B. Bokhonov

The formation of graphite encapsulated metal nanoparticles during mechanical activation and annealing of soot with iron and nickel

Abstract The investigation of morphological and structural changes during high energy ball milling and thermal annealing of the mixtures soot–iron and soot–nickel demonstrated that the activation is accompanied by the formation of nano-sized metal particles (10–50 nm) distributed over the amorphous carbon matrix. Prolonged mechanical activation of the amorphous soot–iron system (for more than 3–5 min) leads to the formation of nano-sized cementite Fe3C phase. Moreover, mechanical activation of the soot–metal compositions causes a substantial decrease in graphitization temperature of the amorphous carbon: for the soot–iron system, the temperature at which the amorphous carbon starts to crystallize is 250–300°C while for the soot–nickel system, the minimal temperature at which the crystallization of the amorphous carbon was observed exceeded 600°C. Morphological characteristics of the annealed, mechanically activated soot–metal samples depend on the time of preliminary mechanical activation. The annealing of soot–metal samples obtained after short-time mechanical activation (1–3 min) causes a crystallization of the amorphous carbon as onion-like graphite-metal structures. Annealing of the soot/metal samples after mechanical treatment for more than 5 min leads to the formation of metal nanoparticles (40–50 nm) encapsulated by graphite. The longer preliminary mechanical activation, the smaller the size of encapsulated particles. In-situ electron microscopic studies of the interaction of metal particles with amorphous carbon thin film showed that the interaction starts in these systems at temperatures about 600°C. The interaction in the systems iron–amorphous carbon film and nickel–amorphous carbon film proceeds via the formation of the carbide phases Fe3C and Ni3C; their decomposition results in the formation of crystal carbon and metal nanoparticles.

Hydrogen interaction with mechanically alloyed magnesium-salt composite materials

The effects of the addition of the inorganic salts NaF, NaCl, MgF2 or CrCl3 on the hydriding and dehydriding behavior of Mg and Mg–Ni mechanical alloys were investigated. It has been found that some inorganic salts, being mechanically alloyed with magnesium or magnesium alloys, not only promote metal powdering but also modify the metal surface. This leads to an acceleration of the hydriding and dehydriding reactions of these materials and to quite high hydrogen capacities (∼5.5–6 wt.%). The investigated salts had different influences on the metal surface properties, which was reflected in reaction kinetics, in particular at first hydriding.

In-situ investigation of the formation of nickel silicides during interaction of single-crystalline and amorphous silicon with nickel

Abstract In situ investigations showed that the sequence of phase formation during interaction of nickel particles with single crystalline (100) silicon and amorphous silicon corresponds to the following sequence of stages during the annealing of thin-film systems: (a) within a temperature range up to 500°C, the first and prevailing phase formed is Ni 2 Si; and (b) annealing at temperatures above 600°C is accompanied by the formation and epitaxial growth of the NiSi 2 phase. The growth of the nickel disilicide crystalline phase is accompanied by the formation of dislocations both in the nickel disilicide phase and in the silicon phase. The interaction of the amorphous silicon film with nickel particles at temperatures above 600°C leads to the crystallization of several silicide phases: NiSi 2 , NiSi, Ni 3 Si 2 . The formation of silicide phases due to the interaction of nickel particles with silicon during annealing did not confirm the formation of an intermediate amorphous silicide that was observed earlier in thin-film nickel–silicon systems. Irradiation with a beam of accelerated electrons in a microscope leads to an increase of the rate of silicide phase formation and to a decrease of the temperature at which the nickel disilicide phase is formed epitaxially, at least to 400°C. In our opinion, the observed effect can be due to the formation of defects in the structure of single crystalline silicon.

The stages of formation of a solid solution during the mechanical alloying of Si and Ge

Abstract The formation of a solid solution during the mechanical alloying (MA) of Si and Ge was studied using electron microscopy and X-ray diffraction. The process occurs via the formation of aggregates in which, under the action of plastic deformation, the mutual diffusion of Si and Ge proceeds to form the solid solution Si 1− x Ge x . According to our data, the observed partial amorphization of the components during MA is due to the formation of amorphous SiO and Si 1− x Ge x O oxides, structurally similar to amorphous Si.

