1. No category

High-pressure neutron
experiments.
What is state of the art?
&
How do we know the
pressure?
Craig L Bull
High Pressure Instrument Scientist
Talk Outline
• High Pressure & what it can do for you
• How do materials behave when
squeezed
• Generating high pressure
• Some science examples
• The Future
•Titanic resting depth of ~4300 m
•Pressure of ~430 bar pressure
(0.43 kbar, 0.043 GPa)
•Requires special submersival
equipment to reach
•Its high pressure but not that
high!
Pressure
•Pressure (P) is defined as force (F) per unit area (A)
P( Nm − 2 ) =
F (N )
A(m 2 )
•Consider a 60kg person in trainers (surface area=0.03 m2)
P~600N/0.03m2 =20000 Nm-2 – simple foot print in mud (0.02 MPa)
•Consider a 60kg person in high heels (surface area=0.0001 m2)
P~600N/0.0001m2=6000000 Nm-2 - deep hole left in mud (6 MPa)
•A 300 fold increase in pressure.
Use same ideas to apply high pressure to our samples
•Atoms and molecules rearrange/ move upon densification – possible
phase transitions
•Can follow rearrangement of atoms using diffraction techniques
•We are interested in pressures at least 1000 times these
The Pressure Scale
• The pressure at centre of earth
~360 GPa
– ~8000 times that at resting
place of Titanic
• The pressure at centre of Jupiter
~4 TPa
– Metallic hydrogen
– ~4000 GPa (~93000 times
that at resting place of
Titanic)
• The pressure at centre of the
sun 108 GPa!!
• ………
High Pressure – What Can it
Do?
• Material synthesis
• Geological studies
• Fundamental physics
• Planetary studies
• Astrophysics
• Biology
• ……
How Do Materials Behave Under Pressure ?
• Consider a box of volume V
• Apply some pressure
• The volume decreases to V’
• Continue squeezing & measure
volume
• The resistance to compression
known as bulk modulus
1.000
0.995
V/Vo
0.990
0.985
0.980
0.975
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Pressure (GPa)
3.5
4.0
4.5
5.0
Bulk Modulus
The higher the bulk modulus – more resistant to compression
Material
Bulk Modulus (GPa)
Glass
35-55 (composition
dependent)
Steel
160
Diamond
443
Water
22
Methanol
8
However – the bulk modulus changes with increasing
pressure (materials generally get stiffer) and changes with
temperature
As you compress over a wider pressure
range – the change in volume is not linear
as expected from our previous description
The bulk modulus generally increases
with pressure
Need to introduce a term that shows the
rate of change in bulk modulus with
pressure – known as bulk modulus
derivative B’
𝐵′
𝑑𝑑
=
𝑑𝑑
B~4 (unit less) for oxides
B>10 for organic/soft materials
B<4 for stiff metals
Materials generally get softer with increasing temperature
Electronic Excitations
• Ruby as found in gem stones Al2O3:Cr
• Excite with a laser and emits light at a
different wavelength (fluorescence)
• Exciting electrons to high energy state
• Results in emission of energy in form of light
The Pressure Dependence
Energy gap pressure dependent
•
•
•
•
Emission of ruby measured on a spectrometer
The emission is pressure dependent
Emission to higher wavelength with increasing pressure
Can relate emission line to pressure (very temperature sensitive)
Ruby Fluorescence In Practice
• Require optical access & laser system
• Window choice important
• Safety implications of laser systems
Up to 0.7 GPa (7 kbar)
•
•
•
•
Can directly apply gas pressure to sample
However, limited pressure as a result of wall thickness ratio
Significant safety implications
Pressure of sample directly that of the applied gas pressure (no need to
