D.A. Zherebtsov

Growing Doped Barium Ferrite Single Crystals Using the Flux Method

Substances exhibiting ferromagnetic properties areused in inductors, devices for recording and reproduction of sound, and information storage devices, as wellas in electronic and electrical appliances.For solving many technical problems, ferromagnets with rather high Curie temperatures (350°С) areof crucial importance. This property is inherent in barium ferrite (BaFe

Three Oxidation States of Manganese in the Barium Hexaferrite BaFe12–xMnxO19

The coexistence of three valence states of Mn ions, namely, +2, +3, and +4, in substituted magnetoplumbite-type BaFe12-xMnxO19 was observed by soft X-ray absorption spectroscopy at the Mn-L2,3 edge. We infer that the occurrence of multiple valence states of Mn situated in the pristine purely iron(III) compound BaFe12O19 is made possible by the fact that the charge disproportionation of Mn3+ into Mn2+ and Mn4+ requires less energy than that of Fe3+ into Fe2+ and Fe4+, related to the smaller effective Coulomb interaction of Mn3+ (d4) compared to Fe3+ (d5). The different chemical environments determine the location of the differently charged ions: with Mn3+ occupying positions with (distorted) octahedral local symmetry, Mn4+ ions prefer octahedrally coordinated sites in order to optimize their covalent bonding. Larger and more ionic bonded Mn2+ ions with a spherical charge distribution accumulate at tetrahedrally coordinated sites. Simulations of the experimental Mn-L2,3 XAS spectra of two different samples with x = 1.5 and x = 1.7 led to Mn2+:Mn3+:Mn4+ atomic ratios of 0.16:0.51:0.33 and 0.19:0.57:0.24.

Composite Micropowders for Selective Laser Sintering

A method is proposed for obtaining composite micropowders. The technology includes the atomization of liquid metal by a gas jet and modification of the micropowder in a planetary mixer in the presence of carbon nanopowder. The particles of the composite micropowders produced by this method are close to spherical in shape. The fact that some parts of the particles’ surface are covered with graphite and some are not shows that it might be possible to obtain a sintered product which has adequate strength and a structure that contains carbon impregnations. The proposed technology makes it possible to produce a wide range of composite powders suitable for use in selective laser sintering.

Magnetic and Structural Properties of Barium Hexaferrite BaFe12O19 from Various Growth Techniques

Barium hexaferrite powder samples with grains in the μm-range were obtained from solid-state sintering, and crystals with sizes up to 5 mm grown from PbO, Na2CO3, and BaB2O4 fluxes, respectively. Carbonate and borate fluxes provide the largest and structurally best crystals at significantly lower growth temperatures of 1533 K compared to flux-free synthesis (1623 K). The maximum synthesis temperature can be further reduced by the application of PbO-containing fluxes (down to 1223 K upon use of 80 at % PbO), however, Pb-substituted crystals Ba1–xPbxFe12O19 with Pb contents in the range of 0.23(2) ≤ x ≤ 0.80(2) form, depending on growth temperature and flux PbO content. The degree of Pb-substitution has only a minor influence on unit cell and magnetic parameters, although the values for Curie temperature, saturation magnetization, as well as the coercivity of these samples are significantly reduced in comparison with those from samples obtained from the other fluxes. Due to the lowest level of impurities, the samples from carbonate flux show superior quality compared to materials obtained using other methods.

Influence of Powder Characteristics on Processability of AlSi12 Alloy Fabricated by Selective Laser Melting

Selective laser melting (SLM) is one of the additive manufacturing technologies that allows for the production of parts with complex shapes from either powder feedstock or from wires. Aluminum alloys have a great potential for use in SLM especially in automotive and aerospace fields. This paper studies the influence of starting powder characteristics on the processability of SLM fabricated AlSi12 alloy. Three different batches of gas atomized powders from different manufacturers were processed by SLM. The powders differ in particle size and its distribution, morphology and chemical composition. Cubic specimens (10 mm × 10 mm × 10 mm) were fabricated by SLM from the three different powder batches using optimized process parameters. The fabrication conditions were kept similar for the three powder batches. The influence of powder characteristics on porosity and microstructure of the obtained specimens were studied in detail. The SLM samples produced from the three different powder batches do not show any significant variations in their structural aspects. However, the microstructural aspects differ and the amount of porosity in these three specimens vary significantly. It shows that both the flowability of the powder and the apparent density have an influential role on the processability of AlSi12 SLM samples.

The Influence of Cold Rolling on Magnetocaloric Properties of Gd100-xYx (x = 0, 5, 10, 15) Alloys

In this work we report the results of investigation of the magnetocaloric effect in Gd100-xYx (x= 0, 5, 10, 15) cold rolled ribbons. It is shown that the magnetocaloric effect exists within a wide temperature interval 258-295 K and it is comparable with the magnetocaloric effect observed in bulk samples of pure gadolinium. The value of the magnetocaloric effect in the rolled samples is reduced in comparison with the bulk samples and strongly depends on a degree of plastic deformation. High temperature heat treatment can restore a value of the magnetocaloric effect in the cold rolled ribbons up to initial ones. Thus, cold rolling is proposed to be a promising technique for producing thin forms of magnetocaloric materials for heat exchangers of magnetic cooling devices.

Structure and adsorption properties of microporous glassy-carbon materials

Products of the calcination of polymer based on furan resin modified by surface-active agent and diluter are studied. Microporous glassy-carbon materials with specific areas of up to 1350 m2/g are obtained. The materials are studied by means of electron microscopy, and their specific surface areas are compared to their structures. The range of solution compositions in which bicontinual structures are formed as three-dimensional networks from polymer drops is determined; the drop size varies from 0.3 to 1.5 μm.

Gas Atomization of the Liquid 82N7HSR Nickel Base Alloy to Produce Micropowder for Additive and Coating Technologies

In this paper an atomization of the Ni-based alloy 82N7HSR in a gas jet was studied. During the series of experiments different combinations of 3 main technological parameters (melting temperature, type of spraying gas, gas pressure in the spray nozzle) were considered. The size and shape of powder particles were studied by scanning electron microscopy and the optical granulomorphometer Occhio 500nano. The suitability of obtained powders 82N7HSR for usage in additive technologies and coating technologies was estimated.

Magnetocaloric Properties of Cold Rolled Gd100-xZrx (x = 0, 1, 2, 3) Intermetallic Alloys

In this work we report the results of experimental investigation of the magnetocaloric effect in Gd100-xZrx (x = 0, 1, 2, 3) cold rolled ribbons. As it is shown the magnetocaloric effect exist within the large temperature interval 279-295 K and comparable with magnetocaloric effect observed on pure Gd. As it shown cold rolling is one of promising techniques for producing thin forms of magnetocaloric materials for heat exchangers of magnetic cooling devices.

Distribution of dopant metals between PbTiO3 crystals and PbO-B2O3 flux

Single crystals of lead titanate PbTiO3 doped with silicon, calcium, chromium, manganese, cobalt, nickel, copper, zinc, and cadmium were grown. The compositions and crystallographic parameters of the crystals were studied. The lowest distribution coefficients of dopants between PbTiO3 crystals and flux were observed with Mn+2 and Co+2 and the highest, with Ca+2. Doping with niobium leads to the formation of solid solutions with the pyrochlore structure A2B2O7 and even higher distribution coefficient. A correlation was found between dopant concentrations and crystal cell parameters.