Enhancement Of Thermoelectric Properties Research Articles

Enhancement of thermoelectric properties of CuFeS2 through formation of spinel-type microprecipitates

Efforts to substitute chromium into chalcopyrite, CuFeS2, lead to preferential formation of chromium-rich spinel-type microprecipitates, which alter the transport properties and increase zT by a factor of three.

Synergistically Optimized Carrier and Phonon Transport Properties in Bi-Cu2S Coalloyed GeTe.

GeTe is an emerging lead-free thermoelectric material, but its excessive carrier concentration and high thermal conductivity severely restrict the enhancement of thermoelectric properties. In this study, the synergistically optimized thermoelectric properties of p-type GeTe through Bi-Cu2S coalloying are reported. It can be found that the donor behavior of Bi and the substitution-interstitial defect pairs of Cu+ ions effectively reduce the hole concentration to an optimal level with carrier mobility less affected. At the same time, Bi-Cu2S coalloying induces many phonon scattering centers involving stacking faults, nanoprecipitations, grain boundaries and tetrahedral dislocations and suppresses the lattice thermal conductivity to 0.64 W m-1 K-1. Consequently, all effects synergistically yield a peak ZT of 1.9 at 770 K with a theoretical conversion efficiency of 14.5% (300-770 K) in the (Ge0.94Bi0.06Te)0.988(Cu2S)0.012 sample, which is very promising for mid-low temperature range waste heat harvest.

Grain boundary engineered, multilayer graphene incorporated LaCoO3 composites with enhanced thermoelectric properties

Enhancement of thermoelectric properties by virtue of decreased electrical resistance through grain boundary engineering is realised in this study. A robust strategy of optimisation of the transport properties by tuning the energy filtering effects at the interfaces by decreasing the interfacial electrical resistance is achieved in LaCoO 3 (LCO). This is accomplished by the incorporation of multilayer graphene within the parent LCO matrix containing multi-scale nano/micro grains. The present work has attained a substantial increment in electrical conductivity from a value of 96 Scm -1 for bare LCO to ∼5300 Scm -1 at 750 K by incorporating 0.08 wt% multilayer graphene in LCO. No significant change in thermal conductivity is observed due to the presence of multilayer graphene in LCO. A zT of 0.33 at 550 K for 0.08 wt% multi-layer graphene incorporated LCO composite is achieved which is the highest thermoelectric figure of merit value for undoped LCO reported until now.

Enhancement of thermoelectric properties at room temperature through isoelectronic ruthenium-substitution in [formula omitted]-type half-Heusler VFeSb compounds

The effects of ruthenium (Ru) substitution on the thermoelectric properties of half-Heusler (HH) V(Fe1−xRux)Sb compounds have been theoretically and experimentally investigated. All V(Fe1−xRux)Sb samples with deficient HH structures reveal deficiencies in V 4a and Fe 4c sites, interstitial Fe occupied 4d sites, and Ru substituted for Fe 4c sites, by Rietveld analysis. Based on the refined crystal structures and electronic structure analysis, the Ru content x with a maximum power factor (PF) is predicted to be around x=0.02. A small amount of Ru-substitution improves the PF by 26% at room temperature for the V(Fe0.98Ru0.02)Sb sample, which is coincident with the predicted range of Ru content for maximum PF. The lattice thermal conductivity κL of the V(Fe1−xRux)Sb samples decreases with an increase in x. The atomic mass and radius of Ru are substantially larger than those of Fe; therefore, κL of the V(Fe1−xRux)Sb samples decreases mainly due to phonon scattering strengthened by the introduction of Ru at the Fe 4c sites. A significantly reduced κL of 5.8 W/Km at room temperature is obtained for the V(Fe0.92Ru0.08)Sb sample, which is a ca. 40% decrease compared with the pristine HH-VFeSb sample. The dimensionless thermoelectric figure-of-merit reaches close to 0.2 at 300 K for the V(Fe0.98Ru0.02)Sb sample.

