Silicon tetrachloride is burned in a flame of
hydrogen and oxygen(at approximately
**00℃) to produce molten spheres of silicon
dioxide(and hydrogen chloride). Depending on process parameters,
the size of these silica spheres can be varied and, in a given
batch, show excellent uniformity(by electron micrograph). The
molten spheres collide and fuse with one another to form branched,
three-dimensional chain-like aggregates. Many aggregates have chains from *0 to *0
spheres in length, or from0.1 to 0.2 microns(um) in length. As the
aggregates cool down below the fusion temperature of
silica(approximately **10℃),further collisions result in some reversible
mechanical entanglement or agglomeration. Further agglomeration
occurs during the collection process; this can be reversed by
proper dispersion in a suitable medium. During the formation of the product, hydroxyl
groups become attached to some of the silicon atoms on the surface
of silica particles, making the surface hydrophilic and capable of
hydrogen bonding with suitable molecules. There are
(estimated)3.**4.5 hydroxyl groups per square millimicron of silica
surface, compared to a theoretical maximum of 7.*5. The structure
of fumed silica is amorphous (as indicated by absence of lines in
its X-ray diffracetion pattern.) The surface area was determined by calculation
using a nitrogen adsorption method of brunauer,and the value used
to calculate particle diameter. The residual hydrogen chloride on the surface
of the fumed silica was reduced to less than **0ppm by calcining.
The moisture content of the product will vary,depending on storage
conditions. moisture adsorbed on the surface can be removed by
evacuation at room temperature (at ***2mm Hg) or by heating at
**0℃,(if the product is heated above
**0℃,it
sinters irreversibly.)
