Increase In Seebeck Coefficient Research Articles

Enhanced thermoelectric properties of layered anisotropic PbBi2Te4 through the suppression of bipolar diffusion effect via Se alloying

The layered ternary PbBi2Te4 is regarded as a promising thermoelectric material due to its intricate crystal structure. However, current research on PbBi2Te4 primarily focuses on theoretical calculations and predictions, with limited investigations conducted on its actual thermoelectric transport properties. In this study, we synthesized the single-phase PbBi2Te4 compound using the high-temperature melting method. The research findings indicate that PbBi2Te4 exhibits significant anisotropic thermoelectric properties and a pronounced bipolar diffusion effect, which can be attributed to its unique layered crystal structure and relatively narrow bandgap. This bipolar diffusion effect remarkably exacerbates the Seebeck coefficient of PbBi2Te4 and amplifies its thermal conductivity. To address these challenges, we effectively manipulated the electronic band structure of PbBi2Te4 and enlarged the bandgap through the incorporation of Se alloying. The thermoelectric performance of PbBi2Te4 has been significantly enhanced as a result of the increase in Seebeck coefficient and reduction in thermal conductivity. Ultimately, a superior ZT value of ∼0.52 at 673 K is achieved in PbBi2(Te0.85Se0.15)4, representing an impressive improvement of ∼ 49% compared to the pure PbBi2Te4. These results demonstrate the promising potential of ternary PbBi2Te4 as a thermoelectric material.

Thermopower in Underpotential Deposition-Based Molecular Junctions.

Underpotential deposition (UPD) is an intriguing means for tailoring the interfacial electronic structure of an adsorbate at a substrate. Here we investigate the impact of UPD on thermoelectricity occurring in molecular tunnel junctions based on alkyl self-assembled monolayers (SAMs). We observed noticeable enhancements in the Seebeck coefficient of alkanoic acid and alkanethiol monolayers, by up to 2- and 4-fold, respectively, upon replacement of a conventional Au electrode with an analogous bimetallic electrode, Cu UPD on Au. Quantum transport calculations indicated that the increased Seebeck coefficients are due to the UPD-induced changes in the shape or position of transmission resonances corresponding to gateway orbitals, which depend on the choice of the anchor group. Our work unveils UPD as a potent means for altering the shape of the tunneling energy barrier at the molecule-electrode contact of alkyl SAM-based junctions and hence enhancing thermoelectric performance.

Electrically Programmed Doping Gradients Optimize the Thermoelectric Power Factor of a Conjugated Polymer

AbstractFunctionally graded materials (FGMs) are widely explored in the context of inorganic thermoelectrics, but not yet in organic thermoelectrics. Here, the impact of doping gradients on the thermoelectric properties of a chemically doped conjugated polymer is studied. The in‐plane drift of counterions in moderate electric fields is used to create lateral doping gradients in films composed of a polythiophene with oligoether side chains, doped with 2,3,5,6‐tetrafluoro‐tetracyanoquinodimethane (F4TCNQ). Raman microscopy reveals that a bias voltage of as little as 5 V across a 50 µm wide channel is sufficient to trigger counterion drift, resulting in doping gradients. The effective electrical conductivity of the graded channel decreases with bias voltage, while an overall increase in Seebeck coefficient is observed, yielding an up to eight‐fold enhancement in power factor. Kinetic Monte Carlo simulations of graded films explain the increase in power factor in terms of a roll‐off of the Seebeck coefficient at high electrical conductivities in combination with a mobility decay due to increased Coulomb scattering at high dopant concentrations. Therefore, the FGM concept is found to be a way to improve the thermoelectric performance of not yet optimally doped organic semiconductors, which may ease the screening of new materials as well as the fabrication of devices.

Enabling Oxidation Protection and Carrier-Type Switching for Bismuth Telluride Nanoribbons via in Situ Organic Molecule Coating.

