Charles Wood

Measurement of Seebeck coefficient using a light pulse

A high temperature (1900 K) Seebeck coefficient apparatus is described in which a small temperature gradient is generated by a light pulse technique and, by employing an analog substraction circuit, the Seebeck coefficient is displayed directly on an X-Y recorder.

Thermoelectric properties of lanthanum sulfide

The Seebeck coefficient, electrical resistivity, thermal conductivity, and Hall effect have been studied in γ‐phase La3−xS4 (LaSy) in the composition range 0.04≤x≤0.3 (1.35≤y≤1.48) in order to ascertain its suitability for high‐temperature (300 to 1400 K) thermoelectric energy conversion. In this temperature and composition range the material behaves as an extrinsic semiconductor whose degenerate carrier concentration is controlled by the stoichiometric ratio of La to S. A maximum figure‐of‐merit (Z) of ∼5×10−4 K−1 at a composition x=0.3, y=1.48 (LaS1.48) was obtained.

Measurement of Seebeck coefficient using a large thermal gradient

The integral method of measuring the Seebeck voltage V(T), in which one end of the sample is held at a fixed temperature TC, and the other end is varied through the temperature T range of interest, has been adapted to short rod‐shaped samples. The Seebeck coefficient S is obtained from the slope of the V(T) vs T curve, i.e., S=dV(T)/dT. The apparatus has been completely automated such that the specimen is automatically cycled through a preselected temperature range, up to a maximum temperature of 1000 °C, and the V(T), T, and TC values are acquired, stored, and analyzed by means of a microcomputer. Simplicity of sample handling and minimal operator involvement make this method well suited to the survey of large numbers of samples.

Effect of high temperature annealing on the thermoelectric properties of GaP doped SiGe

Silicon-germanium alloys doped with GaP are used for thermoelectric energy conversion in the temperature range 300-1000 C. The conversion efficiency depends on Z = S-squared/rho lambda, a materials parameter (the figure of merit), where S is the Seebeck coefficient, rho is the electrical resistivity and lambda is the thermal conductivity. The annealing of several samples in the temperature range of 1100-1300 C resulted in the power factor P (= S-squared/rho) increasing with increased annealing temperature. This increase in P was due to a decrease in rho which was not completely offset by a drop in S-squared suggesting that other changes besides that in the carrier concentration took place. SEM and EDX analysis of the samples indicated the formation of a Ga-P-Ge rich phase as a result of the annealing. It is speculated that this phase is associated with the improved properties. Several reasons which could account for the improvement in the power factor of annealed GaP doped SiGe are given.

Boron carbides as high temperature thermoelectric materials

Narrow band semiconductors, in which the transport occurs by small polaron hopping, appear to be of promise for very high temperature thermoelectric energy conversion. Boron carbide is an example of this class of materials and its parameters relevant to this application will be discussed. The figure of merit was found to be a strongly increasing function of temperature with a value of ∼0.5×10−3 K−1 at 1300 K. The prospects of further improvement in performance will be discussed.

Accurate determination of specific heat at high temperatures using the flash diffusivity method

The flash diffusivity method can be extended, very simply, to measuring simultaneously thermal diffusivity and specific heat and thus obtaining the thermal conductivity directly. This was accomplished by determining the amount of heat absorbed by a sample with a well-known specific heat and then using this to determine the specific heat of any other sample. The key to using this technique was to have identically reproducible surfaces on the standard and the unknowns. This was achieved earlier by sputtering the surfaces of the samples with a thin layer of graphite. However, the accuracy in determining the specific heat was within ±10% and there was considerable scatter in the data. Several improvements in the technique have been made which have improved the accuracy to ±3% and increased the precision. The most important of these changes has been the introduction of a method enabling the samples to be placed in exactly the same position in front of the light source. Also, the control of the thickness and the application of the graphite coating have turned out to be very important. A comparison of specific heats obtained with this improved technique and with results obtained using other techniques has been made for two materials.

Transport properties of boron carbide

Electrical conductivity, Seebeck‐coefficient, and Hall‐effect measurements have been made on single‐phase microcrystalline boron carbides, BxC, in the compositional range 4≤x≤9 between liq. N2 temperature and 2273 K. Thermal cycling effects are also discussed. In addition, conductivity and Seebeck coefficient measurements are reported on a large‐grain polycrystal. The results tend to confirm that boron carbide is a degenerate semiconductor in which the predominant conduction mechanism is small‐bipolaron hopping between carbon atoms at energetically inequivalent sites in highly disordered structures.

Refractory Semiconductors for High Temperature Thermoelectric Energy Conversion

Thermoelectric energy conversion utilizing nuclear heat sources has been employed for several decades to generate power for deep space probes. In the past, lead telluride and, more recently, silicon-germanium alloys have been the prime choices as thermoelectric materials for this application. Currently, a number of refractory semiconductors are under investigation at the Jet Propulsion Laboratory in order to produce power sources of higher conversion efficiency and, thus, lower mass per unit of power output. Included amongst these materials are improved Si-Ge alloys, rare earth compounds and boron-rich borides. The criteria used to select thermoelectric materials, in general, and the above materials, in particular, will be discussed. The current state of the art and the accomplishments to date in thermoelectric materials research will be reviewed.

High temperature Hall-effect apparatus

A high‐temperature Hall‐effect apparatus is described which allows measurements up to temperatures greater than 1200 K using the van der Pauw method. The apparatus was designed for measurements on refractory materials having high charge carrier concentrations and generally low mobilities. Pressure contacts are applied to the samples. Consequently, special contacting methods, peculiar to a specific sample material, are not required. The apparatus has been semiautomated to facilitate measurements. Results are presented on n‐ and p‐type silicon.

Electronic Transport Properties of Hot-Pressed B6Si

Measurements of the electrical conductivity, Seebeck coefficient and Hall mobility from approx.300 K to approx.1300/sup 0/K have been carried out on multiphase hotpressed samples of the nominal composition B/sub 6/Si. In all samples the conductivity and the p-type Seebeck coefficient both increase smoothly with increasing temperature. By themselves, these facts suggest small-polaronic hopping between inequivalent sites. The measured Hall mobilities are always low, but vary in sign. A possible explanation is offered for this anomalous behavior.