Temperature-dependent Seebeck Research Articles

Terbium Ion Doping in Ca3Co4O9: A Step towards High-Performance Thermoelectric Materials

The potential of thermoelectric materials to generate electricity from the waste heat can play a key role in achieving a global sustainable energy future. In order to proceed in this direction, it is essential to have thermoelectric materials that are environmentally friendly and exhibit high figure of merit, ZT. Oxide thermoelectric materials are considered ideal for such applications. High thermoelectric performance has been reported in single crystals of Ca3Co4O9. However, for large scale applications single crystals are not suitable and it is essential to develop high-performance polycrystalline thermoelectric materials. In polycrystalline form, Ca3Co4O9 is known to exhibit much weaker thermoelectric response than in single crystal form. Here, we report the observation of enhanced thermoelectric response in polycrystalline Ca3Co4O9 on doping Tb ions in the material. Polycrystalline Ca3−xTbxCo4O9 (x = 0.0–0.7) samples were prepared by a solid-state reaction technique. Samples were thoroughly characterized using several state of the art techniques including XRD, TEM, SEM and XPS. Temperature dependent Seebeck coefficient, electrical resistivity and thermal conductivity measurements were performed. A record ZT of 0.74 at 800 K was observed for Tb doped Ca3Co4O9 which is the highest value observed till date in any polycrystalline sample of this system.

High Temperature Electronic and Thermal Transport Properties of EuGa2−x In x Sb2

The Zintl phase EuGa_2Sb_2 was synthesized via ball milling followed by hot pressing. The crystal structure of EuGa_2Sb_2 is comprised of a 3-D network of polyanionic [Ga_2Sb_2]^(2−) tunnels filled with Eu cations that provide charge balance (Eu^(2+)[Ga_2Sb_2]^(2−)). Here we report the temperature-dependent resistivity, Hall Effect, Seebeck coefficient and thermal conductivity for EuGa_(2−x)In_xSb_2 (x = 0, 0.05, 0.1) from 300 K to 775 K. Experimental results demonstrate that the material is a p-type semiconductor. However, a small band gap (∼0.1 eV) prevents EuGa_2Sb_2 from having high zT at higher temperatures. Isoelectronic substitution of In on the Ga site leads to point defect scattering of holes and phonons, thus reducing thermal conductivity and resulting in a slight improvement in zT.

Metal-semiconductor transition and Seebeck inversion in CoFe2O4 nanoparticles

A Semiconductor to metal transition has been observed in hydrothermally synthesized CoFe2O4 (CFO) nanoparticles. Temperature dependent Seebeck coefficient was measured in order to confirm the type of majority charge carriers and conductivity vs. temperature measurement was performed to get the activation energy. The CFO nanoparticles shows p-type semiconducting behaviour at lower temperature as confirmed by the sign of Seebeck coefficient and at elevated temperatures a sign inversion in Seebeck voltage was observed. The p-type and n-type semiconducting behaviour have been attributed to hopping of holes and electrons between Co3+–Co2+ and Fe2+–Fe3+ respectively dominating at different temperatures.

Metal-like electrical conductivity in LaxSr2-xTiMoO6 oxides for high temperature thermoelectric power generation.

Increasing electrical conductivity in oxides, which are inherently insulators, can be a potential route in developing oxide-based thermoelectric power generators with higher energy conversion efficiency. In the present work, environmentally friendly non-toxic double perovskite LaxSr2-xTiMoO6 (LSTM) ceramics were synthesized using a solid-state reaction route by optimizing the sintering temperature and atmosphere for high temperature thermoelectric applications. Rietveld refinement of XRD data confirmed a single-phase solid solution with a cubic structure in these double perovskites with the space-group Pm3[combining macron]m. SEM studies showed a highly dense microstructure in these ceramics. High electrical conductivity on the order of 105 S m-1 and large carrier concentration (∼1022 cm-3) were obtained in these materials. The temperature-dependent electrical conductivity measurement showed that the LSTM ceramics exhibit a semiconductor to metal transition. Thermopower (S) measurements demonstrated the conductivity switching from a p-type to n-type behavior at higher temperature. A temperature dependent Seebeck coefficient was further explained using a model for coexistence of both types of charge carriers in these oxides. A conductivity mechanism of these double perovskites was found to be governed by a small polaron hopping model.

