- Nanomaterial scientist with aerospace and laser experienceedit
ABSTRACT Measurements of the vibrational temperature in a shock layer produced by a cylindrical blunt body in an arc jet nitrogen flow are made for both the neutral molecule and the molecular ion. Spectra were obtained at two locations in... more
ABSTRACT Measurements of the vibrational temperature in a shock layer produced by a cylindrical blunt body in an arc jet nitrogen flow are made for both the neutral molecule and the molecular ion. Spectra were obtained at two locations in the shock layer produced by a 20-cm long cylindrical blunt body inserted in the arc jet conical flow. The technique used for determining the temperatures from the spectral radiation involves obtaining ratios of integrated intensities for certain spectral regions of the measured spectrum and making a comparison with computed ratios as a function of temperature for the same spectral regions. The technique is difficult to apply due to radiation overlapping bands from other species. However, using a trial and error procedure, the vibrational and rotational temperatures at the two locations are inferred. The results indicate that the shock layer is significantly out of thermal equilibrium.
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This work focuses on the size and spatial dependence of single-wall carbon nanotubes produced by the pulsed-laser vaporization technique. The study indicates that very long (tens of microns) individual nanotubes form in the vicinity of... more
This work focuses on the size and spatial dependence of single-wall carbon nanotubes produced by the pulsed-laser vaporization technique. The study indicates that very long (tens of microns) individual nanotubes form in the vicinity of the target, and subsequently coalesce into bundles. The role of the inner flow tube is confirmed to restrict plume expansion and improve interactions between carbon atoms resulting in nanotube and rope formation. The effect of the flowing buffer gas seems to influence the dispersion of particulate contaminant material in the nanotube product. More particulate matter is produced at lower oven temperatures.
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Carbon nano structures including fullerenes, nanotubes, nanocones and graphene are gaining a considerable interest for supercapaitors in memory backup devices, electric vehicles, and emergency power supplies. In addition, these structures... more
Carbon nano structures including fullerenes, nanotubes, nanocones and graphene are gaining a considerable interest for supercapaitors in memory backup devices, electric vehicles, and emergency power supplies. In addition, these structures are utilized to improve resolution and power consumption of field emission displays. The energy density and performance of supercapacitors depend on the choice of electrode and electrolyte materials. Our recent
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ABSTRACT We will present possible mechanisms for nanotube production by laser oven process. Spectral emission of excited species during laser ablation of a composite graphite target is compared with that of laser irradiated C60 vapor. The... more
ABSTRACT We will present possible mechanisms for nanotube production by laser oven process. Spectral emission of excited species during laser ablation of a composite graphite target is compared with that of laser irradiated C60 vapor. The similarities in the transient and spectral data suggest that fullerenes are intermediate precursors for nanotube formation. The confinement of the ablation products by means of a 25-mm diameter tube placed upstream of the target seems to improve the production and purity of nanotubes. Repeated laser pulses vaporize the amorphous/graphitic carbon and possibly catalyst particles, and dissociate fullerenes yielding additional feedstock for SWNT growth.
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Radiation is analyzed from a highly nonequilibrium shock layer produced by a blunt body in a low-density arcjet now of nitrogen. Populations of vibrational states are obtained at several locations in the shock layer through fits of basis... more
Radiation is analyzed from a highly nonequilibrium shock layer produced by a blunt body in a low-density arcjet now of nitrogen. Populations of vibrational states are obtained at several locations in the shock layer through fits of basis spectra to measured spectra. Several techniques determine the rotational and vibrational temperature corresponding to Boltzmann fits to low vibrational state populations for N 2 and N + 2 molecules. These techniques include correlations of ratios of intensity integrals, fits of calculated vibrational basis sets to measurements, and minimization of the deviation between calculated and measured spectra. Analyses of basis set fits yield vibrational populations that deviate from a Boltzmann distribution at vibrational quantum numbers greater than about 6-8. Differences in rotational and vibrational temperatures are found, together with temperature differences between the neutral molecule and the ion. Temperature profiles and non-Boltzmann vibrational populations indicate the nonequilibrium character of the layer. Experimentally determined population distributions should interest anyone modeling nonequilibrium vibrational kinetics. These distributions, up to v' = 20, should be of interest, and temperature profiles generally should be useful when validating the gas models used for high-enthalpy nonequilibrium nitrogen flows.
