Studies on the Impact Initiation and Kinetics of Condensed Phase Reactives with Application to the Shock Induced Reaction Synthesis of Cubic Boron Nitride

Matthew T Beason, Purdue University

Abstract

Shock induced reaction synthesis is a complex, scientifically rich field with the potentially to produce novel materials with unique properties. This work seeks to understand the processes governing shock induced reaction synthesis. Particular emphasis is placed on the reaction kinetics of condensed phase reactives under various mechanical and thermal heating rates. This understanding was then applied to the synthesis of cubic boron nitride through shock induced reaction synthesis. Mechanical initiation of reactions in powder systems involve complex interactions that can yield unexpected results. Two materials that exhibit similar thermal responses can behave very differently under the same loading conditions due to differences in their mechanical properties. Reactive composite powders with small characteristic dimensions can exhibit short ignition delays and reduced thermal ignition thresholds; however, a full understanding of the response of these powders to rapid mechanical loading is still unclear. This work seeks to clarify the role of mechanical properties in impact induced ignition by considering the response of nanolaminate (NL) powders and high energy ball milled (HEBM) Ni-Al powders subjected to impact loading. The powders were placed into a windowed enclosure and mechanically loaded using a light gas gun, which allowed the resulting reactions to be observed using high-speed imaging. Even though the thermal ignition temperatures for the two powders are within 30 °C of each other, it was observed that the NL powders reacted on the microsecond timescale, immediately following the compaction wave for a short distance before decoupling from the compaction front. In contrast, the HEBM powders reacted after a several millisecond delay and clearly propagated as a deflagration front. Microindentation showed that the HEBM powders are much more ductile than those of NL. This suggests that the primary difference between the behavior of these materials on impact results from the ability and degree of the material to fracture, illustrating that the mechanical properties of a reactive material can have a dramatic effect on ignition during impact loading. By using the jump equations to understand compaction events, it is easy to think about the compaction wave as a discontinuity, with no structure. In practice this is not the case. Both shock waves and compaction events have been observed to have a structure with a finite thickness. Studies of the propagation of shocks through monolithic solids have shown that the strain rate, which is directly related to the shock width, scales with the pressure rise to the fourth power. Studies of dynamic compaction of porous materials have shown that this relationship is closer to linear. This work seeks to study the effect that increasing the crush strength of the compact has on the width of the compaction wave. Ball milling is used to produce strain hardened powders that are then pressed to form a porous compact. Plate impact experiments are performed to evaluate the equation of state and measure the shock width of both milled and unmilled powders. The results show that a Mie-Gruneisen equation of state accurately predicts the response of all materials tested; however, the compaction width is found to change with milling condition. For all materials tested, the compaction width is found to decrease with increase pressure rise; however, the unmilled material is found to have a longer rise time compared to the ball milled material. This results in a reduction in apparent viscosity with increased crush strength. It is suggested that stress waves percolating ahead of the compaction front (since the velocity of the compaction wave is below the acoustic velocity of the parent material) and their interaction defines the compaction width. In a weaker material, a weaker stress is required to begin compaction, resulting in a broader front compared to a stronger material and an increased viscosity. Despite their widespread use, the reaction pathways of thermite (reduction-oxidation) reactions are relatively unknown. Multilayer thin films produced through magnetron sputtering provide a highly controlled geometry and direct contact between reactives, making them an ideal platform to study atomic-scale processes underlying thermite reactions. This work utilizes the multilayer thin film geometry to study the combustion and reaction pathway of equimolar Al-NiO. The low heating rate kinetics and product phase growth are studied through hot-stage X-ray diffraction and differential scanning calorimetry. The results indicate significant product formation beginning as low as 180°C, and results in the formation of nickel aluminum intermetallic phases. Hot-plate ignition experiments show that ignition occurs in the solid state for fine bilayer thicknesses, with a transition to melt dependent reaction for multilayers with larger bilayer thicknesses. (Abstract shortened by ProQuest.)

Degree

Ph.D.

Advisors

Gunduz, Purdue University.

Subject Area

Mechanical engineering

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