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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Traps in Al2O3 detected by tunneling

A structure, quite different from inelastic tunneling peaks, has been observed in electron-tunneling spectra of MOM junctions. A capacitance peak is associated with this structure, which is attributed to traps in the oxide at energies smaller than those previously reported. The C-V characteristics calculated using a single-energy trap model agree with experimental results; however, no satisfactory explanation has yet been found to account for the strong temperature dependence of the trap energy levels.

Skarlatos, Y.↗
NASA NTRS1978

Controlled vapor growth of small particles of Pd and Fe on thin Al2O3 substrates

An exploratory study, dealing with the preparation of well-defined particulate metal deposits that can be used in model studies of catalytic reactions, was performed. Small metal particles of Fe and Pd were grown in situ in an electron microscope by vapor deposition onto different phase of electron transparent alumina substrates. The results show that characteristic properties of the deposits, such as particle density, size distribution, habit, and orientation, are strongly dependent on the cleanliness, phase, and crystallographic orientation of the alumina substrate; also, the deposition conditions can be chosen in such a way as to reproducibly manipulate the overall deposit structure.

Poppa, H.↗
NASA NTRS1979

Effects of isothermal and cyclic exposures on interface structure and mechanical properties of FPalpha-Al2O3/aluminum composites

The microstructural and interface stability of FPalpha-Al203/Al-Li composites are investigated as a function of isothermal exposure at 500 C or thermal cycling between 140 and 500 C with hold time at Tmax. Interfacial morphology, growth kinetics, crystal structure, and composition of interfacial reaction products are characterized. Strength is monitored in the transverse orientation, and fracture mechanics is analyzed in terms of interface reaction products. The interfacial reaction product in FP/Al is Li2O.5Al2O3. Significant fiber-matrix reaction occurs during fabrication. The number of thermal cycles rather than total time at Tmax is the determining factor in strength degradation, thermal cycling giving rise to voids at the fiber-matrix interface. Extensive interface failures occur at composite fracture stresses below about 128 MPa; above this stress level failure is attributed to ductile matrix fracture.

Kim, W. M.↗
NASA NTRS1981

High temperature (Al2O3) insulation and light weight conductors

The of an aluminum conductor with an aluminum oxide insulation film was investigated. Aluminum oxide insulated wire or strip (with a melting point of 2050 C) is unique for applications in the electronic, missile, atomic reactor, aerospace, and aircraft industries. The oxide film is highly flexible, suitable for all windings of any size and shape of coil (magnetic). Briefly touched upon are the ultraviolet, proton gamma radiation uses, as well as high vacuum and cryogenic applications. Since the film is inorganic and chemically inert, it does not age or deteriorate in storage and has good dielectric properties (1000 volts per mil).

Walker, H.↗
NASA NTRS1984

A thermodynamic model for subsolidus equilibria in the system CaO-MgO-Al2O3-SiO2

It is shown that the high temperature enthalpy of solution data for pure phases and solid solutions in the CMAS system are, for 'gabbroic' and 'peridotitic' compositions, consistent with available phase equilibrium data for the MAS, CAS and CMAS systems. A refined set of values of thermodynamic properties for these phases and solid solutions is tabulated. The small differences between the new data set and that of Helgeson et al. (1978) arise from new data on heat capacity and enthalpy being incorporated. The important constraints applied and the major difficulties in fitting which arose are summarized.

Wood, B. J.↗
NASA NTRS1983

Thermal degradation of the tensile properties of undirectionally reinforced FP-Al2O3/EZ 33 magnesium composites

The effects of isothermal and cyclic exposure on the room temperature axial and transverse tensile strength and dynamic flexural modulus of 35 volume percent and 55 volume percent FP-Al203/EZ 33 magnesium composites were studied. The composite specimens were continuously heated in a sand bath maintained at 350 C for up to 150 hours or thermally cycled between 50 and 250 C or 50 and 350 C for up to 3000 cycles. Each thermal cycle lasted for a total of six minutes with a hold time of two minutes at the maximum temperature. Results indicate no significant loss in the room temperature axial tensile strength and dynamic flexural modulus of composites thermally cycled between 50 and 250 C or of composites isothermally heated at 350 C for up to 150 hours from the strength and modulus data for the untreated, as-fabricated composites. In contrast, thermal cycling between 50 and 350 C caused considerable loss in both room temperature strength and modulus. Fractographic analysis and measurement of composite transverse strength and matrix hardness of thermally cycled and isothermally heated composites indicated matrix softening and fiber/matrix debonding due to void growth at the interface and matrix cracking as the likely causes of the strength and modulus loss behavior. Previously announced in STAR as N82-21260