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Phenolic impregnated carbon ablator (PICA) is a thermal protection system (TPS) material developed at NASA Ames Research Center in the mid-90 s for Discovery missions. It was used on the Stardust return capsule heat shield which... more
Phenolic impregnated carbon ablator (PICA) is a thermal protection system (TPS) material developed at NASA Ames Research Center in the mid-90 s for Discovery missions. It was used on the Stardust return capsule heat shield which successfully executed the highest speed Earth entry to date on January 15, 2006. PICA is a porous fibrous carbon insulation infiltrated with phenolic resin, and is an excellent ablator that is effective for heating rates up to 1000 W/sq cm. It is one of several candidate TPS materials for the next generation of crewed spacecraft for Lunar and Mars missions. We will describe an ongoing research effort at NASA to improve mechanical properties of the phenolic matrix with carbon nanotubes. The aim is two-fold: to increase overall TPS strength during reentry and to improve Micrometeoroid/Orbital Debris (MMOD) protection in space. The former requires at least a good dispersion of nanotubes in phenolic, while the latter also requires covalent bonding between them t...
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Irradiation by 30 keV Ga+ ions was used to create defects in multiwalled carbon nanotubes. Damage to the graphitic structure of the nanotube wall resulting from ion irradiation was observed by a transmission electron microscope which was... more
Irradiation by 30 keV Ga+ ions was used to create defects in multiwalled carbon nanotubes. Damage to the graphitic structure of the nanotube wall resulting from ion irradiation was observed by a transmission electron microscope which was accompanied by corresponding changes in Raman spectra. It was found that ion irradiation at 2 × 1013 ions/cm2 cumulative dose increases the electric double layer capacitance of a multiwalled carbon nanotube electrode by a factor of 2.3, followed by a decrease and saturation at higher (2 × 1014 and 4 × 1014 ions/cm2) doses. This might be a trade-off between the enhancement caused by the tip opening and lowering of the capacitance due to amorphization of carbon nanotubes.
Research Interests: Engineering, Physics, Materials Science, Applied Physics, Raman Spectroscopy, and 14 moreCarbon Nanotube, Transmission Electron Microscopy, Mathematical Sciences, Applied, Physical sciences, Ion, Capacitance, Irradiation, Ion Beam, Transmission Electron Microscope, Raman Spectra, Electric Double Layer, Cumulant, and Ion Irradiation
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The Nobel Prize winning discovery of the Buckuball (C60) in 1985 at Rice University by a group including Dr. Richard Smalley led to the whole new class of carbon allotropes including fullerenes and nanotubes. Especially interesting from... more
The Nobel Prize winning discovery of the Buckuball (C60) in 1985 at Rice University by a group including Dr. Richard Smalley led to the whole new class of carbon allotropes including fullerenes and nanotubes. Especially interesting from many viewpoints are single-walled carbon ...
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Spectra of molecular nitrogen and molecular nitrogen ions are used to infer vibrational and rotational temperatures. Various techniques are used for correlating spectral features with the temperatures. The techniques include determining... more
Spectra of molecular nitrogen and molecular nitrogen ions are used to infer vibrational and rotational temperatures. Various techniques are used for correlating spectral features with the temperatures. The techniques include determining intensity integrals and forming ratios of them which are correlated with rotational and vibrational temperatures. Sensitivity factors are determined from correlation of ratios with temperature. These factors are indicators of the potential accuracy of different regions of the spectrum for determining temperature. In another technique presented, one makes least squares fits of measured spectra to temperature dependent computed spectra. These include either global fits to computed spectra as function of temperature, or fits to vibrational level basis spectra. The accuracy of these techniques are described; and it is shown that the precision is improved by combining the results of several techniques. The overall accuracy for a shock layer point temperature determination is about ±4% for vibrational temperature and about ±10% for rotational temperature when results from various techniques are combined.
