Energetic materials and munitions are used across DoD in mission critical applications such as rockets, missiles, ammunition, and pyrotechnic devices. In these applications, energetic materials and munitions must perform as designed to ensure success in both training and combat operations. Energetic materials consist of fuels and oxidizers which are intimately mixed. The material may also contain other constituents such as binders, plasticizers, stabilizers, pigments, etc. Since the invention of black powder the technology for making solid energetic materials has remained either the physical mixing of solid oxidizers and fuels, referred to as composite energetic materials (e.g., black powder); or the incorporation of oxidizing and fuel moieties into one molecule, referred to as monomolecular energetic materials (e.g., trinitrotoluene, TNT).
In a paper from the Los Alamos National Laboratory, Alex Mueller is leading a team to create the next-generation of explosives using 3D printing. By examining the microstructure and manipulating internal hollow spaces of TNT, the scientists are trying to control and tailor a new form of explosives. TNT is susceptible to unplanned detonation. This led the team at Los Alamos to develop insensitive high explosives (IHE). These explosives can be hit with a hammer, dropped, or thrown into a fire and not detonate. This might ruin some Hollywood movies, but it might also ruin a timed detonation.
Making an explosive more difficult to detonate when there’s an accident also makes it more difficult to detonate intentionally. Explosives such as TNT’s behavior are largely controlled through hot spots. Introducing inclusions, such as air bubbles, into TNT will trap air inside, causing it to compress and rapidly heat up. The uneven flow into and around these bubbles results in points of intense heat called hot spots. These hot spots largely control the energy necessary to initiate detonation in TNT and other high explosives. This is where 3D printing is disrupting explosives. With the ability to control material, and voids, Mueller’s team looks to control the release of energy through a sophisticated arrangement of hot spots. “The ability to tailor sensitivity and the resultant energy release in the chemical reaction zone would be a holy grail in detonation physics research,” says Dana Dattelbaum, a Los Alamos detonation expert. “Control and manipulation of structures at the microscopic scales through 3D printing is an exciting step toward achieving these goals.”
3D printing lets the material be manipulated at a mesoscale, which is difficult to do with conventional methods. Printing in mesoscale layers eliminates variables and unpredictable hot-spot areas. Los Alamos is using fused-deposition modeling and an optically cured method of 3D printing to make explosives safer. However, the real work/excitement isn’t that the team is revolutionizing the process, but that they are working on all new materials. Like many solutions today, the work seems to be in the material science—a trend that is echoed in research over 2,000 miles away.
Limitations of traditional manufacturing
Since the invention of black powder the technology for making solid energetic materials has remained either the physical mixing of solid oxidizers and fuels, referred to as composite energetic materials (e.g., black powder); or the incorporation of oxidizing and fuel moieties into one molecule, referred to as monomolecular energetic materials (e.g., trinitrotoluene, TNT).
The basic distinctions between these prior known energetic composites and energetic materials made from monomolecular approaches are as follows. In composite systems, desired energy properties can be attained through readily varied ratios of oxidizer and fuels. A complete balance between the oxidizer and fuel may be reached to maximize energy density. Current composite energetic materials can store energy as densely as >23 kJ/cm.sup.3. However, due to the granular nature of composite energetic materials, reaction kinetics are typically controlled by the mass transport rates between reactants. Hence, although composites may have extreme energy densities, the release rate of that energy is below that which may be attained in a chemical kinetics controlled process.
In monomolecular energetic materials the rate of energy release is primarily controlled by chemical kinetics, not by mass transport. Therefore, monomulecular materials can have much greater power than composite energetic materials. A major limitation with these monomolecular energetic materials is the total energy density achievable. Currently, the highest energy density for monomolecular materials is approximately 12 kJ/cm.sup.3, about half that achievable in composite systems. The reason for this is that the requirement for a chemically stable material and the current state of the art synthetic procedures limit both the oxidizer-fuel balance and the physical density of the material.
Traditional manufacturing of energetic materials involves processing granular solids into parts. Materials may be pressed or cast to shape. Performance properties are strongly dependent on particle size distribution, surface area of the constituents, and void volume. In many cases achieving fast energy release rates, as well as insensitivity to unintended initiation, necessitates the use of small particles (.ltoreq.100 .mu.m) which are intimately mixed. Reproducibility in performance is adversely affected by the difficulties of synthesizing and processing materials with the same particle morphology and distribution uniformity. Manufacturing these granular substances into complex shapes is often difficult due to limitations in processing highly solid filled materials.
An example of an existing limitation of processing granular solids is in manufacturing energetic materials for detonators. The state-of-the-art now requires the precise synthesis and recrystallization of explosive powders. These powders typically have high surface areas (e.g., >1 m.sup.2/g). The powders are weighed and compacted at high pressures to make pellets. Handling fine grain powders is very difficult.
