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Thin-Film Thermoelectric Devices With High Room-Temperature Figures of Merit
Abstract
Thermoelectric materials are of interest for applications as heat pumps and power generators. The performance of thermoelectric devices is quantified by a figure of merit, ZT, where Z is a measure of a material's thermoelectric properties and T is the absolute temperature. A material with a figure of merit of around unity was first reported over four decades ago, but since then-despite investigation of various approaches-there has been only modest progress in finding materials with enhanced ZT values at room temperature. Here we report thin-film thermoelectric materials that demonstrate a significant enhancement in ZT at 300 K, compared to state-of-the-art bulk Bi2Te3 alloys. This amounts to a maximum observed factor of approximately 2.4 for our p-type Bi2Te3/Sb2Te3 superlattice devices. The enhancement is achieved by controlling the transport of phonons and electrons in the superlattices. Preliminary devices exhibit significant cooling (32 K at around room temperature) and the potential to pump a heat flux of up to 700 W cm-2; the localized cooling and heating occurs some 23,000 times faster than in bulk devices. We anticipate that the combination of performance, power density and speed achieved in these materials will lead to diverse technological applications: for example, in thermochemistry-on-a-chip, DNA microarrays, fibre-optic switches and microelectrothermal systems.
... As one of the renewable energy sources, thermoelectric (TE) power generation is attracting much attention. 2,3) In particular, film TE generator on Si substrate can be a small and stand-alone power source for internet of things sensor. 4) The TE conversion efficiency monotonically increases with increase of a dimensionless figure of merit; ZT = S 2 σT/κ, where S is Seebeck coefficient, σ is electrical conductivity, κ is thermal conductivity, and T is absolute temperature. ...
... 2(a)] displayed the diffraction peaks coming from CaGe 2 -based crystals in addition to the peaks coming from Si(111) substrates [111 Si Kβ (∼25.7°), 111 Si Kα (∼28.4°), and 222 Si (∼58.8°)]. It is considered that the broad peak at ∼44.2°does not originate from Ca(Ge 1−x Sn x )2 because it also appears in Si substrate measurement in the present XRD systems [Fig. 2(a)]. ...
Two-dimensional (2D) material is drawing considerable attention as a promising thermoelectric material. This study establishes the formation method of renewed Ca-intercalated group IV 2D materials, Ca(Ge1−x Sn x )2 crystals including germanene-based 2D layers. The solid phase epitaxy allows us to form epitaxial Ca(Ge1−x Sn x )2 on Si. Atomic force microscopy reveals that the Ca(Ge1−x Sn x )2 has island structures. X-ray diffraction proved the epitaxial growth of the Ca(Ge1−x Sn x )2 island structures and the increase of the c-axis lattice constant with Sn content increase. The formation of this renewed intermetallic compound including group IV 2D layer opens an avenue for high performance thermoelectric generator/Si.
... TEG is a solid-state semiconductor that excites free electrons, small in size, and requires no maintenance [5,6]. Although the current efficiency of the TEG is less than 5% [7], NASA Jet Propulsion Laboratory revealed obtaining 20% TEG conversion efficiency [8] while new TEG materials and superlattice configurations have increased the figure-of-merit (ZT) up to 2.4 [9]. These advances indicate that electrical generation from industrial waste heat is economically practical in the future. ...
... where N fin is the number of fins, A f is the fin area while µ f is the fin efficiency that can be obtained from [7,64]. (9) The parameter L fin is the fin length and m represent the relationship between the thermal conductivity and the heat transfer coefficient of [45]. ...
In the sustainable energy agenda, thermoelectric generators (TEG) can be a central technology for low-cost combined heat and power (CHP) systems. TEG module (TEM) is the combination of TEG cells, heat pipes, heat sinks and copper blocks that produce electrical power and thermal energy for low temperature heating simultaneously. Two TEG cells were used in each TEM for CHP in a bakery factory with a reference waste heat temperature of 250°C. Different designs of TEM affect the heat transfer mechanics through the components. However, actual testing of each design requires high cost and time consuming. Identifying the principal parameters affecting the desired output is indeed important before investing in actual design fabrication. One-dimensional model is developed in this manuscript to evaluate the fundamental interactions between each component. Parametric variation for nine main parameters characterized the steady-state response of each parameter under four novel heat sink configurations. The parameter sweeps approach benefits in designing a novel TEM for optimum system output. An improved TEM with 6 TEG cells was designed and it increased the heat recovery ratio from an initial 14% to 38%. The Reynolds number of streams are the major operating parameter as it influences the heat sink effectiveness. Large heat exchanger frontal area and copper block housing surface area are also significant parameters. Identification of these principle parameters would assist in effective designs of TEM systems for industrial CHP.
... Furthermore, the PbTe exhibit a high coefficient of thermal expansion and a significant thermal expansion is experienced at high temperature [6,7,10,11]. Likewise, the Bi 2 Te 3 needs to operate at low temperature (∼150°C) along with its toxicity and insufficiency limited to use in thermoelectric applications [8,12]. To raise the thermoelectric efficiency the band gap should be wide but Si 1-x Ge x displays a narrower band gap that limits its efficiency. ...
... It is crucial to remember that rather than in regular bulk materials, these high values are frequently obtained in particular compositions, structures, or nanostructured systems. For instance, the highest reported zT value for Bi 2 Te 3 [12] and PbTe (1.5) [63], which are considered to be an efficient thermoelectric materials. Furthermore, the zT value ranging from 0.8 to 1.5 for the half-heusler Alloys (e.g., TiNiSn, ZrNiSn) [64]. ...
Based on first-principles calculations, we investigated the electro-optic and thermoelectric properties of SiX (X = P, As). We find that the SiAs (-30613.62300 Ry) is more favorable than SiP(-1158.99360 Ry) due to higher optimized energies. The positive phonon spectra confirms the dynamical stability of these materials. The SiP and SiAs exhibited the semiconductor nature with the band gap of 2.33 eV, and 2.04 eV, respectively. Interestingly, the SiP possesses a direct band gap, which could be promising for optoelectronic devices. We find that SiX (X = P, As) showed the electron transition from valance band to conduction band owing towards the strong absorption of sharp band edges. The SiX (X = P, As) compound strongly absorbed light of energy 4.0 eV, which suggests it a potential candidate for solar cell applications. Furthermore, the compound exhibited the strong absorption of whole sun spectrum (ultra-violet to infra-red wave length), makes it capable for the applications in optical devices. Additionally, we have computed the thermoelectric properties using Boltztrap code. We have estimated the zT value 0.67 and 0.76 for SiP and SiAs, respectively. Both the SiAs and SiP exhibits a high zT, which could be applicable in the thermoelectric devices. Based on our calculated results, we anticipate that our studied materials could be an encouraging candidate for optical devices and thermoelectric devices.
... In this equation, S stands for Seebeck coefficient, σ for electrical conductivity, k c for charge carrier-contributed thermal conductivity, k p for lattice (phonon) thermal conductivity, and T for absolute temperature. In the previous two decades, there have been substantial attempts and advancements towards improving ZT, particularly for known inorganic materials, by raising the electronic power factor (S 2 ) or lowering the thermal conductivity of lattice [43][44][45][46][47][48][49][50][51][52][53]. Thermoelectric properties vary depending on the temperature and other parameters. ...
... wearable electronic devices. [188] In tandem with the continual innovation of wearable electronics, the demand for power generation devices that ensure prolonged endurance and expanded application scenarios is ever-evolving. Nanogenerators, which transform mechanical energy into electrical energy, come in three primary types: friction nanogenerators (TENGs), [189] piezoelectric nanogenerators, [190] and magnetoelectric nanogenerators. ...
