LLNL-PRES-725837
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. Lawrence Livermore National Security, LLC
Highly Crosslinked Polymers for Transparent Ablators
21st Target Fabrication Meeting 2017LDRD 15-ERD-020
Xavier Lepró
Salmaan Baxamusa, Paul Ehrmann, Joe
Menapace, Johann Lotscher, Swanee
Shin, Richard Meissner, Ted LaurenceMarch 14, 2017
LLNL-PRES-xxxxxx
2
To achieve ignition, symmetry is the key!….not only physically, but chemically too!
Chemically vapor deposited (CVD) polymer makes an ideal ablator capsule: amorphous, low atomic number, and can be easily doped with high-Z atoms
~ 200 m
“Ablator capsule”
• Imploding capsule compresses hydrogen-filled interior, leading to Deuterium+Tritium fusion.
• Capsule must be topographically and chemically symmetric (round, smooth, no bumps, homogenous chemical composition).
2 mm 0
LLNL-PRES-xxxxxx
3
The changes of properties of plasma
polymers with time are related to the
concentration of trapped free radicals.
J. Macromol. Sci. A, 10, 383 (1976)
Plasma CVD polymers are not equivalent to their liquid-phase counterparts!
Plasma CVD polymers do not have a well-defined chemical structure
Advantages of plasma CVD polymers:• Single-step synthesis and casting
• Conformal over complex geometries
• Solvent-free (highly pure)
• Very smooth (nm RMS)
• Highly crosslinked However…
Polym. Degrad. Stab. 122 (2015), 1333
I=13 mW/cm2
C● + O2 COOH
LLNL-PRES-xxxxxx
4
Our Approach: Produce well-defined polymers that can replace amorphous Hydrogenated Carbon
Non-plasma energy sources can be used for gas-phase polymerization!
initiated Chemical Vapor Deposition
iCVD
Macromolecules 39, 2006, 3688-94
By using a thermally liable initiator, monomer molecules remain
unchanged until polymerization
Similar to Traditional polymerization
Photolytic
Pyrolytic
PROPERTIESSTRUCTURE
LLNL-PRES-xxxxxx
5
Ablators have to endure stringent processing conditions
Spherical MandrelConformal coating
(Low stress film)
Pyrolysis
300°C
Hydrocarbon
polymer capsule
ablator
Polish/cleanLaser drill
AssemblyInspection
20 K
Hydrogen solid
monocrystal layerImplosion!
Thermally stable
Mechanical stiff
1.
2.
3.
Chemical inert
Photostable4.Transparent5.Cryostable6.Amorphous CH
Symmetric
No oxygen
Smooth
7.
LLNL-PRES-xxxxxx
6
Ablator manufacturing requires highly resilient, cross-linked polymers
iCVD enables the synthesis andconformal coatings of polymeric filmsof highly crosslinked polymers.
n
CH3
PDVB
Poly(Divinylbenzene)
Divinylbenzene(p-DVB)
• Highly crosslinked
• Optically transparent
• No oxygen content
• Similar to polystyrene
• Stability via aromaticity
• No crystallizable groups
Lack of plasma-induced defects make iCVDPDVB transparent and photo-stable
iCVD - PDVB Plasma Polymer
Infusible and insoluble
LLNL-PRES-xxxxxx
7
Lack of plasma-induced defects make iCVD PDVB transparent and photostable
UV-Vis spectrumTransparent to visible light
PhotochemistryNo visible light photochemistry
Baxamusa et al, Chem. Vap. Dep. 2015
200 250 300 350 400 450 500 550 6000
20
40
60
80
100
Wavelength (nm)
Tra
ns
mis
sio
n %
GDP
PDVB
Plasma polymer
iCVD PDVB
No photon absorption
No photo-oxidation=
0 5x105
1x106
2x106
0
100
200
300
400
3 h
Plasma polymer 1 mW
Plasma polymer 10 mW
85 h
PDVB anneal 280°C 5 mW
PDVB as-deposited 5 mW
[O
H]/
[CH
]1
00
0
Dose (mJ/cm2)
70 h
= 405 nm
iCVD PDVB
(dry air)
LLNL-PRES-xxxxxx
8
200 400 600 800 1000
0.0
0.2
0.4
0.6
0.8
1.0
1-T
wavelength, (nm)
Fresnel Reflection
iCVD PDVB free-standing films are mechanically robust
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0
2
4
6
8
10200°C annealed
Lo
ad
on
sa
mp
le (
mN
)
Displacement into surface (m)
as-deposited PDVB
Young’s Modulus invariant after 200°C anneal
5.7±0.2GPa
Stiffer than plasma polymer!
