Yu. V. Kabirov

Synthesis of Nanostructured Pt x Ni/C and Pt x Co/C Catalysts and Their Activity in the Reaction of Oxygen Electroreduction

Nanostructured PtxNi/C and PtxCo/C catalysts (1 ≤ x ≤ 3) containing from 27 to 40 wt % of metal with the average size of metal (alloy) particles from 2.6 to 4.7 nm are obtained using the methods of liquid-phase synthesis. Addition of a doping component is found to yield a decrease in the average nanoparticle sizes and an improved activity of materials in oxygen electroreduction reactions. The obtained catalysts are characterized by high specific activity as compared to commercial Pt/C materials. The stability of electrocatalysts to selective dissolution of the doping component is evaluated.

Sequence of phase transitions in PbHfO3

The temperature changes in the PbHfO3 structure in the temperature range of 20 < t < 400°C have been studied by X-ray powder diffraction. The sequence of phase changes with an increase in temperature was found to be as follows: orthorhombic phase Pbam (O1), orthorhombic phase C2mm (O2), tetragonal phase P4mm (T), and cubic phase Pm3m (C). The C2mm and P4mm phases are ferroelectric, which is confirmed by measuring the dependences ɛ(t). The similarity of the transition pattern obtained with the known transition sequences for ferroelectric (barium titanate, potassium niobate, and lead titanate) and antiferroelectric (lead zirconate) oxides is analyzed.

Structure Formation and Phase Transitions in Cadmium Titanate

Crystal structure formation in cadmium titanate of the ilmenite and perovskite phases consisting of a stoichiometric mixture of cadmium oxide and titanium dioxide was studied by X-ray powder diffraction. Unit cell parameters and their thermal changes below 1000°C were determined. The mechanism of the ilmenite–perovskite phase transition is discussed.

Magnetoresistivity of La0.7Sr0.3MnO3/Sb2O3 glass composites at room temperature

The ceramic composites La0.7Sr0.3MnO3/Sb2O3 with different weight ratios of the components have been synthesized. Their magnetoresistive properties have been studied in weak magnetic fields. The values of negative isotropic magnetoresistance of the 90%La0.7Sr0.3MnO3/10% Sb2O3 composites at room temperature are found to be 1.2% and 14% for 0.25 and 20 kOe, respectively. The observed increase in magnetoresistance with an increase in the field has specific features of the relaxation process.

Structure and phase transitions of lead zirconate hafnate solid solutions

Solid solutions of two antiferroelectrics, namely, lead zirconate and lead hafnate, i.e., PbZr1 − xHfxO3 (x = 0, 0.3, 0.5, 0.7, 1.0), have been prepared by solid-phase synthesis from simple oxides. The elemental composition of the samples and their structure have been studied using X-ray fluorescence and X-ray diffraction analyses. The phase transformations of the solid solutions have been determined in the temperature range 20°C < T < 300°C.

Phase states in the Y1 − xCaxMnO3 solid solution system

The room-temperature stability region of the tetragonal phase in the Y1 − xCaxMnO3 solid solution system has been found for the first time. The concentration boundaries between this phase and the orthorhombic (x > 0.5) and hexagonal (x < 0.25) phases are sensitive to the sample preparation conditions. The coexistence of the hexagonal and tetragonal phases in the range 0.15 < x < 0.25 indicates that these phases undergo a first-order reconstructive phase transition. Preliminary measurements of magnetic ordering effects for compositions with x = 0.3, 0.5, and 0.7 have been performed.

Magnetic properties of (SrFe12O19)x(CaCu3Ti4O12)1–x composites

New composite materials (SrFe12O19)x(CaCu3Ti4O12)1–x (x = 0, 0.05, 1) have been synthesized. Their magnetic properties are studied in the temperature range 5–300 K using the magnetic resonance and magnetometry methods. It is found that strontium hexaferrite microinclusions in the (SrFe12O19)0.05(CaCu3Ti4O12)0.95 composite “magnetize” CaCu3Ti4O12 at temperatures from 300 to 200 K, forming a ferrimagnetic particle near the SrFe12O19 “core.” The magnetic resonance line below 200 K splits into two lines corresponding to SrFe12O19 and CaCu3Ti4O12. The core effect decoration is manifested in the increase in the Curie–Weiss temperature from 25 K in CaCu3Ti4O12 without the doping ceramics to 80 K in the composite with 5% of SrFe12O19.

Specific features of structural states in the BiFeO3-YMnO3 solid solutions

The (1 − x)BiFeO3−xYMnO3 solid solutions have been found to undergo the following sequence of phase transformations with increasing x: R3c → Pbnm → C2 → Pnma → P63cm. It has been established that the Pbnm and Pnma phases have different orientations of atomic displacements and can exhibit antiferroelectric properties.

Negative magnetoresistance of (1–x)La0.7Sr0.3MnO3/x(GeO2, Li4P2O7) composite ceramics

Ceramics of (1–x)La0.7Sr0.3MnO3/x(GeO2) and (1–x)La0.7Sr0.3MnO3/x(Li4P2O7) compositions with x = 0.1–0.3 have been synthesized that possess high absolute values of isotropic negative magnetoresistance MR at room temperature. In a composite containing 20 mass % GeO2 as the barrier component, the negative magnetoresistance reaches MR = 15% in a magnetic field of 18 kOe, while an analogous composite with Li4P2O7 under identical conditions has MR = 16%.

Specific features of the dielectric spectrum of CaCu3Ti4O12 in the low-frequency range

The dielectric spectra of CaCu3Ti4O12 ceramics with giant values of the dielectric constant have been presented. The spectrum exhibits Debye dispersion with an activation energy of 0.075 eV in the high-frequency range of 106–107 Hz, and a characteristic plateau has been seen in the frequency dependence of the dielectric constant in the range of 104–105 Hz. A considerable increase in the loss factor ɛ″ has been observed in the low-frequency part of the spectrum and a capacitive-to-inductive impedance transition has been revealed in some samples at a frequency below 10−2 Hz. The causes of this transition have been discussed.