Preparation Of Mullite Fiber-based Porous Ceramics By In-situ Conversion Of Organic Precursor

As a crucial part of the hyper-sonic vehicle thermal protection system,high temperature thermal insulation material plays a important role in thermal insulation of the aircraft,designed to minimize the heat transfer so as to ensure the safety of the cabin.On account of the extreme working environment,all the materials of thermal protection system should not only be lightweight and high-strength,but also possess low conductivity and certain deformation resistance.Inorganic fiber-based insulation material with excellent heat resistance and high bending strength of the matrix can cope with extreme flight environment and prevent the cracking of each component due to thermal expansion caused by huge high-temperature thermal effect,so as to eliminate the safety threat of the aircraft.Highly porous mullite fiber-based ceramics with a three-dimensional cross structure were successfully designed and fabricated by in situ pyrolysis of organic precursor.To address the thermal migration problem of silica sol,polysiloxane was introduced in the bonding system as high temperature binder to form more stable nods in the skeleton as opposed to silica sol which would migrate to fibers surface during the sintering process.And the micro structure,phase composition,basic physical properties and thermal insulation properties of the products were systematically and comprehensively studied.Firstly,mullite fibrous ceramics with polyacrylamide and mixed polysiloxane precursor respectively as room temperature and high temperature binder were successfully prepared by filtration,in which procespolycrystalline mullite fiber was used as the matrix and hydroxypropyl methyl cellulose as the aqueous solution dispersant.Due to the lapped structure of mullite fibers,all products with different sintering temperature and binder content showed high compression strength,low density and relatively low thermal conductivity.Amorphous silica was formed between the fibers by in-situ conversion of precursor,and the compression rebound cycle test showed that the products possess good rebound characteristics,which is mainly contributed by the bending and torsional deformation of mullite fibers and binders.Secondly,in order to further reduce the density of porous ceramics and simplify the preparation process,the viscosity of mixed polysiloxane precursor was studied and redesigned so that the precursor could work as both room temperature and high temperature binder.So the preparation process could be greatly simplified by replacing the two-step filtration with one step molding.And it turned out that mullite fiber-based porous ceramics with lighter weight(0.348-0.406g/cm~3),lower heat conductivity(<0.12W/(m·K))and higher resilience(>90%)were successfully prepared,besides,all products still showed relatively high compression strength.