Positive Thermoelectric Power Research Articles

Galvanomagnetic properties and thermoelectric power of ultrathin films of the bismuth-antimony system on a mica substrate

The study of the electronic properties of ultrathin films of pure bismuth and bismuth-antimony alloys is of interest, since an increase in conductivity with decreasing sample thickness was found. This paper presents the results of an experimental study of the structure, electrical, galvanomagnetic and thermoelectric properties of pure bismuth and Bi1-xSbx thin films (x=0.05 and 0.12) on a mica substrate in the thickness range of 10-30 nm. An increase in the conductivity with a decrease in the thickness of the samples was found. It may be due to the presence of topologically protected surface states. It is shown that the features of the manifestation of this effect are significantly influenced by the alloys band structure. The form of the temperature dependences of the Seebeck coefficient casts doubt on the fact that surface states have a positive effect on the thermoelectric efficiency of thin bismuth-antimony films. However, the detection of a positive thermoelectric power in Bi0.88Sb0.12 samples can become an important factor for searching for the possibility of creating a p-branch of thermoelectric converters. Keywords: bismuth, antimony, thin films, surface states, thermoelectric power.

Probing the ‘elephant’: On the essential difference between graphenes and polycyclic aromatic hydrocarbons

There is a crucial difference between the structure of a polycyclic aromatic hydrocarbon (PAH) molecule and that of a single-layer sp2-hybridized carbon cluster; its misunderstanding leads to unresolvable inconsistencies when attempting to rationalize the behavior of graphenes across their impressively wide spectrum of existing or potential applications. The PAHs are not ‘nanographenes’, as is too often assumed, because not all the edges in graphenes are H-saturated. This increasingly obvious fact has profound implications for both ‘physical’ (e.g., magnetism, luminescence, thermoelectric power) and ‘chemical’ properties (e.g., functionalization, adsorption, reactivity, electrocatalysis) of graphene-based carbon materials. As revealed by computational quantum chemistry, the triplet ground state of graphenes, containing carbene-like active sites at the zigzag edge or in basal-plane vacancies, offers a consistent explanation for their wide-ranging physicochemical properties and behavior. The most salient consequences of such σ-π coupling of valence electrons, as well as of tailorable singlet-triplet and HOMO-LUMO gaps, account not only for the creation of holes in the conduction band, and thus positive thermoelectric power, but also for the pH-sensitive basicity of oxygen-free graphene-based materials, as well as the ubiquitous (electro)chemical interactions with molecular oxygen including the transition from a peroxy to dioxiranyl intermediate.

Vapor grown carbon nanofiber based cotton fabrics with negative thermoelectric power

Vapor grown carbon nanofiber (CNF) based ink dispersions were used to dip-coat woven cotton fabrics with different constructional parameters, and their thermoelectric (TE) properties studied at room temperature. Unlike the positive thermoelectric power (TEP) observed in TE textile fabrics produced with similar carbon-based nanostructures, the CNF-based cotton fabrics showed negative TEP, caused by the compensated semimetal character of the CNFs and the highly graphitic nature of their outer layers, which hinders the p-type doping with oxygen groups onto them. A dependence of the electrical conductivity (σ) and TEP as a function of the woven cotton fabric was also observed. The cotton fabric with the largest linear density (tex) showed the best performance with negative TEP values around − 8 μV K−1, a power factor of 1.65 × 10−3 μW m−1 K−2, and a figure of merit of 1.14 × 10−6. Moreover, the possibility of a slight e− charge transfer or n-doping from the cellulose onto the most external CNF graphitic shells was also analysed by computer modelling. This study presents n-type carbon-based TE textile fabrics produced easily and without any functionalization processes to prevent the inherent doping with oxygen, which causes the typical p-type character found in most carbon-based TE materials.

Magnetocaloric and electrical transport properties of selenospinels A0.9Cu0.1Cr2Se4 (A = Cd and Zn)

Magnetization, electrical resistivity and Seebeck coefficient of ACr2Se4 (A = Cd and Zn) spinels with 10% Cu-substitution at A-site have been studied. While Cd0.9Cu0.1Cr2Se4 orders ferromagnetically at 129 K (TC), Zn0.9Cu0.1Cr2Se4 displays complex antiferromagnetic order at TN = 20 K. Magnetocaloric effect (MCE) in the vicinity of the magnetic transition has been estimated in terms of isothermal magnetic entropy change (ΔSm). The maximum values of ΔSm for Cd0.9Cu0.1Cr2Se4 and Zn0.9Cu0.1Cr2Se4 are about −4.2 J/kg K and −3.4 J/kg K respectively at 133 K and 48 K for a field change from 0 to 70 kOe. Electrical resistivity shows Mott variable range hopping behaviour below 50 K. Large positive thermoelectric power values are observed below 300 K in both Cd0.9Cu0.1Cr2Se4 and Zn0.9Cu0.1Cr2Se4.

