Novel Energetic Materials
Novel Energetic Materials consists of fundamental research programs to expand and validate physics-based models and experimental techniques to devise chemical formulations that will enable the design of novel insensitive high-energy propellants and explosives with tailored energy release for revolutionary Future Force lethality and survivability. This program supports demonstration of advanced energetic materials with ability to tune energy release for precision munition & counter-munition applications (e.g., propellants, explosives, thermobarics, multi-purpose warhead, APS).
These energetic materials may have the potential of providing factors of 3 to 4 in increased energy release rate compared with conventional formulations. The Army's Novel Energetic Materials for the Objective Force effort seeks to mature advanced energetic materials to provide a 40% increase in deliverable energy from advanced gun propellant systems and a 20-50% increase in warhead effectiveness (munitions, active protection).
DoD and DOE continue to have very similar requirements for energetic materials. Both agencies desire high explosives with increased or tailored performance and decreased sensitivity, and recent accomplishments have benefited both agencies. Like advanced initiation, improved energetic materials are enabling technology for the next generation of weapon systems that will be safer, smaller and more lethal. Under this program a combination of evolutionary and novel technologies are under development. Conventional chemistry has been used to develop more powerful, less sensitive explosives.
Nano-structured and engineered materials are being explored to increase energy density and energy on target by factors of three or more. Higher risk efforts are also underway to explore the possibility of metastable High Energy Density Materials (HEDM). Using conventional chemistry, a number of new candidate molecules have been synthesized, characterized and formulated. The development of new materials is based on theoretical molecular design. The structure, performance and sensitivity of new molecules are predicted computationally, then synthesis is attempted. The focus is in two areas: molecules with significantly increased energy over current materials and very insensitive materials with reasonable energies.
Energetic materials consist of fuels and oxidizers which are intimately mixed. This is done by incorporating fuels and oxidizers within one molecule or through chemical and physical mixtures of separate fuel and oxidizer ingredients. The material may also contain other constituents such as binders, plasticizers, stabilizers, pigments, etc.
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.
In general, initiation and detonation properties of energetic materials are dramatically affected by their microstructural properties. It is generally known in material science that the mechanical, acoustic, electronic, and optical properties are significantly and favorably altered in materials called "nanostructures," which are made from nanometer-scale building blocks. Modern technology, through sol-gel chemistry, provides an approach to control structures at the nanometer scale, thus enabling the formation of new energetic materials, generally having improved, exceptional, or entirely new properties.
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.
The overall goal is to engineer multi-dimensional nanoscale energetic materials systems whose energy release can be controlled in terms of its type, rate, spatial distribution, and temporal history. The goal is to manipulate individual atoms and molecules and control their assembly into a large-scale bulk energetic material. The possibility exists to build large-scale energetic materials with a very high degree of uniformity (few/no defects, perfect crystalline structure, composites with molecularly engineered uniformity, laminated composites with structures built molecularly controlled and selectable layers - - no stirring, mixing - - all done through self-assembly). It is also possible to embed molecular scale devices within the energetic matrix (embedded smart devices and sensors).
The current emphasis in the nanoscale energetic materials area is on the preparation and characterization of single nanoscale energetic particles. These particles are then utilized in an otherwise conventional composite formulation, incorporating the nanoparticles (typically aluminum, the fuel) in a matrix with micron-sized oxidizer particles. While there is some performance improvement, the full extent of the anticipated performance gains of the nanoscale materials have not been realized. In large measure this is due the incompatibility of the length scales. What is needed is a formulation with all constituents at the nanoscale. If this were accomplished, the reactivity of the material would be characterized by the almost premixed gas-phase reaction rates of the nanomaterials; not limited by the slower, diffusion dominated reactions of micron-sized constituents. It may also be expected that the much smaller crystalline sizes of nanomaterials would be much less susceptible to shear-induced initiation and may be less responsive to some hazards.
