  In order to point out the influence of thermal history (fictive temperature)
and OH content on the elastic properties of silica glass, we have performed
high resolution in situ Brillouin spectra of SiO2 glass from room temperature
to the supercooled liquid at 1773K across the glass transition. The well known
anomalous increase of elastic modulus in the glassy state and in the
supercooled liquid regime is observed. No change of slope in the elastic moduli
of silica appears as a characteristic of glass transition, on the contrary to
what happens in various other glasses. We show that thermal history has a weak
effect on elastic moduli in the glass transition regime for silica glass. The
effect of water content in silica glass is more efficient than the fictive
temperature effect and gives larger changes in the amplitude of elastic modulus
for the same thermal dependence. A singular decrease above 1223K is also
observed in the shear moduli for hydrated samples. Different models explaining
elastic properties temperature dependence in relationship with frozen-in
density fluctuations or with the structure are discussed.
