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6 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2003

Thermal Conductivity and Stability of HfO2-Y2O3 and La2Zr2O7 Evaluated for 1650 Deg C Thermal/Environmental Barrier Coating Applications

HfO2-Y2O3 and La2Zr2O7 are candidate thermal and environmental barrier coating (T/EBC) materials for gas turbine ceramic matrix composite (CMC) combustor applications because of their relatively low thermal conductivity and high temperature capability. In this paper, thermal conductivity and high temperature stability of hot-pressed and plasma sprayed specimens with representative partially-stabilized and fully-cubic HfO2-Y2O3 compositions and La2Zr2O7 were evaluated at temperatures up to 1700 C using a steady-state laser heat-flux technique. Sintering behavior of the plasmasprayed coatings was determined by monitoring the thermal conductivity increases during a 20-hour test period at various temperatures. Durability and failure mechanisms of the HfO2-Y2O3 and La2Zr2O7 coatings on mullite/SiC hexoloy or SiC/SiC CMC substrates were investigated at 1650 C under thermal gradient cyclic conditions. Coating design and testing issues for the 1650 C thermal/environmental barrier coating applications are also discussed.

Zhu, Dong-Ming↗
DOE OSTI2020

Materials Data on La2Zr2O7 by Materials Project

La2Zr2O7 crystallizes in the cubic Fd-3m space group. The structure is three-dimensional. La3+ is bonded in a distorted body-centered cubic geometry to eight O2- atoms. There are two shorter (2.36 Å) and six longer (2.65 Å) La–O bond lengths. Zr4+ is bonded to six equivalent O2- atoms to form corner-sharing ZrO6 octahedra. The corner-sharing octahedral tilt angles are 50°. All Zr–O bond lengths are 2.13 Å. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded to four equivalent La3+ atoms to form corner-sharing OLa4 tetrahedra. In the second O2- site, O2- is bonded in a 4-coordinate geometry to two equivalent La3+ and two equivalent Zr4+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on La2Zr2O7 by Materials Project

La2Zr2O7 crystallizes in the cubic Fd-3m space group. The structure is three-dimensional. La3+ is bonded in a distorted hexagonal planar geometry to six equivalent O2- atoms. All La–O bond lengths are 2.38 Å. Zr4+ is bonded in a body-centered cubic geometry to eight O2- atoms. There are six shorter (2.28 Å) and two longer (2.33 Å) Zr–O bond lengths. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded to two equivalent La3+ and two equivalent Zr4+ atoms to form a mixture of edge and corner-sharing OLa2Zr2 tetrahedra. In the second O2- site, O2- is bonded to four equivalent Zr4+ atoms to form a mixture of edge and corner-sharing OZr4 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on La2Zr2O7 by Materials Project

La2Zr2O7 crystallizes in the orthorhombic Pmma space group. The structure is three-dimensional. there are two inequivalent La3+ sites. In the first La3+ site, La3+ is bonded to six O2- atoms to form a mixture of distorted edge and corner-sharing LaO6 octahedra. The corner-sharing octahedral tilt angles are 75°. There are four shorter (2.40 Å) and two longer (2.43 Å) La–O bond lengths. In the second La3+ site, La3+ is bonded in a 6-coordinate geometry to six O2- atoms. There are a spread of La–O bond distances ranging from 2.29–2.45 Å. There are two inequivalent Zr4+ sites. In the first Zr4+ site, Zr4+ is bonded in a body-centered cubic geometry to eight O2- atoms. There are a spread of Zr–O bond distances ranging from 2.17–2.63 Å. In the second Zr4+ site, Zr4+ is bonded in a body-centered cubic geometry to eight O2- atoms. There are a spread of Zr–O bond distances ranging from 2.21–2.44 Å. There are five inequivalent O2- sites. In the first O2- site, O2- is bonded to two equivalent La3+ and two equivalent Zr4+ atoms to form OLa2Zr2 tetrahedra that share corners with fourteen OLa2Zr2 tetrahedra and edges with five OLa3Zr tetrahedra. In the second O2- site, O2- is bonded to three La3+ and one Zr4+ atom to form a mixture of distorted edge and corner-sharing OLa3Zr tetrahedra. In the third O2- site, O2- is bonded to two equivalent La3+ and two equivalent Zr4+ atoms to form OLa2Zr2 tetrahedra that share corners with fourteen OLa3Zr tetrahedra and edges with five OZr4 tetrahedra. In the fourth O2- site, O2- is bonded to four Zr4+ atoms to form a mixture of distorted edge and corner-sharing OZr4 tetrahedra. In the fifth O2- site, O2- is bonded to one La3+ and three Zr4+ atoms to form OLaZr3 tetrahedra that share corners with fourteen OLa2Zr2 tetrahedra and edges with six OLa3Zr tetrahedra.

36 MATERIALS SCIENCE↗
NASA NTRS2004

Thermal Conductivity and Water Vapor Stability of Ceramic HfO2-Based Coating Materials

HfO2-Y2O3 and La2Zr2O7 are candidate thermal/environmental barrier coating materials for gas turbine ceramic matrix composite (CMC) combustor liner applications because of their relatively low thermal conductivity and high temperature capability. In this paper, thermal conductivity and high temperature phase stability of plasma-sprayed coatings and/or hot-pressed HfO2-5mol%Y2O3, HfO2-15mol%Y2O3 and La2Zr2O7 were evaluated at temperatures up to 1700 C using a steady-state laser heat-flux technique. Sintering behavior of the plasma-sprayed coatings was determined by monitoring the thermal conductivity increases during a 20-hour test period at various temperatures. Durability and failure mechanisms of the HfO2-Y2O3 and La2Zr2O7 coatings on mullite/SiC Hexoloy or CMC substrates were investigated at 1650 C under thermal gradient cyclic conditions. Coating design and testing issues for the 1650 C thermal/environmental barrier coating applications will also be discussed.

Zhu, Dong-Ming↗
NASA NTRS2006

Effects of Doping on Thermal Conductivity of Pyrochlore Oxides for Advanced Thermal Barrier Coatings

Pyrochlore oxides of general composition, A2B2O7, where A is a 3(+) cation (La to Lu) and B is a 4(+) cation (Zr, Hf, Ti, etc.) have high melting point, relatively high coefficient of thermal expansion, and low thermal conductivity which make them suitable for applications as high-temperature thermal barrier coatings. The effect of doping at the A site on the thermal conductivity of a pyrochlore oxide La2Zr2O7, has been investigated. Oxide powders of various compositions La2Zr2O7, La(1.7)Gd(0.3)Zr2O7, La(1.7)Yb(0.3)Zr2O7 and La(1.7)Gd(0.15)Yb(0.15)Zr2O7 were synthesized by the citric acid sol-gel method. These powders were hot pressed into discs and used for thermal conductivity measurements using a steady-state laser heat flux test technique. The rare earth oxide doped pyrochlores La(1.7)Gd(0.3)Zr2O7, La(1.7)Yb(0.3)Zr2O7 and La(1.7)Gd(0.15)Yb(0.15)Zr2O7 had lower thermal conductivity than the un-doped La2Zr2O7. The Gd2O3 and Yb2O3 co-doped composition showed the lowest thermal conductivity.

Bansal, Narottam P.↗