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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Silicon carbide semiconductor device fabrication and characterization

A number of basic building blocks i.e., rectifying and ohmic contacts, implanted junctions, MOS capacitors, pnpn diodes and devices, such as, MESFETs on both alpha and beta SiC films were fabricated and characterized. Gold forms a rectifying contact on beta SiC. Since Au contacts degrade at high temperatures, these are not considered to be suitable for high temperature device applications. However, it was possible to utilize Au contact diodes for electrically characterizing SiC films. Preliminary work indicates that sputtered Pt or Pt/Si contacts on beta SiC films are someways superior to Au contacts. Sputtered Pt layers on alpha SiC films form excellent rectifying contacts, whereas Ni layers following anneal at approximately 1050 C provide an ohmic contact. It has demonstrated that ion implantation of Al in substrates held at 550 C can be successfully employed for the fabrication of rectifying junction diodes. Feasibility of fabricating pnpn diodes and platinum gated MESFETs on alpha SiC films was also demonstrated.

Davis, R. F.↗
NASA NTRS2009

Spacecraft Electrical Connector Selection and Application Processes

This assessment was initiated by the NASA Engineering & Safety Center (NESC) after a number of recent "high profile" connector problems, the most visible and publicized of these being the problem with the Space Shuttle's Engine Cut-Off System cryogenic feed-thru connector. The NESC commissioned a review of NASA's connector selection and application processes for space flight applications, including how lessons learned and past problem records are fed back into the processes to avoid recurring issues. Team members were primarily from the various NASA Centers and included connector and electrical parts specialists. The commissioned study was conducted on spacecraft connector selection and application processes at NASA Centers. The team also compared the NASA spacecraft connector selection and application process to the military process, identified recent high profile connector failures, and analyzed problem report data looking for trends and common occurrences. The team characterized NASA's connector problem experience into a list of top connector issues based on anecdotal evidence of a system's impact and commonality between Centers. These top issues are as follows, in no particular rank order: electrically shorted, bent and/or recessed contact pins, contact pin/socket contamination leading to electrically open or intermittencies, connector plating corrosion or corrosion of connector components, low or inadequate contact pin retention forces, contact crimp failures, unmated connectors and mis-wiring due to workmanship errors during installation or maintenance, loose connectors due to manufacturing defects such as wavy washer and worn bayonet retention, damaged connector elastomeric seals and cryogenic connector failure. A survey was also conducted of SAE Connector AE-8C1 committee members regarding their experience relative to the NASA concerns on connectors. The most common responses in order of occurrence were contact retention, plating issues, worn-out or damaged coupling mechanisms, bent pins, contact crimp barrel cracking and torn seals. In addition to these common themes, responses included issues with markings, dimensional errors on the build, contact/socket damage (handling), manufacturing defects and customer misapplication and mishandling. The NESC team concluded that considering the large quantity and wide variety of connectors successfully flown on human and robotic space applications, the number of failures is quite low. However, "high profile" failures with significant cost, schedule, safety, and/or mission success impacts continue to occur. It was also concluded that connector failures occur throughout a system's life-cycle with the majority of connector issues application related. A number of recommendations were identified for improving NASA connector selection processes and overall space connector reliability and performance.

Iannello, Chris↗
NASA NTRS2003

Contactless Rotary Electrical Couplings

Rotary electrical couplings based on induction (transformer action) rather than conduction between rotating and stationary circuitry have been invented. These couplings provide an alternative to slip rings and contact brushes. Mechanical imperfections of slip-ring and brush contact surfaces and/or dust particles trapped between these surfaces tend to cause momentary interruptions in electrical contact and thereby give rise to electrical noise. This source of noise can be eliminated in the inductive rotary couplings because no direct contact is necessary for transformer action.

Kumagai, Hiroyuki↗
NASA NTRS1972

A reliable all-silver front contact for silicon solar cells

The feasibility of making an adherent and moisture degradation resistant silver-only front contact to silicon solar cells was demonstrated. Optimum fabrication processes and process sequences were determined for making such contacts. These contacts were found to also have good electrical characteristics. A back contact of aluminum-silver was also developed. This proved very satisfactory for low-temperature applications.

