Single-source-precursor Synthesis Of Si-based Functional Ceramics

The polymer-to-ceramic transformation consists of two steps:(i)Cross-linking of the polymers at low temperatures(100?400 ?)leading to infusible organic/inorganic networks;(ii)Ceramization via pyrolysis at temperatures up to 900?1400 ?.Whereas mainly amorphous ceramics are obtained upon pyrolysis,subsequent annealing at high temperatures leads to(poly)crystalline materials.During the polymer-to-ceramic transformation accompanied with the gaseous release,it leads to the formation of porous structure.After annealing at high temperature,some crystalline phase will be formed in ceramics,which endows the obtained ceramics advanced functional properties.In this thesis,the precursors were prepared by modifying the polysilane[AHPCS?PSZ.PSO]with the acetylacetone metal compounds[Fe(acac)3?Ni(acac)2].The introduction of magnetic metal could increase the cross-linking degree of the precursors,which can enhance the evolved pore stability during the polymer-to-ceramic transformation.On the other hand,the existence of magnetic metal endowed the resultant ceramics electromagnetic properties and could also accelerate the crystallization,and enhance the electromagnetic microwave absorption properties of the ceramic.Firstly,Ni/SiOC micro/mesoporous ceramics were successfully fabricated by the pyrolysis(600?)of nickel acetylacetonate(Ni(acac)2)modified allylhydridopolycarbosilane(AHPCS).The resultant Ni/SiOC nanocomposites are comprised of in-situ formed nanoscaled Ni,NiO and nickel silicides embedding in the amorphous SiOC(H)matrix,associated porous structure has an excellent adsorption capacity of a template dye acid fuchsin.The adsorption kinetic curves of AF on the Ni/SiOC(H)nanocomposites with higher Ni content follow the pseudo second-order kinetic model,indicating the typical feature of a chemisorption mechanism.Secondly,the precursor of PFSO was prepared by ferric acetylacetonate[Fe(acac)3]modified polysiloxane(PSO).Porous Fe/SiOC ceramics are formed when pyrolysis at 700 ?.Compared with Ni/SiOC,the distribution of pore size of Fe/SiOC is more concentrated in 2?3nm.The introduction of Fe also promoted the crystallization of SiC,C and ferric silicide(FexSiy)which also endows the resultant ceramics electromagnetic properties.The results show that the effective absorption bandwidth(<-10dB)and the minimum reflection loss has reached 2.6GHz and-38dB.Finally,Fe/SiOCN ceramic nanocomposites were prepared by the pyrolysis of PFSZ which were synthesized by using polysilazane(PSZ)and Fe(acaca)3 as starting materials.The electromagnetic properties and wave-absorbing properties of the ceramics were investigated.Compared with PFSO,Fe(acaca)3 has a higher reactivity with PSZ,and the corresponding pyrolysis products also show different crystallization behavior which results in different wave-absorbing properties.The results show that the effective absorption bandwidth of Fe/SiOCN covered the whole X-band,and the minimum reflection loss reached-47dB.