Small and large signal analysis using circuit model of InGaAs/InP based uni-travel carrier photodiode

An equivalent circuit model of uni-traveling carrier photodiode (UTC-PD) is developed from integral carrier density rate equation and few important properties of the device such as the electrical and optical characteristics are evaluated by employing advanced device physics. Circuit model incorporates chip and package parasitic of the device quite simply to provide practical behaviour of UTC-PD. We have developed small signal ac circuit model which is useful for the analysis of low power modulation characteristics of the device and dc circuit model which is advantageous to find wavelength dependent responsivity fairly accurately. At high optical input power the device bandwidth is found to be increased through enhancement of self-induced field in the absorption region and high output power can be derived from the device when absorption width is large. Such condition calls for large signal analysis. We have developed large signal circuit model by combining few mathematical transformations with small signal circuit model with different circuit element values. Our large signal model is unique that the same circuit can be used for both small and large signal analysis. With large signal model the optical power induced bandwidth improvement and output photocurrent saturation are explained. Large signal model is validated through linearity and IP3 analysis which found close agreement with the measured results.     

Characterization of serpentine: a potential nuclear shielding material

The use of serpentine as a potential nuclear shielding material necessitates a chemical quality control of the samples before its use in reactors. With this in view, characterization of these mineral samples was carried out using inductively coupled plasma atomic emission spectrometry (ICP-AES) and Instrumental neutron activation analysis (INAA) methods. The analytical results obtained by both ICP-AES and NAA techniques were found to be comparable. Na, Cr, Co, Zn, and Cu were found to be present in all samples of Indian origin while Ga, Ag, Ni, and Cd were found to below the limits of detection. A comparison on the detection limits of elements of interest was also carried out by both the analytical techniques and found to be in good agreement. An infrared spectroscopic investigation was also carried out on all the mineral samples. Bands at 3,689 and 3,648 cm⁻¹ were attributed to inner and outer hydroxyl stretching of Mg–OH, respectively. The weak and broad band centered around 3,416 cm⁻¹ was assigned due to the stretching vibrations of the adsorbed water molecules while three bands at 1076, 1022 and 968 cm⁻¹ were prescribed to the vibrations of the SiO₄ tetrahedra.     

Synthesis,Structures, and Fungitoxicity of Novel Organophosphorus Compounds

Abstract

A series of biologically active organophosphorus compounds have been synthesized by the reactions of O,O-diethylchlorophosphate with Schiff bases derived from 5-(phenyl/substituted phenyl)-2-hydrazino-1,3,4-oxadiazole and salicylaldehyde/2-hydroxyacetophenone. The compounds have been characterized on the basis of analyses and spectral (IR, ¹H, ¹³C NMR) data. Fungicidal activities of these derivatives against Colletotrichum falcatum, Fusarium oxysporum, and Curvularia pallescence have been evaluated. All compounds showed moderate to significant antifungal activity.     

Crystallization of Pseudopolymorphs of Some Gamboge Pigments. Pyridine,Dimethylformamide and Dimethylsulfoxide Solvates of Morellic Acid,Gambogic Acid and Guttiferic Acid

Abstract

Morellic acid, gambogic acid and guttiferic acid are related naturally-occurring xanthone pigments that yield X-ray quality crystals only from solvents like pyridine, dimethylformamide (dmf) and dimethyl sulfoxide (dmso). The structures of four of these crystals have been determined and are found to contain solvents of crystallization. The solvents hydrogen bond to the carboxyl groups with O—H…O/N motifs previously seen in other carboxylic acids. Distinctive, however, is the presence of an extended though somewhat diffuse array of C—H…O hydrogen bonds that aggregates the entire solute-solvent assemblage in a multi-point manner. Pyridine and dmf are able to mimic each other with respect to their hydrogen bond donating and accepting characteristics and in this respect play equivalent roles in their solvates with morellic acid and gambogic acid. Dmso is seen to self-associate in its guttiferic acid solvate. It is possible that these solvents with multiple hydrogen bonding donor and acceptor capability can act as hydrogen bond nucleators, providing just enough rigidity to the solutes to ensure crystallization.     

Convenient Synthesis of Bicyclic Lactones Related to the Trichothecenes

Short and convenient synthesis of bicyclic lactones (1) and (2), important intermediates in trichothecene synthesis is described.     

