Mössbauer study of mechanically alloyed powders

Using the⁵⁷Fe Mössbauer spectroscopy, the microscopic behavior of Fe powders has been investigated during and after the mechanical alloying (MA) process for Al?Fe and Ag?Fe systems. A repeated rolling method as well as a conventional ball-milling method are employed in order to understand the microscopic process of the kneading of Al and Fe powders during their MA and the resultant powders show quite similar Mössbauer spectra suggesting that the kneading by an impact between colliding balls is the same process as that of the thickness reduction by cold rolling. Mössbauer spectra show clearly the occurrence of the mutual atomic dispersion for the thermodynamically immiscible Ag?Fe system.     

Study of the photodegradation of brilliant green on mechanically activated powders of zinc oxide

A study of the photocatalytic activity of commercial and mechanically activated zinc oxide powders has been carried out based on the example of the decomposition of Brilliant Green. The goal of this work was to study the effect of the grinding time (0, 1, 3, 5, and 7 min) on the structure of zinc oxide and its photocatalytic activity under visible and ultraviolet (UV) radiation. It has been found that, when UV radiation is used, the constant of the dye oxidation rate for samples activated for 1 min increases compared with unactivated powders, whereas further mechanical activation leads to a decrease in the photocatalytic activity. When using visible radiation, samples activated for 1 min showed the minimum photocatalytic activity and further mechanical activation led to an increase in the efficiency of photocatalysis.     

Spectroscopic study of the surface oxidation of mechanically activated sulphides

Surfaces of chalcopyrite CuFeS₂, sphalerite ZnS, pyrite FeS₂ and galena PbS modified by mechanical activation have been studied by means of infrared (IR) spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray photoelectron spectroscopy (XPS). The different oxidation species were identified on mechanically disordered surfaces of the minerals. The mechanochemical surface oxidation increases with the time of mechanical activation. The sulphides under study are partially covered with oxidised sulphates and carbonate species. They are distinguished by an affinity to the formation of surface compounds. According to the affinity of sulphides for mechanochemical S²⁻→S⁶⁺ surface oxidation the decreasing rate was observed in the following order: FeS₂>PbS>CuFeS₂>ZnS. The ratio between bivalent and hexavalent form of sulphur depends on the degree of mechanochemical surface oxidation of the sample under study.     

Electromagnetic properties and structure of polydisperse mechanically activated Fe-Si materials

Complex investigation of the physical properties and structure of sintered powder materials of the Fe-Si system after grinding of a mixture of powders in two different mechanical aggregates has been performed. It is shown that fragmentation of powders in both a BW-micro vibration grinder and in a Fritsch-type centrifugal planetary mill leads to a nonmonotonic change in the properties of sintered materials. This result is explained by mechanical activation of powders during grinding. Original Russian Text ? G.A. Dorogina, I.A. Kuznetsov, E.S. Gorkunov, Yu.V. Subachev, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 1, pp. 141–144.     

Molecular dynamics study of lattice rearrangement under mechanically activated diffusion

The paper presents a molecular dynamics study to investigate the behavior of materials under loading by friction stir welding (FPW). The loading is simulated by assigning constant angular and forward velocities to a certain group of atoms, being a FPW tool. The joined materials are two defect-free Cu crystallites, Cu and Fe crystallites, and two crystallites of the same solid solution structured as D16 (2024) alloy. It is found that as the tool passes along the weld line, the crystal structure of the materials is rearranged with subsequent mixing of their surface atoms. Under certain loading conditions, the crystal lattice after passage of the tool recovers its regular order. Also analyzed is the influence of vibrations additionally applied to the FPW tool. The simulation results provide a better understanding of the processes involved in mechanically activated diffusion.     

EPR study of nitrogen-doped mesoporous TiO2 powders

Mesoporous titanium dioxide powders were synthesized by the sol-gel route either directly or using the P123 templating agent and nitrogen doping from thiourea. X-ray diffraction studies show that doping and structuring give rise to rutile growth at a relatively low temperature of 400 °C compared to the transition point of pure TiO₂, which is close to 900 °C. Systematic EPR experiments were performed in order to monitor the involved paramagnetic centres with respect to calcination treatments and also to probe the nitrogen doping efficiency in the host media. Several paramagnetic species were found in these mesostructured samples mainly with oxygen radicals for calcinations at 400 °C, while nitrogen centres appear for calcination temperature as high as 500 °C. The effect of material microstructure on the effective incorporation of nitrogen in the TiO₂ lattice is also examined and compared in representative samples.     

