ift\p2006-154
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- PowerPoint PPT PresentationTRANSCRIPT
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)