| Continuous oxide polycrystalline fibers inherent intrinsic features of oxides such as high melting point,chemical stability and high strength,and also have the characteristics of large aspect ratio,flexibility and weavability of fiber materials.All the advantages make them widely used in aviation,aerospace,military defense,civilian industry and other important fields as the key raw materials of fiber reinforced composite materials or high temperature insulation materials.Due to the high melting points of metal oxides,the sol-gel precursor conversion method is a common method for preparing continuous oxide polycrystalline fibers.In the process of preparation and formation of oxide fiber,the phase and microstructure of the fibers will change,so different preparation methods and heat treatment technologies have a great impact on the final fiber quality.In this paper,from the perspective of transformation and formation process of the fibers,the molecular structure of precursor,polymeric evolution process and the effect on rheological behavior were studied,and the spinnability of precursor sol was optimized to obtain high-quality precursor fibers,which could avoid defects that affect fiber quality in the precursor fibers.Since oxide fibers are prepared by the sol-gel method,direct sintering in air will cause the violent decomposition of the organics in precursor fibers and generate a large number of pores and cracks in the fibers,which will seriously affect the fibers quality.Based on the understanding of the molecular structure of the precursor,the hydrolysis and removal procedure of the organics,nucleation and crystallization process in the fibers pretreated at high pressure vapor was adjusted.The transformation process of the internal microstructure of the fibers and the relationship between the microstructure and mechanical properties of the fiber were investigated,and based on which,high-quality continuous polycrystalline oxide fibers were obtained.The main research contents and results are as follows:I.Study on the molecular structure and the relationship between polymerization process and rheological behavior of polyaceticzirconium(PEZ).1.By using cheaper zirconium oxychloride(ZrOCI2·8H2O,ZOC)and potassium acetate(CH3COOK)as raw materials,PEZ precursor sols with different adding amounts of potassium acetate were synthesized,and the structural information of different proportion precursors such as molecular structures and the coordination mode of the ligand were analyzed by means of infrared spectrum,nuclear magnetic resonance spectroscopy and thermogravimetric analysis.PEZ precursor was used acetate radical as the ligand.Meanwhile,according to the differential value between the asymmetric and symmetric stretching vibrations of carboxyl group,it could be seen that the acetate group was bound to Zr by chelating coordination.ZOC was dissolved in methanol to form[Zr4(OH)8(H2O)16]8+ tetrameric clusters.The infrared vibration peak at 1628 cm-1 belonged to water of crystallization,and the intensity of the peak decreased with the increase of the adding amount of potassium acetate,which indicated that the coordination water molecules in the tetrameric clusters were gradually deprotoned and more coordination hydroxyl groups were formed.The tetramers were connected by hydroxyl,which made the precursor have spinnability.The coordination of the organic groups is the key to the formation of stable precursor sols with spinniability.The acetate radicals took the place of the coordinated hydroxyl groups as ligands and adjusted the content of coordinated hydroxyl groups.The PEZ was confined to one-dimension polymerization under the action of the acetate groups as side ligand,forming a linear polymer structure and avoiding the formation of Zr(OH)4 structure with excess hydroxyl groups.2.The relationship between polymerization structure and rheological properties was studied by rheological measurement.The results indicated that when the adding amount of potassium acetate was low,the viscosity of the sol was low and it exhibited the characteristics of Newtonian fluid.The formed[Zr4(OH)]2(H2O)4(CH3COO)2]2+was a positively charged linear molecule structure surrounded by chloride ions.There was a repulsive force between the charged linear molecule structures and they would not entangle with one another.With the increase of the adding amount of potassium acetate,the polymerization structure of Zr4(OH)14(CH3COO)2 was formed.PEZ could polymerize in three dimensions because of insufficient content of acetate,which increased the possibility of linear polymerization structures twining each other.The viscosity of the sol was large,and it showed the characteristics of shear thinning fluid.When the acetate was sufficient,the linear polymerization structure Zr4(OH)12(CH3COO)4 was formed.The linear structure that could be entangled with each other appeared as a shear thinning fluid with moderate viscosity.3.The optimum molar ratio of ZOC and potassium acetate was 1:2,and PEZ precursor with both excellent spinnability and stability was prepared.Each PEZ unit had an acetate group as side ligand and three hydroxyl groups.Two PEZ monomers could polymerize with each other to form a double-chain linear molecular structure through the adjacent OH-.The precursor fiber prepared by dry spinning had good continuity and flexibility.After heat treatment,the surface of the polycrystalline fibers was smooth and defect free with uniform diameter of about 10 μm.