Electronic thin films play a ubiquitous role in microelectronic devices and especially hold great promise for flexible electronics, energy conversion and storage, and biomedical applications. Their characterizations, including ultra‐thin, large‐scale dimensions, stretchability, and conformal ability to curved or 3D structures, present new challenges for thin film fabrication based on the solution method. Electrospray deposition emerges as a feasible method for fabricating large‐area, flexible, and curved films. It offers many advantages such as material adaptability, controlled atomization, tunable film morphology, and shape retention on complex substrates. These advantages make it a key method for fabricating high‐performance films on large‐area, 3D surfaces. This work presents a comprehensive review of the mechanisms, processes, applications, and equipment of electrospray deposition. First, the fundamental principles of electrospray deposition are introduced, focusing on the mechanisms and scaling laws of liquid atomization. Moreover, the control methods for electrospray modes, structures, and film morphology are discussed. These advanced control methods pave the way for the fabrication of smart skins, wearable devices, and energy conversion and storage components. Finally, this work introduces three types of electrospray deposition manufacturing equipment to illustrate the advantages of electrospray deposition for large‐area, and 3D surface manufacturing.
... Thermoelectric (TE) technology enables the direct and reversible conversion between thermal and electrical energies and has emerged as a notable approach regarding environmentally friendly power generation and carbon emission reduction (1)(2)(3)(4)(5)(6). The energy conversion efficiency of TE materials is determined by their dimensionless figure of merit ZT = S 2 σT/κ, where S stands for the Seebeck coefficient, σ is the electrical conductivity, T is the operating temperature, and κ is the total thermal conductivity that comprises both lattice (κ lat ) and electronic (κ e ) contributions (7)(8)(9)(10)(11). ...
P-type Bi 2− x Sb x Te 3 compounds are crucial for thermoelectric applications at room temperature, with Bi 0.5 Sb 1.5 Te 3 demonstrating superior performance, attributed to its maximum density-of-states effective mass ( m *). However, the underlying electronic origin remains obscure, impeding further performance optimization. Herein, we synthesized high-quality Bi 2− x Sb x Te 3 (00 l ) films and performed comprehensive angle-resolved photoemission spectroscopy (ARPES) measurements and band structure calculations to shed light on the electronic structures. ARPES results directly evidenced that the band convergence along the Γ ¯ - M ¯ direction contributes to the maximum m * of Bi 0.5 Sb 1.5 Te 3 . Moreover, strategic manipulation of intrinsic defects optimized the hole density of Bi 0.5 Sb 1.5 Te 3 , allowing the extra valence band along Γ ¯ - K ¯ to contribute to the electrical transport. The synergy of the above two aspects documented the electronic origins of the Bi 0.5 Sb 1.5 Te 3 ’s superior performance that resulted in an extraordinary power factor of ~5.5 milliwatts per meter per square kelvin. The study offers valuable guidance for further performance optimization of p-type Bi 2− x Sb x Te 3 .
... Thin-film and nanostructured thermoelectric materials have been extensively investigated for their potential use as compact and lightweight independent power supplies. [1][2][3][4][5][6][7][8][9][10][11] To be widely adopted, thin-film thermoelectric materials must achieve high ZT at near RT. ...
High-quality epitaxial Mg3Sb2 thin films are promising thermoelectric materials to enable practical applications of compact and environmentally friendly thermoelectric conversion at RT. In this study, high-quality single-crystal Mg3Sb2 with high c-plane orientation was epitaxially grown directly on annealed c-Al2O3 substrates without passive layers. These thin films exhibited about three times higher thermoelectric power factor than any previously reported values due to high carrier mobility. The ultra-smooth surface of the annealed c-Al2O3 substrate facilitated the formation of high-quality Mg3Sb2 thin films without passive layers or polycrystalline interfaces that could be carrier scatters.
... A nanostructured Bi 2 Te 3 /Sb 2 Te 3 superlattice with alternative layer thickness of 1 nm and 5 nm for Bi 2 Te 3 and Sb 2 Te 3 , respectively, is reported to have impressive zT values at 300 K of 2.4 and Weiliang Ma, Marie-Christine Record, and Pascal Boulet have contributed equally to this work. 1.5 for p-type and n-type, 14 respectively, whereas the pure Sb 2 Te 3 film exhibits a zT of 0.26 at 300 K. 15 Numerous materials belonging to the IV-VI and V 2 -VI 3 families exhibit high TE performance. These include lead chalcogenides, 16 Bi 2 Te 3 -based alloys, 17 and recently discovered compounds such as SnSe 18 and GeTe. ...
This paper reports first-principles calculations on nanosheets of Ge–Sb–Te compounds, namely \(\hbox {Sb}_{2}\hbox {Te}_{3}\), \({\hbox {GeSb}_{2}\hbox {Te}_{4}}\) and \({\hbox {Ge}_{2}\hbox {Sb}_{2}\hbox {Te}_{5}}\) under two crystalline atomic stakings S1 and S2. \(\hbox {Sb}_{2}\hbox {Te}_{3}\), \({\hbox {GeSb}_{2}\hbox {Te}_{4}}\) and \({\hbox {Ge}_{2}\hbox {Sb}_{2}\hbox {Te}_{5}}\)-S1 are semiconductors with a narrow band gap ranging between 0.7 and 0.74 eV as evaluated with the HSEsol functional. The transport properties have been investigated by Boltzmann transport theory together with deformation potential theory. The strain effects on their electronic and thermoelectric properties as well as on their dynamical properties have been investigated. A valence band convergence is found in the equilibrium structures, which is an efficient approach to improve the thermoelectric performance of materials. \(\hbox {Sb}_{2}\hbox {Te}_{3}\), \({\hbox {GeSb}_{2}\hbox {Te}_{4}}\) and \({\hbox {Ge}_{2}\hbox {Sb}_{2}\hbox {Te}_{5}}\)-S1 possess high TE performance in a wide range of temperature, and the highest values of zT are 2.94, 2.63 and 2.27, respectively.
... Thermoelectric (TE) thin films have been developed for microelectronics applications; see for example, Rowe et al. [1], Volklein et al. [2], Venkatasubramanian et al. [3], Chen et al. [4], Kogo et al. [5], Corbett et al. [6], Nandihalli [7], and Mele et al. [8]. These TE films can be used to convert the waste heat (for example, the heat generated by the computer processors) into electric power. ...
This paper develops a thermoelectric (TE)–piezoelectric (PE) hybrid structure with the PE layer acting as both a support membrane and a sensor for the TE film for microelectronics applications. The TE and PE layers are assumed to be perfectly bonded mechanically and thermally but electrically shielded and insulated with each other. The thermo-electro-mechanical responses of the hybrid bilayer under the TE generator operation conditions are obtained, and the influence of the PE layer on the TE energy conversion efficiency is investigated. The numerical results for a Bi2Te3/PZT-5H bilayer structure show that large compressive stresses develop in both the PE and TE layers. With a decrease in the PE layer thickness, the magnitude of the maximum compressive stress in the PE layer increases whereas the maximum magnitude of the stress in the TE layer decreases. The numerical result of the TE energy conversion efficiency shows that increasing the PE layer thickness leads to lower energy conversion efficiencies. A nearly 40% reduction in the peak efficiency is observed with a PE layer of the same thickness as that of the TE layer. These results suggest that design of TE films with supporting/sensing membranes must consider both aspects of energy conversion efficiency and the thermomechanical reliability of both the TE and PE layers.
... Other functional thin-film devices can potentially make significant advances in society. One such device is a thermoelectric generator, which converts thermal energy into electricity [16][17][18][19][20]. Conventional thin-film thermoelectric generators are typically fabricated using inorganic materials such as bismuth telluride-based alloys through dry processes such as sputtering, vacuum evaporation, and chemical vapor deposition [21][22][23][24][25][26]. In recent years, the development of lightweight and flexible thermoelectric generators as independent power sources for Internet-of-Things (IoT) sensors, such as wearable devices and wireless sensor nodes, has advanced rapidly [27][28][29][30][31][32][33][34]. ...
