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At least 739 records · Page 41

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NASA NTRS1979

Analysis of the interaction of an electron beam with a solar cell. III - The effect of spacial variations of the number density of recombination centers on SEM measurements

By means of an exactly soluble model the short circuit current generated by a scanning electron microscope in a P-N junction has been determined in cases where the trap density is inhomogeneous. The diffusion length for minority carriers becomes then dependent on the spacial coordinates. It is shown that in this case the dependence of the Isc on characteristic parameters as cell thickness, distance of the beam excitation spot from ohmic contacts, etc., becomes very intricate. This fact precludes the determination of the local diffusion length in the usual manner. Although the model is somewhat simplified in order to make it amenable to exact solutions, it is nevertheless realistic enough to lead to the conclusion that SEM measurements of bulk transport parameters in inhomogeneous semiconductor material are impractical since they may lead to serious errors in the interpretation of the data by customary means.

Von Roos, O.↗

NASA NTRS1980

Testing EKG electrodes on-line

Simple test instrument allows electrocardiograph operator to check individual electrodes while they are attached to subject. Simply by rotating switch and observing meter, operator verifies that each electrode is not short-circuited or open-circuited and does not present excessive contact resistance at its interface with skin. Instrument also makes it convenient to check electrode cables that are subject to frequent bending and wear, such as cables used on patients who are exercising.

Crosier, W. G.↗

NASA NTRS1981

Instrument measures cloud cover

Eight solar sensing cells comprise inexpensive monitoring instrument. Four cells always track Sun while other four face sky and clouds. On overcast day, cloud-irradiance sensors generate as much short-circuit current as Sun sensor cells. As clouds disappear, output of cloud sensors decreases. Ratio of two sensor type outputs determines fractional cloud cover.

Laue, E. G.↗

NASA NTRS1979

Novel approaches for alleviation of electrical hazards of graphite-fiber composites

Four basically different approaches were considered: gasification of fibers, retention in the matrix, clumping to prevent entrainment, and electrical insulation of fibers. The techniques used to achieve them are described in some detail. These involved surface treatment of fibers to improve the wettability of fibers and coating the fibers with the selected substances before laying them up for composite fabrication. Thermogravimetric analyses were performed on the plain and treated fibers in inert (nitrogen, argon) and reactive (air) atmospheres. The treated fibers embedded in epoxy were ignited in a Bunsen flame to determine the efficiency of these treatments. A simple apparatus was assembled to detect the time for the first short circuit (in a typical electrical circuit) when exposed to the combustion products from a graphite fiber composite fire. The state-of-the-art and treated fibers cast in typical epoxy were burned and ranked for potential success. It was inferred that the gasification schemes appear promising when reduction or oxidation is tried. It was also found that some very promising candidates were available for the clumping and for the electrical insulation of fibers.

Ramohalli, K.↗

NASA NTRS1980

Diffusion length measurements of thin GaAs solar cells by means of energetic electrons

A calculation of the short circuit current density (j sub sc) of a thin GaAs solar cell induced by fast electrons is presented. It is shown that in spite of the disparity in thickness between the N-type portion of the junction and the P-type portion of the junction, the measurement of the bulk diffusion length L sub p of the N-type part of the junction is seriously hampered due to the presence of a sizable contribution to the j sub sc from the P-type region of the junction. Corrections of up to 50% had to be made in order to interpret the data correctly. Since these corrections were not amenable to direct measurements it is concluded that the electron beam method for the determination of the bulk minority carrier diffusion length, which works so well for Si solar cells, is a poor method when applied to thin GaAs cells.

Vonross, O.↗

NASA NTRS1980

Driver for solar cell I-V characteristic plots

A bipolar voltage ramp generator which applies a linear voltage through a resistor to a solar cell for plotting its current versus voltage (I-V) characteristic between short circuit and open circuit conditions is disclosed. The generator has automatic stops at the end points. The resistor serves the multiple purpose of providing a current sensing resistor, setting the full-scale current value, and providing a load line with a slope approximately equal to one, such that it will pass through the origin and the approximate center of the I-V curve with about equal distance from that center to each of the end points.

