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

9

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

10

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



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



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)

thermochemistry of rare earth silicates for environmental ...€¦ · • advantages of rare eart h...

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