Enhancement Of Thermoelectric Properties Research Articles

Complex defect formation in Fe doped γ-CuI: Enhancement of thermoelectric properties via band engineering

The doping effect of Fe atoms on CuI, a potential candidate for room temperature thermoelectric materials, was investigated due to the lack of studies for the aspect of band engineering. The addition of Fe atoms was observed to enhance the p-type performance of CuI, although Fe is more suitable as a donor material. Through Mossbauer spectroscopy and Density Functional Theory (DFT) calculations, it was determined that only about 5 % of the added Fe atoms simply substitute Cu sites, while most Fe atoms occupy interstitial sites, acting as defects and promoting Cu vacancies. Despite the trace amount of Fe only contribute to magnetic properties, it was found that multiple defects and deformation in the electronic band structure caused the effective mass to nearly double. As a result, electrical conductivity increased by an order of magnitude, while the decrease in the Seebeck coefficient was less than half, demonstrating a violation of the Pisarenko relation. Additionally, the incorporation of Fe atoms induced nano structuring behavior, increasing boundary scattering in CuI and significantly reducing thermal conductivity. Fe could be suggested as a suitable dopant for the "phonon-glass-electron-crystal" strategy.

Bacterial Cellulose-Based ionogels with synergistically enhanced mechanical and thermoelectric performances for low-grade heat harvesting

Ionogels have a bright prospect for low-grade heat harvesting due to their advantages of non-volatility and high thermal stability. However, it is still challenging to develop ionogels with mechanical robustness and high thermoelectric properties. This work reports a carboxymethylated and Na+ doped bacterial cellulose-based ionogel (CBCIG-Na) with synergistically increased mechanical and thermoelectric performances. The enhancement of thermoelectric properties of ionogels is mainly attributed to the increase of ion transport pathways and the mobility difference between anions and cations in ionic liquid. After the modification, the CBCIG-Na ionogel with 0.5 mol% Na+ (CBCIG-Na0.5%) exhibits high Seebeck coefficient (45.02 mV K−1), ionic conductivity (21.2 mS cm−1) and ionic power factor (4296.82 μW m−1 K−2) when the relative humidity (RH) is 85 %. This is the highest ionic power factor ever reported for bacterial cellulose-based ionogels. Moreover, the CBCIG-Na0.5% shows high tensile strength (7.69 MPa) due to the larger 3D fiber network and enhanced intermolecular force, obvious adhesivity (9.28 kPa) and high transparence (76 %). After that, a flexible i-TE device containing 5 legs and packaged with polydimethylsiloxane (PDMS) was constructed. A high thermovoltage of 264.36 mV is generated when wearing the i-TE device (5 legs) on the forearm at room temperature (ΔT = 1.2 K). The outstanding performances, environmentally friendly and low-cost characters of CBCIG-Na0.5% ionogel indicating its great potential for low-grade heat harvesting.

Enhancement of Thermoelectric Properties in Nanocomposites Through the Synergistic Integration of Zinc and Iron Oxides with Polyaniline

In this study, we systematically varied the weight ratios of zinc and iron oxides (2 wt%, 4 wt%, and 6 wt%) to fabricate nanocomposites consisting of polyaniline (PANI), zinc oxide (ZnO), and iron oxide (Fe3O4) through the sol–gel method. Comprehensive analyses using FTIR, XRD, and SEM were conducted to elucidate the functional groups, particle size, crystal structure, and surface morphologies of PANI/ZnO/Fe3O4 nanocomposites. Furthermore, thermoelectric characteristics were thoroughly investigated. A mechanistic insight into the PANI/ZnO/Fe3O4 nanocomposite formation was proposed based on the FTIR findings. SEM investigations revealed the presence of spherical particles in all nanocomposites. Among the three PANI/ZnO/Fe3O4 nanocomposites characterized, the formulation with 6 wt% exhibited superior thermoelectric performance. The electrical conductivity of the nanocomposites exhibited a notable increase from 23.1 to 42.7 mS/cm when the temperature elevated from 30–90 °C. Concurrently, the thermal conductivity exhibited a decline from 1.229 to 0.704 Wm−1K−1, resulting in an augmented figure of merit of 0.024. This enhancement underscores the positive influence of increasing the weight percentage of ZnO and Fe3O4 with PANI on the TE performances of the nanocomposites.

