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Torbjörn Jonsson

Environmental Scanning Electron Microscopy (ESEM)

Torbjörn Jonsson

High Vacuum –  Clean Specimens –  Dry Specimens (unsuitable for Biological specimens)

Specimen Conductivity –  Charging

Limitations of the conventional SEM

What if we could have higher gas pressures around the specimen?

Challenges: Vacuum at electron source AND in column.

ESEM: Schematic illustration

Torbjörn Jonsson

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Pressure-Limiting Apertures (PLA)

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Beam Gas Path Length (BGPL)

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Scattering

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Scattering

Torbjörn Jonsson

Torbjörn Jonsson

High Vacuum –  Clean Specimens –  Dry Specimens (unsuitable for Biological specimens)

Specimen Conductivity –  Charging

Limitations of the conventional SEM

Torbjörn Jonsson

Charging

Nonuniform charge balance at 1.7 kV Typical charging artifacts at 20 kV

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Environmental Secondary Detector (ESD)

Torbjörn Jonsson

Charge neutralisation

0,2 Torr 0.9 Torr 0.2 Torr

Torbjörn Jonsson

Torbjörn Jonsson

Low Vacuum –  Dirty Specimens –  Wet Specimens (Biological specimens)

Non-conducting Specimen –  Charge neutralization

Possibilities of the Environmental SEM

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Examples - Pollen

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Example of low vacuum - Dissolution of Salt

Torbjörn Jonsson

Athene M, D., Environmental scanning electron microscopy for the study of ‘wet’ systems. Current Opinion in Colloid & Interface Science, 1998. 3(2): p. 143-147.

Torbjörn Jonsson

Example of low vacuum - Dissolution of Salt

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High Vacuum –  No beam scattering –  Resolution depends on accelerating voltage

ESEM (and LV-SEM) –  Beam scattering –  Resolution?

EDX in the ESEM

Torbjörn Jonsson

Scattering

Torbjörn Jonsson

Torbjörn Jonsson

EDX in the SEM

Arnoult, C., J. Di Martino, L. Khouchaf, V. Toniazzo and D. Ruch, Pressure and scattering regime influence on the EDS profile resolution at a composite interface in environmental SEM. Micron, 2011. 42(8): p. 877-883.

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EDX in the ESEM

Torbjörn Jonsson

EDX in the ESEM

Torbjörn Jonsson

Imaging –  The resolution is (almost) not affected by beam skirt

EDX spot analysis –  The resolution is severely affected by the beam skirt (millimeters)

Resolution in the ESEM

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–  No  high  vacuum  required  (also  LV-­‐SEM)  –  Non-­‐conduc;ng  samples  (also  LV-­‐SEM)  –  Wet  samples  –  Dirty  samples  (also  LV-­‐SEM)  –  In-­‐situ  experiments  

•  Phase  transi;ons  -­‐  T  •  Oxida;on  etc  -­‐  environment  

 But  poorer  resolu;on  for  EDX  (also  LV-­‐SEM)  

ESEM - Advantages

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Instrumentation

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Instrumentation – Low temperature

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Torbjörn Jonsson

The challenge

point of contact

point of observation

e- beam

specimen

•  Uncontrolled  distribu;on  of  water  

•  Water  obscures  underlying  structure  

•  Point  of  contact  generally  not  visible  

 

Jansson, A. et al. Novel Method for Controlled Wetting of Materials in the Environmental Scanning Electron Microscope. (Submitted to Microscopy and Microanalysis, Jan 2012)

Torbjörn Jonsson

Novel approach

•  In situ manipulator •  Water reservoir

specimen

cooled support

water

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Experimental

•  Fixture compatible with the FEI Peltier cooled stage

•  Electrically and thermally insulated from the stage

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Torbjörn Jonsson

In situ sample holder

cooled Cu cylinder with condensed water

sample

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Cellulose  fibres  

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Instrumentation - heating experiments

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Instrumentation - heating experiments

Torbjörn Jonsson

1000ºC stage –  Good temperature control up to

1000ºC. Temperature ramp max 50ºC/min. Use heat shield above 500ºC for better temperature control.

