This work describes through semiclassical Boltzmann transport theory and simulation a novel nanostructured material design that can lead to unprecedentedly high thermoelectric power factors, with improvements of more than an order of magnitude compared to optimal bulk material power factors. The design is based on a specific grain/grain-boundary (potential well/barrier) engineering such that: i) carrier energy filtering is achieved using potential barriers, combined with ii) higher than usual doping operating conditions such that high carrier velocities and mean-free-paths are utilized, iii) minimal carrier energy relaxation after passing over the barriers to propagate the high Seebeck coefficient of the barriers into the potential wells, and importantly, iv) the formation of an intermediate dopant-free (depleted) region. The design consists thus of a three-region geometry, in which the high doping resides in the center/core of the potential well, with a dopant-depleted region separating the doped region from the potential barriers. It is shown that the filtering barriers are optimal when they mitigate the reduction in conductivity they introduce, and this can be done primarily when they are clean from dopants during the process of filtering. The potential wells, on the other hand, are optimal when they mitigate the reduced Seebeck they introduce by: i) not allowing carrier energy relaxation, and importantly ii) by mitigating the reduction in mobility that the high concentration of dopant impurities cause. Dopant segregation, with clean dopant-depletion regions around the potential barriers, serves this key purpose of improved mobility towards the phonon-limited mobility levels in the wells. Using quantum transport simulations as well as semi-classical Monte Carlo simulations we also verify the important ingredients and validate this clean-filtering design.
A composite material is considered which consists of conducting nanoparticles separated by tunneling dielectric barriers. The influence of the phonon thermal conductivity of dielectric matrix on the thermoelectric figure of merit of this composite material is theoretically in-vestigated. The range of values and barrier parameters that can lead to the thermoelectric figure of merit greater than unity is estimated. The influence of space charge and nonlinearity of current-voltage relations of tunneling barrier are also discussed.
Nanostructured Zinc oxide (ZnO) was synthesized via a ball milling for 10 hours using high energy planetary ball mill. Phase purity and homogeneity of all the samples have been investigated by X-ray diffraction (XRD) and Field Emission Scanning Electron Microscopy (FE-SEM). All the diffraction peaks can be indexed to the hexagonal phase ZnO with hexagonal symmetry (space group P63mc). Average crystallite size was observed to be 20 nm. There was a remarkable suppression in thermal conductivity (κ) compared to the bulk values by a factor of ~50 at room temperature. This suggests to the possibili…
We consider the effective properties of composite structure that includes nanocontacts between macrocrystallits of bulk material. The model of a such nanostructured composite is developed. In the mean-field approximation we calculate effective values of the thermopower, heat - and the electrical conductivity, thermoelectric figure of merit.
The structural stability, electronic structure, and thermal transport properties of one to six quintuple layers (QLs) of Bi2Se3 are investigated by van der Waals density functional theory and semi-classical Boltzmann theory. The bandgap amounts to 0.41 eV for a single QL and reduces to 0.23 eV when the number of QLs increases to six. A single QL has a significantly higher thermoelectric figure of merit (0.27) than the bulk material (0.10), which can be further enhanced to 0.30 by introducing 2.5% compressive strain. Positive phonon frequencies under strain indicate that the structural stabilit…
Inelastic neutron scattering measurements are utilized to explore relative changes in the generalized phonon density of states of nanocrystalline Si1xGex thermoelectric materials prepared via ball milling and hot-pressing techniques. Dynamic signatures of Ge clustering can be inferred from the data by referencing the resulting spectra to a density functional theoretical model assuming homogeneous alloying via the virtual-crystal approximation. Comparisons are also presented between as-milled Si nanopowder and bulk, polycrystalline Si where a preferential low-energy enhancement and lifetime bro…
Half-Heusler (HH) alloys have attracted considerable interest as promising thermoelectric (TE) materials in the temperature range around 700 K and above, which is close to the temperature range of most industrial waste heat sources. The past few years have seen nanostructuing play an important role in significantly enhancing the TE performance of several HH alloys. In this article, we briefly review the recent progress and advances in these HH nanocomposites. We begin by presenting the structure of HH alloys and the different strategies that have been utilized for improving the TE properties o…
A numerical study of thermoelectric nanocomposites is presented. Thermoelectric properties as a function of average grain size or nanoparticle density are examined by simulating the measurements as they would be done experimentally. In accordance with previous theory and experimental results, we find that the Seebeck coefficient, power factor and figure of merit, zT, can be increased by nanostructuring when energy barriers exist around the grain boundaries or embedded nanoparticles. When we simulate the performance of a thermoelectric cooler with the same material, however, we find that the ma…
Energy filtering has been suggested by many authors as a means to improve thermoelectric properties. The idea is to filter away low-energy charge carriers in order to increase Seebeck coefficient without compromising electronic conductivity. This concept was investigated in the present paper for a specific material (ZnSb) by a combination of first-principles atomic-scale calculations, Boltzmann transport theory, and experimental studies of the same system. The potential of filtering in this material was first quantified, and it was as an example found that the power factor could be enhanced by…
Irreversible effects in thermoelectric materials limit their efficiency and economy for applications in power generation and refrigeration. While electron transport is unavoidably irreversible in bulk materials, here we derive conditions under which reversible diffusive electron transport can be achieved in nanostructured thermoelectric materials via the same physical mechanism utilized in quantum optical heat engines. Our results may provide a physical explanation for the very high efficiencies recently reported for nanostructured thermoelectric materials such as quantum-dot superlattices.
