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All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on Y2Si2O7 by Materials Project

Y2Si2O7 crystallizes in the monoclinic P2_1/m space group. The structure is three-dimensional. there are two inequivalent Y3+ sites. In the first Y3+ site, Y3+ is bonded to eight O2- atoms to form distorted YO8 hexagonal bipyramids that share corners with two equivalent SiO4 tetrahedra, edges with two equivalent YO8 hexagonal bipyramids, edges with two equivalent YO6 octahedra, and edges with four SiO4 tetrahedra. There are a spread of Y–O bond distances ranging from 2.33–2.61 Å. In the second Y3+ site, Y3+ is bonded to six O2- atoms to form YO6 octahedra that share corners with two equivalent YO6 octahedra, corners with six SiO4 tetrahedra, and edges with two equivalent YO8 hexagonal bipyramids. The corner-sharing octahedral tilt angles are 59°. There are four shorter (2.26 Å) and two longer (2.34 Å) Y–O bond lengths. There are two inequivalent Si4+ sites. In the first Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with two equivalent YO8 hexagonal bipyramids, corners with two equivalent YO6 octahedra, a cornercorner with one SiO4 tetrahedra, and edges with two equivalent YO8 hexagonal bipyramids. The corner-sharing octahedral tilt angles are 45°. There is three shorter (1.63 Å) and one longer (1.66 Å) Si–O bond length. In the second Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with four equivalent YO6 octahedra, a cornercorner with one SiO4 tetrahedra, and edges with two equivalent YO8 hexagonal bipyramids. The corner-sharing octahedra tilt angles range from 45–61°. There is three shorter (1.63 Å) and one longer (1.67 Å) Si–O bond length. There are five inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted bent 150 degrees geometry to two equivalent Y3+ and two Si4+ atoms. In the second O2- site, O2- is bonded in a trigonal planar geometry to two equivalent Y3+ and one Si4+ atom. In the third O2- site, O2- is bonded in a 3-coordinate geometry to two Y3+ and one Si4+ atom. In the fourth O2- site, O2- is bonded in a trigonal planar geometry to two equivalent Y3+ and one Si4+ atom. In the fifth O2- site, O2- is bonded in a 3-coordinate geometry to two Y3+ and one Si4+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Y2Si2O7 by Materials Project

Y2Si2O7 crystallizes in the monoclinic P2_1/m space group. The structure is three-dimensional. Y3+ is bonded to seven O2- atoms to form distorted YO7 pentagonal bipyramids that share a cornercorner with one YO7 pentagonal bipyramid, corners with four SiO4 tetrahedra, edges with three equivalent YO7 pentagonal bipyramids, and edges with two SiO4 tetrahedra. There are a spread of Y–O bond distances ranging from 2.23–2.50 Å. There are two inequivalent Si4+ sites. In the first Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with four equivalent YO7 pentagonal bipyramids, a cornercorner with one SiO4 tetrahedra, and edges with two equivalent YO7 pentagonal bipyramids. There are a spread of Si–O bond distances ranging from 1.62–1.70 Å. In the second Si4+ site, Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with four equivalent YO7 pentagonal bipyramids, a cornercorner with one SiO4 tetrahedra, and edges with two equivalent YO7 pentagonal bipyramids. There is three shorter (1.63 Å) and one longer (1.69 Å) Si–O bond length. There are five inequivalent O2- sites. In the first O2- site, O2- is bonded in a 3-coordinate geometry to two equivalent Y3+ and one Si4+ atom. In the second O2- site, O2- is bonded in a 3-coordinate geometry to two equivalent Y3+ and one Si4+ atom. In the third O2- site, O2- is bonded in a distorted bent 150 degrees geometry to two equivalent Y3+ and two Si4+ atoms. In the fourth O2- site, O2- is bonded in a trigonal planar geometry to two equivalent Y3+ and one Si4+ atom. In the fifth O2- site, O2- is bonded in a trigonal planar geometry to two equivalent Y3+ and one Si4+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Y2Si2O7 by Materials Project

Y2Si2O7 crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. Y3+ is bonded to six O2- atoms to form distorted YO6 octahedra that share corners with six equivalent SiO4 tetrahedra and edges with three equivalent YO6 octahedra. There are four shorter (2.27 Å) and two longer (2.34 Å) Y–O bond lengths. Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with six equivalent YO6 octahedra and a cornercorner with one SiO4 tetrahedra. The corner-sharing octahedra tilt angles range from 49–57°. There is one shorter (1.64 Å) and three longer (1.65 Å) Si–O bond length. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted trigonal planar geometry to two equivalent Y3+ and one Si4+ atom. In the second O2- site, O2- is bonded in a linear geometry to two equivalent Si4+ atoms. In the third O2- site, O2- is bonded in a distorted trigonal planar geometry to two equivalent Y3+ and one Si4+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Y2Si2O7 by Materials Project

