Rheological Behavior And Structure Evolution Simulation Of Aluminum Carboxylate Sol In Dry Spinning

With high strength,high modulus,excellent high temperature resistance and oxidation resistance,continuous alumina fibers are widely used as thermal insulation materials for key components in aerospace,national defense and military industry.It is a key strategic material in urgent need of breakthrough in China.Sol-gel dry spinning is an effective way to prepare continuous alumina fibers.The structure and rheological properties of alumina precursor sol are the constraints of colloidal fiber formation.However,due to the lack of quantitative characterization and detection technology of aluminum sol structure,the structure evolution of aluminum sol in dry spinning and its relationship with rheological behavior have not been clear.In this paper,aluminum powder,formic acid and acetic acid were used as raw materials to prepare carboxylic acid aluminum sol,and it’s rheological behavior and structural evolution in dry spinning were studied by means of atomic molecular dynamics simulation,finite element numerical simulation and experimental characterization.This research revealed the rheological properties and influencing factors of aluminum carboxylate sol,clarified the evolution mechanism of sol structure in the rheological process,and explicited the influence law of sol rheological properties on its spinning uniformity,so as to lay a foundation for obtaining high-performance continuous alumina fiber.The main innovative achievements are as follows:(1)The effects of solid content,temperature and spinning additives on the rheological properties of aluminum carboxylate sol were clarified.The elastic modulus of the sol increases rapidly with the shear of the dilute solution,and the viscosity of the sol increases rapidly with the increase of the elastic modulus of the dilute solution.With the increase of temperature,the zero shear viscosity of aluminum carboxylate sol decreases,the shear thinning characteristic decreases,the dynamic storage modulus and dynamic dissipation modulus decrease,and the viscosity characteristic of sol increases relatively.Adding high molecular weight PVP can significantly improve the zero shear viscosity of aluminum carboxylate sol,weaken the shear thinning characteristics of sol,increase the dynamic storage modulus and dynamic dissipation modulus,and enhance the elastic characteristics of sol.(2)Atomic molecular dynamics simulation combined with freezedrying in-situ characterization technology was used to reveal the structural evolution law during the formation of spinnable sol.In carboxylic acid aluminum sol,hydroxy aluminum methoxide tetramer,hydroxy aluminum dicarboxylate tetramer and hydroxy aluminum diacetate tetramer polymerize to form colloidal particles through hydrogen bond and intermolecular force.Colloidal particles in low solid content sol are dispersed in aqueous solvent and are not connected with each other.With the increase of solid content,the colloidal particles are gradually crosslinked to form a network structure,and finally aggregate to form a dense structure.The cohesive energy density of aluminum carboxylate sol increases slowly with the increase of solid content.When the sol begins to undergo gel transformation(solid content is about 29 wt%),the cohesive energy density of the system decreases rapidly.(3)The evolution law of aluminum carboxylate sol structure during shear rheological process was revealed,and the influence mechanism of sol structure on its rheological properties was clarified.The rheological properties of aluminum carboxylate sol depend on the molecular migration ability.The diffusion coefficient of solvent water molecule is two orders of magnitude higher than that of aluminum carboxylate oligomer,which is the key factor affecting the movement ability of sol.With the increase of sol solid content,the volume fraction of water molecules decreases,the diffusion coefficient between water molecules and aluminum carboxylate oligomer decreases greatly,and the sol viscosity increases greatly.Under the condition of shear deformation,the colloidal particles in the low solid content sol deflect in the shear direction,and the water molecules flow in layers with the oligomer;Oligomer molecules in high solid content sol rearrange along the shear direction through local deflection and slip.At low shear rate,the sol can reach equilibrium through molecular motion;At high shear rate,it is difficult for molecules to reach equilibrium through rapid movement,and the attraction between molecules is greatly reduced,resulting in the shear thinning of sol.(4)The effects of process conditions and rheological properties of aluminum carboxylate sol on spinning uniformity were revealed by finite element numerical simulation and dry spinning experiment.Optimizing spinning components and increasing the inlet velocity of spinning solution are conducive to reducing the flow difference between different spinnerets.Under the optimized spinning process conditions,appropriately reducing the zero shear viscosity of sol and weakening the shear thinning characteristics of sol are conducive to improve the spinning uniformity of aluminum carboxylate sol.The aluminum carboxylate sol with a relative molecular weight of 58000 PVP has the most suitable rheological properties.Continuous alumina precursor fibers with small diameter dispersion can be obtained by using the sol.