Thermoelectric materials with high electrical conductivity and low thermal conductivity (e.g., Bi2Te3) can efficiently convert waste heat into electricity; however, in spite of favorable theoretical predictions, individual Bi2Te3 nanostructures tend to perform less efficiently than bulk Bi2Te3. We report a greater-than-order-of-magnitude enhancement in the thermoelectric properties of suspended Bi2Te3 nanoribbons, coated in situ to form a Bi2Te3/F4-TCNQ core-shell nanoribbon without oxidizing the core-shell interface. The shell serves as an oxidation barrier but also directly functions as a strong electron acceptor and p-type carrier donor, switching the majority carriers from a dominant n-type carrier concentration (∼1021 cm-3) to a dominant p-type carrier concentration (∼1020 cm-3). Compared to uncoated Bi2Te3 nanoribbons, our Bi2Te3/F4-TCNQ core-shell nanoribbon demonstrates an effective chemical potential dramatically shifted toward the valence band (by 300-640 meV), robustly increased Seebeck coefficient (∼6× at 250 K), and improved thermoelectric performance (10-20× at 250 K).

First-Principles Calculations of the Phonon, Elastic, and Thermoelectric Properties of a Ti2CO2 Monolayer.

The phonon, elastic, and thermoelectric properties of Ti2CO2 are investigated by first-principles calculations. The dynamic and mechanical stabilities of Ti2CO2 are confirmed. The Ti2CO2 monolayer exhibits strong acoustic-optical coupling with the lowest optical frequency of 122.83 cm-1. The TA mode originates from the contribution of Ti(XY) vibrations and has the largest gruneisen parameter at the Γ point; the LA mode has the main contribution of O(XY) and Ti(XY) vibrations and has the lowest gruneisen parameter at the M point. The analysis of the phonon spectrum indicates that the vibration contributions from C, O, and Ti atoms are mainly located in the low-, middle-, and high-energy regions, respectively. The Seebeck coefficient and electronic conductivity increase with increasing carrier concentration under room temperature. The analysis of mechanical properties shows that Ti2CO2 possesses a larger Young's modulus and bending modulus, which has a better ability to resist deformation. Thermal properties are further investigated.

Highly Conductive n‐Type Polymer Fibers from the Wet‐Spinning of n‐Doped PBDF and Their Application in Thermoelectric Textiles

AbstractThe field of electronic textiles currently lacks n‐type polymer fibers that can complement the more established p‐type polymer fibers. Here, a highly conductive n‐type polymer fiber is obtained via wet‐spinning of n‐doped poly(3,7‐dihydrobenzo[1,2‐b:4,5‐b’]difuran‐2,6‐dione) (n‐PBDF). The electrical conductivity of the fibers increases from 1000 to 1600 S cm−1 with increased draw during processing and correlates well with Young's modulus. Wide‐angle X‐ray scattering reveals the existence of a bimodal orientation of the polymer chains, favoring parallel alignment to the fiber axis with increased draw. After 14 d in 80% humid air, fiber conductivity stabilizes maintaining 81% of the initial conductivity. Although the electrical conductivity drops slightly over time, the Seebeck coefficient increases, resulting in the highest thermoelectric power factor being measured at 91 µW m−1 K−2 for the most drawn fiber 14 d after its fabrication. A proof‐of‐concept two‐couple thermoelectric textile is crafted by embroidering bundles of n‐type PBDF fibers and p‐type PEDOT:PSS fibers. The device generates 2.40 nW at a 22 °C temperature gradient. This work represents the initial steps and a crucial advancement toward fabricating high‐performance n‐type polymer fibers that can complement their p‐type counterparts to close the existing performance gap.

Discoveries on the link between the properties of thermoelectric and infrared radiation

Both thermoelectric conversion materials and infrared (IR) radiation materials enjoy broad applicability in energy utilization and aerospace. Therefore, it is essential to study the relationship between the physical properties of these two materials. This paper investigates the connection between the thermoelectric properties and the IR radiation properties using a SrTiO3-based thermoelectric material. As a result, we have demonstrated the correlation between a material's Seebeck coefficient and IR emissivity using experimental data and theoretical calculations. With increasing Seebeck coefficient, the IR emissivity increases. The material's IR emissivity falls as its electrical conductivity rises, and vice versa. When the material's thermal gradient is low, its IR emissivity influences its IR radiation energy. As the thermal gradient of the material increases, the IR radiation energy incident on the material surface is mainly influenced by the material's thermal conductivity. This work provides a fresh perspective on modulating the IR radiation and thermoelectric properties using the connection found in this study. The results provide theoretical guidance for studies at the intersection of thermoelectric and IR materials.

