Custom Manufacturer Creates Multi-functional Components with Unique Foam Material
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High-tech parts applications for the highly porous foam material range from fuel cells to aerospace components, orthopedic testing devices, and semiconductor manufacturing equipment.
We previously featured the first in a two-part series of Q & A articles on ERG Materials and Aerospace Corporation's open-celled foam material. In Open-celled Foam Evokes New Design Possibilities for High-performance Parts, Bryan Leyda, ERG's engineering manager and chief engineer, discussed some of the unique characteristics of the company's Duocel® foam materials in an interview with David Gaines, contributing writer for Design-2-Part Magazine.
Described as "an open-celled reticulated structure with interconnected porosity," Duocel foam is similar to a three-dimensional mesh and can be made from a variety of base materials, including metals, ceramics, and carbons. One of the advantages of the open-celled foam material, Leyda said, is that it enables the creation of multi-functional components.
The aluminum foam material has been used as a lightweight core structure and heat transfer agent in the solar collectors for a solar-powered vehicle. It was also used as the metal hydride powder support matrix in the hydrogen storage system for the H2Fuel Bus project. The objective of the project, sponsored by a coalition of academic, government, and private industry partners, was to build a prototype hydrogen-fueled, electric-powered transit bus with near-zero emissions. According to ERG, the interconnected porosity of the foam and its excellent thermal conductivity were critical factors in the hydride tank storage system.
Following is the second half of the series on ERG's open-celled foam material, featuring additional excerpts from the interview with Bryan Leyda and ERG's project engineer, Dana Ream.
D2P: Please tell us more about the H2Fuel Bus application.
BL: This was a hydrogen fuel-cell bus that was developed jointly by the Southeastern Research Association and the Savannah River National Laboratory. The bus was running around Atlanta when they had the Olympics there in the mid nineties. The intent was to contain the hydrogen by absorbing it in a hydride powder in a low pressure bottle rather than in a potentially dangerous high-pressure bottle.
These hydride powders are a type of material that will absorb and desorb gases readily. They are somewhat analogous to desiccants used in packaging that absorb water vapor. Hydrides will absorb a lot of hydrogen at relatively low vapor pressures. The major problem is to refuel and get the hydrogen gas into the bus fairly rapidly. This is an exothermic process, so as they absorb the hydrogen, the hydride powder gets very hot and stops absorbing. Accordingly, they had to have a way of cooling the powder. By the same token, when the bus is running, and the hydride is under low pressure, it releases hydrogen gas, which is an endothermic process. During the endothermic process, the gas and hydride gets very cold. If it gets too cold, it stops releasing hydrogen. So they have a certain thermal sweet spot where they can absorb and desorb efficiently.
The key is to temperature-control this hydride powder. To achieve this, they stored the powder in a tank that is filled with our aluminum foam. The aluminum foam provides a number of functions. By heating and cooling the outside of the tank, the temperatures are conducted by the foam to the inside of the tank. The foam, acting as a high surface-area heat exchanger, then helps to control hydride powder temperatures inside the tank. The foam also disrupts the packing factor and provides uniform hydrogen gas flow through the entire bottle.
D2P: Do you see any further applications for Duocel foam in components, modules, or systems in the emerging solar power industry?
BL: Applications in the solar power industry could become more prevalent in the future, both as a structure to support solar cells or as an energy absorber, or even in a combination of solar cells and fuel cells. One of the projects we got involved in was for a large array of mirrors on the ground, which were focused on the top of a pole that had a heat exchanger that was supposed to absorb solar energy. The original finned heat exchanger was basically “fried,” so we used a piece of silicon carbide foam that was actually a spherical ball. Gas was run through this device and it flowed out spherically, as the mirrors would concentrate on the ball of foam to heat it up. In this way, you had a spherical flow heat exchanger. There really wasn't any other way to effectively manufacture this type of heat exchanger.
D2P: How about in other cleantech applications, such as fuel cells and batteries?
BL: We're doing some work with fuel cells right now, where we're working with a company that was able to replace about three or four components using our foam. Fuel cells must have a device that allows hydrogen to pass through a structure into an operative membrane, which has to pull off the water vapor, pick up the electrical power, and also keep the device cool. So our foam was used as a heat exchanger to handle all of these functions. It contacted the membrane to pull off the heat, and then conducted the heat into the foam where it could be kept cool by the passing air. It also acted as an electrical current conductor to bring the energy to single-point pickup electrodes, and it served to isothermalize the entire structure.
