Home – Home – Jornals – Chemistry for Sustainable Development 2009 number 6

Structure and morphology of mechanocomposites based on tungsten with copper, nickel or iron added were studied by means of X-ray diffraction studies, electron microscopy and micro- X-ray spectral analysis. The possibilities of rpessing dense samples from the obtained mechanocomposites were demonstrated.

Novel universal approach to obtaining multilevel nanocomposites with different architectures including interpenetrating structures and nucleus - shell structures is proposed. The approach is based on the deposition of charged aerosol particles with the size 30 nm to 300 μm with the help of electric mass classifier. In view of the small excess charge of particles and the large dipole moments, the coating density after deposition is very low. After the fixation of the porous framework by agglomeration, the relative density of the ceramic coating equal to 0.04 was achieved. Filling of the framework provides the possibility to obtain a broad range of interpenetrating composites. The use of mechanocomposites for making aerosol allows one to obtain multilevel composite materials with required architecture.

In this work the experimental results of the investigation carried out on the use of micronized fly ash as pozzolanic material for production of cement mortars are presented. The milling of fly ash was carried out in planetary ball mill; the size distribution of micronized material was similar to silica fume, normally used to produce high strength conglomerates due to its very high specific surface. Micronized material was also beneficiated reducing the unburned fraction by means of triboelectrostatic separation. The mechanical strength tests of mortars were performed through the substitution of cement or sand (between 10 and 25 % in mass) with Micronized Fly Ash (MFA), Beneficiated Micronized Fly Ash (BMFA) and Silica Fume (SF), respectively. The water/cement ratio (w/c) was regulated to obtain equal workability of the mixtures (from 0.35 to 0.4). The best value of compression strength at 28 days (more than 100 MPa) is obtained with BMFA (in substitution of sand): this value is 34 and 10 % higher than reference  mortar with Ordinary Portland Cement (OPC) and mortar with SF, respectively. On the basis of these results a chemico-physico-mechanical improvement of the fly ash was achieved making reutilisation of this by-product in high strength cement conglomerates feasible.

Alumina-zirconia nanocomposite powder with a particle size around 50 nm was synthesised by mechanochemical activation. Milled powders consisted of CaO and XRD amorphous aluminium and zirconium chloride phases. After annealing the milled powder at 360 ^oC displacement reactions occurred and ZrO₂ and Al₂O₃ nanoparticles were formed in a CaCl₂ matrix which was removed by washing. DTA, XRD and SEM analysis were applied to characterise the powders. The formation of cubic zirconia and α-alumina were observed at very low temperatures.

The austenitic stainless steel materials modified with nano-SiC powder were prepared under the production conditions by foundry method. The corrosion resistance for different amount of SiC added to stainless steel was investigated. The results showed that with the addition of nano-SiC powder, the resistance to chemical corrosion and electrochemical corrosion was markedly improved. The addition of nano-SiC powders can significantly enhance the intergranular corrosion of stainless steel, and the anti-intergranular corrosion of the stainless steel was the result shown both by the metallographic and the bending methods. With the addition of nano-SiC powder, the ferritic and chromium of the stainless steel reduce segregation, so nano-SiC powders can help effectively avoiding the two-phase chromium depleted zones and areas prone to localized corrosion, such as the emergence of the region.

Three different α-Al₂O₃ nanopowder species were characterized; their behaviour under dry uniaxial compacting and agglomeration without applying external pressure was investigated. Ceramics dense at a level of 99 % was obtained at agglomeration temperature 1300 ^оC; hte grain size was about 200 nm. It was established that for low-temperature agglomeration it is reasonable to use the powder containing aggregates that are conserved during filling the press mold at the initial stage of pressure rise but get destroyed at subsequent stages of compacting. It was demonstrated that with the optimal molding method is becomes possible to obtain dense ceramic material even at 1150 ^oC, with the conservation of grain size at a level of 100 nm and less.

Nanodisperse powder of magnesium meta- and ortho-silicates was obtained with the help of the solid-phase exchange reaction of magnesium chloride with sodium silicate (using NaOH or without it) proceeding under mechanical activation. The formation of enstatite and forsterite proceeds through the X-ray amorphous (or weakly crystallized) hydrated compound during the subsequent thermal treatment. A comparison with the known method of obtaining these silicates under mechanical activation of the mixtures of magnesium hydroxide and silica gel was carried out.

The influence of thickness of self-lined layer of processed substances on the parameters of mechanical activation is considered. Theoretically, using quartz as an example, dependencies of impulses of pressure and temperature along the layer thickness are calculated. It is shown that this parameter affects significantly the kinetics of mechanochemical processes.

Possibilities to use atomic force microscopy (AFM) as a tool to study morphological features and structural parameters of mechanochemically synthesized alloys and structural parameters of mechanochemically synthesized alloys and metal cements based on them are presented. It is stressed that the application of AFM to the investigation of diffusion-hardening alloys characterized by long-term structure forming process allows one to investigate the intermediate states of the surface. A comparative analysis of the data obtained with the help of atomic force microscopy and scanning electron microscopy was carried out.

