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.     

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 study of the structure of mechanically activated periclase powders

The EPR method has been used to study mechanically activated periclase powders. The Mn²⁺ and Cr³⁺ spectra in MgO depend on the number of defects. The lines broaden to a degree which depends on the nuclear spin; the ratio between the hyperfine components changes; the line shape changes from Lorentzian to a convoluted form; and the relative intensity of the forbidden transitions in the Mn²⁺ changes.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 12, No. 3, pp. 48–52, March, 1969.     

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.     

Kinetic pathways of diffusion and solid-state reactions in nanostructured thin films

Mass transport and solid-state reactions in nanocrystalline thin films are reviewed. It is illustrated that diffusion along different grain boundaries (GBs) can have important effects on the overall intermixing process between two pure films. These processes can be well characterized by a bimodal GB network, with different (fast and slow) diffusivities. First the atoms migrate along fast GBs and accumulate at the film surface. These accumulated atoms form a secondary diffusion source for back diffusion along slow boundaries. Thus the different GBs of the thin films can be gradually filled up with the diffusing atoms and composition depth profiles reflect the result of these processes. Similar processes can be observed in binary systems with intermetallic layers: instead of nucleation and growth of the reaction layer at the initial interface, the reaction takes place in the GBs and the amount of the product phase grows by the motion of its interfaces perpendicular to the GBs. Thus, the entire layer of the pure parent films can be consumed by this GB diffusion-induced solid-state reaction (GBDIREAC), and a fully homogeneous product layer can be obtained.     

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.     

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.     

A specific heat study of natural haematite around the Morin transition and the effects of entrapped water

The Morin transition in samples of natural haemitite has been examined by specific heat and Mössbauer measurements. Specific heat measurements on as-received samples reveal an anomaly at ～ 273.2 K and irregular behaviour at higher temperatures. However, heat treatment of samples demonstrated that these effects were due to water trapped inside the natural specimens.No anomaly due to the Morin transition was observed in the specific heat although the expected changes in the Mössbauer hyperfine parameters were observed for the different magnetic phases below and above the Morin transition. The results are consistent with a coexistence of the two phases and a spread of temperatures ～ 40 K over which the transitions take place throughout the material of purity 99.6 wt%.     

Width reduction of laser microdrillings by subsequent mechanically induced plastic deformation

This work reports a novel process combination, which is capable of forming large hole matrices (100 pores/mm²) in thin stainless steel foils (10 mfoil<300 m) maintaining high processing rates and pore widths smaller than 5 m. This technique perforates stainless steel foils with high-speed on-the-fly laser perforation (60000 drillings/min) followed by a cold-roll forming of the laser-treated foil. The cold-roll forming leads to a pore-size reduction (in one dimension) perpendicular to the rolling direction. For the laser processing a diode-pumped, q-switched and frequency-tripled Nd:YAG laser (=355 nm, =30 ns), and a flashlamp-pumped Nd:YAG laser (=1064 nm, =57 s) were applied. Using this process combination, minimum pore sizes of 3.5 m have been achieved. At present, the processing efficiency of cold-rolled percussion drillings inserted with nanosecond pulse durations is lower in comparison with single-pulse on-the-fly perforation, but in terms of quality (straight pore channel, low standard deviation of pore widths and pore widths smaller than 5 m) well suited for various fields in filtration (e.g. particle removal) . PACS  81.20.Hy; 42.62.Cf; 81.40.Ef     

Kinetic energy of neon atoms adsorbed on activated carbon

Direct measurements of the kinetic energy of Ne atoms adsorbed on activated carbon (AC) at submonolayer coverage, is reported. The electronvolt spectrometer of the ISIS neutron source that utilizes the neutron Compton scattering technique, was employed. The specific adsorption area of the AC was ∼3000 m²/g. The momentum distribution of adsorbed Neon was measured at 12 K. The kinetic energy of Ne was found to be 66.3±6.2 K, and is much higher than that of solid neon, at the same temperature, being 45.8±4.8 K. This result is considered to be an evidence for the occurrence of a confinement effect of Ne atoms within the pores of the AC. The result was also used for deducing the average pore size of the AC adsorber assumed to have slit shaped pores.     

Dynamic diffractometry of phase transformations during high-temperature synthesis in mechanically activated powder systems in the thermal explosion mode

High-temperature synthesis of mechanically activated powders of the Ti-Al system are studied by means of induction heating in an inert atmosphere and vacuum. The effect of the duration of preliminary mechanical activation of the original mixture on the main synthesis parameters (e.g., time and temperature at the onset of reaction between components, the combustion rate, and the temperature of synthesis initiation) is investigated experimentally.     

Kinetic performance of Li4Ti5O12 anode material synthesized by the solid-state method

Li₄Ti₅O₁₂ anode was successfully synthesized by solid-state method. X-ray diffraction and scanning electron micrographs show that Li₄Ti₅O₁₂ prepared by solid-state method has a purity phase with a uniform particle size in the range of 0.5?C1???m. Cyclic voltammogram reveals that there is a big irreversible capacity for the first cycle. Li₄Ti₅O₁₂ shows a stable cycling stability at 1?C rate. After 152 cycles, the discharge capacity is 213?mAh?g^?1, which keeps 93% of it at the second cycle. Electrochemical impedance spectroscopy shows that the resistance of charge-transfer of Li₄Ti₅O₁₂ electrode decreased with increasing the storage temperatures, and the lithium diffusion coefficient is increased with increasing the storage temperatures, revealing that the kinetics of Li⁺ and electron transfer into the electrodes were much faster at high temperature than that at low temperature. The apparent activation energy of the charge transfer and lithium diffusion can be calculated to be 33.1 and 27.3?kJ?mol^?1, respectively.     

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.     

Kinetic effects in an optically excited gas

A resonance transition in a one-dimensional layer of gas contained between transparent parallel plates and optically excited by external radiation has been treated using kinetic theory. A perturbation method has been used to obtain the “first scattering” results for the velocity distribution and number density of excited atoms and the intensity of radiation at any point in the gas. Two special cases are discussed in detail: broad band excitation with inhomogeneous broadening and monochromatic excitation with homogeneous broadening. The effects of particle streaming and wall quenching are shown to produce boundary layer behavior in the excited level density which scales with the particle mean free path. In addition, line reversal of the radiation reemited from the gas is shown to occur and to be a direct result of particle streaming. Numerical and asymptotic results are presented which show these effects. These results should be pertinent to many laboratory and industrial devices in which the particle and photon mean free paths are comparable and to diagnostic techniques which use resonance fluorescence to infer excited level densities.     

Kinetic studies of the electrochemical nitrogen reduction and incorporation into yttria stabilized zirconia

The kinetics of the electrochemical reduction of molecular nitrogen at gold micro electrodes on yttria stabilized zirconia (YSZ) solid oxide electrolyte is studied by steady state polarization measurements. From the η / lg i plot for both cathodic and anodic polarization the apparent transfer coefficients α_a and α_c are evaluated. The sum of α_a + α_c exceeds unity and thus a multistep electron transfer process is suggested. The concept of the stoichiometric number is applied to the electrode reaction N₂ + 6e⁻ = 2N³⁻ supposing that the overall process involves at least two intermediate species. On the basis of the evaluation of the experimental results the reaction N₂⁻ + e⁻ ⇌ N₂²⁻ is suggested as the rate determining reaction step for the cathodic nitrogen reduction and nitride formation.