Lamneck, J. H., Jr.↗
NASA NTRS1988

Investigation of energy transfer in the ignition mechanism of a NASA standard initiator

The principal objective of the proposed research was to construct a detailed computer model of the NASA Standard Initiator (NSI). The NSI plays a critical role in initiating various pyrotechnic events in the National Space Transportation System and is also used in Shuttle payload applications. Several initiators failed when being tested at very low temperatures (4 to 20 K). During subsequent investigation an unacceptable high failure rate was found even at higher temperatures (100 to 150 K) but the precise cause of failure was not determined. The modelling work was undertaken to investigate reasons for failure and to predict the performance of alternate firing schemes. The work has shown that the most likely cause of failure at low temperature is poor thermal contact between the electrically heated bridgewire and the pyrotechnic charge. This problem may be masked if there is good thermal contact between the bridgewire and the alumina charge cup. The high thermal conductivity of alumina at cryogenic temperatures was overlooked in previous analyses, which assumed that the charge cup acted as a thermal insulator.

Varghese, Philip L.↗
NASA NTRS1978

Sialons as high temperature insulators

Sialons were evaluated for application as high temperature electrical insulators in contact with molybdenum and tungsten components in hard vacuum applications. Both D.C. and variable frequency A.C. resistivity data indicate the sialons to have electrical resistivity similar to common oxide in the 1000 C or higher range. Metallographic evaluations indicate good bonding of the type 15R ALN polytype to molybdenum and tungsten. The beta prime or modified silicon nitride phase was unacceptable in terms of vacuum stability. Additives effect on electrical resistivity. Similar resistivity decreases were produced by additions of molybdenum or tungsten to form cermets. The use of hot pressing at 1800 C with ALN, Al2 O3 and Si3N4 starting powders produced a better product than did a combination of SiO2 and AIN staring powders. It was indicated that sialons will be suitable insulators in the 1600K range in contact with molybdenum or tungsten if they are produced as a pure ceramic and subsequently bonded to the metal components at temperatures in the 1600K range.

Phillips, W. M.↗
NASA NTRS1972

Optical interferometry study of film formation in lubrication of sliding and/or rolling contacts

Seventeen fluids of widely varying physical properties and molecular structure were chosen for study. Film thickness and traction were measured simultaneously in point contacts by interferometry, from which a new theory of traction was proposed. Film thickness was measured in line contacts by interferometry and electrical capacitance to establish correlation between these two methods. An interferometric method for the absolute determination of refractive index in the contact zone was developed and applied to point contact fluid entrapments. Electrical capacitance was used to study the thickness and properties of the soft surface film which sometimes forms near a metal-fluid interface.

Stejskal, E. O.↗
NASA NTRS1971

Development and fabrication of lithium-doped solar cells

The application of contacts and coatings after lithium diffusion provides good electrical output and satisfactory contact adhesion by sintering for short times at temperatures less than the lithium diffusion temperature. High output and repeatability are obtainable from both oxygen-rich and oxygen-lean silicon. These fabrication sequence alterations have led to higher cell output, better appearance, and increased contact strength.

Iles, P. A.↗
NASA NTRS1979

Development of economical improved thick film solar cell contact

Materials were surveyed to provide candidates for an all metal electrode paste system. These consisted of a major constituent metal powder, a low melting metal powder suitable for a liquid phase sintering medium, and a powder material suitable as an etchant for silicon dioxide at sintering temperatures. By means of thermal gravimetric analysis a suitable binder was identified for low temperature fired inks. The all metal ink concept was first demonstrated with the silver system to avoid the problems of limited process windows encountered with base metal systems. A number of solid materials capable of selectively etching silicon dioxide at modest temperatures were identified. A paste with silver fluoride was screened onto N-type silicon with 5 ohm cm resistivity. The resulting contact pads had excellent adhesion but were not electrically ohmic. Metallurgically, these contacts have equal or better grain structure as commercial inks fired at the same temperatures.