As power sources for Internet-of-Things sensors, thermoelectric generators must exhibit compactness, flexibility, and low manufacturing costs. Stretchable and flexible painted thermoelectric generators were fabricated on Japanese paper using inks with dispersed p- and n-type single-walled carbon nanotubes (SWCNTs). The p- and n-type SWCNT inks were dispersed using the anionic surfactant of sodium dodecylbenzene sulfonate and the cationic surfactant of dimethyldioctadecylammonium chloride, respectively. The bundle diameters of the p- and n-type SWCNT layers painted on Japanese paper differed significantly; however, the crystallinities of both types of layers were almost the same. The thermoelectric properties of both types of layers exhibited mostly the same values at 30 °C; however, the properties, particularly the electrical conductivity, of the n-type layer increased linearly, and of the p-type layer decreased as the temperature increased. The p- and n-type SWCNT inks were used to paint striped patterns on Japanese paper. By folding at the boundaries of the patterns, painted generators can shrink and expand, even on curved surfaces. The painted generator (length: 145 mm, height: 13 mm) exhibited an output voltage of 10.4 mV and a maximum power of 0.21 μW with a temperature difference of 64 K at 120 °C on the hot side.
... Reducing fossil fuel consumption and ameliorating the resultant environmental crisis can be facilitated by recovering low-grade heat [7][8][9][10]. Thermoelectric (TE) devices offer a viable avenue for recovering low-temperature waste heat by converting it into electrical energy through the Seebeck effect [11][12][13][14][15]. Due to their solid-state operation, along with high stability and durability, thermoelectric technology presents opportunities for harnessing usable electricity from waste heat [16][17][18]. ...
The current thread of study focuses on tuning the properties of nanostructured Bi2Te3 films for thermoelectric applications, by optimizing electrodeposition conditions such as the deposition potential, pH of the electrolyte, temperature and concentration of the precursors in the solvent. In this paper, small coagulated masses of Bi2Te3 dendritic nanostructures were electrodeposited on a stainless-steel substrate at previously optimized deposition potential and pH conditions while changing the concentration of Bi3+ ions in the electrolyte (varied from 2.5 mM to 15 mM). Cyclic voltammetry studies revealed that the material was deposited irreversibly at all concentrations. The effect of varying the concentration is indicated by a shift in the current peaks for oxidation and reduction. A series of scanning electron micrographs revealed the evolution of beautiful small coagulated masses of dendritic structures as a function of the concentration of electrolyte. Thermoelectric properties such as a Seebeck coefficient up to -45.81 μV/K and power factor up to 311 μW/cmK2 have been achieved for the deposited films. The results reveal significant variation in the thermoelectric properties as a function of the concentration of Bi3+ in the electrolyte through changes in the surface morphology and stoichiometry of the films. The analysis also revealed ideal concentrations of precursor ions to obtain Bi2Te3 as the dominant phase.
... In this equation, S stands for Seebeck coefficient, σ for electrical conductivity, k c for charge carrier-contributed thermal conductivity, k p for lattice (phonon) thermal conductivity, and T for absolute temperature. In the previous two decades, there have been substantial attempts and advancements towards improving ZT, particularly for known inorganic materials, by raising the electronic power factor (S 2 ) or lowering the thermal conductivity of lattice [43][44][45][46][47][48][49][50][51][52][53]. Thermoelectric properties vary depending on the temperature and other parameters. ...
... A notable portion, exceeding 50%, of heat dispersion in industrial settings emanates from low-grade sources. Reducing fossil fuel consumption and ameliorating the resultant environmental crisis can be facilitated by recovering low-grade heat [7][8][9][10]. Thermoelectric (TE) devices offer a viable avenue for recovering low-temperature waste heat by converting it into electrical energy through the Seebeck effect [11][12][13][14][15]. Due to their solid-state operation, along with high stability and durability thermoelectric technology presents opportunities for harnessing usable electricity from waste heat [16][17][18]. ...
The current thread of work focuses on tuning the properties of nano-structured Bi 2 Te 3 films for thermoelectric applications, by optimizing electrodeposition conditions such as deposition potential, pH of electrolyte, temperature and concentration of the precursors in the solvent. In this paper, small coagulated masses of Bi 2 Te 3 dendritic nanostructures were electrodeposited on a stainless-steel substrate under previously optimized deposition potential and pH conditions while changing the concentration of Bi ³⁺ ions in the electrolyte (varied from 2.5 mM to 15 mM). Cyclic Voltammetry studies revealed that the material was deposited irreversibly at all concentrations. The effect of varying the concentration is indicated by a shift in the current peaks for oxidation and reduction peaks. A series of scanning electron micrographs revealed the evolution of beautiful small coagulated masses of dendritic structures as a function of the concentration of electrolyte. Thermoelectric properties such as the Seebeck coefficient up to -45.81 µV/K and power factor up to 311 µW/cmK ² have been achieved for the deposited films. The results reveal significant variation in thermoelectric properties as a function of the concentration of Bi ³⁺ in the electrolyte through changes in the surface morphology and stoichiometry of the films. The analysis also revealed ideal concentrations of precursor ions to obtain Bi 2 Te 3 as the dominant phase.
... Thermoelectric generators have attracted considerable interest owing to the increasing demand for energy harvesting technologies 1,2 . In particular, flexible thermoelectric devices that can be deployed in diverse environments have significant potential for internet of things applications, such as remote sensing [3][4][5][6] . The figure of merit (ZT) of a thermoelectric device is a measure of its heat-to-electricity conversion performance. ...
Studying the properties of thermoelectric materials needs substantial effort owing to the interplay of the trade-off relationships among the influential parameters. In view of this issue, artificial intelligence has recently been used to investigate and optimize thermoelectric materials. Here, we used Bayesian optimization to improve the thermoelectric properties of multicomponent III–V materials; this domain warrants comprehensive investigation due to the need to simultaneously control multiple parameters. We designated the figure of merit ZT as the objective function to improve and search for a five-dimensional space comprising the composition of InGaAsSb thin films, dopant concentration, and film-deposition temperatures. After six Bayesian optimization cycles, ZT exhibited an approximately threefold improvement compared to its values obtained in the random initial experimental trials. Additional analysis employing Gaussian process regression elucidated that a high In composition and low substrate temperature were particularly effective at increasing ZT. The optimal substrate temperature (205 °C) demonstrated the potential for depositing InGaAsSb thermoelectric thin films onto plastic substrates. These findings not only promote the development of thermoelectric devices based on III–V semiconductors but also highlight the effectiveness of using Bayesian optimization for multicomponent materials.
... This approach involves decreasing the size of the material to increase ZT, leveraging the quantum confinement effect on charge carriers. Structuring has proven effective in increasing ZT for materials such as Bi 2 Te 3 -Sb 2 Te 3 superlattices [69] and PbTe-PbSe quantum dots [46]. Lower values of thermal conductivity in low-dimensional materials such as GaAs/AlAs/ErAs [70,71] and Si/Si-Ge [72,73] superlattices, achieved through photon scattering at boundaries and interfaces of nanostructures, significantly contribute to increasing ZT and the power factor. ...
Recent advancements in thermoelectric (TE) materials have opened up new possibilities for efficient waste heat recovery and renewable energy generation. The latest innovations in thermoelectric materials, including novel composites, nanostructured systems, and low-dimensional materials, have significantly enhanced the thermoelectric performance, thereby enabling higher efficiency in waste heat conversion. Integration strategies, such as incorporating thermoelectric modules into industrial exhaust streams, automotive exhausts, and solar thermal collectors, have demonstrated the feasibility and scalability of thermoelectric energy conversion technologies. Moreover, these advancements not only offer a sustainable energy solution but also contribute to reducing greenhouse gas emissions and enhancing overall energy efficiency, aligning with global efforts towards mitigating climate change. Thermoelectric (TE) materials provide an answer by turning excess heat into electricity through the Seebeck effect. It is crucial to have low thermal conductivity (κ) and high thermoelectric power factor (S2σ) when assessing efficiency using the ZT metric. This review explores the latest developments in thermoelectric (TE) materials spanning diverse categories, including the SnSe series, HH alloys, BiTe series, organic-inorganic composites, Cu2Se series, oxides, and GeTe/PbTe series. Through detailed exploration of preparation methods and TE properties for each material, the review offers insights into their potential for efficient waste heat recovery and renewable energy generation. Additionally, it discusses strategies aimed at enhancing the performance of these TE materials, proposing promising avenues for further improving their properties and applicability in sustainable energy technologies.
