ellingham diagram - national energy technology laboratory library/research/coal/energy...
TRANSCRIPT
Ellingham Diagram
Growth rates of selected oxidesCoO>NiO>Cr2O3>Al2O3
Thermodynamic stabilities of selected oxides Al2O3>Cr2O3>CoO>NiO
Effect of Electrical Current on Oxidation Rate
x
Cations
Electrons
Anions
Cation Vacancies
Metal GasOxideMO
MOO2eM 2212 =++ −+
−− =+ 222
1 O2eOor
−− +=+ 2eMOOM 2
−+ += 2eMM 2
or
1Ma 2Op
M O2
o
MO2 2MO;O2M G∆=+Overall reaction:
p'O2
( )M/MO
= exp 2 G∆ MOO
RT a"M
exp ∆GMOO
RT= p"O2
( )1
½
Oxidation Under Open Circuit Conditions
vBN x
Z Fxi
i
A
ii= − +( )
∂µ∂
∂φ∂
1
)(x
FZxN
BCvCj i
i
A
iiiii ∂
∂φ∂∂µ
+−==
2)( FZCRTN
DkTBii
iAii
κ==
Replace Bi in ji
⎟⎠⎞
⎜⎝⎛ +−=
xFZ
xFZj i
i
i
ii ∂
∂φ∂∂µκ
22
Consider a scale growing by cations moving outward.
jZ F x
Z Fxc
c
c
cc= − +⎛
⎝⎜⎞⎠⎟
κ ∂µ∂
∂φ∂2 2
(B.)
jZ F x
Z Fxe
e
e
ee= − +⎛
⎝⎜⎞⎠⎟
κ ∂µ∂
∂φ∂2 2
Electrical Neutrality (C.)Z j Z jc c e e+ = 0
(A.)
(B.)
( )∂φ∂ κ κ
κ ∂µ∂
κ ∂µ∂x F Z x Z xc e
c
c
c e
e
e= −+
+⎡
⎣⎢
⎤
⎦⎥
1
The cation flux now becomes:
( )j
Z F xZZ xc
c e
c c e
c c
e
e= −+
−⎡
⎣⎢
⎤
⎦⎥
κ κκ κ
∂µ∂
∂µ∂2 2
Considering ionic equilibrium and integrating across the oxide
jZ F x
dcc
c e
c eM
M
M= −
+′
″
∫1
2 2
κ κκ κ
µµ
µ
The relation between jC and scale growth rate is
j C dxdtc M=
CM mol cm-3
Comparison with the parabolic rate equation
dxdt
kx
=′
where k’ is the parabolic rate constant in cm2s-1.
′ =+″
′
∫kZ F C
dc M
c e
c eM
M
M12 2
κ κκ κ
µµ
µ
( )∂φ∂ κ κ
κ ∂µ∂
κ ∂µ∂x F Z x Z xc e
c
c
c e
e
e= −+
+⎡
⎣⎢
⎤
⎦⎥
1
The cation flux now becomes:
( )j
Z F xZZ xc
c e
c c e
c c
e
e= −+
−⎡
⎣⎢
⎤
⎦⎥
κ κκ κ
∂µ∂
∂µ∂2 2
Considering ionic equilibrium and integrating across the oxide
jZ F x
dcc
c e
c eM
M
M= −
+′
″
∫1
2 2
κ κκ κ
µµ
µ
The relation between jC and scale growth rate is
j C dxdtc M=
CM mol cm-3
Comparison with the parabolic rate equation
dxdt
kx
=′
where k’ is the parabolic rate constant in cm2s-1.
