Stoichiometric (Ni,Mg)‐Cordierite Glass–Ceramics

Abstract

Both the properties and crystallization behavior of xNiO· (2−x)MgO·2Al2O3·5SiO2 glasses for x=0.0, 0.25, 0.5, 0.75, and 1.0 are studied. The glass transition temperature and dilatometric softening temperature decrease as increasing amounts of nickel are substituted for magnesium. The coefficient of thermal expansion (CTE) of the glasses is largely independent of the glass composition—the CTE for all nickel containing glasses is ca. 4.0 ppm/K, which is slightly higher than the CTE for the nickel‐free glass (ca. 3.8 ppm/K). The crystallization behavior of the glasses is studied using differential thermal analysis data and high‐temperature X‐ray diffraction data. All glasses crystallize to form a β‐quartz solid solution at the surface; this phase transforms to cordierite at higher temperatures and/or longer annealing times. The crystallization temperature of the β‐quartz is related to the occurrence of phase separation. The temperature at which β‐quartz transforms to hexagonal cordierite increases with increasing amounts of nickel. Glasses where x≥0.50 crystallize to form a spinel phase in the bulk of the glass. The spinel crystallization temperature decreases with increasing amounts of nickel substituted for magnesium. The spinel composition is dependent on the initial glass composition and crystallization temperature. Competing transformation processes (spinel in the bulk and cordierite at the surface) in glasses where x≥0.75 result in a compound activation energy value.

Evolution of the Ni2+ Environment During the Formation of a MgO–Al2O3–SiO2 Glass‐Ceramic: A Combined XRD and Diffuse Reflectance Spectroscopy Approach

“…In MAS glasses, in the absence of nucleating agents, the crystallization begins with the formation of β‐quartz, which crystallizes mainly from the surface . On the other hand, in the Ni‐MAS glass, spinel crystallizes at a constant rate (Fig.…”

“…) and, for a plateau temperature above 1100°C, spinel becomes the major crystalline phase. At least 5 mol% of NiO is needed to observe nucleation of spinel and this phase appears in the bulk of the glass …”

“…The question remains on how the Ni 2+ coordination in the initial glass does affect the crystallization pathways and the fraction and site occupancy of Ni 2+ in the spinel phase. A recent study on Ni‐MAS glass‐ceramics has shown that the composition of the spinel is tunable by modifying the initial glass composition and crystallization temperature . However, without data on Ni 2+ site occupancy, it is not possible to relate the spinel structure to the Ni 2+ environment and to the optical properties of the glass‐ceramics.…”

In situ evolution of Ni environment in magnesium aluminosilicate glasses and glass–ceramics–Influence of ZrO2 and TiO2 nucleating agents

“…Indeed, apart from being a potential optically active ion, Ni 2 þ can influence the crystallization processes [32,[56][57][58]. For instance, NiO in MAS glasses with no nucleating agent can induce spinel formation [44,45], if at least 5 mol% NiO is introduced [32,45]. However these glass-ceramics rapidly loses transparency due to significant crystallization of ß-quartz s.s..…”

“…The role of these agents was particularly studied in the MgO-Al 2 O 3 -SiO 2 (MAS) ternary system [33][34][35][36][37][38][39][40][41][42][43], in which nucleation and internal bulk crystallization is achieved using TiO 2 and/or ZrO 2 . The MAS glass-ceramics are also well known to promote spinel crystallization [13,44,45]. However, little is known about the interactions between TM and the nucleating agents, which is relevant in order to identify the optimum environment for Ni 2 þ ions that gives a better control of the crystallization processes.…”