... When the distance between nanostructured interfaces is shorter than the mean free path of phonons but still larger than that of the electron, the engineered interfaces enhance phonon scattering, thereby impeding heat transport, without affecting electronmediated electrical transport. For the quantum-well superlattices Bi 2 Te 3 /Sb 2 Te 3 , a high ZT of 2.4 has been achieved [89]. More recently, simpler structures consisting of weakly bound 2D layers have gained a lot of attention. ...
... In particular, chalcogenide compounds are promising inorganic materials for this purpose. CdTe, [1][2][3][4][5][6] Cu(In,Ga)Se 2 (CIGS), [7][8][9][10][11] ZnSe, [12][13][14] and Bi 2 Te 3 [15,16] are representative chalcogenides used in energy-conversion devices such as photovoltaic solar cells, light-emitting devices, and thermoelectric devices. Among them, chalcopyrite, CuInSe 2, and DOI: 10.1002/aenm.202400087 ...
Aluminum‐based chalcopyrite materials have attracted attention because of the wide controllability range of their material properties and potential for use in energy‐conversion devices. Herein, CuAlSe2‐based thin‐film growth and solar cell device properties are discussed. Ternary CuAlSe2 thin films are relatively unstable and decompose weeks after film growth, even when preserved in a dry box. However, alloying with elemental In led to significant improvements in stability. Cu(In,Al)Se2 (CIAS) solar cells yield better photovoltaic performance than CuInSe2 cells, although the effective range of Al concentration that can improve device performance is narrower than that of elemental Ga in Cu(In,Ga)Se2 (CIGS). A decrease in the alkali metal concentration in CIAS films with increasing Al concentration is observed, indicating that the formation energy of alkali‐metal substitutional defects on the Cu site is high, and/or Al‐related complex defects formed are kinetically stable and difficult to replace with alkali metals once they form. Although the direct observation of alkali metals in the bulk (grain interior) of chalcopyrite CuInSe2 and CIGS films has been difficult to date, this result can serve as indirect evidence of the presence of alkali metals in the bulk of CuInSe2 films.
... Phonons are fundamentally quantized lattice vibrations responsible for a solid's thermal, elastic, acoustic, and infrared properties. Understanding their interactions with electrons and other phonons is of fundamental interest and crucial for many scientific and engineering applications, including ultrafast laser-induced phase transitions (1,2), superconductivity (3), additive manufacturing (4), design of thermo-electric (5) and plasmonic (6) devices, and radiation damage in materials under extreme environments such as inside fusion reactors (7). There have been substantial advances in understanding the phonon transport properties in semiconductors motivated by their role in developing new technologies and devices (8)(9)(10). ...
Phonon scattering in metals is one of the most fundamental processes in materials science. However, understand-
ing such processes has remained challenging and requires detailed information on interactions between phonons
and electrons. We use an ultrafast electron diffuse scattering technique to resolve the nonequilibrium phonon dynamics
in femtosecond–laser-excited tungsten in both time and momentum. We determine transient populations of phonon
modes which show strong momentum dependence initiated by electron-phonon coupling. For phonons near Brillouin
zone border, we observe a transient rise in their population on a timescale of approximately 1 picosecond driven by
the strong electron-phonon coupling, followed by a slow decay on a timescale of approximately 8 picosecond governed
by the weaker phonon-phonon relaxation process. We find that the exceptional harmonicity of tungsten is needed
for isolating the two processes, resulting in long-lived nonequilibrium phonons in a pure metal. Our finding high-
lights that electron-phonon scattering can be the determinant factor in the phonon thermal transport of metals.
Using the density function theory in combination with the non-equilibrium Green’s function method, the thermoelectric properties of molecular devices based on transition metal–terpyridine complexes are investigated. The results show that their thermoelectric properties can be significantly improved by changing the transition metal and the twist angle of the complex molecule, which is caused by shifting the molecular energy levels, resulting in increased coupling strength between the electrodes and the central molecule. The ZT value of the Ru-containing molecular device can reach up to 0.9 at room temperature, which is three orders of magnitude greater than that of the graphene nanoribbons of the same width. In addition, its thermoelectric performance can be further promoted by suppressing phonon thermal conductance through enhanced isotope scattering. The ZT value of doped devices can reach up to 1.0 in the range of 300–700 K. This work may help in the design and fabrication of transition metal-containing twistable molecular devices and provide effective methods to regulate their thermoelectric properties.
We present an order-N quantum transport calculation methodology to evaluate thermoelectric transport coefficients, such as electric conductivity and Seebeck coefficient. Different from a conventional method using the electric conductivity spectrum, it obtains the coefficients directly from the correlation function between heat and electric current based on linear response theory. As an example, we apply the methodology to a two-dimensional square-lattice model with static disorder and confirm that the calculated results are consistent with those obtained by the conventional method. The proposed methodology provides an effective approach to evaluate the thermoelectric performance of micron-scale materials based on quantum mechanics from an atomistic viewpoint.
Low-dimensional materials, in which the electronic and transport properties are drastically modified in comparison to those of three-dimensional bulk materials, yield a key class of thermoelectric materials with high conversion efficiency. Among such materials, the organic compounds may serve peculiar properties owing to their unique molecular-based low-dimensional structures with highly anisotropic molecular orbitals. Here we present the thermoelectric transport properties of the quasi-one-dimensional dimer-Mott insulator β′-(BEDT-TTF)2ICl2, where BEDT-TTF stands for bis(ethylenedithio)-tetrathiafulvalene. We find that the thermopower exhibits typical activation-type temperature variation expected for insulators but its absolute value is anomalously large compared to the expected value from the activation-type temperature dependence of the electrical resistivity. Successively, the Jonker-plot analysis, in which the thermopower is usually scaled by the logarithm of the resistivity, shows an unusual relation among such transport quantities. We discuss a role of the low dimensionality for the enhanced thermopower along with recent observations of such a large thermopower in several low-dimensional materials.
This review highlights the implications of the local crystal structure for phonon dynamics and explores various strategies for enhancing thermoelectric performance in crystalline materials through local structure engineering.
The current study is based on the theoretical investigation of the thermoelectric properties of \(d^{0}\) half-Heusler (HH) alloys GeKCa and GeKSr using Density Functional Theory (DFT). The calculations have been performed using the BoltzTrap code which is based on the semi-classical Boltzmann Transport theory incorporating the rigid band and constant relaxation time approximation. Different transport parameters such as Seebeck coefficient (S), electrical conductivity (\(\sigma /\tau\)), electronic thermal conductivity (\(\kappa _e/\tau\)), power factor (\(S^2\sigma /\tau\)) and figure of merit (zT) are evaluated and their variation with chemical potential (\(\mu\)) is studied in a temperature range of 300–800 K with a gap of 100 K. The variation of these transport parameters have also been studied with respect to temperature. Lattice thermal conductivity of the compounds has also been calculated and found to be quite low at high temperatures. The observed transport properties indicate towards the alloys being p-type in nature. The highest value of S are observed to be 1948 \(\mu\)V/K and 1512 \(\mu\)V/K for GeKCa and GeKSr, respectively, at 300 K for p-type doping. Further, a high value of 0.63 and 0.60 is observed for the figure of merit, zT for GeKCa and GeKSr at 800 K, respectively. The observed thermoelectric properties suggest that both the HH alloys can be considered as good thermoelectric material for medium-temperature thermoelectric power generation applications.