Y2Si2O7 crystallizes in the monoclinic P2_1/c space group. The structure is three-dimensional. Y3+ is bonded to six O2- atoms to form distorted YO6 octahedra that share corners with six equivalent SiO4 tetrahedra and edges with three equivalent YO6 octahedra. There are a spread of Y–O bond distances ranging from 2.27–2.35 Å. Si4+ is bonded to four O2- atoms to form SiO4 tetrahedra that share corners with six equivalent YO6 octahedra and a cornercorner with one SiO4 tetrahedra. The corner-sharing octahedra tilt angles range from 42–61°. There are a spread of Si–O bond distances ranging from 1.64–1.66 Å. There are four inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted trigonal planar geometry to two equivalent Y3+ and one Si4+ atom. In the second O2- site, O2- is bonded in a 3-coordinate geometry to two equivalent Y3+ and one Si4+ atom. In the third O2- site, O2- is bonded in a linear geometry to two equivalent Si4+ atoms. In the fourth O2- site, O2- is bonded in a 3-coordinate geometry to two equivalent Y3+ and one Si4+ atom.

36 MATERIALS SCIENCE↗
NASA NTRSDate not supplied

A Machine Learning-Derived Atomistic Potential for Y2Si2O7

Incorporation of SiC/SiC ceramic matrix composite (CMC) hot section components into aircraft engines promises to increase efficiency and safety. However, SiC/SiC CMCs are subject to water vapor-induced oxidation and recession at the high temperatures of engine operation, and thus environmental barrier coatings (EBCs) are required to reduce this degradation and enable their widespread adoption. An understanding of EBCs failure mechanisms, including thermochemical and thermomechanical mechanisms, is essential as coating degradation leads to reduced CMC component service life. Computational modeling approaches can provide insight into EBC material properties important for coating design. However, density functional theory (DFT) is computationally expensive and atomistic potentials are lacking for materials of interest. In this work, we utilize a machine learning approach and DFT training data to parameterize atomistic potentials for two candidate EBC materials, Y2Si2O7 and Yb2Si2O7. These potentials enable near DFT-accurate calculations of thermodynamic and thermomechanical properties essential to EBC design.

Cameron J Bodenschatz↗
NASA NTRS2023

Influence of Cation Species on Thermal Expansion of Y2Si2O7–Gd2Si2O7 Solid Solutions

Mixtures of Y 2 Si 2 O 7 and Gd 2 Si 2 O 7 were synthesized by solid-state reaction at 1600°C and characterized via in situ x-ray diffraction (XRD) to determine their coefficients of thermal expansion (CTE). All solid solutions within the system exhibited the orthorhombic δ-RE 2 Si 2 O 7 (Pna2 1 ) structure. Thermal expansion measurements of Y 2 Si 2 O 7 and Gd 2 Si 2 O 7 correlated well with reported values in literature, and all synthesized solid solutions exhibited CTEs between Y 2 Si 2 O 7 and Gd 2 Si 2 O 7 . Generally, there was a slight decrease in CTE exhibited by the materials with increasing Gd 2 Si 2 O 7 content, with Gd 2 Si 2 O 7 having the lowest CTEs and Y 2 Si 2 O 7 the highest CTEs. The decrease in CTE was attributed to stronger bonds of Gd-O over Y-O, as determined by calculated crystal orbital Hamilton populations using density functional theory. However, such differences were very small and crystal structure was the dominating factor in CTE trends.

rare earth silicates↗
NASA NTRS2021

Thermal Expansion Coefficients of Ca2Y8(SiO4)6O2 and Ca2Yb8(SiO4)6O2 Apatite-type Silicates

High temperature X-ray diffraction (XRD) scans of Y2Si2O7 and Yb2Si2O7 reactions with calcium-magnesium-aluminosilicates (CMAS) were utilized to determine thermal expansion coefficients (CTEs) of Ca2Y8(SiO4)6O2 and Ca2Yb8(SiO4)6O2 apatite materials. In order to validate these measurements, the CTEs of Y2Si2O7, Yb2Si2O7 and SiO2 were also determined from the same scans. The directional CTEs for γ-Y2Si2O7 were determined to be αa=5.84×10-6/K, αb=6.81×10-6/K and αc=0.81×10-6/K, and β-Yb2Si2O7 was determined to have values of αa=6.89×10-6/K, αb=4.81×10-6/K and αc=2.78×10-6/K. The average CTEs of γ-Y2Si2O7 and β-Yb2Si2O7 were determined to be 4.5×10-6/K and 4.7×10-6/K, respectively, which agreed with previous analyses. Ca2Y8(SiO4)6O2 exhibited directional CTEs of αa=9.36×10-6/K and αc=7.95×10-6/K (averaged between two sets of data), whereas Ca2Yb8(SiO4)6O2 had values that were very similar (αa=9.63×10-6/K, αc=7.45×10-6/K). Both results for the Ca2RE8(SiO4)6O2 apatites correlated well with the limited data on apatite-type silicates available in literature.

