Showing papers on "Seebeck coefficient published in 1997"

Thermoelectric Materials: New Approaches to an Old Problem

Abstract: Thermoelectrics is an old field. In 1823, Thomas Seebeck discovered that a voltage drop appears across a sample that has a temperature gradient. This phenomenon provided the basis for thermocouples used for measuring temperature and for thermoelectric power generators. In 1838, Heinrich Lenz placed a drop of water on the junction of metal wires made of bismuth and antimony. Passing an electric current through the junction in one direction caused the water to freeze, and reversing the current caused the ice to quickly melt; thus thermoelectric refrigeration was demonstrated (figure 1).

Thermoelectric properties of Al-doped ZnO as a promising oxide material for high-temperature thermoelectric conversion

Abstract: The thermoelectric properties of a mixed oxide (Zn 1-x Al x )O (x=0, 0.005, 0.01, 0.02, 0.05) are investigated in terms of materials for high-temperature thermoelectric conversion. The electrical conductivity, σ, of the oxide increases on Al-doping by more than three orders of magnitude up to ca. 10 3 S cm -1 at room temperature, showing metallic behaviour. The Seebeck coefficient, S, of (Zn 1-x Al x )O (x>0) shows a general trend in which the absolute value increases gradually from ca. -100 µV K -1 at room temperature to ca. -200 µV K -1 at 1000 °C. As a consequence, the power factor, S 2 σ, reaches ca. 15×10 -4 W m -1 K -2 , the largest value of all reported oxide materials. The thermal conductivity, κ, of the oxide decreases with increasing temperature, owing to a decrease in the lattice thermal conductivity which is revealed to be dominant in the overall κ. In spite of the considerably large values of κ, the figure of merit, Z=S 2 σ/κ, reaches 0.24×10 -3 K -1 for (Zn 0.98 Al 0.02 )O at 1000 °C. The extremely large power factor of (Zn 1-x Al x )O compared to other metal oxides can be attributed to the high carrier mobility revealed by the Hall measurements, presumably resulting from a relatively covalent character of the Zn–O bond owing to a fairly small difference of the electronegativities of Zn and O. The dimensionless figure of merit,ZT, of 0.30 attained by (Zn 0.98 Al 0.02 )O at 1000 °C demonstrates the potential usefulness of the oxide.

Calculated thermoelectric properties of la-filled skutterudites

Abstract: The thermoelectric properties of La-filled skutterudites are discussed from the point of view of their electronic structures. These are calculated from first principles within the local-density approximation. The electronic structure is in turn used to determine transport related quantities. Virtual-crystal calculations for La(Fe,Co)${}_{4}{\mathrm{Sb}}_{12}$ show that the system obeys near rigid band behavior with varying Co concentration, and has a substantial band gap at a position corresponding to the composition LaFe${}_{3}{\mathrm{CoSb}}_{12}.$ The valence-band maximum occurs at the $\ensuremath{\Gamma}$ point and is due to a singly degenerate dispersive band, which by itself would not be favorable for high thermopower. However, very flat transition-metal-derived bands occur in close proximity and become active as the doping level is increased, giving a nontrivial dependence of the properties on carrier concentration and explaining the favorable thermoelectric properties.

Charge transport of the mesoscopic metallic state in partially crystalline polyanilines

Abstract: Charge transport properties, including temperature-dependent dc conductivity, thermoelectric power, electron paramagnetic resonance, microwave frequency dielectric constant and conductivity, and electric-field-dependent conductance of partially crystalline (``physically'' cross-linked) HCl-doped polyaniline correlated with x-ray structure studies, demonstrate that charge delocalization in physically cross-linked polyaniline systems is structurally controlled. Further, we observe a positive dielectric constant at room temperature which increases (to values $g~{10}^{4})$ with increasing percent crystallinity, the size of crystalline regions, and polymer chain alignment in the disordered regions, supporting the establishment of mesoscopic metallic regions. We propose an inhomogeneous disorder model for this system in which ordered (crystalline) regions, described by three-dimensional metallic states, are connected through amorphous regions of polymer chains where one-dimensional disorder-induced localization is dominant. We utilize the metallic box, interrupted metallic strands, and Nakhmedov's phonon-induced delocalization models to account for the temperature dependence of charge transport properties of the various partially crystalline polyanilines. Analyses for the sample and temperature-dependent electron paramagnetic resonance linewidth and thermoelectric power are presented.

