Elsevier

Combustion and Flame

Volume 140, Issue 4, March 2005, Pages 299-309
Combustion and Flame

Combustion wave speeds of nanocomposite Al/Fe2O3: the effects of Fe2O3 particle synthesis technique

https://doi.org/10.1016/j.combustflame.2004.10.009Get rights and content

Abstract

Combustion wave speeds of nanoscale aluminum (Al) powders mixed with iron oxide (Fe2O3) were measured as a function of Fe2O3 synthesis technique and fuel/oxidizer composition. Three reactant synthesis techniques were examined; two focus on sol–gel processing of nanoscale Fe2O3 particles and the third utilizes commercially available nanoscale Fe2O3 powder. Nanoscale aluminum particles (52 nm in diameter) were combined with each oxidizer in various proportions. Flame propagation was studied by igniting low-density mixtures and taking data photographically with a high-speed camera. Both open and confined burning were examined. Results indicate that the combustion wave speed is a strong function of the stoichiometry of the mixture and a slightly fuel-rich mixture provides an optimum combustion wave speed regardless of oxidizer synthesis technique. Oxidizers processed using sol–gel chemistry originally contained impurities which retarded the combustion wave speeds. When the same oxidizers are annealed at moderate temperatures, the new heat-treated oxidizer shows a dramatic improvement, with combustion wave speeds on the order of 900 m/s.

Introduction

The development of nanostructured energetic materials is a relatively new area of investigation. There are basically three structures already being synthesized: (1) foils composed of alternating layers of fuel and oxidizer nanoscale films [1], [2]; (2) composites of nanoparticles prepared by ultrasonic mixing (MICs) [3], [4]; and (3) composites prepared using sol–gel processing [5], [6], [7], [8], [9]. All three structures offer advantages and disadvantages for thermite applications. For example, foils are highly structured composites that are prepared by deposition of the reactants onto a substrate. The thickness of the deposited film is controlled so that the amount and diffusion distance between reactants can be tailored to optimize the combustion process. Greater uniformity between reactants is achieved at the expense of small-scale production. The procedure can be time-consuming, expensive, and typically only small amounts of material can be synthesized at a single time. Mechanical (or ultrasonic) mixing of individual particles is a more common technique for combining reactants. This process is relatively inexpensive and large quantities of material can be prepared quickly. However, the unstructured nature of the composite causes large and almost uncontrollable variability in the combustion behavior.

Researchers at the Lawrence Livermore National Laboratory (LLNL) have recently invented a new method of making nanostructured energetic materials using sol–gel chemistry [5]. The approach combines fuel and oxidizer through sol–gel chemistry and allows one to uniformly disperse solid fuels within a nanoscale oxidizer framework matrix, while eliminating concentration gradients in the final material due to settling. The sol–gel method is a bulk chemical technique for the synthesis of materials with nanometric dimensions. Fig. 1 is a schematic summary of the sol–gel process. Molecular precursors dissolved in solution undergo hydrolysis and condensation to form a stable sol (suspension of particles 1–1000 nm in diameter in solution) [5], [6], [7], [8], [9]. This process is initiated by changing the pH, temperature, or ionic strength of the solution or through the addition of a catalyst or gelling agent as described in more detail in Refs. [5], [6], [7], [8], [9]. Through manipulation of these conditions the sol can be further linked through condensation of surface groups on the particle surfaces to form a gel. The gel is a rigid three-dimensional structure that has a nanostructured framework and pores with dimensions of 2–100 nm in diameter. The resulting wet gel can be dried under atmospheric conditions to yield a xerogel. A xerogel is a porous material with high surface area (50–500 m2/g) and a moderate density (typically 20–75% of that of the bulk). Alternatively, supercritical extraction of the solvent in the wet gel leads to an aerogel. An aerogel is a highly porous material with a high surface area (100–1000 m2/g) and low density (typically only 1–25% of that of the bulk). These properties make aerogels remarkable thermal insulators.

