Preparation of Polyimide Ablator Coatings Using an Improved Solvent Vapor Smoothing Process

We completed the development of a method for preparing smooth vapor-deposited polyimide ablators up to 160 m thick for NIF target capsules. The process consists of two steps. The first step is vacuum chemical vapor deposition of monomer species, pyromellitic dianhydride and 4,4ʹ-oxidianiline, onto the surface of a spherical shell mandrel where they may react to form polyamic acid. In the second step dimethyl sulfoxide (DMSO) vapor exposure in a gaslevitation smoothing apparatus swells and fluidizes the outer surface. Roughness in the outer fluid layer is reduced by surface-tension-driven flow. The shells are cured in the final smoothing step by heating to 300°C, converting the polyamic acid to polyimide. Recent experiments using X-ray radiography have allowed us to determine the depth of solvent penetration and the solvent concentration over a range of solvent exposure conditions. We found that the rate of penetration is a function of the solvent partial pressure in the flowing vapor stream. The concentration of solvent in the swollen layer is ~0.43 g/cm^3 and is independent of exposure conditions. Using the penetration information we were able to improve the smoothing process by increasing the solvent partial pressure. The optimized vapor smoothing process allowed us to consistently meet the surface smoothness specifications of NIF capsules.