1500ºC stage

–  Good temperature control above 800ºC. Temperature ramp max 50ºC/min. Use heat shield for better temperature control.

Heating stage

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Water cooler

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ESEM in-situ oxidation

Torbjörn Jonsson

A small furnace inside the SEM makes it possible to image the corrosion process at temperature.

What do we study? - Oxidation under the electron beam - Oxidation in low oxygen pressure - …

ESEM in-situ oxidation

Torbjörn Jonsson

Mass gain curve of iron at 500 ºC. Parabolic growth rate! The growth rate is controlled by ion diffusion.

What do we study? - Oxidation under the electron beam - Oxidation in low oxygen pressure - … 0

0,5

1

1,5

2

2,5

0 10 20 30

Mas

s gai

n (m

g/cm

2 )

Exposure time (hours)

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ESEM in-situ oxidation

Torbjörn Jonsson

Calculate the amount of oxygen in the chamber and compare with the oxygen consumed to form an oxide (exercise to do before lab)

What do we study? - Oxidation under the electron beam - Oxidation in low oxygen pressure - …

What is low oxygen content?

Oxidation in the ESEM – High temperature stage experiments

Torbjörn Jonsson

The initial oxidation pure iron –  The effect of water vapour at 500ºC

The effect of KCl on the low alloyed T22 (Fe-2.25Cr) –  Small amounts of KCl at 400ºC

ESEM – High temperature stage

Torbjörn Jonsson

•  Example 1: ESEM in-situ exposures of Fe at 500ºC. - Exposure time: 1 hour. - Three atmospheres: Dry air, wet air (~1% H2O) and H2O.

•  FIB - Cross-section milling and imaging to study the oxide microstructure. •  Furnace exposures as references.

Jonsson, T., B. Pujilaksono, S. Hallström, J. Ågren, J.-E. Svensson, L.-G. Johansson, and M. Halvarsson, An ESEM in-situ investigation of the influence of H2O on iron oxidation at 500 ºC. Corrosion Science, 2009. 51: p. 1914-1924.

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Torbjörn Jonsson

Electron backscattered diffraction (EBSD)

Torbjörn Jonsson

Crystallographic contrast §  EBSD: A very fine polished or (ion) etched surface is needed to be able to perform EBSD.

§  Forward scattered electron imaging: channelling contrast shows the different metal grains without the need for a very fine surface finish. §  Our aim is to keep the same surface treatment before exposure as for furnace exposures → use forward scattered electron imaging.

Torbjörn Jonsson

Forward scattered electron imaging - ESEM in-situ

Dry lab air 500ºC 2,5 Torr

Forward scattered image before exposure

ESEM in-situ after 42 minutes exposure

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Torbjörn Jonsson

ESEM in-situ Dry lab air (Fe)

•  ESEM in-situ •  Pure Fe •  exposed in dry lab air, 2.5T •  500ºC •  1h •  FIB and XRD

Torbjörn Jonsson

ESEM in-situ Dry lab air (Fe)

Dry lab air 500ºC 2,5 Torr

8 min 42 min

ESEM in-situ Dry lab air (Fe) 33 min

Dry lab air 500ºC 2,5 Torr

5 µm

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Torbjörn Jonsson

FIB oxide cross-section - dry lab air (Fe)

Dry lab air 500ºC 2,5 Torr

Torbjörn Jonsson

FIB oxide cross-section - dry lab air (Fe)

Dry lab air 500ºC 2,5 Torr

Torbjörn Jonsson

Iron, 500C, dry lab air, 1h, incidence grazing angle (5 deg), ESEM in-situ exposure

0

100

200

300

400

500

600

700

20 25 30 35 40 45 50 55 60 65

2 theta

Inte

nist

y (c

ps)

H M

H

H, M

M H M

H M

XRD -Dry lab air (Fe)