Enhancement of thermoelectric properties in Sr0.7Ba0.3Nb2O6−δ-based ceramics via nano-sized Ti as additive

A series of Sr0.7Ba0.3Nb2O6−δ/x wt. % Ti (x = 1, 3, 5, and 10) composite ceramic thermoelectric materials were prepared, and the mechanism for improving their thermoelectric properties was explored. The experimental results demonstrate that nano-additive titanium powder undergoes oxidation to form TiO2 during sintering. However, under annealing in a reducing atmosphere, oxidation reactions further deplete the lattice oxygen, leading to an increased generation of oxygen vacancies and enhanced carrier concentration, ultimately leading to successful resistivity reduction. The samples consistently exhibit low thermal conductivity values below 2.0 W m−1 K−1 due to crystal defects, complex structure, and phonon scattering at the grain boundaries. The sample doped with 5 wt. %. Ti exhibits the lowest resistivity and highest PF value (409.3 μW/m K2 at 1073 K). Consequently, the figure of merit of Sr0.7Ba0.3Nb2O6−δ with 5 wt. % Ti attains its maximum value of 0.30 at 1073 K, representing a 50% increase compared to that of the undoped sample Sr0.7Ba0.3Nb2O6−δ (0.20 at 1073 K).

Nickel Oxide Decorated MWCNTs Wrapped Polypyrrole: One Dimensional Ternary Nanocomposites for Enhanced Thermoelectric Performance

AbstractThis study reports on a customized and revised approach to fabricate “nickel oxide decorated multi‐walled carbon nanotubes (MWCNTs) wrapped polypyrrole (PPy)” nanocomposite with enhanced room‐temperature thermoelectric (TE) properties. The nanocomposite is formed through three steps: MWCNTs functionalization via diazonium salt grafting of 5‐amino‐1,2,3‐benzene tricarboxylic acid; in situ generation on their surfaces of NiO nanoparticles with a homogenous distribution; the chemical polymerization of pyrrole using methyl orange as templating and dopant to wrap the MWCNTs‐(COOH)3‐NiO. Various techniques were used as characterization tools, including XRD, TEM, FTIR, Raman, TGA, XPS, and TE measurements. The PPy‐MWCNTs‐(COOH)3‐NiO nanocomposite exhibits significantly higher Seebeck coefficient, electrical conductivity, and power factor than PPy and PPy‐MWCNTs‐(COOH)3. The achieved enhancement in TE properties (figure of merit, ZTPPy‐MWCNTs‐(COOH)3‐NiO=1.51×10−2) is attributed to the presence of NiO, which acts as a dopant and improves the charge carrier density in the nanocomposite. These results offer the potential for waste heat recovery and large‐scale fabrication of high‐performance composites.

Enhancement of thermoelectric properties in rapidly synthesised β-Cu2Se using optimized Cu content and microwave hybrid heating

To our knowledge, this is the first study to successfully synthesise high-temperature-phase copper selenide (β-Cu2Se) at room temperature using rapid microwave hybrid heating (MHH). Controlling the starting Cu/Se ratio is the critical parameter for adjusting the content of α- and β-phases in the as-synthesised sample. The relatively low Cu composition causes impurities to form in the Cu3Se2 phase, deteriorating the thermoelectric (TE) properties of the Cu2Se material. The β phase formation at room temperature promotes electrical conductivity. The thermal conductivities of the Cu2.0Se samples were 0.5–0.8 Wm−1K−1 at 303–673 K. A strong electronic-phonon interaction may potentially couple electronic thermal conductivity (κe) and lattice thermal conductivity (κL), resulting in incomplete separability of κL and κe in the β-Cu2.0Se sample. The Cu2.0Se exhibited a ZT value of 0.65 at 523 K because of its considerably lowered thermal conductivity.

