Atomic and molecular emission following fracture of alkali halides: A dislocation driven process
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. INTRODUCTION The fracture of many materials in vacuum is accompanied by fracto-emission (Refs. 1-8 and references therein), i.e., the emission of photons, charged particles, and neutral species. The photon, electron, and ion emission intensities are readily sampled on ns time scales and, with appropriately instrumented samples, these emissions can be correlated in time with the fracture event. In the vast majority of our previous work, most emissions exhibited their highest emission intensities during the fracture event. Such measurements can provide a great deal of information concerning rapid (km/s) crack growth. In previous work,3"8 we also showed that a number of neutral species are released with fracture. In the case of silicate glasses,8 to within a time resolution of 10~5 s, major atomic and molecular emission occurs during the fracture event. A number of these materials have emissions with properties that are consistent with bond breaking induced decomposition. However, in the case of NaCl and LiF single crystals, we observe no detectable neutral emission during fracture; instead, strong bursts of neutrals are consistently detected after fracture, typically delayed 0.5-250 ms from the time of fracture. We shall argue that these delayed bursts are due to energetic processes accompanying the arrival of dislocations from the near surface region at the fracture surface. These signals could provide a relatively simple way of investigating dislocations generated by fracture, indentation, and tribological loading, as well as the kinetics of dislocation motion following rapid mechanical stimulation. II. EXPERIMENTAL
Single crystals of NaCl and LiF were obtained from Optovac, Incorporated (North Brookfield, MassachuJ. Mater. Res., Vol. 6, No. 1, Jan 1991
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setts). Rectangular samples, 2.5 x 6 x 13 mm3, were cut and mechanically polished to 0.25 ^m diamond grit. The samples were mounted in one of two multiple sample holders, the first a carousel apparatus which allowed the samples to be rotated into place between a loading manipulator and the detectors. The second holder utilized a sample injection mechanism which moved successive samples into position for loading. Both sample holders allowed the rectangular specimens to be loaded in three point bend at a rate of 70 ixva/s. Alkali emission was measured by two techniques. Surface ionization of alkali atoms provided high sensitivity measurements of the total alkali emission, while quadrupole mass spectrometry provided the identities and relative intensities of the major emission components. Figure l(a) shows schematically a surface ionization detector9 used with the carousel. Adsorbed alkali metal atoms and compounds on high work function metal surfaces undergo strong charge transfer. At elevated temperatures, this can result in the evaporation of positive alkali metal ions with a probability given by the Saha-Langmuir equation.10 In our experiments, the ionizing surface was a Pt ribbon, 50 p-m x 8 mm x 20 mm,
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