Superconductivity at 11.3 K induced by cobalt doping in CeOFeAs

Pure phases of a new oxyarsenide superconductor of the nominal composition CeOFe0.9Co0.1As was successfully synthesized by solid state reaction in sealed silica ampoules at 1180 C. It crystallizes in the layered tetragonal ZrCuSiAs type structure (sp gp P4/nmm) with lattice parameter of a = 3.9918(5) angstrom and c = 8.603(1) angstrom. A sharp superconducting transition is observed at 11.31 K with an upper critical field of 45.22 T at ambient pressure. The superconducting transition temperature is drastically lowered (~ 4.5, 4.9 K) on increasing the concentration (x = 0.15, 0.2) of cobalt

An Effective Approach for Turbine Hot Component Failure Analysis

One advantage of computational fluid dynamics (CFD) is its ability to reveal the physics or nature of practical engineering problems in detail, allowing engineers and scientists to develop rigorous, effective, and efficient solutions. In this chapter, an effective approach to investigate gas turbine hot component failure is demonstrated, and the mid-span cracking of nozzle guide vanes (NGVs) is used as an example. It is a two-step approach. In the first step, a 60° combustor sector with simplified NGVs and thermocouples attached is simulated; and in the second step, NGV sectors are simulated, where each NGV sector is comprised of one high-fidelity probe NGV and several dummy NGVs. The former identifies the NGV having the highest thermodynamic load and provides the inlet boundary conditions for the latter. The CFD analysis successfully identified the root causes of the NGV damage pattern and mid-span cracking, i.e., the hot streaks from the combustor and inadequate internal cooling

Multiple-scale modeling of Pt effect on durability of aluminide coatings

The beneficial effect of Pt on the durability of aluminide coatings subjected to thermal cycling was studied by first-principles density functional theory (DFT) calculations, thermal physical analysis, atom diffusion simulation and mechanics-based modeling. This presentation will highlight these analytical approaches and research findings in three subject areas. First, the effect of doping elements (S, Pt, Cr, Y, Zr, Hf) on work adhesion, Wad, of the β-NiAl coating/Al2O3 scale interface was investigated. S was found to significantly reduce, while Y, Zr and Hf to largely increase Wad of the interface, with Pt having a minimal effect. The bonding characteristics of the interface associated with the elemental doping were used to explain the trend of Wad. Furthermore the X-S (X=Pt, Y, Zr, Hf) co-doping effects were also examined to elucidate the potential of these dopants in supressing the detrimental effects of S. Second, the beneficial effect of Pt on reducing the β-NiAl/Al2O3 interfacial tensile stress was assessed. The coefficients of thermal expansion (CTE) of Pt, β-NiAl and β-NiAl+Pt were obtained using DFT calculations and thermal physical formulas. The calculated CTE of the coatings, along with the experimentally measured CTE of Al2O3, were then incorporated to evaluate thermal cyclic tensile stress at the undulated β-NiAl/Al2O3 interface. The results showed that the addition of Pt to b-NiAl coating reduced the interfacial tensile stress, thus contributing to the improvement of thermal cyclic durability of the coating. This beneficial effect of Pt was more pronounced with a thicker oxide scale and a larger ratio of interfacial wave amplitude over wavelength. Third, the beneficial effect of Pt on lowering the diffusivity of S in b-NiAl coating was evaluated. The apparent activation energy and the pre-exponential factor of diffusivity via the next nearest neighbour (NNN) atom transportation as well as via interstitial jumps were analyzed, and the bonding characteristics of S with its surrounding atoms were calculated and compared with experimental results in the literature to elucidate the diffusion process of S. The addition of Pt in b-NiAl was found to significantly reduce the diffusivity of S, thus supressing the detrimental segregation of S to the β-NiAl coating/Al2O3 scale interface. Please click Additional Files below to see the full abstract

Two-Hop Routing with Traffic-Differentiation for QoS Guarantee in Wireless Sensor Networks

This paper proposes a Traffic-Differentiated Two-Hop Routing protocol for Quality of Service (QoS) in Wireless Sensor Networks (WSNs). It targets WSN applications having different types of data traffic with several priorities. The protocol achieves to increase Packet Reception Ratio (PRR) and reduce end-to-end delay while considering multi-queue priority policy, two-hop neighborhood information, link reliability and power efficiency. The protocol is modular and utilizes effective methods for estimating the link metrics. Numerical results show that the proposed protocol is a feasible solution to addresses QoS service differenti- ation for traffic with different priorities.Comment: 13 page

Superconductivity in CeO_{1-x}F_xFeAs with upper critical field of 94 T

We have successfully synthesized Ce based oxypnictide with fluorine doping (CeO_{1-x}F_xFeAs) by a two step solid state reaction method. Detailed XRD and EDX confirm the crystal structure and chemical compositions. We observe that an extremely high Hc2(0) of 94 T can be achieved in the x = 0.1 composition. This increase in Hc2(0) is accompanied by a decrease in transition temperature (38.4 K in x = 0.1 composition) from 42.5 K for the x = 0.2 phase. The in-plane Ginzburg-Landau coherence length is estimated to be ~ 27 A at x = 0.2 suggesting a moderate anisotropy in this class of superconductors. The Seebeck coefficient confirms the majority carrier to be electrons and strong dominance of electron-electron correlations in this multiband superconductor

Study of Ni and Zn doped CeOFeAs: Effect on the structural transition and specific heat capacity

We have systematically studied the substitution of nonmagnetic Zn and magnetic Ni at iron sites in Ce based oxypnictide. The parent compound (CeOFeAs) shows an anomaly in resistivity around 150 K due to structural transition from tetragonal (space group: P4/nmm) to orthorhombic structure (space group: Cmma). Substitution of Zn suppresses this anomaly to lower temperature (~130 K) but Ni substitution does not show any anomaly around this temperature and the compound behaves like a metal. Further, we find that non magnetic (Zn) doping leads to higher impurity scattering as compared to magnetic Ni doping. Similar to the resistivity measurement, the specific heat shows another jump near 4 K for CeOFeAs. This is attributed to the ordering of Ce3+ moments. This peak shifts to 3.8 K for Zn substituted compound and there is no change in the ordering temperature in the Ni substituted CeOFeAs. These peaks are broadened in applied magnetic field (5 T) and the calculated magnetic entropy tends to saturate at the same value for 0 T and 5 T external magnetic field.Comment: 16 pages Text+Fig

Upper critical field, superconducting energy gaps, and Seebeck coefficient in La0.8Th0.2OFeAs

We report the synthesis and characterization of a new electron-doped La-oxypnictide superconductor by partial substitution of lanthanum by thorium. The superconducting transition temperature at about 30.3 K was observed in La0.8Th0.2OFeAs which is the highest in La-based oxypnictide superconductors synthesized at ambient pressure. We find that the decrease in lattice parameters with Th doping in LaOFeAs is more drastic as compared to that obtained by high pressure (6 GPa) synthesis of oxygen deficient LaOFeAs. The resistivity and magnetic susceptibility measurements yield an upper critical field Hc2 (0) of 47 T. Partial substitution of Th in place of La induces electron doping as evidenced by negative Seebeck coefficient. The temperature dependent magnetic penetration depth data provides strong evidence for a nodeless low energy gap of 1.4 meV