Properties of mechanically activated bismuth ferrite

Methods of mechanochemical treatment applied to eliminate the disadvantages of BiFeO₃ that impede its practical use are described. It is shown that the use of certain techniques for the mechanical activation of precursors allows us to enhance the structural and thermal stability of bismuth ferrite, to reduce its electric conductivity, and to improve its electrophysical properties.     

A structural,magnetic and microwave study on mechanically milled Fe-based alloy powders

Fe₉₀M₁₀ powders with M=Fe, Co, Ni, Si, Al, Gd, Dy and Nd were prepared by mechanical milling. Their structure and magnetic properties were investigated. Microwave measurements were performed on the mechanically milled Fe₉₀M₁₀ powders. The results were compared with those of carbonyl Fe powders and coarse Fe powder. It has been shown that fine nanocrystalline Fe-based alloy powders prepared by mechanical milling are promising for microwave applications.     

EPR investigations of SiC and SiOC nanometric powders

EPR investigations are performed in SiC and SiOC nanometric powders annealed between 1200 and 1800°C. By using different EPR frequency bands and a suitable spectra analysis, three quite different paramagnetic defects with well defined $\tilde g_i $ (i=1, 2, 3) and hyperfine $\tilde A_i $ (i=1, 2) tensors account for the EPR signal in these materials. The defects are characterized by $\tilde g_1 $ (g ₁ ^‖ =2.0046(3), g ₁ ^? =2.0023(3)), $\tilde g_2 $ (g ₂ ^‖ =2.0037(3), g ₂ ^? =2.0028(3)) and isotropic $\tilde g_3 $ (2.0030(3)) tensors. In SiC powders, the defects assignment is discussed with respect to the different SiC forms, namely α-SiC and β-SiC polytypes as well as amorphous SiC and carbon present in minor concentration in the network. In SiOC powders, the above defects are evidenced only at high annealing temperature (T _a≥1200°C) when the oxygen contained is highly reduced.     

Kinetic effects in the mechanically activated solid-state reduction of haematite

Nanometer-sized -Al₂O₃-Fe composites were obtained by solid-state reduction in ball mills of Fe₂O₃ and Al in nitrogen and air. Together with -Al₂O₃ and Fe, the formation of Hercynite and clusters of Fe in alumina are observed in both cases.     

Formation of the crystal structure of intermetallic compounds in mechanically activated Ni-Al and Ti-Al systems during self-propagating high-temperature synthesis

The dynamics of structural and phase transitions during heterogeneous reaction in mechanically activated mixtures has been investigated by X-ray diffraction, scanning electron microscopy, and dynamic synchrotron radiation diffraction. It is shown that the mechanisms of formation of the structure of reaction products are significantly different, depending on the temperature mode of the synthesis. Upon slow heating, the process is multistage and includes several intermediate crystalline phases. Upon fast heating, the transformation in the combustion wave occurs much faster, with only one intermediate phase observed.     

Anisotropy models in precise crystallite size determination of mechanically alloyed powders

Nanocrystalline AA 4032 alloy powder was synthesized by high-energy ball milling from elemental powders for 30 h duration. XRD and TEM results reveal that the powder is cubic and nanocrystalline in nature. X-ray peak broadening analysis was used to evaluate the lattice strain and the crystallite size using the Williamson-Hall analysis with three different models viz., uniform deformation, uniform deformation stress and uniform deformation energy density. The root mean square (RMS) strain was calculated from the interplanar spacing and the strain estimated from the three models. The three models yield different strain values due to the anisotropic nature of the material. The energy density model is proposed to be the best fit model among the three as severe lattice strain is associated with ball milled powders.     

EPR study of the peculiarities of incorporating transition metal ions into the diamond structure

In this paper previously obtained data is reviewed and new data is discussed about nickel-containing centers in diamonds. These data are used to suggest interpretation of new data about cobalt-containing centers and to understand the influence of iron on the defects in diamonds grown in the iron system. A newly discovered nickel-nitrogen center has three nitrogen atoms in the first neighbor sphere around the double semi-vacancy and looks like the N3 (P2) center. In diamonds grown in the cobalt system two new types of cobalt-containing centers were found (NLO2 and NWO1). Both centers have electron spinS=1/2 and hyperfine structure from one cobalt ion (I=7/2 with natural abundance 100%). A case can be made for a double semi-vacancy structure for these defects. Special growth of diamond in the system enriched in¹⁵N decreased the line width down to 0.6 G, but gave no direct evidence of the existence of nitrogen in the defect structure. Asymmetrical shapes of the lines in the electron paramagnetic resonance (EPR) spectra of cobalt-containing centers with opposite signs in low and high magnetic field parts of spectra are due to very sensitive spin-Hamiltonian parameters of these defects to the lattice distortions. Annealing of cobalt-containing crystals at 2600 K produces the disappearance of all cobalt-containing EPR spectra, probably due to the capture of an additional nitrogen atom and the creation of a 3d⁶ diamagnetic state. In diamonds grown in the iron system with a high content of nitrogen there is evidence of an influence of ferromagnetic inclusion on the exchange interaction between substitutional nitrogen as an additional channel of indirect exchange interaction.     