Negative thermoelectric power of melt mixed vapor grown carbon nanofiber polypropylene composites

In this work, commercial vapor grown carbon nanofibers (CNF), produced by chemical vapor deposition (CVD), were melt extruded with polypropylene (PP) with the aim of analyzing their thermoelectric properties (i.e., electrical conductivity, thermoelectric power, power factor and figure of merit). Unexpectedly, all PP/CNF composites showed negative thermoelectric power (TEP) values instead of showing the typical positive TEP observed for this type of carbon-based polymer composites. These results can be attributed to the double layer structure surrounding the tubular core of the carbon nanofiber grown by the CVD method at 1100 °C, which may lead the intrinsically negative TEP contribution from the larger inner layer to counteract the positive TEP contribution from the smaller outer layer due to common oxygen doping. Overall, all composites showed negative TEP values around −8.5 μVK−1 and a maximum power factor of 1.75 × 10-3 μW m-1 K−2, corresponding to a figure of merit of 1.2 × 10-6 at room temperature. This study demonstrates that melt mixed polymer composites with large-diameter tubular carbon nanostructures and negative Seebeck coefficients can be directly produced with large-scale processing methods without requiring specific additives and/or deoxygenation treatments.

Nontrivial Phenomena in Magnetic Nanocomposites Co/Al2O3 and Co/SiO2

Magnetic nanocomposites (MNC), in which nanoparticles of ferromagnetic metals are distributed in wide-gap dielectric matrices (Al2O3 or SiO2), are prospective materials for electronics due to their amenability to technological control of the concentration and size of ferromagnetic nanoparticles. Co/Al2O3 and Co/SiO2 MNC layers with Co concentrations below the percolation threshold were deposited on polycor substrates using electron-beam deposition in a vacuum (EB-PVD). Scanning electron microscopy showed the presence of tightly packed Co grains of irregular shape with sizes of 5–50 nm in MNC. Low-temperature measurements of the magnetization for MNC Co/Al2O3 were made in the temperature range 4–300 K and magnetic fields up to 10 kOe. The ‘magnetic exchange bias’ has been detected, and it increases with a rise in Co concentration. By studying the electrical properties of the MNC Co/Al2O3 and Co/SiO2 in the temperature range of 77–280 K, the Mott type electron transport hopping mechanism was established. We first observed the effect of a giant positive thermoelectric power in a magnetic field in MNC Co/Al2O3 and the effect of a negative magnetothermoelectric power in a Co/SiO2.Magnetic nanocomposites (MNC), in which nanoparticles of ferromagnetic metals are distributed in wide-gap dielectric matrices (Al2O3 or SiO2), are prospective materials for electronics due to their amenability to technological control of the concentration and size of ferromagnetic nanoparticles. Co/Al2O3 and Co/SiO2 MNC layers with Co concentrations below the percolation threshold were deposited on polycor substrates using electron-beam deposition in a vacuum (EB-PVD). Scanning electron microscopy showed the presence of tightly packed Co grains of irregular shape with sizes of 5–50 nm in MNC. Low-temperature measurements of the magnetization for MNC Co/Al2O3 were made in the temperature range 4–300 K and magnetic fields up to 10 kOe. The ‘magnetic exchange bias’ has been detected, and it increases with a rise in Co concentration. By studying the electrical properties of the MNC Co/Al2O3 and Co/SiO2 in the temperature range of 77–280 K, the Mott type electron transport hopping mechanism was established. We...