Thermoelectric response of an interacting two-dimensional electron gas in a quantizing magnetic field

Abstract: We present a discussion of the linear thermoelectric response of an interacting electron gas in a quantizing magnetic field. Boundary currents can carry a significant fraction of the net current passing through the system. We derive general expressions for the bulk and boundary components of the number and energy currents. We show that the local current density may be described in terms of ``transport'' and ``internal magnetization'' contributions. The latter carry no net current and are not observable in standard transport experiments. We show that although Onsager relations cannot be applied to the local current, they are valid for the transport currents and hence for the currents observed in standard transport experiments. We relate three of the four thermoelectric response coefficients of a disorder-free interacting two-dimensional electron gas to equilibrium thermodynamic quantities. In particular, we show that the diffusion thermopower is proportional to the entropy per particle, and we compare this result with recent experimental observations.

Transport Properties of Bi2S3 and the Ternary Bismuth Sulfides KBi6.33S10 and K2Bi8S13

Abstract: We have studied transport properties (resistivity, Hall effect, thermopower, and thermal conductivity) of Bi2S3 and two new ternary bismuth sulfides, KBi6.33S10 and K2Bi8S13, to explore their potential for thermoelectric applications. While both Bi2S3 and K2Bi8S13 display an essentially metallic conduction, KBi6.33S10 has a distinctly semiconducting character. From the Hall measurements it follows that the dominant carriers in all three compounds are electrons and that the carrier density in K2Bi8S13 is almost 2 orders of magnitude larger than in Bi2S3 and KBi6.33S10 over the whole temperature range from 4 to 300 K. While the room temperature thermopowers of KBi6.33S10 and K2Bi8S13 are comparable (∼−100 μV/K), the thermopower of Bi2S3 is a factor of 3 larger. All three compounds are poor conductors of heat, in particular the thermal conductivity of KBi6.33S10 is some 25% lower than the thermal conductivity of Bi2Te3, the benchmark thermoelectric material. To make these materials useful thermoelectrics, wa...

Effects of Gd, Co, and Ni doping in La 2 / 3 Ca 1 / 3 MnO 3 : Resistivity, thermopower, and paramagnetic resonance

Abstract: The effect of elemental substitutions on the properties of the ferromagnetic, conducting, highly magnetoresistive compound ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_{3}$ has been studied. The results of Co doping and Ni doping on the Mn site and Gd doping on the La site are reported. These compounds were investigated by x-ray diffraction, magnetization measurements, resistivity measurements, thermopower measurements, and by paramagnetic resonance. The result of replacing La by Gd atoms is to lower the ferromagnetic (or metal-insulator) transition temperature, an effect which is shown to be due to bond bending caused by the lattice adjusting to the size differential between the La and Gd ions. On the other hand, the reduction of the magnetic transition temperature when Mn ions are replaced with Co or Ni ions is not attributed to changes in the size of the ions. Instead, we ascribe the lowering of the ferromagnetic transition temperature to a weakening of the double-exchange interaction between two unlike ions. The resistivity and Seebeck coefficient in these materials have been investigated as a function of elemental substitution. The magnetic polaron theory of Zhang is used, phenomenologically, to provide a quantitative explanation of these transport properties. In addition, the effect of these elemental substitutions on the linewidths of the paramagnetic resonances is studied and is discussed in terms of exchange narrowing and spin-lattice relaxation.

Microstructure and electrical transport properties of pulse-plated nanocrystalline nickel electrodeposits

Abstract: The microstructure and the electrical transport properties (the electrical resistivity, its temperature coefficient and the thermoelectric power) were investigated for pulse-plated nanocrystalline nickel electrodeposits. Transmission and scanning electron microscopy were used to study the microstructure (grain size and lattice defects) and the surface morphology respectively. The samples were prepared from the same bath as used previously for d.c. plating and the deposition current density was constant, in most cases i dep = 20 A dm −2 . In a given series, the pulse length t on was kept constant at 0.001, 0.01, 0.1, 1 or 10 s and the separation between pulses t off was varied from 0.001 s to 10 s. Systematic variations of the electrical transport parameters with t on and t off were observed, which we attempt to explain in terms of the periodic variation due to pulse-plating of the local Ni 2+ concentration at the cathode-electrolyte interface.