Iron Disilicide as High-Temperature Reference Material for Traceable Measurements of Seebeck Coefficient Between 300 K and 800 K

Thermoelectric generators (TEGs) convert heat to electrical energy by means of the Seebeck effect. The Seebeck coefficient is a central thermoelectric material property, measuring the magnitude of the thermovoltage generated in response to a temperature difference across a thermoelectric material. Precise determination of the Seebeck coefficient provides the basis for reliable performance assessment in materials development in the field of thermoelectrics. For several reasons, measurement uncertainties of up to 14% can often be observed in interlaboratory comparisons of temperature-dependent Seebeck coefficient or in error analyses on currently employed instruments. This is still too high for an industrial benchmark and insufficient for many scientific investigations and technological developments. The TESt (thermoelectric standardization) project was launched in 2011, funded by the German Federal Ministry of Education and Research (BMBF), to reduce measurement uncertainties, engineer traceable and precise thermoelectric measurement techniques for materials and TEGs, and develop reference materials (RMs) for temperature-dependent determination of the Seebeck coefficient. We report herein the successful development and qualification of cobalt-doped β-iron disilicide (β-Fe0.95Co0.05Si2) as a RM for high-temperature thermoelectric metrology. A brief survey on technological processes for manufacturing and machining of samples is presented. Focus is placed on metrological qualification of the iron disilicide, results of an international round-robin test, and final certification as a reference material in accordance with ISO-Guide 35 and the “Guide to the expression of uncertainty in measurement” by the Physikalisch-Technische Bundesanstalt, the national metrology institute of Germany.

Thermoelectric signals of state transition in polycrystalline SmB6

Topological Kondo insulator SmB6 has attracted quite a lot of attention from the condensed matter physics community. A number of unique electronic properties, including low-temperature resistivity anomaly, 1D electronic transport and 2D Fermi surfaces have been observed in SmB6. Here, we report on thermoelectric transport properties of polycrystalline SmB6 over a broad temperature from 300 to 2 K. An anomalous transition in the temperature-dependent Seebeck coefficient S from S(T) T−1 to S(T) T was observed around 12 K. Such a transition demonstrates a transition of conductivity from 3D metallic bulk states to 2D metallic surface states with insulating bulk states. Our results suggest that the thermotransport measurements could be used for the characterization of state transition in topological insulators.

N and p-type properties in organo-metal halide perovskites studied by Seebeck effects

Organo-metal halide perovskites can exhibit co-existed electrical polarizations and semiconducting properties respectively from organic and inorganic components. Here, we find that the Seebeck coefficient can be changed between positive and negative values when the concentration of chloride ions is varied between single-halide (CH3NH3PbI3) and mixed-halide structures (CH3NH3PbIxCl3−x). This indicates that varying the concentration of chloride ions can tune the semiconducting properties between the n-type and p-type regimes in the organo-metal halide perovskites. Our temperature-dependent capacitance measurement shows that increasing temperature can cause a change on internal electrical polarization. As a result, we can propose that the internal polarization functions as the underlying mechanism responsible for large temperature-dependent Seebeck coefficients in organo-metal halide perovskites operating between n-type and p-type regimes.

Influence of induced defects on transport properties of the Bridgman-grown Bi2Se3-based single crystals

The impact of induced defects such as Bi intercalants, Bi antisites, and Se vacancies on the transport properties of the Bridgman-grown Bi2Se3-based single crystals in the ab plane were investigated by means of temperature-dependent electrical resistivity, Seebeck coefficient, and thermal conductivity measurements. We found that the Bi2Se3-based crystals can be grown either along the c-axis or perpendicular to it, depending on the different Bi-Se ratios in the starting material. All grown crystals showed a weak metallic behavior with a predominant electron-phonon scattering governing their electrical transport. The absolute value of the Seebeck coefficient of the n-type crystals grown along the c-axis is higher than those grown perpendicular to the c-axis (along the ab plane) at room temperature. The Fermi energy estimated from the Seebeck coefficient data is in the range of 0.20–0.43 eV. The thermal conductivity measurement showed that the lattice phonons dominate thermal transport in these Bi2Se3-based crystals. Analyses of lattice thermal conductivity data of the crystals by the Debye-Callaway approximation revealed that both boundary and point-defect scattering of phonons have a significant effect on the size and shape of the low-temperature phonon peak. These present findings indicate that, in addition to the change in composition, the physical properties of the Bi2Se3-based crystals are also affected significantly by the induced defects. Finally, the near-stoichiometric Bi2.04Se2.99 compound has a room-temperature thermoelectric figure of merit (ZT) of ∼0.12.

