Electrical Interconnect Applications for Carbon Nanotubes in Military Aerospace Systems

Flexible, lightweight, and versatile CNTs are becoming a valuable material in conductor applications for the military and a host of other markets.

Carbon nanotubes are one of the most unique and interesting materials developed in the last decade. These products, widely known as CNTs, can be manufactured by various methods, but are most commonly made using chemical vapor deposition, a high-temperature manufacturing process used to create durable, solid, high-performance materials. The end result is a paper-like, ultra-thin sheet that can be further processed into a variety of forms suitable for a wide range of applications.

This material can become an electrically charged carbon sheet with some very special properties that are of great utility in conductor development. CNT sheets can be spun into fiber-like strands and twisted into various configurations that simulate copper stranding. An insulated jacket is then extruded on top of the carbon nanotube to create a wire. The resulting product is approximately one-eighth the weight of a typical copper conductor wire and has a strength-to-weight ratio 117 times that of steel. Additionally, because the material is fundamentally a type of plastic, it does not have the same fatigue characteristics as copper wire.

CNT technology can also be used to replace copper shields. By simply taking a thin CNT sheet and wrapping it around the wires to form a shield (see Figure 1), designers can achieve substantial weight savings over copper shields.

Challenges to CNT Adoption

Cost

CNT technology has had limited viability for use in many aerospace applications due to its higher initial costs. However, costs are coming down quickly. In the early development years of CNTs, the material cost around 500 times as much as copper. Today, the cost is closer to 10 times that of copper and, within the next five years, CNT is expected to command only a 20–30% price premium.

Electrical Conductivity

The electrical conductivity of CNT technology is vital to its overall success in electrical interconnect applications. In the early research and development phase, the resistivity of electrically charged CNTs was about 200 times that of copper. More recently, that value dropped down to only 20 times that of copper. Now samples from manufacturers are getting closer to 10 times the resistivity of copper. The current objective for leading developers of this technology is to bring it down to five times that of copper. If that happens, it will be a real game-changer.

EMI Protection

Lightweight shielding is very important to the defense industry, and CNTs show great promise here. Currently, CNT shields exhibit performance similar to copper at frequencies above 1GHz, but performance drops off significantly, especially at frequencies below 100Mhz. The weight savings over copper can be as much as 80%, though, so there is a trade-off that largely depends on an application’s EMI requirements. Solutions include using a combination of a CNT shield with a smaller-gauge copper shield, which could still reduce weight and perform reasonably well.

Termination

A lot of investigative work has been done on both crimping and soldering CNT material. To date, the crimp method shows the best results from a viability standpoint. Current mil spec standard tools and crimp contacts

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Connectors

Initial research into using CNTs to advance connector technology has focused on two primary areas: coaxial cables with RF connectors and mil standard 1553 data bus cables with dedicated discrete connectors. There are also efforts to evaluate CNTs with standard 38999 Series III connectors, as well as with miniature push-pull circular connectors commonly used in soldier systems. With the exception of the RF connectors, the preliminary results are encouraging, and show viability. However, this application may require some minor modifications to either the contacts or to the inserts. More research is still needed.

Manufacturers

The development of carbon nanotubes is truly a global effort led by several universities, including Rice University in Houston, that collaborate with CNT producers. Over the last few years, manufacturers in the US, Japan, and other countries have significantly scaled up production to help reduce costs to the point at which CNTs can become a viable multi-use product. These suppliers of CNT yarns and sheets are also working closely with several wire manufacturers to produce primary wire, cables, and RF coaxial cables.

Electrical Applications

One immediate application being evaluated by the defense industry is the replacement of copper-based 1553 database cables with CNT conductors and shields. The weight savings they offer can be upwards of several pounds, which is an especially significant advantage for space applications, as each pound of payload typically costs upwards of $10,000 to launch into space. Electrical testing has already been conducted to determine the signal integrity loss of CNT conductors and shields and has returned surprisingly good results, showing little signal loss degradation on lengths up to 10–15 feet, which will only improve as the conductivity of CNT materials continues to improve.

In addition to the space market, two helicopter manufacturers in the United States are conducting independent test studies to determine the viability of CNT materials for use in the rotary wing market.

A Bright Future for CNTs

CNT’s unique set of properties is helping the material find a place in applications across nearly every industry. CNTs are currently being employed in and evaluated for applications including optical power detectors, radar absorption, microelectronics, transistors, thermal management, solar panels, and even body armor. With this versatility, the future looks extremely bright for carbon-nanotube-based products.

Original story by by Tom Briere on June 20, 2017

Mil-Aero Industries Eye Carbon Nanotubes as They Target Cost Savings

Ultra-lightweight carbon nanotubes may replace copper wires.

Today’s aerospace and aircraft industries focus on size, weight, power, and cost (SWaP-C), and cost is now often figured for program or operational life, which may total thousands of dollars per pound. This gives tremendous impetus and justification to accept high-cost new technology to obtain weight savings.

Reducing F-35 by 20 Pounds Could Provide $230M Savings

Satellites have always paid extra to reduce weight since each payload pound may cost more than $5,000 to launch. Studies by the Center for Strategic and Budgetary Assessments (CSBA) show that the new F-35 has a $4,500 cost per pound over the aircraft’s operational program life

until 2070. (For comparison, the cost per pound for the F-22 is estimated at $3,500.) The F-35 has projected production of2,557 aircraft for the U.S. and nine for export customers scheduled through 2037. Therefore, a weight reduction of just 20lbs per plane could result in savings of $230,000,000! Even if this is off by 50%, the expected benefits already are driving new industry developments.

