Improved thermal properties of Al powders coated with submicron-sized hollow nickel particles
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Hsiao Y. Ng, Pei C. Soon, and Yiew W. Lee DSO National Laboratories, Singapore 118230, Singapore (Received 18 February 2009; accepted 2 July 2009)
This work investigates the coating of nano- to submicron-sized hollow nickel particles onto Al powders to optimize the thermal and combustion properties of Al-based energetic materials. Different wt% of hollow nickel particles coated onto Al particles was synthesized using an autocatalytic aqueous reduction method. X-ray diffraction results showed distinct crystalline Al and Ni phases indicating that a pure coating process was achieved. Homogeneous coating of hollow nickel particles onto individual Al particles was also reflected from the scanning electron microscope images. All the samples showed significant improvements in terms of their ignition and kinetics of combustion as compared to untreated Al. The most promising candidate produced was the 30wt%Ni–Al sample whereby the %complete combustion of Al exhibited the largest improvement. The hollow Ni-coated Al samples were also found to perform better than their physically mixed Ni–Al powder samples, as there is more intimate contact between the two components to hasten the overall combustion kinetics. I. INTRODUCTION
Aluminum (Al) has been widely used as an energetic material in propellants due to its high enthalpy. However, practical uses are limited due to its slow ignition time, which is contributed by the formation of the protective oxide layer (alumina) on the particle surface. Aluminum typically ignites in an oxidizing atmosphere only after heating up to temperatures close to the melting point of alumina at 2300 K.1,2 Furthermore, it has been reported that agglomeration of Al particles has led to the incomplete burning of Al particles and promotes slag formation.3 The full potential of Al has not been realized due to slow kinetics that result in incomplete combustion in the desired time frame. Aluminum particles coated with a thin layer of nickel (Ni) have been shown to reduce particle agglomeration during metalized propellant combustion.4 The exothermic chemical reaction between aluminum and nickel5 is also believed to promote the ignition process. In this work, a novel concept of using nano- to submicron-sized hollow nickel particles coating onto Al powders is proposed. This approach is based on the idea of the collapse of hollow microbubbles where extremely high pressures and high temperatures in the order of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0389
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 10, Oct 2009 Downloaded: 14 Mar 2015
thousands of degrees Kelvin could be locally generated.6,7 Thus, with the use of hollow nickel particles coating, they can actually act as localized “hot spots” for enhanced thermal storage to improve the Al–Ni thermal properties. II. EXPERIMENTAL
The method that was attempted for this study is an autocatalytic aqueous reduction method adopted from the work reported by Deng et al.8 on the synthesis
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