Synthesis Of Zirconium-containing Polymeric Precursors And Preparation Of The Precursor Derived Ceramics

Zr/Si/C ceramics perform excellent oxidation resistance and high temperature resistance,due to their dense ZrO₂/SiO₂ multiphase oxide layer generated in the air.By the flexibility of precursor-derived method,Zr/Si/C ceramics can be fabricated into diverse shapes at relatively low temperatures.Thus,in this paper,zirconium-containing polymers were synthesized by the reaction between chlorosilanes and active lithiozirconocene complex,which could be applied as single-source polymeric precursors for the preparation of Zr/Si/C ceramics.This simple route could be fulfilled in normal pressure and at room temperature,without heating,cooling,pressurizing or electrolyzing.Together with the self-sacrificed templates or emulsion molding method,precursor-derived ceramic foams or precursor-derived microspheres with wave-absorbing and high temperature-resistant properties can be prepared.The main research includes parts as follows.First of all,the lithiation of Me₂ZrCl₂ was achieved to obtain the lithiozirconocene complex with high activity,which could further react with dichlorodimethylsilane,methyltrichlorosilane or dichloromethylvinylsilane monomers and gave PZS-1,PZS-2and PZS-3 in normal pressure and at room temperature.TG,TG-FTIR and TG-GC-MS analyses indicate that the ceramic yields at 800°C of the three polymers are77.14%-55.91%and 78.55%-57.26%under nitrogen and helium respectively.The transformation process from organometallic polymers into ceramics can be classifid into two phases.Namely,25-400°C witnessed the evaporation of vapor and the emission of alkanes and alkenes,while 450-700°C suggested the removal of cyclopentadienyls and their decomposition into ethylene.After annealed at temperatures ranging from 1000°C to 2000°C,conversions in the crystalline composition,morphology and element contents of ceramics were studied by XRD,SEM and EDS.Ceramics obtained at 1000°C are composed by Zr,Si,C and O,and crystals in ceramics annealed at 1000-1600°C consist of SiO₂,ZrO₂ and ZrSiO₄.Carbothermic reduction is presumed to occur over1600°C,leading to the presence of hexagonal ZrC.The content of Zr dominates the ceramics obtained from 1000°C to 2000°C,while the content of Si decreases with the increment of annealing temperature.Oxygen can barely be observed over1800°C,ending with the formation of Zr/Si/C ceramics.Finally,PZS-3 was utilized as precursor to fabricate ceramic microspheres at 1000°C.With the RLmax of-34 dB,such precursor-derived multiphase ceramic microspheres are potential candidates for electromagnetic wave shielding.Also,the economical lithiation of Cp₂ZrCl₂ was achieved to further react with dichlorodimethylsilane,methyltrichlorosilane or dichloromethylvinylsilane monomers and gave PZS-a,PZS-b and PZS-c in normal pressure and at room temperature.TG,TG-FTIR and TG-GC-MS analyses indicated that the ceramic yields of the three polymers were 80.93%-67.53%under nitrogen and 78.55%-57.26%under helium at800°C.The pyrolysis from organometallic polymers into ceramics can be classifid into two phases as well.Namely,the formation of cyclopentadiene?25-300°C?and the decomposition of cyclopentadienyl into propylene,propane,butane,propane and so on?300-800°C?.The precursor-derived ceramics obtained at 10000°C consist of Zr,Si,C and O.The conversions in the crystalline composition,morphology and element contents of ceramics annealed at 1000-2000 were studied by XRD,SEM and EDS,while resemble those of.ceramics derived from PZS-1,PZS-2,PZS-3.Subsequently,precursor-derived ceramic microspheres were obtained by PZS-c via emulsion molding method.Ceramic microspheres obtained at 1000°C could maintain the spheric appearance at 1800°C.Consequently,by the precursor PZS-c cellular Zr/Si/C foamed ceramics were obtained at 1000°C,whose cellular structure and mesoporous microstructure could maintain at 1800°C.The density of Zr/Si/C foamed ceramics prepared at 1000°C was determined as 1.66 g/cm³.Also,such materials performed attractive wave-absorbing properties at frequencies from 15 GHz to 4 GHz,with absorption quantity peaking at 39 dB?1000°C?and 37 dB?1800°C?.These lightweight mesoporous Zr/Si/C foamed ceramics with wave-absorbing and heat-resisting properties are promising candidates for novel electromagnetic wave shield,high-temperature catalyst supports or hot gas filters.