This protocol describes the preparation of gasless nanostructured energetic materials (Ni+Al, Ta+C, Ti+C) using the short-term high-energy ball milling (HEBM) technique. It also describes a high-speed thermal imaging method to study the reactivity of mechanically fabricated nanocomposites. These protocols can be extended to other reactive nanostructured energetic materials.
High-Energy Ball Milling (HEBM) is a ball milling process where a powder mixture placed in the ball mill is subjected to high-energy collisions from the balls. Among other applications, it is a versatile technique that allows for effective preparation of gasless reactive nanostructured materials with high energy density per volume (Ni+Al, Ta+C, Ti+C). The structural transformations of reactive media, which take place during HEBM, define the reaction mechanism in the produced energetic composites. Varying the processing conditions permits fine tuning of the milling-induced microstructures of the fabricated composite particles. In turn, the reactivity, i.e., self-ignition temperature, ignition delay time, as well as reaction kinetics, of high energy density materials depends on its microstructure. Analysis of the milling-induced microstructures suggests that the formation of fresh oxygen-free intimate high surface area contacts between the reagents is responsible for the enhancement of their reactivity. This manifests itself in a reduction of ignition temperature and delay time, an increased rate of chemical reaction, and an overall decrease of the effective activation energy of the reaction. The protocol provides a detailed description for the preparation of reactive nanocomposites with tailored microstructure using short-term HEBM method. It also describes a high-speed thermal imaging technique to determine the ignition/combustion characteristics of the energetic materials. The protocol can be adapted to preparation and characterization of a variety of nanostructured energetic composites.
Classical energetic materials, i.e., explosives, propellants and pyrotechnics are a class of material with a high amount of stored chemical energy that can be released during rapid exothermic reaction1-5. For example, explosives are usually generated by combining fuel and oxidizer groups into one molecule. The energy density of those materials is very high. For example, upon decomposition trinitrotoluene (TNT) releases 7.22 kJ/cm3 and forms 8.36 moles of gases per 100 g (Table 1) in a very short period of time. These materials are composed of micrometer-scale organic and inorganic species (fuels and oxidizers).
Thermite systems, where reactions take place between the inorganic compound, i.e., reducing metals (e.g., Al) and oxides (Fe2O3, CuO, Bi2O3), belong to another type of energetic materials. The energy density (15-21 kJ/cm3) of such systems exceeds that of TNT, however the amount of gas products (0.15-0.6 moles per 100 g) is typically much less than for explosives (Table 1). Also, the nano-thermites may show extremely high velocity of combustion wave propagation (>1,000 m/sec)2-5.
It was recently shown6-12 that a number of gasless heterogeneous reactive systems (Ni+Al, Ti+C, Ti+B) that form intermetallic or refractory compounds could also be considered as energetic materials. The energy densities (kJ/cm3) of those systems are closer or higher than that of TNT (Table 1). At the same time, the absence of gas products during the reaction makes such materials excellent candidates for a variety of applications including synthesis of nanomaterials, reactive bonding of refractory and dissimilar parts, gasless micro power generators, etc.11-17. However, the relatively high ignition temperature of those systems (900-3,000 K, see Table 1) compared to thermites (~1,000 K) hinders their applications. The preparation of engineered nanostructured composites could significantly enhance the ignition and combustion characteristics of gasless heterogeneous systems12-14, 17.
Many methods have been developed to fabricate the engineered energetic nanocomposites, such as ultrasonic mixing18,19, self-assembly approaches5, sol-gel20-22, vapor deposition techniques16,17,23,24, as well as high-energy ball milling (HEBM)1,5. The disadvantage of ultrasonic mixing of nano-powder is that a thick (5-10 nm) oxide shell on metal nanoparticles reduces energy density and degrades the combustion performance of reactive mixtures. Also, the distribution of fuel and oxidizer is not uniform, and the interfacial contact between reactants is not intimate. Sol-gel and self-assembly strategies were developed for preparation of specific thermite nanocomposites. Despite being low-cost techniques, those strategies are not green from an environmental standpoint. Moreover, large amounts of impurities are introduced into prepared composites. Vapor deposition or magnetron sputtering is used to prepare reactive multi-layer foils and core-shell energetic materials. It provides a pore-free and well-defined geometry of composites that simplifies theoretical modeling and enhances accuracy. However, this technology is expensive and difficult to scale up. Furthermore, the prepared layered nanocomposites are unstable in certain conditions.
