Improved Thermoelectric Power Factor in Metal-Based Superlattices
TL;DR: It is shown that metal-based superlattices with tall barriers can achieve a large effective thermoelectric figure of merit (ZT > 5 at room temperature), a key parameter to achieving high performance is the nonconservation of lateral momentum during the thermionic emission process.
Abstract: In this paper we present a detailed theory of electron and thermoelectric transport perpendicular to heterostructure superlattices. This nonlinear transport regime above barriers is also called heterostructure thermionic emission. We show that metal-based superlattices with tall barriers can achieve a large effective thermoelectric figure of merit (ZT > 5 at room temperature). A key parameter to achieving high performance is the nonconservation of lateral momentum during the thermionic emission process. Conservation of lateral momentum is a consequence of translational symmetry in the plane of the superlattice. We also discuss the use of nonplanar barriers and embedded quantum dot structures to achieve high thermoelectric conversion efficiency.
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TL;DR: In this article, the ability to achieve a simultaneous increase in the power factor and a decrease in the thermal conductivity of the same nanocomposite sample and for transport in the same direction is discussed.
Abstract: Many of the recent advances in enhancing the thermoelectric figure of merit are linked to nanoscale phenomena found both in bulk samples containing nanoscale constituents and in nanoscale samples themselves. Prior theoretical and experimental proof-of-principle studies on quantum-well superlattice and quantum-wire samples have now evolved into studies on bulk samples containing nanostructured constituents prepared by chemical or physical approaches. In this Review, nanostructural composites are shown to exhibit nanostructures and properties that show promise for thermoelectric applications, thus bringing together low-dimensional and bulk materials for thermoelectric applications. Particular emphasis is given in this Review to the ability to achieve 1) a simultaneous increase in the power factor and a decrease in the thermal conductivity in the same nanocomposite sample and for transport in the same direction and 2) lower values of the thermal conductivity in these nanocomposites as compared to alloy samples of the same chemical composition. The outlook for future research directions for nanocomposite thermoelectric materials is also discussed.
3,562 citations
TL;DR: The most promising bulk materials with emphasis on results from the last decade are described and the new opportunities for enhanced performance bulk nanostructured composite materials are examined and a look into the not so distant future is attempted.
Abstract: Herein we cover the key concepts in the field of thermoelectric materials research, present the current understanding, and show the latest developments. Current research is aimed at increasing the thermoelectric figure of merit (ZT) by maximizing the power factor and/or minimizing the thermal conductivity. Attempts at maximizing the power factor include the development of new materials, optimization of existing materials by doping, and the exploration of nanoscale materials. The minimization of the thermal conductivity can come through solid-solution alloying, use of materials with intrinsically low thermal conductivity, and nanostructuring. Herein we describe the most promising bulk materials with emphasis on results from the last decade. Single-phase bulk materials are discussed in terms of chemistry, crystal structure, physical properties, and optimization of thermoelectric performance. The new opportunities for enhanced performance bulk nanostructured composite materials are examined and a look into the not so distant future is attempted.
1,951 citations
TL;DR: In this paper, the authors introduce the principles and present status of bulk nanostructured materials, then describe some of the unanswered questions about carrier transport and how current research is addressing these questions.
Abstract: Thermoelectrics have long been recognized as a potentially transformative energy conversion technology due to their ability to convert heat directly into electricity. Despite this potential, thermoelectric devices are not in common use because of their low efficiency, and today they are only used in niche markets where reliability and simplicity are more important than performance. However, the ability to create nanostructured thermoelectric materials has led to remarkable progress in enhancing thermoelectric properties, making it plausible that thermoelectrics could start being used in new settings in the near future. Of the various types of nanostructured materials, bulk nanostructured materials have shown the most promise for commercial use because, unlike many other nanostructured materials, they can be fabricated in large quantities and in a form that is compatible with existing thermoelectric device configurations. The first generation of these materials is currently being developed for commercialization, but creating the second generation will require a fundamental understanding of carrier transport in these complex materials which is presently lacking. In this review we introduce the principles and present status of bulk nanostructured materials, then describe some of the unanswered questions about carrier transport and how current research is addressing these questions. Finally, we discuss several research directions which could lead to the next generation of bulk nanostructured materials.
1,742 citations
TL;DR: This article sets out to summarize and clarify the current understanding in this field; explain the underpinnings of breakthroughs reported in the past decade; and provide a critical review of various concepts and experimental results related to nanostructured thermoelectrics.
