Room-temperature Thermoelectric Power Research Articles

Effects of Pb and carbonate substitutions on the superconducting properties of TlSr4Cu2OzCrO4

We have investigated the effects of Pb and carbonate substitutions on the phase formation and the superconducting properties of the intergrowth superconductor TlSr4Cu2OzCrO4. X-ray diffraction data showed that nearly single-phase samples with tetragonal structure were obtained in the compositions (Tl1-xPbx)Sr4Cu2OzCrO4 (0 ≤ x ≤ 0.5) and (Tl0.5Pb0.5)Sr4Cu2Oz(CrO4)1-y(CO3)y (0 ≤ y ≤ 1.0). Resistivity measurements indicated that the transition temperature decreased significantly as the Pb doping content x increased to x = 0.5. In contrast to the Pb doping effects, the carbonate substitution in the (Tl0.5Pb0.5)Sr4Cu2Oz(CrO4)1-y(CO3)y system resulted in an improvement of the superconducting transition temperature up to y = 0.9. The y = 1.0 sample showed a reduced transition temperature, compared to the x = 0.9 sample. The experimental results are discussed in conjunction with the change in hole carrier concentration due to the Pb and carbonate substitutions, based on the data from room temperature thermoelectric power measurements.

Synthesis and enhanced room-temperature thermoelectric properties of CuO-MWCNT hybrid nanostructured composites.

This work presents the synthesis of novel copper oxide-multiwalled carbon nanotube (CuO-MWCNT) hybrid nanostructured composites and a systematic study of their thermoelectric performance at near-room temperatures as a function of MWCNT wt% in the composite. The CuO-MWCNT hybrid nanostructured composites were synthesized by thermal oxidation of a thin metallic Cu layer pre-deposited on the MWCNT network. This resulted in the complete incorporation of MWCNTs in the nanostructured CuO matrix. The thermoelectric properties of the fabricated CuO-MWCNT composites were compared with the properties of CuO-MWCNT networks prepared by mechanical mixing and with the properties of previously reported thermoelectric [CuO]99.9[SWCNT]0.1 composites. CuO-MWCNT hybrid composites containing MWCNTs below 5 wt% showed an increase in the room-temperature thermoelectric power factor (PF) by ∼2 times compared with a bare CuO nanostructured reference thin film, by 5-50 times compared to mixed CuO-MWCNT networks, and by ∼10 times the PF of [CuO]99.9[SWCNT]0.1. The improvement of the PF was attributed to the changes in charge carrier concentration and mobility due to the processes occurring at the large-area CuO-MWCNT interfaces. The Seebeck coefficient and PF reached by the CuO-MWCNT hybrid nanostructured composites were 688 μV K-1 and ∼4 μW m-1 K-2, which exceeded the recently reported values for similar composites based on MWCNTs and the best near-room temperature inorganic thermoelectric materials such as bismuth and antimony chalcogenides and highlighted the potential of CuO-MWCNT hybrid nanostructured composites for applications related to low-grade waste heat harvesting and conversion to useable electricity.

Atomic layer deposition of Zr-sandwiched ZnO thin films for transparent thermoelectrics

Atomic layer deposited (ALD) transparent thermoelectric materials enable the introduction of energy harvesting and sensing devices onto surfaces of various shapes and sizes in imperceptible manner. Amongst these materials, ZnO has shown promising results in terms of both thermoelectric and optical characteristics. The thermoelectric performance of ZnO can be further optimized by introducing extrinsic doping, to the realization of which ALD provides excellent control. Here, we explore the effects of sandwiching of ZrO2 layers with ZnO on glass substrates. The room-temperature thermoelectric power factor is maximised at 116μW m−1 K−2 with samples containing a 2% nominal percentage of ZrO2. The addition of ZrO2 layers is further shown to reduce the thermal conductivity, resulting in a 20.2% decrease from the undoped ZnO at 2% doping. Our results contribute to increasing the understanding of the effects of Zr inclusion in structural properties and growth of ALD ZnO, as well as the thermal and thermoelectric properties of Zr-doped ZnO films in general.

Room-temperature power factor of CuAlO2 composite tablets enhanced by MWCNTs

CuAlO2 with high theoretical thermoelectric performance has potential applications in thermal energy conversion. Herein, multi-wall carbon nanotubes (MWCNTs)/CuAlO2 composite tablets are prepared by using different amounts of MWCNTs and solid paraffin binder, where MWCNTs served as a conductive agent and rendered three orders of magnitude increase in electrical conductivity. Seebeck coefficient of the composites was reduced with increasing MWCNTs content. Consequently, an optimal room-temperature thermoelectric power factor (PF) of 1.31 μW m−1K−2 has been rendered by MWCNTs/CuAlO2 composite tablet with 1 wt % MWCNTs. Moreover, PF value increased with increasing temperature after a slight decrease at 333 K, which can be ascribed to the modulation of electrical conductivity. Current work provides an effective strategy to improve thermoelectric performance of CuAlO2 materials.

