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Q & A: Frozen and Brittle

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Most recent answer: 10/22/2007
Q:
When a rose touches liquid nitrogen, liquid nitrogen will freeze it, and the rose will become very brittle. What is the hardest material that will turn brittle, when it comes in contact with liquid nitrogen?
- Vivien
A:
Vivien -

Almost anything will become more stiff when you make it cold enough, no matter how hard it is to start off with. Some things stiffen quite spectacularly, such as rose petals and racquetballs, since we do not expect these normally flexible things to become brittle and to shatter when struck while they are cold.

The words "hard", "strong", "stiff", and "brittle" all mean rather different things. Diamond is the "hardest" material known because a sharp diamond point will make a scratch in a smooth surface of any other material. This hardness does not mean that diamond is a strong material for all applications, because it is brittle -- diamond will crack along well-defined crystal planes when struck appropriately, as with a jeweler's tools. When forces are applied to a diamond that are not exactly set up to make it crack, then diamond is very very strong -- it is used in very high-pressure apparatus called diamond-anvil cells.

Diamond probably remains very hard but also brittle along those well-defined axes at lower temperatures too. Steels are strong materials that become brittle at liquid nitrogen temperatures -- they crack more easily.

Some solid materials have properties that do not change enormously with temperature, due to their structure on various distance scales. For example, paper does not get much more brittle at liquid nitrogen temperatures than it is at room temperature -- paper is made of lots of wood fibers loosely mechanically arranged together. Even if the wood fibers become stiff and brittle, the paper can still flex because the attachments between the fibers can flex. A chain may behave in the same way -- the steel will get brittle, but the links will still slip through each other and the chain itself will flex (although if you lay it on a table and hit it with a hammer, the link you hit may chip into lots of pieces -- please don't do this because flying metal chips can cause serious injury!).

What may be a more interesting question is what materials will not get brittle at low temperatures, and why.

Helium is a very, very interesting substace with very interesting low-temperature behavior. Even at the absolute zero of temperature, helium will not settle down into a solid at atmospheric pressure. This is due to the fact that the bonds between helium atoms are so weak and because the helium atom is very light. Essentially Heisenberg's uncertainty principle gets in the way -- to slow down an atom so it stays in a crystal lattice site and to keep it located at that crystal lattice site at atmospheric pressure violates the uncertainty principle which says that the uncertainty in the position of an object times the uncertainty in its momentum has to be greater than Planck's constant (divided by 2*pi).

If you increase the pressure, helium will solidify at low enough temperatures.


-Tamara and Tom

(published on 10/22/2007)

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