Strengths of Sapphire Crystals
THE strength of single crystals of entirely brittle materials should depend on the presence of stress raising flaws. Internally these could be formed by static dislocation arrays, voids or inclusions of foreign material. On the surface the flaws could be formed by mechanical damage or by surface steps as suggested by Marsh1.
The Ideal Strength of Solids
There is an upper limit to the mechanical strength of any material under any particular test conditions. This limit is known as the ideal strength. The present paper attempts to define the ideal strength as closely as is presently possible. To this end, a brief account is first given of some of the highest strengths recorded experimentally. Then follows a short account of the published theoretical calculations of the ideal strength. Finally, the theoretical and experimental values are compared, and it is shown that in several instances these differ by less than a factor 2x.
Growth of Aluminum Oxide Whiskers by Vapor Deposition
The growth of aluminum oxide crystals has been observed in progress at 80X magnification with a stereo‐microscope. Hydrogen is passed over an aluminum‐oxide rod heated by a surrounding tungsten coil. As the hot products diffuse to the cooler region around the reaction zone, the reaction reverses to grow aluminum‐oxide crystals. This system is of particular interest since it allows the screw dislocation theory of crystal growth to be examined in great detail on a high‐temperature system.