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At least 235 records · Page 13

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NASA NTRS1985

Potential productivity benefits of float-zone versus Czochralski crystal growth

Efficient mass production of single-crystal silicon is necessary for the efficient silicon solar arrays needed in the coming decade. However, it is anticipated that there will be difficulty growing such volumes of crystals using conventional Czochralski (Cz) methods. While the productivity of single crystals might increase with a crystal diameter increase, there are two obstacles to the mass production of large diameter Czochralski crystals, the long production cycle due to slow growth rate and the high heat requirements of the furnaces. Also counterproductive would be the large resistivity gradient along the growth direction of the crystals due to impurity concentration. Comparison between Float zone (FZ) and Cz crystal growth on the basis of a crystal 150 mm in diameter is on an order of two to four times in favor of the FZ method. This advantage results from high growth rates and steady-state growth while maintaining a dislocation-free condition and impurity segregation.

Abe, T.↗

NASA NTRS1986

1700 deg C optical temperature sensor

A new gas temperature sensor was developed that shows promise of sufficient ruggedness to be useful as a gas turbine temperature sensor. The sensor is in the form of a single-crystal aluminum oxide ceramic, ground to a cone shape and given an emissive coating. A lens and an optical fiber conduct the thermally emitted light to a remote and near-infrared photodetector assembly. Being optically coupled and passive, the sensor is highly immune to all types of electrical interference. Candidate sensors were analyzed for optical sensor performance, heat transfer characteristics, stress from gas loading. This led to the selection of the conical shape as the most promising for the gas turbine environment. One uncoated and two coated sensing elements were prepared for testing. Testing was conducted to an indicated 1750 C in a propane-air flame. Comparison with the referee optical pyrometer shows an accuracy of + or - 25 C at 1700 C for this initial development. One hundred cycles from room temperature to 1700 C left the sapphire cone intact, but some loss of the platinum, 6% rhodium coating was observed. Several areas for improving the overall performance and durability are identified.

Mossey, P. W.↗

NASA NTRS1986

Effect of crystallographical and geometrical changes of a ferrite head on magnetic signals during the sliding process with magnetic tape

This paper reviews changes in the crystalline structure and geometry of lapped Mn-Zn ferrite heads in sliding contact with magnetic tape and the effects of these changes on magnetic signals. A highly textured, polycrystalline structure was produced on the surface of a single-crystal Mn-Zn ferrite head when it was finished with an aluminum oxide lapping tape. Sliding this lapped surface against a magnetic tape produced a nearly amorphous structure. The sliding process led to a degradation in readback signal of 1 to 2 dB (short-wavelength recording). Furthermore, wear of the magnetic head caused geometrical changes in the head surface. The signal read back with the worn magnetic head was sensitive to operating parameters such as head displacement and tape tension. A change in operating parameters created head-to-tape spacings and, consequently, excessive gains or losses in the readback signal.

Miyoshi, K.↗

NASA NTRS1985

Temperature dependence of gamma-gamma prime lattice mismatch in nickel-base superalloys

High temperature X-ray diffraction techniques were used to determine the gamma-gamma prime lattice mismatch of three different nickel-base superalloys at temperatures between 18 and 1000 C. The measurements were performed on oriented single-crystal disks which had been aged to produce a semicoherent gamma-gamma prime structure. The thermal expansion of the lattice parameters of the gamma and gamma-prime phases was described by a second-order polynomial expression. The expansion of the gamma-prime phase was consistently smaller than that of the gamma phase, which caused the lattice mismatch to become more negative at higher temperatures. It was also shown that high values of lattice mismatch resulted in increased rates of directional gamma-prime coarsening during elevated temperature creep exposure.

Nathal, M. V.↗

NASA NTRS1986

Effect of counterions on the formation of ohmic contact between p-Si and poly(pyrrole) film - An ac impedance analysis

Conditions under which poly(pyrrole) (PP) films form ohmic contact with single-crystal p-Si are described. Counterions affect both the conductivity and flatband potential, V(FB), values of poly(pyrrole). While paratoluene-sulfonate-doped PP acts like a switch, the impedance behavior of PP films doped with ClO4(-), BF4(-), or PF6(-) allows evaluation of the V(FB) of these films. The formation of 'quasi-ohmic' and 'nonohmic' contacts, respectively, of PP (ClO4) and PP films doped with other counterions, with p-Si, are explained in terms of conductivity of these films and V(FB) of PP films with respect to that of p-Si. PP film seems to passivate or block intrinsic surface states present on p-Si surface.

