Novel micron- and nano-scale energetic materials for advanced gun propulsion, their material properties, and their effects on ballistic performance

This dissertation focused on the investigation of novel materials that are both energetic and inert in their micron- and nano-scale crystalline form. The characterization of the materials properties and its effects on the ballistic performance when incorporated into a composite material were evaluated as a gun propellant for application in a future weapon system for the US Army. Some of these materials may find dual use in civilian applications. Applications in small and medium arms, artillery, tank, aircraft, and shipboard gun systems will all benefit from these advancements. Not only will gun system performance be improved for greater stand-off range and accuracy, but the ability to perform consistently across a broad temperature range. Additionally, an improved performance and longer gun barrel life achievable by tailoring the combustion products, lowering the propellant flame temperature, minimum sensitivity of burning velocity to pressure, temperature and gas velocity (erosive burning) and with munitions that are insensitive to outside stimulus attack will give such systems a significant advantage during military use. In addition, green chemistry and lower lifecycle cost were taken into consideration during this research. The approach to be taken was to incorporate these novel materials into a gun propellant formulation by using nitramine-based micron scale cyclotrimethylene trinitramine (RDX) explosives in combination with synthesized novel ingredients in nanoscale crystalline form, characterize the material properties and predict the ballistic performance across the ballistic temperature range. The nano-scale crystalline materials evaluated consisted of polymeric nitrogen stabilized in single wall carbon nanotubes (SWNTs), nitrogenated boron nanotubes / nanofibers (BNNTs/BNNFs), nano-aluminum, and titanium dioxide. The polymeric nitrogen and the nitrogenated boron nanotubes / nanofibers (BNNTs/BNNFs), should provide an enhancement in the propellant burn rate by achieving the burn rate differential goal of 3:1 between the fast and the slow burning propellant and at the same time improve the gun propellant performance by lowering the CO/CO2 ratio and raising the N2 / CO ratio for mitigating gun bore wear and erosion, respectively. For the synthesis approaches of polymeric nitrogen stabilized in carbon nanotubes, the following synthesis method were performed, optimized and compared: Electrochemical Reactions, Microwave Induced Electrochemical Chemical Reactions and Plasma Enhanced Chemical Vapor Deposition (PE-CVD). The Electrochemical Reaction process has proven to be the most efficient synthesis approach for the polymeric nitrogen based on analytical results obtained through Raman Spectroscopy, Laser Ablation Mass Spectroscopy, Scanning Electron Microscope, Fourier Transform Infrared-Attenuated Total Reflectance (FTIR-ATR) and Differential Scanning Calorimeter/Thermal Gravimetric Analysis (DSC/TGA). The PE-CVD is the second recommended synthesis approach to synthesize the polymeric nitrogen although a cost benefit economic analysis has to be performed which is beyond the objectives of this research work. For the synthesis of the nitrogenated boron nanotubes, the use of the magnesium borohydride to initiate the reaction has proven to be the most optimized process due to a much lower reaction temperature which is approximately 500°C when compared with the reaction temperature of 950°C when using Magnesium Boride (MgB2) in the thermally induced CVD process. The small scale synthesis of boron nanotubes /nanofibers carried out using MgB2 powder, Nickel Boride (Ni2B) powder catalysts and mesostructured hexagonal framework zeolite powder was successfully achieved at 950C. The quality of the nanotubes produced was checked by Raman spectroscopy and transmission electron microscope analysis. The TEM data shows the production of 10-20 nm boron nanotubes using the MgB2, Ni2B and Mobile Crystalline Material (MCM-41) in the synthesis process

Formulation development and characterization of cellulose acetate nitrate based propellants for improved insensitive munitions properties

