Enhancement Of Thermoelectric Figure Research Articles

Effect of bismuth doping and SiC nanodispersion on the thermoelectric properties of solution-processed PbTe

Thermoelectric materials are promising as they found numerous applications in electrical power generation and solid-state cooling. In the last decade, Lead Telluride (PbTe) has emerged as a potential thermoelectric candidate for electrical power generation in the medium temperature range. In the present work, we demonstrate nearly 78% enhancement in thermoelectric figure of merit (zT) of nanostructured PbTe compound through doping with bismuth and dispersing 50 nm SiC nanoparticles. Bismuth doping induces n-type conduction and improves the electrical conductivity, while the SiC nanoinclusions modulate the Seebeck coefficient by carrier energy filtering, in addition to suppressing the lattice thermal conductivity. The significant phonon scattering at multi-scale scattering centers resulted in a minimum lattice thermal conductivity (κL) of 0.69 W/m-K in Bi0.02-Pb0.98Te-8% SiC nanocomposite sample. Finally, the reduced lattice thermal conductivity in conjunction with moderate electrical conductivity and high Seebeck coefficient leads to a zTmax value of 0.32 at 590 K in the sample with 6% SiC content. Hence, the partial substitution of the host atom and the presence of a secondary phase can be another promising strategy to enhance the thermoelectric performance.

Local ferroelectric polarization switching driven by nanoscale distortions in thermoelectric {text {Sn}}_{0.7}{text {Ge}}_{0.3}{text {Te}}

A remarkable decrease in the lattice thermal conductivity and enhancement of thermoelectric figure of merit were recently observed in rock-salt cubic SnTe, when doped with germanium (Ge). Primarily, based on theoretical analysis, the decrease in lattice thermal conductivity was attributed to local ferroelectric fluctuations induced softening of the optical phonons which may strongly scatter the heat carrying acoustic phonons. Although the previous structural analysis indicated that the local ferroelectric transition temperature would be near room temperature in {text {Sn}}_{0.7}{text {Ge}}_{0.3}{text {Te}}, a direct evidence of local ferroelectricity remained elusive. Here we report a direct evidence of local nanoscale ferroelectric domains and their switching in {text {Sn}}_{0.7}{text {Ge}}_{0.3}{text {Te}} using piezoeresponse force microscopy(PFM) and switching spectroscopy over a range of temperatures near the room temperature. From temperature dependent (250–300 K) synchrotron X-ray pair distribution function (PDF) analysis, we show the presence of local off-centering distortion of Ge along the rhombohedral direction in global cubic {text {Sn}}_{0.7}{text {Ge}}_{0.3}{text {Te}}. The length scale of the {text {Ge}}^{2+} off-centering is 0.25–0.10 Å near the room temperatures (250–300 K). This local emphatic behaviour of cation is the cause for the observed local ferroelectric instability, thereby low lattice thermal conductivity in {text {Sn}}_{0.7}{text {Ge}}_{0.3}{text {Te}}.

Enhancement in thermoelectric figure of merit of bismuth telluride system due to tin and selenium co-doping

Thermoelectric performance of tin and selenium co-doped bismuth telluride in the temperature range 10–300 K, prepared by solid-state reaction is reported in the current work. The powder X-ray diffraction study reveals hexagonal crystal structure with R3‾m space group. Energy dispersive X-ray analysis confirms elemental composition within the experimental limits and Field Emission Scanning Electron Microscopy (FESEM) shows uniform grain density and porosity on the surface of the pristine and doped samples. The electrical resistivity shows quasi degenerate semiconducting behavior and the temperature dependent Seebeck coefficient confirms n-type semiconducting nature of the pristine as well as doped samples. The carrier concentration and carrier mobility are of the order 1025/m3 and 10−4 m2/Vs respectively. A significant reduction in the thermal conductivity has been found in the (Bi0.98Sn0.02)2Te2.7Se0.3 compound, leading to an enhancement in the power factor (PF) and thermoelectric figure of merit (ZT) by 3.2 and 13.5 times respectively as compared to that of the pristine sample Bi2Te3 at 300 K. The highest ZT value of about 0.27 is achieved for (Bi0.98Sn0.02)2Te2.7Se0.3 at 300 K.

