Glass Division , CSIR-Central Glass and Ceramic Research Institute , 700032 Kolkata , India.
Department of Materials and Ceramic Engineering, CICECO , University of Aveiro , 3810-193 Aveiro , Portugal.
J Phys Chem B. 2018 May 3;122(17):4737-4747. doi: 10.1021/acs.jpcb.8b01811. Epub 2018 Apr 20.
Aluminosilicate glasses are considered to follow the Al-avoidance principle, which states that Al-O-Al linkages are energetically less favorable, such that, if there is a possibility for Si-O-Al linkages to occur in a glass composition, Al-O-Al linkages are not formed. The current paper shows that breaching of the Al-avoidance principle is essential for understanding the distribution of network-forming AlO and SiO structural units in alkaline-earth aluminosilicate glasses. The present study proposes a new modified random network (NMRN) model, which accepts Al-O-Al linkages for aluminosilicate glasses. The NMRN model consists of two regions, a network structure region (NS-Region) composed of well-separated homonuclear and heteronuclear framework species and a channel region (C-Region) of nonbridging oxygens (NBOs) and nonframework cations. The NMRN model accounts for the structural changes and devitrification behavior of aluminosilicate glasses. A parent Ca- and Al-rich melilite-based CaO-MgO-AlO-SiO (CMAS) glass composition was modified by substituting MgO for CaO and SiO for AlO to understand variations in the distribution of network-forming structural units in the NS-region and devitrification behavior upon heat treating. The structural features of the glass and glass-ceramics (GCs) were meticulously assessed by advanced characterization techniques including neutron diffraction (ND), powder X-ray diffraction (XRD), Si and Al magic angle spinning (MAS)-nuclear magnetic resonance (NMR), and in situ Raman spectroscopy. ND revealed the formation of SiO and AlO tetrahedral units in all the glass compositions. Simulations of chemical glass compositions based on deconvolution of Si MAS NMR spectral analysis indicate the preferred formation of Si-O-Al over Si-O-Si and Al-O-Al linkages and the presence of a high concentration of nonbridging oxygens leading to the formation of a separate NS-region containing both SiO and AlO tetrahedra (Si/Al) (heteronuclear) in addition to the presence of Al-O-Al bonds; this region coexists with a predominantly SiO-containing (homonuclear) NS-region. In GCs, obtained after heat treatment at 850 °C for 250 h, the formation of crystalline phases, as revealed from Rietveld refinement of XRD data, may be understood on the basis of the distribution of SiO and AlO structural units in the NS-region. The in situ Raman spectra of the GCs confirmed the formation of a Si/Al structural region, as well as indicating interaction between the Al/Si region and SiO-rich region at higher temperatures, leading to the formation of additional crystalline phases.
铝硅酸盐玻璃被认为遵循铝回避原则，该原则指出 Al-O-Al 键合在能量上是不利的，因此，如果玻璃成分中有可能形成 Si-O-Al 键合，则不会形成 Al-O-Al 键合。目前的论文表明，违反铝回避原则对于理解碱性土铝硅酸盐玻璃中网络形成的 AlO 和 SiO 结构单元的分布是至关重要的。本研究提出了一种新的改进随机网络（NMRN）模型，该模型接受铝硅酸盐玻璃中的 Al-O-Al 键合。NMRN 模型由两个区域组成，一个是由分离的同核和杂核骨架物种组成的网络结构区域（NS-Region），另一个是非桥氧（NBO）和非骨架阳离子的通道区域（C-Region）。NMRN 模型解释了铝硅酸盐玻璃的结构变化和析晶行为。通过用 MgO 代替 CaO，用 SiO 代替 AlO 来改性富 Ca 和 Al 的尖晶石基 CaO-MgO-AlO-SiO（CMAS）玻璃组成，以了解 NS 区域中网络形成结构单元的分布变化以及热处理时的析晶行为。通过包括中子衍射（ND）、粉末 X 射线衍射（XRD）、Si 和 Al 魔角旋转（MAS）-核磁共振（NMR）以及原位拉曼光谱在内的先进表征技术，对玻璃和玻璃陶瓷（GC）的结构特征进行了细致评估。ND 表明所有玻璃组成中都形成了 SiO 和 AlO 四面体单元。基于 Si MAS NMR 光谱分析的解卷积对化学玻璃组成的模拟表明，Si-O-Al 比 Si-O-Si 和 Al-O-Al 键合更优先形成，并且存在大量的非桥氧，导致形成一个单独的 NS 区域，其中包含 SiO 和 AlO 四面体（Si/Al）（杂核），此外还存在 Al-O-Al 键；该区域与主要含有 SiO 的 NS 区域（同核）共存。在 850°C 热处理 250 小时后获得的 GCs 中，从 XRD 数据的 Rietveld 精修中可以看出，晶体相的形成可以根据 NS 区域中 SiO 和 AlO 结构单元的分布来理解。GCs 的原位拉曼光谱证实了 Si/Al 结构区域的形成，并表明在较高温度下 Al/Si 区域与富 SiO 区域之间的相互作用，导致形成额外的晶体相。
