Elsevier

Combustion and Flame

Volume 159, Issue 1, January 2012, Pages 2-15
Combustion and Flame

Reactive sintering: An important component in the combustion of nanocomposite thermites

https://doi.org/10.1016/j.combustflame.2011.07.015Get rights and content

Abstract

One of the open questions in understanding the reactivity of nanometric metal/metal oxide particulate composites is the relative role of gas–solid vs. condensed state reactions. This work is an investigation of several nano-Al based thermites subjected to very rapid heating rates. The ignition temperature of thermites, as measured by the onset of optical emission, was measured using a rapidly heated Pt wire. Generally, ignition was seen to occur above the melting temperature of aluminum. The exception was Al/Bi2O3 which ignited slightly below this point. Samples were also rapidly heated in situ within electron microscopes to provide direct imaging before and after heating. The sintering of aggregated and/or agglomerated particles into characteristically larger structures was experimentally observed in all cases, and the fuel and oxidizer were found to be in surface contact suggesting that a condensed-phase reactive sintering mechanism had occurred. High resolution image sequences of thermites ignited on the Pt wire were collected using a real time phase contrast imaging technique at the Advanced Photon Source of Argonne National Lab. The results suggest that reactive sintering occurs on a fast timescale, and relatively early in the reaction, leading to rapid melting and coalescence of aggregated particles. This dramatically changes the initial size and morphology of the constituents before the remainder of the material burns. The results question the idea that a decrease in particle size will necessarily lead to an enhancement in reactivity, since large amounts of sintering occurs early in the reaction, and alters the morphology. It is suggested that improvements in reactivity can be achieved by designing architectures to improve the interfacial contact upon sintering, as well as by selecting oxidizers based on their ability to liberate and transport oxygen in the condensed phase, while producing volatile species to assist in convective energy transport.

Introduction

Nanocomposite thermites, or metastable intermolecular composites (MICs) are intimate mixtures of metal/metal oxide nanoparticles, and typically have the consistency of a loose powder. Using nanoparticles greatly reduces mass diffusion lengths between the fuel and oxidizer, and also increases the interfacial contact and homogeneity of mixing. Upon ignition, these materials give rise to a self-propagating reaction with a characteristically high temperature, and low to moderate gas production. Research on MICs can be traced back about 15 years to when Aumann et al. [1] showed that using nanoparticles of Al/MoO3 resulted in several orders of magnitude increase in combustion characteristics over similar mixtures with micron-sized particles. Since then, research efforts have increased to understand the ignition and combustion mechanism, so that improvements in safety and performance can be achieved.

MICs have been experimentally shown to exhibit pressures and flame velocities somewhere in between propellants and explosives [2]. Flame velocities range between 10’s to 1000’s of meters per second, while the pressures range between a few to nearly 1000 atmospheres. The pressure and flame velocity in MICs is something that can be tuned through easily-adjusted parameters, such as changing the method and uniformity of mixing [3], [4], [5], particle size and distribution [6], [7], [8], choice of materials and stoichiometry [9], mixture density [10], or by other techniques such as electrostatic assembly [11] or creating new types of core–shell oxidizers [12]. This tunability, along with other attributes such as high mass/volumetric energy densities and the production of environmentally benign products, make MICs very attractive energetic systems. Nano-sized aluminum is a fuel which exhibits a high reactivity, and nano-Al based MICs are currently being investigated for uses in propellants, explosives, and pyrotechnics, along with other more recent applications, such as antimicrobial energetic systems designed to combat harmful biological agents [13].

Despite the amount of experimental results available in the literature, the ignition and combustion mechanism remains poorly understood. A major problem has been designing experimental techniques which can probe the intrinsic reaction while replicating the environment these materials are subjected to during the self-heating in a freely propagating reaction. This means very rapid and uniform heating, speculated to be somewhere in the range of 4 × 104 K/s (Martirosyan et al. [14]) to upwards of 108 K/s, predicted by an ad hoc calculation assuming thermites can reach an ignition temperature of ∼1000 K in 10 μs, an experimentally observed pressure rise time [15]. Furthermore, in order to understand the thermite mechanism, the ignition and combustion mechanism of nano-Al itself must first be well understood.

