D. N. Chitanov

Preparation of 2000NN ferrite ceramics by a complete and a short radiation-enhanced thermal sintering process

Abstract2000NN Ni-Zn ferrites have been produced by radiation-enhanced thermal sintering. Mössbauer spectroscopy and X-ray diffraction have been used to investigate general aspects of phase transformations during the radiation-enhanced thermal sintering of green compacts. We have studied distinctive features of the phase transformations that occur during the radiation-enhanced thermal sintering of green compacts prepared from a starting oxide mixture (short procedure) and from ferrite powder presynthesized from oxides. The effect of basic composition on the electromagnetic properties of ferrite products has been examined. The addition of surfactants has been shown to be effective in raising the density of green compacts and improving the performance of the ferrites.

Influence of technological factors and chemical composition on the thermal stability and electromagnetic properties of M-type hexaferrites

The possibility of increasing the coercive force of barium hexaferrite by the isomorphous substitution of iron oxide with alumina, gallium and scandium oxides, and by impurity doping is shown. The results of the synthesis and study of new a (Me+2Me+4)xMey + 3Fe12–x–yO19 hexagonal ferrites with thermally stable anisotropy field and coercive force are presented. It is shown that doping with metal vapors of Me+2Me+4 = Ni+2Ti+4, Ni+2Ge+4, Ni+2Cu+2Ge+4, Me+3 = Al+3, Cr+3 allows improvement in the reproducibility of the properties and the thermal stability of barium hexaferrite.

Obtaining barium hexaferrite brand 7BI215 with improved isotropic properties

Currently, the global production of permanent magnets is about 150000 tons per annum, in which the share of ceramic magnets made of barium and strontium hexaferrites is more than 90%. Anisotropic hexaferrites have outstanding magnetic properties; in these materials, the texture is formed by pressing in the magnetic field, i.e., the hexagonal axes of flaky powder particles are oriented in the direction of the magnetic field; thus, the magnetic properties in this direction increase and decrease in the other directions. However, in a great number of applications, cheaper isotropic magnets, which have inferior magnetic properties, are successfully used; in these magnets, the magnetic axes of the particles are distributed uniformly in all directions, so magnetic properties are the same in all directions. The well-known technologies for fabricating isotropic magnets do not provide sufficient isotropy of magnetic properties, since the texture, which is due to the orientation of flaky hexaferrite particles, is formed in the process of pressing, which, in turn, reduces the magnetic energy of ring magnets in the radial direction. In this paper, we investigate the possibility of obtaining barium hexaferrite with isotropic properties by using a short manufacturing scheme, which includes no operations of diffusion annealing and grinding. It is shown that such a manufacturing scheme, which involves pressing the mixture (granulated with a binder) of the initial components with the near-spherical shape of particles, makes it possible to improve the isotropic properties of the magnet.

Studying the features of hexaferrite film fabrication using a slurry casting for the substrates of subminiature microstrip ferrite decoupling devices in the short-wave range of millimeter wavelengths

Using polyelectrolytes, ecologically safe film elements from hexaferrites have been designed for the first time with water slurry casting instead of existing processes with the use of organic solvent binders. The use of polyelectrolyte substances as binders has made it possible to reduce the energy consumption of drying by 20–30% during the process of film casting.

The effect of the base composition and microstructure of nickel-zinc ferrites on the level of absorption of electromagnetic radiation

