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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Lattice model calculation of elastic and thermodynamic properties at high pressure and temperature

The elastic constants and the entire frequency spectrum were calculated up to high pressure for the alkali halides in the NaCl lattice, based on an assumed functional form of the inter-atomic potential. The quasiharmonic approximation is used to calculate the vibrational contribution to the pressure and the elastic constants at arbitrary temperature. By explicitly accounting for the effect of thermal and zero point motion, the adjustable parameters in the potential are determined to a high degree of accuracy from the elastic constants and their pressure derivatives measured at zero pressure. The calculated Gruneisen parameter, the elastic constants and their pressure derivatives are in good agreement with experimental results up to about 600 K. The model predicts that for some alkali halides the Grunesen parameter may decrease monotonically with pressure, while for others it may increase with pressure, after an initial decrease.

Demarest, H. H., Jr.↗
NASA NTRS1989

Electrical properties of materials for high temperature strain gage applications

A study was done on the electrical resistance of materials that are potentially useful as resistance strain gages at high temperatures under static strain conditions. Initially a number of binary alloys were investigated. Later, third elements were added to these alloys, all of which were prepared by arc melting. Several transition metals were selected for experimentation, most prepared as thin films. Difficulties with electrical contacts thwarted efforts to extend measurements to the targeted 1000 C, but results obtained did suggest ways of improving the electrical resistance characteristics of certain materials.

Brittain, John O.↗
NASA NTRS1990

Tribological properties of PM212 - A high temperature, self-lubricating, powder metallurgy composite

This paper describes a research program to develop and evaluate a new high temperature, self-lubricating powder metallurgy composite, PM212. PM212 has the same composition as the plasma-sprayed coating, PS212, which contains 70 wt percent metal-bonded chromium carbide, 15 wt percent silver and 15 wt percent barium fluoride/calcium fluoride eutectic. The carbide acts as a wear resistant matrix and the silver and fluorides act as low and high temperature lubricants, respectively. The material is prepared by sequential cold press, cold isostatic pressing and sintering techniques. In this study, hemispherically tipped wear pins of PM212 were prepared and slid against superalloy disks at temperatures from 25 to 850 C in air in a pin-on-disk tribometer. Friction coefficients range from 0.29 to 0.38 and the wear of both the composite pins and superalloy disks was moderate to low in the 10(exp -5) to 10(exp -6) cubic mm/N-m range. Preliminary tests indicate that the material has a compressive strength of at least 130 MPa over the entire temperature range of 25 to 900 C. This material has promise for use as seal inserts, bushings, small inside diameter parts and other applications where plasma-sprayed coatings are impractical or too costly.

Dellacorte, Christopher↗
NASA NTRS1994

Electrical Properties Of Capacitors At High Temperatures

Brief report describes results of experiments in which capacitance and dielectric loss of glass, metallized-polytetrafluoroethylene, and solid-tantalum capacitor measured at temperatures from 20 degrees C to 200 degrees C. Conclusions drawn concerning suitability of capacitors for use at high temperatures; such as in nuclear powerplants, aircraft, equipment for extracting geothermal energy, switching power supplies, and automotive integrated engine electronics.

Baumann, E. D.↗
NASA NTRS1997

Single-Tow Minicomposite Test Used to Determine the Stressed-Oxidation Durability of SiC/SiC Composites

SiC-fiber-reinforced SiC-matrix composites are considered future materials for high temperature (less than 1200 C), air-breathing applications. For these materials to be successful, they must be able to maintain desirable mechanical properties at high temperatures while existing in highly corrosive environments. The critical constituent of a ceramic matrix composite is a thin interphase layer between the fiber and matrix which enables matrix cracks to deflect around the fibers, that is, to perform even when damaged. Unfortunately, the only interphase materials (to date) that offer the desired properties are carbon and boron nitride. Both of these materials react with oxidizing environments to form gaseous or liquid oxidation products that can lead to fiber-strength degradation or strong bonding between the fiber and the matrix at temperatures above approx. 600 C.

Morscher, Gregory N.↗
NASA NTRS1982

High-temperature resins

The basic chemistry, cure processes, properties, and applications of high temperature resins known as polyimides are surveyed. Condensation aromatic polymides are prepared by reacting aromatic diamines with aromatic dianhydrides, aromatic tetracarboxylic acids, or with dialkyl esters of aromatic tetracarboxylic acids, depending on the intended end use. The first is for coatings or films while the latter two are more suitable for polyimide matrix resins. Prepreg solutions are made by dissolving reactants in an aprotic solvent, and advances in the addition of a diamine on the double bond and radical polymerization of the double bond are noted to have yielded a final cure product with void-free characteristics. Attention is given to properties of the Skybond, Pyralin, and NR-150B polyimide prepreg materials and characteristics of aging in the NP-150 polyimides. Finally, features of the NASA-developed PMR polyimides are reviewed.

