Thermoelectric Energy Conversion Efficiency Research Articles

Influence of substituting Sn for Sb on the thermoelectric transport properties of CoSb3-based skutterudites

Band structure calculations that incorporate impurity effects suggest that a band resonant state may be formed in p-type CoSb3-based skutterudites by replacing Sb atoms with Sn dopant atoms. Such resonant states have the potential to greatly improve thermoelectric energy conversion efficiency by increasing the density of states variation near the Fermi level, thereby increasing the Seebeck coefficient at a given carrier concentration. Through transport measurements of the Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient, we show that a practical band resonant state is not achieved by Sn doping. Compared to undoped CoSb3, the dimensionless figure of merit (ZT) in Sn-doped CoSb3 is enhanced slightly at high temperatures to a value of 0.2, mostly due to a reduction in thermal conductivity. The Fermi level is calculated not to reach the band resonant state induced by Sn impurity atoms within the range of Sn concentrations examined here.

Nanoscale thermal transport. II. 2003–2012

A diverse spectrum of technology drivers such as improved thermal barriers, higher efficiency thermoelectric energy conversion, phase-change memory, heat-assisted magnetic recording, thermal management of nanoscale electronics, and nanoparticles for thermal medical therapies are motivating studies of the applied physics of thermal transport at the nanoscale. This review emphasizes developments in experiment, theory, and computation in the past ten years and summarizes the present status of the field. Interfaces become increasingly important on small length scales. Research during the past decade has extended studies of interfaces between simple metals and inorganic crystals to interfaces with molecular materials and liquids with systematic control of interface chemistry and physics. At separations on the order of ∼1 nm, the science of radiative transport through nanoscale gaps overlaps with thermal conduction by the coupling of electronic and vibrational excitations across weakly bonded or rough interfaces between materials. Major advances in the physics of phonons include first principles calculation of the phonon lifetimes of simple crystals and application of the predicted scattering rates in parameter-free calculations of the thermal conductivity. Progress in the control of thermal transport at the nanoscale is critical to continued advances in the density of information that can be stored in phase change memory devices and new generations of magnetic storage that will use highly localized heat sources to reduce the coercivity of magnetic media. Ultralow thermal conductivity—thermal conductivity below the conventionally predicted minimum thermal conductivity—has been observed in nanolaminates and disordered crystals with strong anisotropy. Advances in metrology by time-domain thermoreflectance have made measurements of the thermal conductivity of a thin layer with micron-scale spatial resolution relatively routine. Scanning thermal microscopy and thermal analysis using proximal probes has achieved spatial resolution of 10 nm, temperature precision of 50 mK, sensitivity to heat flows of 10 pW, and the capability for thermal analysis of sub-femtogram samples.

Numerical simulation of the transport properties of indium antimonide

A systematic investigation of the behavior of the transport coefficients of n-InSb over wide ranges of temperatures and concentrations of dopant atoms has been performed using the numerical solution of the Boltzmann transport equation. The thermoelectric characteristics of indium antimonide have been analyzed. The influence of different mechanisms of scattering of charge carriers on the transport coefficients and efficiency of thermoelectric energy conversion has been considered. The nature of the specific features of the temperature and concentration dependence of the transport and thermoelectric properties of n-InSb has been revealed.

The key role of charge carriers scattering on polar optical phonons in semiconductors for thermoelectric energy conversion

The Boltzmann equation for charge carriers in n-type InSb is solved by numerical procedure. Temperature and donor atoms concentration dependences of kinetic coefficients are studied with respect to the thermoelectric energy conversion efficiency. It is found that the mechanism of the charge carriers scattering on polar optical phonons is of crucial importance for thermoelectric figure of merit of semiconductors. High thermoelectric efficiency of compounds and alloys comprising such heavy atoms as Pb or Bi is explained by weakening of the above mentioned scattering mechanism due to gigantic values of dielectric constants of substances caused by high polarizability of heavy atoms.

Heat Transfer in Thermoelectric Materials and Devices

Solid-state thermoelectric devices are currently used in applications ranging from thermocouple sensors to power generators in space missions, to portable air-conditioners and refrigerators. With the ever-rising demand throughout the world for energy consumption and CO2 reduction, thermoelectric energy conversion has been receiving intensified attention as a potential candidate for waste-heat harvesting as well as for power generation from renewable sources. Efficient thermoelectric energy conversion critically depends on the performance of thermoelectric materials and devices. In this review, we discuss heat transfer in thermoelectric materials and devices, especially phonon engineering to reduce the lattice thermal conductivity of thermoelectric materials, which requires a fundamental understanding of nanoscale heat conduction physics.

