Unmanned airborne drones have played an important role in the war against terrorism. Less known are drones that were deployed underwater in the early stages of the war, searching for mines off the coast of Iraq.
The sonar- and video-equipped drones perform similar duties to those carried out by the Seabees in World War II.
An interesting materials technology provides the necessary buoyancy to carry the heavy equipment underwater with minimal power — an 11 kW-hr rechargeable Li-ion battery pack. Typical mission duration for the Remus 6000 underwater drone is 22 hours, subject to speed and sensor configuration. The autonomous underwater vehicle can go up to five knots.
"There are 13 syntactic foam pieces on the Remus 6000," says Thomas J. Murray, a senior materials engineer who cofounded CMT Materials of Attleboro, MA to explore new applications opportunities for the foam technology.
Syntactic foams are composite materials whose matrix is embedded with preformed particles such as glass or ceramic microspheres. They are called syntactic because the spheres provide an ordered structure.
"Sometimes we like to refer to it as reinforced air," says Murray. The reference to "reinforced air" is half humorous, but it gives syntactic foam unique characteristics compared to foam that is blown with chemicals or gases.
The hollow particles constitute half or more the volume of the composite, resulting in lower density, higher strength, resistance to compressive stress and a lower coefficient of thermal expansion.
One of the big benefits of syntactic foam composites is their ability to be tailored for specific applications. The matrix material can be many different types of metals, polymers or ceramics. The microballoons can be made from glass, carbon and polymers. Some of the most commonly used products are glass bubbles from 3M.
CMT Materials developed thermoplastic matrix materials in the 1990s for use as plug-assists in thermoforming dies.
"The thermoplastic materials allow much thinner sections because the epoxy composites can be quite brittle," says Murray. As a result, thermoformed packages used for electronics or medical devices can be made with thinner sections.