Nano-Al forms a passivating oxide shell when exposed to air. This shell is amorphous and uniform [16], and typically has a thickness of 2–3 nm [17]. The oxide shell can occupy a relatively large portion of the particle’s mass, and in some cases can even exceed 50 Wt% [18]. One result generally observed is that the measured ignition temperature of nano-Al is much closer to the melting point of Al, and far below the melting point of Al2O3, which is where large aluminum is reported to ignite [17]. This observation has led to much speculation that the interaction between the low melting point core (933 K) and the high melting point shell (2327 K) is critical to elucidate the underlying mechanism leading to ignition.

Several authors have focused on understanding the ignition mechanism of nano-Al from the standpoint of the core/shell interaction. Trunov et al. [17], [19] used thermal analysis (∼40 K/min) and developed an ignition mechanism suggesting that outwards transport of aluminum can occur as the oxide shell undergoes polymorphic phase transformations, rendering it permeable to aluminum. Using hot stage microscopy at similar heating rates and under vacuum, Rai et al. [20] have observed the outwards flow of aluminum through the oxide shell above the melting temperature. In a separate work, it was shown using molecular dynamics simulations that the effective transport rate of aluminum through the shell can significantly be enhanced by built-in electric fields [21]. At even higher heating rates (∼106  108 K/s), Levitas et al. [22], [23], [24], [25] have suggested that a “Melt Dispersion Mechanism” can occur. In such a mechanism, the rapid melting of the aluminum core induces significant stress in the oxide shell to completely rupture it, followed by the subsequent unloading of the aluminum core via tensile stresses. The authors have suggested that in such a mechanism, the kinetics would not be limited by diffusion. A key parameter for this mechanism is the relative thickness of the oxide shell to the aluminum core, and the authors have shown in some cases that increasing the shell thickness can lead to a higher flame velocity, a result which is consistent with such a mechanism.

Understanding the oxidation mechanism of nano-Al is even more challenging when one considers that the mechanism is strongly contingent upon understanding the relevant ignition mechanism. Also, experimental techniques which employ uniform and rapid heating rates are necessary to replicate a true combusting environment. Rai et al. [27] have investigated nano-Al oxidation at intermediate heating rates (∼103 K/s) using a furnace, and have suggested that oxidation is diffusion-controlled via a shrinking core mechanism [28], which involves inwards diffusion of O2 through the oxide shell. At even higher heating rates, (∼106  108 K/s), Bazyn et al. have conducted several experiments of nano-Al burning in varying environments inside a shock tube [29]. The authors used optical pyrometry to measure the combustion temperature of the particles as a function of pressure and gas composition, and suggest that the burning cannot be modeled by a droplet burning model, but instead large heat losses characteristic of nanoparticles cause the flame to sit much closer, if not directly on the particle surface. This suggests that heterogeneous reactions between the gas and the particle are prominent in the combustion mechanism. The authors have also investigated the ignition and combustion of nanocomposite Al/Fe2O3 and Al/MoO3 using the same technique, and measured the ignition temperatures in an inert environment to be 1400 and 1800 K, respectively [30]. It should be noted that these ignition temperatures are significantly higher than the melting temperature of Al, which has been experimentally observed to be approximately where nano-Al ignites in a gaseous oxidizing environment [17]. The authors also measure the combustion temperatures of the composites to be in the range of 2750–3350 K, and find that combusting in an oxygenated environment can raise the temperature several hundred degrees, indicating some degree of reaction with the gas.

Besides the aforementioned experiments, there have been limited other studies of the ignition and combustion of nano-Al and nano-Al composite materials which:

  • (a)

    Avoid the negative effects of studying a bulk sample such as packing density, mixing, differences in heating, etc.

  • (b)

    Probe intrinsic properties.

  • (c)

    Uniformly and rapidly heat the samples.

These considerations have led to the development of temperature jump (T-Jump) techniques, which can ramp the temperature of a small amount of sample very quickly. In these experiments, a thin wire or filament (coated with sample) is supplied a tunable voltage pulse and rapidly heats (∼105  106 K/s) through resistive heating. The ignition and combustion event can be monitored optically [31], [32], or in a mass spectrometer [33] to probe transient species evolution. Chowdhury et al. [31] used this setup to examine the ignition delay in a nano-Al/CuO composite as a function of aluminum oxide shell thickness. The authors concluded that the diffusion of Al through the oxide shell was responsible for the delay, since an increasing delay time was measured with increasing oxide shell thickness.

