Investigation On The Preparation And Properties Of The Near-infrared Reflective Oxide Fibers By Precursor Method

With the rapid development of cutting-edge science and technology,the research and development of new aircraft and reusable high-performance spacecraft have always been the research hotspot and the high point in the field of space science and technology in various countries,among which the thermal protection materials involved in the thermal protection system are one of the key factors affecting its success or failure.Advanced thermal protection materials are developing towards the direction of comprehensive properties such as lightweight,high efficiency,high temperature resistance,ablation resistance,oxidation resistance and reusability.In the field of high temperature industrial,the performance of current heat insulation materials is poor.There are prominent problems such as thick insulation layer,high density,low space utilization rate,poor uniformity of temperature field and high energy consumption of equipment.It is hard to meet the urgent demand of the high-temperature industrial technology industry for more high-temperature resistant and efficient insulation materials,which seriously restricts the upgrading of technology and industrial equipment in relevant fieldsHeat transfer including three ways:conduction,convection and radiation.The heat transfer in solid at high temperatures is dominated by heat conduction and heat radiation,the thermal radiation wavelength is mainly concentrated in the near-infrared band.High-strength oxide fiber has excellent self-supporting properties,and its flexible and rigid products have the characteristics of low density and high porosity.Since the thermal conductivity of gas is much lower than that of solid,the thermal conductivity of fiber products can be significantly reduced when comparing with intrinsic thermal conductivity.According to the heat transfer mechanism and the requirements of the aerospace and high temperature industries for the performance and application of thermal protection materials,this work screened suitable high temperature resistant oxides,designed and prepared corresponding fiber materials,and developed fiber material with high near-infrared reflection,light weight,high temperature resistance,and low thermal conductivity.Near-infrared reflective materials mainly include metals and metal oxides.Metal materials mainly include silver foil,copper foil,nickel foil,stainless steel foil,etc.;metal oxides mainly include ZrO2,TiO2,Ni2O3,MnO2,Al2O3,Y2O3,Ce2O3,and their composite oxides,among which fiber materials such as zirconium oxide,aluminum oxide,yttrium oxide,etc.also have the advantages of lightweight,high strength,high temperature resistance,ablation resistance,oxidation resistance,and efficient heat insulation.Based on the research group has developed high-strength zirconia continuous fibers and sub-micron fibers and their products,this thesis is devoted to exploring the preparation of high-efficiency thermal insulation fiber materials with high near-infrared reflectance.Taking the yttrium oxide fibers,yttrium oxide-zirconia fiber,and yttrium aluminum garnet fiber as the research object,the systemic research starts from the structure design and synthesis of precursor,fiber preparation,crystal transformation and structure adjustment,the near-infrared reflection,and heat insulation performance,aiming to provide theory,technology,process and material support for the development of advanced thermal protection materialsFirstly,the precursors of yttrium-acetylacetone complexes with different structures were synthesized via adding different proportions of acetylacetone to YCl3 6H2O.The relationship between the structure of the precursor and the content of the ligand was systematically analyzed,and the polyacetylacetone-yttrium(PAY)precursor suitable for spinning was successfully selected.The PAY precursor uses Y(acac)(OH)2 as the monomer,polymerizes to form a linear structure through the interaction between Y-OH,and has excellent stability and spinnability.In air,the decomposition of PAY precursors includes the volatilization of trace residual solvents,the decomposition of ligands,the carbonization of organics,the oxidation of carbides,and the removal of residual carbides.The final oxide yield was 42%.Using PAY as the yttrium source,the flexible yttrium oxide nanofiber membrane was prepared by electrospinning technology.The average diameter of the fiber was 330 