Thermoelectric effects in ion conducting membranes and perspectives for thermoelectric energy conversion

Thermoelectric phenomena in ion conducting membranes have only been studied superficially in the literature and have, to the best of our knowledge, not been considered for direct thermoelectric conversion of heat into electricity. In the present article, it is shown that there exist two types of thermoelectric effects in membranes, isobaric and non-isobaric, that differ by experimental constraints of the thermoosmotic volume flux. In literature only isobaric thermoelectric effects have been investigated. Using the Onsager relations and the phenomenological transport equations, we derive a relation between the isobaric Seebeck coefficient (ηS) and the non-isobaric Seebeck coefficient (ηS⁎) in a similar manners as for the Saxén relations. Experimental literature data suggests that the absolute value of ηS is of the same order as in aqueous electrolyte solutions∼1.0mVK−1. Based on literature values of measured transport properties the absolute value of the non-isobaric Seebeck coefficient ηS⁎is estimated for a few membrane systems. It is found that ηS⁎has a larger variation than ηS and can attain absolute values above 10mVK−1. This makes efficient thermoelectric energy conversion in ion conducting membranes more realistic. It is expected that large values of ηS⁎may be found in charged/ion-conducting nano-porous membranes with pore sizes in the range 5–200nm.

Periodic heating amplifies the efficiency of thermoelectric energy conversion

We show that the use of a periodic heat source, instead of a constant heat source, can improve the conversion efficiency of a thermoelectric power generator (TPG). A periodic heat source drives a periodic temperature difference across the thermoelectric with an amplitude ΔT. While the time average of ΔT is identical to the temperature difference under a constant heat source with equivalent energy input, the time average of (ΔT)2 is larger, resulting in improved conversion efficiency. Here we present experimental measurements on a commercial thermoelectric device (bismuth telluride based) to validate analytical and numerical models. These models show that maximum efficiency is achieved when the period of the heat source is much larger than the thermal time constant of the TPG. Under this quasi-steady condition, the thermoelectric figure of merit ZT is still the relevant parameter for material optimization. A conventional thermoelectric material with ZT = 1, operated with sinusoidal and square-wave heat sources (ΔT = 100 K, TCold = 300 K), can achieve 140% and 180% of the constant heat source efficiency; or otherwise stated, can perform like advanced materials with ZT of 1.6 and 2.8. Even greater improvement, inaccessible through materials-based ZT enhancements, can be achieved with low duty cycle heat sources.

An overview of the engineered graphene nanostructures and nanocomposites

This critical review focuses on the property analysis of graphene and graphene nanocomposites (GNCs) and their demonstrated superior performances in energy storage and conversion, electrochemical- and bio-sensing, environmental remediation and flame retardant application and atomic thickness membrane separation. The performances of graphene and GNCs are strongly dependent on their chemical component, synthetic method, nanoscale morphology, and assembling structure of the hybrids. The current progress in supercapacitor energy storage density, solar cell power conversion efficiency, thermoelectric energy conversion efficiency, electrochemical sensing capability, biosensor sensitivity, heavy metal adsorption capacity and efficiency, photocatalytic degradation rate of organic dye, flame retardant polymer nanocomposites, graphene and porous graphene membranes is discussed with detailed examples through extensive analysis of the literature.

Development of a Miniaturized Energy Converter Without Moving Parts

New developments in portable electrical and mechanical devices have created demand for increasing amounts of energy and thus new ways of supplying energy. The high energy density of hydrocarbon fuels are a possible way to solve this issue. This paper deals with the development of an adapted thermodynamic concept for a micro energy converter based on the thermoelectric effect. Developing a PowerMEMS device that does not contain any moving parts is the main design feature. In the proposed concept liquid hydrocarbon fuel, such as methanol, is evaporated in a micro evaporator, mixed with air, and combusted in a micro combustion chamber. The combustion process is assisted by catalytically coated microfibers. Electrical power can be generated by a thermoelectric generator, which is located between the hot combustion zone and the cold micro evaporator. This arrangement leads to large temperature differences between hot and cold junctions, which is necessary for efficient thermoelectric energy conversion and hence power generation. For a more detailed investigation of thermal boundary conditions and interior thermal management, in-situ temperature measurements of the combustor walls are performed using thermographic phosphors.