rare earth↗
NASA NTRS1989

Microstructural changes in beta-silicon nitride grains upon crystallizing the grain-boundary glass

Crystallizing the grain-boundary glass of a liquid-phase-sintered Si3N4 ceramic for 2 h or less at 1500 C led to formation of delta-Y2Si2O7. After 5 h at 1500 C, the delta-Y2Si2O7 had transformed to beta-Y2Si2O7 with a concurrent dramatic increase in dislocation density within beta-Si3N4 grains. Reasons for the increased dislocation density are discussed. Annealing for 20 h at 1500 C reduced dislocation densities to the levels found in as-sintered material.

Lee, William E.↗
NASA NTRS1991

Microstructural changes in Beta-silicon nitride grains upon crystallizing the grain-boundary glass

Crystallizing the grain boundary glass of a liquid phase sintered Si3N4 ceramic for 2 h or less at 1500 C led to formation of gamma Y2Si2O7. After 5 h at 1500 C, the gamma Y2Si2O7 had transformed to beta Y2Si2O7 with a concurrent dramatic increase in dislocation density within beta Si3N4 grains. Reasons for the increased dislocation density is discussed. Annealing for 20 h at 1500 C reduced dislocation densities to the levels found in as-sintered materials.

Lee, William E.↗
NASA NTRS1988

Crystallization and characterization of Y2O3-SiO2 glasses

Glasses in the yttria-silica system with 20-40 mol pct Y2O3 have been subjected to recrystallization studies after melting at 1900-2100 C in W crucibles in 1 and 50 atm N2. The TEM and XRD results obtained indicate the presence of the delta, gamma, gamma-prime, and beta-Y2Si2O7 crystalline phases, depending on melting and quenching conditions. Heat-treatment in air at 1100-1600 C increased the amount of crystallization, and led to the formation of Y2SiO5, cristobalite, and polymorphs of Y2Si2O7. Also investigated were the effects of 5 and 10 wt pct zirconia additions.

Drummond, C. H., III↗
NASA NTRS1991

Crystallization and characterization of Y2O3-SiO2 glasses

Glasses in the yttria-silica system with 20 to 40 mol pct Y2O3 were subjected to recrystallization studies after melting at 1900 to 2100 C in W crucibles in 1 and 50 atm N2. The TEM and XRD results obtained indicate the presence of the delta, gamma, gamma prime, and beta-Y2Si2O7 crystalline phases, depending on melting and quenching conditions. Heat treatment in air at 1100 to 1600 C increased the amount of crystallization, and led to the formation of Y2SiO5, cristabalite, and polymorphs of Y2Si2O7. Also investigated were the effects of 5 and 10 wt pct zirconia additions.

Drummond, Charles H., III↗
NASA NTRSDate not supplied

Machine Learning Approaches for Rare-Earth Silicate Environmental Barrier Coating Thermochemical and Thermomechanical Property Predictions

