Thermochemistry of Rare Earth Silicates for Environmental ...€¦ · • Advantages of Rare eart h...
Transcript of Thermochemistry of Rare Earth Silicates for Environmental ...€¦ · • Advantages of Rare eart h...
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Thermochemistry of Rare Earth Silicates for Environmental Barrier Coatings
Gustavo Costa and Nathan JacobsonNASA Glenn Research Center
Cleveland, [email protected]
ICACC, Daytona Beach, Jan 2015
https://ntrs.nasa.gov/search.jsp?R=20150004112 2020-06-15T09:59:35+00:00Z
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Outline of Presentation
• Advantages of Rare earth silicates as Environmental Barrier Coatings (EBCs)
• Essential parameter – a(SiO2)
• Methods to measure silica activity– Thermodynamics of rare earth silicates: Knudsen Effusion Mass
Spectrometry
• Results of Y2O3-SiO2 and Yb2O3-SiO2 systems
• Benefits of coating - example
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Applications:
3
Ceramics in non-moving parts:• Combustor liners• Exhaust nozzles
Eventually moving parts!
Sensors and Electronic Circuits in Extreme Environments: Space and Deep Oil Exploration
• Stable to high temperatures• Disilicate: Appropriate thermal expansion match to SiC and SiC
composites• Lower reactivity than pure SiO2
Rare Earth Silicates as Coatings for Silicon-based Ceramics
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Low Reactivity of Rare earth Silicates
4
Si(OH)4(g) , MOH(g)H2O(g)
SiO2, MO(Underline indicates in solution)
SiC + 3/2 O2(g) = SiO2 + CO(g)
• SiO2 + 2 H2O(g) = Si(OH)4(g)
Meschter and Opila., Annu Rev Mater Res 43, 559 (2013) N. S. Jacobson, J Am Ceram Soc 97, 1959 (2014)
P[Si(OH)4] = K a SiO2 [P(H2O)]2
Na2O(s) + SiO2(s) = Na2O·xSiO2
• Molten Salt Reaction
Y and Yb silicatesNeed to be measured!
Lower a(SiO2) –less reaction
Exposure Time (hrs)0102030405060708090100
Wt. Change (mg/cm2 )
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
CVI (ACI)[1232 C]
Ml[1232 C]
CVI (ACI)[1274 C]
MI[1107 C]
SiC/SiC CMC HPBR Paralinear Weight Change(1100 -1300 C, 6 atm; Robinson/Smialek 1998)
Si(OH)4 volatility (Opila et al., 1998-2006)
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Key Parameters in Boundary Layer Limited Transport Modeling
• SiO2(pure or in silicate soln) + 2 H2O(g) = Si(OH)4(g)
• Reduce a(SiO2) reduce recession. Recession drives need for coatings
4)(
2)(
33.0
)(
5.0
)()(
33.0
)(
5.0
Si(OH) ofy diffusivit phase gasviscositydimension sticcharacteridensity gas stream free velocitystream free
664.0
664.0
4
22
4
4
44
4
OHSi
OHSiOOHSi
OHSi
OHSiOHSi
OHSi
DL
PaKLTR
DD
L
LTRPD
DLFlux
L
Combustion Gases
Gas Boundary Layer
Si(OH)4(g)
SiO2
ν = 0
Free Stream Gas Velocity ν
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Calculated Y2O3-SiO2 Phase Diagram: Fabrichnaya-Seifert Database
6
Indirect evidence suggests that the SiO2 thermodynamic activity is lower in the Y2O3-Y2SiO5 and Y2SiO5-Y2Si2O7 regions
But there are no direct measurements!
1600
1800
2000
2200
2400
2600
2800
TEM
PER
ATU
RE_
KEL
VIN
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0MOLE_FRACTION SIO2
THERMO-CALC (2010.08.10:09.24) : DATABASE:USER AC(O)=1, N=1, P=1.01325E5;
Y2O3 + MSY2O3 + MS
MS
+ D
S
MS
+ D
S MS
+ S
iO2
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Methods to measure silica activity
• Oxidation-reduction equilibrium using gas mixtures or electrochemical cells
• High temperature reaction calorimetry
• Knudsen Effusion Mass Spectrometry
vapor
sample
Effusion orifice
Mass spectrometer: I(SiO) P(SiO) a(SiO2)
Y and Yb silicates + *reducing agent to boost vaporization of SiO2 without changing solid composition!
