Enhancement Of Thermoelectric Efficiency Research Articles

Enhancement of thermoelectric efficiency by quantum interference effects in trilayer silicene flakes

In recent years, the enhancement of thermoelectric efficiencies has been accomplished in nanoscale systems by making use of quantum effects. We exploit the presence of quantum interference phenomena such as bound states in the continuum and Fano antiresonances in trilayer silicene flakes to produce sharp changes in the electronic transmission of the system. By applying symmetric gate voltages the thermoelectric properties can be tuned and, for particular flake lengths, a great enhancement of the figure of merit can be achieved. We show that the most favorable configurations are those in which the electronic transmission is dominated by the coupling of bound states to the continuum, tuned by an external gate.

Designing width-modulated Si nanowires for enhanced thermoelectric efficiency

Efficient thermoelectric energy conversion requires good electronic properties and low thermal conductivity. Width-modulated nanowires have been proposed for thermoelectric efficiency enhancement. We have used the Monte Carlo simulation technique to study the effect of width modulation on the thermoelectric transport properties of Si nanowires. We have found that it is possible to achieve significant decrease of the thermal conductivity without deteriorating the electrical transport properties. Our simulations predict enhanced thermoelectric efficiency in Si nanowires modulated by a single constriction compared to the corresponding non-modulated nanowires.

Spectacular enhancement of thermoelectric phenomena in chemically synthesized graphene nanoribbons with substitution atoms.

We analyze theoretically the transport and thermoelectric properties of graphene nanoribbons of a specific geometry, which have been synthesized recently from polymers [Cai, et al., Nature, 2011, 466, 470]. When such nanoribbons are modified at one of the two edges by Al or N substitutions, they acquire a ferromagnetic moment localized at the modified edge. We present numerical results on the electronic structure and thermoelectric properties (including also spin thermoelectricity) of the modified nanoribbons. The results show that such nanoribbons can display large thermoelectric efficiency in certain regions of chemical potential, where the corresponding electric and spin figures of merit achieve unusually large values. The enhancement of thermoelectric efficiency follows from a reduced phonon heat conductance of the nanoribbons and from their peculiar electronic band structure. Thus, such nanoribbons are promising for practical applications in nanoelectronic and spintronic devices.

Enhancement of thermoelectric efficiency in non-uniform semiconductor nanowires

Thermoelectric efficiency has been limited to a non-practical range because of the electronic material properties that are not easily controlled. Therefore, limiting thermal transport has been the best approach to enhance efficiency, and efforts to reduce the thermal conductivity even further have been extensively reported. In this work, we investigate thermoelectric transport coefficients of uniformly straight and sinusoidally undulated nanowires and compare the resulting thermoelectric figure-of-merit. The improved efficiency should suggest another strategy to effectively control thermal transport in semiconductor nanostructures.

Electronic tuning of the transport properties of off-stoichiometric PbxSn1−xTe thermoelectric alloys by Bi2Te3 doping

The recent energy demands affected by the dilution of conventional energy resources and the growing awareness of environmental considerations had motivated many researchers to seek for novel renewable energy conversion methods. Thermoelectric direct conversion of thermal into electrical energies is such a method, in which common compositions include IV-VI semiconducting compounds (e.g., PbTe and SnTe) and their alloys. For approaching practical thermoelectric devices, the current research is focused on electronic optimization of off-stoichiometric p-type PbxSn1−xTe alloys by tuning of Bi2Te3 doping and/or SnTe alloying levels, while avoiding the less mechanically favorable Na dopant. It was shown that upon such doping/alloying, higher ZTs, compared to those of previously reported undoped Pb0.5Sn0.5Te alloy, were obtained at temperatures lower than 210–340 °C, depending of the exact doping/alloying level. It was demonstrated that upon optimal grading of the carrier concentration, a maximal thermoelectric efficiency enhancement of ∼38%, compared to that of an undoped material, is expected.

