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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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS2007

Tin Whisker Electrical Short Circuit Characteristics Part 2

Existing risk simulations make the assumption that when a free tin whisker has bridged two adjacent exposed electrical conductors, the result is an electrical short circuit. This conservative assumption is made because shorting is a random event that has a currently unknown probability associated with it. Due to contact resistance electrical shorts may not occur at lower voltage levels. In this experiment, we study the effect of varying voltage on the breakdown of the contact resistance which leads to a short circuit. From this data we can estimate the probability of an electrical short, as a function of voltage, given that a free tin whisker has bridged two adjacent exposed electrical conductors. In addition, three tin whiskers grown from the same Space Shuttle Orbiter card guide used in the aforementioned experiment were cross-sectioned and studied using a focused ion beam (FIB).

Courey, Karim J.↗
NASA NTRS1965

Boron carbide whiskers produced by vapor deposition

Boron carbide whiskers have an excellent combination of properties for use as a reinforcement material. They are produced by vaporizing boron carbide powder and condensing the vapors on a substrate. Certain catalysts promote the growth rate and size of the whiskers.

Source record↗
NASA NTRS1972

Whiskers on the moon

The existence of whiskers on lunar soil grains was observed. It is hypothesized that the fibers on these particles are whiskers that grew from clouds of vaporized lunar rock during macro-sized cratering events.

Brownlee, D. E.↗
NASA NTRS1972

Investigation of crystal growth in zero gravity environment and investigation of metallic whiskers

Theoretical and experimental work reported relates to the effects of near-zero gravity on growths of crystals and metallic whiskers during Skylab and Apollo flight experiments. Studies on growth and characterization of candidate materials for flight experiments cover indium-bismuth compounds, bismuth single crystals, gallium arsenide films and single crystals, and cadmium whiskers.

Davis, J. H.↗
NASA NTRS1983

Pyroxene whiskers and platelets in interplanetary dust - Evidence of vapour phase growth

Enstatite crystals with whisker and platelet morphologies have been observed within chondritic porous micrometeorites. Samples of such crystals collected from the stratosphere are described. The samples display unique crystal morphologies and microstructures, such as axial screw dislocations, that strongly suggest that they are primary vapor phase condensates which could have formed either in the solar nebula or in presolar environments. The relationship between the crystal polymorphism and the thermal conditions of growth is discussed, adopting the view that protoenstatite and orthoenstatite are the stable high and low temperature forms, respectively. The kinetic aspects and mechanisms of whisker and platelet growth are considered, and the nature of the likely growth medium for the crystals is addressed. It is concluded that growth from a relatively low-pressure vapor phase is the most likely mode of growth.

Bradley, J. P.↗
NASA NTRS1985

Cone formation as a result of whisker growth on ion bombarded metal surfaces

It is shown experimentally that the seed metals which can cause cone development on ion bombarded metal targets need not to have a lower sputtering yield as widely believed but rather a higher melting point. By lowering the bombarding ion energy to near sputtering threshold values (less than 100 eV) the growth of genuine single crystal whiskers which point in all directions not related to the direction of ion incidence is observed. It is concluded that (intentional or unintentional) seed cones are the result of an interplay among whisker growth, surface movement of atoms, and the effects of sputtering.

Wehner, G. K.↗
NASA NTRS1988

Whisker-reinforced ceramic composites for heat engine components

Much work was undertaken to develop techniques of incorporating SiC whiskers into either a Si3N4 or SiC matrix. The result was the fabrication of ceramic composites with ever-increasing fracture toughness and strength. To complement this research effort, the fracture behavior of whisker-reinforced ceramics is studied so as to develop methodologies for the analysis of structural components fabricated from this toughened material. The results, outlined herein, focus on the following areas: the use of micromechanics to predict thermoelastic properties, theoretical aspects of fracture behavior, and reliability analysis.

