•MSiO4 phases crystallize from the reaction of Hf/ZrO2 with acidic silicate melts.•Reactive crystallization is more favorable in HfO2 relative to ZrO2 based systems.•YO1.5 addition degrades MSiO4 ...formation in favor of non-equilibrium (M,Y)O2.•A secondary phase can constrain grain growth, preventing microcracking in HfO2.•Dissolution and diffusion kinetics of Hf4+ and Zr4+ are quantified and compared.
Hafnia is of interest in thermal and environmental barrier coatings, but little is known about its response to molten silicate attack. This article investigates that response using two model silicate melts, compares it with pure ZrO2 and examines the effect of YO1.5 additions. HfO2 was found to form HfSiO4 with acidic melts but undergoes grain boundary penetration in basic melts, which do not exhibit reactive crystallization. The latter can be exacerbated by microcracking resulting from the thermal expansion anisotropy of monoclinic HfO2. Y additions generally degrade the ability to form hafnon (and zircon), and exacerbate grain boundary penetration, especially in HfO2 where Y is present as a fluorite second phase. The fluorite controls grain growth in monoclinic HfO2 and suppresses microcracking, but dissolves faster, especially in basic melts. The results are presented in the context of the relevant thermodynamics and kinetics. The implications for coating applications are discussed.
Rare earth hafnates and zirconates are candidate materials for thermal and environmental barrier coatings (T/EBC) to protect gas turbine engine components from various environmental threats, ...including molten silicates derived from ingested mineral debris. This article examines the reactions of Gd2Hf2O7 (GHO) and Gd2Zr2O7 (GZO) to exemplary acidic and basic silicate melts. Exposure experiments at 1400°C reveal that both materials react to form mixed layers of apatite and fluorite. These layers largely hinder melt penetration of grain boundaries in GZO for exposures up to 4 h. However, extensive intergranular melt penetration occurs into GHO below the reaction layer within 1 h for the acidic melt and within 4 h for the basic melt. Shorter exposures (1–5 min) of Gd‐lean versions of the two compounds, viz. Gd0.2Hf0.8O1.9 and Gd0.2Zr0.8O1.9, are used to probe differences in the dissolution and diffusion rates. While both oxides form fluorite, the HfO2‐based one reacts more slowly than that based on ZrO2. Analysis of composition profiles across the solid/melt boundary reveals that Hf4+ diffuses more slowly than Zr4+ and that the hafnate dissolves more slowly than the zirconate; in both cases dissolution is diffusion‐controlled. The implications for the efficacy of reactive crystallization as a silicate mitigation strategy are discussed.
Ru-based B2 phases present an opportunity to design two-phase BCC + B2 refractory multi-principal element alloys (RMPEAs) with higher temperature stability compared to B2 phases observed in RMPEAs. ...In this investigation, seven equiatomic Ru-containing RMPEAs were characterized in the as-cast and annealed conditions. Of the two Hf-free alloys, Mo
25
Nb
25
Ta
25
Ru
25
was determined to be a single-phase B2 alloy and Mo
20
Nb
20
Ta
20
W
20
Ru
20
was single-phase BCC. Within all five Hf-containing alloys, phases formed during solidification included HfRu–B2, disordered BCC, and HfO
2
phases. The Hf-containing alloys also precipitated B2 nanoparticles within the BCC phases after further cooling in the solid. All phases were still present after annealing at 1500
∘
C to 1600
∘
C. The HfRu–B2 nanoparticles in as-cast Hf
20
Mo
20
Nb
20
Ta
20
Ru
20
were characterized by transmission electron microscopy (TEM), and a lattice misfit of < 1 pct between the BCC phase and B2 nanoparticles was calculated. Room-temperature micropillar compression tests were performed on BCC + B2 nanoparticle regions in annealed Hf
20
Mo
20
Nb
20
Ta
20
Ru
20
. Post-mortem TEM analysis revealed precipitate shearing by dislocations, resulting in paired dislocations, along with bowing of dislocations around precipitates. Utilizing the insights from this investigation, compositions for RMPEAs with solutionable B2 precipitates stable above 1200
∘
C are suggested.
A three-step method to create dense polycrystalline semiconductor thin films from nanocrystal liquid dispersions is described. First, suitable substrates are coated with nanocrystals using ...aerosol-jet printing. Second, the porous nanocrystal coatings are compacted using a weighted roller or a hydraulic press to increase the coating density. Finally, the resulting coating is annealed for grain growth. The approach is demonstrated for making polycrystalline films of copper zinc tin sulfide (CZTS), a new solar absorber composed of earth-abundant elements. The range of coating morphologies accessible through aerosol-jet printing is examined and their formation mechanisms are revealed. Crack-free albeit porous films are obtained if most of the solvent in the aerosolized dispersion droplets containing the nanocrystals evaporates before they impinge on the substrate. In this case, nanocrystals agglomerate in flight and arrive at the substrate as solid spherical agglomerates. These porous coatings are mechanically compacted, and the density of the coating increases with compaction pressure. Dense coatings annealed in sulfur produce large-grain (>1 μm) polycrystalline CZTS films with microstructure suitable for thin-film solar cells.
