The principles of thermionic refrigeration are explained. The simplest device is composed of two metal plates separated by a vacuum gap. This device is efficient at room temperature only for plates with small work functions (eø <0.3 eV). Small barriers are available in semiconductor-semiconductor interfaces (band off-sets) and in metal-semiconductor interfaces(Schottky barriers). An efficient refrigerator is modeled for these systems. Possible realizations are discussed.

The energy conversion between electrons and phonons in a heterostructure is studied for thermionic cooling by using the hot electron approximation. Calculations show that the energy exchange rate between electrons and phonons should be small for a net cooling power, and the layer length should be in the range of submicron for effective cooling.

Experimental evidence for a significant thermal conductivity reduction have been reported in recent years for GaAs/AlAs, Si/Ge, and Bi 2Te3/Sb2Te3 superlattices. In this work, we present preliminary experimental results on the reduction of the in-plane and cross-plane thermal conductivity for a symmetric Si/Ge superlattice. A differential 2-wire 3ω method is developed to perform the anisotropic thermal conductivity measurements. In this technique, a patterned heater with a width much larger than the film thickness yields the cross-plane thermal conductivity of the film. The in-plane thin film thermal conductivity is inferred from the temperature rise of a narrow width heater that can create more heat spreading in the in-plane direction of the thin film. A differential method to measure the temperature drop across the film is employed in order to increase the accuracy of the measurement.

Various possible mechanisms for the recently discovered enhanced Seebeck coefficient S in PbTe/Te superlattices relative to the corresponding PbTe bulk are investigated. Among the various mechanisms which can account for the enhanced S, the energy dependent τ model (τ ∼ ɛr) seems the most plausible. Here the effective scattering parameter r is preferably increased due to the extra scattering by the periodic Te layers introduced in the superlattice. Other transport properties including the longitudinal magnetoresistance are also discussed.

A quantitative description of the power factor for thermoelectric transport in quantum well and quantum wire superlattices has been developed. The size dependence of the carrier scattering rates as well as carrier tunneling between layers are included, and results are obtained for full three-dimensional superlattice systems. In addition, model calculations of the lattice thermal conductivity of free standing wells and wires have been obtained, and their implications for the thermoelectric figure of merit are discussed. Illustrative results are given for GaAs and PbTe systems.

Advanced thermoelectric microdevices integrated into thermal management packages and low power, electrical source systems are of interest for a variety of space and terrestrial applications. By shrinking the size of the thermoelements, or legs, of these devices, it becomes possible to handle much higher heat fluxes, as well as operate at much lower currents and higher voltages that are more compatible with electronic components. The miniaturization of state-of-the-art thermoelectric module technology based on Bi2Te3 alloys is limited due to mechanical and manufacturing constraints for both leg dimensions (100–200 μm thick minimum) and the number of legs (100–200 legs maximum). We are investigating the development of novel microdevices combining high thermal conductivity substrate materials such as diamond, thin film metallization and patterning technology, and electrochemical deposition of thick thermoelectric films. It is anticipated that thermoelectric microcoolers with thousands of thermocouples and capable of pumping more than 200 W/cm2 over a 30 to 60 K temperature diffrence can be fabricated. In this paper, we report on our progress in developing an electrochemical deposition process for obtaining 10–50 μm thick films of Bi2Te3 and its solid solutions. Results presented here indicate that good quality n-type Bi2Te3, n-type Bi2Te2.95Se0.05 and p-type Bi0.5Sb1.5Te3 thick films can be deposited by this technique. Some details about the fabrication of the miniature thermoelements are also described.

The Harman method was applied to measure thermal conductivity κ of thermoelectric materials, and the reliability of the measured κ was investigated. The quantitative κ requires a highly sensitive technique to measure minute Peltier heat. Temperature difference by Peltier heat pumping was successfully measured by developing the DC method of resistance measurement. κ of n-type Bi2Te3 sintered compact and n-type PbTe boules was measured at 295K by the Harman method. Static comparative method was also applied to obtain the standard value of κ. In the case of Bi2Te3, the κ by the Harman method agreed well with the standard value. In the case of PbTe in the electron concentration ne range <5 × 1024/m3, the κ almost agreed with the standard value. However, PbTe in the ne range ≥1 × 1025/m3 showed a larger κ than the standard value. The Harman method has an error to give the larger κ for the material with a large carrier component κ, of κ This error is due to the fast conduction of Peltier heat by the carrier. The reliable κ can be measured for the material with a small κ,.

Ultrarapid quenching processes have been extensively studied during the past few years. particularly for glasses. In the case of polycrystalline materials, the problem is slightly different. In this case, the microstructures obtained by this process are very different from those obtained by normal cooling and they induce new physical properties.

The present study was aimed at demonstrating the possibility of producing a graded charge carrier concentration in a PbTe crystal by taking advantage of the concentration profile that is set up by the diffusion of In from an external source. Doping by indium generates deep impurity levels lying close to the edge of the conduction band. The Fermi level pinning effect and the electron population of the In impurity levels, which reduces the minority carrier concentration at elevated temperature, significantly improve the thermoelectric behavior of the resulting material. The penetration profiles of In, originating from an external gaseous or liquid source, were determined using Seebeck coefficient measurements in p- and n-type PbTe crystals. In the p-type crystal, the Seebeck coefficient changed sign as the In concentration induced a change from p-type to n-type character. The thermovoltage of a PbTe crystal in which an In concentration profile, generated by In diffiusing from a gaseous source had been established, was determined in the 50 to 430 °C temperature range. The constant Seebeck coefficient that was observed over the whole temperature range provides the experimental support for the underlying premises of this study.