N-type Seebeck Coefficient Research Articles

Synthesis and Transport Properties of ZnSnP2-yAsy Chalcopyrite Solid Solutions.

This work focuses on the synthesis and properties of quaternary ZnSnP2-yAsy chalcopyrite solid solutions. Full miscibility of the solid solution is achieved using ball milling followed by hot press sintering. The measured electrical conductivity increases substantially with As substitution from 0.03 S cm-1 for ZnSnP2 to 10.3 S cm-1 for ZnSnAs2 at 715 K. Band gaps calculated from the activation energies show a steady decrease with increasing As concentration from 1.4 eV for ZnSnP2 to 0.7 eV for ZnSnAs2. The Seebeck coefficient decreases significantly with As substitution from nearly 1000 μV K-1 for ZnSnP2 to -100 μV K-1 for ZnSnAs2 at 650 K. Thermal conductivity is decreased for the solid solutions due to alloy phonon scattering, compared to the end members with y = 0 and y = 2, with the y = 0.5 and y = 1.0 samples exhibiting the lowest values of 1.4 W m-1 K-1 at 825 K. Figure of merit values are increased for the undoped solid solutions at lower temperatures when compared to the end members due to the decreased thermal conductivity, with the y = 0.5 sample reaching zT = 1.6 × 10-3 and y = 1 reaching 2.1 × 10-3 at 700 K. The largest values of the figure of merit zT for the undoped series was found for y = 2 with zT = 2.8 × 10-3 at 700 K due to the increasing n-type Seebeck coefficient. Boltztrap calculations reveal that p-doping could yield zT values above unity at 800 K in case of ZnSnAs2, comparable with ZnSnP2.

Synergistic Interactions in Sequential Process Doping of Polymer/Single-Walled Carbon Nanotube Nanocomposites for Enhanced n-Type Thermoelectric Performance.

This study focuses on the fabrication of nanocomposite thermoelectric devices by blending either a naphthalene-diimide (NDI)-based conjugated polymer (NDI-T1 or NDI-T2), or an isoindigo (IID)-based conjugated polymer (IID-T2), with single-walled carbon nanotubes (SWCNTs). This is followed by sequential process doping method with the small molecule 4-(2,3-dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI) to provide the nanocomposite with n-type thermoelectric properties. Experiments in which the concentrations of the N-DMBI dopant are varied demonstrate the successful conversion of all three polymer/SWCNT nanocomposites from p-type to n-type behavior. Comprehensive spectroscopic, microstructural, and morphological analyses of the pristine polymers and the various N-DMBI-doped polymer/SWCNT nanocomposites are performed in order to gain insights into the effects of various interactions between the polymers and SWCNTs on the doping outcomes. Among the obtained nanocomposites, the NDI-T1/SWCNT exhibits the highest n-type Seebeck coefficient and power factor of -57.7 µV K-1 and 240.6 µW m-1 K-2 , respectively. However, because the undoped NDI-T2/SWCNT exhibits a slightly higher p-type performance, an integral p-n thermoelectric generator is fabricated using the doped and undoped NDI-T2/SWCNT nanocomposite. This device is shown to provide an output power of 27.2 nW at a temperature difference of 20 K.

Stretchable Thermoelectric Fibers with Three-Dimensional Interconnected Porous Network for Low-Grade Body Heat Energy Harvesting.

Electricity generation from body heat has garnered significant interest as a sustainable power source for wearable bioelectronics. In this work, we report stretchable n-type thermoelectric fibers based on the hybrid of Ti3C2Tx MXene nanoflakes and polyurethane (MP) through a wet-spinning process. The proposed fibers are designed with a 3D interconnected porous network to achieve satisfactory electrical conductivity (σ), thermal conductivity (κ), and stretchability simultaneously. We systematically optimize the thermoelectric and mechanical traits of the MP fibers and the MP-60 (with 60 wt % MXene content) exhibits a high σ of 1.25 × 103 S m-1, an n-type Seebeck coefficient of -8.3 μV K-1, and a notably low κ of 0.19 W m-1 K-1. Additionally, the MP-60 fibers possess great stretchability and mechanical strength with a tensile strain of 434% and a breaking stress of 11.8 MPa. Toward practical application, a textile thermoelectric generator is constructed based on the MP-60 fibers and achieves a voltage of 3.6 mV with a temperature gradient between the body skin and ambient environment, highlighting the enormous potential of low-grade body heat energy harvesting.

