Study of a Glass-Ceramic Seal Porosity

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 the installation of the seal by deposition of the suspension on the areas to be sealed. Then, a heat treatment is applied to shape the material and ensure the sealing of the system. The use of a composition that tends to crystallize is implemented to improve, a priori, the thermal and mechanical properties. The reference heat treatment consists in a first temperature rise followed by a crystallization stage. As the temperature rises, the organic solvents evaporate producing CO2 and gas present in the atmosphere of the furnace is trapped during sintering, which causes porosity to appear. 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. Indeed, a large and interconnected porosity can create preferential leakage paths and decrease the sealing performances of the stack. Moreover, a dense material is known to be mechanically more resistant than a porous one. Indeed, a pore can initiate a crack that will eventually propagate through the seal. This paper will present in particular the study of the material porosity. A quantification by SEM image analysis allows to determine the evolution of the porosity (percentage, size, number) as a function of time and temperature. Swelling, coalescence and rising of the pores are observed at high temperature. Glass powder and slurry are studied. Figure 1