A promising thermoelectric response of fully compensated ferrimagnetic spin gapless semiconducting Heusler alloy Zr2MnAl at high temperature: DFT study

In recent times, Heusler compounds have emerged as potential candidates for spintronics and thermoelectric materials owing to their interesting properties such as gapless semiconducting nature with fully compensated ferrimagnetic behavior, 100% spin polarization with high mobility of charge carriers and better thermoelectric figure of merit. Motivated by this fact, we have carried out a systematic theoretical investigation of structural, electronic, magnetic, thermoelectric and lattice dynamical properties of spin gapless Heusler alloy Zr2MnAl within the frame work of density functional theory based first principle calculation. Our ground state total energy calculations confirms the structural stability of antiferromagnetic phase of Zr2MnAl over ferromagnetic phase, which is in agreement with previous study. The positive value for Cauchy’s pressure (C12–C44) predicts ductile nature of the material. The band structure calculation reveals indirect band gap in spin down channel and zero band gap in spin up channel of valence and conduction bands confirming the spin gapless semiconducting nature of the compound. The real frequency of phonon modes throughout the Brillouin zone confirms the structural and dynamical stability of antiferromagnetic phase of Zr2MnAl, which is reported for the first time as per our knowledge. Calculated Seebeck coefficient in spin up and spin down channel reveals that the Zr2MnAl behaves as both n and p type thermoelectric materials with better output efficiency. The calculated thermoelectric figure of merit of 1.4 at 800 K, favors use of an alloy in recovery of waste heat at higher temperature and thermoelectric spin generators. The comprehensive analysis of various chemical potential dependent thermoelectric coefficients for enhancing thermoelectric performance for the material has been carried out. The obtained results propose Zr2MnAl as a potential candidate for high temperature thermoelectric devices. Higher optical gap in the phonon dispersive curve is responsible for decrease in lattice thermal conductivity with temperature, resulting in enhancement in thermoelectric figure of merit.

Maximization of transporting bands for high-performance SnTe alloy thermoelectrics

Environment-friendly thermoelectric SnTe has recently attracted much attention as a top candidate for replacing conventional p-type PbTe. Effective strategies leading to great advancements in this material are typified by manipulation of valence bands and chemical defects, among of which MgTe- and Cu2Te-alloying are particularly successful, respectively, for valence band convergence and lattice thermal conductivity minimization. However, pristine SnTe enables a MgTe solubility of only ∼12%, which might limit the full convergence of possibly transporting bands for an electronic performance maximization. In this work, we show an approach to increase the MgTe solubility up to ∼20%, leading to an involvement of one more highly degenerated valence band Λ for charge transport in addition to existing L and Σ bands. Such a collection of many transporting valence bands enables a great improvement in electronic performance, as well as a significant enhancement in thermoelectric figure of merit with a well-reduced lattice thermal conductivity by the point defects introduced in these SnTe alloys.

Enhanced thermoelectric properties by effective decoupling of electrical, thermal properties and seebeck coefficient through the addition of rare earth sesquioxides in p-type BiSbTe alloys

The decoupling between the electrical conductivity and thermal conductivity is a crucial challenge to elevate the figure of merit of a given thermoelectric material. In this research, thermoelectric properties of the rare-earths sesquioxides added Bi-Sb-Te material has been investigated systematically. The electrical conductivity (σ) is increased by increase in carrier concentration due to progressive movement of reduced Fermi level deeper into the valance band. Meanwhile, an effective mass of the rare earth added BiSbTe composites has been increased and contributed to the resonant distortion in density of states (DOS), that gives rises to the enhancement of Seebeck coefficient (α). Owing to the increment of both factors (σ &α), a considerable enhancement in power factor of 14% was achieved for the 2 wt% added BiSbTe/(x-wt%) Nd2O3 composites. The low content addition of rare earth oxide particles are potentially acts as scattering centers for mid to long wavelength phonons and reduced the thermal conductivity by 7%. The effective decoupling of the electrical and thermal properties by rare earths sesquioxides (Nd2O3) provides a 15% enhancement in thermoelectric figure of merit, ZT for 2 wt% added BiSbTe/(x-wt%)Nd2O3 composites.

