Lithium-metasilicate crystals

This metallic silver is the nucleating agent for the lithium metasilicate Li2Si03 phase. As a result, this crystal phase can be precipitated by controlled crystallization at 600 C (Beall 1993). The lithium metasilicate crystals are easily etched with dilute hydrofluoric acid (HF). Thus, defined structures can be etched into the finished products. These specially structured glassy bodies... 

Droplets, Silver Clusters and Lithium-Metasilicate Crystals... 

Crystallisation at a temperature of 600°C (10 K higher than 590°C ) would also lead only to the formation of the lithium metasilicate crystal phase,... 

Figure 6.11 XRD spectrum of the crystallized glass-ceramic showing the presence of cristobalite, lithium metasilicate, lithium disilicate, and lithium orthophosphate phases.

Rgure 6.12 Transmission electron micrograph showing the epitaxial growth of lithium metasilicate (LS) on a lithium orthophosphate (IP) crystal in a glass-ceramic heated through a typical thermal cyde. ... 

The neutral silver is formed by this reaction. During the subsequent heat treatment of the glass at approximately 600 C, colloids of metallic silver are formed (Beall 1992). This colloidal silver forms heterogeneous nuclei of approximately 80 A for the subsequent crystallization of a lithium metasilicate primary crystal phase, Li2Si03. Lithium metasilicate possesses a chain silicate structure. The crystallization of this compound proceeds dendritically. The dentritic growth of crystals in glass-ceramics is discussed in Section 3.2.4. 

The lithium disilicate layered silicate phase, Li2Si205, is the primary crystal phase. In addition, the growth of lithium disilicate may be initiated by the primary crystallization of the precusor chain silicate, lithium metasilicate, Li2Si03. 

Figure 3-7 SEM image showing dendritic crystallization of lithium metasilicate in Fotoform glass-ceramic. Size of dendrites = approx. 2 microns.

Jacquin J.R. and Tomozawa M., "Crystallization of Lithium Metasilicate from Lithium Disilicate Glass," / Non-Cryst. Solids, 190, 233-37 (1995). 

T.I. Barry, D. Clinton, L.A. Lay, R.A. Mercer, R.P. Miller The crystallization of glasses based on eutectic compositions in the system Li20-Al203-Si02 part 1 Lithium metasilicate - /5-spodumene , J. Mat. Sci. 4, 596-612 (1969)... 

Nucleated by gold and/or silver nanoparticles, themselves photonucleated with the aid of UV-absorbing cerium (3 ) ions Crystallized (at about 600°C) to dendritic form of lithium metasilicate (LijSiOj), which is soluble in HF add Photonucleation plus heat treatment at higher temperatures (850°C) produces stable hthium disihcate (Li2Si205) and a-quartz... 

X-ray diffraction (Fig. 9.5) was used to confirm that the crystal phase in both cases is lithium metasilicate (Li20 Si02). Normally this crystal phase is white, however, because of the presence of agglomerates of silver atoms acting as nuclei, these crystals have brown colour. The intensity of the colour as well as colour shifts from yellow to brown and green depend not only on the number of nuclei but also the size and shape of the LMS crystals. 

Quantitative X-ray diffractometry allows the determination of the real amount of LMS crystals that formed within the glass (128). It requires a calibration of the XRD-curve. This was performed using mixtures of synthetic lithium metasilicate and the exactly composed glass phase as described above [128]. Ehrhardt estimated that only 15mass% lithimn metasUicate crystals form, which is significantly lower than the theoretically possible crystalline phase. This discrepancy is due to the diffusion limitation of the lithium ions in the glass matrix. 

The LMS crystals (Fig. 9.10) are anisotropic having a columnar/dendritic structure. The crystal phase is rhombic-pseudohexagonal. Lithium metasilicate melts congruently at a temperature of 1201°C [226]. The coefficient of thermal expansion (GTE) depends on the crystal axis. In the direction of the columnar axis the GTE is a20-400°c = 9.31 x 10 however perpendicular to this direction the GTE is a2o-4oo°c = 14.82 x 10 K . A sintered body with randomly oriented single crystals has a GTE of O20-400°c = 10.34 X 10- K-i [128]. 

A high content of lithium metasilicate in the partially crystallised areas is desired, because of the high rate of etching of this phase. The LMS crystals should be in contact with each other. 

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