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place photo here. Quantrainx50 Module 7.2Peltier. Feb 2011. Cold Stage Use. Keep wet samples wet Increase contrast in non-conductive, hydro-phillic samples Perform dynamic experimentation. 15. Liquid phase. Solid phase. 10. Pressure - Torr. Gaseous phase. 5. 0. -10. 0. 10. 20. - PowerPoint PPT Presentation
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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)
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 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