Fabrication of Polyimide Shells by Vapor Phase Deposition for Use as ICF Targets

Hollow polyimide shells, for use as ICF targets, were fabricated by co-depositing monomer precursors from the vapor phase onto bounced spherical mandrels. The process involved two stages: first, the deposited monomers (pyromellitic dianhydride and 4,4′-oxydianiline) reacted on the mandrel surface to form polyamic acid; second, the mandrel was heated to 300°C to imidize the polyamic acid and to decompose the mandrel. During this latter process the decomposed mandrel diffused through the thermally stable coating, leaving a polyimide shell. Depositions were performed under low (∼10⁻³ Torr) and high (∼10⁻⁶ Torr) vacuum. Also, flat witness films of polyimide deposited on Si wafers and NaCl allowed the mechanical properties and chemical composition of the film during the heating cycle to be measured. Polyimide shells with diameters ranging from 700 to 950 μm and wall thicknesses ranging from 2 to 13 μm were produced. The shell's sphericity was greater than 99%. Burst and buckle pressure tests on these shells yielded the estimated mechanical strength properties. The elastic modulus and tensile strength were ∼15 GPa and ∼300 MPa, respectively. The permeability of D₂ through polyamic acid at 25°C was 7.4 × 10⁻¹⁷ mol·m/m²·Pa·s and increased to 6.4 × 10⁻¹⁶ mol·m/m²·Pa·s at 25°C upon curing the shell to 150°C. The permeability of D₂ at 25°C through vapor-deposited polyimide flat films was measured to be 240 times greater than through the as-deposited polyamic acid, and about 7 times greater than through commer ially available solution-cast Kapton.