Enhancement in electrical and magnetodielectric properties of Ca‐ and Ba‐doped BiFeO3 polycrystalline ceramics

We carried out a comparative study on the electrical and magnetodielectric properties of polycrystalline BiFeO₃, Bi_0.9Ca_0.1FeO_2.95, Bi_0.9Ba_0.05Ca_0.05FeO_2.95, and Bi_0.9Ba_0.1FeO_2.95 ceramics. The two dielectric anomalies, near 25 K and 281 K, are observed for BiFeO₃. Interestingly, the anomaly near 25 K shifts towards a higher temperature above 60 K with Ca and/or Ba doping, attributed to the doping induced chemical pressure. In addition, the room temperature switchable magnetodielectric effect is witnessed for the doped BiFeO₃ compounds, due to the quadratic magnetoelectric coupling. This indicates the improved magnetoelectric coupling in BiFeO₃ with the Ca and Ba doping. This is essentially due to the enhanced magnetic ordering and reduced leakage current in BiFeO₃ after the doping.     

Time-resolved photoluminescence of type-II Ga(As)Sb/GaAs quantum dots embedded in an InGaAs quantum well

Optical properties and carrier dynamics in type-II Ga(As)Sb/GaAs quantum dots (QDs) embedded in an InGaAs quantum well (QW) are reported. A large blueshift of the photoluminescence (PL) peak is observed with increased excitation densities. This blueshift is due to the Coulomb interaction between physically separated electrons and holes characteristic of the type-II band alignment, along with a band-filling effect of electrons in the QW. Low-temperature (4?K) time-resolved PL measurements show a decay time of [Formula: see text]?ns from the transition between Ga(As)Sb QDs and InGaAs QW which is longer than that of the transition between Ga(As)Sb QDs and GaAs two-dimensional electron gas ([Formula: see text]?ns).     

Settlement, Sliding, and Liquefaction Remediation of Layered Soil

A series of highly instrumented, large-scale centrifuge models have been tested to investigate the extent of remediation required to control settlement and lateral sliding of soil deposits at a hypothetical bridge site. The baseline model represents a prototype with a 9-m-thick layer of fine sand having a relative density (Dr) of 50%. The sand layer is overlain by clay floodplains with a free face at a river channel. One nearly level floodplain surface supports a bridge abutment. The other floodplain has a 9% slope toward the river. In different models, different amounts of the 50% relative density sand was densified to Dr = 80%. Full depth improvement reduced settlements and lateral sliding of the sand by about a factor of 3. Due to the effects at the clay-sand interface, lateral sliding of the surficial clay deposit was not controlled by densification of the sand. Tests in which the width of the densified zone was only about 75% of the thickness of the loose sand indicated that relatively narrow zones of improvement can control settlement and sliding of the sand. Differences in shear resistance, pore pressures, dilatancy, and energy dissipation in loose and dense sands are presented.     

Synthesis and spectral characterization of surface‐enriched polymer‐supported phase transfer catalysts and their effects on alkylation of phenylacetone

Polymer‐supported phase transfer catalysts with active sites mostly on the surface were prepared by suspension copolymerization of styrene (St), divinylbenzene (DVB), and vinylbenzyl chloride (VBC) with AIBN, followed by the quaternization of the resulting copolymer beads with triethylamine. Active sites on the surface were achieved by the delayed addition of functional monomer (VBC) to the partially copolymerized St/DVB. Polymer beads enriched with active sites were characterized by SEM, EDAX, FTIR, and ESCA. The electron micrographs showed that the exterior surface of delayed‐addition functional monomer catalysts (type 1) has a large number of nodules attached to the surface compared to the smooth surface exhibited by the conventional type 2 catalyst upon the simultaneous addition of all three monomers. In the EDAX analysis up to a depth of 100 Å, the surface chloride of type 1 peak intensity is greater (compared with type 2), indicating the  CH₂Cl enrichment on the surface. In FTIR, the peak intensities of the C N stretching (quaternary onium group) in type 1 are greater than those of type 2, confirming the evidence of more quaternization on the surface than in the bulk. From ESCA analysis to a depth of about 30 Å, it was found that type 1 (beads) contains 26% and type 2 contains 14% of covalent chloride on the surface, which strongly supports the grafting of VBC on St/DVB. In the estimation found by the Volhard method, type 1 has 4.73 m eq g⁻¹ and type 2 has 2.29 m eq g⁻¹ of ionic chloride, thus supporting the surface grafting of VBC. The catalytic activity of these two different catalysts was tested by studying the reaction, that is, the C‐alkylation of phenylacetone. The rate constants of this reaction for type 1 are almost twofold greater than those of type 2, a finding that could uphold the preceding experimental observations. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 408–418, 2000     