Simultaneous enhancement in thermopower and electrical conductivity of SnTe-Te nano heterostructures

SnTe nanomaterials are investigated as the potential “Lead” free thermoelectric material owing to their low thermal conductivity induced enhanced figure of merit in comparison with their bulk counterpart. In this work, SnTe and its SnTe-Te heterostructures were synthesized for the first time by changing the protocol of heating time using microwave-assisted solvothermal processes. The confocal and scanning electron microscope images demonstrate the nano-flower-like morphology of SnTe-Te heterostructures. Multi-phase Reitveld refinement structural analysis exhibits a significant increase in tellurium occupancies, whereas elemental mapping confirms the presence of SnTe-Te heterostructure. Interestingly, room temperature thermoelectric power of SnTe-Te heterostructures (~60 μV/K) is increased by ~55% when compared to SnTe nanostructures (~45 μV / K). Simultaneous enhancement of both electrical conductivity and Seebeck coefficient are attributed to the increment of carrier mobility and effective mass respectively which is due to the novel strategy of SnTe-Te heterostructures formation that is promising for large scale p-type thermoelectric applications.

Improved room-temperature thermoelectric characteristics in F4TCNQ-doped CNT yarn/P3HT composite by controlled doping

Abstract High room-temperature thermoelectric performance is important for low-grade waste heat power generation as there are plenty of heat thrown away uselessly in our daily life, most of which are below 100 °C. However, most of the thermoelectric materials are limited to high temperature application. In this work, room-temperature thermoelectric power factor of carbon nanotube (CNT) yarn is improved by controlled doping, which is achieved by making composite with poly 3-hexylthiophene −2, 5-diyl (P3HT) followed by doping with 2, 3, 5, 6-tetrafluo-7, 7, 8, 8-tetracyanoquinodimethane (F4TCNQ). The temperature-dependent Seebeck coefficient based on power–law model suggests that P3HT shifts the Fermi energy of CNT yarn towards the valence band edge, and reduces the ionic scattering and carrier relaxation time. As a result, the Seebeck coefficient is increased while the variation of Seebeck coefficient with temperature is reduced, and hence, the room-temperature thermoelectric power factor is improved. With controlled doping, the power factor of CNT yarn/P3HT composite reaches to 1640–2160 μW m−1K−2 at the temperature range of 25–100 °C, which is higher than that of CNT yarn alone.

Substrate temperature-dependent thermoelectric figure of merit of nanocrystalline Bi2Te3 and Bi2Te2.7Se0.3 prepared using pulsed laser deposition supported by DFT study

This work reports the pulsed laser deposition of n-type selenium (Se) doped bismuth telluride (Bi2Te2.7Se0.3) and n-type bismuth telluride (Bi2Te3) nanostructures under varying substrate temperatures. The influence of the substrate temperature during deposition on the structural, morphological and thermoelectric properties for each phase was investigated. Density functional theory (DFT) simulations were employed to study the electronic structures of the unit-cells of the compounds as well as their corresponding partial and total densities of states. Surface and structural characterization results revealed highly crystalline nanostructures with abundant grain boundaries. Systematic comparative analysis to determine the effect of Se inclusion into the Bi2Te3 matrix on the thermoelectric properties is highlighted. The dependence of the thermoelectric figure of merit (ZT) of the nanostructures on the substrate temperatures during deposition was demonstrated. The remarkable room temperature thermoelectric power factor (PF) of 2765 μW/mK2 and 3179 μW/mK2 for pure and Se-doped Bi2Te3 compounds respectively, signifies their potential of being useful in cooling and power generation purposes. The room temperature ZT values of the Se-doped Bi2Te3 was found to be 0.92, about 30% enhancement as compared with the pure phase, which evidently results from the suppressed thermal conductivity in the doped species caused by phonon scattering at the interfaces.

