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We report a composite capable of advanced manufacturing and damage repair. Microwave energy is used to induce thermal reversible polymerization of the matrix allowing for microwave assisted composite welding and repair. Composites can be bonded together in just a few minutes through microwave welding. Lap shear testing demonstrates that microwave welded composites exhibit 40% bond strength relative to composites bonded with epoxy resin. Double cantilever beam testing shows 60% recovery in delamination strength after microwave assisted composite repair. The interfacial adhesion and composite repair after microwave exposure are examined by X-ray computed tomography. The microwave processing is shown to be reproducible and consistent. The ability to perform scalable manufacturing is demonstrated by the construction of a large structure from smaller components.

Metallic single-wall carbon nanotubes, synthesized through a pulsed-laser vaporization process, were selectively reacted with dodecyl-oxybenzene-diazonium tetrafluoroborate to yield tetrahydrofuran (THF) suspensions of nanotubes enriched in metallic content. The nanotube material that did not suspend in THF displayed a lower D/G ratio in Raman spectroscopy indicating less covalent functionalization and corresponds to an increase in semiconducting nanotube population. After the THF suspendable nanotubes were extracted from the unsuspendable nanotube material, the two separate nanotube populations were stripped of the dodecyloxybenzene functional groups through an annealing process. In this way, the functionalization process was made to be reversible whereby the nanotubes from both semiconducting and metallically enriched populations could have their original band gap properties restored.

Pulsed laser vaporization synthesis of single-wall carbon nanotubes on Co/Ni and Rh/Pd catalysts was explored with respect to variations in the production temperature. The nanotube type populations were determined via photoluminescence, UV-Vis-NIR absorption and Raman spectroscopy. It was found that lowered production temperature leads to smaller nanotube diameters and exceptionally narrow (n, m) type distributions, with marked preference towards large chiral angles for both catalysts. Interestingly, larger nanotube diameters tend to be associated with larger chiral angles. These results demonstrate that PLV production technique can provide at least partial control over the nanotube (n, m) populations. In addition, these results have implications for the understanding the nanotube nucleation mechanism in the laser oven. SWCNT synthesized at lower temperatures appear quite attractive as a starting material for nanotube type separation experiments.

In the interest of developing a highly sensitive, low power radiation dosimeter, a series of tests were performed on single-wall carbon nanotube (SWCNT)-based nanomaterials to monitor their respons...

Metal impurities within as-produced single-walled carbon nanotubes (SWCNTs) are generally coated with thick shells of graphitic carbon, which prevent the acid treatments commonly used in SWCNT purification from attacking the metals. The purpose of this study was to determine a more effective and efficient means for the removal of metal impurities from SWCNTs produced by pulsed laser vaporization (PLV). The method investigated was a modified version of a soft-bake purification procedure developed at Rice University, and in the current work, the soft-bake temperature suitable for PLV-produced SWCNTs was determined, and several samples were purified. The quality of material was determined using the standard protocol developed at NASA Johnson Space Center (JSC), and data was collected for several samples in order to determine the consistency and reproducibility of the purification yield and purity. The properties and quality of soft-bake purified material were compared with those of several equivalent materia...

Samples from nanotubes produced by various methods are analyzed by laser desorption-ionization ion mobility time-of-flight mass spectrometry (LDI-IM-TOF-MS). The measurements find mostly positive f...

Single-walled carbon nanotubes (SWCNTs) were added to aluminium using friction stir processing (FSP). The SWCNTs survived the thermal and stress cycles involved with friction stir processing. The Raman spectroscopy and SEM results are presented. Potential applications of nanotubes inserted into metals by this method are discussed.

Stopping powers of energetic Si and P ions are measured in thin GaAs films. A new technique derived from molecular beam epitaxy (MBE) was developed to prepare near stoichiometric GaAs films on thin carbon layers for use in transmission ion beam experiments. The GaAs films were characterized using X-ray photoelectron spectroscopy (XPS) and particle induced X-ray emission (PIXE). Stopping power data are presented for the first time for Si and P in GaAs.

An important issue in field emission vacuum microelectronics is the stability of the field emitters with the residual ambient gas. Particularly important is that the field emitter tips made of refractory metals like molybdenum, niobium and tungsten are susceptible to oxidation. The corresponding metal oxides are insulating and adversely affect the emission current characteristic by increasing the width of the effective tunneling barrier. With this perspective, we studied iridium oxide field emitters to evaluate the characteristics of conductive oxide tips. We studied the field emission characteristics of iridium and thermally prepared iridium oxide field emitters using field emission microscopy and current–voltage measurements. We found that, upon oxidation, the voltage required to achieve the desired emission current desire dropped significantly. In addition, oxidation led to a decrease of emission current fluctuations. The development of stable conductive oxide field emitters should improve the performa...

