Thermodynamics And Kinetics Of Formation Of Calcium Aluminate Glass

Although there are many important breakthroughs in glass science,the nature of glass and the formation mechanism of glass are still a major issue in the field of glass.Exploring the thermodynamics and kinetics is critical for understanding the issue.However,the metastable state of the glass and the complexity of the structure have brought great challenges to research.In order to explore the thermodynamics and kinetics during the glass formation,the CaO-Al₂O₃ binary and CaO-Al₂O₃-SiO₂ ternary systems due to the wide application of calcium aluminate glass were studied with a novel containerless laser heating aerodynamic levitation furnace.The properties of glass such as viscosity,density,hardness and glass transition temperature were evaluated.Raman spectroscopy and NMR spectra were used to characterize the internal structure of the glass.Therefore,the evolution of structure and properties of the glass with changes in CaO content was explored,and the structural origin of the evolution of the properties was analyzed.The results are as follows:（1）Using sol-gel method combined with a novel laser heating aerodynamic levitation furnace,CaO-Al₂O₃ binary glasses were successfully prepared in a wide range of composition from CA2 to C5A for the first time.With the substitution of Al₂O₃by CaO,the content of Al^IV and non-bridge oxygen increases,while the content of highly coordinated Al decreases,and degree of structural connectivity increases first and then decreases,causing an extremum at C12A7.This structural evolution gives rise to non-monotonic variations of the characteristic temperatures,density,thermal expansion,viscosity and extremums occur at C12A7.Through the determination of fragility,it is found that CaO-Al₂O₃ liquids undergo a dynamic fragile-to-strong transition（F-S transition）during the formation of glass,and the extent of transiton is greatest at C12A7.We infer that the F-S transition phenomenon may be related to the structural evolution of medium range order.In addition,the study of the crystallization behavior of binary glass shows that there is a change from two-dimension to three-dimension during the crystallization of C12A7 glass,which has the lowest crystallization activation energy in the system.（2）For the CaO-Al₂O₃-SiO₂ ternary system,the effect of CaO on the structure is different from that of the binary system.When Al³⁺forms tetrahedra,Ca²⁺is needed for charge compensating.Therefore,when the ratio of Ca/Al is less than 1/2,the increase of CaO content will promote the formation of[AlO₄]tetrahedra and enhance the network connection.And the degree of network connection will decrease with the addition of CaO when Ca/Al ratio is greater than 1/2.The evolution of structure leads to the fact that the variations of parameters of thermodynamic and kinetic and physical properties for the ternary glasses with CaO content differ from that of the binary system.And the extreme values occur in the vicinity of Ca/Al=1/2.Through the determination of fragility,it is found that the CaO-Al₂O₃-SiO₂ ternary system also undergo a dynamic F-S transition during the formation of glass.The extent of transition gradually decreases with the increase of CaO,which is different from that of the binary system.This may be related to the competition between Al³⁺and Si⁴⁺forming tetrahedra.This work clarified the effect of CaO content change on the thermodynamics and kinetics and physical properties of the calcium aluminate glass,and found that the CaO-Al₂O₃ binary and CaO-Al₂O₃-SiO₂ ternary systems both underwent a fragile-to-strong transition during the formation of glass which provided a reference for understanding the nature of the glass transition.