Magnetic properties of multiwalled carbon nanotubes as a function of acid treatment

We investigate the electronic properties of multiwalled carbon nanotubes both before and after acid treatment with concentrated sulphuric and nitric acids. Magnetic susceptibility measurements were performed using a SQUID magnetometer and show that there is a considerable enhancement in the density of states at the Fermi level. The data shows that the diamagnetic influence from the graphitic nanotubes dominates. We experimentally observe, after acid treatment, that the diamagnetic susceptibility remains unchanged at 5 K but notably decreases at 77 K. We propose the acid treatment has increased the Van Vleck paramagnetic contribution lowering the diamagnetic response from the π-electron orbital magnetisation. The Van Vleck paramagnetic contribution is finite-temperature dependent with a diminishing contribution at higher fields.     

Cluster Build-up Reactions using Di-gold Cationic Fragments: Synthesis and Structural Characterization of [Os7(CO)19(Au2dppm)]

The reaction of the di-gold cation [Au₂(dppx)]²⁺ with the heptanuclear cluster dianion [Os₇(CO)₂₀]^2– affords the mixed metal cluster [Os₇(CO)₂₀{Au₂(dppx)}] (x=m (1), e (2), b (3)). On standing, in solution, this complex undergoes decarbonylation to give the cluster [Os₇(CO)₁₉{Au₂(dppx)}] (x=m (4), e (5), b (6)). The complexes have been characterised spectroscopically, and an X-ray structure determination of the dppm derivative shows that it contains a metal core based on an Os₇ edge-bridged bicapped tetrahedron with the two ₃-Au atoms capping adjacent triangular Os₃ faces of the central tetrahedron. In an analogous reaction, the carbido anion [Os₇(H)C(CO)₁₉]^– affords the neutral cluster [Os₇C(CO)₁₉{Au₂(dppm)}] (7) when treated with [Au₂(dppm)]²⁺ in the presence of base.     

Effect of ionic strength on the self-assembly, morphology and gelation of pH responsive β-sheet tape-forming peptides

Molecular self-assembly is an intrinsic property of proteins central to their biological functionality. One important industrially interesting property is the ability to control and switch on and off self-assembly using a variety of external chemical and physical triggers. Model peptides have been developed with significantly reduced chemical and structural complexity compared to biological proteins. These are ideal systems for exposing the fundamental principles that drive protein-like self-assembly, as well as for establishing in a quantitative manner their structure-function relationship. We investigate simple, short model peptides that adopt a purely β-strand conformation, align in an antiparallel manner and self-assemble in one dimension in solution into long β-sheet nanotapes and higher order aggregates with no other conformation (i.e., helices, turns or random coils) present in the aggregates. These micrometre-long nanostructures gel in solutions at concentrations as low as 0.2% v/v. Their gel-fluid transition has been previously shown to be controlled by pH, temperature, or by mixing with complementary peptides. Here we show the dramatic effect of another chemical trigger, that of physiological-like salt concentration, on the self-assembly, morphology and gelation of a series of systematically designed charged self-assembling tape-forming peptides, each 11 amino acid residues in length, in the pH range of 2-14. This study provides a detailed understanding of the self-assembly of this class of peptides in aqueous solutions of biologically relevant pH and ionic strength. This insight has led to the development of injectable self-assembling peptide lubricants as potential therapeutics for the treatment of early stage knee joint osteoarthritis.     

