Slim P‐E hysteresis loop and anomalous dielectric response in sol-gel derived antiferroelectric PbZrO3 thin films

Sol-gel derived PbZrO3 (PZ) thin films have been deposited on Pt(111)/Ti/SiO2/Si substrate and according to the pseudotetragonal symmetry of PZ, the relatively preferred (110)t oriented phase formation has been noticed. The room temperature P‐E hysteresis loops have been observed to be slim by nature. The slim hysteresis loops are attributed to the [110]t directional antiparallel lattice motion of Pb ions and by the directionality of the applied electric field. Pure PZ formation has been characterized by the dielectric phase transition at 235 °C and antiferroelectric P‐E hysteresis loops at room temperature. Dielectric response has been characterized within a frequency domain of 100 Hz–1 MHz at various temperatures ranging from 40 to 350 °C. Though frequency dispersion of dielectric behaves like a Maxwell–Wagner type of relaxation, ω2 dependency of ac conductivity indicates that there must be G‐C equivalent circuit dominance at high frequency. The presence of trap charges in PZ has been determined by Arrhenius plots of ac conductivity. The temperature dependent n (calculated from the universal power law of ac conductivity) values indicate an anomalous behavior of the trapped charges. This anomaly has been explained by strongly and weakly correlated potential wells of trapped charges and their behavior on thermal activation. The dominance of circuit∕circuits resembling Maxwell–Wagner type has been investigated by logarithmic Nyquist plots at various temperatures and it has been justified that the dielectric dispersion is not from the actual Maxwell–Wagner-type response.

A Model of Anomalous Enzyme-Catalyzed Gel Degradation Kinetics

We show that a model of target location involving n noninteracting particles moving subdiffusively along a line segment (a generalization of a model introduced by Sokolov et al. [Biophys. J. 2005, 89, 895.]) provides a basis for understanding recent experiments by Pelta et al. [Phys. Rev. Lett. 2007, 98, 228302.] on the kinetics of diffusion-limited gel degradation. These experiments find that the time t(c) taken by the enzyme thermolysin to completely hydrolyze a gel varies inversely as roughly the 3/2 power of the initial enzyme concentration [E]. In general, however, this time would be expected to vary either as [E](-1) or as [E](-2), depending on whether the Brownian diffusion of the enzyme to the site of cleavage took place along the network chains (1-d diffusion) or through the pore spaces (3-d diffusion). In our model, the unusual dependence of t(c) on [E] is explained in terms of a reaction-diffusion equation that is formulated in terms of fractional rather than ordinary time derivatives.

Structural and Biochemical Bases for the Redox Sensitivity of Mycobacterium tuberculosis RslA

An effective transcriptional response to redox stimuli is of particular importance for Mycobacterium tuberculosis, as it adapts to the environment of host alveoli and macrophages. The M. tuberculosis a factor sigma(L) regulates the expression of genes involved in cell-wall and polyketide syntheses. sigma(L) interacts with the cytosolic anti-sigma domain of a membrane-associated protein, RslA. Here we demonstrate that RslA binds Zn2+ and can sequester sigma(L) in a reducing environment. In response to an oxidative stimulus, proximal cysteines in the CXXC motif of RslA form a disulfide bond, releasing bound Zn2+. This results in a substantial rearrangement of the sigma(L)/RslA complex, leading to an 8-fold decrease in the affinity of RslA for sigma(L). The crystal structure of the -35-element recognition domain of sigma(L), sigma(L)(4), bound to RslA reveals that RslA inactivates sigma(L) by sterically occluding promoter DNA and RNpolymerase binding sites. The crystal structure further reveals that the cysteine residues that coordinate Zn2+ in RslA are solvent exposed in the complex, thus providing a structural basis for the redox sensitivity of RslA. The biophysical parameters of sigma(L)/RslA interactions provide a template for understanding how variations in the rate of Zn2+ release and associated conformational changes could regulate the activity of a Zn2+-associated anti-sigma factor. (C) 2010 Elsevier Ltd. All rights reserved.

