Preparation of finely macroporous SiOC foams with high mechanical properties and with hierarchical porosity via pyrolysis of a siloxane/epoxide composite

Mechanically strong SiOC foams were prepared via pyrolysis of polysiloxane composites, into which 20–70 wt% of epoxy powder wereincorporated as sacrificialfiller. The chosen epoxyfiller degrades practically quantitatively during the pyrolysis in nitrogen, due to a high contentof polyoxypropylene chains. The pyrolysis of the composite precursors yielded SiOC foams with apparent densities between 1.31 and 0.62 g/cm3,which corresponded to macro-porosities of 35–69%. A hierarchical porosity was obtained due to a relatively wide distribution of the sacrificialfiller grains' size. Compressive strength between 15 and 38 MPa was achieved, depending on the apparent density of the SiOC foams: the bestresult was obtained for the foam with 56% porosity. In the case of denser foams, their strength was reduced by the presence of pyrolysis cracks,while at 70% porosity, the thinner pore walls led to a reduced strength. The elastic modulus of the most promising foam (56% porosity) wasfound to be 10 GPa.