Opposed round jets issuing into a small aspect ratio channel cross flow

Round jets (diameter D) discharging into a confined cross flow (dimension 3.16D × 21.05D) are investigated experimentally. Two configurations are considered: (1) a single jet (momentum flux ratio, J = 155) and (2) two opposed jets with two different momentum flux ratios (J = 60, and 155). A two-component laser-Doppler anemometer is used to make a detailed map of the normal stresses and mean velocities in the symmetry plane of the jets. In addition, smoke-wire and laser-sheet visualization are used to study the flow.

The rate of bending of the single confined jet is found to be higher than the rate of bending of an unconfined jet with the same momentum flux ratio. In the far field, the jet centerline velocity is observed to decay more slowly than the unconfined jet, indicating poor turbulent diffusion of linear momentum. Annular shear layer vortices are visualized on the upstream edge of the jet in the near field. In the far field, the flow visualization suggests that the jet loses its integrity and fragments into independent regions that are convected by the cross flow.

In the opposed jet configuration at the high momentum flux ratio (J = 155), the jets impinge in the center of the duct, and a pair of vortices is observed upstream of the impingement region. The flow visualization implies that the impingement vortices form quasi periodically and have a finite life span. In the impingement region, the jets are observed to penetrate alternately beyond the symmetry plane of the duct. In the two-jet configuration with J = 60, the jets do not impinge on each other owing to the higher rate of bending. Instead, the flow visualization indicates that the shear layers of the jets penetrate to the central region and periodically pinch off regions of the potential-like cross-flow fluid where they meet. The pinch-off regions of cross-flow fluid are convected by the turbulent flow for large distances, yet remain essentially unmixed.     

Distinguishing nanocrystalline solids from fine-particle ensembles using Mössbauer spectroscopy

Nanocrystalline solids, formed by compacting fine powders, have been proclaimed to possess a new structure. Specific characteristics have been reported from Mössbauer studies, and these appear to confirm and reinforce a model of crystalline cores isolated by disordered, low-density grain boundaries. In measurements of three compacted samples, we were unable to reproduce the characteristics expected. Reasons are suggested as to why the results differ between similarly-prepared samples. It is pertinent to enquire whether any of the previously reported Mössbauer characteristics of fine-particle compacts require a new structural model, or whether all the results can be simply understood in terms of particle agglomerates. Reference to the literature reveals that there is no distinction.     

Quantum key distribution using quantum-correlated photon sources

Quantum key exchanges using weak coherent (Poissonian) single-photon sources are open to attack by a variety of eavesdropping techniques. Quantum-correlated photon sources provide a means of flagging potentially insecure multiple-photon emissions and thus extending the secure quantum key channel capacity and the secure key distribution range. We present indicative photon-counting statistics for a fully correlated Poissonian multibeam photon source in which the transmitted beam is conditioned by photon number measurements on the remaining beams with non-ideal multiphoton counters. We show that significant rejection of insecure photon pulses from a twin-beam source cannot be obtained with a detector having a realistic quantum efficiency. However quantum-correlated (quadruplet or octuplet) multiplet photon sources conditioned by high efficiency multiphoton counters could provide large improvements in the secure channel capacity and the secure distribution range of high loss systems such as those using the low earth orbit satellite links proposed for global quantum key distribution. Received 14 July and Received in final form 20 November 2001     

Light‐Driven Rotary Molecular Motors on Gold Nanoparticles

We report the synthesis of unidirectional light‐driven rotary molecular motors based on chiral overcrowded alkenes and their immobilisation on the surface of gold nanoparticles through two anchors. Using a combination of ¹H and ¹³C NMR, UV/Vis and CD spectroscopy, we show that these motors preserve their photochemical and thermal behaviour after they have been attached to gold nanoparticles. Furthermore, we describe the synthesis of ²H‐ and ¹³C‐labelled derivatives that were used to verify the unidirectionality of the rotary cycle of these motors both in solution and while grafted to gold nanoparticles. Taken together, these data support the conclusion that these motors maintain their unidirectional rotary cycle when grafted to the surface of small (ca. 2 nm) gold nanoparticles. Thus, continuous irradiation of the system under appropriate conditions leads to unidirectional rotation of the upper half of the molecules relative to the entire nanoparticle.     