Measurement of the high-temperature Seebeck coefficient of thin films by means of an epitaxially regrown thermometric reference material

The Seebeck coefficient of a typical thermoelectric material, silicon-doped InGaAs lattice-matched to InP, is measured over a temperature range from 300 K to 550 K. By depositing and patterning a thermometric reference bar of silicon-doped InP adjacent to a bar of the material under test, temperature differences are measured directly. This is in contrast to conventional two-thermocouple techniques that subtract two large temperatures to yield a small temperature difference, a procedure prone to errors. The proposed technique retains the simple instrumentation of two-thermocouple techniques while eliminating the critical dependence of the latter on good thermal contact. The repeatability of the proposed technique is demonstrated to be ±2.6% over three temperature sweeps, while the repeatability of two-thermocouple measurements is about ±5%. The improved repeatability is significant for reliable reporting of the ZT figure of merit, which is proportional to the square of the Seebeck coefficient. The accuracy of the proposed technique depends on the accuracy with which the high-temperature Seebeck coefficient of the reference material may be computed or measured. In this work, the Seebeck coefficient of the reference material, n+ InP, is computed by rigorous solution of the Boltzmann transport equation. The accuracy and repeatability of the proposed technique can be systematically improved by scaling, and the method is easily extensible to other material systems currently being investigated for high thermoelectric energy conversion efficiency.

A simple synthesis of long nanostructured arrays of crystalline strontium titanates at low-temperatures

Long aligned arrays of crystalline strontium titanate (SrTiO3) nanostructures were synthesized by using simple low-temperature processes that incorporate strontium into titanium oxides. Tubular nanostructures are often confine energy carriers that result in extraordinary transport behaviors in various semiconductors including strontium titanates, which are promising for developing efficient thermoelectric energy conversion materials. However, synthesizing a micron-to-milimeter scale array of one-dimensional ternary nanostructures has been difficult. Moreover, ternary compounds are often obtained as disordered cubic-shape particles at the end of complicated and/or long reactions. In this study, a two-step process—anodization for preparing amorphous titanium oxides and a subsequent thermal annealing process in a mixture of strontium hydroxide, ammonia, and water—was employed. Typical diameter and length of the tubes are ~150 nm and ~160 μm, respectively. It has been found that the amorphous structure of titanium oxides plays an important role in obtaining high-purity long strontium titanate nanotubes at low temperatures (90 and 180 °C) with short reaction times. Comparative and systematic studies with different sample pre-treatments, etching times, temperatures, reaction times, and strontium concentrations revealed reaction mechanisms and key synthesis parameters, which may be utilized to obtain other ternary or quaternary nanostructured compounds such as barium or lead titanates.

Femtosecond laser-induced nanostructure formation in Sb2Te3

We report femtosecond laser-induced nanotracks in highly absorbing Sb2Te3. Groups of nanotracks are observed with widths ∼50 nm and periodicity ∼130 nm, their area of coverage extending with the increase of laser fluence. We demonstrate that under a narrow range of laser fluences and laser irradiation times, long highly aligned nanotracks can be formed in Sb2Te3. The results suggest a promising avenue for laser nanostructuring of chalcogenide thermoelectrics, with implications for high efficiency thermoelectric energy conversion.

Analysis of thermoelectric energy conversion efficiency with linear and nonlinear temperature dependence in material properties

A novel approach to estimate energy conversion efficiency for a power-generating thermoelectric element, whose material properties possess both linear (first order) and nonlinear (second order) dependence on temperature, is developed by solving the differential equation governing its temperature distribution, which includes both the Joule heat and the Thomson effect. In order to obtain analytic expressions for power output and energy conversion efficiency, several steps of simplification are taken. Most notably, the material properties are evaluated with a linear temperature profile between the hot and cold ends. The model is further applied to a high-performance n-type half-Heusler alloy, matching the results of direct numerical analysis. The close correspondence between the proposed model and the numerical solution indeed proves that the approximations we have made are valid. The effect of linear and nonlinear components in the temperature dependence of material properties on the energy conversion efficiency is analyzed both qualitatively and quantitatively with the model. The results suggest that the accurate inclusion of the Thomson effect is essential to understand even the qualitative behavior of thermoelectric energy conversion.

Atomic-Scale Field-Effect Transistor as a Thermoelectric Power Generator and Self-Powered Device

Using first-principles approaches, we have investigated the thermoelectric properties and energy conversion efficiency of the paired metal–Br–Al junction. Owing to the narrow states in the vicinity of the chemical potential, the nanojunction has large Seebeck coefficients such that it can be considered an efficient thermoelectric power generator. We also consider the nanojunction in a three-terminal geometry, where the current, voltage, power, and efficiency can be efficiently modulated by the gate voltages. Such current–voltage characteristics could be useful in the design of nanoscale electronic devices such as a transistor or switch. Notably, the nanojunction as a transistor with a fixed finite temperature difference between electrodes can power itself using the Seebeck effect.

