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.

Ocellus, Inc.

Progress Toward Air-Dried RF Shells

John Karnes1, Jon Streit1, Don Bittner1, Nicole Petta1,

Shannan Downey2, Mike Droege2

1Schafer Corporation, Livermore, CA2Ocellus, 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

wet gels

CP Dry

Air Dry

wet gels

CP Dry

Air Dry

• 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

rp

cos2

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 angler = 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.

pre

ssu

re

temperature

Ocellus, Inc.

Approach: Increase Cluster Size to Achieve Larger Pores

nucleation clustering 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.1

1. 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 Dried

Air Dried

Pores sizes ~ 100 nmParticle sizes ~ 50 nm

Pores sizes ~ 200 nmParticle 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

7 days

4 days 4 days

save 29 hours

2 days

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