Introduction

For many years, propellant and explosive formulators have investigated the use of nanomaterials to increase performance and modify reactivity of energetic materials and related systems.  The potential energetic performance enhancements offered by nanoscale metal fuels include enhanced burning rates, easy ignition, higher specific impulse, improved combustion efficiency, and a greater potential for tuning performance through particle loading and size control [1–4].  The key advantages of a reduced particle size are tied to a high surface-to-volume ratio and short oxidation diffusion length, leading to enhanced reactivity.  With nanoscale particles, the rate of reaction is determined by chemical kinetics rather than mass transport.  Oxide formation plays less of a role in controlling combustion rate.  Consequently, the nanoparticles will react quickly at potentially lower temperatures than micron-scale particles.  The nanoparticles are more likely to react completely rather than leaving behind unspent fuel with an oxidized shell.  In some cases, these nanoparticles can increase the energetic yield of the system.

The small particle size can be a disadvantage, however, in that it creates new challenges with process scalability, long-term nanoparticle stability, safe handling, and particle agglomeration [1, 5, 6].  Because the particles are more reactive, they oxidize readily through contact with moisture and elevated temperature; they are subject to premature ignition by electrostatic discharge (ESD), impact, and friction; and they are nearly impossible to keep from agglomerating.  Primary particle sizes on the order of 20 to 50 nm are not uncommon, but there is usually a notable size distribution with agglomerates on the order of 200 to 300 nm or greater.  This size variation, which leads to performance variation, makes it extremely difficult to predict or control the particle morphology to better than a 50- to 100-nm resolution.  Additionally, the loading levels of nanoparticles in a given system are limited, as their high surface area leads to drastic viscosity increases that hinder, and sometimes prohibit, mixing.  Still, the promise for increased performance for energetics and power generation devices (such as batteries and fuel cells) drives continued investigation into new materials and processes.

In the energetics arena, the focus has mainly been on solid propellants, where the incorporation of nanoscale materials promises increased energy density and controlled energy release, while potentially improving sensitivity, environmental impact, and long-term stability.  In addition to the current use of nanoparticles to increase propellant burning rates, and reduce agglomeration of aluminum (thereby increasing combustion efficiency through heat feedback to the burning surface and reduction of agglomerates in the exhaust), these materials may soon be used in radical new propellant approaches that use three-dimensional nanostructures to control energy release and provide on/off capability.  Such characteristics would be of great interest to explosives formulators and warhead designers as well [7].

Polymer Nanocomposites

While the promise of the significant benefits of nanomaterials remains, capitalizing on these benefits has been somewhat elusive, primarily due to the lack of affordable, scalable processes for manufacturing high-quality nanoparticles in sufficient quantities to support high-volume production of nanomodified energetic materials.

There are many natural examples of organic and inorganic components combined at the nanoscale to construct materials with remarkable properties.  Examples include bone, crustacean carapaces, and mollusk shells.  Inspired by these natural materials, scientists are developing new synthesis strategies to produce multifunctional nanoscale materials.  In recent decades, material scientists have spent considerable effort investigating ways to combine an organic phase, typically a polymer, with inorganic nanoparticles, since research had shown that the addition of well-dispersed particles at the nanoscale allows significant tailoring of material properties, resulting in a new class of materials generally referred to as nanocomposites.  Polymer nanocomposites, composed of solid, inorganic structures uniformly dispersed at the nanoscale in a polymer matrix, have taken on high importance in a variety of industries, as scientists gain the capability to define nanoparticle characteristics such as shape, size, uniformity of dispersion, and loading [8].

Synthesis Routes

In general, polymer nanocomposites are prepared either by in situ synthesis of inorganic particles or by dispersion of fillers in a polymer matrix.  The processing technique is critical to obtaining nanomaterials exhibiting the desired properties.  Synthesis techniques are characterized as either bottom-up or top-down.  In a top-down approach, nanoparticles are synthesized by breaking down bulk materials gradually into smaller sizes, or patterning using physical methods, such as the dispersion of layered silicates in polymer matrices.  Examples of top-down processing include high-energy ball milling, cryochemical processing, and combustion synthesis [9].

