Metal-based reactive nanomaterials
Introduction
Commonly used energetic materials are based on monomolecular compounds, such as TNT, RDX, HMX, CL-20, etc. [1], [2]. The maximum heat of combustion of such materials is generally limited by the enthalpy of formation of their reaction products, CO2 and H2O upon complete oxidation. The monomolecular energetic materials enable an exothermic reaction to occur very rapidly, with the rate controlled primarily by the chemical kinetics processes for the molecule decomposition [3], [4]. On the other hand, the energy densities of such materials are relatively low. Higher combustion energies and thus higher energy densities can be obtained from combusting metal fuels, such as Mg, Al, B, Ti, and others, as illustrated in Fig. 1. Shown are the maximum gravimetric and volumetric reaction enthalpies for selected monomolecular energetic compounds and for some metals. The advantages of metal fuels are clear and become more significant when volumetric reaction enthalpies are compared. The main drawback of using such fuels is associated with relatively low rates of energy release. Micron-sized metal particles ignite after a fairly long delay as compared to the initiation of monomolecular energetic compounds. Such delays are usually controlled by relatively slow heterogeneous reactions leading up to the self-sustaining combustion of the metal particles [5], [6], [7], [8]. Furthermore, the rates of combustion of metal particles are often not sufficiently high to fully utilize their energetic benefits in the applications involving explosives, propellants, and pyrotechnics. For micron-sized particles such rates are commonly limited by the gas phase oxygen transport to the burning particle surface.
Combinations of conventional, micron-sized metal powders with condensed oxidizers, such as relatively unstable metal oxides in thermite compositions or ammonium perchlorate (AP) in solid propellants, do not result in significant acceleration of the metal ignition and combustion rates compared to the metal ignition in gaseous oxidizers. The heterogeneous processes controlling the metal ignition delays are usually associated with diffusion of oxidizer and/or fuel through the protective layers of metal oxide. Such layers always form on the surface of the metal oxidizing at a low temperature (prior to its ignition) so that the concentration of oxidizer outside the metal particle has only a limited effect on the rate of the critical diffusion processes. In addition, decomposition of the oxidizer typically occurs much sooner than the metal particles ignite, so that the igniting and burning metal particles are nearly always surrounded by a vapor phase oxidizer. The delayed ignition often causes further problems, making metal combustion less efficient. For example, an issue of critical importance for metallized solid propellants is the agglomeration of unignited aluminum particles [9], [10]. Such particles, initially mixed with AP and binder, melt and agglomerate before they ignite. The resulting large size agglomerates may never ignite or they ignite after very long delays. Because of delayed ignition, such agglomerates often cannot burn during the limited time they fly through the propulsion chamber. Thus, a large portion of the aluminum additive remains unburned, reducing dramatically both the efficiency of the propulsion system and obtained specific impulse.
Similar issues also explain the very limited range of application of conventional thermites. Initiation of a metal–metal oxide redox reaction is quite difficult for mixed micron-sized powders and requires extended pre-heating of a relatively large or a well heat-insulated sample. Due to the high thermal conductivity of metal–metal oxide mixture, small, poorly insulated samples lose heat very rapidly, and for such samples the initial heterogeneous reaction never becomes self-sustaining.
An idealized metal-oxidizer system similar to the monomolecular energetic compound can be described: a metastable, homogeneous metal-oxidizer solution in which the components are not chemically bonded. Thus, the reaction rate would not be limited by heterogeneous transport processes and can be dramatically accelerated. It was observed that metastable metal–gas solutions form naturally inside combusting metal particles [11], [12]. Once such compounds form, they indeed react very rapidly resulting in micro-explosions and disruptive particle combustion [13], [14], [15], [16]. However, it is anticipated that a relatively strong chemical bonding occurs in such solutions which would limit their energy density and thus respective practical applications.
It appears that a practically optimized metal-based energetic material would have the reactive components mixed on a scale as fine as possible, as long as significant chemical bonding between components is prevented. This naturally leads to the idea of using materials with high specific surface area, or materials divided down to the nanoscale in order to reduce ignition delays and accelerate combustion of metals, as proposed in Ref. [17].
