carnegie mellon university, july 8-10, 2015 nano-scale...

Nano-scale resolution X-ray computed tomography

1

Shawn Litster, Ph.D.Associate Professor, Department of Mechanical Engineering

Carnegie Mellon [email protected], 412 268 3050

Students: Siddharth Komini Babu, Sarah Frisco, Pratiti Mandal, William Epting, Arjun Kumar, Tim Hsu, Alex Mohammed

Collaborators: Paul Salvador, Jay Whitacre, Ryan Sullivan, and Jessica Zhang (CMU); Hoon Chung and Piotr Zelenay (LANL)

Workshop on 3D Microstructural StudiesCarnegie Mellon University, July 8-10, 2015

Outline

1. X-ray CT and nano-scale resolution X-ray CT (nano-CT)

2. Application of nano-CT to low temperature fuel cells for vehicles

3. Application of nano-CT to Li-ion batteries

2

Micro/Nano X-ray Computed Tomography

Slide 3

X-rayoptics

Micro X-ray CTConical beam focus~1 µm resolution

Nano X-ray CTX-ray optics50 nm resolution

Synchrotron light souce X-ray CTParallel, high intensity beamResolution of 15 nm with optics, limited depth of focus

Bone Photonic crystal Scaffold Shale RockIntegrated circuits

Images: Xradia.com

Why X-ray CT?• Three-dimensional: Important for anisotropic

materials (Compare to SEM or TEM)

• Internal imaging: Reveal inner structure within otherwise opaque materials (Compare to SEM and AFM)

• Non-destructive: Same sample can be imaged multiple times in different imaging modes and following various physicochemical events (Compare to destructive FIB-SEM).

• Ambient and controlled environments: No vacuum required, enabling in-situand in-operando experiments (Compare to SEM and TEM)

• Large field of view CT: Larger samples and more representative statistics (Compare to TEM-CT)

Slide 4

X-ray Attenuation and Radiographs

5

• Radiographs created by sample attenuating the X-ray

• Two main mechanisms for X-ray attenuation– Photoelectric absorption (dominant for 8 keV nano-CT)

– Compton scattering (hard X-rays)

• Photoelectric absorption scales as ~Z4/E3

• Beer’s Law

expo i i

i

I I x

Attenuated intensity

Initial intensity

Linear attenuation coefficientf(Z, density)

Thickness

Summation for material in series

http://www.nist.gov/pml/data/xraycoef/

Pt (Z = 78)

C (Z =6)

/r

[cm

2/g

]

Density normalized /r

Hand of W.C. Rӧntgen’s wife

Photon energy [MeV]

ThickerDenserHigher Z

More attenuation

X-ray Computed Tomography (CT)

6

• Transmission images collected over 180o

rotation with collimated beam

• Sample rotates in nano-CT

• 2D example for 4 radiographs

• Detector image intensity a function of sample density, atomic number Z, and thickness

Detector

Source

Filtered Back Projection Reconstruction

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• Radiographs are computationally back projected into sample space

• High pass filtered to remove low frequency blurring

• Typically 400-900 projections for high resolution

Slice

Stacked slices 3D Volume

Nano-CT at Carnegie Mellon

Slide 8

• Xradia, Inc.’s UltraXRM-L200 Nano-CT

• Highest level resolution outside of a synchrotron using proprietary optics

• Laboratory 8 keV Cu rotating anode X-ray source

• Non-destructive imaging in ambient and controlled environments

• 4D imaging (space and time) for material evolution studies

• Fluid phase distributions

• 16 nm voxels, 50 nm resolution

NSF MRI award 1229090PI: Litster; Co-PIs: De Graef, Fedder, Feinberg, Sullivan

Nano-CT with X-ray Optics

Slide 9

Micro X-ray CT

Optics needed for resolutions better than 500 nm in X-ray CT

X-ray Source: Rotating Cu Anode• X-ray energy depends on target material’s emission

lines

• Trade-offs in photon energy:• Too low, no transmission

• Too high, low absorption contrast

• 8 keV of Cu anode balances absorption contrast and X-ray penetration depth for high resolution

