Engineering topics
Publications and source records attributed to Wood, C..
Microcomputer-controlled system speeds up measurements of Hall voltage, Seebeck coefficient, and thermal diffusivity in semiconductor compounds for thermoelectric-generator applications. With microcomputer system, large data base of these parameters gathered over wide temperature range. Microcomputer increases measurement accuracy, improves operator productivity, and reduces test time.
Boron-carbon compounds have been deposited by the flow of carbon tetrachloride and boron trichloride, in the presence of a large excess of hydrogen, over a graphite surface maintained at 1000-1300 C. Deposits were formed on either an RF-heated disc or a tube or insert heated by a resistance furnace. Crystalline materials ranging in composition from B2C to B17C have been obtained.
Accuracy at high temperature improved. Furnace heats specimen to experimental temperature, and flash tube raises specimen temperature by small amount and for short time so diffusivity (composite property of heat capacity and conductivity) determined. Specimen mount ensures minimum heat loss during temperature-rise measurement. Measurement temperatures up to 1,000 degrees C realized with fused-quartz light pipe and up to 1,600 degrees C with sapphire light pipe.
The Seebeck coefficient, electrical resistivity, thermal conductivity, and Hall effect have been studied in gamma-phase La(3-x)S4(LaS/y/) for compositions with x in the range from 0.04 to 0.3 (y in the range from 1.35 to 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 approximately 0.0005 per K at a composition x = 0.3, y = 1.48 (LaS/1.48/) was obtained.
Compact furnace minimizes thermal gradients and electrical noise. Semiautomatic Hall-effect apparatus takes measurements on refractory semiconductors at temperatures as high as 1,100 degrees C. Intended especially for use with samples of high conductivity and low chargecarrier mobility that exhibit low signal-to-noise ratios, apparatus carefully constructed to avoid spurious electromagnetic and thermoelectric effects that further degrade measurements.
Knowledge of the thermal conductivity of boron carbide is necessary to evaluate its potential for high-temperature thermoelectric energy conversion applications. Measurements have been conducted of the thermal diffusivity of hot-pressed boron carbide BxC samples as a function of composition (x in the range from 4 to 9), temperature (300-1700 K), and temperature cycling. These data, in concert with density and specific-heat data, yield the thermal conductivities of these materials. The results are discussed in terms of a structural model that has been previously advanced to explain the electronic transport data. Some novel mechanisms for thermal conduction are briefly discussed.
A high-temperature (1900 K) Seebeck coefficient apparatus is described in which small thermal gradients are generated in a sample by light pulses transmitted via light pipes. By employing an analog subtraction circuit, the Seebeck coefficient is displayed directly on an X-Y recorder. This technique presents a convenient, accurate, and rapid method for measuring the Seebeck coefficient in highly doped semiconductors as a function of temperature. The nature of the resulting display (X-Y recording) is a valuable tool in determining validity of the data. A straight line results (i.e., a minimum of hysteresis) only if all potential experimental errors are minimized. Under these conditions, the error of measurements of the Seebeck coefficient is estimated to be less than + or - 1 percent.
Two heat-pulse techniques eliminate uncertainties due to radiation losses. Flashlamp supplies pulse of heat to one side of specimen. Temperatures of illumninated surface and of opposite surface monitored photometrically. In second method, pulse of electrons supplies heat to one side of specimen. Temperature of opposite side monitored by photon detector. Techniques needed because radiation heat losses in this temperature range introduce uncertainties into conventional heat-flux measurements.
Sustained arc plasma chemical reactor fabricates very-fine-grained inorganic solids having low thermal conductivity. Powder fabrication method, based on plasma tube technique produces pure solids without contamination commonly produced by grinding.
The rare earth chalcogenides are important thermoelectric materials due to their high melting points, self-doping capabilities, and low thermal conductivities. Lanthanum sulfides and lanthanum tellurides have been synthesized in quartz ampules, hot-pressed into samples, and measured. The n-type Seebeck coefficients, electrical resistivities, and power factors generally all increased as the temperature increased from 200 to 1000 C. The figure-of-merit for nonstoichiometric lanthanum telluride was 0.001/deg C at 1000 C, considerably higher than for silicon-germanium. Thermoelectric measurements were made for LaTe(2) and YbS(1.4), and p-type behavior was observed for these compounds from 300 to 1100 C.
