Aluminum Cluster Materials Are the Future of Energetics

Cluster 2

By Dr. Dennis Mayo

Former Marine Corps Commandant Michael Hagee described energetics as critical to “battlespace dominance.” Over the past century, researchers have sought to increase the energy outputs of explosive and propellant materials in weapons. Traditional monopropellants (e.g., TNT, RDX, and HMX) are organic molecules—compounds composed of carbon, hydrogen, nitrogen, and oxygen. The principles of organic chemistry tell us that there are many ways of arranging these elements within a molecule. The big challenge scientists and engineers have faced is that although there are many possible permutations of molecular structure in organic compounds, their energy outputs are limited by their densities and heats of formation. Therefore, other novel materials such as fast-reacting metals are needed to realize greater range, quicker delivery, and increased energy imparted to targets. Higher energetic output could result in smaller and lighter munitions and increased range and lethality in current weapons.

The discovery of metal clusters (subnanometerscale compounds containing a core of metal atoms surrounded by an organic outer shell) and the applications arising from their unique physical and chemical characteristics is an active field of academic research. What is largely unexplored is the strong potential use of metal clusters as critical ingredients for weapon systems. To that end, our research team at Naval Surface Warfare Center Indian Head Explosive Ordnance Disposal Technology Division collaborates with our colleagues at Johns Hopkins University, the University of Maryland, Naval Postgraduate School, Naval Air Warfare Center China Lake, and Naval Research Laboratory to study that potential.

Our basic research program is relatively young (less than six years old) and already has made significant advances. We have fabricated specialized instrumentation required to study these materials. We also have made and characterized small quantities of the largest aluminum cluster compound (a molecule with 77 aluminum atoms arranged in a series of shells) using a different methodology than the original report. And we have investigated the basic properties of these materials, finding that they can be chemically compatible with chemical environments such as those in explosive fills and demonstrating these materials’ potential as formulation additives.

We need energetic materials that have high energy density and a rapid kinetic release of that energy on combustion that packs the biggest punch we can muster. Metal clusters have a great potential for that use.

Higher Potential: Away from CHNO

The underlying idea in our research is to move away from combinations of only carbon, hydrogen, nitrogen, and oxygen (CHNO) to increase combustive output. The majority of elements in the periodic table are metals, and when most metals sit out in the open, they form oxides. This transformation—metal and oxygen combining to form metal oxide—is thermodynamically favored and often exothermic. That exothermicity, or the release of heat upon oxidation, is useful from an energetics standpoint. For most metals, the heat of combustion is high, and for some elements—namely boron, silicon, and aluminum—it results in extremely high energy density. These elements are great candidates for energetic fill materials, and some have been put to use in current energetic formulations.

Ideally, one would get as much energy as possible out of these materials on a timescale that is useful for detonation or combustion. There are scientific challenges, however, to the energy output available from bulk metals. For example, decades of research were put into developing boranes (boron hydride materials) for energetics use, only to discover that boron compounds do not fully combust in air because of a kinetically stable partial combustion product—i.e., they do not burn all the way down to boron oxide, and they do not release all of their potential energy as heat. The challenge for aluminum energetics is different. Aluminum combusts well under the right conditions, but it has a native oxide layer that makes the kinetics of aluminum combustion slow. That oxide layer can be advantageous (its presence is why the aluminum foil in your kitchen and your laptop at work do not spontaneously combust or decompose), but it also is a significant challenge from an energetics standpoint. If you watch thermite reactions on YouTube, you will see that the heat release of aluminum oxidation is quite impressive, yet rather slow.

The kinetic barrier to aluminum combustion has been a challenge for a long time, and there have been many attempts to get around this obstacle. One such method is making smaller and smaller aluminum particles with the hope that they may burn more quickly. This method is not without its own challenges: when aluminum particles are a few nanometers in diameter, the oxide layer represents the majority of the aluminum in the particle. The nonoxide (or fuel) portion of the particle becomes increasingly smaller in terms of mass percentage of the particle. We need to come up with clever ways to circumvent this challenge.

One possible way around it involves what we refer to as aluminum cluster materials, or molecular species that are smaller than nanoparticles, with roughly the same molecular size as traditional CHNO fuels. The aluminum cluster materials reported in the literature bear a structural resemblance to bulk aluminum metal: the coordination around the aluminum atoms is often only slightly distorted from that of the bulk metal. That structural resemblance gives us hope that the materials may behave similarly to the bulk metal.

Cluster 3

Synthesis through Aluminum

Aluminum cluster materials are highly challenging to synthesize and characterize, and one specific synthetic method has been crucial to their discovery in gram quantities. German researchers at the Karlsruhe Institute of Technology, led by Professor Hangeorg Schnöckel, have reported on the synthesis of a few incredibly interesting cluster materials containing shells of aluminum atoms: little balls of fuel waiting to have their energy released. These cluster compounds, which have as many as 77 aluminum atoms and up to four shell layers, are prepared using an aluminum monohalide starting material. These materials have been calculated to have high volumetric heats of combustion and specific impulse values, and should release their potential aluminum oxidation energy quickly. Aluminum monohalide starting materials are quite unusual—aluminum prefers to be in either the 0 (metallic) or +3 (aluminum oxide) oxidation state; intermediate oxidation states of aluminum are exceedingly rare. Monohalides serve as a source of aluminum in the +1 oxidation state and are the only reliable synthetic pathway to these multi shell cluster compounds in the condensed phase.