Mechanically Adaptative and Environmentally Stable Ionogels for Energy Harvest.

Converting building and environment heat into electricity is a promising strategy for energy harvest to tackle global energy and environmental problems. The processing challenges, mechanical brittleness, and low environmental tolerance of typical thermoelectric materials, however, prevent them from realizing their full potential when employed in outdoor building systems. Herein, a general concept based on synergistic ionic associations to significantly improve the mechanical properties and harsh environment stability for high-performance ionic-type thermoelectric (i-TE) gels is explored. They demonstrate extraordinarily high stretchability (1300-2100%), fast self-healing (120 s), temperature insensitivity, and great water-proof performance, and could be painted on a variety of surfaces. The n-type ionic Seebeck coefficient is up to -8.8mV K-1 and the ionic conductivity is more than 0.14 mS cm-1 . Both exhibit remarkable thermal and humidity stability (293-333 K, 20-100 RH%), which are rarely achieved in previous studies. Even on a cloudy day, the open-circuit thermovoltage for a painted i-TE array with an area of about 8.5 × 10-3 m2 is above 2V. This research offers a promising approach for gathering significant waste heat and even solar energy on outside building surfaces in an effective and sustainable manner.

Crystal structure, magnetism, and thermoelectric properties of Nd1−x Sr x FeO3−δ (0.1 ≤ x ≤ 0.9)

Polycrystalline Nd1−x Sr x FeO3−δ (0.1 ≤ x ≤ 0.9) samples exhibit a single-phase perovskite-type crystal structure and G-type antiferromagnetism with small ferromagnetic order. Assuming that the Fe site is in the mixed-valence state, the spin state of Fe changes from a low-spin (LS) or intermediate spin Fe3+-dominant state at x ≤ 0.5 to a LS Fe4+ dominant state at x ≥ 0.6. This strongly suggests that the charge carriers change from holes to electrons. In fact, at temperatures less than 500 K, these samples show a p-type Seebeck coefficient for 0.1 ≤ x ≤ 0.5 and an n-type Seebeck coefficient for 0.6 ≤ x ≤ 0.9. However, for 0.6 ≤ x ≤ 0.9, Fe4+ is reduced to Fe3+ because of an oxygen deficiency in the 500–600 K temperature range. In particular, Nd0.7Sr0.3FeO2.99(1) shows the largest p-type ZT = 0.025 at 765 K, whereas compositions with high n-type ZT could not be identified.

Ultra-long air-stability of n-type carbon nanotube films with low thermal conductivity and all-carbon thermoelectric generators

This report presents n-type single-walled carbon nanotubes (SWCNT) films with ultra-long air stability using a cationic surfactant and demonstrates that the n-type Seebeck coefficient can be maintained for more than two years, which is the highest stability reported thus far to the best of our knowledge. Furthermore, the SWCNT films exhibit an extremely low thermal conductivity of 0.62 ± 0.08 W/(m·K) in the in-plane direction, which is very useful for thin-film TEGs. We fabricated all-carbon-nanotube TEGs, which use p-type SWCNT films and the n-type SWCNT films developed, and their air-stability was investigated. The TEGs did not degrade for 160 days and exhibited an output voltage of 24 mV, with a maximum power of 0.4 µW at a temperature difference of 60 K. These results open a pathway to enable the widespread use of carbon nanotube TEGs as power sources in IoT sensors.

