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 synthesized 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 (B 3 N 3 H 6 ) at 60 °C under static argon in an autoclave. They displayed a chemical formula of [B 3.0 N 3.5 H 4.5 ] n, a glass transition in the temperature range 88−120 °C, a controlled dimensional 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 °C into disk-shaped bodies with 13 mm in diameter in a stable operation and (ii) retention of the polymer shape during the further pyrolysis at 1450 °C in a nitrogen atmosphere. Disk-shaped B 3.0 N 2.95 workpieces with a relative density of 86.3% were obtained. Solid-state 11 B NMR has been used to investigate the polyborazylene-to-BN conversion while XRD, SEM, and He-pycnometry have been performed on workpieces pyrolyzed between 1450 and 1800 °C. Densification occurred in this range to generate bulk boron nitride (B 3.0 N 2.9 ) with a relative density as high as ∼93%.