Dependent Thermoelectric Power Research Articles

Thermoelectric power hysteresis in semi-parabolic quantum well

The influence of semi-parabolic surface potential on quantum well thermoelectric power is investigated. The thermoelectric power dependence on the Fermi level and quantum well parameters is determined. It is shown that semi-parabolic quantum well thermoelectric power exhibits hysteresis features. The hysteresic loops and sign change in thermoelectric power dependence on the surface potential are ascribed to properties of electrons at the edges and electron localization.

Effect of high energy electron beam irradiation on the structural properties, electrical resistivity and thermopower of La0.5Sr0.5MnO3 manganites

Bulk samples of La0.5Sr0.5MnO3 manganite have been subjected to electron beam irradiation of different dosages. As a consequence, the structural and electronic transport properties viz resistivity and thermoelectric power are affected to some extent. The stoichiometric content of the sample is confirmed through Energy Dispersive X-ray spectroscopy. X-ray diffraction study of the samples (unexposed and exposed to electron beam radiation) show that the samples crystallize in a single phase rhombohedral structure. Insulating behavior is displayed in the temperature range 10–300 K investigation for all the samples. Temperature dependent thermoelectric power data reveals n-type conductivity in the entire temperature zone. At high temperatures, it is concluded through detailed analysis that both resistivity and thermopower conform to small polaron hopping model. On the other hand, variable range polaron hopping model elucidates the transport characteristics at low temperatures.

Monitoring the Changes in Titanium Defect Structure during Titanium Hydrogen Saturation

Hydrogen saturated samples of technically pure titanium have been studied by the electron-positron annihilation method (EPA), coupled with the thermoelectric power measurements performed in these samples saturated by different amount of hydrogen. The structure of the hydrogenated samples was additionally investigated by X-ray diffraction. The complete coincidence of the moment of occurrence of a change in the structure of hydrogenated titanium depending on the amount of introduced hydrogen has been established. The intensity of positron annihilation drops with increasing hydrogen concentration in α-titanium to 0.04 wt % and then remains unchanged up to values of 0.05 wt % ($$\alpha + \delta $$) -titanium to increases afterwards. At the same time, a sharp change in the values of the thermoelectric power occurs in this range. In the region of 0.05%, the annihilation rate stabilizes and begins to increase, while the thermoelectric power begins to decrease slowly. The inflection point on the dependence of thermoelectric power on hydrogen concentration corresponds to the onset of the formation of titanium $$\delta $$-hydrides. An increase in the positron lifetime is observed in the concentration range of 0.05–0.08 wt %, then the lifetime stays stable up to concentrations of 0.08–0.12 wt %. A transition from ($$\alpha + \beta $$) to ($$\alpha + \delta $$) phase is formed in this range. Next, the positron lifetime increases, as does the number of defects, while the thermoelectric power gradually drops (to a concentration of 0.24 wt %). This is followed by a stabilization mode of all the above parameters to 0.35 wt %.

2D Materials for Large‐Area Flexible Thermoelectric Devices

AbstractThe rapid development of the concept of the “Internet of Things (IoT)” requires wearable devices with maintenance‐free batteries, and thermoelectric energy conversion based on large‐area flexible materials has attracted much attention. Among large‐area flexible materials, 2D materials, such as graphene and related materials, are promising for thermoelectric applications due to their excellent transport properties and large power factors. In this Review, both single‐crystalline and polycrystalline 2D materials are surveyed using the experimental reports on thermoelectric devices of graphene, black phosphorus, transition metal dichalcogenides, and other 2D materials. In particular, their carrier‐density dependent thermoelectric properties and power factors maximized by Fermi level tuning techniques are focused. The comparison of the relevant performances between 2D materials and commonly used thermoelectric materials reveals the significantly enhanced power factors in 2D materials. Moreover, the current progress in thermoelectric module applications using large‐area 2D material thin films is summarized, which consequently offers great potential for the use of 2D materials in large‐area flexible thermoelectric device applications. Finally, important remaining issues and future perspectives, such as preparation methods, thermal transports, device designs, and promising effects in 2D materials, are discussed.

