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At least 253 records · Page 14

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NASA NTRS2001

Purification Procedures for Single-Wall Carbon Nanotubes

This report summarizes the comparison of a variety of procedures used to purify carbon nanotubes. Carbon nanotube material is produced by the arc process and laser oven process. Most of the procedures are tested using laser-grown, single-wall nanotube (SWNT) material. The material is characterized at each step of the purification procedures by using different techniques including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), Raman, X-ray diffractometry (XRD), thermogravimetric analysis (TGA), nuclear magnetic resonance (NMR), and high-performance liquid chromatography (HPLC). The identified impurities are amorphous and graphitic carbon, catalyst particle aggregates, fullerenes, and hydrocarbons. Solvent extraction and low-temperature annealing are used to reduce the amount of volatile hydrocarbons and dissolve fullerenes. Metal catalysts and amorphous as well as graphitic carbon are oxidized by reflux in acids including HCl, HNO3 and HF and other oxidizers such as H2O2. High-temperature annealing in vacuum and in inert atmosphere helps to improve the quality of SWNTs by increasing crystallinity and reducing intercalation.

Gorelik, Olga P.↗

NASA NTRS1984

Structural Characterization of Solar Silicon

Compared to single crystal silicon, solar silicon generally contains large residual stresses and numerous structural defects. During subsequent processing, the defect structure can undergo further changes since, at the high temperatures required for diffusion, dislocations are sufficiently mobile to rearrange themselves in patterns which reduce long range residual stresses. This process, which is similar to the polygonization of strained metals, has no counterpart in single crystal silicon. Many of the solar silicon materials are grown from graphite dies and crucibles and therefore contain carbon concentrations in excess of 1E18. The interactions between carbon, crystal defects, intrinsic point defects, oxygen and diffusing dopants are discussed.

Ast, D. G.↗

NASA NTRS2003

Quantitating Iron in Serum Ferritin by Use of ICP-MS

A laboratory method has been devised to enable measurement of the concentration of iron bound in ferritin from small samples of blood (serum). Derived partly from a prior method that depends on large samples of blood, this method involves the use of an inductively-coupled-plasma mass spectrometer (ICP-MS). Ferritin is a complex of iron with the protein apoferritin. Heretofore, measurements of the concentration of serum ferritin (as distinguished from direct measurements of the concentration of iron in serum ferritin) have been used to assess iron stores in humans. Low levels of serum ferritin could indicate the first stage of iron depletion. High levels of serum ferritin could indicate high levels of iron (for example, in connection with hereditary hemochromatosis an iron-overload illness that is characterized by progressive organ damage and can be fatal). However, the picture is complicated: A high level of serum ferritin could also indicate stress and/or inflammation instead of (or in addition to) iron overload, and low serum iron concentration could indicate inflammation rather than iron deficiency. Only when concentrations of both serum iron and serum ferritin increase and decrease together can the patient s iron status be assessed accurately. Hence, in enabling accurate measurement of the iron content of serum ferritin, the present method can improve the diagnosis of the patient s iron status. The prior method of measuring the concentration of iron involves the use of an atomic-absorption spectrophotometer with a graphite furnace. The present method incorporates a modified version of the sample- preparation process of the prior method. First, ferritin is isolated; more specifically, it is immobilized by immunoprecipitation with rabbit antihuman polyclonal antibody bound to agarose beads. The ferritin is then separated from other iron-containing proteins and free iron by a series of centrifugation and wash steps. Next, the ferritin is digested with nitric acid to extract its iron content. Finally, a micronebulizer is used to inject the sample of the product of the digestion into the ICPMS for analysis of its iron content. The sensitivity of the ICP-MS is high enough to enable it to characterize samples smaller than those required in the prior method (samples can be 0.15 to 0.60 mL).

Smith, Scott M.↗

NASA NTRS2001

Equivalent-Continuum Modeling of Nano-Structured Materials

A method has been developed for modeling structure-property relationships of nano-structured materials. This method serves as a link between computational chemistry and solid mechanics by substituting discrete molecular structures with an equivalent-continuum model. It has been shown that this substitution may be accomplished by equating the vibrational potential energy of a nano-structured material with the strain energy of representative truss and continuum models. As an important example with direct application to the development and characterization of single-walled carbon nanotubes, the model has been applied to determine the effective continuum geometry of a graphene sheet. A representative volume element of the equivalent-continuum model has been developed with an effective thickness. This effective thickness has been shown to be similar to, but slightly smaller than, the interatomic spacing of graphite.