• In between soft and hard X-rays

• Rotating anode for higher power

Slide 10

Emission lines of lab Cu X-ray source

X-rays

Monocapillary Condenser• Elliptical capillary focuses X-rays on sample

• High efficiency mono-capillary condenser instead of Fresnel zone plate condenser

• ~90% efficiency to enable high resolution with low intensity lab sources versus synchrotron beamlines

• Some inherent filtering above 10 keV

Slide 11

Fresnel Zone Plate Objective

Slide 12

• Diffractive X-ray lenses

• Circular grating with varying pitch focuses X-rays by constructive interference

• Rayleigh resolution depends on width of outermost zone

• For DRn = 35 nm, d = 43 nm

• Depth of focus (DOF) of ±16 µm for 8 keVphotons (λ = 0.15 nm)

• Optimized for 8 keV

• Long working distance (f) of ~2 cm

35 nmnRD

Gold rings

Inside the Nano-CT

Slide 13

The Optics – Two Modes

Slide 14

• M Feser, et al. Meas. Sci. Technol. 2008, 19, 094001

Pin hole Phase ring

Fresnel zone plate

Visible light microscope High Resolution (HRES):

Field of view: 16.3 µmResolution: 50 nm

Pixel resolution: 16 nm

Large Field of View (LFOV): Field of view: 65.6 µm

Resolution: 100-150 nmPixel resolution: 64 nm

• Each resolution mode has own set of optics (2 phase rings, 2 FZP, etc.)• Automated switching between LFOV/HRES and absorption/phase contrast

Resolution Test Pattern Images

Slide 15

Large Field of View – Phase Contrast High Resolution – Absorption Contrast

Full field of view

65 µm100 nm spacing at tips

50 nm spacing at tips

Dark areas = ~100 nm thick gold trace

http://www.lanl.gov/mst/mst7/patterson/nanoCTspec.shtml

Length for 1/e transmission dropCarbon (Z=6), 10 mm

Platinum (Z=78), 2.3 µm

3

4400, 13, 2200C C C

Pt Pt Pt

L Z Z

L Z Z

Penetration depth scales ~Z3

Zernike Phase Contrast for Low Z Materials• Used for low absorption contrast materials

– Organic materials: polymers, porous carbon, tissue/cells

– Other low Z materials (Be, Li, Li2O2)

• Achieved by inserting Au phase ring after FZP objective

• Bright/dark halos at material interfaces

• Challenge: intensity not strongly dependent on density or material composition, challenging the segmentation.

Li-ion Graphite Anode (Panasonic 18650)Absorption contrast Zernike phase contrast

• Higher signal to noise ratio for pore/solid interface• Better resolved fine features of layered graphite• Little contrast between bulk solid and bulk air

Physics-Based Phase Contrast Correction Algorithm• Developed new physic based correction for Zernike phase

contrast to remove artifacts by modeling diffraction optics of the system

• Transforms phase contrast image to equivalent absorption image

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Uncorrected Corrected

Correction applied to PTFE coated carbon fiber for PEFC gas diffusion layer

Kumar, Mandal, Zhang, and Litster, J. Applied Physics, in press, 2015

Radiographs and Reconstruction• Combustion particle analysis by Prof. Ryan Sullivan & Kyle

Gorkowski, CMU

• Particles captured on Kapton mesh sample mount inside inertial sampler

• Mesh directly transferred into nano-CT without further sample preparation

Slide 19

Kapton mesh imaged by nano-CT’s visual light microscope

Transmission Radiographs 3D Reconstruction of Particles in Field of View

Start

180o rotation of sample

Finish

Sample requirements• Sample must be small to fit within the field of

view during entire rotation (Not strict, but recommended for quality)• Max. dia. of 65 µm for LFOV

• Max. dia. of 16 µm for HRES

• Not an issue for thin-film (<5 µm)

– e.g., porous polycarbonate

• Ideal sample is a cylinder with diameter of the FOV width

• Sample must be stable• Hygroscopic materials can be an issue due to swelling

• Software can correct for some rigid body motion with gold fiducial particle.

