Open-celled Foam Evokes New Design Possibilities for High-performance Parts
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Heat exchangers, fluid control devices, and orthopedic products are some of the high-tech applications for a unique, highly porous material that can be customized to individual requirements.
This article is the first of a two-part series on ERG's open-celled foam.
ERG Materials and Aerospace Corporation, located in Oakland, California, has manufactured custom parts for the aerospace, defense, semiconductor manufacturing, biotech, and other high-tech industries since 1967. To meet the demanding requirements of these applications, ERG (the company's corporate name is Energy Research and Generation, Inc.) relies on the independently variable properties of a material that provides-among other advantages-low density, high surface area to unit volume, high porosity, and a high strength-to-weight ratio.
“The secret to our success is our proprietary Duocel® open-cell foam materials,” the company states on its website (www.ergaerospace.com). “These unique materials allow us to design and manufacture components that meet engineering challenges not possible with conventional materials.”
Rather than viewing Duocel as a replacement material for existing parts, ERG refers to it as a “new basic material” with unique properties that open up a whole new way of designing. But the company does not supply Duocel as a raw foam material for customers to use as they choose. Instead, the company designs and produces custom engineered components that incorporate Duocel material to meet the high-performance requirements of specific applications.
David Gaines, contributing writer for Design-2-Part Magazine, spoke with ERG's engineering manager and chief engineer, Bryan Leyda, about the company's Duocel foam materials. Also in on the discussion were ERG's Eric Benson, operations manager; and Dana Ream, project engineer. Following are edited excerpts from the interview, the remainder of which will be published in D2P's May issue.
D2P: How does Duocel foam material open up a whole new way of designing?
Brian Leyda (BL): Duocel foams open up a new way of designing because you’re dealing with an open-celled porous material that you can flow fluids through, yet it has structural strength. It’s basically more of a biological design process, where you can create products that have multiple functions, like an optical bench for lasers that is also a heat exchanger. Conventional materials and technologies typically don’t allow for multiple functions as readily.
D2P: In what ways does Duocel foam material exceed conventional technologies?
BL: Duocel foam exceeds conventional technologies in that it provides a structure that is different from other types of structures. It’s a foam, so it’s a lightweight material. We can make this foam out of a metal, ceramic, carbon, or virtually any material. This gives us a much broader range of physical and mechanical properties than you get with standard solid materials or plastic foams. By using rigid materials like ceramics and metals, you have a whole range of new capabilities.
The foams that we make are open-cell foams; closed-cell foams have limited uses. What we do is take closed-cell foams and open them up with reticulation. With open-celled foams, a fluid can be flowed through it. Now the foam can be used as a substrate for processing. Now you can braze it, coat it, and plate it.
The other way it exceeds closed-cell technology is that you can create structures and products. From a technical standpoint, they are not materials, like copper, aluminum, or Inconel, but are basically structures, like a truss bridge. With closed-cell foams, like [polystyrene foam], you are basically limited to an energy absorption or floatation structure. Once you open up the foam, you can create a lightweight, honeycomb core to use as a heat exchanger or a fuel tank.
For example, with the leading edge of an aircraft wing you could use a closed-cell core, but it would only be a simple structure. But if you made the same leading edge with an open-celled foam, you could make a wet wing to store fuel, or you could flow fuel through it to heat the wing for anti-icing purposes. So multiple applications are possible with open-cell foam; it enables multi-functional components.
While the physical properties, like specific heat and melt temperature, come from the material that it’s made from, the bulk of the material characteristics come from its structure. The Duocel structure is the same polygon structure you will get from soap suds or beer foam or from porous volcanic lava.
D2P: Why does Duocel lend itself to so many base materials?
BL: Because we’re using a variety of foaming processes to make these structures, we can use metals, ceramics, and carbons. There are companies who make products similar to ours, but to the best of my knowledge, there isn’t anyone who is doing exactly what we’re doing. A lot of the other firms start with a basic foam and then coat it with other things, like powdered copper. We foam the raw material directly. Like a geodesic dome, or Buckyball structure, each one of the ligaments is a solid, pure metal, or other material. Its characteristics and performance can be much more easily designed and predicted this way.
