Control of ignition and reaction behavior in gasless reactive systems via microstructural modification

Robert V Reeves, Purdue University

Abstract

Gasless reactive systems are attractive for use in many energetic material applications due to their extremely high-energy density. However, they have many limitations that reduce their usefulness, namely their low reaction propagation rate and the large energy input typically necessary for reaction initiation. Microstructural modification of these systems through methods like shock processing has been found to increase the reaction rate and sensitivity of these systems. In this work, the theory that similar microstructural modification through more precise methods like high-energy ball milling (HEBM) and particle size reduction can provide similar benefits but in a controlled manner. To elucidate the processes of rapid reaction initiation in gasless systems, a method of directly monitoring the kinetics of phase transition and subsequent reactions in the Tungsten/Silicon gasless reactive system is developed. The method utilizes synchrotron radiation in transmission geometry to create images of internal microstructural changes with 10 μm spatial resolution and 27.75 μs temporal resolution. Details of the Si melt and the reactions between W and Si, unable to be seen with visible-light imaging, were revealed. The processes include phenomena affecting reactant mixing and the subsequent reaction. A theoretical model for accurately identifying the progress of chemical reaction in both spatial and temporal domains is also proposed. Investigation of the mechanisms of reaction initiation in gasless reactive systems was also performed. Thermal initiation was investigated with Differential Thermal Analysis (DTA), as well as electro-thermal explosion (ETE) techniques. Mechanical impact initiation testing was performed through shear impact testing powered by a light gas gun. HEBM materials and mixtures of Ni/Al nanopowders were tested. DTA and ETE showed the HEBM materials reacted at temperatures below the eutectic point; however, these materials were less sensitive to mechanical initiation. The nanopowder mixtures reacted at higher temperature in the DTA, but ignited under much lower levels of mechanical stimulation than were applied to the HEBM samples. The nanopowder mixtures also exhibited a high-speed reaction mode when experiencing extremely high energy impacts. The high-speed mode was not selectable in the HEBM materials. These observations indicate the methods of particle size reduction and HEBM have differing effects on the thermal and mechanical sensitivities of the Ni/Al reactive system. It is believed that the increased interfacial area of the HEBM materials is responsible for the significant reduction the material's ignition temperature. Alternately, the fine porosity exhibited by the nanopowder samples provides more efficient conversion of kinetic energy from the impact to thermal energy that is usable by the system.

Degree

Ph.D.

Advisors

Son, Purdue University.

Subject Area

Mechanical engineering

COinS