Shock Modification

Shock Modification and Shock Activation Enhanced Solid State Reactivity 

In this chapter shock modification of powders (their specific area, x-ray diffraction lines, and point defects) measurements via analytical electron microscopy, magnetization and Mossbauer spectroscopy shock activation of catalysis, solution, solid-state chemical reactions, sintering, and structural transformations enhanced solid-state reactivity. 

The reported shock-modification observations show that shock-treated powders are substantially modified in their defect structures. From the defect point of view they are essentially new materials. Concentrations of point, line, and higher-order defects are found to be as large as those achieved by any 

The measures of solid state reactivity to be described include experiments on solid-gas, solid-liquid, and solid-solid chemical reaction, solid-solid structural transitions, and hot pressing-sintering in the solid state. These conditions are achieved in catalytic activity measurements of rutile and zinc oxide, in studies of the dissolution of silicon nitride and rutile, the reaction of lead oxide and zirconia to form lead zirconate, the monoclinic to tetragonal transformation in zirconia, the theta-to-alpha transformation in alumina, and the hot pressing of aluminum nitride and aluminum oxide. 

Results of investigations of shock-induced specific surface changes are summarized in Table 7.1. In the table, the data are summarized in terms of the maximum value of specific area observed and the pressure at which the maximum is observed. The specific surface at the highest shock pressure is also indicated. 

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Chapter 7. Shock Modification and Shock Activation Table 7. . Shock-modified powders Specific surface areas. 

Fig. 7.4. Residual strain and crystallite size are compared for TiC powders subjected to wet milling and shock modification. Significant differences are observed in the anisotropies of both features (after Morosin and co-workers [86M02]).

In one of the most significant observations, small amounts of recrystallized material were observed in rutile at shock pressure of 16 GPa and 500 °C. Earlier studies in which shock-modified rutile were annealed showed that recovery was preferred to recrystallization. Such recrystallization is characteristic of heavily deformed ceramics. There has been speculation that, as the dislocation density increases, amorphous materials would be produced by shock deformation. Apparently, the behavior actually observed is that of recrystallization there is no evidence in any of the work for the formation of amorphous materials due to shock modification. Similar recrystallization behavior has also been observed in shock-modified zirconia. 

As shown in Fig. 7.6, the Mossbauer data show a reduction in Morin transition temperature with increasing shock severity. At temperatures below the transition, increasing shock severity causes greater retention of the higher temperature, weak ferromagnetic contribution. The measure of weak ferromagnetic (WF) fraction (the high temperature form) is a sensitive indication of shock modification. 

Fig. 7.6. The weak ferromagnetic (WF) fraction (high temperature form) of hematite provides a sensitive measure of shock modification. Sample 31G836 is an 8 GPa experiment. Sample 29G836 is a 17 GPa experiment, while 17G846 is a 27 GPa sample (after Williamson et al. [86W03]).

A number of ferrites have been subjected to shock modification and studied with x-ray diffraction as well as static magnetization and Mossbauer spectroscopy [87V01], Studies were carried out on cobalt, nickel, and copper ferrites as well as magnetite (iron ferrite). 

The barium ferrite was found to have an increase in magnetic anisotropy, as in the nickel ferrite, but its overall effect on magnetization was less because of greater magnetocrystalline anisotropy. The shock modification caused reduced crystallite size and local damage that resulted in increased microwave absorption. 

The various studies of shock-modified powders provide clear indications of the principal characteristics of shock modification. The picture is one in which the powders have been extensively plastically deformed and defect levels are extraordinarily large. The extreme nature of the plastic deformation in these brittle materials is clearly evident in the optical microscopy of spherical alumina [85B01]. In these defect states their solid state reactivities would be expected to achieve values as large as possible in their particular morphologies greatly enhanced solid state reactivity is to be expected. 

Fig. 7.9. Measurements of the degree of conversion of ct p silicon nitride at a fixed time and various temperatures are thought to show the strong influence of shock modification on the high temperature dissolution [84B01].

Fig. 7.11. The consolidation behavior in hot pressing of shock-modified AIN is found by Beauchamp and co-workers to be strongly influenced by shock modification [87B04].

Fig. 7.12. The monoclinic to tetragonal conversion of shock-modified zirconia was studied with DTA by Hammetter and co-workers. The conversion temperature was found to be strongly changed and dependent on shock-modification conditions. The higher-pressure behavior was found to be strongly correlated with reduction in crystallite size [84H01],...

Fig. 7.13. The conversion of theta- to alpha-phase alumina was found to be strongly affected by shock modification in work of Beauchamp and co-workers [90B01]. Whereas the unshocked powder showed evidence for an incubation period of 60 min, the shock-modified materials show immediate conversion typical of the presence of shock-formed nuclei.

Fig. 8.3. The yield of shock-synthesized zinc ferrite is found to be strongly dependent on the early loading history. This characteristic is thought to be an indication of shock modification on subsequent chemical reaction.

SHOCK-MODIFICATION CAUSES EXTENSIVE MIXING BETWEEN THE Ai AND Nl POWDERS... 

Fig. 8.7. The influence of powder morphology (configuration) on shock modification controlling initiation of reaction is shown by the thermal response of mixed Ni-Al powders of different morphologies. The preinitiation event shown in Fig. 8.5 is observed to be strongly influenced by morphology at fixed shoek condition. The eoarse-medium mixture shows the largest preinitiation event [91D01]. The data show mueh larger preinitiation events for the flaky and fine morphologies.

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