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IFT\P2006-154

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IFT\P2006-154

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  1. IFE Target Fabrication UpdatePresented by Jared Hund1J. Bousquet1, Bob Cook1, D. Goodin1, R. Luo1, B. McQuillan1, R. Paguio1, R. Petzoldt1, N. Petta2, N. Ravelo1, D. Schroen1, J. Streit2, B. Vermillion1, W. Holloway3, N. Robertson3, M. Weber31General Atomics, Inertial Fusion Technology, San Diego, CA2Schafer Corporation, Livermore, CA3UC San Diego, San Diego, CAHAPL WorkshopPrinceton, New JerseyDecember 12-13, 2006 IFT\P2006-154

  2. We have demonstrated basic feasibility of the foam shell (Aug 06) The current challenge is developing the HAPL specified CH coating Gas tight Smooth (50 nm RMS) The current HAPL target design is a 4.6mm foam capsule Thin (300-1200 Å) High Z coating 5 m CH Overcoat Foam + DT DT DT Vapor ~ 2.3 mm rad Foam layer: ~0.18 mm divinyl benzene (DVB)

  3. Achieving this is a hard problem because • Low buckle and burst strength of shells Fabrication Impacts Permeation Filling Layering • Covering large pores of DVB • Foam has pores of ~1μm width that coating must cover • Smoothness • Related to covering porous structure

  4. Current strategies for improving the CH overcoating • Keep the interfacial coatings from breaking • Reduce Δpressure in interfacial polymerization fabrication (PVP) • Osmotic pressure: Solvent exchanges – eliminate IPA step • Better control pressure drops in CO2 dryer • Improve 2 layer coating by making a better interfacial layer – modify chemistries to better cover large pores and make a smoother interface for dual layer coating • “Repair” damage to the interfacial coating layer • Parylene coating • Smoothing – make everything smoother in the end

  5. A challenge of fabricating a continuous overcoat is the low buckle strength of any 5μm polymer coating Calculated Buckle Strength of Parylene • Buckle Strength*: Material Constant Buckle Pressure (atm) w = coating thickness r = radius This term is similar for most types of polymers that can be used Wall thickness (μm) Elastic (tensile) modulus (E) of various polymers Alternate form from Roark* sugests buckle may be even less *Roark and Young, Formulas for Stress and Strain (1982) Topic #1 Reduced ΔP

  6. The buckle strength of DVB shells with thick coatings has been measured Buckle Data of GDP/PVP Coated DVB Shells • Buckle Strength: Material Constant Curve fits based on buckle equation 4.1mm dia 4.6mm dia w = coating thickness r = radius • The Burst Strength is higher: The buckle pressure of a HAPL target will be ~0.1 atm* ~2-5atm S = tensile strength *assuming no foam contribution Topic #1 Reduced ΔP

  7. There are several process steps that contribute to pressure differentials across the capsule wall • The early process steps can create microcracks that are “healed” with GDP Dual Layer Process GDP or Parylene Coating CO2 drying Solvent exchange PVP coating DEP IPA DEP IPA IPA CO2 CO2 Osmotic Buckle Pressure Buckle and (venting) Burst Pressures Buckle and Burst Pressures If we can control Δpressure better we may improve gas retention DEP – diethyl phthalate IPA – isopropyl alcohol Topic #1 Reduced ΔP

  8. X DEP (1-X) IPA X-DXDEP (1-X+DX) IPA DEP flow IPA flow The solvent exchanges (DEP to IPA) can generate huge pressure differences across overcoat. DPosmotic is the pressure difference which stops flow across the overcoat • Assuming DEP diffuses much faster than IPA: DPosmotic = 85 atm (DX/XDEP) XDEP = mole fraction of the diffusing solvent (DEP);DX = X(inside) - X(outside) • One needs very small steps ofDX/XDEP • Exact diffusion rates are unknown • To be absolutely safe, long exchange times- >400 days could be needed It is best to avoid DEP-IPA-CO2; go from DEP to CO2 directly Topic #1 Reduced ΔP

  9. 5) Heat CO2 to supercritical fluid (90 atm, 38°C) Coated capsules are more sensitive to pressure changes in the CO2 drying process than bare foam shells Possible problem steps: • In step 2, bubbles nucleated in the liquid-and possibly in foam/overcoat • Steps 2-4 Osmotic pressure (CO2 diffusion vs. IPA diffusion) • Step 6 is a vent that can subject the shells to a large pressure differential Osmotic Pressures 2) Drain liquid CO2 3) Refill liquid CO2 1) Pressurize with liquid CO2 4) Repeat steps 2&3 (~25x) CO2 (l) Pressure vessel CO2 (g) IPA vial IPA IPA/CO2 mix shells Pressure Differentials 6) Vent CO2 (SCF) Vent rate ~9 hrs corresponds to ~3 atm burst pressure Topic #1 Reduced ΔP