nm,and the fiber surface was compact without obvious cracks,holes,and other defects.The fiber membrane was composed of yttrium oxide nanofibers arranged disordered.The fiber membrane has a tensile strength of 1.6 MPa after heat-treated at 800?.It had good flexibility and could be folded,wound and bent.The average near-infrared reflectance of the fiber membrane was higher than 92%in the range of 750-2500 nm.In order to verify the near-infrared reflective heat insulation performance of yttrium oxide fiber membrane,a single layer fiber membrane with a thickness of 100 microns was used as a high reflective surface,combining with a low density of 20 mm thick zirconia fiber-board.When the hot side temperature was 1500?,the temperature of the cold surface was reduced from 376? to 359?.Increasing the number of layers did not further reduce the temperature of the cold surface,illustrating the yttrium oxide nanometer fiber membrane has good near infrared reflection effect.Secondly,using PAY and polyacetylacetonate zirconium(PAZ)as the yttrium source and zirconium source,respectively,yttrium-zirconia oxide fibers with different yttrium-zirconium molar ratios were prepared.The influence of the molar ratio on the grain growth rate was systematically studied from the perspective of configuration entropy and oxygen deficiency.It was found that the yttrium zirconate fiber formed with the molar ratio of 1:1 had a slower grain growth rate.The yttrium zirconate fibrous membrane(YZFM)was prepared by electrospinning technology,and the composition,structure,mechanics,heat insulation,sound absorption,and other properties of the YZFM were studied.The results showed that the fibrous membrane was composed of the polycrystalline Y2Zr2O7 phase.Macroscopically,it was a multilayer structure composed of an almost transparent monolayer fibrous membrane,with a tensile strength of 8.27 MPa and a density of 44 mg/cm3.The average near-infrared reflectance was higher than 96%in the wavelength range of 750-2500 nm.After heat treatment at 1200?,the thermal conductivity of the fibrous membrane was 38 mW·m-1K-1.When the hot surface temperature is 1260?,the temperature of the cold surface of the fibrous membrane with 30 mm thickness could be stably controlled at 185?.In addition,the noise reduction coefficient(NRC)of YZFM was 0.6,which is better than the commercial melamine sound insulation foam and nonwovens.Based on the above work,two yttrium-aluminum oligomer precursors with good spinning performance were synthesized using water and alcohol as solvents,respectively.During the crystallization process,the precursors prepared in the two solvents directly transform to crystallized yttrium-aluminum garnet phase from amorphous,and no intermediate phase produced.Yttrium aluminum garnet continuous fiber was prepared by dry spinning.The fiber has a length of more than 50 cm,a diameter of about 23 ?m,a tensile strength of 851 MPa,and Young's modulus of 150 GPa.It has high strength and excellent toughness.The yttrium aluminum garnet nanofibers were prepared by electrospinning.The average near-infrared reflectance in the range of 750-2500 nm was over 92%.The yttrium aluminum garnet fibrous membrane was prepared using a precursor synthesized from the alcohol solvent system.The tensile strength was 2.43 MPa,had good flexibility,and it could almost fold in half after heat treatment at 1200?.The density of the yttrium aluminum garnet fibrous membrane was 28.8 mg/cm3,and the thermal conductivity of the fibrous membrane was 26 mW·m-1K-1.When the hot surface is 1220?,the cold surface temperature could be controlled at 318? by an yttrium aluminum garnet fibrous membrane with 10 mm thickness,showing excellent thermal insulation performanceFinally,based on the successful preparation of yttrium oxide nanofibers,the catalysts for dry gas reforming of methane with carbon dioxide were successfully prepared by loading metal nickel on yttrium oxide nanofibers.The effect of calcination temperature on the structure and properties of the catalysts were studied The Ni@Y2O3 NFs-400 catalyst with high catalytic activity and stability was obtained.When the reaction temperature was 600?,the conversion of methane and carbon dioxide were 73.58%and 39.31%,respectively.After ten cycles,the conversion of methane and carbon dioxide at 600? remained at 55.44%and 33.90%.The nickel catalyst supported by yttrium oxide nanofibers provides a new material form for the carbon dioxide reforming reaction of methane.