Serafini, T. T.↗
NASA NTRS1989

Characterizations of electrical properties of highT(sub c) superconducting materials

The automated data acquisition system developed in the Space Science Laboratory at Marshall Space Flight Center for measuring electrical properties of high temperature superconductors is described. The acquisition system, consisting of a computer and computer-controlled hardware, allows large numbers of voltage, current, temperature, and magnetic measurements to be performed on bulk and thin film samples. Typical results are shown characterizing transition temperature (T sub c), critical current density (J sub c), and magnetic properties of bulk high T(sub c) materials as a function of temperature.

Sisk, R. C.↗
NASA NTRS1988

Prediction of high temperature metal matrix composite ply properties

The application of the finite element method (superelement technique) in conjunction with basic concepts from mechanics of materials theory is demonstrated to predict the thermomechanical behavior of high temperature metal matrix composites (HTMMC). The simulated behavior is used as a basis to establish characteristic properties of a unidirectional composite idealized an as equivalent homogeneous material. The ply properties predicted include: thermal properties (thermal conductivities and thermal expansion coefficients) and mechanical properties (moduli and Poisson's ratio). These properties are compared with those predicted by a simplified, analytical composite micromechanics model. The predictive capabilities of the finite element method and the simplified model are illustrated through the simulation of the thermomechanical behavior of a P100-graphite/copper unidirectional composite at room temperature and near matrix melting temperature. The advantage of the finite element analysis approach is its ability to more precisely represent the composite local geometry and hence capture the subtle effects that are dependent on this. The closed form micromechanics model does a good job at representing the average behavior of the constituents to predict composite behavior.

Caruso, J. J.↗
NASA NTRS1984

Mechanical and spectroscopic properties of metal containing polyimides

The incorporation of specific metal ions into polyimides is described. Detailed studies have included various compounds of copper, lithium, and palladium as dopants. Addition of the metal during polymermzation or after formation of the polyamic acid precedes the thermal imidization step. With many dianhydride-diamine-dopant combinations high quality variously colored films are produced. Many metal doped films exhibit (1) improved high temperature adhesive properties, (2) increased electrical conductivity, (3) excellent thermal stability, (4) improved acid/base resistance, (5) increased modulus in flexible films and (6) excellent high temperature tensile strength. X-ray photo-electron spectroscopic study of these films suggests that many of the additives undergo chemical modification during thermal imidization. Palladium dopants appear to be partially reduced to the metallic state, while lithium and copper dopants are probably converted to their oxides. Ion etching experiments with Auger electron spectroscopy monitoring are discussed.

Taylor, L. T.↗
NASA NTRS1983

Mechanical and spectroscopic properties of metal-containing polyimides

The incorporation of specific metal ions into polyimides is described. Detailed studies have included various compounds of copper, lithium, and palladium as dopants. Addition of the metal during polymerization or after formation of the polyamic acid precedes the thermal imidization step. With many dianhydride-diamine-dopant combinations high quality variously colored films are produced. Many metal doped films exhibit (1) improved high temperature adhesive properties, (2) increased electrical conductivity, (3) excellent thermal stability, (4) improved acid/base resistance, (5) increased modulus in flexible films and (6) excellent high temperature tensile strength. X-ray photo-electron spectroscopic study of these films suggests that many of the additives undergo chemical modification during thermal imidization. Palladium dopants appear to be partially reduced to the metallic state, while lithium and copper dopants are probably converted to their oxides. Ion etching experiments with Auger electron spectroscopy monitoring are discussed.

Taylor, L. T.↗
NASA NTRS1993

Microwave techniques for physical property measurements

Industrial processing of metals and ceramics is now being streamlined by the development of theoretical models. High temperature thermophysical properties of these materials are required to successfully apply these theories. Unfortunately, there is insufficient experimental data available for many of these properties, particularly in the molten state. Microwave fields can be used to measure specific heat, thermal diffusivity, thermal conductivity and dielectric constants at high temperatures. We propose to (1) develop a microwave flash method (analogous to the laser flash technique) that can simultaneously measure the thermal diffusivity and specific heat of insulators and semiconductors at high temperatures, (2) an appropriate theory and experimental apparatus to demonstrate the measurement of the specific heat of a metal using a new microwave ac specific heat technique, and (3) experimental methods for noncontact measurement of the real and imaginary dielectric constants.

Barmatz, M.↗
NASA NTRS1998

Affordable Fabrication and Properties of Silicon Carbide-Based Interpenetrating Phase Composites

An affordable processing technique for the fabrication of silicon carbide-based interpenetrating phase composites (IPCs) is presented. This process consists of the production of microporous carbon preforms and subsequent infiltration with liquid silicon or silicon-refractory metal alloys. The microporous preforms are made by the pyrolysis of a polymerized resin mixture for which methods to control pore volume and pore size have been established. The process gives good control of microstructure and morphology of silicon carbide-based composite materials. Room and high temperature mechanical properties (flexural strength, compressive strength, and flexural creep) of low and high silicon-silicon carbide composites will be discussed.

