Showing papers on "Thermoelectric effect published in 1999"

Thermoelectric Cooling and Power Generation

Abstract: In a typical thermoelectric device, a junction is formed from two different conducting materials, one containing positive charge carriers (holes) and the other negative charge carriers (electrons). When an electric current is passed in the appropriate direction through the junction, both types of charge carriers move away from the junction and convey heat away, thus cooling the junction. Similarly, a heat source at the junction causes carriers to flow away from the junction, making an electrical generator. Such devices have the advantage of containing no moving parts, but low efficiencies have limited their use to specialty applications, such as cooling laser diodes. The principles of thermoelectric devices are reviewed and strategies for increasing the efficiency of novel materials are explored. Improved materials would not only help to cool advanced electronics but could also provide energy benefits in refrigeration and when using waste heat to generate electrical power.

Holey and Unholey Semiconductors

Abstract: Thermoelectric materials, which have applications in refrigeration and power generation, are experiencing a surge in research activity. Many different materials are investigated with the goal of maximizing electrical conductivity while minimizing thermal conductivity, which is required for good thermoelectric performance. Only recently was a 30-year deadlock in thermoelectric performance overcome. Predictions for the new materials suggest that important further improvements are on the horizon, with promising applications in the computer and other high-tech industries.

Estimation of the thermal band gap of a semiconductor from Seebeck measurements

Abstract: It is shown that the magnitude of the Seebeck coefficient of a semiconductor has a maximum value that is close to one-half the energy gap divided by eT. An expression for the position of the Fermi level at which the Seebeck coefficient has a maximum or minimum value is derived, with account taken of the mobility and effective mass ratios. It is concluded that measurement of the Seebeck coefficient as a function of temperature on any novel semiconductor is one of the simplest ways of estimating its band gap.

Transport properties of pure and doped mnisn (m=zr, hf)

Abstract: We have studied the transport properties in a family of pure and doped intermetallics of the form MNiSi (M=Zr, Hf), the structures known as the half-Heusler alloys. We have shown that the transport is very sensitive to structural arrangements of the constituent atoms, and this can be manipulated by annealing, isostructural alloying, and doping. The unusual transport properties are viewed in the context of a semimetal-semiconductor transition that in pure alloys sets in near 150 K. Doping with indium can shift the transition upward towards 200 K. The high-temperature transport is dominated by the presence of heavy electrons that are responsible for surprisingly large values of thermopower. Minute amount of antimony (n-type doping) have a spectacular influence on the nature of transport and drive the electrical resistivity and Hall effect to be metal-like at all temperatures. Sb-doped alloys display very high thermoelectric power factors, but the thermal conductivity is still too high to make the material a prospective thermoelectric.

Efficient dopants for ZrNiSn-based thermoelectric materials

Abstract: Four efficient n-type dopants have been found for ZrNiSn-based thermoelectric materials. These are Nb or Ta at the zirconium sites, and Sb or Bi at the tin sites. No suitable dopant was found for the nickel sites. In a alloy, a power factor of and a thermal conductivity of were measured at 300 K, resulting in a dimensionless figure of merit ZT = 0.12. These values are increased to and at 700 K.

High temperature thermoelectric properties of oxide Ca9Co12O28

Abstract: The electrical conductivity, Seebeck coefficient and thermal conductivity of oxide Ca 9 Co 12 O 28 with Ca 2 Co 2 O 5 -type structure are 84 S cm –1 , 118 µV K –1 and 1.73 W m –1 K –1 respectively at 700 °C, and its figure of merit is 0.67×10 –4 K –1 , showing that Ca 9 Co 12 O 28 is a potential material for high temperature thermoelectric energy conversion.

Thermoelectric heat exchanger

Abstract: Disclosed is a system for thermally conditioning and pumping a fluid. The system includes a thermoelectric heat exchanger having a thermoelectric device configured to pump heat. Heat exchangers are provided for transferring heat to and from the thermoelectric device and for generating a fluid flow across the thermoelectric device. The conditioned fluid may be placed in thermal communication with a variety of objects, such as a vehicle seat, or anywhere localized heating and cooling are desired. Thermal isolation may also be provided in the direction of flow to enhance efficiency.