β”-(CNB-EDT-TTF)4BF4; Anion Disorder Effects in Bilayer Molecular Metals

The preparation and characterization of new salts based on the dissymmetrical TTF derivative CNB-EDT-TTF (cyanobenzene-ethylenedithio-tetrathiafulvalene) and BF4− anions, are reported. Depending on the electrocrystallization conditions salts with different stoichiometries, (CNB-EDT-TTF)BF4 and β”-(CNB-EDT-TTF)4BF4, can be obtained. The 1:1 salt is an electrical insulator isostructural to the ClO4 analogue previously described. The 4:1 salt is a new member of the family of 2D metals of this donor with different small anions X, (CNB-EDT-TTF)4X, characterized by a bilayer arrangement of the donors and it was obtained in a monoclinic polymorph with a β”-type donor packing pattern. The small anions in this compound are severely disordered between the donor bilayers, which present slightly larger lattice parameters than the isostructural ClO4 analogue. Both electrical conductivity and thermoelectric power measurements in single crystals denote metallic properties as predicted by electronic band structure calculations. As a consequence of the anion disorder the metallic regime of the electrical conductivity denotes electronic localization effects with a progressive increase of resistivity below ~25 K. Because of the larger lattice parameters the intermolecular interactions and electronic bandwidth are decreased compared to other (CNB-EDT-TTF)4X salts. The large and positive thermoelectric power S of this compound (~110 μV/K in the range 100–330 K) and its electrical conductivity σ = 20 S/cm at room temperature lead to a power factor S2σ = 24 μW/K2m, quite large among molecular conductors, placing these compounds as potential candidates for thermoelectric materials.

Structural, electrical, magnetic and thermal properties of Pr0.8-xDyxSr0.2MnO3 with (x = 0, 0.2 and 0.25)

In the present work we report structural, electrical, magnetic and thermo-power of Pr0.8-xDyxSr0.2MnO3 with (x = 0, 0.2 and 0.25) manganites. It is observed that Curie temperature TC decreases with Dy doping and also ZFC magnetization curves decrease more sharply with Dy from magnetization study. Though, Dy3+ ion has large magnetic moment μ = 10.65μB, because of its smaller ionic radius that of Pr3+ ion, changes in magnetic properties have observed. The zero field resistivity for pristine sample shows a small peak at around 157 K, near the metal-insulator transition temperature (TMI), which is also closer to the Curie temperature TC and a more pronounced peak at (Tf) around 95 K. All samples show the positive thermo-electric power throughout the temperature range.

Electrical properties of the LaLiyCo1 – yO3 – δ (0 ≤ y ≤ 0.10) oxides

The effect of the Li ion concentration on the phase composition, the electrical conductivity, and the thermoelectric power of the LaLi y Co1–yO3–δ (0 ≤ y ≤ 0.1) oxides synthesized by cocrystallization has been studied. It is found that the region of the perovskite-like solid solution LaLi y Co1–yO3–δ is no higher than y = 0.037. In the temperature range 300–1020 K, lithium alloying leads to an increase in the electrical conductivity and a decrease in the positive thermoelectric power of the single-phase samples compared to LaCoO3–δ. The results are discussed using the density of states model proposed by Senarus Rodriguez and Goodenough for LaCoO3–δ and La1–xSr x CoO3–δ and using the Mott theory of noncrystalline substances.

Preparation and characterization of chemically deposited nickel sulphide film and its application as a potential counter electrode

Nickel sulphide (NiS) film has emerged as a counter electrode in many applications, such as thin film batteries, dye sensitized solar cells, and supercapacitors. In this regard, we report the direct liquid coating of pure hexagonal NiS films on glass using a precursor solution of nickel–thiourea complex. A uniform and void free film is observed using scanning electron microscopy. The room temperature electrical conductivity of ∼5 × 103 S cm−1 and the positive thermoelectric power (+6 μV K−1) specify p-type conduction. The temperature variation conductivity in the range 77–300 K depicts the transition of NiS films from conducting to semi-conducting behaviour at certain transition temperatures. Preliminary results from a cyclic voltammetry study shows the feasibility of NiS films as counter electrodes.

Magnetic and transport properties of Gd1−xCaxCrO3 (x=0.0–0.3): Effect of orbital degeneracy in thermoelectric power

Detailed magnetic and transport properties of orthorhombic perovskites ceramics Gd1−xCaxCrO3 (x=0.0–0.3) have been reported. Magnetization measurements under both field cooled cooling (FCC) and field cooled heating (FCH) protocol reveal magnetization reversal at low temperature and low magnetic field. This indicates clear evidence of two magnetic sublattices aligned opposite to each other. The effect of Gd3+ ion on the low temperature magnetization decreases largely above x=0.2. The nature of temperature dependent dc magnetic susceptibility (χ) was Curie–Weiss like above the transition temperature (Tmax) with an effective magnetic moment (μeff) that decreased with the increase of Ca concentration. These orthochromites exhibit a temperature independent, large and positive thermoelectric power or Seebeck coefficient (S) value above 200K. Upon Ca2+ substitution, a concomitant decrease of the S value is found. The evolution of S as a function of hole concentration (Cr4+) can be well fitted by the modified Heikes formula taking into account the orbital degeneracy related to Cr3+ and Cr4+.