Ross, B.↗
NASA NTRS1972

A reliable all-silver front contact for silicon solar cells.

Methods have been found to apply an adherent and humidity resistant silver-only front contact to n on p silicon solar cells. The critical processing step was the removal of the diffused oxide layer. Humidity resistance is superior to present day commercial titanium-silver contacts and the electrical characteristics of the cells are excellent. Titanium, which is degraded by humidity and which is a source of heavy metal poisoning, has been eliminated from the contact.

Lamneck, J. H., Jr.↗
NASA NTRS1982

Rotary Transformer Seals Power In

Rotary transformer originally developed for spacecraft transfers electrical power from stationary primary winding to rotating secondary without sliding contacts and very little leakage of electromagnetic radiation. Transformer has two stationary primary windings connected in parallel. Secondary, mounted on a shaft that extends out of housing, rotates between two windings of primary. Shaft of secondary is composed of electrically conducting inner and outer parts separated by an insulator. Electrical contact is made from secondary winding, through shaft, to external leads.

Studer, P. A.↗
NASA NTRS1986

A heater made from graphite composite material for potential deicing application

A surface heater was developed using a graphite fiber-epoxy composite as the heating element. This heater can be thin, highly electrically and thermally conductive, and can conform to an irregular surface. Therefore it may be used in an aircraft's thermal deicing system to quickly and uniformly heat the aircraft surface. One-ply of unidirectional graphite fiber-epoxy composite was laminated between two plies of fiber glass-epoxy composite, with nickel foil contacting the end portions of the composite and partly exposed beyond the composites for electrical contact. The model heater used brominated P-100 fibers from Amoco. The fiber's electrical resistivity, thermal conductivity and density were 50 micro ohms per centimeter, 270 W/m-K and 2.30 gm/cubic cm, respectively. The electricity was found to penetrate through the composite in the transverse direction to make an acceptably low foil-composite contact resistance. When conducting current, the heater temperature increase reached 50 percent of the steady state value within 20 sec. There was no overheating at the ends of the heater provided there was no water corrosion. If the foil-composite bonding failed during storage, liquid water exposure was found to oxidize the foil. Such bonding failure may be avoided if perforated nickel foil is used, so that the composite plies can bond to each other through the perforated holes and therefore lock the foil in place.

Hung, C. C.↗
NASA NTRS1987

A heater made from graphite composite material for potential deicing application

A surface heater was developed using a graphite fiber-epoxy composite as the heating element. This heater can be thin, highly electrically and thermally conductive, and can conform to an irregular surface. Therefore it may be used in an aircraft's thermal deicing system to quickly and uniformly heat the aircraft surface. One-ply of unidirectional graphite fiber-epoxy composite was laminated between two plies of fiber glass-epoxy composite, with nickel foil contacting the end portions of the composite and partly exposed beyond the composites for electrical contact. The model heater used brominated P-100 fibers from Amoco. The fiber's electrical resistivity, thermal conductivity and density were 50 micro ohms per centimenter, 270 W/m-K and 2.30 gm/cubic cm, respectively. The electricity was found to penetrate through the composite in the transverse direction to make an acceptably low foil-composite contact resistance. When conducting current, the heater temperature increase reached 50 percent of the steady state value within 20 sec. There was no overheating at the ends of the heater provided there was no water corrosion. If the foil-composite bonding failed during storage, liquid water exposure was found to oxidize the foil. Such bonding failure may be avoided if perforated nickel foil is used, so that the composite plies can bond to each other through the perforated holes and therefore lock the foil in place.

Hung, Ching-Cheh↗
NASA NTRS1984

Effect of Ion Sputtering on Interface Chemistry and Electrical Properties of an Gaas (100) Schottky Contacts

Auger and electron photoelectron spectroscopy were used to measure the extent of As depletion during 1 keV to 5 keV argon sputtering of GaAs surfaces. This depletion was correlated with a general decrease in the barrier height of the rectifying Au contact deposited in situ. However, nondestructive angle resolved XPS measurements showed As was depleted at the outer surface more by 1 keV than 3 keV argon. These effects are explained based on a combined work effective work function model and creation of a donor like surface damage layer. The donor layer was correlated with As depletion by sputtering. Deep level trap formation and annealing of sputtering effects were studied.