In addition to fighter aircraft, each ounce is also critical in future soldier wearables, UAVs, portable radars, vehicle communications, and other equipment to increase survivability, mission endurance, and success.

Interconnect weight savings are being obtained by incorporating higher contact density, composite materials, combinational multi-port connectors, and other approaches. However, a new technology involving carbon nanotubes (CNTs) is emerging and offers a lightweight alternative to copper wire and other conductive shielding materials. A carbon nanotube is produced as a layer of carbon atoms in a tubular configuration, in single- or multiple-walled versions.

CNTs are being mixed with polymers to create high-strength, lightweight composite materials. CNT fibers can be made into conductive sheets and tapes, which offer a myriad of potentials. Optimal performance may result from spinning CNT fibers into conductive threads (referred to as yarn) to potentially replace copper wires in harnesses, motor windings, and shields.

Another important gain is reliability. CNT fibers and yarn can withstand millions of bending cycles, while standard fiber/wire would have yielded many times. The minimum bending radius requirements of today’s cable is not applicable for CNT fibers and cables.

Market potentials for CNT technology are bringing new companies into the forefront. Nanocomp Technologies offers commercial CNT fibers created using a carbon vapor deposition (CVD) reactor and then formed into sheets or fibers that can be twisted into shields or primary conductors. Another supplier is Syscom Advanced Materials Inc., which provides a variety of metal-clad fibers.

DexMat Inc. in Houston produces CNT fiber using a wet acid process that draws multiple fibers that can be shaped into a shield or primary conductor, and future developments for  include flat tape. The company boasts a strong Ph.D. cadre from nearby Rice University where they have successfully fabricated coaxial cable inner and outer conductors by coating a solution of CNTs in chlorosulfonic acid to achieve a two-times better conductivity than seen previously. This may prove an attractive alternative to commercial coax cable using tin-coated-copper with comparable attenuation and greater mechanical durability with 97% reduced mass, according to the company.

Usually, the outer conductor is the heaviest portion of today’s cables. In coax, the outer conductor provides both signal transmission and electromagnetic shielding. While shielding does not require high conductivity in the outer conductor, signal loss (i.e., signal attenuation) through the transmission line is significantly affected by the conductivity and architecture of the outer conductor. The new solution-coated CNT outer conductors offer near-term application potentials. Several connector companies are reportedly studying termination techniques.

Carbon Nanotube Materials Provide Shielding

TE Connectivity has been working to use CNT materials for shielding and data transmission cables. In a paper presented at the 2012 IWCS Conference, Dr. Stefanie Harvey, senior manager for corporate strategy, reported that they had achieved greater than 50dB shielding effectiveness in the GHz range, and their “data transmission cables using a yarn format perform comparably to MIL-STD-1553.” In the January 5, 2016 issue of ASSEMBLY, Dr. Harvey reviewed how replacing the braid in RG-58 cable would reduce weight from 38.8 grams per meter (g/m) to 11.5g/m, while replacing the center conductor with CNT yarn would further reduce weight to 7.3g/m for a combined weight reduction of 80%.

Composites are used to replace heavy copper wire with metal plated aramid fibers for use in wire and cable EMI shielding. EMI shielding made with plated aramid fibers can reduce weight by as much as 80%, leading to major weight reduction depending on the size of the aircraft or satellite. Aramid fibers are a class of strong, heat-resistant synthetic fibers, the best known of which is DuPont™ Kevlar®, used in ballistic-rated body armor.

Carlisle Interconnect Technologies (formerly Micro-Coax Inc.) provides a unique weight-reducing EMI/RFI shielding solution using their proprietary high-strength ARACON® brand metal clad fibers. Ron Souders, technical director, Carlisle Interconnect Technologies, advises that, for typical applications, switching to ARACON allows a weight savings of 80% when compared to traditional metal braided or woven EMI shielding products. This offers the conductivity of an outer metal coating with the strength, light weight, and flexibility of aramid fiber.

*Note: DexMat also provides products for shielding applications not mentioned in this article.

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Ron Souders further explained that the specific gravity of aramid fiber is only 1.44g/cc, compared to copper at 8.9g/cc, and that, even with the addition of metal coatings, the specific gravity of ARACON fibers ranges from 3 – 5g/cc. The tensile strength (measured in kilopounds per square inch, or Ksi) of the aramid core (350Ksi) is from three to 10 times higher than that of traditional or high-strength copper cores, which typically span 35 to 95Ksi. Since ARACON fibers behave like a textile, they are far more flexible and compliant than metal.

Industry Standardization is Underway

The benefits offered by CNT fiber, whether as EMI/RFI shielding, signal or coaxial cable, or other new components, have prompted the Naval Air Systems Command (NAVAIR) in Patuxent River, Maryland, to sponsor the establishment of suitable “Military Specification for Conductive Carbon Conductors used in Aircraft Wiring,” eventually with QPL sources. The proposed formal qualification program should stabilize components and materials for future use.

CNT technology also was included in a recent multiple-day RF coordination meeting held in February by the Defense Logistics Agency (DLA) at the Defense Supply Center Columbus (DSCC). Suppliers of basic CNT materials, wire, cables, cable assembles, and signal and RF/microwave connectors are now working on both application-specific and generalized products to achieve the weight reduction and reliability benefits offered by CNT and other metallized fibers.

Original story by David Shaff – April 28, 2017