Stability of oxides/environmental barrier coating candidate materials in hightemperature, high-velocity steam

Stability of oxides/Environmental Barrier Coating (EBC) candidate materials in high-temperature, high-velocity steam has been characterized using a steam-jet furnace modeled after Lucato et al [1]. The objective of this work is to quantify stability of oxides for use as coatings on SiC-based composites in turbine engine environments with the long term goal of developing thermochemical life prediction models for EBCs. SiO2, TiO2, Y2O3, and rare earth silicates were exposed in one atmosphere steam flowing at approximately 170 m/s at temperatures between 1200 and 1400°C for times up to 375 h. Oxide recession, attributed to formation of volatile metal hydroxides, was measured for SiO2, TiO2 and Y2O3. The SiO2 recession rates were consistent with values predicted assuming loss of material was limited by transport of Si(OH)4(g) through a laminar gas boundary layer. TiO2 single crystal recession was slightly less than SiO2 but too rapid for use in a turbine environment. Y2O3 recession was not measureable within the sensitivity of techniques used here. Y2Si2O7 exposed in the steam-jet furnace was selectively depleted of SiO2 by the reaction: Y2Si2O7 + 2H2O(g) = Y2SiO5 + Si(OH)4(g) (1) A porous surface layer of Y2SiO5 formed after exposure of Y2Si2O7 and was confirmed by X-ray Diffraction Analysis (XRD), Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS). Key microstructural features observed in addition to the porosity include grain refinement, faceting, and grain fall out. The growth rate of the porous layer decreased with time at 1300°C, although the depletion depth varied significantly across the surface, possibly due to preferred crystallographic orientations for the depletion reaction. The silica depletion depth decreased with increasing temperature. The depletion depths were uniform at 1200°C as shown in Figure 1. At 1400°C the porous surface layers sintered rapidly, closing off paths for water vapor ingress into the material and thus minimizing SiO2 depletion by Reaction (1). Y2SiO5 was significantly more stable than Y2Si2O7. Significant SiO2 depletion of the monosilicate was not observed within the sensitivity of the techniques used here

Today's Children, Tomorrow's America: Six Experts Face the Facts

Compiles essays about trends in family structure; how federal, state, and local budget deficits and projected cuts affect child poverty rates and health; and their long-term implications of reduced investment in children. Includes policy recommendations

Teaching-the purpose of repeatable testing: Response to comment of J. D. Edmonds

It is not clear that the comment Grading and Regrading \u27 has very much to do with our article.\u27 The comment seems rather to be a nostalgic statement recalling the good old days when with glee we cracked the whip and the students either shaped up or shipped out. Our purpose in doing repeatable testing was not to become a do your own thing force, to raise everyone\u27s grades, to achieve well defined educational objectives like teaching the solution to the inclined plane problem, to do away with certification, and most definitely our purpose was not to replace teachers with computer-programmed instruction. Our purpose was in fact really rather simple. It was to try to teach more physics. Dr. Edmond\u27s comment has not addressed the premise of our paper, which is that people learn from their mistakes (even in the real world). We have used nothing but the old guard establishment tool of testing as a measure of content mastery. Once the test is established as the measure of content mastery, how large a jump is it to the statement that if you do better on the next exam you will have mastered more content? If we can improve the teaching of physics and make the students feel good about learning at the same time, we should be willing to give up a few of our old sadistic pleasures

Deletion of Tsc2 in nociceptors reduces target innervation, ion channel expression, and sensitivity to heat

