Magnitude Of Thermopower Research Articles

Computational approach to enhance thermoelectric performance of Ag2Se by S and Te substitutions.

Designing an n-type thermoelectric material with a high thermoelectric figure of merit at near room temperature is extremely challenging. Generally, pristine Ag2Se reveals unusually low thermal conductivity along with a high electrical conductivity and Seebeck coefficient, which leads to high thermoelectric performance (n-type) at room temperature. Herein, we report a pseudo-ternary phase (Ag2Se0.5Te0.25S0.25) that exhibits significantly high thermoelectric performance (zT ∼ 2.1) even at 400 K. First-principles calculation reveals that the Rashba type of spin-dependent band spitting, which originates due to sulfur and tellurium substitution, helps to improve the thermopower magnitude. We also show that the intrinsic carrier mobility is not only controlled by the carrier effective mass but is substantially limited by longitudinal acoustic and optical phonon modes, which is an extension of the deformation potential theory. Locally off-center sulfur atoms, together with the increase in configurational entropy via substitution of Te and S atoms in Ag2Se, lead to a drastic reduction in the lattice thermal conductivity (klat ∼ 0.34 W m-1 K-1 at 400 K). The Rashba effect coupled with the configurational entropy synergistically results in a high thermoelectric figure of merit in the n-type thermoelectric material working in the near-room-temperature regime.

Nickel-Fullerene Nanocomposites as Thermoelectric Materials.

Nickel films with nanovoids filled with fullerene molecules have been fabricated. The thermoelectric properties of the nanocomposites have been measured from room temperature down to about 30 K. The main idea is that the phonon scattering can be enhanced at the C60/matrix heterointerface. The distribution of atoms within the Ni and Ni-C60 layers has been characterized by Auger depth profiling. The morphology of the grown samples has been checked using cross-sectional scanning electron microscopy (SEM). The Seebeck coefficient and electrical conductivity have been addressed employing an automatic home-built measuring system. It has been found that nanostructuring using Ar+ ion treatment increases the thermopower magnitude over the entire temperature range. Incorporating C60 into the resulting voids further increased the thermopower magnitude below ≈200 K. A maximum increase in the Seebeck coefficient has been measured up to four times in different fabricated samples. This effect is attributed to enhanced scattering of charge carriers and phonons at the Ni/C60 boundary.

The study of structural, electronic and thermoelectric properties of Ca1−xYbxZn2Sb2 (x = 0, 0.25, 0.5, 0.75, 1) Zintl compounds

Based on the electronic structure, the physical properties of [Formula: see text] ([Formula: see text], 0.25, 0.5, 0.75, 1) Zintl compounds are studied. The transport properties can be significantly changed by varying the composition [Formula: see text]. The materials under study are more metallic with increasing [Formula: see text] and behaves like a semiconductor when [Formula: see text] decreases. It is found that [Formula: see text] exhibits a larger thermopower magnitude ([Formula: see text] at [Formula: see text] and the Seebeck coefficient decreases as [Formula: see text] increases. The calculated figure of merit factor of [Formula: see text] is found to be low, this is explained by the fact that its structure is very compact and its bandgap is small which lead to high electrical and thermal conductivity due to high carrier concentration ([Formula: see text] at [Formula: see text]). On other hand a narrow-gap (0.46 eV for [Formula: see text]), provides a balance between a high Seebeck coefficient and low electronic thermal conductivity, with a slight increase in the carrier concentration when the temperature increases ([Formula: see text] at 600 K). As a consequence, [Formula: see text] compound is predicted to have good performance for thermoelectric applications. The electrical [Formula: see text] and the thermal [Formula: see text] conductivity for [Formula: see text] compound in both directions (along [Formula: see text] and [Formula: see text]-axes) are calculated. It is obtained that [Formula: see text] is 120% of [Formula: see text] at high-temperature, whereas [Formula: see text] Seebeck coefficient was higher than [Formula: see text] especially at [Formula: see text] ([Formula: see text]. The large value of [Formula: see text] showed that the transport is dominated by zz-axis.

Vacancy defect control of colossal thermopower in FeSb2

Iron diantimonide is a material with the highest known thermoelectric power. By combining scanning transmission electron microscopic study with electronic transport neutron, X-ray scattering, and first principle calculation, we identify atomic defects that control colossal thermopower magnitude and nanoprecipitate clusters with Sb vacancy ordering, which induce additional phonon scattering and substantially reduce thermal conductivity. Defects are found to cause rather weak but important monoclinic distortion of the unit cell Pnnm → Pm. The absence of Sb along [010] for high defect concentration forms conducting path due to Fe d orbital overlap. The connection between atomic defect anisotropy and colossal thermopower in FeSb2 paves the way for the understanding and tailoring of giant thermopower in related materials.