Large enhancement of thermoelectric properties of CoSb3 tuned by uniaxial strain

Skutterudite (CoSb3), as a promising thermoelectric material for medium-temperature applications, has attracted considerable interest. However, improving its thermoelectric properties is still an important problem. In this work, first-principles calculations combined with semiclassical Boltzmann theory are adopted to investigate the electronic transport and thermoelectric properties of CoSb3 under external strain. It is found that both tensile strain and compressive strain reduce the bandgap of the CoSb3 system and influence the Co_d orbitals near the Fermi level. Further, the steepness of the valence band near the Fermi level along the M–G and G–R directions are changed by external strain to different extents, which directly affects the Seebeck coefficient and the ratio of conductivity to relaxation time; both of these values are optimized under tensile and compressive strain. Therefore, the power factor of CoSb3 is greatly enhanced under external strain. At 700 K, the maximum power factors under strains of − 1% and 3% are 23.77% and 47.63% higher than those without strain, respectively. Thus, the thermoelectric properties of CoSb3 can be greatly improved by external strain.

Thermoelectric performance of hydrothermally synthesized micro/nano Cu2-xS

Among the state-of-the-art thermoelectric materials, copper sulfides (Cu2S) have been predicted as promising thermoelectric materials due to their low intrinsic thermal conductivity. However, they still confront the problems with lower electrical properties, which distinctly restrict their thermoelectric application. Significant enhancement of thermoelectric properties is a great challenge owing to the common interdependence of electrical and thermal conductivity. Herein, a micro/nano Cu2-xS composite, with significantly enhanced thermoelectric properties, is prepared via a simple hydrothermal method. Due to the synergistic effect of the introduced nanostructure and the increased Cu1.96S contents, the micro/nano Cu2-xS bulk samples present a power factor of 10.1 μW cm−1 K−2 at 773 K. Meanwhile, a decrease of the thermal conductivity to 0.69 W m−1 K−1 is obtained, originating from the strong phonon scattering of micro/nano structure. Remarkably, a ZTmax value of 1.1 at 773 K is obtained, which is higher than the reported Cu2-xS thermoelectric material using chemical methods. This study proposes a facile microstructure engineering strategy for the development of high-performance thermoelectric materials.

Wet-spun PEDOT:PSS/CNT composite fibers for wearable thermoelectric energy harvesting

Fiber-based thermoelectric materials have received increased attention as they are light weight and can be sewn and woven for portable and wearable thermoelectric generators. In this work, Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)/single-walled carbon nanotube (SWCNT) composite fibers were fabricated by a continuous wet spinning process using concentrated sulfuric acid as a coagulation bath, which could effectively remove the excess PSS and result in improved electric conductivity. With a SWCNT content of 50 wt%, the PEDOT:PSS/SWCNT composite fibers show a high electric conductivity of 2982 S cm−1 and a Seebeck coefficient of 27.1 μV K−1, leading to a promising power factor of 219 μW m−1 K−2. By further treated with sodium hydroxide aqueous solution, the PEDOT:PSS/SWCNT fibers exhibit an enhanced Seebeck coefficient of 36.4 μV K−1 and a power factor of 280 μW m−1 K−2, which is higher than most PEDOT:PSS-based thermoelectric fibers. The mechanism of the enhancement of thermoelectric properties for the composite fibers was discussed. A flexible thermoelectric generator with five pairs of PEDOT:PSS/SWCNT fibers and copper wires were assembled and the electrical power generation capability were evaluated, demonstrating its potential application in wearable electronic devices.

Enhancement in thermoelectric properties of ZrNiSn-based alloys by Ta doping and Hf substitution

Though half-Heusler alloys are listed among the mid-high temperature thermoelectric materials with excellent thermoelectric and mechanical properties, their thermal conductivities remain high and limit their practical applications. In this work, the thermal conductivities and electrical conductivities of ZrNiSn-based alloys were synergistically optimized by Ta doping and Hf substitution. The samples were fabricated using levitation melting combined with spark plasma sintering. When compared with the pristine ZrNiSn, the electrical conductivity in Zr0.68Hf0.3Ta0.02NiSn increases from 7.3 × 104 to 12.9 × 104 S·m−1, and the lattice thermal conductivity declines from 4.1 to 2.7 W·m−1·K−1 at 923 K. As a result, an enhanced ZT of 0.94 for Zr0.68Hf0.3Ta0.02NiSn is achieved, which is a nearly 56% enhancement over pristine ZrNiSn. Furthermore, the microhardness, Young's modulus, and shear modulus in Zr1-x-yHfxTayNiSn (x = 0, 0.2, 0.3, 0.4; y = 0, 0.02) alloys are substantially improved upon increasing the Ta and/or Hf content, far exceeding those in other conventional thermoelectric materials.

