| Zirconia polycrystalline fiber is a promising thermal insulation material and composite reinforced material for its high melting point,high corrosion resistance,high temperature oxidation resistance,good chemical stability and low thermal conductivity.In the past two decades,our group has made a big breakthrough for the fabrication of zirconia polycrystalline fiber,which has realized industrialization in a large scale.Therefore,performance optimization and application development are putting on the agenda for further study.The decrease of mechanical performance and high temperature creep resistance for polycrystalline zirconia fiber at high temperatures restrict its applications in ultra-high temperatures,which makes it urgent for improving the mechanical performance and high temperature creep resistance of zirconia fibers.Furthermore,potential threats are existed for the intrinsic defect of zirconia,such as low temperature degradation(LTD)in moisture and fast grain growth for the high oxygen ion migration.It is necessary to explore new polycrystalline oxide fibers for dealing with different harsh environments.The performance optimization and application exploration of zirconia fibers,and the preparation of novel zirconate polycrystalline fibers are the main contents for the present study.?.Grain growth control for zirconia polycrystalline fibers1.The effects of water vapor treatment and the additive of phase stabilizer Y(NO3)3·6H2O on the thermal decomposition,crystallization,grain growth and phase transformation of zirconia precursor polyacetylacetonatozirconium(PAZ)were studied in detail.The results suggested that intermedium zirconium acetate was formed in the thermal decomposition process of PAZ.Compared with air atmosphere,water vapor heat treatment could promote the thermal decomposition of PAZ at a lower temperature for the hydrolysis of acetylacetone.Furthermore,water vapor could further promote the crystallization of PAZ and phase transformation of zirconia from tetragonal to monoclinic.With the additive of phase stabilizer Y(NO3)3 ·6H2O,the oxidation of NO3-decreased the amount of carbon residual and Y2O3 hindered the phase transformation and grain growth of zirconia.2.The effect of preheat treatment temperature in water vapor on the grain size and grain size distribution of zirconia fiber with rapid sintering was studied by scanning electronic microscopy(SEM).The SEM results suggested that water vapor pretreatment could facilitate the shift of grain to a smaller size and increase the degree of concentration at 1300?.3.The second phase in ceramic could hinder the grain growth by phase competition and decrease the thermal conductivity by the new phonon scattering center.In the work,the influence of second phase on the grain growth and thermal conductivity of zirconia fiber at 1400? was studied by small amount additive of La2O3 in the precursor fiber.The X-ray diffraction(XRD)and Raman Spectroscopy results confirmed that pyrochlore La2Zr2O7 was formed in the zirconia fiber.SEM and grain size distribution evidenced that the existence of small amount of La2Zr2O7 could hinder the grain growth of zirconia obviously and decrease the grain size distribution range to a lower degree.Furthermore,the thermal conductivity of zirconia fiber with different amount of La2Zr2O7 was also characterized.The results suggested that the zirconia fiber pellet with 3 mol%of La2Zr207 had the lowest thermal conductivity.4.Fiber diameter could affect the operating temperature and mechanical properties of zirconia fiber.Zirconia fibers with different diameters were prepared by centrifugal spinning and electrospinning.The grain size and flexibility of the fiber heat-treated at 1000?,1200? and 1400? were simply studied.The results suggested that the grain size had the decrease trends with the increase of the fiber diameter when the grain size of the fibers was lower than the diameter of the fibers.When the grain size was obviously lower than the fiber diameter,the smaller of the fiber diameter,the more flexible of the fiber.The flexible stability was obviously decreased as the grain size was similar to the fiber diameter.?.Mesoporous zirconia fiber preparation as supporting materials for catalyst.In the present part,mesoporous zirconia fibers were prepared with cetyltrimethyl ammonium bromide(CTAB)as the soft