Complex microstructure induced high thermoelectric performances of p-type Bi–Sb–Te alloys

In this research, we have fabricated the complex microstructure features along with the high density of twin boundaries using the combination of magnetic pulse compaction (MPC) and spark plasma sintering (SPS), which are exclusively improved the thermoelectric transport properties of p-type Bi0.5Sb1.5Te3 alloys via effective scattering of carriers and phonons at the grain boundary interfaces. The texture analysis confirmed the formation of bimodal (mixed grains) microstructural features in MPC + SPS specimens sintered at 350 °C, and 400 °C respectively due to the adhesion of fine grains (grain growth) and initiation of recrystallization at high temperatures. In addition to high density of twin structure, high-angle grain boundaries have also been identified. The power factor of 3.53 mW/m.K2 was obtained for the MPC + SPS at 400 °C due to increased Seebeck coefficient and larger electrical conductivity values. The maximum reduced thermal conductivity of 1.14W/mK was recorded due to a significant reduction in lattice thermal conductivity, which is mainly owing to strong scattering of phonons at the formation of mixed grain microstructures with high-density dislocations. As a result, maximum figure of merit, ZT = 1.17was achieved for MPC + SPS at 400 °C temperature.

Realizing the thermoelectric performance in cobalt doped p-CuO semiconductor by valance band flattening and effective phonon scattering approach

In this work we investigated the thermoelectric properties of CuO microcrystals through band structure modification and effective scattering of charge carriers and phonons. Synergetic improvement in carrier concentration, band effective mass and magnetic moment of CuO through ferromagnetic cobalt doping has extensive impact for the simultaneous increase of electrical conductivity and Seebeck coefficient. Band conduction model was employed to study the conduction mechanism in detail. Modification in the electronic band structure through band flattening results average S2σ of 5.52 μW/mK2 in the temperature range of 300–873 K for an optimum doping level of 0.5 at% of Co, this is 3.5 times superior to undoped CuO. Moreover enormous phonon scattering centers such as point defects suppressed the lattice thermal conductivity from 4.43 W/mK to 1.92 W/mK at 873 K. These viable strategies aided to enhance the thermoelectric efficiency of our prepared samples to efficiently utilize non-exploited thermal energy into green energy production.

Effective decoupling of grain boundaries and secondary phase interfaces for enhanced thermoelectric performance of Cu1.8S/WS2 nanocomposites

Copper Sulfide (Cu1.8S) has influentially concerned the increasing attention in mid-temperature thermoelectric materials. The inherent low Seebeck coefficient and high thermal conductivity of Cu1.8S hinders the thermoelectric performance. The superionic conductivity exhibited by Cu1.8S phase possess characteristic phonon-liquid electron-crystal property eliminate transverse phonon vibrations. In this study, improved thermoelectric performance for the Cu1.8S material was obtained through the efficient strategy of compositing with WS2 nanoparticles. A minimum thermal conductivity of 1.4 W/mK at 753 K for the Cu1.8S/5 wt. % WS2 nanocomposite. Along with the increased Seebeck coefficient values, the thermal conductivity is suppressed by WS2 composite due to the formation of Cu2WS4 secondary phase which strengthens the phonon scattering. The evident reduction in the thermal conductivity, coupled with less affected Seebeck coefficient, led to increased figure of merit (zT) of 0.46 at 753 K, which is 95 % higher than that of pure Cu1.8S. This work provides a distinguished improvement in the performance of Cu1.8S thermoelectric material thereby enabling its employment in the application of waste heat harvesting.

Tuning the Radius Ratio to Enhance Thermoelectric Properties in the Zintl Compounds AM2Sb2 (A = Ba, Sr; M = Zn, Cd)

