Neutron Imaging of Fuel Cells at NIST: Present and Future Plans

Neutron scintillator• Converts neutrons to light 6LiF/ZnS:Cu,Al,Au• Note that ZnS was used by Rutherford over 100 years ago to image

alpha particles backscattered from the gold nucleus• 6Li absorbs neutrons, then promptly splits apart into energetic charged

particles• Neutron absorption cross section for 6Li is huge (940 barns)• 0.3 mm thickness absorbs 20 % of the neutrons• Nuclear reaction produces energetic charged particles• Charged particles come to rest in 10 – 15 microns in the ZnS• ZnS:Cu,Al,Au produces green light • Unfortunately light easily propagates through the screen expanding to a

200 micron blob that degrades the spatial resolution

6Li + n0 4He + 3H + 4.8 MeV

Scintillator

Neutrons inGreen light out

Real-Time Detector Technology• Amorphous silicon • Radiation hard• High frame rate (30 fps)• 127 micron spatial resolution• Picture is of water with He bubbling

through it• No optics – scintillator directly couples

to the sensor to optimize light input efficiency

• Data rate is 42 Megabytes per second (160 gigabytes per hour)

• Most users opt for lower data rates due to the enormous pressure to download the data during and after the experiment

Neutron beam

scintillator

aSi sensor

Side view

Readout electronics

Scintillator aSi sensor

Front viewHelium through water at 30 fps

How Detectors Work

• Scintillator produces after absorbing a neutron (uncertainty of 0.2 mm).

• Light sensors record light distribution• Basic principle has been the same for 100

years.• Radical new method developed in a

collaborative effort here at NIST will improve spatial resolution to 0.025 mm – 0.015 mm.

Microchannel Plate DetectorsThe general scheme is photon conversion (photocathode) or direct detection (ions/e-), 1, 2 or 3 MCPs to provide gain, and then some type of readout.For Neutron detection and imaging we have used and open face detector with MCP triple stacks and an event counting/imaging cross delay line anode

Anode

Window/cathode

MCPs

25mm cross delay line anode detector showing anode (left), and neutron sensitive MCPs (right)

Absorption of NeutronSecondary(s) reaching surfaceEmission of photoelectronElectron gain above electronic threshold

n + 10B 7Li (1.0 MeV) + 4He (1.8 MeV) 7%

n + 10B 7Li (0.83 MeV) + 4He (1.47 MeV) + γ (0.48 MeV) 93%

σ = 2100 b at 1 Å

n + 157Gd 158Gd + γ's + X-rays + e- (29 keV - 182 keV, ~75%) σ = 70,000 b at 1 Å

n + 155Gd 156Gd + γ's + X-rays + e- (39 keV - 199 keV; ~75%) σ = 17,000 b at 1 Å

HB4 MCP types use Boron

B14 MCP types use Gadolinium

Detection of Neutrons in MCPs

Ultra High Resolution

• Idea proposed by NIST (Greg Downing)

• Goes beyond the latest high resolution advancement

• Innovative design based on a very different concept

Neutron Converter

Encoder

Encoder

Time-of-Flight (ToF) Coincidence

Neutron Beam

The reaction gives a unique coordinate solutionKnown:• Mass of each particle• Initial energy of each particle• Stopping power of converter• Stopping rate for each particle is different

Measure:• The unique time of flight (ToF) for each particle pair • Two PSD encoders establish the x-y coordinates for each pair

Calculate:• TOF Residual energy for each particle pair unique depth (x) of each reaction• Position sensitive encoder establishes a unique (y,z) position for the reaction• Variation in time/energy/stopping power/x-y position give spatial uncertainty• List mode output

Impose conditions:• Min./Max. delta time window for the coincidence pair• Line segment must pass through detector volume• Particle pair must yield a unique depth• A Jacobian Transformation defines unique angular emission & confirms measured angle

t1

t2

Water Sensitivity

0.001

0.01

0.1

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

s/<m

t>

Laminar Water Thickness (mm)

0.5s5s

25s50s

250s500s

-2.00E-02

0.00E+00

2.00E-02

4.00E-02

6.00E-02

8.00E-02

1.00E-01

0 0.2 0.4 0.6 0.8 1 1.2

Fractional Distance Down Cell

Addit

ional

Wate

r volu

me (

mL)

100 mA/cm2

650 mA/cm2

1250 mA/cm2

The highest water content is not always observed at the greatest current density. There is a competition between water generation and local heating.