Promising absorbing materials include Ni—Zn ferrites, as they quite intensively absorb electromagnetic waves in the frequency range from 50 to 1000 MHz. The electromagnetic properties of Ni—Zn ferrite absorbing materials obtained by different technological methods were studied in this paper. A model making it possible to evaluate the dielectric permeability of the ferrite material, depending on the microstructure parameters and electrophysical properties of grain boundaries, was proposed. The influence of base composition and microstructure on the amount of absorption of electromagnetic radiation by Ni—Zn ferrite absorbing materials was determined. It was stated that the increase of the content of excess Fe2O3 to 51.0 mol % leads to the shift of the frequency range of the absorption of electromagnetic radiation towards lower frequencies. It can be explained by the increase of the dielectric and magnetic permeability of ferrite. Moreover, the introduction of an excess of Fe2O3 in the grinding stage of the synthesized burden is more efficient. It was revealed that increasing the sintering temperature to 1350°C also shifts the frequency range of absorption of electromagnetic radiation towards lower frequencies. Probably it is caused by the increase of the dielectric and magnetic permeability of ferrite and the shift of the resonance frequency of domain walls as a result of the formation of a coarse-grained structure.

Obtaining Hexagonal Ferrites for Substrates Microstrip Microwave Devices of mm-Range of LTCC-technology

In the work by the method low temperature co-fired ceramics (LTCC) obtained samples of isotropic and anisotropic polycrystalline hexaferrite BaFe12O19 and SrFe12O19. Using in the LTCC-technology the pressing operation for samples (tablets) in a magnetic field produces anisotropic hexaferrites, pressing without a magnetic field isotropic hexaferrites. Application in the LTCC-technology molding process tape produces exclusively isotropic samples.

Analysis of the effect of crushing of strontium hexaferrite powders in a vibratory mill on the properties of magnets on their basis

We analyze the effect of duration of dry crushing of strontium hexaferrite powders (manufactured at joint-stock corporation Olkon) in a vibratory mill on the degree of disaggregation of powders and on the properties of articles on their basis. It is shown that an increase in the vibrational frequency of the vibromill from 25 to 50 Hz with the amplitude of vibrations maintained in the limits 3–4 mm makes it possible to reduce by 5 times the crushing time ensuring complete disaggregation of powders. An increase in the impact energy of balls with increasing vibrational frequency activates the powders due to microdistortions appearing in the crystal lattice. An increase in the degree of disaggregation of powders leads to sintering and formation of a dense fine-grain microstructure ensuring an increase in the maximal energy product (BH)max of magnets by 15–20%.

EFFECT OF THE BASE COMPOSITION AND MICROSTRUCTURE ON THE LEVEL OF ABSORPTION OF ELECTROMAGNETIC RADIATION IN NICKEL−ZINC FERRITE

Promising absorbing materials include Ni—Zn−ferrites, as they quite intensively absorb electromagnetic waves in the 50 MHz to 1000 MHz frequency range. In this paper we have studied the electromagnetic properties of Ni—Zn ferrite absorbing materials obtained in different technological modes. We propose a model that allows one to evaluate the dielectric constant of the ferrite material depending on the parameters of the microstructure and electrical properties of grain boundaries. Influence of base composition and microstructure on the level of absorption of electromagnetic radiation by Ni—Zn ferrite absorbing materials has been found. An increase in Fe₂O₃ excess to 51 % has been found to shift the frequency interval of electromagnetic radiation absorption towards lower frequencies, and this effect can be explained by an increase in the dielectric and magnetic constants of ferrite. Introduction of excess Fe₂O₃ in step 2 of grinding proved to be more efficient. An increase in the sintering temperature to 1350 °C also provides for a shift of electromagnetic radiation absorption frequency interval towards lower frequencies, which can be explained by an increase of the dielectric and magnetic constants of ferrite and resonance frequency shift of domain walls due to the formation of a coarse−grained structure.

Soft-magnetic Mg-Zn ferrite ceramics comparable in performance to 600NN Ni-Zn ferrite: Fabrication by radiation-enhanced thermal sintering

Mg-Zn ferrites comparable in performance to 600NN Ni-Zn ferrite have been produced by radiation-enhanced thermal sintering. We have studied the effect of their basic composition and doping on their electromagnetic properties. The results demonstrate that doping with bismuth oxide as a low-melting-point material allows one to activate sintering processes, thereby improving the performance of the material. The addition of surfactants is shown to be effective in raising the density of green compacts for the preparation of the ferrites.