1. Divinylbenzene (DVB) Shells High Average Power Laser Program Workshop Princeton Plasma Physics Laboratory Princeton, NJ October 27th-28th, 2004 Jon Streit, Diana Schroen Schafer Corporation Reny Paguio, Brian Vermillion General Atomics

2. Review 4 mm Diameter Foam Shell • 300 micron DVB Foam Wall • CH Polymer • ~1-3 Micron Cell Size • 20 - 120 mg/cc • 1-5 micron Carbon Overcoat • Status at last review: • Lowest average batch NC was 4 %. Further improvement needed. • Smoothness of PVP overcoat improved, shrinkage still problematic. • Investigation of alternatives to PVP initiated. • Shell formed through microencapsulation • Overcoat applied with interfacial polycondensation

3. Shell Production Status Formation / Gelation We have switched to a horizontal flask configuration which has lowered average batch nonconcentricity to 3%. Individual shells have been characterized with NC <2%. 3% NC can be achieved with GA’s dual initiator method at 30 °C or with a single initiator at 80 °C. The combination of the different shell flow pattern and a longer gelation time (>1 hr) has lowered NC. Overcoat To try to reduce shrinkage of the PVP overcoat, PVP has been combined with small amounts to up to 50% of two different tri-functional monomers. Diethylenetriamine and melamine formaldehyde have been investigated as possible PVP alternatives.

4. Nonconcentricity and Density Matching Density matching appears to have an effect with a horizontal flask at 75 °C to 80 °C, lowering NC about 2%. It does not seem to have an effect with the two initiator system at 30 °C.

5. Accuracy of Density Matching Increased Liquid Level Average Temperature Calculated for Area Oil Bath To increase the accuracy of density matching, an IR camera is being used to more accurately determine the temperature in the flask. The temperature in the flask has been found to be lower than previously thought, which has resulted in adjustments being made to increase DM accuracy.

6. Nonconcentricity and Agitation Three different flask configurations have been used with very different results. Flow Patterns Horizontal, Side View Bottom View

7. Flask Dimensions • Since flask fill level and orientation appears to have a dramatic effect on NC, the effect of flask dimensions on NC is being examined. • Flow patterns are different for each flask – the frequency and degree of deformation may affect NC. • Initial results have not shown a dramatic effect on NC.

8. PVP Shrinkage • To reduce PVP shrinkage several methods have been tried: • PVP with low concentration of trifunctional monomers • PVP with 50% trifunctional monomers • Flash trifunctional coatings followed by PVP coatings • Flash PVP coatings followed by trifunctional coatings Smoothest PVP/5%DET Coating (RMS surface roughness = 25 nm) • PVP shrinkage occurred in all of the above methods. Surface finish tends to be good despite shrinkage.

9. 2. Place Shell in MF Solution at 70 °C 3. Shell is then exchanged to IPA & CO2 Dried Melamine Formaldehyde Coating Process Finished • Melamine formaldehyde was selected for investigation because of its straightforward processing conditions and reports of its smoothness • An aqueous solution of melamine and formaldehyde at elevated pH is heated to produce a precondensate • A gelled shell is placed in the precondensate solution and the pH is lowered – MF polymer then condenses on the organic surface

10. PVP / MF Surface Comparison 5000x, 20K PVP, 5 min 5000x, highly crosslinked MF, 1 Hour The MF has a generally smooth appearance with clusters of excess polymer attached. Can the excess polymer be removed through washing or prevented?

11. 0.5  MF Drawbacks • Two problems have been encountered with the MF coating: • Coating thickness is thin – about 0.5 microns. Can thickness be increased? • Micro cracks can occur during the drying process. Could these be eliminated with a thicker coating?

12. Melamine Formaldehyde Surface Finish Interferometer surface roughness measurement of a 60 x 46 micron area of Melamine Formaldehyde. Surface roughness is 32 nm RMS.

13. Future Work • Continue to reduce nonconcentricity through careful control of temperature, density matching, agitation and gelation time. • Continue experimentation with additives to try to reduce PVP shrinkage. • Continue to investigate overcoat layering and PVP alternatives. • Attempt to increase thickness of melamine formaldehyde coating to eliminate micro-cracks.