Studies Of Several Fundamental Problems In The Fabrication Of Oxide Crystalline Fibers

The oxide crystalline fibers have high melting point, high hardness, corrosion and oxidation resistance, excellent photoelectric and magnetic properties. They have important applications in the fields of aviation, aerospace, military and civilian industries. In this dissertation, the fundamental problems in the fabrication of oxide crystalline fibers were studied. They are the precursor polymerization, the fiber crystallization and the optimization of fiber spun technology. The understanding of the above problems is good for the controllable fabrication of the oxide crystalline fibers.(I) ZrO2 continuous fibers are the toughening materials in ceramics matrix composites used in aviation and aerospace field. In our group, during the last decades, the ZrO2 continuous fibers with the tensile strength up to 2.8 GPa, longer than 1 meter have been successfully obtained through polyacetonatozirconium precursor method. The spinneret used was 50 holes. In this charpter, several fundamental problems in the fabrication of ZrO2 continuous fibers were discussed. Continuous fiber spinning using spinneret with 500 holes was realized firstly. The main content and conclusions draw in this charpter are as followed:(1) The effect of synthesis temperature on the structure of PAZ was studied. The results demonstrate that the increasing temperature facilitates the dissociation of acetonate (Hacac) group from Zr4+ion. In the unit of PAZ, one Zr4+ ion connect to an Hacac and three -OH groups. The bonding between Hacac and Zr4+ ion is coordinate bonding. The Hacac group was considered that confined the PAZ polymerizing into linear structure which is the origin of spinnability. In our research, the dissociation of Hacac group from Zr4+ ion would deteriorate the spinnability. Based on the above conclusion, the sythesis temperature should be low.(2) The polymerization structure of PAZ in methonal solution was studied through rheological measurement. The results demonstrate that as the concentration of PAZ methanol solution increasing, the solution was pseudoplastic fluid. The fluid became newtonian fluid as the temperature increasing and concentration descreasing. The results demonstrate the polymerization occurred inducing by the high concentration and low temperature.(3) The rheological parameters of the spin dopes were optimized through regulating the concentration and the temperature of the spin dopes. The infinite dry spinning of PAZ continuous fibers was realized. The dry spinning using spinneret with 500 holes was realized through the addition of CTAB in precursor solutions. The role of CTAB in dry spinning was analyzed. Exfoliation of the spin dopes from the spinneret was enhanced due to the addition of CTAB in spin dopes.(4) The phase and microstructure of the PAZ continuous fibers was characterized. The results demonstrate that the precursor continuous fibers can be obtained in large scale through the modified dry spinning. The diameter of the fibers are uniform and about 30 μm. The precursor fibers are amorphous.(5) The morphology of the ZrO2 grain heat treated at different temperatures was observed using HRTEM and SEM. The results demonstrate that when the fibers heat treated at 500℃, the fibers were composed of nano ZrO2 crystal grain, amorphous component and pores.(6) The effect of CTAB in precursor on the grain size of ZrO2 was discussed. The results demonstrate that the grain size of the fibers with addition of 2% wt CTAB are the largest. The grain size of the fibers without addition of CTAB are the smallest. As an activation agent of self-propagation combustion, the CTAB in the precursor can facilitate the removal of the carbon in the precursors. The driving force for sintering of the Z1O2 grains increased due to the decreasing of the carbon in precursor. However, with the increasing of CTAB in precursor, the content of carbon would be increased.(II) Phase transition in ZrO2 occurred when the temperature changing which would induce inner stress and the lose of mechanical properties. In the research of this chapter, the CaO doped ZrO2 fibers were fabricated through the addition of CaCl2 in PAZ methanol solutions. The effect of CaO dopant on the crystal phase of ZrO2 was discussed. When the content of dopant are large, new phase of CaZrO3 was observed. The CaZrO3 fibers were fabricated by using precursor method and the high temperature ablation was measured.(1) The CaO doped ZrO2 short fibers were obtained through centrifugal spinning the PAZ methanol solution with different CaCl2 dopant into gel fibers followed by heat treatment. The crystal phase of the ZrO2 was studied using XRD and Raman. The results demonstrate the ZrO2 fibers were monoclinic and tetragonal phase when the content of CaO is below 10% mol. The ZrO2 were tetragonal and cubic phase when the content of CaO is above 10% mol.(2) The CaO stabilized ZrO2 were heat treated at 1600℃ for 2 h. The fibers with 10% mol CaO dopant transformed from tetragonal to monoclinic with the appearance of crack. The reason was that the Ca2+ ions were removed from ZrO2 grains at high temperature.(3) CaZrO3 fiber were fabricated using PAZ, calcium nitrate and citric acid as starting materials. The pyrolysis mechanism were studied by IR, TG-DTA and XRD. The microstructure of the fibers were characterized by SEM. The high temperature ablation property were characterized by coal gas-oxygen flame. The results demonstrate:The obtained fibers are pure CaZrO3 phase. The crystal phased is stable under the high temperature. However the fiber brick is little shrink.(Ⅲ) The ZnO is one of the most important semiconductor materials, featuring the excellent photo-electron properties. One dimension ZnO has large aspect ratio, special electron properties. It can be used as photo-electron device, photocatalysis. In the research of this chapter, based on the structure of PAZ, the polycitratezinc(PCZ) with linear structure was designed as ZnO fiber precursor. The crystallization of ZnO from the amorphous precursor was discussed. The properties of photocatalysis was tested. The ZnO nanorods were fabricated by using layer zinc hydroxide precursor. The crystal growth mechanism of ZnO nanorods from the precursor was discussed.(1) The citrate acid was chosen as the ligand. The PCZ precursor was synthesized through molecular design. The molecular structure and polymerization of PCZ were studied using NMR and IR. The results demonstrate that in PCZ, the uncoordinated-OH in CA has tendency to coordinate the Zn2+ion. As a result, the PCZ polymerized into linear structure with good spinnability.(2) The pyrolysis mechanism of PCZ were studied using TG-DSC, IR and XRD. The results demonstrate that the thermal decomposition of PCZ has five steps, that is, the dissocitation of NH/, the dissocitation of-OH group in CA, the carbonization of CA ligand, the crystallization of ZnO, the combustion of carbon. The ZnO fibers can be obtain after the thermal decomposition of the precursor. The microstructure of the ZnO fibers was studied using SEM. Owning to the escape of gas, the ZnO fibers is inner hollow. As the temperature increasing, the fibers became dense due to the growth of crystal grains.(3) The photocatalytic degrading MB aqueous under the illuminating of Xe lampe using ZnO fibers was tested. The ZnO powders was used as comparison. The results demonstrate that ZnO fibers can photocatalytic degrade the MB solution efficiently.(4) ZnO nanorods were assembledonto the surface of TiO2 fiber. The ZnO nanorods@TiO2 fibers heterostructure were obtained. The growth mechanism was studied. The results demonstrate that first the layer zinc hydroxide precursor absorbed onto the surface of the TiO2 fibers through electrostatic adsorption. Then the precursor would transformed into ZnO nanorods in chemical bath.