One other phenomenon which has received little attention in nanoparticle combustion studies, and is relevant to this work, is the sintering of aggregated particles. This directly impacts the question of size dependence on reactivity, and what is the “effective” particle size of the reacting material. Commercially available nanoparticles are almost always highly aggregated, and the size specified by a supplier often corresponds to the average size of the primary particles within these aggregates. Surface tension forces will of course drive the particles to coalesce if the temperature is sufficiently high to make the particles liquid-like [34], [35]. In a reacting thermite, nanoparticles can be rapidly heated by hot interstitial gases, and can be further heated by the energy liberated during an exothermic reaction. If the exothermic reaction is primarily responsible for inducing the sintering processes, it can be referred to as reactive sintering, a phenomenon which has been shown to be important in Al/Ni reacting systems [36].

One key point we will have to consider is how the kinetic timescale for sintering compares with reactive timescales we observe experimentally. If sintering occurs on a comparable timescale, or faster, than the chemical reaction, it raises two very important questions:

  • 1)

    Do nanoparticles maintain their high surface area morphology during combustion, and if not, then what is the appropriate “size” to report?

  • 2)

    Is there an advantage of using aggregated nanoparticles below a certain critical size?

The current work is a compilation of various experiments investigating the reaction mechanism of both nano-Al and nano-Al thermites. The studies are all performed at high heating rates (105  106 K/s), by passing a tunable voltage pulse through either a thin Pt wire or a specially fabricated electron microscopy grid. Several different thermite systems were investigated, and not all were studied using every experimental technique, largely due to time constraints on borrowed equipment or facility usage. The particular thermite studied in each case, therefore, was selected based on what would give the clearest representation of the steps involved in the nanocomposite thermite reaction for the particular experimental technique. The experimental results will suggest that a general similarity for all systems studied is that at least some amount of condensed-phase reaction and sintering occurs.

Section snippets

Experimental

The nano-Al used in this work is termed “50 nm ALEX”, and was purchased from the Argonide Corporation. The average primary particle size is specified by the supplier to be 50 nm, and the elemental portion of the particles was found to be 70% by mass, measured using thermo gravimetric analysis (TGA). A representative image of the nano-Al is shown in Fig. 1. The primary particles are largely spherical in nature, and are highly aggregated. One of the samples of CuO, which we term “6nmCuO”, was

T-Jump/PMT ignition temperature

The ignition temperature, is summarized for various thermite systems in Table 3. Also included in the table is the bulk melting point of the oxidizer. While both WO3 and Bi2O3 do melt, the melting of CuO and Fe2O3 involves the decomposition to Cu2O and Fe3O4 (and FeO at higher T), coupled with the release of O2 gas. We have recently argued, through temporally resolved mass spectrometry, that the O2 release for these particular oxidizers plays an important role in the ignition and combustion

Characteristic reaction and sintering times

Up to this point, we have shown visual evidence that sintering is occurring, however, we have only really discussed it in the context of a reactive sintering mechanism. That is, the exothermic reaction initiates, and this causes rapid melting/fusion of adjacent particles. In general, particle sintering is a thermally activated process, and will occur even in the absence of an exothermic reaction, as long as the temperature is sufficiently high. The following section investigates the kinetic

Conclusion

This reaction mechanism of nano-Al based thermites using several high heating techniques was investigated. First, thermites were rapidly heated on an ultra thin Pt wire, and the optical emission was monitored to determine the ignition temperature. It was found that three nano-Al based thermites (CuO, Fe2O3, WO3) ignited above the melting temperature of Al, with Al/Bi2O3 igniting just below this temperature.

High heating rate microscopy experiments were conducted for pure nano-Al and CuO, along

Acknowledgments

We thank the Army Research Office and the Defense Threat Reduction Agency for their financial support. We acknowledge the support of the University of Maryland’s Nanocenter and its NISPLAB. The NISP laboratory is operated jointly by the Maryland Nanocenter and the NSF MRSEC as a shared experimental facility. We also thank Rich Fiore and Protochips, Inc. for their technical help and for supplying the heating holder and grids. We thank Dr. Wen-An Chiou for his help with the operation of the

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