ESEM LTU training 9

roastdismalMechanics

Oct 27, 2013 (4 years and 17 days ago)

105 views

Confidential

Quantrainx50

Module 7.2Peltier

Feb 2011

place photo here

2

Cold Stage Use


Keep wet samples wet


Increase contrast in non
-
conductive, hydro
-
phillic samples


Perform dynamic experimentation

3

Pressure/temp Phase Diagram for H
2
O

Temperature
-

Celsius

0

5

10

15

-
10

30

0

10

20

Solid

phase

Liquid phase

Gaseous phase

Pressure
-

Torr

4

Peltier
-

cooled stage

Connector flange

Stage platform

Specimen holder

Temperature range:


-
5
°

-

+60
°
C

Relative humidity:


0
-
100% achievable

Copper braid

(no water cooling)

Water cooling

5

ESEM
-

Applications

1

2

3

4

5

0

5

-
10

0

10

20

s

l

g

1

2

3

4

5

T

P (Torr)

Suberabsorbents

6

Cold Stage use


Keep wet samples wet


Increase contrast in non
-
conductive, hydro
-
phillic samples


Perform dynamic experimentation

7

Pressure / Temp phase diagram for
H
2
O

Pressure
-

Pa

0

600

1200

1800

-
10
°

30
°

0
°

10
°

20
°

Solid

phase

Liquid phase

Gaseous phase

Temperature
-

°
Celsius

ESEM

LOW VAC

8

Keep samples WET during pump down cycle


Desired final environment:


5ºC, 850 Pa (6.5 torr)



Use cyclic pumping and flooding:


Cycle between 850
-

1300 Pa



Put some extra water droplets inside chamber



9

Live animals

10

Water in an SEM…

11

Maintaining WET Samples During
Pumpdown Cycle


Desired final environment: 3 ºC, 5.5 Torr


Use cyclic pumping and flooding

Initial pump


5.5 Torr


First flood


9.6 Torr

Second pump


5.5 Torr

(perform 8x)


Final flood


9.8 Torr


Final pump


5.5 Torr


12

Dry to Wet to Dry Experimentation

13

Keeping Wet Samples Wet Can
Preserve Natural Structures

14

Basics of Chemistry and Physics


Water in a pure state behaves differently than when it has a
soluble substance in it


Solutes generally change the vapor dynamics of a material


Any system will have a unique vapor identity which will
determine the ease of driving off water or keeping it around


Water activity is a measure of the total water attraction of a
substance

15

Water Activity


Definition from book with reference

16


Fact:


wet, soft & squishy specimens look ‘better’ & remain stable at
pressures lower than prescribed by the SVP curve for water



Why is this?


We need to consider the thermodynamic equilibria and kinetics
of the specimen!

Imaging Hydrated Specimens

17

Vapour: a dynamic phase

Condensing

Evaporating

Stable

18

Specimen equilibria


Vapour pressure is proportional to mole fraction of solute
(Raoult’s law)


Consequence:


Vapour pressure of aqueous phase is
less

than that of pure water

19

Osmotic pressure


Thermodynamics, Van’t Hoff:

20

‘ESEM phase diagram’

Condensin
g

Low vacuum mode

ESEM mode

Evaporatin
g

21

Condensing

Evaporating

Stable

Warmer

Colder

Vapour: a dynamic phase

22

-
0.4

-
0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0

2

4

6

8

10

Vapour Pressure (torr)

Mass Loss (mg
-
mm

-
2

-
sec

-
1

)

0 ºC

5 ºC

10 ºC

20 ºC

30 ºC

Water loss rate is greater the greater the
temp

23

Condensing

ESEM mode

Evaporating

Low vacuum mode

Thermodynamics & kinetics

24

Thermal Gradients


Samples have differing thermal properties in addition to their
hydrophilic properties which must be considered


Conductive samples will have a thermal gradient from the side
temperature is applied to (samples will be more different on
the side farthest from the source)


Samples which are not thermally conductive will have a larger
temperature gradient than thermally conductive samples


Mounting can play an important part in minimizing this
difference and making an experiment successful

25

Dealing with thermal gradients


In an SEM the surface to be imaged is the most
important and needs to be at the proper condition


It may be necessary to over apply temperature in order
to achieve a desired surface condition


It may be necessary to wrap thermally
-
conductive
materials around non
-
thermally
-
conductive samples


It may be necessary to innovate with mounting
schemes to place a sample in a colder space


Vacuum is a great insulator and will provide a vapor
gradient around a sample (where it is coldest it will
have a higher vapor pressure and when warmest the
pressure will be lower)

26

Mounting suggestions for thermal stability

Placing a sample in a tight thermally
-
conductive space will help
keep it uniformly cold as the conduction area is in more contact
with the sample surface. Placing samples in a well or covering
with a washer can often be beneficial to keeping a sample at a
desired condition. The tighter the contact the better the
conduction.

27

X
-
ray can be done in ESEM mode

Brine

Oil

28

Contrast Enhancement can sometimes be
achieved on hydrophilic samples

25C (~25% RH)


5C (~95%RH)

29

29

STEM
-
2 :

High
-
resolution EDS

30

End of Quantrain 7.2 Peltier.ppt