Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.     

Metal Oxide Film Anodes in the Electrolysis of Water or Brine

Metal (M) oxide (M: Ir, Os, Pd, Pt, Rh, Ru) together with MaO₂ and MnO₂ alone, were coated on SnO₂ films and the anode behavior was examined in 1.0 N H₂SO₄, 1.0 N NaOH and 1.0, N NaCl aqueous solutions at 25°. The results are compared with those of DSA and of metallic Pt.     

The Stabilities and Thermodynamic Properties of Manganese(II) and Nickel(II) Chelates of Trans-1,2-Diaminocyclohexane-N,N, N', N'-Tetraacetic Acid

The interaction between trans-1,2-diaminocyclohexane-N, N, N', N'-tetraacetic acid (H₁DCTA or H₄Z) and Mn(II) and Ni(II) ions has been investigated. The formation constants of the hydrogen chelates (MHZ^?), and of the normal chelates (MZ^2?), have been measured at 20, 30 and 40°C, and an ionic strength of 0.10 (KNO₃). The former was done by a titration technique and the latter by a mercury indicator electrode method. Enthalpy and entropy changes characterizing the formation of the normal chelates have been calculated at 25°C.     

Studies on 1,2-Bis(O-Aminophenoxy)Ethane-N,N,N′,N′;-Tetraacetic Acid Chelates of the Tripositive Lanthanide Ions

The acid dissociation constants of 1,2-bis(cis-aminophenoxy)ethane-N,N,N′,N′;-tetraacetic acid (H₄BAPAT or H₄Z), and the stability constants of its chelates with tripositive rare-earth metal ions have been determined by the potentiometric titration and mercury indicator electrode methods at 15°, 25′ and 35°C and an ionic strength of 0.1 (KNO₃). The existence of a monohydrogen chelate species, LnHZ, and the normal chelate, LnZ^?, is illustrated. Enthalpy and entropy changes characterizing the formation of the normal chelates and the dissociation of the last two protons of chelating acid have been calculated at 25°C. These functions have been compared with corresponding values for related chelating agents.     

Spectrophotometric Studies of Cerium(III) and Thorium(IV) Chelates of Diethylenetriaminepentaacetic Acid (DTPA)

The interaction between diethylenetriaminepentaacetic acid (DTPA or H_sZ) and Ce(III) and Th(IV) ions has been investigated spectrophotometrically in aqueous solution at an ionic strength of 0.1 and for various temperatures. It has been found that the Ce(III)-DTPA chelate (1:1) exhibited a characteristic absorption maximum at 297 nm, and the optimum pH range is between 3.4 to 4.4. The absorption of Ce(III)-DTPA chelate is considerably diminished by adding small amounts of Th(IV) ions. This phenomenon was used to evaluate the formation constant of Th(IV)-DTPA chelate (1:1). The formation constants and the thermodynamic properties characterizing the formation of the chelates have been calculated at 25°. The results are as follows:      

The influence of a transverse magnetic field on a subnormal glow discharge in air

In subnormal glow discharge under d.c. excitation at different pressure in a varying transverse magnetic field (0 to 30 G) some measurements have been carried out for various initial average tube currents. The voltage across the discharge increases and average tube current and residual current decreases in the magnetic field. With the help of Beckman’s expression [4] for the axial field and the electron density distribution in a transverse magnetic field the observed variation of current and voltage can be satisfactorily explained. The variation of axial electric field with transverse magnetic field can be represented to a fair degree of accuracy by the derived equation. The behaviour of residual current with magnetic field has been observed in these oscillations.     

Integral equation formulation for buoyancy-driven convection problems

An integral equation formulation for buoyancy-driven convection problems is developed and illustrated. Buoyancy-driven convection in a bounded cylindrical geometry with a free surface is studied for a range of aspect ratios and Nusselt numbers. The critical Rayleigh number, the nature of the cellular motion, and the heat transfer enhancement are computed using linear theory. Green's functions are used to convert the linear problem into linear Fredholm integral equations. Theorems are proved which establish the properties of the eigenvalues and eigenfunctions of the linear integral operator which appears in these equations.     