Efficiency of thermoelectric energy conversion in biphenyl-dithiol junctions: Effect of electron-phonon interactions

The electron-phonon interaction is the dominant mechanism of inelastic scattering in molecular junctions. Here we report on its effect on the thermoelectric properties of single-molecule devices. Using density functional theory and the nonequilibrium Green's function formalism we calculate the thermoelectric figure of merit for a biphenyl-dithiol molecule between two Al electrodes under an applied gate voltage. We find that the effect of electron-phonon coupling on the thermoelectric characteristics strongly varies with the molecular geometry. Two molecular configurations characterized by the torsion angles between the two phenyl rings of ${30}^{\ifmmode^\circ\else\textdegree\fi{}}$ and ${90}^{\ifmmode^\circ\else\textdegree\fi{}}$ exhibit significantly different responses to the inelastic scattering. We also use molecular dynamics calculations to investigate the torsional stability of the biphenyl-dithiol molecule and the phonon thermal transport in the junction.

Nanocomposite Materials for Thermoelectric Energy Conversion: A Brief Survey of Recent Patents

The past few decades have witnessed an increasing demand for renewable energy technology, including that of direct thermal to electrical energy conversion via thermoelectricity. Central to high efficient thermoelectric (TE) energy conversion is a high figure of merit, ZT, of the TE material, which requires high electrical conductivity, high thermopower and low thermal conductivity. For simple bulk materials it is hard to simultaneously satisfy these criteria because these physical quantities are inter-dependent: optimizing one quantity often adversely affects the others. Over the past decade, the pursuit of higher ZT materials has culminated into a new paradigm, namely, nanocomposite thermoelectric materials (NcTMs). A NcTM is typically multi-phased, and the characteristic length scale of at least one constituent is on the order of nanometers. The resulting classical and quantum size effects arising from the nanophase(s) and at the interfaces help decouple the inter-dependence of those TE properties, leading to outstanding TE performance. In this paper, we present a brief survey of relevant patent disclosures for NcTMs, focusing on the preparation methods and the peculiar micromorphologies. Keywords: Renewable energy, energy conversion, heat management, thermoelectrics, nanocomposite, micro-morphology, interface, self-assembly

Efficient thermoelectric energy conversion on quasi-localized electron states in diameter modulated nanowires

It is known that the thermoelectric efficiency of nanowires increases when their diameter decreases. Recently, we proposed that increase of the thermoelectric efficiency could be achieved by modulating the diameter of the nanowires. We showed that the electron thermoelectric properties depend strongly on the geometry of the diameter modulation. Moreover, it has been shown by another group that the phonon conductivity decreases in nanowires when they are modulated by dots. Here, the thermoelectric efficiency of diameter modulated nanowires is estimated, in the ballistic regime, by taking into account the electron and phonon transmission properties. It is demonstrated that quasi-localized states can be formed that are prosperous for efficient thermoelectric energy conversion.

Diameter-modulated nanowires as candidates for high thermoelectric energy conversion efficiency

High optimal thermoelectric efficiencies are theoretically demonstrated in ballistic nanowires with diameter modulation. The physics underlying the good thermoelectric performance of diameter-modulated nanowires is the strong energy dependence of their transmission coefficients. It is shown that the thermoelectric efficiency is directly related to the geometry of the diameter modulation. It becomes evident that geometry optimization can lead to efficient thermoelectric devices based on modulated nanowires.

Thermoelectric properties of silicon nanostructures

Semiconductor nanostructures are promising candidates for efficient thermoelectric energy conversion, with applications in solid-state refrigeration and power generation. The design of efficient semiconductor thermocouples requires a thorough understanding of both charge and heat transport; therefore, thermoelectricity in silicon-based nanostructures requires that both electronic and thermal transport be treated on an equal footing. In this paper, we present semiclassical simulation of carrier and phonon transport in ultrathin silicon nanomembranes and gated nanoribbons. We show that the thermoelectric response of Si-membrane-based nanostructures can be improved by employing the anisotropy of the lattice thermal conductivity, revealed in ultrathin Si due to boundary scattering, or by using a gate to provide additional carrier confinement and enhance the thermoelectric power factor.

Numerical Simulations of Non-equilibrium Energy Transport in Nanostructures using Boltzmann Transport Equation

Heat transfer in nanostructures differ significantly from that in the bulk materials since the characteristic length scales associated with heat carriers, i.e., the mean free path and the wavelength, are comparable to the characteristic length of the nanostructures. Nanostructure materials hold the promise of novel phenomena, properties, and functions in the areas of thermoelectric energy conversion and micro/nano electronic devices. One of the major challenges in micro/nano electronic devices is to study the ‘hot spot’ generation by accurately modeling the carrier-optical phonon-acoustic phonon interactions. Thermoelectric properties are among the properties that may drastically change at nanoscale. During the last decade, advances have been made in increasing the efficiency of thermoelectric energy conversion using nanostructures. In this paper, the non-equilibrium interaction between carriers and phonons in semiconductor thin films is modeled using the Boltzmann transport model (BTM) for studying the t...