Correlation transports at p-/n-types in electron metastable perovskite family of rare-earth nickelates

It is important to achieve both donor and acceptor doping for correlated oxide semiconductors to cater for elementary device constructions, e.g., establishing a p-n junction and a thermal couple or a thermoelectric π-joint in correlated electronics. The perovskite family of rare-earth nickelates (ReNiO3) exhibits correlated transportation characters (e.g., metal to insulator transitions and thermistor transportations) dominated by electron conductions, as indicated by their negative thermopower for n-type materials. Herein, we demonstrate the presence of positive magnitude of thermopower as achieved in GdNiO3 single crystalline thin films, indicating a hole dominated transportation for p-type correlated semiconductors. Probing the Ni-L edge of GdNiO3 via near edge x-ray absorption fine structure indicates its distinguished intermediate acceptor energy states that are more easily occupied by the thermal excited valence band electrons. The hole-dominated transportation for GdNiO3 was further confirmed by its increased localization length and opposite sign in magnetoresistance, which are derived from the general tendency of ReNiO3, owing to the larger effective mass of holes compared to electrons. The discovery of p-types in GdNiO3 will further promote establishing ReNiO3-based elementary semiconductive devices in the field of correlated electronics.

A d-Band Electron Correlated Thermoelectric Thermistor Established in Metastable Perovskite Family of Rare-Earth Nickelates.

The d-band electron correlations shed a light on bridging multiple functionalities within one material system, and this further extends the horizon in material designs and their emerging device applications. Herein, we demonstrate the combination of thermoelectric and thermistor functionalities within the perovskite family of correlated rare-earth nickelates (ReNiO3) having small rare-earth elements (i.e., YNiO3 and DyNiO3), in addition to their already known metal-to-insulator transitions. In contrast to conventional semiconductive materials, the electronic band structure of ReNiO3 split within the hybridized Ni3d-O2p is closely coupled to the structure of NiO6 octahedron. Based on such a distinguished feature, it is possible to achieve the coexistence of a large magnitude of thermopower (S) and negative temperature coefficient of resistance (NTCR) in the insulating phase of ReNiO3 with small Re and more distorted NiO6 octahedron. This establishes a thermoelectric thermistor that can be used for sensing the thermal perturbations by integrating the two distinguished detection modes within one system: the active mode utilizing the high NTCR, and the passive mode utilizing the large S. It is worth noticing that as-achieved S-NTCR relationship in ReNiO3 differs form the one for conventional semiconductors, in which cases enlarging the band gap enlarges S but reduces NTCR. As achieved thermoelectric thermistor combing thermistor and thermoelectric functionalities via electron correlation opens up a new direction to explore emerging energy/electronic devices for sensing the thermal perturbations. The temperature range that keeps a high thermoelectric thermistor performance (i.e., |TCR | >2%K-1 and meanwhile S > 100 μVK-1) of ReNiO3 with a small rare-earth radius is possible to cover most of the outdoor conditions on earth (i.e., -50 to 150 °C).

Effect of the edge states on the conductance and thermopower in zigzag phosphorene nanoribbons

We study numerically the effect of the edge states on the conductance and thermopower in zigzag phosphorene nanoribbons (ZPNRs) based on the tight-binding model and the scattering-matrix method. It is interesting to find that the band dispersion, conductance, and thermopower can be modulated by applying a bias voltage and boundary potentials to the two layers of the ZPNRs. Under a certain bias voltage, the twofold-degenerate quasi-flat-edge bands split perfectly. The conductance can be switched off, and the thermopower around zero energy increases. In addition, when only the boundary potential of the top layer or bottom layer is adjusted, only one edge band bends and merges into the bulk band. The first conductance plateau is strongly decreased to ${e}^{2}/h$ around zero energy. In particular, when the two boundary potentials are adjusted, all the edge bands bend and fully merge into the bulk band, and the bulk energy gap is maximized. More interestingly, a pronounced conductance plateau with $G=0$ is found around zero energy, which is attributable to the opening of the bulk energy gap between the valence and conduction bands. Meanwhile, the thermopower can be enhanced more than twice compared to that of the perfect ZPNRs. The large magnitude of thermopower is ascribed to the appearance of the bulk energy gap around zero energy. Our results show that the modulated ZPNRs are more reliable in a thermoelectric application.

Giant oscillating thermopower at oxide interfaces.

Understanding the nature of charge carriers at the LaAlO3/SrTiO3 interface is one of the major open issues in the full comprehension of the charge confinement phenomenon in oxide heterostructures. Here, we investigate thermopower to study the electronic structure in LaAlO3/SrTiO3 at low temperature as a function of gate field. In particular, under large negative gate voltage, corresponding to the strongly depleted charge density regime, thermopower displays high negative values of the order of 104–105μVK−1, oscillating at regular intervals as a function of the gate voltage. The huge thermopower magnitude can be attributed to the phonon-drag contribution, while the oscillations map the progressive depletion and the Fermi level descent across a dense array of localized states lying at the bottom of the Ti 3d conduction band. This study provides direct evidence of a localized Anderson tail in the two-dimensional electron liquid at the LaAlO3/SrTiO3 interface.