Polarographic and redox potential studies on copper(I) and copper(II) and their complexes. Part III. Copper (I and II)-ethylene diamine and edta complexes and theory of formation of step reduction waves in cupric copper systems

Polarographic and redox potential measurements on the cupric and cuprous complexes of ethylenediamine and EDTA have been carried out. From the ratio of the stability constants of the cupric and cuprous complexes, and the stability constant of the cupric complex, the stability constant of the cuprous-ethylenediamine complex is obtained. In the case of the EDTA complex it has been possible to obtain only βic/β2ous from the equilibrium concentrations of the cuprous and cupric complexes and the disproportionation constant. The inequalities for the appearance of step reduction waves have been given. The values of the stability constants of the cupric and cuprous complexes determined by the polarographic-redox potential method have been used to explain the appearance of step reduction waves in some systems and the non-appearance in other systems.

Polarographic and redox potential studies on copper(I) and copper(II) and their complexes. Part I. Copper(I and II)-ammonia and methylamine complexes

The equilibrium between cuprous ion, cupric ion and metallic copper has been studied using polarographic and redox potential measurements, by reducing cupric ion with copper gauze until equilibrium. Using the well-defined anodic diffusion current plateau, an amperometric method for estimating cuprous copper based on the titration of cuprous ion with dichromate or permanganate has been developed. The diffusion current constant and the disproportionation constant of cuprous ion and the standard potential for the reduction reaction of Cu2+ → Cu+ have been determined. Polarograms have been taken after reducing cupric complexes of ammonia and methylamine with copper until equilibrium. In the case of the copper-ammonia system, reduction to the cuprous state is practically complete while in the case of the cupric-methylamine system, the first cathodic wave occurs to some extent. A new method, called the polarographic-redox potential method, for determining the stability constants of cuprous and cupric complexes has been developed. The method depends upon the determination of the concentration of complexes by polarographic wave heights, and free cupric anc cuprous ions by redox potentials. The stability constants of the following complexes have been obtained: Cu(NH3)2+4, Cu(NH3)+2, Cu(CH3NH2)2(OH)2, Cu(CH3NH2)+2. The stability constants determined by the new method and the half-wave potential shift method agree and the value for the cupric-ammonia complex is in good agreement with Bjerrum method, indicating the reliability of this method.

Experimental evidence of the cavity theory of sorption-desorption hysteresis by a study of the effect of electric discharge on entrapped water in silica gel

With the use of the quartz fiber spring balance, sorptions and desorptions of water on silica gel at 30°C were studied and the permanent and reproducible hysteresis loop was obtained. At different points on the desorption curve forming the loop, the gel was subjected to high tension glow electric discharge. As a result of the electric discharge, the gel at any point on the desorption curve shifts to a corresponding point on the sorption curve. This is due to the release from the cavities of gel of the entrapped water held in a metastable state. The electric discharge has no effect on the gel at different points on portions of the desorption curve which coincide with the sorption curve and also on the sorption curve itself, indicating the absence of entrapped water in the gel in these regions. The results afford direct experimental evidence of the reality of the cavity theory of sorption-desorption hysteresis.

Template-Assisted Sol-Gel Synthesis of Nanocrystalline BaTiO3

Nanocrystalline perovskite barium titanate with an average particle size less than similar to 10 nm is produced using sol-gel route involving ligand-assisted templating. BaTiO3 is obtained by the controlled hydrolysis and condensation reaction of barium acetate (Ba(CH3COO)(2)) with titanium tetra chloride (TiCl4) in the reverse micelles of dodecylamine (DDA) which is used as the template. Our attempts to produce mesoporous BaTiO3 have resulted in the formation of nanocrystalline BaTiO3. The synthesis of nanostructured BaTiO3 is carried out using the ligand-assisted templating approach which proceeds through the sol-gel route. Dodecylamine is used as the template. The sol-gel process in general presents inherent advantages because the nanostructure of the desired materials can be controlled together with their porous structure. Ligand-assisted templating approach involves the formation of covalent bond between the inorganic analogue and the template. Ba(CH3COO)(2) and TiCl4 are used as barium-source and titanium-source respectively. The reaction between Ba(CH3COO)(2) and TiCl4 is found to take place deliberately on the pre-assembled species which acts as the template or occurring with in them which in turn will lead to the generation of the desired nanoscale structure (nanopores or nanoparticles).