Enhancement of thermoelectric property of carbon nanotubes by p-type doping with judicious molecular design of spiro-bifluorene derivatives

Although thermoelectric composites of carbon nanotubes (CNTs) with organic polymers or small organic molecules have witnessed explosive achievements, judicious molecular design still remains a great challenge. Here, p-type doping using spiro-bifluorene derivatives is proposed to significantly improve single-walled CNT (SWCNT) thermoelectric performance. The composite shows an increase of ∼108% in room-temperature power factor relative to the pristine SWCNT (112.6 ± 4.3 μW m-1 K-2), with the maximum of 239.2 ± 4.2 μW m-1 K-2. The corresponding mechanism of effective p-type doping and strong interfacial interaction are elucidated by calculated energy levels, SWCNT surface coating and photophysical spectroscopic measurements. Finally, the thermoelectric device connected in series display an open-circuit voltage and an output power of 9.77 mV and 57.3 nW, respectively at a temperature gradient of 30 K. The results will benefit the exploitation of novel CNT-based TE composites and their applications.

Enhancement of thermoelectric properties of low-toxic and earth-abundant copper selenide thermoelectric material by microwave annealing

Our current work aims to enhance the thermoelectric properties of p-type copper selenide (Cu2Se), a low toxic, earth-abundant intermediate material for thermoelectric applications by a low-cost, simple and fast microwave-assisted metallurgy route. Hot pressed copper selenide pellets were treated at various temperatures in the range of 250 oC to 425 oC in a microwave furnace. We analyze the variation of several characteristics of the samples, such as electrical, thermal, structural, microscopic, dielectric, and mechanical properties, when the samples are exposed to different annealing conditions. Broadband dielectric spectroscopy (BDS) analysis facilitated the comprehensive study of variation of AC electrical characteristics of the samples such as AC conductivity, dielectric permittivity storage, dielectric loss, and AC capacitance, with temperature and frequency. The results indicate exceptional electrical, thermal, and mechanical properties for samples annealed at 375 oC with high room temperature electrical conductivity of 538.3 S/cm and ultra-low thermal conductivity of 0.59 W/mK. The crystallinity improved as the annealing temperature increased, as did the formation of oxides over 400 oC. The samples have a high strength with maximum nanohardnesss and compression strength of 3.75 GPa and 326.75 MPa, respectively that has been a significant improvement over the existing studies on copper selenides.

Enhancement of thermoelectric properties and first-principles calculations of Ag / Ti co-substituted Ca3Co4O9

Thermoelectric materials of Ca3Co4O9 with excellent comprehensive performance have stagnated the research of thermoelectric conversion devices due to their low thermoelectric conversion efficiency. Ca3-xAgxCo4-yTiyO9 was synthesized by double substitution at Ca and Co sites via a traditional solid-state method to change the microstructure and improve the thermoelectric properties in this paper. Ag and Ti co-substituted increase the porosity, reduce the concentration of Co4+ ions, and improve the Seebeck coefficient and power factor. The optimum substitute amount is x = 0.2, y = 0.2, and the Seebeck coefficient and power factor are 238.4 μV/K and 1481.9 μW/K2m. The first-principles calculation results show that the addition of Ti increases the covalent bond strength between Co–O atoms, and the addition of Ag narrows the gap, which is beneficial to the improvement of thermoelectric properties.

Thermoelectric Enhancement in Single Organic Radical Molecules.

Organic thermoelectric materials have potential for wearable heating, cooling, and energy generation devices at room temperature. For this to be technologically viable, high-conductance (G) and high-Seebeck-coefficient (S) materials are needed. For most semiconductors, the increase in S is accompanied by a decrease in G. Here, using a combined experimental and theoretical investigation, we demonstrate that a simultaneous enhancement of S and G can be achieved in single organic radical molecules, thanks to their intrinsic spin state. A counterintuitive quantum interference (QI) effect is also observed in stable Blatter radical molecules, where constructive QI occurs for a meta-connected radical, leading to further enhancement of thermoelectric properties. Compared to an analogous closed-shell molecule, the power factor is enhanced by more than 1 order of magnitude in radicals. These results open a new avenue for the development of organic thermoelectric materials operating at room temperature.

