Showing papers on "Seebeck coefficient published in 2014"

A review on thermoelectric renewable energy: Principle parameters that affect their performance

Abstract: Developing thermoelectric materials with superior performance means tailoring interrelated thermoelectric physical parameters – electrical conductivities, Seebeck coefficients, and thermal conductivities – for a crystalline system. High electrical conductivity, low thermal conductivity, and a high Seebeck coefficient are desirable for thermoelectric materials. Therefore, knowledge of the relation between electrical conductivity and thermal conductivity is essential to improve thermoelectric properties. In general, research in recent years has focused on developing thermoelectric structures and materials of high efficiency. The importance of this parameter is universally recognized; it is an established, ubiquitous, routinely used tool for material, device, equipment and process characterization both in the thermoelectric industry and in research. In this paper, basic knowledge of thermoelectric materials and an overview of parameters that affect the figure of merit ZT are provided. The prospects for the optimization of thermoelectric materials and their applications are also discussed.

Two-dimensional molybdenum carbides: potential thermoelectric materials of the MXene family

Abstract: A newly synthesized family of two-dimensional transition metal carbides and nitrides, so-called MXenes, exhibit metallic or semiconducting properties upon appropriate surface functionalization. Owing to their intrinsic ceramic nature, MXenes may be suitable for energy conversion applications at high temperature. Using the Boltzmann theory and first-principles electronic structure calculations, we explore the thermoelectric properties of monolayer and multilayer M2C (M = Sc, Ti, V, Zr, Nb, Mo, Hf, and Ta) and M2N (M = Ti, Zr, and Hf) MXenes functionalized with F, OH, and O groups. From our calculations, it turns out that monolayer and multilayer nanosheets of Mo2C acquire superior power factors to other MXenes upon any type of functionalization. We therefore propose the functionalized Mo2C nanosheets as potential thermoelectric materials of the MXene family. The exceptional thermoelectric properties of the functionalized Mo2C nanosheets are attributed to the peculiar t2g band shapes, which are a combination of flat and dispersive portions. These types of band shapes allow Mo2C to gain a large Seebeck coefficient and simultaneously a good electrical conductivity at low carrier concentrations.

Assessment of the thermoelectric performance of polycrystalline p-type SnSe

Abstract: We report the evaluation of the thermoelectric performance of polycrystalline p-type SnSe, a material in which unprecedented values of the thermoelectric figure of merit ZT have been recently discovered in single crystals Besides anisotropic transport properties, our results confirm that this compound exhibits intrinsically very low thermal conductivity values The electrical properties show trends typical of lightly doped, intrinsic semiconductors with thermopower values reaching 500 μV K−1 in a broad temperature range An orthorhombic-to-orthorhombic transition sets in at 823 K, a temperature at which the power factor reaches its maximum value A maximum ZT of 05 was obtained at 823 K, suggesting that proper optimization of the transport properties of SnSe might lead to higher ZT values These findings indicate that this system represents an interesting experimental platform for the search of highly efficient thermoelectric materials

Origin of the high performance in GeTe-based thermoelectric materials upon Bi2Te3 doping.

Abstract: As a lead-free material, GeTe has drawn growing attention in thermoelectrics, and a figure of merit (ZT) close to unity was previously obtained via traditional doping/alloying, largely through hole carrier concentration tuning. In this report, we show that a remarkably high ZT of ∼1.9 can be achieved at 773 K in Ge0.87Pb0.13Te upon the introduction of 3 mol % Bi2Te3. Bismuth telluride promotes the solubility of PbTe in the GeTe matrix, thus leading to a significantly reduced thermal conductivity. At the same time, it enhances the thermopower by activating a much higher fraction of charge transport from the highly degenerate Σ valence band, as evidenced by density functional theory calculations. These mechanisms are incorporated and discussed in a three-band (L + Σ + C) model and are found to explain the experimental results well. Analysis of the detailed microstructure (including rhombohedral twin structures) in Ge0.87Pb0.13Te + 3 mol % Bi2Te3 was carried out using transmission electron microscopy and crystallographic group theory. The complex microstructure explains the reduced lattice thermal conductivity and electrical conductivity as well.