The sol–gel method provides a means of controlling microstructural properties, such as particle size and morphology, as well as the chemical composition of the composite. All of these properties will affect the combustion behavior of the final mixture. For sol–gel energetic nanocomposites, the gel oxidizer framework acts to capture the suspended fuel nanoparticles by growing around them. Although the ultimate goal is to combine the fuel with oxidizer in solution through sol–gel synthesis, this study will only synthesize the oxidizer via sol–gel processes. An aerogel, xerogel, and commercially obtained oxidizer were individually mixed with aluminum fuel particles using an ultrasonification process. By comparing sol–gel synthesized oxidizers with a commercial oxidizer, a thorough understanding of the effect of oxidizer on combustion performance will be achieved. The results of this study could lead to the development of improved sol–gel synthesized energetic composites.

This study will examine the combustion performance of a classic thermite, Al + Fe2O3, synthesized using ultrasonic mixing of fuel and oxidizer particles. Objectives are to determine the optimum stoichiometry based on combustion wave speed for the Al/Fe2O3 composites and determine the combustion behavior of the powder in confined and open environments. Nanoscale Al powder is mixed with three separate Fe2O3 oxidizers. The first two were prepared at LLNL and are sol–gel synthesized xerogel and aerogel. The third is a commercially obtained Fe2O3 powder (NANOCAT, from MACH I, King of Prussia, PA) and is composed of 3-nm-average-diameter particles. The NANOCAT was used as a reference oxidizer for comparison to the sol–gel synthesized oxidizers. All oxidizers have similar thermodynamic properties but are different in their physical properties. For example, the NANOCAT and aerogel oxidizers are fluffy and can easily become airborne. The xerogel oxidizer is not fluffy and will not become airborne easily. Fig. 2 illustrates the physical nature though photographs of the same sample mass of each oxidizer.

Section snippets

Sample preparation

The oxidizers vary according to preparation technique and are described in Table 1. The aluminum was obtained from Technanogy Inc. (Irvine, CA) and has a 52-nm average particle diameter. This diameter is calculated from surface area measurements made using a Micrometerics ASAP2000 gas adsorption analyzer using BET (Brauner, Emmett, and Teller) theory and five data points between relative pressure 0.05 and 0.2 at 77 K. The adsorption gas was nitrogen. Aluminum particles are pyrophoric and are

Open burn experiments

The optimum combustion wave speed for the given stoichiometry was determined from performing a series of experiments varying the equivalence ratio from 0.9 (fuel-lean) to 4.0 (fuel-rich). Fig. 5A shows a sequence of still frame images illustrating flame propagation along the channel for a mixture of 52 nm Al and Fe2O3 NANOCAT at an equivalence ratio of 1.4. The camera is recording visible light emission images at 32,000 frames/s. Frame 0 illustrates a ruler reference for the experiment and

Discussion

Fig. 5A illustrates the nature of flame propagation through an unconfined powder. The images were captured at 32,000 fps and the aperture on the camera lens was almost fully closed. This sequence of images indicates that the flame is highly luminescent and radiation transport may be significant in flame propagation. In fact, the plume appears to precede the flame front and may contribute to preheating the particulate bed prior to reaction. Thermal equilibrium calculations for the adiabatic

Conclusions

Combustion wave speeds are a strong function of the oxidizer synthesis technique. The sol–gel prepared aerogel and xerogel oxidizers contain impurities that act as heat sinks during flame propagation and retard the combustion wave speed. By heat-treating the oxidizers to remove the impurities (i.e., bonded water) in the aerogel and xerogel, a significant increase in combustion wave speeds was observed. The composites with aerogel showed the largest increase in combustion wave speed from ∼10 to

Acknowledgments

The authors gratefully acknowledge the Army Research Office (Contract DAAD19-02-1-0214) and our ARO program manager, Dr. David Mann. Dr. Gash acknowledges the Office of Munitions, Memorandum of Understanding Programs. Some of this work was done under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore Laboratory under Contract W-7405-Eng-48. We are grateful to Dr. Steven Son for helpful discussions and Dr. Michael Hiskey for the donation of the

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