Sulfurization temperature induced enhancement in thermoelectric properties of polycrystalline WS2 nanomaterials

We have presented an interesting study about the evolution of vibrational modes in WS2 nanostructures and their effect on material thermoelectric performance. High purity polycrystalline nanostructures of WS2 were prepared by sulfurizing the hydrothermally prepared WS2 powders at different temperatures. The effect of sulfurization temperature on the structure of WS2 nanomaterials was probed by XRD and Raman Spectroscopy. XRD data confirmed the crystal structure of WS2 along with the presence of WO3 based secondary phases. An experimental investigation of the phase changes with sulfurization temperature has been made by Raman scattering methods. Both in Plane (E12g) and out of plane (A1g) Raman modes exhibited red shift by 45 and 11 cm−1 respectively and variation in Raman peak intensity with sulfurization temperature was also observed. For all samples, the room temperature thermoelectric measurements reveal that an increase in sulfurization temperature from 300 to 823 k resulted in the enhancement of electrical conductivity and the Seebeck coefficient. The observed thermoelectric data was linked with the structural changes occurred during the increase of sulfurization temperature. Power factor of WS2 nanostructures increase up to 5.09x 10−3 Wm−1K−2 with increasing sulfurizing temperature.

3D cellulose fiber networks modified by PEDOT:PSS/graphene nanoplatelets for thermoelectric applications

Organic materials have attracted considerable attention for thermoelectric (TE) applications. Given their potential as wearable power generators, there is an urgent need to develop organic TE materials that possess superior electronic properties as well as excellent mechanical and environmental stability. Here, we develop paper-based TE materials using the poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS), graphene nanoplatelets (GNPs), and a starch-based biopolymer as a binder for GNPs. The device fabrication consists of spraying the biopolymer/GNP ink onto the cellulose paper followed by spraying the PEDOT:PSS solution. Further enhancement of TE properties was obtained by adding an ionic liquid (IL), bis(trifluoromethane)sulfonimide lithium salt to the PEDOT:PSS solution. Upon addition of the IL, the electrical conductivity of as-fabricated PEDOT:PSS films increased nearly two orders of magnitude. The electrical conductivity increases with GNPs' content due to formation of an effective electrical percolation network. Interestingly, incorporating GNPs simultaneously improves the Seebeck coefficient. Raman measurements suggest that the concurrent enhancement of the Seebeck coefficient and electrical conductivity might be related to the chemical bonding between the conducting polymer chains and the filler. In addition, these composites display remarkable flexibility at various bending angles and environmental stability without losing their original conductivity after three months of exposure to ambient conditions.

Impact of impurities and time on thermoelectric performance of MgAgSb

In this work the thermoelectric performance of MgAgSb was improved by enhancing electrical conductivity, Seebeck coefficient and reducing thermal conductivity simultaneously. We synthesized MgAgSb at various sintering temperatures and researched its thermoelectric properties in detail. The effect of impurities associated with MgAgSb on the thermoelectric properties shows that their presence can deteriorate the performance. The resultant samples were tested after 2, 3 and 8 months from its preparation for thermoelectric performance and a significant enhancement in thermoelectric properties are observed. The resultant power factor improved from 8.5 μWcm-1K-2to 15.8 μWcm-1K-2 and the thermal conductivity is reduced from 1.12 Wm-1K-1 to 0.86Wm-1K-1 and we find an improved ZT~0.91. The internal mechanism for the reducing thermal conductivity is presented which plays an important role in improving ZT value of MgAgSb.Furthermore, we discussed that the strength of the chemical bonding among the atoms may also be reduced with the passage of time in thermoelectric materials and resulting in decrease in localized frequency of vibration, which has an ultimate effect in reducing the thermal conductivity of the material.

