On the 25th of August 1989, radio signals from the unmanned Voyager II spacecraft produced high resolution radio displays of the planet Neptune and its moon, Triton, on the monitors of Caltech's Jet Propulsion Laboratory. Thus ended the 12-year NASA Voyager I and II missions to the giant outer planets of our solar systems. Somewhat overlooked in the excitement over the impressive scientific findings from Voyager's epic planetary encounters was the power system aboard the spacecraft for data collection and signal transmission. This was the multi-hundred watt, radioisotope fueled thermoelectric generator using thermocouple modules made of Si-Ge alloys. The first section of this paper presents a historical review of the research program at the RCA Laboratories in Princeton, N.J. during the 4-year period required for the conception and development of Si-Ge thermoelements for power generation. This includes RCA's strategy in thermoelectric materials research from the viewpoint of device principles and requirements, scientific issues relating to the growth and thermoelectric characterization of Si-Ce alloys, and device feasibility studies. The performance of Si-Ge thermoelements in the power source of several space missions is also discussed. Finally, a perspective is presented on the research management of this remarkably successful program. The second section of the paper describes the results of more recent research on the thermoelectric properties of hot-pressed, sintered Si-Ge alloys and, in particular, the effects of grain size and additions of GaP. In the final section, consideration is given to future areas of research based, in part, on the results of the earlier studies of Si-Ge technology for thermoelectric power generation.

The thermal conductivity of various systematically disordered crystals has been measured. It has been found that the lowest thermal conductivity above ∼100 K that can be achieved is equal to that of the amorphous phase. This experimentally observed lower limit of the thermal conductivity can be described with a model proposed by Einstein in which the elastic energy propagates in a random walk among the atoms which are vibrating with random phases, and which is called the minimum thermal conductivity. This heat transport mechanism resembles somewhat that commonly accepted in liquids.

A theoretical study of composite thermoelectric media has resulted in the development of a number of simple approximations, as well as some exact results. The latter include exact upper and lower bounds on the bulk effective thermoelectric transport coefficients of the composite and upper bounds on the bulk effective thermoelectric quality factor Ze. In particular, as a result of some exact theorems and computer simulations we conclude that Ze can never be greater than the largest value of Z in the different components that make up the composite.

This paper deals with the modeling of materials in which electric, thermal and cross effects between these are important. Also the effect of a magnetic field is included. A basic model and its FEM implementation are discussed. Some numerical test result are given.

An attempt is made to study the thermoelectric power in ultrathin films of semiconductors under magnetic quantization by including all types of aniso tropies in the energy spectrum within the domain of theory, and taking n-Cd3As2 as an example. It is found that, the magne to-thermopower decreases with increasing surface electron concentration and also changes in an oscillatory manner with film thickness respectively. The theoretical results are in agreement with the experimental observations as reported elsewhere.

In this paper we have studied the thermoelectric power under strong magnetic field in degenerate semiconductors on the basis of fourth order in effective mass theory and taking into account the interactions of the conduction electrons, heavy-holes, light-holes and split-off holes respectively. The results obtained are then compared to those derived on the basis of the well-known three-band Kane model. It is found, taking n-Hg1-xCdxTe as an example, that the magneto-thermo power increases with decreasing electron concentration and increasing magnetic field respectively for both the models in an oscillatory way. The oscillations are due to SdH effects and the theoretical snelysis in accordance with fourth order in effective mass theory is in agreement with the experimental observation as reported elsewhere. In addition, the corresponding results for parabolic energy bands have also been obtained as special cases of our generalized foriulations.

A new method for the measurement of the thermal diffusivity in metals, using transient conditions, is demonstrated. This method is using the principle of superposition of thermal loads. Only time measurements are required, and no accurate temperature and heat flow rate measurements are needed. The method utilizes the Seebeck effect for the measurement of the time dependence of the temperature. A solution of the heat transfer equation for the model pertinent to the described method is derived, and results obtained for several materials are presented.