Bottom-up methods, such as template synthesis, chemical (reactive) precipitation, chemical vapor deposition,  supercritical fluid processing, and sol-gel synthesis, result in the buildup of nanoparticles atom by atom, or molecule by molecule.  In other words, precursors are used to construct and grow well-organized structures at the nanometric level.

Figure 1 illustrates the difference between top-down and bottom-up production processes in terms of their relative effects on material properties and performance.

Figure 1: Bottom-Up vs. Top-Down Nanoparticle Production Approaches [10].

 

Top-down methodologies have disadvantages associated with high cost and significant potential for damage to the nanoparticles produced.  To achieve high-quality, affordable nanoparticles, bottom-up strategies seem to be the preferred approach.  Generally, there are two methods used to produce bottom-up nanocomposites:  (1) in situ polymerization in the presence of existing nanoparticles, or (2) in situ synthesis of inorganic nanoparticles in the presence of a polymer [8].

Recent Advances in In Situ Nanocomposite Processing

Two examples of organizations currently using the in situ synthesis approach to produce polymer nanocomposites for energetic materials are the Helicon Chemical Company in Orlando, FL, and the Cornerstone Research Group (CRG) in Dayton, OH.  These small businesses are involved in the research, development, and commercialization of advanced material and processing technology solutions to a variety of engineering problems.  Each is independently researching and producing various nanoparticle-doped polymer systems using (in the case of Helicon) liquid-phase formation and (in the case of CRG) reactive gas-vacuum evacuation techniques.  Between these organizations, a wide variety of particles can be produced on a scale large enough for large-scale propellant and explosive formulation.  Helicon and CRG can produce materials in 200-g and kilogram-sized batches, respectively, with further scale-up planned.  Particles can be produced in polymer matrices with a wide variety of sizes, providing a solution to the formulation difficulties of introducing dry nanoparticles into propellant mixes.

Helicon has developed a liquid phase chemical process to grow nanoparticles in situ in existing polymer binders.  The company’s process uses optimized reaction conditions that allow polymers and nanoparticle molecular precursors to combine in single-phase solutions.  The nanoparticles are then grown in situ by the bottom-up process of homogeneous nucleation.  The nascent particles become coated by the surrounding polymer, which limits particle growth and prevents aggregation and agglomeration.  This process generates homogeneous nanoparticle dispersions within the host polymer.  Because the nanoparticles are pre-formed in the polymer and never exist in a free state, the typical difficulties associated with nanoparticle mixing, handling, and safety are greatly reduced or eliminated [11].

For energetic materials applications, the company is developing products based on in-situ-grown nanoaluminum (nAl) and metal-oxide nanoparticles, such as TiO₂.  Hydroxyl-terminated polybutadiene (HTPB) R45M is the typical binder of choice for these materials, but the processes are adaptable to a wide variety of other polymers.  The in situ nAl accelerates burning rates and increases combustion efficiency of solid fuels and propellants.  By virtue of their small size, homogeneous dispersion, and lack of oxide coating, the nAl particles ignite and burn rapidly and completely.  This nAl combustion provides intense heat feedback to the surface of the fuel or propellant, which accelerates the burning rate.  By a different mechanism, TiO₂ increases the burning rate of composite propellants by catalyzing the gas-phase reactions of the oxidizer ammonium perchlorate (AP).  The in-situ-formed TiO₂ particles have far greater specific surface area and dispersion uniformity than conventional nanopowders, which allows them to exhibit greater catalytic effect at lower particle loading.  In both cases, the in situ nanoparticle-loaded liquid polymer/prepolymer binders are intended to be used as direct replacements for conventional binders (i.e., HTPB) in existing propellant mixing operations [11].