Ⅱ.Effect of high pressure vapor pretreatment on microstructure evolution and tensile strength of zirconia fibers1.Taking PEZ continuous precursor fibers as research objects,the effects of high pressure vapor pretreatment on the decomposition of organic ligands,ZrO2 crystallization and the internal phase composition of the fibers were studied.According to the infrared spectral analysis,the infrared peaks of the precursor fibers pretreated at high pressure vapor in the range of 1555 to 1030 cm-1 were attributed to acetate group,and the intensity of the peaks were significantly lower than that of fibers heat-treated directly in air,indicating that high pressure vapor pretreatment could promote the decomposition of organic ligands.The positions and intensities of the peaks measured from the fibers heat-treated in air underwent dramatic changes.However,the peak locations of the fibers pretreated at high pressure vapor did no change,which indicated that the acetate ligands were decomposed in the form of molecules,avoiding the violent oxidative decomposition of organics during heat treatment in air.The acetate ligands in the pretreated PEZ precursor were removed by hydrolysis reaction,and the PEZ was converted into a new structure Zr(OH)4.By TG analysis,compared with 50.4%mass loss of the untreated fiber,the mass loss of the fibers pretreated at 200℃ for 3 h was 24%,and only 9.5%for the fibers pretreated at 250℃.The decomposition of ligands in the fibers was basically completed at this point.The weight loss curve of the pretreated fibers was gentler,which indicated that the high pressure vapor pretreatment was a moderate decomposition method,and was beneficial to maintaining the integrity of the fiber matrix.2.The molecular structures of the precursor were verified by XPS spectroscopy.The results showed that the bridging oxygen structure(hydroxyl group)was converted to the ordered oxygen stacking in crystalline ZrO2 with the increase of pretreatment time or temperature,indicating that the high pressure vapor pretreatment promoted the crystallization of ZrO2.The content of the terminal hydroxyl Zr-OH decreased with the increase of pretreatment time or temperature,which indicated that the transformation from Zr(OH)4 to crystalline ZrO2.X-ray diffraction(XRD)results showed that the high pressure vapor pretreatment promoted the crystallization of ZrO2,and the crystallization temperature of ZrO2 decreased from 400℃ to 200℃.The crystal phase was tetragonal and the crystal size was about 6 nm calculated by Scheler formula.The high pressure vapor reduced the critical nucleation free energy required for the formation of stable ZrO2 crystal nucleus,and promoted nucleation and crystallization of the precursor fibers at low temperatures3.The morphology of the fibers after heat treatment was observed by scanning electron microscopy(SEM).All the fibers surfaces were smooth and dense without obvious defects.The fibers at high pretreatment temperatures had smaller grain size,more uniform distribution and denser than those at the low pretreatment temperatures.After conducting tensile strength measurements,it was found that the tensile strength of fibers at higher pretreatment temperatures was lower than that of fibers at low pretreatment temperatures,which was contrary to the theory that the fine-grained and dense fibers have high tensile strength.The relationship between the microstructure,the composition and the mechanical properties of the fiber was studied by means of infrared spectrum,XPS and transmission electron microscopy(TEM).It could be seen from the infrared spectrum and XPS that the new structure Zr(OH)4 connected to each other through oxygen or hydroxyl groups,which could enhance the strength of the fibers matrix and maintain the integrity of the fibers.From the TEM images,it could be seen intuitively that the spherical ZrO2 nanocrystals about 5 nm were surrounded by the amorphous structure in the low temperature pretreatment fibers.As the pretreatment temperature increases,the amorphous structures gradually transformed into spherical nanocrystals.At high pretreatment temperatures,all the fibers were composed of spherical nanocrystals.There were more amorphous structures and fewer spherical nanocrystals in the fibers pretreated at low temperatures.During heat treatment,the amorphous structure played a cross-linking role to maintain the integrity of the fibers,and then decomposed into crystalline ZrO2 to fill the fibers.The fibers maintained integrity at all times,so they had a high tensile strength in the end.For the high pretreatment temperature fibers composed of spherical nanocrystals,the spaces between the spherical nanocrystals made the combining