Slide 20

Shearing et al., J.Euro.Ceram. Soc., 2010

Sample mounting

Slide 21

Clip type sample holder

Flat back sample holder Pin vise sample holder

1

32

Sample Preparation• Sample mounting ranges from very simple (film clip )

to challenging (FIB)

• Majority of samples are hand prepared at CMU and at Xradia’s applications lab

• Some options:• Microtome to thin polymer samples

• Hand cut sample to triangle so tip fits in FOV

• Laser milling

• Epoxy small and flimsy samples to pin

• Focused ion beam lift out

• Gold fiducial particles for HRES reconstruction

Slide 22

Catalyst particles on Kapton cut to fine tip

FIB lift out of SOFC electrode

Laser milled cylinder of shale rock

Setup for mounting gold fiducial particles

Sample Preparation – Miro Laser Milling

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• Laser micromill with 1 µm accurate control and beam width to prepare sample columns from MEAs

• Prepare through-thickness samples with maximum sample size in 3D image

Pillar of intact catalyst layer from MEA Membrane

surface

100 µm

Pillar of non-precious metal catalyst fuel

cell electrode

SOFC – current collector, cathode,

electrolyte, and anode

Laser extracted End of Life PEFC MEA with GDL

LFOV Zernike mosaicradiograph

Epoxy on pin

Sample

Laser mill Pin mounted column

Polymer Electrolyte Fuel Cells (PEFCs)

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Cathode

Anode

Bipolar plate

Gas diffusion layer (GDL)

Catalyst layer(10-20 m)

Membrane(15-50 m)

Hydrogen flow

Air flow

2H+

2e-

2e-

Overall fuel cell reactionH2 + 1/2O2 → H2O + Electricity + Heat

1 mm

0.3 mmH2

H2O

1/2O2

Oxygen reduction reaction1/2O2 + 2H+ + 2e- → H2O

Hydrogen oxidation reactionH2 → 2H+ + 2e-

Hyundai ix Tucson FCEV• 100 kW PEFC• Li-ion battery (21 kW)• Range: 650 km at 70 MPa• Mileage: 72 mpgge combined

• High efficiency, long range, low emission fuel cell vehicles• Low temperature, high power density fuel cell• Acidic polymer electrolyte membrane• Pt and Pt-alloy catalyst

Toyota Mirai FCEV• 114 kW PEFC• Range: 650 km at 70 Mpa• 3 min refueling time• Mileage: 72 mpgge combined

Gas Diffusion Layer(GDL)

Pt catalyst

Carbon support

3-5 nm

40 nm H2OH+

O2

e-Agglomerate

Nafionbinder

Pores• Litster and McLean, J. Power Sources, 130, 61-76, 2004.• Uchida et al., J. Electrochem. Soc, 142, 4143-4149, 1995.

Polymer Electrolyte Fuel Cell (PEFC) CathodesCarbon supported Pt catalyst

Oxygen reduction reaction (ORR)

1

2

3

4

Imaging Catalyst Aggregates 50 nm features on Pt/C aggregates are visible in nano-CT

Vulcan XC-72R aggregate

Soboleva et al., Applied Materials & Interfaces, 2010

SEM – After sputtering 20 nm Au coating1 2

3 4

NanoCT – Before Au sputtering

Carbon supported Pt aggregates with Nafion coating

Kapton film

1

2

3

4

Non-Precious Metal Catalyst (NPMC) Cathodes

SEM: K. More & D. Cullen, ORNL

CM-PANI-Fe (35% Nafion)

CM-PANI-Fe: Air/H2 at 1 atm, 80oC

Images: Zelenay et al., DOE AMR, 2014

Projected stack cost

Pt price instability (2000-2013)