This review is focused on such metal-based reactive nanomaterials, which became available recently as a result of active developments in the fields of materials synthesis and characterization. While the scale of mixing of metal and oxidizer in such nanocomposite materials remains coarser than can be achieved in a true metal–oxidizer solution, the enthalpy of formation of the bulk nanocomposite can remain close to that of the individual compounds. In addition to the metal–oxidizer systems, materials with metal–metal and metal–metalloid components have also been developed, with the common feature that the starting components are always capable of a highly exothermic reaction. The products of this initial reaction, intermetallic alloys or metal–metalloid compounds, often continue oxidizing if the reaction is initiated in an oxidizing gaseous environment.
Reactive nanomaterials quickly attracted a lot of interest among potential users and many different compositions were recently prepared, characterized, and evaluated. Such applications as percussion or electric primers, explosive additives, propellant rate modifiers, and others became feasible. Fundamentally new and exotic applications for metal-based energetic materials, such as MEMS energy sources [18], are also being discussed in conjunction with development of new metal-based reactive nanomaterials.
This review will discuss the methods used for preparation of reactive metal-based nanomaterials, techniques developed for their laboratory testing, and some of the published results describing mechanisms of their ignition and combustion.
Section snippets
Nanosized aluminum
Nanosized aluminum powder or nanoaluminum (n-Al) is the most common component of metal-based reactive nanomaterials, while other nanopowders, e.g., boron, magnesium, or zirconium have also been considered [19]. The popularity of n-Al is understandable because it is often considered as a potential replacement for the conventional aluminum powders and flakes widely used in explosives, propellants, and pyrotechnics. In fact, the rapid acceleration of research in the area of reactive nanocomposite
Powder mixing
Given the availability of n-Al powders, most metal-based reactive nanomaterials are prepared by mixing such powders with additional components capable of highly exothermic reactions with aluminum. Literature data are available for many mixes of n-Al with nanosized metal oxide powders; including Fe2O3, MoO3, CuO, Bi2O3, and others. These thermite compositions made of nanosized components were termed MIC (for metastable intermolecular composites or metastable interstitial composites) or
Materials properties
Knowledge of material properties is essential for understanding their reaction mechanisms, for prediction of their performance in energetic formulations, and for designing practical systems and components employing such materials. Some of the material properties commonly used for micron-sized reactive powders and structures will need to be redefined for materials with nanoscale features. For example, many reaction features for conventional metal fuels are affected by such thermodynamic
Exothermic heterogeneous reactions
The exothermic heterogeneous reactions are of primary interest as both driving ignition and affecting aging of reactive nanomaterials. Among such reactions, oxidation of nanoaluminum is the most fundamental and occurs in all such materials containing aluminum. This oxidation process was recently studied extensively and its mechanisms continue to be the subject of considerable debate. It will be discussed first, followed by a discussion of more complex, relevant solid–solid reactions.
Ignition studies
The exothermic heterogeneous reactions reviewed in the previous section are expected to be responsible for ignition of reactive nanocomposite materials under practically useful conditions. It is therefore expected that detailed reaction models based on thermo-analytical measurements combined with analysis of changes in the material morphology, structure, and composition, will be capable of predicting the ignition kinetics in such materials. However, given the complexity of processes involved,
Laboratory tests
In the early work on reactive nanocomposite materials, the primary motivation for most of the laboratory combustion experiments was to establish a clear and reproducible difference between combustion of such materials and that of reference materials using the same elemental or molecular compounds which were not mixed on the nanoscale. This led to a large number of semi-qualitative measures of reactivity, primarily involving one or another variant of an open tray burn experiment, e.g., Refs. [92]
Concluding remarks and future research
In the last 5–10 years, research has been very active in development and evaluation of many different types of reactive nanomaterials. Several approaches, including those based on mixed nanopowders, multilayered nanofoils, and three-dimensional micron-sized nanocomposite powders have been extensively investigated for a broad range of material compositions. It is anticipated that future research will focus on the development of scalable and cost-effective manufacturing approaches in which the
Acknowledgement
This work was primarily funded by Defense Threat Reduction Agency (DTRA) and interest, encouragement, and support of Drs. W. Wilson and S. Peiris of DTRA are very much appreciated. Additional funding was provided by RDECOM ARDEC, Picatinny (Mr. P. Redner). Multiple contributions from the graduate students and research staff members of the NJIT group are gratefully acknowledged. Discussions with Drs. M. Schoenitz and M. A. Trunov are acknowledged in particular.
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