Electrical conduction of SnBi4Se7

Polycrystalline samples of Bi₂Se₃ and stoichiometric ternary compounds in the quasi-binary system SnSe-Bi₂Se₃ were characterized by measurements of temperature and field dependence of electrical conductivity. The current density–electric field characteristics were found to be non-linear, especially when the applied electric field exceeds a certain value which is dependent on the temperature T. Furthermore, the electrical conductivity can be enhanced by the applied electric field. The characteristic length a(T) seemed to be enhanced with increasing temperature. Electrical conductivity measurements elucidated the semiconducting behaviour of both compounds, especially when the temperature of measurement exceeds a certain value for SnBi₄Se₇, and hopping and band type conduction are dominant at low and high ranges of temperature, respectively. Below 200 K, the electrical conductivity of SnBi₄Se₇ decreases with increasing temperature. Meanwhile, additional scattering and hopping seemed to characterize the behaviour of SnBi₄Se₇ due to the Sn doping of Bi₂Se₃ resulting in additional states at the Fermi level. PACS 72.20.-i; 72.15.-v     

Topological Dirac surface states in ternary compounds GeBi2Te4, SnBi2Te4 and Sn0.571Bi2.286Se4

Using high-resolution angle-resolved and time-resolved photoemission spectroscopy, we have studied the low-energy band structures in occupied and unoccupied states of three ternary compounds GeBi₂Te₄, SnBi₂Te₄ and Sn_0.571Bi_2.286Se₄ near the Fermi level. In previously confirmed topological insulator GeBi₂Te₄ compounds, we confirmed the existence of the Dirac surface state and found that the bulk energy gap is much larger than that in the first-principles calculations. In SnBi₂Te₄ compounds, the Dirac surface state was observed, consistent with the first-principles calculations, indicating that it is a topological insulator. The experimental detected bulk gap is a little bit larger than that in calculations. In Sn_0.571Bi_2.286Se₄ compounds, our measurements suggest that this nonstoichiometric compound is a topological insulator although the stoichiometric SnBi₂Se₄ compound was proposed to be topological trivial.     

Anisotropic thermoelectric power of TTF-TCNQ

We have measured the thermoelectric power along the a-axis as well as the highly conducting b-axis in TTF-TCNQ crystals. Measurements were taken both parallel and perpendicular to the long axis on two sets of crystals. One set grew along the a-axis and the other along the b-axis. The thermopowers are of opposite sign except near 60°K where both cross zero at slightly different temperatures. The a-axis thermopower is consistent with non-metallic diffusive transport in the a direction.     

Se substitution and micro-nano-scale porosity enhancing thermoelectric Cu_2Te

Binary Cu-based chalcogenide thermoelectric materials have attracted a great deal of attention due to their outstanding physical properties and fascinating phase sequence.However,the relatively low figure of merit z T restricts their practical applications in power generation.A general approach to enhancing z T value is to produce nanostructured grains,while one disadvantage of such a method is the expansion of grain size in heating-up process.Here,we report a prominent improvement of z T in Cu_2Te_(0.2)Se_(0.8),which is several times larger than that of the matrix.This significant enhancement in thermoelectric performance is attributed to the formation of abundant porosity via cold press.These pores with nano-to micrometer size can manipulate phonon transport simultaneously,resulting in an apparent suppression of thermal conductivity.Moreover,the Se substitution triggers a rapid promotion of power factor,which compensates for the reduction of electrical properties due to carriers scattering by pores.Our strategy of porosity engineering by phonon scattering can also be highly applicable in enhancing the performances of other thermoelectric systems.     

High pressure synthesis and thermoelectric performances of Cu2Se compounds

The Cu₂Se samples were synthesized by high pressure directly at room temperature in several minutes. The composition evolution under high pressure demonstrates that the critical conditions to synthesize Cu₂Se are the pressure of 1 GPa and the reaction time of 5 min. The synthetic pressure can effectively tune the morphology, carrier concentration and the electrical transport properties. The low lattice thermal conductivity less than 0.5 Wm^?1 K^?1 is obtained because of the intrinsic superionic character and the microstructures by high pressure including abundant micropores and lattice defects. A maximum zT of 0.92 at 783 K is achieved for Cu₂Se synthesized at 1 GPa. This work indicates the potentiality of high pressure technique to further enhance the thermoelectric properties of Cu₂Se materials.     

自启动太阳能温差发电系统

基于现有的温差发电理论,建立太阳能热水器与温差发电片组结合的自启动温差发电系统模型.作为大学生物理演示实验仪器的制作实践,设计了其温差发电片组在温差条件变化情况下的发电输出功率和维持稳定温差所需水泵的消耗功率、水泵流量和体系热散失关系的物理实验,并验证了自启动温差发电模型的可行性.     

Ti4+ and Ge4+ ionic substitution in Cu-ferrite,electrical conductivity and thermoelectric power

Both the DC conductivity and the thermoelectric power of Cu_1+xGe_xFe_2−2xO₄ and Cu_1+xTi_xFe_2−2xO₄ ferrites, for 0⩽x⩽0.4, were measured in a wide range of temperature from RT up to 773 K. The measurements showed that the substitution of both tetravalent ions (Ge⁴⁺ and Ti⁴⁺) tend to convert Cu-ferrite from n- to p-type semiconductor. The results were analyzed on the basis of the small polaron model. The cation distribution for each system was proposed. The activation energy ΔE, Fermi energy E_F, the density of charge carriers n or p and the carriers mobility μ were determined for both systems. In addition, an energy band schemes of Cu–Ge and Cu–Ti ferrites were schematically represented.     