Introduction of porous structure: A feasible and promising method for improving thermoelectric performance of Bi2Te3 based bulks

The porous p-type Bi0.4Sb1.6Te3 bulks containing irregularly and randomly oriented pores were obtained by artificially controlling the relative density of sintered samples during resistance pressing sintering process. It is demonstrated that the thermoelectric performances are significantly affected by the porous structure, especially for the electrical and thermal conductivity due to the enhanced carrier scattering and phonon scattering. The increasing porosity resulted in the obvious decrease in electrical and thermal conductivity, and little change in Seebeck coefficients. It is encouraging that the reduction of thermal conductivity can compensate for the deterioration of electrical performance, leading to the enhancement in thermoelectric figure of merit (ZT). The maximum ZT value of 1.0 was obtained for the sample with a relative density of 90% at 333 K. Unfortunately, the increase in porosity also brought in obvious degradations in Vickers hardness from 51.71 to 27.74 HV. It is worth mentioning that although the Vickers hardness of the sample with a relative density of 90% decreased to 40.12 HV, it was still about twice as high as that of the zone melting sample (21.25 HV). To summarize, introducing pores structure into bulks properly not only enhances the ZT value of Bi2Te3 based alloys, but also reduces the use of raw materials and saves production cost.

Solid-State Explosive Reaction for Nanoporous Bulk Thermoelectric Materials.

High-performance thermoelectric materials require ultralow lattice thermal conductivity typically through either shortening the phonon mean free path or reducing the specific heat. Beyond these two approaches, a new unique, simple, yet ultrafast solid-state explosive reaction is proposed to fabricate nanoporous bulk thermoelectric materials with well-controlled pore sizes and distributions to suppress thermal conductivity. By investigating a wide variety of functional materials, general criteria for solid-state explosive reactions are built upon both thermodynamics and kinetics, and then successfully used to tailor material's microstructures and porosity. A drastic decrease in lattice thermal conductivity down below the minimum value of the fully densified materials and enhancement in thermoelectric figure of merit are achieved in porous bulk materials. This work demonstrates that controlling materials' porosity is a very effective strategy and is easy to be combined with other approaches for optimizing thermoelectric performance.

Nanostructured polypyrrole: enhancement in thermoelectric figure of merit through suppression of thermal conductivity

Semi-crystalline polypyrrole (PPy) nanotubes were synthesized through a chemical polymerization route using methyl orange-ferric chloride (MO-FeCl3) as a template for growth. The thermoelectric properties of these PPy nanotubes have been studied in the temperature range 300–380 K after treatment with various dopants such as hydrochloric acid (HCl), p-toluene-sulphonic acid monohydrate (ToS), and tetrabutyl ammonium hexaflurophosphate (PF6). It has been observed that these dopants affect the electrical and thermal transport properties of PPy nanotubes in different ways. The temperature dependence of electrical resistivity suggests that pure PPy and ToS-doped PPy nanotubes exhibit a critical regime of metal-to-insulator transition, and doping with HCl drives them into the metallic regime. In contrast, PF6 doping of PPy nanotubes carries them into the insulating regime and these samples exhibited the highest figure of merit of ~3.4 × 10‒3 at 380 K, which was 240% higher than the value obtained in the case of pristine PPy nanotubes. Strongly repressed thermal conductivity along with moderately high Seebeck coefficient and electrical conductivity have been found to be responsible for the high figure of merit observed in PF6-doped PPy nanotubes. The suppression of thermal conductivity in PF6-doped PPy nanotubes is attributed to the scattering of the spectrum of phonons via hierarchical length-scale defect structures present in the sample.

Influence of the pore shape and dimension on the enhancement of thermoelectric performance of graphene nanoribbons

Abstract

A synergistic combination of atomic scale structural engineering and panoscopic approach in p-type ZrCoSb-based half-Heusler thermoelectric materials for achieving high ZT

In situsynthesis of composites employing ASSET in combination with panoscopic approach has been demonstrated in p-type ZrCoSb-based half-Heusler thermoelectric materials for significant enhancement in thermoelectric figure of merit.