Decrease of Ca3Co4O9+δ thermal conductivity by Yb-doping

In this study, the effect of Yb-substitution on the structural, electrical and thermal transport properties of the Ca3Co4O9+δ system has been investigated in the low temperature region (between 10 and 300K). The resistivity of samples increases with raising the Yb-concentration in the system. All the samples show a metal-semiconductor transition below 85K. In the analysis based on Strongly Correlated Fermi Liquid Model, an increased bandwidth and a reduced electronic correlation are found. It has also been found that the energy gap, E0, value also decreases with Yb-substitution. The samples show positive thermoelectric power, indicating that dominant charge carriers are holes in all the samples. The thermoelectric power value decreases with Yb-substitution. From Mott equation, it is determined that the Fermi energy and hole concentration decrease in the Yb-susbtituted samples, compared to the undoped ones. Thermal conductivity, κ, is decreased in about 50% of the measured in undoped samples for the 0.01 and 0.03 Yb-doped samples at 300K. Highest figure of merit, ZT, value is found to be 5.4 10−3 at 300K for the unsubstituted sample and the ZT value decreases by the substitution.

Seebeck Effect and Mechanical Properties of Carbon Nanotube-Carbon Fiber/Cement Nanocomposites

Carbon nanotubes (CNTs) were homogeneously dispersed in a mixed solution by surface-active agents and ultrasonics, and then mixed with short carbon fibers before added into cement paste. Seebeck effect, microstructure, and mechanical properties of the fabricated composites were measured. Results show that the addition of CNTs to carbon fiber/cement composites results in improvement in the compressive and splitting tensile strengths of the composites. The addition of 0.5% by weight of cement of CNTs to carbon fiber/cement composites results in the highest increase in the compressive and splitting tensile strengths by 18.4% and 21.8%, respectively. The doping of CNTs leads to a highest increase (up to 260%) reaching 23.5 μV K−1 in the positive thermoelectric power of the carbon fiber/cement composites, when CNTs addition reaches 0.5% by weight of cement. The microscopic observation shows that CNTs act as fillers, which result in a dense microstructure of cement paste.

The effects of high energy probe sonication on the thermoelectric power of large diameter multiwalled carbon nanotubes synthesized by chemical vapor deposition

The thermoelectric properties of large diameter multiwalled carbon nanotubes synthesized by chemical vapor deposition have been measured after successively longer periods of high energy probe sonication. Electrical conductivity decreases with increased sonication time due to shortening of the nanotubes. The initial Seebeck coefficient before sonication is −2.5μV/K, but increases to 12μV/K for sonication periods longer than 12h. This effect is attributed to the introduction of defects and shortening of the nanotubes during sonication which exposes previously shielded carbon from internal walls to oxygen doping from exposure to atmosphere which results in increased p-type doping and a positive thermoelectric power. Temperature dependent measurements indicate that this oxygen doping of the exposed nanotube ends has a thermally activated charge transfer energy since low temperature thermoelectric power measurements of the sonicated samples approach that of the raw n-type nanotubes.

Investigation of thermoelectric power with modification of two band model with linear T term for superconductors and thermal conductivity study of Se added YBCO samples

Abstract In this study, the electrical and thermal transport properties such as thermoelectric power and thermal conductivity of the Se added YBCO system was investigated. The samples fabricated under different heat treatment conditions exhibited the T c value between 90.1 K and 80.6 K. Positive thermoelectric power for all the samples was obtained over all measurement temperature range which indicates the hole type conductivity. Thermoelectric power data was analyzed by replacing term ( T – T * ) by T in “ Modified two band model with linear T-term ” which is good agreement with experimental TEP data for all the samples. T * obtained by fitting corresponds to a temperature close to T 0 obtained from resistivity measurement. Temperature dependence of thermal conductivity of the samples showed small peak with broad maximum just below T c . Thermal conductivity of the samples prepared was analyzed by the lattice thermal conductivity component by “ The modified Callaway model ” and the electronic component by “ Wiedemann –Franz law ”.