Significant enhancement in thermoelectric properties of half-Heusler compound TiNiSn by grain boundary engineering

Half-Heusler compounds MNiSn (M = Ti, Zr, Hf) have attracted increasing attentions in the field of thermoelectrics, since their constituent elements are non-toxic and abundant. Meanwhile, it possesses good thermal stability as well as high mechanical strength. Herein, a substantial enhancement to the thermoelectric performance of intrinsic TiNiSn is achieved by passing the pulverized powders through the sieve prior to sintering. The Seebeck coefficient has increased sharply owing to the energy filtering effect, which is induced by the reduction in the average grain sizes, combined with a slightly decreased electrical conductivity, leading to a significant improvement in power factor. Moreover, the lattice thermal conductivity can be further reduced via the enhancement of the grain boundary scattering. Consequently, a maximum figure-of-merit (zT) value of 0.84 at 820 K is achieved for un-doped TiNiSn. This work indicates that the grain boundary engineering may serve as an effective method for improving the thermoelectric performance of other half-Heusler compounds.

Enhancement of thermoelectric properties of Mg2Sn single crystals via lattice-defect engineering

Enhancement of thermoelectric properties of Mg₂Sn single crystals via lattice-defect engineering

Enhancement in thermoelectric properties of n-type (La0.7Sr0.3MnO3)0.5.(NiO)0.5: composite and nano-structure effect

In the enhancement of thermoelectric properties of material for energy harvesting applications, the role of nano-composite made of two different materials plays a very crucial role. When materials with different crystal and electronic structure are synthesized in nano-crystalline phase to form a composite, they provide large power factor via energy filtering effect, whereas the lattice mismatching and grain boundaries of two phase scatter the phonons having wide range of frequencies, and effectively maintain the low thermal conductivity. In the present work, we have reported the structural and thermoelectric properties of (La0.7Sr0.3MnO3)0.5(NiO)0.5 composite synthesized by sol-gel method. Rietveld refinement of the XRD pattern confirms the presence of both phases. Thermoelectric properties are measured in 300–600 K temperature range. The magnitude of Seebeck coefficients, electrical resistivity, and thermal conductivity found to be monotonically increase with temperature upto the ∼450 K, which decrease gradually with further increase in temperature. Such variation in transport properties are resulted from the combine effect from the composite materials formed with semiconducting/metallic, and insulating electronic structure mixed in a 50:50 weight ratio. The calculated ‘figure-of-merit’ (ZT) shows an increasing trend with temperature. The value of ZT is found to be ∼0.05 at 600 K, which is nearly three times larger than that of La0.7Sr0.3MnO3 (ZT = 0.013) reported earlier. Our strategy of making nano-composite two materials with different electronic structures enhanced the ZT by ∼285%. The improvement in thermoelectric properties of n-type oxide materials can be useful to consider this material for applications in mid-temperature range.

Enhancement of Thermoelectric Properties in n-type NbCoSn Half-Heusler Compounds via Ta Alloying

Enhancement of Thermoelectric Properties in n-type NbCoSn Half-Heusler Compounds via Ta Alloying

Enhancement of thermoelectric properties of D‐A conjugated polymer through constructing random copolymers with more electronic donors

AbstractDeveloping new D‐A conjugated polymer system for thermoelectric (TE) application is highly desirable. Herein, a series of random copolymers by incorporating 3,4‐ethylenedioxythiophene (EDOT) electron rich units into a diketopyrrolopyrrole (DPP) D‐A conjugated polymer were designed and synthesized. Compared to the alternating conjugated copolymer PDPP‐3T, the HOMO level of the random copolymers are increased as part of the electron deficiency acceptor DPP units in the polymer chain were superseded by electron rich EDOT, which could contribute to effective p‐doping. Moreover, through incorporating EDOT to construct random copolymers, it can also induce an orientation change from face‐on dominated to edge‐on dominated orientation as well as enhance the packing of copolymer chains, which is beneficial to the charge transport. Under same doping condition, the electrical conductivities of the doped polymers increase and the Seebeck coefficient decrease as the increasing of EDOT content, resulting in an optimized power factor of 6.4 μW m−1 K−2 for the random polymer with EDOT content of 40% which is four times higher than that of alternating conjugated copolymer PDPP‐3T. These results demonstrated that constructing random copolymers by incorporating more electronic donors into D‐A conjugated polymers may be a promising strategy for developing TE conjugated polymers.