Structural Refinement in Al-MoO 3 Nanocomposites Prepared by Arrested Reactive Milling
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Structural Refinement in Al-MoO3 Nanocomposites Prepared by Arrested Reactive Milling Swati.M. Umbrajkar, Mirko Schoenitz and Edward L. Dreizin New Jersey Institute of Technology, Newark, NJ 07109 U.S.A ABSTRACT Al-MoO3 nanocomposites were synthesized based on arrested reactive milling. Altering the milling parameters leads to different scales of refinement in the structure of the reactive nanocomposites. The objective of this work is to determine the range in which the degree of structural refinement can be changed in a controlled manner. The milling intensity was controlled by using different milling media along with varying amounts of process control agent (PCA). XRD, SEM, DSC and wire-ignition tests were performed to analyze the Al-MoO3 nanocomposites. Results indicate that there is a decrease in the crystallite size with increase in the milling intensity. However increase in milling intensity also stimulates the undesired reaction between Al and MoO3. Milling conditions resulting in the highest structural refinement and lowest ignition temperatures were identified. INTRODUCTION Thermites are used for many applications ranging from igniters in automobile airbags to welding. Using compositions based on Al and metal oxides, different types of reactive nanocomposites were produced, including mixtures of nanopowders (or metastable intermolecular composites, MIC) [1, 2], nanofoils [3] and nanocomposite powders prepared by arrested reactive milling (ARM) [4, 5]. Such materials are characterized by higher reaction rates as a result of a higher reactive surface to volume ratio. Nanocomposites synthesized by ARM are micron-sized powders, in which reactive ingredients are mixed on the nanoscale within each particle. They offer unique advantages over the competing materials because of ease of their handling and low level of passivation and thus a higher energy density. In addition, they are inexpensive to produce and can be made with a wide variety of reactive components. The specific thermite system 2Al +MoO3 is well known and respective MIC materials were studied previously [1,2, 4 - 7]. In this study, Al-MoO3 nanocomposites were synthesized using ARM [4, 5], a technique derived from reactive milling [8, 9]. In ARM, the reactants are ball milled, and the milling process is interrupted before a spontaneous reaction is mechanically triggered. The milling time at which the reaction is mechanically triggered effectively sets a limit to the achievable degree of refinement. This refinement can also be influenced by the specific milling parameters chosen, such as material and size of the milling media, mass ratio of sample to milling media, and the use of process control agents (PCA). The scientific goal of this work is to establish a correlation between the milling parameters that influence the synthesis of the nanocomposites and the structure, properties, and performance of the nanocomposites. The practical goal is to optimize the synthesis of the Al-MoO3 nanocomposites for use in propellants and explosives. EX
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