Structural correlation and structural relaxation at the final stage of gel-to-glass transition in silica

In this work more detailed information is obtained about the final stage of the gel-to-glass transition and the interrelations between the glass network structure and the mechanical properties of thr material are revealed. The structural modification of silica xerogels was followed by measuring Vickers hardness and Young's modulus in parallel with measurements of the network structure by means of IR reflection and low-frequency Raman spectroscopies, Two correlations of material properties and scales have been found: (i) at annealing temperature, T-a= 1000 degrees C, the relaxation of the silica. network produces a definite correlation sphere and an increase of Young's modulus to that for vitreous SiO2; (ii) at T-a= 1160 degrees C, a decrease of the mobility of glass-forming units causes a decrease of the correlation radius and an increase of Vickers hardness to that specific for vitreous SiO2.