A group of our people invented this technology in the late sixties. One of the reasons that it’s not well known is that it was used for many years for Cold War military products, like nuclear weapons and reconnaissance satellites. Consequently, very few people in commercial industry knew about it until the mid-nineties. So now it’s getting into commercial aviation and semiconductors, and other industries.
D2P: What allows Duocel to work so well for high-performance, high-tech applications?
BL: It can do things that you can’t do with other types of material technologies. One simple case would be to create a very high-temperature structure. Let’s say you’re making a composite panel out of an epoxy-bonded honeycomb. The skins might be aluminum, but the bonding agent is some type of adhesive. When you get that type of structure up to 300 to 500 degrees, it is going to disintegrate rather rapidly. It’s also going to be outgassing hydrocarbons as the epoxy adhesives begin to degrade.
We’ve seen situations where there have been lightweight honeycomb structures with skins that were built for satellites and went into environmental test chambers prior to launch. When heated by sun simulators, they started to outgas hydrocarbons that would re-condense onto the satellite and destroy some of the optics or sensing devices. So they had to scrap those composite designs.
We made a design that was essentially identical, using lightweight aluminum foam. Because of the open-celled nature of our foam, we can braze these structures. So, even though we’re creating a composite structure, with a lightweight core and skins, we can actually use brazing technologies. With brazing, we can take the structure up to about 1000 degrees. So you don’t have a weak thermal link or hydrocarbon outgassing.
Taking the same type of structure that I just described, it can be turned into a very lightweight mirror for aircraft or spacecraft applications. In fact, there was a set of ERG mirrors that were used on a NASA satellite called Stardust. It was flown through the tail of a comet to get particles to bring back to Earth. The mirrors that guided the satellite through the debris tail of the comet were made of our aluminum foam, with aluminum skins polished as a mirror. Most mirrors in the aerospace industry are made of aluminum or other lightweight metals, since glass is too heavy for most applications.
When the satellite flew through the tail of the comet, it would be going roughly 20,000 feet per second, or about 8 times the velocity of a bullet from a high-powered rifle. If a particle hits a glass mirror at that velocity, it could shatter it. If a high-performance, open-celled, aluminum structure is used, it would just puncture a little hole through it, but it would still keep on functioning. To make these mirrors, we took aluminum foam and brazed solid aluminum skins to them, and then we polished the skins to make a very lightweight, damage-resistant mirror.
D2P: What makes Duocel useful in such a diverse marketplace?
BL: Our foams can be used in a diverse marketplace….for a lot of applications. Duocel can be used to make lightweight composite structures, or you can use it as a gas diffuser, or as an energy absorber. We make it for a lot of military systems and missiles that need energy absorption structures.
Duocel is a porous material that essentially mimics the trabecular bone that’s in our own skeletal structure. It’s a very natural, organic cellular structure that’s encountered in coral, wood, or bone, so it tends to find itself in many different applications. One example would be a heat exchanger, somewhat like the gills of a fish or the lungs of our own body. You can see this type of porous structure in many areas of Mother Nature. So, we’re essentially using Mother Nature’s structural models, but with materials that she doesn’t use, like metals, carbons, and ceramics.
D2P: Your website states that “Messiah College’s entry into the Sun Race Solar Powered Vehicle Competition in 1995 utilized aluminum foam as a core structure and heat transfer agent in the solar collectors.” What were the benefits of using aluminum foam as a core structure and heat transfer agent?
BL: Many electromagnetic devices, like solar cells or radar antennas, are very temperature-sensitive in terms of their performance. Therefore, if you can keep a solar cell or radar antenna cool, you can either get more energy out of it, or, in the case of the antenna, you can get a stronger electronic pulse or a better signal. In the case of the solar cells for the Sun Race solar vehicle, what we did was provide a lightweight aluminum core structure that allowed the students to pull air into the vehicle so the driver could breathe. It also cooled the solar panels through the hollow core that the solar cells were attached to so that they would produce higher voltages. This is a good example of the multi-functionality offered by this type of foam.
For more on ERG Materials and Aerospace Corporation, visit www.ergaerospace.com.
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