Evidence of mechanically activated processes in slow granular flows

We study how a shear band in a granular medium dramatically changes the mechanical behavior of the material further in the non sheared region. To this end, we carry out a microrheology experiment, where a constant force F is applied to a small rod immersed outside the shear band. In the absence of a shear band, a critical force F(c) is necessary to move the intruder. When a shear band exists, the intruder moves even for a force F less than the critical force F(c). We systematically study how the creep velocity V(creep) of the rod varies with F(c) - F and with the distance to the shear band, and show that the behavior can be described by an Eyring-like activated process.     

EPR theoretical study of local lattice structure in ZnS:Cr system

By analyzing the EPR parameters a, D and F of Cr²⁺ ion located at tetrahedral site in ZnS, the local structure around Cr²⁺ in the crystal has been investigated on the basis of the complete energy matrix for a d⁴ configuration in a tetragonal ligand-field within a strong-field-representation. It is shown that there exists an expansion distortion in the local lattice structure. From EPR calculation, the distortion parameters ΔR=0.13 Å and Δθ=1.417° are determined.     

EPR theoretical study of local lattice structure in YAG:Mn system

By diagonalizing a set of complete energy matrices constructed for a d⁵ configuration ion in a trigonal ligand field, a reasonable interpretation is obtained for the EPR zero-field splitting of Mn²⁺ ions located at octahedral sites in yttrium aluminum garnet. It is shown that the local lattice structure around an octahedrally coordinated Mn²⁺ center has an expansion distortion, which may be attributed to the fact that the radius of Mn²⁺ ion is larger than that of Al³⁺ ion, and the Mn²⁺ ion will push the oxygen ligands outwards. Simultaneously, the local lattice structure distortion parameters ΔR=0.1825-0.2158 A and Δθ=1.220°-1.315° for the octahedral Mn²⁺ center in the crystal are determined, respectively. Meanwhile, we also demonstrated that the empirical impurity-ligand distance is not suitable for the YAG:Mn²⁺ system which has been approximately taken in previous works.     

Thermally activated hopping in granular films and powders

A mechanism involving carrier-carrier interaction is proposed for intergrain hopping of electrons in thin films and fine particle powders. The calculated hopping rate yields large broadening of electron levels in fine particle samples.     

A study of the molecular structure of bacterial lysodektose by quantum-chemical calculations and EPR spectroscopy

The standard redox potentials of the sequential oxidation of lysodektose to the corresponding nitrone were estimated by quantum chemistry methods. It follows from these estimates that the experimentally observed accumulation of the intermediate nitroxyl radical in substantial amounts during the oxidation of lysodektose can be explained by high medium reorganization energy in the oxidation of the nitrosyl radical with simultaneous proton abstraction. The EPR spectra of the radical lysodektose form were modeled. Arguments in favor of the suggestion that one nonequivalent proton appeared in the formation of an intramolecular H-bond were presented. Quantum-chemical calculations of the hyperfine structure constants were in satisfactory agreement with experiment.     

Hot-pressed and die-upset Pr-Co magnets produced from mechanically activated alloys

Anisotropic die-upset PrCo₅-based magnets were produced by hot pressing and subsequent die-upsetting of alloy powders activated by high-energy ball milling. In addition to the PrCo₅ major phase the magnets contained Pr₂Co₁₇, even when prepared from a stoichiometric 1:5 alloy. Hard magnetic properties can be improved by partial Sm substitution for Pr and partial Cu substitution for Co. A higher cobalt/rare-earth ratio as well as the addition of Fe was found to be unfavorable for the deformation-induced anisotropy. The largest values of remanence, intrinsic coercivity and maximum energy product in hot-pressed magnets were obtained for Pr_0.7Sm_0.3Co_5.5 (7.8 kG, 14.1 kOe and of 13.1 MGOe) and in die-upset magnets for Pr_0.9Sm_0.1(Co_0.98Cu_0.02)₅ (9.2 kG, 10.5 kOe and 16.7 MGOe).