A New Route for the Synthesis of Dialkyl Tellurides and Dialkyl Ditellurides

Sodium cleaves elemental tellurium, Te_X, quantitatively to Te^2– ₂ and Te^2- in anhydrous tetrahydrofuran in the presence of catalytic amounts of naphthalene. Subsequent addition of alkylating agent affords dialkyl tellurides and dialkyl ditellurides in excellent yields.     

Role of alkyl substituent in room temperature ionic liquid on the electrochemical behavior of uranium ion and its local environment

A systematic study was carried out to understand the effect of structural modification of C_nmim⁺ moiety of C_nmimBr (n = 6, 8 or 10) on the electrochemical behavior of uranium. The cyclic voltammetric study of the above room-temperature ionic liquids (RTIL) media revealed that with increase in the chain length the electrochemical window extended more towards the negative potential. This resulted in the possibility of conversion of U(VI) to U(III) or even possibly to, U metal via U(IV) (as UO₂) when n ≥ 10. The diffusion coefficient of U(VI) was found to decrease from n = 6 to 8 to 10 due to the increasing order of viscosity of the RTIL. As a consequence, the activation energy was found to follow reverse order i.e. E _act(C₆mimBr) < E _act(C₈mimBr) < E _act(C₁₀mimBr). The conversion of UO₂ ²⁺ to UO₂ was found to be quasi reversible and also exothermic while the entropy was found to decrease due to the reduction reaction. An UV–Vis spectroscopic study was also carried out to understand the local environment around uranium in aqueous and RTIL media. Among several cationic and anionic species, the predominance of UO₂Cl₄ ^2? in 7 M HCl with D_4h coordination symmetry was observed. The decrease in symmetric stretching frequency of UO₂ ²⁺ in RTILs in comparison with aqueous system indicates that the bond strength of UO₂ ²⁺ in aqueous is less than that of UO₂ ²⁺ in RTILs.     

A comparative study of the complexation of Am(III) and Eu(III) with TODGA in room temperature ionic liquid

On the growing awareness of the environmental impact associated with the use of volatile organic diluents, room temperature ionic liquid gained world-wild acceptance as environmentally benign diluents for actinide partitioning. The observed unusual behavior of less extraction efficiency of Eu with TODGA in RTIL in comparison with that of Am-TODGA was addressed in this paper. The stoichiometry of Am-TODGA complex was found to be 1:2 while that of Eu-TODGA was 1:1. More the ligand molecules associated in the metal ligand complex, the organophilicity of the complex will be more and the solubility of the metal–ligand complex in RTIL will be more which reflects in the higher distribution ratio for Am. In RTIL both Am and Eu showed slower kinetics of extraction with TODGA which can be attributed to the high viscosity coefficient of RTIL compared to the molecular diluents. The observed slower kinetics of extraction was quantified and found to follow first order kinetics with the rate constant of 5.5 × 10^?4 s^?1. The formation constant of Am-TODGA complex was found to be more (4.18 × 10⁸ M^?1) than Eu-TODGA complex (3.31 × 10⁸ M^?1) in RTIL. The parameters viz. diffusion coefficient, activation energy for Eu(III)/Eu(II) were determined and found to be 3.08 × 10^?8/cm² s^?1 (at 303 K) and 39.34 kJ mol^?1 respectively. The thermodynamic parameters ΔG, ΔH and ΔS for the reaction were evaluated using the linear regression of the plot of E ^0* versus T. The redox reaction was found to be exothermic with decrease in entropy value.     

Application of hollow fiber supported liquid membrane for the separation of americium from the analytical waste

Americium from analytical solid waste containing U and metallic impurities was separated using hollow fiber supported liquid membrane (HFSLM) technique impregnated with DHOA–TODGA from nitric acid medium. An aliquot of 5 g of the solid waste containing Am (19.95 mg) as minor actinide and of U (2,588 mg), Fe (1,360 mg), Ca (1,810 mg) and Na (3,130 mg) as major impurities was processed. The feed solution obtained after the dissolution of the residue in ~4 M HNO₃ was passed through HFSLM module. In the first stage using 1 M DHOA–dodecane U was recovered while Am and other impurities were left in the raffinate. In the second stage, 0.5 M DHOA + 0.1 M TODGA/dodecane was used for the separation of Am from other impurities. Though, majority of the elements were separated in this cycle, Ca was co extracted along with the americium. CMPO extraction chromatographic technique was used for further separation of americium from Ca. Significant decontamination factors were achieved in this three step separation process with respect to U, Fe, Na and Ca with ~77 % recovery of americium.