Preparation of Polyborazylene-Derived Bulk Boron Nitride with Tunable Properties by Warm-Pressing and Pressureless Pyrolysis

The polymer-derived ceramics (PDCs) route has been combined with a warm-pressing process to synthesize for the first time boron nitride workpieces avoiding the conventional sintering process. A polyborazylene was synthesize with a controlled degree of cross-linking suitable to be warm-pressed into cylindrical green bodies which were pressureless pyrolyzed into disk-shaped boron nitride workpieces. On the basis of elemental analyses, differential scanning calorimetry (DSC), and thermomechanical analyses (TMA), the best polyborazylene candidates are those prepared by self-condensation of borazine (B3N3H6) at 60 degrees C under argon in an autoclave. They displayed a chemical formula of [B3.0N3.5H4.5](n,) a glass transition in the temperature range 88-120 degrees C, a controlled dismensional change upon heating under load and a high ceramic yield. Such properties provided (i) adjusted viscoelastic properties and sufficient plasticity to be warm-pressed under 74 MPa at 60 degrees C into disk-shaped bodies with 13 mm in diameter in a stable operation and (ii) rentention of the polymer shape during the further pyrolysis at 1450 degrees C in a nitrogen atmosphere. Disk-shaped B3.0N2.95 workpieces with a relative density of 86.3% were obtained. Solid-state B-11 NMR has been used to investigate the polyborazylene-to-BN conversion while XRD, SEM, and He-pyenometry have been performed on workpieces pyrolyzed between 1450 and 1800 degrees C. Densification occurred in this range to generate bulk boron nitride (B3.0N2.9) with a relative density as high as similar to 93%.