Adsorption and kinetic behavior of uranium and thorium in seawater-sediment system

The adsorption and kinetic mechanism of uranium (U) and thorium (Th) in seawater-sediments system of Mumbai Harbour Bay (MHB) has been studied by K _d values of two sets of experimental determinations using a batch experiment. After equilibrium time (7 days), under static conditions, K _d for U and Th ranged from 25,030 to 55,662 mL/g (mean: 42,140 mL/g) and 24,926 to 38,561 mL/g (mean: 34,256 L/kg), respectively. Extraction studies showed that both U and Th were strongly bound to sediment components due to insignificant difference in their mean concentration in equilibrium solution. Rate constants (k) for transfer between seawater and the exchangeable fraction were found to be similar for the sediments as 1.02 ± 0.03 day^?1 for uptake of U and Th. The resulting adsorption data were fitted to Freundlich, linear and Langmuir isotherm models. All the three models showed a significant correlation (R ² >0.9), indicating that there is more complex relationships with adsorption behavior of U and Th on sediment surface. Since, the Freundlich constant (n) for U and Th was found to be closer to unity. Therefore linear model was observed to be highly suitable. Based on the linear model, the distribution coefficient (k _d) of Th was about 1.5 times higher than U. According to Freundlich model, sorption of U in sediments observed to be higher than Th. However, Langmuir model considered in opposite to Freundlich and showed a higher value of K _L constant for Th than U. The pH (water) of the sediments ranged from 7.8 to 8.2 and the estimated total carbon (determined by C H N S–O elemental analyser) ranged from 1.7 to 3.4 %.     

Tandem regioselective synthesis of tetrazoles and related heterocycles using iodine

A one-pot, tandem process has been developed for the synthesis of a library of tetrazoles from aryl isothiocyanates. Condensation of aryl isothiocyanates with ammonia, and aryl amines (R-NH(2)) provided mono, 1,3-disubstituted symmetrical and unsymmetrical thioureas, which on desulfurization with molecular iodine (I(2)) led to formation of the corresponding heterocumulene (cyanamides or carbodiimides). The in situ generated heterocumulene on subsequent treatment with sodium azide at room temperature gave corresponding tetrazoles. The product regioselectivity for unsymmetrical 1,3-disubstituted thioureas was found to be correlated with the basicities (pK(a)'s) of the parent amines attached to the thiourea. Aryl-sec-alkyl unsymmetrical thioureas gave thioamido guanidino products rather than the 5-aminotetrazoles produced by HgCl(2) mediation of the reaction. Bis-thioureas derived from aryl isothiocyanates and hydrazine gave thiadiazoles exclusively.     

Doping dependent tunneling conductance in SDW ordered copper oxide superconductors

It is observed that doping suppresses the long range anti-ferromagnetic order and induces superconducting phase for a suitable doping. In order to study this effect, we present a model study of the doping dependence of the tunneling conductance in high-T_c systems. The system is described by the Hamiltonian consisting of spin density wave (SDW) and s-wave type superconducting interaction in presence of varying impurity concentrations. The gap equations are calculated by using Green’s functions technique of Zubarev. The gap equations and the chemical potential are solved self-consistently. The imaginary part of the electron Green’s functions shows the quasi-particle density of states which represent the tunneling conductance observed by the scanning tunneling microscopy (STM). We investigate the effect of impurity on the gap equations as well as on the tunneling conductance. The results will be discussed based on the experimental observations.     

The effect of band Jahn-Teller distortion on the magnetoresistivity of manganites: a model study

We present a model study of magnetoresistance through the interplay of magnetisation, structural distortion and external magnetic field for the manganite systems. The manganite system is described by the Hamiltonian which consists of the s-d type double exchange interaction, Heisenberg spin-spin interaction among the core electrons, and the static and dynamic band Jahn-Teller (JT) interaction in the e(g) band. The relaxation time of the e(g) electron is found from the imaginary part of the Green's function using the total Hamiltonian consisting of the interactions due to the electron and phonon. The calculated resistivity exhibits a peak in the pure JT distorted insulating phase separating the low temperature metallic ferromagnetic phase and the high temperature paramagnetic phase. The resistivity is suppressed with the increase of the external magnetic field. The e(g) electron band splitting and its effect on magnetoresistivity is reported here.     