template.The effects of air atmosphere,water vapor atmosphere and additive of SiO2 on the preparation and specific surface area of mesoporous zirconia fibers was fully studied.The specific activity of catalysts with mesoporous zirconia fiber as supporting materials toward the bromination of phenol red was also characterized.1.Mesoporous zirconia ceramic fibers were fabricated by electrospinning method with CTAB as the template and polyacetatozirconium(PEZ)as zirconia source.The thermal decomposition and crystallization process of the mesoporous zirconia fibers were fully studied by TG/DSC,IR spectra and XRD.Nitrogen adsorption-desorption isotherms and SEM results confirmed that the formation of mesoporous structure in the fiber.The highest surface area of the mesoporous zirconia fiber was about 120 m2/g at 300?.With zirconia fibers heated treated at 300? as supporting materials,the effect of the loading of tungstate on the specific activity of catalysts were investigated.The results indicated that the catalytic activity of catalysts indicated that the catalytic activity of catalysts showed a trend of increase and then decrease.The optimal loadings of tungstate were 6 wt%for the supported fibers with a specific activity of 1.26 mmol h-1g-1.2.Water vapor treatment was found to be a facile template removal strategy for the fabrication of mesoporous fibers.With CTAB as the template,zirconia precursor fibers were prepared with electro spinning.The effect of water vapor on the crystallization,phase transformation and microstructure of the mesoporous zirconia fibers were studied and compared with that of air.The bromination catalytic activity of tungstate supported mesoporous zirconia fibers to phenol red were characterized.The electrosp1n zirconia fibers possessed a high surface area of 199 m2/g in water vapor at 300?.Furthermore,water vapor promoted the crystallization and phase transformation of mesoporous zirconia fibers.When mesoporous zirconia fibers were used as the supporting materials,the bromination specific activity of tungstate supported zirconia catalyst was 1.68 mmol h-1g-1,which was higher than the previous works.The higher specific activity was mainly attributed to the higher surface area and more active sites.3.Tetraethyl orthosilicate(TEOS,SiO2 source)was added into the zincoria precursor fibers using CTAB as the template,and the precursor fibers were pretreated to 350? in water vapor.The effects of air and water vapor on the crystallization,phase transformation and microstructural change for the further thermal treatment.The formation of the mesoporous structure and evolution mechanism were investigated.The addition of SiO2 enhanced the thermal stability of the porous structure by providing network building blocks and hindering the crystallization of zirconia fibers.Mesoporous zirconia nanofibers with a high surface area of 332 m2/g were obtained by water vapor pretreatment and the addition of SiO2 at 350?.The pretreatment by water vapor could promote the hydrolysis of TEOS and the mild thermal decomposition of CTAB.At the same time,with the volatilization of the decomposition product,water vapor replaced the evaporated vapor phases and filled in the slit-like mesopores.The increased contraction pressure from the water vapor hindered the shrinkage of mesopores and thus enhanced the surface areas and thermal stability of the pores.Furthermore,the addition of SiO2 provided the framework for the mesoporous structure,hindered the crystallization,phase transformation and growth of the grain,and improved the high temperature stability of the pores.With increasing of the temperatures,the smaller slit-like mesopore and the neighboured ones merged into each other by crystallization of the pore walls to form the longer slit-like mesopores.At higher temperatures,further consolidation and extenstion of the pore walls which was induced by grain growth and coalesence led to the formation of interconnecting mesopores.4.With the additive of SiO2,and the high surface area of zirconia fibers as the supporting material,the effects of activation temperature,loading amounts of tungstate and reaction temperature on the tungstate supported zirconia catalysts were fully characterized.The results showed that a high specific activity of bromination of phenol red up to 2.13 mmol h-1g-1 at 45? was obtained with the