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Textile Capacitor

The Air Force (AF) is evolving from a Cold War-era force with a large, containment- focused infrastructure to a smaller, more responsive and affordable Air and Space Expeditionary Force. In support of this transformation, AFRL is developing affordable, sustainable, and scalable force applications, including directed energy weapons, kinetic energy weapons, electromagnetic guns and launchers, and high-power microwaves.

Capacitors are an essential component in all of these systems because of their ability to store prime electrical energy and expel it in short, fast energy pulses. If they were to be made using conventional technology, however, the capacitors required to support such advanced systems would be bulky and could weigh thousands of pounds. Consequently, AFRL researchers are exploring ways to integrate load-bearing capacitor fibers into air vehicle structures to reduce airframe weight, free up valuable space, and offer fuel cost savings.

Three AFRL scientists, Mr. William Baron, Dr. Maxwell Blair, and Mrs. Sandra Fries-Carr, recently received a patent entitled “Airframe Structure- Integrated Capacitor” (see inset). The term “structural capacitance” implies that in addition to carrying load, the aircraft or spacecraft structure maintains a capacitive charge that permits energy storage and power conditioning for use in a variety of applications, both pulsed and continuous. The specific objectives of the scientists’ work effort were to identify a plausible design concept, conduct experimental trials, and characterize the concept’s structural and electrical efficiency.

In selecting from available capacitor types, the team considered only those configurations capable of carrying normal, shear, and bending loads. Two common capacitors— parallel plate and cylindrical—met this requirement. While evaluating parallel plate capacitors, the researchers considered laminated structural systems constructed from metal and dielectric material; they envisioned bonding sheets of aluminum to the chosen dielectric material. Commercially available, structural precedents such as Arall™ use this procedure. Although not designed for electrical purposes, Arall is a structurally durable material constructed from impregnated aramid fibers and bonded with aluminum sheets into a laminate. Technicians apply this design approach to the structural capacitor application by laminating a good dielectric with a conductive lamina. Because the parallel plate capacitor concept is quite efficient, the team members initially believed it had significant potential. However, they later abandoned the concept because of associated developmental challenges, including the control of dielectric layer properties, damage tolerance, and repair issues.

The team then focused on an alternative approach—designing a cylindrical capacitor capable of reacting structural loads. To fabricate this concept, technicians would cover a dielectric-coated conductor with a conductive metal layer. Next, they would integrate the coaxial system into a hybridized, composite weave with carbon tow for additional reinforcement. Finally, they would lay up the material on a tool and inject it with resin to create an air vehicle’s primary load-bearing structure. Dielectric characteristics, which are the Achilles’ heel of any electrostructural system, govern capacitor performance. While the cylindrical capacitor approach enables precise control over dielectric layer quality, designing the energy’s ingress and egress paths into the structure is more complicated than simply using the parallel plate capacitor concept. Nevertheless, using the cylindrical capacitor composite allows engineers to integrate tens of thousands of feet of capacitor into an air vehicle’s structure.

To demonstrate the basic concept, AFRL researchers used a commercially available copper wire with Kapton™, an aromatic polyamide dielectric material available in numerous manufactured forms. Kapton material has voltage breakdown strengths of around 4500 V/mil and offers low electromagnetic losses. While designers did not intend for this material to form the basis of a transitionable solution, they did believe it capable of showing concept feasibility and establishing future research activity parameters. For the purposes of the initial feasibility investigation, technicians fabricated the first specimens with a conductive copper paint offering a uniform coat and good adhesion. The team conducted an initial capacitor test to examine performance using a Hewlett-Packard 4284A precision impedance meter. Results showed good capacitance, inductance, and resistance from 1 kHz to 1 MHz. The team considered these results significant, given the limited time spent optimizing the system.

The team then integrated the design into an experimental piece of carbon fabric material, evaluated its ability to maintain structural capacitance, and demonstrated the weaving process viability. This process required a significant amount of capacitive wire, since 1.0 sq ft of plain weave fabric requires 120 ft of cylindrical capacitor when used only in the fabric warp direction at 10 pics/in. The team created a small, taskspecific weaving machine to produce a structural capacitor fabric coupon measuring 8.0 in. wide by 14.0 in. long. Testing this first-generation specimen (see Figure 1) verified the capacitor concept’s preliminary feasibility. Researchers subsequently used this demonstrated performance in an integrated conceptual air vehicle fuselage design that ultimately generated 168.92 μF at high-voltage operation.

