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At least 793 records · Page 44

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NASA NTRS1987

Preparing for the 90s using today's communications assets

Such existing NASA/U.S. facilities and spacecraft as those of the Deep Space Network, VLA, and Arecibo are presently judged capable, at modest additional investment during the next five years, to acquire unique space science data, to generate mission planning data for missions to be launched in the early 1990s, and to evaluate and demonstrate communications and navigation technology for missions of the late 1990s and beyond. The more ambitious of these efforts will contribute the continuation of space research attractiveness for students, as well as furnish an important part of their scientific training.

Posner, Edward C.↗

NASA NTRS1988

Poly(imide-siloxane) segmented copolymer structural adhesives prepared by bulk and solution thermal imidization

The improved properties that have been demonstrated through thermal solution imidization in the case of polyimides and poly(imide-siloxane) segmented copolymers suggests significant potential for application of these new materials. Specifically, the enhancement in solubility, moisture reduction, and processability observed through this solution technique is quite dramatic. Previous work has shown that the presence of low amounts of siloxane does not detract significantly from the lap shear strength of these materials to titanium in the case of bulk thermal imidization synthesis. In addition, the siloxane incorporation results in the added advantage of resistance to hot, wet environments. This added durability is presumably due to the hydrophobic siloxane segments preventing the uptake of water at the critical interphase between the adhesive and the adherend. This paper discusses the extension of this work to the solution imidization synthesis technique recently developed in our laboratory. Results dealing with the absolute bond strengths as well as durability and failure surface analysis will be presented.

Bott, R. H.↗

NASA NTRS1988

Preparation for microgravity - The role of the Microgravity Material Science Laboratory

Experiments at the NASA Lewis Research Center's Microgravity Material Science Laboratory using physical and mathematical models to delineate the effects of gravity on processes of scientific and commercial interest are discussed. Where possible, transparent model systems are used to visually track convection, settling, crystal growth, phase separation, agglomeration, vapor transport, diffusive flow, and polymer reactions. Materials studied include metals, alloys, salts, glasses, ceramics, and polymers. Specific technologies discussed include the General Purpose furnace used in the study of metals and crystal growth, the isothermal dendrite growth apparatus, the electromagnetic levitator/instrumented drop tube, the high temperature directional solidification furnace, the ceramics and polymer laboratories and the center's computing facilities.

Johnston, J. Christopher↗

NASA NTRS1988

Characterization of Ba2YCu3O(7-x) prepared in an inert atmosphere

Reaction data, X-ray data, and critical temperature measurements have been used to show that superconducting Ba2YCu3O(7-x) can be produced in an inert atmosphere using BaO2 as the barium source. Weight loss data are presented for samples produced in O2 and N2. The use of BaO2 is found to result in decreased levels of intergrain BaCO3. The necessity for total inert atmosphere handling and tighter control of the carbon content of starting materials is pointed out. It is shown that materials made with BaO2 and handled in inert atmospheres can be used to improve the J sub c of melt processed materials.

Hepp, A. F.↗

NASA NTRS1988

Comparison of sulfuric and oxalic acid anodizing for preparation of thermal control coatings for spacecraft

The development of thermal control surfaces, which maintain stable solar absorptivity and infrared emissivity over long periods, is challenging due to severe conditions in low-Earth orbit (LEO). Some candidate coatings are second-surface silver-coated Teflon; second-surface, silvered optical solar reflectors made of glass or quartz; and anodized aluminum. Sulfuric acid anodized and oxalic acid anodized aluminum was evaluated under simulated LEO conditions. Oxalic acid anodizing shows promise of greater stability in LEO over long missions, such as the 30 years planned for the Space Station. However, sulfuric acid anodizing shows lower solar absorptivity.

Le, Huong G.↗

NASA NTRS1988

Preparation of non-metals properties for data base

The development of non-metallic material properties data bases is discussed. The data bases consist of the non-metallic material classes of adhesives, adhesive/sealants, plastics, and elastomers. A specifications data base was also developed to incorporate material specifications data as a supplement to the Elastomers Data Base. Examples of the forms used are provided to show the properties of the materials which appear in the data base.

