Diffraction patterns obtained from atomically clean germanium surfaces contained half-integral order beams in (110) azimuths for both (100) and (110) surfaces and in all azimuths for the (111) surface. These results are considered to be due to displacements of surface atoms from their normal bulk lattice positions in the surface plane. Adsorption of oxygen on all of these surfaces extinguished all of the diffraction beams which were not integral order.In addition to the normal surface lattice spacings of clean (111) and (100) surfaces of silicon, there were surface structures with larger spacings, most of which depended on the conditions of ion bombardment and/or subsequent heat treatment. Two such structures have been observed for the (100) surface of silicon; one is a double-spaced lattice in the (110) azimuth, similar to that for germanium, and the other has a spacing about 8% greater than that of normal silicon and was obtained onlv after radiation quenching of the crystal from 1000°C. Two large-spaced
An experiment was performed to measure the total impulse transferred to a surface by a high-power pulsed 10.6-μ laser under atmospheric conditions, and also to determine the degree to which the observed plasma decouples the laser energy from the surface. Specific interest was directed toward conditions under which a ``radiation-driven detonation wave'' may form due to the presence of a surface. The momentum transferred in dyn sec/J decreases with an increase in the ratio of incident energy to spot area since a larger fraction of the energy is decoupled from the surface.
It should also be emphasiZed that rubber is not ideally elastic, especially in, the region of strain approaching breaking point, and the analogy with brittle fracture, although useful'in that it may lead to an explanation of the observed maximum speeds of perfectly smooth crack propagation, could be misleading if the slightest step formation occurred at high strains since the consequent energy dissipation could be very much greater than for glass.In this respect a better analogy is with anelastic materials such as polycrystalline metals. Gilman 16 has calculated.tli~ energy dissipated in plastic deformation at a,:>tep and obtains values of the order of 10 6 ergs/sq cm ·~f fracture surface for a typical step height of 1000 A. The work required to produce a unit area of fractured surface thus greatly exceeds the surface energy. This is also the case for rubber4 where the hysteretic loss in the highly sheared material around the step apparently takes the place of the energy dissipated by plastic shearing.The general pattern of fracture propagation in both normal and highly elastic solids would appear to be as follows. First, at low speeds, a mode of quasi-static 18 J. J, Gilman. J, App!. Phys, 27, 1262Phys, 27, (1956 propagation, generally with smooth surfaces, obtained by carefully controlling the boundary conditions to maintain the stability of the stress distribution around the leading edge of the fracture and hence the steadiness of the propagation. Secondly, there is the region of more or less unsteady propagation in which rough surfaces are produced and an average rate of propagation is obtained as a compromise between the tendency to accelerate toan upper limit (as in the third region) and the tendency to instability, i.e., for the fracture to develop on two or more planes, and to use the available energy in shearing between these fracture planes. Thirdly, there is a region where the speed of propagation is a maximum (excluding shock wave phenomena), being limited by the speed of elastic waves in the material as in Mott's theory. ACKNOWLEDGMENTSThe author expresses his gratitude to Mr. J. E.The ion-bombardment cleaning method has been successfully applied to the (100) faces of germanium, silicon, and nickel, and to the (0001) face of titanium. Conditions and precautions necessary for the production of clean surfaces are described. Tests have been made for contamination from the ambient during the cleaning procedure for germanium. It has been shown that contamination approximating one-half monolayer does not occur under the conditions which were obtained, and that the method is capable of producing surfaces which are atomically clean.Results for clean (100), (111), and (110) germanium faces and the (100) silicon face indicate that the atomic positions in the surface planes are not the same as the corresponding positions in the bulk structure. For (100) nickel and (0001) titaniuIil, the positions of the atoms in the surface planes are in agreement with x-ray data. In the case of titanium, an unidentified surf...
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