Ocellus, Inc. Progress Toward Air-Dried RF Shells John Karnes 1, Jon Streit 1, Don Bittner 1, Nicole Petta 1, Shannan Downey 2, Mike Droege 2 1 Schafer Corporation, Livermore, CA 2 Ocellus, Inc., Livermore, CA

Ocellus, Inc. Introduction: Fabrication of ICF/IFE Foams Fragile, low density aerogel materials are produced for ICF/IFE research –The foams are synthesized via wet chemistry (sol-gel) –The foam’s pore spaces are small and solvent-filled –A drying step is required to produce the desired foam However, during typical evaporative drying, large capillary forces arise and cause the collapse of the foam –Density increases –Pores space lost –Foam morphology destroyed

Ocellus, Inc. Introduction: Drying of ICF/IFE Foams Traditional solution to drying ICF/IFE foams is supercritical processing –The wet gels are dried by moving through the solvent’s critical point eliminates liquid-gas interface and minimizes capillary pressure Supercritical process requires elevated temperatures and pressures –Pressure vessel processing is required capital expense –Processing is inherently batch-wise time-consuming process cycle –Ultimately impacts cost of ICF/IFE foam production

Ocellus, Inc. Introduction: Efficient Production of ICF/IFE Foams A simple, efficient foam drying method is desired for cost-effective foam production –Ambient temperature, evaporative drying –Minimizes capital equipment expense –Increases production throughput –Lower cost, efficient foam processing

Ocellus, Inc. Background: Traditional Critical Point Drying During evaporative drying, capillary forces can collapse the pores and destroy the structure Critical point drying of wet gels occurs by moving through the solvent’s critical point, eliminating any liquid-gas interface

Ocellus, Inc. Approach to Efficient Foam Production: Air Drying In critical point drying, we dry the RF by moving through the supercritical phase. This avoids the capillary forces that would collapse the aerogel structure: no gas- liquid interface ever exists. p =capillary pressure  =surface tension  =contact angle r = pore radius To reduce this capillary pressure, we can increase the pore size. Since we would like water to be our solvent, the rest of the governing equation is constant. pressure temperature

Ocellus, Inc. Approach: Increase Cluster Size to Achieve Larger Pores nucleationclustering crosslinking Low R/C High R/C Increasing the ratio of resorcinol to sodium carbonate (R/C) reduces the number of sites where the dissolved monomer grows into clusters. The pore size therefore increases as in dense packed sphere models. Bulk density and pore volume remain constant but pore sizes increase.

Ocellus, Inc. Results: One-Step Air Dry Synthesis has a Minimum Density Limit Reducing the theoretical density of this high R/C RF does not produce the expected low density aerogel R. Petricevic et al., J Non-Cryst. Solids, 1998, 225, 41. Changing R/C ratios gave a lower density limit of about 200 mg/cc Attempts to produce even lower density RF foams were unsuccessful A new synthetic method was then developed –Very low density, air- dried RF foams were successfully produced

Ocellus, Inc. Results: We Successfully Synthesized 100mg/cc RF Foam using Simple Evaporative Drying A 95mg/cc RF aerogel produced by evaporative drying SEM shows morphology similar to sol-gel produced low-density foam SEM image of a low density, air- dried RF aerogel (95mg/cc).

Ocellus, Inc. Results: Air Dried RF has Slightly Larger Scale than Critical Point Dried RF Foam Supercritically DriedAir Dried Pores sizes ~ 100 nm Particle sizes ~ 50 nm Pores sizes ~ 200 nm Particle sizes ~ 100 nm

Ocellus, Inc. Summary: Evaporative Drying of ICF/IFE Foams Traditional critical point drying is time- consuming and expensive We successfully fabricated 95 mg/cc RF foam using a simple evaporative drying step –New synthetic method developed SEM shows similar morphologies between critical point dried and air-dried foams –Air-dried foam have a slightly larger scale Evaporative, air drying of very low density RF foams is feasible

Ocellus, Inc. Summary: Potential Economic Impact of the Air Drying Approach Synthesis Solvent exchange Air Drying Savings 3 hours 12 hours 4 hours 24 hours 7 days 4 days save 29 hours 2 days save 10 days Elimination of the supercritical drying step could reduce lead time by 50% and man hours by 75% Lead Time Man Hours CP Dried RF Air Dried RF