Transparent Hafnia-Containing β-Quartz Glass Ceramics: Nucleation and Crystallization Behavior

Heterogeneous nucleation and crystallization of lithium alumosilicate glasses with hafnia-containing nucleation agents was explored. Kinetic, thermodynamic, and structural data were considered to assess nucleation efficiency and to characterize the crystallization process. It is shown how lattice parameters and, particularly, anisotropy of the nuclei phase depend on the amount of ZrO₂–HfO₂ substitution in a specific base glass. For a given crystallization treatment, the size of the derived crystallites of β-quartz structure is used as a measure of nucleation rates. A nonlinear dependence of nucleation efficiency on HfO₂ content was established, with the supposedly most efficient nucleation corresponding to the lowest degree of anisotropy of the nuclei crystallites, i.e., when about 20%–30% of ZrO₂ was substituted by HfO₂. Apparent activation energies and estimates of the Avrami coefficient were determined from nonisothermal crystallization experiments for selected compositions to highlight the differences between hafnia and zirconia. Hafnia can be used alone or in combination with other agents to nucleate nanocrystalline glass ceramics with a low coefficient of thermal expansion.