Synthesis of chemically coprecipitated hexagonal strontium-ferrite and its characterization

Highly uniform submicrometre size particles of hexagonal strontium ferrite (SrFe₁₂O₁₉) have been synthesized by chemical coprecipitation technique at pH ⋍ 13. Chemical coprecipitation technique has helped in bringing down the ferritization temperature from 1300 to 925° C which is revealed by DTA-TG and XRD studies. Reproducible uniform single domain particle size and its distribution has been observed by scanning electron microscopy. X-ray and Mossbauer studies have identified single phase ferrite with Fe³⁺ ions occupying the proper crystallographic sites. The performance parameters of the sintered isotropic strontium ferrite magnets have proved to be superior by about 20% over the ferrites prepared by conventional ceramic technique.     

Nickel incorporated carbon nanotube/nanofiber composites as counter electrodes for dye-sensitized solar cells

A nickel incorporated carbon nanotube/nanofiber composite (Ni-CNT-CNF) was used as a low cost alternative to Pt as counter electrode (CE) for dye-sensitized solar cells (DSCs). Measurements based on energy dispersive X-rays spectroscopy (EDX) showed that the majority of the composite CE was carbon at 88.49 wt%, while the amount of Ni nanoparticles was about 11.51 wt%. Measurements based on electrochemical impedance spectroscopy (EIS) showed that the charge transfer resistance (R(ct)) of the Ni-CNT-CNF composite electrode was 0.71 Ω cm(2), much lower than that of the Pt electrode (1.81 Ω cm(2)). Such a low value of R(ct) indicated that the Ni-CNT-CNF composite carried a higher catalytic activity than the traditional Pt CE. By mixing with CNTs and Ni nanoparticles, series resistance (R(s)) of the Ni-CNT-CNF electrode was measured as 5.96 Ω cm(2), which was close to the R(s) of 5.77 Ω cm(2) of the Pt electrode, despite the significant difference in their thicknesses: ～22 μm for Ni-CNT-CNF composite, while ～40 nm for Pt film. This indicated that use of a thick layer (tens of microns) of Ni-CNT-CNF counter electrode does not add a significant amount of resistance to the total series resistance (R(s-tot)) in DSCs. The DSCs based on the Ni-CNT-CNF composite CEs yielded an efficiency of 7.96% with a short circuit current density (J(sc)) of 15.83 mA cm(-2), open circuit voltage (V(oc)) of 0.80 V, and fill factor (FF) of 0.63, which was comparable to the device based on Pt, that exhibited an efficiency of 8.32% with J(sc) of 15.01 mA cm(-2), V(oc) of 0.83, and FF of 0.67.     

Prediction of semiconducting behavior in minority spin of Co2CrZ (Z = Ga, Ge, As): LSDA

Volume optimization was performed to obtain the theoretical lattice constants by using the generalized gradient approximation (GGA). The electronic and magnetic properties of Heusler alloys Co₂CrZ (Z = Ga, Ge, As) were investigated by using local spin density approximation (LSDA). Amongst the systems under investigation, Co₂CrGe and Co₂CrGa give 100% spin polarization at the Fermi level (E_F). Co₂CrGe and Co₂CrGa are the most stable half-metallic ferromagnets (HMFs); their E_F lie exactly at the gap of 0.24 eV and 0.38 eV, respectively, in the spin-down channel. Even though Co₂CrAs gives a distinct and bigger gap as compared to Co₂CrGa and Co₂CrGe, its E_F is not located at the middle of the gap in the spin-down channel. We have also found that the total magnetic moments increase as the Z goes from Ga to As. The calculated density of states and band structures show the HMF character for Co₂CrGe and Co₂CrGa.     

Omni-directional bands in multilayer structure containing exponentially graded materials

The omni-directional reflection (ODR) in a one-dimensional multilayered structure containing exponentially graded material is studied theoretically using the transfer matrix method. We propose a periodic multiplayer structure containing alternate layers of the exponentially graded refractive index and layers of constant refractive index. The reflectance for TE- and TM-modes of the structure are studied for different angles of incidence. We obtained two ODR bands, one in the visible and the other in the infrared region for both polarizations. The behavior of the ODR band in the infrared region is different from the usual Bragg ODR band in the visible region. The width of the ODR band for the TM-mode is larger that for the TE-mode in the wavelength range 850 nm–1050 nm. Such a structure with a large ODR band may be useful in the design of a broad infrared reflector.     

Acute encephalopathy and seizure rates in children under age two years in Oregon and Washington state