Environmental barrier coatings (EBCs) are a necessary enabling technology for the transition from superalloys to silicon carbide (SiC) ceramic matrix composites (CMCs) in gas turbine engines for increased efficiency and decreased fuel costs. SiC-based CMCs are prone to oxidation-based degradation in the engine hot section, and rare-earth (RE) silicates are promising candidates for EBCs due to their close thermal expansion match to the composite substrate and oxidation resistance. However, the design of EBCs is hindered by the large chemical space of candidate materials and the difficulty in obtaining material properties for engineering optimization. This is especially difficult as research continues into mixed-cation or “high-entropy” RE silicates. First-principles computational methods such as density functional theory (DFT) are highly effective at calculating material properties to guide coating design but are limited by their computational cost. Atomistic simulations have the potential to both accelerate property calculations and expand the properties able to be calculated due to their lower computational compared to DFT. However, they require interatomic potentials (IAPs) specific to the material system of interest, and, to our knowledge, there are no suitable IAPs for RE silicates. Machine learning (ML) is a promising technique to accelerate material property predictions indirectly by generating IAPs for atomistic simulations or via direct prediction. In this work, we present two ML approaches to accelerate the calculation of RE silicate properties relevant to EBC design: 1) a ML-derived interatomic potential (IAP) for atomistic simulations of yttrium disilicate (Y2Si2O7) from DFT training data, and 2) a neural network (NN) model to directly predict thermochemical properties of RE silicates and oxides directly from easily obtainable unit cell parameters. Classical MD simulations using the IAP yield lattice properties and bond lengths in good agreement with both DFT and experimental results from x-ray diffraction. Thermodynamic properties calculated using the finite-displacement phonon method and quasi-harmonic approximation were orders of magnitude faster than DFT with good agreement to the DFT results. The IAP was also used to calculate properties such as coefficient of thermal expansion (CTE) that require large simulation supercells and are therefore difficult with DFT. The IAP correctly predicted the anisotropic nature of the CTE in three different phases of Y2Si2O7. The NN model predicts constant pressure heat capacity, Cp, orders of magnitude faster than DFT calculations, which can enable its use as a surrogate model for multiscale simulations. The two methods presented in this work demonstrate the utility of ML for accelerating the prediction of RE silicate properties, which can in turn accelerate EBC design and optimization.

machine learning↗
DOE OSTI2024

Accelerated Design of Cost-Effective Thermal/Environmental Barrier Coatings based on High-Entropy Rare Earth Disilicates: A First-Principles Study

This project aims to design cost-effective thermal/environmental barrier coatings (TEBC) based on high entropy rare earth disilicates to protect SiC-based ceramic matrix composites from chemical and thermal attack for better performance of components in the hot section of gas turbine engines. To accelerate the alloy design, we utilize first-principles density functional theory (DFT) together with combinatorial chemistry methodology to predict key properties including phase stability, apparent bulk coefficient of thermal expansion (ABCTE), intrinsic lattice thermal conductivity, and temperature-dependent elastic constants. Specifically, this project focuses on β-RE2Si2O7 (RE=Yb, Y, Er, Lu, La, Ce,) with β-Yb2Si2O7 and β-Y2Si2O7 as the benchmark. Our DFT calculations predict that Er1/4Lu1/4Y3/4Yb3/4Si2O7 and Er1/2Lu1/2Y1/2Yb1/2Si2O7 have ultralow lattice thermal conductivity < 0.23 W/m/K at 1500 K and a good match of average ABCTE (5.1 - 5.2×10-6 K-1) with SiC. Owing to the low cost and abundant supply of Ce and La, the A- and G-La2Si2O7/Ce2Si2O7 disilicates are also studied. Our study shows that G-phase Ce2Si2O7 has an ultralow thermal conductivity (0.26 W/m/K at 1500 K) and the apparent bulk ABCTE (≈6.9×10-6 K-1) slightly higher than SiC, demonstrating great potential as low-cost high-performance T/EBC. However, La2Si2O7 and Ce2Si2O7 undergo an A-phase to G-phase polymorphic transition at around 1470 K.

environmental barrier coatings↗
NASA NTRS1986

Effect of grain-boundary crystallization on the high-temperature strength of silicon nitride

Si3N4 specimens having the composition 88.7 wt pct Si3N4-4.9 wt pct SiO2-6.4 wt pct Y2O3 were sintered at 2140 C under 25 atm N2 for 1 h and then subjected to a 5 h anneal at 1500 C. Crystallization of an amorphous grain-boundary phase resulted in the formation of Y2Si2O7. The short-time 1370 C strength of this material was compared with that of material of the same composition having no annealing treatment. No change in strength was noted. This is attributed to the refractory nature of the yttrium-rich grain-boundary phase (apparently identical in both glassy and crystalline phases) and the subsequent domination of the failure process by common processing flaws.

Pierce, L. A.↗
NASA NTRS1991

Glass formation and crystallization in high-temperature glass-ceramics and Si3N4

The softening of glassy grain boundaries in ceramic matrix composites and Si3N4 at high temperatures reduces mechanical strength and the upper-use temperature. By crystallizing this glass to a more refractory crystalline phase, a material which performs at higher temperatures may result. Three systems were examined: a cordierite composition with ZrO2 as a nucleating agent; celsian compositions; and yttrium silicate glasses both in bulk and intergranular in Si3N4. For the cordierite compositions, a series of metastable phases was obtained. The crystallization of these compositions was summarized in terms of metastable ternary isothermal sections. Zircon formed at the expense of ZrO2 and spinel. In SiC composites, the transformations were slower. In celsian, two polymorphs were crystallized. One phase, hexacelsian, which always crystallized, even when metastable, had an undesirable volume change. The other phase, celsian, was very difficult to crystallize. In yttrium silicate bulk glasses, similar in composition to the intergranular glass in Si3N4, a number of polymorphs of Y2Si2O7 were crystallized. The conditions under which these polymorphs formed are compared with crystallization in Si3N4.