Negative thermoelectric power from large diameter multiwalled carbon nanotubes grown at high chemical vapor deposition temperatures

Multiwalled carbon nanotubes (MWNTs) have been grown using a standard chemical vapor deposition method, except for varying the growth temperature. Nanotubes grown below 770 °C exhibit typical positive thermoelectric powers, while those grown above have negative values. This behavior is attributed to the larger nanotube diameters observed at higher growth temperatures. Below 770 °C, the average nanotube diameter is about 50 nm, while above, nanotubes reach diameters of 300 nm. This increase in diameter and number of inner shells leads to the intrinsic negative thermoelectric power of the inner nanotube shells becoming larger than the positive thermoelectric power due to oxygen doping on the outer surface of the nanotube. The overall negative thermopower (about −6 μV/K, compared to +7 μV/K for smaller diameter nanotubes) can be understood in terms of a parallel conduction model. Our large-diameter multiwalled carbon nanotubes allow the intrinsic negative thermopower of MWNTs to be accessed without requiring specific deoxygenation treatment.

Enhancing the Thermoelectric Properties in Carbon Fiber/Cement Composites by Using Steel Slag

Thermoelectric properties of steel slag-carbon fiber/cement composites were studied in this paper. The effect of steel slag content on thermoelectric properties was focused on especially. The experimental results show that the addition of steel slag leads to an increase in the positive thermoelectric power of the cabon fiber/cement composites. The highest absolute thermoelectric power of carbon fiber/cement composites was rendered as positive as 14.4µV/°C by using steel slag, which had a high concentration of holes. Beside, a good linear relationship was observed between thermoelectric power and temperature differential on the specimen.

Low temperature electrical and thermal transport properties of the Ca3−xSbxCo4O9 system

In this study, the Ca3−xSbxCo4O9 (x = 0.0, 0.5, 0.75 and 1.0) system was fabricated using solid-state reaction technique. From the XRD analysis, it was found that increase of the Sb concentration in the system caused formation of impurity phases and therefore a distortion in the main lattice. The SEM-EDX results showed that the grain size reduced with increasing Sb in the Ca3Co4O9 system. The resistivity, ρ, value increased with increasing the Sb-content and insulating behavior was obtained as the temperature was decreased. The samples fabricated have positive thermoelectric power, S, value, suggesting that holes are dominant charge carriers in the samples. S value decreased with decreasing the temperature. The Fermi energy, EF, and the carrier concentration, n, decreased by the Sb-substitution. The Sb-substitution caused a decrease of thermal conductivity, κ which is due to reduction of the phonon thermal conductivity, κph, in the system. Figure of merit, ZT, value decreased significantly due to strongly increase in ρ. The impurities, defects and distortion with substitution of Sb are responsible for large variation on electrical and thermal transport properties.

Thermoelectric power factor of RuSr2GdCu2O8

The influence of thermal treatment between 1273 and 1333 K in an oxygen atmosphere on the perovskite system RuSr2GdCu2O8 was studied by X-ray diffraction (XRD), scanning electron microscopy, electrical resistivity and Seebeck coefficient measurements. The XRD patterns revealed that the phase purity depends on the sintering process. The samples were found to exhibit very small crystallite sizes in the nanometer range. The electrical resistivity was found to be strongly correlated with the heat treatment. The temperature dependence of electrical resistivity for one sample revealed semiconducting behavior, whereas the others exhibit metallic behavior. All the investigated samples exhibit a positive thermoelectric power, indicating the predominance of positive charge carriers. It was found that the power factor reaches a maximum value of 0.4 × 10−4 W m−1 K−2 at 300 K.

Electronic transport and specific heat of

The results of low temperature thermoelectric power and the specific heat of 1T-VSe2 (Vanadium diselenide) have been reported along with the electrical resistivity, and Hall coefficient of the compound. The Charge Density Wave (CDW) transition is observed near 110K temperature in all these properties. The Thermoelectric power has been measured from 15K to 300K spanning the incommensurate and commensurate CDW regions. We observed a weak anomaly at the CDW transition for the first time in the specific heat of VSe2. The linear temperature dependence of resistivity and thermoelectric power at higher temperatures suggests a normal metallic behavior and electron-phonon scattering above the CDW transition. The positive thermoelectric power and negative Hall coefficient along with strongly temperature dependent behavior in the CDW phase suggest a mixed conduction related to the strongly hybridized s-p-d bands in this compound.