Dependence of thermoelectric power and electrical conductivity on structural order of PEDOT-Tos-graphene nanocomposite via charge carrier mobility

Nanocomposites of tosylate doped poly (3, 4 ethylenedioxythiophene) (PEDOT-Tos) and graphene with improved structural order were synthesized and investigated as organic thermoelectric (TE) materials. A TE figure of merit ZT > 0.05 is reported. Gradual incorporation of graphene results in an increase in the degree of structural order of the samples. A consequent increase in the figure of merit is observed, the maximum value crossing 0.05 (higher than that reported for bulk nanostructured polymer based composites) for a graphene content of 25%. The tuning of the ordered structure boosts up the charge carrier mobility, which in turn improves the thermoelectric power (S) simultaneously with the electrical conductivity (σ). Consequently there is an increase in the TE power factor. Moreover, the thermal conductivity of the nanocomposites is very low due to selective phonon scattering by the introduction of the nanointerfaces designed in the PEDOT-Tos-graphene nanocomposites, which further contributes to the enhancement in ZT. An opposite trend is observed in structure and ZT, for a graphene content of 40%; arising probably from reduced π-stacking interactions between graphene and the PEDOT units. This study proves to be an effective way and can escort the future design and synthesis of high-performance thermoelectric conjugated polymers.

Determination of parameters of a system of charge carriers in Y1–2x Ca x Pr x Ba2Cu3O y within a combined analysis of temperature dependences of thermoelectric power and Nernst–Ettingshausen coefficients

The temperature dependences of thermoelectric power S and transverse Nernst–Ettingshausen effect Q in high-temperature superconductors of the Y1–2xCaxPrxBa2Cu3Oy system with different concentrations of dopants have been investigated experimentally. It has been shown that all the specific features revealed in the behavior of the two kinetic coefficients under investigation can be described using the narrowband model with a single concept of the structure of an energy spectrum and a common set of model parameters that characterize the structure of the spectrum and properties of a system of charge carriers. Based on the quantitative analysis of the dependences S(T) and Q(T), the values of these parameters for samples of the studied composition have been determined and the character of their changes with an increase in the impurity content has been analyzed. It has been demonstrated that, for the explanation of the obtained results, it is necessary to take into account the interaction of calcium and praseodymium ions upon their simultaneous introduction into the lattice. It has been found that the mobility of charge carriers has extremely low values and almost does not depend on the level of doping, which is associated with the character of changes in the structure of the energy spectrum of Y1–2xCaxPrxBa2Cu3Oy upon doping. It has been concluded that the structure of the energy spectrum of high-temperature superconductors in the yttrium system is characterized by an asymmetry of the dispersion curve.

Theoretical study of magnetotransport properties of colossal magnetoresistive manganites (Re1−xAxMnO3): A variational treatment

Abstract The magnetotransport properties like electrical resistivity and Thermoelectric power of rare earth manganites doped with alkaline earths namely Re 1− x A x MnO 3 (where Re=La, Pr, Nd etc., and A=Ca, Sr, Ba etc.) which exhibit colossal magnetoresistance (CMR), metal–insulator transition and many other poorly understood phenomena, have been studied in this work by means of a Variational method. We have used two band (l-b) model for manganites in the strong electron–lattice Jahn–Teller (JT) coupling regime, an approach similar to the two-fluid models along with (l-b) hybridization. We have already used this Variational method to study the zero field electrical resistivity ρ ( T ) of doped CMR manganites. In the present study we find that the resistivity decreases with increasing magnetic field and the metal–insulator transition temperature ( T ρ ) shifts towards higher temperature region. This may happen because of the delocalized charge carriers, induced by applied magnetic field, decreasing the resistivity which in turn leads to the local ordering of the electron spins. Due to this ordering, the ferromagnetic metallic state might have suppressed the paramagnetic insulating regime resulting in the observed increase in T ρ under applied magnetic field. We have also shown the temperature dependent magneto-resistance (MR) ratio at different magnetic fields. We obtained large MR value at low temperatures. An interesting feature of the Magnetic field dependent thermoelectric power Q ( T ) is that it rises gradually with decreasing temperatures and a broad peak appears at low temperatures and as the temperature is lowered further, the value of Q ( T ) drops sharply (i.e. Q →0 as T →0 ), which is suggested to originate from the weak carrier localization effect. In the external magnetic field, the phonon-drag effect is expected to become weaker so that value of Q ( T ) reduces.