Band gap engineering of Si-Ge alloys for mid-temperature thermoelectric applications

The viability of Si-Ge alloys in thermoelectric applications lies in its high figure-of-merit, non-toxicity and earth-abundance. However, what restricts its wider acceptance is its operation temperature (above 1000 K) which is primarily due to its electronic band gap. By means of density functional theory calculations, we propose that iso-electronic Sn substitutions in Si-Ge can not only lower its operation to mid-temperature range but also deliver a high thermoelectric performance. While calculations find a near invariance in the magnitude of thermopower, empirical models indicate that the materials thermal conductivity would also reduce, thereby substantiating that Si-Ge-Sn alloys are promising mid-temperature thermoelectrics.

Thermoelectric power factor enhancement of textured ferroelectric SrxBa1–xNb2O6–δceramics

Abstract

Magneto-thermopower of parent compound LaFeAsO

We report the effect of magnetic field on the transport properties of polycrystalline parent compound LaFeAsO. The magnetic field has no significant influence on the structural phase transitions or spin density wave (SDW) ordering of small moments on iron, but causes large positive magento-resistance below 100 K. Meanwhile, the absolute magnitude of thermopower, which is negative, increases remarkably with increasing magnetic field below 100 K. Such a large positive magneto-thermopower was first reported in this iron arsenide. The result suggests that there could exist an enhancement of SDW gapping in holelike Fermi surface.

Thermopower and thermal conductance for a Kondo correlated quantum dot

We study the thermopower and thermal conductivity of a gate-defined quantum dot, with a very strong Coulomb repulsion inside the dot, employing the X-boson approach for the impurity Anderson model. Our results show a change in the sign of the thermopower as function of the energy level of the quantum dot (gate voltage), which is associated with an oscillatory behavior and a suppression of the thermopower magnitude at low temperatures. We identify two relevant energy scales: a low temperature scale dominated by the Kondo effect and a T ∼ Δ temperature scale characterized by charge fluctuations. We also discuss the Wiedemann–Franz relation and the thermoelectric figure of merit. Our results are in qualitative agreement with recent experimental reports and other theoretical treatments.

The family of molecular conductors [(n-Bu)4N]2[M(dcbdt)2]5, M = Cu, Ni, Au; band filling and stacking modulation effects

Single crystals of the partially oxidised complexes [(n-Bu)4N]2[M(dcbdt)2]5 with M = Au, Ni and Cu (dcbdt = 4,5-dicyanobenzene-1,2-dithiolate) have been obtained by electrocrystallisation techniques. These compounds are isomorphous and present a triclinic structure, space groupP. Their crystal structures were solved for the Au compound at 120 K and 298 K and for Ni compound at 120 K and consist of pentamerised segregated stacks of the partially oxidised complex along [−2,1,0], arranged in layers alternating with cation layers. Their physical properties (electrical conductivity and thermopower) are compared and related on the basis of extended Hückel calculations to the different electronic bandfillings imposed by the different metals and their slightly different structures. The lower electrical conductivity, σRT = 0.15 S cm−1, with a larger activation energy, Ea = 176 meV, as well as the larger magnitude of thermopower of the Ni compound, are consequences of both a more pronounced modulation of the stacking and electronic bandfilling with a Fermi level at a distortion induced gap. Both Au and Cu compounds present higher conductivity values, σRT = 10 S cm−1, with very small activation energy (27 and 16 meV respectively) and in the Au compound a clear metallic regime is induced down to 100 K under a pressure of 9 kbar.

Thermopower and electrical conductivity of Mn1.68−XCu0.6+X+Y+ZCo0.24−YNi0.48−Z thin film oxides obtained through metal organic decomposition processing

Compositional variations of spinel oxides to a reference composition Mn1.68−XCu0.6+X+Y+ZCo0.24−YNi0.48−ZO4 (x and z=0.2, 0.4 and y=+∕−0.1) are made. Thermopower and electrical conductivity of these materials are measured in thin film forms obtained through metal organic decomposition. This low-temperature fabrication technique allows the concentration of metastable Mn4+ and Cu1+ ions, which are essential to charge transport in the oxide spinel, to be controlled. All compositions have n-type behavior, and the conduction occurs via polaron hopping between neighboring Mn3+oct and Mn4+oct sites. Compositional variations alter the Mn and Cu oxidation state concentrations, and modify the conductivity and thermopower. Charge balance models are used in some cases to interpret the influences that doping and fluctuating oxygen content have on the variable valence Mn and Cu. After accounting for changes in oxygen concentration, we found that replacing portions of Co with Cu raise both the conductivity and thermopower magnitude, and substituting small amounts of Cu for Ni reduces these thermoelectric factors. Surprisingly, the effects of doping show parallel rises or declines in conductivity and thermopower magnitude. This implies that conductivity is more strongly impacted by the activated carrier mobility than by the number of charge carriers in these oxides.