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ABSTRACT A laser-oven set up is modified to collect individual carbon SWNTs and small ropes. Material is collected at different locations in the laser oven for a variety of flow and temperature conditions. The lengths and diameter... more
ABSTRACT A laser-oven set up is modified to collect individual carbon SWNTs and small ropes. Material is collected at different locations in the laser oven for a variety of flow and temperature conditions. The lengths and diameter distributions of the nanotubes are measured directly (without additional processing steps) using AFM, Raman and NIR spectroscopic techniques. The study confirmed the presence of long (several microns) individual single wall nano tubes (SWNTs) closer to the target and small SWNT ropes upstream (inside the inner tube) of the flow tube. The role of the ``inner-tube'' of the flow-tube set up is confirmed to restrict plume expansion and improve interactions between carbon atoms resulting in formation of ropes [1]. Flowing buffer gas that improves the interactions of the carbon plume products seems to influence the dispersion of particulate material in the nanotube product. Implications of the measured length distribution and possible nanotube growth dynamics will be discussed based on these initial studies. [1] S. Arepalli et al., Appl. Phys. A. 70, 125 (2000). Work carried out at NASA - Johnson Space Center under NASA contract NAS9-19100
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An optical fiber is used to collect radiation from the plenum of an arcjet wind tunnel. Studying the spectra, the electronic excitation temperatures are determined from the Boltzmann plots of atomic oxygen and nitrogen emission lines. In... more
An optical fiber is used to collect radiation from the plenum of an arcjet wind tunnel. Studying the spectra, the electronic excitation temperatures are determined from the Boltzmann plots of atomic oxygen and nitrogen emission lines. In the case of atomic oxygen, the temperature is found to be about 15,000 ± 3400 K, and that of nitrogen is found to be 7600 ± 1500 K. Determination of molecular vibrational-rotational temperature is made by comparing experimental and theoretical spectra of the N 2 + molecular radiation. The temperature is estimated to be 9700 ± 1200 K, using an integrals ratio method.
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Spectral measurements in the plenum of an arcjet wind tunnel. Mohammad A Rob, Larry HMack AIAA, Thermophysics Conference, 30 th, San Diego, CA, 1995. We have used an optical fiber to collect radiation from the plenum of an arcjet wind... more
Spectral measurements in the plenum of an arcjet wind tunnel. Mohammad A Rob, Larry HMack AIAA, Thermophysics Conference, 30 th, San Diego, CA, 1995. We have used an optical fiber to collect radiation from the plenum of an arcjet wind tunnel. ...
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Spectral radiation in the range of 200 to 900 nm is collected across a section of the plenum of an arc jet wind tunnel using four optical fibers. The spectra contained line radiation from oxygen and nitrogen atoms and molecular radiation... more
Spectral radiation in the range of 200 to 900 nm is collected across a section of the plenum of an arc jet wind tunnel using four optical fibers. The spectra contained line radiation from oxygen and nitrogen atoms and molecular radiation from N2, N2(+), and NO. The Abel inversion ...
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Using the High Pressure carbon monoxide (HiPco) reactor we conducted an experiment on the effects of changing the catalyst concentration. With each catalyst concentration tested the resulting raw HiPco material was characterized for... more
Using the High Pressure carbon monoxide (HiPco) reactor we conducted an experiment on the effects of changing the catalyst concentration. With each catalyst concentration tested the resulting raw HiPco material was characterized for average SWNT lengths, SWNT diameters, residual iron particle size, and large fullerene content. We were able to determine trends in each of these characteristics as the catalyst concentration was changed. As the catalyst concentration was decreased SWNT lengths increased, SWNT diameters increased, the residual iron particle size increased, and the large fullerene content decreased. From these trends we have developed a Competitive Growth model for nucleation and growth of SWNTs via the HiPco process.