Zur elektrolytischen Bestimmung der Salpeters?ure

Ohne Zusammenfassung     

Mixed convection boundary-layer flow over a vertical surface embedded in a porous medium

In this paper we have numerically investigated the existence and uniqueness of a vertically flowing fluid passed a model of a thin vertical fin in a saturated porous media. We have assumed the two-dimensional mixed convection from a fin, which is modelled as a fixed, semi-infinite vertical surface, embedded in a fluid-saturated porous media under the boundary-layer approximation. We have taken the temperature, in excess of the constant temperature in the ambient fluid on the fin, to vary as  , where is measured from the leading edge of the plate and λ is a fixed constant. The Rayleigh number is assumed to be large so that the boundary-layer approximation may be made and the fluid velocity at the edge of the boundary-layer is assumed to vary as . The problem then depends on two parameters, namely λ and , the ratio of the Rayleigh to Péclet numbers. It is found that when λ>0 (<0) there are (is) dual (unique) solution(s) when is grater than some negative values of (which depends on λ). When λ<0 there is a range of negative value of (which depends on λ) for which dual solutions exist and for both λ>0 and λ<0 there is a negative value of (which depends on λ) for which there is no solution. Finally, solutions for 0<1 and 1 have been obtained.     

Investigation on the flameholding mechanisms in supersonic flows: backward-facing step and cavity flameholder

Abstract  

As effective devices to extend the fuel residence time in supersonic flow and prolong the duration time for hypersonic vehicles cruising in the near-space with power, the backward-facing step and the cavity are widely employed in hypersonic airbreathing propulsive systems as flameholders. The two-dimensional coupled implicit RANS equations, the standard k-ε turbulence model, and the finite-rate/eddy-dissipation reaction model have been used to generate the flow field structures in the scramjet combustors with the backward-facing step and the cavity flameholders. The flameholding mechanism in the combustor has been investigated by comparing the flow field in the corner region of the backward-facing step with that around the cavity flameholder. The obtained results show that the numerical simulation results are in good agreement with the experimental data, and the different grid scales make only a slight difference to the numerical results. The vortices formed in the corner region of the backward-facing step, in the cavity and upstream of the fuel injector make a large difference to the enhancement of the mixing between the fuel and the free airstream, and they can prolong the residence time of the mixture and improve the combustion efficiency in the supersonic flow. The size of the recirculation zone in the scramjet combustor partially depends on the distance between the injection and the leading edge of the cavity. Further, the shock waves in the scramjet combustor with the cavity flameholder are much stronger than those that occur in the scramjet combustor with the backward-facing step, and this causes a large increase in the static pressure along the walls of the combustor.     

Fluid Flows Through Two-Dimensional Channels of Composite Materials

The present analysis relates to the study of the full two-dimensional Brinkman equation representing the fluid flow through porous medium. The steady, incompressible fluid flow, with a negligible gravitational force, is constrained to flow in an infinitely long channel in which the height assumes a series of piecewise constant values. The control volume method is used to solve the Brinkman equation which involves the parameter, =/Da, where Da is the Darcy number and is the ratio of the fluid viscosity _f to the effective viscosity . An analytical study in the fully developed section of the composite channel is presented when the channel is of constant height and composed of several layers of porous media, each of uniform porosity. In the fully developed flow regime the analytical and numerical solutions are graphically indistinguishable. A geometrical configuration involving several discontinuities of channel height, and where the entry and exit sections are layered, is considered and the effect of different permeabilities is demonstrated. Further, numerical investigations are performed to evaluate the behaviour of fluid flow through regions which mathematically model some geological structures of various sizes, positions and permeability, for example a fault or a fracture, where the outlet channel is offset at different levels. The effect on the overall pressure gradient is also considered.     

Measurement of thermal stresses within an experimental nuclear-reactor vessel head

This paper describes certain tests and techniques employed in measuring stresses within an experimental nuclear-reactor head of unusual design. The incorporation of certain desired design features necessitated that the head be extremely thick. Due to the thickness and its complex geometry, it was considered desirable to determine stress distribution within the head under conditions of steady-state pressure combined with rapid heating and cooling transients within the reactor, in order to determine safe limits for the operation of the head. A photoelastic study of a three-dimensional model of the reactor head was completed in 1956; this study permitted prediction of the stress distribution throughout the head as a function of internal pressure, but it was not possible to assimilate the head thermal stresses by photoelastic means. It is the purpose of this paper to describe the technique employed in measuring thermal stresses in the interior of the head, under simulated operating conditions of steady-state pressure and temperature transients.