On the formation and properties of helical dislocations

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Abstract
The factors influencing the development of helical dislocations found in CaF2 by Bontinck and Amelinckx are discussed. At temperatures near and below 700°c the principal electrolytic current in CaF2 appears to be carried by the negative ions. Thus climb of dislocations is difficult to produce as a result of processes inducing electrolytic flow. It is proposed that the positive ions carry a larger component of the current near 1200°c and that maintenance of the electrolytic currents induced during additive coloration produce what might be termed mild climb. Under this climb dislocations having a strong screw component are induced to assume a helical or spiral prismatic form as proposed earlier by Seitz. The model of F-centres in CaF2 and their equilibrium characteristics are discussed. It is pointed out that helical dislocations should be generated in many materials under a wide variety of circumstances in which climb occurs and that such dislocations are, in principle at lead, capable of generating whiskers when they occur sufficiently near the surface. Such whiskers would grow from the base and should possess many of the attributes found in the whiskers of this type which are observed in practice. It is proposed that controlled climb of any material may be produced with the use of an appropriate temperature gradient if the production or annihilation of lattice defects at the dislocations engenders climb and if the production lies in an attainable temperature range.

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