High thermoelectric performance realized in a BiCuSeO system by improving carrier mobility through 3D modulation doping.

Abstract: We report a greatly enhanced thermoelectric performance in a BiCuSeO system, realized by improving carrier mobility through modulation doping. The heterostructures of the modulation doped sample make charge carriers transport preferentially in the low carrier concentration area, which increases carrier mobility by a factor of 2 while maintaining the carrier concentration similar to that in the uniformly doped sample. The improved electrical conductivity and retained Seebeck coefficient synergistically lead to a broad, high power factor ranging from 5 to 10 μW cm(-1) K(-2). Coupling the extraordinarily high power factor with the extremely low thermal conductivity of ∼0.25 W m(-1) K(-1) at 923 K, a high ZT ≈ 1.4 is achieved in a BiCuSeO system.

Abnormally enhanced thermoelectric transport properties of SWNT/PANI hybrid films by the strengthened PANI molecular ordering

Abstract: Single-walled carbon nanotube (SWNT)/polyaniline (PANI) hybrid films were prepared by casting the suspension containing well-dispersed SWNTs and CSA-doped PANI. The electrical conductivity of the SWNT/PANI film at first increased with the increasing SWNT content and then decreased at high SWNT content, whereas the Seebeck coefficient increased monotonically in the present SWNT content range. Moreover, the electrical conductivity values of the SWNT/PANI composites were much higher than the values calculated based on the series-connected two-component mixture model, whereas the dependence of the Seebeck coefficient on the SWNT content fitted well with the mixture model. Thermal conductivities increased with the SWNT content, but the rate of increase was much lower than the values estimated using the mixture model. The maximum values of electrical conductivity and Seebeck coefficient of hybrid films were up to 769 S cm−1 and 65 μV K−1. Consequently, the maximum thermoelectric power factor and ZT value at room temperature reached 176 μW m−1 K−2 and 0.12, respectively. The optimal TE property of the SWNT/PANI hybrid film was remarkably higher than those of either individual component of the composite, and exhibits the highest values for inorganic–organic composite materials reported so far. XRD and Raman analyses revealed that the PANI molecules in the composite film had a more expanded conformation, and were more orderly arranged compared with both pure PANI bulk and pure PANI film. The abnormally enhanced thermoelectric performance is attributed to the highly ordered PANI interface layer on the SWNT surface, which formed by the synergetic effect of chain expansion by the chemical interactions between PANI and the solvent and chain-ordering due to the π–π conjugation between PANI and CNTs.

Enhanced thermoelectric performance of phosphorene by strain-induced band convergence

Abstract: The newly emerging monolayer phosphorene was recently predicted to be a promising thermoelectric material. In this work, we propose to further enhance the thermoelectric performance of phosphorene using the strain-induced band convergence. The effect of the uniaxial strain on the thermoelectric properties of phosphorene was investigated by using the first-principles calculations combined with the semiclassical Boltzmann theory. When the zigzag-direction strain is applied, the Seebeck coefficient and electrical conductivity in the zigzag direction can simultaneously be greatly enhanced at the critical strain of 5%, at which the band convergence is achieved. The largest $ZT$ value of 1.65 at 300 K is then conservatively estimated by using the bulk lattice thermal conductivity. When the armchair-direction strain of 8% is applied, the room-temperature $ZT$ value can reach 2.12 in the armchair direction of phosphorene. Our results indicate that strain-induced band convergence could be an effective method to enhance the thermoelectric performance of phosphorene.

