Study On Influence Factors Of SiOC Ceramics Derived From Heterogrneous Elemental Hf Modified Polysiloxane

Polysiloxane（PSO）is considered as an ideal raw material for the preparation of cost-effective ceramic materials due to the advantages such as low price,good stability in air and high ceramic yield.The PSO-derived SiOC ceramics exhibits higher thermal stability than quartz ceramics owing to its unique ternary substable structure.However,at temperatures above 1250℃,the rearrangement of Si-O and Si-C bonds within the ceramics leads to phase separation.Subsequently,the phase separation product[SiO₄]and free carbon in ceramics will undergo carbothermal reductions in which result gas releas,structural instability and degradation of mechanical properties at 1400℃.It is reported that the introduction of heterogeneous elements M（B,Al,Zr,Hf,etc.）into the polymer precursors significantly improved the thermal stability of SiOC ceramics,thus,the modification of SiOC ceramics by doping with heterogeneous elements to improve their thermal stability has become a hot research topic.However,the researches on the influence of raw material type and elemental content on the modification of SiOC ceramics with heterogeneous elements is not systematic.Therefore,in this paper,based on previous studies,the effects of raw material type,elemental content and sample morphology on the cracking behavior of Hf-containing precursors and the high-temperature structural evolution of SiHfOC ceramics are investigated in detail,taking Hf-element-modified SiOC ceramics,which have been less studied so far,as the subject investigated.The main contents of this paper are as follows.Firstly,MK resin with all methyl side chains and SR249 resin with more phenyl side chains were used as the precursors,respectively,and organic hafnium acetylacetonate(C₂₀H₂₈HfO₈)and inorganic hafnium dichloride oxide octahydrate(Hf OCl_2·8H₂O)were used as Hf sources.A series of different Hf-containing polysiloxane precursors were synthesized by the sol-gel method,and the effects of feedstock type and elemental content on the cleavage behavior of the precursors were investigated.The results show that organic Hf alkoxides were more likely to undergo condensation reactions with silicone resins to form Si-O-Hf bond structures,and the formation of this structure improved the stability of the precursor systems and changes the cleavage behavior of the precursors.In contrast,the inorganic Hf alkoxides did not show the formation of this structure until Hf:Si=0.2.The thermal mass spectrometry tests showed that the introduction of Hf elements significantly changed the cleavage behavior of the precursors.Secondly,the SiHfOC ceramics obtained by cleavage of Hf-containing polysiloxanes at 1000℃ were subjected to heat treatment at 1200～1600℃ under inert atmosphere,and the high-temperature evolution behavior of the composition and structure of SiHfOC ceramics was further investigated.The results suggested that the introduction of Hf elements precipitated HfO₂grains at high temperatures,which could hinder the diffusion of free carbon within the ceramics,thus enhancing the thermal stability of SiOC ceramics to different degrees.Thirdly,in terms of raw material types,MK resin with lower C content and organic Hf alkoxides derived-SiHfOC ceramics presented best thermal stability due to formation of Si-O-Hf bonds.In terms of elemental content,a higher amount of Hf promoted the formation of Si-O-Hf bonds,which in turn produced more uniformly dispersed HfO₂grains during the precursor cracking process and significantly improved the phase stability of the ceramics.From the sample morphology,the bulk samples exhibited better thermal stability than the powder samples.