Interaction of lithium-titanium ceramics with air during sintering

Interaction of lithium-titanium polycrystalline ferrites with ambient air during their sintering is investigated using the methods of electroconductivity and thermal gravimetry within the temperature interval T = 600–1300 K. It is found out that the temperatures exceeding T = 1100 K correspond to effective course of reducing reactions in this material, which results in the formation of an oxygen-deficient ceramic structure in the stage of isothermal tempering (T = 1280 K). It is shown that in the final cooling stage the process of exchange of the ferrite ceramics with the ambient atmosphere exhibits a pronounced oxidative character. The role of these redox processes in the formation of the oxygen non-stoichiometry of the sintered specimen and the depth of its distribution are analyzed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 35–40, February, 2007.     

Magnetic and Gas Sensing Property of Nanosized NiFe2O4 Powders Synthesized by Pulsed Wire Discharge

Nickel ferrite (NiFe₂O₄) powders were synthesized by pulsed wire discharge method. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses show that only nickel ferrite spinel and no other phase was observed in the powders. Mean size of the obtained particles strongly depended on the oxygen pressure: the higher oxygen pressure corresponds to larger powder size, as determined by Brunauer–Emmet–Teller (BET). The room temperature saturation magnetization of the synthesized powders was 42–46 emu/g depending on the powder size. These powders also showed high chlorine sensitivity at about 280–360°C, and a good linear sensitivity with chlorine concentration.     

Inverse magnetocaloric effect in sol–gel derived nanosized cobalt ferrite

The magnetocaloric properties of cobalt ferrite nanoparticles were investigated to evaluate the potential of these materials as magnetic refrigerants. Nanosized cobalt ferrites were synthesized by the method of sol–gel combustion. The nanoparticles were found to be spherical with an average crystallite size of 14 nm. The magnetic entropy change (ΔS _m) calculated indirectly from magnetization isotherms in the temperature region 170–320 K was found to be negative, signifying an inverse magnetocaloric effect in the nanoparticles. The magnitudes of the ΔS _m values were found to be larger when compared to the reported values in the literature for the corresponding ferrite materials in the nanoregime.     

Investigation into the special features of the dielectric and polarization properties of lithiumtitanium ferrite ceramics

Loop-shaped dependences of the electric polarization on the electric field strength (the dielectric hysteresis) are registered for the first time for polycrystalline Li–Ti ferrite. Temperature evolution of the hysteresis loop parameters is investigated for ferrite samples. The results obtained can be interpreted from the viewpoint of the Maxwell–Wagner relaxation polarization or induced ferroelectric-like state in the electric ferrite subsystem.     

Determination of the Oxygen Diffusion Coeficient in Polycrystalline Li-Ti Ferrites

Two independent methods, the isotope method based on nuclear microanalysis and the method based on measuring the electronic-conductivity activation energy, are used to determine the grain-boundary diffusion and volume diffusion of an oxygen isotope ¹⁸O in a polycrystalline lithium-titanium ferrite at the thermal annealing temperature 1073 K. A comparative analysis is conducted of the potential of the methods in studying oxygen diffusion in the material concerned. It is shown that the technique for obtaining the diffusion parameters from the electronic conductivity measurements allows a comparatively precise determination of both the volume and grain-boundary diffusion coefficients of oxygen in polycrystalline ferrites.     

Temperature dependences of the dielectric properties of lithium-titanium ferrite ceramics

Temperature dependences of the real ?′ and imaginary ?″ parts of the complex permittivity of lithium-titanium ferrite ceramics are measured in the frequency range 10²?10⁶ Hz at different test-signal amplitudes and dc bias voltages. It is found that the dielectric characteristics of the ceramic samples drastically change in narrow temperature ranges. The assumption is made that relaxators whose reorientation is due to tunneling transitions of electrons inside “bivalent iron ion-trivalent metal ion” pairs are involved in polarization processes. Under certain conditions, the reorientation of relaxators can have collective character.     

On the causes of formation of non-optimal structures in a pressure-treated low-carbon steel

An electron microscopy investigation of structural-phase and stress-strained state of a hot-forged material workpiece is performed. It is found out that the scalar dislocation density in ferrite grains and ferrite interlayers of pearlite of steel from the forged piece fractured after technological processing is one and half times higher than that in the commercial material. The metal in this state has a higher content of sulfides of lamellar morphology. The volume fraction of pearlite in it is 1.5–2 times larger, with the lamellar pearlite prevailing, and the local long-range stresses being comparable with the yield stress. It is found out that the reason for formation of an unfavorable structural-phase state is the elevated carbon content, which resulted in overheating of the metal both during pressure treatment and final thermal treatment. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 8–12, October, 2007.     

