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

Abstract

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 warmpressed 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 selfcondensation 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%.

Low‐Temperature Synthesis of Highly Crystallized Hexagonal Boron Nitride Sheets with Li3N as Additive Agent

“…[23][24][25][26] This method consists of the synthesis of a polymeric precursor, which is converted into ceramic after shaping. Among available precursors, recent works have shown that the use of polyborazylene (PBN) is the best alternative [27][28][29][30] because of its easy and reproducible synthesis, combined with its relatively high ceramic yield and high purity. Tailoring the condensation rate by altering the condensation time allows the polymer viscosity to be adjusted.…”

The Synthetic Strategy for Developing Mesoporous Materials through Nanocasting Route

“…Another interesting material, Boran carbide (B 4 C) can be synthesized by wet infi ltration technique [79]. As reported in literature, SBA-15 was infi ltrated with bisdecaboranyl-hexane precursor solution in ethanol and left overnight for evaporation.…”

“…It has low temperature reducibility which is responsible for the excellent catalytic performance oxidation of typical volatile organic compounds such as formaldehyde, acetone, and methanol [78]. Cr 2 O 3 -PMA nanocomposites exhibit high catalytic activity and durability in oxidation of 1-phenylethanol using H 2 O 2 as an oxidant [79]. 5 wt.% mesoporous NiO milled together with MgH 2 shows faster H 2 desorption rates (7 times faster) together with slightly lower sorption activation energies compared with pure MgH 2 .…”

Influence of Monomer Structures on the High Temperature Properties of Poly[(Alkylamino)Borazine)]s Derived BN

“…Due to its high melting point, good thermal conductivity, high temperature stability and superior oxidation resistance, boron nitride (BN) has been a potential non-oxide ceramic material for aircraft and space applications [1,2]. The most promising approach for the fabrication of shaped BN such as fiber, film, and mesoporous material is polymer-derived ceramics (PDCs) method [3][4][5][6].…”