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Strategies in the search for new TBC materials HT-TBC project review on April 14. 2008

Strategies in the search for new TBC materials HT-TBC project review on April 14. 2008. Plan of work and proposal. Proposal Several classes HT-TBC1, HT-TBC2, and HT-TBC3 materials (s. Chapter 2) will be checked in Phase I:

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Strategies in the search for new TBC materials HT-TBC project review on April 14. 2008

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  1. Strategies in the search for new TBC materials HT-TBC project review on April 14. 2008

  2. Plan of work and proposal Proposal Several classes HT-TBC1, HT-TBC2, and HT-TBC3 materials (s. Chapter 2) will be checked in Phase I: (Mat-1) ZrO2 with Y-content above 15 wt% (retaining single-phase cubic structure at HT)(Mat-2) ZrO2 co-doped with oxides of trivalent and pentavalent (refractory) metals (e.g. Y2O3 + Ta2O5) (Mat-3) ZrO2 multiply stabilized with different RE elements, leading to cluster formation (TC lowering)(Mat-4) ZrO2 stabilized with MgO and/or CaO (Mat-5) CaZrO3/MgAl2O4 composites. Materials to be checked (30.11.07) (Mat-6) CaSZ or (Ca)YSZ (related to Mat-4 of proposal) Plan (Mat-7) (Ti)YSZ (see recent publication by Levy’s group [1]) of (Mat-8) (Ta)YSZ (related to Mat-2 of proposal) talk (Mat-9) 14YSZ (studied in detail in TBC PLUS project [2])

  3. Mg-Zr-O, Ca-Zr-O, Sr-Zr-O H. Yokokawa et al 1993 Calculated 2000 oC Polymorphism Polymorphism 1400 oC Y. Du et al 1991 Calculated Y. Yin et al 1993 T ss - based on t-ZrO2 Experimental C ss - based on c-ZrO2 Study of solid solutions and solid solutions/YSZ Polymorphism

  4. Mg-PSZ, Ca-PSZ Could be bought: • - Increased Ca-content? • Study of ZrCaO3 • Compatibility of ZrCaO3 and YSZ

  5. Mg-Y-Zr-O, Ca-Y-Zr-O, Sr-Y-Zr-O: double stabilization Sr-Y-Zr-O phase diagram not found 1600 oC 1300 oC L M Lopato et al 1991L M Lopato et al 1988 Experimental Experimental -Y4Zr3O12 forms below 1380 oCCaZrO3 in eqilibrium with all phases except t-ZrO2 No ternary compounds Study of solid solution MO+YSZ

  6. Durability (toughening) of non-transformable t'-YSZ by titania Schaedler et al. (UCSB) 2007 • Durability (toughening) can be enhanced by controlling • tetragonality (through change of chemical composition) due to • mechanism of ferroelastic toughening • - Increase to durability relative to materials in cubic phase field • Concomitant enhancement in toughness, phase stability, • sintering resistance • - Resistance to monoclinic transformation • - No TC decrease is expected: mA(Ti4+) < mA(Zr4+) • APS processability is OK APS (Movchan) • HT stability needs additional studies

  7. Effect of Ta2O5, Nb2O5, HfO5 alloying on transformability of t-YSZ Kim 1990 Ta2O5 content in YSZ Ternary alloy content c/a axial ration • - ZrO2 stabilizers: alkaline-earth, rare-earth, actinide, TM oxides • Residual stress as driven force for transformability. •  (CTA anisotropy) can be used as measure of transformability • Criterion 1 Criterion 2 • Ta2O5, Nb2O5, HfO5 enhanced transformability of ZrO2 and YSZ • (Ta2O5, Nb2O5, HfO5 increase tetragonal distortion of c (F) lattice) • Small stabilizers concentrations were used

  8. Ta,Y-SZ Decrease of of stability fields of both Cub ss and Tet ss with regards to binary ZrO2-Y2O3 system as a result of Ta2O5 addition Kim 1990 Schaedler et al. 2007

  9. Ta,Y-SZ • 16.6%YO1.5+16.6%TaO2.5-SZ is tetragonal and stable at 1500 oC • Only light corrosion of Ta,Y-SZ after 500 h (50-100 h for YSZ) • Small Ta5+ addition can be detrimental due to annihilation of • anion vacancies responsible • for stabilization • - Narrower Tet ss stability field • 16.6%YO1.5+16.6%TaO2.5-SZ • Synthesis: • reverse co-precipitation of • ZrOCl2.8H2O • Y(NO3)3.6H2O • TaCl5Pitek et al. 2007

  10. Y partition ~8wt% ~14wt% 8-YSZ/14SZ system • Fully stabilized c-SZ has poor cycling life • 13SZ has bad toughness • Y partitions between c and t (1200 oC) • No decrease of total Y • Y content in t decreases with the • service time • Y-content in t can not be lower • than 3.5% • 14%-YO1.5 in c is constant after • prolonged annealing at 1400 oC • YO1.5-content in t can be used as • marker of TBC state • 14-SZ layer to stabilize 8YSZ • underlayer as Y-source • - Study of interface 8-YSZ/14SZ J. Ilavsky et al 2001

  11. Toughness of t'-YSZ Mercer et al 2007 • Mechanisms affected by toughness: • Delamination due to strain misfit • with substrate (especially • under cycling conditions) • Materials loss due to FOD • Delamination due to CMAS • Enhancing toughness by • promoting tetragonality relative • to cubic YSZ TZP - partially stabilized zirconia, 2-phase (c/t) material made by sintering