Odegard, Gregory M.↗

NASA NTRS1992

Material characterization of composite laminates using low-frequency plate wave dispersion data

A recently developed ultrasonic techique which has been successful in monitoring the integrity of advanced structures is described. The technique is based on a two-transducer contact type arrangement that can be used to determine the dispersion curves of guided waves generated within the specimen. The phase velocity of antisymmetric plate waves is determined through spectral analysis of signals recorded from a lead break source on the surface of graphite epoxy laminates. The influence of all five stiffness constants on the dispersion curves of both symmetric and antisymmetric plate waves is theoretically investigated for propagation at 0, 45, and 90 deg to the fibers. All but c12 were found to have a strong influence on the dispersion curves in this frequency range. The fit with the theoretically predicted dispersion curves was excellent for propagation parallel and perpendicular to the fibers in the unidirectional specimen and good for the other cases considered.

Mal, Ajit K.↗

NASA NTRS1977

Fracture surface characteristics of off-axis composites

The fracture surface characteristics of off-axis high-modulus graphite-fiber/epoxy composite specimens were studied using a scanning electron microscope (SEM). The specimens were subjected to tensile loading at various angles (0-90 deg) to the fiber direction. SEM photomicrographs of the fractured surfaces revealed three different load angle regions with distinct fracture characteristics. Based on these revelations, criteria were established which can be used to characterize fracture surfaces with respect to a predominant 'single-stress' fracture mode.

Sinclair, J. H.↗

NASA NTRS2008

John F. Kennedy Space Center's Technology Development and Application 2006-2007 Report

Topics covered include: Reversible Chemochromic Hydrogen Detectors; Determining Trajectory of Triboelectrically Charged Particles, Using Discrete Element Modeling; Using Indium Tin Oxide To Mitigate Dust on Viewing Ports; High-Performance Polyimide Powder Coatings; Controlled-Release Microcapsules for Smart Coatings for Corrosion Applications; Aerocoat 7 Replacement Coatings; Photocatalytic Coatings for Exploration and Spaceport Design; New Materials for the Repair of Polyimide Electrical Wire Insulation; Commodity-Free Calibration; Novel Ice Mitigation Methods; Crack Offset Measurement With the Projected Laser Target Device; New Materials for Structural Composites and Protective Coatings; Fire Chemistry Testing of Spray-On Foam Insulation (SOFI); Using Aerogel-Based Insulation Material To Prevent Foam Loss on the Liquid-Hydrogen Intertank; Particle Ejection and Levitation Technology (PELT); Electrostatic Characterization of Lunar Dust; Numerical Analysis of Rocket Exhaust Cratering; RESOLVE Projects: Lunar Water Resource Demonstration and Regolith Volatile Characterization; Tribocharging Lunar Soil for Electrostatic Beneficiation; Numerically Modeling the Erosion of Lunar Soil by Rocket Exhaust Plumes; Trajectory Model of Lunar Dust Particles; Using Lunar Module Shadows To Scale the Effects of Rocket Exhaust Plumes; Predicting the Acoustic Environment Induced by the Launch of the Ares I Vehicle; Measuring Ultrasonic Acoustic Velocity in a Thin Sheet of Graphite Epoxy Composite; Hail Size Distribution Mapping; Launch Pad 39 Hail Monitor Array System; Autonomous Flight Safety System - Phase III; The Photogrammetry Cube; Bird Vision System; Automating Range Surveillance Through Radio Interferometry and Field Strength Mapping Techniques; Next-Generation Telemetry Workstation; GPS Metric Tracking Unit; and Space-Based Range.

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NASA NTRS1977

Fracture surface characteristics of off-axis composites

The fracture surface characteristics of off-axis high-modulus graphite-fiber/epoxy composite specimens were studied, using a scanning electron microscope (SEM). The specimens were subjected to tensile loading at various angles (0 deg - 90 deg) to the fiber direction. SEM photomicrographs of the fractured surfaces revealed three different load angle regions with distinct fracture characteristics. Based on these revelations, criteria were established which can be used to characterize fracture surfaces with respect to a predominant single stress fracture mode.

