William R. Pogue

Multifunctional structure-battery composites for marine systems

Multifunctional structure-battery composites were developed using fiber reinforced marine composites for structure function and rechargeable lithium-ion cells for energy storage and structure function. Laminate, sandwich, and modular beam configurations were fabricated and tested to determine flexural stiffness and strength, energy storage capacity versus discharge rate, and buoyancy (density). The structure-battery composites exhibited higher flexural stiffness but lower strength than equivalent unifunctional designs, energy storage capacities between 40 and 60 Wh/L, and buoyancies bracketing the unifunctional specimen values. Issues requiring further attention include: improved bending strength, simplified fabrication, reversible attachments for modular components, electrical wiring and connections, and battery management circuitry.

Tiled Composite Laminates

This article describes our efforts to develop a meso-scale in-plane tiling technique for fiber-reinforced composite laminates. Such a technique expands the design space of the laminate, and the ability to tailor local laminate properties may provide a means, for example, to mitigate stress concentrations that arise in places such as along free edges of the laminate. Preliminary fabrication of tiled laminates produced material with an elastic modulus that is nearly equal to that of continuously reinforced laminates; however, the strength of these first specimens was significantly reduced. Finite element analyses were then performed to characterize the effects of features unique to tiled composite laminates, such as the existence of resin-rich tile-to-tile interfaces, and to explore the effects of relative arrangement of tiles through the thickness of the laminate. This led to a novel composite joint geometry as well as recommendations to minimize strength reduction. Strength retention of laminates fabricated using the new design guidelines was experimentally found to exceed 92% in comparison with the traditional analogs. Finally, we discuss the potential application of composite tiling for the suppression of free-edge delamination.

Performance Characterization of Multifunctional Structure-Battery Composites for Marine Applications

The aim of our work is to design, fabricate and characterize multifunctional structure-power composites for marine applications such as in unmanned underwater vehicles. Three types of structure-power (or structure-battery (SB)) specimens were fabricated using fiber-reinforced polymers and closed-cell foam as the structural components, and commercial-off-the-shelf lithium-polymer cells as the power-plus-structure component. This paper details the mechanical and electrical characterization of the S-B composites while a companion paper deals with the design and fabrication issues. The three multifunctional designs are: integrated SB laminate with lithium-polymer pouch cells embedded on one side, SB sandwich with cells embedded within a closed-cell polymeric foam along the neutral axis, and a SB modular stiffener that can be attached and removed from a host-structure. Unifunctional composites (i.e. without embedded cells) were also fabricated for comparison with the multifunctional composites. The embedded cells show identical charge-discharge electrical performance as their un-embedded counterparts, thus, indicating that the composite fabrication procedures did not adversely affect their electrical performance. Ragone curves (energy density vs. power density) of the S-B composites show that the targeted energy density of 50 Wh/L was achieved in the SB modular stiffener design. The bending stiffnesses of the integrated SB and SB modular stiffener designs were ∼7x greater than the unifunctional design while the multifunctional sandwich specimens were ∼17% stiffer than their unifunctional counterparts. Tests are currently being conducted to determine the affect of mechanical flexure (constant displacement) on embedded cell discharge and charge characteristics, and conversely, cell discharge and charge on the load and deflection during flexure.Copyright

Autophagous structure-power systems

Novel autophagous (self-consuming) systems combining structure and power functionalities are under development for improved material utilization and performance enhancement in electric unmanned air vehicles (UAVs). Much of the mass of typical aircraft is devoted separately to the functions of structure and fuel-energy. Several methods are proposed to extract structure function from materials that can also serve as fuel for combustion or as a source of hydrogen. Combustion heat is converted to electrical energy by thermoelectric generation, and hydrogen gas is used in fuel cells to provide electrical energy. The development and implementation of these structure-fuels are discussed in the context of three specific designs of autophagous wing spars. The designs are analyzed with respect to mechanical performance and energy storage. Results indicate a high potential for these systems to provide enhanced performance in electric UAVs.

Design and Fabrication of Multifunctional Structure-Power Composites for Marine Applications

Various subsystems in marine applications, especially unmanned underwater vehicles, compete for space. Multifunctional structure-battery (SB) composites combine structure and power functions through the use of high-performance fiber reinforced polymer layers and lithium ion cell batteries, to create volumetric opportunities for increase in overall power generation capacity and/or payload. This paper focuses on the design and fabrication aspects of the SB composites. The design objectives are to achieve or exceed structural performance of traditional marine composites while attaining a volumetric energy density of 50 Wh/L with similar buoyancy levels and dimensional sizes. The design process reveals that all four objectives can be achieved only if the components related to energy storage have the same mechanical and physical properties as the material being replaced. With commercially available batteries, at least three out of four objectives are met for the proposed SB designs. Selection of materials and fabrication methods are heavily influenced by the temperature limit of the battery cells, cell surface preparation for adhesion and load transfer, and power bussing layout. A companion paper addresses multifunctional performance characterization of these composites.Copyright

Electrospray Ionization of Polymers: Evaporation, Drop Fission, and Deposited Particle Morphology

The fundamental challenge of nanomanufacturing is to create, control, and place immense quantities of nanoscale objects controllably over large surface areas. Electrospray ionization (ESI) has the potential to address this challenge due to its simplicity, applicability to a broad range of materials, and intrinsic scalability. However, the interactions between electrospray parameters and final deposited morphology are not well understood. Experimental results are combined with physics-based models to explain how observed particle size distributions are caused in the spray by evaporation and Coulomb fission of drops with solute concentration gradients.

