Controlling the Pore Size and Gelation Time of Resorcinol Formaldehyde Foam for Fabrication of Direct Drive Targets for ICF Experiments

Resorcinol Formaldehyde (R/F) foam has been used in the fabrication of direct drive shell targets for Inertial Fusion Confinement (ICF) experiments at the University of Rochester's Laboratory for Laser Energetics (LLE). Recent cryogenic experiments at LLE using R/F shells have shown the necessity of larger pore foam compared to the standard R/F formulation. In this paper, we report controlling the pore size of R/F foam with concomitant control of the gelation time, which is crucial for successful shell fabrication. The "standard" formulation, with pores of <100 nm, was modified by decreasing the base catalyst to resorcinol concentration ratio creating a large pore R/F foam (~ >0.5 m) through reaction limited aggregation. However, this formulation decreased the gelation time, which decreased the yield of shells with proper wall uniformity (~ 30%) to an unacceptable level of <1%. We developed a technique to achieve control over the gelation time, while keeping the large pore characteristics of R/F to improve shell non-uniformity and increasing the yield to an acceptable level. We also developed a new technique for large pore formation involving changes to the acid catalyst concentration. The effects of this new formulation on the wall uniformity of shells are discussed. The pore distributions obtained using these new R/F foams were characterized using a variety of techniques, including electron microscopy, nitrogen gas adsorption, visible spectroscopy, and small angle x-ray scattering and compared to the standard small pore formulation.