Enhancement of electronic and thermoelectric properties of armchair bilayer graphene nanoribbons by chemical derivation and strain

Because of decreasing lattice thermal conductivity of GNR by chemical derivation and strain, enhancement of the thermoelectric properties of electron part in the systems can be one of important methods in approach “phonon-glass electron-crystal” (PGEC). In this paper, we have suggested the synthesized hybrid systems formed by chemical derivation into middle parts of b-AGNRs, and then investigated the band structures and thermoelectric properties of electron part as function of out-layer strain and elastic strain along ribbon direction. The calculated results reveal that all hybrid systems are stable structurally and the band gaps of the systems increase remarkably from 0.04 to 1.59 eV under different strains. When the strain along periodical direction change 4 % and 12 %, the thermoelectric characteristics are noticeably enhanced than other strains. Especially, the systems formed by graphane exhibit the electronic figure of merit (ZTe) of 0.95. In the case of hc < h0, the dependence of the thermoelectric properties of electron part in hybrid b-AGNRs on the out-layer strain is insignificant.

Electrical Conductance and Thermopower of β-Substituted Porphyrin Molecular Junctions─Synthesis and Transport.

Molecular junctions offer significant potential for enhancing thermoelectric power generation. Quantum interference effects and associated sharp features in electron transmission are expected to enable the tuning and enhancement of thermoelectric properties in molecular junctions. To systematically explore the effect of quantum interferences, we designed and synthesized two new classes of porphyrins, P1 and P2, with two methylthio anchoring groups in the 2,13- and 2,12-positions, respectively, and their Zn complexes, Zn-P1 and Zn-P2. Past theory suggests that P1 and Zn-P1 feature destructive quantum interference in single-molecule junctions with gold electrodes and may thus show high thermopower, while P2 and Zn-P2 do not. Our detailed experimental single-molecule break-junction studies of conductance and thermopower, the latter being the first ever performed on porphyrin molecular junctions, revealed that the electrical conductance of the P1 and Zn-P1 junctions is relatively close, and the same holds for P2 and Zn-P2, while there is a 6 times reduction in the electrical conductance between P1 and P2 type junctions. Further, we observed that the thermopower of P1 junctions is slightly larger than for P2 junctions, while Zn-P1 junctions show the largest thermopower and Zn-P2 junctions show the lowest. We relate the experimental results to quantum transport theory using first-principles approaches. While the conductance of P1 and Zn-P1 junctions is robustly predicted to be larger than those of P2 and Zn-P2, computed thermopowers depend sensitively on the level of theory and the single-molecule junction geometry. However, the predicted large difference in conductance and thermopower values between Zn-P1 and Zn-P2 derivatives, suggested in previous model calculations, is not supported by our experimental and theoretical findings.

Towards an efficient p-n heterojunction in ternary composite material based polypyrrole, metal oxide and surface functionalized graphene for thermoelectric properties enhancement

The thermoelectric properties of ternary p-n heterojunction composites based on [p-type polymer/surface treated graphene/metal oxide in ceramic form] were investigated in this study. In the presence of ceramic tungsten oxide (WO3) particles and diazonium salt surface treated graphene, a simple in-situ pyrrole polymerization method was carried out. FTIR, Raman scattering, X-ray diffraction analysis (XRD), diffuse reflectance UV-Vis spectroscopy, I-V characteristics, and scanning electron microscopy - energy dispersive spectrometer (SEM-EDS) have all been used to characterize composites. Based on the comparison of observed thermoelectric parameters, the ternary composite created by PPy-FrGO (DS-SO3H)-WO3 based on functionalized graphene with diazonium salt (DS-SO3H) was chosen as having superior thermoelectric capabilities than the other samples. This advancement was made possible by combining two strategies: graphene functionalization with a conjugated diazonium salt and thermochemical treatment of tungsten oxide with the sol-gel method and sintering procedure. Functionalization reduced thermal conductivity (= 0.127 W.m−1. K−1) while adding ceramic tungsten oxide boosted electrical conductivity ( = 1087 S/m), resulting in a merit coefficient (ZT) of 1.63 × 10−4.

