Transcript ppt - UC San Diego

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, CA
2Schafer Corporation, Livermore, CA
3UC San Diego, San Diego, CA
HAPL Workshop
Princeton, New Jersey
December 12-13, 2006
IFT\P2006-154
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
• 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)
Foam layer:
~0.18 mm divinyl benzene (DVB)
Achieving this is a hard problem because
• Low buckle and burst strength of shells
Impacts
Fabrication
Permeation 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
•Buckle Strength*:
Material
Constant
Pbuckle 
2E

3 1 
2

2
w
r 
w = coating thickness
r = radius
This term is similar for
most types of
polymers that can
be used
Buckle Pressure (atm)
Calculated Buckle Strength of Parylene
Wall thickness (μm)
Elastic (tensile) modulus (E) of various polymers
Alternate form from Roark* sugests
buckle may be even less
Pbuckle  0.365 E  w
r
2
*Roark and Young, Formulas for Stress
and Strain (1982)
Polymer
E (kpsi)
Polystyrene
260-490
Polyimide
189 - 580
Parylene
348
Topic #1 Reduced ΔP
The buckle strength of DVB shells with thick coatings
has been measured
•Buckle Strength:
Pbuckle 

3 1 
2

 wr 
2
w = coating thickness
r = radius
Buckle strength (atm)
2.5
Material
Constant
2E
Buckle Data of GDP/PVP
Coated DVB Shells
Curve fits
based on
buckle
equation
2
1.5
1
4.1mm dia
0.5
4.6mm dia
0
5
7
9
11
13
15
17
Thickness of coating (microns)
•The Burst Strength is higher:
The buckle pressure of a
HAPL target will be ~0.1 atm*
Pburst  S  2rw
*assuming no foam contribution
Topic #1 Reduced ΔP
~2-5atm
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
Process
PVP coating
DEP
Solvent
exchange
CO2
drying
DEP
IPA
IPA
DEP – diethyl phthalate
IPA – isopropyl alcohol
IPA
Osmotic
Buckle
Pressure
CO2
CO2
Buckle and
(venting)
Burst
Pressures
GDP or
Parylene
Coating
Buckle
and Burst
Pressures
If we can control Δpressure better
we may improve gas retention
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-X
DEP
(1-X+X) IPA
IPA flow
•One needs very small steps
of X/XDEP
X
DEP
•Exact diffusion rates are
(1-X) IPA
unknown
•To be absolutely safe, long
DEP flow
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
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
1) Pressurize with liquid CO2
Pressure
vessel
vial
2) Drain liquid CO2
CO2
(l)
IPA
5) Heat CO2 to
supercritical fluid
(90 atm, 38°C)
CO2 (g)
IPA
3) Refill liquid CO2
4) Repeat steps
2&3 (~25x)
IPA/CO2 mix
shells
6) Vent
CO2 (SCF)
Topic #1 Reduced ΔP
Pressure
Differentials
Vent rate ~9 hrs
corresponds to ~3
atm burst pressure
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
Backpressure
regulator
CO2
(l)
Vent
Vent
Sample
chamber
Liquid
drain
Dead
volume
A 29 hour vent is required so that no more
than a 1 atm buckle pressure is applied
Topic #1 Reduced ΔP
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)
Polymer
Coating
Organic
reactant
PVP
isophthalyol
dichloride
Polyvinyl alcohol
(PVA)
isophthalyol
dichloride
PVA
sebacoyl
chloride
PVA
benzoyl chloride
Melamineformaldehyde
None
Resorcinol
Isophthalyol
Hydroxyethyl
cellulose
isophthalyol
dichloride
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.
p-chlorotoluene
Hydrogen
bonding value
diethyl phthalate
0.0
dimethyl maleate
8.5
11.8
Shells wet
50 μm
50 μm
50 μm
Shells dry
not yet dry
50 μm
50 μm
More interfacial polymerization experiments are underway
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
PVP/GDP Dual Layer Gas Retention
thicker than specification 12
Gas Retention
to hold gas
10
GDP thickness (m)
5 µm
Yield
>=50%
<50%
0%
8
PVP
6
GDP
4
DVB
2
Current Spec
0
Cross section of coated DVB shell
0
1
Topic #3 Top layer coating
2
3
4
5
PVP thickness (m)
6
7
8
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
Parylene overcoated PVP/DVB shell
0.5 μm
SEM of a
parylene
overcoated
DVB shell
1 mm
Topic #3 Top layer coating
stalk
Smoothness specification is also a challenge
• The smoothness specification is 50nm RMS
(over lengths of 50 to 100 μm)
• Possible ways of meeting spec
1. Make an inherently smooth coating
2. Vapor smooth the coating
3. Mechanical polish
Topic #4 Smoothing
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
Expose
to solvent
vapor
Wyko
re-measure
Topic #4 Smoothing
The solvents tried either had no surface effect, or
wicked into the foam and compromised the shell
* Roughness data is reported in rms (nm).
Solvent
ShellTop layer
Toluene
Dichlorohexane
CH2Cl2
RMS* RMS*
Before After
RMS* RMS*
Before After
RMS*
Before
RMS*
After
DVB-PVP
612
1920
434
444
DVB-PVP-GDP
326
1610
323
320
RF-GDP
79
1320
728
739
GDP alone
18
15
79
1320
18
16
It is unlikely a suitable vapor smoothing solvent
can be found for GDP or PVP.
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
Topic #4 Smoothing
Power Spectrum of GDP
Coated RF shell
~900 μm dia shell
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)