D. Mehandjiev

Mechanism of the decomposition of cobaltous compounds in vacuo

Abstract The decomposition of cobaltous compounds (nitrate, hydroxide, carbonate, oxalate and oxyhydroxide) in vacuo has been investigated by magnetic methods and thermogravimetry. The mechanism is shown to be similar to that in air and begins with the formation of Co 3+ ions in octahedral coordination. The formation of different final products (Co 3 O 4 , CoO and Co) has been attributed to the nature of the initial cobaltous compound. The results are in agreement with thermodynamic calculations.

A magnetic study of the formation of NiCO2O4

Abstract The mechanism of NiCo 2 O 4 formation was investigated by magnetic methods and thermogravimetry. This compound was obtained by simultaneous decomposition of cobalt(II) and nickel(II) salts in air at temperatures up to 350°C. It was shown that NiCo 2 O 4 has the spinel structure of Co 2+ [Ni 3+ Co 3+ ]O 4 . The decomposition process passes through the formation of paramagnetic Co 3+ , Co 2+ and Ni 3+ ions, which were stabilized in the ferrimagnetic spinel lattice of NiCo 2 O 4 .

Study of copper-cobaltite spinel formation during simultaneous decomposition of copper and cobaltous nitrates

Abstract The formation of a copper-cobaltite spinel, Cu 0.9 Co 2.1 O 4 , during simultaneous decomposition of copper and cobaltous nitrates has been investigated by magnetic, EPR and DTA methods and diffusive reflectance spectroscopy. It has been shown that the spinel appears as a result of a solid-state reaction between basic copper nitrate and anhydrous cobaltous nitrate which are intermediate products of the decomposition of the initial nitrate salts. During the solid-state reaction, an intermediate pre-spinel structure containing copper ions in the tetrahedral sites only is formed. This structure transforms into a spinel with partially inverse distribution of the copper cations in the tetrahedral and octahedral sites.

On the mechanism of preparation of Co3O4 from various salts

Abstract The mechanism of preparation of Co 3 O 4 by thermal decomposition of cobaltous salts (oxalate, nitrate, carbonate) was studied using magnetic methods and thermogravimetry. The effective magnetic moment of the Co ion passes through a minimum (2,2—2,3 BM) before the formation of Co 3 O 4 (2.55 BM). It is shown that the first decomposition stage of cobaltous salts in air is the formation of Co 3+ ions in octahedral coordination without passing through the intermediate stage of metallic Co and CoO. The results from the magnetic studies are in agreement with thermodynamic calculations.

Study of the interglobular structure of metal powders obtained by a borohydride reduction process

Abstract The effect of borohydride reduction duration and magnetic field in the specific surface area, metal particle size and form, and form and dimensions of the textural arrangement of pores in metal powders is investigated. It is established that the particle chains formed involve interglobular voids, i.e. the chains may be considered as consisting of successivly ordered globule layers. The interglobular void is found to possess definite dimensions and configuration. Powders obtained in a magnetic field have tetrahedral voids, while octahedral voids correspond to powders prepared without a magnetic field. The magnetic parameters of the powders have displayed a similar difference which needs additional studies.

Mechanism of borohydride reduction process and properties of obtained nanosized ferromagnetic particles

Abstract The mechanism of the chemical borohydride reduction process and the hydrodynamic conditions in the solutions predetermine the contents of bounded boron and hydrogen atoms. The influence of the technological conditions and the equipment used on the compositions, on the structures — amorphous or nanocrystalline — and on the magnetic properties of the particles and powders are established.

Preparation and properties of magnetic metal powders obtained by a borohydride reduction process

Abstract Experimental data are presented on pairs of magnetic metal powders with different compositions and magnetic properties obtained by a borohydride method in the presence (type I powders) and absence (type M powders) of a magnetic field. The density of the chains is found to be 0.957 (equal to that of Kondorskii) for type I and 0.743 (the value corresponding to the closest packing of spheres) for type M. From the measured magnetic parameters of the metal powders, the constant of surface anisotropy and the shape coercivity are calculated. The application of surface-active substances the the preparation of the powders causes no change in the type of the chains formed. The crystallization of the powders, which contain considerable amounts of boron and hydrogen, leads to the appearance of magnetocrystalline anisotropy and a change in the coefficient of surface anisotropy, which becomes negative.

Amorphous magnetic powders — Some special peculiarities

Abstract The amorphous powders prepared by a high-rate borohydride reduction process propose the possibility for some principal conclusions concerning the occurence of closest packing of the monodomain magnetic globules. The simulataneous nitrogenation, pressing and determination of the thermal effects of crystallization provided for both kinds of probes - Fe x Co y B 1−( x + y ) (compositionally equal), obtained with or without a magnetic field show differences in the structure and energy of the interglobular voids. Therefore, the differences between the predominating amount of tetrahedral or octahedral voids determine some special peculiarities of this material.

On the possibility of formation of 3d-transition metal mixed oxides with spinel structure

Abstract A thermodynamic approach is used to predict the possibility of formation of 3 d -transition metal mixed oxides with spinel structure on the basis of data on the thermodynamic stability of the single oxides. The temperature regions of the most probable formation of spinel chromites, ferrites, manganites and cobaltites were outlined. The theoretical considerations are compared with experimental results for copper and nickel cobaltites.

Crystallographic phases in nanosized ferromagnetic particles obtained by two different methods

Abstract The phases in ferromagnetic cobalt powders obtained by two methods of borohydride reduction process are investigated. Two kinds of powders, amorphous and nanocrystalline, are produced. The influence of the heat-treatment procedures on the phase transformation is studied. The lattice parameters of phase Co 3 B are calculated and their shrinkage is evident. This fact is due to the high content of bounded hydrogen in powders and it is assumed that a cobalt borides hydrides compound is produced.