IFE Target Fabrication Update

Presented by Jared Hund1

J. 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. Weber3

1General Atomics, Inertial Fusion Technology, San Diego, CA2Schafer Corporation, Livermore, CA

3UC San Diego, San Diego, CA

HAPL WorkshopPrinceton, New JerseyDecember 12-13, 2006

IFT\P2006-154

The current HAPL target design is a 4.6mm foam capsule

DT Vapor

Foam + DT

Thin (300-1200 Å)High Z coating

~ 2

.3 m

m r

ad

5 m CH Overcoat

DT

Foam layer: ~0.18 mm divinyl benzene (DVB)

• 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)

Achieving this is a hard problem because

• Low buckle and burst strength of shells

ImpactsFabricationPermeation Filling

Layering• Covering large pores of DVB

– Foam has pores of ~1μm width that coating must cover

• Smoothness– Related to covering porous structure

Current strategies for improving the CH overcoating

1. 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

2. 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

3. “Repair” damage to the interfacial coating layer – Parylene coating

4. Smoothing – make everything smoother in the end

A challenge of fabricating a continuous overcoat is the low buckle strength of any 5μm polymer coating

Material Constant

w = coating thicknessr = radius

•Buckle Strength*:

22

13

2rwP

Ebuckle

This term is similar for most types of polymers that can be used

Calculated Buckle Strength of Parylene

Topic #1 Reduced ΔP

Bu

ckle

Pre

ssu

re (

atm

)

Polymer E (kpsi)Polystyrene 260-490

Polyimide 189 - 580

Parylene 348

Elastic (tensile) modulus (E) of various polymers

Wall thickness (μm)

2365.0 rwEPbuckle

Alternate form from Roark* sugests buckle may be even less

*Roark and Young, Formulas for Stress and Strain (1982)

0

0.5

1

1.5

2

2.5

5 7 9 11 13 15 17

Thickness of coating (microns)Bu

ckl

e s

trength

(atm

)

The buckle strength of DVB shells with thick coatings has been measured

4.1mm dia4.6mm dia

Topic #1 Reduced ΔP

The buckle pressure of a HAPL target will be ~0.1 atm*

rw

burst SP 2 ~2-5atm

•The Burst Strength is higher:

*assuming no foam contribution

Buckle Data of GDP/PVP Coated DVB Shells

Curve fits based on buckle equation

Material Constant

w = coating thicknessr = radius

•Buckle Strength:

22

13

2rwP

Ebuckle

S = tensile

strength

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 ProcessPVP coating

Solvent exchange

IPA

Osmotic Buckle

Pressure

CO2 drying

Buckle and (venting)

Burst Pressures

GDP or Parylene Coating

DEP – diethyl phthalateIPA – isopropyl alcohol

If we can control Δpressure better we may improve gas retention

DEPDEP

IPA CO2

IPA

CO2

Buckle and Burst Pressures

Topic #1 Reduced ΔP

The solvent exchanges (DEP to IPA) can generate huge pressure differences across overcoat.Posmotic is the pressure difference which

stops flow across the overcoat– Assuming DEP diffuses much faster than IPA:

Posmotic = 85 atm (X/XDEP)

XDEP = mole fraction of the diffusing solvent (DEP);X = X(inside) - X(outside)

X DEP(1-X) IPA

X-X DEP(1-X+X) IPA

DEP flowIPA flow

•One needs very small steps of X/XDEP

•Exact diffusion rates are unknown•To be absolutely safe, long exchange times- >400 days could be needed

Topic #1 Reduced ΔP

It is best to avoid DEP-IPA-CO2; go from DEP to CO2 directly

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

IPA

CO2 (l)

1) Pressurize with liquid CO2

3) Refill liquid CO24) Repeat

steps 2&3 (~25x)

2) Drain liquid CO2

CO2 (g)

5) Heat CO2 to supercritical fluid

(90 atm, 38°C)

6) Vent

CO2 (SCF)

Osmotic Pressures

Pressure Differential

s

IPAshells

Vent rate ~9 hrs corresponds to ~3 atm burst pressure

Pressure vessel

vial

IPA/CO2 mix

Topic #1 Reduced ΔP

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

A 29 hour vent is required so that no more than a 1 atm buckle pressure is applied

Sample chamber

CO2 (l)

Dead volume

VentLiquid drain

Backpressure regulator

VentVent

Topic #1 Reduced ΔP

By creating a smoother under coating, we may be able to improve gas retention

•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)

Polymer CoatingOrganic

reactant

PVPisophthalyol

dichloride

Polyvinyl alcohol (PVA)

isophthalyol dichloride

PVAsebacoyl

chloride

PVA benzoyl chloride

Melamine-formaldehyde None

Resorcinol Isophthalyol

Hydroxyethyl cellulose

isophthalyol dichloride

Coatings currently investigated:

Topic #2 Improve Interfacial Layer

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

p-chlorotoluene

diethyl phthalate

dimethyl maleate

Hydrogen bonding value

0.0 8.5 11.8

More interfacial polymerization experiments are underway

50 μm

50 μm50 μm

50 μm 50 μm

Topic #2 Improve Interfacial Layer

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

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7 8PVP thickness (m)

GD

P th

ickn

ess

( m

)

>=50%<50%0%

Topic #3 Top layer coating

Current Spec

Cross section of coated DVB shell

PVP

DVBGDP

5 µm

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

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

0.5 μm

SEM of a parylene overcoated DVB shell

1 mm

stalk

Parylene overcoated PVP/DVB shell

Topic #3 Top layer coating

Smoothness specification is also a challenge

• The smoothness specification is 50nm RMS (over lengths of 50 to 100 μm)

• Possible ways of meeting spec1. Make an inherently smooth coating2. Vapor smooth the coating3. Mechanical polish

Topic #4 Smoothing

A series of basic vapor smoothing experiments were performed

WykoExposeto solventvapor

re-measure

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

Topic #4 Smoothing

Wyko

The solvents tried either had no surface effect, or wicked into the foam and compromised the shell

Solvent

Shell-Top layer

Toluene Dichloro-hexane

CH2Cl2

RMS* Before

RMS* After

RMS* Before

RMS* After

RMS* Before

RMS* After

DVB-PVP 612 1920 434 444

DVB-PVP-GDP

326 1610 323 320

RF-GDP 79 1320 728 739 79 1320

GDP alone 18 15 18 16It is unlikely a suitable vapor smoothing solvent can be found for GDP or PVP.

* Roughness data is reported in rms (nm).

Topic #4 Smoothing

The best surface finish on foam capsules so far is on resorcinol formaldehyde shells

• 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

Power Spectrum of GDP Coated RF shell

~900 μm dia shell

Topic #4 Smoothing

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

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)