Platium Film Electrodes (1) Platium Film on Ti or TiO2-Covered Ti

A platium film was formed on a Ti(Pt-Ti) and on a TiO₂(Pt-TiO₂-Ti) substrate by the conventional electroplating method (ELP) and by the electroless plating technique (ELSP). The effective minimum film thickness was found to be 0.5 μm judging from the maximum electrocatalytic capablity in alkaline (1 M NaOH) water electrolysis. The film obtained from ELP is superior to that obtained from ELSP, being more tightly bound to the substrate, showing better coverage of the subsrate surface, and also being mechanically stronger. Comparisons with bright Pt and with Pt-black electrodes were made and it is concluded that the electrodes newly prepared using ELP will work as well as Pt-black electrods. The electrodes from ELSP are useful only as cathodes, and are not suitable as anodes.     

Crystal and Molecular Structures of (η4-Cyclohexa-1,3-Diene) (2-Tert-Butyl-1,1,4,4-Tetrafluoro-1,4-Disilabutene) Metal Carbonyls. M?Fe,W

Molecular structures of the two title compounds were investigated to reveal the relationship between the structure of the reaction intermediates and the reaction pathways in the transition metal-mediated cycloaddition reaction between tetrafluorodisilacyclobutene and conjugated dienes. The structures were determined by X-ray diffraction. Both cyrstals are orthorhombic. Compound I. C₁₄H₁₈O₂Si₂F₄Fe. is of space group P2₁2₁2₁ with Z=4; a=6.862(7) Å. b=10.9560(10) Å. c=23.597(25) Å. and Dc=1.562 gcm^?1. Compound II, C₁₅H₁₈O₃Si₂F₄W, is of space group Pbca with Z=8; a=16.999(8) Å, b=15.948(6), c=14.045(7) Å, and Dc=1.962 gcm^?1. Both structures were solved by heavy-atom methods and refined by full-matrix least-squares using anisotropic temperature factors for all non-hydrogen atoms to R values of 0.092 for 1544 observed reflections and 0.078 for 2751 observed reflections, respectively, for compound I and compound II. The butadiene segment of the cyclohexadiene ligand is essentially planar in compound I while it is twisted with a torsional angle of 22° (2) in compound II. The five-membered disilametallacycle ring is planar in compound II while ii is in an envelope comformation in compound I. The coordination geometry of the iron atom may be considered as a distorted tetragonal-pyramidal with the diene unit occupying the apical site, That of tungsten is a distorted seven-coordinated pentagonal-bipyramidal with the two axial carbonyl groups displaced toward the disilabutene ligand. The disilabutene of compound I lies almost parallel and cis to the plane of the butadiene segment while that of compound II orients almost perpendicular to the diene group. The differences in the structural features of the two complexes explain clearly their product patterns and the reactions are apparently stereo-controlled.     

Synthesis and Photochemistry of syn- and anti-9,10-Epoxy-1, 4-dihydro-1,4-ethanonaphthalene-2,3-diones

The title epoxydiketones were prepared stereoselectively, direct epoxidation of 3a-c with MCPBA produced syn isomers la-c whereas epoxidation of 4a-d followed by saponification of the spirolactone rings gave anti isomers 2a-c. The stereochemistry of 1a and 2a was established by X-ray diffraction, whereas that of the remaining epoxydiketones was determined from the correlation of visible, ¹H, and ¹³C NMR spectral data; the differences between spectra of the corresponding syn and anti isomers are explained in terms of through-space interaction and steric effects. Photolysis of syn isomers 1a-c afforded the corresponding naphthalenes 8a-c in almost quantitative yields; in contrast, irradiation of anti isomers 2a-c gave complicated mixtures. The quantum yield of disappearance of 1a was 52 times that of 2a. Reaction mechanisms are proposed to account for the product formation. The differences in the photochemical behavior of the title syn and anti isomers are rationalized in terms of stereoelectronic effects of the epoxy rings in the syn isomers.