PI: Piotr Zelenay

• Significant opportunity for reduced PEFC cost with non-precious metal cathodes (NPMCs)

• Great improvements in activity and durability with carbon-supported nitrogen transition metal catalysts (e.g., C-Fe-N, C-Co-N)

• Transport increasingly important• Low volumetric activity relative to Pt/C

• Thick electrodes (50 – 100 µm)

• Large transport losses

• New NPMC electrode structures

• Model development and transport loss analysis

• Effect of Nafion of loading

Carbon supported Cyanamide-Polyaniline-Fe Catalyst

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Two high temperature heat treatment synthesisFeN4 or Fe2N5 moieties embedded into carbon

High microporesurface area

Lefevre et al., Science, 2009

Holby et al., J. Phys. Chem. C, 2014

Nano-CT Imaging of NPMC Cathode

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• Multi-mode imaging combining separate phase contrast and absorption contrast scans. Large field of view (65 nm voxels).

• Two 3D images are registered and overlaid in post-processing

• Grey scale slices shows carbon structure from Zernike phase contrast scan.

• Orange volume rendering shows high Z Fe from absorption contrast scan.

Effect of Nafion Loading on 1 atm Air Performance

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• Low Nafion loading reduces activity Lower voltage at low current

• High Nafion loading increase transport loss Large voltage drop at high current

0 1 2 3 4 5 60

5

10

15

20

Diameter [(m)]

Vo

lum

e P

erc

en

tag

e [%

]

CM-PANI-Fe 35wt% Nafion



0.05 0.1 0.15 0.2 0.25 0.3 0.350

2

4

6

8

10

12

14

Diameter [(m)]

Vo

lum

e P

erc

en

tag

e [%

]




Hierarchical Morphology Properties

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• Wide variety of morphological properties characterized, including effective length scales• Characterization effective diameter of various electrodes for pore and solid domains

0.1 0.2 0.3 0.40

2

4

6

8

10

12

Diameter [(m)]

Vo

lum

e P

erc

en

tag

e [%

]

CM-PANI-FE 35wt% Nafion



0 2 4 6 80

5

10

15

20

25

30

35

Diameter [(m)]

Vo

lum

e P

erc

en

tag

e [%

]




Pore SolidLarge Field of View Scan

High Resolution Scan

10 µm

5 µm

Macro-pore porosities35% wt. = 0.40 50% wt. = 0.1760% wt. = 0.21

Macro-solid vol. fractions35% wt. = 0.60 50% wt. = 0.8360% wt. = 0.79

Meso-pore porosities35% wt. = 0.4750% wt. = 0.4960% wt. = 0.46

Meso-solid vol. fractions35% wt. = 0.5350% wt. = 0.5160% wt. = 0.54

65 nm voxels

16 nm voxels

Imaging 3D Nafion Distribution• Ion-exchange protons (H+) in Nafion with

high Z cesium ions (Cs+) by submerging sample in Cs2SO4 solution

• Imaged in phase and absorption contrast mode

• Absorption contrast shows Nafion (red/orange) and Fe (yellow/white)

• Phase contrast shows solid structure (black/grey)

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Ion exchange Nafion with Cs2SO4

H+ → Cs+

35% wt versus 60% wt.

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Intensities normalized for density comparison by gold fiducial and air absorption intensity

35% 60%

3D Nafion Distribution with Varying Loading

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• Nafion absorption intensity normalized by gold fiducial and air intensity values for comparison between scans.

