Structural modification of synthetic opals during thermal treatment

Abstract

The density and porosity of synthetic opals with spheres 315 and 1000 nm in diameter were measured in relation to the annealing temperature. At annealing temperatures of up to 500°C, the seeming density and porosity remain almost unchanged. Then, at temperatures of up to 950°C, the density increases gradually and, accordingly, the porosity decreases due to the collapse of nanopores caused by the sphere substructure. As the annealing temperature increases further, the opal density increases sharply up to 2.22 g/cm 3 (which corresponds to the density of amorphous silica) and the open microporosity due to the voids between spheres disappears. Differential thermal and thermogravimetric analyses showed that SiO 2 powders with particles with average size of 315 and 1000 nm can have, respectively, two-and three-level systems of micro-and nanopores.