Effects of negative response of electron transport to thermoelectric properties of Bi2O2Se

In thermoelectrics, the negative response of the electron transport can be observed and can block the flow of electrons leading to the reduced thermal current. As a result, the thermoelectric device’s efficiency decreases accordingly. Because of interesting properties and superior stability of Bi2O2Se at elevated temperature, it is used in this work to investigate the effects of temperature on thermoelectric properties. In the first part of calculations, density functional theory (DFT) and semiclassical Mott formula are introduced to compute density of states (DOS) and energy dependent thermoelectric properties, respectively. Then the hopping model is employed to investigate temperature-dependent thermoelectric properties, such as, electrical conductivity (σ), Seebeck coefficient (S), and power factor (PF=S2σ). The obtained results agree well with the experimental data in literature where the maximum PF=1.1×10−4W/(mK2) occurs around 800K. This investigation can provide basic knowledge and better understandings of the negative response of electron transport to temperature-dependent thermoelectric properties.

Study on Transport Property of VO2 thin Film Observed by Temperature and Magnetic Field Dependence of Hall Effect and Temperature Dependence of Seebeck Effect

Here, we investigated the temperature and magnetic-field-dependent Hall effect and temperature-dependent Seebeck effect in VO₂ thin films by using physical property measurement systems. It was observed from the ordinary Hall effect measurement from room temperature to 370K that the Hall voltage decreased with increasing external magnetic field and dominant carriers changed near the critical temperature where an insulator-to-metal transition in VO₂ was triggered. The decrease of the Hall voltage implies the weakening of the strong electron correlation with increasing applied magnetic field. In the case of the Seebeck coefficient in VO₂ thin films, its negative slope variation was obtained with respect to the temperature gradient between two ends of the VO₂ film at room temperature. The Seebeck coefficient could be evaluated as -250 and -28㎶/K at 300 and 360K, respectively. Near the critical temperature of VO₂, the rate of change in the Seebeck coefficient with respect to the temperature gradient showed a gradual decrease in its magnitude and then changed its sign with increasing temperature gradient. This peculiar variation of the rate of change in the Seebeck coefficient in the vicinity of the critical temperature seems to stem from the percolated motion of the structural phase transition in VO₂.

Large anisotropic thermal transport properties observed in bulk single crystal black phosphorus

The anisotropy of thermal transport properties for bulk black phosphorus (BP) single crystal, which might be of particular interest in the fabrication of thermoelectric/optoelectronic devices, was investigated by using angular dependent thermal conductivity and Seebeck coefficient measurements at various temperatures. We found that the maximum thermal conductivities in x (zigzag), y (armchair), and z (perpendicular to the puckered layers) directions are 34, 17, and 5 W m−1 K−1, respectively, exhibiting large anisotropy. At temperature around 200 K, a large Seebeck coefficient up to +487 ± 10 μV/K has been obtained in x direction, which is 1.5 times higher than that in z direction. The large anisotropy of thermal transport properties can be understood from the crystal structure and bonding characters of BP. In addition, the energy gap has been obtained from nuclear spin lattice relaxation measurements, which is consistent with the value derived from temperature-dependent Seebeck coefficient measurements.

Thermoelectric properties of PbTe with indium and bismuth secondary phase

Lead telluride (PbTe) with indium (In) and bismuth (Bi) as micrometer sized secondary phases dispersed throughout the bulk has been prepared by matrix encapsulation method. In and Bi are not found to substitute in PbTe as shown by Rietveld and room temperature Raman studies but are present as secondary phases. Increased values of temperature dependent electrical resistivity and Seebeck coefficient show the effect of interfaces on electronic transport. As expected, thermal conductivity is found to reduce on addition of secondary phases due to a reduced electronic contribution, further confirming that electron scattering at interfaces is more important than phonon scattering in such systems for thermoelectric properties. However, due to the reduction in the power factor of the In and Bi added samples from that of the parent PbTe, the overall thermoelectric figure of merit (zT) does not increase beyond that of PbTe, for which the highest value of 0.7 is obtained at 778 K.