Source record↗

NASA NTRS1988

Preparative electrophoresis for space

A premise of continuous flow electrophoresis is that removal of buoyance-induced thermal convection caused by axial and lateral temperature gradients results in ideal performance of these instruments in space. Although these gravity dependent phenomena disturb the rectilinear flow in the separation chamber when high voltage gradients or thick chamber are used, distortion of the injected sample stream due to electrodynamic effects cause major broadening of the separated bands. The electrophoresis separation process is simple, however flow local to the sample filament produced by the applied electric field were not considered. These electrohydrodynamic flows distort the sample stream and limit the separation. Also, electroosmosis and viscous flow combine to further distort the process. A moving wall concept is being proposed for space which will eliminate and control the disturbances. The moving wall entrains the fluid to move as a rigid body and produces a constant residence time for all samples distributed across the chamber thickness. The moving wall electrophoresis chamber can only be operated in space because there is no viscous flow in the chamber to stabilize against thermal convection.

Rhodes, Percy H.↗

NASA NTRS1988

Preparation and evaluation of silicon nitride matrices for silicon nitride-SiC fiber composites

Continuous silicon carbide (SiC) fiber was added to three types of silicon nitride (Si3N4) matrices. Efforts were aimed at producing a dense Si3N4 matrix from reaction-bonded silicon nitride (RBSN) by hot-isostatic-pressing (HIP) and pressureless sintering, and from Si3N4 powder by hot-pressing. The sintering additives utilized were chosen to allow for densification, while not causing severe degradation of the fiber. The ceramic microstructures were evaluated using scanning optical microscopy. Vickers indentation was used to determine the microhardness and fracture toughness values of the matrices. The RBSN matrices in this study did not reach more than 80 percent of theoretical density after sintering at various temperatures, pressures, and additive levels. Hot-pressing Si3N4 powder produced the highest density matrices; hardness and toughness values were within an order of magnitude of the best literature values. The best sintering aid composition chosen included Y2O3, SiO2, and Al2O3 or AlN. Photomicrographs demonstrate a significant reduction of fiber attack by this additive composition.

Axelson, Scott R.↗

NASA NTRS1988

The preparation of new perfluoro ether fluids exhibiting excellent thermal-oxidative stabilities

A series of low molecular weight perfluoroalkyl ethers (PFAEs) were synthesized by direct fluorination. Viscosity-temperature properties and oxidation stabilities were determined. Viscosity-temperature correlations indicated that increases in branching and increases in the size of the branching substituent caused a deterioration in viscometric properties (i.e., an increase in ASTM slope). In addition, increasing the ratio of carbon to oxygen in these compounds also increased the ASTM slope. Preliminary oxidation stability measurements indicated that highly branched PFAE fluids (i.e., those containing quaternary carbons) may be less stable than either those containing a single trifluoromethyl pendant group or those containing no branching at all.

Jones, William R., Jr.↗

NASA NTRS1988

Dispersion strengthening of precipitation hardened Al-Cu-Mg alloys prepared by rapid solidification and mechanical alloying

Several Al-4Cu-1Mg-1.5Fe-0.75Ce alloys have been processed from either rapidly solidified or mechanically alloyed powder using various vacuum degassing parameters and consolidation techniques. Strengthening by the fine subgrains, grains, and the dispersoids individually or in combination is more effective when the alloys contain shearable precipitates; consequently, the strength of the alloys is higher in the naturally aged rather than the artificially aged condition. The strengths of the mechanically alloyed variants are greater than those produced from prealloyed powder. Properties and microstructural features of these dispersion strengthened alloys are discussed in regards to their processing histories.

Gilman, P. S.↗

NASA NTRS1990

Preparation Of High-Temperature Reactive Oligomers

Very reactive materials form very-heat-stable polymers. Recent research directed toward synthesis of polyimides soluble in common organic solvents, melt-processable, and thermally curable without evolution of volatile by-products. Diels-Alder polymerization yields compounds that maintain integrities and toughnesses during long exposure times at high temperatures. High-temperature polymers synthesized by use of technique. Films and perhaps fibers fabricated from prepolymer in solution. Major potential at this stage of research limited to aerospace applications.

Ottenbrite, Raphael M.↗

NASA NTRS1989

Human serum albumin crystals and method of preparation

Human serum albumin (HSA) crystals are provided in the form of tetragonal plates having the space groups P42(sub 1)2, the crystals being grown to sizes in excess of 0.5 mm in two dimensions and a thickness of 0.1 mm. Growth of the crystals is carried out by a hanging drop method wherein a precipitant solution containing polyethylene glycol (PEG) and a phosphate buffer is mixed with an HSA solution, and a droplet of mixed solution is suspended over a well of precipitant solution. Crystals grow to the desired size in 3 to 7 days. Concentration of reagents, pH and other parameters are controlled within prescribed limits. The resulting crystals exhibit a size and quality such as to allow performance of x ray diffraction studies and enable the conduct of drug binding studies as well as genetic engineering studies.

