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					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

                 Neutrons in
                                                Green light out



                                Scintillator
    Real-Time Detector Technology
•   Amorphous silicon                           Helium through
•   Radiation hard                              water at 30 fps      Front view
•   High frame rate (30 fps)
•   127 micron spatial resolution
•   Picture is of water with He bubbling
    through it
                                                                  Scintillator   Readout
•   No optics – scintillator directly couples                     aSi sensor     electronics
    to the sensor to optimize light input
    efficiency
•   Data rate is 42 Megabytes per second                               Side view
    (160 gigabytes per hour)
•   Most users opt for lower data rates
                                                                             Neutron
    due to the enormous pressure to
                                                                             beam
    download the data during and after the
    experiment                                                               scintillator
                                                                       aSi sensor
       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 Detectors
The general scheme is photon conversion
(photocathode) or direct detection                    Window/cathode
(ions/e-), 1, 2 or 3 MCPs to provide gain,
                                                          MCPs
and then some type of readout.
For Neutron detection and imaging we
                                                           Anode
have used and open face detector with
MCP triple stacks and an event
counting/imaging cross delay line anode




                              25mm cross delay
                              line anode detector
                              showing anode (left),
                              and neutron sensitive
                              MCPs (right)
Detection of Neutrons in MCPs
                            Absorption of Neutron
                            Secondary(s) reaching surface
                            Emission of photoelectron
                            Electron gain above electronic
                             threshold
                             B14 MCP types use Gadolinium
                 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
         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
       Ultra High Resolution
• Idea proposed by NIST (Greg Downing)
• Goes beyond the latest high resolution
  advancement
• Innovative design based on a very
  different concept
  Time-of-Flight (ToF) Coincidence

                     Encoder




Neutron Beam
                                   Neutron
                                   Converter




                               Encoder
                   The reaction gives a unique coordinate solution
Known:
• Mass of each particle
• Initial energy of each particle
• Stopping power of converter                                                 t2
• Stopping rate for each particle is different
                                                      t1
Measure:
• The unique time of flight (ToF) for each particle pair
• Two PSD encoders establish the x-y coordinates for each pair

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

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
             Water Sensitivity
             1
                                         0.5s
                                           5s
                                         25 s
                                         50 s
            0.1                         250s
 /<m t>




                                        500s
s




           0.01



          0.001
                  0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
                       Laminar Water Thickness (mm)
  Additional Water Content Due to Current
                              1.00E-01
                                              60°C, 100% RH, 2 stoic @ 1.5A/cm2                   100 mA/cm2               Wet
                                                                              100 mA/cm2
                              8.00E-02                                        650 mA/cm2
Additional (mL) volume (mL)




                                                                              1250 mA/cm2


                              6.00E-02
           Water




                              4.00E-02                                                            650 mA/cm2
                 H2O




                              2.00E-02
    DV




                              0.00E+00
                                          0     0.2     0.4     0.6     0.8        1        1.2
                                                                                                   1250 mA/cm2
                              -2.00E-02
                                                   Fractional Distance Down Cell
                                                    fractional distance from inlet

                                                                                                                           Dry
        The highest water content is not always observed
        at the greatest current density. There is a
        competition between water generation and local
        heating.                                                                                  Collaborator: Sandia National Lab
 Down-channel condensation model at Bulk Cell
           Temperature of 60°C
       mv1  mliq1
                                                        mvN  mliq N
                                                               
2                                             N 1                        Logical test applied at the exit of each
          
          mair                                                
                                                              mair
                                                                              volume:
                          VolumeN                                             If (  N 1 > m axN 1 ) Then VolumeN = Saturated
mv2  mv1  mliq1
                                            mv N 1  mvN  mliq N
                                                             
                                                                                             MWH 2O         Psat (Texit )
                                                                                    max2               
                                                                                             MWAir Ptot 2  Psat (Texit )




                                     I  ADV
                          mliq N 
                          
                                       2F
 1             2      3                            1            2         3                            1         2         3
 6             5      4                            6            5         4                            6         5         4
 7             8      9                            7            8         9                            7         8         9
 12           11     10                           12            11       10                           12        11        10
 13           14     15                           13            14       15                           13        14        15
 18           17     16                           18            17       16                           18        17        16
 19           20     21                           19            20       21                           19        20        21
 24           23     22                           24            23       22                           24        23        22
 25           26     27                           25            26       27                           25        26        27
 30           29     28                           30            29       28                           30        29        28
 31           32     33                           31            32       33                           31        32        33

 0.5 A/cm2                                        1.0 A/cm2                                           1.5 A/cm2
 cell 2 – predicted                               cell 4 – predicted                                  cell 7 – predicted
 cell 2 – actual                                  cell 5 – actual                                     cell 8 – actual
                                                                                              Collaborator: Sandia National Lab
MEA Hydration Characterization



Initial Water Content   Water after 20 min purge with        Water after 40 min purge with
                                Dry Nitrogen                         Dry Nitrogen
                                      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
                                 Collaborator: Rensselaer Polytechnic Institute, Plug Power
      Capillary properties of GDLs and Catalyst
           layers via Neutron Radiography
                                                                                                  50                  Capillary Pressure of GDLs
                                                                                                  45                                                            10AA Imbibition
                                                                                                                                                                10AA Drainage
                                                                                                  40
                                    Neutron Detector/                                                                                                           10BA Imbibition




                                                                          Pc (=P_g-P_l), mm H2O
  Neutron beam                                                                                    35
                                      Imaging Device                                                                                                            10BA Drainage
                                                                                                  30
 GDL sample
                                                                                                  25

  Sample holder                                                                                   20
                                                                                                  15
                                    Water reservoir
                                                                                                  10
                                                                                                    5
Sketch of Capillary Pressure Experiment
                                                                                                    0
                                                                                                        0.0    0.1   0.2   0.3   0.4       0.5    0.6     0.7     0.8    0.9      1.0
                                                                                                                                       Saturation

                                     High Flow Rate
                                                                                                              Capillary Pressure of Thickened Catalysts
                                                                                            10.0
                               DP         Low Flow Rate                                           9.0
Gas                      Gas
                                                                                                  8.0
In                       Out
                                                          Pc (=P_g-P_l), mm H2O
                                                                                                                                                                 Imbibition
                                                                                                  7.0
                                                                                                                                                                 Drainage
                                                                                                  6.0
              DP                                                                                  5.0
                                           Time
              (a)                         (b)                                                     4.0
                                                                                                  3.0
Gas Permeability versus saturation                                                                2.0
                                                                                                  1.0
                                                                                                  0.0
                                                                                                        0.0    0.1   0.2   0.3   0.4       0.5      0.6   0.7     0.8     0.9     1.0
  In collaboration with T.V. Nguyen, et al                                                                                             Saturation
     Modeling a single serpentine




                                                       Fluent Model
        Neutron Imaging Data



In collaboration with X. Li and J. Park, U. Waterloo
First Data with 0.025 mm resolution
Change in water thickness (mm)


                                                                                     Current Density
                                                                                               -2
                                 0.2                                                  0.05 A cm-2
                                            Anode               Cathode               0.10 A cm
                                                                                               -2
                                                                                      0.20 A cm

                                 0.1



                                  0


                                   -1.5    -1    -0.5    0     0.5     1       1.5
                                          Distance from membrane center (mm)
• 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
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

				
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posted:3/9/2012
language:English
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