Magnetic and high-pressure magnetotransport properties of cesium-substituted lanthanum calcium manganites

The nature of the double-exchange (DE) interaction in lanthanum manganites is studied through chemical substitutions, Cs for La, and high-pressure measurements. Static and high-frequency magnetic measurements and high-pressure electrical transport studies were carried out on bulk polycrystalline and radio-frequency sputtered thin films of La_0.7-xCs_xCa_0.3MnO₃ for x=0-0.1. The samples are found to be cubic. Curie temperature T_c measurements provide evidence for bond-length-related weakening of DE as x is increased from 0 to 0.03. For higher x, the bond-angle-related changes lead to an increase in the strength of DE. High-pressure mangetoresistance data indicate both bond length and bond-angle-related increase of 10–20 K/GPa in T_c with pressure, with the largest increase measured for x=0.03. The rate of increase in the Curie temperature with pressure decreases with increasing T_c. Anomalies are observed in the magnetic parameters for x=0.03. The Cs-concentration dependence of the low-temperature saturation magnetization shows a minimum close to x=0.03. Ferromagnetic resonance studies at x-band reveal a 5% decrease in the g-value for x=0.03 relative to the end members (x=0 and 0.1). The low-field magnetostriction for x=0.03 indicates a relatively strong electron–phonon spin coupling compared to neighboring compositions. Received: 15 May 2000 / Accepted: 24 July 2000 / Published online: 9 November 2000     

A novel intramolecular reversible reaction between the hydroxyl group and isobutenylene chain in a cyclophane-type macrocycle

A novel Hg²⁺ ion induced reversible ring contraction was achieved employing the intramolecular reaction of isobutylene with an aromatic hydroxyl group of cyclophane; reversibility of the reaction was facilitated by excess addition of NaBH₄ which also resulted in complexation. The ring contraction and expansion was monitored by UV-VIS absorption, and by fluorescence and ¹H NMR spectra. Switchable fluorescence behavior (on—off—on) was observed when the ring-size was tuned from a 19-membered ring to an 18-membered and vice versa. This fine tuning has the potential to be applied in the construction of new supramolecular devices.     

Structural investigations of synthetic ferrihydrite nanoparticles doped with Si

X-ray diffraction (XRD), transmission electron microscopy (TEM), and photoacoustic/Fourier transform infrared spectroscopy (PA/FTIR) are employed to monitor the changes in the structural aspects of two-line ferrihydrite (FHYD) nanoparticles doped with silica viz. Si_xFe_1−xOOH·nH₂O for x=0, 0.02, 0.05, 0.10, 0.15, 0.20, 0.26, 0.40 and 0.50. In XRD, crystallinity decreases and d-spacings increase with increase in x. TEM studies show that the particle size increases systematically with increase in x, from 3.7 nm for x=0 to 5.7 nm for x=0.50. In PA/FTIR, two new bands appear, band F near 3700 cm⁻¹ identified with the surface Si-O-H group and band A near 900 cm⁻¹ identified with Si-O-Fe group, which shifts to higher wavenumbers with increase in x. These results are used to propose a model in which doped Si⁴⁺ ions do not displace Fe³⁺ ions but are chemisorbed on the FHYD surface making a shell of silica for higher doping. This model is consistent with the reported changes in the magnetic properties of FHYD with Si doping.     

Nafion based amperometric hydrogen sensor

A Nafion based amperometric hydrogen sensor that operates at room temperature has been developed. The electrolyte used in the sensor is Nafion 117, which is a proton conducting solid polymer electrolyte. Palladium catalyst was used on the sensing side and platinum supported on carbon on the air side. The sensor functions as fuel cell, H₂/Pd//Nafion//Pt/O₂ and the short circuit current is measured. The short circuit current is found to be linear with respect to concentration of hydrogen on the sensing side. The sensor is able to detect the concentration of hydrogen in argon down to ppb level. Details of assembly of the sensor, response behavior and applications are discussed. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Primary processes in the photosensitized polymerization of cationic monomers

A detailed time-resolved laser spectroscopy investigation has been carried out on the electron transfer reactions of substituted thioxanthone derivatives with diphenyliodonium (Ph-I⁺) salts having different metal halide counterions (MX^?_n). Quenching of thioxanthones' triplet state has been followed under various conditions, by changing the number and nature of substituents on the thioxanthone skeletone, using anion with different nucleophilicity and employing different solvents, namely methanol and acetonitrile. A Photosensitization mechanism is proposed involving an electron transfer from thioxanthone to diphenyliodonium salt. The absorption spectra of the thioxanthone's excited state and the formed new transient are recorded and the rate constants of the excited state processes are measured. The triplet state of thioxanthone derivatives has been quenched by cationically polymerizable monomers and the quantum yield of the major processes has been evaluated. Photolytic experiments have been performed to measure the extent of acid formation. Form photopolymerization experiments using different photoinitiating systems, the rate of polymerization and percentage of monomer conversion have been determined. Both the reactivity in the excited states and the nucleophilicity of the anions affect the efficiency of the photopolymerization reaction.     

Metal ion doped polymers as media for optical memories

Polymer materials are making an impact on optical storage technology to develop high information density and fast access type memories with a high read-out efficiency. The principle and advantages of three-dimensional (3-D) data storage in the form of interference patterns (holograms) have been outlined. Three different information storage materials have been developed by doping metal ions such as Cr(VI) and Fe(III) in water-soluble polymers, namely poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA), which need no further thermal or chemical treatment. Volume transmission holograms have been recorded in dichromated poly(vinyl alcohol) (DCPVA), ferric chloride doped poly(vinyl alcohol) (FePVA) and dichromated poly(acrylic acid) (DCPAA), with and without an electron donor (dimethyl formamide), and xanthene dyes (fluorescein, eosin Y and Rose Bengal). Different parameters influencing the holographic performance have been optimized to achieve a high real-time diffraction efficiency (～70%). An electron transfer process from the polymer matrix to Cr(VI)/Fe(III), leading to the photocrosslinking of the polymer in the form of an interference pattern, has been suggested as the mechanism of information storage (hologram recording) in these materials.     

Versatile Nutraceutical Potentials of Watermelon—A Modest Fruit Loaded with Pharmaceutically Valuable Phytochemicals

Watermelon (Citrulus lantus) is an important horticultural crop which belongs to the Curcubitaceae family. The nutraceutical potential of watermelon has been illustrated by several researchers, which makes it a better choice of functional food. Watermelon has been used to treat various ailments, such as cardio-vascular diseases, aging related ailments, obesity, diabetes, ulcers, and various types of cancers. The medicinal properties of watermelon are attributed by the presence of important phytochemicals with pharmaceutical values such as lycopene, citrulline, and other polyphenolic compounds. Watermelon acts as vital source of l-citrulline, a neutral-alpha amino acid which is the precursor of l-arginine, an essential amino acid necessary for protein synthesis. Supplementation of l-citrulline and lycopene displayed numerous health benefits in in vitro and in vivo studies. Similarly, the dietary intake of watermelon has proven benefits as functional food in humans for weight management. Apart from the fruits, the extracts prepared from the seeds, sprouts, and leaves also evidenced medicinal properties. The present review provides a comprehensive overview of benefits of watermelon for the treatment of various ailments.