N.N. Oleynikov

Synthesis of nanocrystalline TiO2 powders from aqueous TiOSO4 solutions under hydrothermal conditions

Abstract Nanocrystalline anatase powders with particle sizes from 8 to 38 nm have been synthesized by hydrothermal treatment of aqueous TiOSO4 solutions and TiO2·nH2O amorphous gel. The products have been characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and BET method.

On the stability region and structure of the Nd1+xBa2−xCu3Oy solid solution

Abstract We studied the low and upper stability boundaries and the structure evolution of the Nd 1+ x Ba 2− x Cu 3 O z solid solution along its single-phase field. It was found that in oxygen the limit of the solid solution extends up to Nd 1.9 Ba 1.1 Cu 3 O z at 950–1050°C while at lower and higher temperatures it becomes narrower. In nitrogen atmosphere the substitution range is much smaller and x does not exceed 0.3 at 800°C. The replacement of Ba 2+ by Nd 3+ is accompanied by decreasing both a and c lattice constants of quenched samples. At the same time in oxygen at x ≥ 0.6 ordering phenomena occur leading to the orthorhombic distortion of the tetragonal subcell. A model of the solid solution crystal structure with a B-centered unit cell and lattice parameters a = 2 a sub , b = b sub and c = 2 c sub was suggested and refined.

Pressure-induced dimerization of fullerene C60: a kinetic study

Abstract The kinetics of pressure-induced dimerization of fullerite C60 at 1.5 GPa in the 373–473 K temperature interval was studied by X-ray diffraction, infrared and Raman spectroscopy. Kinetic curves of the dimerization reaction in the fcc and sc phases of C60 were obtained by monitoring the dimeric (C60)2 IR line at 796 cm−1. The value of the dimerization activation energy was determined to be E a ( dim ) =134±6 kJ mol −1 , assuming the second order irreversible reaction. The peculiarities of the dimerization processes in the fcc and sc phases of C60 fullerite are also discussed.

Structural disorder and superconductivity suppression in NdBa2Cu3Oz (z∼7)

Abstract Structural features of a low-Tc nearly stoichiometric Nd1+xBa2−xCu3Oz phase (x⩽0.05, z∼6.9, a=3.897–3.899 A , b=3.902–3.908 A , c=11.707–11.719 A ) have been investigated for the first time by XRD, EXAFS structure refinement techniques and Raman spectroscopy. It has been found that the abnormal lattice parameters and the low superconductivity transition temperature (55 K) of such a phase are related to structural disorder in the Ba and Nd sites resulting in oxygen disordering which cannot be avoided by a standard oxidation procedure. It is shown that longer annealing at high temperatures is an important factor for reducing the oxygen disordering and hence enhancing superconducting properties after oxidation.

Structural disorder and degradation of superconductivity in the NdBa2Cu3Oz (z∼7) defects simulations

Abstract This short paper is an extension of our previous studies of low- T c stoichiometric NdBa 2 Cu 3 O z prepared by polymerizable complex method and via nitrates melting technique. The most recent results describing the effect of annealing time and temperature on the superconducting transition for the samples prepared from the fine precursor are presented. The defects simulation was applied to understand the previously unexplained structural data discovered by EXAFS.

Phase equilibria and the thermodynamics of coexisting phases in the system iron-lithium-oxygen☆

Abstract Phase equilibria in the system Fe 2 O 3 FeOLi 0.5 Fe 0.5 O have been studied over a broad range of temperatures (900–1200°C) and partial pressure of oxygen (10 −15 – 1 atm) both by quench methods and solid-electrolyte electromotive force measurements together with chemical and X-ray analyses. Equilibrium conditions have been determined for the formation of iron lithium spinel, Li x Fe 3− x O 4+γ , which changes its non-stoichiometric character according to the amount of oxygen present from an excess of the latter (γ > 0) when x ≈ 0 to a deficity (γ x approaches 0.5. The activity of the components in coexisting spinel and wustite phases has been calculated. It is shown that solutions of lithium ferrite with magnetite in equilibrium with wustite are characterized by small deviations from ideality—positive in the region which is rich in magnetite and negative in the concentration range bordering on lithium ferrite. Furthermore, ΔH is approximately zero, i.e., the solutions are athermal. Solutions of lithium ferrite and magnetite in equilibrium with hematite and solutions of wustite and lithium orthoferrite in equilibrium with the spinel phase are characterized by marked deviations from ideality.