Ab initio study of electronic and thermoelectric properties of a new B2CO monolayer

In this work, we present the thermoelectric properties of a novel two-dimensional (2D) B2CO monolayer obtained using first principles calculations. We investigated the electronic band structure and density of states based on density functional theory and using the QUANTUM ESPRESSO computational package. Thermoelectric properties were calculated using the semiclassical Boltzmann transport equation in relaxation time approximation and within Boltztrap computational package. These electrical transport properties include the electrical conductivity coefficients (sigma), thermal conductivity (κe), the Seebeck coefficient (S) and dimensionless figure of merit (ZT), which are suitable for designing thermoelectric devices. According to our results, two-dimensional (2D) B2CO monolayer indicates an indirect band gap semiconductor with value of 1.68 eV. The numerical results show almost isotropic transport properties for the 2D B2CO monolayer. In particular, the thermoelectric study shows a good thermoelectric performance of the 2D B2CO monolayer with high figure of merit, so that, the nanostructure of B2CO is an n-type semiconductor and Seebeck coefficient and figure of merit at room temperature were obtained as -2595 µV/K and 1, respectively.

Interfacial engineering effect and bipolar conduction of Ni- doped MoS2 nanostructures for thermoelectric application

Transition Metal Chalcogenides play an important role in the thermoelectric (TE) application due to their unique properties such as stability, easily tunable carrier transport and charge carrier mobility which enhance the power factor. Molybdenum disulphide (MoS2) is a hopeful TE material with a layered structure, but it possesses low electrical conductivity. Herein, our present work focuses to improve the electrical conductivity of MoS2 by Ni-doping and the effect of Ni-concentration was studied extensively. Undoped and Ni-doped MoS2 samples were successfully prepared by hydrothermal route. The hexagonal structured 2H-MoS2 and the formation of nickel sulphide (NiS) secondary phase at 10 at% of Ni-concentration was confirmed by XRD analysis. HRTEM confirmed the influence of Ni content in MoS2 layers which create the bonding between the layers and reduces the interlayer distance of MoS2. The reduced thermal conductivity was observed around 0.48 Wm−1K−1 for 7.5 at% of Ni-doped MoS2 which is two times lower than that of undoped MoS2. Meanwhile, the enhancement of electrical conductivity of Ni-doped samples from 0.012 S/cm to 0.049 S/cm observed for 7.5 at% of Ni-doped MoS2 sample. Because of NiS, the conduction behavior was changed from p-type to n-type Seebeck coefficient. The maximum zT was obtained as 0.28 × 10−3 for undoped sample. However, Ni-doping was experimentally proved to the simultaneous optimization of enhanced electrical conductivity and the suppressed thermal conductivity, the change in carrier type due to nickel sulphide (NiS) phase reduced the figure of merit.

Promising Thermoelectric Performance in Two-Dimensional Semiconducting Boron Monolayer.

A heavy element is a special character for high thermoelectric performance since it generally guarantees a low lattice thermal conductivity. Here, we unexpectedly found a promising thermoelectric performance in a two-dimensional semiconducting monolayer consisting of a light boron element. Using first-principles combined with the Boltzmann transport theory, we have shown that in contrast to graphene or black phosphorus, the boron monolayer has a low lattice thermal conductivity arising from its complex crystal of hexagonal vacancies. The conduction band with an intrinsic camelback shape leads to the high DOS and a high n-type Seebeck coefficient, while the highly degenerate valence band along with the small hole effective mass contributes to the high p-type power factor. As a result, we obtained the p-type thermoelectric figure of merit up to 0.96 at 300 K, indicating that the boron monolayer is a promising p-type thermoelectric material.