′ =+″
′
∫kZ F C
dc M
c e
c eM
M
M12 2
κ κκ κ
µµ
µ
( )∂φ∂ κ κ
κ ∂µ∂
κ ∂µ∂x F Z x Z xc e
c
c
c e
e
e= −+
+⎡
⎣⎢
⎤
⎦⎥
1
The cation flux now becomes:
( )j
Z F xZZ xc
c e
c c e
c c
e
e= −+
−⎡
⎣⎢
⎤
⎦⎥
κ κκ κ
∂µ∂
∂µ∂2 2
Considering ionic equilibrium and integrating across the oxide
jZ F x
dcc
c e
c eM
M
M= −
+′
″
∫1
2 2
κ κκ κ
µµ
µ
The relation between jC and scale growth rate is
j C dxdtc M=
CM mol cm-3
Comparison with the parabolic rate equation
dxdt
kx
=′
where k’ is the parabolic rate constant in cm2s-1.
′ =+″
′
∫kZ F C
dc M
c e
c eM
M
M12 2
κ κκ κ
µµ
µ
( )∂φ∂ κ κ
κ ∂µ∂
κ ∂µ∂x F Z x Z xc e
c
c
c e
e
e= −+
+⎡
⎣⎢
⎤
⎦⎥
1
The cation flux now becomes:
( )j
Z F xZZ xc
c e
c c e
c c
e
e= −+
−⎡
⎣⎢
⎤
⎦⎥
κ κκ κ
∂µ∂
∂µ∂2 2
Considering ionic equilibrium and integrating across the oxide
jZ F x
dcc
c e
c eM
M
M= −
+′
″
∫1
2 2
κ κκ κ
µµ
µ
The relation between jC and scale growth rate is
j C dxdtc M=
dxdt
kx
=′
where k’ is the parabolic rate constant in cm2s-1.
′ =+″
′
∫kZ F C
dc M
c e
c eM
M
M12 2
κ κκ κ
µµ
µ
Oxidation in the Presence of an Impressed CurrentEquations (A) and (B) are unchanged. Equation (C) is written:
iFjZFjZ eecc =+
( ) ( )ec
e
e
ec
c
c
ec
ixZxZFx κκ∂
∂µκ∂∂µκ
κκ∂∂φ
+−⎥
⎦
⎤⎢⎣
⎡++
+−=
1
κκ
∂∂µ
∂∂µ
κκκ
FZi
xZZ
xFZj
c
ce
e
cc
c
ecc +⎥
⎦
⎤⎢⎣
⎡−−= 22
xFZi M
c
eStop
µκ ∆=
Note: If the external oxygen pressure is 1 atm, oMOM G∆=∆µ
⎥⎥⎦
⎤
⎢⎢⎣
⎡−′=′
StopOC i
ikk 1
For chromia at 900oC: κe = 10-2 (ohm cm)-1 Park & Natesan, 1990∆Go = -820,000 J/mole
For a 1 µm thick oxide: iStop = -140 A/cm2
For a typical Fuel Cell current density of 1 A/cm2,OCkk ′=′
(Note: if Park & Natesan’s 2-probe data are correct there could be asubstantial effect of current on chromia growth.)
