Preparation and magnetic properties of the Sb-doped Bi---Pb---Sr---Ca---Cu---O superconductors

Two groups of samples were prepared by the wet method with nominal composition Bi_x1Pb_x2Sb_x3Sr₂Ca₃Cu₄O_z. In the first group x₁ + x₂ + x₃ = 2. Here x₁ ranges from 1.2 to 1.6 with x₂ varying from 0.72 to 0.12. The second group is characterized by x₁ + x₂ = 2. Here x₁ was chosen to be 1.4 and 1.6 while x₃ ranges from 0.08 to 0.16. The results show: (i) high Sb content reduces the transition temperature; (ii) for small Sb contents the transition temperature is relatively constant but the transition becomes sharper, (iii) the best sample (with x₁ = 1.4, x₂ = 0.48 and x₃ = 0.12) exhibits a very sharp transition with a peak in the imaginary component of the ac susceptibility at 108.4 K; abd (iv) the transition of lightly Sb-doped material is strongly influenced by small external magnetic fields.     

Separation of isomers of nitro-polycyclic aromatic hydrocarbons

Since positional isomers of several nitro-polycyclic aromatic hydrocarbons (nitro-PAH) contain widely different mutagenic properties, methods for separating the isomers were investigated. High performance liquid chromatography with dichloromethane in hexane on silica gave the best resolution for the majority of the compounds. A few groups of isomers were better resolved with other modifiers or with reversed phases. Of the reversed phase systems, methanol-water gave better resolution than acetonitrile-water. With dichloromethane/hexane on silica, the retention was found to depend on the presence of a bay nitro group (first), the number of H atoms peri to the nitro group (second), and the length/breadth ratio of the molecule (third). Front electron density calculations were combined with the structure-retention relations in making tentative structure assignments of minor isomers of nitro-PAH.     

Determination of indium in standard rocks by neutron activation analysis

A rapid neutron activation method for the determination of indium in rocks, based on 54 min (116m)In, is described. The method has been applied to a series of geochemical standards including granite G-1 and diabase W-1. The precision is better than +/- 5% for samples containing more than 5 x 10(-10)g indium. Good agreement with previously published values for G-1 and W-1 has been obtained.     

Determination of copper,gallium and zinc in “standard rocks” by neutron activation

A neutron activation method for the simultaneous determination of copper, gallium and zinc in rocks is described. The method is based on anion-exchange separation steps and measurement of γ-activity. Chemical yields were determined by re-activation. Results for a series of igneous “standard rocks” are presented. The accuracy of the mean values is ca. 5% for concentrations exceeding 10 p.p.m.     

Water-in-crude oil emulsions from the Norwegian continental shelf

A destabilization study of water-in-crude oil emulsions based on crude oils from the Norwegian Continental Shelf is reported. It is found that medium-chain alcohols (1-butanol and benzyl alcohol) and amines are speeding up the separation of water. The destabilization mechanisms in these two cases seem to be fundamentally different. The alcohols seem to modify the rigidity of the interfacial film by a diffusion/partitioning process while the amines show a strong and specific interaction with interfacial groups, hence hydrophilizing the whole film. Observed trends in the time-dependence of the interfacial tension upon addition of alcohols and amines support these suggestions to destabilization mechanisms. Data for a commercial demulsifier Dissolvan 4455 are also given.     

Colloidal crystallization and transport in stripes and mazes

We study guided crystallization and transport of paramagnetic spheres on top of a magnetic film that arranges its domains into stripes or mazes. In the absence of liquid flow, the paramagnetic spheres are confined within the magnetic domains, and it is shown how the particles self-assemble into several interesting phases depending on the complexity of the domain patterns. We also find that colloids guided through a complex maze exhibit structured patterns that can be controlled by an external magnetic field. The results presented here could help us understand both static and dynamic properties of pattern formation in confined geometries of tunable complexity.     

Assembling and manipulating two-dimensional colloidal crystals with movable nanomagnets

We study crystallization of paramagnetic beads in a magnetic field gradient generated by one-dimensional nanomagnets. The pressure in such a system depends on both the magnetic forces and the hydrodynamic flow, and we estimate the flow threshold for disassembling the crystal near the magnetic potential barrier. A number of different defects have been observed which fluctuate in shape or propagate along the crystal, and it is found that the defect density increases away from the nanomagnet. We also study the melting of the crystal/fluid system after removal of the nanomagnet and demonstrate that the bond-oriental order parameter decreases with time. The nanomagnet can be moved in a controlled manner by a weak external magnetic field, and at sufficiently large driving velocities we observe self-healing crack formation characterized by a roughening of the lattice as well as gap formation. Finally, when confined between two oscillating nanomagnets, the colloidal crystal is shown to break up and form dipolar chains above a certain oscillation frequency.