Enhancement of thermoelectric efficiency by embedding hexagonal boron-nitride cells in zigzag graphene nanoribbons

We study thermoelectric properties of zigzag graphene nanoribbon (ZGNR)–boron nitride (BN) junctions coupled to square electrodes using nonequilibrium Green function formalism in the linear response regime. The embedding of hexagonal BN cells into the ZGNR results in the change of the thermoelectric properties with the length and position of BN cells. The influence of the width variation on the electrical conductance and the Seebeck coefficient of the ZGNR–BN junctions is examined. Also, the coupling of asymmetric electrodes to the ZGNR–BN junctions and the pristine ZGNR is considered. It is observed that the asymmetric electrodes lead to the increase of the Seebeck coefficients of both structures, while the phonon thermal conductance is decreased because of the reduction of the phonon transport in inhomogeneous structures. Our results predict that the thermoelectric efficiency of the system is increased by embedding the hexagonal BN cells, as well as coupling to the asymmetric electrodes.

Enhancement of thermoelectric efficiency of doped PCDTBT polymer films

PCDTBT films doped with FeCl3 present a large power factor of 24 μW m−1 K−2 at 150 °C and a thermal conductivity of 1 W m−1 K−1 for the same film. This improvement is mainly achieved by up to 9 orders of magnitude increased in the electric conduction.

Enhancement of thermoelectric efficiency of CoSb3-based skutterudites by double filling with K and Tl.

The high-temperature thermoelectric properties of thallium (Tl) and potassium (K) double-filled cobalt antimonide (CoSb3)-based skutterudites with nominal compositions TlxK0.3Co4Sb12 (x = 0.1 − 0.3) were investigated. The filling fraction of Tl in CoSb3 was enhanced by co-filling with K, which resulted in all of the samples showing the filled-skutterudite single phase. Owing to the high filling ratio, the carrier concentration in the sample with x = 0.3 was as high as 4.3 × 1020 cm−3 at room temperature. Furthermore, quite low lattice thermal conductivity (as low as 0.9 Wm−1K−1) was obtained for the sample with x = 0.3, probably because of strong phonon scattering by the Tl and K co-rattling effect, which resulted in a maximum zT of around one at 773 K.

Thermoelectric properties of Sr and Mg double-substituted LaCoO3 at room temperature

A strategy to improve the thermoelectric properties of LaCoO3 by the double substitution of Sr and Mg in La1−xSrxCo1−xMgxO3 (x=0, 0.025, 0.05, 0.1) is presented. Effects of either Sr or Mg substitution are also investigated separately in the series of La1−xSrxCoO3 and LaCo1−xMgxO3 (x=0, 0.02, 0.05, 0.1, 0.2). All the samples were successfully synthesized by the citrate sol–gel method. Replacing La by a small amount of Sr increases electronic conductivity but decreases the Seebeck coefficient; the decrease in the Seebeck coefficient is reduced when Mg is simultaneously substituted for Co. The employment of both Sr and Mg substitutions benefit to the enhancement in thermoelectric efficiency since both electrical conductivity and Seebeck coefficient are enhanced while thermal conductivity is suppressed. With this strategy, a sample with a small amount of double substitution, which is typically La0.975Sr0.025Co0.0975Mg0.025O3, exhibits a ZT of 0.047 at room temperature.

Impurities-contributed abnormal thermoelectric effect in a parallel double quantum dot structure

We discuss the influence of local impurities on the thermoelectric effect in a parallel double quantum dot (QD) structure. It is first found that in this structure, the Fano effect contributes significantly to the enhancement of thermoelectric efficiency, especially in the case of ϕ = π. Next, impurities are introduced to couple to the QDs, respectively. We readily find that regardless of which QD is coupled to a local impurity, the thermoelectric efficiency can be enhanced by the strengthening of impurity–QD coupling. This means that the destruction of the Fano interference is not the necessary condition to suppress the thermoelectric effect. Accordingly, we hope that the numerical results can help to understand the role of impurities in adjusting the thermoelectric properties of the QD structure.

Enhancement of thermoelectric efficiency in triple quantum dots by the Dicke effect

We theoretically study thermoelectric transport through a quantum dot coupled to two side quantum dots in the linear response regime by the nonequilibrium Green's function technique. Our results show that thermoelectric properties are strongly influenced by the Dicke effect. At low temperature, for small (very large) level shift and large interdot coupling the subradiant state contributes a sharp peak to the Dicke spectral line in the electrical and thermal conductance spectrum and leads to a strong violation of the Wiedemann-Franz law. As a result, the thermoelectric efficiency is strongly enhanced near the subradiant state. At relatively high temperature, the large tunneling coupling may be used to compensate the attenuation of the interference effect so that a considerable thermoelectric efficiency can still be obtained around the subradiant state. Moreover, the thermoelectric efficiency is also strongly dependent on the asymmetry parameter and intradot Coulomb interaction.