Fine structure of vibratory spectra and cation distribution in nonstoichiometric Mn-Zn ferrites

We have studied vibratory spectra in the range 200–700 cm⁻¹ of Mn−Zn ferrite crystal lattices containing both an iron excess or an iron deficit of up to 8 mole% relative to the stoichiometric compound. We performed a Kramers-Kronig analysis of the reflectance spectra, and a dispersion analysis of the spectra of the dielectric functions. The vibratory spectral band is quite complex. The compositional dependence of the individual component intensities is explained by a cation distribution model. According to this model the manganese cations in the octahedral sites are +3 valence in both iron-poor and iron-rich ferrites. Kazanskii State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 34–40, October, 1996.     

Tunneling interference of counterpropagating waves in the region of negative magnetic permeability

The interference of two coherent ultrahigh-frequency waves incident normally on opposite faces of a transversely magnetized ferrite layer with negligibly small losses is investigated in the region of negative effective magnetic permeability. It is shown that a coordinate-independent energy flux is formed inside the layer as a result of tunneling interference. The ferrite layer can be made antireflective for appropriate values of the external field and the phases and amplitudes of the incident waves. Zh. Tekh. Fiz. 67, 77–80 (October 1997)     

Microwave and MM-wave magnetoelectric interactions in ferrite-ferroelectric bilayers

Measurements of the strength of magnetoelectric (ME) interactions at microwave and millimeter-wave frequencies have been carried out on layered ferrite-piezoelectric oxides. An electric field E applied to the composite produces a mechanical deformation, resulting in a field shift δH_E or a frequency shift δf_E in the resonance. A stripline structure or a cavity resonator was used. The strength of ME coupling is obtained from data on δH_E or δf_E vs. E. Studies were performed at 1–110 GHz on bilayers of single crystal nickel zinc ferrite or hexagonal ferrites and single crystal lead magnesium niobate-lead titanate, lead zinc niobate-lead titanate or polycrystalline lead zirconium titanate. The coupling strength has been found to be dependent on the nature of piezoelectric phase, magnetic field orientation and volume for both phases. The ME coupling strength is on the order of 1–2 Oe cm/kV (or 3–6 MHz cm/kV) and is an order of magnitude stronger than in polycrystalline ferrite-piezoelectric bilayers. The high frequency ME effect is of importance for dual electric and magnetic field tunable ferrite-piezoelectric devices.     

Bragg reflection during the propagation of magnetoelastic microwave pulses in a structure consisting of a ferrite thin film on a dielectric substrate

The propagation of a microwave pulse in a ferrite thin film-substrate structure in a regime of rereflections (“ringing”) of the acoustic component of the substrate is studied theoretically. It is shown that as a result of the interaction of microwave pulses with the boundaries of the substrate, propagation of a microwave excitation in this system can be regarded as a propagation of a wave packet in a periodic nonuniform medium. The basic characteristics of a propagating wave packet are obtained. Zh. Tekh. Fiz. 69, 82–86 (December 1999)     

Lead zirconate titanate-nickel zink ferrite thick-film composites: obtaining by the screen printing technique and magnetoelectric properties

Layered thick-film composites containing one lead zirconate titanate (PZT) layer, one nickel zinc ferrite (NZF) layer, two PZT-NZF layers, or three PZT-NZF-PZT layers each 40–50 μm thick are prepared. The layers are applied by screen printing on a ceramic aluminum oxide substrate with a preformed contact (conducting) layer. The dielectric properties of the composites are studied in the temperature interval 80–900 K and the frequency interval 25 Hz-1 MHz. Polarized samples exhibit piezoelectric, pyroelectric, and magnetoelectric effects. In tangentially magnetized two- and three-layer composites, the magnetoelectric conversion factor equals 57 kV/(m T) at low frequencies and reaches 2000 kV/(m T) at the mechanical resonance frequency.     