Macroscale formulations of energetic materials that preserve the intrinsic nanoscale structure of the individual components are needed to realize the true potential of nanoscale energetic materials. An optimal material may be a macroscale three-dimensional, ordered array of nanoscale constituents, with spacing and interstitial/bonding materials chosen to optimize both stability and reactivity. At a minimum, these macroscale units would be on the order of millimeter size, which could then be processed into the centimeter to meter sizes needed for practical propellants and explosives. The advantages of this "bottom-up" approach to energetic materials are: a) developing a fundamental understanding of the evolution of properties with the size of the system, b) understanding the effects of the interaction of matter at different molecular-length scale with external stimuli, and c) developing a detailed understanding of the functionalities of matter at molecular-length scale.
The chemistry, physics and materials science of nanoscale energetic material preparation need to be developed, focusing on those processes that lead to well ordered structures, e.g. self-assembly, vapor deposition, etc. Computational methods are needed to assess the reactivity of candidate structures and to predict the stability of the energetic material structure, to both hazards (shock, spark, etc.) and to long-term degradation. These computations should also provide guidance to and receive validation from the experimental aspects of the program, specifically the formulation and characterization activities.
Experimental methods of characterizing nanoenergetic structures are needed to verify structure and performance. This includes developments of techniques capable of the determination of the threedimensional structure of the nanoscale assembly and the orientation and bonding of the constituents. Characterization of reaction front progress through the nanostucture is also desirable.
This research program will enable molecularly manipulated energetic materials and formulations with tuned chemical and physical properties, high performance, low sensitivity, and multifunctionality (a single smart material that can function as a structural material, embedded sensor, and have real-time selectable propellant, explosive, or non-lethal functionality within a precision munition).
Novel Energetic Materials science and technology is recognized as a critical enabler in support of changing force structure for advanced weapons platforms. Nanoenergetic materials are a key area highlighted by the DDR&E/ OUSD (S&T) "National Advanced Energetics Program". That initiative is funding the first generation materials. The program supports the ARL-led STO IV.WP.2003.01 "Novel Energetic Materials for the Objective Force", the DTO WE.70 "Novel Energetics" in which ARL is a partner, the provisional SRO "Insensitive High Energy Materials" which ARL leads, the OSD Office of Munitions efforts on "Insensitive Munitions", and DTAP/Reliance "Advanced Gun Propulsion" which ARL leads. This program will provide the next generation novel energetic materials.
One new explosive under development is LLM-105. It is dense, thermally stable and very insensitive. With 30% more energy than TNT it has possible detonator and booster applications and is an alternative to TATB in special purpose weapons such as hard target penetrators that have to survive high shock loading. The synthesis, scale-up, and characterization of this material have been completed and its use as insensitive booster material for Navy weapons applications is now being evaluated. Efforts to crystallize the pure form of a newly synthesized energetic material with predicted energy greater than CL-20, LLM-121 continued in FY 2001.
Two other very fast burning materials, BTATz and DHT, have been successfully synthesized and are under evaluation as enhanced performance gun and rocket propellant ingredients. Metastable Intermolecular Composites (MIC) developed under this program were the first successful examples of nano-structured energetic materials with significantly enhanced performance. They demonstrated that tailored, ultra-fine reactant particles could dramatically increase the energy release rate of thermite-like materials and provide twice the total energy of high explosives. The first application of this technology is for lead-free percussion primers for small arms ammunition, and this program is now in engineering development under SERDP funding. The current focus is on the optimization of this material for other weapons applications via better diagnostic and measurement methods.
A new bulk process for manufacturing nano-structured energetic materials using sol-gel chemistry has been developed with the promise of precise control of material homogeneity, properties, and geometry. Samples of this material were manufactured this year for testing and evaluation in support of reactive warheads that better couple energy to the target and applications that require very high thermal loading. Extended solid HEDMs are also under development. This work uses intense pressure and temperature to force elements into highly energetic bonding states that can be recovered to ambient conditions. Current synthesis techniques have produced CO-derived solids and a family of novel nitrogen materials, but in very small quantities. These materials are expected to be highly energetic, but characterizing them, and particularly verifying the energy content, has been difficult due to the microscopic quantities of material available.