Phase‐transfer catalysis: Free‐radical polymerization of acrylonitrile using potassium peroxomonosulphate– tetrabutyl phosphonium chloride catalyst system: A kinetic study

This article presents the systematic study of kinetics and mechanism of phase‐transfer‐catalyzed free‐radical polymerization of acrylonitrile (AN) and water‐soluble initiator potassium peroxomonosulphate (PMS) coupled with tetrabutyl phosphonium chloride (TBPC) in ethyl acetate/water biphase system in the temperature range 45–55°C at fixed pH and ionic strength. The rate of polymerization increases with an increase in concentrations of AN, PMS, and phase transfer catalyst, PTC. It was observed that R_p is proportional to [AN]^1.5, [KHSO5₅⁻]^0.5, and [TBPC]^0.5. A suitable kinetic scheme has been proposed to account for the experimental observations and its significance was discussed. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1564–1571, 2000     

Effect of functionality levels and compatibility of polycarbonate blends with maleic anhydride grafted ABS

ABS was melt grafted with maleic anhydride at three different levels of 1, 2, and 3 wt %. These three different modified ABSs were melt blended with polycarbonate using a single‐screw extruder to choose a suitable maleic anhydride grafting level on ABS for better performance. For the compatibility study, binary blends of polycarbonate with ABS and maleic anhydride‐grafted ABS were prepared over the entire range of compositions. Compatibility of these blends was studied using a Dynamic Mechanical Analyzer and by Differential Scanning Calorimetry. Both techniques suggest more partial compatibility for modified blend systems. In addition to this, DSC thermograms show multiple peaks between the transition points of ABS fractions and polycarbonate fractions for the polycarbonate/maleic anhydride‐grafted ABS blends. These multiple peaks are characteristic of better partial compatibility with fine microstructure. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2102–2110, 1999     

Temperature-jump apparatus with Raman detection based on a solid-state tunable (1.80-2.05 microm) kHz optical parametric oscillator laser

The operating characteristics of a pulsed (10 ns) tunable near-infrared (NIR) laser source are described for temperature-jump (T-jump) applications. A Q-switched Nd:YLF laser (approximately 10 ns pulses) with a 1 kHz repetition rate is used to pump a potassium titanyl arsenate (KTA) crystal-based optical parametric oscillator (OPO), producing approximately 1 mJ NIR pulses that are tunable (1.80-2.05 microm) across the 1.9 microm vibrational overtone band of water. This T-jump source has been coupled to a deep ultraviolet (UV) probe laser for Raman studies of protein dynamics. T-jumps of up to 30 degrees C, as measured via the O-H stretching Raman band of water, are readily achieved. Application to cytochrome c unfolding is demonstrated.     

Ferroelectric order in individual nanometre-scale crystals

Ferroelectricity in finite-dimensional systems continues to arouse interest, motivated by predictions of vortex polarization states and the utility of ferroelectric nanomaterials in memory devices, actuators and other applications. Critical to these areas of research are the nanoscale polarization structure and scaling limit of ferroelectric order, which are determined here in individual nanocrystals comprising a single ferroelectric domain. Maps of ferroelectric structural distortions obtained from aberration-corrected transmission electron microscopy, combined with holographic polarization imaging, indicate the persistence of a linearly ordered and monodomain polarization state at nanometre dimensions. Room-temperature polarization switching is demonstrated down to ~5?nm dimensions. Ferroelectric coherence is facilitated in part by control of particle morphology, which along with electrostatic boundary conditions is found to determine the spatial extent of cooperative ferroelectric distortions. This work points the way to multi-Tbit/in(2) memories and provides a glimpse of the structural and electrical manifestations of ferroelectricity down to its ultimate limits.     

Studies on the Behaviour of Thiourea in Bright Nickel Plating

An attempt has been made to discuss the various phenomena taking place at a nickel electrode during the deposition of bright nickel from a Watts bath containing different concentrations of thiourea. The potential, polarization and capacity measurements indicate that the decomposition products rather than thiourea itself are responsible for the increased polarization arid the decrease in capacity and so for the consequent inhibition. Spectrophotometric measurements in the ultra violet region indicate the presence of another absorption range, in addition to that corresponding to thiourea, for the solution of thiourea in a Watts bath. It remains to be confirmed, however, whether it corresponds to the nickel-thiourea complex or to the decomposition products of thiourea. Spectrophotometric measurements have also indicated that the consumption of thiourea at the cathode is greater than at the anode and the difference is still greater in the presence of oxygen.