Confidential

Quantrainx50Module 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 H2O

Temperature - Celsius

0

5

10

15

-10 300 10 20

Solidphase

Liquid phase

Gaseous phase

Pre

ssu

re -

To

rr

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

123

4

5

0

5

-10 0 10 20

s

l

g

1234

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 H2O

Pre

ssu

re -

Pa

0

600

1200

1800

-10° 30°0° 10° 20°

Solidphase

Liquid phase

Gaseous phase

Temperature - °Celsius

ESEM

LOW VAC

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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

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Water in an SEM…

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Maintaining WET Samples During Pumpdown Cycle

• Desired final environment: 3 ºC, 5.5 Torr

• Use cyclic pumping and floodingInitial 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

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Dry to Wet to Dry Experimentation

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Keeping Wet Samples Wet Can Preserve Natural Structures

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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

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Water Activity

• Definition from book with reference

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• 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

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Vapour: a dynamic phase

Condensing EvaporatingStable

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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

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Osmotic pressure

• Thermodynamics, Van’t Hoff:

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‘ESEM phase diagram’

0

2

4

6

8

10

12

0 2 4 6 8 10Temperature ºC

Vap

our P

ress

ure

(tor

r)

Condensing

Low vacuum mode

ESEM mode

Evaporating

21

Condensing EvaporatingStable

Warmer

Colder

Vapour: a dynamic phase

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-0.4-0.2

00.20.40.6

0.81

1.21.4

1.6

0 2 4 6 8 10Vapour Pressure (torr)

Mas

s L

oss

(m

g-m

m-2

-sec

-1)

0 ºC

5 ºC

10 ºC

20 ºC

30 ºC

Water loss rate is greater the greater the temp

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0

2

4

6

8

10

12

0 2 4 6 8 10

Temperature ºC

Vap

our P

ress

ure

(tor

r)

Condensing

ESEM mode

Evaporating

Low vacuum mode

Thermodynamics & kinetics

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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

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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)

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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.

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X-ray can be done in ESEM modeBrine Oil

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Contrast Enhancement can sometimes be achieved on hydrophilic samples

25C (~25% RH) 5C (~95%RH)

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STEM-2 : High-resolution EDS

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End of Quantrain 7.2 Peltier.ppt