In-situ investigation of the formation of eutectic alloys in the systems silicon-silver and silicon-copper

Abstract An in-situ electron microscopic investigation of the formation of eutectic alloys in the systems amorphous Si/particle Ag and amorphous Si/particle Cu showed that the formation of eutectics is preceded by metal diffusion into amorphous silicon with the formation of metastable amorphous metal silicide. Decomposition of the formed metastable amorphous metal silicide results in the evolution of polycrystalline silicon. An oriented formation of liquid eutectic alloy is observed in the systems crystalline (100)Si/particle Ag and (100)Si/particle Cu. In these cases, at the metal–silicon eutectic/silicon crystal interface the formation of silicide phases and a dislocation structure are observed. Investigations allowed us to propose a scheme for the formation of eutectic alloys in the studied systems. The scheme involves the diffusion of metal atoms into the silicon crystal structure, leading to the formation of a metastable silicide or solid solution, the occurrence of strain at the interface and, as a consequence, the formation of defects of different types. The subsequent process is accompanied by synchronous dissolution of metal atoms into the liquid eutectics and by diffusion from the liquid eutectics to the solid crystalline silicon phase, giving rise to metastable silicide, so that an intermediate diffusion zone is always present at the interface between the liquid eutectics and crystalline silicon.

Hydriding properties of mechanically alloyed icosahedral phase Ti45Zr38Ni17

Abstract The interaction with hydrogen was investigated for mechanochemically synthesised icosahedral phase of the composition Ti 45 Zr 38 Ni 17 . It was demonstrated that, unlike icosahedral phases of the same composition formed as a result of rapid quenching from melt or the annealing of alloys, hydriding of mechanochemically synthesized icosahedral phase at 503 K is not preceded by an induction period and starts at a maximal rate even when hydrogen pressure is less than 0.1 MPa. A diagram of the composition of icosahedral phase versus equilibrium hydrogen pressure ( P – T – C diagram) at 503 K was experimentally obtained. Based on published data and the data obtained in our experiments, an assumption was made on the existence of two types of hydrogen positions in the icosahedral structure of the hydrogenated phase. The first type is more energetically bound hydrogen occupying the positions within the structure-forming icosahedral Bergman clusters. The concentration of such hydrogen is approximately corresponding to the relation H/M=1.1. The second type is less bound hydrogen occupying the linkages between these clusters as solid solution.

Stage formation of quasi-crystals during mechanical treatment of the cubic Frank-Kasper phase Mg32(Zn, Al)49

Abstract Using high resolution electron microscopy, the stage formation of the icosahedral phase during the process of mechanochemical treatment of the cubic Mg 3Z (Zn, Al) 49 crystals has been studied. It has been shown that in the initial stages the icosahedral phase forms locally owing to the formation of rotation defects and disorder of the crystal planes due to the action of plastic deformation. The size of the disordered regions in the icosahedral phase increases with increase in the time of mechanical treatment. An orientation correspondence has also been revealed between the initial cubic and icosahedral phases; this is apparently due to the similarity in the structural designs of the cubic and icosahedral phases.

Nanocrystalline powder formation during mechanical alloying of W and Si

Abstract The present study of mechanical alloying in the system W-23.33wt.%Si has shown that the process goes through the formation of material of nanocrystalline composition. In this case a rather intimate intergranular contact forms between silicon and tungsten particles, which enables the comparatively easy preparation of tungsten disilicide by mechanical alloying under the action of plastic deformation and by low temperature annealing.

Comparative study of first hydriding of Mg–NaF and Mg–NaCl mechanical alloys

Abstract A kinetic and electron microscopy study of first hydriding of magnesium preliminary mechanically alloyed with the addition of NaF or NaCl salt has been performed. The salts have been found to modify the magnesium particle surfaces in a different way in the course of both mechanical alloying and hydriding. The action of NaCl consists in the local destruction of the oxide layer on the magnesium surface, facilitating the hydride nucleation process, whereas formation of NaMgF 3 has been observed at the very initial stages of hydriding of the Mg–NaF mechanical alloy. This ternary fluoride has been shown to play an active role in the process of first hydriding affecting the reaction kinetics and altering the overall course of the reaction.