In 2001 the energy content of laboratory produced high energy density material was preliminarily measured. A special press was installed for production of milligram sample sizes, which can then be characterized more accurately using standard and improved techniques. The creation of the thermochemical code Cheetah represents a major accomplishment of this program. The code predicts the performance of energetic materials including high explosives, propellants and pyrotechnics and reduces the number of tests necessary to develop a new material. Cheetah 3.0 was released in 2001 to DoD, DOE and DoD contractor users. This version includes new equations of state resulting in greatly enhanced stability and accuracy of the code. A major effort is also underway to develop a suite of codes for use in predicting the response of energetic materials in weapon systems subjected to thermal and mechanical insult. The objective is to reduce the number and cost of the current go/no-go insensitive munitions test protocols required to qualify a new system for military use and to improve our understanding of the physical mechanisms and safety margins.
A collaborative effort with the Navy was initiated to experimentally assess and validate codes for use in predicting the response of weapon systems including the violence of reaction in cookoff accidents. Quantitative data on cookoff violence have been generated by both the Navy in small-scale experiments and by DOE in the scaled thermal explosion experiments. Data on both HMX based explosives and PBX-109 have been obtained for use in establishing the accuracy and range of validity of the predictive models. The measured properties were used this year to successfully predict the time to explosion in cookoff tests performed by the Navy. In order to preserve and transition the energetic materials technology generated under this program, two explosives databases have been distributed to government laboratories and contractors. One database, HEAT1, contains over 3,000 chemical structures, and is a compilation of measured heats of formation for a wide range of organic molecules of interest to researchers in the weapons community. A second database is APEX, A Pure Explosives Database. This database contains over 500 energetic materials of different molecular structure to guide the synthesis of new materials and ensure the retention of important characterization data.
Work in energetic materials was aligned with the recommendations from the DoD 2000 Weapons Technology Area Review and Assessment (TARA) and, in particular, was coordinated with the national initiative in advanced energetic materials. Concern from the DoD 2000 Weapons TARA regarding the need to maintain weapon lethality as weapon and platform size decrease continued to be addressed in efforts to synthesize, characterize and scale-up new energetic materials with increased or tailored performance and decreased sensitivity. The development and characterization of new insensitive and new high-energy, high power materials continued with synthesis based on theoretical molecular design.
Efforts to crystallize the new high energy molecule, LLM-121, in its pure form continued. Efforts sponsored under this program continued to exploit opportunities in nano-energetics by developing nano-structured and engineered energetic materials, including sol-gel derived materials, and evaluating their effectiveness and utility for warhead applications. With the completion of the LLM-105 synthesis and scale-up work, efforts will focused on formulation for evaluation and eventual qualification as a Navy booster material. The creation of new HEDMs continued, along with the development and implementation of accurate techniques for determining crystal structure and energy content of the newly synthesized materials.
With the installation of a special press in FY 2001 designed to produce sample sizes of 100mm3, the feasibility of bulk synthesis on CO-derived and nitrogen HEDMs was demonstrated and initial measurements of their energy content with larger sample sizes was completed. The synthesis of additional extended solid HEDMs was explored.
With the release of Cheetah 3.0, the emphasis in Cheetah development will turn towards implementing more sophisticated kinetic models into the code that account for differences in explosive microstructure including explosive particle morphology and towards generating more accurate equations of state for detonation products. To support this work, a new impulsive stimulated light scattering spectrometer was used to conduct measurements in a diamond anvil cell to monitor the onset of chemical reactivity at extreme conditions with great accuracy. Efforts to develop and validate computational tools for predicting munition system response to operational threat and accident environments will continue. The first generation of simulation tools for munitions response to accident environments will be exercised against test data to validate the codes and expand their ability to predict weapon system performance and response in accident situations. The joint experimental program with Navy to measure the violence of reaction in cookoff accidents will be expanded to testing and analyses of a full weapon systems. Experiments to determine mechanical property of both fielded high explosives and their constituents continued for development and validation of high explosive mechanical response models.
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