Enhanced thermoelectric properties of PEDOT:PSS nanofilms by a chemical dedoping process

Abstract: We report that a simple chemical dedoping treatment of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) nanofilms enhances the thermoelectric properties of the polymer nanofilms. The dedoping process was done by over-coating a mixture of dimethyl sulfoxide (DMSO) and hydrazine (HZ), a strong chemical reducing agent, onto the PEDOT:PSS nanofilms. This additional step led to the removal of excess PSS chains and the formation of neutral states of PEDOT chains, resulting in an improvement in the Seebeck coefficient, from 30 μV K−1 to 142 μV K−1, and a decrease in the electrical conductivity from 726 S cm−1 to 2 S cm−1. By controlling the concentration of HZ, we obtained an optimized power factor of 112 μW m−1 K−2 at 0.0175 wt% of HZ in DMSO at room temperature. The corresponding electrical conductivity and Seebeck coefficient under optimized conditions were 578 S cm−1 and 67 μV K−1, respectively. We expect that this simple dedoping process can be applied to general thermoelectric nanofilms based on chemically doped polymers in order to enhance the power factor.

Improvement of the Seebeck coefficient of PEDOT:PSS by chemical reduction combined with a novel method for its transfer using free-standing thin films

Abstract: A simple method for improving the Seebeck coefficient of PEDOT:PSS up to 161 μV K−1 is presented and combined with a new process for transferring thick (>10 μm) films of PEDOT:PSS onto substrates with various shapes, and in particular onto flexible substrates. These reduced transferred films have been used in combination with a nickel ethylenetetrathiolate coordination polymer to fabricate cheap and flexible heat flux sensors.

Thermoelectrics with earth abundant elements: low thermal conductivity and high thermopower in doped SnS

Abstract: The thermoelectric properties of Ag-doped SnS samples synthesized by mechanical alloying followed by spark plasma sintering were studied. We report that SnS possesses a high Seebeck coefficient of >+400 μV K−1 and Ag doping increases the carrier concentration by more than four orders of magnitude giving significantly improving electrical conductivity. The thermal conductivity falls below 0.5 W m−1 K−1 at 873 K and leads to a high ZT of 0.6. The data indicate that earth-abundant and environmentally friendly SnS is a promising candidate for thermoelectric applications despite its relatively wide bandgap of 1.2 eV.

Optimization of thermoelectric efficiency in SnTe: the case for the light band

Abstract: p-Type PbTe is an outstanding high temperature thermoelectric material with zT of 2 at high temperatures due to its complex band structure which leads to high valley degeneracy. Lead-free SnTe has a similar electronic band structure, which suggests that it may also be a good thermoelectric material. However, stoichiometric SnTe is a strongly p-type semiconductor with a carrier concentration of about 1 × 1020 cm−3, which corresponds to a minimum Seebeck coefficient and zT. While in the case of p-PbTe (and n-type La3Te4) one would normally achieve higher zT by using high carrier density in order to populate the secondary band with higher valley degeneracy, SnTe behaves differently. It has a very light, upper valence band which is shown in this work to provide higher zT than doping towards the heavier second band. Therefore, decreasing the hole concentration to maximize the performance of the light band results in higher zT than doping into the high degeneracy heavy band. Here we tune the electrical transport properties of SnTe by decreasing the carrier concentration with iodine doping, and increasing the carrier concentration with Gd doping or by making the samples Te deficient. A peak zT value of 0.6 at 700 K was obtained for SnTe0.985I0.015 which optimizes the light, upper valence band, which is about 50% higher than the other peak zT value of 0.4 for GdzSn1−zTe and SnTe1+y which utilize the high valley degeneracy secondary valence band.

Longitudinal spin Seebeck effect: from fundamentals to applications

Abstract: The spin Seebeck effect refers to the generation of spin voltage as a result of a temperature gradient in ferromagnetic or ferrimagnetic materials. When a conductor is attached to a magnet under a temperature gradient, the thermally generated spin voltage in the magnet injects a spin current into the conductor, which in turn produces electric voltage owing to the spin-orbit interaction. The spin Seebeck effect is of increasing importance in spintronics, since it enables direct generation of a spin current from heat and appears in a variety of magnets ranging from metals and semiconductors to insulators. Recent studies on the spin Seebeck effect have been conducted mainly in paramagnetic metal/ferrimagnetic insulator junction systems in the longitudinal configuration in which a spin current flowing parallel to the temperature gradient is measured. This 'longitudinal spin Seebeck effect' (LSSE) has been observed in various sample systems and exclusively established by separating the spin-current contribution from extrinsic artefacts, such as conventional thermoelectric and magnetic proximity effects. The LSSE in insulators also provides a novel and versatile pathway to thermoelectric generation in combination of the inverse spin-Hall effects. In this paper, we review basic experiments on the LSSE and discuss its potential thermoelectric applications with several demonstrations.

