Study On Properties Of Fly Ash Based SiO₂ Aerogel Composite Insulation Material

With the rapid development of chemical,construction,national defense and other industries,the demand for efficient thermal insulation materials is more and more urgent.SiO₂aerogel is one of the most promising materials in the field of thermal insulation due to its very low thermal conductivity,high porosity and outstanding specific surface area.However,at present,SiO₂ aerogel has high production cost,poor mechanical properties,insufficient toughness and other shortcomings,which seriously limits its development.Therefore,in this thesis,fly ash rich in silicon and low in price will be used as the silicon source of SiO₂aerogel,and the sol-gel method and atmospheric pressure drying technology will be used to synthesize SiO₂ aerogel.Fiber is introduced as the reinforcing phase to overcome the problems of poor mechanical properties and insufficient toughness of SiO₂ aerogel,and research on the preparation and performance testing of materials is carried out.In this thesis,the sinter-acid leaching method was used to achieve the separation of Si and Al from fly ash.When sintering temperature is 850℃,sulfuric acid concentration is 30 wt.%,and acid leaching time is 1 h,aluminum dissolution rate can reach 91.17%.The Box-Behnken response surface was used to optimize the process of preparing sodium silicate solution from separated silicon slag.The results showed that there were interactions among reaction time,alkali-Si mass ratio and NaOH concentration.The influence degree was alkali to silicon mass ratio>reaction time>NaOH concentration.The optimum conditions are reaction time 5 h,mass ratio of alkali to silicon 1.5,concentration of NaOH 15 wt.%and modulus of sodium silicate up to1.32.The synthesized fly ash based SiO₂aerogel is mainly mesoporous and has good thermal stability.The effects of silicon-water volume ratio and gel pH on aerogel properties were investigated.The results show that when the silica water volume ratio is 1:2,gel pH is 5.5,the performance of SiO₂ aerogel samples is the best,the corresponding density is 0.108 g/cm³,gel time is 39 min,and the porosity is 95.07%.The effects of gel aging on the properties of SiO₂ aerogel were investigated.The results show that aging can improve the strength of aerogel skeleton to a certain extent and reduce the shrinkage in the drying process.The aerogel has the best performance when the aging time is 24 h and the aging temperature is 40℃.Surface modification is the key to atmospheric drying fly ash based SiO₂ aerogel.The Si-OH structure of SiO₂aerogel molecules is replaced by Si-CH₃ by silicon-containing functional groups in surface modifiers,which makes the SiO₂ aerogel with strong hydrophobicity and prevents the condensation of adjacent-OH on the molecular surface of SiO₂aerogel in drying,so as to avoid structural collapse.When TMCS:HMDSO=3:2,the performance of fly ash based SiO₂ aerogel is the best,the specific surface area is 795.81 m²/g,the porosity is 95.8%,and the water contact angle is 146.64°.SiO₂ aerogel composite insulation material was prepared by composite preparation method with fly ash based SiO₂ aerogel as matrix and fiber as reinforcing phase.The mechanical properties and thermal insulation properties of the composites were studied with the amount of fiber added.The fiber content increased from 10%to 30%,the three-point flexural strength of the composite insulation material increased from 0.67 MPa to1.64 MPa,the uniaxial compression strength increased from 0.06 MPa to 0.18 MPa,and the thermal conductivity increased from 0.023 W/（m·K）to 0.047 W/（m·K）.Based on the theory of material microstructure and heat transfer,combined with the performance parameters of material structural units and the assumption of material homogenization,the multi-physics coupling steady-state solution was carried out by using COMSOL Multiphysics,and the calculation model of effective thermal conductivity of fly ash based SiO₂aerogel composite was established.The influence law of material temperature field distribution and gas-solid coupling thermal conductivity and radiation thermal conductivity with temperature is obtained.The results show that temperature mainly affects the radiation heat transfer of materials.By comparing the simulation results with the experimental results,the effects of different temperatures on the effective thermal conductivity and the temperature difference between hot and cold surfaces were verified to verify the accuracy of the simulation model.