Abstract: The field of thermoelectrics has progressed enormously and is now growing steadily because of recently demonstrated advances and strong global demand for cost-effective, pollution-free forms of energy conversion. Rapid growth and exciting innovative breakthroughs in the field over the last 10-15 years have occurred in large part due to a new fundamental focus on nanostructured materials. As a result of the greatly increased research activity in this field, a substantial amount of new data--especially related to materials--have been generated. Although this has led to stronger insight and understanding of thermoelectric principles, it has also resulted in misconceptions and misunderstanding about some fundamental issues. This article sets out to summarize and clarify the current understanding in this field; explain the underpinnings of breakthroughs reported in the past decade; and provide a critical review of various concepts and experimental results related to nanostructured thermoelectrics. We believe recent achievements in the field augur great possibilities for thermoelectric power generation and cooling, and discuss future paths forward that build on these exciting nanostructuring concepts.
1,268 citations
TL;DR: In this article, the authors present recent progress in understanding and manipulation of energy dissipation and transport in nanoscale solid-state structures, including silicon transistors, carbon nanostructures, and semiconductor nanowires.
Abstract: Understanding energy dissipation and transport in nanoscale structures is of great importance for the design of energy-efficient circuits and energy-conversion systems. This is also a rich domain for fundamental discoveries at the intersection of electron, lattice (phonon), and optical (photon) interactions. This review presents recent progress in understanding and manipulation of energy dissipation and transport in nanoscale solid-state structures. First, the landscape of power usage from nanoscale transistors (∼10−8 W) to massive data centers (∼109 W) is surveyed. Then, focus is given to energy dissipation in nanoscale circuits, silicon transistors, carbon nanostructures, and semiconductor nanowires. Concepts of steady-state and transient thermal transport are also reviewed in the context of nanoscale devices with sub-nanosecond switching times. Finally, recent directions regarding energy transport are reviewed, including electrical and thermal conductivity of nanostructures, thermal rectification, and the role of ubiquitous material interfaces.
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...In addition, such asymmetric heating can also be observed at metal– semiconductor contacts [41, 42], at contact energy barriers between dissimilar semiconductors [43–45], and even at more subtle transitions between regions with different densities of states (DOS)....
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References
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TL;DR: Th thin-film thermoelectric materials are reported that demonstrate a significant enhancement in ZT at 300 K, compared to state-of-the-art bulk Bi2Te3 alloys, and the combination of performance, power density and speed achieved in these materials will lead to diverse technological applications.
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.
4,921 citations
TL;DR: It is demonstrated that improved cooling values relative to the conventional bulk (Bi,Sb)2(Se,Te)3thermoelectric materials using a n-type film in a one-leg thermoelectrics device test setup, which cooled the cold junction 43.7 K below the room temperature hot junction temperature of 299.8 K.
Abstract: PbSeTe-based quantum dot superlattice structures grown by molecular beam epitaxy have been investigated for applications in thermoelectrics. We demonstrate improved cooling values relative to the conventional bulk (Bi,Sb) 2 (Se,Te) 3 thermoelectric materials using a n-type film in a one-leg thermoelectric device test setup, which cooled the cold junction 43.7 K below the room temperature hot junction temperature of 299.7 K. The typical device consists of a substrate-free, bulk-like (typically 0.1 millimeter in thickness, 10 millimeters in width, and 5 millimeters in length) slab of nanostructured PbSeTe/PbTe as the n-type leg and a metal wire as the p-type leg.
2,371 citations
TL;DR: In this paper, a single-stage room temperature cooling of high power electronic and optoelectronic devices is achieved by selective emission of hot electrons over a barrier layer from the cathode to the anode.
Abstract: 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.
343 citations
TL;DR: In this paper, a new method of refrigeration is proposed by thermionic emission of electrons over Schottky barriers between metals and semiconductors, which can have only a small temperature difference.
Abstract: 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 $(\ensuremath{\sim}1\mathrm{K})$. 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.
310 citations
TL;DR: In this article, a detailed theory of nonisothermal electron transport perpendicular to multilayer superlattice structures is presented, and the currentvoltage and cooling power density are calculated using Fermi-Dirac statistics, density-of-states for a finite quantum well and the quantum mechanical reflection coefficient.
Abstract: A detailed theory of nonisothermal electron transport perpendicular to multilayer superlattice structures is presented. The current–voltage (I–V) characteristics and the cooling power density are calculated using Fermi–Dirac statistics, density-of-states for a finite quantum well and the quantum mechanical reflection coefficient. The resulting equations are valid in a wide range of temperatures and electric fields. It is shown that conservation of lateral momentum plays an important role in the device characteristics. If the lateral momentum of the hot electrons is conserved in the thermionic emission process, only carriers with sufficiently large kinetic energy perpendicular to the barrier can pass over it and cool the emitter junction. However, if there is no conservation of lateral momentum, the number of electrons participating in a thermionic emission will increase. This has a significant effect on the I–V measurements as well as the cooling characteristics. Theoretical calculations are compared with...
154 citations