Shock-wave processing of powder mixtures

Citation

Krueger, Barry Robert (1991) Shock-wave processing of powder mixtures. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/f01e-9k60. https://resolver.caltech.edu/CaltechETD:etd-06222007-081112

Abstract

The effects of shock waves on the initiation of exothermic chemical reactions in mixtures of powders is explored experimentally and compared to thermal initiation at atmospheric pressure in this thesis. A full understanding of shock initiated chemical reactions and shock compaction of composites requires knowledge of the Hugoniot of the mixture. A model for calculation of the shock Hugoniot of non-reacting solid or powder mixtures up to moderate pressures using only thermodynamic properties of the components is presented. In addition, conditions for the production of dense, bulk samples of a metallic glass from the metastable powder are determined. Previous models for the Hugoniot of a mixture assume the components in the shock front are in mutual thermal equilibrium, and use measured or calculated Hugoniot data for the components. The model proposed in this thesis does not presuppose either the relative magnitude of the thermal and elastic energies or temperature equilibrium between the components. It assumes the components are at equal pressures and have equal particle velocities. For a mixture, it is shown that the conservation equations define a Hugoniot surface, and that the ratio of the thermal energy of the components determines where on that surface the shocked state of the mixture lies. This ratio, which may strongly affect shock initiated chemical reactions and the properties of consolidated mixtures, is found to have only a minor effect on the Hugoniot. It is also found that the Hugoniots of solids and solid mixtures are sensitive to the pressure derivative of the isentropic bulk modulii of the components at constant entropy. The initiation of the reaction forming the compound NiSi from elemental powders by shock waves of varying energy and pressure and by thermal initiation at atmospheric pressure was investigated. Using plane wave shock geometry with well-defined shock pressure and energy, it was determined that a sharp energy threshold, between 384 and 396 J/g, exists for the initiation of the reaction (with 20 µm to 45 µm Ni and -325 mesh Si). The threshold energy range heats the powder mixture to a temperature between 631 and 648° C (with no chemical reaction) after local thermal equilibration is achieved. The reaction goes to completion when the shock energy is above the threshold energy, and melting of the compound is indicated. Differential thermal analyses (DTA) of powder mixtures of Ni and Si (1:1 atomic ratio) at atmospheric pressure show the reaction starts at a temperature which depends upon the porosity of the mixture. Higher porosities give higher initiation temperatures. Reaction starts at about 900° C in a mixture with 50% porosity and at about 650° C in a sample statically pressed to 23% porosity. The sharp energy threshold for the initiation of the reaction, and the correlation with the shock temperature and the reaction initiation temperature in the DTA indicates that the homogeneous temperature determines whether or not the reaction occurs rather than local particle conditions of temperature or pressure as has been proposed in the literature. The conditions for initiation and propagation of the reaction forming Ti5Si3 from elemental powders (5:3 atomic ratio) of varying porosity have been investigated using shock waves of different pressure in vacuum, and using hot wire ignition in an argon atmosphere. In powders with a high initial porosity, evacuated to 0.1 torr, a low energy regime (producing low shock pressures) triggers the reaction in the presence of residual oxygen while no reaction is observed with a 128% higher shock energy and a lower initial porosity (producing a higher shock pressure) in an inert residual gas. Hot wire ignition of porous powder at room temperature initiates a self-propagating high temperature reaction (SHS) in air or (less readily) in an Ar atmosphere, while the Ni/Si powder must be heated to allow the reaction to propagate in high or low porosity mixtures. These observations are compared to published work on self-sustaining reactions in multilayer films.

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