Five novel Zintl phase solid solutions in the Ba1–xSrxZn2–yCdySb2 (0 ≤ x ≤ 0.13(1); 0 ≤ y ≤ 0.32(2)) system were successfully synthesized by the molten Pb metal-flux method, and the powder X-ray diffraction and single-crystal X-ray diffraction analyses proved that all five title compounds adopted the BaCu2S2-type phase having the orthorhombic Pnma space group (Z = 4, Pearson code oP20) with five crystallographically independent atomic sites. The previously studied BaCu2S2-type antimonides demonstrated a limited tolerance for doping in contrast to the CaAl2Si2-type antimonides. To understand the relatively narrower phase width and limited dopability of the title BaCu2S2-type phase than the CaAl2Si2-type phase in the overall Ba1–xSrxZn2–yCdySb2 system, the radius ratio of cations and anionic elements r+/r– for two structure types were thoroughly investigated. For the first time, the r+/r– ratio was identified as a critical factor for the phase selectivity: (1) r+/r– > 1 favored the BaCu2S2-type phase, and (2) r+/r– < 1 favored the CaAl2Si2-type phase. We also revealed the structural transformation mechanism from the more widely observed CaAl2Si2-type phase to the title BaCu2S2-type phase as the relatively larger cationic elements were introduced to the system. A series of DFT calculations using the three hypothetical models indicated that a resonance peak near EF in the density of states curves was descended from the relatively flat band structure at several special symmetry points rationalizing the enhanced Seebeck coefficients of Ba0.94(1)Sr0.06Zn1.86(3)Cd0.14Sb2 and Ba0.96(1)Sr0.04Zn1.68(2)Cd0.32Sb2. Electron localization function analysis rationalized the correlation between the polarity change of anionic Zn/Cd–Sb bonds and the charge carrier mobility on the anionic frameworks. Temperature-dependent thermoelectric properties were studied for the four title compounds, and the results proved that the Sr and Cd doping in the title Ba1–xSrxZn2–yCdySb2 system successfully enhanced the ZT values through the increased Seebeck coefficients and the reduced total thermal conductivities.

Sputtering Codeposition and Metal-Induced Crystallization to Enhance the Power Factor of Nanocrystalline Silicon.

The power factor of highly boron-doped nanocrystalline Si thin films with controlled doping concentration is investigated. We achieve a high degree of tuning of boron content with a charge carrier concentration from 1018 to 1021/cm3 and with the electrical conductivity by varying the boron magnetron power from 10 to 60 W while maintaining the power of a SiB source constant during codeposition from two independent sputtering sources. Along with the increase in the electrical conductivity with increased boron doping, we observe a steady decrease in the Seebeck coefficient from 500 to 100 μV/K. These values result in power factors that exhibit a marked maximum of 5 mW/K2m for a carrier concentration of around 1021/cm3 at room temperature. Temperature-dependent measurements up to 650 °C show, with increasing doping concentration, a change of the resistivity from a semiconducting to a metallic behavior and an increase of both Seebeck coefficient and power factor, with this last one peaking at 9.8 mW/K2m in the 350-550 °C temperature range. For higher concentrations, scanning electron microscopy and energy-dispersive X-ray spectroscopy show a partial segregation of boron on particles on the surface. These results exemplify the great advantage of sputtering codeposition methods to easily tune and optimize the thermoelectric performance in thin films, obtaining in our specific case highly competitive power factors in a simple and reliable manner.

Effect of Zn atomic diffusion due to pulsed electric field treatment on the thermoelectric properties of Zn–Sb films

Zn4Sb3 has brought into sharp focus by its low cost and excellent thermoelectric (TE) properties in the medium temperature range. Zn atoms can diffuse in the Zn4Sb3 matrix under the treatment of temperature or current. This provides a way to control its TE performance. In this study, the TE properties of the Zn4Sb3 films were improved by the Zn atomic diffusion under the treatment of periodic pulsed electric field. The Zn atomic diffusion is caused by the electrostatic interaction of the pulsed electric field and the diffusion mode is related to the pulse period. When the pulse period is smaller than 10 s, the interstitial Zn atom diffuses first. The Zn vacancy increases, leading to the increase of the carrier concentration and the decrease of the resistivity. When the pulse period increases to 100 s, more Zn(1) atoms diffuse through the interstitial position, resulting in a small amount of ZnSb phase. Thereby in the Zn4Sb3 films, the carrier concentration decreases and the Seebeck coefficient increases. This kind of the synergistic effect decreases the resistivity and increases the Seebeck coefficient. The power factor of the Zn4Sb3 films treated with a pulse period of 100 s is more than 4500 μW m−1 K−2 in the range of 373 K–573 K. This study presents a new method to control atomic diffusion by periodic pulsed electric field, and the TE properties of the Zn4Sb3 films are improved.