A significant enhancement in thermoelectric power factor of In2Te3 thin films by Se doping and Te composition tuning

Thermoelectric power factor of a material significantly relies on its electrical conductivity, thermal conductivity, and Seebeck coefficient. Herein, an attempt has been made to enhance the thermoelectric power factor of In₂Te₃ thin films by tuning their Te composition and via Se doping. The optimum Se-doping concentration and Te composition enhanced the power factor of pristine In₂Te₃ films by 14 and 7.4 times, respectively. The modified chemical composition, structural characteristics, and surface morphological features of In₂Te₃ films are observed to be pivotal in improving their thermoelectric power factor. Overall, this study offers a facile approach to control the thermoelectric power factor of In₂Te₃ thin films which is significant for their futuristic applications.     

Module-level design and characterization of thermoelectric power generator

Thermoelectric power generation provides us the unique capability to explore the deep space and holds promise for harvesting the waste heat and providing a battery-free power supply for IoTs. The past years have witnessed massive progress in thermoelectric materials, while the module-level development is still lagged behind. We would like to shine some light on the module-level design and characterization of thermoelectric power generators (TEGs). In the module-level design, we review material selection, thermal management, and the determination of structural parameters. We also look into the module-level characterization, with particular attention on the heat flux measurement. Finally, the challenge in the optimal design and reliable characterization of thermoelectric power generators is discussed, together with a calling to establish a standard test procedure.     

High‐temperature thermoelectric properties of Cu2In4Te7

Cu₂Ga₄Te₇ has recently been reported to have a relatively high thermoelectric (TE) figure of merit (ZT). However, the TE properties of Cu₂In₄Te₇, which has the same defect zinc‐blende structure as Cu₂Ga₄Te₇, have been hardly investigated. Here, we demonstrate that Cu₂In₄Te₇ has relatively high ZT values that are similar to those of Cu₂Ga₄Te₇. High‐density polycrystalline bulk samples of Cu₂In₄Te₇ were prepared and their electrical resistivity (?), Seebeck coefficient (S), and thermal conductivity (κ) were measured. Cu₂In₄Te₇ has a maximum ZT of 0.3 at 700 K, with ?, S, and κ values of 62.1 × 10^–5 Ω m, 394 μV K^–1, and 0.61 W m^–1 K^–1, respectively. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optimization of thermoelectric properties of Bi2Se3 using insertion techniques

In this work we investigate the optimization of the performance of thermoelectric materials using insertion techniques. Our results indicate that in cases such as Bi₂Se₃ compounds, where insertion of foreign species in the lattice is possible due to the particular crystal structure, the insertion technique could be proven to be a valuable easy to apply and cost effective technique, an alternative to doping, that can produce homogeneous materials with fine tuning of the electronic and thermoelectric properties. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Enhanced thermoelectric power in dual-gated bilayer graphene

The thermoelectric power of a material, typically governed by its band structure and carrier density, can be varied by chemical doping that is often restricted by solubility of the dopant. Materials showing large thermoelectric power are useful for many industrial applications, such as the heat-to-electricity conversion and the thermoelectric cooling device. Here we show a full electric-field tuning of thermoelectric power in a dual-gated bilayer graphene device resulting from the opening of a band gap by applying a perpendicular electric field on bilayer graphene. We uncover a large enhancement in thermoelectric power at a low temperature, which may open up a new possibility in low temperature thermoelectric application using graphene-based device.     

Improved thermoelectric power factor in metal-based superlattices

In this paper we present a detailed theory of electron and thermoelectric transport perpendicular to heterostructure superlattices. This nonlinear transport regime above barriers is also called heterostructure thermionic emission. We show that metal-based superlattices with tall barriers can achieve a large effective thermoelectric figure of merit (ZT > 5 at room temperature). A key parameter to achieving high performance is the nonconservation of lateral momentum during the thermionic emission process. Conservation of lateral momentum is a consequence of translational symmetry in the plane of the superlattice. We also discuss the use of nonplanar barriers and embedded quantum dot structures to achieve high thermoelectric conversion efficiency.     

Modulation of thermoelectric power of individual carbon nanotubes

Thermoelectric power (TEP) of individual single walled carbon nanotubes (SWNTs) has been measured at mesoscopic scales using a microfabricated heater and thermometers. Gate electric field dependent TEP modulation has been observed. The measured TEP of SWNTs is well correlated to the electrical conductance across the SWNT according to the Mott formula. Strong modulations of TEP were observed in the single-electron conduction limit. In addition, semiconducting SWNTs exhibit large values of TEP due to the Schottky barriers at SWNT-metal junctions.     

The absolute thermoelectric power of polyvalent liquid metals

Results on the measurement of the absolute thermoelectric power of ten polyvalent liquid metals (Al, Bi, Cd, Ga, Hg, In, Pb, Sn, Tl, Zn) from their melting points to about 750°C are reported. The electrical resistivities and the absolute thermoelectric powers of these metals have been calculated using the latest available data on structure factor and Harrison and Animalu form factors. These are compared with the experimental values. It is seen that whereas the predicted and experimental values of the electrical resistivities are in reasonable agreement, those for the absolute thermoelectric power are not. It is suggested that the experimental data on the absolute thermoelectric powers and the resistivities of liquid metals may be used to find the magnitude of the form factor at K = 2k _F.