Vladimir Šepelák

Enhanced magnetisation in nanocrystalline high-energy milled MgFe2O4

The changes in magnesium ferrite (MgFe2O4) caused by high-energy milling are investigated by means of Mossbauer spectroscopy, magnetisation measurements, and electron microscopy. The observed enhancement of the magnetisation in nanoscale milled MgFe2O4 is discussed with respect to the mechanically induced cation redistribution and spin canting.

Structural disorder in the high-energy milled magnesium ferrite

The structural and magnetic evolution in magnesium ferrite (MgFe2O4) caused by high-energy milling are investigated by Mossbauer spectroscopy. It is found that the nanostructural state of the milled MgFe2O4 is characterized by a mechanically induced cation redistribution between tetrahedral (A) and octahedral [B] sites. The reduced concentration of iron ions at (A) sites in the mechanically treated samples leads to the variation in the number of magnetic and nonmagnetic (A)-site ions as nearest neighbors of the Fe3+[B] ions. This results in a broad distribution of magnetic hyperfine fields at the [B] sites. In addition to the local magnetic fields B(6), B(5), and B(4) characteristic of nonactivated ferrite and corresponding to Fe3+[B] ions with n=6, 5, and 4 nearest (A)-site iron neighbors, respectively, the distribution curves of mechanically treated samples show additional components at smaller magnetic fields. The weight of the B(6) field decreases with increasing milling time, and the B(5) field becom...

Transformations in oxides induced by high-energy ball-milling

This paper, by no means exhaustive, focuses on high-energy ball-milling of oxides, on their mechanically induced changes and on the consequences of such changes on their physical and chemical properties. High-energy ball-milling offers a fortunate combination of technical simplicity and of complexity both of physical mechanisms which act during milling and of mechanosynthesized materials. Its basic interest, which stems from the large diversity of routes it offers to prepare oxides either directly or indirectly, is illustrated with various families of oxides. The direct path is to be favoured when as-milled oxides are of interest per se because of their nanocrystalline characteristics, their defects or their modified structures which result from mechanically driven phase transformations. The indirect path consists of a sequence of steps starting with mechanically activated oxides which may be subsequently just annealed or submitted to a combination of thermal treatments, with the possible occurrence of various chemical reactions, to prepare the sought-after materials with potential gains in processing temperatures and times. High energy ball-milling of oxides is more and more currently used to activate powders and to prepare nano-oxides at moderate temperatures. The interest of an activation step is well illustrated by the broad development of doped titania powders, synthesized by heat treatment of pre-ground reactants, for photocatalytic applications or to develop antibacterial materials. Another important class of applications of high-energy ball-milling is the formation of composites. It is exemplified here with the case of oxide-dispersed strengthened alloys whose properties are considerably improved by a dispersion of ultra-stable nanosized oxides whose formation mechanisms were recently described. The basic understanding of the mechanisms by which oxides or oxide mixtures evolve by high-energy ball-milling appears to be less advanced than it is for metallic materials essentially because of the overall complexity of the oxide structures, of their surfaces, of their defects and of their mechanical behavior.

Microwave Absorption Properties of Mn–Co–Sn Doped Barium Ferrite Nanoparticles

Substituted barium ferrite BaFe₉(Mn_0.5Co_0.5Sn)_3/2O₁₉ was prepared by sol-gel method. X-ray diffraction (XRD), transmission electron microscope (TEM), AC susceptometer, vibrating sample magnetometer (VSM) and vector network analyzer were used to analyze the structure, static and dynamic magnetic properties of the prepared samples. The prepared ferrite particles possess irregular non spherical shape with a broad size distribution. The substitution was very effective in reducing coercive field of the material. Based on microwave measurements of reflectivity, BaFe₉(Mn_0.5Co_0.5Sn)_3/2O₁₉ is a good candidate for electromagnetic compatibility and other practical applications at high frequencies.

Mechanically induced cation redistribution in ZnFe2O4 and its thermal stability

The changes in zinc ferrite caused by high-energy ball-milling are investigated. Formation of spin arrangement in the ball-milled ZnFe2O4 is caused by the onset of the exchange interaction of the Fe3+ (A)O2−Fe3+[B] type, taking place due to the mechanically induced inversion as well as by the onset of the interaction of the Fe3+[B]O2−Fe3+[B] type with deformed bond angle. Structural metastability of the milled ZnFe2O4 is manifested by the gradual recrystallization terminating at 900 K by a total recovery of the structure.

Mechanochemical reduction of nickel ferrite

Abstract The changes in nickel ferrite (NiFe2O4) due to high-energy milling in a stainless steel vial have been investigated by Mossbauer spectroscopy, X-ray diffraction (XRD), and thermal analysis. The milling process reduces the average crystallite size of NiFe2O4 to the nanometer range. The Mossbauer spectra show the formation of metallic iron and of a phase containing Fe2+ ions. The fraction of the reduced products increases with increasing milling time. The range of thermal stability of the metastable milled reduction products has been determined by studying their response to changes in temperature.

Thiourea leaching of silver from mechanically activated tetrahedrite

Abstract The thiourea leaching of silver from a tetrahedrite concentrate mechanically activated in a planetary mill or an attritor was studied. It was found that the two types of equipment gave rise to different rates of new surface formation and of crystal structure disordering. The rate of thiourea leaching of silver from tetrahedrite (Cu,Ag)10(Zn,Fe)2(Sb,As)4S13 is a structure-sensitive quantity, while the dependence of the rate constant of leaching on the empirical coefficient S A (1 − R) (SA = specific surface, R = disordering of tetrahedrite structure) exhibits a linear character with equal slope for both types of mills. The results are also of prognostic character because they enable us to propose suitable equipment for intensive grinding depending on the demand for fineness or reactivity of the solid substances.

Mechanically induced cation redistribution in magnesium ferrite and its thermal stability

Abstract The changes in magnesium ferrite (MgFe 2 O 4 ) caused by high-energy milling are investigated. Mechanical treatment reduces the average crystallite size of MgFe 2 O 4 to the nanometer range and induces cation redistribution between tetrahedral and octahedral sites. The degree of inversion of the mechanically treated ferrite is compared with that of MgFe 2 O 4 quenched from high temperatures. The range of thermal stability of mechanically induced metastable states is determined by studying the response of the metastable mechanically activated MgFe 2 O 4 to changes in temperature.

Nanocrystalline materials prepared by homogeneous and heterogeneous mechanochemical reactions

Abstract At present, the high-energy milling method becomes widely used for the preparation of nanocrystalline materials due to its relative simplicity and availability. In this overview, selected examples are presented of the preparation of nanoscale materials by homogeneous and heterogeneous mechanochemical reactions in spinel ferrites. Despite numerous efforts, the understanding of the nonequilibrium mechanochemical processes is considered to be far from complete, leaving large scope for further research in this exciting field.

Mechanically Induced Cation Redistribution and Spin Canting in Nickel Ferrite

The structural and magnetic evolution in nickel ferrite (NiFe2O4) caused by high-energy milling are investigated by Mössbauer spectroscopy. It is found that the nanostructural state of the milled NiFe2O4 is characterized by a reduced concentration of iron ions on tetrahedral sites. The degree of inversion in NiFe2O4 is calculated from the subspectral area ratio of both high- and zero-field Mössbauer spectra. Several interesting features are involved in the work, e.g., superparamagnetic relaxation, mechanically induced cation redistribution, and spin-canting effect.