Carrier transport phenomenon and thermoelectric properties in melt-grown tellurium doped n-type Bi0.88Sb0.12 alloy

Abstract We report the carrier transport phenomenon and thermoelectric properties in tellurium (Te) doped Bi 0.88 Sb 0.12 . Two types of samples were prepared with 0.5 and 1 mol% Te. The structural analysis was carried out by XRD, transmission electron microscope, and energy dispersive spectroscopic techniques. The transport properties were evaluated through the temperature dependent resistivity and thermopower measurements with room temperature Hall and thermal conductivity. The negative values of Hall coefficients and degenerate behavior showed the effectiveness of Te as an n -type dopant in Bi 0.88 Sb 0.12 . The room temperature dimensionless figure of merit was found to be in the range of 5.8 × 10 −3 –5.71 × 10 −2 . The enhancement in thermoelectric figure of merit by one order of magnitude is found due to lower doping of Te. The temperature dependent behavior of the resistivity and thermopower was correlated and explained by the thermal excitation of the dopant atoms from the unionized to ionized states.

Significantly enhanced thermoelectric figure of merit through Cu, Sb co-substitutions for Te in Ga2Te3

Here, we used two elements Cu and Sb as the substitutes for Te in Ga2Te3 and observed a giant enhancement in thermoelectric figure of merit (ZT). This potential substituted Ga2Cu0.05SbxTe2.95−x (x = 0.2) gives the maximum ZT value of 0.5 at 700 K, more than 3 times as high as that of intrinsic Ga2Te3. The enhancement in ZT is attributed to the significantly reduced thermal conductivity caused mainly by the presence of Cu at Ga vacancy planes, and remarkably improved electrical conductivity by the occupation of Sb at Te sites, which increases the carrier concentration and conserves the high mobility.

The effects of dual doping on the thermoelectric properties of Ca3−xMxCo4−yCuyO9 (M = Na, La)

The effects of dual doping on the thermoelectric properties of Ca3−xMxCo3.8Cu0.2O9 (M=Na, La, x=0.1, 0.2, 0.3) have been systematically investigated, with Ca3−xMxCo3.8Cu0.2O9 powders synthesized using sol–gel method, and then consolidated into bulk ceramics by spark plasma sintering. It is found that dual doping by Cu and La results in substantial increase in electric conductivity and small decrease in Seebeck coefficient, while dual doping by Cu and Na increase Seebeck coefficient and decrease electric conductivity slightly. Both types of doping enhance power factor and reduce thermal conductivity simultaneously in general, with substantial reduction in thermal conductivity observed in dual doping of Cu and La. These lead to enhancement in thermoelectric figure of merit by both types of doping in general, with ZT value of Ca2.8La0.2Co3.8Cu0.2O9 78.07% higher than Ca3Co4O9 and 57.36% higher than Ca3Co3.8Cu0.2O9 near 773K. These results suggest that dual doping by Cu and La is effective in enhancing thermoelectric figure of merit of Ca3Co4O9.

Enhancement in Thermoelectric Figure of Merit in Nanostructured Bi2Te3 with Semimetal Nanoinclusions

AbstractThe effect of Bi (semimetal) nanoinclusions in nanostructured Bi2Te3 matrices is investigated. Bismuth nanoparticles synthesized by a low temperature solvothermal method are incorporated into Bi2Te3 matrix phases, synthesized by planetary ball milling. High density pellets of the Bi nanoparticle/Bi2Te3 nanocomposites are created by hot pressing the powders at 200 °C and 100 MPa. The effect of different volume fractions (0–7%) of Bi semimetal nanoparticles on the Seebeck coefficient, electrical conductivity, thermal conductivity and carrier concentration is reported. Our results show that the incorporation of semimetal nanoparticles results in a reduction in the lattice thermal conductivity in all the samples. A significant enhancement in power factor is observed for Bi nanoparticle volume fraction of 5% and 7%. We show that it is possible to reduce the lattice thermal conductivity and increase the power factor resulting in an increase in figure of merit by a factor of 2 (from ZT = 0.2 to 0.4). Seebeck coefficient and electrical conductivity as a function of carrier concentration data are consistent with the electron filtering effect, where low‐energy electrons are preferentially scattered by the barrier potentials set up at the semimetal nanoparticle/semiconductor interfaces.