Structural And Rheological Study Of High-temperature Melts Of Alkaline Earth Aluminosilicate Glasses For High-performance Fibers

High-performance glass fiber has the advantages of high specific strength,large elongation,and good resistance,and is widely used in aerospace,weapons,nuclear energy,industrial equipment and other fields.The high-strength glass fibers are mainly from alkaline earth aluminosilicate glass system,which possess high strength,high temperature resistance,corrosion resistance and other characteristics.Nowadays,the high-strength glass fibers at home and abroad have super high melting and forming temperatures,which brings about obstacles to their production.At present,although theories,experiments,and computer simulations were applied in research work,the structure of fiber melt is still missing;at the same time,full attentions should be emphasized on scientific research and industry on the processing parameters of drawing glass fiber.In response to the above problems,this dissertation has studied the structural and rheological properties for high-temperature melt,and explored the relationship between the composition and the rheology properties of fiber melt on the basis of the structure.High-temperature viscometer,dilatometer,DSC,Raman and molecular dynamics simulation were applied to study the effects from mixed alkaline earth effect caused by alkaline earth metal oxides（RO）,RO/Al₂O₃ ratio and RO/SiO₂ ratio on the structure,the high-temperature rheology of glass melt,the drawing process parameters and the comparison of the calculated and experimental properties at low,melting and fiber-spinning temperatures.The results show that:Adjustment of ratio within RO（CaO replacing MgO）:The main structure at low temperature and high temperature is composed of[SiO₄]and[AlO₄],with other cations filling in the voids of the network.The bond length and strength of different ions from PDF,the coordination,bond angle distribution of Si⁴⁺,Al³⁺,Mg²⁺and Ca²⁺are consistent with the experimental data.Raman test shows that the content of Q³ and Q⁴ in the silicon-oxygen network decreased firstly and then increased,while the structural parameter（NBO/T）first increased and then decreased,indicating the network loosened first and then aggregated.The monotone changing in Qⁿ at high temperature was related to the weakening of the mixed alkaline-earth effect.The characteristic temperatures,viscosity,activation energy,and fragility of the melt at low temperature first decreased and then increased,while the workability and expansion coefficient changed oppositely.The parameter change at high temperature is related to the linear increase of free volume.The mixed-alkaline effect led to only little reduction from1872.80K to 1862.40K in glass melting temperature,with only small impact in high-temperature zone,and more modification should be applied;while the relationship between fragility and the melting temperature is negative,which could help discussing the melting temperature through the relation of NBO/T and fragility.A relationship can be established between NBO/T and the fragility（independent variable x）:when Ca/（Ca+Mg）is less than 0.5,the equation is y=-9.653·x+55.31;otherwise,the equation equals y=-21.705·x+68.12.The fiber forming parametersΔT and K_fib both decreased first and then increased,while the relationship between the fragility,m,and the melt rigidity parameter,D,indicated that this group of melts is not easy for drawing fibers.The drawing experiment also confirmed that K_fibcan accurately describe the fiber forming ability,and obtained fibers with diameters around30μm with strength of about 2000MPa.According to the simulation,the value of NBO/T can also build a relationship with K_fib（independent variable x）:when NBO/T is less than 0.7,the equation of y=-10.984·x+12.86 can be used,and R² is 0.939;when NBO/T is greater than 0.7,the equation equals y=0.238·x+0.99,and R² is 0.995.The glass melt network in which RO replaces Al₂O₃ is also mainly composed of[SiO₄]and[AlO₄],and the aluminum-oxygen coordination changes from a polyhedron to a tetrahedral structure.When Al₂O₃ reduced,and the aluminum oxide polyhedron in the overall structure also reduced,the PDF,ion coordination number and bond angle distribution from calculation are also consistent with the experimental results,but the high temperature led the bond length to become larger,the coordination platform to narrow,and the bond connection to become weaker.The decrease in Al₂O₃ content would cause the decrease in BO and the increase in NBO and FO.The Qⁿ distribution and the increase in NBO/T also indicated the gradual depolymerization of the glass network.The replacement of Al₂O₃ by RO leads to a more open structure and a lower degree of polymerization for the glass network.The characteristic temperatures,such as melting temperature and activation energy,viscosity dropped rapidly,while the fragility,workability,and the coefficient of expansion increased significantly,and the free volume increased linearly at high temperatures.The relationship between NBO/T and fragility（independent variable x）is a positive linear relationship,with the relationship of y=12.775·x+31.44,and the correlation coefficient is 0.824.The replacement of Al₂O₃ by RO caused a significant decrease inΔT and K_fib;the relationship between m and D showed that the melt is in the‘fragile region’,which is not conducive to fiber drawing.Similarly,the experiment also proved that only K_fib can describe the fiber forming ability,and the NBO/T from simulation has an exponential relationship with K_fib（independent variable x）,and the relationship is y=758.16·e^-6.866x+0.51,and the coefficient of fitting is 0.96.The drawn fiber diameters are still about 30μm,and the strength decreases with the decrease of Al₂O₃,with the highest at about2000MPa.The change of RO gradually replacing SiO₂ is similar to replacing Al₂O₃.The coordination of Al gradually changes from five to four.The content of Q⁴ in the aluminum oxide network increased,but it has no significant effect on the coordination of Al³⁺.The PDF,coordination number and bond angles for all cations indicated that the extra RO would destroy glass network.The characteristic temperatures,viscosity,and activation energy decreased rapidly with the decrease in SiO₂,while the fragility,workability,and expansion coefficient increased significantly.The whole glass network possesses a more open structure and gradually depolymerizes,and the NBO/T also increases;while high-temperature structural changes are mainly related to the increase in free volume.The relationship between NBO/T and fragility（independent variable x）is:y=77.503·x-29.29,and R² is 0.936.When referring the fiber forming parameters,it can be seen that the reduction of SiO₂ causes structural depolymerization,which also leads to a significant decrease inΔT and K_fib;the relationship between fragility and melt rigidity parameters varies with the content of SiO_2,which indicates the melt is impossible for drawing fibers.It is confirmed that K_fib can accurately describe the fiber forming ability;the experiment obtained a fiber with a strength of 1800 MPa and a diameter of 30μm.The relationship between the structure parameter NBO/T and the fiber forming parameter K_fib（independent variable x）is y=4.36×10¹¹·e^-18.91x+0.041,and R² is 0.984.This dissertation would calculate the melt structure through molecular dynamic simulation,and combine the structural parameter（NBO/T）with the fragility and/or fiber forming parameter quantitatively to predict fragility,melting temperature and fiber drawing parameter,greatly improving the efficiency of research work.