High-Energy Ball Milling (HEBM) is an environmentally friendly, easily scalable approach that allows effective fabrication of nanostructured energetic composites5,9-14. HEBM is inexpensive and can be used with various reactive material compositions (e.g., thermites, reactions that form intermetallics, carbides, borides, etc.).
The protocol provides a detailed description for preparation of reactive energetic (Ni+Al, Ti+C, Ta+C) nanocomposites with tailored microstructure by using the short-term HEBM method. It also describes a high-speed thermal imaging technique to determine the ignition/combustion characteristics of as-fabricated energetic materials. Finally it shows the analysis of the microstructure of the nanocomposites using Field Emission Scanning Electron Microscope (FESEM) Equipped by Focused Ion Beam (FIB). The protocol is an important guide for the preparation of different energetic nanomaterials (gasless and thermite systems) that could be used as either high energy density sources or for synthesis and processing of advanced nanomaterials by combustion-based approaches.
1. High-energy Ball Milling
2. Reactivity Characterization of Energetic Materials
3. Microstructure Analysis Using Field Emission Scanning Electron Microscope (FESEM) Equipped by Focus Ion Beam (FIB)
To prepare nanostructured energetic composites, a mixture of desired powdered components (typically micrometer-sized) is mechanically treated under preset milling conditions. Processing time (typically minutes) is accurately controlled to generate the compositionally homogenized nanocomposite particles but not permitting the self-sustaining chemical reaction to initiate during milling.
Figure 1 and Video 1 show that contact surface area between the reactants in composite particles increases by orders of magnitude compared to the initial mixture. After HEBM each component is incorporated into the matrix of another component. In most cases, the obtained nanostructured energetic composites are fully dense with high contact area between reactants (Figure 2). Moreover, the reactants can be mixed on a scale of less than 100 nm. It is also important that tuning HEBM conditions allows regulation of the internal microstructure of composites. It is seen in Figure 2 that different mixing degrees between the reactant may be achieved in the same system. Moreover, HEBM forms fresh (oxygen-free) contacts between reactants. Figure 3 illustrates that HEBM effectively removes the protective oxide layer on the initial metal (e.g., Al) particles. Dark field (DF) image of transmission electron microscopy (TEM) analysis coupled with energy-dispersive X-ray spectroscopy (EDX) in Ni/Al composite particles clearly indicate new boundaries between the reactants that are oxygen-free.
Despite tuning of the internal microstructure of composite particles HEBM enables the regulation of the size of the particles. For example, this could be achieved by changing the rotation ratio (k) of sun wheel (1,400 rpm) and milling jar (from 700 to 1,300 rpm). Video imaging show that several HEBM regimes may occur depending on k ratio. The mixture of balls and powder k ≤ 1.5 is “sliding” on the surface of jar (Video 2). In 1.85 ≤ k < 1.5 interval intensive collisions of balls take places (Video 3). Figure 4 indicate that such different HEBM regimes significantly influence the sizes of particles, i.e., coarse particles (100-150 µm) are formed in the “sliding” regime, while many fine particles (10-50 µm) could be prepared in the collision regime.
Along with preparation of energetic composite particles, the protocol describes their characterization techniques. Such an approach reveals the important links between preparation of materials, their microstructure and reactivity of composite particles. For example, detailed microstructural investigation of Ti/C composite particles revealed that, after 3 min of HEBM, a carbon-rich layer formed between the flattened titanium layers11 due to the cold welding. TEM images in Figure 5 indicate that the carbon layer contains uniformly distributed titanium nanoparticles and titanium carbide (TiC) nuclei.
Temperature-time profiles recorded by infrared imaging for Ti/C composite particles are shown in Figure 5C.The particles were placed on a hot plate with a temperature of ~600 K. A High speed Thermo-Vision system was used to monitor the time-temperature history of the particle. The selected temperature measurement range was 600-1,200 K. It can be seen that the materials, after 2 min of mechanical treatment, cannot be self-ignited under the investigated conditions. The self-ignition temperature after 3 min of HEBM is about 600 K, while after 5 and 7.5 min of treatment Tig is well below 600 K. It is interesting that the ignition temperature is again above 600 K for a milling time of 9 min. This effect is explained by the formation of an amount of the TiC phase in the milling jar. It should be noted that that during combustion of a conventional Ti + C mixture, the microstructure of the reaction medium remains unchanged upon appearance of a liquid metal phase (1,941 K) and the exothermic reaction initiates at ~2,000 K. These results indicate that a direct link exists between the microstructure formed during the HEBM and ignition temperature. Formation of intimate oxygen-free contacts between reactants and product nuclei makes the Ti/C composites extremely reactive as the ignition temperature drops from 2,000 to 600 K. HEBM also significantly influences the ignition delay time, i.e., the time after immersing the particle in a furnace and until reaction initiation, as well as combustion front propagation velocity. Temperature-time profiles in Figure 5C shows that the ignition delay time also decreases with an increase of milling time.