Challenges associated with damage tolerance in polymer matrix composites must be successfully addressed in order to ensure highly reliable structures with significant weight savings. Self-healing materials provide a viable means to surmount damage tolerance concerns, thereby allowing for the realization of the mass reduction such structures have promised but not yet achieved. Introduction of multifunctional properties into self-healing composites can further extend their usefulness. This study examines the incorporation of carbon nanotubes into a self-healing composite in order to achieve this. Composite panels were fabricated with carbon fibers, a bismaleimide tetrafuran (2MEP4F) polymer resin, and various carbon nanotube materials. The composites exhibit enhancement in electrical, mechanical, and thermal properties. The healing mechanism is a thermally activated reversible polymerization of the 2MEP4F resin. The proposed method of heating exploits the enhanced microwave absorption inherent to carbon nanotubes to provide the thermal energy required for the reversible polymerization. Microwave testing demonstrated that the heating efficiency is increased, allowing uniform heating to the required temperature for polymer healing. Impacted composites show localized heating at the damage site, which implies that microwave heating can also be used as a means for damage detection and potential structural health monitoring.

A simple instrument to determine field emission tip work functions and shape functions from simultaneous current–voltage (I–V) characteristics and field emission energy distributions of field emitter tips and tip arrays is described. This instrument uses a cylindrical energy analyzer with a few correcting elements to simulate a hemispherical analyzer and provides a low cost and more compact alternative to a commercial hemispherical spectrometer. I–V curves and energy distributions may be automatically obtained as a function of time to study field emission tip degradation with usage and/or exposure to gases of interest.

The work function of hydrogen-terminated polycrystalline diamond films deposited by electrophoresis on molybdenum was studied using ultraviolet photoelectron spectroscopy with 21.2 eV photons for average grain sizes ranging from 0.32 to 108 μm. The work function has a maximum of about 5.1 eV at 0.32 μm, then decreases with increasing grain size to a minimum of about 3.2 eV at an average grain size of about 4 μm and then increases to a value of about 4.8 eV at a grain size of 108 μm. The results are consistent with a model in which the work function is controlled by the work function of single crystal diamond (111) at the larger grain sizes, graphitic carbon at the smaller grain sizes, and by a negative electron affinity that increases with decreasing grain size due to defects near diamond (111) crystallite edges for the intervening grain sizes. The large change in work function (almost a factor of 2) could be useful to make conductors with different work functions for microelectronic gate structures.

The emission current in a molybdenum field emission array can decrease by 50% in 1000 s at an oxygen pressure of 10−6 Torr. To overcome this disadvantage of molybdenum microtips, the effectiveness of dynamic surface cleaning has been investigated in a single-aperture gated-diode configuration. For dynamic surface cleaning, tip surface oxide buildup is balanced by tip oxide removal due to sputtering by ions created in ionizing collisions with field-emitted electrons. The present results demonstrate stable dynamic cleaning with clean and partially oxidized molybdenum tips for currents ranging from 10−11 to 10−9 A with oxygen exposures of up to 1000 L. For currents above 10−9 A, ion bombardment causes the tip shape to become unstable leading to failure with increasing oxygen exposure.

The thermal stability and the resiliency of molybdenum carbide field-emission tips deposited at room temperature by electrophoresis have been studied. The field emission from Mo2C films deposited on Mo tips does not change after being heated to 800 °C while exposed to 360 L of air, although MoO2, MoO3, and possibly MoO, are present in the films. The field-emission thresholds agree with photoelectric work functions determined from photoelectron spectroscopy measurements of similarly grown flat samples. These films are found to exist in three distinct phases as a function of temperature after formation by room-temperature electrophoresis. From room temperature to 500 °C, MoO3 is the dominant oxide, from 500 to 775 °C, MoC2 is the dominant oxide, and above 825 °C both oxides have virtually disappeared.

We report a decrease in field emission threshold from 3.8 to 3.4 eV for room temperature electrophoresis grown polycrystalline diamond films on molybdenum tips as the diamond average grain size increases from 0.25 to 6 μm. The field emission thresholds agree with photoelectric work functions determined from photoelectron spectroscopy measurements of similarly grown flat samples. In addition, diamond surface states are observed at 0.4, 0.9, and 1.8 eV above the valence band. The results are consistent with an increasing negative electron affinity with grain size due to increased surface hydrogen bonding and with perhaps a contribution from surface defect states.

The effect of thermal growth conditions on the morphology and surface work function of iridium oxide thin films grown by annealing Ir thin films in an O2 ambient is presented. The samples were analyzed using x-ray diffraction, x-ray photoelectron spectroscopy, atomic force microscopy, and photoelectric work function measurements. It is found that, with increasing temperature, IrO2 changes from (110) oriented to a mixture of (110) and (200) during the oxide growth. This is manifested as a sharpening of the photoelectric energy distributions at 800 °C. The surface work function was determined to be 4.23 eV using ultraviolet photoelectron spectroscopy. X-ray photoelectron spectroscopy analysis shows that IrO2 starts to form at 600 °C accompanied by surface roughening. Annealing the Ir film at 900 °C in O2 ambient leads to almost complete desorption of the film.