Air stability of n-type single-walled carbon nanotube films with anionic surfactants investigated using molecular dynamics

The semiconducting property and air-stability of n-type single-walled carbon nanotube (SWCNT) are important for manufacturing all-carbon thermoelectric generators. We fabricated n-type SWCNT films with different anionic surfactants via drop-casting followed by heat treatment. Among the surfactants used (sodium dodecyl sulfate (SDS), sodium cholate (SC), and sodium dodecylbenzene sulfonate (SDBS)), the SWCNT films with SDBS surfactants exhibited an n-type Seebeck coefficient for 35 days. Structural analyses of the films showed that the SWCNT bundles and SDBS surfactants were homogeneously mixed. To investigate the mechanism behind the air-stability of the n-type property in SWCNT films, molecular dynamics (MD) calculations were performed. The MD calculations revealed that the adsorbability of SDBS molecules to SWCNT was higher than that of the other surfactants, which was consistent with the results of structural analyses. The sodium atoms in the vicinity of the SWCNT surface were vital in imparting n-type properties to the SWCNT. The SWCNT films with SDBS surfactants exhibited n-type properties for a long period of time because the surfactant molecules homogeneously covered the SWCNT surface, which enhanced the transfer of electrons from the sodium atoms to the SWCNT. The findings will aid toward further improving the air-stability of n-type SWCNT films using new surfactants.

Origins of Minimized Lattice Thermal Conductivity and Enhanced Thermoelectric Performance in WS2/WSe2 Lateral Superlattice.

We report a configuration strategy for improving the thermoelectric (TE) performance of two-dimensional transition metal dichalcogenide WS2 based on the experimentally prepared WS2/WSe2 lateral superlattice (LS) crystal. On the basis of density function theory combined with a Boltzmann transport equation, we show that the TE figure of merit zT of monolayer WS2 is remarkably enhanced when forming into a WS2/WSe2 LS crystal. This is primarily ascribed to the almost halved lattice thermal conductivity due to the enhanced anharmonic processes. Electronic transport properties parallel (xx) and perpendicular (yy) to the superlattice period are highly symmetric for both p- and n-doped LS owing to the nearly isotropic lifetime of charger carriers. The spin-orbital effect causes a significant split of conduction band and leads to three-fold degenerate sub-bands and high density of states (DOS), which offers opportunity to obtain a high n-type Seebeck coefficient (S). Interestingly, the separated degenerate sub-bands and upper conduction band in monolayer WS2 form a remarkable stair-like DOS, yielding a higher S. The hole carriers with much higher mobility than electrons reveal the high p-type power factor, and the potential to be good p-type TE materials with optimal zT exceeds 1 at 400 K in WS2/WSe2 LS.

Origin of n type properties in single wall carbon nanotube films with anionic surfactants investigated by experimental and theoretical analyses

We investigated the origin of n-type thermoelectric properties in single-wall carbon nanotube (SWCNT) films with anionic surfactants via experimental analyses and first-principles calculations. Several types of anionic surfactants were employed to fabricate SWCNT films via drop-casting, followed by heat treatment at various temperatures. In particular, SWCNT films with sodium dodecylbenzene sulfonate (SDBS) surfactant heated to 350 °C exhibited a longer retention period, wherein the n-type Seebeck coefficient lasted for a maximum of 35 days. In x-ray photoelectron spectroscopy, SWCNT films with SDBS surfactant exhibited a larger amount of sodium than oxygen on the SWCNT surface. The electronic band structure and density of states of SWCNTs with oxygen atoms, oxygen molecules, water molecules, sulfur atoms, and sodium atoms were analyzed using first-principles calculations. The calculations showed that sodium atoms and oxygen molecules moved the Fermi level closer to the conduction and valence bands, respectively. The water molecules, oxygen, and sulfur atoms did not affect the Fermi level. Therefore, SWCNT films exhibited n-type thermoelectric properties when the interaction between the sodium atoms and the SWCNTs was larger than that between the oxygen molecules and the SWCNTs.