Microstructural and Mechanical Characterization of Carbon Nanofiber Reinforced Composites

Carbon nanofibers, such as single walled carbon nanotubes (SWNT), multiwalled carbon nanotubes (MWNT) and vapor-grown carbon nanofibers (VGCF or VGCNF) are routinely compounded with polymers to create thermally and electrically conductive polymer nanocomposites. Our group is interested in combining the conduction with structural functionality by reinforcing a high-performance thermotropic liquid crystal polymer (LCP) matrix with vapor-grown carbon nanofibers and single walled carbon nanotubes. High strength and stiffness can be achieved in LCPs through the alignment of molecular domains during high-shear mixing and extrusion. Further strength and stiffness enhancements are potentially possible if the carbon nanofibers could also be aligned, perhaps, with the assistance of the aligned domains of the LCP matrix. However, the geometrical structure of VGCF is quite different and the diameter is one to two orders of magnitude larger than that of SWNT. Therefore, the processing conditions and the interactions between the LCP domains and the nanofibers are expected to lead to different dispersion and alignment characteristics of VGCF and SWNT within the LCP matrix. In this work, twin-screw and Maxwell-type mixer-extruders were used to produce neat LCP filaments and LCP-nanofiber composite filaments with various concentrations of VGCF and SWNT. The dispersion and orientation of the VGCF and SWNT reinforcements were determined by X-ray diffraction and electron microscopy. The filaments were loaded in quasi-static uniaxial tension until fracture to determine the tensile modulus, strength and strain-to-failure. The mechanical properties showed a strong dependence on the filament diameter, nanofiber concentration and processing parameters. A significant increase in mechanical performance was observed with decreasing filament diameter irrespective of the carbon nanofiber concentration. Fracture surfaces examined under electron microscopy revealed hierarchical features at multiple length scales. At the macroscopic scale, a skin-core configuration was observed in the filament cross-section with the skin possessing a greater degree of LCP molecular alignment and nanofiber alignment than the core. The mechanical and electrical properties of the LCP, LCP-VGCF and LCP-SWNT nanocomposite filaments will be described and related to processing parameters, the type of carbon nanofibers (VGCF or SWNT) and the resulting composite microstructure.Copyright

Suppression of Edge Delamination Through Meso-Scale Structuring

We are developing a new class of fiber-reinforced polymer composite materials to facilitate imbedding multifunctional features and devices in material systems, and to manage interlaminar stresses at free edges and cut-outs. The idea is centered on introducing one more level of design space by composing plies with individual tiles possessing the same degrees of design freedom that are associated with individual plies. In this work, we have focused on tiling schemes that will allow blending of laminates (lay-ups), where a lay-up suitable for suppressing interlaminar stresses could be placed at necessary locations whereas another lay-up could be used for the main objective. This results in the introduction of matrix-rich tile-to-tile interface pockets in the blending region. Preliminary mechanical testing shows that uniaxially reinforced tiled composites attain stiffness levels near those of their traditional counterparts, yet with a potential degradation of strength. We used the finite element method to investigate the effects of resin-rich pocket size, the use of supporting continuous layers, tile size, and tile overlapping (interface stacking) schemes on stress distribution around interfaces in uniaxially reinforced tiled composites, with the aim to identify parameters controlling overall strength. We discovered that alignment of the resin-rich pockets through the thickness exacerbates stress-concentration and that outer continuous layers on the composite may help in better load transfer. As a first step in the application of this technique for the suppression of delamination at the free edges of holes in laminates, a bilaminate material was modeled, and the concept was shown to be effective in the suppression of edge delamination.Copyright

Microstructure-Property Relation in a Liquid Crystalline Polymer-Carbon Nanofiber Composite

Microstructure-property relationship is being examined in a polymer matrix composite system consisting of vapor grown carbon nanofibers (VGCF) mixed in a thermotropic liquid crystalline polymer (LCP) matrix. These nanocomposites show an inherent hierarchical structuring, which we hope to utilize in the development of multifunctional structure-conduction composites with improved performance. Among unfilled polymers, extruded LCPs show relatively high strength and high stiffness that have been attributed in the literature to the preferential molecular alignment along the extrusion direction and the hierarchical nature of LCPs. Further, as is typical for polymers, LCPs have poor thermal and electrical conductivity relative to metals. By contrast, carbon nanofibers are known to possess high strength, high stiffness and high conductivity in the axial direction. It is expected that the combination of the extrusion process and the similarity of the length-scales of LCP fibrils and carbon nanofibers will lead to improved axial alignment of both phases within the nanocomposite filaments. This simultaneous alignment of the LCP matrix and that of the carbon nanofibers is expected to lead to interesting mechanical and conductive behavior in the nanocomposite filaments through hierarchical interactions at the nanometer to micrometer scale levels. Carbon nanofibers, 60-150 nm in diameter, were mixed with Vectra A950 LCP and the mixture was extruded as 0.5–2 mm diameter filaments. Nanocomposite filaments with 1%, 2%, 5% and 10 wt.% VGCF were characterized via tensile testing and fractography. The tensile modulus, failure strength and strain-to-failure were found to be sensitive to filament diameter, carbon nanofiber content and extrusion process. There was a noticeable increase in mechanical performance with decreasing filament diameter irrespective of carbon nanofiber content. Fracture surfaces showed hierarchical features from nanometer to micrometer scale and processing defects in the form of voids. The results of this research will be used to fabricate composite components that exploit structural hierarchy from nano-to macro-scale.Copyright