Mass Transfer Effects on Unsteady Free Convective Flow Past an Infinite, Vertical Porous Plate with Variable Suction

A two-dimensional unsteady flow of a viscous incompressible dissipative fluid past an infinite, vertical porous plate with variable suction, is studied. Approximate solutions to the coupled non linear equations governing the flow are derived and expressions for the fluctuating parts of the velocity, the transient velocity, temperature and concentration, the amplitude and the phase of the skin-friction, and the rate of heat transfer, are derived. The effects of w(Omega)(frequency), Gr (Grashof number), Gc (modified Grashof number), Sc (Schmidt number), P (Prandtl number) and A (variable suction parameter), on the above physical quantities are calculated numerically and presented in figures and table. Problems of this nature find place in ablative cooling, transpiration and film cooling of rocket and jet engines

Comparative study of EB-PVD gadolinium-zirconate and yttria-rich zirconia coatings performance against Fe-containing calcium-magnesium-aluminosilicate (CMAS) infiltration

This detailed study compare and contrast the calcium-magnesium-aluminosilicate (CMAS) infiltration resistance behavior of electron-beam physical vapor deposition (EB-PVD) produced gadolinium zirconate (GZO) and yttria rich zirconia (65YZ, 65 wt % Y2O3 rest zirconia) coatings. The infiltration kinetics, as well as the stability and protective nature of different reaction products, was studied by performing long term infiltration tests (up to 50 h) at 1250 °C. The results reveal that for the specific microstructures used in this study, 65YZ has a higher infiltration resistance and forms a thinner reaction layer compared to GZO. The analysis indicates that the better performance of 65YZ is associated with a synergetic reaction mechanism, which includes the formation of Carich apatite and a uniform layer of a garnet phase. The formation of apatite requires more rare-earth (RE) in the case of GZO than its 65YZ counterpart, meaning that more Gd would be dissolved before forming apatite crystals, which leads to higher consumption of the GZO layer compared to that of 65YZ. The implications of these mechanisms are discussed in detail concerning the tendency of garnet formation, equilibration of the apatite phase with Ca and RE content, and the effects of the reduction in viscosity due to the RE dissolution into the glass. However, microstructural differences in the coatings used in this study might also affect the diverging infiltration resistance and reaction kinetics and need to be considere

Total synthesis of naturally occurring cephalosporolides E/F

A modular total synthesis of cephalosporolides E/F featuring sequential epoxide–alkyne coupling and subsequent highly regioselective gold catalyzed alkynolcycloisomerization of the resulting alkynetetrol to construct the central spiroketal core has been documented

Interaction and infiltration behavior of Eyjafjallajökull, Sakurajima volcanic ashes and a synthetic CMAS containing FeO with/in EB-PVD ZrO2-65 wt% Y2 O3 coating at high temperature

Yttria rich-zirconia thermal barrier coatings (TBCs) with a nominal composition of 65 wt % Y2O3 balanced ZrO2 were deposited by electron-beam physical vapor deposition (EB-PVD) and tested for calciummagnesium- aluminum-silicate (CMAS) infiltration resistance. The infiltration studies were performed with a set of one synthetized CMAS composition and two real volcanic ashes from the Eyjafjallaj€okull volcano located in Iceland and the Sakurajima volcano located in southern Japan. The coatings were tested at 1250 �C for short term (5 min) and long term (intervals from 1 to 20 h). The results indicate a significantly different reaction process for the synthesized CMAS compared with the natural volcanic ashes. The yttria-rich zirconia coatings demonstrate promising results against infiltration by vigorously reacting against the molten glass inducing its crystallization by forming apatite and garnet phases. The formed reaction products effectively sealed the columnar gaps of the TBC and generated a uniform reaction layer that prevented further infiltration

Target cum flexibility: Synthesis of C(3′)-spiroannulated nucleosides

We report a simple strategy for the synthesis of a collection of C(3′)-spirodihydroisobenzo-furannulated nucleosides featuring a [2+2+2]-cyclotrimerization as the key reaction. The cyclotrimerization reactions are facile with the unprotected nucleosides having a diyne unit. When both alkynes of the diyne are terminal, the regioselectivity is poor. However, when one of the terminal alkynes is additionally substituted, the cyclotrimerizations are highly diastereoselective. Since the key bicycloannulation is the final step, this strategy provides flexibility in terms of the alkynes and is thus amenable for the synthesis of a focussed small molecule library

Growth, characterization and performance of bulk and nanoengineered molybdenum oxides for electrochemical energy storage and conversion