In the era of ubiquitous computing with flourished visual displays in our surroundings, the application of haptic feedback technology still remains in its infancy. Bridging the gap between haptic technology and the real world to enable ambient haptic feedback on various physical surfaces is a grand challenge in the field of human-computer interaction. This paper presents the concept of an active electronic skin, characterized by three features: richness (multi-modal haptic stimuli), interactivity (bi-directional sensing and actuation capabilities), and invisibility (transparent, ultra-thin, flexible, and stretchable). By deploying this skin on physical surfaces, dynamic and versatile multi-modal haptic display, as well as tactile sensing, can be achieved. The potential applications of this skin include two categories: skin for the physical world (such as intelligent home, intelligent car, and intelligent museum), and skin for the digital world (such as haptic screen, wearable device, and bare-hand device). Furthermore, existing skin-based haptic display technologies including texture, thermal, and vibrotactile feedback are surveyed, as well as multidimensional tactile sensing techniques. By analyzing the gaps between current technologies and the goal of ambient haptics, future research topics are proposed, encompassing fundamental theoretical research on the physiological and psychological perception mechanisms of human skin, spatial-temporal registration among multimodal haptic stimuli, integration between sensing and actuation, and spatial-temporal registration between visual and haptic display. This concept of active electronic skin is promising for advancing the field of ambient haptics, enabling seamless integration of touch into our digital and physical surroundings.
Sb2Te3-based thermoelectric (TE) ink was synthesized by mixing different Sb2Te3 microsizes with a ChaM-based solution. A thick-film TE was fabricated via a screen-printing technique on SiO2/Si-wafer substrates. The thickness of the film was controlled at 200 µm, the film was dried on hotplates at 433 K for 30 min, and the film was annealed at 523 K under vacuum for 30 min. The crystal structure, morphology, chemical composition, Seebeck coefficient, electrical resistivity, thermal conductivity, and ZT were evaluated for the annealed film samples. The small powder size of Sb2Te3 was found to be in good condition, and a maximum ZT value of 1.04 was obtained at 468 K, which is more than three times that of the large size at the same temperature.
The results of a study of biphase ceramics SrTiO3–TiO2, previously proposed as a promising n-type thermoelectric material, obtained using synchrotron radiation techniques at the shared research center “Siberian Synchrotron and Terahertz Radiation Center”, are presented. In particular, it has been demonstrated by in-situ heating X-ray diffraction that the reaction between the powder components SrCO3 (strontianite) and TiO2 (anatase) to obtain SrTiO3 (tausonite) is not the driving force in the preparation of ceramics by spark plasma sintering of the reaction mixture. For two spectral methods – X-ray luminescence and XANES spectroscopy, the spectrum of biphasic ceramics was compared with a model spectrum obtained from the spectra of single-phase ceramics as reference samples. The X-ray luminescence method revealed a shift to the high-energy region and a narrowing of the spectrum of biphase ceramics, which may indicate size quantization (the presence of a two-dimensional electron gas) in the system. Changes were found in the XANES spectrum of biphase ceramics in the region in which its shape can significantly depend on the symmetry of the nearest environment of Ti4+ atoms. However, it is difficult to interpret these data without numerical simulation.
In the present work, the thermoelectric properties of PbTe embedded with spherical Sb nanoscale inclusions were calculated in detail, following the idea that energy-selective carrier scattering can effectively increase the Seebeck coefficient. The quantitative relationships between such nanostructures in PbTe and thermoelectric properties indicated that interface potential barrier induced by Sb nanoinclusions results in a significant enhancement of the Seebeck coefficient, especially when around room temperature. Furthermore, the optimal parameters for boosting the thermoelectric performance of PbTe were found to be 4 nm-radius Sb nanoinclusions with high concentration.
Surface mounted thin film thermoelectric (TE) devices for localized cooling, power generation, and sensing are topics of immense current interest. Here, we establish the superior TE performance of thin film junctions made of topological insulators (TIs) Bi85Sb15 (BiSb) and Sb2Te3 by comparing their performance with those of Bi2Te3 and Sb2Te3. Thin films of these TIs were first evaluated for their carrier concentration, Hall mobility, resistivity, and thermopower. The Seebeck coefficient of BiSb, Sb2Te3, and Bi2Te3 measured against copper at ambient temperature is −100, +160, and −70 (±10) μV/K, while their power factors are 0.5, 0.5, and 0.45 (±0.05) 10−3 W m−1 K−2, respectively. Single TE junctions of BiSb–Sb2Te3 and Bi2Te3–Sb2Te3 yield a response of 272 and 240 (±10) μV/K, respectively. This comparative study shows that BiSb is a superior n-type counter electrode for Sb2Te3 compared to the n-type Bi2Te3. Moreover, Bi2Te3 is prone to tellurium antisite disorder, which affects its TE properties significantly.
MXenes exhibit significant potential in thermoelectric materials owing to their exceptional electrical conductivity; however, their limited number of semiconductors restricts their application. Thus, it is highly desirable to expand the MXene family beyond carbides and nitrides to broaden their applications in thermoelectricity. In this work, we systematically investigate the thermoelectric transport of Ti2OX2 (X = F, Cl) MOene through comprehensively evaluating the electron–phonon coupling (EPC) from first principles. Our findings first emphasize the limitations of the deformation potential theory method and stress the importance of considering EPC. Ti2OF2 (Ti2OCl2) monolayer exhibits exceptional electronic transport, with Seebeck coefficients reaching 1483.87 (1206.22) μV/K and electrical conductivity reaching 9.5 × 105 (7.6 × 105) Ω−1 m−1 at room temperature for its N-type counterpart. Additionally, the presence of degenerate multiple valleys and peaks significantly enhances their electronic transport. For phonon transport, EPC results in a significant reduction in lattice thermal conductivity (kL) [e.g., at 300 K with 1.44 × 1015 (1.68 × 1015) cm−2 of hole, the reduction is 86.3% (73.3%) for Ti2OF2 (Ti2OCl2)]. Additionally, their kL demonstrates a strong correlation with the density of states at corresponding Fermi levels. Moreover, the kL and total thermal conductivity of P-type Ti2OF2 show T-independence, making it suitable for applications in aviation and thermal insulation materials. Finally, N-type Ti2OF2 and Ti2OCl2 demonstrate superior zT values of 0.63 and 0.9 at 900 K, respectively. This study provides in-depth insights into the superior thermoelectric properties of Ti2OX2 (X = F, Cl) MOene with considering EPC, providing a novel platform for the next-generation thermoelectric field.
This study reveals exceptionally large Nernst coefficients in two-dimensional materials at room temperature by employing first principles calculations. Notably, ABA-stacked trilayer graphene exhibits a Nernst coefficient as high as 112 μV (KT) ⁻¹ .
This study improves the thermoelectric performance of single-walled carbon nanotube (SWCNT) films with different concentrations of SWCNT dispersions with an anionic surfactant. The SWCNT dispersions were prepared by adding SWCNT powders and sodium dodecylbenzene sulfonate (SDBS) as the anionic surfactant in deionized water, followed by ultrasonic dispersion using a homogenizer. The dispersion concentration, which is defined as the weight ratio of SWCNTs and SDBS to deionized water, was varied from 0.1 to 0.3 wt%. The freestanding SWCNT films (Buckypaper) were prepared by vacuum filtration. In-plane thermoelectric properties, including the Seebeck coefficient, electrical conductivity, and power factor, of the SWCNT films were measured at approximately 300 K. As a result, all thermoelectric properties showed the highest values at a concentration of 0.2 wt%. These results indicate that the dispersion condition is a key factor in improving thermoelectric performance, and the findings in this study are useful for the improvement of various functional materials using dispersion processes.
A new method of refrigeration is proposed. Efficient cooling is obtained by thermionic emission of electrons over Schottky barriers between metals and semiconductors. Since the barriers have to be thin, each barrier can have only a small temperature difference (∼1 K). Macroscopic cooling is obtained with a multilayer device. The same device is also an efficient generator of electrical power. A complete analytic theory is provided.
