using uniaxial pressure. The obtained pellet was sintered in air at 1,150 °C for 24 h. XRD patterns showed the crystal structure of the as-sintered CuAl1-xZnxO2 (0x0.05) belonging to rhombohedral, space groupalong with the CuO and CuAl2O4 phases. From SEM micrographs, the grain size slightly decreases when the amount of Zn in CuAl1-xZnxO2 samples increases. Hall mobility and hole concentration of the as-sintered samples were obtained from Hall effect measurements in van der Pauw configuration at room temperature. The electrical resistivity measurements as a function of temperature were performed. The variation of Seebeck coefficient and power factor as a function of temperature was also investigated. In the present study, the Zn-doped CuAlO2 ceramics probably have advantages for use in high temperature thermoelectric devices.

Seebeck Coefficient of Nanolayer Growth of Anatase TiO2-x/Al-foil by Atomic Layer Deposition

ABSTRACTNon-stoichiometric and impurity doped titanium dioxide materials are good candidates for use in high temperature thermoelectric devices. Nanolayers of non-stoichiometric (TiO2-x) thin films were deposited on Al-foil by atomic layer deposition growth method. X-ray diffraction experiments showed anatase phase for these nanolayers. This crystal structure was maintained even after an annealing treatment of 600 °C for 60 minutes under an O2 pressure of ∼ 10 psi. This investigation presents for the first time how Al-foil can be functionalized by manipulating the Seebeck coefficient of these TiO2-x nanolayers.

Synthesis, characterization, and thermoelectric properties of nanostructured bulk p-type MnSi1.73, MnSi1.75, and MnSi1.77

P-type higher manganese silicide (HMS) has attracted considerable interest due to its remarkable thermoelectric (TE) properties and potential applications at intermediate and high temperature TE devices. In this study, a series of nanostructured bulk p-type HMS materials with different compositions of MnSix (where x=1.73, 1.75 and 1.77) were synthesized via mechanical ball milling and hot-press sintering. The X-ray diffraction analysis of the synthesized materials showed that increasing the Si contents yields to a slight shift to higher diffraction angles. The increase in Si content further resulted in a decrease in electrical conductivity and increase in Seebeck coefficient. The power factor of all three compositions are approximately identical. However, the lowest thermal conductivity was achieved in MnSi1.75 and resulted in the highest figure-of-merit among all the compositions.

Determining the effect of Ru substitution on the thermal stability of CeFe 4− xRu xSb 12

The ternary, rare-earth filled (RE) Skutterudites (REM 4 Pn 12; M = Fe–Os; Pn = P–Sb) have been proposed for use in high-temperature thermoelectric devices to convert waste heat to useful power. CeFe 4Sb 12 has been one of the most popular materials proposed for this application; however, it oxidizes at relatively low temperatures. The thermal stability of Skutterudites can be enhanced by selective substitution of the constituent elements and Eu(Fe,Ru) 4Sb 12 variants have been found to oxidize at temperatures above that of CeFe 4Sb 12. Unfortunately, these materials have poor thermoelectric properties. In this study, the thermal stability of CeFe 4− x Ru x Sb 12 was examined depending on the value of x. (These compounds have similar thermoelectric properties to those of CeFe 4Sb 12.) It has been found by use of TGA and XANES that the temperature at which point CeFe 4− x Ru x Sb 12 oxidizes increases with greater Ru substitution. XANES was also used to confirm the general charge assignment of Ce 3+Fe 4− x 2+Ru x 2+Sb 12 1−.

Fluxless Bonding of Bismuth Telluride Chips to Alumina Using Ag–In System for High Temperature Thermoelectric Devices

Bismuth telluride (Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) and its alloys are the most commonly used materials for thermoelectric devices. In this paper, the fluxless bonding process was developed to bond Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> chips to alumina substrates for high temperature applications. The silver-indium (Ag-In) system was chosen for the process development. To work with this system, the Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> chips were coated with 100 nm titanium (Ti) and 100 nm gold (Au) as barrier layer and plated with 10 μm Ag layer. The Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> samples were annealed at 250 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C for 200 h. No interdiffusion between Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and Ag was detected. The Ti/Au barrier layer was not affected either. It showed exceptional step coverage on the rough Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> surface even after the annealing process. To prepare for bonding, alumina substrates with 40 nm TiW and 2.5 μm Au were plated with 60 μm Ag, followed by 5 μm In and thin Ag cap layer for oxidation prevention. The Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> chips were bonded to alumina substrates at 180 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. No flux was used. The resulting void-free joint consists of five regions: Ag, (Ag), Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> In, (Ag) and Ag. (Ag) is Ag-rich solid solution. The joint has a melting temperature higher than 660 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. Due to the thick ductile Ag layer on alumina, the Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> chip did not break after bonding despite its significant coefficient of thermal expansion mismatch with alumina.

High temperature thermoelectric properties and energy transfer devices of Ca 3Co 4− xAg xO 9 and Ca 1− ySm yMnO 3

The high temperature oxide thermoelectric materials of p-type Ca 3Co 4− x Ag x O 9 (denoted as p-Co349/Ag x ) and n-type Ca 1− y Sm y MnO 3 (denoted as n-Mn113/Sm y ) were prepared by the self-ignition method combined with a sintering technique. The influence of doping Ag and Sm on the thermoelectric properties of the corresponding materials was evaluated. The figures of merit, ZT, for the p-Co349/Ag 0.2 and n-Mn113/Sm 0.02 materials reached maxima of 0.20 and 0.15 at 973 K, respectively. The performances of thermoelectric devices constructed with the p- and n-type pairs were evaluated in terms of the maximum output power ( P max) and manufacturing factor. The P max and volume power density for the four-leg devices reached 36.8 mW and 81.9 mW cm −3 at Δ T of 523 K, respectively.