Structural, magnetic and electronic properties on the Li-doped manganites

We present results of a comprehensive investigation of the structural, frequency dependent ac susceptibility, dc magnetization, magnetoresistance and thermoelectric power measurements on polycrystalline samples of La1−xLixMnO3 (0.05≤x≤0.30). All samples undergo ferromagnetic to paramagnetic transition and metal to semiconductor transition. A cusp in the zero field-cooled in dc magnetization and a frequency-dependent peak in the ac susceptibility reveal the glassy behaviors. The out of phase component of the ac susceptibility shows frequency-dependent peaks below the Curie temperature (indicative of glassy behavior) which have previously been interpreted in terms of freezing of clusters. The grain boundaries play a dominant role in the conduction process. The framework of the magnon and phonon drag concept analyzed thermoelectric power data at low temperature, while small polaron conduction mechanism explained thermoelectric power data at high temperature. It has been found that the percolation threshold sample is x=0.10, so the author’s point of view refers to make spot on this sample in the future works.

Effects of molecular adsorption on carrier transport properties of large-size graphene

The temperature dependent resistance and thermoelectric power of macroscopic graphenes are studied in various gas environments. The temperature dependent slope of resistance is weaker in gas environments with heavier molecules. Following the temperature dependent slopes of normalized resistance, one can identify the molecular mass of the environmental gas. This is relative to the atomically sharp potential modification due to adsorbed gas molecules on the graphene. The temperature dependent thermopower increases as the mass of the gas molecules increases. A universal relationship between resistance and thermoelectric power in various gas environments further confirms that the transport mechanism is dominated by the adsorbed gas molecules on the graphene surface.

Ambipolar Surface State Thermoelectric Power of Topological Insulator Bi2Se3

We measure gate-tuned thermoelectric power of mechanically exfoliated Bi2Se3 thin films in the topological insulator regime. The sign of the thermoelectric power changes across the charge neutrality point as the majority carrier type switches from electron to hole, consistent with the ambipolar electric field effect observed in conductivity and Hall effect measurements. Near the charge neutrality point and at low temperatures, the gate-dependent thermoelectric power follows the semiclassical Mott relation using the expected surface state density of states but is larger than expected at high electron doping, possibly reflecting a large density of states in the bulk gap. The thermoelectric power factor shows significant enhancement near the electron-hole puddle carrier density ∼0.5 × 10(12) cm(-2) per surface at all temperatures. Together with the expected reduction of lattice thermal conductivity in low-dimensional structures, the results demonstrate that nanostructuring and Fermi level tuning of three-dimensional topological insulators can be promising routes to realize efficient thermoelectric devices.

Electrical property measurements of Cr-N codoped TiO2 epitaxial thin films grown by pulsed laser deposition

The temperature dependent resistivity and thermo-electric power of Cr-N codoped TiO2 were compared with that of single element N and Cr doped and undoped TiO2 using epitaxial anatase thin films grown by pulsed laser deposition on (100) LaAlO3 substrates. The resistivity plots and especially the thermoelectric power data confirm that codoping is not a simple sum of single element doping. However, the negative sign of the Seebeck coefficient indicates electron dominated transport independent of doping. The narrowing distinction among the effects of different doping methods combined with increasing resistivity of the films with improving crystalline quality of TiO2 suggest that structural defects play a critical role in the doping process.

Magnetic-field-tuned quantum criticality of the heavy-fermion system YbPtBi

In this paper, we present the systematic measurements of the temperature and magnetic field dependences of the thermodynamic and transport properties of the Yb-based heavy fermion YbPtBi for temperatures down to 0.02 K with magnetic fields up to 140 kOe to address the possible existence of a field-tuned quantum critical point. Measurements of magnetic field and temperature dependent resistivity, specific heat, thermal expansion, Hall effect, and thermoelectric power indicate that the AFM order can be suppressed by applied magnetic field of $H_{c}$ $\sim$ 4 kOe. In the $H-T$ phase diagram of YbPtBi, three regimes of its low temperature states emerges: (I) AFM state, characterized by spin density wave (SDW) like feature, which can be suppressed to $T$ = 0 by the relatively small magnetic field of $H_{c}$ $\sim$ 4\,kOe, (II) field induced anomalous state in which the electrical resistivity follows $\Delta\rho(T) \propto T^{1.5}$ between $H_{c}$ and $\sim$ 8 kOe, and (III) Fermi liquid (FL) state in which $\Delta\rho(T) \propto T^{2}$ for $H \geq$ 8 kOe. Regions I and II are separated at $T$ = 0 by what appears to be a quantum critical point. Whereas region III appears to be a FL associated with the hybridized 4$f$ states of Yb, region II may be a manifestation of a spin liquid state.