Thermoelectric phase diagram of the SrTiO3-LaTiO3 solid-solution system through a metal to Mott insulator transition

Systematic clarification of thermoelectric properties of a solid-solution system is essentially important, especially if the materials have different crystal and electronic structures. Here, we report the thermoelectric phase diagram of the full-range Sr1 − xLaxTiO3 solid solutions composed of a band insulator SrTiO3 (cubic perovskite) and a Mott insulator LaTiO3 (distorted perovskite), which shows the room temperature thermoelectric power factor changing pattern with the La substitution (x) in the SrTiO3 system. We measured the room temperature thermoelectric properties of Sr1 − xLaxTiO3 (0.01 ≤ x ≤ 1) epitaxial films grown by the pulsed laser deposition and found that there are two thermoelectric phase boundaries at x ∼ 0.5 and x ∼ 0.7 in the Sr1 − xLaxTiO3 system. The former is attributed to the lattice distortion from cubic to orthorhombic variation, whereas the latter is due to the electronic phase transition from metal to Mott insulator. The thermoelectric phase diagram obtained in our study will be fruitful for the future design of n-type thermoelectric materials based on the electron-doped SrTiO3.

Defect‐Controlled Copper Iodide: A Promising and Ecofriendly Thermoelectric Material

AbstractEarth‐abundant and non‐toxic materials are important in the utilization of thermoelectric technology. Here, the thermoelectric behavior of copper iodide (CuI), which is prepared in bulk using a simple synthetic approach, is reported. The as‐prepared CuI has a high Seebeck coefficient of ≈431 μV K−1 with a resistivity ≈0.26 Ω cm−1, resulting in a room temperature thermoelectric power factor of ≈70 μW m−1 K−2. The thermoelectric properties are tuned by changing the defect chemistry via thermal annealing. Annealing at moderate temperatures (≈400 K) improves thermoelectric power factor, from its room temperature value of ≈142 μW m−1 K−2 to ≈160 μW m−1 K−2 at 353 K. Detailed analysis reveals that this is caused by iodine depletion upon annealing, which compensates for the intrinsic defects present in the form of copper vacancies that are responsible for the p‐type conductivity of the CuI.

Crystal Structure of (Lu1−x Ca x )Ba2Cu3O7−z (x = 0, 0.2) Superconducting Compounds Studied by Neutron Powder Diffraction

The crystalline structures of two polycrystalline samples of (Lu1−x Ca x )Ba2Cu3O7−z with x = 0 (pure sample) and x = 0.2 (Ca-doped sample) have been analyzed by using neutron powder diffraction techniques to address the structural mechanisms associated with changes in superconducting behavior induced by Ca doping. The pure and Ca-doped samples exhibit superconductivity around 87 and 32 K, respectively. The room-temperature thermoelectric power measurements indicate that both the x = 0 and 0.2 samples are in overdoped state. The Rietveld refinements of the neutron diffraction data collected at room temperature for the pure and Ca-doped samples show that the higher level of oxygen vacancies is located on both the CuO2 planes and the apical oxygen sites for the pure sample compared to those of the Ca-doped sample. The present results suggest that the decrease in T c observed in the Ca-doped sample is mainly caused by the hole overdoping effect rather than by oxygen vacancies in the CuO2 planes.

Enhanced thermoelectric transport in modulation-doped GaN/AlGaN core/shell nanowires

The thermoelectric properties of unintentionally n-doped core GaN/AlGaN core/shell N-face nanowires are reported. We found that the temperature dependence of the electrical conductivity is consistent with thermally activated carriers with two distinctive donor energies. The Seebeck coefficient of GaN/AlGaN nanowires is more than twice as large as that for the GaN nanowires alone. However, an outer layer of GaN deposited onto the GaN/AlGaN core/shell nanowires decreases the Seebeck coefficient at room temperature, while the temperature dependence of the electrical conductivity remains the same. We attribute these observations to the formation of an electron gas channel within the heavily-doped GaN core of the GaN/AlGaN nanowires. The room-temperature thermoelectric power factor for the GaN/AlGaN nanowires can be four times higher than the GaN nanowires. Selective doping in bandgap engineered core/shell nanowires is proposed for enhancing the thermoelectric power.

Enhanced Thermoelectric Performance of Nanostructured Bi2Te3 through Significant Phonon Scattering.

N-type Bi2Te3 nanostructures were synthesized using a solvothermal method and in turn sintered using sparking plasma sintering. The sintered n-type Bi2Te3 pellets reserved nanosized grains and showed an ultralow lattice thermal conductivity (∼0.2 W m(-1) K(-1)), which benefits from high-density small-angle grain boundaries accommodated by dislocations. Such a high phonon scattering leads an enhanced ZT of 0.88 at 400 K. This study provides an efficient method to enhance thermoelectric performance of thermoelectric nanomaterials through nanostructure engineering, making the as-prepared n-type nanostructured Bi2Te3 as a promising candidate for room-temperature thermoelectric power generation and Peltier cooling.