Nagsubramanian, G.↗

NASA NTRS1986

The evaluation of single crystal superalloys for turbopump blades in the SSME

This paper discusses single-crystal nickel-base superalloys for use in gaseous hydrogen environments, like the Space Shuttle Main Engine (SSME). PWA 1480E was chosen as a candidate alloy based on strength retention in hydrogen environments. Selection of single-crystal-alloy primary and secondary crystallographic axes allows tailoring of the airfoil resonant frequency as a function of orientation. PWA 1480E 111-line primary orientation proved to be the most hydrogen-resistant orientation. By choosing the proper primary and secondary orientation combinations, the fourth excitation mode of the SSME may be avoided.

Bowen, K.↗

NASA NTRS1986

Dry etching of beta-SiC in CF4 and CF4 + O2 mixtures

Dry etching of cubic (100) beta-SiC single-crystal thin films produced via chemical-vapor deposition (CVD) has been performed in CF4 and CF4 + O2 mixtures, in both the reactive-ion-etching (RIE) and plasma-etching modes. The latter process yielded measurable etch rates, but produced a dark surface layer which appears, from the results of secondary-ion mass spectrometry, to be residual SiC. The RIE samples had no residual layer, but Auger electron spectroscopy did reveal a C-rich surface. The optimal RIE conditions were obtained with 10 sccm of pure CF4 at 40 mtorr and a power density of 0.548 W/sq cm, giving an etch rate of 23.3 nm/min. Neither the increase of temperature between 293 and 573 K, nor the incremental addition of O2 to CF4 to 50 percent, produced any strong effect on the etch rates of SiC during RIE. Pictorial evidence of fine line structures produced by RIE of beta-SiC films are also presented.

Palmour, J. W.↗

NASA NTRS1987

Study of Silicon-Web Growth

Study of dendritic-web growth of single-crystal silicon casts doubts on previously accepted physical concepts of process.

Richter, Robert↗

NASA NTRS1986

Point contact silicon solar cells

The construction of a 22.2% efficient single-crystal silicon solar cell fabricated at Stanford University is described. The cell dimensions were 3 x 5 mm and 100 microns thick with a base lifetime of 500 microseconds. The cell featured light trapping between a texturized top surface and a reflective bottom surface, small point contact diffusions, alternating between n-type and p-type in a polka-dot pattern on the bottom surface, and a surface passivation on all surfaces between contact regions.

Swanson, R. M.↗

NASA NTRS1986

Protein crystal growth; Proceedings of the First International Conference, Stanford University, CA, August 14-16, 1985

Papers are presented on mechanisms of nucleation and growth of protein crystals, the role of purification in the crystallization of proteins and nucleic acids, and the effect of chemical impurities in polyethylene glycol on macromolecular crystallization. Also considered are growth kinetics of tetragonal lysozyme crystals, thermodynamic and kinetic considerations for crystal growth of complex molecules from solution, protein single-crystal growth under microgravity, and growth of organic crystals in a microgravity environment. Papers are also presented on preliminary investigations of protein crystal growth using the Space Shuttle, convective diffusion in protein crystal growth, and the growth and characterization of membrane protein crystals.

Feigelson, R. S.↗

NASA NTRS1986

Plastic deformation of a magnesium oxide 001-plane surface produced by cavitation

An investigation was conducted to examine plastic deformation of a cleaved single-crystal magnesium oxide 001-plane surface exposed to cavitation. Cavitation damage experiments were carried out in distilled water at 25 C by using a magnetostrictive oscillator in close proximity (2 mm) to the surface of the cleaved specimen. The dislocation-etch-pit patterns induced by cavitation were examined and compared with that of microhardness indentations. The results revealed that dislocation-etch-pit patterns around hardness indentations contain both screw and edge dislocations, while the etch-pit patterns on the surface exposed to cavitation contain only screw dislocations. During cavitation, deformation occurred in a thin surface layer, accompanied by work-hardening of the ceramic. The row of screw dislocations underwent a stable growth, which was analyzed crystallographically.

Hattori, S.↗

NASA NTRS1986

Reflectivity of silicon carbide in the extreme ultraviolet

Reflectivity and scattering measurements on samples of SiC manufactured by a variety of processes. Measurements were made from near-normal to grazing incidence at wavelengths in the range of 114 to 1216 A. CVD SiC displays the highest reflectivity at EUV wavelengths and normal incidence. At grazing incidence, Si-rich samples show reflectivity cutoff identical to polycrystalline Si. From the limited data available for single-crystal material, it is concluded that CVD material has comparable performance.