New materials and devices for thermoelectric applications

Abstract: New high efficiency thermoelectric materials with ZT values substantially larger than 1.0 have been recently developed. The successful development of a segmented thermoelectric generator utilizing a combination of state-of-the-art thermoelectric materials and these novel thermoelectric materials could result in a high thermal-to-electric materials conversion efficiency over 19%. This is because the new thermoelectric materials have a higher average ZT value and the generator can be operated in a temperature range wider than possible with state-of-the-art (SOA) materials. Such improved power generators could be used for a variety of applications including waste heat recovery. This could also lead to the development of high performance thermoelectric generators operating in a relatively narrow temperature range for specific applications. However, there are also new low power and thermal management applications for small thermoelectric devices using SOA materials. If more efficient thermoelectric materials are incorporated, the performance of these new devices will be even higher.

Thermomagnetic transport properties of Nd 1.85 Ce 0.15 CuO 4 + δ films: Evidence for two types of charge carriers

Abstract: We report measurements of the thermomagnetic properties of Nd{sub 1.85}Ce{sub 0.15}CuO{sub 4{plus_minus}{delta}} thin films as a function of oxygen content. The variation of the Nernst coefficient with carrier doping{emdash}and its large magnitude{emdash}cannot be explained by a conventional single-carrier model. These measurements along with the variation of the Hall coefficient and the thermoelectric power with oxygen doping suggest that a two-carrier model might be suitable to describe the transport properties of these {ital electron-doped} cuprate superconductors. The ratio of the electrical and thermal Hall mobilities is presented as further evidence for the applicability of a two-carrier model. We finally show the correlation between the doping dependence of the Nernst coefficient and the magnetoresistance. {copyright} {ital 1997} {ital The American Physical Society}

Transport Properties of Heavy Fermion Compounds

Abstract: A technique for measuring the electrical resistivity and absolute thermopower is presented for pressures up to 30 GPa, temperatures down to 25 mK and magnetic fields up to 10 T. With the examples of CeCu2Ge2 and CeCu2Si2 we focus on the interplay of normal phase and superconducting properties. With increasing pres- sure, the behaviour of CeCu2Ge2 evolves from that of an antiferromagnetically ordered Kondo system to that characteristic of an intermediate valence compound as the Kondo temperature increases by about two orders of magnitude. In the pressure window 8-10 < P < 20 GPa, a superconducting phase occurs which com- petes at low pressure with magnetic ordering. For CeCu2Si2 the effective mass of carriers is probed by both the coefficient of the Fermi liquid law and the ini- tial slope of the upper critical field. The magnetic instability is studied no- tably for CeRu2Ge2 and Yb-based compounds for which pressure-induced magnetic ordering tends to develop. Finally, contrary to conventional wisdom, we argue that in heavy fermions a large part of the residual resistivity is most likely not independent of temperature; tentatively ascribed to Kondo hole, it can be very pressure as well as sample dependent. [electrical resistivity, thermoelectric power, heavy fermion, magnetic order, superconductivity]

Thermoelectric materials -- New directions and approaches. Materials Research Society symposium proceedings, Volume 478

Abstract: Thermoelectric materials are utilized in a wide variety of applications related to solid-state refrigeration or small-scale power generation. Thermoelectric cooling is an environmentally friendly method of small-scale cooling in specific applications such as cooling computer chips and laser diodes. Thermoelectric materials are used in a wide range of applications from beverage coolers to power generation for deep-space probes such as the Voyager missions. Over the past thirty years, alloys based on the Bi-Te systems {l{underscore}brace}(Bi{sub 1{minus}x}Sb{sub x}){sub 2} (Te{sub 1{minus}x}Se{sub x}){sub 3}{r{underscore}brace} and Si{sub 1{minus}x}Ge{sub x} systems have been extensively studied and optimized for their use as thermoelectric materials to perform a variety of solid-state thermoelectric refrigeration and power generation tasks. Despite this extensive investigation of the traditional thermoelectric materials, there is still a substantial need and room for improvement, and thus, entirely new classes of compounds will have to be investigated. Over the past two-to-three years, research in the field of thermoelectric materials has been undergoing a rapid rebirth. The enhanced interest in better thermoelectric materials has been driven by the need for much higher performance and new temperature regimes for thermoelectric devices in many applications. The essence of a good thermoelectric is given by the determination of themore » material's dimensionless figure of merit, ZT = ({alpha}{sup 2}{sigma}/{lambda})T, where {alpha} is the Seebeck coefficient, {sigma} the electrical conductivity and {lambda} the total thermal conductivity. The best thermoelectric materials have a value of ZT = 1. This ZT = 1 has been an upper limit for more than 30 years, yet no theoretical or thermodynamic reason exits for why it can not be larger. The focus of the symposium is embodied in the title, Thermoelectric Materials: New Directions and Approaches. Many of the researchers in the field believe that future advances in thermoelectric applications will come through research in new materials. The authors have many new methods of materials synthesis and much more rapid characterization of these materials than were available 20--30 years ago. They have tried to focus the symposium on new directions and new materials such as skutterudites, quantum well and superlattice structures, new metal chalcogenides, rare earth systems, and quasicrystals. Other new materials are also presented in these proceedings. Separate abstracts were prepared for all the papers in this volume.« less

