PYROPHORIC PASTA: PROMISING NEW MATERIALS AND PROCESSES FOR THE IR BATTLEFIELD
By Kendall Mills, Aaron Stern, and Clinton Plaza
The Advanced Materials and Technology Branch (AMTB) at the U.S. Army’s Combat Capabilities Development Command Armaments Center (DEVCOM-AC) at Picatinny Arsenal, NJ, has been quietly innovating with a special class of materials known as pyrophoric substances and finding interesting ways to exploit their properties. Pyrophoric materials, such as pyrophoric iron, spontaneously burn in contact with air, and the AMTB team specializes in harnessing this power. Though not obvious to the casual observer, the formation of rust (iron oxides) on the surface of iron is an exothermic reaction, meaning heat is produced. The reaction is not exactly energetic enough for one to notice his/her car slowly heating up as it rusts in the driveway; however, things get can much more interesting when a lot of rust is made in a very small space very quickly.
CUE THE NANOTECHNOLOGY
Nanomaterials have structures and features that are 100 nm or smaller in size (a nanometer is 1 billionth of a meter). Not only is this size much smaller than a human hair, it’s also much smaller than the wavelength of visible light. In addition, nanopowders and nanostructured materials have an exceedingly high surface area-to-mass ratio. To better understand why this is so, one can consider a cube of iron. If the cube is cut into finer and finer pieces, more surface area is exposed, even though the same amount of material is present. Moreover, if taken to the extreme, cutting the cube into 100-nm cubes exposes hundreds, if not thousands, of square meters of reactive surface area. In this way, the surface area-to-mass ratio increases exponentially as the material gets smaller. And nano-sized iron materials have so much surface that nearly every last iron molecule can, in a literal flash, spontaneously convert to rust when exposed to air.
Pyrophoric iron is actually already used in military applications for generating infrared (IR) light (such as shown in Figure 1), but the AMTB team has discovered uniquely cost-effective and environmentally friendly precursor materials to generate such materials. The group has also focused on innovation in bulk manufacturing these materials in tunable form factors and the products using them. And because the Armaments Center mission focuses on the scale-up and advanced manufacturing of new weapons technology rather than on pure basic science, Picatinny Arsenal is well-poised to help bring nanotechnology-based materials to the battlefield at strategic scale.
The thermal IR spectrum is an acutely important part of the electromagnetic (EM) spectrum as it is used for detection and tracking of many systems and targets, including aircraft. Most readers are familiar with gun camera footage using forward-looking infrared (FLIR) pods in which many systems and missiles (e.g., Hellfires) can home in on their target, even tracking them through the thickest battlefield smoke. One can readily see how a hot engine can easily be distinguished against the relative cold of the night sky. It is this thermal IR emission that is directly detected by “heat-seeking” systems, which are typically low cost, widely available, and easily portable. Thus, these systems have enjoyed significant proliferation and represent a real and present danger to adversarial aircraft.
This threat was arguably first made obvious to the public in the 1980s, as the Mujahidin successfully repelled the Soviet Union forces in Afghanistan. (Interestingly, as this article is being written, one can watch nearly the same thing happing on the news in near real time.) The threat is real; the proliferation of IR search and track (IRST) passive detection systems, even on fourth-generation fighters and air defense systems, is a particularly challenging development. IRST capabilities allow threats to track aircraft in an entirely undetectable manner, which adds an additional wrinkle to the battlespace. Accordingly, new materials and countermeasures are being prepared to address many aspects of the rapidly advancing IR battlefield.
Pyrophoric iron is traditionally produced by the caustic etching of bulk iron into a nanostructure. The etching process requires the use of extremely large quantities of toxic and dangerous chemicals, which produces waste products that have to be carefully handled to mitigate environmental impacts. This handling, of course, adds significantly to production costs. Alternatively, the aforementioned new materials (shown in Figures 2 and 3) sidestep this issue by heat treating precursor iron salts (compounds containing iron atoms but not metallic iron) and converting them directly to nanostructured iron through heat. And the only byproducts of this “activation” are carbon dioxide and water vapor, which can be safely vented directly into the atmosphere.