Mrowka, Stanley↗

NASA NTRS1987

The stability of lamellar gamma-gamma-prime structures

The stability of stress-annealed gamma/gamma-prime lamellar structures were investigated using three nickel-base single-crystal alloys (the NASAIR 100 and two similar alloys, E and F, containing 5 and 10 wt pct Co, respectively) stress-annealed at 1000 C to form lamellae perpendicular to the applied stress. The rate of the lamellar thickening under various thermal and creep exposures was examined by SEM. For unstressed aging at 1100 C, the lamellar structures of the NASAIR and the E alloys exhibited continuous but slow lamellar coarsening, whereas the lamellae of the alloy F showed pronounced thickening plus spheroidization. Resistance to lamellar thickening was correlated with high magnitudes of lattice mismatch, which promoted a more regular lamellar structure and a finer spacing of misfit dislocations. Specimens which were tension-annealed prior to compressive creep testing exhibited an earlier onset of tertiary creep in comparison with only heat-treated specimens. This was associated with accelerated lamellar coarsening in the stress-annealed specimens.

Nathal, M. V.↗

NASA NTRS1987

Development of a new generation of high-temperature composite materials

There are ever-increasing demands to develop low-density materials that maintain high strength and stiffness properties at elevated temperatures. Such materials are essential if the requirements for advanced aircraft, space power generation, and space station plans are to be realized. Metal matrix composites and intermetallic matrix composites are currently being investigated at NASA Lewis for such applications because they offer potential increases in strength, stiffness, and use temperature at a lower density than the most advanced single-crystal superalloys presently available. Today's discussion centers around the intermetallic matrix composites proposed by Lewis for meeting advanced aeropropulsion requirements. The fabrication process currently being used at Lewis to produce intermetallic matrix composites will be reviewed, and the properties of one such composite, SiC/Ti3Al+Nb, will be presented. In addition, the direction of future research will be outlined, including plans for enhanced fabrication of aluminide composites by the arc spray technique and fiber development by the floating-zone process.

Brindley, Pamela K.↗

NASA NTRS1987

Remote gate capacitance-voltage studies for noninvasive surface characterization

A measurement technique has been developed which allows noncontact capacitance-voltage measurements to be made using a gate electrode located remote from the semiconductor surface under study. With gate electrodes about 0.5 mm in diameter and gate to semiconductor separations of about 1500 A, it was possible to generate data entirely comparable to that obtained with integrated MIS structures but with the advantage that there was access directly to the free-semiconductor surface. This technique was applied to bulk single-crystal Si and InP samples.

Chang, R. R.↗

NASA NTRS1987

Effect of initial gamma prime size on the elevated temperature creep properties of single crystal nickel base superalloys

The influence of initial gamma-prime size and shape on the high-temperature creep properties of two single-crystal Ni-base superalloys was investigated. The two alloys were chosen to represent different magnitudes of gamma/gamma-prime lattice mismatch. A range of initial microstructures was produced by various quenching and aging treatments. Creep-rupture testing at 1000 C was performed under stresses where gamma-prime directionally coarsens to form gamma/gamma-prime lamellae in the early portion of the creep life. Both alloys exhibited a peak in creep resistance as a function of initial gamma-prime size. The peak corresponded to an initial microstructure consisting of cuboidal precipitates aligned along 001 line directions. These aligned cuboidal gamma-prime particles directionally coarsened into a relatively perfect lamellar gamma/gamma-prime structure in the early stages of creep, whereas the more irregularly shaped and distributed gamma-prime particles in both under- and overaged material formed more irregular lamellae with more imperfections. The alloy with a lower magnitude of mismatch was less sensitive to initial gamma-prime size and shape.

Nathal, M. V.↗

NASA NTRS1987

Electrochemical studies of thin films of conducting polymers and conducting polymer composites deposited on metal and semiconductor electrodes

Electrochemical studies indicate that poly(Isothianaphthene) or PITN, can be p-doped only. Electrochemical properties of PITN and Nafion-PITN in acetonitrile solutions containing tetra-phenyl Phosphonium chloride as supporting electrolyte are compared. In both cases, the electrochemical behavior of thin films are different from that of thick films. In addition, Nafion does not seem to alter the electrochemical properties of PITN. Cyclic voltammetric and chronocoulometric measurements were made to compute the diffusion coefficient of the counter ions. Electrochemical behavior of both PITN and Nafion-PITN in acetonitrile solution containing different counter ions are described. PITN, when electrochemically deposited, apparently neither passivates surface states present nor forms ohmic contacts with p-Si or p(+)Si single-crystal electrodes.

Nagasubramanian, G.↗

NASA NTRS1987

Isothermal dendritic growth - A low gravity experiment

The Isothermal Dendritic Growth Experiment has been designed to test dendritic growth theory at low undercoolings, under microgravity conditions in the Space Shuttle Cargo Bay-borne Material Science Laboratory. The experiment will be essentially autonomous, although limited in-flight interaction through a computer interface is planned. A crystal growth chamber able to yield oriented single-crystal dendritic growth will be incorporated; 'seeding' the chamber with a crystal of the requisite orientation will not in itself meet this requirement.

Glicksman, M. E.↗