Wang, Y. X.↗
NASA NTRS1984

Effect of ion sputtering on interface chemistry and electrical properties of Au GaAs(100) Schottky contacts

The effects of sputtering n-type (100)GaAs with 1 to 5 keV argon ions have been studied in ultrahigh vacuum. Auger electron and X-ray photoelectron spectroscopies have been used to characterize the surface before and after sputtering. Gold Schottky contacts were prepared in situ to avoid contamination at the metal/semiconductor interface. The data show that sputtering depletes the surface in As, and the extent of depletion increased as the ion energy was increased. However, angular-resolved photoelectron data showed As was enriched on the outer atom layer by 5 keV but depleted by 1 keV Ar(+) sputtering. Current and capacitance versus voltage data showed that sputtering always lowered the Schottky barrier height and generally increased the ideality factor. However, the barrier height after 3 keV sputtering was higher than after 1 keV sputtering. Those effects are explained based on a combined effective work function model and creation of a donorlike surface damage layer. The donor layer was correlated with As depletion by sputtering. Deep level trap formation and annealing of sputtering effects were studied.

Wang, Y.-X.↗
NASA NTRS2010

Electrically Variable Resistive Memory Devices

Nonvolatile electronic memory devices that store data in the form of electrical- resistance values, and memory circuits based on such devices, have been invented. These devices and circuits exploit an electrically-variable-resistance phenomenon that occurs in thin films of certain oxides that exhibit the colossal magnetoresistive (CMR) effect. It is worth emphasizing that, as stated in the immediately preceding article, these devices function at room temperature and do not depend on externally applied magnetic fields. A device of this type is basically a thin film resistor: it consists of a thin film of a CMR material located between, and in contact with, two electrical conductors. The application of a short-duration, low-voltage current pulse via the terminals changes the electrical resistance of the film. The amount of the change in resistance depends on the size of the pulse. The direction of change (increase or decrease of resistance) depends on the polarity of the pulse. Hence, a datum can be written (or a prior datum overwritten) in the memory device by applying a pulse of size and polarity tailored to set the resistance at a value that represents a specific numerical value. To read the datum, one applies a smaller pulse - one that is large enough to enable accurate measurement of resistance, but small enough so as not to change the resistance. In writing, the resistance can be set to any value within the dynamic range of the CMR film. Typically, the value would be one of several discrete resistance values that represent logic levels or digits. Because the number of levels can exceed 2, a memory device of this type is not limited to binary data. Like other memory devices, devices of this type can be incorporated into a memory integrated circuit by laying them out on a substrate in rows and columns, along with row and column conductors for electrically addressing them individually or collectively.

Liu, Shangqing↗
NASA NTRS1988

Aging behavior of Au-based ohmic contacts to GaAs

Gold based alloys, commonly used as ohmic contacts for solar cells, are known to react readily with GaAs. It is shown that the contact interaction with the underlying GaAs can continue even at room temperature upon aging, altering both the electrical characteristics of the contacts and the nearby pn junction. Au-Ge-Ni as-deposited (no heat treatment) contacts made to thin emitter (0.15 micrometer) GaAs diodes have shown severe shunting of the pn junction upon aging for several months at room temperature. The heat-treated contacts, despite showing degradation in contact resistance did not affect the underlying pn junction. Au-Zn-Au contacts to p-GaAs emitter (0.2 micrometer) diodes, however, showed slight improvement in contact resistance upon 200 C isothermal annealing for several months, without degrading the pn junction. The effect of aging on electrical characteristics of the as-deposited and heat-treated contacts and the nearby pn junction, as well as on the surface morphology of the contacts are presented.

Fatemi, Navid S.↗