Mass transfer control in multilayer EBC systems at high temperatures

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Microstructure control of multi-layered EBC prepared by dual electron beam PVD

Environmental barrier coatings (EBCs) can play important roles in enabling SiC fiber reinforced SiC matrix composites (SiC/SiC) for application to advanced hot-section components in airplane engines. EBCs must exhibit superior environmental shielding durability and excellent volatilization resistance in the combustion environment containing water vapor. Therefore, a multilayered structure is applied when designing EBCs. Yb silicates are promising substances for EBC use. Volatilization resistance of Yb2SiO5 is superior to Yb2Si2O7. Thermal expansion coefficient of Yb2Si2O7 is closer to SiC/SiC composites than Yb2SiO5 and it exhibits a single phase up to about 1873 K. Mullite has higher oxygen shielding performance than Yb2Si2O7. Thus, we design EBC which is composed of a bond layer, dense mullite oxygen shielding layer, compositional-gradient dense Yb silicate layer showing water vapor shielding function, and Yb2SiO5 layer with segmented structure. The function of the Yb2SiO5 layer is reduction of thermal stresses during thermal cycling. Such an environmental shielding capability depends greatly on both the compositions and microstructures of the layers. Therefore, the coating processes used to make the EBCs should allow precise control of these factors. While it is difficult to strictly control the compositions of such complex oxides using conventional plasma spray deposition techniques due to incongruent evaporation of raw powders, dual electron beam physical vapor deposition (EB-PVD) is a potential process for constructing the complex oxides layer with controlled compositions as well as microstructures. We recently reported that formation of Yb2Si2O7 layer by dual EB-PVD [1]. In the present study, we investigated the in-situ formation of a dense mullite layer, compositional gradient layer from Yb2Si2O7 to Yb2SiO5 with dense structure, and Yb2SiO5 layer with segmented structure via dual EB-PVD, simultaneously heating the substrate. Please click Additional Files below to see the full abstract

Residual stress measurement of YB silicates by Raman Spectroscopy: First-principles and experimental studies

Components of next-generation gas turbines made from lightweight SiC-based ceramics need environmental barrier coatings (EBCs) to protect from water vapor at high temperature because Si-based ceramics vaporize in such environments. Yb silicates Yb2SiO5 and Yb2Si2O7 are promising EBC materials. In EBCs, residual stresses develop during thermal cycling due to mismatch between the thermal expansion coefficients of the silicate and the underlying ceramics, resulting in critical fatigue of the coating structure [1]. Raman microscopy is one method for measuring stress distributions in coating materials and has the potential to be used for diagnosing EBCs. Its suitability for analyzing stress states of Yb silicates has been unknown. In this study, we examine Raman spectra of Yb2SiO5, and Yb2Si2O7 under hydrostatic pressure based on first-principles calculations based on the density functional theory and we also examine the spectra of Yb2Si2O7 under uniaxial compressive stress in experiments using polycrystalline samples. When no external pressures applied, good agreement between calculated and experimental spectra is obtained as shown in Figure 1. The differences in the spectra between the silicates demonstrate the utility of using Raman microscopy to detect compositional changes in Yb-silicate coatings. From the calculations, lattice vibrations associated with a Raman peak are identified as exemplified by the characteristic mode of Si2O7 units in Yb2Si2O7 shown in figure 1(a). Please click Additional Files below to see the full abstract

Advanced design of EBC based on mass-transfer mechanisms in oxides under oxygen potential gradients at high temperatures