International audienceMolybdenum oxides (MoOy) exhibit quite interesting structural, chemical, electrical, optical and electrochemical properties, which are often dependent on the synthetic procedures and fabrication conditions. The MoOy materiails are promising in numerous current and emerging technological applications, which include nanoelectronics, optoelectronics, energy storage and micromechanics. However, fundamental understanding of the crystal structure and engineering the phase and microstructure is the key to achieving the desired properties and performance in all of these applications. Therefore, in this review, an attempt made to provide a comprehensive review by considering the illustrative examples to highlight the fundamental scientific issues, challenges, and opportunities as related to various Mo-oxides applicable to electrochemical energy applications. In the course of development of lithium batteries delivering high-power and high-energy density for powering electric vehicles, here in this paper, we examine the performances of Mo-oxides, which are candidates as electrodes materials primarily for lithium-ion batteries (LIBs), while some aspects considered in sodium-ion batteries (SIBs) or electrochemical supercapacitors (ECs). Due to the wide range of oxidation states (from +6 to +2) they are promising as both positive (cathode) and negative (anode) electrodes of electrochemical cells. Based on their specific structural, chemical, electrical, and optical properties, which are dependent on the growth conditions and the fabrication technique, this review highlights the progress made in improving and understanding the electrochemical performance of MoOy compounds. Various materials (2.0 ≤ y ≤ 3.0) including anhydrous, hydrates, nanorods, nanobelts, composites and thin films of MoOy are considered. Due to their higher oxidation states, MoOy compounds undergo reversible topotactic lithium intercalation reactions; however, electrochemical features appear strongly dependent on the crystal quality and structural arrangement in the host lattice. Using in-situ and ex-situ X-ray diffraction and Raman spectroscopic data, structural characteristics of various MoOy are discussed. While the reasons for first-cycle irreversible capacity losses identified and discussed elaborately, the approaches adopted for enhanced performance and/or improvements also summarized. Several sub-stoichiometric MoOy positive electrodes exhibit excellent cycle life (up to 300 cycles) with high initial coulombic efficiency (80–90%) and large reversible capacity (>300 mAh g−1). Molybdenum oxides also categorized as one of the conversion-type transition-metal oxides and applied as negative electrodes for LIBs and SIBs with a specific capacity approaching 1000 mAh g−1. In addition to the discussion of the key aspects of crystal growth, characterization, and structure-property relationships, the future prospects to design Mo-oxide materials to enhance the structural stability and electrochemical performance are presented and discussed

A proposed new type of arsenian pyrite: Composition, nanostructure and geological significance

This report describes a new form of arsenian pyrite, called As3+-pyrite, in which As substitutes for Fe [(Fe,As)S2], in contrast to the more common form of arsenian pyrite, As1--pyrite, in which As1- substitutes for S [Fe(As,S)2]. As3+-pyrite has been observed as colloformic overgrowths on As-free pyrite in a hydrothermal gold deposit at Yanacocha, Peru. XPS analyses of the As3+-pyrite confirm that As is present largely as As3+. EMPA analyses show that As3+-pyrite incorporates up to 3.05 at % of As and 0.53 at. %, 0.1 at. %, 0.27 at. %, 0.22 at. %, 0.08 at. % and 0.04 at. % of Pb, Au, Cu, Zn, Ni, and Co, respectively. Incorporation of As3+ in the pyrite could be written like: As3 + + y Au+ + 1 - y (□) ⇔ 2 Fe2 +; where Au+ and vacancy (□) help to maintain the excess charge. HRTEM observations reveal a sharp boundary between As-free pyrite and the first overgrowth of As3+-pyrite (20-40 nm thick) and co-linear lattice fringes indicating epitaxial growth of As3+-pyrite on As-free pyrite. Overgrowths of As3+-pyrite onto As-free pyrite can be divided into three groups on the basis of crystal size, 8-20 nm, 100-300 nm and 400-900 nm, and the smaller the crystal size the higher the concentration of toxic arsenic and trace metals. The Yanacocha deposit, in which As3+-pyrite was found, formed under relatively oxidizing conditions in which the dominant form of dissolved As in the stability field of pyrite is As3+; in contrast, reducing conditions are typical of most environments that host As1--pyrite. As3+-pyrite will likely be found in other oxidizing hydrothermal and diagenetic environments, including high-sulfidation epithermal deposits and shallow groundwater systems, where probably kinetically controlled formation of nanoscale crystals such as observed here would be a major control on incorporation and release of As3+ and toxic heavy metals in oxidizing natural systems