Bhatt, R.T.↗
NASA NTRS1990

Experimentally determined wear behavior of an Al2O3-SiC composite from 25 to 1200 C

The sliding wear behavior of a self-mated alumina-silicon carbide whisker toughened composite was studied using optical, scanning electron (SEM) and transmission electron (TEM) microscopy. Because of its excellent strength and toughness properties this composite material is under consideration for use in heat engine applications for sliding contacts which operate at elevated temperatures. The composite's wear behavior and especially its wear mechanisms are not well understood. Pin-on-disk specimens were slid in air at 2.7 m/s sliding velocity, under a 26.5-N load, at temperatures 25 to 1200 C. Pin wear increased with increasing temperature. Based upon the microscopic analyses, the wear mechanism seems to be loosening of the reinforcing whiskers due to frictional and bulk heating. This leads to whisker pullout and increased wear.

Dellacorte, Christopher↗
NASA NTRS1989

An investigation of a mathematical model of an optically pumped Ti(3+):Al2O3 laser system

During the last several years, solid state lasers were developed that have the potential for meeting rigorous performance requirements for space-based remote sensing of the atmosphere. In order to design a stable and efficient laser and to understand the effect on laser output of changes in the physical and design parameters, an understanding of the development of the dynamical processes of the laser is necessary. Typically, the dynamical processes in a laser system are investigated via rate equations describing the evolution of the occupancy in the electronic levels and of the photon density in the laser cavity. There are two approaches to this type of study. Most often, for the sake of simplicity, the spatial variations of the dynamic variables in the laser system are disregarded and the mathematical model consists of a system of first order nonlinear ordinary differential equations (ODE). The second approach is to take into account both spatial and temporal variations in the dynamic variables in the laser cavity. The resulting model consists of a first order semilinear system of partial differential equations (PDE). The model which was studied was studied was generic in the sense that it was a four-level laser system, but the parameters used in the numerical study were specific to Titanium-doped sapphire. For simplicity, a constant, spatially uniform pumping scheme was considered. In addition, a simplification of the model was made so that it treats a single lasing wavelength with a narrow bandwidth. The purpose was to investigate both versions of the mathematical model and to determine whether the numerical solutions are similar both qualitatively and quantitatively. The systems of ordinary differential equations were solved numerically using a Runge-Kutta-Fehlberg algorithm which was very efficient for typical values of the physical parameters. A numerical scheme, based on the Modified Euler method, for computing solutions to the system of partial differential equations was developed and implemented. The PDE model was solved numerically at the expense of greatly increased computer time.

Roberts, Lila F.↗
NASA NTRS1991

Combustion synthesis of TiB2-Al2O3-Al composite materials

The oxide-aluminum exothermic reduction reaction is presently used in the combustion-synthesis of ceramic/metal composites. An excess of Al is used in the reacting materials, which rapidly generate enough heat to exceed Al's melting point. The molten Al thus evolved is allowed to infiltrate the porous ceramic matrix as the exothermic reaction proceeds; this feature of the process turns the disadvantage of high porosity levels in combustion-synthesized materials into an advantage. Attention is given to the system obtained with 3TiO2 + 3B2O3 + (10-x)Al starting materials.