Strontium(II)-added CoAl2O4 nanocatalysts for the selective oxidation of alcohols

Strontium added CoAl2O4 nanocatalysts with spinel structure were prepared by a modified sol–gel method using ethylene diamine followed by sintering at 900 °C. The samples were labeled as CoSA1-900, CoSA2-900, CoSA3-900, CoSA4-900, CoSA5-900, CoSA6-900, where the molar ratios of Co:Sr were 1.0:0.0, 0.9:0.1, 0.8:0.2, 0.7:0.3, 0.6:0.4 and 0.5:0.5 and the aluminum molar ratio was kept constant, 900 referring to the sintering temperature. The effect of Sr addition on the structural and morphological properties of cobalt aluminate nanocatalysts was investigated by X-ray diffraction, Fourier transform infrared spectra, high resolution scanning electron microscopy, energy dispersive X-ray analysis, nitrogen adsorption/desorption isotherms, temperature dependent conductance measurements and thermoelectric power measurements. The addition of Sr improves the performance of the nano cobalt aluminate catalyst towards the selective oxidation of alcohols and decreases the grain size. The effect of solvent, oxidant and reaction time on the Sr(II)-added cobalt aluminate nanocatalysts for the oxidation of benzyl alcohol was studied. Higher activity was obtained for the conversion of benzyl alcohol to benzaldehyde for 0.3 molar percentage Sr(II) added cobalt aluminate catalyst (CoSA4-900) which was used for the selective oxidation of other alcohols. The stability and reusability of the catalyst were also investigated.

Temperature dependent thermoelectric material power factor measurement system

Thermoelectric materials can be used for cooling/heating applications, or converting waste heat into electricity. Novel thermoelectric materials have been discovered in recent years. Characterization of an electrical conductivity and thermopower of a sample from room temperature to > or = 900 K is often necessary for thermoelectric materials. This paper describes a system built for measurement of the power factor of thermoelectric materials from 300 to 1273 K. Characterization results of the system are also presented.

The impurities of iron and cobalt in mercury selenide: Localization effects of hybridized electronic states in the temperature dependences of thermoelectric power

The temperature dependences of thermoelectric power coefficient have been studied in mercury selenide with small concentrations of iron and cobalt impurities at low temperatures. The considerable anomalies (sharp non-monotonic regions of the temperature dependences), related to the influence of the impurities, have been revealed. On the base of the comparison with the data of the other experiments, obtained before, and interpretation of the anomalies, proposed by us, it has been shown that in crystals with iron impurities the specific behaviors above are explained by manifesting the hybridized electronic states on donor impurities. As a result of discussing the data, obtained on the crystal with cobalt impurities, it was established that the revealed anomalies of thermoelectric power are the conclusive evidences of hybridizing the states on donor cobalt impurities.

Thermoelectric power and Hall coefficient measurements onBa(Fe1−xTx)2As2(T=Coand Cu)

Temperature dependent thermoelectric power (TEP) data on Ba(Fe1-xTMx)2As2 (TM = Co and Cu), complemented by the Hall coefficient data on the samples from the same batches, have been measured. For Co-doping we clearly see a change in the temperature dependent TEP and Hall coefficient data when the sample is doped to sufficient $e$ (the number of extra electrons associated with the TM doping) so as to stabilize low temperature superconductivity. Remarkably, a similar change is found in the Cu-doped samples at comparable e-value, even though these compounds do not superconduct. These changes possibly point to a significant modification of the Fermi surface / band structure of Ba(Fe1-xTM_x)2As2 at small electron doping, that in the case of Co-doping is just before, and probably allows for, the onset of superconductivity. These data further suggest that suppression of the structural / magnetic phase transition and the establishment of a proper e-value are each necessary but, individually, not sufficient conditions for superconductivity.

Temperature dependent conductivity and thermoelectric power of the iodine doped poly(vinyl alcohol)–Cu 2+ chelate