In the future, superior dielectrics that offer improved structural and electrical performance will increase the structure’s energy density storage capacity. The AFRL team is working with researchers from the University of Dayton Research Institute (UDRI) to develop highperformance resins for achieving dielectric performance with good structural properties. This work involves doping resins to improve the voltage breakdown strength, provide low electromagnetic losses, and increase the material dielectric constant. In addition, the team is developing material that is compatible with the injected structural resin and also has low moisture uptake, since the dielectric is not hermetically sealed. The researchers are synthesizing this resin material for use in the precision dielectric coating pultrusion process that AFRL is currently developing to produce the next generation of coaxial structural capacitors. Figure 2 shows a scanning electron microscope image of the doped resin.

Researchers have also made significant progress in lowering the structural capacitor concept’s specific weight and improving its durability. The first improvement was in eliminating the copper paint electrode. This step involved thorough consideration of several options, including metal foil bonding, electroless plating, metal coating flame spray, vapor deposition, and braiding of a conductive surface with composite fibers. For the second-generation device, researchers used a conductive fiber overbraid to improve structural and electrical performance. They tested several varieties of this overbraid, including variations fabricated from titanium, copper zirconium alloy, and carbon tow. These braids provided an overall thickness of approximately 4 mils, enabling a small profile for integration into the composite architecture. Researchers evaluated these materials based on cost, electrical resistance, weight, and ease of fabrication, ultimately choosing the carbon braid electrode (see Figure 3) for future development activity. The team also evaluated copper zirconium options for the center electrode. Although the zirconium contained in the copper improved structural fatigue resistance with minimal impact to electrical conductivity, the desire to further reduce weight has since inspired additional efforts to fabricate the center electrode using carbon fiber tow in a pultrusion process.

Finally, researchers used the Textile Composite Analysis for Design (TEXCAD) computer program to model the plain weave unit cell structure used in the feasibility study. TEXCAD is a general-purpose micromechanics code that models yarn architecture to predict three-dimensional thermal and mechanical properties, damage initiation and progression, strength under tension, compression, and shear. These analyses guided the development of an advanced textile architecture and resulted in a new weave that provided significant fiber volume fraction increase and improved weight efficiency. Researchers then worked with the UDRI team to model this new weave using a novel finite element approach in ABAQUS, a finite element analysis software suite that uses embedded elements. ABAQUS’ embedded elements enabled detailed modeling of the complex architecture, which established mechanical performance estimates that the team will use to baseline empirical results and update vehicle conceptual design application studies. In addition, the researchers performed preliminary analyses characterizing the Lorentz forces induced during structural capacitor system discharge. These forces arise from electrostatic and electromagnetic fields and can induce significant mechanical stresses in the dielectric layers. The team’s parametric studies characterized both physical and operational effects on the stress levels induced into the dielectric material.

The researchers also developed and demonstrated several approaches for busing energy into and out of the structure. These methods encompass various attempts at designing an integrated bus structure capable of transferring critical loads through joints to prevent disruption of the load path. Technicians have fabricated prototype panels to validate these approaches, and researchers are using validation results to optimize structural performance and develop improved fabrication methods and bus integration designs. It is likely that implementing this technology in the future will involve integrating an active switching network into the bus system. This type of network would allow the creation of parallel networks of series circuits to provide power handling tailored to weapon system requirements. This network would offer another advantage as well: if any loadbearing capacitor elements were shorted due to in-service structural damage, the active network could eliminate those components from the circuit.

Developing this technology is an important step toward fulfilling future AF needs. One day, engineers will use AFRL-developed capabilities to design smaller air vehicles capable of delivering short pulses of electrical energy to power directed energy weapons and provide necessary power for air vehicle subsystems. These future air vehicles will play an important role in tomorrow’s AF and will forever change the face of warfare.

Mr. William Baron and Ms. Melissa Withrow (Azimuth Corporation), of the Air Force Research Laboratory’s Air Vehicles Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp. Reference document VA-H-06-06.

Posted in: Briefs, Electronics & Computers
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Posted in: Briefs, Materials
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Posted in: Briefs, Materials
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Posted in: Briefs, Materials
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Posted in: Briefs, Photonics
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Posted in: Briefs, Medical
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Posted in: Briefs, Software
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Posted in: Briefs, Medical
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Posted in: Briefs, Software
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Posted in: Briefs, Electronics & Computers
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