Dual-Drug Delivery System Based on In Situ Gel-Forming Nanosuspension of Forskolin to Enhance Antiglaucoma Efficacy

The present study was designed to improve the bioavailability of forskolin by the influence of precorneal residence time and dissolution characteristics. Nanosizing is an advanced approach to overcome the issue of poor aqueous solubility of active pharmaceutical ingredients. Forskolin nanocrystals have been successfully manufactured and stabilized by poloxamer 407. These nanocrystals have been characterized in terms of particle size by scanning electron microscopy and dynamic light scattering. By formulating Noveon AA-1 polycarbophil/poloxamer 407 platforms, at specific concentrations, it was possible to obtain a pH and thermoreversible gel with a pH(gel)/T-gel close to eye pH/temperature. The addition of forskolin nanocrystals did not alter the gelation properties of Noveon AA-1 polycarbophil/poloxamer 407 and nanocrystal properties of forskolin. The formulation was stable over a period of 6 months at room temperature. In vitro release experiments indicated that the optimized platform was able to prolong and control forskolin release for more than 5 h. The in vivo studies on dexamethasone-induced glaucomatous rabbits indicated that the intraocular pressure lowering efficacy for nanosuspension/hydrogel systems was 31% and lasted for 12 h, which is significantly better than the effect of traditional eye suspension (18%, 4-6 h). Hence, our investigations successfully prove that the pH and thermoreversible polymeric in situ gel-forming nanosuspension with ability of controlled drug release exhibits a greater potential for glaucoma therapy.

Sol-Gel Synthesis, characterization and optical properties of Tio2 thin films deposited on ITO/Glass substrates

TiO2 thin films have been deposited on glass and indium tin oxide (ITO) coated glass substrates by sol-gel technique. the influence of annealing temperature on the structural , morphological and optical properties has been examined. X-ray diffraction (XRD) results reveal the amorphous nature of the as-deposited film whereas the annealed films are found to be in the crystalline anatase phase. The surface morphology of the films at different annealing temperatures has been examined by atomic force microscopy (AFM). The in situ surface morphology of the as-deposited and annealed TiO2 films has also been examined by optical polaromicrograph (OPM). TiO2 films infatuated different structural and surface features with variation of annealing temperature. The optical studies on these films suggest their possible usage in sun-shielding applications.

Novel Nanocomposites Made of Boron Nitride Nanotubes and a Physical Gel

This article describes successful incorporation of multiwalled boron nitride nanotubes (BNNTs) and various functionalized BNNTs by Lewis bases such as trioctylamine (TOA), tributylamine (TBA), and triphenylphosphine (TPP), etc., in organogels formed by triphenylenevinylene (TPV)-based low molecular weight gelator (LMWG) in toluene and consequent characterization of the resulting gel nanocomposites. Functionalized BNNTs were synthesized first,and the presence of tubular structures with high aspect ratio and increased diameter compared to the starting BNNTs was confirmed by SEM. TEM, and Raman spectroscopy. The micrographs of composites of I and BNNTs showed evidence of wrapping of the gelator molecules on to the BNNT surface presumably brought about by pi-pi stacking and van der Waals interactions, This leads to the formation of densely packed and directionally aligned fibrous networks. Such ``reinforced'' aggregation of the gelator molecules in presence of doped BNNTs led to an increase in the sot-to-gel transition temperature and the solidification temperature of the gel nanocomposites as revealed from differential scanning calorimetry. Rheological investigations of the gel nanocomposites indicate that the flow properties of the resulting materials become resistant to applied stress upon incorporation of even a very low wt % of BNNTs. Finally, the increase in thermal conductivity of the nanocomposite compared to the gelator alone was observed for the temperature range of 0-60 degrees C which may make these composites potentially useful in various applications depending on the choice and the amount of BNNT loading in the composite.