between adjacent nanocrystals more difficult,resulting in the loss of fibers strength.The high pressure vapor pretreatment fibers were directly sintered in air to obtain continuous polycrystalline ZrO2 fibers with an average tensile strength of 1.299 GPa.III.Preparation of polyacetylacetonetitanium precursor and the effect of high pressure vapor on titanium oxide fibers.1.Tetrabutyl titanate(TBOT),glacial acetic acid and acetylacetone were used as raw materials to synthesize the polyacetylacetonetitanium(PAT)precursor.The effects of different amounts of acetylacetone on the molecular structure and polymerization process of PAT were analyzed by infrared spectrum and nuclear magnetic resonance.The results showed that the binding force of acetylacetone and Ti4+ was stronge,which avoided the hydrolysis reaction between percursor and water in air when acetate was used as the ligand.The acetylacetone group as ligand confined the PAT precursors polymerizing into linear structure with well spinnability.The PAT precursor had better stability with the increase of the adding amount of acetylacetone.When the optimum molar ratio of acetylacetone to Ti was 1:1,a linear polymer structure with good spinnability,better stability and simple synthesis could be obtained.One Ti4+ ion connect with an acetylacetone and three hydroxyl groups,and the molecular structure of the PAT precursor is Ti(acac)(OH)3.2.The thermal decomposition and crystallization of PAT precursor in air were studied by infrared spectrum,XRD and thermogravimetric and differential thermal analysis(TG/DTA).The results showed that:In the thermal decomposition process,acetylacetone ligand was decomposed at 200℃ and converted to acetate,and then the acetate was decomposed by carbonization and combustion.The decomposition of organics in the PAT precursor was basically completed at 500℃.The PAT precursor crystallized into TiO2 with the anatasc phase at 400℃,and the phase transition from anatase to rutile phase occurred at 800℃.The microstructure of TiO2 fibers after heat treatment was observed by SEM.The results showed that the fibers diameter was in the range of 400-800 nm,and the surface was smooth without obvious defects.The grain size of the fibers grew with the increase of heat treatment temperature,which made the surface coarser,and led to the decrease of the fibers tensile strength when the temperature exceeded 700℃.3.The near-infrared reflectance properties of TiO2 nanofibers film was studied by diffuse reflectance spectrum and thermal insulation experiment.The fibers heat-treated at 700℃ had average reflectance of 91.3%in the wavelength range of 500-2500 nm.With the increase of heat treatment temperature,the average reflectance of the fibers decreased because of the coarse grain and the rough surface.In the thermal insulation experiment,the temperature of the cold surface could be reduced from 421℃ to 408℃by using a single layer of 100 μm TiO2 nanofiber membrane on the hot surface at 1200℃.The similarly effect of multilayers on cold surface temperature indicated that the TiO2 nanofiber membrane had insulation effect by means of reflection.The nanofiber membrane placed on the hot surface for 3 h still maintained good fiber morphology and high average reflectance of 87.9%4.Using PAT as the titanium source,continuous precursor fibers were prepared by extrusion spinning.The effects of high pressure vapor pretreatment on the molecular structure,composition and crystallization of PAT precursors were studied by infrared spectrum XPS,XRD,TEM and TG.The results showed that high pressure vapor could effectively remove the organics by promoting the hydrolysis of the precursor,and the new structure of Ti(OH)4 formed by hydrolysis reaction was easily converted into crystalline TiO2 in the subsequent heat treatment.It could avoid the gas produced by the violent oxidative decomposition to damage the fiber matrix.High pressure vapor pretreatment also promoted the crystallization of TiO2,reducing the crystallization temperature of anatase from 400℃ to 120 0C5.The continuous TiO2 polycrystalline fibers were obtained after heat treatment at 700℃ and the tensile strengths were measured.The results showed that the fibers pretreated at 150 ℃ for 1 h had the highest average tensile strength of 385 MPa.This was because the high pressure vapor caused the fiber to dissolution-precipitation process when it promoted the decomposition of organics,which would generate defects and reduced the tensile strength.When the decomposition amount of organics is constant,the defects generated by long-term exposure in vapor could be avoided by improving the decomposition efficiency,and obtained high strength fibers.XRD showed that the crystalline phase of the pretreated fiber at 120℃ was anatase,while the crystalline phase of the pretreated fiber at 150℃ was a mixed phase of anatase and rutile.This might be because the pore defects in the fibers at low pretreated temperatures inhibited the growth of anatase grains,so that the anatase phase could be maintained. |
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