Structural, Transport, and Magnetic Properties of (Ru0.9Nb0.1)Sr2(Gd0.67R0.67Ce0.66)Cu2O z (R = Nd, Gd, and Tb)

Samples with nominal compositions of (Ru0.9Nb0.1)Sr2(Gd0.67R0.67Ce0.66)Cu2Oz (R = Nd, Gd, Tb) were prepared, and the influence of doped rare-earth element on the structural, magnetic and superconducting properties of these samples has been investigated. Resistivity and room-temperature thermoelectric power measurements showed that the superconductivity is mainly affected by the change in the hole concentration induced by doping of the rare-earth element. All of these samples exhibited weak ferromagnetic behavior at temperatures below approximately 90 K with the branching of zero-field-cooled (ZFC) and field-cooled (FC) magnetization. In the ZFC magnetization measurements, diamagnetic transition was observed at temperatures below approximately 15 K for the R=Gd and Nd samples. Magnetization measurements also revealed that the partial substitution of Tb for Gd results in a significant enhancement of weak-ferromagnetic component of the FC magnetization, as well as an increase in the magnetic ordering temperature up to 20 K. A quite opposite behavior was observed in the FC magnetization for the case of the R=Nd sample. The experimental results are discussed in conjunction with the local structural changes in the Ru sublattice, which are induced by doping of the rare-earth element, based on the results of Rietveld refinement of the X-ray diffraction data.

Synthesis and Thermoelectric Studies in Cr Doped Cobalt Ferrites Nano Particles

Samples of Cr doped cobalt ferrite were prepared by co-precipitation route. These particles were characterized by X-ray diffraction (XRD) at room temperature. The structural properties were observed before and after sintering. The FCC spinel structure was confirmed by XRD patterns of the samples. The crystallite sizes lie in the range of 37-60 nm. DC electrical properties as a function composition were measured. Scanning electron microscopy was used in order to investigate the surface morphology of the prepared samples. The system for thermoelectric power measurement was designed, developed and calibrated in the laboratory. The room temperature thermoelectric power was measured for the prepared samples. The magnitude of Seebeck coefficient depends on the composition and resistivity of the samples. The obtained values of Seebeck coefficient for CoFe2O4 are in good agreement to the reported values. Determined values of Seebeck coefficient for other studied compositions are an addition to the literature.

Thermodynamic and thermoelectric properties of (Ga,Mn)As and related compounds

Various experimental results providing information on thermodynamic density of states in (Ga,Mn)As are analyzed theoretically assuming that holes occupy GaAs-like valence bands. Allowing for Gaussian fluctuations of magnetization, the employed model correctly describes a critical behavior of magnetic specific heat found experimentally in (Ga,Mn)As near the Curie temperature ${T}_{\text{C}}$ [S. Yuldashev et al., Appl. Phys. Express 3, 073005 (2010)]. The magnitudes of room-temperature thermoelectric power, as measured for GaAs:Be and (Ga,Mn)As [M. A. Mayer et al., Phys. Rev. B 81, 045205 (2010)], are consistent with the model for the expected energy dependencies of the hole mobility. The same approach also describes temperature variations of conductance specific to the Anderson-Mott localization, found for various dimensionality (Ga,Mn)As nanostructures at subkelvin temperatures [D. Neumaier et al., Phys. Rev. Lett. 103, 087203 (2009)]. We conclude that the examined phenomena do not provide evidence for an enhancement of density of states by the presence of an impurity band at the Fermi energy in ferromagnetic (Ga,Mn)As. Furthermore, for (Ga,Mn)As we provide expected values of both electronic specific heat at low temperatures $T\ensuremath{\ll}{T}_{\text{C}}$ and magnetization as a function of the magnetic field at ${T}_{\text{C}}$.

Electrical and thermal properties of Pr2/3(Ba1−xCsx)1/3MnO3 manganites

Electrical and thermal properties of Pr2/3(Ba1−x Cs x )1/3MnO3 (0 ≤ x ≤ 0.25) manganite perovskites are reported here. Two insulator-metal (I-M) transitions (T P1 &T P2) are observed in the electrical resistivity (ρ) of the pristine Pr2/3Ba1/3MnO3 (PBMO) sample, and they are systematically shifted to lower temperatures with increasing Cs substitution. An upturn in ρ is noticed below 50 K in these perovskites, presumably due to the combined effect of weak localization, electron-electron and electron-phonon scattering. It is found that the absolute value of room-temperature thermoelectric power (TEP) gradually decreases with increasing Cs content, implying the annihilation of the charge carriers with doping. An analysis of the electrical resistivity and thermoelectric power data indicates that the paramagnetic insulating state above T P1 is governed by the small polaron hopping due to a non-adiabatic process. It is argued that the electron-magnon scattering processes are responsible for low temperature metallic behavior of TEP. A distinct specific heat peak below T P1 is observed, attributed to the magnetic ordering, and its broadening with Cs-doping corresponds to the increase of magnetic inhomogeneity. Further, the temperature variation of thermal conductivity and the low temperature plateau in κ has been associated with the delocalization of Jahn-Teller polarons and transition from Umklapp scattering to a defect-limited scattering, respectively.