A Pyrazine‐Based Polymer for Fast‐Charge Batteries

The lack of high‐power and stable cathodes prohibits the development of rechargeable metal (Na, Mg, Al) batteries. Herein, poly(hexaazatrinaphthalene) (PHATN), an environmentally benign, abundant and sustainable polymer, is employed as a universal cathode material for these batteries. In Na‐ion batteries (NIBs), PHATN delivers a reversible capacity of 220 mAh g^?1 at 50 mA g^?1, corresponding to the energy density of 440 Wh kg^?1, and still retains 100 mAh g^?1 at 10 Ag^?1 after 50 000 cycles, which is among the best performances in NIBs. Such an exceptional performance is also observed in more challenging Mg and Al batteries. PHATN retains reversible capacities of 110 mAh g^?1 after 200 cycles in Mg batteries and 92 mAh g^?1 after 100 cycles in Al batteries. DFT calculations, X‐ray photoelectron spectroscopy, Raman, and FTIR show that the electron‐deficient pyrazine sites in PHATN are the redox centers to reversibly react with metal ions.     

The structure and magnetic properties of chromium(III) molybdate

Magnetic susceptibility measurements have shown that Cr₂(MoO₄)₃ orders magnetically at 42 K. Powder neutron diffraction experiments at 295 and 5 K indicate that Cr₂(MoO₄)₃ is chemically and magnetically isostructural with the L-type ferrimagnet Fe₂(MoO₄)₃, and has a magnetic moment of 2.5 ± 0.2 μ_B per cation at 5 K. The limitations imposed on powder neutron diffraction methods by particle-size effects are discussed.     

The nexus between alternatives assessment and green chemistry: supporting the development and adoption of safer chemicals

ABSTRACT

Alternatives assessment and green chemistry share a common goal of supporting the transition to safer, more sustainable chemicals, materials, and products. Yet the two fields, and their respective scientific communities, are not well integrated. To better understand the nexus between alternatives assessment and green chemistry as complementary approaches to support the development and adoption of safer, more sustainable chemicals for specific functional uses, this article discusses the foundations of the two fields and examines two case examples in which companies have utilized the tools and approaches of both disciplines in developing safer chemical solutions. This research demonstrates the importance and utility of the overlapping skillsets and tools of the two disciplines and the potential benefit of educational opportunities and collaborative spaces in jointly strengthening both fields. Additionally, the literature and case examples identify a number of research and practice needs that would bolster the application of both alternatives assessment and green chemistry in supporting the transition to safer, more sustainable chemistry, including: clearer definitions and criteria of what is ‘safer’; improved approaches to evaluate potential unintended consequences of chemical applications; and more effective tools to evaluate toxicity, consider inherent exposure trade-offs, and combine multiple attributes to make an informed decision.     

The metal complexes of some azo and azomethine dyestuffs : Determination of stability constants,and preparation of solid complexes

The stability constants of the complexes of 4-(2-pyridylazo)-resorcinol (I) with copper(II), cobalt-(II), zinc(Il), lead(ll), and uranium(VI) were determined by potentiometric titration in aqueous media, and were shown to have very high values, the stability constant of the copper(ll) complex approaching that of the copper(ll)-EDTA complex. The nickel(II) complex was shown to behave anomalously. The stability constants were also determined by this method in 1:1 dioxan/water, and the values obtained compared with those for the complexes of salicylidene-2-aminopyridine (II), 2-(o-hydroxy-phenyl-imino-methyl)-pyridine(III), and benzeneazoresorcino (IV). It is shown that chelation by (1) is terdentate, involving the pyridine nitrogen, the o-hydroxyl group and the azo nitrogen farthest from the heterocycle.The solid copper(II) complexes of these four ligands were prepared; in the solid complexes the azo nitrogen nearest the heterocycle plays a greater part than in the complexes in solution.