Preparation And Properties Of Boron Nitride Foam

Boron nitride（BN）foam not only possesses the features of foam materials,such as high porosity,low density and high specific surface area,but also inherits good mechanical properties,high thermal stability and chemical inertness from traditional BN materials,thus showing a wide application prospect in energy absorption,catalyst carrier,environmental protection and so on.This thesis aims to construct macroscopic 3D BN foam by the self-assembly of porous BN microfibers with high aspect ratios.The influence of the synthetic conditions on the structure and mechanical properties of the developed BN foams have been studied in detail.Then the BN foam-based composite materials have also been prepared.The main contents of this thesis are as follows.1.The foam precursor consisting of cross-linked melamine diborate（M·2B）fibers has been prepared via an ultrasonic-assisted growth method using boric acid and melamine as raw materials.After high temperature pyrolysis treatment,BN foam with low density（15mg/cm³）,high porosity（99.3%）and controllable size can be obtained through the self-assembly of porous BN fiber with high aspect ratios.The synthesized BN foam has a very low thermal conductivity(0.0351 W·K^-1m^-1),showing good thermal insulation performance.The study of growth mechanism indicates that the ultrasonic-assisted treatment plays an important role for the self-assembly of precursor fibers into a 3D framework structure.2.The influence of the synthetic conditions,including ultrasonic frequencies,protective atmospheres,and the pyrolysis temperatures on the microstructures and properties of BN foams have been studied in detail.The results indicate that the ultrasonic-assisted treatment has a significant effect on the cross linking and self-assembly of BN fibers,which can lead to different specific surface areas and mechanical properties of BN foams.The sample prepared under 45 kHz ultrasonic treatment shows the best elasticity.The permanent deformation is¹5%after compressing and releasing cycles.The mechanical properties of BN foams prepared by high temperature pyrolysis of M·2B precursors in different atmospheres have obvious distinction.During the compression tests,the sample prepared in nitrogen shows higher stress,while the elasticity of the sample treated in ammonia is better.In addition,with the heat treatment temperature increasing,the crystallinity of the BN foam becomes better and the specific surface area also increases.As the heating rate increases,the specific surface area of the sample also greatly increases.3.With the aim of improving the mechanical properties of BN foams,composite materials are prepared using BN foam and Polyhydroxyalkanoate（PHA）polymers.The chloroform/PHA solution with desired concentration and BN foam are used to obtain a PHA/BN foam composite with a high stress（35 kPa）and a high thermal conductivity(0.0525 W·K^-1m^-1).The composite conditions show that both the concentration of the chloroform/PHA solution and the composite time have important influence on the mechanical properties and thermal conductivities of the BN foam/PHA composites material.