• Ratios of volume integration of normalized intensity matches loading ratios

• Low penetration of Nafion into catalyst particles consistent with lower observed activity

35 % wt. Nafion loading 60 % wt. Nafion loading

Computational PEFC Model with NPMC Cathode• Two-dimensional PEFC model of channel

cross-section

• Established models for multi-phase PEFC simulations

• Special treatment of NPMC using multi-scale characterization from nano-CT

• Implementation of a microstructurallyconsistent agglomerate model

Schematic of model domains and distribution of physical models

10 µm

AgglomerateNano-CTimage

Catalyst agglomerate model

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Characterizing Hierarchical Electrode Structures

CM-PANI-Fe 35% wt Nafion

Large field of view scan processed to extract only large marcoporesLight gray = pore/voidDark grey = porous solid

Macropore & agglomerate

Micropore & intra-agglomerate

Computed microporeand intra-agglomerate parameters

• Two length-scales of imaging and analysis

• Macropore & agglomerate

• Micropore & intra-agglomerate

• Separated morphological characterization

• Characterized electrodes with 35% and 50% wt. Nafion

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Agglomerate Identification and Sizing

Slide 37

Large Field of View Nano-CT ImageMorphological separation of distinct

solid domains

0 2 4 6 80

5

10

15

20

25

30

35

Diameter [m]

Vo

lum

e P

erc

en

tag

e [%

]



Macro-solid vol. fractions35% wt. = 0.60 50% wt. = 0.83

Size distributions

Diameter from volume to surface area ratio of each separated solid

Macropore volume fraction• 40% for 35% wt. Nafion• 17% for 50% wt. Nafion

2

1

, ,2

,

2, ,

1 14

i agg i aggO

i ORR

r c agg lO Oagg i agg i agg

r rc RTj F

H E k D ka r r

d

d d

NPMC Agglomerate Model

Inputs from ex-situ& in-situ data

,

3

i agg c

eff

r k

D

Thiele modulus

O2 diffusivity in agglomerate updated for micro-pore liquid saturation

Inputs from imaging

1 1 1

tanh 3 3rE

Effectiveness factor

Ratio of actual to ideal reaction rate

2

1exp exp

4

s s c c cc ref

Oagg

aR J F Fk

c RT RTF

Transport-free reaction rate constant Butler-Volmer eqn

Film diffusion + interface

Henry’s law

Volumetric ORR current density in each size binORR in Agg.

0 2 4 6 80

5

10

15

20

25

30

35

Diameter [(m)]

Vo

lum

e P

erc

en

tag

e [%

]




Volumetric catalyst activity in solidRs Js = Site density*Site activity

Agglomerate model applied to size distribution

Idealized model of transport and reaction in dense region

,ORR i i ORRj V j

Volumetric ORR is sum of volume fraction and ORR current of each size in histogram

Agglomerate size distribution from nano-CT

Method: Epting and Litster, Int. J. Hydrogen Energy, 2012 38

Effective Diffusivity inside Agglomerates

8 ( )

3

pore

Kn pore

d RTD d

M

1 1 1( ) ( )DG pore Kn pore mD d D d D

Bosanquet formulation for diffusivity in pore

Knudsen diffusivity

0DGD c

• Pore-scale simulation of Knudsen transition-regime diffusion in micro-pore domain

• Effective intra-agglomerate diffusivity

CM-PANI-Fe 35% wt Nafion

Local diffusivity based on pore size, DDG

Concentration and pathlines

Method: Litster et al., Fuel Cells, 2013

Micropore size distribution

Integrated flux over faces for given concentration difference provides effective diffusivity

Diffusion with non-uniform diffusivity

eff

Face

eff

Face

cJ jdA AD

L

LD jdA

A c

D

D

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Results and Comparison to Experiment• Simulations for 35% wt and 50% wt Nafion loading

• Only morphological parameters modified according to characterization (table)

• Consistent trends with experiments and similar limiting currents

• Increased mass transport loss at high current density with 50 % wt. Nafion

Simulation

Experiment

4 mg/cm2 CM-PANI-Fe85 µm thick cathode80oC, 1 atm H2/AirNafion 211

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Effect of Agglomerate Size Distribution

Slide 41

Distribution of agglomerate diameter

Single meandiameter2.4 [µm]

0 2 4 6 80

5

10

15

20

25

30

35

Diameter [m]

Vo

lum

e P

erc

en

tag

e [%

]



