Font Size: a A A

Preparation And Properties Of Mullite Whisker Frame/Silica Aerogel Composites

Posted on:2021-07-13Degree:MasterType:Thesis
Country:ChinaCandidate:Y GaoFull Text:PDF
GTID:2481306470991479Subject:Materials engineering
Abstract/Summary:PDF Full Text Request
SiO2 aerogel has the advantages of high porosity,high specific surface and low thermal conductivity,and so on,which make it great attraction in heat insulation,sound insulation,adsorption and other application fields.However,its low strength and high brittleness restricts its application in engineering.It is of great engineering significance to improve the mechanical properties of SiO2 aerogel and reduce the restriction of its application through introducing the second phase composite.In this paper,a three-dimensional frame network with high strength is designed,and SiO2 aerogel prepared in-situ is filled pores among the network frame.High-strength frame is used to reduce the load of silica aerogel.Moreover,the high-strength frame structure affects the brittle crack propagation of silica aerogel to reduce its cracking,and the composite is expected to be machined into all kinds of shapes and remain the nano-pore structure of silica aerogel.This design is expected to greatly facilitate the engineering service of SiO2 aerogel.Mullite is well known for its high temperature strength,good thermal stability and excellent thermal shock resistance.Mullite whisker frame structure has been paid more attention to because of its lightweight and high strength.Mullite whisker frame was prepared by optimizing the porosity of mullite whisker frame to meet the requirements of vacuum infiltration,in-situ gel and ambient pressure drying during the formation of SiO2 aerogel/MWF eventually.First,the process conditions for the synthesis of mullite whisker frame based on gas-solid mechanism are optimized.The effect of sintering temperature from 1100℃~1500℃on the structure and thermal conductivity of mullite whisker frame were studied with Al2O3,TEOS and Al F3 as raw materials.With the increase of sintering temperature,the linear shrinkage rate of MWF increased,the porosity decreased,the bulk density increased.The mass loss of reactant accelerated,the proportion of mullite phase increased,and the thermal conductivity of MWF increased.The open porosity of MWF was improved by introducing carbon pore forming agent.The mesophase carbon microsphere(MCMB)was introduced into the initial raw material,MWF were formed at 1300℃and then the MCMB was removed by calcination at 900℃in air atmosphere.MWF is constructed with needle-like mullite whiskers interlacing,which contains a large number of uniform distributed and isolated spherical pores from heredity of MCMB.The porosity of MWF is greatly increased to 90.93%by the addition of spherical pores.The room temperature thermal conductivity of MWF decreases with the increase of porosity.In addition,the thermal conductivity of MWF increases with the increase of temperature.When the large flake graphite(CFG)was as pore-forming agent,MWF was sintered at 1500℃and then calcined in 800℃in air for removal of CFG.The max porosity of MWF is 84.9%.The fine needle-like mullite whiskers are cross-linked and the interconnected lamellar pores inherited from CFG appear.These pores’width are about 15μm~38μm.The thermal conductivity of lightweight MWF increases with the increase of temperature.And the increase of porosity also reduces the room temperature thermal conductivity of MWF due to introduction of CFG.Different kinds of MWF with high porosity were impregnated by SiO2 sol with different precursor ratio in vacuum.Subsequently,the processes of gelation,aging,solvent exchange,surface modification and ambient drying were used for in-situ formation of SiO2 aerogels in MWF pores.SiO2aerogels are not observed in the isolated spherical pores in MWF used MCMB Ethanol,n-hexane and other solvents were difficult to replace the water in gel nano pores by solvent exchange process mainly because of the small pore size between whiskers and poor connectivity of pores.The gel shrank and collapsed sharply in the drying process and could not retain the aerogel morphology.The fracture surface SEM of composite show that SiO2aerogels can be filled into continuous open pores in MWF,and the diffuse reflection peak of XRD confirms the SiO2 aerogels in composite.Because of the superlight of SiO2 aerogel,the density of the composite is similar to that of MWF.However,the specific surface area of the composite material has been greatly improved.Aerogels are introduced into MWF macropores.The heat transfer mechanism includes the heat transfer of whiskers,the heat conduction of the aerogel network and the heat conduction at the composite interface,which will increase the heat transfer of the composites.The contact angle between the composite and water is 148°significantly higher than that of mullite whisker frame 0°,which imply the composites’super hydrophobic.
Keywords/Search Tags:mullite whisker frame, SiO2 aerogel, high porosity, thermal insulation material
PDF Full Text Request
Related items