Oleg Vyunov

Effects of Chemical Composition and Sintering Temperature on the Structure of La1 – xSrxMnO3 ± γ Solid Solutions

AbstractThe structural parameters of La1 – xSrxMnO3 ± γ solid solutions sintered at different temperatures are refined using chemical analysis, x-ray fluorescence, and Rietveld profile analysis data. The samples are shown to consist of two rhombohedrally distorted perovskite phases (sp. gr. R

Effect of Isovalent Ba-Site Substitutions on the Properties of (Ba1 – x – yMyYx)TiO3(M = Ca, Sr, Pb) PTCR Ceramics

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Lithium Ion Conductors Based on System (Li,Na,La){Ti,Nb,Та}O with Perovskite Structure

c and R3m) identical in chemical composition and containing excess oxygen. The electrical properties and magnetoresistive response of the Sr-doped lanthanum manganite ceramics depend on the relative amounts of these phases. The origin of structural disordering in the materials studied is discussed.

Complex Impedance Analyses of Ba1-xLi0.5xBi0.5xTiO3 Solid Solution PTCR Ceramics

The structural parameters of yttrium-doped (Ba,Pb)TiO3 solid solutions containing low-melting B2O3–PbO–SiO2 (BPS) glass additions were determined by Rietveld powder-diffraction analysis. It is shown that the introduction of BPS enables dense, semiconducting ceramics containing up to 60 mol % PbTiO3 to be produced by solid-state reactions followed by sintering at markedly reduced temperatures. Increasing the Pb content increases the lattice strain in the solid solutions, while BPS additions have the opposite effect. Sintering of BPS-containing materials leads to the formation of (Ва1 – χ – yPbχYy )(Ti1 – δPbδ)O3 solid solutions on grain surfaces (core–shell structure).

Evidence for changes on the lithium conduction dimensionality of Li0.5−yNayLa0.5Nb2O6 (0 ≤ y ≤ 0.5) perovskites

An approach is proposed for fabricating fine-grained, low-resistivity BaTiO3-based PTCR ceramics via partial isovalent substitutions on the Ba site. The grain size of the ceramics thus prepared is shown to decrease as the ratio of ionic radii r(Ba2+)/r(M2+) (M = Ca, Sr, Pb) increases. Isovalent substitutions on the Ba site narrow down the range of donor dopant (yttrium) concentrations in which PTCR materials can be prepared. The experimental results agree well with thermodynamic calculations under the assumption that the materials contain the Y3+Ti3+O3 phase, as suggested by ESR data, which point to the presence of Y3+–Ti3+ associates. Partial calcium, strontium, and lead substitutions on the Ba site reduce the average grain size of PTCR ceramics, which is probably due to the lattice strain arising from the isovalent substitution. Partial replacement of Ba2+ with mixtures of different isovalent elements (e.g., Sr2+ and Pb2+) offers the possibility of obtaining fine-grained, low-resistivity PTCR ceramics, without changing the phase transition temperature.

Some aspects of charge transport in Li0.5-xNaxLa0.5TiO3 (x = 0, 0.25) ceramics

The electrical properties and microstructure of (Ba,Y)TiO3 PTCR ceramics were studied. The results indicate that the Mn ions increase the intergranular barrier height and produce a high-resistance layer on the grain surface. The temperature-dependent resistances of the grain bulk, surface layer, and grain boundaries, the temperature coefficient of resistance, and the magnitude of the varistor effect were assessed as a function of Mn content.

(1-x)BaTiO 3 -x(Li 0.5 Bi 0.5 )TiO 3 PTCR solid solution

Solid solutions with defect perovskite structure have been obtained in the systems Li0,5-уNayLa0,5TiO3, Li0.5-yNayLa0.5◊Nb2O6 and Li0.5-yNayLa0.5◊Та2O6 at 0 ≤ y ≤ 0.5. Their structure has been shown to undergo partial disordering with increasing sodium content in the system Li0.5-yNayLa0.5◊Nb2O6 as in the system Li0.5-yNayLa0.5◊Та2O6 structure has been ordering. Lithium diffusion in system Li0,5-уNayLa0,5TiO3 follows a percolation model for the lithium diffusion. The ionic conductivity as a function of sodium content in the system Li0.5-yNayLa0.5◊Nb2O6 has a maximum. The ionic conductivity of Li0.5-yNayLa0.5◊Ta2O6 samples decreases with sodium content.

Structure and Properties of Nonstoichiometric La1 – xNa x MnO3 ± γ Solid Solutions

Conditions for the formation of (1-x)BaTiO3–xLi0.5Bi0.5TiO3 (0 ≤ x ≤0.6) solid solutions with positive temperature coefficient of resistance (PTCR) effect were studied. Solid solutions were prepared by solid state reaction technique. Samples were sintered under reducing atmosphere N2/H2 in the temperature range 1200–1450 °C with subsequent oxidation in air. The phase composition was investigated by X-ray powder diffraction method. It was found that samples of (1-x)BaTiO3–xLi0.5Bi0.5TiO3 (0 ≤ x ≤0.6) solid solutions at room temperature exhibit perovskite structure. Unit cell parameters of unstable at the room temperature compound Li0.5Bi0.5TiO3 were determined by extrapolation of concentration dependence of the unit cell parameters in the (1-x)BaTiO3–xLi0.5Bi0.5TiO3 system. It was shown that minimum value of resistivity ρmin rises with increase in x value. Complex impedance method shown that ceramic grains of (1-x)BaTiO3–xLi0.5Bi0.5TiO3 materials consist of three areas with different electrical properties. Boundary and outerlayer region of grains make the main contribution to the PTCR effect in lithium-containing solid solutions. It was shown that magnitude of the potential barriers decreases with increasing x.