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Rare-Earth Magnetic Materials

#1427


New Methods of Synthesis of Exchange-coupled High-Coercive Hard Magnetic Materials on the Base of Rare-Earth Ferromagnetics Using Mechanically-Induced Reactions in Solids

Tech Area / Field

  • MAT-ALL/High Performance Metals and Alloys/Materials

Status
3 Approved without Funding

Registration date
17.12.1998

Leading Institute
NPO Mayak, Russia, Chelyabinsk reg., Oziorsk

Supporting institutes

  • Ural Branch of RAS / Institute of Metal Physics, Russia, Sverdlovsk reg., Ekaterinburg\nVNIITF, Russia, Chelyabinsk reg., Snezhinsk

Collaborators

  • Institut für Festkörper und Werkstofforschung, Germany, Dresden\nUniversity of North Carolina / North Carolina State University, USA, NC, Raleigh

Project summary

The purpose of the Project is to improve the magnetic properties of hard magnetic materials, and to search for new methods of synthesis of magnetic materials with high hysteresis properties through application of mechanically-induced reactions in solids.

The scientific goal of the Project is obtaining new information about the physical mechanisms of formation of mechanically-induced non-equilibrium, metastable (polycrystalline, amorphous etc.) structural states of magnetically-ordered solids, defects formation and evolution in the crystalline structure of material in the process and after treatment, and structural-phase and chemical transformations at various stages of mechanical alloying and milling.

Implementation of the presented Project will be aimed at development of the new methods and modification of the available methods of optimization of the structural state and magnetic properties of hard magnetic alloys based on rare-earth metals (REM), synthesized by the method of mechanical alloying (including chemical reductants application, e.g. in case oxide-based REM compounds are used), and creation of efficient technologies for new magnetic materials manufacture.

At that, the practical goal realization will be based on the application of the fundamental research results carried out under this Project.

The investigation area, to which the presented Project will be dedicated, covering the physical mechanisms of formation of mechanically-induced non-equilibrium, metastable magnetically-ordered phases (nanocrystalline, amorphous etc.) in solids, is comparatively new. The first intensive investigation of this problem began when magnetic amorphous materials were obtained by the method of super-rapid quenching. Yet even now, when many aspects of the said problem have been clarified, hundreds of experimental and theoretical papers published, the physical picture of the widely spread hysteresis properties formation, even in the soft magnetic amorphous and nanocrystalline magnetic materials, is far from being complete. The available knowledge does not allow to reliably predict the magnetic properties in relation to the concrete structure, and mechanical and phase composition of magnetically-ordered compounds obtained as a result of super-rapid quenching and subsequent treatment conditions regulation. The information gap is specifically wide where it concerns realization of high hysteresis properties in nanocrystalline and amorphous hard magnetic materials. This allows only very generalized schematic planning of the today's methods of synthesis and modification of hard magnetic materials, e.g. on the base of rare-earth metals, especially with the use of the new alternative techniques, such as mechanical alloying based on the methods of reaction chemistry. The experimental results and the theoretical qualitative analysis of the problem of non-equilibrium (amorphous, nanocrystalline) phases realization through mechanical alloying and milling date back to the early 80s. Nevertheless, up to the present moment the experimental and theoretical developments in the field of mechanical alloying application in synthesis of hard magnetic materials, especially of REM-based materials with low (below 10 %) rare-earth components content, or the so-called exchange-coupled ferromagnetics, are limited and scarce. The mechanisms and kinetics of phase transformations going in solid-phase reactions, including those in intermetallides containing rare-earth and 3d-metals, are insufficiently studied. The physical reasons and the conditions of formation in metastable phases of the unique magnetic hysteresis properties - the high residual magnetic moment values and the coercive force - still have to be clarified. It should be noted at the same time that compared with the known traditional techniques, mechanical alloying offers considerable advantages from the point of view of manufacture of exchange-coupled permanent magnets with high-level properties on the base of rare-earth components with nanocrystalline structure.

Experimental data are particularly scarce on type Nd-Fe-B and Sm-Co magnetic materials synthesis with the use of mechanically-induced reactions of rare-earth oxides reduction to pure metals and alloys in the presence of chemical reductants and alloying elements. Controlled mechanical-chemical reactions, going with synthesis of the wanted chemical products, involving reductants and 3d-metals and other alloying additions, may certainly present scientific and practical interest.

This direction may prove to be very promising in synthesis of hard magnetic materials on the base of REM with high hysteresis properties, the more so, when certain positive results have already been obtained in direct reduction of metals.

By changing the conditions (milling intensity, temperature etc) of materials treatment in energy-intensive mills, the energy that may be stored in a solid is comparable to the energy of chemical bond, which allows us to speak of the opportunity of realizing the reaction of dissociation of higher oxides, and initial components reduction to pure metals and alloys, without using reductants. Successful solution to these problems will allow implementing a new technological process of rare-earth metals reduction and synthesis of rare-earth magnetic compounds (Nd-Fe-B, Sm-Co and others) for various practical applications.

The above-mentioned new phenomenon, mechanically-induced chemical instability of initial alloys, that is, variation of chemical composition of an alloy, particularly, change of local concentration of its components (or gaseous component removal from a binary oxide-based compound, e.g.), deserve close experimental and theoretical study.

In solving the formulated problem of instability, special attention will be given to theoretical study of diffusion as a mechanism ensuring various solid-phase reactions in the process of treatment. For example, the effect of mechanically-induced and chemical instability may be a kinetic process observed in the presence in the crystal of ordered flows of point defects, with non-uniform interaction of.these defects with the alloy components having various mobility. It should be noted that the effect of phase or chemical instability requires considerable diffusion mass transfer of components, suppressed at low temperatures take place. However, the mechanically-stimulated diffusion may enhance chemical decomposition of compounds (including metastable phases) at temperatures, at which normal thermal disintegration cannot take place. Thermodynamic substantiation of occurrence of such reactions, with a detailed analysis of the main driving forces of the reaction, will be given.

The main idea is the formation of a definite structural-kinetic state as an accommodational transformation mode. In this connection, a most important factor defining the nature and type of transformation, its future evolution, is the kinetics of the process and difference in time of accommodation of alternative structural transformations. Hence, the rate of energy dissipation in solids will set the route in a sequence of transformations.

Thus, controversy between the various structural and phase transformations may be interpreted as different ways of critical structure relaxation, with characteristic time periods of accommodational processes taking place in solids with the formation of nanocrystals, amorphous states, cracks formation, diffusion separation etc., in the process of mechanical milling.

Complex research in this field will allow studying the influence of the processes of interaction between mechanically-induced defects and structural-phase transformations in nanocrystalline alloys. Besides, judgements can be made with regard to the physical criteria and parameters of such interaction, based on experimental analysis and theoretical description of the kinetics of phase transformations and chemical instability of studied alloys.


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