One of the main trends of the thermoelectric instrument-making development is the microminiaturization of instruments and devices. Thermoelectric converter compactness is achieved by this. As thermoelement geometric dimentions reduce, temperature gradients in materials increase. From this it follows the research problem directed at the investigation of transport phenomena in thermoelectric materials at sufficiently large heat fluxes. Enthusiasm on investigation statement is warmed by the consideration about possible improvement of the efficiency at thermoelectric convertion when in thermoelectric materials temperature gradients of 106 - 109 K/cm/1/ have been realized.

The thermoelemernts based on the Seebeck coefficlent anisotropy in homogeneous media are known as well as classical thormoelements and thermopiles using Seebeck effact in the circuit of haterogeneous materials [1] Some our variants of such thormoelement have been patented particularly in the USA [2,3]. The simplest of this kind thermoelements called anisotropic is represented in figtl. The voltage produced by the thermoelement.

The lattice thermal conductivity of 80-20 Si-Ge is treated theoretically for the case of the Fermi energy positioned for optimum figure of merit. The spectral distribution of the lattice conductivity is limited by anharmonic interactions, by the randomness of the Si-Ge lattice and, at low frequencies, by the interaction with free carriers and neutral donors. The two latter processes dominate over grain boundary scattering. The spectral conductivity is sharply peaked around 0.1 of the Debye frequency. A further reduction in lattice conductivity can be obtained by small insulating inclusions. This is partially offset by a reduction in electronic conductivity, but results in some improvement in the figure of merit.

A model is presented for the high temperature transport properties of large grain size, heavily doped p-type silicon-germanium alloys. Good agreement with experiment (±10%) is found by considering acoustic phonon and ionized impurity scattering for holes and phonon-phonon, point defect and hole-phonon scattering for phonons. Phonon scattering by holes is found to be substantially weaker than phonon scattering by electrons, which accounts for the larger thermal conductivity values of ptype silicon-germanium alloys compared to similarly doped n-type silicongermanium alloys. The relatively weak scattering of long-wavelength phonons by holes raises the possibility that p-type silicon-germanium alloys may be improved for thermoelectric applications by the addition of an additional phonon scattering mechanism which is effective on intermediate and long-wavelength phonons. Calculations indicate improvements in the thermoelectric figure of merit up to 40% may be possible by incorporating several volume percent of 20 Å radius inclusions into p-type silicon-germanium alloys.

The objective of the work reported here is to reduce the thermal conductivity of thermoelectric materials in order to improve their figureof- merit and conversion efficiency. Theory predicts that the addition of ultra-fine, inert, phonon-scattering centers to thermoelectric materials will reduce their thermal conductivity [1]. To investigate this prediction, ultra-fine particulates (20Å to 120Å) of silicon nitride have been added to boron doped, p-type, 80/20 SiGe. All of the SiGe samples produced from ultra-fine powder have lower thermal conductivities, than that for standard SiGe, but high temperature heat treatment increases the thermal conductivity back to the value for standard SiGe. However, the SiGe samples with silicon nitride, inert, phonon-scattering centers, retained the lower thermal conductivity after several heat treatments. A reduction of approximately 25% in thermal conductivity has been achieved in these samples.

A neutron activation study was performed to follow the total oxygen content during the preparation sequence of mechanically alloyed Si-20 at.% Ge n-type alloys using both elemental powders and chunk starting materials. The Si-20 at. % Ge alloys were doped with 1.6 at. % GaP and 3.4.at. % P and the total oxygen concentration was measured in the starting materials, after six hours of mechanical alloying in a helium environment, after hot pressing, and after a short 1100°C soak in fused silica ampoules. The alloys containing high oxygen levels showed low carrier mobility and low thermal conductivity whereas those containing low oxygen showed high mobility and thermal conductivity. The microstructure, as observed by optical metallography and SEM, was found to differ greatly with oxygen content as the low oxygen alloys showed relatively large, well defined grains and the high oxygen alloys showed evidence of poor sintering and limited grain growth.

SiGe alloys have been fabricated with the hiqghest carrier concentrations achieved to date. Values in excess of 4.0 × 1020 have been observed. The thermoelectric behavior of these samples has been characterized, both in the as-pressed state and as a function of 3anneal time and temperature. Figures-of-merit between 0.85 and 0.90 × 10−3 K−1 have been reproducibly achieved. Further improvements will be achieved by reducing the carrier concentration.

