Foaming performance of the closed-pore foamed ceramics using silicon nitride as a foaming agent and sodium tetraborate as a modifier of foaming

Using silicon nitride as a foaming agent and sodium tetraborate as a modifier of foaming, foamed ceramics (FCs) with porous internal structure and dense surface were prepared by a high temperature foaming method based on the foaming mechanism which is simultaneous occurrence of oxygen diffusion from the air into the molten matrix balanced with the generation of the oxidation-derived nitrogen from silicon nitride at a specific molten matrix viscosity. The foaming performance of the FCs was improved significantly by establishing the conditions for the best synergy between the generation rate of the oxidation-derived nitrogen and the viscosity of the molten matrix during sintering. By synergistically changing the proportion of raw materials and the sintering time at 1150-1250 degrees C, the FCs demonstrated excellent overall properties with a high closed porosity of 89 %, a low bulk density of 0.32 g/cm(3) and a relatively high compressive strength of 6.2 MPa.