Enhancement of thermoelectric power factor in composite thermoelectrics

Abstract: The analytical properties of macroscopic transport coefficients of two-component composites are first used to discuss the thermoelectric power factor of such a composite. It is found that the macroscopic power factor can sometimes be greater than the power factors of both of the pure components, with the greatest enhancement always achieved in a parallel slabs microstructure with definite volume fractions for the two components. Some interesting examples of actual mixtures are then considered, where the components are a “high quality thermoelectric” and a “benign metal,” leading to the conclusion that considerable enhancement of the power factor is often possible, with but a modest reduction in the thermoelectric figure of merit, compared to those of the high quality thermoelectric component. Two possibilities for fabricating real composites with such improved thermoelectric properties emerge from this study: a parallel slabs microstructure of benign metal and high quality thermoelectric, and a sintered collection of benign metal grains, each of them coated by a thin shell of high quality thermoelectric.

Theoretical modeling of thermoelectricity in Bi nanowires

Abstract: A theoretical model based on the basic electronic band structure of bulk Bi is developed to predict the dependence of the band structure and thermoelectric properties on nanowire width. By carefully tailoring the Bi wire size and carrier concentration, substantial enhancement in the thermoelectric figure of merit is expected for small nanowire widths.

Microfabricated thermoelectric power-generation devices

Abstract: A device for generating power to run an electronic component. The device includes a heat-conducting substrate (composed, e.g., of diamond or another high thermal conductivity material) disposed in thermal contact with a high temperature region. During operation, heat flows from the high temperature region into the heat-conducting substrate, from which the heat flows into the electrical power generator. A thermoelectric material (e.g., a BiTe alloy-based film or other thermoelectric material) is placed in thermal contact with the heat-conducting substrate. A low temperature region is located on the side of the thermoelectric material opposite that of the high temperature region. The thermal gradient generates electrical power and drives an electrical component.

Cooling performance of integrated thermoelectric microcooler

Abstract: A theoretical model for calculating the cooling performance of an integrated thermoelectric microcooler (ITM), which takes into account the effects of substrate thermal bypass, convection and radiation heat loads and electrical and thermal contact resistances, is developed based on a recently proposed device configuration. The model is used to obtain the optimum thermoelement length for achieving maximum cooling performance of an ITM. It is concluded that an ITM with adequate cooling performance for microelectronics applications can be made using currently available integrated circuit fabrication technology.

Antimonides with the half-Heusler structure: New thermoelectric materials

Abstract: The thermoelectric properties near ambient temperature of half-Heusler alloys based on LnPdSb, where Ln=Ho, Er, and Dy are reported. The Seebeck coefficients are large, between 60 and 250 μV/K, and the materials are p type. The resistivities are between 0.6 and 20 mΩ cm. Thermal conductivities are between approximately 5.0 and 3.5 W/mK at 300 K, and are smallest in intentionally disordered materials. The highest ambient temperature ZT obtained is 0.06. Band-structure calculations are presented for LuPdSb. It is suggested that half-Heusler alloys with 18 electrons per formula unit may represent a large class of thermoelectric materials.

Thermoelectric power and conductivity of different types of polypyrrole

Abstract: We have measured the thermoelectric power and conductivity as a function of temperature of a wide range of polypyrrole samples, including a film of soluble polypyrrole synthesized chemically, and wrinkled films synthesized using indium–tin oxide electrodes; other samples investigated include high-conductivity polypyrrole films synthesized at different temperatures and current densities, films grown on nonconducting substrates, and polypyrrole gas sensors. The thermoelectric powers are remarkably similar and metal-like for the medium and high conductivity samples but show nonzero extrapolations to zero temperature for wrinkled samples. The temperature dependence of conductivity tends to be greater for samples of lower conductivity. In contrast to polyaniline and polyacetylene, a crossover to metallic sign for the temperature dependence of conductivity at higher temperatures is not observed in any of our samples; the fluctuation-induced tunnelling and variable-range hopping expressions account for nearly all our conductivity data except for low-temperature anomalies. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 953–960, 1999

