Study of the Crystallization in a Glass-Ceramic Seal

Abstract

Sealing is a major issue in high temperature electrolyzers used for hydrogen production. The specifications that seals must meet are particularly demanding. In addition to sealing, the material must have mechanical, thermomechanical and chemical properties, as well as sufficient electrical resistivity. The geometries are complex and the lengths to be sealed are important. These numerous constraints lead to the use of glass powder suspended in organic solvents. This process allows for the installation of the seal by simply depositing the suspension on the areas to be sealed. A heat treatment is then applied to shape the material and ensure the sealing of the system. The use of a composition that tends to crystallize allows, a priori, improving the thermal and mechanical properties.The reference heat treatment consists of a first temperature rise followed by a crystallization stage. As the temperature rises, the organic solvents evaporate, producing CO2, which causes porosity to appear. Sintering of the glass powder also takes place during this stage. Early surface crystallization can compete with the sintering phenomenon and prevent the maximum densification of the glass ceramic by freezing the structure. The objective of the study is to obtain a seal material with maximum density, thus minimum porosity, by controlling the removal of organic solvents, sintering and crystallization.This paper will present in particular the study of the material crystallization: phases identification and evolution of crystal morphology as a function of temperature (SEM, XRD) (Figure 1), crystal surface fraction as a function of time and temperature (determined by image analysis). A poikilitic texture phenomenon, particularly important for the crystalline fraction quantification, is also detailed (SEM-EBSD). These data allow to obtain the equilibrium crystalline fraction as a function of temperature, and to further optimize the thermal treatment. Figure 1