The effect of surface modification of aluminum powder on its flowability, combustion and reactivity☆
Graphical Abstract
Surface modification of aluminum powders using dry coating nano-particles of silica, titania and carbon black and chemical treatment via methyltrichlorosilane shows improved flowability which also improves the combustion behavior of aluminum powder if the powder treatment does not add an inert component to the surface (e.g. oxides).
Introduction
Aluminum powder is used as an additive in energetic materials for various applications, due to its high calorific value [1], [2], [3], [4]. However, agglomeration of aluminum particles before and during the combustion process reduces the burning rate and therefore reduces combustion efficiency when residence time of aluminum particles in the combustion device is limited [5]. Extensive research has been done to investigate and improve the combustion behavior of fine aluminum powder [6], [7], [8]. An effective way to increase the reactivity of aluminum powder is to reduce the particle size, but the effectiveness of this method is limited by the fact that fine aluminum particles agglomerate naturally. The agglomerations occur due to the very strong inter-particle cohesion, mainly ascribed to van der Waals forces [9] dominating the particle weight [10]. Agglomeration of dry aluminum powders hinders mixing of such powders with energetic binders; it also poses a problem in several applications where aluminum powders are directly injected into an oxidizer flow. The latter situation exists, for example, in aluminum–water propulsion devices [11], [12], including water ramjet designs [13].
In order to improve the flow of fine aluminum particles as well as to reduce their agglomeration upon their injection into a combustion chamber, various approaches have been developed, e.g., encapsulating the aluminum particle with organic or inorganic materials [14], [15]. It is proposed here that a dry coating technique [16] may also be used to improve flowability, reduce agglomeration, and improve combustion behavior of aluminum powders. Dry coating was shown to be efficient in reducing the inter-particle adhesion forces and to improve the flowability [17] and fluidizabilty [18] of fine powders by precisely depositing small amount of nano-sized particles (called guest particles) on the surface of primarily cohesive powders (or host material). The deposits artificially generate nanoscale roughness, which can reduce the area of contact when two surfaces are in touch with each other. Yang et al. [17] found that the adhesion force between dry coated fine particles is reduced in proportion to the ratio of the guest particle radius to the average asperity radius of the host particle, and is a major factor that contributes to the flow improvement. An alternative approach to improve the powder flowability is chemical modification of its surface. In particular, silane treatment of aluminum powders is considered in this effort. The modification of spherical, micron-sized aluminum powders using different surface modification techniques is attempted as a means to reduce aluminum agglomeration. Our main objective is to determine whether surface modification by silane treatment or by dry coating of nano-silica, titania, and carbon black improves flowability, and thereby enhances aluminum powder combustion behavior.
Section snippets
Materials and methods of surface modifications
The starting materials used in this study are two batches of H-5 aluminum powders from Valimet Inc, silica powder (hydrophobic) from Evonik, and carbon black and titania from Cabot Inc., USA. Methyltrichlorosilane 99%, CAS 75-79-6, purchased from Dow Corning, USA. Properties of the raw materials are summarized in Table 1. Particle size measurements are described below.
Two batches of H-5 aluminum were used in this study in order to determine if there is a difference in their flowability and
SEM images
Images showing the particle morphology, the presence of guest particles on the surface of dry coated host particles, the roughness of the surfaces of the uncoated and silane treated particles, and morphology of the agglomerates of the original material were obtained with a LEO 1530 field emission SEM. Samples were mounted on aluminum stubs using a double sided carbon tape and sputter coated with carbon. Representative SEM images are shown in Fig. 1, Fig. 2. Fig. 1 shows the images of uncoated
Discussion
Flowability of powders is affected by cohesion/adhesion, and two major factors influencing adhesion are surface energy (defined in the form of work of adhesion/cohesion) and surface roughness. Extremely smooth surfaces or very rough surfaces (wide spacing between asperities) have a large adhesion force due to the large true area of contact [34]. However, intermediate values of surface asperity, in particular those at nanoscale, yield a lower adhesion force [17], [18]. Such asperity spacing is
Conclusion
Dry particle coating and surface silanization methods used to modify the surface of aluminum particles are found to be effective in the improvement of flowability through the reduction of particle cohesion. For dry particle coating, cohesion is reduced through the introduction of nanoscale roughness. For surface silanization, cohesion is reduced by chemically modifying the particle surface and lowering the surface energy. The surface modification methods also led to a small reduction in the
Acknowledgements
This work has been supported by the U.S. Naval Undersea Warfare Center, the National Science Foundation through the ERC (EEC-0540855) awards, and the Defense Advanced Research Projects Agency. Authors also thank Aveka, Inc., Woodbury, MN, for providing use of the MAIC device. We thank Roger Sullivan and Brian Zentner (Naval Air Warfare Center) for assistance with the silane treatment process, and Gregory Ostrom for conducting the ion chromatography analysis.
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