Magnetic nanoparticles in MCM-41 type mesoporous silica

Structural phase transformations and magnetic properties of mesoporous MCM-41 template modified with iron and nickel salts were studied by nitrogen physisorption, X-ray diffraction, Mossbauer spectroscopy and transmission electron microscopy. The FeNi-oxide or the bimetallic crystal structure is formed for low and high Ni concentrations, respectively. The average size of nanoparticles is about 10 nm. About 70% of particles exist in a superparamagnetic state at room temperature

Permeation of CO2 and N2 through glassy poly(dimethyl phenylene) oxide under steady- and presteady-state conditions

Glassy polymers are often used for gas separations because of their high selectivity. Although the dual-mode permeation model correctly fits their sorption and permeation isotherms, its physical interpretation is disputed, and it does not describe permeation far from steady state, a condition expected when separations involve intermittent renewable energy sources. To develop a more comprehensive permeation model, we combine experiment, molecular dynamics, and multiscale reaction–diffusion modeling to characterize the time-dependent permeation of N2 and CO2 through a glassy poly(dimethyl phenylene oxide) membrane, a model system. Simulations of experimental time-dependent permeation data for both gases in the presteady-state and steady-state regimes show that both single- and dual-mode reaction–diffusion models reproduce the experimental observations, and that sorbed gas concentrations lag the external pressure rise. The results point to environment-sensitive diffusion coefficients as a vital characteristic of transport in glassy polymers

Tailoring the oxidation state of cobalt through halide functionality in sol-gel silica

The functionality or oxidation state of cobalt within a silica matrix can be tailored through the use of cationic surfactants and their halide counter ions during the sol-gel synthesis. Simply by adding surfactant we could significantly increase the amount of cobalt existing as Co3O4 within the silica from 44% to 77%, without varying the cobalt precursor concentration. However, once the surfactant to cobalt ratio exceeded 1, further addition resulted in an inhibitory mechanism whereby the altered pyrolysis of the surfactant decreased Co3O4 production. These findings have significant implications for the production of cobalt/silica composites where maximizing the functional Co3O4 phase remains the goal for a broad range of catalytic, sensing and materials applications

Testing CPT symmetry in ortho-positronium decays with positronium annihilation tomography

Charged lepton system symmetry under combined charge, parity, and time-reversal transformation (CPT) remains scarcely tested. Despite stringent quantum-electrodynamic limits, discrepancies in predictions for the electron–positron bound state (positronium atom) motivate further investigation, including fundamental symmetry tests. While CPT noninvariance effects could be manifested in non-vanishing angular correlations between final-state photons and spin of annihilating positronium, measurements were previously limited by knowledge of the latter. Here, we demonstrate tomographic reconstruction techniques applied to three-photon annihilations of ortho-positronium atoms to estimate their spin polarisation without magnetic field or polarised positronium source. We use a plastic-scintillator-based positron-emission-tomography scanner to record ortho-positronium (o-Ps) annihilations with single-event estimation of o-Ps spin and determine the complete spectrum of an angular correlation operator sensitive to CPT-violating effects. We find no violation at the precision level of 10−4, with an over threefold improvement on the previous measurement

Adsorption Properties of Hydrated and Thermally Modified Silica Gel

The adsorption process of benzene + cyclohexane mixtures on silica gel with modified surface has been studied. Silica gel was thermally modified at 473 K, 823 K and 1023 K. Another set of modified silicas was prepared by adsorption of different amounts of water on the surface. The characteristics of the silicas were established on the basis of adsorption isotherms of nitrogen at 78 K and thermogravimetric measurements. The stability of the adsorbed layer of water in the adsorption process from the liquid phase was examined by using the Karl-Fischer method. Excess adsorption isotherms of benzene from cyclohexane were discussed in terms of a theoretical formulation concerning the adsorption on heterogeneous solid surfaces. It has been found that energetical homogeneity of the silica gel surface increases from the silica gel activated at 473 K through silicas deactivated at 823 K and 1023 K to silica gels with physically adsorbed water

Effects of Surface Heterogeneity in Adsorption from Binary Solutions on Modified Silica Gels

The adsorption of benzene from cyclohexane solutions on modified silica gels is analysed using expressions derived from the model of adsorption of binary solutions on heterogeneous solid surfaces. The adsorption equation is found to represent the data for several systems and leads to a method for estimating the energetical heterogeneity of the adsorbent surface. The energy distribution functions for investigated systems are calculated. The same adsorption data are analysed using the simple form of isotherm equation for adsorption of binary liquid mixtures on heterogeneous solid surfaces. The results obtained by using two different methods for the same adsorption systems are discussed