Cross-plane superlattices composed of nanoscale layers of alternating potential wells and barriers have attracted great attention for their potential to provide thermoelectric power factor improvements and higher ZT figure of merit. Previous theoretical works have shown that the presence of optimized potential barriers could provide improvements to the Seebeck coefficient through carrier energy filtering, which improves the power factor by up to 40%. However, experimental corroboration of this prediction has been extremely scant. In this work, we employ quantum mechanical electronic transport …
The field of thermoelectric materials has undergone a revolutionary transformation over the last couple of decades as a result of the ability to nanostructure and synthesize myriads of materials and their alloys. The ZT figure of merit, which quantifies the performance of a thermoelectric material has more than doubled after decades of inactivity, reaching values larger than two, consistently across materials and temperatures. Central to this ZT improvement is the drastic reduction in the material thermal conductivity due to the scattering of phonons on the numerous interfaces, boundaries, dis…
We present the full thermoelectric characterization of nanostructured bulk PbTe and PbTe-PbSe samples fabricated from colloidal core-shell nanoparticles followed by spark plasma sintering. An unusually large thermopower is found in both materials, and the possibility of energy filtering as opposed to grain boundary scattering as an explanation is discussed. A decreased Debye temperature and an increased molar specific heat are in accordance with recent predictions for nanostructured materials. On the basis of these results we propose suitable core-shell material combinations for future thermoe…
Hot wall technique was used to grow block single crystal films of Bi_2Te_3 and solid solutions of Bi_(0.5)Sb_(1.5)Te_3 on mica (muscovite) substrates. X-ray diffraction studies demonstrated that the crystalline c-axis in the films was normal to the substrate plane. Seebeck coefficient, electrical conductivity and magnetoresistivity tensor components were measured at various orientations of magnetic and electric fields in the temperature interval 77-300 K and magnetic field up to 14 T. Scattering mechanism of charge carriers in the films were studied using temperature dependences of the degener…
Advances in solid-state device design now allow the spectrum of transmitted electrons in thermionic and thermoelectric devices to be engineered in ways that were not previously possible. Here we show that the shape of the electron energy spectrum in these devices has a significant impact on their performance. We distinguish between traditional thermionic devices where electron momentum is filtered in the direction of transport only and a second type, in which the electron filtering occurs according to total electron momentum. Such 'total momentum filtered' kr thermionic devices could potential…
Thermoelectric effects enable the direct conversion between thermal and electrical energy and provide an alternative route for power generation and refrigeration. The clathrate Ba8Ga16Ge30 has the highest figure of merit (ZT ~ 1) among other members in the family of type-I inorganic clathrates. Enhancement of the thermoelectric properties have been observed in multilayered superlattices, quantum wires and in nanostructured materials, either due to the increase in power-factor (S^{2}σ) or due to the reduction of lattice thermal conductivity (κ). Here, we investigate the thermoelectric propertie…
Intermetallic clathrates are candidate materials for thermoelectric applications above room temperature. Here we explore whether their intrinsically low lattice thermal conductivities can be further reduced by nanostructuring and whether this can further enhance their thermoelectric performance.}{As bulk nanostructuring routes we studied melt spinning and ball milling. To optimize the compaction process and/or stabilize the nanostructure we varied the process parameters, used additives, and studied clathrate-based composites. Initial results on clathate nanowires as simpler model nanostructure…
Sb-doped Bi2Te3 is known since the 1950s as the best thermoelectric material for near-room temperature operation. Improvements in material performance are expected from nanostructuring procedures. We present a straightforward and fast method to synthesize already nanostructured pellets that show an enhanced ZT due to a remarkably low thermal conductivity and unusually high Seebeck coefficient for a nominal composition optimized for arc-melting: Bi0.35Sb1.65Te3. We provide a detailed structural analysis of the Bi2-xSbxTe3 series based on neutron powder diffraction as a function of composition a…
Hierarchically nanostructured materials, where disorder is introduced in various length scales (at the atomic scale, the nanoscale, and the mesoscale) is one of the most promising directions to achieve extremely low thermal conductivities and improve thermoelectric performance. Here we theoretically investigate one such system, a nanocrystalline material with nanopores that are introduced between the crystalline regions. We use the Nonequilibrium Greens Function method for electronic transport and Molecular Dynamics for phonon transport.
