quantrainx50 module 7.2peltier

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

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 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

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 EvaporatingStable

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’

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

22

-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

23

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

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)

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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.

27

X-ray can be done in ESEM modeBrine Oil

28

Contrast Enhancement can sometimes be achieved on hydrophilic samples

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

29

STEM-2 : High-resolution EDS

30

End of Quantrain 7.2 Peltier.ppt