Preparation and characterization of rubidium-beta-alumina by the gel-to-crystallite conversion method

Preparation of Rb-beta -alumina was realized by the gel-to-crystallite conversion method. Reaction of hydrated aluminum hydroxide gel with RbOH in ethanol medium gave rise to the Rb+-inserted pseudoboehmite precursor under wet chemical conditions. The thermal decomposition of the precursor yielded Rb-beta -alumina. The Rb2O:Al2O3 ratio of monophasic Rb-beta -alumina ranged from 1:10 to 1:22. The extended stability in the compositional range is due to the fact that the conduction planes containing Rb+ and O2- ions can have lower occupancy of Rb+ ions for larger sized alkali ions, permitting the steric separation of the adjoining spinel blocks. High-resolution electron microscopy revealed that the decreasing occupancy of alkali ions in the conduction plane is balanced by changing widths of spinel blocks arising from the shift of tetrahedral Al3+ ions to octahedral sites and an accompanying increase in stacking defects. (C) 2000 Elsevier Science Ltd. All rights reserved.

Enhanced reducibility of Ce1-xTixO2 compared to that of CeO2 and higher redox catalytic activity of Ce1-x-yTixPtyO2-delta compared to that of Ce1-xPtxO2-delta

Nanocrystalline Ce1-xTixO2 (0 <= x <= 0.4) and Ce1-xTixPtyO2-delta (x = 0.15, gamma = 0.01, 0.02) solid solutions crystallizing in fluorite structure have been prepared by a single step solution combustion method. Temperature programmed reduction and XPS study of Ce1-xTixO2 (x = 0.0-04) show complete reduction of Ti4+ to Ti3+ and reduction of similar to 20% Ce4+ to Ce3+ state compared to 8% Ce4+ to Ce3+ in the case of pure CeO2 below 675 degrees C. The substitution of Ti ions in CeO2 enhances the reducibility of CeO2. Ce0.84Ti0.15Pt0.01O2-delta crystallizes in fluorite structure and Pt is ionically substituted with 2+ and 4+ oxidation states. The H/Pt atomic ratio at 30 degrees C over Ce0.84Ti0.15Pt0.01O2-delta is 5 and that over Ce0.99Pt0.01O2-delta is 4 against just 0.078 for 8 nm Pt metal particles. Carbon monoxide and hydrocarbon oxidation activity are much higher over Ce1-x-yTixPtyO2 (x = 0.15, 0.01, 0.02) compared to Ce1-xPtxO2 (x = 0.01, 0.02). Synergistic involvement of Pt2+/Pt degrees and Ti4+/Ti3+ redox couples in addition to Ce4+/Ce3+ due to the overlap of Pt(5d), Ti(3d), and Ce(4f) bands near E-F is shown to be responsible for improved redox property and higher catalytic activity.

Effect of La modification on antiferroelectricity and dielectric phase transition in sol-gel grown PbZrO3 thin films

Antiferroelectricity of sol-gel grown pure and La modified PbZrO3 thin films, with a maximum extent of 6 mol%, has been characterized by temperature dependent P-E hysteresis loops within the applied electric field of 60 MV/m. It has been seen that on extent of La modification electric field induced phase transformation can be altered and at 40 degrees C its maximum value has been observed at +/- 38 MV/m on 6 mol% modifications whereas the minimum value is +/- 22 MV/m on 1 mol%. On La modification the variation of electric field induced phase transformations at 40 degrees C has been correlated with the temperature of ntiferroelectric phase condensation on cooling. The critical electric fields for saturated P-E hysteresis loops have been defined from field dependent maximum polarizations and their variations on La modification show a similar trend as found in their dielectric phase transition temperatures. (C) 2010 Elsevier Ltd. All rights reserved.