Study On Sol Design And Pre-sintering Mechanism Of The Alumina-mullite Precursor Fibers

Continuous alumina-mullite fibers possess outstanding thermal insulation properties and high-temperature mechanical properties,making them suitable as thermal structural materials in high-end equipment such as hypersonic aircraft and aerospace engines.Given the strategic value of continuous alumina-mullite fibers,developed countries in Europe and America have imposed strict technical blockades and product embargoes on China.The production of high-performance continuous alumina-mullite fibers is dependent on highly spinnable sol and precursor fiber thermal decomposition schemes,but relevant research is scarce.This study employs a sol-gel method combined with dry spinning technique,utilizing raw materials including aluminum powder,formic acid（HCOOH）,acetic acid（CH₃COOH）,nitric acid（HNO₃）,tetraethyl orthosilicate（TEOS）,and water to manufacture continuous alumina-mullite fibers.The objective was to systematically investigate the formation mechanism of alumina-mullite fiber precursor sol,clarify the thermal decomposition behavior and influencing factors of aluminum-mullite precursor fibers,as well as reveal the impact mechanism of thermal decomposition residues on the phase transformation and microstructure evolution of alumina-mullite fibers.This study produces exceptional results,providing important insights into the preparation of high-performance continuous alumina-mullite fibers,which has significant guiding meaning for realizing the production of high-performance continuous alumina-mullite fibers in China.The main achievements of this paper were as follows:（1）The influences of HCOOH,CH₃COOH,HNO₃ and TEOS on the formation and spinnability of alumina-mullite precursor sol were revealed.The study found that the amount of Al O₆ oligomer had a significant impact on the spinnability of carboxylic acid aluminum sol.Compared with HCOOH and HNO₃,CH₃COOH could promote the formation of Al O6oligomer in the sol.However,when the proportion of CH₃COOH in the total acid exceeded 60%,Al（OH）（OOCCH₃）₂ and Al（OOCCH₃）₃precipitated.When TEOS was used as the silicon source,it could be mixed with carboxylic acid aluminum sol to form a uniform alumina-mullite fiber precursor sol.During the preparation of the precursor sol,increasing the proportion of HCOOH could lead to cracking and peeling of the spun precursor fibers surface,while increasing the proportion of HNO₃ could result in moisture absorption and fracture of the spun precursor fiber.A mole ratio of Al:HCOOH:CH₃COOH:HNO₃:Si of 1:0.67:0.6:0.35:0.15could obtain an alumina-mullite fibers precursor sol with excellent spinnability.（2）The thermodynamic mechanism of the decomposition of alumina-mullite precursor fibers was clarified,and the influence of heat treatment temperature and atmosphere on the thermal decomposition behavior of precursor fibers was revealed.As the thermal decomposition temperature increased,there was a sequential removal of free water,decomposition of NO₃^-,decomposition of HCOO^-,removal of-OH,and decomposition of CH3COO^-from the precursor fiber.Under a nitrogen atmosphere,the CH3COO^-removal mode in the precursor fiber shifts from oxidation decomposition to anaerobic cracking.Under a steam atmosphere,the HCOO^-and CH3COO^-removal modes in the precursor fibers change from oxidation decomposition to hydrolysis removal.In the range of350～700℃,increasing the heat treatment temperature could effectively reduce the amount of residual substances in the fibers,but could also cause dense sintering of the fibers cortex,resulting in an uneven sintered microstructure.（3）The influence mechanism of thermal decomposition residual carbon and residual hydroxyl on the phase transformation and microstructure evolution of alumina-mullite fibers during high temperature sintering was clarified.The alumina-mullite pretreated fibers directly generate tetragonal mullite at a sintering temperature of 800℃,and the orthorhombic mullite was formed at a sintering temperature of 1300℃,whileθ-Al₂O₃ andα-Al₂O₃ were precipitated at the same time.When the alumina-mullite pretreated fibers contain～10 wt.%residual carbon or hydroxy group,γ-Al₂O₃ phase is preferentially generated at a sintering temperature of 800℃,andθ-Al₂O₃ and orthorhombic mullite are generated starting at 1300℃.The presence of residual carbon or hydroxy groups contributed to obtaining dense and fine-grained alumina-mullite fibers.Systematic studies have found that the existence of residual carbon and hydroxy groups can effectively prevent the formation of Al-O-Si bonds in the fibers during sintering and inhibit the formation of mullite at low temperatures.