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The effects of high hydrostatic pressures (P) and magnetic fields (H) in a broad temperature range (T = 77-325 K) on the electric resistance (R) and magnetoresistance (DeltaR/R-0) was studied in la ...

Comprehensive study of the structure and properties of ferrite materials prepared from powders with different specific surface (0.4 m2/g

The clustered inhomogeneity observed in ferromagnetic materials deepens our concepts of the actual structure of solids and opens new possibilities for controlling their properties. These investigations were made for the purpose of establishment of the relationship between clusterization and magnetic properties of SrO-nFe{sub 2}O{sub 3}, where 5.4 < n < 6.2, metal oxide magnetically hard strontium ferrites.

Mossbauer, electromagnetic, and gravimetric studies have been used to examine the influence of ferrite powder specific surfaces, the isostatic compacting pressure, and the sintering temperature on a set of physical properties for isotropic barium ferrites. The optimum production parameters have been determined, such that Ssp=103 m2/kg, P=0.6−0.7 GPa and Ts=1240°C. A correlation has been obtained between the microstresses, the fine crystalline structure parameters and the electromagnetic properties for sintered specimens as a function of the process conditions. It has been shown that the Fe57 nuclei states located in the trigonal bipyramid of a magnetic plumbate network of barium hexaferrites are most sensitive to temperature changes. High isostatic pressures together with other optimized process conditions enable the isotropic barium ferrites quality to be increased to that of the 7BI215 belt quality.

Magnetoresistive ceramic and thick-and thin-film La0.6Sr0.3Mn1.1−x Fe x O3±δ(x=0,0.04) samples are studied by X-ray diffraction, 55Mn nuclear magnetic resonance (NMR), resistivity measurements, and magnetic measurements. Their rhombohedrally distorted ( $$R\overline 3 c$$ ) perovskite structure is found to contain anion and cation vacancies and nanocluster defects. Their broad asymmetric 55Mn NMR spectra support high-frequency electron-hole exchange between Mn3+ and Mn4+ and the fact that their environments are different due to a high defect concentration and high structural inhomogeneity. Iron doping and an increase in the annealing temperature result in a decrease in the temperatures of the metal-semiconductor and ferromagnet-paramagnet phase transitions and an increase in the magnetoresistive effect (MRE). The low-field MRE in the low-temperature region (∼100 K) in the ceramics and thick film is explained by tunneling in crystallite boundaries. An analysis of the effect of iron and the annealing temperature on the activation energy confirms the conclusion regarding a defect system of the perovskite structure and the presence of several mechanisms of activation processes.

Data yielded by x-ray diffraction, magnetic, metallographic, pycnometric, gravimetric, and dilatometric investigations into the kinetics of formation and sintering of ferrites with various crystal structures have enabled the mechanisms of these processes to be established. It is shown that the formation of a new (ferrite) phase is effected on the basis of the lattice of a more stable oxide, with preferential diffusion of cations of the other oxide. The processes of formation and sintering of ferrites taking place under nonequilibrium conditions are satisfactorily described by a nonequilibrium thermodynamics equation. The ferrite synthesis and sintering processes have comparatively small energies of activation, which is due to the high degree of defectiveness of the ferrite crystal lattice.

The degree of oxidation of ferrite powders from a high-quality batch is lower and that of parts sintered from them higher compared with a low-quality batch. Manganese-zinc ferrospinel parts of better quality are characterized by higher degrees of nonstoichiometry and inversion in the presence of manganese and iron cations of different valences. The lower electrical conductivity of these specimens is attributable to a complex character of reactions of cation-anion and electron-hole defects. To obtain high-quality manganese-zinc ferrites, it is necessary to create conditions preventing overoxidation during the stage of coprecipitation and synthesis of ferrite powders and enabling sintered parts to be produced whose spinel phase deviates slightly from stoichiometry in the direction of an excess of oxygen.

The temperature ranges of the physicochemical transformations occurring during heating in compacts from manganese-zinc ferrite powders produced by different methods have been established. The effect of method of preparation of ferrite powders on the character of processes taking place in compacts during heating is demonstrated. The degree of decomposition of the spinel at a given rate of heating is lower in powders produced by the carbonate coprecipitation method than in powders obtained from an oxide mixture. The reason for this is that the structure of carbonate-grade powders has comparatively few defects. Lowering the rate of heating of specimens in the temperature range 550–650‡C increases the degree of decomposition of the spinel, ensures a better reproducibility of properties, and improves the quality of ferrite parts.

The concept of effective coordinate charge is used to show that the relative shift in the K absorption edge of ions of transition metals in simple oxides and ferrites is determined by their valence. Formulas are obtained linking the shortwave shift of the K-edge with the valence of the ions in ferrites having the structure of a spinel. A method is proposed for determining the mean valence of ions of iron and manganese in manganese-bearing ferrites. The method is used to study cooling of commercial MnZn-ferrite with different oxygen partial pressures.

Changing the pressure in the hot pressing of manganese-zinc ferrites affects both the composition and state of the solid-phase system and the structural characteristics of specimens. With rise in hot-pressing pressure, the Fe2+ concentration, density, Curie temperature, and degree of inversion increase; there is also a tendency for the initial magnetic permeability to fall and for the crystallite size to grow. An interrelationship is shown to exist between the Fe2+ content of manganese-zinc ferrospinel on the one hand and the degrees of nonstoichiometry and inversion on the other.

