JES 12-5-02 1

Production of Divinylbenzene (DVB) Shells

High Average Power Laser Program Workshop

Naval Research Laboratory

Washington, DC

December 5, 2002

Jon Streit

Diana Schroen

JES 12-5-02 2

Review

• Refurbished droplet generator to accommodate larger shells

• Polymerized DVB shells ~ 4 mm in diameter at 100 mg/cc

• Obtained characterization data for first six batches of shells

– Concentricity primary problem

• Created gelled, overcoated beads in a single step

• 4 mm Diameter Foam Shell• 300 micron DVB Foam Wall

CH Polymer ~ 1 micron Cell Size 20 - 100 mg/cc

• 1 micron Carbon Overcoat• 0.03 micron Metallic Coating

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Density, Interfacial Tension, and Agitation

• Work has focused on density matching.– Density matching helps center the inner and outer drop, but density

is a function of temperature. If the inner water core of the shell has the same density at room temperature as the surrounding organic layer, the densities will not be the same at the elevated gelling temperature because the thermal expansion is not the same for the different solutions.

• Influences of interfacial tension and agitation need to be studied further as will be seen.

• M. Takagi, R. Cook, R. Stephens, J. Gibson, S. Paguio, “Decreasing Out-Of-Round in Poly(α-Methyl Styrene) Mandrels by Increasing Interfacial Tension,” Fusion Technology, Vol. 38, p. 46 July 2000.

• T. Norimatsu, Y. Izawa, K. Mima, P. M. Gresho, “Modeling of the Centering Force in a Compound Emulsion to Make Uniform Plastic Shells for Laser Fusion Targets,” Fusion Technology, Vol. 35, p. 146, March 1999.

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Initial Density Matching Results in 5% PVA

• Chart represents the calculated room temperature density difference between the inner water core and the organic phase compared with the average measured nonconcentricity percent.

• Initial results indicated that the optimal density difference would occur at about 0.012 g/cc.

• Shell characterization provided by GA.

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Additional Density Matching Results in 5% PVA

• After more data was collected in the 0.010 to 0.012 g/cc range, the trend appeared to be less clear. Optimum appeared to be about 0.019 g/cc.

• A “V” shape would be expected when the data is plotted in this manner.

0

10

20

30

40

50

60

0.000 0.005 0.010 0.015 0.020

Density Difference (mg/cc)

No

nco

ncen

tric

ity %

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Reinterpreted Density Matching Data in 5% PVA

• Examining the results for a greater than 0.015 g/cc density difference should help clarify the situation.

• Other factors to reduce nonconcentricity are being investigated (temperature, interfacial tension, agitation).

0

10

20

30

40

50

60

0.000 0.005 0.010 0.015 0.020

Density Difference (mg/cc)

No

nco

ncen

tric

ity %

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Initial 0.05% PAA Density Matching Results

• PAA has been investigated as a replacement for PVA. PAA has been shown to increase interfacial tension leading to decreased out-of-round in the PAMS shell system.

• Initial results in PAA suggest that density matching may not be the determining nonconcentricity factor.

0

10

20

30

40

50

60

70

80

0 0.005 0.01 0.015

Density Difference (mg/cc)

No

nc

on

ce

ntr

icit

y %

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Temperature Dependence of Density

• Temperature in gelation flask was found to vary from 65-73 C. This causes a fluctuation in the density match.

• A temperature controller was obtained to minimize temperature fluctuations.

• DVB polymerization experiments using UV initiation performed. Polymerization achieved, but presently too slow to be useful.

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Agitation

• A cylindrical flask has replaced a pear shaped flask – more uniform agitation.

• Centering forces have been shown to be generated with shell deformation.

• Shells travel in an undisturbed circular pattern when the flask is completely full of solution.

• Filling the flask with less solution adds disruption to shell path.

Bottom view of shell path in full flask Bottom view of shell path in 2/3 full flask

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Shell Overcoating Has Begun

INTERNAL WATER PHASE

IPA RINSE

POLYMER AND ORGANIC PHASE

The shell is placed in a reactive aqueous solution. A wall is built at the interface.

The shell is placed in organic solvent + acid chloride.

WATER PHASE + REACTANT

ORGANIC + ACID CHLORIDE

ORGANIC + ACID CHLORIDE

LIQUID CO2

The organic phase and internal water phase must be removed.

The shell is rinsed with IPA, then liquid CO2. The CO2 is taken supercritical and vented.

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Overcoating Characterization

• Confocal image of shell.

• 900X magnification.

• Image is 100 microns across.

• All features are submicron in depth.

Uncoated Shell Coated Shell

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Shell Cracking

• Shells do not ship well - ~25% broken or cracked after shipment.

• ~95% broken or cracked after exchanging to DBP for characterization (estimates and image from GA). Handling needs to be minimized.

• Cracking less problematic when exchanging into IPA – needs to be monitored.

JES 12-5-02 13

Radiographic Characterization

• X-ray radiograph of shell.

• System is being modified for foam needs: optimizing energy and adding rotation.

• System currently has ~28 micron resolution.

• Also designing tomographic system with ~10 micron resolution – planned for FY04.

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Future Work

• Determine optimal density matching and its significance.• Determine advantage of using PVA or PAA system.• Examine influence of agitation.• Determine methods to reduce shell cracking.• Develop x-ray radiography characterization system.• Develop wet characterization system at Sandia.