Feng, H. J.↗
NASA NTRS1992

An analytical approach for the prediction of gamma-to-alpha phase transformation of aluminum oxide (Al2O3) particles in the Space Shuttle ASRM and RSRM exhausts

The analytical approach developed here utilizes the flow-field output from industry standard nozzle and plume codes as input into a particle phase conversion code which predicts the amount of gamma-to-alpha conversion in SRM exhausts. Sixty different cases were considered which varied such parameters as particle size, degree of undercooling, motor type, and altitude. On-centerline calculations were made for both the ASRM and RSRM at an altitude of 100,000 feet with particle sizes varying from 3.5 to 9.1 micron radius and undercooling varying from 0 to 20 percent. Additional calculations were made for the ASRM at 100,000 feet off centerline and at an altitude of 60,000 feet on centerline. The results indicate that significant amounts of metastable alumina will be present in ASRM and RSRM exhausts. Though not significant to motor performance, this may be important in such issues as environmental effects of rocket exhausts, plume radiative heating predictions, and particle size determination by laser scattering.

Oliver, S. M.↗
NASA NTRS1995

CVD silicon carbide monofilament reinforced SrO-Al2O3-2SiO2 (SAS) glass-ceramic composites

Unidirectional CVD SiC fiber-reinforced SrO.Al2O3.2SiO2 (SAS) glass-ceramic matrix composites have been fabricated by hot pressing at various combinations of temperature, pressure and time. Both carbon-rich surface coated SCS-6 and uncoated SCS-0 fibers were used as reinforcements. Almost fully dense composites have been obtained. Monoclinic celsian, SrAl2Si2O8, was the only crystalline phase observed in the matrix from x-ray diffraction. During three point flexure testing of composites, a test span to thickness ratio of approximately 25 or greater was necessary to avoid sample delamination. Strong and tough SCS-6/SAS composites having a first matrix crack stress of approximately 300 MPa and an ultimate bend strength of approximately 825 MPa were fabricated. No chemical reaction between the SCS-6 fibers and the SAS matrix was observed after high temperature processing. The uncoated SCS-0 fiber-reinforced SAS composites showed only limited improvement in strength over SAS monolithic. The SCS-0/SAS composite having a fiber volume fraction of 0.24 and hot pressed at 1400 deg C exhibited a first matrix cracking stress of approximately 231 +/- 20 MPa and ultimate strength of 265 +/- 17 MPa. From fiber push-out tests, the fiber/matrix interfacial debonding strength (tau(sub debond)) and frictional sliding stress (tau(sub friction)) in the SCS-6/SAS system were evaluated to be approximately 6.7 +/- 2.3 MPa and 4.3 +/- 0.6 MPa, respectively, indicating a weak interface. However, for the SCS-0/SAS composite, much higher values of approximately 17.5 +/- 2.7 MPa for tau(sub debond) and 11.3 +/- 1.6 MPa for tau(sub friction) respectively, were observed; some of the fibers were so strongly bonded to the matrix that they could not be pushed out. Examination of fracture surfaces revealed limited short pull-out length of SCS-0 fibers. The applicability of various micromechanical models for predicting the values of first matrix cracking stress and ultimate strength of these composites were examined.

Bansal, Narottam P.↗
NASA NTRS1996

Tribological evaluation of PS300: A new chrome oxide based solid lubricant coating sliding against Al2O3 From 25 to 650 C

This paper presents the tribological characteristics of Al203 sliding against PS300; a chrome oxide based self lubricating coating. Al203 pins were slid against PS300 coated superalloy disks in air, under a 4.9 N load at velocities of 1 to 8 m/s. At a sliding velocity of 1 m/s, friction ranged from 0.6 at 25 C to 0.2 at 650 C. Wear factors for the Al203 pins were in the 10(exp -7) mm(exp 3)/N-m range and for the PS300 coating was in the 10(exp -5) mm(exp 3)/N-m range. The test results suggest that increased surface temperature resulting from either frictional heating, generated by increased sliding velocity, or ambient heating caused a reduction in friction and wear of the sliding couple. Based upon these results, the tested material combination is a promising candidate for high temperature wear applications.

DellaCorte, C.↗
NASA NTRS1998

Development of a Ti:Al2O3 Laser for Remote Sensing of the Atmosphere

During the past year, the following research has been conducted on the Titanium Sapphire Laser system development project: (1) Characterization of the laser system; (2) Gain measurement; (3) Second and third harmonic generation; (4) Laser temporal characteristics; and (5) Laser modeling. Figures and descriptions of the topics above are presented.

Elsayed-Ali, Hani↗