Optical vortices in the scattering field of magnetic domain holograms

Experimental investigations and theoretical-model analyses have been made of the magnetooptic diffraction of light at ferrite garnet magnetic films with a banded domain structure which includes isolated magnetic grating defects in the form of “forks” and “breaks.” An analysis of the magnetic grating structure and the light diffraction field shows that in terms of its action on laser radiation, a banded domain grating is similar to a computer-synthesized phase hologram of an isolated pure screw-wavefront dislocation. It is shown that as a result of magnetooptic diffraction at a magnetic hologram, optical vortices may be reconstructed with a helicoidal wavefront carrying the topological charge l=±1,±2. Zh. Tekh. Fiz. 68, 54–58 (December 1998)     

Magnetic anisotropy of a system of nanocrystalline BaO·6Fe2O3 particles

The anisotropy of a system of barium ferrite particles with an average diameter of 60 nm has been studied. The effective anisotropy constant has been determined in the temperature range from 4.2 K to T _c by the law governing approach to saturation magnetization. The observed deviation from the magnetocrystalline anisotropy constant is explained as due to a negative contribution of the “surface” anisotropy constant. An estimate is made of this contribution as a function of particle size. Fiz. Tverd. Tela (St. Petersburg) 40, 1894–1897 (October 1998)     

Preparation and characterization of complex ferrite nanoparticles by a polymer-pyrolysis route

The polymer-pyrolysis route used in this work was to synthesize the copolymeric precursor of the mixed metallic ions and then to pyrolyze the precursor into complex spinel ferrite nanoparticles. Thermogravimetric analysis (TGA) showed that the complex ferrite nanoparticles could be obtained by calcination of their precursors at 500°C. The structures, elemental analyses and particle morphology of the as-calcined products were characterized by powder X-ray diffraction (XRD), ICP-AES, transmission electron microscope (TEM) and electron diffraction (ED) pattern. The results revealed that the as-calcined powders were complex spinel ferrites and the size of those nanoparticles ranged from 10 to 20 nm. Magnetic measurements were carried out at room temperature using a vibrating sample magnetometer (VSM). The saturation magnetization of the Mn–Zn ferrites was related to the molar ratio of Mn to Zn and increased with the increase of Mn. The complex Co–Mn–Zn ferrite nanoparticles showed a high magnetization of 58 emu/g at the applied field of 10 kOe and a low coercivity of 30 Oe, which indicated that this materials exhibited characteristics of soft ferromagnetism.     

Nonlinear surface magnetic polaritons in a nonuniform magnetic field

High-frequency surface magnetic polaritons of finite amplitude propagating along the interface between a ferrite and a nonlinear insulator in a weakly nonuniform, shaft-shaped external magnetic field are investigated theoretically. The analysis is based on employment of the variational method together with bilinear relations having the form of Lorentz’s lemma. It is shown that the wave dispersion and the transverse profile of a wave along the field nonuniformity depend significantly on the amplitude of the wave. Zh. Tekh. Fiz. 68, 92–95 (September 1998)     

Features of the plastic deformation of steels with lamellar perlite structure

The evolution of the perlite structure in the course of plastic deformation of U8 and St70 steels has been studied. With increasing degree of cold working, the dislocation density increases near the interphase boundary in the ferrite interstices of perlite, which forms a dislocational “fringe”, which in turn forms a pseudocellular structure. Under thermal deformation conditions, a subgrain structure arises in the ferrite interstices of lamellar perlite. The most dangerous localized flow channels in which plasticity is exhausted for the first time and cracks appear, are the deformation bands. The narrower the deformation bands and the larger the value of the relative displacement of the subcolumn blocks adjacent to them, the more dangerous they are with regard to failure. The features revealed in the development of deformation and failure provides a basis for controlling steel treatment regimes in order to produce perlite with the optimum structural parameters. Novosibirsk State Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 3–10, July, 1996.     

Nonlinear surface magnetostatic waves in a ferrite semiconductor structure

A theoretical analysis is made of the propagation of a nonlinear surface magnetostatic wave in a planar ferrite semiconductor structure as a function of the carrier concentration in the semiconductor layer. It is shown that for certain concentrations the surface magnetostatic wave is unstable with respect to longitudinal perturbations and may propagate perpendicular to the magnetic field in the form of solitons. Zh. Tekh. Fiz. 69, 119–121 (June 1999)     

Comparative analysis of the wave characteristics of circular and rectangular waveguides with gyrotropic filling

A comparative analysis of the propagation constants of the structurally similar H₀₁-mode of a circular waveguide and the H₁₀-mode of a rectangular waveguide with transversely magnetized ferrite fillings is presented, based on a rigorous electrodynamic calculation. Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 80–82, April, 1997.     

Influence of frustration on magnetization processes in the ferrite CoFeCrO4

The behavior of isotherms of the magnetization σ(H) and the longitudinal λ_‖ and transverse λ_⊥ magnetostriction of the polycrystalline ferrite CoFeCrO₄ with frustrated magnetic structure has been investigated for the first time in magnetic fields up to H=50 kOe at T=4.2 K. It is found that the magnetization grows with increasing field due to two different paraprocesses. Fiz. Tverd. Tela (St. Petersburg) 41, 2042–2043 (November 1999)