Fabrication of nanostructured non-stoichiometric TiO2-x by spark plasma sintering for enhancing its thermoelectric properties

In this study, nanostructured non-stoichiometric TiO2-x compacts were prepared by the in-situ reduction of rutile titanium oxide (TiO2) powder with urea powder via spark plasma sintering (SPS). The crystal structure and particle size of the prepared compacts were examined. The XRD patterns revealed that TiO2 could be reduced easily by the urea powder to obtain non-stoichiometric TiO2-x, and the compacts still possessed rutile crystal structures. The average particle sizes of the compacts were less than 250 nm, successfully obtaining the non-stoichiometric TiO2-x with uniform nanostructures at the sintering temperature of 1073 K. In addition, nanostructured TiO2-x compacts with Magnéli phase TinO2n-1 (n = 2, 4, 8) were fabricated by varying the volume fraction of Ti powder in a urea environment via SPS. The results suggested that addition of Ti powder contributed to the formation of Magnéli phases TinO2n-1, and the value of n decreased with an increase in the volume fraction of the Ti powder. Furthermore, the thermoelectric properties of the compacts sintered with and without Ti powder were both investigated. The TiO2–U13.3-Ti10 compact displayed the highest power factor of 5.04 μWcm−1K−2 at 973 K. A lower thermal conductivity was achieved by TiO2–U13.3-Ti10 compact in the temperature range of 373–973 K, approximately 3 Wm−1K−1, due to the nanostructures and Magnéli phases. The highest ZT value of 0.146 was obtained for the TiO2–U13.3-Ti10 compact at 973 K, achieving a reasonable enhancement of thermoelectric properties.

Enhancement of thermoelectric properties of transition metals, nickel and copper dually doped ZnO

Thermoelectricity is one of the potential sustainable energy sources, that can be treated as a remedy for the present energy scarcity as it is a clean-green technology. ZnO is a promising multifunctional metal oxide and its thermoelectric conversion property is highly exploited in the field of waste heat recovery. Herein, the impact of nickel and copper substitution on the thermoelectric properties of ZnO is investigated. The structural and morphological analysis of the doped materials confirmed the hexagonal wurtzite structure of ZnO. The strain induced in the lattice and the variation of crystallite size established the substitution of the dopants into the host matrix. The optical band gap energy shows a reduction from 3.1 eV to 2.15 eV with a redshift in the absorption edge. The electrical conductivity and Seebeck coefficient of ZnO increased upon doping. The most remarkable result of nickel and copper doping is the enhancement of the Seebeck coefficient to 1318 µV/K. As a result of the lattice distortion created due to the dual substitution of the dopants, the thermal conductivity is considerably reduced to 8.36 W/mK. The power factor enhancement and degradation of thermal conductivity give rise to a six-fold increase in the thermoelectric figure of merit of the material.

