Preparation Of Boride Ceramic Hollow Microspheres By Precursor Conversion/Emulsion Method

Boride ceramics have the advantages of low atomic number,high neutron absorption cross-section,good thermal stability,excellent oxidation resistance and corrosion resistance.They can be used as target materials in laser-driven Inertial Confinement Fusion,which is expected to solve the energy crisis.However,the preparation of hollow ceramic microspheres is difficult.At present,it is challenging to obtain thick-walled microspheres by magnetron sputtering.The sphericity and concentricity of the microspheres prepared by gel-casting are not good.Moreover,neither of them can achieve batch preparation.This paper aimed to solve the above issues.PAN-based carbon hollow microspheres were prepared by the precursor conversion/emulsion method,and related research was carried out.Then,reaction boronization of carbon microspheres and the boronation blending of the PAN phase in emulsion method were explored to prepare boride ceramic microspheres.O₁/PAN（DMF）double droplets were generated using a coaxial co-current device,then they were collected in silicone oil and formed into solid gel shells by rotary evaporation.The obtained microspheres had sphericity higher than 98%;the concentricity and wall thickness uniformity were mostly distributed between 99-100%and 75%-95%,separately.By changing the concentration of PAN and the flow rate ratio of two phases,the adjustment of specific values of the microsphere within the diameter of 1800-2300μm and the wall thickness of 60-120μm was achieved.Following this,the internal silicone oil was exchanged with the solvent using the principle of Soxhlet extraction.Then,the solvent was evaporated and removed to form hollow microspheres.The study found that the silicon content in the shell could be reduced to 0.2-2.5 at%after extraction by n-hexane for 40 cycles.This method worked with microspheres with different sizes.And the microspheres still had good morphology parameters.The PAN microspheres were sequentially implemented to pre-oxidation in air and carbonization in inert atmosphere to prepare hollow carbon microspheres.The effects of process parameters on the morphology and composition of microspheres were analyzed.Gradient heating to 280°C and holding for 480-960 min was an appropriate procedure which was conducive to sufficient pre-oxidation on the basis of ensuring a proper size and shell uniformity.The"skin-core-skin"structure caused by the heterogeneity of pre-oxidation resulted in different reactions on different positions of shells during carbonization.Consequently,carbon microspheres had defects such as wrinkles,holes,and cracks.However,this pheonmenon would be improved with the extension of pre-oxidation insulation time.After carbonization,the diameter of the microspheres was still on the scale of millimeter,and their sphericity was above 96%.The preparation method of boride ceramic microspheres was preliminarily investigated.The solid-solid reaction of boron powder and carbon spheres at 1800°C could form B-C on the surface of microspheres.B₂O₃ could be used as a boron source in the form of a gas phase at 1600°C to react with carbon microspheres.The surface of the microsphere was introduced in B-B,B-N and B-O merely.The increase of B₂O₃ and the decrease of reaction space were beneficial to increase the boron content.However,the structure of the microspheres was loose after CO escape.The third way was to blend B₄C nanopowder in PAN（DMF）to prepare polymer microspheres by emulsion method,followed by ceramization.The sphericity,concentricity,and shell compactness of the obtained microspheres were excellent.The surface roughness caused by the agglomeration of the nanopowder was improved by reducing the amount of PAN to a mixing ratio of 0.67.