Impurity centers in a barium titanate ceramic doped with rare-earth ions

The ceramic BaTiO₃ doped with rare-earth ions Y, La, Nd, Sm, Lu, and Dy to 0.1–0.5 at. % was studied in the temperature range 160<T<480 K by ESR. Several ESR spectra were observed. The most intense spectra have g factors close to 5.5 and 1.96. The dependences of their intensity, g factor, and width on the concentration of rare-earth ions and temperature were studied. Analysis of the data obtained made it possible to determine the critical concentration of the rare-earth ions x _c =0.2–0.3 at. %. It is characterized by the fact that for x<x _c or x>x _c all rare-earth ions, except Lu, replace predominantly Ba⁴⁺ or Ti⁴⁺, respectively. Models of paramagnetic centers were established: Fe³⁺-V _O (g≈5.5) and Ti³⁺-Ln³⁺ (g≈1.96), where V _O is an oxygen vacancy and Ln is a rare-earth ion. An abrupt change in axial symmetry to cubic for a Fe₃₊-V _O center at a phase transition from the tetragonal into the cubic phase was observed for the first time. The role of new centers in the appearance of the posistor effect is discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 1838–1842 (October 1999)     

Orientational order of a nematic polymer grafted on polytetrafluoroethylene

The orientation behavior of a nematic side‐chain polymer grafted on polytetrafluoroethylene (PTFE) is studied. The sandwich‐like composite was prepared by the graft post‐polymerization of 4‐cyan‐4′‐(biphenyloxy)butyl acrylate on a PTFE film pretreated by vacuum ultraviolet irradiation (VUV). The oriented liquid crystal layer was prepared by mechanical stretching of the composite film. Orientation degree was measured by means of FT‐IR spectroscopy. The orientation function increases with the increasing strain and attains the upper limit value at the strain Δl/l > 1.0. The temperature dependence of the orientation degree is completely reversible under cooling and heating. Copyright © 2000 John Wiley & Sons, Ltd.     

Polymer composites of nano-sized particles isolated in matrix

The kinetic and mechanism of thermal decay of transition metal acrylates or its cocrystallites are studied. At 200–300°C the rate of thermal decay can be described by first-order rate equations. The thermal transformations of the metal-containing monomers under study involve dehydration, solid-phase polymerization and thermolysis processes. The composition of products of the thermal transformations are studied. The products of the decay are analyzed by optical and electron microscopy as well as by magnetic measurements, IR and mass spectroscopy. The main solid-state products of decomposition are nanometer-sized particles of metal or its oxide and ferrites with a narrow size distribution stabilized by the polymeric matrix. The average particle sizes, L_av, are 6.0–13.0 nm in the case of FeAcr₃ and 6.0 nm for cocrystallites FeCoAcr and Fe₂CoAcr. © 1998 John Wiley & Sons, Ltd.     

(Co)polymerization of metal-containing monomers as a way of MMC synthesis

The principal advances and problems of the synthesis, of poly-, co- and graft- polymerization of metal-containing monomers (MCM) are analysed. The MCMs are classified on the type of the metal bond with the organic part of the molecule, viz. MCM with σ-, nv-, and π-bonds. The effect of a transition metal on both the polymerization and on the properties of the products formed is of special interest. The basic fields of application of metal-containing polymers have been summed up.     

On the solvability of the Cauchy problem with growing initial data for a class of anisotropic parabolic equations

We study the Cauchy problem for a doubly nonlinear parabolic equation with anisotropic degeneration in the case where the initial data are locally finite Radon measures growing, generally saying, at infinity. The weak solution of the problem is obtained as the limit of regular solutions with smoothed initial data.     

Reaction of 2-methyl-3,4-dihydro-β-carbolin-2-ium iodide with acylmethyl halides controlled by electronic effects: a new route to 1,2-dihydroazepino[4,5-b]indoles

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