Macrokinetic characteristics of the combustion process in reaction mixtures Cr-B and Cr-Ti-B after preliminary mechanical activation were investigated. It was established that mechanical activation has a substantial effect on the composition and structure of the initial mixtures and therefore on temperature and rate of combustion of the mechanically activated mixtures. It was shown that the interaction between the reagents at the combustion temperature follows the solid-phase mechanism. The mechanisms of phase formation in the combustion wave were studied with the help of stopped-front method. With an increase in the concentration of oxygen in the mixture as a consequence of mechanical activation, the role of gas transport of boron to the surface of the metal increases; reaction diffusion becomes the limiting stage of the interaction of a metal with boron. It was established using X-ray structural analysis that the products of combustion of the mixture Cr-Ti-B contain previously unknown ternary compounds like Cr₄Ti₉B and Ti₂CrB₂.

Effect of attritor mechanical activation on the structure and reactivity of powder mixture Fe-45 mass % Al for self-propagating high-temperature synthesis (SHS) of powders based on iron aluminides was studied. It was established that the decisive influence on the change of the mechanisms of phase formation during SHS is exerted by the level of micro-heterogeneity of the reaction mixture and the formation of nanometer-sized iron aluminides В2-FeAl and/or Fe₂Al₅ in mechanically activated powder mixtures.

X-ray diffraction, Mossbauer and Auger electron spectroscopy, electron and atomic force microscopy were applied to study the sequence of structural-phase transformations during mechanical alloying (MA) and annealing of system iron - titanium - carbon obtained using the mixtures of different compositions: powders of pure elements Fe (70 at. %), Ti (15 at. %), graphite (15 at. %); powders Fe (70 at. %) and TiC (30 at. %); powders Fe (70 at. %), Ti (15 at. %) and toluene. It was demonstrated that in all the cases MA results in the formation of nanocomposite powders with complicated phase composition: a solid solution based on Fe, X-ray amorphous phase based on carbides TiC and Fe₃C, carbide TiC. Annealing leads to the crystallization of the amorphous phase, decomposition of the solid solution and formation of nanocomposite Fe + TiC + Fe₃C. The dispersity, number of composition of carbide phases depend on MA conditions. In the case of the mixture of Fe, Ti and toluene, by varying MA time one may obtain the two-phase system Fe + TiC with the uniform distribution of nanocrystalline phases over the sample.

The possibility of the mechanochemical synthesis of nanopowders of magnesium and zinc oxides for use as modifying additives to polymers was studied. The process of obtaining nanopowder includes four stages: dehydration of the initial salt, treatment in the planetary mill, washing by means of decantation, and drying. Manganese and zinc chlorides were used as the initial salt. Dehydration was carried out at a temperature corresponding to the removal of adsorption and crystallization water as determined by means of TGA. Then the salt was treated in a planetary mill for 2-3 h in the presence of a phase separating agent (sodium nitrite NaNO₂). The treatment process was accompanied by powder dispersing and the mechanochemical reaction between the initial chloride and the salt matrix of NaNO₂. The moment of reaction completion was determined on the basis of pressure change inside the reaction volume with time. The mixture of oxides and water-soluble salts, obtained as a result of the mechanochemical reaction, was washed by centrifuging and then the formed precipitate was dried. The resulting oxides were investigate with the help of X-ray structural analysis, scanning electron microscopy, measurements of specific surface with the help of BET procedure, laser analyzer of particle size. It was established that the proposed procedure allows obtaining oxide particles with the average size of 60-140 nm.

The features of phase and structural transformations in the powder of graphite-like boron nitride occurring as a result of the treatment in attritor are considered. It is established that attritor treatment causes an increase in the specific surface of boron nitride particles by more than an order of magnitude. An increase in the time of mechanical activation of the powder causes the transformation of the substructure of BN from the crystal to nanocrystalline and amorphous. It is shown that phase transformations with the formation of rhombohedral BN and high-pressure phases - wurtzite and cubic BN - with the particle size of submicrometer range proceed in boron nitride. When the optimal intensity and duration of treatment are exceeded, reverse transfer of cubic boron nitride into graphite-like (hexagonal) boron nitride with recrystallization of the latter is observed.

The zones of normal-tangential indenting (N-TI) of the single crystals of apatite, quartz and lithium fluoride were examined by means of scanning and high-resolution electron microscopy. The texture, structural and phase transformations thus detected were conventionally related to the "deformation" and "diffusion" processes of plastic deformation. Two levels of structural transformations were revealed in the zones of N-TI in signle crystals; a sharp boundary between them occurs at the strain equal to the theoretical strength limit (σ_tsl). In the upper zone of scratches within the strain range from microhardness value H_ц to the σ_tsl value, the substance undergoes profound structural and phase transformations with the formation of the amorphous state, "paracrystalline", "liquid crystal" and "two-dimensional" nanoparticles. The substance in the zone of scratches is a model of mechanically activated substance. In the bed of scratches at the strain below σ_tsl we observe fragmentation of single crystals with the formation of blocks and steps; this is a model of the grinding process. A wave model is proposed for the transformation of the input mechanical energy for indention with the specific power approximately equal to 3 · 10¹¹ J/(m³ · s), which is close to the level of atom bond strength in the activated zone of N-TI. As a result of excitation of chemical bonds, phase and structural transformations occur in the activated zone of N-TI.