The aluminum monohalides involved in this work are not accessible under standard atmospheric conditions; successful formation involves a high-vacuum/hight-emperature reaction chamber. In this chamber, aluminum metal is placed inside a furnace, the atmosphere evacuated, and the furnace heated to around 1,800 degrees Fahrenheit. Once the furnace is hot, hydrogen chloride gas is passed over the surface of the metal and gas-phase aluminum monochloride is formed along with hydrogen gas. The generated aluminum is then deposited along with solvent on the liquid nitrogen-cooled walls of the chamber. After a few hours the reaction is complete, the oven is turned off, and a cold solution of aluminum monohalide is drained into a cold bottle and stored at -112 degrees Fahrenheit for later use.

Screening, Planning, Projecting

The aluminum cluster materials discovered by the Germans are limited in terms of the reactants (ligands) that were mixed with the aluminum monohalides. Most contained one specific bis(trimethylsilyl)amide ligand that carries a negative charge on a nitrogen atom that binds directly to the aluminum clusters. This ligand has proven to work very well in stabilizing aluminum cluster compounds. In fact, it stabilizes the largest aluminum cluster compound reported to date—a four-layer molecule with 77 aluminum atoms. And depending on the ratio of aluminum to ligand used and the heat applied, different products form with this same ligand. In collaboration with the University of Maryland and Johns Hopkins University, we reproduced the largest of these cluster materials. The reproduction of the anionic Al77 cluster represents a huge (and unexpected) step: the precursor materials used by the Germans and used in our lab are different, meaning the 77-atom aluminum architecture may have some special stability relative to other possible molecular configurations, and may be synthetically accessible through other, more accessible methods.

In addition to reproducing materials from the literature, we thought it would be advantageous to expand the scope of ligands employed in these systems, both in terms of the kind of ligands used (e.g., what element is bonded to aluminum) and the ligands’ size (affecting how close the ligands can be to one another). By doing that, we hoped to find new cluster compounds. With that in mind, we have looked into a variety of systems to try and understand the influence of ligands on cluster size and shape.

Our initial use of phosphorous-based ligands supported what the German researchers found: that phosphorous-carbon bonds will break down in the presence of aluminum cluster compounds, resulting in oxidized aluminum and ligand fragmentation (i.e., no multilayer clusters are formed). Since that discovery, we have been working with theoretical physicists at the Naval Postgraduate School to understand better the electronic effects of the ligands used in our systems. This will help us eliminate what appear to be less promising systems before we mix any reagents together, saving time and effort.

The materials we are studying are quite novel, making their fundamental properties difficult to assess. Related compounds in the literature are limited, meaning there is not a lot out there about chemical compatibilities between these aluminum clusters and organic functional groups that are used frequently in energetics. To test these compatibilities, we made a series of ligands with a variety of organic functional groups and mixed them with aluminum monohalide solutions. We synthetically modified the functional groups in the ligand and left everything else in the system identical to minimize the variables involved with the studies. What we found is that a whole lot of functional groups, including nitro groups, are compatible with reduced aluminum materials under certain conditions. This is a really exciting result—it demonstrates a baseline of compatibility between very reactive aluminum compounds with some of the functional groups that are common in energetic fills. The results of these studies have given insight into what elements reduced aluminum prefers to bond to, and led us to investigate an entirely different class of ligand compounds. This information will be highly useful as we develop cluster materials for later use.

Next Steps

Over the course of the past five years of research, a tremendous amount of work has gone into understanding the conditions under which we can make aluminum cluster materials. We have discovered new compounds, reproduced others from the literature, and are investigating other methods by which we can synthesize and characterize cluster materials. Going forward, we aim to prepare novel materials and scale them up to investigate their fundamental properties (e.g., combustion behavior and sensitivities). Ultimately, synthesis of cluster materials will be achieved through scalable means.

Our collaborators at Johns Hopkins and the University of Maryland have recently been awarded a Multidisciplinary University Research Initiative grant through the Office of Naval Research, providing a sustained source of funding to an expert group of researchers to study methods to generate ordered assemblies of related metal cluster materials. We plan to work closely with them, building on our years of collaboration.

This work on metal-rich cluster chemistry is out at the frontiers of what is synthetically possible; it is, in a sense, exploration on a molecular scale. Every new compound that we have discovered gives us another reference point. From there, we work to find similarities and trends between materials and find a way to put these materials to use, ultimately getting to a stage where we are fielding novel materials for energetics usage in the field. The field of cluster science is fascinating, exciting, and incredibly challenging. These new metal-based materials should have higher energy content than traditional explosives, resulting in greater firepower for warfighters, whether that is manifested as greater ordnance range, lighter weapons, or a bigger “boom” in existing platforms.

About the author:

Dr. Mayo is an American Society for Engineering Education Science, Mathematics, and Research for Transformation fellowship recipient and scientist at Naval Surface Warfare Center Indian Head Explosive Ordnance Disposal Technology Division.