Research Interests: Engineering, Chemical Engineering, Materials Science, Technology, Catalysis, and 15 moreCarbon Nanotube, Transmission Electron Microscopy, Nanotechnology, Medicine, Fullerenes, Carbon Monoxide, Nanoscience, Nucleation, Nanostructures, CHEMICAL SCIENCES, Pressure, Particle Size, Normal Distribution, Nanotubes Carbon, and Nanoscience and nanotechnology
Promise of applications of carbon nanotubes has led to an intense effort at NASA/JSC, especially in the area of nanotube composites. Using the extraordinary mechanical strength of nanotubes, NASA hopes to design this revolutionary... more
Promise of applications of carbon nanotubes has led to an intense effort at NASA/JSC, especially in the area of nanotube composites. Using the extraordinary mechanical strength of nanotubes, NASA hopes to design this revolutionary lightweight material for use in ...
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One of the most attractive applications of single-wall carbon nanotubes (SWNT) is found in the area of structural materials. Nanotubes have a unique combination of high strength, modulus, and elongation to failure, and therefore have... more
One of the most attractive applications of single-wall carbon nanotubes (SWNT) is found in the area of structural materials. Nanotubes have a unique combination of high strength, modulus, and elongation to failure, and therefore have potential to significantly enhance ...
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Metallic single-wall carbon nanotubes, synthesized through a pulsed-laser vaporization (PLV) process, were selectively reacted with dodecyl-oxybenzene-diazonium tetrafluoroborate to yield tetrahydrofuran (THF) suspensions of nanotubes... more
Metallic single-wall carbon nanotubes, synthesized through a pulsed-laser vaporization (PLV) process, were selectively reacted with dodecyl-oxybenzene-diazonium tetrafluoroborate to yield tetrahydrofuran (THF) suspensions of nanotubes enriched in ...
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Raman spectroscopy of a carbon nanotube–reinforced phenolic resin is used to study the interaction of nanotubes with a host matrix. The observed sublinear dependence of the Raman G-band shift on the matrix strain, accompanied by... more
Raman spectroscopy of a carbon nanotube–reinforced phenolic resin is used to study the interaction of nanotubes with a host matrix. The observed sublinear dependence of the Raman G-band shift on the matrix strain, accompanied by inhomogeneous broadening of ...
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Recent interest in developing new applications for carbon nanotubes (CNT) has fueled the need to use accurate macroscopic and nanoscopic techniques to characterize and understand their chemistry. X-ray photoelectron spectroscopy (XPS) has... more
Recent interest in developing new applications for carbon nanotubes (CNT) has fueled the need to use accurate macroscopic and nanoscopic techniques to characterize and understand their chemistry. X-ray photoelectron spectroscopy (XPS) has proved to be a useful analytical tool for nanoscale surface characterization of materials including carbon nanotubes. Recent nanotechnology research at NASA Johnson Space Center (NASA-JSC) helped to establish a characterization protocol for quality assessment for single wall carbon nanotubes (SWCNTs). Here, a review of some of the major factors of the XPS technique that can influence the quality of analytical data, suggestions for methods to maximize the quality of data obtained by XPS, and the development of a protocol for XPS characterization as a complementary technique for analyzing the purity and surface characteristics of SWCNTs is presented. The XPS protocol is then applied to a number of experiments including impurity analysis and the study...
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The advances in large scale applications of carbon nanotubes demand a reliable supply of raw and processed materials. It is imperative to have a consistent quality control of these nanomaterials to distinguish material inconsistency from... more
The advances in large scale applications of carbon nanotubes demand a reliable supply of raw and processed materials. It is imperative to have a consistent quality control of these nanomaterials to distinguish material inconsistency from the modifications induced by processing of nanotubes for any application. NASA Johnson Space Center realized this need five years back and started a program to standardize the characterization methods. The JSC team conducted two workshops (2003 and 2005) in collaboration with NIST focusing on purity and dispersion measurement issues of carbon nanotubes [1]. In 2004, the NASA-JSC protocol was developed by combining analytical techniques of SEM, TEM, UV-VIS-NIR absorption, Raman, and TGA [2]. This protocol is routinely used by several researchers across the world as a first step in characterizing raw and purified carbon nanotubes. A suggested practice guide consisting of detailed chapters on TGA, Raman, electron microscopy and NIR absorption is in the...