Turner, G. B.↗

NASA NTRS1980

Preliminary results from a four-working space, double-acting piston, Stirling engine controls model

A four working space, double acting piston, Stirling engine simulation is being developed for controls studies. The development method is to construct two simulations, one for detailed fluid behavior, and a second model with simple fluid behaviour but containing the four working space aspects and engine inertias, validate these models separately, then upgrade the four working space model by incorporating the detailed fluid behaviour model for all four working spaces. The single working space (SWS) model contains the detailed fluid dynamics. It has seven control volumes in which continuity, energy, and pressure loss effects are simulated. Comparison of the SWS model with experimental data shows reasonable agreement in net power versus speed characteristics for various mean pressure levels in the working space. The four working space (FWS) model was built to observe the behaviour of the whole engine. The drive dynamics and vehicle inertia effects are simulated. To reduce calculation time, only three volumes are used in each working space and the gas temperature are fixed (no energy equation). Comparison of the FWS model predicted power with experimental data shows reasonable agreement. Since all four working spaces are simulated, the unique capabilities of the model are exercised to look at working fluid supply transients, short circuit transients, and piston ring leakage effects.

Daniele, C. J.↗

NASA NTRS1979

Some TEM observations of Al2O3 scales formed on NiCrAl alloys

The microstructural development of Al2O3 scales on NiCrAl alloys has been examined by transmission electron microscopy. Voids have been observed within grains in scales formed on a pure NiCrAl alloy. Both voids and oxide grains grew measurably with oxidation time at 1100 C. The size and amount of porosity decreased towards the oxide-metal growth interface. It was postulated that the voids resulted from an excess number of oxygen vacancies near the oxide-metal interface. Short-circuit diffusion paths were discussed in reference to current growth stress models for oxide scales. Transient oxidations of pure, Y-doped, and Zr-doped NiCrAl was also examined. Oriented alpha-(Al,Cr)2O3 and Ni(Al,Cr)2O4 scales often coexisted in layered structures on all three alloys. Close-packed oxygen planes and directions in the corundum and spinel layers were parallel. The close relationships between oxide layers provided a gradual transition from initial transient scales to steady state Al2O3 growth.

Smialek, J.↗

NASA NTRS1980

The spectral response of a front surface field solar cell

A calculation of the short-circuit current as a function of wavelength of the incident light, induced in a front surface field solar cell, is presented. The cell consists of a P(+)-P-N(+) junction. The electric field present in the P(+) layer situated at the front surface (the surface exposed to the sun) boosts the minority carrier collection at the P-N(+) junction sufficiently to make this structure a viable alternative to ordinary solar cells. A distinct advantage derives from the placement of all ohmic contacts on the back-surface facilitating array assemblage.

Von Roos, O.↗

NASA NTRS1980

Determination of deep level capture cross sections in wide band-gap semiconductors by means of an electron beam

The paper presents a general theory for the determination of the short circuit current generated by a sinusoidally amplitude-modulated electron beam in the presence of recombination centers with an arbitrary number of charge states. It is shown that a measurement of the coherent phase shift with respect to the incident beam as a function of the modulation frequency of the beam ascertains the magnitudes of electron and hole capture cross sections. The surface exposed to the electron beam must have a negligible surface recombination velocity for the measurements to be unambiguous.

Von Roos, O.↗

NASA NTRS1980

Electric energy production by particle thermionic-thermoelectric power generators

Thermionic-thermoelectric power generators, composed of a thin layer of porous, low work function material separating a heated emitter electrode and a cooler collector electrode, have extremely large Seebeck coefficients of over 2 mV/K and can provide significant output power. Preliminary experiments with 20-micron thick (Ba Sr Ca)O coatings, limited by evaporative loss to temperatures below 1400 K, have yielded short circuit current densities of 500 mA/sq cm and power densities of 60 mW/ sq cm. Substantially more output is expected with cesium-coated refractory oxide particle coatings operating at higher temperatures. Practical generators will have thermal-to-electrical efficiencies of 10 to 20%. Further increases can be gained by cascading these high-temperature devices with lower temperature conventional thermoelectric generators.

