New insights on the high-temperature nanostructure evolution of SiOC and B-doped SiBOC polymer-derived glasses

SiOC and SiBOC polymer-derived glasses show a complex nanostructure in which nanocrystalline beta-SiC and sp(2) C coexists with nanoclusters of amorphous SiO2 or SiO2-B2O3 and with mixed silicon oxycarbide and boron oxycarbide units. The characterization of the nanostructure is performed with a multiple technique approach on SiOC and SiBOC glasses before and after HF etching. The acid attack dissolves the silica-based nanoclusters and allows indirect information on their size and amount to be obtained. By increasing the pyrolysis temperature from 1200 up to 1500 degrees C the oxide clusters grow in size and amount in both SiOC and SiBOC glasses. Compared to the B-free SiOC glass, SiBOC is more easily etched and develops higher porosity and larger pore size. Boron has also an important effect on the ordering of the sp(2) C phase: it leads to thicker sp(2) C nanocrystals. This effect is remarkable because it starts at low temperature ( 1500 degrees C) compared to the usual temperature, 1800 degrees C, reported in the literature for different forms of nanocrystalline carbon. Raman analysis of SiBOC glasses pyrolyzed at 1500 degrees C clearly shows the presence of the D' band at 1617 cm(-1). From this experimental result it is postulated that B enters into the graphene layers forming BC3 units. Finally, the shift toward lower wavenumbers of the Raman spectra recorded on SiOC and SiBOC glasses after etching seems to reveal a high compressive stress acting along the basal sp(2) C planes of the graphite nanocrystals.