Figure 2 shows the in situ nAl and TiO₂ in HTPB, while Figure 3 presents transmission electron microscopy (TEM) cross sections of cured HTPB binder containing the in situ nAl and conventional powdered nAl for comparison.  The in situ binders are currently being produced at the hundreds of grams per batch scale, with near-term scale-up plans to kilogram-level in support of currently funded programs [10].

Figure 2: In-Situ-Produced nAl in HTPB (top) and nTiO2 in HTPB (bottom) [10].

 

Figure 3: Comparison of In Situ nAl With Conventional Micron Aluminum in HTPB [10].

 

In a Navy-sponsored Small Business Innovation Research (SBIR) project, Helicon is developing an AP/HTPB composite propellant with equivalent performance to the double-base propellants currently used in ejection-seat rocket motors and cartridges [10].  The goal of this propellant development is to eliminate the safety hazard associated with double-base propellant nitrate ester (NE) stabilizer depletion from prolonged high-temperature exposure, while duplicating, as closely as possible, performance of the incumbent propellant.

This new nanocomposite synthesis and processing technology is being used to create homogeneous nanoparticle/polymer composites.  The propellant development effort combines multiple materials and technologies to achieve the desired effects.  HTPB binders containing Helicon’s homogeneous nAl and nTiO₂ produce composite propellants with the unique performance characteristics required for this effort.  Table 1 lists the specific properties brought to the propellant formulation by each of the nanomaterials employed.

Table 1: Properties of Al/HTPB Polymer Nanocomposites [10].

nAl dispersed in HTPB	nTiO2 dispersed in HTPB
Oxide-free	Highly active anatase crystal structure
Homogeneous dispersion in the binder	Homogeneous dispersion in the binder
Extremely rapid ignition
Complete combustion	High burning rates due to AP reaction catalysis
Not catalytic toward AP decomposition
High burning rates due to heat feedback mechanism

Formulations matching specific impulse, burning rate, plateau behavior, temperature sensitivity, and thermal stability goals have been developed and tested.  Current formulation efforts are focused on extending the burning rate plateau to a higher pressure regime.  Figure 4 illustrates the significant increase in burning rate achieved using a relatively low loading of the novel nAl.

Figure 4: Effect of Helicon’s nAl on Propellant Burning Rate [10].

 

In contrast, CRG has adapted a much different in situ nanocomposite manufacturing method to address the challenge of nanoparticle incorporation in a variety of energetic applications, including gun propellant, nanothermite, and rocket propellant applications.  This process, depicted in Figure 5, converts an adsorbed gaseous precursor to the nanoparticles of interest using the molecular free volume of the polymer as a template.  Typically, the particles are on the order of 5 to 10 nm in size, although it is possible to grow the particles larger by repeating the process.  The particles are monodisperse, uniform in size and shape, and of high purity, provided that quality precursor materials are used.  Because the particles form directly from the gas phase, already trapped in a polymer binder, there is no route for particle agglomeration and the particles are provided a degree of oxidation resistance.

Figure 5: Solid Phase In Situ Nanocomposite Manufacturing Process [1].

 

In situ nanocomposite manufacturing replaces both the synthesis and mixing steps of traditional nanocomposite fabrication.  The significant challenges associated with incorporating nanoparticles into a polymer binder are eliminated because the nanoparticles form already dispersed in the polymer.  A polymer matrix is used as a template for growing the particles in situ.  There are no freestanding nanoparticles.  They do not exist until they are trapped in a polymer that is easy and safe to handle.  Figure 6 illustrates the basic configuration of CRG’s nanomanufacturing system, in this case a reaction vessel designed for remote operation to allow evaluation of energetic polymers.

Figure 6: The CRG Nanocomposite Manufacturing System [12].