Enhanced thermoelectric properties for p-type BiSbTe by incorporating multiferroic BiFeO3 nanoparticles

Thermoelectric materials can directly convert electrical energy and thermal energy by regulating carrier and phonon transport under electrical and thermal fields. However, further enhancement of thermoelectric properties is hindered by limited available methods that can synergistically tune transport properties. Here, multiferroic BiFeO3 nanoparticles with spontaneous electrical and magnetic polarization are synthesized and incorporated into p-type Bi0.5Sb1.5Te3, to probe the impact of simultaneous local electrical field and magnetic field on the transport properties of the BiFeO3/Bi0.5Sb1.5Te3 nanocomposites. The BiFeO3/Bi0.5Sb1.5Te3 atomic-resolution interfacial structure is revealed using aberration-corrected scanning transmission electron microscopy. The selective reaction of Sb cations leads to reduced carrier concentration and decreased electrical conductivity. The Seebeck coefficient is enhanced by additional scattering from local electrical and magnetic field. Phonon scattering is boosted by the nanostructures from interfacial reaction and local lattice distortion caused by the polarization. As a result, the ZT values and the cooling performance are improved by 9% and 31% for the nanocomposites, respectively.

Enormous enhancement of thermoelectric properties via piezo-gating effect

The thermoelectric effect is one of the most promising techniques to generate electricity from waste heat energy. The commonly adopted approaches to enhance the thermoelectric performance limit the power factor because of the contradictory behavior between the Seebeck coefficient and electrical conductivity. This hurdle can be overcome via piezo-gating effect, which can modify the electronic band structure of active materials through strain. In this study, we developed piezo-gated flexible transistor (PGFT) comprised of ZnO thin-film (TFPGFT) and ZnO nanowires (NWPGFT) on polyethylene terephthalate (PET) as a flexible substrate and demonstrate the simultaneous enhancement of the Seebeck coefficient and conductivity by incorporating piezo-gating effect. The TFPGFT shows a gauge factor of ∼115, much higher than NWPGFTs. Further, the TFPGFT shows a higher Seebeck coefficient enhancement ∼12 times (∼0.13 to ∼1.76 µV/K) under strain resulting from the dominating piezo-gating effect. Therefore, the synergistic piezo-gating and thermoelectric effect dramatically boosts the power factor of TFPGFT by approximately 400 times (from ∼0.36 mW/K2 to ∼140.3 mW/K2), which can improve the performance of thermoelectric devices to a new dimension. Finally, we propose a self-powered single-dimension strain sensor based on thermoelectric power, reaching a gauge factor of ∼16.

Ab initio study of uniaxial strain-induced thermoelectric property tuning of individual single-wall carbon nanotubes

Tunability and enhancement of thermoelectric properties of single-wall carbon nanotubes (SWCNTs) were acheived through uniaxial strain.

Synergistic Optimization of Thermoelectric Performance in SnSe2 through co-doping: Anionic Vacancy Formation and Band Engineering

SnSe2 is a layered crystal structure material that is abundant in the Earth's crust and considered non-toxic. However, its thermoelectric properties are anisotropic due to the differences in its interlayer and intralayer electrical and thermal transport properties. The intrinsically poor thermoelectric performance of SnSe2 can be attributed to its lower electrical transport properties in its pristine condition. To address this, we developed a method involving combined mechanical alloying (MA) and spark plasma sintering (SPS) to synthesize n-type Sn-rich Cu-Br co-doped SnSe2 polycrystals. Optimization of Sn enrichment facilitated superior material selection for subsequent doping. The resulting substitutional doping induced a substantial rise in carrier concentration, leading to improved thermoelectric performance. Notably, the power factor displayed a significant increase, reaching approximately 795 µWm-1K-2 at 765 K through Cu-Br co-doping. Furthermore, density functional theory (DFT) analysis elucidated a reduced bandgap and increased degeneracy within the electronic band structure and density of states, affirming the enhancement of thermoelectric properties in Cu-Br co-doped Sn-rich SnSe2 polycrystals. Finally, a maximum figure of merit (ZT) value of 0.46 was achieved at 765 K for the Sn0.985Cu0.015Se1.92Br0.03 sample, perpendicular to the SPS pressing direction, which was nearly threefold higher than the pure SnSe1.95. This compelling outcome highlights the improved thermoelectric performance of the co-doped SnSe2 polycrystals.

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.