Fluence dependant formation of <Emphasis Type="Italic">β</Emphasis>-SiC by ion implantation and thermal annealing

The β-SiC nanocrystals were synthesized by the implantation of carbon ions (C⁻) into silicon followed by high-temperature annealing. The carbon fluences of 1×10¹⁷, 2×10¹⁷, 5×10¹⁷, and 8×10¹⁷ atoms/cm² were implanted at an ion energy of 65 keV. It was observed that the average size of β-SiC crystals decreased and the amount of β-SiC crystals increased with the increase in the implanted fluences when the samples were annealed at 1100 °C for 1 h. However, it was observed that the amount of β-SiC linearly increased with the implanted fluences up to 5×10¹⁷ atoms/cm². Above this fluence the amount of β-SiC appears to saturate. The Fourier Transform Infrared Spectroscopy (FTIR), Raman Spectroscopy, and X-ray diffraction (XRD) techniques were used to characterize the samples.     

Luminescence studies on photostimulable phosphor CsBr:Eu2+

Photostimulable phosphor CsBr:Eu²⁺ is prepared through a solid-state reaction. The effect of annealing atmosphere on photoluminescence and photostimulated luminescence was investigated. Optimum luminescence intensity was obtained when samples were prepared at 350°C in air atmosphere. The effect of irradiation of gamma and neutron had resulted in the formation of optically stimulable traps with different trap depths. The role of monovalent and divalent dopants on thermoluminescence dosimetric properties has been discussed.     

Creaming enhancement in a liter scale ultrasonic reactor at selected transducer configurations and frequencies

Recent research has shown that high frequency ultrasound (0.4–3 MHz), can enhance milkfat separation in small scale systems able to treat only a few milliliters of sample. In this work, the effect of ultrasonic standing waves on milkfat creaming was studied in a 6 L reactor and the influence of different frequencies and transducer configurations in direct contact with the fluid was investigated. A recombined coarse milk emulsion with fat globules stained with oil-red-O dye was selected for the separation trials. Runs were performed with one or two transducers placed in vertical (parallel or perpendicular) and horizontal positions (at the reactor base) at 0.4, 1 and/or 2 MHz (specific energy 8.5 ± 0.6 kJ/kg per transducer). Creaming behavior was assessed by measuring the thickness of the separated cream layer. Other methods supporting this assessment included the measurement of fat content, backscattering, particle size distribution, and microscopy of samples taken at the bottom and top of the reactor. Most efficient creaming was found after treatment at 0.4 MHz in single and double vertical transducer configurations. Among these configurations, a higher separation rate was obtained when sonicating at 0.4 MHz in a vertical perpendicular double transducer setup. The horizontal transducer configuration promoted creaming at 2 MHz only. Fat globule size increase was observed when creaming occurred. This research highlights the potential for enhanced separation of milkfat in larger scale systems from selected transducer configurations in contact with a dairy emulsion, or emulsion splitting in general.     

Structural and proton conductivity study of BaZr1-xRExO3-δ(RE = Dy,Sm) ceramics for intermediate temperature solid oxide fuel cell electrolyte

Journal of Solid State Electrochemistry - Proton-conducting BaZr1-xRExO3-δ (RE = Dy, Sm) (x = 0.05, 0.10, 0.15, 0.20) ceramics were synthesized via a conventional mixed...     

The role of CDW gap on the magnetic phase transition in CMR materials

We propose a model to study the magnetic phase transition in the Colossal-Magneto-Resistance (CMR) material of general type R_1−x A _x MnO₃ (R = La, Sm, Nd; A = Ca, Sr, Ba). The model Hamiltonian consists of a Charge Density Wave (CDW) gap in the e _g -band and the strong magnetic field due to the spin ordering in the localized t _g core electrons. The Hamiltonian is solved by using Zubarev’s Green’s function technique to calculate CDW gap (Δ) and magnetization (M ^d ) in t _2g band. Both of them are solved self-consistently. Their combined effect on the temperature dependent magnetization (M ^c ) due to the e _g band electrons is investigated. Both the magnitude and the transition temperature of (M ^c ) are strongly influenced by both Δ and M ^d . Hence the hopping of the band electrons are strongly controlled by these two long range interactions. The results are discussed by varying the model parameters of the manganite system.      

The influence of band Jahn-Teller effect and magnetic order on the magneto-resistance in manganite systems

A model calculation is presented in order to study the magneto-resistivity through the interplay between magnetic and structural transitions for the manganite systems. The model consists of an orbitally doubly degenerate conduction band and a periodic array of local moments of the t_2g electrons. The band electrons interact with the local t_2g electrons via the s-f hybridization. The phonons interact with the band electrons through static and dynamic band Jahn-Teller (J-T) interaction. The model Hamiltonian including the above terms is solved for the single particle Green's functions and the imaginary part of the self-energy gives the electron relaxation time. Thus the magneto-resistivity (MR) is calculated from the Drude formula. The MR effect is explained near the magnetic and structural transition temperatures.