Parabolic Rate Constants 900C dry air
y = 1.23E-11x
y = 2.61E-11x
0
2 E -0 6
4 E -0 6
6 E -0 6
8 E -0 6
1 E -0 5
1 E -0 5
0 5 0 0 0 0 1 0 0 0 0 0 1 5 0 0 0 0 2 0 0 0 0 0 2 5 0 0 0 0 3 0 0 0 0 0 3 5 0 0 0 0 4 0 0 0 0 0 4 5 0 0 0 0 5 0 0 0 0 0seconds
g2/cm4
Nickel
Sr-doped Nickel
Parabolic Rate ConstantNickel 2.61x10-11 g2/cm4sSr-doped Nickel 2.23x10-11 g2/cm4s
Surface Micrographs
Nickel 900C Sr-doped Nickel 900C
Planar SOFC Configuration
Repeat Components
⎟⎟⎠
⎞⎜⎜⎝
⎛−= C
O
AO
pp
FRTV
2
2ln40
voltsV 7.0≈
2/1 cmAmpi ≈Electrolyte – YSZ Cathode – La/Sr-Manganite (LSM) Anode – Ni/YSZ Interconnect – Metallic
Oxidized Ni-50 Cr AlloyCr-Rich Second Phase has Dissolved in the Cr-Depleted Zone
Cr O32
Cr-depleted zone
Alloy
H2-3%H2OAir
Air
AirAir
H2-3%H2O
Thermocouple
Exhaust
Materials:
• 10 mil thick Ag tube
Experimental Arrangement
Materials:
• 10 mil thick Ag tube
Variables:
• Temperatures, 700, 600, 500oC;
• Simultaneous exposures to H2+3%H2O and air;
Variables:
• Temperatures, 700, 600, 500oC;
• Simultaneous exposures to H2+3%H2O and air;Courtesy of Dr. Prabhakar
Singh, PNNL
Diagram of Apparatus
Dry Air Exposures – 700oCTime vs. Mass Change / Area (700oC, dry air)
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0 200 400 600 800 1000 1200 1400 1600 1800 2000Exposure (Cycles)
Mas
s C
hang
e / A
rea
(mg/
cm2 )
Crofer - 1
Crofer - 2
E-brite - 1
E-brite - 2
AL453 - 1
AL453 - 2
ZMG232 - 1
ZMG232 - 2
Simulated Anode Gas (Ar-4%H2, H2O) Exposures – 700oC
Time vs. Mass Change / Area for Crofer, E-brite, AL453, & Ni (700oC, Ar/H2/H20)
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Exposure (cycles)
Mas
s C
hang
e / A
rea
(mg/
cm2 )
Crofer - 1Crofer - 2E-brite - 1E-brite - 2E-brite - 3AL453 - 1AL453 - 2Ni
Microstructural and Phase IdentificationCrofer 700oC
Cr2O3MnCr2O4
Dry Air2000 cycles
Internal Al2O3Wet Air1017 cycles
Internal Al2O3
SAG2000 cycles
Internal Al2O3
Cr2O3
Cr2O3
MnCr2O4
Si rich oxide Si rich oxide
MnCr2O4
Si rich oxide
Degradation Mechanism
H2(g) = 2[H]Ag
[H]Ag
O2(g)= 2[O]Ag [O]Ag
[O]Ag+[H]A==H2O(g)
H2(g)Air
Reaction Steps1. (H2)g = 2(H)Ag Ag/Fuel interface
2. (O2) g = 2(O)Ag Ag/ Air Interface
3. 2(H) Ag + (O)Ag = (H2O) g Bulk Ag
Bulk Degradation Related to :1.Dissociation and dissolution of H and O in the bulk metal2. Interaction between dissolved H & O to form High pressure H2O gas3. Nucleation and growth of steam bubbles/voids at GB/defects
Courtesy of Dr. Prabhakar Singh, PNNL
Pressure of CrO3
-18
-16
-14
-12
-10
-8
-6
0.7 0.75 0.8 0.85 0.9 0.95 1 1.051/T*103 (K-1)
log
p CrO
3 (at
m)
log p(Cr) log p(Mn)(A) log p(La) log p(Mn)(B)
Cr2O3
LaCrO3
MnCr2O4
Partial pressures of CrO3 in Equilibrium with
Cr2O3, MnO-saturated MnCr2O4, and LaCrO3
Note: similar reductions would be achieved in the pressure of CrO2(OH)2
Crofer oxidized in contact with LaSrMnO4(cathode) for 88hrs at 900oC in air + 0.1atm H2O
Crofer (side in contact with cathode)
MnCr2O4with ~1.5% Sr and ~2.6% La
Cr2O3 with ~2% Sr, ~4% La, and ~4.7% Mn
~74.9% Fe ~20% Cr ~2.2% La ~2% Mn ~0.9% Sr
Crofer (side opposite cathode)
MnCr2O4
Cr2O3
After exposure, the cathode contained ~0.9% Cr and ~1.8% Al