Drummond, Charles H., III↗
NASA NTRS2013

Calcium-Magnesium-Aluminosilicate (CMAS) Reactions and Degradation Mechanisms of Advanced Environmental Barrier Coatings

The thermochemical reactions between calcium-magnesium-aluminosilicate- (CMAS-) based road sand and several advanced turbine engine environmental barrier coating (EBC) materials were studied. The phase stability, reaction kinetics and degradation mechanisms of rare earth (RE)-silicates Yb2SiO5, Y2Si2O7, and RE-oxide doped HfO2 and ZrO2 under the CMAS infiltration condition at 1500 C were investigated, and the microstructure and phase characteristics of CMAS-EBC specimens were examined using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). Experimental results showed that the CMAS dissolved RE-silicates to form crystalline, highly non-stoichiometric apatite phases, and in particular attacking the silicate grain boundaries. Cross-section images show that the CMAS reacted with specimens and deeply penetrated into the EBC grain boundaries and formed extensive low-melting eutectic phases, causing grain boundary recession with increasing testing time in the silicate materials. The preliminary results also showed that CMAS reactions also formed low melting grain boundary phases in the higher concentration RE-oxide doped HfO2 systems. The effect of the test temperature on CMAS reactions of the EBC materials will also be discussed. The faster diffusion exhibited by apatite and RE-doped oxide phases and the formation of extensive grain boundary low-melting phases may limit the CMAS resistance of some of the environmental barrier coatings at high temperatures.

Ahlborg, Nadia L.↗
NASA NTRS2019

Thermochemistry of Protective Coatings and Molten Silicate Debris

The durability of gas-turbine engine components can be significantly affected by the ingestion of siliceous particles, which can melt at high temperature and corrode protective coatings that are essential for long life requirements. The silicate debris consists mainly of CaO-MgO-Al2O3-SiO2 (CMAS) and is usually ingested by aircraft engines during and after take-off, sticking to their hot surfaces and resulting in the formation of calcium rare-earth silicate oxyapatites. The thermochemistry of coatings and their reaction products with molten silicate debris are crucial to understand in order to improve the durability of gas-turbine engines. Here we discuss results of high temperature drop solution calorimetry, drop-and-catch calorimetry (DnC) and differential thermal analysis (DTA) techniques for the thermodynamic properties of both thermal barrier coatings (TBCs) and environmental barrier coatings (EBCs) and their reaction with CMAS compositions. The enthalpies of solution of Y2Si2O7, Yb2Si2O7, 31YSZ, and 16RESZ based coatings and the oxyapatite are moderately positive. However, oxyapatite formation is only favorable over coating dissolution in terms of enthalpy for 7YSZ. The enthalpies of mixing between the coatings and the molten silicate are less exothermic for Yb2Si2O7 and CaYb4Si3O13 than for 7YSZ, indicating lower energetic stability of the latter against molten silicate corrosion. We also report for the first time the calorimetric measurements of the enthalpies of formation of rare-earth silicate based EBC coatings and oxyapatites (rare-earth, RE = Y, Yb, Gd, Dy, Er, Nd and Sm).

Costa, Gustavo↗
DOE OSTI2023

Influence of cation species on thermal expansion of Y 2 Si 2 O 7 –Gd 2 Si 2 O 7 solid solutions

Mixtures of Y 2 Si 2 O 7 and Gd 2 Si 2 O 7 were synthesized by solid-state reaction at 1600°C and characterized via in situ x-ray diffraction (XRD) to determine their coefficients of thermal expansion (CTE). All solid solutions within the system exhibited the orthorhombic δ-RE 2 Si 2 O 7 (Pna2 1 ) structure. Thermal expansion measurements of Y 2 Si 2 O 7 and Gd 2 Si 2 O 7 correlated well with reported values in literature, and all synthesized solid solutions exhibited CTEs between Y 2 Si 2 O 7 and Gd 2 Si 2 O 7 . Generally, there was a slight decrease in CTE exhibited by the materials with increasing Gd 2 Si 2 O 7 content, with Gd 2 Si 2 O 7 having the lowest CTEs and Y 2 Si 2 O 7 the highest CTEs. Here, the decrease in CTE was attributed to stronger bonds of Gd-O over Y-O, as determined by calculated crystal orbital Hamilton populations using density functional theory. However, such differences were very small and crystal structure was the dominating factor in CTE trends.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