Elsevier

Powder Technology

Volume 204, Issue 1, 10 December 2010, Pages 63-70
Powder Technology

The effect of surface modification of aluminum powder on its flowability, combustion and reactivity

https://doi.org/10.1016/j.powtec.2010.07.017Get rights and content

Abstract

Surface modification of aluminum powders for the purpose of flow improvement was performed and several samples were prepared. Correlations between the flowability and reactivity for these powders as well as for the initial untreated aluminum powder were established. The powders were characterized using Scanning Electron Microscope (SEM), particle size distribution, angle of repose flowability test, Constant Volume Explosion (CVE) combustion test, and Thermo-Gravimetric Analysis (TGA). The surface modification of micron-sized aluminum powders was done by: (1) dry coating nano-particles of silica, titania and carbon black onto the surface of spherical aluminum powders and (2) chemically and physically altering the surface properties of the same powders with methyltrichlorosilane. All surface modifications improved flowability of the powders. CVE measurements indicate that powders with an improved flowability exhibit improved combustion characteristics if the powder treatment does not add an inert component to aluminum. The TGA results do not show significant differences in the reactivity of various powders. Based on combined flowability and CVE characteristics, the silane modified material gave the best results followed by the powders dry coated with carbon, titania and silica, respectively.

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).

  1. Download : Download full-size image

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.

References (35)

  • Y. Kwon et al.

    Combust. Flame

    (2003)
  • A. Hahma et al.

    Combust. Flame

    (2006)
  • M. Schoenitz et al.

    Proc. Combust. Inst.

    (2005)
  • J.Q. Feng et al.

    Powder Technol.

    (2003)
  • A.B. Yu et al.

    Powder Technol.

    (2003)
  • J. Yang et al.

    Powder Technol.

    (2005)
  • I. Piwonski et al.

    Appl. Surf. Sci.

    (2006)
  • J.B. Donnet et al.

    Carbon

    (2002)
  • E. Papirer et al.

    Carbon

    (1999)
  • Y. Chen et al.

    Colloids Surf. A Physicochem. Eng. Aspects

    (2010)
  • O. Demirbas¸ et al.

    J. Hazard. Mater.

    (2007)
  • M.A. Trunov et al.

    Combust. Flame

    (2005)
  • M. Hertzberg et al.

    Proc. Combust. Inst.

    (1992)
  • P.E. Pokhil et al.
    (1972)
  • A.P. Ilyin et al.

    Combust. Explosion Shock Waves

    (2001)
  • E.W. Price et al.
  • Cited by (70)

    View all citing articles on Scopus

    The views, opinions, and/or findings contained in this article/presentation are those of the author/presenter and should not be interpreted as representing the official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of Defense.

    View full text