It is worth noting that combustion of mechanically fabricated composite shows great advantages for synthesis of nanostructured materials. During the combustion of conventional media, control over the microstructure of product is extremely difficult. For example, the reaction onset temperature of conventional Ni+Al coincides with the lowest eutectic temperature of the system (~910 K). The liquid phase formed during the reaction significantly changes the microstructure of the initial mixture (Figure 6). In the mechanically fabricated composites, the reactions proceed below the eutectic temperature of the system, which completely eliminates formation of liquid phases, i.e., a true solid-state combustion, so-called solid flame, takes place. This is evidenced by an onset reaction temperature as low as 470 K, whereas the lowest eutectic temperature in this system occurs at 910 K; this implies that a significant conversion must occur due to a purely solid-state reaction. Samples prepared from such composite particles retain their shape and microstructure (Figure 6).
Figure 1. The transformation of the heterogeneous reactive media microstructure during high-energy ball milling: Schematic representation of the transformation of micrometer size particles of individual reactants to a layered composite particles (A), and the formation of Ni/Al composite particles by using HEBM of nickel and aluminum reactants (B). Please click here to view a larger version of this figure.
Figure 2. Tuning the contact area between the reactants by varying HEBM conditions for different systems: Ni/Al (A–C), Ti/C (D,E) and Ta/C (F). Please click here to view a larger version of this figure.
Figure 3. Formation of oxygen-free contacts between the reactants: Schematic representation (A), Bright field image of a Ni/Al boundary formed by HEBM (B) and Energy dispersive X-ray Spectroscopy (EDS) profiles of nickel, aluminum and oxygen (C). Please click here to view a larger version of this figure.
Figure 4. Preparation of composite particles with different sizes by tuning the ratio (k) of sun wheel and milling jar rotation speeds: k ≤ 1.5 (A) and 1.8 ≤ k < 1.5 (B). Please click here to view a larger version of this figure.
Figure 5. The relationship between microstructure and reactivity of composite particles: A TEM image of Ti/C composite particle (A), high resolution TEM image of TiC nanoparticles (B) and time-temperature ignition profiles (C) of Ti/C composite particles various milling times (2, 3, 5, 7.5, 9 min). Please click here to view a larger version of this figure.
Figure 6. Synthesis of materials with pre-designed microstructure by using combustion of nano-structured composite particles: Microstructures of NiAl intermetallics using conventional media (A) and mechanically fabricated composite particles (B). Please click here to view a larger version of this figure.
Mole gas released per 100 g | Energy densities per volume, kJ/cm3 | Ignition temperature, K | |
Trinitrotoluene (TNT) | 8.36 | -7.22 | 510 |
Termites | |||
2Al + 3CuO | 0.54 | -20.8 | 900-1,100 |
2Al + Fe2O3 | 0.14 | -16.4 | |
2Al+ Bi2O3 | 0.47 | -15.2 | |
Gasless Systems | |||
Al + Ni | 0 | -7.13 | 910/520 |
Ta + C | 0 | -10.9 | 3,000/1,500 |
Ti + C | 0 | -15.2 | 2,000/900 |
Table 1. Some characteristics of energetic materials.
Video 1. “Slice and view” imaging of a Ni/Al composite particle.
Video 2. “Sliding” regime of HEBM at k ≤ 1.5.
Video 3. Intensive collisions of balls in 1.85 ≤ k < 1.5 interval.
The protocol provides a detailed description for preparation of reactive energetic (Ti+C, Ta+C, Ni+Al) nanocomposites with tailored microstructure by using the short-term HEBM method. HEBM of gasless heterogeneous mixtures involve their processing in a high-speed planetary ball mill, where the particles of the mixture are subjected to mechanical impact with a force sufficient for breakdown of brittle components (e.g. graphite) and deformation of plastic components (e.g., Al, Ti, Ta, Ni). Brittle reactants are milled to finer particles and may become amorphous, while plastic metals are subjected to multiple deformations and cold welding, forming composites particles. Small fragments of brittle components are often found inside the particles of plastic reactants. Fine tuning of the HEBM conditions allow for control of the composite particle sizes and their intrinsic microstructure. It should be noted that such a degree of control in microstructure cannot be achieved in most other techniques currently available for preparation of nanostructured energetic composites. Thus the energy released in mechanically fabricated energetic composites could be precisely controlled by their microstructure through fine tuning of HEBM conditions.