Different concentrations of Ti4+ as a donor and electronic properties of Bi2-xTixO3

Bi(2-x)TixO3 (x = 0, 0.01, 0.03. &amp;amp; 0.05) (BO-xT) ceramics are prepared by conventional solid-state route followed by low sintering temperatures. X-ray diffraction analyses show the presence of the monoclinic phase of Bi2O3. The electrical conductivities at room temperature concerning the frequency (ranging from 25 kHz to 5 MHz) and Seebeck Coefficient ranging from 50°C to 400°C were measured. With an increase in Ti (dopant) content, the conductivity and Seebeck Coefficient increased with the temperature increment. The BO-0.03T has the highest Seebeck value (47 μV/°C), which shows a higher carrier concentration. In terms of electrical conductivities, the BO-0.05T ceramic shows the maximum electrical conductivity, i.e. 2.0 × 10−9 μS/m as compared to other samples, which exhibit the presence of free electrons. Moreover, relative permittivity (dielectric constant) and dielectric loss are also measured concerning the frequency at room temperature to investigate the dielectric behaviour of the ceramics. This low-temperature sintering ceramics will open new applications in the domain of electronic materials.

Photo-Seebeck measurement of Bi-doped amorphous germanium telluride oxide film

Seebeck measurements of n-type amorphous germanium telluride thin films (100 nm) containing oxygen and implanted with Bismuth (Bi) have been studied under dark and monochromatic light conditions from 1800 to 400 nm. The Bi-doped film has a negative Seebeck coefficient indicating its n-type nature that results from the implantation compared to the p-type undoped films. Upon illumination with monochromatic light across the near-infrared and visible region the magnitude of the measured Seebeck voltage increases (becomes more negative). This increase in Seebeck coefficient displays distinctive behaviours in different spectral regions and is caused by the inclusion of Bi ions as foreign impurities. Furthermore, the Seebeck coefficients are used to determine the film electrical properties and enable, along with complementary characterisation including X-ray photoelectron spectroscopy, electronic band diagrams to be proposed for before and after Bi ion implantation. The photo-Seebeck technique is utilised for the first time to probe the trap states created due to the implantation, providing an understanding of the mechanisms behind non-equilibrium carrier-type reversal in an amorphous system, including the modification of electronic and optoelectronic properties such as the optical bandgap.

Realize high thermoelectric properties in n-type Bi2Te2.7Se0.3 materials via ZrO2 ceramic nanoparticles mediated heterogeneous interface

Bismuth telluride (Bi2Te3)-based thermoelectric materials have been commercially utilized for refrigeration and energy conversion at room/near room temperature. However, n-type Bi2Te3 compounds always exhibit inferior performance compared with their p-type counterparts, which is pronouncedly impeding Bi2Te3-based energy converters from further optimizations. Herein, a promising ZT value (ZT = 1.33) was obtained in n-type Bi2Te2.7Se0.3 (BTS) materials by using an interface engineering strategy to synergistically optimize the electron and phonon transport properties. In detail, ZrO2 ceramic nanoparticles (NPs) were entered into BTS matrix to construct high-density heterogeneous interfaces. The heterogeneous interfaces strongly scatter phonons and lower energy carriers, leading to decreased thermal conductivity and increased Seebeck coefficient, while the electrical conductivity is not sacrificed due to the compromise of the slightly reduced carrier mobility by interfacial scattering and the increased carrier concentration by ZrO2 NPs doping. A planar module consisting of 127 pairs of p-n single legs were assembled. At the temperature difference of 200 K, the output power and energy conversion efficiency reached 9.6 W and 6.2%, respectively. This strategy provides a feasible way to rationally design advanced n-type Bi2Te2.7Se0.3 materials with excellent thermoelectric and mechanical properties induced by high-density heterogeneous interfaces.

Bi2Te3 nanowires tuning PEDOT:PSS structure for significant enhancing electrical transport property

We developed Bi2Te3 nanowires (NWs)/poly (3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS) with a high power factor (PF) value of 170 μW·m−1·K−2 by a simple pulling method, which is an 85-fold improvement over PEDOT:PSS. The XRD, FTIR, UV–vis-NIR, XPS, FESEM, and NanoMap co-confirm the significant increase in electrical transport performance mainly attribute to two vital factors. First, the π bond of PEDOT:PSS interacts with the van der Waals force between the Te atom layer of Bi2Te3 NWs in the composite film, which makes PEDOT:PSS change from benzene type structure to quinone type structure with high conductivity. Second, the energy filtering effect generated by the interfacial barrier between Bi2Te3 NWs and PEDOT:PSS promotes the increase of seebeck coefficient.

