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)
