Crystal growth velocities of a highly anisotropic phase obtained via surface and volume crystallization of barium–strontium–zinc silicate glasses

In the past few years, a new phase, Ba 0.5 Sr 0.5 Zn 2 Si 2 O 7 with negative thermal expansion has been described in the literature. Some excess of SiO 2 is necessary to produce glasses from which the Ba 0.5 Sr 0.5 Zn 2 Si 2 O 7 phase can be crystallized. Unfortunately, in such glasses usually surface crystallization occurs; however, the addition of nucleating agents such as trace quantities of platinum or relatively high quantities of ZrO 2 is necessary to achieve bulk nucleation. These additional components also affect the crystal growth velocity, which furthermore is different for crystal growth from the surface and in the bulk. In this paper, three different chemical compositions containing different ZrO 2 concentrations, where one composition additionally contains 100 ppm platinum, are studied with respect to their crystallization behaviour. Although the compositions do not differ much, the crystallization behaviour and also the crystal growth velocities are surprisingly different.