The Rheological Behavior And Ceramization Process Of Polyaluminocarbosilane Prepared By High-pressure Method

The preparation of SiC fibers by precursor conversion is a great breakthrough and a dominant method in recent years.Introducing the heterogeneous element Al into the precursor plays a role of sintering aids and inhibiting grain growth at high temperature,thus obtaining high performance SiC fibers with near stoichiometric ratio,which is of great significance in aerospace and nuclear energy fields.For the first time,polyaluminocarbosilane?PACS?was synthesized under high temperature and pressure conditions using polysilacarbosilane?PSCS?and aluminium?III?acetylacetonate?Al?AcAc?₃?.It was found that the molecular weight and its polydispersity,the pressure of the system,and the branching extent of the molecular structure of PACS increase with the reaction time and temperature and aluminum conten.Combining the results of GC-MS,the reaction mechanism was proposed.It concludes three main reactions:?1?Cleavage and rearrangement reaction of PSCS.?2?Silicon free radicals react with Al?AcAc?₃,leading to cleavage of O=C and/or O-C bonds and formation of AlOx ligands.?3?Conversion reaction of Al ligands from AlO₆into AlO₅ and AlO₄.Furthermore,PACS prepared by normal and high pressure methods were compared.It was found that PACS by the high pressure method has a lower degree of branching in its molecular structure in comparison to its analogue prepared under ambient pressure conditions.which is beneficial for the processing?e.g.spinning?and the overall properties of the final product.The structural forms of component elements for each precursor are quantitatively determined by ¹H NMR,²⁹Si NMR,¹³C NMR,²⁷Al MAS NMR and elemental analyses.The spinning rheological properties of these PACS precursors are investigated on a monofilament spinning device.It was found that as Al content increases,the weight percentage of SiC₃O group increases,yet the weight percentage of SiC₄ group remains almost unchanged.The CH₂ and CH contents also increase with increasing Al content.These lead to increase of the branching degree of the molecular structure.PACS is a typical non-Newtonian fluid,which apparent viscosity decreases with the increases of the shear rate.The apparent viscosity increases and the spinnability of the precursor weakens with increase of Al content,while the non-Newtonian index becomes larger.The pyrolysis process of PACS and the existence forms of heterogeneous element Al were studied in detail by means of TG-DTG,FTIR,Raman,XRD and ²⁷Al MAS NMR.It was found that the final product which was heat treated in 1800?were consisted of highly crystalline?-SiC with a small amount of?-SiC and graphite,at the same time,there was still a small amount of SiO₂.The Al-O bond disappeared substantially,and Al mainly existed in the form of Al-C bonds.