Correlation between the air stability of n-type thermoelectric properties and defects in single-walled carbon nanotubes with anionic surfactants

Single-walled carbon nanotubes (SWCNTs) are promising thermoelectric materials for use as sustainable power sources for the Internet of Things technology due to their flexibility and excellent thermoelectric properties near 300 K. One of the most important challenges in the development of SWCNTs is achieving n-type thermoelectric properties with long air stability. Here, we investigated the correlation between the air stability of the n-type property and the defects of SWCNTs using two types of SWCNTs with different defect densities. SWCNT films with anionic surfactants were prepared using drop-casting, followed by heat treatment. Both types of SWCNT films exhibited approximately the same n-type Seebeck coefficient values at appropriate heat treatment temperatures. The SWCNT films with low defect density exhibited high electrical conductivity, but the n-type Seebeck coefficient was converted into a p-type one at 14 d. Conversely, the SWCNT films with high defect density exhibited low electrical conductivities but maintained the n-type Seebeck coefficient for 35 d. Therefore, the defect density of SWCNTs impacted the air-stability of the thermoelectric properties. This phenomenon possibly indicates that SWCNT films with high defect density were homogeneously coated with surfactants, thus preventing oxygen atoms from adhering to the film.

Anionic conduction mediated giant n-type Seebeck coefficient in doped Poly(3-hexylthiophene) free-standing films

The present work demonstrates a totally radical change in conduction nature from typical p-type to n-type through ionic transport in FeCl3 doped free-standing poly(3-hexylthiophene) (P3HT) films. The thermodiffusion of Cl− ions generated a giant negative Seebeck coefficient (∼2.7 mV/K) and a moderately high electrical conductivity (∼1 S/cm); an unprecedented level in polymers. A thermoelectric power generator fabricated using these P3HT films delivered an output electrical power of 25 μW with open circuit voltage of 128 mV for a ΔT of 46 °C. Though continuous operation reduced the output power due to inability of the ions to pass through the interface between doped P3HT and metallic contact, yet the generated voltage was found to be quite stable over a period of 1 h under load. With such a high n-type Seebeck coefficient, free-standing P3HT films show a great potential for energy harvesting from intermittent heat sources as well as in supercapacitor charging for futuristic energy storage devices.

Flexible thermoelectric films formed using integrated nanocomposites with single-wall carbon nanotubes and Bi2Te3 nanoplates via solvothermal synthesis

Single-wall carbon nanotubes (SWCNTs) and Bi2Te3 nanoplates are very promising thermoelectric materials for energy harvesting. When these two materials are combined, the resulting nanocomposites exhibit high thermoelectric performance and excellent flexibility. However, simple mixing of these materials is not effective in realizing high performance. Therefore, we fabricated integrated nanocomposites by adding SWCNTs during solvothermal synthesis for the crystallization of Bi2Te3 nanoplates and prepared flexible integrated nanocomposite films by drop-casting. The integrated nanocomposite films exhibited high electrical conductivity and an n-type Seebeck coefficient owing to the low contact resistance between the nanoplates and SWCNTs. The maximum power factor was 1.38 μW/(cm K2), which was 23 times higher than that of a simple nanocomposite film formed by mixing SWCNTs during drop-casting, but excluding solvothermal synthesis. Moreover, the integrated nanocomposite films maintained their thermoelectric properties through 500 bending cycles.

Facile preparation of air-stable n-type thermoelectric single-wall carbon nanotube films with anionic surfactants

Thermoelectric generators based on single-wall carbon nanotubes (SWCNTs) have great potential for use in wearable and skin electronics because of their lightweight and mechanically soft structure. However, the fabrication of air-stable n-type thermoelectric SWCNTs using conventional processes is challenging. Herein, we propose a facile process for fabricating air-stable n-type SWCNT films with anionic surfactants via drop casting followed by heat treatment. We examined different surfactants (Sodium Dodecyl Sulfate, Sodium Dodecylbenzene Sulfonate, and Sodium Cholate) and heat-treatment temperatures. The optimal SWCNT film maintained the n-type Seebeck coefficient for 35 days. Moreover, to further extend the n-type Seebeck coefficient maintenance, we periodically reheated the SWCNT film with a surfactant that had returned to the p-type Seebeck coefficient. The reheated film recovered the n-type Seebeck coefficient, and the effect of the reheating treatment lasted for several reheating cycles. Finally, we elucidated a simple mechanism for realizing an air-stable n-type Seebeck coefficient based on spectroscopic analyses of the SWCNT films.