Duffy, Stephen F.↗
NASA NTRS1987

Zirconia toughened SiC whisker reinforced alumina composites small business innovation research

The objective of this phase 1 project was to develop a ceramic composite with superior fracture toughness and high strength, based on combining two toughness inducing materials: zirconia for transformation toughening and SiC whiskers for reinforcement, in a controlled microstructure alumina matrix. The controlled matrix microstructure is obtained by controlling the nucleation frequency of the alumina gel with seeds (submicron alpha-alumina). The results demonstrate the technical feasibility of producing superior binary composites (Al2O3-ZrO2) and tertiary composites (Al2O3-ZrO2-SiC). Thirty-two composites were prepared, consolidated, and fracture toughness tested. Statistical analysis of the results showed that: (1) the SiC type is the key statistically significant factor for increased toughness; (2) sol-gel processing with a-alumina seed had a statistically significant effect on increasing toughness of the binary and tertiary composites compared to the corresponding mixed powder processing; and (3) ZrO2 content within the range investigated had a minor effect. Binary composites with an average critical fracture toughness of 6.6MPam sup 1/2, were obtained. Tertiary composites with critical fracture toughness in the range of 9.3 to 10.1 MPam sup 1/2 were obtained. Results indicate that these composites are superior to zirconia toughened alumina and SiC whisker reinforced alumina ceramic composites produced by conventional techniques with similar composition from published data.

Loutfy, R. O.↗
NASA NTRS1989

Analysis of whisker-toughened ceramic components - A design engineer's viewpoint

The analysis of components fabricated from whisker-toughened ceramic matrix composites requires a departure from the 'factor-of-safety' design philosophy prevalent in the design of metallic structural component, which are more tolerant of flaws. A public-domain computer algorithm has been developed which, in conjunction with a general-purposed FEM program, can predict the fast-fracture reliability of a structural component under multiaxial loading conditions. The present version of the algorithm, designated 'Toughened Ceramics Analysis and Reliability Evaluation of Structures', accounts for material symmetry imposed by whisker orientation; the processes of crack deflection and crack pinning are also addressed.

Duffy, Stephen F.↗
NASA NTRS1989

Aging effects on the fracture toughness of SiC whisker reinforced 2XXX aluminum alloys

The effect of aging (at 150 C) time on the fracture toughness behavior of a 2XXX alloy (Al-3.55Cu-1.29Mg-0.01Fe-trace Mn) reinforced with 5 vol pct F-8 SiC whiskers was investigated by measuring hardness and electrical conductivity followed by fracture toughness tests on center-cracked specimens. The ageing time-hardening response plots showed that, independent of whisker orientation, the initial rapid increase in hardness was followed by a more gradual increase, with a broad hardness peak between 32 and 128 hrs of aging. Coincident with the hardness changes, the electrical conductivity initially decreased, reached a minimum, and then increased at aging times beyond 32 hrs. Examination by SEM indicated that the initial increase in hardness and decrease in conductivity was due to the GPB zone formation, while the subsequent increase in electrical conductivity and decrease in hardness (overaging) was due to S nucleation and growth.

Ratnaparkhi, P. L.↗
NASA NTRS1990

Noninteractive macroscopic reliability model for whisker-reinforced ceramic composites

Considerable research is underway in the field of material science focusing on incorporating silicon carbide whiskers into silicon nitride and alumina matrices. These composites show the requisite thermal stability and thermal shock resistance necessary for use as components in advanced gas turbines and heat exchangers. This paper presents a macroscopic noninteractive reliability model for whisker-reinforced ceramic composites. The theory is multiaxial and is applicable to composites that can be characterized as transversely isotropic. Enough processing data exists to suggest this idealization encompasses a significantly large class of fabricated components. A qualitative assessment of the model is made by presenting reliability surfaces in several different stress spaces and for different values of model parameters.