Oxidation of emitter surfaces can be a serious problem for Mo field emitter arrays. We studied the oxidation and related changes in the electronic properties of Mo thin films as a function of annealing temperature. Experiments were done on Mo thin films prepared on Si and sodalime glass substrates. These films were thermally oxidized and characterized using a variety of techniques including x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and thermal desorption spectroscopy (TPD) methods. For films oxidized below 400°C, partial oxidation was observed, with MoO3(110) being the principal oxide phase. However, at a temperature of 500°C and above, oxidation of the film was complete. Electrical characteristics of the films undergo a rapid transition from semiconductive to highly insulating at temperatures between 475 to 500°C. Temperature programmed desorption spectra showed that the oxides are stable at elevated temperature with only a principal O2 desorption peak at approximately 786°C.

Field emission current-voltage characteristics and simultaneous field emission electron energy distributions have been measured using single tip gate diodes. An energy distribution is generated at each step of a current-voltage characteristic using a compact low-cost simulated hemispherical energy analyzer. A PC programmed with graphics-based data acquisition software is used for data acquisition and control. The PC is connected to a CAMAC crate and a picoammeter through a GPIB interface. The picoammeter measures the current leaving the tip and the field emission electrons are energy analyzed, detected and processed in the CAMAC crate. The CAMAC crate also sends control voltages. to the gate anode and the energy analyzer. This apparatus was used to measure tip work functions and Fowler-Nordheim tip shape parameters for Mo and IrO2 field emission tips. Work function measurements from field emission tips are compared to photoelectric work function measurements from flat surfaces.

Molybdenum carbide and diamond films were deposited at room temperature by dielectrophoresis on molybdenum foil and field emission tips. Films deposited on flat surfaces were characterized with XPS, UPS and SEM. Field emission Fowler-Nordheim current-voltage characteristics and electron energy distribution measurements were obtained using films deposited on field emission tips as part of a single aperture gated diode in a VG ESCALAB II system. Molybdenum trioxide was present after molybdenum carbide deposition at room temperature on all samples. At a temperature of about 650 °C, the dominant oxide changes to molybdenum dioxide. At about 800 °C, the oxygen content of the films significantly decreases and molybdenum carbide with significant graphite content remains. The UPS data for these samples were obtained at different annealing temperatures and a number of photon energies to determine photoelectric work functions for molybdenum trioxide, molybdenum dioxide and molybdenum carbide. The same kind of data was obtained for diamond deposited on molybdenum and molybdenum carbide coated foils and field emission tips.

We report a significant decrease in the photoelectric threshold of chemical vapor deposition grown diamond films as the fraction of sp3 carbon to sp2 plus sp3 carbon in the films decreases. Raman spectroscopy and x-ray photoelectron spectroscopy are used to characterize the different forms of carbon in the films and the sp3/(sp2 + sp3) carbon fraction at the surface. We observe a decrease in the photoelectric threshold from 4.5 eV to 3.9 eV as the sp3/(sp2 + sp3) carbon fraction at the surface decreases from 71% to 55%. Ultraviolet photoelectron spectroscopy of the films shows that they have a negative electron affinity surface. Therefore, the work function of the films decreases from 4.5 eV to 3.9 eV. We propose that the decrease in photoelectric threshold is due to a decrease in the band gap of sp2-sp3 carbon networks at the grain boundaries. The observed decrease in photoelectric threshold can be used to tailor the electronic properties of diamond films for specific applications.

Boron doped polycrystalline diamond films grown on p-type single-crystal Si substrates using chemical vapor deposition with a gas mixture of hydrogen, methane and diborane were characterized with scanning electron microscopy, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Raman spectroscopy and photoelectric current measurements. The energy distributions are not sensitive to boron doping for diborane concentrations from 0 to 4.75 ppm, although the boron doping modifies the surface morphology and the photoemission intensity. The photoemission intensity is high where the microcrystalline content is highest (at diborane concentrations of 2.91 and 4.75 ppm. The photoelectric threshold is found to be at 4.38 eV, in agreement with earlier measurements. The present results are characteristic of valence band emission at 4.38, 4.63, 4.92, 5.12 and 5.30 eV for incident photons between 4.87 and 5.63 eV.

Many potential applications of carbon nanotubes are expected to benefit from the availability of single-walled carbon nanotube materials enriched in metallic species, and specifically armchair nanotubes. The present work focuses on the modification of the pulsed laser vaporization (PLV) technique to selectively produce certain carbon nanotube structures. Nanotube nuclei were “warmed-up” with an additional laser pulse, timed to coincide approximately with the nucleation event. The effect of the second laser on the carbon vapor temperature was studied by emission spectroscopy. Nanotube type populations with and without warm-up were compared by means of absorption, photoluminescence, and Raman spectroscopy. It was found that the warm-up of nanotube nuclei with a laser pulse has a noticeable, albeit small, effect on the nanotube population. The intensity of spectral features associated with (9,7) nanotube and its large chiral angle neighbors increased, while small chiral angle nanotubes decreased, with exception of the (15,0) tube. This experiment demonstrates that nanotube population during PLV synthesis can be manipulated in a controlled fashion.

Single 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.