Hydrogen permeance studies in ordered ternary Cu–Pd alloys

Considerable attention has recently been given to bimetallic systems such as Cu–Pd alloys as hydrogen storage material. These alloys are attractive alternatives to pure Pd membranes because of their sulphur tolerance and high permeability to hydrogen. The density functional theory based full potential linearized augmented plane wave and projector augmented wave methods have been implemented to understand the phenomenon of hydrogen diffusion in Cu–Pd alloys. The AuCu3 structure of CuPd3 and Cu3Pd allows intercalation of hydrogen atoms in its octahedral sites. The lattice parameters and volume of the unit cell expand on hydrogenation, without phase change.We have studied the formation and stability of hydrides of the ternary alloys. The heats of formation support the hydrogenation of CuPd3 alloy whereas the endothermic reactions indicate that hydrogen will be absorbed in the Cu3Pd alloy at the expense of energy. The phonon dispersion relations of the hydrides of CuPd3 exhibits instability due to the Pd–H bonds which leads to distortion of Pd-octahedra in the unit cell. Softening of acoustic modes is not observed in the Cu3Pd alloy; instead the optical modes due to H–H bonds have very high vibrational frequencies.The permeance of H atoms in the alloy can be quantified from the extent of hybridization between H s-states, Cu and Pd d-states. From the density of states, the movement of the H s-band and centre of d-bands towards the Fermi energy level EF was observed. Both the factors indicated that the monohydride of CuPd3 was most stable. The temperature dependent Hall and Seebeck coefficients reflect the modifications in the electronic structure due to formation of hydrides. At ambient temperatures, the free charges carriers (electrons) are found to increase in the monohydride of Cu3Pd, whereas it decreases in the monohydride of CuPd3. The electronic component of specific heats γ obtained from low temperature specific heats is found to decrease in the hydrides of both the alloys, which implies an increase in the lattice contribution through the electron-phonon coupling. From the optical properties, it is possible to monitor the hydrogen uptake in the alloys by studying the variations in reflectance, absorption coefficients in the visible regions.The electronic and dynamical studies suggest that the alloy with excess Pd has potential applications as hydrogen storage materials. The monohydride of CuPd3 appears to be appropriately stable for applications such as electrode material in rechargeable batteries.

Thermoelectric characteristics of textured KSr2Nb5O15 ceramics

Effect of crystal anisotropy on thermoelectric properties of textured KSr2Nb5O15 ferroelectric ceramics is studied for the first time. Temperature dependent conductivity and Seebeck coefficient were measured in directions parallel and perpendicular to the c direction. A transition in electrical conduction from semiconductor to metallic behavior was observed at around 610K. The power factor was remarkably improved depending on measurement directions such that it was 390μW/mK2 in parallel and 283μW/mK2 in perpendicular to the c direction at 1120K. Textured KSr2Nb5O15 ceramics are promising candidates to be used as n-type thermoelectric materials at high temperatures and oxidizing environments.

Electrical transport properties of Ge-doped GaN nanowires

The conductivity and charge carrier concentration of single GaN nanowires (NWs) doped with different concentrations of Ge were determined by four-point resistivity and temperature-dependent Seebeck coefficient measurements. We observed high carrier concentrations ranging from 9.1 × 1018 to 5.5 × 1019 cm−3, well above the Mott density of 1.6 × 1018 cm−3, and conductivities up to 625 S cm−1 almost independent of the NW diameter. The weak temperature dependence of the conductivity between 2 and 10 K could be assigned to the formation of an impurity band. For the sample with the highest conductivity metallic behaviour was found, indicated by a positive temperature coefficient of the resistivity. The near band edge emission analyzed by micro-photoluminescence spectroscopy showed only a small increase of the peak width up to 70 meV and no spectral shift for carrier concentrations up to 5.5 × 1019 cm−3. The latter was attributed to the simultaneous influence of band filling, band gap renormalization, and strain.