The thermoelectric properties of nanostructured silicon are not fully understood despite their initial promise. While the anomalously low thermal conductivity has attracted much work, the impact of nanostructuring on the power factor has mostly escaped attention. While initial reports did not find any significant changes to the power factor compared to the bulk, subsequent detailed measurements on p-type silicon nanowires showed a stark reduction in the Seebeck coefficient when compared to similarly doped bulk. The reduction is consistent with the disappearance of the phonon drag contribution,…
We theoretically investigate the thermoelectric properties of zigzag graphene nanoribbons in the presence of extended line defects, substrate impurities and edge roughness along the nanoribbon's length. A nearest-neighbor tight-binding model for the electronic structure and a fourth nearest- neighbor force constant model for the phonon bandstructure are used. For transport we employ quantum mechanical non-equilibrium Green's function simulations. Starting from the pristine zigzag nanoribbon structure that exhibits very poor thermoelectric performance, we demonstrate how after a series of engin…
Half-Heusler alloys (MgAgSb structure) are promising thermoelectric materials. RNiSn half-Heusler phases (R=Hf, Zr, Ti) are the most studied in view of their thermal stability. The highest dimensionless figure of merit (ZT) obtained is ~1 in the temperature range ~450-900oC, primarily achieved in nanostructured alloys. Through proper annealing, ZT~1.2 has been obtained in a previous ZT~1 n-type (Hf,Zr)NiSn phase without the nanostructure. There is an appreciable increase in the power factor, decrease in charge carrier density, and increase in carrier mobility. The findings are attributed to th…
Nanostructured materials have emerged as an alternative to enhance the figure of merit (ZT) of thermoelectric (TE) devices. Graphene exhibits a high electrical conductivity (in-plane) that is necessary for a high ZT; however, this effect is countered by its impressive thermal conductivity. In this work TE layered devices composed of electrochemically exfoliated graphene (EEG) and a phonon blocking material such as poly (3,4-ethylenedioxythiophene)polystyrene sulfonate (PEDOT:PSS), polyaniline (PANI) and gold nanoparticles (AuNPs) at the interface were prepared. The figure of merit, ZT, of each…
Herein, we report the use of nanostructured crystalline Si as a thermoelectric material and its integration into thermoelectric harvesters. The proof-of-concept relies on the partial suppression of lattice thermal transport by introducing pores with dimensions scaling between the electron mean free path and the phonon mean free path. In other words, we artificially aimed at the electron crystal phonon glass tradeoff targeted for thermoelectric efficiency. The devices were fabricated using CMOS compatible processes and exhibited power generation from a few microWatts per cm^2 to a few milliWatt…
Nanostructured semiconducting materials are promising candidates for thermoelectrics due to their potential to suppress phonon transport while preserving electrical properties. Modeling phonon-boundary scattering in complex geometries is crucial for predicting materials with high conversion efficiency. However, the simultaneous presence of ballistic and diffusive phonons challenges the development of models that are both accurate and computationally tractable. Using the recently developed first-principles Boltzmann transport equation (BTE) approach, we investigate diffusive phonons in nanomate…
Nano-structured molybdenum-copper composites have been produced through severe plastic deformation of liquid-metal infiltrated Cu30Mo70 and Cu50Mo50 (wt.%) starting materials. Processing was carried out using high pressure torsion at room temperature with no subsequent sintering treatment, producing a porosity-free, ultrafine grained composite. Extensive deformation of the Cu50Mo50 composite via two-step high-pressure torsion produced equiaxed nanoscale grains of Mo and Cu with a grain size of 10-15 nm. Identical treatment of Cu30Mo70 produced a ultrafine, lamellar structure, comprised of Cu a…
Tuning thermal transport in nanostructured materials is a powerful approach to develop high-efficiency thermoelectric materials. Using a recently developed approach based on the phonon mean free path dependent Boltzmann transport equation, we compute the effective thermal conductivity of nanoporous materials with pores of various shapes and arrangements. We assess the importance of pore-pore distance in suppressing thermal transport, and identify the pore arrangement that minimizes the thermal conductivity, composed of a periodic arrangement of two misaligned rows of triangular pores. Such a c…
Recently, the superionic thermoelectrics, which typify the novel `phonon-liquid electron-crystal' concept, have attracted enormous attention due to their ultralow thermal conductivity and high figure-of-merit (zT). However, their high zT is generally obtained deep inside the superionic phase, e.g., near 1000~K in the Cu$_2$X (X: chalcogen atom) family where the superionic transition is close to 400~K. At such high temperatures, the liquid-like flow of the metal ions under an electric field or a temperature gradient, both of which are integral to the working of a thermoelectric device, results …
For simultaneously achieving the high-power factor and low lattice thermal conductivity of Si-Ge based thermoelectric materials, we employed, in this study, constructively modifying the electronic structure near the chemical potential and nano-structuring by low temperature and high-pressure sintering on nano-crystalline powders. Nickel was doped to create the impurity states near the edge of the valence band for enhancing the power factor with boron for tuning the carrier concentration. The nanostructured samples with the nominal composition of Si0.65-xGe0.32Ni0.03Bx (x = 0.01, 0.02, 0.03, an…
