JES Divinylbenzene (DVB) Shells Meeting Specifications High Average Power Laser Program Workshop Georgia Institute of Technology Atlanta, GA February 5-6, 2004 Jon Streit Diana Schroen

JES Review Status at last review: –Major concentricity problem. Agitation study begins. –In-house characterization begins. –First overcoated shells successfully dried. Low drying yields. –Characterization of overcoat limited to SEM. 300 micron DVB Foam Wall – CH Polymer – ~1-3 Micron Cell Size – mg/cc 1 micron Carbon Overcoat Shell formed through microencapsulation Overcoat applied with polycondensation reaction 4 mm Diameter Foam Shell

JES Shell Production Status Formation / Gelation Nonconcentricity problematic. Correct agitation and density matching greatly reduces nonconcentricity. New agitation apparatus being developed. Characterization In-house characterization developed and utilized. Characterization includes physical dimensions, concentricity, and out of round. Overcoating Very time consuming. Surface finish has improved. Shrinkage has been noted. Exchanging method has been improved. Supercritical Drying Low yield when drying overcoated shells. Switching to Polaron to dry shells.

JES Production Timeline 1. Gelation2. Exchange to Benzyl Salicylate 3. Characterize4. Exchange to IPA 5. Exchange to Chlorotoluene 6. Overcoat7. Exchange to IPA 8. Supercritical Drying 9. Characterize Overcoat 10. Finished. 6 week minimum for entire process 2 Days 2 Weeks 1 Week 1 Day 1 Week

JES Agitation Changing flow pattern by filling flask 2/3 full has resulted in better concentricity A prototype design of a submerged coil has been constructed. Tubing is pinched to deform shells to facilitate core centering. Six units could be placed in bath allowing shells to remain in coil for one hour. Bottom view of shell path in full flask Bottom view of shell path in 2/3 full flask Drafting: Chris Russell Machining: Jim Metzler

JES Agitation Results Agitation with flask 2/3 full generally results in better concentricity Flask 2/3 Full (5 best) Batch Density Offset Batch NC % 1JS150A JS158C JS8A JS1B JS7A0.016 Flask Full (5 best) Batch Density Offset Batch NC % 1JS147A JS6A JS36B JS159A JS159C Density matching, time to gelation also factors

JES Surface Changes with Reaction Time 10 min 15 min 20 min 30 min

JES Small Molecule and Surface Finish PVP without Triamine 25000x PVP with Triamine 25000x Triamine provides additional crosslinking which increases dimensional stability of the overcoat.

JES Overcoat Profile Approximately 1.9 micron thickness (10 minute reaction time)

JES Surface Roughness Interferometer surface roughness measurement of overcoat. PVP reacted for 15 minutes, RMS = 49.5 nm

JES Final IPA Exchange / Supercritical Drying Yields during the final IPA exchange and supercritical drying are still very low. Exchanging from 4-chlorotoluene to IPA instead of water to IPA is helpful. Also, slowing time of exchange (2 weeks +) leads to good yields before drying. Low yields after the supercritical drying process continue to be a problem. We have switched back to the Polaron drier from the automated drier. This switch makes it easier to study failure modes.

JES Uniformity Specification Summary ParameterValueBest ResultComments Diameter4 mm4.006 mm with standard deviation of 24; 4026 average Achieved using manual stripping fluid flow control Wall Thickness 300 μ302 μ standard deviation of 5; 304 μ average Density100 mg/cc~100 mg/ccMeasured from bulk Out Of Round<1% of radius0.9 % averageSome exceed Overcoat Thickness 1 – 5 μ1.9 μ Measured from SEM of 10 minute reaction Partially controlled with reaction time Non- concentricity <1% of wall thickness 2% individual shell; 4% batchImproved with increased agitation Surface Finish~20 nm RMS50 nm RMSPreviously unable to scan

JES We Have Made A Foam For LANL Cryo Experiments. The side view shows the band of foam in the ground out cylinder. There is a crack that I believe is due to the mismatch of thermal expansion. A top view shows the uniformity of the foam layer. The crack is at 4:00.

JES We are starting to work with GA to develop a flow through system. The idea is to take the shells as they emerge from the droplet generator and keep them in tubing through as many processes as possible. There are four advantages: –The shells will have less handling damage and be cleaner as they will not be exposed to ambient particles. –Agglomeration will be greatly reduced. We may need to add an air bubble between shells in the overcoating line to completely eliminate it in this step. –Volumes of solutions are greatly reduced. –Lends itself to an automated mass production process.

JES The individual steps can be developed separately. Droplet Generator Heated Coil IPA Rinse Benzyl Salicylate Microscopes (2 Views) Fail Pass Optical Cell Shells Sorted Water Wash Removes Excess Organic PVP Solution 10% HCl Rinse Water Rinse Gelation Overcoat Characterization To CO 2 Drying

JES If a foam overcoated target is required we will have to rethink the flow through design. Two overcoating techniques have been visualized. 1.A second microencapsulation technique. This has been done in prototyping experiments by Takagi, but with much smaller shells. 2.Casting with or without an outer layer. I have serious concerns about the microencapsulation technique. It is difficult to produce highly concentric capsules when they have a density matched inner core. When the core is a foam shell with a permeation barrier it will make density matching and deformation more difficult. I have found that nature is kind when trying the casting technique.

JES A foam overcoated target could be cast in two ways. The outer foam layer could be cast, then another overcoating done, the capsule dried and a metallic coating applied. OR The mold could be coated with the metallic coating, the lower foam cast, the remaining solution and upper mold added, the shell dried and removed from the mold. The seam can be smoothed by a heated point source.

JES Surface tension tries to center the capsule. We have been looking at this technique using 5 mm capsules. The solution meniscus wants to center the capsule. The height of the capsule can be set by the amount of solution. This was done with a full density capsule. It needs to be tried with an overcoated foam shell, preferably filled with a low density solvent.

JES Future Work Continue to study nonconcentricity. Continue to study the overcoating process and study the effects of chemistry and reaction conditions on overcoat surface roughness. Try to convert this step to a flow though process. Explore methods to eliminate shell rupture problem during supercritical drying. Continue exploring techniques for producing a foam overcoated target.