Fuel cells are usually hot at one end and cool at the other, depending on which way they are bringing in the gases. Our foam structure isothermalizes the membrane, so the whole thing is operating at the same electrical potential. The foam also acts like a wicking material to wick the water moisture out of the fuel cell. So again, open-celled foam provides multifunctional properties.
D2P: Is this type of foam the only thing like it on the market right now?
BL: There are a number of people in the porous materials business, but we are all using the technology in different ways. The market for ordinary plastic foams, like for ice chests and seat cushions, is huge, but the market for our foams is fairly small. Our foams tend to be expensive, so they are only used in very special applications, such as custom-designed, high-tech, high-precision applications for the military. Our foams are also used by the aerospace industry and for semiconductor manufacturing equipment.
D2P: How is this product manufactured?
BL: In terms of the actual manufacturing of the foams, it is a proprietary process. There are a variety of manufacturing processes we use to make these foams, but I can't really get into what they are. Copper foam, for example, can be produced using two or three different technologies. And we do use many different secondary processes, such as heat treating, brazing, and highly-modified machining processes, depending on the nature of the foam.
One of the nice things about heat-treating this material is that you can get a very uniform quench. For example, with a piece of solid aluminum, you can very easily quench the outer portion of the metal, but the inner sections cool much more slowly, so you get a very non-uniform heat treatment. Whereas with a large billet of our foam, you can get a very even heat treat because there are no ligaments in the structure that are more than four thousandths of an inch in diameter. So you don't get the kind of residual stresses you would get in normal heat-treating. We also perform cryogenic treating for products like the optical mirrors to minimize residual stress.
In terms of how we make the foams, we first resolve the material to a liquid state if we are making a primary foam, say, a copper or aluminum foam. If a customer needs an expensive gold or platinum foam, there is no reason to set up a whole production line to make it. We would make a secondary foam structure by first making a primary foam, like aluminum, and then plating the gold or platinum onto that foam structure, which usually only requires a thin layer a couple of angstroms thick.
D2P: Since this is not a mass-produced product, do you offer design and engineering for your customers?
BL: Yes, we do offer design and engineering services. We don't sell bulk, raw foams because the design and fabrication characteristics tend to be so unique to our foams, it would be a waste of time to sell it to someone. It's better for them to use our 40 years of experience to design something that we know is going to work. We have a Ph.D. on staff for thermal work, and mechanical engineers for everything else.
We start the process by talking to a customer--not by picking a material--to determine what type of function they are trying to perform. The first reason is to determine if it even makes sense to use foam structures. We've been doing this for a long time, so we can very quickly determine if this will work for them, either technically or economically. We would then set out to design the part for them.
D2P: How do you make a design engineer's job easier?
BL: We can make a design engineer's job easier because it gives them a much wider range of design options with so many materials to choose from, and the ability to vary the pore size and the relative density of the materials. The other thing is, since we don't sell the raw foam, but rather offer customers our design services, all a customer's design engineer has to say is, 'I'm trying to perform a particular function, what can you do for me, and does foam make any sense for this application based on my performance requirements?' If it does, we can quickly develop a design solution for his part or component, and then make prototypes for quick testing.
D2P: What's your biggest market?
BL: Our largest markets include heat exchangers for semiconductor, aerospace, and aircraft applications; optical devices; and fuel cells for several industries. We're also starting to produce more structures for bone simulations for orthopedic applications.
D2P: What types of orthopedic applications has ERG served?
Dana Ream: As far as the orthopedic applications are concerned, we haven't made anything that is implanted in a person. Rather, several of the different structures that we make are used to test orthopedic products because they can simulate the trabecular bone. Our foams can be used to simulate bone material, which is very expensive and difficult to obtain.
For example, a customer might have a new, adhesive bone putty that's used to attach an artificial joint, and would want to test the product to make sure it will flow through the trabecular bone. Or perhaps they might want to test their technique for injecting it. There was even one study where they tested some bone putties that give off heat when they cure. If they give off too much heat, it can kill some of the osteoplasts inside the bone. So they performed a test where they injected a fixed amount of putty into the bone to see how much heat it was giving off. They will use our aluminum foam product for this test because it conducts heat so well. So our foams allow them to use their tools and instruments, or to study the behavior of bone putties and attachment methods.
For more on ERG Materials and Aerospace Corporation, visit www.ergaerospace.com.
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