| Magnesium aluminosilicate (MAS) glass-ceramics containing spinel and high-/low quartz solid solution, own good chemical stability, low thermal expansion coefficient, excellent dielectrical property and good mechanical properties. In the dissertation, the compositions, processes, structure and properties of MAS glass-ceramics were investigated by XRD, DSC, horizontal dual-rod dilatometer, rotating crucible viscometer FE-SEM and UV-Vis-NIR. The results show that:The MAS glass with5%TiO2+3%ZrO2(wt%, the same as below) as nucleating agents is reasonable to prepare transparent glass-ceramic. When nucleated at800℃for4h and then crystallized at950℃for1.5h, the nearly colorless transparent glass-ceramic with spinel as the main crystal phase was prepared, and the transmittance in the visible spectra region reached86%. The mean particle size of the transparent glass-ceramic was about34nm. As the MAS glass using only TiO2as nucleating agent, the prepared transparent glass-ceramic, containing MgTi2O5and MgAl2O4crystalline phases, was slight purple and with83%transmittance in visible spectra region. As using3-8%ZrO2as nucleating agent, only the ZrO2concentration reached8%, the transparent glass-ceramics could prepared by nucleated at900℃for6h and then crystallized at1060℃for6h. The glass-ceramic was absolutely colorless and with indialite as the main crystal phase. It could not be ignored that this compostion glass was difficult to melt, high temperature and long time to heat-treat.In the MgO-Al2O3-SiO2system glass, the0～4%ZnO was partially substituted for MgO. The transparent glass-ceramics were prepared after two-step heat-treated schedule800℃/4h+950℃/1.5h, and the main crystal phase were spinel solid solution. With the increasing of ZnO content, the proportion of gahnite in the spinel solid solution increased. Meanwhile, the prepared MAS glass-ceramics were all with over85%transmittance in visible spectra region when the substitution content below3%. However, when the ZnO content increased to4%, the transmittance of glass-cermamic decreased to79%. In the UV spectral region, the UV cut-off wavelength shifted to the short wavelength direction with the increasing ZnO content.When the ZnO content was equal or greater than2%, it was effective to refine crystal size and make the gains uniformly distribution. The mean grain size of the glass-ceramics containg2%ZnO was only29.4nm, and the Sd was7.5. And the MAS glass-ceramics kept transparent even heat-treated800℃/4h+1000℃/1.5h. As to the same composition MAS glss-ceramic, the UV cut-off wavelength moved towards long wavelength with the increasing of crystallization temperature.The effects of alkali metal oxide on the viscosity and crystallization properties of MAS glass-ceramics were studied. The primary MAS glass contained2%Na20, and the0.5%,1.0%,1.5%and2%Li2O were used to substitute for Na2O. The results showed that the viscosities of MAS glasses first decreased and then increased with the increasing of the Li2O-concentrations. As1%Na2O was replaced by the Li2O, the MAS glass had the lowest viscosity and melting temperature. The activation energy for viscous flow of MAS glass with2%Na2O,1%Li2O+1%Na2O,1%K20+1%Na20were306.04kJ/mol、284.01kJ/mol、288.78kJ/mol respectively, and the melting temperature were1612℃,1508℃,1573℃respectively. As compared to the glass containing2%Na2O, the introduction of Li2O changed the crystal phase precipitation mode from two-dimensional to bulk crystallization, but had little effect on the activation energy of crystallization. It also changed the main crytal phase to the quartz solid solution. The introduction of K2O decreased the activation energy of crystallization from370.1kJ mol-1to275.3kJ mol-1, but had little effect on the crystal phase precipitation mode. The MAS glass containing1%K2O+1%Na2O could prepare the transparent glass ceramic at the crystallization temperature range from880℃to1000℃.The effects of Y2O3on the physical and crystallization properties of MgO-Al2O3-SiO2glass were investigated. As the Y2O3-concerntration grows from0to1.5mol%, the transformation temperature (Tg) varied slightly, the dilatometric softening temperature (Ts) increased from791℃to814℃, the CTE increased from41×10-7/℃to45×10-7/℃, and the micro hardness increased from605kg/mm2to652kg/mm2. The MAS glasses with0～0.01mol yttria precipitated the quartz solid solution when crystallized at the temperatures range from840℃to1120℃. As the glass was crystallized above1000℃, the high-to low quartz solid solution phase transformation was occured during the glass-ceramic cooling. The MAS glass doped with0.015mol Y2O3was still amorphous when heat-treated at840℃, at940℃～1120℃, the quartz solid solution phase presented, and at1060℃the phase transition of the high-to low-quartz solid solution was observed. The MAS glass doped with0.005mol and O.Olmol Y2O3were transparent with the transmittance about41%after heat-treated at840℃for1.5h. |
PDF Full Text Request