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

Neutron scintillator Converts neutrons to light 6 LiF/ZnS:Cu,Al,Au Note that ZnS was used by Rutherford over 100 years ago to image alpha particles backscattered from the gold nucleus 6 Li absorbs neutrons, then promptly splits apart into energetic charged particles Neutron absorption cross section for 6 Li 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 6 Li + n 0  4 He + 3 H MeV Scintillator Neutrons in Green 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 view Helium 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 mm – mm.

Microchannel Plate Detectors The 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 Neutron Secondary(s) reaching surface Emission of photoelectron Electron gain above electronic threshold n + 10 B  7 Li (1.0 MeV) + 4 He (1.8 MeV) 7% n + 10 B  7 Li (0.83 MeV) + 4 He (1.47 MeV) + γ (0.48 MeV) 93% σ = 2100 b at 1 Å n Gd  158 Gd + γ's + X-rays + e- (29 keV keV, ~75%) σ = 70,000 b at 1 Å n Gd  156 Gd + γ's + X-rays + e- (39 keV 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 solution Known: 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 t1t1 t2t2

Water Sensitivity

The highest water content is not always observed at the greatest current density. There is a competition between water generation and local heating.  V H2O (mL) fractional distance from inlet 60°C, 100% RH, 2 1.5A/cm2 Additional Water Content Due to Current Dry Wet 100 mA/cm mA/cm mA/cm 2 Collaborator: Sandia National Lab

Down-channel condensation model at Bulk Cell Temperature of 60°C A/cm 2 cell 2 – predicted cell 2 – actual 1.0 A/cm 2 cell 4 – predicted cell 5 – actual 1.5 A/cm 2 cell 7 – predicted cell 8 – actual Volume N If ( > ) Then Volume N = Saturated 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 Sketch of Capillary Pressure Experiment Capillary Pressure of GDLs Capillary Pressure of Thickened Catalysts PP Time Low Flow Rate High Flow Rate (b) Gas In Gas Out (a) P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 Model Neutron Imaging Data

First Data with 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

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