ZBLAN in Space: A Comparison of Phase Transitions in Microgravity versus Earth's Gravity

Understanding how certain materials behave in space is vital for establishing a sustainable manufacturing base in low-earth-orbit, which is currently spearheaded by the International-Space-Station (ISS) National Lab and NASA's In-Space Production Applications Portfolio. This paper presents phase transition measurements of ZBLAN glass on the International Space Station (ISS) , which provides key insights into spontaneous symmetry breaking in microgravity and earth's gravity. Despite the presence of vibrations, we find the glass to be more resistant to bulk-crystallization phase-changes in space. However, the surface continues to be vulnerable under ambient conditions. We conducted a set of 10 differential-calorimetry tests in the Microgravity Science Glovebox, including 2 sets of controls. A set of similar nearly identical samples were studied on the ground. Glass transitions manifest as endothermic events in our differential calorimeters, and crystallization is marked by exothermic inflections for each sample. On an average our space sample shows significantly less bulk-crystallization in the viscosity and temperature regime most significant for drawing optical fibers. All samples were subjected to temperatures exceeding $425^o C$ and they all crystallized to varying degrees. The crystals that grew in space tend to be about one hundred times larger and well faceted. We find $\alpha-BaZr_2F_{10}$ to be the most commonly occurring crystalline-species and forms at lower temperatures close to around $320^\circ C$. $\beta-BaZr_2F_{10}$ is also common and forms around the same temperature. There is also some indication that less of the $\beta-BaZr_2F_{10}$ polytype forms in space. Overall a deep understanding of the phase-diagrams and crystallization-reaction pathways is key for developing targeted mitigation strategies for future space-stations and orbital free-flyers that will serve as manufacturing platforms.