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The reactivity between rare-earth (RE-) oxide stabilized ZrO 2 or HfO 2 thermal barrier coatings (TBCs) and a calcium-magnesium-aluminum-silicate (CMAS) melt was studied at 1310°C. These reactions are representative of the ingestion of siliceous materials by the intake air of gas turbines (e.g., in aircraft engines) at high temperatures (>1200°C). These materials can melt and react with coated components in the hot section, resulting in premature failure. The goal of this work was to probe the effect of various RE (RE = Y, Yb, Dy, Gd, Nd, and Sm) oxides in the melt phase equilibrium and stability of the top-coating system. Thermodynamic calculations of the phase assemblage of the (1Àx) ZrO 2 -xY 2 O 3 coating materials and CMAS melt are compared with the experimental findings.CMAS was found to penetrate the samples at the grain boundaries and dissolve the coating materials to form silicate phases containing the RE elements. Furthermore, apatite and garnet crystalline phases formed in the samples with total REoxide content higher than 16 mol% in the reaction zone for the ZrO 2 system. In general, samples with nominal compositions ZrO 2 -9Dy 2 O 3 , HfO 2 -7Dy 2 O 3 , ZrO 2 -8Y 2 O 3 , HfO 2 -6Er 2 O 3 , ZrO 2 -9.5Y 2 O 3 -2.25Gd 2 O 3 -2.25Yb 2 O 3 , and ZrO 2 -30Y 2 O 3 exhibited lower reactivity, or more resistance, to CMAS than the other coating compositions. K E Y W O R D S coatings, degradation, rare earths, thermal barrier coatings

The analytical utility of a micro-hollow cathode glow discharge plasma for detection of varied hydrocarbons was tested using acetone, ethanol, heptane, nitrobenzene, and toluene. Differences in fragmentation pathways, reflecting parent compound molecular structure, led to differences in optical emission patterns that can then potentially serve as signatures for the species of interest. Spectral simulations were performed emphasizing the CH (A(2)Δ-X(2)Π), CH (C(2)Σ-X(2)Π), and OH (A(2)Σ(+)-X(2)Π) electronic systems. The analytical utility of selected emission lines is demonstrated by a linear relationship between optical emission spectroscopy and parent compound concentration over a wide range, with detection limits extending down to parts per billion (ppb) levels.

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A comparison is made between SnO2, ZnO, and TiO2 single-crystal nanowires and SnO2 polycrystalline nanofibers for gas sensing. Both nanostructures possess a one-dimensional morphology. Different synthesis methods are used to produce these materials: thermal evaporation-condensation (TEC), controlled oxidation, and electrospinning. Advantages and limitations of each technique are listed. Practical issues associated with harvesting, purification, and integration of these materials into sensing devices are detailed. For comparison to the nascent form, these sensing materials are surface coated with Pd and Pt nanoparticles. Gas sensing tests, with respect to H2, are conducted at ambient and elevated temperatures. Comparative normalized responses and time constants for the catalyst and noncatalyst systems provide a basis for identification of the superior metal-oxide nanostructure and catalyst combination. With temperature-dependent data, Arrhenius analyses are made to determine activation energies for the catalyst-assisted systems.

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The lower Venusian atmosphere is the region from the surface to the cloud deck or about 0–50 km. Early modeling studies of the atmosphere were primarily based on thermodynamics; more recent modeling studies are based on kinetics of the elementary reactions. In this paper, we take the accepted nominal composition of near-surface gases at ∼42 km and show some of the constituents are indeed at thermodynamic equilibrium. We impose a small oxygen gradient and use a thermodynamic free energy minimization code to describe the vertical gradients of mixing ratios for the primary gases in the lower atmosphere. The oxygen gradient is within the measurement errors on oxygen and thus maintains mass conservation. Reasonable agreement is found between our calculations and the vertical profiles of H2O, H2SO4, OCS, H2S, and S n (n = 1–8). We then did a kinetic analysis of kinetic expressions for the formation of these species. Consistent with other investigators, we find that very few if any reactions should be at equilibrium in the lower atmosphere. Yet our equilibrium calculations do show some agreement with observations. We conclude that the available kinetic expressions likely need improvement and factors such as catalysis must be included to reflect actual Venus conditions.

Tantalum pentoxide (Ta2O5) and its solid solution phases are candidate coatings for components to be used in combustion environments. Thus, it is important to understand the response of Ta2O5 to high‐temperature water vapor, a product of combustion. Thermogravimetric methods are used to examine the oxide in reactant streams of controlled water vapor contents at 1250°C‐1450°C. The observed weight loss indicates a reaction of the general form ½ Ta2O5(s) + x H2O(g)=TaOy(OH)x(g). Methodical variation in the water vapor pressure suggests the products are a mix of TaO(OH)3(g) and Ta(OH)5(g). Evidence of TaO(OH)3(g) was observed with a sampling mass spectrometer. The measured hydroxide and oxyhyroxide vapor fluxes from Ta2O5 are compared with calculated vapor fluxes from SiO2 and Al2O3. Ta2O5 exhibits fluxes similar to those from SiO2 due to gaseous metal hydroxide formation.