Two approaches
*A. I Zaitsev and B. M. Mogutnov, J Mater Chem 5, 1063 (1995)
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Knudsen Effusion Mass Spectrometry (KEMS)
• 90 magnetic sector; non-magnetic ion source ion counting detector no mass discrimination• Cross axis electron impact ionizer• Resistance heated cell; multiple Knudsen cell system• Measurements to 2000 C, Pressure to 1 x 10-10 bar
section cross ionization (K) eTemperatur
constant instrument i component of pressure
i
i
iii
ST kp
STIkp
Use Multi-Cell Flange for a(SiO2)
Design of E. Copland 2002
I(SiO) P(SiO) a(SiO2)
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• Vapor pressure of SiO2 too low to measure in temperature range of interest
• Need measurable signal for SiO2—use reducing agent to make excess SiO(g). Tried several, selected Mo or Ta
– For a(SiO2) < ~0.02• 2Ta(s) + 2SiO2(soln) = 2SiO(g) + TaO(g) + TaO2(g)
– For a(SiO2) > ~0.02• Mo(s) + 3SiO2(soln) = 3SiO(g) + MoO3(g)
– Note reducing agent must not change solid phase composition• Monosilicates + disilicates +Ta – leads to tantalates
• Need to account for non-equilibrium vaporization
• SiO overlaps with CO2 (m/e = 44)– Use LN2 cold finger for improved pumping– Shutter to distinguish vapor from cell and background– Gettering pump for CO2
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Issues with Measuring a(SiO2) in RE Silicates
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Approaches use two phase regions
Two cells:• Au• 3Ta + Y2O3 + Y2O3 SiO2
2Ta(s) + 3SiO2(soln) = 3SiO(g) + TaO(g) + TaO2(g)
- Using Peq(SiO) and FactSage (free energy minimization)
- Correction for non-equilibrium vaporization
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1 – Monosilicate + RE2O3 2 – Monosilicate + Disilicate
33.0
33
33
2
33.0
33
2
32
33.0
33
2
32
32
33
)()()()()(
)()()(
33
)()(1)(
33)(
)()(
MoOISiOIMoOISiOISiOa
KMoOPSiOPSiOa
MoOSiOSiOMo
KMoOPSiOPSiOa
MoOSiOSiOMoSiOa
MoOPSiOPK
oo
oo
Three cells:• Au (reference)• 3Mo + Y2O3 2SiO2 + Y2O3 SiO2
• 3Mo + SiO2
Mo(s) + 3SiO2(soln) = 3SiO(g) + MoO3(g)- Compare cells 1 and 2- Less data processing than with Ta- Correction is not needed.
Cell 3
Cell 2
1 23
Cells are part of the system
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Monosilicate + RE2O3
1600
1800
2000
2200
2400
2600
2800
TEM
PER
ATU
RE_
KEL
VIN
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0MOLE_FRACTION SIO2
THERMO-CALC (2010.08.10:09.24) : DATABASE:USER AC(O)=1, N=1, P=1.01325E5;
Y2O3-SiO2 Yb2O3-SiO2
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XRD after KEMS Measurements of RE Monosilicates + RE2O3 + Ta:
Sample: 4-7-12APosition [°2Theta] (Copper (Cu))
10 20 30 40 50 60 70
0
2500
10000
22500
NJ 4-7-12A
Y2O3
Y2O3.(SiO2)
TaTa3Si
414944
wt (%)Phase
Yb2O3
Yb2O3.(SiO2)
TaTa2Si
246622
wt (%)Phase
Yttrium monosilicate + Y2O3 + Ta Ytterbium monosilicate + Yb2O3 + Ta
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Raw Data—log (IT) vs 1/T Cell (1): Au Reference Cell (2): Ta + Y2O3 + MS
Tabulated 346.3 kJ/mol
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Y2O3 + Y2O3.(SiO2)
*Liang et al. “Enthalpy of formation of rare-earth silicates Y2SiO5 and Yb2SiO5 and N-containing silicate Y10(SiO4)6N2”, J. Mater. Res. 14 [4], 1181-1185. **J. A. Duff, J. Phys. Chem. A 110, 13245 (2006)
Yb2O3 + Yb2O3.(SiO2)
RE2O3(s, 1600 K) + SiO2(s, 1600 K) RE2SiO5(s, 1600 K) H1 = measured in this workRE2SiO5(s, 1600 K) RE2SiO5(s, 298 K) H2 = H1600 K – H298 K
