Fabrication of porous boron nitride by using polyborazylene as precursor, polymethylmeth-acrylate as reaction agent

Abstract

A novel process for the production of micro-molecular boron nitride foams has been developed through foaming of boron nitride preceramic polymer with the help of sacrificial microbeads of polymethylmethacrylate (PMMA), followed by pyrolysis under inert atmosphere. The process parameters of porous BN ceramics were studied by thermo gravimetric analysis, thermo mechanical analysis, X-ray diffraction, scanning electron microscopy, mercury porosimetry and elemental analysis. The microstructure and element composition of the porous ceramics were also studied by the above methods. The results showed that: when using polyborazylene from thermolization of borazine at 60°C as boron nitride precursor, PMMA microbeads with pore size of 25μm as sacrificial microbeads, adding into polyborazylene powder with 80 wt% in the foaming mixture. After warm-pressing the powders under 74MPa at 60°C, subsequently pyrolyzing the powders under ammonia to 1000°C then to 1450°C under nitrogen atmosphere, we can obtain boron nitride foams. Through scanning electron microscopy observation, X-ray diffraction analysis and porosity measurement, it was confirmed that the obtained boron nitride pieces were microcellular foams with homogeneous distributed pore size average at ∼19.15μm and had a mostly interconnected porosity of 79.06 vol%. The temperature should be increased to 1700-1800°C to prepare porous BN ceramic with a certain degree of crystallinity (layer spacing d002 = 0.335-0.333 nm)