Yongcai Song

Effect of Temperature on the Composition and Properties of Poly[(alkylamino)borazine] Precursor to Boron Nitride

A novel poly[(alkylamino)borazine] (PPAB) precursor to boron nitride (BN) was synthesized and the effect of synthesis temperature (Tsyn) on its composition and properties was investigated. The results showed that with elevation of temperature, the precursor turned from a colorless transparent liquid to a yellow foamed solid. The softening point (Ts) can be adjusted according to the equation Ts = 1.43Tsyn – 125.7, in which the Tsyn was between 130°C and 170°C. The PPAB was composed of B, C, N, and H elements and the C content in PPAB decreased gradually with an increase of Tsyn. The soluble and fusible precursor, with a softening point of 95°C, provided good processability into polymer fibers. The weight loss of PPAB in N2 can be divided into three ranges: below 240°C, 240∼700°C, and 700∼1200°C. The ceramic yield of PPAB was improved from 43 wt% to 55 wt%, when the Tsyn was elevated from 110°C to 190°C. Furthermore, the residue from PPAB pyrolyzed under N2 showed characteristic IR spectra of hexagonal BN.

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

Preceramic polymers to boron nitride (BN) have been synthesized by thermal condensation of different (alkylamino)borazine monomers. Crystalline BN was obtained by pyrolysis of those polymeric precursors in NH3 and then in Ar. The influence of monomer structure on the chemical composition, microstructure, oxidation resistance and high temperature stability of BN was investigated by a combination of elemental analysis (EA), density measurement, FTIR, XPS, XRD, HRTEM and TGA. With similar chemical composition, BN derived from symmetric monomer exhibited a higher crystallinity, better oxidation resistance and higher temperature stability. This was because that the structure of the precursor polymerized from symmetric molecular was close to that of hexagonal BN (h-BN).

A Novel Liquid Poly[(Alkylamino)Borazine] for Boron Nitride

A novel liquid polymeric precursor for boron nitride (BN), which can be directly used as an impregnant for ceramic matrix composites, was synthesized by condensation of the substituent from 2,4,6-trichloroborazine and n-propylamine. The structure and composition of the precursor were characterized by nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and chemical analysis. The preceramic polymer was composed of B3N3 hexagons linked mainly with =NCH2– units and had a chemical formula of BC2.4N1.63H8.6. Pyrolysis of the cured precursor to 1,200°C in NH3 yielded turbostratic BN almost without carbon impurity. Moreover, the BN displayed good anti-oxidation property in air up to 950°C.

Effects of pyrolysis time on properties of hollow silica fibre reinforced BN–Si3N4 matrix composites

Abstract Hollow silica fibre reinforced BN–Si3N4 matrix composites (SiO2f/BN–Si3N4 composites) were fabricated by repeated infiltration and pyrolysis from a hybrid precursor. The effects of pyrolysis time on densification behaviour, mechanical properties, dielectric properties and microstructures of the composites were investigated. With increasing pyrolysis time, the density of SiO2f/BN–Si3N4 composites increases, the elastic modulus increases accordingly; however, the flexural strength and the dielectric properties decrease. The composites prepared at 400°C with the pyrolysis time of 1 h exhibit a maximum flexural strength of 132·4 MPa, a low dielectric constant of 2·78 and a low loss angle tangent value of 1·4 × 10–3. The high performance of the composites results from the good state of the silica fibres, controlled fibre/matrix interfacial microstructures, high purity hollow silica fibres with excellent dielectric properties and low density non-carbon nitride matrix. The calculation results of dielectric performance show that the composites prepared with the pyrolysis time of 1 h have broadband transmission properties.