Oettinger, P. E.↗

NASA NTRS1980

Base drive for paralleled inverter systems

In a paralleled inverter system, a positive feedback current derived from the total current from all of the modules of the inverter system is applied to the base drive of each of the power transistors of all modules, thereby to provide all modules protection against open or short circuit faults occurring in any of the modules, and force equal current sharing among the modules during turn on of the power transistors.

Nagano, S.↗

NASA NTRS1980

A four-cylinder Stirling engine controls model

A four working space, double acting piston, Stirling engine simulation was developed for controls studies. Two simulations, one for detailed fluid behavior, and a second model with simple fluid behavior but containing the four working space aspects and engine inertias, validate these models separately, then upgrade the four working space model by incorporating the detailed fluid behavior model for all four working spaces. The single working space model contains the detailed fluid dynamics. The four working space (FWS) model was built to observe the behavior of the whole engine. The drive dynamics and vehicle inertia effects are simulated. The capabilities of the model are exercised to look at working fluid supply transients, short circuit transients, and piston ring leakage effects.

Lorenzo, C. F.↗

NASA NTRS1980

Radiation tolerance of vertical junction solar cells

Extensive radiation testing of vertical junction (VJ) solar cells demonstrated a radiation tolerance better than both planar silicon cells and at least one type of (AlGa)As-GaAs cell. Due to tradeoffs between short circuit current and open circuit voltage, the end of life (10 to the 16th power 1 MeV electrons/sq cm) maximum power point is nearly independent of bulk resistivity between 2 and 10 ohm cm, increases slightly with increasing wafer thickness between 3 and 11 mils, and increases slightly with increasing groove depth between 1 and 3 mils.

Schelnine, A.↗

NASA NTRS1981

Li/SO2 cell performance

The development of a Li/SO2 cell exhibiting intermittent storage capability at 98 C and instantaneous voltage response is discussed. Results of short circuit, cell puncture, and forced discharge testing are presented along with elevated temperature storage and temperature cycling responses.

Cloyd, J. S.↗

NASA NTRS1981

Safety testing of lithium cells

The testing consisted of a forced discharge to zero volts constant current under isothermal conditions. The temperature range was -40 to 65 C. Short circuit tests, drop tests, and puncture tests were run to determine how a cell might behave if it developed a leak. Once the sulfur dioxide is exhausted, a lithium acetontirile reaction occurs. An excess of sulfur dioxide must be maintained in order to avoid chemical explosions.

Bene, J.↗

NASA NTRS1981

The effect of minority carrier mobility variations on solar cell spectral response

Analysis of multistep diffused, high voltage 0.1 ohm-cm solar cells suggests that the increased voltage capability of these cells is correlated with localized variations in the base minority carrier mobility. An attempt to calculate the behavior of those cells revealed unexpected results. It is shown, contrary to what was expected, that spatial variations in the mobility effects severe changes in the short-circuit current and the spectral response. Variations in cell output as a result of imposing abrupt, linear, and exponential mobility variations are presented.

Weizer, V. G.↗

NASA NTRS1981

Reduced annealing temperatures in silicon solar cells

Cells irradiated to a fluence of 5x10,000,000,000,000/square cm showed short circuit current on annealing at 200 C, with complete annealing occurring at 275 C. Cells irradiated to 100,000,000,000,000/square cm showed a reduction in annealing temperature from the usual 500 to 300 C. Annealing kinetic studies yield an activation energy of (1.5 + or - 2) eV for the low fluence, low temperature anneal. Comparison with activation energies previously obtained indicate that the presently obtained activation energy is consistent with the presence of either the divacancy or the carbon interstitial carbon substitutional pair, a result which agrees with the conclusion based on defect behavior in boron-doped silicon.

Weinberg, I.↗