We have investigated the temperature dependence of electrical conductivity and thermoelectric power (TEP) at 1.7 K < T < 300 K in an organo metallic complex, the iodine doped poly(vinyl alcohol)–Cu 2+ chelate. We observed intrinsic metallic temperature dependence of resistivity from room temperature to 68 K with a broad minimum [ ρ(68 K)/ ρ(300 K) ∼0.75], which has not been observed previously in similar organo metallic complexes. There occurs an unusual metal-insulator transition at T ∼68 K and the resisitivity increases upon cooling below 68 K. However, the low temperature resistivity becomes finite (instead of going to infinity), [ρ(1.7 K)/ ρ(300 K) ∼0.98] indicating that a quantum mechanical tunneling conduction is dominant at this low temperature. It is remarkable that the resistivity at 1.7 K is as small as that of room temperature. Such unusual temperature dependence of conductivity could be understood as thermally assisted hopping conduction between metallic islands. However, the observed intrinsic metallic temperature dependence of resistivity implies that such hopping conduction barrier is not important at high temperature ( T > 68 K). The intrinsic metallic characteristics are confirmed by the quasi-linear temperature dependence of TEP for the whole measured temperature range (1.7 K < T < 300 K) with a small slope change at low temperature, T < 68 K, which is understood as an effect of variable range hopping (VRH) conduction at low temperature. The results of magneto resistance (MR) and magneto thermoelectric power (MTEP) are consistent with the above interpretation.

Magnon drag contribution to thermopower of Nd0.67Sr0.33MnO3 nanocrystalline manganites

With a view to investigate the influence of nanosize on thermopower behavior of Nd0.67Sr0.33MnO3 manganites, a systematic investigation of their temperature dependent thermoelectric power (TEP) studies has been undertaken. For this purpose, the samples with different particle sizes were prepared by the sol-gel method by sintering at four different temperatures (viz., 800, 900, 1000, and 1100 °C). The average crystallite sizes were calculated using a modified Scherrer’s formula. The ferromagnetic metallic (FMM) part of the TEP data were fitted to an equation containing diffusion, magnon drag, and phonon drag terms. The relative weights of these contributions were estimated and it has been found that the magnon drag contribution predominantly contributes to the TEP in the FMM region. The phonon drag process in the samples of the present investigation is found to be dominant along the direction of cell parameter “a” than the other two directions. Finally, the paramagnetic insulting part of the TEP data has been analyzed by using small polaron hopping mechanism.

High temperature thermoelectric power study on calcium substituted lanthanum manganites

Compositional and temperature dependent thermoelectric power measurements have been performed on the polycrystalline manganite perovskite system: La1−xCaxMnO3+δ (x = 0.00, 0.25, 0.33, 0.50 and 1.00), in the paramagnetic state (T > Tc) (300–673 K). The Seebeck coefficient (α) has been used to calculate the Fermi energy, the activation energy (Eα), the Mn4+-ion concentration and the oxygen content (δ), for different compositions. The observed decrease in Eα with increase in Ca-concentration and a large value of Eα as compared with a relatively high temperature sintered material of the same compositions have been explained in the light of the decrease in the interatomic distances and grain growth with Ca-substitution. It is shown that [Mn4+][Mn3+] ion concentration is not the only factor that governs conduction in the manganite perovskites. The significantly small values of α (−22 µV K−1) at high temperature can be explained on the basis of (Mn3+–Mn3+) charge disproportionation into stable Mn2+–Mn4+pairs in the system rather than on the basis of nominal Mn3+/4+ valence arguments. Finally, nearly temperature independent variation of α, charge-carrier concentration at high temperature and low value of mobility are hallmarks of small polaron hopping.

Structural, electrical, and optical characterizations of epitaxial Zn1−xGaxO films grown on sapphire (0001) substrate

In this paper we report the structural, electrical, and optical properties of epitaxial Zn1−xGaxO films (x=0–0.05) grown on single crystal sapphire (0001) substrate by pulsed laser deposition technique. Structural and elemental analysis was performed using high-resolution x-ray diffraction (θ-2θ and Φ scan) and energy dispersive x-ray spectroscopy. Temperature dependent electrical resistivity and thermoelectric power measurements were performed over the temperature range of 77–300K and 296–373K, respectively. Hall effect and optical transmission measurements were preformed at room temperature. All these studies showed that the structural, electrical as well as the optical characteristics of Zn1−xGaxO films depend very sensitively on the Ga contents. As the Ga doping concentration is increased, initially an increase in carrier concentration and optical band gap is observed (until x=0.04), which is followed by a decrease at higher concentrations. These features were attributed to the combined effect of band filling (Burstein-Moss effect), electronic correlation, and epitaxial strain present in the system. Above parameters also affected the electrical properties of the films quite significantly. Zn1−xGaxO films with 1% of Ga doping (x=0.01) showed metal-like electrical resistivity. However, for higher doping levels, enhanced scattering potential, arising from randomly distributed impurity atoms, resulted in the Anderson localization of electronic states.