High ZT in p-type (PbTe)1-2x(PbSe)x(PbS)x thermoelectric materials.

Abstract: Lead chalcogenide thermoelectric systems have been shown to reach record high figure of merit values via modification of the band structure to increase the power factor or via nanostructuring to reduce the thermal conductivity. Recently, (PbTe)1-x(PbSe)x was reported to reach high power factors via a delayed onset of interband crossing. Conversely, the (PbTe)1-x(PbS)x was reported to achieve low thermal conductivities arising from extensive nanostructuring. Here we report the thermoelectric properties of the pseudoternary 2% Na-doped (PbTe)1-2x(PbSe)x(PbS)x system. The (PbTe)1-2x(PbSe)x(PbS)x system is an excellent platform to study phase competition between entropically driven atomic mixing (solid solution behavior) and enthalpy-driven phase separation. We observe that the thermoelectric properties of the PbTe-PbSe-PbS 2% Na doped are superior to those of 2% Na-doped PbTe-PbSe and PbTe-PbS, respectively, achieving a ZT ≈2.0 at 800 K. The material exhibits an increased the power factor by virtue of valence band modification combined with a very reduced lattice thermal conductivity deriving from alloy scattering and point defects. The presence of sulfide ions in the rock-salt structure alters the band structure and creates a plateau in the electrical conductivity and thermopower from 600 to 800 K giving a power factor of 27 μW/cmK(2). The very low total thermal conductivity values of 1.1 W/m·K of the x = 0.07 composition is accounted for essentially by phonon scattering from solid solution defects rather than the assistance of endotaxial nanostructures.

Large thermoelectricity via variable range hopping in chemical vapor deposition grown single-layer MoS2.

Abstract: Ultrathin layers of semiconducting molybdenum disulfide (MoS2) offer significant prospects in future electronic and optoelectronic applications. Although an increasing number of experiments bring light into the electronic transport properties of these crystals, their thermoelectric properties are much less known. In particular, thermoelectricity in chemical vapor deposition grown MoS2, which is more practical for wafer-scale applications, still remains unexplored. Here, for the first time, we investigate these properties in grown single layer MoS2. Microfabricated heaters and thermometers are used to measure both electrical conductivity and thermopower. Large values of up to ∼30 mV/K at room temperature are observed, which are much larger than those observed in other two-dimensional crystals and bulk MoS2. The thermopower is strongly dependent on temperature and applied gate voltage with a large enhancement at the vicinity of the conduction band edge. We also show that the Seebeck coefficient follows S ∼ T(1/3), suggesting a two-dimensional variable range hopping mechanism in the system, which is consistent with electrical transport measurements. Our results help to understand the physics behind the electrical and thermal transports in MoS2 and the high thermopower value is of interest to future thermoelectronic research and application.

Enhanced thermoelectric performance of PEDOT with different counter-ions optimized by chemical reduction