Growth of Bi2Te3 and Sb2Te3 thin films by MOCVD

Abstract: Metal organic chemical vapor deposition (MOCVD) has been investigated for elaboration of Bi 2 Te 3 and Sb 2 Te 3 using TMBi (Trimethylbismuth), TESb (Triethylantimony) and DETe (Diethyltellurium) as metal–organic sources. Their thermoelectric and physical properties were studied versus growth conditions. The MOCVD elaboration of Bi 2 Te 3 and Sb 2 Te 3 was carried out in an horizontal reactor for a temperature varying from 400 to 500°C, a total hydrogen flow rate D T varying from 3 to 6 l mm −1 and ( R VI/V ) ratio ranging from 1.5 to 15. The thin films were deposited on pyrex and silicon substrates. The partial pressure of the V element varied between 0.5 10 −4 to 2 10 −4 atm to obtain high growth rate for micro-peltier applications. The cristallinity was investigated by X-ray diffraction and we observed a typical preferential c -orientation. The SEM micrographs show the layers quality and confirms the hexagonal structure. The microprobe data indicate that the stoichiometry of Bi 2 Te 3 and Sb 2 Te 3 is constant for all thickness of the epitaxial films (0.3–7 μm). The films are always n-type conduction for Bi 2 Te 3 and p-type for Sb 2 Te 3 . Seebeck coefficient and the minimum values of the resistivity were found close to −210 and +110 μV K −1 , 9 and 3.5 μΩ.m for Bi 2 Te 3 and Sb 2 Te 3 , respectively. Electrical measurements (mobility and carrier density) were performed by Van der Pauw method. For the two materials, the best values of thermoelectrical properties were obtained at a growth temperature closed to 450°C and a VI/V ratio varying from 2 to 8. The thermoelectric properties of the two materials stay constant when the growth rate is increasing to value higher than 1.5 μm h −1 . This result is very interesting for thick film applications. The previous objective of these experimental results has been to perform the thermoelectric properties of n- and p-type films by establishing first suitable deposition conditions and the elaboration of ternary alloys is now possible.

Thermionic emission cooling in single barrier heterostructures

Abstract: Nonisothermal transport in InGaAsP-based heterostructure integrated thermionic coolers is investigated experimentally. Cooling on the order of a degree over 1 μm thick barriers has been observed. This method can be used to enhance thermoelectric properties of semiconductors beyond what can be achieved with the conventional Peltier effect.

Why clathrates are good thermoelectrics: A theoretical study of Sr8Ga16Ge30

Abstract: Recent measurements have shown that the inorganic clathrate Sr8Ga16Ge30 has good thermoelectric properties. This discovery has caused intense experimental activity to synthesize and test other compounds in this class. It has been conjectured that clathrates may be good thermoelectrics if they satisfy several conditions. The Sr atoms, trapped inside the clathrate cages, scatter phonons efficiently, leading to low thermal conductivity. Electric conductivity takes place mostly through the clathrate frame and the conduction electrons are not scattered by Sr vibrations. The compounds, being made of atoms that are semiconductors in the solid state, may have a high Seebeck coefficient. There has been no direct evidence, experimental or theoretical, for this scenario. By performing density functional calculations we show that these ideas are correct. The Sr atoms are weakly bound to the cage and do undergo large-amplitude motion. An analysis of conductivity shows that the largest contribution comes from a band in...

The promise of low-dimensional thermoelectric materials

Abstract: The study of thermoelectric materials has recently been revived as an active research field, in part due to the recent demonstration of enhancement in the thermoelectric figure of merit of a two-dimensional (2D) PbTe quantum well system, relative to its three-dimensional (3D) bulk counterpart. Calculations suggest that the thermoelectric performance of any 3D material should show an enhanced thermoelectric figure of merit, when prepared as a 2D multi-quantum well superlattice, utilizing the enhanced density of states at the onset of each electronic subband, and the increased scattering of vibrational waves at the boundary between the quantum well and the adjacent barrier of the superlattice. In principle, low dimensionality also allows certain materials such as bismuth, which are poor thermoelectrics in 3D, to become good thermoelectrics. Thus, the successful fabrication of 1D bismuth nanowires offers new possibilities for the study of 1D systems for possible thermoelectric applications.