VH

2O (

mL

)

fractional distance from inlet

60°C, 100% RH, 2 stoic @ 1.5A/cm2

Additional Water Content Due to Current

Dry

Wet100 mA/cm2

650 mA/cm2

1250 mA/cm2

Collaborator: Sandia National Lab

Down-channel condensation model at Bulk Cell Temperature of 60°C

1 2 3

6 5 4

7 8 9

12 11 10

13 14 15

18 17 16

19 20 21

24 23 22

25 26 27

30 29 28

31 32 33

1 2 3

6 5 4

7 8 9

12 11 10

13 14 15

18 17 16

19 20 21

24 23 22

25 26 27

30 29 28

31 32 33

1 2 3

6 5 4

7 8 9

12 11 10

13 14 15

18 17 16

19 20 21

24 23 22

25 26 27

30 29 28

31 32 33

0.5 A/cm2

cell 2 – predictedcell 2 – actual

1.0 A/cm2

cell 4 – predictedcell 5 – actual

1.5 A/cm2

cell 7 – predictedcell 8 – actual

VolumeN

F

AIm DV

liqN 2

air

liqv

m

mm

11

2

112 liqvv mmm

air

liqvN m

mmNN

1

NNN liqvv mmm 1

)(

)(

2

2

2maxexitsattot

exitsat

Air

OH

TPP

TP

MW

MW

If ( > ) Then VolumeN = Saturated1max N1N

Logical test applied at the exit of each volume:

Collaborator: Sandia National Lab

Assume the water content underneath the gaskets is due solely to MEA water

Can evaluate membrane hydration without interference from GDL or channel water

Red is average active area water content, Blue is average water content under gasket

Future studies planned to assess the method

Accepted in Journal of Power Sources

Initial Water Content Water after 20 min purge with Dry Nitrogen

Water after 40 min purge with Dry Nitrogen

MEA Hydration Characterization

Collaborator: Rensselaer Polytechnic Institute, Plug Power

Capillary properties of GDLs and Catalyst layers via Neutron Radiography

GDL sample

Sample holder

Water reservoir

Neutron beam Neutron Detector/ Imaging Device

0

5

10

15

20

25

30

35

40

45

50

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Saturation

Pc

(=P

_g-P

_l),

mm

H2O

10AA Imbibition

10AA Drainage

10BA Imbibition

10BA Drainage

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Saturation

Pc

(=P

_g-P

_l),

mm

H2O Imbibition

Drainage

Sketch of Capillary Pressure Experiment

Capillary Pressure of GDLs

Capillary Pressure of Thickened CatalystsP

Time

Low Flow Rate

High Flow Rate

(b)

GasIn

GasOut

(a)

P

Gas Permeability versus saturation

In collaboration with T.V. Nguyen, et al

Modeling a single serpentine

In collaboration with X. Li and J. Park, U. Waterloo

Fluent ModelNeutron Imaging Data

First Data with 0.025 mm resolution

• Membrane swelling complicates data analysis• Use 0.02 A cm-2 as the reference state to analyze

change in water content• Improved mounting scheme will eliminate the issue

0

0.1

0.2

-1.5 -1 -0.5 0 0.5 1 1.5

Chang

e in

wate

r th

ickn

ess

(m

m)

Distance from membrane center (mm)

Anode Cathode

Current Density0.05 A cm-2

0.10 A cm-2

0.20 A cm-2

Future Plans, Freeze Chamber

– Manufacturer, Thermal Product Solutions– -40 C to +50 C, +/- 1 C stabilization– 1000 kW cooling at -40 C– 32” W, 24” H, 18” D sample volume– Hydrogen safety features

• Explosion proof components• Hydrogen sensor in return, will tie into Facility E-stop• Nitrogen gas as cooling/heating fluid

– Remote Control Panel– Air handling unit to reside permanently inside BT2– Install hopefully during Feb. shutdown, definite

operation by April