  12. Effect of YSZ composition Effect of BC and YSZ composition • YSZ with 12 wt% Y2O3 is single phase • Y2O3 level was decreased to 6-8 wt% • where ceramic has 2-3 phases and • thus is tougher Improved TBC, burning ring test Burning ring test ZrO2-12Y2O3/Ni16Cr6Al0.6Y 1.8CaO.SiO2/Ni16Cr6Al0.6Y, duplex Cermet with MgO, higher TC Number of cycles to spallation Levine et al 1980

  13. PhD work Application to SNF for PhD • Title: “New high-temperature stable oxide systems with low • thermal conductivity” • - Pyrochlores and -phase zirconates • - Lanthanum molibdates • - Lanthanum hexaaluminates • Theoretical estimation of TC of multicomponent systems • from structural data • Radiation component of TC at high temperature • (influence of grain boundaries, structure non-homogenieties) • Correlations FWHM of Raman lines - phonon lifetime - TC • (using structural data) • Decision: Fall 2008

  14. PhD work Pyrochlores M2Zr2O7,-phase M4Zr3O12 Pyrochlores of La-Gd, stable to 1500 oC Yokokawa et al 1990 Levi 2004 -Phases, Eu-Lu TC decrease by mixing Zr and Hf (higher atomic mass) 1300 oC -phase in system ZrO2-Y2O3 HfO2-ZrO2-Y2O3

  15. PhD work Pyrochlore La2Zr2O7 as top layer? • Andrievskya et al 1999 • Pyrochlore La2Zr2O7 is in equlibrium with m-, t- and c-ZrO2 • Phase diagram La-Zr-Y-O at 1200-1600 oC? • What is eqiulibrium for La2Zr2O7 -YSZ at T1400 oC? • Pyrochlore La2Zr2O7 1600 oC Perovskite LaYO3 Narrow homogenieity range

  16. PhD work La-molibdate La2Mo2O9 and related systems • TC of La2Mo2O9 is lower than of any other crystalline compound • No TC change in spite of phase transitions • Reason 1 for low TC: large mean atomic mass • Reason 2 for low TC: “structural vacancies”, disordering (both • for O and Mo). 312 symmetrically independent atoms! • But: -La2Mo2O9is stable only to ~1000 oC. Move further in • MoO3-La2O3 and WO3- La2O3 phase diagrams

  17. PhD work Ternary La-aluminates LaMgAl11O19 • C. Friedrich et al 2001 • LaMgAl11O19 or La-magnetoplumbite LaM1+xQxAl11-2xO19, • where Ln3+=LnGd, M2+=Sr, MnZn, Q4+=Ti, Si • Related to Pb(Fe,Mn)12O19 • Long-term stability up to 1400 oC • =0.8-2.6 W/m K (=0.6-2.3 W/m K for YSZ) • Only one publication? • Narrow homogeneity range? 1400 oC

  18. La-aluminates La-MP (La-magnetoplumbite), e.g. LaMgAl11O19 Friedrich et al 2001 PhD work • La-MP have plate-like structure, • Anisotropy of TC and diffusion • properties is expected • Phase composition is not • optimized CTE of APS La-MP and YSZ Shrin- kage, % La-MP YSZ Aging behavior of APS La-MP and YSZ

  19. Some multilayer concepts Levy 2004 Delamination of top layer (CTE mismatch)** A Al2O3 based top-layer 7YSZ under-layer Erosion, corrosion protection Durability* Minimum layer thickness > MFP, morphological instability B Alternating layers (e.g. Al2O3 and YSZ) Enhancing thermal insulation (phonon scattering) Layering internal porosity Stability of pore structure? C By deposition conditions D YSZ/TGO interlayer Stability? * TGO compatible. ** Functionally graded structure is suggested

  20. Multilayer concepts: one of the methods • - Equilibrium alloys (slow) J.-C. Zhao et al 2002 • Diffusion multiples (known • long time, but not exploited. • Nanoindentation needs only • 4-25 µm2) • Screening diffusion barriers • Mapping phase diagrams • (experimental data for • theoretical predictions) • Several interfaces! • - HIP, 4 h, 1100 oC, 200 MPa • To cause interdiffusion: • 1500 h, 1100 oC • Interfacial contact structure is closer to TBC then in loose powder mixtures!

  21. 8YSZ 14YSZ CaAsZ TaSZ TiSZ Multilayer concepts: one of the methods • Simultaneous welding (e.g. 1150 oC, 24 h, • 200 MPa) • Simultaneous HT diffusion • SEM, EDX (EPMA) studies of at least • four important interfaces Importance of in-situ XRD study D. Zhu et al 2002

  22. Studies of new low-TC materials, Raman spectra • Observation: Low-TC materials have broader Raman lines • If phonon life-time  FWHM of Raman lines • And phonon MFP  phonon life-time, then • Correlation between TC and FWHM of Raman lines is • expected (if phonon velocity • remains unchanged) Image: Horiba scientific

  23. Concluding remarks • Initial part of the project is finished • Synthetic and analytical methods are checked • (except TC) • We have potential to accelerate the work • Communication ETHZ-Alstom can be improved • Back to the short interim 1-month reports • Do we need in-house PS chamber? (UCSB has)

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