Joycelyn S. Harrison

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.

Electrical properties of single wall carbon nanotube reinforced polyimide composites

Electrical properties of single wall carbon nanotube (SWNT) reinforced polyimide composites were investigated as a function of SWNT concentration. AC and DC conductivities were measured, and the frequency behavior of the specific admittance was investigated. The experimental conductivity was found to obey a percolation-like power law with a relatively low percolation threshold. The current-voltage measurement results exhibited a non-ohmic behavior, indicating a quantum tunneling conduction mechanism. Analytical modeling and numerical simulation using high aspect ratio, rigid, spherocylinders as models for the SWNT were carried out to aid in understanding these results. The predictions were in good agreement with the experimental results. Published by Elsevier Ltd.

Very Long Single and Few-Walled Boron Nitride Nanotubes via the Pressurized Vapor/Condenser Method

A new method for producing long, small-diameter, single- and few-walled, boron nitride nanotubes (BNNTs) in macroscopic quantities is reported. The pressurized vapor/condenser (PVC) method produces, without catalysts, highly crystalline, very long, small-diameter, BNNTs. Palm-sized, cotton-like masses of BNNT raw material were grown by this technique and spun directly into centimeters-long yarn. Nanotube lengths were observed to be 100 times that of those grown by the most closely related method. Self-assembly and growth models for these long BNNTs are discussed.

Electrospinning of a micro-air vehicle wing skin

Electrospinning was utilized to create lightweight, electrically responsive wing skins for micro-air vehicle (MAV) wing frame designs. Various compositions of an electroactive polymer were investigated to determine the appropriate electrospinning conditions for these materials. Electrospun mats of these materials were characterized via optical microscopy and scanning electron microscopy. Tensile properties of the electrospun fibers were also measured. An optimal polymer composition was electrospun onto MAV wing frames to create a bird wing-like texture. Preliminary testing of electroactivity of these prototype MAV wings is reported here.

Electrostrictive Grafr Elastomers and Applications

Efficient actuators that are lightweight, high performance and compact are needed to support telerobotic requirements for future NASA missions. In this work, we present a new class of electromechanically active polymers that can potentially be used as actuators to meet many NASA needs. The materials are graft elastomers that offer high strain under an applied electric field. Due to its higher mechanical modulus, this elastomer also has a higher strain energy density as compared to previously reported electrostrictive polyurethane elastomers. The dielectric, mechanical and electromechanical properties of this new electrostrictive elastomer have been studied as a function of temperature and frequency. Combined with structural analysis using x-ray diffraction and differential scanning calorimetry on the new elastomer, structure-property interrelationship and mechanisms of the electric field induced strain in the graft elastomer have also been investigated. This electroactive polymer (EAP) has demonstrated high actuation strain and high mechanical energy density. The combination of these properties with its tailorable molecular composition and excellent processability makes it attractive for a variety of actuation tasks. The experimental results and applications will be presented.

Adhesion study of polyimide to single-wall carbon nanotube bundles by energy-filtered transmission electron microscopy

High-resolution electron microscopy and energy-filtered imaging methods were used to examine single-wall carbon nanotubes and nanotube reinforced polyimide composites. The nanotubes were studied alone, in a polyimide matrix composite, and after deposition of composite material previously dissolved in a solvent. Energy-filtered images based on nitrogen core loss excitations were used to discern the presence of polyimide. Elemental maps of nanotubes extracted from the composite revealed good wetting of the nanotube surfaces by the polyimide.

Evidence of Piezoelectricity in SWNT-Polyimide and SWNT-PZT-Polyimide Composites

Nanotechnology offers opportunities to reenergize the area of smart materials by addressing their current shortfalls and expanding their application range. For example, sensors based on polymer nanocomposites would provide a new paradigm for lightweight structural health monitoring for broad aeronautics and space applications. Deployable structures such as inflatable antennae and space mirrors will benefit from the incorporation of multifunctional lenses employing smart, articulating materials. In this paper, an approach to enhance the piezoelectricity of polyimides through the addition of lead zirconate titanate (PZT) particles and single-wall carbon nanotubes (SWNT) is presented. The dielectric and electrical properties of the composites are investigated as a function of SWNT volume content. The dynamic and static mechanical properties are presented to assess the effect of the inclusions on the macro-scale properties of the nanocomposites. It is found that the SWNTs increase the dielectric, piezoelectric, and mechanical properties of the polyimide matrix. Addition of the SWNT in the PZT/polyimide composites facilitates poling and results in an increase of the piezoelectric properties of the three-phase composite.

Electromechanically active polymer blends for actuation

Actuator mechanisms that are lightweight, durable, and efficient are needed to support telerobotic requirements, for future NASA missions. In this work, we present a series of electromechanically active polymer blends that can potentially be used as actuators for a variety of applications. This polymer blend combines an electrostrictive graft-elastomer with a ferroelectric poly (vinylidene fluoride-trifluoroethylene) polymer. Mechanical and piezoelectric properties of the blends as a function of temperature, frequency and relative composition of the two constituents in the blends have been studied. Electric field induced strain response of the blend films has also been studied as a function of the relative composition. A bending actuator device was developed incorporating the use of the polymer blend materials. The results and the possible effects of the combination of piezoelectricity and electrostriction in a material system are presented and discussed. This type of analysis may enable the design of blend compositions with optimal strain, mechanical, and dielectric properties for specific actuator applications.

Flexible low-mass robotic arm actuated by electroactive polymers

Miniature, lightweight, low-cost actuators that consume low- power can be used to develop unmatched robotic devices to make an impact on many technology areas. Electroactive polymers (EAP) actuators offer the potential to produce such devices and they induce relatively large bending and longitudinal actuation strains. This reported study is concentrating on the development of effective EAPs and the resultant enabling mechanisms employing their unique characteristics. Several EAP driven mechanisms, which emulate human hand, were developed including a gripper, manipulator arm and surface wiper. The manipulator arm was made of a composite rod with a lifting actuator consisting of a scrolled rope that is activated longitudinally by an electrostatic field. A gripper was made to serve as an end effector and it consisted of multiple bending EAP fingers for grabbing and holding such objects as rocks. An EAP surface wiper was developed to operate like a human finger and to demonstrate the potential to remove dust from optical and IR windows as well as solar cells. These EAP driven devices are taking advantage of the large actuation displacement of these materials for applications that have limited requirement for actuation force capability.

Aircraft Morphing program

In the last decade smart technologies have become enablers that cut across traditional boundaries in materials science and engineering. Here we define smart to mean embedded actuation, sensing, and control logic in a tightly coupled feedback loop. While multiple successes have been achieved in the laboratory, we have yet to see the general applicability of smart devices to real aircraft systems. The NASA Aircraft Morphing program is an attempt to couple research across a wide range of disciplines to integrate smart technologies into high payoff aircraft applications. The program bridges research in seven individual disciplines and combines the effort into activities in three primary program thrusts. System studies are used to assess the highest-payoff program objectives, and specific research activities are defined to address the technologies required for development of smart aircraft systems. In this paper we address the overall program goals and programmatic structure, and discuss the challenges associated with bringing the technologies to fruition.