An environmental barrier coating (EBC) must exhibit excellent oxygen/water vapor shielding and thermomechanical durability in severe combustion environments. Thus, a multilayered structure, in which the individual layers have a particular function, is generally adopted when designing EBCs to enhance their overall performance. As an example, some EBCs incorporate a bonding layer on a SiC/SiC substrate, followed by an oxygen shielding layer and a water vapor shielding layer. The top coating of such systems is fabricated with a segmented structure to reduce thermal stresses during temperature cycling. Naturally, the oxide layers that provide the excellent gas shielding required for EBCs, which should be achieved by using fully dense coatings, are exposed to a large oxygen chemical potential gradient (dµO) at high temperatures. This results in an inward diffusion of oxygen ions and an outward diffusion of cations, as described by the Gibbs-Duhem equation. It should be noted that cation transport induces decomposition of the oxides and collapse of the layered EBC structure. Therefore, to develop robust EBCs with excellent gas shielding, it is very important to elucidate and control mass transfer within them. However, quantitative information on the oxygen shielding and mass transfer mechanisms of EBC candidate materials remains insufficient, and in particular, no diffusion data under a dµO has been reported. In the present study, as a prelude to improving the environmental shielding and structural stability of EBCs, we evaluated the oxygen permeability of polycrystalline Yb2Si2O7 (Y2S2) and Al4+2xSi2-2xO10-x (mullite) wafers, which served as models for the EBC layers, using an oxygen tracer gas (18O2) [1,2]. Wafers with thicknesses of several hundred micrometers were exposed to a dµO at high temperatures, with each surface of the wafer deliberately subjected to a different oxygen partial pressure. Oxygen permeation occurred along grain boundaries (GBs) in both oxides. For Y2S2, the oxygen permeability constant normalized by the GB density, which was independent of grain size, was about ten times larger than that for mullite. However, the water vapor volatilization resistance of Y2S2 is significantly better than that of mullite. Hence, Y2S2 is suitable as the water vapor shielding layer, and mullite is suitable as the underlying oxygen shielding layer. Oxygen permeation occurred by GB diffusion of oxygen ions from the high-oxygen-partial-pressure (high-PO2) surface to the low-PO2 surface, with simultaneous GB diffusion in the opposite direction of Yb ions for Y2S2 and Al ions for mullite. Oxygen permeation related to the GB diffusion of Si ions in both oxides was negligibly small compared to that due to the GB diffusion of other cations, resulting in structural instability of the oxides near both PO2 surfaces. The oxygen and cation fluxes at the outflow side in the oxides were significantly larger than those at the inflow side, in accordance with dominant cation transport at the high-PO2 side and dominant oxygen transport at the low-PO2 side. The structural stability of a mullite layer under a dµO can be improved by utilizing an underlying bonding layer that can function as an Al reservoir. Thus, even if outward diffusion of Al ions occurs in the mullite layer, the Al-containing bonding layer can supply Al ions to the Al-deficient zone in the mullite layer. Another approach is to utilize an upper Y2S2 water vapor shielding layer with limited oxygen shielding. In other words, if the PO2 at the interface between the mullite and Y2S2 layers, which is in equilibrium with the corresponding µO, falls below 10-5 Pa at 1673 K, the Al flux in the mullite layer will be significantly reduced. Structural stabilization of the Y2S2 layer can also be achieved by decreasing the driving force for outward diffusion of Yb ions in the Y2S2 layer to set an upper layer of Yb2SiO5. [1] S. Kitaoka et al., J.Am.Ceram.Soc., DOI: 10.1111/jace.14834 [2] M. Wada et al, Acta Materialia, 135, 372 (2017

Three-dimensional alignment of cellulose II microcrystals under a strong magnetic field

In this study, enzymatic synthesis was conducted using cellodextrin phosphorylase (CDP), sucrose phosphorylase (SP), and sucrose with 1-azido-1-deoxy-β-glucoside (β-glucosyl azide) as the acceptor in phosphate buffer at pH 7.0. This yielded cellulose oligomers (degree of polymerization, DP ≈ 10) with azido groups at the reducing end as a white precipitate. A suspension of cellulose microcrystals with exposed azido groups on the surface was obtained via dissolution and recrystallization of the synthetic products dispersed in water by heating. The flat, ribbon-like cellulose microcrystals were a crystalline form of cellulose II and were several micrometers in length and several hundred nanometers in width. The microcrystals were 5.1–5.2 nm thick, which is equivalent to the chain length of cellulose oligomers with DP ≈ 10. When the cellulose II microcrystal suspensions were dried under a horizontal static magnetic field of 8 T, oriented films were obtained, wherein the microcrystals were aligned three-dimensionally. Synchrotron X-ray diffraction studies of the films revealed that the easy and intermediate axes (χ₁ and χ₂, respectively) of the cellulose II crystals corresponded approximately to the [1 1 0] and [1 ̅₁ 0] directions, respectively

Perturbation study on the spin and charge susceptibilities of the two-dimensional Hubbard model

We investigate the spin and charge susceptibilities of the two-dimensional Hubbard model based upon the perturbative calculation in the strength of correlation $U$. For $U$ comparable to a bare bandwidth, the charge susceptibility decreases near the half-filling as hole-doping approaches zero. This behavior suggesting the precursor of the Mott-Hubbard gap formation cannot be obtained without the vertex corrections beyond the random phase approximation. In the low-temperature region, the spin susceptibility deviates from the Curie-Weiss-like law and finally turns to decrease with the decrease of temperature. This spin-gap-like behavior is originating from the van Hove singularity in the density of states.Comment: Revtex file + 11 figures, to appear in Phys. Rev.