Kinetics of Solid State Reactions With Fractal Reagent

In the present research we theoretically studied the kinetics of nucleation-limited solid state reactions as influenced by the fractal properties of solid reagent. We consider the model of equal-sized primary particles assembled in fractal cluster. The geometry of such an object is assumed to be described solely by its fractal dimension D and by upper (Rmax) and lower (Rmin) cutoffs of fractality further identified with the overall size of the object and the size of the primary particle correspondingly. Depending on the ratio between Rmax, Rmin and the radius of the critical nucleus Rnucl the following cases are considered: (1) Rmax∼ Rnucl. In this case the reaction kinetics is described by the equation: α = 1 − B{ln(k′ τ + 1)}D/(D−3), where B, k′ are constants. Numerical solution of this equation gives rise to n-order reaction kinetics with n & 1. (2) Rmin ≪ Rnucl ≪ Rmax. In this case under certain conditions there can exist non-trivial critical density ρcrit ≠ 0, 1 that favors the formation of the critical nuclei of the new phase. The asymptotic kinetic equation for large times corresponds to n-order reaction with n = (D + 3)/(D + 1). (3) Rmin ≪ Rnucl ∼ Rmax′. In this case the reaction follows the first-order kinetics with D-dependent rate constant.

Fast synthesis of YBa2Cu3Oz superconductor at low temperatures of its orthorhombic modification existence using mechanically activated and densified two-powder precursors

Abstract The possibility of developing a fast (10–30 min) and low-temperature (500–600°C) synthesis of YBa 2 Cu 3 O z superconductor with a T c up to 90 K appearing immediately after preparation is discussed. Mechanically activated and densified two-powder precursors have reproducibly yielded 80–90% XRD-pure YBa 2 Cu 3 O z phase when using internal oxygenation of soft-chemistry produced Cu 2 O by YBa 2 O 3.5 (containing BaO 2 ) in their mixture (Y:Ba:Cu=1:2:3) mechanically activated in a planetary mill and warmly compacted at 250°C into a dense pellet. Such advantages allowed us to obtain trial samples of aluminum-sheathed chemistry tapes with the YBa 2 Cu 3 O z core.

Synthesis of Nanostructured Iron Oxide(III) Powders by Rapid Expansion of Supercritical Fluid Solutions

Nanostructured a-Fe 2 O 3 powders were generated by rapid expansion of supercritical fluid solutions (RESS, T=773 K, P=100 MPa) and by rapid thermal decomposition of precursors in solution ( RTDS, T=623 K, P=100 MPa) on lab RESS-setup from 0,040 M and 0,10 M aqueous solutions of Fe(NO 3 ) 3 . The size of subcrystallites is about 22-29 nm. Comparison of reactivity of α-Fe 2 O 3 powders in a model solid state reaction between a-Fe 2 O 3 powders (generated by RESS from 0,040 M solution) and Li 2 CO 3 (mole ratio 1:1) with literature data on a-Fe 2 O 3 powders produced by other methods shows that its reactivity is markedly higher. A basic essence possibility of zinc ferrite ZnFe 2 O 4 formation immediately at the stage of the rapid expansion (T=773K; P=100 MPa) of a supercritical aqueous solution of zinc and iron nitrates (molar ratio Zn:Fe=1:2; C=0. 1 M) was shown.

Visible spectra of fractal particles in colloidal solutions

The visible spectra of fractal colloidal particles are analysed. The considerable broadening of the light absorption peak for colloidal solutions is supposed to be due to additional oscillatory degrees of freedom arising in a fractal cluster. The shape of the low-frequency edge of the absorption peak is shown to obey the following formula: I(λ) ∝ λ−D, where I is the intensity of the absorbed light, λ is the wavelength and D is the fractal dimension of the particles in colloidal solution.