Morphological effects on the electronic transport properties of three-phase thermoelectric materials

Multi-phase thermoelectric materials are widely investigated due to their high potential for thermoelectric efficiency enhancement as a result of lattice thermal conductivity reduction by phonon scattering from the involved interfaces. Yet, the electronic mutual influence of the involved phases on the thermoelectric efficiency has to date usually been neglected in designing novel multi-phase thermoelectric materials. In the current research, the influence of distribution and morphology alignment on the electronic thermoelectric properties of three-phase composite materials was analyzed. Such an analysis, based on defining interaction volumes of the relevant thermoelectric properties, for any distribution and relative amount of any three phases with known individual properties, can be utilized for thermoelectric efficiency enhancement by intentional alignment of the phases along the preferred thermoelectric direction. For the p-type Gex(SnyPb1−y)1−xTe thermoelectric alloys, following a phase separation reaction it was found that serially relative alignment between the involved phases is optimal from the thermoelectric point of view.

Thermoelectric Properties in a Parallel Double Quantum Dot Structure Modulated by the Fano Interferences

We discuss the thermoelectric properties assisted by the Fano effect of a parallel double quantum dot (QD) structure. By adjusting the couplings between the QDs and leads, we facilitate the nonresonant and resonant channels for the Fano interference. It is found that at low temperature, Fano lineshapes appear in the electronic and thermal conductance spectra, which can also be reversed by an applied local magnetic flux with its phase factor φ=π. And, the Fano effect contributes significantly to the enhancement of thermoelectric efficiency. However, at the same temperature, the thermoelectric effect in the case of φ=π is much more apparent, compared with the case of zero magnetic flux. And, with the temperature increase, the thermoelectric effect in the case of φ=π seems to be more robust. Using the concept of Feynman path, we analyze the difference between the quantum interferences in the cases of φ=0 and π. It is seen that in the absence of magnetic flux the Fano interference originates from the quantum interference among infinite-order Feynman paths, but it occurs only between two lowest-order Feynman paths when φ=π. The increase of temperature inevitably destroys the electron coherent transmission in each paths. So, in the case of zero magnetic field, the thermoelectric effect assisted by the Fano interference is easy to weaken by a little increase of temperature.

Enhanced Seebeck Coefficients of Thermoelectric Bi2Te3 Nanowires as a Result of an Optimized Annealing Process

Although an annealing approach for bulk or thick Bi–Te material had been done for the enhancement of thermoelectric efficiency, this is the first time to report an annealing process for a Bi–Te nanostructure with a high aspect ratio. As is well-known, the Seebeck coefficient, one of the thermoelectric efficiency factors, is strongly influenced by the chemical composition of the material. However, tellurium element tends to be easily evaporated in the high-temperature range (more than 400 °C) due to its low melting point and high evaporation pressure. Furthermore, it is more serious in such a nanostructure with a high aspect ratio. In this article, we suggested the way to prevent the evaporation of tellurium element in the high annealing temperature range (300–623 °C) and investigated why the Seebeck coefficients for annealed Bi–Te nanowires were enhanced. Seebeck coefficients for Bi–Te nanowires were improved 3 times higher than those for the as-prepared ones at the optimized annealing process due to increased crystal defect concentrations, especially for edge dislocations. Consequently, this annealing process can be applied for nanostructured TE devices for the enhancement of thermopower (S).

Enhancement of thermoelectric efficiency and violation of the Wiedemann-Franz law due to Fano effect

We consider the thermoelectric properties of a double-quantum-dot molecule coupled in parallel to metal electrodes with a magnetic flux threading the ring. By means of the Sommerfeld expansion we obtain analytical expressions for the electric and thermal conductances, thermopower and figure of merit for arbitrary values of the magnetic flux. We neglect electronic correlations. The Fano antiresonances in transmission demand that terms usually discarded in the Sommerfeld expansion are taken into account. We also explore the behavior of the Lorenz ratio L = κ/σT, where κ and σ are the thermal and electrical conductances and T the absolute temperature, and we discuss the reasons why the Wiedemann-Franz law fails in presence of Fano antiresonances.