Thin polycrystalline films of the solar absorber copper–zinc–tin–sulfide (CZTS) were formed by annealing coatings deposited on molybdenum-coated soda lime glass via ultrasonic spraying of aerosol ...droplets from colloidal CZTS nanocrystal dispersions. Production of uniform continuous nanocrystal coatings with ultrasonic spraying requires that the evaporation time is longer than the aerosol flight time from the spray nozzle to the substrate such that the aerosol droplets still have low enough viscosity to smooth the impact craters that form on the coating surface. In this work, evaporation was slowed by adding a high boiling point cosolvent, cyclohexanone, to toluene as the dispersing liquid. We analyzed, quantitatively, the effects of the solvent composition on the aerosol and coating drying dynamics using an aerosol evaporation model. Annealing coatings in sulfur vapor converts them into polycrystalline films with micrometer size grains, but the grains form continuous films only when Na is present during annealing to enhance grain growth. Continuous films are easier to form when the average nanocrystal size is 15 nm: using larger nanocrystals (e.g., 20 nm) sacrifices film continuity.
•Experimental approach based on 1D diffusion couples betweenYSZand silicate melts.•YSZdissolves slowly; the relevant kinetics depend strongly on melt composition.•Finite transient whereinZr and Y ...build up at the interfacebefore reprecipitation.•Phase separation in YSZ yields new insight on destabilization mechanism under CMAS.•Dissolution within TBC porosity is largely controlled by interfacial detachment.
The degradation of thermal barrier coatings (TBCs) by molten silicates (CMAS) represents a fundamental barrier to progress in gas turbine technology, requiring a mechanistic understanding of the problem to guide the development of improved coatings. This article investigates the dissolution of yttria-stabilized zirconia (7YSZ and 20YSZ) into two model silicate melts at 1300–1400 °C. The approach involves the 1D dissolution of YSZ into a semi-infinite melt, characterizing the dissolution rates of YSZ and the diffusion rates of Zr4+ and Y3+ therein. The assessed kinetics of YSZ dissolution and diffusion were then applied to modeling the same phenomena on TBC-relevant length scales. These findings provide fundamental insight into (i) the dissolution mechanism of YSZ, (ii) the subsequent reprecipitation upon saturation, (iii) the quantitative effects of temperature and melt composition on the dissolution and diffusion kinetics, and (iv) how the measured kinetics manifests on the scale of flow channels present in TBCs.
The article examines the chemical interactions between HfO2/HfSiO4 composites and melts that originate from siliceous debris ingested into gas turbine engines. Pellets with hafnon volume fractions of ...50%, 70% and 100% were synthesized from powders of the pure components and exposed to two types of quinary siliceous deposits (one acidic and one basic) at 1400 °C for times ranging from 10 min to 4 h. Scanning and transmission electron microscopy examinations of reacted pellets show extensive melt penetration without evidence of an effective mitigating mechanism. Acidic melts preferentially react with hafnia to form hafnon while basic melts dissolve hafnon to form hafnia; in both cases, however, the melts penetrate extensively along grain and interphase boundaries. These processes are accompanied by swelling of the reaction layer followed by blistering and exfoliation of the affected coating material. The thermodynamics of the reactions, mechanisms of melt penetration, and implications for coating applications are discussed.
BackgroundNeural mechanisms may play an important role in non-eosinophilic asthma (NEA). This study compared airway sensory nerve reactivity, using capsaicin challenge, in eosinophilic asthma (EA) ...and NEA and non-asthmatics.MethodsThirty-eight asthmatics and 19 non-asthmatics (aged 14–21 years) underwent combined hypertonic saline challenge/sputum induction, fractional exhaled nitric oxide, atopy and spirometry tests, followed by capsaicin challenge. EA and NEA were defined using a sputum eosinophil cut-point of 2.5%. Airway hyperreactivity was defined as a ≥15% drop in FEV1 during saline challenge. Sensory nerve reactivity was defined as the lowest capsaicin concentration that evoked 5 (C5) coughs.ResultsNon-eosinophilic asthmatics (n=20) had heightened capsaicin sensitivity (lower C5) compared with non-asthmatics (n=19) (geometric mean C5: 58.3 µM, 95% CI 24.1 to 141.5 vs 193.6 µM, 82.2 to 456.0; p<0.05). NEA tended to also have greater capsaicin sensitivity than EA, with the difference in capsaicin sensitivity between NEA and EA being of similar magnitude (58.3 µM, 24.1 to 141.5 vs 191.0 µM, 70.9 to 514.0) to that observed between NEA and non-asthmatics; however, this did not reach statistical significance (p=0.07). FEV1 was significantly reduced from baseline following capsaicin inhalation in both asthmatics and non-asthmatics but no differences were found between subgroups. No associations with capsaicin sensitivity and atopy, sputum eosinophils, blood eosinophils, asthma control or treatment were observed.ConclusionNEA, but not EA, showed enhanced capsaicin sensitivity compared with non-asthmatics. Sensory nerve reactivity may therefore play an important role in the pathophysiology of NEA.