Anomalies in the low-temperature thermoelectric power of p -Bi 2 Te 3 and Te associated with topological electronic transitions under pressure

Abstract: Thermoelectric power measurements have been carried out for the narrow-band semiconductors p-Bi2Te3 and Te under pressures up to 2.5 GPa at liquid-helium temperatures. The dependences observed correlate with the data obtained by oscillation methods. These correlations allow one to use the thermoelectric power to search for topological electronic transitions in semiconductors.

Thermoelectric properties of CuIn1-xGaxTe2 single crystals

Abstract: In this paper we report on crystal growth an the electrical characterization of the compounds CuGaTe 2 , CuInTe 2 and their solid solutions CuIn 1-x Ga x Te 2 . Single crystals were obtained by zone melting and a vertical Bridgman technique. From resistivity and Hall effect measurements in the temperature range of 35 to 400 K the carrier concentrations and mobilities were determined. All samples showed p-type conductivity. The hole densities at room temperature were approximately p = 10 20 cm -3 , which indicates that these materials are degenerate semiconductors. For thermoelectric applications weakly degenerate semiconductors are required. In order to investigate whether these materials are suitable for thermoelectric applications we measured the Seebeck coefficient between 90 and 400 K and the thermal conductivity in the range of 80 to 360 K. From the analysis of the obtained data we were able to calculate the temperature dependence of the Fermi level, the density-of-states effective mass of holes, the electronic contribution to the thermal conductivity and the figure of merit for a thermoelectric material. With a theoretical estimation it is shown that there is a optimum doping concentration which leads to a maximum figure of merit Z for this material group. Further possibilities to improve Z are discussed.

Test structures to measure the Seebeck coefficient of CMOS IC polysilicon

Abstract: We report on two thermal characterization structures to measure the Seebeck coefficient /spl alpha/ of CMOS IC polysilicon thin films relevant for integrated thermal microtransducers. The test structures were fabricated using a commercial 1.2 /spl mu/m CMOS process of Austria Mikro Systeme (AMS). The fabrication of the first structure relies on silicon micromachining. In contrast the second, planar, structure is ready for measurement after IC fabrication. The temperature dependent /spl alpha/ of the two polysilicon layers of the AMS process was measured with both devices. The agreement between the thermoelectric coefficients obtained with the two types of structures is better than 2.1 /spl mu/V at 300 K.

Transport properties of FeSi

Abstract: We have measured thermoelectric power, thermal conductivity and electric resistivity on FeSi samples, single crystal and polycrystals, with different qualities. Our data can be understood in terms of classic semiconductor physics. Good agreement is found with the results from recent band structure calculations.

n-Type conduction in Pb doped Se-In chalcogenide glasses

Abstract: This paper reports on the p to n transition in Pb doped Se–In chalcogenide glasses. Measurements of thermoelectric power in the temperature range 300 K⩽T⩽315 K, dc conductivity in the temperature range 100⩽T⩽300 K, and optical band gap (Egopt) have been carried out for Se75In25−xPbx (x=0,5,10,15) samples. The p-n transition occurs with very low addition of Pb impurity (5 at. %). The conductivity and pre-exponential factor also change by five to six orders of magnitude with Pb doping. Results are explained on the basis of the formation of ionic Pb–Se bonds, instead of covalent bonds. Formation of ionic bonds disturbs the equilibrium between the charged defect states of Se–In glass and unpins the Fermi level and thus leads to n-type conduction in these glasses.

