Sharon E. Lowther

Dispersion of single wall carbon nanotubes by in situ polymerization under sonication

Single wall nanotube reinforced polyimide nanocomposites were synthesized by in situ polymerization of monomers of interest in the presence of sonication. This process enabled uniform dispersion of single wall carbon nanotube (SWNT) bundles in the polymer matrix. The resultant SWNT-polyimide nanocomposite films were electrically conductive (antistatic) and optically transparent with significant conductivity enhancement (10 orders of magnitude) at a very low loading (0.1 vol%). Mechanical properties as well as thermal stability were also improved with the incorporation of the SWNT.

Subsurface characterization of carbon nanotubes in polymer composites via quantitative electric force microscopy

Subsurface characterization of carbon nanotubes (CNTs) dispersed in free-standing polymer composite films was achieved via quantitative electric force microscopy (EFM). The effects of relative humidity, EFM probe geometry, tip-sample distance and bias voltage on the EFM contrast were studied. Non-parabolic voltage dependence of the EFM signal of subsurface CNTs in polymer composites was observed and a new mechanism was proposed taking consideration of capacitive coupling as well as coulombic coupling. We anticipate that this quantitative EFM technique will be a useful tool for non-destructive subsurface characterization of high dielectric constant nanostructures in low dielectric constant matrices.

Polyimide–silica hybrids containing novel phenylethynyl imide silanes as coupling agents for surface-treated titanium alloy

Polyimide-silica hybrids composed of an organic precursor containing a novel phenylethynyl imide silane and an inorganic precursor were evaluated as an adhesion-promoting interphase between surface-treated titanium alloy and a phenylethynyl-containing imide adhesive. The phenylethynyl groups present in the organic precursor, either as a pendent or end group, can bond chemically with a phenylethynyl-containing imide adhesive during processing, while the silane groups of the organic precursor would react chemically with the inorganic precursor. In addition, the inorganic precursor is able to react with the titanium alloy to form a stable bond with the metal oxide. Bond strength and durability were evaluated by single lap shear tests at various conditions. Lap shear specimens exhibited predominantly cohesive failure after a 3-d water boil with 92% retention of the initial room temperature strength. Morphology and chemical composition of the hybrid interphase were investigated with scanning electron microscopy, X-ray photoelectron spectroscopy, and Auger electron spectroscopy, which revealed development of a silicon-gradient, hybrid structure between the metal substrate and the adhesive.

Multifunctional Electroactive Nanocomposites Based on Piezoelectric Boron Nitride Nanotubes

Space exploration missions require sensors and devices capable of stable operation in harsh environments such as those that include high thermal fluctuation, atomic oxygen, and high-energy ionizing radiation. However, conventional or state-of-the-art electroactive materials like lead zirconate titanate, poly(vinylidene fluoride), and carbon nanotube (CNT)-doped polyimides have limitations on use in those extreme applications. Theoretical studies have shown that boron nitride nanotubes (BNNTs) have strength-to-weight ratios comparable to those of CNTs, excellent high-temperature stability (to 800 °C in air), large electroactive characteristics, and excellent neutron radiation shielding capability. In this study, we demonstrated the experimental electroactive characteristics of BNNTs in novel multifunctional electroactive nanocomposites. Upon application of an external electric field, the 2 wt % BNNT/polyimide composite was found to exhibit electroactive strain composed of a superposition of linear piezoelectric and nonlinear electrostrictive components. When the BNNTs were aligned by stretching the 2 wt % BNNT/polyimide composite, electroactive characteristics increased by about 460% compared to the nonstretched sample. An all-nanotube actuator consisting of a BNNT buckypaper layer between two single-walled carbon nanotube buckypaper electrode layers was found to have much larger electroactive properties. The additional neutron radiation shielding properties and ultraviolet/visible/near-infrared optical properties of the BNNT composites make them excellent candidates for use in the extreme environments of space missions.

Boron nitride nanotube: synthesis and applications

Scientists have predicted that carbon’s immediate neighbors on the periodic chart, boron and nitrogen, may also form perfect nanotubes, since the advent of carbon nanotubes (CNTs) in 1991. First proposed then synthesized by researchers at UC Berkeley in the mid 1990’s, the boron nitride nanotube (BNNT) has proven very difficult to make until now. Herein we provide an update on a catalyst-free method for synthesizing highly crystalline, small diameter BNNTs with a high aspect ratio using a high power laser under a high pressure and high temperature environment first discovered jointly by NASA/NIA/JSA. Progress in purification methods, dispersion studies, BNNT mat and composite formation, and modeling and diagnostics will also be presented. The white BNNTs offer extraordinary properties including neutron radiation shielding, piezoelectricity, thermal oxidative stability (> 800°C in air), mechanical strength, and toughness. The characteristics of the novel BNNTs and BNNT polymer composites and their potential applications are discussed.

Optimization of Surface Treatment and Adhesive Selection for Bond Durability in Ti-15-3 Laminates

Abstract The promising mechanical performance of a baseline Hybrid Titanium Composite Laminate (HTCL) inspired an investigation into maximizing the strength and environmental performance of this new aerospace material. This research focused upon finding the strongest and most durably combination of three commercially-available titanium surface treatments (i.e., Pasa-Jell 107TM, Boeings Sol-Gel, and Turco 5578R) and two polyimide adhesives (i.e., LaRCTM-IAX and FM5R) for use in HTCL. The tests employed the cracked-lap shear (CLS) specimen geometry for fatigue crack growth measurements and also for fracture toughness analyses of the bonded specimens. The CLS geometry models several bonded applications found in the aerospace industry, and it also represents the debonding characteristics of a cracked titanium foil in HTCL. The environmental performance of these six material combinations has been evaluated after 5,000 hours of continuous exposure to either a Hot/Wet environment that subjected the bonded speci...

THE EFFECT OF SINGLE WALL CARBON NANOTUBES ON THE DIPOLE ORIENTATION AND PIEZOELECTRIC PROPERTIES OF POLYMERIC NANOCOMPOSITES

Recently, a series of single wall carbon nanotube (SWNT) polyimide nanocomposites were developed since the demand of electroactive polymeric materials as sensors and actuators for use in high temperature applications has been growing. Adding SWNTs into electroactive polyimides enhanced their electrostrictive strain as well as their mechanical integrities and chemical stability. Although an increase in piezoelectricity resulting from the incorporation of SWNTs could be expected, there has been no systematic study detailing the effect of SWNTs on piezoelectricity. In this article, the effects of various types and concentrations of SWNT on the dipole orientation and piezoelectricity were investigated using a thermally stimulated current (TSC) technique and a modified Rheovibron. It was found that the barely modified SWNTs led to a more substantial increase in the remanent polarization (Pr) than the highly modified SWNTs did. As the loading level of SWNTs increased, Pr increased. However, excessive loading of SWNTs showed a reduction in Pr since the actual poling field decreased due to a large leakage of current. The trend of the piezoelectric strain coefficient, d31, was consistent with that of Pr. The increase in interfacial polarization caused by adding SWNT was believed to be primarily responsible for the increase of Pr and d31.