Synthesis Of SiCN Ceramics With Tailored Structures Via The Non-Oxide Sol-Gel Reactions

Silicon carbonitride ceramics (SiCN) are known to possess a variety of unique properties such as high temperature resistance, excellent thermal and thermo- chemical stabilities, high temperature oxidation resistance, high temperate creep resistance and compositional adjustable electronic conductivities, which have potential applications in high-temperature environment, devices and battery materials.In 1991, German researcher Riedel et al. reported the synthesis of bulk dense SiCN ceramics by crosslinking-forming-pyrolysis of a polysilazane, and, later, discovered the non-oxygen sol-gel reactions based on the reaction of chlorosilanes and bis(trimethylsilyl)carbodiimide (BTSC), that enabled to produce SiCN gels. This opens a way to synthesize SiCN ceramics via the sol-gel routes that have been widely used for producing various oxide ceramics. Up to now, however, only single source of chlorosilanes was reacted with BTSC and three dimensional SiCN ceramics have not yet been realized from this process.In this thesis, we used the vinyl containing chlorosilane to react with BTSC to increase the ceramic yield by which we achieved dense bulk SiCN ceramics. Moreover, we used dichlorosilanes and trichlorosilanes to react with BTSC to produce the heterogeneous structures of SiCN gels consisting of the building block of SiCN from the trichlorosilanes and the linear inserts provided by the dichlorosilanes, by which, we achieved highly porous SiCN ceramics with adjustable pore shapes and porosities from pyrolyzing the heterogenous structures of the SiCN gels. The main studies and results are as following:1. The sol-gel reaction of the Vinyltrichlorosilane (CH2=CHSiCl₃) and bis(trimethylsilyl)carbodiimide( Me3Si-N=C=N-SiMe3(BTSC)) in toluene produced highly cross-linked and stable silicon carbonitride gel at room temperature, which was dried at 150℃in argon to form the silicon carbonitride xerogel. The SiCN xerogel was pyrolysed at 800℃and transformed to dense amorphous SiCN ceramics with a high ceramic yield of 68 wt% at 1200℃.Further annealing the ceramics up to1400℃led to form the rod-likeα-Si₃N₄ in the dense body.2. Reaction of the mixture of vinyltrichlorosilane(CH2=CHSiCl₃) and dimethyldichlorosilane((CH₃)2SiCl2) and BTSC in argon, without solvent led to the formation of white SiCN gels that exhibited no apparent phase separation at 35℃. Upon pyrolysis at 1200℃, the silicon carbonitride xerogel transformed directly into porous SiCN ceramics.The pore size decreased with increasing of mole ratio of CH2=CHSiCl₃ and (CH₃)₂SiCl₂, and the porosity increasing with the increasing mole ratio of CH2=CHSiCl₃ and (CH₃)₂SiCl₂. Our study demonstrated that it is possibility to produce porous SiCN ceramics by the sol-gel reactions.Furthermore, the pore size and porosity of SiCN ceramics allows the change of mixture of trichlorosilanes and dichlorosilanes .3. Replace of vinyltrichlorosilane with dichloromethylsilane(CH₃HSiCl2) in the mixture of chlorosilanes in reaction with BTSC at 35℃produced highly transparent SiCN gels. Pyrolysis of the SiCN gel at 1200℃produced highly porous SiCN ceramics with the porosity as high as which is 92%, much higher than that of from the sacrificing template method which produced the porosity normaly below 80%.Our study suggests that it is possible to fabricate silicon carbonitride ceramics of either dense structures or porous structures of adjustable pore size and porosity through the molecular design using the non-oxide sol-gel reaction. This strategy could also be extended to other ceramic systems based on the sol-gel reactions.