RE2O3(s, 298 K) RE2O3(s, 1600 K) H3
SiO2(s, 298 K) SiO2(s, 1600 K) H4
2 RE(s, 298 K) + 3/2 O2(g, 298 K) RE2O3(s, 298 K) H5
Si(s, 298 K) + O2(g, 298 K) SiO2(s, 298 K) H6
2 RE(s, 298 K) + Si(s, 298 K) + 5/2 O2(g, 298 K) RE2SiO5(s, 298 K) H7 =
H(SiO2, 1600 K) = (5200.26)·R·2.303 = 99.57 kJ/mol
Y2O3.(SiO2) -2907 ± 16 -2868.54 ± 5.34Yb2O3.(SiO2) -2744 ± 11 -2774.75 ±16.48
KEMS Calorimetry*
Hf, RE silicate, 298 K (kJ/mol)
a(SiO2), 1650 KOptical basicity**0.786
0.729
0.000804
6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
log[
a(Si
O2)]
T-110-4(K-1)
y = -1412.60(1/T)-1.67
1650 1600 1550 1500T (K)
H(SiO2, 1600 K) = (1412.60)·R·2.303 = 27.05 kJ/mol
0.00298
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Monosilicate + Disilicate
1600
1800
2000
2200
2400
2600
2800
TEM
PER
ATU
RE_
KEL
VIN
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0MOLE_FRACTION SIO2
THERMO-CALC (2010.08.10:09.24) : DATABASE:USER AC(O)=1, N=1, P=1.01325E5;
Y2O3-SiO2+ Y2O3-2SiO2 Yb2O3-SiO2+ Yb2O3-2SiO2
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XRD after KEMS Measurements of RE Monosilicates + Disilicates + Mo:
Y2O3.(SiO2)Phase
Yb2O3.(SiO2)
Mo
56368
wt (%)Phase
Ytterbium monosilicate + disilicate + Mo
Yb2O3.2(SiO2)
Yttrium monosilicate + disilicate +Mo
Position [°2Theta] (Copper (Cu))
10 20 30 40 50 60 70
Counts
0
400
1600
3600
6400
NJ 2-07824
Peak List
Y2 ( Si O4 ) O; Monoclinic; 04-007-4730
Mo; Cubic; 04-004-8483
Y2 Si2 O7; Monoclinic; 00-038-0440
Ln2 Si2 O7; Monoclinic; 00-021-1014
Y2 Si2 O7; Monoclinic; 00-042-0167
Y2O3.2(SiO2)
Mo
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Y2O3.(SiO2) + Y2O3.2(SiO2) Yb2O3.(SiO2) + Yb2O3.2(SiO2)
5.30 5.35 5.40 5.45 5.50 5.55 5.60 5.65-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
log[
a(Si
O2)]
T-1 10-4 (K-1)
1880 1860 1840 1820 1800 1780T (K)
a(SiO2), 1650 KOptical basicity**
0.786
0.729 0.194
Y2O3.(SiO2)
Yb2O3.(SiO2)
Y2O3.2(SiO2)
0.699
0.657
Yb2O3.2(SiO2)
0.281
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L
Combustion Gases
Gas Boundary Layer
Si(OH)4(g)
SiO2
ν = 0
Free Stream Gas Velocity ν
Now Have the Needed Quantities for Modeling Recession
3.86E-4
0.13488
0.00143
0.09312
0.48
0.0
0.1
0.2
0.3
0.4
0.5
J Si(O
H) 4(m
g/cm
2 -h)
– T = 1300 C; P = 10 bar; P(H2O) = 1 bar– v = 20 m/s– L = 10 cm– = 5 x 10-4 g/cm-s– = 2.2 x 10-3 g/cc– DSi(OH)4 = 0.19 cm2/s– log K = -2851.2/T – 3.5249 (Si(OH)4(g) transpiration measurements)– a(SiO2) from activity measurements
2)(
33.0
)(
5.0
22
4
4
664.0 OHSiOOHSi
OHSi
PaKLTR
DD
LFlux
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Summary: Thermochemistry of EBC materials
• The reduced SiO2 activity in Rare-earth silicates should limit their reactivity with water vapor
• Solid State rare earth oxides—activity of SiO2
– Vapor pressure techniques—Knudsen effusion mass spectrometry– Need reducing agent to obtain a measurable signal for SiO(g), which in turn relates to
activity of SiO2. Reducing agent must not change solid phase composition.– Method and choice of reducing agent depends on particular silicate
• Thermodynamic data for gas phase hydroxides and solid candidate coating recession modeling input data
19
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Acknowledgements
• Helpful discussions with B. Opila (Formerly NASA Glenn now Univ of Virginia)
• Multiple cell and sampling system improvements to mass spectrometer: E. Copland (formerly NASA Glenn now CSIRO, Sydney, Australia)
• XRD: R. Rogers (NASA Glenn)