Singh, Mrityunjay↗
NASA NTRS1985

Theoretical Studies of the Surface Tension of Liquid Metal System

A major goal of this project is to understand the surface tension and other thermophysical properties of liquid metals and alloys from a fundamental viewpoint. The approach is to calculate these quantities by a first principles technique which combines the statistical-mechanical theory of the liquid state with an electronic pseudopotential theory of electrons in metals. The inhomogeneity of the surface is treated using an ionic-density-functional formalism developed with the support of NASA. Of particular interest are the variation of surface tension with temperature and impurity concentration: such variations strongly influence the types of convection which make take place in a low-gravity environment. Some progress has already been achieved in computing the reduction of surface tension due to the presence of low-surface-tension impurities, and the corresponding surface segregation of such impurities. In the coming year, it is planned to concentrate on the surface properties of materials of particular interest to the MSA program: Si, Ga and GaSn alloys. An additional goal is to gain some theoretical understanding of the high temperature thermophysical properties of liquid metals, particularly high melting point materials which have not been studied extensively from a theoretical viewpoint.

Stroud, D. G.↗
NASA NTRS1991

Viscoelastic properties of addition-cured polyimides used in high temperature polymer matrix composites

The viscoelastic properties of an addition-cured polyimide, PMR-15, were evaluated through dynamic mechanical and stress relaxation testing. Below the glass transition temperature, the dynamic mechanical properties of the composites are strongly affected by the absorbed moisture in the resin. At temperature 20 C and more above the glass transition temperature, the storage modulus increases continuously with time, indicating that additional crosslinking is occurring in the resin. For resin moisture contents less than 2 percent, stress relaxation curves measured at different temperatures can be superimposed using horizontal shifts along the log(time) axis with only small shifts along the vertical axis.

Roberts, Gary D.↗
NASA NTRS1991

Probabilistic micromechanics for metal matrix composites

A probabilistic micromechanics-based nonlinear analysis procedure is developed to predict and quantify the variability in the properties of high temperature metal matrix composites. Monte Carlo simulation is used to model the probabilistic distributions of the constituent level properties including fiber, matrix, and interphase properties, volume and void ratios, strengths, fiber misalignment, and nonlinear empirical parameters. The procedure predicts the resultant ply properties and quantifies their statistical scatter. Graphite copper and Silicon Carbide Titanlum Aluminide (SCS-6 TI15) unidirectional plies are considered to demonstrate the predictive capabilities. The procedure is believed to have a high potential for use in material characterization and selection to precede and assist in experimental studies of new high temperature metal matrix composites.

Engelstad, S. P.↗
NASA NTRS1992

Electrical properties of teflon and ceramic capacitors at high temperatures

Space power systems and components are often required to operate efficiently and reliably in harsh environments where stresses, such as high temperature, are encountered. These systems must, therefore, withstand exposure to high temperature while still providing good electrical and other functional properties. Experiments were carried out to evaluate Teflon and ceramic capacitors for potential use in high temperature applications. The capacitors were characterized in terms of their capacitance and dielectric loss as a function of temperature, up to 200 C. At a given temperature, these properties were obtained in a frequency range of 50 Hz to 100 kHz. DC leakage current measurements were also performed in a temperature range from 25 to 200 C. The results obtained are discussed and conclusions are made concerning the suitability of the capacitors studied for high temperature applications.

Hammoud, A. N.↗
NASA NTRS2017

Quantitative Characterization of Structural and Mechanical Properties of Boron Nitride Nanotubes in High Temperature Environments

The structural stability and mechanical integrity of boron nitride nanotubes (BNNTs) in high temperature environments are of importance in pursuit of their applications that are involved with extreme thermal processing and/or working conditions, but remain not well understood. In this paper, we perform an extensive study of the impacts of high temperature exposure on the structural and mechanical properties of BNNTs with a full structural size spectrum from nano- to micro- to macro-scale by using a variety of in situ and ex situ material characterization techniques. Atomic force microscopy (AFM) and high resolution transmission electron microscopy measurements reveal that the structures of individual BNNTs can survive at up to 850 °C in air and capture the signs of their structural degradation at 900 °C or above. In situ Raman spectroscopy measurements reveal that the BN bonds in BNNT micro-fibrils undergo substantial softening at elevated temperatures of up to 900 °C. The AFM-based nanomechanical compression measurements demonstrate that the mechanical integrity of individual BNNTs remain intact after being thermally baked at up to 850 °C in air. The studies reveal that BNNTs are structurally and mechanically stable materials in high temperature environments, which enables their usages in high temperature applications.

Chen, Xiaoming↗