A comprehensive approach to crystallization sequence in novel MgO/CaO-Al2O3-SiO2 glass-ceramics by Ostwald's step rule and composition fluctuation model

A new perspective for nucleation and nanocrystallization from interfacial energy and nanoscale composition fluctuations in glasses

“…Nowadays, the existence of spatially and compositionally heterogeneous structures in multicomponent oxide glasses has been well established through experiments and simulations 17–27 . The present authors’ group 28–32 extended the fragility concept proposed by Angell 33,34 to local composition fluctuations in multicomponent oxide glasses, that is, the DNCF model. That is, multicomponent oxide glasses consist of nanoscale network former‐rich strong region and nanoscale network modifier‐rich fragile region and nucleation is initiated preferentially in the nanoscale fragile region.…”

“…One expected crystallization sequence based on Ostwald's step rule is as follows: metastable crystalline phase → high‐temperature type stable crystalline phase → low‐temperature type stable crystalline phase. Indeed, for example, cordierite Mg 2 Al 4 Si 5 O 18 glass exhibits the crystallization sequence expected by Ostwald's step rule as shown in Figure 6 32,53 . Therefore, it is expected that the difference in the free energy between the base cordierite glass/SCL ( G SCL ) and the metastable cordierite‐like crystalline phase ( G MCP ), Δ G SCL→MCP , would be small compared with the stable low‐temperature type cordierite crystalline phase ( G SCP ), that is, Δ G SCL→MCP < Δ G SCL→SCP .…”

“…proposed that nucleation processes may proceed with detectable rates only in “liquid‐like regions” and are suppressed in “solid‐like regions.” The terms of “floppy regions” and “liquid‐like regions” would correspond to “nanoscale fragile regions,” and “rigid regions” and “solid‐like regions” would have the same structural meaning as “strong regions.” The free energy landscape model for SCLs has been widely accepted for the analysis of their glass transition and structural relaxation phenomena, in which SCLs consist of many basins (valleys) with different depths, that is, the local region with shallow basins and the local region with deep basins 48–50 . Very recently, the present authors 32 proposed that the local region with shallow basins would correspond to the local fragile region in the DNCF model, whereas the local region with deep basins would correspond to the local strong region, consequently an activation energy for crystal nucleation in the local fragile region would be low compared with the local strong region.…”

“…For an understanding of the nucleation behavior and crystal growth in glasses/SCLs, we need to know the initial crystalline phase and the crystallization sequence. Recently, the present authors 31,32 proposed a comprehensive approach to the crystallization sequence in some SiO 2 ‐based glasses, such as Li 2 O–2SiO 2 and MgO/CaO–Al 2 O 3 –SiO 2 glass‐ceramics, from Ostwald's step rule 51,52 and the DNCF model. Ostwald formulated that metastable SCLs do not transform directly into the most stable crystalline phases with the lowest Gibbs free energies, but prefer to have crystalline phases with slightly lower (or closer) Gibbs free energies compared with the original SCLs as intermediate stages.…”

“…On the other hand, the MRO structure in glasses is related to the linkage style of SRO structural units and is characterized by “density fluctuations” and “composition fluctuations” at nanoscale levels, that is, “nanoscale heterogeneous structures.” 17–27 Nowadays, it is very important to clarify concretely the nanoscale heterogeneous structure, for example, their chemical composition, fraction, and connectivity, in a given glass. Recently, the present authors’ group 28–32 tried to explain the nucleation process and crystallization sequence (the formation order of crystalline phases) in various multicomponent oxide glasses by using the model of the distribution of nanoscale composition fluctuations (DNCF) and the concept of Ostwald's step rule (rule of stages).…”