Abstract: This work reports on the synthesis of the intrinsically conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with several counter-ions, ClO4, PF6 and bis(trifluoromethylsulfonyl)imide (BTFMSI), by electro-polymerization and its thermoelectric properties. We show that, depending on the counter-ion size, the thermoelectric efficiency of PEDOT can be increased up to two orders of magnitude. A further chemical reduction with hydrazine optimizes the power factor (PF). By changing the counter-ions, we were able to increase the electrical conductivity (σ) of PEDOT by a factor of three, while the Seebeck coefficient remains at the same order of magnitude in the three polymers. The best thermoelectric efficiency has been observed in PEDOT:BTFMSI. From the measurement of the Seebeck coefficient and σ, a PF of 147 μW m−1 K−2 has been deduced, while the measured thermal conductivity is κ = 0.19 W m−1 K−1, resulting in a ZT ∼ 0.22 at room temperature, one of the highest values reported in the literature for polymers. The increase in σ with the change of the counter-ion is mainly due to the stretching of the polymer chains. In this work, we provide a chemical route to further improve ZT in polymers and demonstrate a method of synthesis based on the electro-polymerization on gold. After removing the gold layer, a very thin semiconducting polymer film can be isolated.

Thermoelectric properties of Weyl and Dirac semimetals

Abstract: We study the electronic contribution to the thermal conductivity and the thermopower of Weyl and Dirac semimetals using a semiclassical Boltzmann approach. We investigate the effect of various relaxation processes including disorder and interactions on the thermoelectric properties, and also consider doping away from the Weyl or Dirac point. We find that the thermal conductivity and thermopower have an interesting dependence on the chemical potential that is characteristic of the linear electronic dispersion, and that the electron-electron interactions modify the Lorenz number. For the interacting system, we also use the Kubo formalism to obtain the transport coefficients. We find exact agreement between the Kubo and Boltzmann approaches at high temperatures. We also consider the effect of electric and magnetic fields on the thermal conductivity in various orientations with respect to the temperature gradient. Notably, when the temperature gradient and magnetic field are parallel, we find a large contribution to the longitudinal thermal conductivity that is quadratic in the magnetic field strength, similar to the magnetic field dependence of the longitudinal electrical conductivity due to the presence of the chiral anomaly when no thermal gradient is present.

Transparent and flexible organic semiconductor nanofilms with enhanced thermoelectric efficiency

Abstract: Sequential doping and dedoping increased the conductivity and optimized the oxidation level of transparent and flexible poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonic acid) (PEDOT:PSS) films, resulting in an improvement in the thermoelectric figure of merit ZT. The electrical conductivity (σ) increased from 970 to 1260 S cm−1 and the power factor from 66.5 to 70.7 μW mK−2 at the optimum concentration of the chemical dopant p-toluenesulfonic acid monohydrate (TSA). Then, the doped PEDOT:PSS films were treated with hydrazine/DMSO solutions with different hydrazine concentrations to precisely control the oxidation level. During the hydrazine/DMSO treatment (dedoping), σ of the films continuously decreased from 1647 to 783 S cm−1 due to a decrease in the carrier concentration, whereas the Seebeck coefficient (S) steeply increased from 28 to 49.3 μV K−1 at the optimum oxidation level. A power factor of 318.4 μW mK−2 (σ = 1310 S cm−1, S = 49.3 μV K−1), the highest among all existing thermoelectric nanofilms, was achieved while maintaining polymer film flexibility and transparency (88.3% of optical transmittance). In addition, the thermal conductivity (κ) of the PEDOT:PSS films decreased from 0.38 to 0.30 W mK−1 upon removal of PSS. At the lowest κ value, a high ZT value of 0.31 was achieved at room temperature.

Giant Seebeck coefficient in semiconducting single-wall carbon nanotube film

Abstract: We found a giant Seebeck effect in semiconducting single-wall carbon nanotube (SWCNT) films, which exhibited a performance comparable to that of commercial Bi2Te3 alloys. Carrier doping of semiconducting SWCNT films further improved the thermoelectric performance. These results were reproduced well by first-principles transport simulations based on a simple SWCNT junction model. These findings suggest strategies that pave the way for emerging printed, all-carbon, flexible thermoelectric devices.

