V. A. Zelenskii

The effect of surfactant admixture during milling on pressing, sintering, and magnetic properties of FeCrCoMoW alloy

The effect of surfactant admixture (stearic and oleic acids) with a variety of milling (dry milling, milling in water and ethanol) on pressing and sintering and magnetic properties of the Fe-30Cr-20Co-2Mo-2W (wt %) hard magnetic powder alloy was studied. The addition of stearic and oleic acids in the amount of up to 2 wt % during dry or wet milling increases the density of pressed samples. The use of surfactants during milling in water and ethanol increases the density of sintered samples. X-ray diffraction shows that only accumulation of defects takes place in the crystal structure of the particles, and no additional phases form during dry or wet milling of powder mixtures containing a surfactant. The best magnetic hysteresis properties were obtained with samples milled in ethanol with 1% stearic acid.

Synthesis of micron particles with Fe–Fe4N core–shell structure at low-temperature gaseous nitriding of iron powder in a stream of ammonia

Synthesis of the single-phase γ′-Fe4N on the surface of the micron-sized particles of iron at low-temperature gaseous nitriding of carbonyl iron powder in a stream of ammonia is studied. It is shown that synthesis of particles with such structure is possible with simultaneous control of the number of process parameters: temperature, degree of dissociation of ammonia, and treatment time. It is found that, at temperature T = 400°C and nitriding potential of the atmosphere rN ≈ 1.3 atm−1/2, the shells with a thickness of about 1 μm are formed on the particles within ~15–20 min and the powder consists of the γ-Fe4N phase within ~60 min of treatment. The mechanisms of formation of microparticles with a core–shell structure are considered. A qualitative model for the thermochemical treatment of the micron iron powder with consideration of the diffusion processes of the transport of ammonia molecules in the pore space of the powder and atomic nitrogen diffusion inside the particles is developed. Geometric and dimensional effects at nitriding of iron powders are discussed.

Low-temperature synthesis of micron nitride powders of the Fe-N system

The practical possibility of synthesizing micron-size corrosion-resistant powder materials based on lower and higher nitrides of the Fe-N system in the form of both a bulk composition and a “core-shell” system was implemented at relatively low temperatures. Information on the elemental and chemical composition of iron nitrides was obtained using the methods of Auger electron spectroscopy and X-ray quantitative analysis. Fe3N and Fe4N phases were identified in the volume of microcrystals in the form of compositions with unreacted α-Fe without foreign impurities in the temperature range of 300–350°C. The highest mass concentration of the Fe4N phase was observed at 350°C and a nitriding time of 60 min (α-Fe, 14.70%; Fe3N, 24.50%; and Fe4N, 60.80%). Porous nanocoatings were synthesized on the surfaces of carbonyl iron microparticles at 165°C and a synthesis time of 420 min. This nanocoating has a thickness of ∼10–15 nm and, in the first approximation, corresponds to the ∼Fe15.7N2 compound.

Sintering diagram of titanium carbide powders

Sintering diagrams, which determine the dominating mechanism of powder sintering at a given temperature, the size of powder grains, and the neck between the grains, may be used to explain the experiments on sintering and solve some practical problems concerning the sintering of metals and ceramics. In this work we have built sintering diagrams for titanium carbide powders of various dispersities. Using such diagrams, we have established that the dominating mechanisms of sintering titanium carbide micro- and nanopowders are the surface and grain-boundary diffusion of substance to the neck between the particles. It is found that the contribution of sintering basic mechanisms in the case of titanium carbide does not depend on the dispersity of powders.

Passivation of Nickel Nanoparticles at Temperatures below 0°C

It has been experimentally demonstrated that, at temperatures below 0°C, nickel nanopowder does not ignite in dry air; however, passivation takes place, thus providing the composition stability of nickel nanoparticles in air at ambient temperature.

Investigation into the dependence of time characteristics of ignition and combustion of iron nanopowders on air on the duration of passivation after synthesis

A method of estimating the extent of passivation of iron nanopowders based on color high-speed filming is suggested. It is established that the number of primary centers of combustion and the time of their emergence depend on the passivation time: the more the passivation time is, the lower the number of primary centers of combustion is. This allows using the dependence for estimating both the extent of passivation and the minimum time of complete passivation. A method for calculating the time of complete passivation for the sample of arbitrary thickness is suggested.

Preparation and characterization of iron nanoparticles protected by an oxide film

Iron nanopowders ranging in particle size from 20 to 100 nm have been synthesized by reducing a 1-mm-thick iron(III) hydroxide layer in flowing hydrogen at 400°C and then passivated for 6–60 min in flowing argon containing 3% air. Our results demonstrate that the passivated iron nanopowders do not oxidize in air for six months. The iron nanoparticles have been characterized by X-ray diffraction (crystallite size evaluation), Auger electron spectroscopy, and polymolecular adsorption. The passivated iron nanoparticles have been shown to consist of a metallic core and oxide shell 2–4 nm in thickness.

Self-organization processes and synthesis of nanostructured silver microparticles in AgNO3 solutions with variable pH values

The morphology of micrometer-sized silver particles obtained by liquid-phase chemical reduction of silver nitrate with ascorbic acid depends appreciably on the solution pH. The synthesis carried out at 100°C for 20 min at pH < 4 or pH > 9 yields anisotropic faceted nanocrystalline particles, while the synthesis at pH = 5–8 results in self-assembly to give microspheres representing close-packed aggregates of a huge number of silver nanoparticles with a cauliflower structure.

Improvement of magnetic properties by hot rolling of sintered powder alloy in the Fe–Cr–Co system

A method to produce a technically suitable, hard-magnetic material of composition Fe–26% Cr–16% Co–2% Mo–2% W from a powder raw material was proposed. A distinguishing feature of the method is a reduced temperature of sintering of powder compacts (1200°C). The sintered samples are subjected to hot rolling. The magnetic and strength characteristics of the thus obtained magnetic material are on the level of those of alloys produced by powder metallurgy at a sintering temperature of about 1400°C and their cast analogs. Data on the magnetic hysteresis and strength properties of the synthesized material were presented.

Effect that powder dispersity and sintering medium have on acoustic-emission activity

It has been revealed that replacing a rough copper powder with a nanopowder increases the acoustic emission (AE) upon sintering the pressings of the W + 20% Cu structure. Hydrogen sintering of the W + 20% Ag structure is accompanied by an elevated level of acoustic emission when compared to vacuum. At the same time, the frequency spectra of hydrogen and vacuum processes are similar. For the binary powder systems with an infusible base, the AE after the crystallization of a fusible component has been explained by the relaxation of the internal mechanical stresses which emerge in the material upon cooling.