The thermal conductivity of Si–Ge superlattices with superlattice periods 30≪L≪300 Å, and a Si <sub> 0.85 </sub> Ge <sub> 0.15 </sub> thin film alloy is measured using the 3 ω method. The alloy film shows a conductivity comparable to bulk SiGe alloys while the superlattice samples have a thermal conductivity that is smaller than the alloy. For 30≪L≪70 Å, the thermal conductivity decreases with decreasing L ; these data provide a lower limit to the interface thermal conductance G of epitaxial Si–Ge interfaces: G ≫ 2 × 10 <sup> 9 </sup> W m <sup> -2 </sup> K <sup> -1 </sup> at 200 K. Superlattices with relatively longer periods, L≫130 Å, have smaller thermal conductivities than the short-period samples. This unexpected result is attributed to a strong disruption of the lattice vibrations by extended defects produced during lattice-mismatched growth. © 1997 American Institute of Physics.
Among the unusual transport properties predicted for disordered materials is the Anderson localization1 phenomenon. This is a disorder-induced phase transition in the electron-transport behaviour from the classical diffusion regime, in which the well-known Ohm's law holds, to a localized state in which the material behaves as an insulator. The effect finds its origin in the interference of electrons that have undergone multiple scattering by defects in the solid. A similar phenomenon is anticipated for multiple scattering of electromagnetic waves, but with one important simplification: unlike electrons, photons do not interact with one another. This makes transport of photons in disordered materials an ideal model system in which to study Anderson localization. Here we report direct experimental evidence for Anderson localization of light in optical experiments performed on very strongly scattering semiconductor powders.
Monolithically integrated active cooling is an attractive way for thermal management and temperature stabilization of microelectronic and optoelectronic devices. SiGeC can be lattice matched to Si and is a promising material for integrated coolers. SiGeC/Si superlattice structures were grown on Si substrates by molecular beam epitaxy. Thermal conductivity was measured by the 3omega method. SiGeC/Si superlattice microcoolers with dimensions as small as 40×40 µm^2 were fabricated and characterized. Cooling by as much as 2.8 and 6.9 K was measured at 25 °C and 100 °C, respectively, corresponding to maximum spot cooling power densities on the order of 1000 W/cm^2.
Measurements of the thermal conductivity above 30 K of mixed crystals with controlled disorder, (KBr)1-x(KCN)x, (NaCl)1-x, (NaCn)x Zr1-xYxO2-x/2, and Ba1-xLaxF2+x, support the idea of a lower limit to the thermal conductivity of disordered solids. In each case, as x is increased, the data approach the calculated minimum conductivity based on a model originally due to Einstein. These measurements support the claim that the lattice vibrations of these disordered crystals are essentially the same as those of an amorphous solid.
Thermoelectric (Peltier) heat pumps are capable of refrigerating solid or fluid objects, and unlike conventional vapor compressor systems, they can be miniaturized without loss of efficiency. More efficient thermoelectric materials need to be identified, especially for low-temperature applications in electronics and devices. The material CsBi(4)Te(6) has been synthesized and its properties have been studied. When doped appropriately, it exhibits a high thermoelectric figure of merit below room temperature (ZT(max) approximately 0.8 at 225 kelvin). At cryogenic temperatures, the thermoelectric properties of CsBi(4)Te(6) appear to match or exceed those of Bi(2-x)Sb(x)Te(3-y)Se(y) alloys.
Fifteen years ago, Esaki proposed to build artificial semiconductor superlattices to obtain new transport properties. In Esaki’s pioneering work [1–2], electrons in the superlattice were supposed to be completely delocalized. The main purpose was to obtain oscillatory features with Bragg reflections at electron minizone boundary, i.e. at d in the kz direction, d being the super-period. Although the pure Bloch oscillation in this system has been discussed by Zak [3], the existence of extended states in the growth direction is presently a relevant question, since many future devices may use electrical transport perpendicular to these superlattices. The present paper starts from the most ideal situation on theoretical aspects, and ends with experimental results and discussion. The first part deals with the near equilibrium mobility as limited by polar optical phonons, acoustical phonons, and epitaxial irregularities. Two different approaches of that mobility are made: either extended state diffusive transport in ideal superlattices, or hopping, i.e. phonon-assisted tunneling from layer to layer considering one or a finite number of periods. The next part considers the problem of limited thickness for real epitaxial layers. Since the superlattice is imbedded with other contact layers, a careful analysis has to be made based upon an effective medium theory. These considerations will help to extract “debugged” (as far as possible) information giving microscopic parameters of the superlattice. The main purpose is to measure the mobility in the growth direction; this parameter may be obtained from two different types of information s the current-voltage variation and the photocurrent response to short optical pulses. These experimental methods will be described in the last part and comparison of experiment and theories will be the final conclusion.
The zone folding in the acoustic-phonon dispersion curve of GaAs/AlGaAs superlattices is demonstrated using superconducting tunnel junctions as sources and detectors of quasimonochromatic phonons. We present data on selective transmission of high-frequency phonons due to narrow band reflection determined by the superlattice period.
There is widespread interest in the search for materials that would allow the fabrication of more efficient thermoelectric devices for cooling and power generation applications. Here, we report a large increase in the thermoelectric power of p-doped antimony bismuth telluride alloys upon pressure tuning under nonhydrostatic compression conditions. Together with measurements of the electrical conductivity and an upper bound estimated for the thermal conductivity under pressure, these results indicate that values of the dimensionless thermoelectric figure of merit, ZT, in excess of 2 have been achieved, substantially larger than the best observed values in bulk materials to date. We suggest an explanation for the observed behavior and strategies for attempting to reproduce it at ambient pressure.
The resistivity, Hall coefficients and low field magnetoresistance coefficients associated with current flow in the cleavage planes have been measured at 77°K on a number of n-type specimens of bismuth telluride. The resistivity and Hall coefficient measurements were extended up to room temperature. The experimental results are given and are shown to be reasonably consistent with a many-valley model of the band structure in which the energy minima are situated on the reflection planes. The parameters associated with this model are evaluated and used to relate the two Hall coefficients to the density of carriers. These relations are used to obtain the conductivity mobility of electrons for current flow in the cleavage planes. This mobility varies as T-1 68 over the temperature range 150°K to 300°K and has a value of 1250 cm2sec-1V-1 at room temperature.
After a brief review of the transport and thermoelectric properties of filled skutterudite antimonides, the authors present resonant ultrasound, specific heat, and inelastic neutron scattering results that establish the existence of two low-energy vibrational modes in the filled skutterudite LaFe{sub 3}CoSb{sub 12}. It is likely that at least one of these modes represents the localized, incoherent vibrations of the La ion in an oversized atomic {open_quotes}cage{close_quotes}. These results support the usefulness of weakly bound, {open_quotes}rattling{close_quotes} ions for the improvement of thermoelectric performance.
A study of thermal conductivities perpendicular to the interfaces in Bi2Te3/Sb2Te3 superlattices is presented. The lattice thermal conductivities in these short-period superlattices are less than those in homogeneous solid-solution alloys and exhibit a minimum for a period of ∼50 Å. For periods less than 50 Å, the adjoining layers of the superlattice apparently become coupled and, in effect, make their thermal conductivities approach that of an alloy. Using the mean free path from kinetic theory, a diffusive transport analysis suggests a low-frequency cutoff (ωcutoff) in the spectrum of heat-conducting phonons. A physical model based on the coherent backscattering of phonon waves at the superlattice interfaces is outlined for the reduction of lattice thermal conductivity; this suggests conditions of localizationlike behavior for the low-frequency phonons. The ωcutoff from the diffusive transport model is comparable to that estimated from applying the Anderson criterion to the potential localization of phonon waves. The general behavior of localizationlike effects is not unique to Bi2Te3/Sb2Te3 superlattices; it is also apparent in the thermal conductivities of Si/Ge superlattices. These superlattice structures offer a scope for studying the phonon localization phenomena while the lattice thermal conductivity reduction could lead to high-performance thermoelectric materials.
