Department of Materials, Imperial College London, London SW7 2AZ, UK.
Dent Mater. 2011 Oct;27(10):990-6. doi: 10.1016/j.dental.2011.06.005. Epub 2011 Aug 30.
This study reports real time neutron diffraction on the Empress II glass-ceramic system.
The commercial glass-ceramics was characterized by real time neutron diffraction, ³¹P and ²⁹Si solid-state MAS-NMR, DSC and XRD.
On heating, the as-received glass ceramic contained lithium disilicate (Li₂Si₂O₅), which melted with increasing temperature. This was revealed by neutron diffraction which showed the Bragg peaks for this phase had disappeared by 958°C in agreement with thermal analysis. On cooling lithium metasilicate (Li₂SiO₃) started to form at around 916°C and a minor phase of cristobalite at around 852°C. The unit cell volume of both Li-silicate phases increased linearly with temperature at a rate of +17×10⁻³ ų.°C⁻¹. Room temperature powder X-ray diffraction (XRD) of the material after cooling confirms presence of the lithium metasilicate and cristobalite as the main phases and shows, in addition, small amount of lithium disilicate and orthophosphate. ³¹P MAS-NMR reveals presence of the lithiorthophosphate (Li₃PO₄) before and after heat treatment. The melting of lithium disilicate on heating and crystallisation of lithium metasilicate on cooling agree with endothermic and exotermic features respectively observed by DSC. ²⁹Si MAS-NMR shows presence of lithium disilicate phase in the as-received glass-ceramic, though not in the major proportion, and lithium metasilicate in the material after heat treatment. Both phases have significantly long T₁ relaxation time, especially the lithium metasilicate, therefore, a quantitative analysis of the ²⁹Si MAS-NMR spectra was not attempted. Significance. The findings of the present work demonstrate importance of the commercially designed processing parameters in order to preserve desired characteristics of the material. Processing the Empress II at a rate slower than recommended 60°C min⁻¹ or long isothermal hold at the maximal processing temperature 920°C can cause crystallization of lithium metasilicate and cristobalite instead of lithium disilicate as major phase.
本研究报告了 Empress II 微晶玻璃系统的实时中子衍射。
通过实时中子衍射、³¹P 和 ²⁹Si 固态 MAS-NMR、DSC 和 XRD 对商业微晶玻璃进行了表征。
在加热过程中，所研究的微晶玻璃包含锂辉石（Li₂Si₂O₅），随着温度的升高而熔化。这一点通过中子衍射得到证实，在 958°C 时，该相的布拉格峰消失，与热分析一致。在冷却过程中，大约在 916°C 时开始形成锂硅酸钠（Li₂SiO₃），大约在 852°C 时形成少量的方石英。两种锂硅酸盐相的单元体积随温度线性增加，速率为+17×10⁻³ ų。°C⁻¹。冷却后材料的室温粉末 X 射线衍射（XRD）证实了锂硅酸钠和方石英是主要相，此外还存在少量的锂辉石和正磷酸盐。³¹P MAS-NMR 表明，热处理前后均存在锂磷氧（Li₃PO₄）。加热时锂辉石的熔化和冷却时锂硅酸钠的结晶与 DSC 分别观察到的吸热和放热特征一致。²⁹Si MAS-NMR 表明，原始微晶玻璃中存在锂辉石相，尽管比例不大，但热处理后材料中存在锂硅酸钠相。两种相都具有显著长的 T₁弛豫时间，特别是锂硅酸钠，因此，未尝试对 ²⁹Si MAS-NMR 谱进行定量分析。意义。本工作的发现证明了商业设计的加工参数的重要性，以便保留材料的理想特性。以低于推荐的 60°C min⁻¹ 的速率或在 920°C 的最大加工温度下长时间等温保持对 Empress II 进行处理，可能会导致锂硅酸钠和方石英而不是锂辉石作为主要相结晶。