Admittedly, thermal decomposition of metal salts into high-surface-area metal is not new. However, the team has experimented with numerous additives and binders to alter or enhance the properties and aid in processing bulk quantities, ultimately finding that carbon-based binders such as high-gluten flour offered some unique and symbiotic benefits. So, leveraging the amateur chef pretensions of some of the AMTB group, a commercial pasta machine was purchased, and a new class of material (shown in Figure 4) was developed. This material—which some have affectionately referred to as pyrophoric “pasta” or even “Chaff Boyardee”—can be produced in quantity without elaborate and expensive manufacturing facilities.
One piece of equipment that may not be as familiar to some people is the inert gas glove box. As the nanomaterials react spontaneously with oxygen, it is necessary to perform all the manufacturing steps for activated materials in the inert gas atmosphere. One can think of a glove box as a hermetically pressurized container with an atmosphere scrubbing unit. Material enters and exits through an air lock, and the operator uses arm-length gloves to manipulate objects in the box. These glove boxes are widely used in numerous industries, the most notable being lithium battery production, as lithium will spontaneously react with oxygen and moisture in the atmosphere.
PROTOTYPING AND TESTING
The team has developed the capability to rapidly prototype pyrophoric products by leveraging advances in additive manufacturing (i.e., 3-D printing), as well as scalable industrial processes. The team’s processes and supporting facilities have enabled rapid design iteration and the capability to make kilogram-sized quantities of materials in several chemical and physical configurations in a single day. Furthermore, scalable production techniques have been a priority for the team as it focuses on small-scale equipment (e.g., pasta extruders and pharmaceutical granulation equipment) with easy upgrade paths to mass production (see Figure 5). In fact, research is being conducted on the smallest available equipment that will allow testable quantities to be produced and that will guarantee that equipment exists to manufacture (literally) tons of material at a time at production-efficient speeds. Furthermore, several patents have been granted to cover the subject materials, production, and component configurations, allowing the technology to remain entirely under the ownership of the U.S. Government.
The slowest and most expensive part of the entire endeavor described herein is testing, whether it be instrumented radiometric analysis or flight testing. While laboratory tests can provide useful information, use-case-representative flight testing provides critical data that cannot otherwise be collected. In addition, the team is currently looking for interested partners who have testing and collaboration opportunities that could assist in expediting development.
The new pyrophoric compounds mentioned herein (and shown in Figure 6) have additional benefits beyond just the previously described environmental waste, cost, and flexibility advantages. The carbon-bonded materials are also extremely lightweight, which can reduce the expulsion forces that can be a concern in some applications. The use of these materials could also open the door to equipping unmanned aerial systems with countermeasures without significantly impacting loiter times and payload and with minimal effect on size, weight, and power. Another benefit is that these materials are not pyrotechnic (usually categorized as a class 1 explosives), thus significantly increasing their safety and logistical footprints. As class 4 flammable solids, they are much safer and easier to handle and transport.
Flour, water, salt, heat—though there’s obviously more to the new pyrophoric pasta “recipe” than just these ingredients, the production process for these promising new materials has proven to be remarkably simple and flexible. In addition, the materials’ IR properties and performance can easily be adjusted by simply modifying the ingredients and ratios and/or changing the dimensions of the materials produced (much like the differences between linguini, spaghetti, and couscous). And the juxtaposition of cutting-edge nanotechnology and advanced manufacturing at the Army’s premier weapons research facility with the humble phenomenon of rusting iron has truly led to remarkable results. Who knew so much from the kitchen could be used on (and above) the battlefield?
ABOUT THE AUTHORS
Mr. Kendall Mills is a materials engineer and acting Branch Chief of the Advanced Materials and Technology Branch (AMTB) at the U.S. Army’s Combat Capabilities Development Command Armaments Center (DEVCOM-AC). He is experienced in synthesis and characterization of nanomaterials for a wide range of applications, from armor to countermeasures. Mr. Mills holds a bachelor’s and master’s degree in materials science and engineering from Rutgers University and a professional certificate in electromagnetic warfare technology from the Georgia Tech Research Institute.
Mr. Aaron Stern is a chemical engineer in AMTB, where he specializes in pyrophoric materials, composites, and materials characterization. He holds a bachelor’s degree in chemical engineering from Rutgers University and is currently enrolled in the Rutgers master’s program in materials science and engineering.
Mr. Clinton Plaza is a computer engineer in the Countermeasures and Flares Branch in DEVCOM-AC’s Pyrotechnics Technology Division, overseeing modeling and simulation and aircraft-based flight and integration testing of expendable infrared countermeasures. He holds a bachelor’s degree in computer engineering from Georgia Tech.