A Multi-Scale Study of Solid Phase Reactions: Unraveling Mechanical-Chemical Interactions

University Of Nebraska-Lincoln, Lincoln NE

Investigators

Abstract

ABSTRACT PI: H. J. Viljoen Institution: University of Nebraska at Lincoln Proposal Number: 0096381 Certain solid materials, outside the class of classic explosives, can react in an extremely violent manner if they are hydrostatically compressed. The elastic potential energy in the solid's lattice is "unlocked" in an avalanche-like manner and if this energy release is synchronized with a chemical reaction, a shock wave of extraordinary intensity could form in the solid structure. The existence of solid phase detonation becomes a real possibility. The conditions in such a shock front could be contemplated: presses in excess of one million atmospheres and shock wave velocities of 5-10 km/s. Although such pressures and velocities are routinely reached in implosion experiments and conventional explosive, energy density in a solid phase detonation exceeds conventional processes by several orders of magnitude. This high-energy wave constitutes a non-equilibrium state of the material, but simultaneously it finds a harmonic interaction across atomic, mesoscopic and macroscopic scales. It is the goal of this project to investigate events of energy transfer on these three scales. A hierarchical approach is planned. Small clusters of single element/compound of a few thousand atoms will be studies by molecular dynamics simulation. Energy distributions and characteristic times to equilibrate are calculated for a variety of dynamic and hydrostatic loadings. Clusters of different compounds introduce chemistry as an additional form of potential energy to the system. When pre-compressed clusters are brought in contact and lattice collapse is initiated at one face, the symbiotic interaction between chemistry and elastic potential energy release will be elucidated. The mesoscopic models are used to study energy transfer on a particle level. Although time scales are still small enough to study non-equilibrium states, atom-atom interaction is limited to nearest neighbors to expand the length scale. Waves begin to build up across several atomic layers. When sufficient energy is supplied during build-up, a soliton is created. This particular wave type traps elastic potential energy. At heterogeneities some energy is transferred to thermal energy, but local spalling also occurs and lattice collapse is initiated. The inverse scattering theory will be employed to analyze formation of solitons. Contiuum models need refinement, because kinetics depends not on the concentration of reactants, but on their surface area. Combinatorics provides expectation values of common surface area and dependency on surface area due to soliton action, melting and inhibition by condensed phase products. Descriptions of the state of species must be included: particle size and surface area. This links the macroscopic model with mesoscopic models. In addition, a series of experiments are planned to test the proposed mechanisms. Reactive mixtures are loaded in a Bridgman anvil. The amount of potential energy loaded in the system is varied and beyond critical load a fast reaction with active participation of the elastic potential energy in the reaction process is anticipated. Metal/oxide and metal/non-oxide systems with different physical properties will be explored. The electrothermal explosion method will be combined with the Bridgman anvil to study kinetics under pressure.