Sinclair, J. H.↗

NASA NTRS1984

Embedded gage impact study

Impact damage in graphite/epoxy laminates was characterized and transient strain history during impact was correlated. The material investigated was AS-4/3501-6 graphite/epoxy. Eight-ply and sixteen-ply quasi-isotropic laminates of 45/0/-45/90 sub s and 45/0/-45/90 sub 2s layups were fabricated with strain gages embedded between plies during the strain gages and leads from the highly conductive graphite fibers. The specimens were circular plates 12.7 cm (5 in.) in diameter and clamped along their circumference. The specimens were impacted with a 185 gm impactor, dropped from heights of 1.20 m and 1.65 m. An accelerometer was attached to the back surface of the specimen opposite the impact point and was used to trigger the recording instrumentation. The transient strain data were recorded with an eight channel waveform digitizer capable of sampling data at 0.5 microsec intervals. The data were stored, processed, and plotted by means of a microcomputer. Transient strain data were correlated with results from ultrasonic inspection of the specimens.

Schramm, S. W.↗

NASA NTRS2021

Random vibration, exported vibration, and passive isolation testing of the Ricor K508N cryocooler

The Ricor K508N is a rotary Stirling Cryocooler whose predecessor, the K508, has extensive flight heritage. The K508N has significant potential as a low-cost option for CubeSat infrared electro-optical payload applications. This paper will explore the work done by the ATA project on the exported vibration characterization of a set of standard K508N cryocoolers (35 atm) and the implementation and performance comparison of several passive vibration isolation and damping technologies to help mitigate the jitter effect of COTS cryocoolers. Kistler Force dynamometers and accelerometers were used to measure the K508N’s body and cold tip Multicomponent Exported Forces and Torques (EFT). Wire rope and Pyrolytic Graphite Sheet thermal straps are used as vibration isolation technologies. The K508N also underwent sine sweep resonant and peak profile testing as well as random vibration testing to GEVS launch loads in a NanoRacks soft stow configuration.

Mok, M↗

NASA NTRS1985

Measurements of the response of transport aircraft ceiling panels to fuel pool fires

Tests were performed to characterize the responses of various aircraft ceiling panel configurations to a simulated post-crash fire. Attention was given to one currently used and four new ceiling configurations exposed to a fuel pool fire in a circulated air enclosure. The tests were controlled to accurately represent conditions in a real fire. The panels were constructed of fiberglass-epoxy, graphite-phenolic resin, fiberglass-phenolic resin, Kevlar-epoxy, and Kevlar-phenolic resin materials. The phenolic resin-backed sheets performed the best under the circumstances, except when combined with Kevlar, which became porous when charred.

Bankston, C. P.↗

NASA NTRS2022

Building and Breaking Carbon Composites with REACTER

Carbon-based composites have become indispensable materials in aerospace and other high-performance applications, yet obtaining a detailed, nanoscale understanding of their morphology and failure mechanisms using only experimental methods remains a difficult challenge. REACTER is a versatile computational modeling tool for atomistic molecular dynamics simulations designed to model chemical reactions at the speed and length scales of classical force fields. In this work, several recent features of REACTER were applied to the creation and subsequent mechanical testing of two classes of carbon composites: carbon nanotube (CNT) composites and carbon fiber reinforced polymers (CFRP). A network of CNTs was grown dynamically using the new ‘create atoms’ feature of REACTER. The CNT filler was embedded into a polyarylacetylene (PAA) matrix by simulated in situ polymerization to obtain the final composite model. To generate the second class of carbon composite, fully carbonized (graphitic) carbon fiber morphologies were created by the method of Desai et al. [1], but using the advanced reaction constraints framework of REACTER. Two fiber models were created, representing a circular carbon fiber core and a flat surface, and similarly infiltrated with resin to obtain the final CFRP structure. Failure mechanisms were elucidated by simulating mechanically induced bond breaking, as characterized by third order DFT-based tight-binding simulations, via a reaction constraint on the total potential energy of the involved atoms.

Polymer↗

NASA NTRS1984

Composite Fracture Toughness and Impact Characterization

The basic threat to the more widespread use of composites in aircraft primary structure is the problem of residual strength in the presence of damage. Specific problem areas that need to be addressed include the following: (1) modeling/understanding composite failure (impact: relate strain energy release rate to strain-to-failure, open hole compression: relate shear crippling to strain-to-failure, and account for variables such as thickness and stacking sequence); (2) micromechanics models; (3) consider the use of hybrid combinations, both interply and intraply, using graphite, Kevlar, and glass; and (4) modeling/understanding the role of the interface. Solving these problems involves a multidisciplinary approach including dynamics, structural stability, composite mechanics, and fracture mechanics.