Macro-solid vol. fractions35% wt. = 0.60 50% wt. = 0.83

2.4 µm agglomerates

• Using single, volume-averaged diameter significantly changes predicted performance

• Significant losses in large agglomerates

Surface area based diameter

Lithium ion batteries imaging

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Image: http://www.scs.illinois.edu/murphy/Ran/research/energystorage.html

• Battery collaborations with Professors Jay Whitacre and Venkat Viswanathan at CMU

• Evaluation of material structure and degradation mechanisms

• Imaging of carbon additive distribution in cathode in addition to higher Z active material

• Imaging of commercial graphite anodes at beginning of life and end of life

• Current work on electroplating and dendrite phenomena with metal anodes and graphite

Commercial LiCoO2 Cathode (MTI)

Slide 43

• LiCoO2 active particles (95.7 %)• Carbon black conductive additive• PVdF binder• 80 µm thick electrode

• Study on distribution and connectivity of carbon conductive additive

• Prior work focused on absorption• Influences current distribution and

mechanical integrity

Material Separation by Imaging Mode

Slide 44

Absorption Contrast Phase ContrastLiCoO2

Carbon + PVdF

Attenuations lengths at 8 keV: LiCoO2 = 10 µm and carbon = 1 mm

Transmission radiographs in high resolution mode

Combined Phase and Absorption Images

45

Komini Babu, Mohammed, Whitacre, and Litster, J. Power Sources, 2015

• Images are registered and overlaid in post processing

• Separate segmentation of absorption image enables clean segmentation of LiCoO2 particles

Analysis of LiCoO2/carbon Additive Contract

Slide 46

Distribution of LiCoO2/Carbon contact area sizePhase contrast artifacts require image processing to identify and segment LiCoO2/carbon contact

Segmented carbon additive with contact correction

Komini Babu, Mohammed, Whitacre, and Litster, J. Power Sources, 2015

• Evaluate electrode connectivity and mechanical properties

• Analysis of current hot spots on active material surface

Graphite Anode Degradation Imaging

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• Imaging of automotive-relevant commercial 18650 Li-ion battery

• Imaged at beginning of life (BOL) and at end of life (EOL)

• Evaluation of morphology change, including solid electrolyte interphase growth and transport restriction at EOL

• Depth of discharge effect

BOL

EOL

Zernike Phase Contrast Imaging

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Beginning of Life End of Life

Fine pore structure filled in end of life image

Absorption contrast imaging for inorganic SEI signal

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Absorption contrast showing higher Z components in SEI Absorption (green) combined with phase contrast

Identification of excessive SEI coating on pore walls and filling of smaller internal pore

Water and Transport Visualization

Slide 50

Air

65 µm

Paper edge

Paper fibers

Water filled paper

Radiographs of water drying from common paper (25 s exposure)

Summary• Application of nano-CT to broad range of electrochemical

energy materials

• Multi-mode imaging of materials with high and low Z components

• Material morphological characterization and phase mapping

• Synthesis and fabrication evaluation

• Micro-scale transport simulation

• Transport property extraction for macro-scale device modeling

Future: In-operando (4D) imaging of electrochemical cells

51

AcknowledgmentsStudents: Siddharth Komini Babu, Sarah Frisco, Billy Epting, Pratiti Mandal, Arjun Kumar, Tim Hsu, and Alex Mohammed

Collaborators: Hoon Chung and Piotr Zelenay (LANL); Paul Salvador, Jay Whitacre, Ryan Sullivan, and Jessica Zhang (CMU)

52

US National Science Foundation (NSF) • NSF CAREER Award for electrode analysis• Major Research Infrastructure award for nano-CT acquisition

Co-Pis: Marc De Graef, Gary Fedder, Adam Feinberg, Ryan Sullivan

US Dept. of Energy, EERE• Nano-CT characterization of non-PGM PEFC cathodes

• Li-ion battery imaging• SOFC microstructural analysis

SOFC microstructural analysis

carnegie mellon university, july 8-10, 2015 nano-scale...

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