| MgO-Al2O3-SiO2?MAS?system glass-ceramics have the properties of high mechanical strength,low dielectric loss,excellent chemical stability and thermal stability,showing great application value in electron,military,architecture and other fields.In this dissertation,the relationships among composition,process,structure and properties of MAS glass-ceramics were investigated by DTA,XRF,XRD,SEM,EDS,FT-IR,thermal dilatometer,hardness meter,bending strength tester and UV-Vis-NIR.The glass-ceramics with large grain size,high crystalline phase content,excellent mechanical properties and light transmittance were prepared by optimizing the composition,crystalline phase and heat treatment.The results show that:?1?The influence of composition on crystallization behavior,microstructure and properties of glass-ceramics was studied.Transparent glass-ceramics were prepared by controlling the precipitation of solid solution crystalline phase.The initial crystalline phase of glass-ceramics is Mg0.6Al1.2Si1.8O6 solid solution,and the final crystalline phase is cordierite.The crystallization activation energy of Mg0.6Al1.2Si1.8O6 phase decreases from 499 KJ/mol to 345 KJ/mol,cordierite phase increases from 439 KJ/mol to610 KJ/mol when SiO2/MgO molar ratio increases from 2.5 to 3.3.The thermal stability of metastable phase Mg0.6Al1.2Si1.8O6 enhances with the increase of SiO2/MgO molar ratio,thus causing a difficulty in cordierite crystallization and slowing down the phase transition process.Compared with cordierite,the refractive index of Mg0.6Al1.2Si1.8O6 phase is closer to that of glass phase.The suitable glass composition is determined as 19MgO-19Al2O3-62SiO2?mol%?.When nucleated at 850°C for 10 h and then crystallized at 1020°C for 2 h,transparent glass-ceramics of Mg0.6Al1.2Si1.8O6 solid solution are obtained,and the average grain size is 8.33?m.?2?The effect of heat treatment on the crystalline phase content,microstructure and properties of glass-ceramics was researched.High-performance transparent glass-ceramics were prepared by improving the Mg0.6Al1.2Si1.8O6 crystalline phase content.The optimum heat treatment schedule is as follows:nucleation temperature is 750°C,nucleation time is 20h,crystallization temperature is 1020°C,and crystallization time is 8 h.After heat treated at 750°C/20 h+1020°C/8 h,the glass-ceramics with average grain size of 26.68?m and the crystalline phase content of 94.6 wt%are obtained.The crystalline phase content of glass-ceramics increases from 42.9 wt%to97.5 wt%with the increase of crystallization time from 2 h to 10 h.The light transmittance of glass-ceramics slowly decreases with prolonging crystallization time.The thermal expansion coefficient slowly increases with prolonging crystallization time.The Vickers hardness increases first and then flattens with prolonging crystallization time,and the bending strength increases first and then decreases with prolonging crystallization time.When heat-treated at optimum heat treatment schedule,the glass-ceramics have the best comprehensive properties,which have light transmittance of 83%in visible light region,thermal expansion coefficient of 3.857×10-6/°C?600°C?,Vickers hardness of 10.2 GPa,and bending strength of 200 MPa.?3?The crack formation of glass-ceramics caused by the phase transition of Mg0.6Al1.2Si1.8O6 to cordierite was inhibited by introducing oxides with different roles on networks.The preparation of transparent glass-ceramics with the co-existence of Mg0.6Al1.2Si1.8O6 and cordierite was preliminarily explored.Compared with the glass-ceramics without additive,P2O5,B2O3,ZnO,BaO and K2O promote the crystallization amount of cordierite to different extents,in which ZnO>B2O3>K2O>BaO>P2O5.The network intermediate ZnO and network modifiers BaO,K2O do not achieve the purpose of suppressing the crack,due to increasing the thermal expansion coefficient of basic glass.However,the network formers B2O3 and P2O5 decrease the thermal expansion coefficient of basic glass,which plays a role in suppressing crack during the crystallization process of cordierite.After heat treated at850°C/10 h+1050°C/2 h,the translucent crack-free glass-ceramics with the co-existence of Mg0.6Al1.2Si1.8O6 and cordierite are prepared by adding 2mol%P2O5 as the additive.The relative crystalline phase content of Mg0.6Al1.2Si1.8O6 and cordierite is 70.2%and 29.8%,respectively.The thermal expansion coefficient of glass-ceramics is 3.168×10-6/°C?600°C?,bending strength is 253 MPa,and Vickers hardness is 10.0 GPa.The light transmittance of glass-ceramics in visible light region is more than 60%. |
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