Heavy doping of n-type Si-Ge alloys is necessary for improving their high temperature thermoelectric properties. Because of the limited solid solubility of the best dopant available, phosphorus, simultaneous additions of gallium were started several years ago. The substantially higher carrier concentration values obtained in such super-saturated, hot-pressed SiGe/GaP materials point to significant changes in dopant solid solubilities. To better understand the behavior of these alloys, investigation of homogeneous single crystalline materials were needed. As a near-thermodynamic equilibrium technique with processing temperatures well below the melting point of these materials, liquid-phase epitaxy (LPE) was particularly suited to the study of the mechanisms of multidoping. Based on successful crystal growth of Si1-xGex thin films using metals such as Ga, In, Sn and Bi for solvents, several experiments were designed to grow multi-doped SiGe layers with III-V dopant combinations. Knowledge of ternary and quaternary phase diagrams is essential to develop the LPE process. Ternary Si-Ge-M systems computations in good agreement with experimental determinations were used to calculate some of the necessary multicomponent phase diagrams and assess the strength of the various III–V interactions.

An apparatus for measuring electrical resistivity and Hall coefficient on both thin films and bulk material over a temperature range of 300K to 1300K has been built. A unique alumina fixture, with four molybdenum probes, allows arbitrarily shaped samples, up to 2.5 cm diameter, to be measured using van der Pauw's method. The system is fully automated and is constructed with commercially available components. Measurements of the electrical properties of doped and undoped Si-Ge thin films, grown by liquid phase epitaxy reported here, are to illustrate the capabilities of the apparatus.

Boron carbides are refractory solids with potential for application as very high temperature p-type thermoelectrics in power conversion applications. The thermoelectric properties of boron carbides are unconventional. In particular, the electrical conductivity is consistent with the thermally activated hopping of a high density (≈ 1021/cm3) of bipolarons; the Seebeck coefficient is anomalously large and increases with increasing temperature; and the thermal conductivity is surprisingly low. In this paper, these unusual properties and their relationship to the unusual structure and bonding present in boron carbides are reviewed. Finally, the potential for utilization of boron carbides at very high temperatures (up to 2200°C) and for preparing n-type materials is discussed.

The electronic structure of Ru2Si3 has been calculated with the empirical tight binding method and the recursion procedure. The calculation strongly indicates that there exists a gap in the structure, which makes Ru2Si3 semiconducting, as found experimentally and explains the stability of the chimney-ladder phases for a valence electron concentration per transition metal atom smaller than 14.

Over the past few years, there has been a trend to use thermoelectrics for generating power from waste heat or coupling the thermoelectrics with fossil fuel driven appliances or Diesel engines to make them more efficient and/or independent of outside power. PbTe based alloys are being considered for some of these applications. Presented below are some of the techniques that have been pursued in making electrical contacts. Because PbTe can be used up to ∼ 600°C, these alloys offer more efficiency than the (Bi,Sb)2 (SeTe)3 based alloys which are only useful to <300° C. Unfortunately, at these h~gher temperatures, the PbTe based alloys need to be encapsulated to prevent oxidation. This latter step is not addressed in this paper, but can significantly add to the price of a PbTe generator.

The development of growing markets for thermoelectric devices strongly depends upon improving the performance of the Peltier effect alloys. Breaking out of the specialty niches requires doubling present figures of merit, Z, of commercial alloys though each incremental gain potentially opens additional specialty niches.

The alloys are polycomponent, heavily doped, N and P type semiconductors. Optimization to the highest Z's requires controlling bulk phase interactions of phase diagram, compositional, crystal growth, and processing variables influencing the imperfection structures impacting on alloy quality. From the first, the system complexity reeds recognition so the performance optimizing variables become clearly identified. This is crucial to commercial production.

The (BiSb)2 (TeSe)3 provide both N and P type alloy model systems useful in understanding 1he general challenge of performance optimization. Illustrations of the imperfection structural chemical limitations on attainable performance by varied technologies are given.

In seeking other superior performance alloys the experimental designs for exploratory research need immediately to address the identification of the dominating imperfection variables in order to recognize quickly the potentials present.