Band gap estimation from temperature dependent Seebeck measurement—Deviations from the 2e|S|maxTmax relation

In characterizing thermoelectric materials, electrical and thermal transport measurements are often used to estimate electronic band structure properties such as the effective mass and band gap. The Goldsmid-Sharp band gap, Eg = 2e|S|maxTmax, is a tool widely employed to estimate the band gap from temperature dependent Seebeck coefficient measurements. However, significant deviations of more than a factor of two are now known to occur. We find that this is when either the majority-to-minority weighted mobility ratio (A) becomes very different from 1.0 or as the band gap (Eg) becomes significantly smaller than 10 kBT. For narrow gaps (Eg ≲ 6 kBT), the Maxwell-Boltzmann statistics applied by Goldsmid-Sharp break down and Fermi-Dirac statistics are required. We generate a chart that can be used to quickly estimate the expected correction to the Goldsmid-Sharp band gap depending on A and Smax; however, additional errors can occur for S < 150 μV/K due to degenerate behavior.

A unified physical model of Seebeck coefficient in amorphous oxide semiconductor thin-film transistors

A unified physical model for Seebeck coefficient was presented based on the multiple-trapping and release theory for amorphous oxide semiconductor thin-film transistors. According to the proposed model, the Seebeck coefficient is attributed to the Fermi-Dirac statistics combined with the energy dependent trap density of states and the gate-voltage dependence of the quasi-Fermi level. The simulation results show that the gate voltage, energy disorder, and temperature dependent Seebeck coefficient can be well described. The calculation also shows a good agreement with the experimental data in amorphous In-Ga-Zn-O thin-film transistor.

Low-temperature soft-chemical synthesis and thermoelectric properties of barium-filled p-type skutterudite nanocrystals

Bulk skutterudites, such as cobalt triantimonide (CoSb3) are promising inorganic materials for thermoelectric power generation at high temperatures. Generally, bulk CoSb3 is synthesized by high temperature solid state reactions. Herein, we demonstrate the low temperature solution phase synthesis of p-type nanocrystalline CoSb3 and Ba-filled CoSb3. Increase in the temperature dependent Seebeck coefficient with simultaneous increase in temperature dependent electrical conductivity has been observed in the present nanocrystalline samples, which is unusual in the case of bulk CoSb3. Efficient phonon scattering by nanoscale grain boundaries and the additional phonon damping due to the rattling of Ba in the void of nanocrystalline CoSb3 give rise to low thermal conductivity, which results in improved thermoelectric performance in nanocrystalline p-type Ba0.048CoSb3.

Thermoelectric performance of intermetallic FeGa3 with Co doping

Investigation on temperature-dependent electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) of intermetallic Fe1−xCoxGa3 (0.005⩽x⩽0.5) compounds are carried out to probe their thermoelectric performance. From resistivity study, it is observed that increase in number of valence electrons introduced by Co doping leads to a change from semiconducting to metallic behavior, which occurs between x=0.05 and 0.125. The characteristics of the Seebeck coefficient show a substantial decrease with the Co doping, due to the modifications in the band gap and the Fermi-level density of states. Analyses of thermal conductivity of the Co doped FeGa3 compounds reveal that thermal transport is essentially due to the lattice phonons. It is also noticed that the low-temperature peak in the lattice thermal conductivity of these compounds is reduced significantly with the increase in Co content, attributing to the enhanced scattering of phonons by point-defects. The value of the figure-of-merit, ZT=(S2/ρκ)T, is estimated for all compounds, and the maximum room-temperature ZT value of about 0.02 was achieved for Fe0.95Co0.05Ga3, and increased further with temperature to the value of about ∼0.05 at 400K.

Thermoelectric Properties of Transition Metal Deposited MWCNT Buckypaper

In this work, we report the preparation of transition metal deposited flexible multiwalled carbon nanotube buckypaper for thermoelectric applications. MWCNT buckypaper prepared by dispersion and filtration method was then deposited with the transition metals such as silver (Ag) and copper (Cu) by the electrodeposition method. We measured the voltage yield of Ag and Cu-doped buckypaper by making the temperature gradient along the sample. We established the temperature dependent Seebeck coefficient for Ag and Cu-doped buckypaper and found significant increase in the S(T). It is also revealed that remarkable rise in the value of S(T) and output voltage by connecting 3-sheets of BP in series. Here we determined the enhancement of Seebeck coefficient by increasing the number of BP sheets, thereby improving the thermoelectric efficiency. Furthermore, these paper-like CNT films show good flexibility, which makes them possible to be widely applied in various flexible energy conversion devices.