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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.