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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

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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– 20 - 120 mg/cc

• 1 micron Carbon Overcoat

• Shell formed through microencapsulation

• Overcoat applied with polycondensation reaction

4 mm Diameter Foam Shell

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Shell Production Status

Formation / Gelation

Nonconcentricity problematic. Correct agitation and density matching greatly reduces nonconcentricity. New agitation apparatus being developed.

CharacterizationIn-house characterization developed and utilized. Characterization includes physical dimensions, concentricity, and out of round.

OvercoatingVery time consuming. Surface finish has improved. Shrinkage has been noted. Exchanging method has been improved.

Supercritical DryingLow yield when drying overcoated shells. Switching to Polaron to dry shells.

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Production Timeline

1. Gelation 2. Exchange to Benzyl

Salicylate

3. Characterize 4. Exchange to IPA

5. Exchange to Chlorotoluene

6. Overcoat 7. Exchange to IPA

8. Supercritical Drying

9. Characterize Overcoat

10. Finished.

6 week minimum for entire process

2 Days

2 Weeks

1 Week1 Week

1 Day

1 Day

1 Day

1 Day

1 Week

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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

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Agitation Results

• Agitation with flask 2/3 full generally results in better concentricity

Flask 2/3 Full (5 best)

BatchDensity Offset

Batch

NC %

1JS150A 0.0085 4

1JS158C 0.01 5

2JS8A 0.01 5

2JS1B 0.0085 6

2JS7A 0.01 6

Flask Full (5 best)

BatchDensity Offset

Batch

NC %

1JS147A 0.002 15

2JS6A 0.012 21

2JS36B 0.006 41

1JS159A 0.0085 46

1JS159C 0.0085 49

• Density matching, time to gelation also factors

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Surface Changes with Reaction Time

10 min

15 min

20 min

30 min

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Small Molecule and Surface Finish

PVP without Triamine 25000x

PVP with Triamine 25000x

Triamine provides additional crosslinking which increases dimensional stability of the overcoat.

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Overcoat Profile

Approximately 1.9 micron thickness (10 minute reaction time)

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Surface Roughness

Interferometer surface roughness measurement of overcoat.

PVP reacted for 15 minutes, RMS = 49.5 nm

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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.

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Uniformity Specification Summary

Parameter Value Best Result Comments

Diameter 4 mm 4.006 mm with standard deviation of 24; 4026 average

Achieved using manual stripping

fluid flow control Wall Thickness

300 μ 302 μ standard deviation of 5; 304 μ average

Density 100 mg/cc ~100 mg/cc Measured from bulk

Out Of Round <1% of radius 0.9 % average Some exceed

Overcoat Thickness

1 – 5 μ 1.9 μ Measured from SEM of 10 minute reaction

Partially controlled with reaction time

Non-concentricity

<1% of wall thickness

2% individual shell; 4% batch Improved with increased agitation

Surface Finish ~20 nm RMS 50 nm RMS Previously unable to scan

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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.

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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.

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The individual steps can be developed separately.

Droplet Generator Heated Coil

IPA Rinse

Benzyl Salicylate

Microscopes (2 Views)

Fail

PassOptical

Cell

Shells Sorted

Water Wash Removes Excess

Organic

PVP

Solution 10% HCl

RinseWater Rinse

Water Rinse

Gelation

Overcoat

Characterization

To CO2 Drying

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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.

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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.

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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.

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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.