JPL has been leading a concentrated effort to develop improved thermoelectric materials for space applications. Thermoelectric generators are an attractive source of electrical energy for space power because of lack of moving parts and slow degradation of performance. Thermoelectric material is characterized by: Seebeck coefficient, electrical resistivity and thermal conductivity. To measure the high temperature thermal conductivity is experimentally very difficult. However, it can be calculated from the specific heat and thermal diffusivity which are easier to measure at high temperatures, especially using the flash method. Data acquisition and analysis for this experiment were automated at JPL using inexpensive microcomputer equipment. This approach is superior to tedious and less accurate manual analysis of data. It is also preferred to previously developed systems utilizing expensive minicomputers or mainframes.
Electrical conductivity, Seebeck-coefficient, and Hall-effect measurements have been made on single-phase boron carbides, B(1-x)C(x), in the compositional range from 0.1 to 0.2 X, and between room temperature and 1273 K. The results indicate that the predominant conduction mechanism is small-polaron hopping between carbon atoms at geometrically inequivalent sites.
The electronic and thermal transport properties of the most promising material systems for high temperature thermoelectric energy conversion are discussed from a theoretical and experimental viewpoint. Rare earth chalcogenides and boron-rich borides are considered in terms of thermal conductivity and transport mechanisms. Rare earth chalcogenides are generally n-type semiconductors and boron-rich borides are generally p-type semiconductors.
A simple and inexpensive thermal diffusivity apparatus is described for measurement up to 1600 K. The novel features of apparatus include a light pipe, a long furnace, and a differential thermocouple. A low heat-load sample holder for clamping the sample in a vertical position is also described. The results of measurements on AXM-5Q graphite are reported.
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.
Theoretical work of two decades ago adequately explained the transport behavior and effectively guided the development of thermoelectric materials of high conversion efficiencies of conventional semiconductors (e.g., SiGe alloys). The more significant contributions involved the estimaiation of optimum doping concentrations, the reduction of thermal conductivity by solid solution doping and the development of a variety of materials with ZT approx. 1 in the temperature range 300 K to 1200 K. ZT approx. 1 is not a theoretical limitation although, experimentally, values in excess of one were not achieved. Work has continued with emphasis on higher temperature energy conversion. A number of promising materials have been discovered in which it appears that ZT 1 is realizable. These materials are divided into two classes: (1) the rare-earth chalcogenides which behave as itinerant highly-degenerate n-type semiconductors at room-temperature, and (2) the boron-rich borides, which exhibit p-type small-polaronic hopping conductivity.
The high-temperature thermoelectric energy-conversion theory is examined. It is shown that semiconductors are the logical choices for high-figure of merit-value materials, but the requirements for optimization differ depending on whether the material is classed as a broad-band or narrow-band semiconductor. Optimization is discussed as a function of temperature. Phonon scattering mechanisms are considered; and for the reduction of thermal conductivity, basic material guidelines are given. Although experimental results indicate that ZT (where Z is the figure of merit and T is the temperature) of about 1 represents an upper limit, there is in fact, no theoretical limitation. It is unlikely that the Z values will be significantly reduced by radiation transfer at temperatures lower than 2000 K. The current status of materials research is also considered. The rare-earth chalcogenides which behave as n-type semi-conductors and the boron-rich borides which exhibit hopping p-type conductivity behavior are the materials that show promise for high-temperature Z values. The reasons for low thermal conductivities in these materials is discussed.
A joint JPL-NASA program to develop an automated system to measure the thermoelectric properties of newly developed materials is described. Consideration is given to the difficulties created by signal drift in measurements of Hall voltage and the Large Delta T Seebeck coefficient. The benefits of a computerized system were examined with respect to error reduction and time savings for human operators. It is shown that the time required to measure Hall voltage can be reduced by a factor of 10 when a computer is used to fit a curve to the ratio of the measured signal and its standard deviation. The accuracy of measurements of the Large Delta T Seebeck coefficient and thermal diffusivity was also enhanced by the use of computers.