On The Styrene Assisted Organosilicon Monomer Route For Porous Silicoboron Carbonitride Ceramics

Silicoboron carbonitride(SiBCN)ceramics have been demonstrated to possess high temperature stability,oxidation resistance,and corrosion resistance,as well as to be potentially serviceable under harsh conditions and environments.Light weight SiBCN ceramics may be achievable by simply introducing porous structures,which adds diversified surface properties into bulk ceramics,enriching their performances in thermal insulation,adsorption,filtration,etc.The most common way to prepare porous SiBCN ceramics is the sacrificial templating,which strongly limits by the pore distributions because of the nonadjustable templates introduced,making it difficult to control the chemical compositions and microstructures of the final products.It is,therefore,of scientific significance and technological importance to develop a new method for synthesis of porous SiBCN ceramics with controllable pore structures and chemical compositions.In this work,two organosilicon monomers of bis(trimethylsilyl)carbodiimide(BTSC)and tris(dichloromethylsilylethyl)borane(TCB)containing the target elements of silicon(Si),carbon(C),nitrogen(N)and boron(B)were selected and prepared.The as-prepared organosilicon monomers were reacted to form the product with the desirable element composition and the synthesis methods were explored to obtain the precursor derived SiBCN ceramics without or with styrene.Accordingly,a series of porous SiBCN ceramics were prepared via a novel styrene assisted monomer route.The effects of the styrene amount and sequence on the microstructures and chemical compositions of the ceramics were systematically investigated.The mechanisms of pore structures and chemical compositions affected by styrene are discussed.The results indicated that styrene played a critical role in controls of porosity and oxygen content of SiBCN ceramics.In the absence of styrene,BTSC and TCB reacted directly,resulted in the open porosity of 40%.After the chemical reaction,the introduction of styrene tended to enter the spaces within the precursor and formed polystyrene to construct hierarchical pores,led to the open porosity of 58%and oxygen content of 14.77%.When styrene was mixed with BTSC and TCB,the chemical reaction took place inhomogeneously since styrene could be dispersed in the mixture to separate BTSC and TCB.The formed polystyrene acted as a "itemplate" to construct macroporous pores.The open porosity of 62%,oxygen content of 2.81%and thermal conductivity of 0.114 W/(m·K)were obtained with the mass ratio of styrene to the total raw materials x=0.41.However,the packing of spherical particles was observed at x=0.58.The chemical composition could be controlled at a molecular level by this styrene assisted monomer route through an organic to inorganic transformation from precursor to ceramic.A large amount of C=C and C-C was incorporated into the precursor with styrene.However,when x=0.41,the carbon content decreased from 67.40%in the precursor to 29.60%in the ceramic.At the same time,significant amounts of C-Si and C-B were consumed,while C=C in phenyl ring was transformed to C=C in free carbon.On the other hand,the evolutions of silicon,nitrogen and boron from SiCnN4-n and BCnO3-n(n?1?3)in the precursor to Si-N and B-N in the ceramic were also observed.When x=0.58,boron was coordinated to carbon forming carbon deficient BC?(?<3)site in the precursor.Oxygen was encountered during the synthesis process mainly in the forms of O-B?O-Si.