Research Of Preparation Of Continuous Alumina Fibers By Sol-Gel Method

Alumina fiber is an important non-oxide ceramic fiber that can be used in various applications such as high-temperature insulation materials, catalyst support in high temperature reactions, fire protection, and as reinforcement for resins, metals and ceramics for its outstanding physical and chemical stability and good thermal insulation. Sol-gel technique is one of the most versatile methods for producing alumina fibers.The present work is mainly focused on the basic research on the fabrication of alumina fibers by sol-gel method, which will be a valuble direction for preparing continous alumina fibers.Spinnable alumina precursor gels were prepared from aluminum isopropaxide (AIP), aluminum nitrate (AN) and distilled water (H₂O) by refluxing and concentration. Continous gel fibers were drawn by hand. After drying and sintering,α-alumina fibers with the length and diameter of about 20cm and 20-30μm, respectively, were obtained. PH analysis, particle size analysis, FTIR and NMR were used to trace the reactions occurred in the sol-gel process and the corresponding changes of the particle structures. The hydrolysis and condensation reactions exist in the sol-gel prosess simultaneously. At initial stage, hydrolysis reaction takes the dominant place and then condensation reaction replaces. There are only octahedral Al species in the system at first. With the development of the reactions, tetrahedrally coordinated Al species are appeared. The gelation occures by condensation reactions between more-symmetric species to form less-symmetric and higher-molecular weight species. Then the further condensation makes the linear polymers change to the network structure.Single factor alternation method was used to study the synthesis process. The optimized parameters were as follows: the mole ratio of AIP and AN 3/1, the mole ratio of H₂O and AN 100, lactic acid 4wt%, condensation temperature 80℃. Under such parameters, the well spinnable gel with high solid content was synthesized easily. The composition and structure of the obtained gel were studied. The gel is composed of aluminum, oxygen, carbon, nitrogen and hydrogen. Nitrigen is existed in the form of NO3-. Carbon is mainly distributed in the isopropyl and lactic acid. Aluminum is connected with oxygen only in the form of Al-O-Al. The backbone of the gel is Al-O-Al or Al-OH-Al with some residual alkoxyls. Otherwise, Al-O-Al shows tetrahedron or octahedron sterically.The necessary structure for the spinnable gel is high- molecular- weight linear polymers with some cross-linking and no over hydrolyzed precipitation. The range of PH for spinnable gel is 3.05-3.45. The spinnability is highly relative with the viscosity of gels. Only when the viscosity is distributed among 30-60 Pa·s, continous gel fibers could be obtained. In such viscosity range, the diameters of gel fibers increase with the viscosity. The temperature and humity of the environment, expecialy humity also influence the fabrication of the gel fibers. The transparent, flexible and quite tough gel fibers with no distinct defects could be obtained in the environment with the humity 65%-75% and the temperature 25-30℃. These fibers are as long as 2-3m and 30-40μm in diameters.The drying of gel fibers is nearly inflounced by the drying conditions such as temperature, humity and pressure. The rate of evaporation of water increases with the ascent temperature, descent humity and degrassive pressure. The experiment shows that 25℃-40℃for temperature and 45%-65% for humity are the appropriate conditions in which the good gel fibers with only a few defects could be obtained.The heating process of the gel fibers was investigated by TG, DSC, MS, FTIR and XRD. The inorganic conversion of the dry gel fibers mainly involves three stages: (I) Below 120℃, the free water in the gel fibers are volatilized during this step. (II) At 120⁶00℃, the thermolysis of the organic groups and nitrate occurs, leading to escape of hydrocarbons nitroxide and hydrogen and the mineralization is completed. (III) At 600¹300℃, the mass loss stops and crystallization take place.Kinetic study of the crystallization process indicates that the apparent activation energy(Ea) forγ-Al₂O₃ crystallization is 253.31KJ/mol and forα-Al₂O₃ is 301.60KJ/mol. Besides, Ea forγ-Al₂O₃ increases with the increase of the crystallized fraction and Ea forα-Al₂O₃ is practically constant at the fraction range which indicate that the crystallization ofγ-Al₂O₃ is more complex thanα-Al₂O₃.The high performance alumina fibers which are typically fine, dense, with few detects, fine equiaxed grains and homogenous structure could be obtained at the optimized heating system: heated to 600℃at the rate of 1℃/min with the holding time of 2h and then quickly heated up to 1300℃at the rate of 10℃/min with the holding time of 1h. Besides, fibers sintered on tension showed better property and fewer defects than traditional sintered fibers.