Duffy, Stephen F.↗
NASA NTRS1991

Processing and properties of SiC whisker- and particulate-reinforced reaction bonded Si3N4

The microstructure and mechanical properties of reaction bonded Si3N4 (RBSN) reinforced with SiC whiskers of particles were investigated using RBSN composites made from colloidally pressed octanol dispersions of high-purity Si powders mixed with either SiC whiskers or alpha-SiC particles. Results of investigations, revealing high conversions of Si to Si3N4, specific surface areas, and constant relative densities and strengths, showed that the uniform microstructure and small flaw size of the matrix were maintained in the composites and that no degradation of the reinforcements was taking place. Neither the monolithic nor the composite materials exhibited R-curve behavior. A modest increase in fracture toughness was observed only in the RBSN containing 33 vol pct SiC(p).

Lightfoot, A.↗
NASA NTRS1992

Strength, toughness and R-curve behaviour of SiC whisker-reinforced composite Si3N4 with reference to monolithic Si3N4

The flexural strength and fracture toughness of 30 vol pct SiC whisker-reinforced Si3N4 material were determined as a function of temperature from 25 to 1400 C in an air environment. It was found that both strength and toughness of the composite material were almost the same as those of the monolithic counterpart. The room-temperature strength was retained up to 1100 C; however, appreciable strength degradation started at 1200 C and reached a maximum at 1400 C due to stable crack growth. In contrast, the fracture toughness of the two materials was independent of temperature with an average value of 5.66 MPa sq rt m. It was also observed that the composite material exhibited no rising R-curve behavior at room temperature, as was the case for the monolithic material. These results indicate that SiC whisker addition to the Si3N4 matrix did not provide any favorable effects on strength, toughness and R-curve behavior.

Choi, S. R.↗
NASA NTRS1992

Whisker growth studies under conditions which resemble those available on an orbiting space station

Minimal funding was provided by NASA with one designated 'mission' being the clear demonstration of the relevance of previously supported whisker growth studies to microgravity research. While in one sense this work has shown the converse, namely, that ambient gravitational fields as high as 1 Earth normal have no relevance to growth of whiskers by hydrogen reduction of metal halides, a case is made that this does not demonstrate lack of relevance to microgravity research. On the contrary, the driving forces for this growth are precisely those which must be understood in order to understand growth in microgravity. The results described suggest that knowledge gained from this work may be highly fundamental to our understanding of the genesis of metal crystals. Time and money ran out before this work could be considered complete. At least another year's study and analysis will be required before publications could be justified.

Hobbs, Herman H.↗
NASA NTRS2007

Space Shuttle Program Tin Whisker Mitigation

The discovery of tin whiskers (TW) on space shuttle hardware led to a program to investigate and removal and mitigation of the source of the tin whiskers. A Flight Control System (FCS) avionics box failed during vehicle testing, and was routed to the NASA Shuttle Logistics Depot for testing and disassembly. The internal inspection of the box revealed TW growth visible without magnification. The results of the Tiger Team that was assembled to investigate and develop recommendations are reviewed in this viewgraph presentation.

Nishimi, Keith↗
NASA NTRS2009

Tin Whisker Electrical Short Circuit Characteristics

Existing risk simulations make the assumption that when a free tin whisker has bridged two adjacent exposed electrical conductors, the result is an electrical short circuit. This conservative assumption is made because shorting is a random event that has an unknown probability associated with it. Note however that due to contact resistance electrical shorts may not occur at lower voltage levels. In our first article we developed an empirical probability model for tin whisker shorting. In this paper, we develop a more comprehensive empirical model using a refined experiment with a larger sample size, in which we studied the effect of varying voltage on the breakdown of the contact resistance which leads to a short circuit. From the resulting data we estimated the probability distribution of an electrical short, as a function of voltage. In addition, the unexpected polycrystalline structure seen in the focused ion beam (FIB) cross section in the first experiment was confirmed in this experiment using transmission electron microscopy (TEM). The FIB was also used to cross section two card guides to facilitate the measurement of the grain size of each card guide's tin plating to determine its finish.

Courey, Karim J.↗