measure pressure any other way)
6 kbar
SXD, HRPD, inelastic experiments……
7.36 kbar
Generating static high
pressure beyond 1 GPa
• Use the idea of stiletto
heel effect
– Diamond anvil cell
– Large force on a small
area
– High pressure (upto
~350 GPa, norm – <100
GPa)
– But small sample volume
• X-ray Synchrotron /
spectroscopic
techniques
• Current neutron source
– Need for larger
volume
» Limits pressure
range
A solution - Paris-Edinburgh
Press
•Traditional 4 tie-rod variant
2 00 m m
•250 tonne press
•30 GPa - anvil dependent – sintered
diamond
•30-100mm3 – suitable for neutron
diffraction
But optical access not easy
Need to use idea of diffraction to
measure sample pressure
•Hydrostatic Loadings with encapsulated
null scattering (TiZr) gaskets
Measuring Pressure by
Diffraction
• Remember idea of the compressed
box
• Atoms in materials generally also
sit/arranged in regular boxes
• Diffraction techniques gives measure
of atomic box size
Measuring Pressure by
Diffraction
• Compress sample and measure change in “crystal box” (crystallographic unit cell)
volume
• Known bulk modulus
• Can determine pressure by measured
change in “crystal box” volume
∆𝑉
B = ∆𝑃
𝑉
Pressure Standards
• Choice of pressure marker important
–
–
–
–
–
–
Inert
Simple structure
Low B
No phase transitions
Good n scatter
Low n absorber
• NaCl
• MgO
• Pb
Perovskites at Pressure
4.72(13) GPa
Intensity (Arb. Units)
3.33(8) GPa
2.49(8) GPa
1.79(11) GPa
1.16(11) GPa
0.90(9) GPa
0.72(8) GPa
0.59(8) GPa
0.35(8) GPa
0.21(8) GPa
0.09(7) GPa
0.08(7) GPa
1.0
1.5
2.0
2.5
3.0
3.5
2.0
4.0
2.1
2.4
2.5
1.000
0.995
0.990
V/Vo
La2NiMnO6
Capacitors
Pb pressure marker
Clear phase
transition
• Can determine
samples bulk
modulus
2.3
d-spacing (Å)
d-spacing (Å)
•
•
•
•
2.2
0.985
0.980
0.975
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Pressure (GPa)
3.5
4.0
4.5
5.0
Urea Clathrates
• Gas cell
• D2 pressure
media
• Complex setup
• Gas pressure
gives sample
pressure
• Gas loading clamp for
PE press – higher
sample pressure
• 2 kbar D2 loading
• Further compressed
by PE press
• Observe phase
transition
• Hydrostatic
Internal heater design
Measuring temperature
Anvil
Anvil
6 GPa and 1200 K
PTFE ring
Pyrophyllite
gasket
New High T setup
10 GPa and 1200 K
Stefan Klotz et al, Appl.
Phys. Lett. 93, 091904
(2008)
Pressure Determination at
High Temperature
• Know temperature by the resonance technique
• MgO present as part of the gasket (and visible to neutrons)
• The unit cell size of the MgO determined by diffraction technique
• The bulk modulus behaviour at temperature known by extrapolation
• Can therefore determine sample pressure for a given temperature
60 tonnes
Intensity (Arb. Units)
800 W
700 W
600 W
500 W
400 W
300 W
200 W
150 W
100 W
50 W
0W
1.75
2.00
2.25
2.50
d-spacing (Å)
2.75
New High T setup
T=678 K, P=8.2
GPa
The α−γ−ε triple point of iron investigated by high
pressure–hightemperature neutron
scatteringStefan Klotz et al, Appl. Phys.
Lett. 93, 091904 (2008)
Diamond Anvils Cells – The
Future
• Accepted maximum currenty for neutrons is ~ 28 GPa
• However -
DAC Technology
95 GPa!!!!
Permits
• Higher pressure
• Optical access
• Measurement of pressure instantly
using ruby fluorescence
Boehler et al 2013
Acknowledgements
• ISIS, UK
–
–
–
–
–
–
–
Dr William Marshall
Dr Matt Tucker
Dr Kevin Knight
Dr Helen Playford
Mr Chris Goodway
Mr Mark Kibble
Dr Oleg Kirichek
• UPMC, Paris
– Dr Stefan Klotz
– Mr Gerard Hamel
• ESS, Sweden
– Dr Malcolm Guthrie
• CSEC, Edinburgh
– Dr John Loveday
– Miss Mary-Ellen Donelly