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Pure germanium is known to spontaneously extrude very long screw dislocations. They are one of the primary reasons for the failure of older diodes and transistors made from germanium; depending on what they eventually touch, they may lead to an electrical short. -Wikipedia

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I'm not sure what is going on in the below figure: the caption is confusing.

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FIG. 7. Point contact between tungsten-molybdenum whisker and germanium surface: (a) Before forming; (b) After forming; (field of view measures 50 μm from left to right; Smith 1956).

Silicon Carbide also form screw dislocations which are more pipe-like. Pictures of SiC screw dislocation.

A team led by University of Wisconsin-Madison chemist Song Jin, writing this week (April 23, 2010) in the journal Science, shows that a simple crystal defect known as a "screw dislocation" drives the growth of hollow zinc oxide nanotubes just a few millionths of a centimeter thick.

Dislocations are fundamental to the growth and characteristics of all crystalline materials. As their name implies, these defects prompt the creation of spiral steps on an otherwise flawless crystal face. As atoms alight on the crystal surface, they form a structure strikingly similar in appearance to the spiral ramps of multistory parking structures. In earlier work, Jin and his research group showed that screw dislocations drive the growth of one-dimensional nanowire structures that looked like tiny pine trees. That, says Jin, was a critical clue to understanding the kinetics of spontaneous nanotube growth.

It turns out that "making the structure hollow and making it twist are two good ways of relieving such strain and stress," Jin explains. "In some cases, the large screw dislocation strain energy contained within the nanomaterial dictates that the material hollow out its center around the dislocation, thus resulting in the spontaneous formation of nanotubes."

Is it stress that causes screw dislocations in Germanium? What causes the stress?

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Perhaps this is similar to "tin whiskers" that afflict lead-free solder joints. In that case stresses may be due to the way cooling proceeds from molten to solid states or due to external mechanical stresses. –  RedGrittyBrick Feb 18 '13 at 17:40
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