Zirconia Key To Continuous Ceramic Fiber Preparation Technology

In the chapter one, the conception and application of continuous ceramic fiber is firstly introduced. Then, the research progress involves zirconia precursor and the preparation techniques of zirconia fiber are reviewed in details. The sol-gel technique is suitable for the preparation of continuous zirconia fibers. Whereafter, the chemical principle of the sol-gel process is introduced. The procedures involved in sol-gel process including the hydrolysis and polycondensation of the metal alkoxide, gelation, aging, drying and heat treatment are given in details. A great deal of attentions had been paid to the effects of the processing parameters on the sol and the gel of every step, and the corresponding characterization techniques are given. Afterwards, the application of sol-gel process in the preparation of metal oxide ceramic fibers is reviewed as well. In chapter one, we also describe the purpose of this thesis and the main results obtained. In this chapter, 43 references are cited.In chapter two, the crack-free polycrystalline yttria stabilized tetragonal zirconia (T-ZrO₂) continuous fibers were first obtained by pyrolysis of gel fibers using zirconium oxychloride octahydrate as raw material. The spinnable zirconia sol is prepared by electrolyzing the zirconium oxychloride octahydrate solution in the presence of acetic acid and sugar (sucrose, glucose or fructose), in which the molar ratio of CH₃COOH/ZrOCl₂·8H₂O and sugar/ZrOCl₂·8H₂O was in the range of 1.0-4.0 and 0.2-0.4, respectively. The sugar is used to improve the stability of sol based on the formation of the zirconyl-sugar complex.During electrolyzing, zirconyl-sugar complex decomposes and the zirconium acetate complex is formed through the coordination of the carboxyl groups to Zr(IV),which favors the formation of long linear polymers and spinnable sol. The IR result of gel fibers suggests that the carboxyl group coordinates to zirconium by bidentate bridging mode. The coordination of acetate to Zr(IV) is important in slowing the hydrolysis of Zr(IV) and assisting the complex condensation or polymerization to form the linear polymers for the preparation of the spinnable sol. The relation of spinnability to the shape of colloidal particle is discussed, indicating that particles maintained the linear shape in sol. The as-prepared zirconia fibers sintered at different temperatures show smooth and crack-free surface with the diameter of 5-10 μm. Slow heating rate below 600℃ and then sintering at 1400℃ for 30 minutes are necessary to obtain the dense tetragonal zirconia ceramic fibers, the size of the particles composed the fibers is -150 nm.Then, polycrystalline T-ZrO₂ continuous fiber was obtained by pyrolysis of precursor fibers from citrate-acetate-zirconium system. In this process, the aqueous solution containing citrate-acetate-zirconium is concentrated to prepare the high viscous spinnable sol. The viscous sol with good spinnability was prepared by aging the starting solution of ZrOCl₂·8H₂O (ZOC) in the presence of acetic acid (HA) and citric acid (CA), and the optimal molar ratio of HA/ZOC is 1.5-2.0 and CA/ZOC is 0.3-0.5, respectively. The stable zirconium citrate complexes are predominant species in precursor solution, the coordination of carboxyl groups to zirconium tetramer in the solution favors the formation of the stable sol and is prefer to the formation of linear polymers, which is critical to the sol spinnability. From the IR spectra of the gel fibers, it can be concluded that the carboxyiate species bond to Zr(IV) of the tetramer [Zr₄(OH)₈(H₂O)₁₆]⁸⁺ cations in bidentate bridging mode. And the coordinated carboxyiate favors the formation of the linear sol particles. All sols show no spinnability if aging time is less than 6 days, indicating that a period of aging time is needed to obtain linear zirconium polymer species and a long gelation time. Raising the aging temperature results in the formation of spinnable sol in a short time. The polycrystalline tetragonal zirconia fibers sintered at 1400℃ are crack-free with smooth surface and ca .5-10μm in diameter.In chapter three, firstly, polycrystalline Al₂O₃-3Y-TZP tetragonal zirconia fibers are obtained by pyrolysis of gel fibers using zirconium oxychloride octahydrate (ZOC) as raw material. The addition of 5wt % alumina favors the sintering process and the formation of fine grains. The zirconia grains grow rapidly at 1200¹500°C from SEM observation, and the size of the crystals composed the fibers is ⁹0nm at 1200°C and -230nm at 1500℃. More defects are observed on surface of fibers sintered at 1500°C, and rapid grain growth occurs after annealing for 5 min. Pretreating the gel fibers at 800 ℃ in N₂ atmosphere followed by sintering at hingher temperature, such as 1400 ℃ for 30 min in air atmosphere, the dense ZrO₂ fibers were obtained. For the obtained dense zirconia ceramic fibers, the tensile strength is up to 2.1 GPa.Secondly, sub-micron Al₂O₃-3Y-TZP ceramic continuous fiber was successively prepared by the electrolysis combined sol-gel process followed by the electrospinning. Sintered fibers show high densification at 1200℃ after pretreatment in N₂ atmosphere at 800℃ followed by sintering at hingher temperature in air atmosphere. No pores and defects can be observed and the average size of the zirconia grains is -100nm.From BET measurements, the surface area of sintered fibers decreased rapidly with the calcination temperature increased from 600 to 1200℃.Thirdly, based on the experiments in chapter 2 and 3, the drying and sintering of the Al₂O₃-3Y-TZP gel fibers were investgated. The effect of the pretreatment in N₂ atmosphere for Al₂O₃-3Y-TZP fiber is discussed. And the differences between 3Y-TZP fiber and Al₂O₃-3Y-TZP fiber are also given. The effect of sintering temperature and time on the grain growth in 3Y-TZP fiber is investgated. Both of the pretreatment in N₂ atmosphere and addition of aluminium favor the sintering and the formation of the fibers composed of the fine grains. Sintered at 1400℃ for 30 min in air atmosphere after pretreated in N₂ atmosphere at 800℃, the Al₂O₃-3Y-TZP fibers composed of the -200nm zirconia grains can be obtained.In chapter 4, the sol-gel dry spinning technique is introduced briefly. Al₂O₃-3Y-TZP continuous ceramic fibers are successively prepared by sol-gel dry spinning technique. Polymer and surfactant are applied to improve the spinnability of the sol. The adhesive force between sol and spinneret decreases upon the addition of the surfactant, which also increases the yield of gel fibers. The effect of the spinning condition is investigated, and the optimal conditions are given: the viscosity of sol is 1-30Pa·S, spinning temperature is 20-35℃, inner diameter of the spinneret is 0.1mm, spinning pressure is 0.2-0.4 MPa, relative humidity is 30-50%, hot air temperature is 60-80℃. Under the conditions, the continuous gel fibers with diameter of 15-30nm are obtained.