Cronin B. Vining

A model for the high-temperature transport properties of heavily doped n-type silicon-germanium alloys

A model is presented for the high‐temperature transport properties of large‐grain‐size, heavily doped n‐type silicon‐germanium alloys. Electron and phonon transport coefficients are calculated using standard Boltzmann equation expressions in the relaxation time approximation. Good agreement with experiment is found by considering acoustic phonon and ionized impurity scattering for electrons, and phonon‐phonon, point defect, and electron‐phonon scattering for phonons. The parameters describing electron transport in heavily doped and lightly doped materials are significantly different and suggest that most carriers in heavily doped materials are in a band formed largely from impurity states. The maximum dimensionless thermoelectric figure of merit for single‐crystal, n‐type Si0.8Ge0.2 at 1300 K is estimated at ZT≂1.13 with an optimum carrier concentration of n≂2.9×1020 cm−3.

Thermoelectric properties of pressure‐sintered Si0.8Ge0.2 thermoelectric alloys

The thermoelectric properties of 28 sintered Si0.8 Ge0.2 alloys, heavily doped with either boron or phosphorus and prepared from powders with median particle sizes ranging from about 1 μm to over 100 μm, have been determined from 300 to 1300 K. The thermal conductivity decreases with decreasing particle size, however, the figure of merit is not significantly increased due to a compensating reduction in the electrical conductivity. The thermoelectric figure of merit is in good agreement with results of Dismukes et al. [J. Appl. Phys. 10, 2899 (1964)] on similarly doped alloys prepared by zone‐leveling techniques. The electrical and thermal conductivity are found to be sensitive to preparation procedure while the Seebeck coefficient and figure of merit are much less sensitive. The high‐temperature electrical properties are consistent with charge carrier scattering by acoustic or optical phonons.

Comparison of solid-state thermionic refrigeration with thermoelectric refrigeration

A theoretical analysis of single-barrier thermionic emission cooling in semiconducting materials is performed using Fermi–Dirac statistics. Both maximum cooling and coefficient of performance are evaluated. It is shown that the performance of a thermionic refrigerator is governed by the same materials factor as thermoelectric devices. For all known materials, single-barrier thermionic refrigeration is less effective and less efficient than thermoelectric refrigeration.

Extrapolated thermoelectric figure of merit of ruthenium silicide

Recently, single crystals have been grown of a promising new thermoelectric material, ruthenium silicide (Ru2Si3). Although high figure of merit values have not actually been achieved as yet, the intrinsic properties of Ru2Si3 appear to be very favorable for thermoelectric applications. In this paper, the properties of undoped ruthenium silicide are extrapolated to heavily doped materials in order to provide some estimate of the thermoelectric figure of merit which might be achieved with optimal doping. In order to estimate the effect of doping several unverified assumptions have been made. First, is assumed that the carrier mobility of ruthenium silicide scales with doping level in the same way as the carrier mobility of silicon, decreasing substantially at the higher doping levels. It is assumed that the lattice component of the thermal conductivity does not vary doping level. All other effects of doping are accounted for using a simple, non‐degenerate two band model for the transport properties. It is ...

Determination of the thermal diffusivity and specific heat using an exponential heat pulse, including heat-loss effects

The one-dimensional heat diffusion equation has been solved analytically for the case of a heat pulse of the form F(t) = exp(−t/τ)/τ applied to the front face of a homogeneous body including the effects of heat loss from the front and back faces. Approximate expressions are presented which yield a simple, accurate technique for the determination of the thermal diffusivity and specific heat, suitable to a wide range of heat-pulse time constant and heat-loss parameters, without recourse to graphical techniques or requiring further computer analysis. A procedure is described for the determination of an effective time constant to allow application of the present results to the case of a nonexponential heat pulse. Experimental results supporting the theoretical analysis are presented for five samples of silicon germanium alloys of various thicknesses, determined using a xenon flash tube heat-pulse exhibiting an exponential dependence. Proper consideration of the experimental heat pulse shape is shown to lead to reliable corrections to the apparent thermal diffusivity, even for relatively long heat-pulse times.

A promising new thermoelectric material: Ruthenium silicide

Experimental and theoretical efforts directed toward increasing thermoelectric figure of merit values (ZT=σS2T/λ, where σ=electrical condcutivity, S=Seeback coefficient and λ=thermal conductivity) by a factor of two or three have been encouraging in several respects. An accurate and detailed theoretical model developed for n‐type silicon‐germanium (SiGe) indicates that ZT values several times higher than currently available are expected under certain conditions. These new, high ZT materials are expected to be significantly different from SiGe, but not unreasonably so. Several promising candidate materials have been identified which may meet the conditions required by theory. One such candidate, ruthenium silicide, currently under development at the Jet Propulsion Laboratory, has been estimated to have the potential to exhibit figure of merit values four times higher than conventional SiGe materials. Recent results are summarized.

Effect of contact resistance in solid-state thermionic refrigeration

An analytical model of thermionic emission cooling that includes contact resistance is presented. The electrical current density necessary for peak operation of thermionic emission coolers is such that even the slightest resistance in the contacts to the devices will significantly reduce the cooling and coefficient of the performance. The effect of contact resistance is analyzed numerically using a model of thermionic emission cooling based on Fermi–Dirac statistics. The cooling and coefficient of performance are shown to be reduced dramatically by even the slightest contact resistance.

Multiple doping of silicon-germanium alloys for thermoelectric applications

It is shown that heavy doping of n-type Si/Ge alloys with phosphorus and arsenic (V-V doping interaction) by diffusion leads to a significant enhancement of their carrier concentration and possible improvement of the thermoelectric figure of merit. High carrier concentrations were achieved by arsenic doping alone, but for a same doping level higher carrier mobilities and lower resistivities are obtained through phosphorus doping. By combining the two dopants with the proper diffusion treatments, it was possible to optimize the different properties, obtaining high carrier concentration, good carrier mobility and low electrical resistivity. Similar experiments, using the III-V doping interaction, were conducted on boron-doped p-type samples and showed the possibility of overcompensating the samples by diffusing arsenic, in order to get n-type behavior.<<ETX>>

High Figure Of Merit Thermoelectrics: Theoretical Considerations

The thermoelectric figure of merit of a semiconductor, ZT, can be calculated from a small number of microscopic material parameters, the material composition, the doping level, and the temperature. The functional dependence of ZT on these parameters has been studied for a range of material parameters using a recently developed model which accurately and self-consistently describes the thermoelectric properties of n-type silicon-germanium alloys. ZT values several times larger than current state-of-the-art values of ZT of about 1 are shown to be entirely consistent with existing theory, even using material parameters already observed. A search for materials with much higher figure of merit values therefore remains of interest, in spite of several decades of relatively slow progress in this area.

Milliwatt isotope power source for microspacecraft

Miniature spacecraft offer the potential to greatly reduce mission costs, but today there is no flight qualified power source that could operate a microspacecraft during a journey to the outer planets. This paper describes the Milliwatt Isotope Power Source (MIPS), a concept capable of reliable, long term electrical power generation in the milliwatt range. Utilizing existing Radioisotope Heater Unit (RHU) heat source technology and proven thermoelectric energy conversion module technology, a MIPS package about the size of a D‐cell battery could deliver about 30 milliwatts of electrical power for several decades and weigh 70 grams. Such a power source could be used to power miniature instruments such as seismometers, propel a microrover or provide decentralized power aboard a more conventional spacecraft. Also, reliance on flight‐qualified heat source technology and the small radioisotope inventory required are attractive safety considerations.