Lead-free thermoelectrics: promising thermoelectric performance in p-type SnTe1−xSex system

Abstract: Lead chalcogenides are the best performers for thermoelectric power generation at mid/high temperatures; however, environmental concern about Pb prevents its use in large-scale thermoelectric applications. SnTe, a Pb-free IV–VI narrow band gap semiconductor, has the potential to be a good thermoelectric material due to its crystal structure and valence band characteristics being similar to those of PbTe. Here, we report the promising thermoelectric performance in high quality crystalline ingots of In-doped SnTe1−xSex (x = 0–0.15) synthesized by a simple vacuum sealed tube melting reaction. First, we have optimized the lattice thermal conductivity of SnTe by solid solution alloying with SnSe. Resonance level formation in the valence band through In doping along with the increase in the contribution of the heavy hole valence band through solid solution alloying significantly improved the Seebeck coefficient, resulting in a promising ZT of ∼0.8 at 860 K in the Pb-free p-type 1.5 mol% In doped SnTe0.85Se0.15 sample.

Side-Chain Effects on the Conductivity, Morphology, and Thermoelectric Properties of Self-Doped Narrow-Band-Gap Conjugated Polyelectrolytes

Abstract: This contribution reports a series of anionic narrow-band-gap self-doped conjugated polyelectrolytes (CPEs) with π-conjugated cyclopenta-[2,1-b;3,4-b′]-dithiophene-alt-4,7-(2,1,3-benzothiadiazole) backbones, but with different counterions (Na+, K+, vs tetrabutylammonium) and lengths of alkyl chains (C4 vs C3). These materials were doped to provide air-stable, water-soluble conductive materials. Solid-state electrical conductivity, thermopower, and thermal conductivity were measured and compared. CPEs with smaller counterions and shorter side chains exhibit higher doping levels and form more ordered films. The smallest countercation (Na+) provides thin films with higher electrical conductivity, but a comparable thermopower, compared to those with larger counterions, thereby leading to a higher power factor. Chemical modifications of the pendant side chains do not influence out of plane thermal conductivity. These studies introduce a novel approach to understand thermoelectric performance by structural modi...

Giant Seebeck coefficient in semiconducting single-wall carbon nanotube film

Abstract: We found a giant Seebeck effect in semiconducting single-wall carbon nanotube (SWCNT) films, which exhibited a performance comparable to that of commercial Bi2Te3 alloys. Carrier doping of semiconducting SWCNT films further improved the thermoelectric performance. These results were reproduced well by first-principles transport simulations based on a simple SWCNT junction model. These findings suggest strategies that pave the way for emerging printed, all-carbon, flexible thermoelectric devices.

Elemental tellurium as a chiral p-type thermoelectric material

Abstract: The thermoelectric transport properties of elemental tellurium are investigated by density functional theory combined with the Boltzmann transport equation in the rigid band approximation. We find that the thermoelectric transport properties parallel and perpendicular to the helical chains are highly asymmetric (almost symmetric) for p- (n-) type doped tellurium due to the anisotropic (isotropic) hole (electron) pockets of the Fermi surface. The electronic band structure shows that the lone-pair derived uppermost heavy-hole and extremely light-hole lower valence bands offer the opportunity to obtain both a high Seebeck coefficient and electrical conductivity along the chains through Sb or Bi doping. Furthermore, the stairlike density of states yields a large asymmetry for the transport distribution function relative to the Fermi energy which leads to large thermopower. The calculations reveal that tellurium has the potential to be a good p-type thermoelectric material with an optimum figure of merit zT of 0.31 (0.56) at room temperature (500 K) at a hole concentration around 1×10^19 cm^−3. Exploiting the rich chemistry of lone pairs in chiral solids may have important implications for the discovery of high-zT polychalcogenide-based thermoelectric materials.