Carrier pocket engineering applied to “strained” Si/Ge superlattices to design useful thermoelectric materials

Abstract: The concept of carrier pocket engineering is applied to strained Si/Ge superlattices to obtain a large thermoelectric figure of merit ZT. In this system, the effect of the lattice strain at the Si/Ge interfaces provides another degree of freedom to control the conduction band structure of the superlattice. We explore various geometries and structures to optimize ZT for the whole three-dimensional superlattice. The resultant ZT, calculated for a symmetrized Si(20 A)/Ge(20 A) superlattice grown on a (111) oriented Si0.5Ge0.5 substrate, is 0.96 at 300 K and is shown to increase significantly at elevated temperatures. Such a superlattice can be grown using molecular beam epitaxy.

Thermoelectric cooling apparatus and method for maximizing energy transport

Abstract: Apparatus and method for sub-ambient cooling using thermoelectric dynamics in conjunction with novel configuration schemes to maximize energy transport to thereby increase the efficiency of thermoelectric cooling. In one form, a junction maximizes energy transport being positioned between and coupled to thermoelectric elements having minimal spacing to provide efficient thermoelectric cooling. Preferable implementations provide thermoelectric elements configured such that thermal energy is transferred away from the junction and dissipated by thermal sinks coupled to thermoelectric elements

PbTe/Te superlattice structures with enhanced thermoelectric figures of merit

Abstract: The thermoelectric properties of PbTe/Te superlattice structures grown by molecular beam epitaxy have been investigated. The in-plane Seebeck coefficient, Hall coefficient, and electrical resistivity have been measured at 300K. The data show that a significant enhancement of the in-plane Seebeck coefficient, thermoelectric power factor and figure of merit has been achieved.

Thermoelectric materials 1998 -- The next generation materials for small-scale refrigeration and power generation applications

Abstract: Thermoelectric materials are used in a wide variety of applications related to small-scale solid-state refrigeration or power generation. Over the past 30 years, alloys based on the Bi-Te compounds (refrigeration) [(Bi[sub 1[minus]x]Sb[sub x])[sub 2] (Te[sub 1[minus]x]Se[sub x])[sub 3]] and Si[sub 1[minus]x]Ge[sub x] compounds (power generation) have been extensively studied and optimized for their use as thermoelectric materials. Thermoelectric cooling is an environmentally friendly method of small-scale cooling in specific applications such as cooling computer chips and laser diodes. Currently, one of the most common uses of thermoelectric refrigeration materials is in small beverage coolers. Another very important application of thermoelectric materials is in power generation for deep-space probes such as in the Voyager and Cassini missions. Despite the extensive investigation of these traditional thermoelectric materials, there is still substantial room for improvement, and thus, entirely new classes of compounds will have to be investigated. The essence of a good thermoelectric is given by the determination of the material's dimensionless figure of merit, ZT = ([alpha][sup 2][sigma]/[lambda])T, where [alpha] is the Seebeck coefficient, [sigma] is the electrical conductivity, and [lambda] is the total thermal conductivity. The thermal conductivity consists of two parts: the electronic and lattice thermal conductivity. Many of themore » papers presented in this proceedings revolve around either maximizing the numerator of ZT, called the power factor, PF = [alpha][sup 2][sigma]/[lambda], or by minimizing the lattice thermal conductivity. As previously described by Glen Slack, a promising thermoelectric material should possess the thermal properties of a glass and the electronic properties of a crystal, i.e., a phonon-glass and electron-crystal (PGEC). This theme is quite prevalent in the many papers presented in this symposium. Separate abstracts were prepared for 69 papers in this book.« less

