Greinar (Vísindafélag Íslendinga) - 01.01.1977, Blaðsíða 96

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Germania glass is thought to have a very similar constitu-
tion, on the basis of these density ratios: tridymite/quartz
= 2.31 / 2.65 = 0.872; germania glass / quartz-like Ge-crystal
= 3.65 / 4.2 = 0.870. In Table I we add, for further orienta-
tion, the densities for silica forms, after the same source. The
density of silica glass seems to mean that the packing is not
dense, whereas the above ratios indicate dense packing in ger-
mania glass.
TABLE I
Form Density
silica glass ................................ 2.20
tridymite ................................... 2.31
crystobalite ................................ 2.32
keatite 2.49
quartz 2.65
coesite 3.01
stishovite .................................. 4.28
Crystals in the above range of size we shall, in the following,
call nanocrystals. Nanocrystals are remarkable i.a. for the fact
that a majority of their atoms belong to the crystal surface.
Consider for sake of simplicity a cubical crystal with n atoms
on the edge. By n=10, over 50% of the atoms belong to the
surface, by n=5, practically all belong to the surface. The first
one might be a 20 angstrom crystal, with a lattice constant
of close to 2 angstrom.
The significance of these cirumstances might be that the sur-
rounding medium acts directly on a high percentage of the
atoms in each crystal, and the lattice responds easily to the
surrounding conditions.
A second peculiarity of nanocrystals can be derived from
a theorem in elasticity theory. Suppose a spherical pore of
radius r0 in an elastic medium, and fill the pore with a fluid
of (high) pressure p0. Then the radial pressure p(r) at di-