The heating and cooling of a Mn-Zn ferrite during sintering are accompanied by solid-phase oxidation-reduction reactions, additional ferritization, and deferritization, which lead to further changes in the volume and density of the solid-phase system. Structural defects forming during the synthesis of a ferrite powder have a marked effect on its sintering and on solid-phase reactions associated with it. The sintering activity of an “oxide” ferrite powder manifests itself in greater shrinkage, higher density, and a larger grain size, and is due to its more defective structure. The sintering of a Mn-Zn ferrite can be arbitrarily divided into three consecutive stages in which superplasticity, diffusion-viscous flow of material, and vacancy pore dissolution are the dominant mechanisms. The healing of pores during the sintering of a ferrite powder is accompanied by a Frenkel' effect, which is due to the difference in the diffusion coefficients of the components forming the ferrite.

Results of computations of interior subsonic ideal gas flows with a twist in axisymmetric channels with a short flow turn are quoted. The computations were made in an orthogonal coordinate system ψ, f , using an iterative method. Examples are given.

X-ray and Mossbauer spectroscopy is used to investigate the effect of vacuum cooling on the valence of cations of manganese and iron, nonstoichiometry and the degree of inversion of manganese-zinc ferrites. The dependence of the valency of the cations of manganese and iron, the degree of inversion, and the degree of nonstoichiometry on the partial pressure of oxygen produced in the cooling stage is established. An interrelationship is found between the defects and the degree of inversion of manganese-zinc ferrites.

The viability of mouse kidney tissues, assessed quantitatively by growth in a plasma-free medium, was shown to depend on the state of the donor. Growth of the cultures was inhibited after whole-body x-ray irradiation of the donors, prolonged starvation, or poisoning with cadmium chloride. Increased growth of the culture was observed if compensatory hypertrophy of the kidney had developed as a result of unilateral nephrectomy, and also after subcutaneous inflammation.

The viability of tissue explants of mouse kidneys, estimated quantitatively by growth in plasma-free culture, was shown to depend on several factors connected with the conditions of culture. The effect of methods of treatment of the serum, pH of the medium, and the character of distribution of the tissue explant during culture, the size of the explants, and the firmness of attachment of the tissue fragments to the substrate on this index was demonstrated.

In the formulation of new ferrite materials for magnetic head cores starting raw materials and conditions of preparation of ferrite powders must be chosen with regard to the conditions prevailing in the hot-pressing process. The differences in the properties of the two hot-pressed ferrite batches investigated are attributable to the difference in the activities of the ferric oxide grades used (MM2 and SP) in the solid-phase synthesis and sintering reactions. Use of grade MM2 ferric oxide in the preparation of ferrite powders and hot pressing and annealing under optimum conditions (psp = 40 MPa, Thp = 1663°K, τih = 2.4 ksec, Tann = 1613°K and τann = 7.2 ksec) enable specimens to be obtained combining high values of initial magnetic permeability (μin = 10,000) and density.

It has been established that the mechanism of formation of new crystalline structures can be studied by determining the variation of the most probable crystallite size as a function of the ratio between the amounts of the starting phases. It is shown that magnesium and nickel ferrites with spinel structures form on the bases of the thermally-more-stable magnesium and nickel oxide lattices, with preferential diffusion of iron cations. To the initial stages of formation of these ferrites there will correspond a nonstoichiometric solid solution of increased divalent cation concentration. The crystal lattices of such spinel solid solutions are deficient in oxygen and contain interstitial cations.

The particle size distribution of a high-permeability manganese-zinc ferrite powder has a pronounced effect on the properties of hot-pressed parts. Ferrite powders from which high-density specimens with μo≥ 10,000 were produced by hot-pressing were of uniform particle size, with a predominance of particles of 1- to 2-μm size. Hot-pressed specimens obtained from a ferrite powder of nonuniform particle size were characterized by lower density and lower initial magnetic permeability.

Viability of mouse kidney tissue explants as estimated quantitatively according to their growth in plasmafree culture depends on some factors connected with cultivation conditions. Methods of serum preparation, pH of the medium, as well as peculiarities of tissue explants distribution in cultivation, their size and fastness of attachment to the substrate proved to influence the quantitative estimation of the tissue explants viability.

Alcoholics' blood serum produces a greater stimulation of the growth of human embryonic cells as compared with the serum from healthy individuals. In the course of treatment, the serum stimulating influence on the growth of human embryonic cells tended to diminish, approaching that of control. The authors ascertained a statistically significant positive correlation between the intensity of the embryonic cells growth under the influence of blood serum from alcoholic patients on the one hand and some hematologic parameters of these subjects on the other. Sensitizing therapy with teturam had no substantial effect on either the biological properties of alcoholics' serum or on the reproductive activity of fibroblasts, whereas the administration of neuroleptics was associated with a statistically significant decrease in the proliferation of fibroblasts.

It was shown that the vital capacity of the mouse kidney tissues quantitatively estimated by the in vitro growth in a plasma-free medium depended upon the condition of the donor organism. Depression of the culture growth was noted after the general X-irradiation of the animals, as well as following prolonged starvation and chloric cadmium poisoning. An increased growth of the kidney tissues was observed both in compensatory hypertrophy caused by unilateral nephrectomy and in subcutaneous inflammation.

High isostatic pressures were found to have a beneficial effect on the sintering and magnetic properties of nickel-zinc ferrite parts. Raising the pressure to 20 kbar: 1) increased the density of specimens by 10%, decreasing the closed and open porosities by 33 and 70%, respectively; 2) increased the initial magnetic permeability and its relative thermal stability by 19%; 3) diminished the relative tangent of losses by 24% in a field of 0.8 A/m and by 17% in a field of 8 A/m.