Large Thermoelectricity via Variable Range Hopping in Chemical Vapor Deposition Grown Single-layer MoS2

Abstract: Ultrathin layers of semiconducting molybdenum disulfide (MoS2) offer significant prospects in future electronic and optoelectronic applications. Although an increasing number of experiments bring light into the electronic transport properties of these crystals, their thermoelectric properties are much less known. In particular, thermoelectricity in chemical vapor deposition grown MoS2, which is more practical for wafer-scale applications, still remains unexplored. Here, for the first time, we investigate these properties in grown single layer MoS2. Micro-fabricated heaters and thermometers are used to measure both electrical conductivity and thermopower. Large values of up to ~30 mV/K at room temperature are observed, which are much larger than those observed in other two dimensional crystals and bulk MoS2. The thermopower is strongly dependent on temperature and applied gate voltage with a large enhancement at the vicinity of the conduction band edge. We also show that the Seebeck coefficient follows S~T^1/3 suggesting a two-dimensional variable range hopping mechanism in the system, which is consistent with electrical transport measurements. Our results help to understand the physics behind the electrical and thermal transports in MoS2 and the high thermopower value is of interest to future thermoelectronic research and application.

Strong enhancement of phonon scattering through nanoscale grains in lead sulfide thermoelectrics

Abstract: We present nanocrystalline PbS, which was prepared using a solvothermal method followed by spark plasma sintering, as a promising thermoelectric material The effects of grains with different length scales on phonon scattering of PbS samples, and therefore on the thermal conductivity of these samples, were studied using transmission electron microscopy and theoretical calculations We found that a high density of nanoscale grain boundaries dramatically lowered the thermal conductivity by effectively scattering long-wavelength phonons The thermal conductivity at room temperature was reduced from 25 W m−1 K−1 for ingot-PbS (grain size >200 μm) to 23 W m−1 K−1 for micro-PbS (grain size >04 μm); remarkably, thermal conductivity was reduced to 085 W m−1 K−1 for nano-PbS (grain size ∼30 nm) Considering the full phonon spectrum of the material, a theoretical model based on a combination of first-principles calculations and semiempirical phonon scattering rates was proposed to explain this effective enhancement The results show that the high density of nanoscale grains could cause effective phonon scattering of almost 61% These findings shed light on developing high-performance thermoelectrics via nanograins at the intermediate temperature range Thermoelectric materials that transform waste heat generated by equipment or buildings into electricity are emerging as an important green energy technology Currently, researchers are trying to improve thermoelectric substances by embedding within them nanoscale precipitates that allow these materials to capture more heat An international team led by Jiaqing He from the South University of Science and Technology of China has now discovered a way to improve this process by systematically introducing nanoscale crystal structure defects, or ‘nanograins’, into lead sulfide (PbS) particles Their approach tripled the thermoelectric performance of this low-cost mineral from its bulk state without introducing charge-disrupting centers commonly associated with nanoscale precipitates Detailed analysis revealed that the densely packed nanograins trap heat by scattering solid-state vibrations, or phonons, while simultaneously suppressing ‘bipolar’ interactions between charge carriers that can diminish thermoelectric power We report here on the effects of grains of PbS with different length scales on thermal conductivity reduction and bipolar effect ‘suppression’ through macro-properties/microstructure analysis We found that nanograins can achieve the above goals simultaneously Combining experimental results and theoretical calculations, we found that the effect of nanograins on the reduction in lattice thermal conductivity can surpass that of nanoprecipitates Improved properties corresponding to the lowest lattice thermal conductivity in a PbQ (Q=Te, Se, S) system (05 W m K−1 at 923 K) and the highest ZT value in PbQ nanocrystalline materials were achieved by the nanograin method

Fe2MnSixGe1−x: influence thermoelectric properties of varying the germanium content