Origin of the increase in resistivity of manganese–zinc ferrite polycrystals with oxygen partial pressure

Abstract: In our present study, the origin of the increase in resistivity of polycrystalline Mn0.47Zn0.47Fe2.06O4 with increasing oxygen partial pressure was investigated by measuring thermoelectric power and electrical resistivity, and through analysis of grain size. The ferrous ion (Fe2+) concentration of the samples was estimated using the thermoelectric power data and it indicated that the increase of oxygen partial pressure accompanied only a 0.5 wt. % decrease in the concentration of Fe2+. The decrease in Fe2+ concentration failed to explain the order of magnitude increase in resistivity. Preferential oxidation of the grain boundaries did not contribute to the increase in resistivity since all the samples were cooled under the same conditions, i.e., constant oxidation potential. Impedance spectroscopy revealed that the increase in resistivity arose from the increase in resistivity of the grain boundary; this is discussed in terms of the microscopic shape factor, the brick-layer model, and the Maxwell–Wagner m...

Off-diagonal Seebeck effect and anisotropic thermopower in Bi2Sr2CaCu2O8 thin films

Abstract: The anisotropy ΔS of the thermopower in thin films of the high-Tc superconductor Bi2Sr2CaCu2O8 is investigated using off–c-axis epitaxial film growth and the off-diagonal Seebeck effect. The measurements represent a new method for the investigation of anisotropic transport properties in solids, taking advantage of the availability of oriented grown crystalline thin films instead of using bulk crystals. First results for the temperature dependence ΔS(T) in Bi2Sr2CaCu2O8 thin films are presented and described within a two-band model. Subjecting the films to heat treatments in different gases at relatively low temperatures, ΔS could reversibly be changed in a wide range.

Thermoelectric Properties of (ZnO)5In2O3Thin Films Prepared by r.f. Sputtering Method

Abstract: We have reported that the thermoelectric figures of merit (Z) of the sintered bodies of layered structured (ZnO)mIn2O3, (m=5, 7, and 9) are comparatively large among the semiconducting oxides. In this work, (ZnO)5In2O3 thin films were prepared by r.f. (radio frequency) sputtering method to clarify their anisotropic nature. Under optimum sputtering conditions, (ZnO)5In2O3 thin films developed either (0021) or (110) crystallographic preferred orientation and had dense columnar structures. For c-axis and ab-plane oriented (ZnO)5In2O3 thin films, Seebeck coefficient (α) and electrical conductivity (σ) along the sheet direction were measured at 573-973K. Electrical conductivity of a c-axis oriented thin film was about an order of magnitude higher than that of an ab-plane oriented thin film, while their Seebeck coefficients were substantially the same. This observation suggests that the carrier mobility of (ZnO)5In2O3 along the c-plane is larger than along the ab-plane.

Thermal conductivity of thin films-experimental methods and theoretical interpretation

Abstract: The determination of the thermal conductivity of thin films is of great interest both for understanding the structure and conduction mechanism and for numerous technical applications of these films. The thermal conductivity /spl lambda/ is a crucial term of the thermoelectric figure of merit z=/spl alpha//sup 2//spl sigma///spl lambda/ and consequently an important parameter for the design of thermoelectric thin film devices. Usually the film properties differ considerably from the bulk. Recently research activities are focused on thermoelectric thin film materials, since high z values can be expected in low-dimensional structures. Standard methods for the investigation of the Seebeck coefficient /spl alpha/ and the electrical conductivity /spl sigma/ are well established. However, measurements of the thermal conductivity of thin films are sophisticated and associated with various problems. New methods for the measurement of the thermal conductivity of thin films are reviewed. The problems of stationary and transient measuring techniques are discussed. The results are interpreted with models of surface scattering and grain boundary scattering of charge carriers and phonons.

Metallic thin-film thermocouple for thermoelectric microgenerators

Abstract: A new metallic thin-film thermocouple orientated towards thermoelectric microgenerators has been developed. It consists of a 3 μm thick NiCr/SiO 2 /Sb multilayer structure sputter deposited onto a thermally oxidized silicon substrate. A relative Seebeck coefficient of α ab = 76 μ V K −1 and an optimal figure of merit of z ab = 0.08 × 10 −3 K −1 have been measured for this material combination. Both parameters are very close to the theoretical values.