We describe a simple, yet phenomenologically very different, low-temperature modification to the conventional metal–organic chemical vapor deposition. It has been applied to the epitaxy of hexagonal-phased Bi2Te3/Sb2Te3 superlattices on zinc-blende GaAs substrates. The modification enables a two-dimensional, layer-by-layer, epitaxy instead of a three-dimensional islanded growth. Therefore, this approach is of generic importance to the epitaxy of many electronic and magnetic materials and their superlattices. High-resolution transmission electron microscopy studies indicate that the interface between the GaAs substrate and Bi2Te3 film is qualitatively defect free and that periodic structures are formed in the Bi2Te3/Sb2Te3 superlattices, with one of the individual layers as small as 10 Å. Such ultra-short-period superlattices offer significantly higher carrier mobilities than their respective solid-solution alloys, apparently due to the elimination of alloy scattering and the minimal effects of random interface scattering on carrier transport. This represents one of the successful observations of enhanced carrier mobilities in monolayer-range superlattices. © 1999 American Institute of Physics.
Thermionic emission in heterostructures is proposed for integrated cooling of high power electronic and optoelectronic devices. This evaporative cooling is achieved by selective emission of hot electrons over a barrier layer from the cathode to the anode. It is shown that with available high electron mobility and low thermal conductivity materials, and with optimized conduction band offsets in heterostructures, single-stage room temperature cooling of up to 20°–40° over thicknesses of the order of microns is possible. © 1997 American Institute of Physics.
PREVIOUS publications from this Laboratory have described the progress made with bismuth telluride in connexion with thermoelectric refrigeration or generation. With a thermo-junction between p-type bismuth telluride and bismuth the maximum temperature difference obtained1 was 26?? C., and between p- and n-type bismuth telluride2 40??C. This corresponded with a figure of merit 0.0335 degree???1/2. Here ?? is the thermoelectric power, ?? the electrical conductivity, and K the thermal conductivity.
Thin-film superlattice (SL) structures in thermoelectric materials are shown to be a promising approach to obtaining an enhanced figure-of-merit, ZT, compared to conventional, state-of-the-art bulk alloyed materials. In this paper we describe experimental results on Bi2Te3/Sb2Te3 and Si/Ge SL structures, relevant to thermoelectric cooling and power conversion, respectively. The short-period Bi2Te3/Sb2Te3 and Si/Ge SL structures appear to indicate reduced thermal conductivities compared to alloys of these materials. From the observed behavior of thermal conductivity values in the Bi2Te3/Sb2Te3 SL structures, a distinction is made where certain types of periodic structures may correspond to an ordered alloy rather than an SL, and therefore, do not offer a significant reduction in thermal conductivity values. Our study also indicates that SL structures, with little or weak quantum-confinement, also offer an improvement in thermoelectric power factor over conventional alloys. We present power factor and electrical transport data in the plane of the SL interfaces to provide preliminary support for our arguments on reduced alloy scattering and impurity scattering in Bi2Te3/Sb2Te3 and Si/Ge SL structures. These results, though tentative due to the possible role of the substrate and the developmental nature of the 3-ω method used to determine thermal conductivity values, suggest that the short-period SL structures potentially offer factorial improvements in the three-dimensional figure-of-merit (ZT3D) compared to current state-of-the-art bulk alloys. An approach to a thin-film thermoelectric device called a Bipolarity-Assembled, Series-Inter-Connected Thin- Film Thermoelectric Device (BASIC-TFTD) is introduced to take advantage of these thin-film SL structures.
There is widespread interest in the search for materials that would allow the fabrication of more efficient thermoelectric devices for cooling and power generation applications. Here, we report a large increase in the thermoelectric power of p-doped antimony bismuth telluride alloys upon pressure tuning under nonhydrostatic compression conditions. Together with measurements of the electrical conductivity and an upper bound estimated for the thermal conductivity under pressure, these results indicate that values of the dimensionless thermoelectric figure of merit, ZT, in excess of 2 have been achieved, substantially larger than the best observed values in bulk materials to date. We suggest an explanation for the observed behavior and strategies for attempting to reproduce it at ambient pressure.
Polycrystalline p‐type samples of IrSb 3 and Ir 0.5 Rh 0.5 Sb 3 have been made by hot isostatic pressing of powders. A number of properties such as thermal expansion coefficient, sound velocity, thermal conductivity, electrical conductivity, Seebeck coefficient, and carrier concentration have been measured. These compounds show promise as thermoelectric materials.
A special technique for the accurate measurement of thermal conductivity is discussed. The method involves use of the Peltier heat to maintain a temperature gradient along the specimen. Straightforward measurements allow calculation of the absolute value of the thermoelectric power, thermal conductivity, and electrical resistivity. An especially useful feature of the method is that the thermoelectric figure of merit is given in terms of the ratio of two voltages. The theory is presented for the case in which the radiative heat transfer is important. The method has been tested experimentally at 300°K only, but analysis suggests that accurate measurements of thermal conductivity can be made by this technique on low thermal conductivity materials of small dimensions up to 1000°K.
Two basic models for rectangular contacts to planar devices, the Kennedy-Murley Model (KMM) and the Transmission Line Model (TLM) are discussed and compared. The KMM does not take into account the interface resistance between metal and semiconductor, whereas the TLM disregards the vertical structure of the semiconductor layer. An extension of the TLM is derived (ETLM), which approximately considers this vertical structure. KMM and TLM thus appear as special cases of the ETLM. The calibration of the latter on the KMM then yields a simple quantitative criterion for the applicability of the KMM or the pure TLM. Measurement results on typical aluminum-silicon contacts are described satisfactorily by the (E)TLM. Concurrently with the applicability criterion, the KMM proves inadequate for these contacts due to the disregard of interface resistance. Conclusions are derived from the TLM pertaining to current distribution over the contact area and to contact resistance. In particular, the contacts are classified according to their operation mode. Finally, the TLM approach is applied also to circular contacts.
The thermoelectric and thermomagnetic properties of undoped Bi-Sb alloys have been investigated with highly homogeneous single crystals throughout the entire alloy composition as functions of temperature (77–300°K), magnetic field (0–7·5 kilo-oersteds), and crystallographic direction (∥ and ⊥ to the trigonal axis).The undoped, n-type Bi85Sb15 alloy gave the largest magneto-thermoelectric figure of merit (11 × 10−3 deg−1) at 100°K and the same value at 80°K in transverse fields of 3 and 1·3 kilo-oersteds, respectively. Within the range of field intensity used, the largest thermomagnetic figure of merit (4·1 × 10−3 deg−1) was obtained at 100°K with undoped n-types Bi99Sb1 in a field of 7·5 kilo-oersteds.The possibility has been explored of obtaining p-type Bi-Sb alloys through additions of acceptor impurities and by applying transverse magnetic fields. The results showed that a Sn-doped Bi88Sb12 alloy yields a p-type figure of merit of 2·3 × 10−3 deg−1 at 85°K in 7·5 kilo-oersteds.The difference between the adiabatic and isothermal conditions was reviewed in some detail in connection with measurements of various transport coefficients that enter into the calculation of the figures of merit. Also, a comparison was given on the difference between the magneto-Peltier and the Ettingshausen cooling.With a hybrid cooling unit based on the combination of the Peltier and magneto-Peltier effects in telluride alloys and BiSb alloys, respectively, a continuous cooling as large as 172°K, from room temperature down to 128°K, has been achieved.
Quantum and Classical Waves.- Wave Scattering and the Coherent Potential Approximation.- Coherent Waves and Effective Media.- Diffusive Waves.- The Coherent Backscattering Effect.- Renormalized Diffusion.- The Scaling Theory of Localization.- Localized States and the Approach to Localization.- Localization Phenomena in Electronic Systems.- Mesoscopic Phenomena.