Abstract: The semi-classical Boltzmann theory, as implemented in the BoltzTraP code, was used to study the influence of varying the germanium content on the thermoelectric properties of the Heusler compounds, Fe2MnSi and Fe2MnGe. The electrical conductivity (σ/τ), the Seebeck coefficient (S), the electronic power factor (S2σ), the electronic thermal conductivity (κe), the electronic heat capacity cel(Tel), and the Hall coefficient (RH), as a function of temperature at certain values of chemical potential (μ) with constant relaxation time (τ), were evaluated on the basis of the calculated band structure using the standard Boltzmann kinetic transport theory and the rigid band approach. The increase/reduction in the electrical conductivity (σ = neμ) of Fe2MnSixGe1−x alloys is attributed to the density of charge carriers (n) and their mobility (μ = eτ/me). The S for Fe2MnGe is negative over the entire temperature range, which represents the n-type concentration. Whereas Fe2MnSi shows a positive S up to 250 K and then drops to negative values, which confirms the existence of the p-type concentration between 100–250 K. Fe2MnSi0.25Ge0.75/Fe2MnSi0.5Ge0.5/Fe2MnSi0.75Ge0.25 possess positive S up to 270/230/320 K and then drop to negative values. The power factor of Fe2MnGe rapidly increases with increasing temperature, while for Fe2MnSi it is zero up to 300 K, and then rapidly increases with increasing temperature. The S2σ of Fe2MnSi0.25Ge0.75 is zero between 250–350 K, whereas Fe2MnSi0.5Ge0.5 possesses a zero S2σ of up to 320 K. Fe2MnSi0.75Ge0.25 has a zero S2σ between 200 and 500 K. The electronic thermal conductivity (κe) and the electronic heat capacity cel(Tel) increases with increasing temperature. The parent compounds (Fe2MnGe and Fe2MnSi) show the highest positive value of the Hall coefficient RH at 100 K, and then drop to negative values at 260 K. On the other hand, the RH for Fe2MnSi0.25Ge0.75, Fe2MnSi0.5Ge0.5 and Fe2MnSi0.75Ge0.25 alloys exhibit negative RH along the temperature scale. The behavior of RH is attributed to the concentration of the charge carriers and their mobility.

Improved thermoelectric performance of hot pressed nanostructured n-type SiGe bulk alloys

Abstract: Silicon germanium alloys (Si80Ge20) have been used in thermoelectric generators for deep space missions to convert radioisotope heat into electricity. This work demonstrates the highest value of thermoelectric figure-of-merit (ZT) ∼1.84 at 1073 K for n-type SiGe nanostructured bulk alloys, which is 34% higher than the reported record value for n-type SiGe alloys. The optimized samples exhibit a Seebeck coefficient of ∼284 μV K−1, resistivity of ∼45 μΩ m and thermal conductivity of ∼0.93 W m−1 K−1 at 1073 K. The main contributing factor for the enhanced ZT is very low and almost temperature independent thermal conductivity, which overcomes the low power factor of the material. Significant reduction of the thermal conductivity is caused by the scattering of low, medium and high wavelength phonons by atomic size defects, dislocations, and grain boundaries that are present due to the formation of nanocrystalline grains in the bulk material.

Thermoelectric transport of Se-rich Ag2Se in normal phases and phase transitions

Abstract: Small amount of Se atoms are used to tune the carrier concentrations (nH) and electrical transport in Ag2Se. Significant enhancements in power factor and thermoelectric figure of merit (zT) are observed in the compositions of Ag2Se1.06 and Ag2Se1.08. The excessive Se atoms do not change the intrinsically electron-conducting character in Ag2Se. The detailed analysis reveals the experiment optimum carrier concentration in Ag2Se is around 5 × 1018 cm−3. We also investigate the temperature of maximum zT and the thermoelectric transport during the first order phase transitions using the recently developed measurement system.

The roles of Na doping in BiCuSeO oxyselenides as a thermoelectric material

Abstract: The thermoelectric properties of the Bi1−xNaxCuSeO (0.0 ≤ x ≤ 0.02) system have been investigated in the temperature range 300–923 K. Na doping significantly increased the carrier concentration to ∼0.92 × 1020 cm−3 at the doping amount of x = 0.02. Furthermore, a relatively high carrier mobility and a slight Seebeck coefficient enhancement was seen, thus resulting in a high power factor of 8.0 μW cm−1 K−2 at room temperature. Coupled with a low thermal conductivity reduced by point defects scattering, this leads to a ZT of 0.91 at 923 K for Bi0.985Na0.015CuSeO which is nearly twice the value observed in pristine BiCuSeO.