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Equations of state R. Wentzcovitch U. of Minnesota VLab tutorial. EoS relates P,V,T in materials EoS of minerals are necessary to build Earth models In this lecture: isothermal EoS only (Eos parameters are functions of T). - PowerPoint PPT Presentation
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Equations of stateR. WentzcovitchU. of MinnesotaVLab tutorial
-EoS relates P,V,T in materials
-EoS of minerals are necessary to build Earth models
-In this lecture: isothermal EoS only (Eos parameters are functions of T)
Poirier’s “Introduction to the Physics of the Earth Interior”, Cambridge Press
The definition of the bulk modulus offers an EoS
ln
dP dPK V
dV d V
0
00
V P
Vo
dV dP
V K
PV V exp
K
(with K=cte=K0)
-This is only a naive example of how to generate EoS.
-K is not cte. It varies with P, except for really ifinitesimal volume changes.
Murnaghan EoS• It can be similarly derive assuming
'0 0K K K P
'0 0
dPK K P V
dV
0
0
1'
0
00 0 0 0
'1
'
KV P
V
K PdV dP V
V K K P V K
0 '
0 0
0
1'
KK V
PK V
0 'K is cte
Strains• Eulerian strain (f>0 for compression)
• Lagrangian strain (ε<0 for compression)
OK for ε→0
• Hencky strain (logarithmic strain)
0
1
20
(1 )
(1 2 )
l l f
l l f
0
1
20
(1 )
(1 2 )
l l
l l
0 0
lnl
H H
l
dl ld d
l l
0
1ln3H
V
V
23
011
2
Vf
V
23
0
11
2
V
V
For hydrostatic compression
• One more relation to be used:
320
11 3
1 2
Vf
V f
03dV
Vdf
0 00
00
0
lim lim
1 1lim
3 3
V V V V
P
dP VPK V
dV dV
dFK
f V df
0 00
19 lim
P
dFK V
f df
For f → 0
Therefore
Bulk modulus
3dV
fV
dFP
dV
with
Now we will expand the free energy in term of (eulerian)strains and derive relationships P(V), K(V), K’(V)…
F=af2+bf3+cf+…
Birch Murnaghan EoS (2nd order)
• 2nd order expansion of the free energy F=af2
• Recall that
• Therefore
0 0 0
19 limP
T
FK V
f f
0 0
9
2a K V
T
F F fP
V f V
23
011
2
Vf
V
with
with 52
0
11 2
3
Ff
V V
7 53 3
502
00 0
33 1 2
2
KP K f f
52
0 (1 7 )(1 2 )K K f f
7 53 3
0
0 0 0
0
7 52
KdP dPK
dd
32
0
/ (1 2 )w f
0
12 49'
3 21
K K f fK
P f P f
Therefore for f→0 0 ' 4K
2
( ' 4)
ndBM
M
PR
P K
LM assemblage
Murnaghan EoS overestimate P for non-infinitesimal strain
Birch-Murnaghan 3rd order2 3F af bf
T
F F fP
V f V
52
0
11 2
3
ff
V V
Take into account:
with
Then one gets:
At P=0 (f=0), K=K0
K’=K0’
0 0 0
19 limP
T
FK V
f f
52
0
33 1 2 1
2
bfP K f f
a
→ 2 eq.s for 2 unknowns, a and b
7 5 23 3 3
0
0 0 0
31 1
2
KP
0
3' 4
4K
If K0’=4 we recover the 2nd order BM
with
• One needs measurements in a larger pressure range to fit a 3rd order EoS
• There are trade offs between Ko and Ko’
• If the pressure range is small Ko’ is usually constrained to 4.
Vinet EoS
• This EoS is based on a different expansion of F
l is a scaling length
• Defining and changing variables (r→V) in F
• Replace a, l, K0, and K0’ from relations above and get
0
0
(1 )exp( )
( )
F F a a
r ra
l
34
3V r
02
00 2
0
exp( )4
4
FFP a a
V lrF
Kl r
00
2' 1
3
rK
l
1 12/3 3 3
0 00 0 0
33 1 exp ' 1 1
2
V V VP K K
V V V
Logarithmic EoS
• Expand F in powers of Hencky strains εH like Birch-Murnaghan
• To 2nd order in εH one gets
• And to 3rd order one gets
00 0
lnP K
00
0 0 0
' 2ln 1 ln
2
KP K
Comparison between parameters offered by various EoS
Summary
• EoSs based on expansions of F in terms of strain (finite strain EoSs) give order dependent parameters (trade offs).
• High order EoS require data in larger pressure ranges.
• The Vinet EoS is good for any pressure range.