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ABSTRACTSingle wall carbon nanotubes (SWCNT) have previously been considered potential catalyst supports in proton exchange membrane fuel cells (PEMFC) [1]. Earlier research and development of SWCNT for PEMFC catalyst supports has been... more
ABSTRACTSingle wall carbon nanotubes (SWCNT) have previously been considered potential catalyst supports in proton exchange membrane fuel cells (PEMFC) [1]. Earlier research and development of SWCNT for PEMFC catalyst supports has been advanced by utilizing differing SWCNT purities and carbon blacks in differing ratios. This study validates the performance of SWCNT and introduces new concepts for SWCNT membrane fabrication.
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Single-walled carbon nanotubes (SWCNT) are new materials of emerging technological importance. As SWCNT are introduced into the life cycle of commercial products, their effects on human health and environment should be addressed. We... more
Single-walled carbon nanotubes (SWCNT) are new materials of emerging technological importance. As SWCNT are introduced into the life cycle of commercial products, their effects on human health and environment should be addressed. We demonstrated that pharyngeal aspiration of SWCNT elicited unusual pulmonary effects in C57BL/6 mice that combined a robust but acute inflammation with early onset yet progressive fibrosis and granulomas. A dose-dependent increase in the protein, LDH, and γ-glutamyl transferase activities in bronchoalveolar lavage were found along with accumulation of 4-hydroxynonenal (oxidative biomarker) and depletion of glutathione in lungs. An early neutrophils accumulation ( day 1), followed by lymphocyte ( day 3) and macrophage ( day 7) influx, was accompanied by early elevation of proinflammatory cytokines (TNF-α, IL-1β; day 1) followed by fibrogenic transforming growth factor (TGF)-β1 (peaked on day 7). A rapid progressive fibrosis found in mice exhibited two dist...
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The rate constant for the reaction of O(/sup 3/P)+D/sub 2/ was measured over the temperature range 343 to 446 K using the laser photolysis--resonance fluorescence technique. The results can be fit well with a two-parameter Arrhenius... more
The rate constant for the reaction of O(/sup 3/P)+D/sub 2/ was measured over the temperature range 343 to 446 K using the laser photolysis--resonance fluorescence technique. The results can be fit well with a two-parameter Arrhenius function and are in excellent agreement with the previous measurements of Presser and Gordon and with the recent high temperature measurements of Michael. The data are also in very good agreement with the theoretical calculations of Joseph et al. and of Bowman et al. and demonstrate that tunneling makes a significant contribution to the rate coefficient over the entire range of temperatures studied in this experiment. The Arrhenius behavior of the rate constant supports the conclusions of Robie et al. in their study of the intramolecular isotope effect in O(/sup 3/P)+HD, where it was found that tunneling plays a major role at low temperatures.
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ABSTRACT Multiphoton ionization spectra have been obtained and analyzed for excitation in the 215–360 nm region from the X 3Σ−g, a 1Δg, and b 1Σ+g states of O2. The 0–0 band of the C 1Πg state is reported for the first time. Measurements... more
ABSTRACT Multiphoton ionization spectra have been obtained and analyzed for excitation in the 215–360 nm region from the X 3Σ−g, a 1Δg, and b 1Σ+g states of O2. The 0–0 band of the C 1Πg state is reported for the first time. Measurements of other vibrational bands terminating in the C 3Πg and d 1Πg states are in good agreement with determinations by other groups. Several vibrational levels (v′=0–5) of the 3dπg Rydberg complex have been assigned on the basis of (1) an analysis of the spin–orbit couplings between the (Λ,S) basis‐set states, (2) spectral simulation, and (3) the behavior of the states when the excitation radiation is changed from linear to circular polarization.