Interplay between magnetic properties and thermoelectricity in misfit and Na cobaltates

We present a comparative study of $\mathrm{Co}{\mathrm{O}}_{2}$ layers in the Bi misfit and ${\mathrm{Na}}_{x}\mathrm{Co}{\mathrm{O}}_{2}$ cobaltates. Co NMR measures the intrinsic susceptibility of the Co layers and is not affected by spurious contributions. At low dopings where room-temperature thermoelectric power (TEP) is large, Curie-Weiss susceptibilities are observed in both materials. But NMR and muon-spin resonance experiments find neither charge nor spin order down to low temperatures in Bi misfit cobaltates in contrast to the case of ${\mathrm{Na}}_{x}\mathrm{Co}{\mathrm{O}}_{2}$. This demonstrates that metallicity, charge, and magnetic order are specific to the Na layers in ${\mathrm{Na}}_{x}\mathrm{Co}{\mathrm{O}}_{2}$ whereas strong correlations are generic for cobaltate physics and could explain the large TEP.

Intergrowth and thermoelectric properties in the Bi–Ca–Co–O system

Single crystals of the Bi–Ca–Co–O system have been grown using the flux method with cooling from 900 and 950 ∘ C , respectively. The single crystals are characterized by transmission electron microscopy and X-ray diffraction. The misfit cobaltite [ Ca 2 Bi 1.4 Co 0.6 O 4 ] RS [ CoO 2 ] 1.69 single crystals with quadruple ( n = 4 ) rocksalt (RS) layer are achieved with cooling from 900 ∘ C . Such a crystal exhibits room-temperature thermoelectric power (TEP) of 180 μ V / K , much larger than that in Sr-based misfit cobaltites with quadruple RS layer. However, intergrowth of single crystals of quadruple ( n = 4 ) and triple ( n = 3 ) RS-type layer-based misfit cobaltites is observed with cooling from 950 ∘ C . Both of TEP and resistivity were obviously enhanced by the intergrowth compared to [ Ca 2 Bi 1.4 Co 0.6 O 4 ] RS [ CoO 2 ] 1.69 single crystal, while the power factor at room temperature remains unchanged.

Cu-intercalated (Bi0.5Sb1.5Te3)0.96(Bi2Se3)0.04 single crystals

A procedure is described for converting the conductivity type (p → n) of single crystals of the Bi0.5Sb1.5Te3-4 mol % Bi2Se3 solid solution via copper intercalation. Using this procedure, we have produced high-performance single-crystal n-legs with a room-temperature thermoelectric power α = −200 μV/K. This procedure facilitates the fabrication of thermoelectric coolers because the same solid solution can be used to produce p-and n-legs, the only difference being the presence of copper in the n-legs. We have fabricated thermolements and determined their characteristics in the range 100–300 K.

Superconductivity in the (Ga 1− yCo y)Sr 2(Tm 1− xCa x)Cu 2O z compounds synthesized under ambient pressure

A search for superconductivity has been performed with (Ga 0.5Co 0.5)Sr 2(Tm 1− x Ca x )Cu 2O z (0 ⩽ x ⩽ 0.5) and (Ga 1− y Co y )Sr 2(Tm 0.6Ca 0.4)Cu 2O z (0.5 ⩽ y ⩽ 1.0) compounds synthesized under ambient pressure. Materials are characterized by X-ray diffraction, electrical transport, magnetic susceptibility measurements. The X-ray diffraction pattern reveals that all samples have an orthorhombic Ima2 structure. We find that the Co substitution for Ga results in an increase of the solubility limit of Ca of GaSr 2(Tm 1− x Ca x )Cu 2O z , and thereby superconductivity is induced by the co-substitution of Ca for Tm and Co for Ga. Bulk superconductivity with a T c of 35 K is observed in a sample with nominal composition of (Ga 0.5Co 0.5)Sr 2(Tm 0.6Ca 0.4)Cu 2O z . The variation of the room-temperature thermoelectric power shows that the carrier concentration on the superconducting planes is sensitive to the substitution of Co for Ga in addition to the Ca substitution for Tm.