Electrochemical Synthesis of Bi1–xSbx Nanowires with Simultaneous Control on Size, Composition, and Surface Roughness

We present the synthesis of Bi1–xSbx nanowires with controlled composition between x = 0.05 and x = 0.40 over a wide range of diameters (20–100 nm) interesting for the investigation of the enhancement of thermoelectric efficiency via quantum size effects. We find that the relative concentration of Bi and Sb ions in the electrolyte, together with the deposition potential, determines the resulting nanowire composition. Morphology and composition were analyzed using X-ray diffraction, electron microscopy, and energy dispersive X-ray analysis. Two different templates were fabricated to enable the synthesis of nanowires with smooth and rough contour.

Colossal enhancement in thermoelectric efficiency of weakly coupled double quantum dot system

A colossal enhancement in thermoelectric efficiency, ZT, of coupled double quantum dot (DQD) system in series configuration, have been predicted in the Coulomb blockade regime for a non-magnetic case, as a result of very weak interdot tunnel coupling and highly asymmetric dot-lead couplings using Keldysh nonequilibrium Green function approach. Such an enhancement (which leads to saturation in the extreme case) of ZT as a result of tuning of coupling parameters is understood in terms of bipolar effect. These findings suggest that highly asymmetric DQD system, with very weak interdot tunnel coupling can be used to design more efficient thermoelectric devices.

Enhancement of thermoelectric efficiency in type-VIII clathrate Ba8Ga16Sn30 by Al substitution for Ga

Single-crystalline samples of type-VIII clathrate Ba8Ga16−xAlxSn30 (0≤x≤12) were grown from Sn flux to characterize the structural and thermoelectric properties from 300 to 600 K. The lattice parameter increases by 0.5% as x is increased to 10.5 whose value is the solubility limit of Al. The Seebeck coefficients of all samples are largely negative and the absolute values increase to approximately 300 μV/K on heating to 600 K. This large thermopower coexists with the metallic behavior in the electrical resistivity. The values of resistivity for 1≤x≤6 at 300 K are in the range 3.3–3.8 mΩ cm which is 70% of that for x=0. As a result, the power factor for x=4 and 6 has a rather large maximum of 1.83×10−3 W/m K2 at 480 K. The thermal conductivity stays at a low level of 0.72 W/mK up to 480 K, and the sample with x=6 reaches a ZT value of 1.2 at 500 K.

Enhancement of thermoelectric efficiency in (Ca,Dy)MnO3–(Ca,Yb)MnO3 solid solutions

Transport and thermoelectric (TE) properties have been investigated for the (Ca,Dy)MnO3–(Ca,Yb)MnO3 solid solutions. Resistivity is found to be closely correlated with the structural distortions in this system. The enhanced TE efficiency in the solid solutions can be well attributed to the distortion of electronic density of states along with a strong point defect scattering. Such band structure engineering and phonon engineering have resulted in an effective improvement of the TE performance in these solid solutions and could be applied to more systems.

Improved Thermoelectric Properties of La1−xSrxCoO3 Nanowires

Nanotechnology has brought novel and controllable thermoelectric (TE) characteristics to conventional materials. Here we report a simple method to synthesize single-crystalline La1−xSrxCoO3 nanowire compacts with an improved TE efficiency. The figure of merit ZT in the nanowires can reach 0.19 at 300 K, which is nearly twice that in the bulk, and this room-temperature ZT is very high among oxide materials. The enhancement of TE efficiency can be attributed to both extrinsic and intrinsic factors: First, the unique nanowire structure together with numerous interfaces in the compact samples strongly suppresses the phonon thermal transport. Second, the small dimensions of nanowires give rise to an unexpected increase in the thermopower due to a reduction of the effective bandwidth and variation of the electron configuration. Our results indicate that the TE response of conventional oxides can be effectively improved by the form of nanowires, which can be synthesized much easier than single crystals and epita...