Ionic magnetic fluid based on cobalt ferrite nanoparticles: Influence of hydrothermal treatment on the nanoparticle size

Magnetic fluid based on cobalt ferrite nanoparticles was obtained using a hydrothermal treatment added to the Massart procedure. This treatment increases the average size of the nanoparticles from 11.9 to 18.7 nm and also improves the dispersity and crystallinity of the cobalt ferrite particles. The nanoparticles obtained after the hydrothermal treatment were dispersed in aqueous solvent by the classical procedure for ionic magnetic fluids. The ferrofluid thus obtained is stable at pH 7 and may be useful for hyperthermia applications.     

Infrared ion dip and ultraviolet spectroscopy of 4-phenyl imidazole, its tautomer, 5-phenyl imidazole, and its multiply hydrated clusters

Mass-resolved resonant two photon ionisation (R2PI) and infrared ion dip spectra have been recorded for 4-phenylimidazole (4PI) and its singly and multiply hydrated clusters 4PI(H₂O)_{n = 0 - 4}, under supersonic expansion conditions. In the case of 4PI(H₂O)_0,1, it has also been possible to record infrared spectra in both the ground (S₀) and excited (S₁) states. Combining the experimental data with the results of ab initio calculations has led to the structural assignment of each cluster. In each case, the water molecules bind primarily to the NH site of the imidazole ring. Clusters with n≥ 2 incorporate linear water chains, in which the proton donating terminus bridges either to the π-electron system (n = 2) or to the >N: atom site (n = 3, 4) on the imidazole ring. Despite the creation of a “water wire”, connecting the donor and acceptor sites of imidazole, there is no evidence of proton transfer in either the ground or excited state. Received 20 December 2001 Published online 13 September 2002     

Excitation-resolved hyperspectral fluorescence lifetime imaging using a UV-extended supercontinuum source

We present a time-gated, optically sectioned, hyperspectral fluorescence lifetime imaging (FLIM) microscope incorporating a tunable supercontinuum excitation source extending into the UV. The system is capable of resolving the excitation spectrum, emission spectrum, and fluorescence decays in an optically sectioned image.     

Local control of light polarization with low-temperature fiber optics

A fiber-optic-based polarization control system that uses a backreflection measurement scheme at low temperatures has been developed. This provides a stringent test of the light polarization state at the output of the fiber, allowing for determination and control of the degree of circular polarization; i.e., it can generate linear, right, or left circular polarization with cryogenic fibers. This polarization controller is paving the way toward the control and manipulation of nuclear spins in semiconductors via the optical Overhauser effect and could be used, for example, for the purpose of quantum information processing with the large nuclear spins of GaAs.     

Copper-Catalyzed Borylative Couplings with C−N Electrophiles

Copper-catalyzed borylative multicomponent reactions (MCRs) involving olefins and C−N electrophiles are a powerful tool to rapidly build up molecular complexity. The products from these reactions contain multiple functionalities, such as amino, cyano and boronate groups, that are ubiquitous in medicinal and process chemistry programs. Copper-catalyzed MCRs are particularly attractive because they use a relatively abundant and non-toxic catalyst to selectively deliver high-value products from simple feedstocks such as olefins. In this Minireview, we explore this rapidly emerging field and survey the borylative union of allenes, dienes, styrenes and other olefins, with imines, nitriles and related C−N electrophiles.     

Taking an Item‐Level Approach to Measuring Change With the Force and Motion Conceptual Evaluation: An Application of Item Response Theory

In order to evaluate the effectiveness of curricular or instructional innovations, researchers often attempt to measure change in students' conceptual understanding of the target subject matter. The measurement of change is therefore a critical endeavor. Often, this is accomplished through pre–post testing using an assessment such as a concept inventory, and aggregate test scores are compared from pre to post‐test in order to characterize gains. These comparisons of raw or normalized scores are most often made under the assumptions of Classical Test Theory (CTT). This study argues that measuring change at the item level (rather than the person level) on the Force and Motion Conceptual Evaluation (FMCE) can provide a more detailed insight into the observed change in students' Newtonian thinking. Further, such an approach is more warranted under the assumptions of Item Response Theory (IRT). In comparing item‐level measures of change under CTT and IRT measurement models, it was found that the inferences drawn from each analysis are similar, but those derived from IRT modeling stand on a stronger foundation statistically. Second, the IRT approach leads to analyzing common item groupings which provide further information about change at the item and topic level.     

Structure dependent luminescence characterization of green–yellow emitting Sr2SiO4:Eu2+ phosphors for near UV LEDs

This paper reports on the luminescence properties of mixtures of α- and β-(Sr_0.97Eu_0.03)₂SiO₄ phosphors. These phosphors were prepared by 3 different synthesis techniques: a modified sol–gel/Pechini method, a co-precipitation method and a combustion method. The structural and optical properties of these phosphors were compared to those of solid state synthesized powders. The emission spectra consist of a weak broad blue band centered near 460 nm and a strong broad green–yellow band centered between 543 and 573 nm depending on the crystal structure. The green–yellow emission peak blue-shifts as the amount of β phase increases and the photoluminescence emission intensity and quantum efficiency of the mixed phase powders is greater than those of predominant α-phase powders when excited between 370 and 410 nm. Thus, (Sr_1?xEu_x)₂SiO₄ with larger proportion of the β phase are more promising candidates than single α-phase powders for use as a green–yellow emitting phosphor for near UV LED applications. Finally the phosphors prepared by the sol–gel/Pechini method, which have larger amount of β phase, have a higher emission intensity and quantum efficiency than those prepared by co-precipitation or combustion synthesis.     

Time-resolved velocity and concentration measurements in variable-viscosity turbulent jet flow

This paper addresses the ability to reliably measure the fluctuating velocity field in variable-viscosity flows (herein, a propane–air mixture), using hot-wire anemometry. Because the latter is sensitive to both velocity and concentration fluctuations, the instantaneous concentration field also needs to be inferred experimentally. To overcome this difficulty, we show that the hot-wire response becomes insensitive to the concentration of the field, when a small amount of neon is added to the air. In this way, velocity measurements can be made independently of the concentration field. Although not necessary to velocity measurements, Rayleigh light-scattering technique is also used to infer the local (fluctuating) concentration, and, therefore, the viscosity of the fluid. Velocity and concentration measurements are performed in a turbulent propane jet discharging into an air–neon co-flow, for which the density and viscosity ratios are 1.52 and 1/5.5, respectively. The Reynolds number (based on injection diameter and velocity) is 15400. These measurements are first validated: the axial decay of the mean velocity and concentration, as well as the lateral mean and RMS profiles of velocity and concentration, is in full agreement with the existing literature. The variable-viscosity flow along the axis of the round jet is then characterized and compared with a turbulent air jet discharging into still air, for which the Reynolds number (based on injection diameter and velocity) is 5400. Both flows have the same initial jet momentum. As mixing with the viscous co-flow is enhanced with increasing downstream position, the viscosity of the fluid increases rapidly for the case of the propane jet. In comparison with the air jet, the propane jet exhibits: (1) a lower local Reynolds number based on the Taylor microscale (by a factor of four); (2) a reduced range of scales present in the flow; (3) the isotropic form of the mean energy dissipation rate is first more enhanced and then drastically diminishes and (4) a progressively increasing local Schmidt number (from 1.36 to 7.5) for increasing downstream positions. Therefore, the scalar spectra exhibit an increasingly prominent Batchelor regime with a ~ k ^?1 scaling law. The experimental technique developed herein provides a reliable method for the study of variable-viscosity flows.