  10. The CO2 dryer has been recently improved to minimize pressure differences across the shell walls • An automated venting system reduces the delta P at final vent to prevent bursting • A dead volume avoids bubble nucleation cause of buckling Vent Vent Backpressure regulator CO2 (l) Vent Liquid drain Dead volume Sample chamber A 29 hour vent is required so that no more than a 1 atm buckle pressure is applied Topic #1 Reduced ΔP

  11. By creating a smoother under coating, we may be able to improve gas retention Coatings currently investigated: • Shells are being fabricated using several interfacial chemistries • Organic reactant can play a role in reaction speed • Literature*suggests that the properties of the solvent can effect surface finish *Fusion Technology 31, 391 (1997) Topic #2 Improve Interfacial Layer

  12. 50 μm 50 μm 50 μm 50 μm 50 μm To study the effect of solvent on the PVP coating, 3 solvents with different solubility parameters were chosen • The original solvent was p-chlorotoluene. Shells wet Shells dry not yet dry More interfacial polymerization experiments are underway Topic #2 Improve Interfacial Layer

  13. 5 µm PVP GDP DVB Cross section of coated DVB shell Our baseline method has been to create an interfacial polymerization layer and cover with GDP • Poly vinyl phenol (PVP) covers the porous foam and glow discharge polymer is deposited on top • To date, this technique requires coatings much thicker than specification to hold gas PVP/GDP Dual Layer Gas Retention Gas Retention Yield Current Spec Topic #3 Top layer coating

  14. Parylene is an alternative coating or secondary coating for repairing damage in under layer • Advantages: • Covers dry shell, so no problems with solvent exchanges or drying • Only one pressurization (venting) step at end of process • More conformal than GDP • Can be used as a coating over interfacial polymerization layer (similar to PVP/GDP) • Disadvantage • Will it be able to meet smoothness spec? • Sticking during coating? • Others? Topic #3 Top layer coating

  15. Stalk mounted DVB shells have been test coated with parylene • Initial coated capsules collapsed due to fast vent (good sign that the shells hold gas) • Now have better control over vent rate so that more overcoated shells survive • Gas testing in progress Parylene overcoated PVP/DVB shell 1 mm SEM of a parylene overcoated DVB shell 0.5 μm stalk Topic #3 Top layer coating

  16. Smoothness specification is also a challenge • The smoothness specification is 50nm RMS (over lengths of 50 to 100 μm) • Possible ways of meeting spec • Make an inherently smooth coating • Vapor smooth the coating • Mechanical polish Topic #4 Smoothing

  17. A series of basic vapor smoothing experiments were performed Vapor smoothing is a process in which a solvent is used to swell the polymer to help asperities sink back into the surface due to surface tension. Basic experiment of solvent effects on dry, coated shells Wyko Wyko Expose to solvent vapor re-measure Topic #4 Smoothing

  18. The solvents tried either had no surface effect, or wicked into the foam and compromised the shell * Roughness data is reported in rms (nm). It is unlikely a suitable vapor smoothing solvent can be found for GDP or PVP. Topic #4 Smoothing

  19. The best surface finish on foam capsules so far is on resorcinol formaldehyde shells Power Spectrum of GDP Coated RF shell • Roughness Spec can be met on solid polymer shells without post coating smoothing • Creating a smooth coating on rough foam substrate is more difficult DVB coated with PVP, GDP/PVP or parylene is typically 300-1000 nm RMS over patches ~200 x 300 μm ~900 μm dia shell Topic #4 Smoothing

  20. Timeline – what’s next? • Reduce delta p • Will have sets of shells though the new drying process by February 07 • Alternate interfacial polymerizations • PVP solvent experiments: Jan 07 • Parylene testing • Coating tests (stalk mounted): Jan-Mar. 07 • If promising results work on freestanding coated shells • HAPL scale RF shell • Fabricate and GDP coat first set of HAPL scale shells: Feb. 07

  21. Conclusion • We are refining our process to reduce the delta pressure • The baseline design has a 0.1 atm buckle strength (extrapolated from data) • We are evaluating alternate interfacial chemistries • Trying new ways to repair coatings in 2 layer process - (GDP, Parylene) • We have evaluated chemical smoothing • Result: Not feasible for PVP or GDP on DVB shells • Trial fabrication of small pore foam with a single layer overcoating - (GDP on RF)

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