Advanced ceramic interconnect material for solid oxide fuel cells: Electrical and thermal properties of calcium- and nickel-doped yttrium chromites

The structural, thermal and electrical characteristics of calcium- and nickel-doped yttrium chromites were studied for potential use as the interconnect material in high temperature solid oxide fuel cells (SOFCs) and other high temperature electrochemical and thermoelectric devices. The Y 0.8Ca 0.2Cr 1− x Ni x O 3± δ compositions with x = 0–0.15 showed single phase orthorhombic perovskite structures between 25 and 1200 °C over a wide range of oxygen partial pressures. Nickel doping remarkably enhanced sintering behavior of otherwise refractory chromites, and densities 94% of theoretical density were obtained after sintering at 1400 °C in air with 15 at.% Ni. The thermal expansion coefficient (TEC) was increased with nickel content to closely match that of an 8 mol% yttria-stabilized zirconia (YSZ) electrolyte for 0.05 ≤ x ≤ 0.15. Nickel doping significantly improved the electrical conductivity in both oxidizing and reducing atmospheres. Undesirable oxygen ion “leakage” current was insignificant in dual atmosphere conditions. No interfacial interactions with YSZ were detected after firing at 1400 °C.

Calcium- and Cobalt-Doped Yttrium Chromites as an Interconnect Material for Solid Oxide Fuel Cells

The structural, thermal, and electrical characteristics of calcium- and cobalt-doped yttrium chromites were studied for potential use as interconnect material in high temperature solid oxide fuel cells as well as other high temperature electrochemical and thermoelectric devices. The compositions formed single-phase orthorhombic perovskite structures in a wide range of oxygen pressures. Sintering behavior was remarkably enhanced as a result of cobalt substitution for chromium, and densities 95 and 97% of theoretical density were obtained after sintering at in air, when was 0.2 and 0.3, respectively. The electrical conductivity in both oxidizing and reducing atmospheres was significantly improved with cobalt additions, and was 49 and 10 S/cm at and 55 and 14 S/cm at in air and forming gas, respectively, for . The conductivity increase upon cobalt substitution for chromium was attributed to the charge carrier density increase as confirmed by Seebeck measurements. The thermal expansion coefficient was increased with cobalt content and closely matched to that of an 8 mol % yttria-stabilized zirconia (YSZ) electrolyte for . For , and YSZ were found to be chemically compatible for firing temperatures to at least .

Enhancement of the thermoelectric figure of merit of Si 1− xGe x quantum wires due to spatial confinement of acoustic phonons

We have theoretically investigated the thermoelectric figure of merit of Si 1− x Ge x quantum wires rigorously taking into account spatial confinement of both electrons and phonons. The calculations were carried out for cylindrical quantum wires with radius 1.5 nm<a<15 nm and infinite potential barriers. A significant enhancement of the thermoelectric figure of merit is predicted despite the decrease of the carrier mobility in very narrow-quantum wires. The enhancement is mostly due to the drop in the lattice thermal conductivity caused by the spatial confinement of acoustic phonons and the corresponding increase in phonon relaxation rates. The predicted increase is important for the anticipated applications of Si 1− x Ge x nanostructured materials for high-temperature thermoelectric devices.

Critical J-Integral Value of Injection-Molded Polycarbonate Using a Low Thermal Inertia Mold

Abstract A low thermal inertia mold (LTIM) is evaluated based on its ability to reduce residual stress and molecular orientation in thin polycarbonate parts. The mold employs a special cavity design which incorporates the use of high-temperature (above 350 °C) thermoelectric devices. Residual stresses are compared in LTIM and conventionally molded parts via an estimation of critical J-integral values (Jc)-the essential energy needed for crack initiation per unit area. The estimated values of Jc for LTIM and conventional parts are 6.3 and 7.3 kJ/m2, respectively. This difference may be due to the elevated LTIM cavity temperature which alleviates residual stresses and in turn reduces the estimated value of Jc. Observations of birefringence distributions were used to compare molecular orientation. The amount of birefringence is greatly reduced in a large central region of the LTIM parts, indicating low levels of orientation. Conventional parts show distinct color fringes along the melt flow path, indicative ...

Thermoelectric properties of boron phosphide

This paper describes the thermoelectric properties of boron phosphide wafers prepared using chemical vapour deposition and the thermal constants of a sintered boron phosphide specimen prepared using the r.f. hot pressing technique. The electric conductivity of the n-type boron phosphide wafers increases with temperature, becomes constant and then begins to decrease with temperature. In p-type wafers the conductivity increases with temperature and with a further increase in temperature the intrinsic conduction region is reached. The absolute thermoelectric power of n-type materials has an almost constant value of 500 μV K −1, but it begins to decrease at a temperature of 650 K owing to the creation of acceptors. The thermoelectric power of p-type materials decreases with increasing temperature, indicating that acceptors and donors contribute to the conduction. Thermal constants were obtained by the laser flash method and were measured using sintered discs as standards. Thermal diffusivity, heat capacity and the corresponding calculated thermal conductivity have an accuracy within ± 5% up to 700 °C. The calculated thermoelectric figure of merit is of the order of 10 −4 K −1. Therefore, boron phosphide is a promising material for high temperature thermoelectric devices.