Thermoelectric effects of an Aharonov-Bohm interferometer with an embedded quantum dot in the Kondo regime

Thermoelectric effects are studied in an Aharonov-Bohm (AB) interferometer with an embedded quantum dot in the Kondo regime. The AB flux-dependent transmission probability has an asymmetrical shape arising from the Fano interference between the direct tunneling path and the Kondo-resonant tunneling path through a quantum dot. The sign and magnitude of thermopower can be modulated by the AB flux and the direct tunneling amplitude. In addition, the thermopower is anomalously enhanced by the Kondo correlation in the quantum dot near the Kondo temperature ${(T}_{K}).$ The Kondo correlation in the quantum dot also leads to crossover behavior in diagonal transport coefficients as a function of temperature. The amplitude of an AB oscillation in electric and thermal conductances is small at temperatures far above ${T}_{K},$ but becomes enhanced as the system is cooled below ${T}_{K}.$ The AB oscillation is strong in the thermopower and the Lorenz number within the crossover region near the Kondo temperature.

Conductors based on metal-bisdicyanobenzodithiolate complexes

The physical properties of the partially oxidised complexes [( n-Bu) 4N] 2 [M(dcbdt) 2] 5with M=Au and Ni (dcbdt=4,5-dicyanobenzene-1,2-dithiolate) are compared and related to the different electronic bandfilling and their slightly different structure. Both compounds present a triclinic structure, space group P-1, with pentamerised segregated stacks of the complex along [-2 1 0]. The lower electrical conductivity with a larger activation energy, as well as the larger magnitude of thermopower and paramagnetic susceptibility, of the Ni compound are a consequence of both its different electronic bandfilling and more pronounced modulation of the stacking.

Thermoelectric Properties of the Brownmillerite Oxide Ca2-yLayCo2-xAlxO5

We prepared the brownmillerite oxide Ca2-yLayCo2-xAlxO5 and found that it was an n-type conductor. The thermopower and the resistivity of the single crystal are respectively -90 µV/K and 68 mΩcm along the ab direction at 440 K, which suggest relatively good thermoelectrical properties, compared with other transition-metal oxides. Their temperature dependences are of activation type, and the activation energies are 0.2 eV for the resistivity and 0.04 eV for the thermopower. These energies differ by one order in magnitude, which implies that a polaron dominates the charge transport. The sign of the thermopower of the polycrystals changes from negative to positive at 500 K, indicating that holes are excited thermally to decrease the magnitude of thermopower.

Thermopower of a semiconductor film with parabolic potential in a strong magnetic field

Relationships for the transverse magnetothermopower of a semiconductor film with parabolic potential are derived for the cases of nondegenerate statistics and strong degeneracy. It is demonstrated that, in the case of strong degeneracy, quantum levels lying below the Fermi level intersect the Fermi level with an increase in the magnetic field, which leads to jumpwise oscillations of the thermopower magnitude.

Figure of merit of quasicrystals: the case of Al–Cu–Fe

Quasicrystals exhibit low thermal and moderate electrical conductivity, accompanied by a relatively high absolute magnitude of thermopower, which makes them suitable candidates for use as thermoelectric devices. Theoretical explanations of electrical and thermal conductivity and thermopower of two representative members of the Al–Cu–Fe icosahedral quasicrystal family ( i-Al 62Cu 25.5Fe 12.5 and i-Al 63Cu 25Fe 12) are given. Their figures of merit are relatively low compared with the best-known thermoelectric materials.

Thermopower and resistivity of La-doped thallium 1201 and bismuth 2204 superconductors

We have measured the thermopower and resistivity of two series of cuprate superconductor, Tl0.5Pb0.5Sr2-xLaxCuO5 and Bi2Sr2-xLaxCuO6+y, which possess a single CuO2 plane. In each series, the hole concentration can be varied by changing the La content, permitting investigation of the overdoped, maximum-Tc and underdoped regimes. The thermopower follows a pattern similar to that seen in superconductor series with more than one CuO2 plane, namely an approximately linear decrease as temperature increases well above Tc. However, the thermopower shows a minimum above Tc followed by a maximum as temperature increases for samples with a small thermopower and high Tc. The magnitude of thermopower as a function of Tc is in general agreement with the universal behaviour found for other cuprate superconductors.