A new mechanism for strong Anderson localization of photons in carefully prepared disordered dielectric superlattices with an everywhere real positive dielectric constant is described. In three dimensions, two photon mobility edges separate high- and low-frequency extended states from an intermediate-frequency pseudogap of localized states arising from remnant geometric Bragg resonances. Experimentally observable consequences are discussed.
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Contains reports on ten research projects.
A class of thermoelectric materials has been synthesized with a thermoelectric figure of merit ZT (where T is temperature and Z is a function of thermopower, electrical resistivity, and thermal conductivity) near 1 at 800 kelvin. Although these materials
have not been optimized, this value is comparable to the best ZT values obtained for any previously studied thermoelectric material. Calculations indicate that the optimized material should
have ZT values of 1.4. These ternary semiconductors have the general formula RM4X12 (where R is lanthanum, cerium, praseodymium, neodymium, or europium; M is iron, ruthenium, or osmium; and X is phosphorus,
arsenic, or antimony) and represent a new approach to creating improved thermoelectric materials. Several alloys in the composition
range CeFe4−xCoxSb12 or LaFe4−xCoxSb12 (0 < x < 4) have large values of ZT.
Currently the materials with the highest thermoelectric figure of merit Z are Bi2Te3 alloys. Therefore these compounds are the best thermoelectric refrigeration elements. However, since the 1960s only slow progress has been made in enhancing Z, either in Bi2Te3 alloys or in other thermoelectric materials. So far, the materials used in applications have all been in bulk form. In this paper, it is proposed that it may be possible to increase Z of certain materials by preparing them in quantum-well superlattice structures. Calculations have been done to investigate the potential for such an approach, and also to evaluate the effect of anisotropy on the figure of merit. The calculations show that layering has the potential to increase significantly the figure of merit of a highly anisotropic material such as Bi2Te3, provided that the superlattice multilayers are made in a particular orientation. This result opens the possibility of using quantum-well superlattice structures to enhance the performance of thermoelectric coolers.
The phonon thermal conductivity of a multilayer is calculated for transport perpendicular to the layers. There is a crossover between particle transport for thick layers to wave transport for thin layers. The calculations show that the conductivity has a minimum value for a layer thickness somewhat smaller then the mean free path of the phonons.
The pseudo-ternary alloy of
(Bi<sub>2</sub>Te<sub>3</sub>)(Sb<sub>2</sub>Te<sub>3</sub>)(Sb<sub>2
</sub>Se<sub>3</sub>) has been explored for over twenty-five years with
little progress in the figure of merit, p-type
3.4×10<sup>-3</sup>/K and n-type 3.2×10<sup>-3</sup>/K.
Using multiple dopants, Te and SbI<sub>3</sub>, higher figure of merit
material can be achieved without creating more of the deleterious pure
Te commonly found as a second phase in the p-type alloy,
(Bi<sub>2</sub>Te<sub>3</sub>)<sub>25</sub>(Sb<sub>2</sub>Te<sub>3
</sub>)<sub>72</sub>(Sb<sub>2</sub>Se<sub>3</sub>)<sub>3</sub>. Using a
combination of the two dopants figures of merit as high as 3.7×10
<sup>-3</sup>/K have been achieved. Using two dopants has also permitted
the creation of an n-type alloy with a composition of (Bi<sub>2</sub>Te
<sub>3</sub>)<sub>70</sub>(Sb<sub>2</sub>Te<sub>3</sub>)<sub>25</sub>(Sb
<sub>2</sub>Se<sub>3</sub>)<sub>5</sub>. Previously n-type doping could
not be achieved with a single dopant because the alloy as grown always
exhibits p-type conductivity. Using Te and SbI<sub>3</sub> together as
dopants, or SbI<sub>3</sub> by itself in this alloy produces n-type
material with a figure of merit of 3.4×10<sup>-3</sup>/K
New thermoelectric materials with superior transport properties at
high temperatures have been discovered. These materials are part of the
large family of skutterudites, a class of compounds which have shown a
good potential for thermoelectric applications. The composition of these
novel materials, called filled skutterudites, is derived from the
skutterudite crystal structure and can be represented by the formula LnT
<sub>4</sub>Pn<sub>12</sub> (Ln=rare earth, Th; T=Fe, Rn, Os, Co, Rh,
Ir; Pn=P, As, Sb). In these compounds, the empty octants of the
skutterudite structure which are formed in the TPn<sub>3</sub> (~T<sub>4
</sub>Pn<sub>12</sub>) framework are filled with a rare earth element.
Some of these compositions, based on CeFe<sub>4</sub>Sb<sub>12</sub>,
have been prepared by a combination of melting and powder metallurgy
techniques and have shown exceptional thermoelectric properties in the
350-700°C temperature range. At room temperature, CeFe<sub>4</sub>Sb
<sub>12</sub> behaves as a p-type semimetal, but with a low thermal
conductivity and surprisingly large Seebeck coefficient. These results
are consistent with some recent band structure calculations on these
compounds. Replacing Fe with Co in CeFe<sub>4</sub>Sb<sub>12</sub> and
increasing the Co:Fe atomic ratio resulted in an increase in the Seebeck
coefficient values. The possibility of obtaining n-type conductivity
filled skutterudites for Co:Fe values higher than 1:3 is currently being
investigated. Measurements on bulk samples with a CeFe<sub>3.5</sub>Co
<sub>0.5</sub>Sb<sub>12</sub> atomic composition and p-type conductivity
resulted in dimensionless figure of merit ZT values of 1.4 at 600°C
New results on the physics of tunneling in quantum well heterostructures and its device applications are discussed. Following a general review of the field in the Introduction, in the second section resonant tunneling through double barriers is investigated. Recent conflicting interpretations of this effect in terms of a Fabry-Perot mechanism or sequential tunneling are reconciled via an analysis of scattering. It is shown that the ratio of the intrinsic resonance width to the total scattering width (collision broadening) determines which of the two mechanisms controls resonant tunneling. The role of symmetry is quantitatively analyzed and two recently proposed resonant tunneling transistor structures are discussed. The third section deals with perpendicular transport in superlattices. A simple expression for the low field mobility in the miniband conduction regime is derived; localization effects, hopping conduction, and effective mass filtering are discussed. In the following section, experimental results on tunneling superlattice photoconductors based on effective mass filtering are presented. In the fifth section, negative differential resistance resulting from localization in a high electric field is discussed. In the last section, the observation of sequential resonant tunneling in superlattices is reported. We point out a remarkable analogy between this phenomenon and paramagnetic spin resonance. New tunable infrared semiconductor lasers and wavelength selective detectors based on this effect are discussed.
Thin-®lm Thermoelectric Cooling and Heating Devices for DNA Genomics/ Proteomics, Thermo-Optical Switching-Circuits, and IR Tags (US Patent Filing
- R Venkatasubramanian
Venkatasubramanian, R. Thin-®lm Thermoelectric Cooling and Heating Devices for DNA Genomics/ Proteomics, Thermo-Optical Switching-Circuits, and IR Tags (US Patent Filing, Ser. No. 60/282,185, 2001).
Thin-®lm superlattice and quantum-well structuresÐa new approach to high-performance thermoelectric materials
- R Venkatasubramanian
Venkatasubramanian, R. Thin-®lm superlattice and quantum-well structuresÐa new approach to high-performance thermoelectric materials. Naval Res. Rev. 58, 31±40 (1996).
Si superlattice microcoolers
- X Fan
Fan, X. et al. SiGeC/Si superlattice microcoolers. Appl. Phys. Lett. 78, 1580±1582 (2001).
Low temperature chemical vapor deposition and etching apparatus and method
- R Venkatasubramanian
Venkatasubramanian, R. Low temperature chemical vapor deposition and etching apparatus and method. US Patent No. 6071351 (6 June 2000).
Thin-film thermoelectric device and fabrication method of same
- R Venkatasubramanian