Phonon wave heat conduction in thin films and superlattices

Abstract: Heat conduction in thin films and superlattices is important for many engineering applications such as thin-film based microelectronic, photonic, thermoelectric, and thermionic divides. Past modeling efforts on the thermal conductivity of thin films were based on solving the Boltzmann transport equation that treats phonons as particles. The effects of phonon interference and tunneling on the heat conduction and the thermal conductivity of thin films and superlattices remain to be explored. In this work, the wave effects on the heat conduction in thin films and superlattices are studied based on the consideration of the acoustic wave propagation in thin film structures and neglecting the internal scattering. A transfer matrix method is used to calculate the phonon transmission and heat conduction through these structures. The effects considered in this work include the phonon interference, tunneling, and confinement. The phonon dispersion is considered by introducing frequency-dependent Lamb constants. A ray-tracing method that treats phonons as particles is also developed for comparison. Sample calculations are performed on double heterojunction structures resembling Ge/Si/Ge and n-period superlattices similar to Ge/Si/n(Si/Ge)/Ge. It is found that phonon confinements caused by the phonon spectra mismatch and by the total internal reflection create a dramatic decrease of the overall thermalmore » conductance of thin films. The phonon interference in a single layer does not have a strong effect on its thermal conductance but for superlattice structures, the stop bands created by the interface effects can further reduce the thermal conductance. Tunneling of phonon waves occurs when the constituent layers are 1--3 monolayer thick and causes a slight recovery in the thermal conductance when compared to thicker layers. The thermal conductance obtained from the ray tracing and the wave methods approaches the same results for a single layer. For superlattices, however, the wave method leads to a finite thermal conductance even for infinitely thick superlattices while the ray tracing method gives a thermal conductance that decreases with increasing number of layers. Implications of these results on explaining the recent thermal conductivity data of superlattices are explored.« less

Modelling of a microelectromechanical thermoelectric cooler

Abstract: We represent the modelling of a thermoelectric cooler, which is designed by using micromachining and thin film technology. The cooler fabrication is compatible with standard semiconductor technology. Therefore, it can be integrated in microelectronic circuits. The most important parameters of the device like cooling power, maximum temperature difference and optimum current density are calculated. By using thermoelectric thin films with high efficiency and very thin SiC/Si3N4-membranes, a cooling power of a few milliWatts or maximum temperature difference of 30–50 K can be achieved.

Thermoelectromechanical refrigeration based on transient thermoelectric effects

Abstract: This letter introduces the concept of a thermoelectromechanical cooler (TEMC), which modifies a traditional thermoelectric cooler (TEC) by using intermittent contact of a mechanical element synchronized with an applied pulsed current. Using Bi2Te3 as the thermoelectric material, it is predicted that the maximum temperature drop across a TEMC operated under zero applied heat flux is about 35% higher than that of a TEC. This effectively increases the thermoelectric figure of merit for maximum temperature differential applications by a factor of 1.8.

Thermoelectric properties of sintered polycrystalline ZnIn2S4

Abstract: Ceramic compacts of spinel-type ZnIn2S4 and IIIa-ZnIn2S4 polytype with a layer structure were synthesized by the reaction-sintering of mixed powders of ZnS and In2S3 at 723 K and 1073 K in Ar (containing 1° H2) atmosphere, respectively. The thermoelectric properties were investigated in the temperature range from 473 to 873 K. Thermoelectric figure of merit of the IIIa type was much larger than that of the spinel type, and it was slightly higher than the figure of merit of (ZnO)9In2O3, which is known to show the largest value among the oxide homologous compounds. To improve the thermoelectric properties, a c-plane-oriented sintered body of the IIIa polytype was successfully fabricated by a usual ceramic process. The figure of merit in the direction on the c plane was larger than on the ab plane due to higher electrical conductivity on the c plane and increased with increasing temperature showing the largest value of 1.3 × 10−4 K−1 at 873 K.

New micro thermoelectric devices based on bismuth telluride-type thin solid films

Abstract: The sputtering technology for deposition of thermoelectric effective bismuth telluride-type material thin films on foils including pattern generation by photolithography was developed by D.T.S. The properties as electrical conductivity, thermoelectric power and the power factor of such p- and n-type films are presented and the advantages of the thin film technology are explained in relation to the bulk material manufacturing. Based on thin films the cost saving production of new micro thermoelectric devices as Low Power Thermoelectric Generators (LPTG) and infrared sensors (IRS) is possible. The LPTG are energy sources with a power output of a few 10 /spl mu/W for self-sufficient micro and sensor systems and the IRS are fast infrared detectors working after the thermopile principle. The technical parameters of the LPTG as output voltage and power in dependence on the temperature difference, internal resistance, integration density and of IRS, e.g. detectivity and time constant are given in detail. Especially with the LPTG the possibility for the construction of new autonomous sensor and micro systems is now opened. Numerous examples for the applications of the devices are indicated. Improvements for the technology and the output parameters of the devices are suggested.