AbstractCellulose acetate nitrate (CAN) was used as an insensitive energetic binder to improve the insensitive munitions (IM) properties of gun propellants to replace the M1 propellant used in 105 mm artillery charges. CAN contains the energetic nitro groups found in nitrocellulose (NC), but also acetyl functionalities, which lowered the polymer's sensitivity to heat and shock, and therefore improved its IM properties relative to NC. The formulation, development and small-scale characterization testing of several CAN-based propellants were done. The formulations, using insensitive energetic solid fillers and high-nitrogen modifiers in place of nitramine were completed. The small scale characterization testing, such as closed bomb testing, small scale sensitivity, thermal stability, and chemical compatibility were done. The mechanical response of the propellants under high-rate uni-axial compression at, hot, cold, and ambient temperatures were also completed. Critical diameter testing, hot fragment conductive ignition (HFCI) tests were done to evaluate the propellants' responses to thermal and shock stimuli. Utilizing the propellant chemical composition, theoretical predictions of erosivity were completed. All the small scale test results were utilized to down-select the promising CAN based formulations for large scale demonstration testing such as the ballistic performance and fragment impact testing in the 105 mm M67 artillery charge configurations. The test results completed in the small and large scale testing are discussed

Enhanced Propellant Performance via Environmentally Friendly Curable Surface Coating

Surface coating of granular propellants is widely used in a multiplicity of propellants for small, medium and large caliber ammunition. All small caliber ball propellants exhibit burning progressivity due to application of effective deterrent coatings. Large perforated propellant grains have also begun utilizing plasticizing and impregnated deterrent coatings with the purpose of increasing charge weights for greater energy and velocity for the projectile. The deterrent coating and impregnation process utilizes volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) which results in propellants that need to be forced air dried which impacts air quality. Propellants undergo temperature fluctuations during their life. Diffusion coefficients vary exponentially with variations in temperature. A small temperature increase can induce a faster migration, even over a short period of time, which can lead to large deviations in the concentration. This large concentration change in the ammunition becomes a safety or performance liability. The presence of both polymeric deterrents and nitroglycerin(NG) in the nitrocellulose matrix and organic solvents leads to higher diffusion rates. This results in continued emissions of VOCs and HAPs. Conventional polymers tend to partition within the propellant matrix. In other words, localized mixing can occur between the polymer and underlying propellant. This is due to solvent induced softening of the polymer vehicle over the propellant grain. In effect this creates a path where migration can occur. Since nitrate esters, like NG, are relatively small, it can exude to the surface and create a highly unstable and dangerous situation for the warfighter. Curable polymers do not suffer from this partitioning due to “melting” because no VOC solvents are present. They remain surface coated. The small scale characterization testing, such as closed bomb testing, small scale sensitivity, thermal stability, and chemical compatibility, will be presented. The 30 mm gun demonstration firing data at hot, cold, and ambient temperatures will also be presented

Innovative boron nitride-doped propellants

The U.S. military has a need for more powerful propellants with balanced/stoichiometric amounts of fuel and oxidants. However, balanced and more powerful propellants lead to accelerated gun barrel erosion and markedly shortened useful barrel life. Boron nitride (BN) is an interesting potential additive for propellants that could reduce gun wear effects in advanced propellants (US patent pending 2015-026P). Hexagonal boron nitride is a good lubricant that can provide wear resistance and lower flame temperatures for gun barrels. Further, boron can dope steel, which drastically improves its strength and wear resistance, and can block the formation of softer carbides. A scalable synthesis method for producing boron nitride nano-particles that can be readily dispersed into propellants has been developed. Even dispersion of the nano-particles in a double-base propellant has been demonstrated using a solvent-based processing approach. Stability of a composite propellant with the BN additive was verified. In this paper, results from propellant testing of boron nitride nano-composite propellants are presented, including closed bomb and wear and erosion testing. Detailed characterization of the erosion tester substrates before and after firing was obtained by electron microscopy, inductively coupled plasma and x-ray photoelectron spectroscopy. This promising boron nitride additive shows the ability to improve gun wear and erosion resistance without any destabilizing effects to the propellant. Potential applications could include less erosive propellants in propellant ammunition for large, medium and small diameter fire arms