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Progress Toward Air-Dried RF Shells

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. Introduction: Fabrication of ICF/IFE Foams.

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Progress Toward Air-Dried RF Shells

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  1. Progress Toward Air-Dried RF Shells John Karnes1, Jon Streit1, Don Bittner1, Nicole Petta1, Shannan Downey2, Mike Droege2 1Schafer Corporation, Livermore, CA 2Ocellus, Inc., Livermore, CA

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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. pressure temperature 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. • p =capillary pressure • g =surface tension • =contact angle r = pore radius

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

  8. Results: One-Step Air Dry Synthesis has a Minimum Density Limit • 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 Reducing the theoretical density of this high R/C RF does not produce the expected low density aerogel.1 1. R. Petricevic et al., J Non-Cryst. Solids, 1998, 225, 41.

  9. Results: We Successfully Synthesized 100mg/cc RF Foam using Simple 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). A 95mg/cc RF aerogel produced by evaporative drying

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

  11. 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

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

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