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NASA NTRS1974

Advances in the analysis and design of adhesive-bonded joints in composite aerospace structures

Several aspects of adhesive-bonded joint analysis and design are presented from the reference of size of structure or load intensity. This integrates the individual characterizations of double-lap, single-lap, stepped-lap, tapered-lap and scarf joints. The paper includes an overview of bonded joint selection from the standpoints of design, fabrication, and processing, each bearing in mind the influence of such considerations on the strength of the joint. A case study is presented of the optimization of a specific relatively thick titanium-to-graphite epoxy stepped-lap joint, using the digital computer analysis program A4EG. The factors accounted for are adhesive plasticity, adherend stiffness imbalance, adherend thermal mismatch, and change of material properties within the range of temperature environment and with load direction. The strength increases obtainable by refining the initial design are demonstrated.

Hart-Smith, L. J.↗

NASA NTRS1977

Mechanical behavior and fracture characteristics of off-axis fiber composites. 1: Experimental investigation

The mechanical behavior, fracture surfaces, and fracture modes of unidirectional high-modulus graphite-fiber/epoxy composites subjected to off-axis tensile loads were investigated experimentally. The investigation included the generation of stress-strain-to-fracture data and scanning electron microscope studies of the fractured surfaces. The results led to the identification of fracture modes and distinct fracture surface characteristics for off-axis tensile loading. The results also led to the formulation of critical for identifying and characterizing these fracture modes and their associated fracture surfaces. The results presented and discussed herein were used in the theoretical investigation and comparisons described in Part 2. These results should also provide a good foundation for identifying, characterizing, and quantifying fracture modes in both off-axis and angle-plied laminates.

Sinclair, J. H.↗

NASA NTRS1986

Quantitative thermal characterization of thin plates

The principles of quantitative thermal testing of the mechanical integrity of platelike structures are discussed. In this technique, a scanned laser pulse is used to inject a thermal pattern in a sample, and a thermal IR imager is used to document the evolution of the pattern subsequent to injection. The measurements are remote, noncontacting, and one-sided, and the algorithms used to obtain diffusivity are all emissivity-independent. The results yielded by application of the three analytical techniques, used to obtain diffusivity values from the image data on stainless steel, a graphite-epoxy composite, brass, and aluminum 2024 samples, are presented.

Welch, C. S.↗

NASA NTRS2002

Carbon Nanotubes Growth by CVD on Graphite Fibers

Due to the superior electrical and mechanical properties of carbon nanotubes (CNT), synthesizing CNT on various substances for electronics devices and reinforced composites have been engaged in many efforts for applications. This presentation will illustrate CNT synthesized on graphite fibers by thermal CVD. On the fiber surface, iron nanoparticles as catalysts for CNT growth are coated. The growth temperature ranges from 600 to 1000 C and the pressure ranges from 100 Torr to one atmosphere. Methane and hydrogen gases with methane content of 10% to 100% are used for the CNT synthesis. At high growth temperatures (greater than or equal to 900 C), the rapid inter-diffusion of the transition metal iron on the graphite surface results in the rough fiber surface without any CNT grown on it. When the growth temperature is relative low (650-800 C), CNT with catalytic particles on the nanotube top ends are fabricated on the graphite surface. (Methane and hydrogen gases with methane content of 10% to 100% are used for the CNT synthesis.) (By measuring the samples) Using micro Raman spectroscopy in the breath mode region, single-walled or multi-walled CNT (MWCNT), depending on growth concentrations, are found. Morphology, length and diameter of these MWCNT are determined by scanning electron microscopy and Raman spectroscopy. The detailed results of syntheses and characterizations will be discussed in the presentation.

Zhu, Shen↗

NASA NTRS1981

An endochronic theory for transversely isotropic fibrous composites

A rational methodology of modelling both nonlinear and elastic dissipative response of transversely isotropic fibrous composites is developed and illustrated with the aid of the observed response of graphite-polyimide off-axis coupons. The methodology is based on the internal variable formalism employed within the text of classical irreversible thermodynamics and entails extension of Valanis' endochronic theory to transversely isotropic media. Applicability of the theory to prediction of various response characteristics of fibrous composites is illustrated by accurately modelling such often observed phenomena as: stiffening reversible behavior along fiber direction; dissipative response in shear and transverse tension characterized by power-laws with different hardening exponents; permanent strain accumulation; nonlinear unloading and reloading; and stress-interaction effects.

Pindera, M. J.↗