Dry lab air 500ºC 2,5 Torr

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Torbjörn Jonsson

ESEM in-situ Lab air (Fe)

•  ESEM in-situ •  Pure Fe •  exposed in lab air, 2.5T •  500ºC •  1h •  FIB and XRD

Torbjörn Jonsson

ESEM in-situ lab air (Fe)

0 min 60 min Lab air 500ºC 2,5 Torr

ESEM in-situ lab air (Fe) 11 min

Lab air 500ºC 2,5 Torr

5 µm

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ESEM in-situ lab air (Fe) 41 min

Lab air 500ºC 2,5 Torr

5 µm

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FIB oxide cross-section - lab air (Fe)

Lab air 500ºC 2,5 Torr

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ESEM in-situ H2O (Fe)

•  ESEM in-situ •  Pure Fe •  exposed in H2O, 2.5T •  500ºC •  1h •  FIB and XRD

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Torbjörn Jonsson

FIB oxide cross-section - H2O (Fe)

H2O 500ºC 2,5 Torr

Torbjörn Jonsson

FIB oxide cross-section

H2O 500ºC 2,5 Torr

Torbjörn Jonsson

Iron, 500C, water vapor, 1h, incidence grazing angle (1 deg), ESEM in-situ exposure: No hematite formation!

0

50

100

150

200

250

20 25 30 35 40 45 50 55 60 65

2 theta

Inte

nist

y (c

ps)

M

M

M

M M

M

XRD

H2O 500ºC 2,5 Torr

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Torbjörn Jonsson

Summary pure iron

§  Water vapour accelerates the oxidation of iron.

§  The influence of water vapour increases with increasing temperature. §  The ESEM in-situ technique provides unique information about the initial oxidation. The results from ESEM exposures are consistent with the furnace exposures. §  The oxide growth varies on different metal grains (different orientation)."

Torbjörn Jonsson

1 µm Metal

Fe3O4

Fe3O4

Fe2O3

Oxide

Metal Metal grain boundary

Top

Cross- section

1 µm Metal

Fe3O4

Fe3O4

Fe2O3

Oxide

Metal

Top

Cross- section

ESEM in-situ 1h, dry air 500°C, 2.5 Torr Ramp time: 50°C/min

Furnace (TG) 1h, O2 525°C, 1 atm Ramp time: 100°C/min

Torbjörn Jonsson

ESEM in-situ Oxidation

•  Example 2: T22 (2,25Cr-Fe) at 400ºC with KCl(s).

- Exposure time: ≤1 hour.

•  A KCl particle is observed during the exposure at 400ºC. The corrosion products formed at the position of the initial KCl particle is the studied in detail after the exposure.

Jonsson, T., N. Folkeson, et al. (2011). "An ESEM in situ investigation of initial stages of the KCl induced high temperature corrosion of a Fe-2.25Cr-1Mo steel at 400 °C." Corrosion Science 53(6): 2233-2246.

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Torbjörn Jonsson

T22 with KCl(s) Before exposure

30 µm

330°C

KCl

355°C

KCl

22

360°C

370°C

375°C

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400°C 0 min at temp.

400°C 2 min at temp.

400°C 20 min at temp.

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330°C 355°C 360°C

370°C 375°C

KCl

400°C (2 min) 400°C (7 min)

400°C (0 min)

400°C (20 min)

10 µm

KCl

Torbjörn Jonsson

330°C

KCl

355°C

KCl

The sudden onset of rapid hematite growth at 355oC is suggested to be caused by the formation of a FeCl2/KCl eutectic melt on the surface

The minimum melting point in the FeCl2/KCl system is about 350oC:

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O

Fe

Cl

K

SEM/EDX

Torbjörn Jonsson

Low Vacuum –  Dirty Specimens –  Wet Specimens

Specimen Conductivity –  No charging, even at high acceleration voltage

Oxidation in-situ experiments –  Both low and high temperature…

Summary ESEM