The unique HEBM conditions also allow one to produce the metastable non-equilibrium supersaturated solutions, which enable reactions to occur at significantly lower temperatures than conventional powder mixtures. Moreover, in some cases the reactions proceed below the eutectic temperature of the system, which completely eliminates formation of liquid phases. Samples prepared from such composite particles retain their shape and microstructure.
One use of HEBM is in production of highly reactive, energetic nanocomposites. This process is simple, highly economical, and easily scaled. There are two major issues with this process, however. The first is safety issues; this process creates nanocomposites that are highly reactive, and as such, the operator must follow all safety procedures. This includes general safety procedures relating to the operation of the machine itself and to more specific safety procedures relating to the compounds being utilized. Because of the highly reactive nature of these nanocomposites; a limited quantity of this material should be produced until knowledge about the safety of the specific system is ascertained. Finally, impurities relating to the vessel can be introduced. This can lead to simple contamination or even undesired side reactions. Third, preparation of pore-free composites (e.g., coatings, films) is difficult and requires additional steps (cold spraying or rolling) 26.
The protocol also provides in-depth information on the characterization of mechanically fabricated nanostructured energetic composites. The use of high-speed infrared techniques allows for accurate spatial (2 μm), thermal (5 K), and temporal resolution (15,000 fps). This enables accurate characterization of the composite particles, including their time-temperature history, ignition temperature, delay time, and propagation velocity.
The protocol is an important guide for the preparation of different energetic nanomaterials (gasless) that could be used as either high energy density sources or for synthesis and processing of advanced nanomaterials by combustion-based approaches. It can be easily modified to apply to thermite systems, and other energetic materials such as metal-polymer composites.
Critical steps within the protocol include the initial preparation of the nanocomposites, starting from the weighing of the powders and choosing the appropriate charge ratio. Additionally, it is of key importance to ensure that the internal atmosphere of the jar is inert through Ar purging. Selection of the ball milling parameters, including the revolution speed and total milling time are necessary for tailoring the microstructure. Finally, the exposure, collection, and classification of the powder with a safe procedure are important, lest the experiment is ruined. Preparation of the powders for the experimentation by pressing determines the data that can be collected, followed by accurate analysis of the data. Use of the FIB S&V program to generate a 3D dataset for analysis is also of importance.
The authors have nothing to disclose.
This material is based upon work supported by the Department of Energy, National Nuclear Security Administration, under Award Number DE-NA0002377. Funding from the Defense Threat Reduction Agency (DTRA), Grant Number HDTRA1-10-1-0119. Counter-WMD basic research program, Dr. Suhithi M. Peiris, program director is gratefully acknowledged. This work was also supported by the Ministry of Education and Science and Education of the Russian Federation in the framework of Increase Competitiveness Program of NUST “MISIS” grant No. K2-2014-001.
Titanium | Alfa Aesar | 42624 | Particle size: -325 mesh | Purity, 99.5% |
Graphite | Alfa Aesar | 46304 | Particle size: 7-11 micron | Purity, 99% |
Nickel | Alfa Aesar | 10256 | Particle size: 3-7 micron | Purity, 99.9% |
Aluminum | Alfa Aesar | 11067 | Particle size: -325 mesh | Purity, 99.5% |
Tantalum | Materion advanced chemicals | T-2017 | Particle size: 325 mesh | Purity, 99.9% |
Carbon lampblack | Fisher scientific | C198-500 | Particle size: 0.1 micron | Purity, 99.9% |
Tungsten wire | Mcmaster Carr | n/a | 0.032" diameter | n/a |
Planetary Ball Mill | Retsch GmbH, Germany | n/a | n/a | n/a |
Uniaxial press | Carver Hydraulic | n/a | n/a | n/a |
Sieve shaker | Gilson performer | n/a | 5mm diameter | n/a |
Cylindrical stainless steel press die | Action Machine | n/a | n/a | n/a |
Stainless steel sieves | Mcmaster Carr | Type 304 | n/a | n/a |
High-speed thermal camera (SC6000) | FLIR | n/a | n/a | n/a |
Helios NanoLab 600, Field Emission Scanning Electron Microscope (FESEM) Equipped by Focus Ion Beam (FIB) | FEI | n/a | n/a | n/a |
Cylindrical reactor with a vacuum pomp | Action Machine | n/a | n/a | n/a |
Autoslice and View (S&V) | FEI | n/a | n/a | n/a |
Avizo Fire | FEI | n/a | n/a | n/a |