 

The in situ nanomanufacturing process is highly scalable.  CRG can currently produce 1–5-kg batches, but there is equipment currently under development to allow larger-scale processing (including a large-scale reactor that will be able to process 30-kg batches).

In a Phase I SBIR effort with the U.S. Air Force Research Laboratory (AFRL), CRG has demonstrated the feasibility of using this in situ nanocomposite manufacturing process to fabricate core-shell fuel and oxidizer materials to meet the military’s future demands for high-energy density energetic material compounds for a variety of applications.  This effort is expected to enable high-volume production of a material whose theoretical value has not been realized because of production limitations.

Successful processing of core-shell particles in multiple polymers, including cellulose acetate butyrate (CAB) and several fluoropolymers, has been reported.  CAB was selected based on previous projects in which high-quality aluminum nanoparticles were produced with consistency.  The fluoropolymers were selected as stable, high-temperature materials with high free volume and a ready supply of free-flowing powder to act as the template for nanoparticle growth.

On the AFRL-sponsored project, this nanocomposite processing method has been used to produce core-shell particle morphologies with a technique that has already been demonstrated at 1-kg batch sizes and can easily be scaled to 100-kg and 1,000-kg batches.  The core-shell structures have been imaged in the nanomodified fluoropolymer materials.  It is believed that the core-shell particles were produced in CAB as well, but challenges with TEM sample preparation restricted the team’s ability to image the particles in that polymer system.  Particle size varies with the polymer matrix, and phase identification of the iron oxide is challenging in the characterization for all materials.

Early work on the formation of iron oxide on aluminum core-shell particles in a fluoropolymer binder has indicated success in forming some core-shell particles, as shown in Figure 7, while also forming particles of aluminum, iron, and what is thought to be an Al/Fe intermetallic compound.

Figure 7. Core of Aluminum Surrounded by Iron Oxide Shell [1].

 

While the feasibility of producing core-shell iron oxide on aluminum has been established, optimization of these materials is still needed.  The core-shell nanothermite particles appear to vary in size, with the majority of particles measuring less than 25 nm.  Particles produced include a mix of iron, aluminum, and core-shell morphologies.  The particles appear well-dispersed and are already useful for tuning performance in energetic formulations containing polymer binder systems.  Further characterization with TEM on these particles will confirm the phase identity of fuel and oxidizer, the relative ratios of core and shell chemistries, the particle formation process, and the size and distribution of particles.  The oxidizer and fuel layers need further improvement through optimization of the current process conditions and possible pretreatment of the polymer.

Future Research and Technology Commercialization

Multiple Department of Defense organizations have expressed interest in these new nanocomposite manufacturing techniques and have funded research projects to evaluate the resultant nanomaterials in a range of potential applications.  These applications include the aforementioned solid rocket propellants, as well as high explosives, solid gun propellants, solid fuels for ramjet combustors, and mitigation concepts to address insensitive munition (IM) requirements for fast and slow cookoff.  Other applications have been proposed in the energetics arena, such as reactive materials for warhead liners and cases, as well as ignition materials with increased reactivity.

The Helicon technology was originally developed with energetic materials applications in mind, especially for the use of in situ nAl as a high-performance fuel and propellant ballistic modifier.  Much of the company’s commercialization efforts remain focused in this area.  These efforts include the development of new composite propellants with advanced performance characteristics, such as plateau burning propellants; hybrid rocket fuels; and IM.  Benefits are also anticipated in explosives applications, and efforts are underway to adapt this process to liquid hydrocarbon fuels and hypergolic propellants.

Most of CRG’s development work has likewise focused on various energetic materials applications.  The company believes there is considerable promise in that space for tailoring burn rates, increasing energy density, improving combustion efficiency, and reducing sensitivity.

The ultimate commercialization potential of these high-quality, affordable, polymer nanocomposites, however, likely extends beyond energetics applications and into large market spaces, such as optical materials, solid electrolyte batteries, capacitors, photovoltaics, and fuel cells [11, 12].