The use of small-volume injections of hypertonic saline solutions (HSS) in resuscitation from hemorrhagic shock is accompanied by well-maintained and pronounced increases in coronary blood flow (CBF) and by increases in myocardial contractility. The present study was performed in open-chest, anesthetized dogs to evaluate the contribution of direct coronary vasodilator and positive inotropic effects of HSS to these therapeutic responses. The left anterior descending coronary artery (LAD) was cannulated and perfused at constant pressure (100 mm Hg) with normal arterial blood. CBF in LAD was measured electromagnetically, and used to calculate myocardial oxygen consumption (MVO2) and coronary arterial plasma osmolality. Percent segmental shortening in LAD bed (% SS) was evaluated with ultrasonic crystals. Measurements were obtained during infusion into LAD of 2.5, 5.0, and 7.5% HSS at 2 ml/min. These HSS solutions yielded calculated plasma osmolalities of 329 +/- 3, 361 +/- 8, and 378 +/- 10 mOsm/kg, respectively. The increases in plasma osmolality by 2.5% HSS were in the therapeutic range, whereas those by 5.0 and 7.5% HSS were supertherapeutic. HSS caused initial peak increases in CBF (reflecting decreases in coronary vascular resistance), which waned rapidly to achieve modest steady-state increases within 2-3 min. The magnitude of the peak and steady-state increases in CBF by HSS correlated to osmolality. The 2.5% HSS had no effect on MVO2 and % SS, whereas the 5.0% and 7.5% HSS increased these variables in an osmolality-dependent manner. Conclusions: (1) intracoronary infusions of HSS caused modest steady-state coronary vasodilation, (2) Supertherapeutic elevations of plasma osmolality by HSS were required for direct positive inotropic effects, and (3) the present findings suggest that the direct cardiac actions of HSS contribute minimally to the increases in coronary blood flow and myocardial contractility that follow the use of these solutions for resuscitation from hemorrhagic shock.     

Asymmetric photoreactions within zeolites: role of confinement and alkali metal ions

In this Account strategies using zeolites as media to achieve chiral induction are presented. Diastereomeric excesses as high as 90% and enantiomeric excesses up to 78% have been obtained with selected systems within zeolites. The same systems show no asymmetric induction in solution. Chiral induction is dependent on the alkali ions present in the zeolites. Alkali ions control not only the extent of asymmetric induction but often the isomer being enhanced. Results of ab initio computations have allowed us to gain an insight into the observed selectivity within zeolites.     

Adsorption and desorption of phosphate in some semi-arid tropical Indian Vertisols

Adsorption and desorption of phosphorus in soils are among the key processes governing its availability to crops. There have been very few studies on the phosphorus adsorption and desorption characteristics of Vertisols. The P adsorption and desorption characteristics of four Vertisols belonging to three agriculturally important soil series were studied. The amounts of P adsorbed by the soils at 0.2µg ml^–1 equilibrium solution P concentration was low and ranged from 34.3 to 79.5µg g^–1 soil. The phosphate adsorption was very well described by Langmuir and Freundlich isotherms. The P adsorbed by a Vertisol (BR-1) fertilized with different rates of P in the previous season (0, 10, 20 and 40 kg P ha^–1) was similar (34.3–41.3µg g^–1 soil) indicating little effect of fertilization on P adsorption. The correlation studies indicated that the DTPA-extractable Fe was the most important factor accounting for P adsorption in these soils. Clay and CaCO₃ content were found to be relatively less important factors affecting P adsorption in the soils studied.The capacity of the two extractants and EUF (electro-ultrafiltration) to desorb the adsorbed P followed the order: EUF (400V, 80°C)>sodium bicarbonate>EUF (200V, 20°C)>calcium chloride. The average amounts of P desorbed from the four Vertisols using these methods were 74, 63, 50, and 3% respectively of the adsorbed P. In the Begamganj soil, the amount of P desorbed by EUF (400V, 80°C) exceeded 100%, indicating that all of the adsorbed P was desorbable including some native P.In conclusion the results of our study show that the Vertisols studied have low phosphate adsorption capacity and that the P they adsorbed is easily desorbable.Approved for publication as Journal Article No. 983 by International Crops Research Institute for the Semi-Arid Tropics (ICRISAT).     

Modification of chitosan towards a biomaterial with improved physico‐chemical properties

Modification of chitosan (CTN), a natural biomaterial, leading to improved physical and chemical properties makes the polymer overcome the limitations on its application in biomedical fields. We have modified CTN using polyvinylpyrrolidone (PVP) in three ways, i.e., by blending (CTN : PVP), by graft copolymerization [poly(CTN‐g‐PVP)], and by free radical induced crosslinking [poly(CTN‐cl‐PVP)], varying the CTN to PVP ratio. The properties of all the modified and unmodified CTN studied included structural, thermal, mechanical, and water uptake capacity. Modification by blending imparted enhanced strength, toughness, glass transition temperatures, and thermal stability to the dry membranes. The equilibrium water content (EWC) of the membrane gels was investigated as a function of pH and ionic strength. Swelling ratio of the chemically modified membranes were found to be less sensitive to pH, with higher % EWC maintaining the gel integrity in both acidic and neutral media unlike the plain CTN. The hydrogen bonding and free volume of the films were found to play a major role in determining the properties of the type of modification. CTN membranes with enhanced water uptake capacities clubbed with optimum mechanical properties shall be a suitable choice for biomedical and controlled release applications. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 63–69, 2006     

Rapid synthesis of nanostructured porous silicon carbide from biogenic silica

Nanostructured silicon carbide (SiC) is an exceptional material with numerous applications, for example, in catalysis, biomedicine, high-performance composites, and sensing. In this study, a fast and scalable method of producing nanostructured SiC from plant materials by magnesiothermic reduction via self-propagating high-temperature synthesis (SHS) route was developed. The produced biogenic material possessed a high surface area above 200 m²/g with a SiC crystallite size below 10 nm, which has not been done previously by SHS. This method enables affordable synthesis of the material plant-based precursors in a reaction that only takes a few seconds, thereby paving a way for nanostructured SiC production in high volumes using renewable resources. The material was also functionalized with carboxylic acid and bisphosphonate moieties, and its use as metal adsorbent in applications such as wastewater remediation was demonstrated.