Dimensional and mechanical tolerances may be very poor as the pellets may contain little or no binder. Changes in the density and dimensions of the pellets affect both initiation and detonation properties. Manufacturing rates are also low as the process is usually done one at a time. Certification of material is typically done by expensive, end-use detonation performance testing and not solely by chemical and physical characterization of the explosive powder. As these detonators or initiating explosives are sensitive, machining to shape pressed pellets is typically not done.
Another current limitation is producing precise intimate mixtures of fuels and oxidizers. The energy release rates of energetic materials are determined by the overall chemical reaction rate. Monomolecular energetic materials have the highest power as the energy release rates are primarily determined by intramolecular reactions. However, energy densities can be significantly higher in composite energetic materials. Reaction rates (power) in these systems are typically controlled by mass transport rates of reactants.
3D Printing with Explosives
Researchers at Purdue have developed a method that deposits tiny amounts of energetic materials (explosives, propellants, and pyrotechnics) using the same technology as an inkjet printer. This research combines the university’s expertise in both energetic materials and 3D printing. Many micromechanical systems incorporate energetic materials in their operation. For example, an automotive airbag deploys using a small amount of solid propellant. But as devices get smaller, the need for micro-level energetics becomes more critical.
“It is really just an assembly of commercial-off-the-shelf technologies,” says Jeffrey Rhoads, professor of mechanical engineering and principal investigator on the project. “The key is formulating the proper ‘ink’ and then integrating these components in a way that allows for appropriate mixing, precise printed geometries, etc. “Our solution was to combine two components as we print them,” “We can have a fuel—aluminum—and an oxidizer—copper oxide—in two separate suspensions, which are largely inert. Then, with our custom inkjet printer, we can deposit the two in a specific overlapping pattern, combining them on a substrate to form nanothermite.”
The mount of explosives is only in the picoliters, so it was a challenge to achieve the right droplet volume and pattern. The other challenge was designing a printer that could deposit droplets accurately. The researchers’ solution was to use a tube surrounded by material that flexes when a voltage is applied. The flexing action squeezes the tube, creating picoliter droplets. Varying the voltage varies the size of the droplets and users can increase or decrease the size of the droplet in 0.1-micron increments.
The modified piezoelectric inkjet printer moves with one-tenth-micron precision by holding the nozzle still and moving the print bed. Working in picoliter volumes means that controlling droplet size is imperative. Deformation of the piezoelectric material is controlled through the voltage applied to it. The piezoelectric material is connected to a tube; by altering the voltage, researchers control the droplet size. The Purdue machine holds the tube and nozzle stationary and moves a stage below it to form whatever shape is required. The stage can move with 0.1 micron precision. The resulting nanothermite reacts just as quickly and powerfully as thermites applied in traditional ways. It burns at 2,500 Kelvin (over 4,000 degrees Fahrenheit), generates a lot of thrust and heat, and makes a nice loud shockwave.
Thermite produces high heat and thrust. Using 3D printing to control it could lead to applications we haven’t discovered yet. For now, it could be used in airbag ignition systems, and many applications in industrial and military defense.
DSTL UK, has started to develop 3D printed explosives
The Defence Science and Technology Laboratory (DSTL), an executive agency sponsored by the Ministry of Defence (MOD) of the UK, has started to develop 3D printed explosives. The project aims to create new possibilities for various explosive effects using intricate designs enabled by 3D printing, and also to reduce transportation and storage costs. Applying additive manufacturing to produce new energetic formulations is part of the MOD’s Future Energetics Project, established to train experts and develop new technologies. 3D printing explosives offers numerous benefits for potential users, including reducing storage and transport costs, and enhanced performance with reproducibility. Charges can be printed on demand, bespoke to requirements, in novel and intricate designs previously impossible to manufacture.
To reduce cost and improve weaponry efficiency, the Australian government has awarded USD $2 million to research institutions to develop methods to 3D print energetic materials. Professor David Lewis, a polymer specialist at the awarded Flinders University, said, “The ability to develop systems like this is the next generation of additive manufacturing and is the key to this technology becoming mainstream.” In Indiana, professors of Purdue University have 3D printed energetic materials without voids. Professor Jeffrey Rhoads commented, “We have shown that we can print these energetic materials without voids, which is key. Voids are bad in energetic materials because they typically lead to inconsistent, sometimes catastrophic burns.”
The energetic formulations for 3D printing are manufactured in a LabRAM resonant acoustic mixer, which uses acoustic energy rather than physical blades to mix materials, making it safer and more efficient to use. Many organisations are looking at different stages of 3D printing, but Dstl is the only place in the UK that is working on an end-to-end process of this kind with high explosives. The 3D printing project is currently in testing stages, mainly focusing printer capabilities and material extrusion to then move on to examining explosive characteristics of a print including utilising charge geometry to create different explosive effects. Understanding what shape has what effect could lead to bespoke printing for individual missions in a warzone, providing an amplification of an effect with less material. As 3D printers give limitless possibilities, this is a real breakaway from traditional explosives.