Stable density changes only ~1%
~25 µm thick films
0 1 2 3 40
1
2
3
4
25°C
50°C
100°C
125°C
150°C
175°C
200°C
225°C
250°C
280°C
PD
VB
mass (
mg
)
Volume (mm3)
200°C
=1.06 g/cm3
EGDP ~3.5GPavs.
LLNL-PRES-xxxxxx
9
PDVB is thermally stable with no glass transition
ThermogravimetryStable at 300oC
0 50 100 150 200 250 300 350 40090
91
92
93
94
95
96
97
98
99
100
Temperature (oC)
Ma
ss
(%
)
<1.5% volatilesConfirmed DVB via mass spec
Mass Spectrometry
CH2
CH2
CH2
CH2
CH2
CH3CH3
CH2 Reversible thermal expansion
50 100 150 200 250
1020
1040
1060
1080
1100
thic
kn
es
s (
nm
)
Temperature (°C)
heating
coolin
g
in-situ ellipsometryRevealed no crystallinity or
glass transition
LLNL-PRES-xxxxxx
10
iCVD-PDVB is strong, transparent, thermally resistant… but can it be used to make capsules?
LLNL-PRES-xxxxxx
11
Film stress on coatingscan produce crippling,substrate warping ordelamination
Is film stress low enough?
It is important to measure film stress!
Undesirable for smooth & symmetrical ablators
LLNL-PRES-xxxxxx
12
Measuring film stressLow stress coatings are a key requirement to make ablators
Deposition of a thin film generatesstress on a substrate which bendsaccordingly
Substrate changes radius of curvature, R
Measured by Interferometry
𝜎𝑓 =𝐹𝑓
𝑑𝑓𝑤=
Ys𝑑𝑠2
6𝑅 1 − 𝜈𝑠 𝑑𝑓
Stoney equation
Generated stress is related solely to substrate properties and film thickness
2 in PDVB coated wafers
f : Film stress
Ys : Substrate Young’s Modulus
ds : Substrate thickness
s : Substrate Poison’s ratio
LLNL-PRES-xxxxxx
13
Thick PDVB films show near-zero intrinsic stressArbitrary film thickness are possible!
0 5 10 15 20 25
0
10
20
30
40
50
60
280°C
200°C
100°C
as-deposited
Str
ess
(M
Pa)
thickness (m)
50°C
PDVB intrinsic film stress
LLNL-PRES-xxxxxx
14
Film stress is related to unreacted monomer… and can be removed by evaporation
Annealing removes unreacted monomer
0 50 100 150 200 250 300
-10
0
10
20
Str
es
s c
ha
ng
e,
(M
Pa
)
Temperature, T (°C)
1st annealing
Tb DVB=192°C at 1 atm
0 50 100 150 200 250 300
-40
-30
-20
-10
02
nd annealing
Str
es
s c
ha
ng
e,
(M
Pa
)
Temperature, T (°C)
LLNL-PRES-xxxxxx
15
We have successfully incorporated Si dopantPolymer design allows doping
Current monomer:
+ Si dopant
Approach:
Si-doped PDVB
Si-PDVB
~1 µm thick film
1600 1400 1200 1000 800 600
IR Inte
nsity
wavenumber (cm-1)
Bands associated
with Silicon
LLNL-PRES-xxxxxx
16
We can coat flat surfaces, but what about spheres?In the works: Devising substrate-shaker alternatives
Coated Uncoated
Glass spheres
Results so far are promising:
LLNL-PRES-xxxxxx
17
Summary
• iCVD allows polymer design.
• Highly crosslinked polymers such as PDVB can be conformationally synthesized.
• PDVB polymers have shown to have high transparency, induce low film stress while being mechanically strong.
• PDVB is more chemically stable than GDP and does not photo-oxidize as easy.
• High-Z elements can be used for polymer doping: Si-doped PDVB.
SEM
Film Cross-section
25.2 m
Si wafer
PDVB
LLNL-PRES-xxxxxx
19
Current technology: CVD plasma polymerizationProduces plastic ablators of amorphous C-H materials (GDP)
Given that plasma breaks organic molecules at different locations , plasma “polymers” do not have a defined chemical composition, are prone to aging and react with humidity and light.
Gas feed
Helical resonator
Vacuum chamber
Plasma glow discharge
Glow discharge
Organic gas
Plasma “polymer”
or Glow Discharge Polymer
LLNL-PRES-xxxxxx
20
Fragmentation in Plasma Polymers leads to chemical instability
Plasma CVD polymer
Goal
J. Pol. Sci. A., 32, 1399 (1994)
▪
▪
▪
▪
▪
BUT…
▪
▪
▪
• Oxygen uptake in polymers hinders ignition.• Not optically clear• Limits control over material properties.
GDP