Carter, Daniel C.↗

NASA NTRS1990

Preparing technicians for engineering materials technology

A long held principle is that for every engineer and scientist there is a need for ten technicians to maximize the efficiency of the technology team for meeting needs of industry and government. Developing an adequate supply of technicians to meet the requirements of the materials related industry will be a challenge and difficult to accomplish. A variety of agencies feel the need and wish to support development of engineering materials technology programs. In a joint effort among Battelle Laboratories, the Department of Energy (DOE) and Northwest College and University Association for Science (NORCUS), the development of an engineering materials technology program for vocational programs and community colleges for the Pacific Northwest Region was recently completed. This effort has implications for a national model. The model Associate of Applied Science degree in Engineering Materials Technology shown provides a general structure. It purposely has course titles which need delimiting while also including a core of courses necessary to develop cognitive, affective and psychomotor skills with the underlining principles of math, science and technology so students have job entry skills, and so that students can learn about and adapt to evolving technology.

Jacobs, James A.↗

NASA NTRS1989

Shuttle remote manipulator system mission preparation and operations

The preflight planning, analysis, procedures development, and operations support for the Space Transportation System payload deployment and retrieval missions utilizing the Shuttle Remote Manipulator System are summarized. Analysis of the normal operational loads and failure induced loads and motion are factored into all procedures. Both the astronaut flight crews and the Mission Control Center flight control teams receive considerable training for standard and mission specific operations. The real time flight control team activities are described.

Smith, Ernest E., Jr.↗

NASA NTRS1989

The use of metalorganics in the preparation of semiconductor materials. VIII - Feasibility studies of the growth of Group III-Group V compounds of boron by MOCVD

The MOCVD growth of B-As and B-P films on Si, sapphire, and Si-on-sapphire substrates is described; in this process, trimethylborane (TMB) or triethylborane (TEB) is pyrolyzed in the presence of AsH3 or PH3 in an H2 atmosphere. The procedures employed are outlined, and the results are presented in graphs, tables, and micrographs. It is found that the growth rate of the primarily amorphous films is dependent on the TMB or TEB concentration but approximately constant for TEB and AsH3 at 550-900 C. The nominal compositions of films grown using TMB are given as B(12-16)As2 and B(1-1.3)P. Carbon impurities and significant stress, bowing, and crazing are observed in the films grown on Si substrates, with the highest carbon content in the films grown from TMB and PH3.

Manasevit, H. M.↗

NASA NTRS1990

Preliminary investigation of a NiAl composite prepared by cryomilling

An attempt has been made to improve the high temperature mechanical strength of the B2 cubic crystal structure intermetallic NiAl by dispersion strengthening. Prealloyed Ni-51 at. pct Al was cryomilled with a Y2O3 addition to form an yttria dispersoid within the intermetallic matrix. Following milling, the powder was hot extruded to full density and machined into test coupons. Compression testing between 1200 and 1400 K indicated that the cryogenic process yielded the strongest NiAl based material tested to date. Creep resistance was six times better than NiAl and twice that of a NiAl particulate composite containing 10 vol pct TiB2. Surprisingly, transmission electron microscopy revealed that the second phase was inhomogeneously distributed. Furthermore, X-ray analysis indicated that the second phase was not Y2O3 but rather AlN.

Whittenberger, J. Daniel↗

NASA NTRS1990

Preparation of a second station for the measurement of solar oscillations of low degree 'l'

An observing station to detect low-degree global solar oscillations is already operational at the Jet Propulsion Laboratory. A second station for continuative measurements of such oscillations has recently been installed and successfully tested in Rome. The high transmission and stability of the magneto-optical filter (MOF) coupled with the lock-in amplifier technique allow analog and real-time detection of oscillation modes with a noise level of only a few cm/s. Observing runs and estimates of the signal-to-noise ratio are shown in time and frequency domains. Routine observations will establish whether the MOF sensitivity and stability are suitable for the detection of stellar oscillations.

Cacciani, A.↗

NASA NTRS1991

PEM-West trajectory climatology and photochemical model sensitivity study prepared using retrospective meteorological data

Trajectory and photochemical model calculations based on retrospective meteorological data for the operations areas of the NASA Pacific Exploratory Mission (PEM)-West mission are summarized. The trajectory climatology discussed here is intended to provide guidance for flight planning and initial data interpretation during the field phase of the expedition by indicating the most probable path air parcels are likely to take to reach various points in the area. The photochemical model calculations which are discussed indicate the sensitivity of the chemical environment to various initial chemical concentrations and to conditions along the trajectory. In the post-expedition analysis these calculations will be used to provide a climatological context for the meteorological conditions which are encountered in the field.

Merrill, John T.↗