optimal activation temperature of 400? and loadings of 10 wt%tungstate.The specific activity of the catalyst could keep 59%for the fifth cycles.?.Study on the fabrication of zirconate binary oxide fibers1.By the use of PEZ as zinrconium source,LaCl3·7H2O,La(NO3)3·6H2O,and La(CH3COO)3·4H2O were used as three different La2O3 precursors for the fabrication of La2Zr2O7 fibers via electrospinning method.Phase transformation of La2Zr2O7 ceramic fibers were characterized by XRD and Raman spectra.The influence of different La2O3 precursors on the thermal decomposition process,crystallization,phase transformation,microstructure of the precursor fibers were studied.XRD and Raman spectra evidenced that defect fluorite La2Zr2O7 was formed at 800? and the phase transformation of La2Zr2O7 from defect fluorite to pyrochlore started at about 1000?.TG/DSC,IR and SEM results showed that slow weight loss process led to the formation of dense La2Zr2O7 ceramic fibers with LaCl3·7H2O as the precursor,while pores appeared on the surface of the fibers with La(NO3)3·6H2O and La(CH3COO)3·4H2O as the precursor at 1200?.Compared with LaCl3·7H2O as the precursor,thermal conductivity results showed that the fiber pallets with La(NO3)3·6H2O and La(CH3COO)3·4H2O as the precursor showed lower thermal conductivity,which may attribute to the pores in the fiber.2.By the use of PEZ as the zinrconium source,barium carbonate as the barium source,glacial acetic acid and methanol as solvents,BaZrO3 fibers were fabricated simply and cost-effectively by centrifugal spining technology.The thermal decomposition,crystallization,solid state reaction and microstructure change process were studied detailly.The high temperature phase stability of barium zirconate fibers and the thermal stability of the fiber brick were tested.The analyses of thermal decomposition and crystallization of the fiber indicated that the crystallization of BaZrO3 fiber was a simultaneous process with the decomposition of BaCO3 which was formed during the pyrolysis process of the precursor fiber.According to the results of the thermal analysis and microstructural change,the appearance of pores in the fiber at high temperatures was mainly due to the thermal decomposition of BaCO3.The high temperature phase stability(XRD)results showed that BaZrO3 ceramic fibers were up to 1500?.The high-temperature resistance test exhibited that the shrinkage of the bricks was both smaller than 2.5%at 1200? and 1300?,and larger than 2.5%at 1400?.Therefore,the barium zinconate fibers fabricated by centrifugal spining method could be used as long-term ceramic fibers at 1300?.3.In order to obtain dense ceramic fiber,it is necessary to optimize the microstructure of BaZrO3 fiber by avoiding the formation of BaCO3.With the barium acetate and PEZ as the precursors,lactic acid and trifluoroacetic acid were used as complex agents for the preparation of dense barium zirconate fibers with electrospinning method.The influence of complex agents on the preparation,thermal decomposition,crystallization,solid state reaction and microstructure of BaZrO3 precursor fibers were fully analyzed.1)By the use of lactic acid and trifluoroacetic acid as the complex agents,barium lactate and barium trifluoroacetate complexes were formed,respectively.The higher solubility of barium acetate in methanol and stable sol indicated that lactic acid and trifluoroacetic acid were effective complex agents for acetate barium.2)Dense BaZrO3 fibers were obtained with lactic acid as the complex agent.With the formation of lactate complex,a milder thermal decomposition process was obtained for the higher thermal stability of lactate precursor and no BaCO3 intermedium was detected.3)With trifluoroacetic acid as the complex agent,high temperature stable intermediate phase of barium fluoride(BaF2)was appeared in air.Obvious pores were formed in the fiber at higher temperatures and the formation of pores was mainly attributed to the pyrolysis of BaF2 and fluoric gas volatilization.4)With trifluoroacetic acid as the complex agent,BaF2 as high temperature stable intermediate phase was avoided by water vapor pretreatment and dense BaZrO3 fibers were obtained.It further confirmed that pores in the BaZrO3 fibers were mainly due to the gas volatilization from the thermal decomposition of BaCO3 and BaF2 intermedium. |
PDF Full Text Request