LLNL-PRES-725737

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

Coating hollow 3D objects with uniform low-

density films by combining EPD and ALD

D. Malone, M. Worsley, M. Wang, M. Biener

March 13, 2017

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Fabrication of a foam liner on the inside of a hollow cylinder (=hohlraum)• sub 50 mg/cm3

• ~200 micron thick

Goal

Peak power

Au material contour

Peak power

20 mg/cm3 Ta2O5, 300 umhohlraum liner

low fill hohlraum(0.03 – 0.6 mg/cm3 Helium)

Why do we need this?

Foam liner reduces wall motion and increases shape control

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electrophoresis

+ -

ElectrophoreticDeposition (EPD)

Coattemplate

removetemplate

Atomic LayerDeposition (ALD)

Burn/wash

electrode PS bead layer

Low density foam with controlled density/pore size

Approach: EPD + ALD

2) Transfer process to cylindrical geometry

Counter Electrode

Deposition ElectrodePolystyrene Suspension

1) Deposit thin layers of foams on flat geometry

0 1 2 3 4 5 6 7 8 9 10

0

20

40

60 2 nm Al2O3

5 nm Al2O3

10 nm Al2O3

de

nsity

of te

mp

late

d m

ate

rial m

g/c

c

diameter of sphere in micron

Projected density:

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Tasks and Challenges

Deposit PS beads on Au flat by EPD • Finding the right beads and EPD conditions• Surface smoothness• Drying/cracking• Thickness (200 micron)

Homogeneously coat PS deposit with ALD• Diffusion of ALD precursor into polymer• ALD temperature (deformation, crack formation)

Remove PS template• Complete removal of PS• Crack formation

Transfer to cylindrical geometry

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vendor X linked carboxylated bead size

ST X 1 um

ST X 1 um

ST 710 nm

PS X X 1 um

PS 1 um

Proper bead selection is critical for coating uniformity

EPD wellST: Spherotech, PS: Polyscience

Parameters explored:• Bead concentration• Flow rate• pH• Electrode pretreatment• Voltage

PS beads result in unconformal coating under various conditions:

Identifying the right beads for EPD requires testing

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Control of EPD layer thickness

0

100

200

300

400

500

600

700

800

0 5 10 15 20 25 30 35 40

Esti

mate

d D

ep

osit

Th

ickn

ess (

um

)

Time (min)

1mAmpthickness

2mAmpthickness

1 mA

2 mA

Spherotech beads (710 nm) deposit: thickness over deposition time

Induction period

Good control of EPD layer thickness by linear growth rate

desired thickness

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Prevention of crack formation during drying

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Slow drying/freeze drying:

ST/Bang beads after EPD (no ALD coating)Dried in air (3 hours)

Ambient drying:

ST beads after EPD (no ALD coating)Dried in ethanol vapor (1 week)

Cracks can be avoided by carefully controlling the drying conditions (slow drying or freeze drying)

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Surface roughness

Ra of ~1.8 um over mm2 has been achieved for a 140 micron thick EPD film

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PS beads: carboxylated15c Al2O3 after burnout

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PS beads: not functionalized15c Al2O3 after burnout

10 micron

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ALD coating on different bead typesMass gain analysis on bulk pieces after 15 c Al2O3 @ 90C (expected ~3 nm)

X-

linked

carbox

ylated

Bead

size

Mass

gain %

Equivalent

ALD

coating

thickness

Density of

material

(mg/cm3)

Expected

density of

material

(mg/cm3)

ST X 1 um 25 10.5 nm 174 50

ST X 1 um 25 10.6 nm 177 50

ST 710 nm 30 9.1 nm 212 70

PS X X 1 um 10 4.4 nm 73 50

PS 1 um 22 9.5 nm 157 50

ST: Spherotech, PS: Polyscience

ALD growth rate depends on the type of beads (resulting foam density varies by a factor of 1.5 to 3.5 for same ALD conditions)

x 3.5

x 3.5

x 3.0

x 1.5

x 3.1

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Polyscience beads, non functionalized

Density: 157 mg/cm3

(~3x higher than expected)caused by subsurface deposition

“on surface”

“sub surface”

Parsons, ALD on Soft Materials, Wiley-VCH Verlag GmbH 2011, pp. 271.

ALD on

surface

functional

polymer

ALD on

surface-inert

polymer

ALD on polymers: diffuse or abrupt interface?

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ST 6.8 um 4.2 12 nm 29 10

ST 5.3 um 8.3 18 nm 58 13

Reduce density by using larger beads 20c Al2O3 @ 90C (expected: ~4 nm)

Mass gain analysis15c Al2O3 @ 90C (expected: ~3nm)

ST: Spherotech, PS: Polyscience

For 15c Al2O3 on 6.8 um beads: 22 mg/cm3

X-

linked

carbox

ylated

Bead

size

Mass

gain %

Equivalent

ALD

coating

thickness

Density of

material

(mg/cm3)

Expected

density of

material

(mg/cm3)

ST X 1 um 25 10.5 nm 174 50

ST X 1 um 25 10.6 nm 177 50

ST 710 nm 30 9.1 nm 212 70

PS X X 1 um 10 4.4 nm 73 50

PS 1 um 22 9.5 nm 157 50

reduce

x 3.5

x 3.5

x 3.0

x 1.5

x 3.1

x 2.9

x 4.5

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ST beads 6.8 um, 20c Al2O3, 450C/air/2h, residual PS mass 1.6%

large cracks

Some beads pop

open,

Some have “vent

holes”

Some beads

“shrivel”

Burning leads to PS removal but also large crack formation

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0

20

40

60

80

100

0 20 40 60 80

PS

Perc

ent

(%)

Time (hr)

Percent of PS Plasma Etched over Time (20c ALD)

Only minor cracks

Some beads are not etched

or only partially etched

Beads at cracks are

completely etched

ST beads 6.8 um, 20c Al2O3, residual PS mass 16%

Air plasma drastically reduces cracks, but no complete PS removal (even after 72 h)

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ST beads 6.8 u, 15c Al2O3, air plasma for 48 hours, 1.3 % of PS mass left

no visible PS

cores left

Minor cracks

(< 10 micron)

Complete PS removal for thinner ALD coatings

Plasma etching efficiently removes PS without major crack formation

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Cylindrical geometryEPD successfully transferred to cylindrical geometry

7 mm

3D printed sample holder

Polystyrene

Suspension

Stainless steel tube

with counter electrode

5.75 mm

Current Challenges:

• Inhomogeneous coating

• Side effects from

inhomogeneous field

• delamination of PS

coating~50 micron thick PS

bead deposit on

inside of SS cylinder

Work in progress:

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Demonstrated flat PS bead deposits by EPD on Au electrodes, 200 microns thick, without cracks

Templated PS films with ALD

Successfully removed the PS core with only minor crack formation

Achieved a Al2O3 foam density of ~20 mg/cm3

Working on optimization of 3D setup

Need: PS bead synthesis capability in-house

Summary

0 1 2 3 4 5 6 7 8 9 10

0

20

40

60 2 nm Al2O3

5 nm Al2O3

10 nm Al2O3

den

sity

of

tem

pla

ted

ma

teria

l mg

/cc

diameter of sphere in micron