Structural, magnetic and thermoelectric properties of double perovskite Nd2Co1-xCrxMnO6

Double perovskite Nd2Co1-xCrxMnO6 (x = 0, 0.1, 0.2, 0.3) samples were synthesized by a sol-gel method, and their structural, magnetic and thermoelectric properties were investigated. X-ray diffraction analysis showed that all crystallized in a monoclinic P21/n structure. The lattice parameters and unit cell volume increase with increasing Cr content. Two ferromagnetic transitions were observed in the Cr-doped samples, which can be attributed to the exchange interactions of the ordered Co2+–O–Mn4+ and the disordered Cr3+/Co3+–O–Mn3+ clusters, respectively. A large coercive field of 10.6 kOe was achieved at 10 K for the sample with x = 0.2. The magnetic transition temperatures and the magnetic moments decrease with increasing Cr content, which combined with the Raman analysis indicated a decrease in the degree of the B-site ordering. Both the electrical conductivity (σ) and Seebeck coefficient (S) increase over the temperature range of 190–1100 K with Cr doping. The undoped Nd2CoMnO6 exhibits a colossal S of 2273 μV/K at 220 K and has a negative S between 300 and 700 K with a maximum of − 24 μV/K at 325 K. All the Cr-doped samples are p-type semiconductors. The conduction mechanism follows the small polaron hopping model.

Optimization of thermoelectric properties of carbon nanotube veils by defect engineering.

Carbon nanotubes (CNTs), with their combination of excellent electrical conductivity, Seebeck coefficient, mechanical robustness and environmental stability are highly desired as thermoelectric (TE) materials for a wide range of fields including Internet of Things, health monitoring and environmental remediation solutions. However, their high thermal conductivity (κ) is an obstacle to practical TE applications. Herein, we present a novel method to reduce the κ of CNT veils, by introducing defects, while preserving their Seebeck coefficient and electrical conductivity. Solid-state drawing of a CNT veil embedded within two polycarbonate films generates CNT veil fragments of reducing size with increasing draw ratio. A successive heat treatment, at above the polycarbonate glass-to-rubber transition temperature, spontaneously reconnects the CNT veils fragments electrically but not thermally. Stretching to a draw ratio of 1.5 and heat repairing at 170 °C leads to a dramatic 3.5-fold decrease in κ (from 46 to 13 W m-1 K-1), in contrast with a decrease in electrical conductivity of only 26% and an increase in Seebeck coefficient of 10%. To clarify the mechanism of reduction in thermal conductivity, a large-scale mesoscopic simulation of CNT veils under uniaxial stretching has also been used. This work shows that defect engineering can be a valuable strategy to optimize TE properties of CNT veils and, potentially, other thermoelectric materials.

Annealing effect on the thermoelectric properties of multiwall carbon nanotubes

In this article, it is demonstrated that transport properties and thus thermoelectric performance of multiwall carbon nanotubes (MWCNTs) can be enhanced significantly by annealing. Two different samples of MWCNTs with small and large diameters were investigated. The as-prepared MWCNT samples were treated at different conditions of annealing. The pristine and annealed MWCNTs were investigated using x-ray diffraction, Raman spectroscopy and scanning electron microscope in order to investigate the microstructure, defect content and morphology before and after the annealing process. The electrical conductivity, Seebeck coefficient and thermal conductivity were measured over the temperature range of (300–450 K) and were found to correlate with the tube diameter and the annealing conditions. The electrical conductivity and Seebeck coefficient measurements show that the conduction mechanism in MWCNTs is thermal activation as the samples have degenerate semiconductor-like behavior. Upon annealing, the Seebeck coefficient increases, followed by a reduction in the thermal conductivity. The thermoelectric power factor was enhanced by annealing to realize the highest value of 1.06 μW m −1 K −2 at 475 K for the sample with the larger diameter. The thermoelectric figure of merit ZT was calculated to determine the performance of the materials. ZT of the sample of the larger tube's diameter was enhanced by about 247% at 450 K. • Pristine and annealed MWCNTs were investigated using x-ray diffraction, Raman spectroscopy and scanning electron microscope. • Thermoelectric performance of multiwall carbon nanotubes (MWNTs) is significantly enhanced by annealing. • The thermoelectric figure of merit ZT was enhanced by about 247% at 450 K.