Enhancing the efficiency of gas turbine engines requires higher operation temperatures and the materials capable of surviving the increasingly challenging environment. Ceramic barrier coatings, with ...carefully engineered microstructures, protect the structural components within the hottest sections of the engine. However, these coatings are susceptible to damage mechanisms arises from the ingestion of siliceous debris, which can melt and deposit on the coating’s surface. Thermomechanical strains develop that are either mitigated or exacerbated by the thermochemical interactions between the coating and the melt. This work investigates these thermochemical interactions and their pertinent kinetics and thermodynamics.Dense compacts or single crystalline pieces of barrier coating oxides were placed into a semi-infinite 1D diffusion couple geometry with one of two synthetic silicate melts at 1200–1400 °C. Concentration profiles within the melt were obtained and fit to partial differential equations quantitatively describing the coating dissolution rate into the melt and diffusivities therein. Cation diffusivities were most affected by the melt composition, whereas the ratio of rare-earth (RE3+) oxides to ZrO2 or HfO2 most strongly affected the initial detachment rate of barrier oxides into the melt. Ultimately, the dissolution kinetics were sufficiently slow to delay melt saturation and the nucleation of reprecipitated or reaction phases that limit coating degradation. This delay was worse for barrier oxides with low concentration of RE3+ elements. Finite element models—using the gathered kinetic data but applied to small length scales relevant for real coatings—suggest this delay will be controlled primarily by the initial interface detachment rate in practice.After the initial dissolution transient period, the crystallization of reprecipitated and reaction phases was investigated qualitatively using electron microscopy and chemical analysis techniques. The presence of only a small amount of RE3+ oxide in the dissolving material (e.g., 7%) kinetically hindered the crystallization of reaction products—even those based on Zr4+ or Hf4+—favoring instead reprecipitated phases, deviating from the expected thermodynamic response predicted by CALPHAD databases. Conversely, those barrier oxides free of RE3+ (e.g., HfO2) more readily crystallized reaction products such as (Zr,Hf)SiO4, or Ca2HfSi4O12; those containing a substantial amount of RE3+ (e.g., Gd2Zr2O7) rapidly crystallized a RE-apatite, nominally Ca2RE8(SiO4)6O2.Finally, the thermodynamics of Y-Al-Fe-garnet formation, i.e., the solid-solubility limits of substitutional cations Ca2+, Mg2+, Fe2+, and Si4+, their crystallographic site preference, and the competition between garnet and other phases was investigated. Long duration heat treatments afforded equilibrated samples, for which the phase assemblage was analyzed using X-ray diffraction, electron microscopy, and standardized chemical analysis techniques. A key factor in the stability of garnet was the Fe:Al ratio of the system. Indeed, increasing the Fe:Al ratio of the as-synthesized powder significantly increased the Ca2+ and Si4+ solubility and the quantity of garnet present, with a concomitant decrease to the quantity of other important reaction phases such as apatite.This dissertation advances the understanding of thermochemical interactions between protective barrier coatings and molten silicates, which is critical to design robust coatings. The quantitative kinetic data and thermodynamic information enables computational approaches to coating design.
Motivated by the variation in reported lattice parameters of floating-zone-grown Nd 2 Zr 2 O 7 crystals, we have performed a detailed study of the relationship between synthesis environment, ...structural disorder, and magnetic properties. Using a combination of polycrystalline standards, electron-probe microanalysis, and scattering techniques, we show that crystals grown under atmospheric conditions have a reduced lattice parameter relative to pristine polycrystalline powders due to occupation of the Nd site by excess Zr (i.e., negative stuffing). In contrast, crystals grown under high-pressure Ar are nearly stoichiometric with an average lattice parameter approaching the polycrystalline value. While minimal disorder of the oxygen sublattices is observed on the scale of the average structure, neutron pair-distribution function analysis indicates a highly local disorder of the oxygen coordination, which is only weakly dependent on growth environment. Most importantly, our magnetization, heat capacity, and single-crystal neutron scattering data show that the magnetic properties of crystals grown under high-pressure Ar match closely with those of stoichiometric powders. Neutron scattering measurements reveal that the signature of magnetic moment fragmentation—the coexistence of all-in-all-out (AIAO) magnetic Bragg peaks and diffuse pinch-point scattering due to spin-ice correlations–persists in these nearly stoichiometric crystals. However, in addition to an increased AIAO transition temperature, the diffuse signal is seemingly stabilized and remains nearly unchanged upon warming to 800 mK. This behavior indicates that both the AIAO magnetic order and spin-ice correlations are sensitive to deviations of the Nd stoichiometry.
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