Strain tunable pudding-mold-type band structure and thermoelectric properties of SnP3 monolayer

Recent studies indicated the interesting metal-to-semiconductor transition when layered bulk GeP3 and SnP3 are restricted to the monolayer or bilayer, and the SnP3 monolayer has been predicted to possess high carrier mobility and promising thermoelectric performance. Here, we investigate the biaxial strain effect on the electronic and thermoelectric properties of the SnP3 monolayer. Our first-principles calculations combined with Boltzmann transport theory indicate that the SnP3 monolayer has the “pudding-mold-type” valence band structure, giving rise to a large p-type Seebeck coefficient and a high p-type power factor. The compressive biaxial strain can decrease the energy gap and result in metallicity. In contrast, the tensile biaxial strain increases the energy gap, increases the n-type Seebeck coefficient, and decreases the n-type electrical conductivity. Although the lattice thermal conductivity becomes larger at a tensile biaxial strain due to the increased maximum frequency of the acoustic phonon modes and the increased phonon group velocity, it is still low, e.g., only 4.1 W m−1 K−1, at room temperature with 6% tensile strain. The tensile strain decreases the figure of merit, but the value is still considerable, and it can reach 2.01 for p-type doping at 700 K with 6% tensile strain. Therefore, the SnP3 monolayer is a good thermoelectric material with low lattice thermal conductivity and promising figure of merit even at 6% tensile strain.

Double doping approach for unusually stable and large n-type thermoelectric voltage from p-type multi-walled carbon nanotube mats

Multi-walled carbon nanotubes (MWCNTs) have shown low n-type Seebeck coefficients (−10 μV K−1), which are not good enough to prepare high performance low-cost MWCNT based mechanically flexible thermoelectric devices.

Seebeck Tensor Analysis of (p × n)-type Transverse Thermoelectric Materials

ABSTRACTSingle-leg (p × n)-type transverse thermoelectrics (TTE) are reviewed as an alternative to conventional or “longitudinal” double-leg thermoelectrics for applications at room temperature and below. As the name suggests, this unique behavior of (p × n)-type transverse thermoelectrics results from choosing ambipolar anisotropic materials that have a Seebeck tensor with orthogonal p- and n-type Seebeck coefficients, leading to transverse relation between net heat and net electrical current. One feature of such materials is that they can operate near intrinsic doping and, therefore will not suffer from dopant freeze-out, opening the possibility of new cryogenic operation for solid state cooling. In this work, a Seebeck tensor analysis of thermoelectric materials is presented. To compare the performance of transverse thermoelectric materials, a transverse power factor PF⊥ is introduced. Materials searches based on these simple criteria reveal that over 1/4 of the database of about 48,000 inorganic materials could potentially function as (p × n)-type TTE’s, demonstrating the underappreciated prevalence of this class of materials.

P-N Conversion in a Water-Ionic Liquid Binary System for Nonredox Thermocapacitive Converters.

An intriguing p-n conversion of thermoelectric property was observed in a water-ionic liquid ([EMIm][Ac]) binary system with precise control over water content. The highest p-type and n-type Seebeck coefficient were optimized at water-[EMIm][Ac] molar ratio of 2:1 and 4:1, respectively. DFT calculation illustrates that a configuration of solvent separation ion pairs is preferred at the water-[EMIm][Ac] molar ratio of 4:1, leading to the p-n conversion through weakening interaction between anion clusters and gold electrodes. Furthermore, p-n thermocapacitive converters were integrated to enhance the output Seebeck voltages. This work opens up new perspectives for harvesting low grade heat with the use of fluidic materials.