CELAJOAS

Since asbestos was forbidden in France in 1997, no materials have replaced this mineral for high temperature applications in the range [500°C, 1000°C]. This is particularly true in the field of sealing technology. This project is focused on the development of high temperature (up to 900 °C) gaskets for nuclear and solar energy, space motors, oil refining chemistry. All theses industries needs gaskets systems applied to severe environments (high pressure and temperature). The aim of the project is to develop a process for making gaskets based on inorganic lamellar materials for high temperature applications where graphite is not usable. Severals inorganic materials can be used: phyllosilicates such as vermiculite, pyrophilite, saponite, sepiolite, xonolithe, and talc or boron nitride. We will develop a process to form a compact and impermeable material from a mineral powder based on lamellar and possibly exfoliated clay. This material should keep far superior sealing characteristics with leak rates lower than 10-4 sccm cm-1 at 900°C, and compressive stress of approximately 20 MPa. The gaskets will be formed through uniaxial pressing or by impregnation of a mineral powder in order to obtain a cohesive, airtight, and compact material. The first step of the process will consist in the preparation by sonochemistry of a powder with a controlled surface chemistry and particle size. In order to increase the formability, inorganic polymer additives bonding the aluminosilicates particles will be added. The porosity of the seal material will be decreased by adding oxides nanoparticles or other minerals such as glass or salts. The porous structure of the gaskets will be studied as a function of the temperature and pressure by nitrogen adsorption measurements at 77 K (BET specific surface area, pore size distribution), scanning electron microscopy (SEM), diffusion coefficient measurement (obtained by proton NMR), and density measurements. The structure and microstructure depending on P and T will be also studied in situ by X-ray and neutron small angle scattering and diffraction. The helium leak rates and elastic recovery will be tested in the pressure (P<20 MPa) and temperature range (T<900°C) for all the prepared gaskets. We will develop models to correlate the properties to the multi-scale structure. These models will be first developed on graphite materials and then applied to the new composite materials based on lamellar aluminosilicates. These models will enable to correlate the multi-scale structure of the gaskets to their properties calculated by numerical simulations of the elastic recovery and the leak rate at P and T chosen in the range of use. The models will predict the performance of use of the gaskets. Moreover, the models and numerical simulations will direct the research toward the most promising materials and lead the working-out process to tend to the optimised densification. At the end of the research work, we will propose an optimised seal material base on lamellar silicates with given physics and chemical characteristics: particle size distribution, chemical composition, surface chemistry, porous structure, etc... Then the research process will be transferred to an industrial pilot level by the Garlock Company to product real commercial gaskets.