AbstractThe mechanistic target of rapamycin complex 1 (mTORC1) is known to regulate cellular growth pathways, and its genetic activation is sufficient to enhance regenerative axon growth following injury to the central or peripheral nervous systems. However, excess mTORC1 activation may promote innervation defects, and mTORC1 activity mediates injury-induced hypersensitivity, reducing enthusiasm for the pathway as a therapeutic target. While mTORC1 activity is required for full expression of some pain modalities, the effects of pathway activation on nociceptor phenotypes and sensory behaviors are currently unknown. To address this, we genetically activated mTORC1 in mouse peripheral sensory neurons by conditional deletion of its negative regulator Tuberous Sclerosis Complex 2 (Tsc2). Consistent with the well-known role of mTORC1 in regulating cell size, soma size and axon diameter of C-nociceptors were increased in Tsc2-deleted mice. Glabrous skin and spinal cord innervation by C-fiber neurons were also disrupted. Transcriptional profiling of nociceptors enriched by fluorescence-associated cell sorting (FACS) revealed downregulation of multiple classes of ion channels as well as reduced expression of markers for peptidergic nociceptors in Tsc2-deleted mice. In addition to these changes in innervation and gene expression, Tsc2-deleted mice exhibited reduced noxious heat sensitivity and decreased injury-induced cold hypersensitivity, but normal baseline sensitivity to cold and mechanical stimuli. Together, these data show that excess mTORC1 activity in sensory neurons produces changes in gene expression, neuron morphology and sensory behavior.</jats:p

Microstructural evolution of environmental barrier coatings in high-temperature steam

Environmental Barrier Coatings (EBCs) are required for use of SiC-based composites in hot sections of gas turbine engines. The primary function of EBCs is to limit the interaction of SiC with steam in the combustion environment. Due to the additional constraints of thermal expansion match and chemical compatibility with the SiC substrate, state-of-the art EBCs are typically fabricated from complex silicates. These silicates typically have sufficiently high silica activities that they also react with the steam by the following generic reaction. Please click Additional Files below to see the full abstract

Who’s in Charge, Anyway – A Proposal for Community-Based Legal Services (with Raymond H. Brescia and Robin Golden)

For over one hundred years, some of our country\u27s most dedicatedlawyers have struggled to provide legal services to poor people.The road has not been an easy one. Richard Nixon vetoed alegal services bill over the issue of presidential appointments, thensigned the Legal Services Corporation Act just before resigning.Nixon\u27s Vice-President, Spiro Agnew, was a vocal opponent of federally-funded legal services. Ronald Reagan submitted eight consecutivebudgets seeking to eliminate all federal funding for theLegal Services Corporation ( LSC ). Simultaneously, he appointeda hostile LSC board of directors. Bill Clinton\u27s election,however, brought new hope to advocates. Hillary Clinton is a formerpresident of the LSC Board. The early Clinton budgets includedan increase in LSC funding, but they were countereddramatically by the severe cuts and restrictions imposed by the1994 Republican-controlled Congress

Periodic Optical Variability of Radio Detected Ultracool Dwarfs

A fraction of very low mass stars and brown dwarfs are known to be radio active, in some cases producing periodic pulses. Extensive studies of two such objects have also revealed optical periodic variability and the nature of this variability remains unclear. Here we report on multi-epoch optical photometric monitoring of six radio detected dwarfs, spanning the $\sim$M8 - L3.5 spectral range, conducted to investigate the ubiquity of periodic optical variability in radio detected ultracool dwarfs. This survey is the most sensitive ground-based study carried out to date in search of periodic optical variability from late-type dwarfs, where we obtained 250 hours of monitoring, delivering photometric precision as low as $\sim$0.15%. Five of the six targets exhibit clear periodicity, in all cases likely associated with the rotation period of the dwarf, with a marginal detection found for the sixth. Our data points to a likely association between radio and optical periodic variability in late-M/early-L dwarfs, although the underlying physical cause of this correlation remains unclear. In one case, we have multiple epochs of monitoring of the archetype of pulsing radio dwarfs, the M9 TVLM 513-46546, spanning a period of 5 years, which is sufficiently stable in phase to allow us to establish a period of 1.95958 $\pm$ 0.00005 hours. This phase stability may be associated with a large-scale stable magnetic field, further strengthening the correlation between radio activity and periodic optical variability. Finally, we find a tentative spin-orbit alignment of one component of the very low mass binary LP 349-25.Comment: Accepted to The Astrophysical Journal; 22 pages; 12 figure