Home – Home – Jornals – Chemistry for Sustainable Development 2005 number 2

The impulses of mechanoluminescence caused by stressing of fine quartz particles in the stirred mill were analysed. An obtained distribution of light impulse can be described by means of the powder function with two different exponents, one for low and another one for high impulse amplitude. The critical amplitude in between remains constant for all the investigated process parameters. The observed difference in exponents reflects different behaviour under applied stress for fine and coarse particles. An intensive production of pores and microcracks occurs during the wet treatment of fine particles. The light emission from stressed particle decreases with growth of pores inside of particles. Consequently, the number of counted stress events at a given impulse amplitude is reduced. Also, the frequency of impulse decreases with growth of pores. A ratio of current to starting impulse frequency reflects the extent of development of pore structure. An intensive particle size reduction occurs when this ratio achieves a critical magnitude. Particle size distribution becomes three-modal instead of bimodal at start of grinding. The third mode with maximum at 100 nm appears due to fracture of porous particles.

The microstructure evolution of Cu-nanostructured powders versus the ball milling conditions was investigated using X-ray diffraction analysis. The characteristics of as-milled Cu powder microstructure in terms of crystallite size, dislocations and twin boundary densities were determined by fitting the simulated X-ray diffraction peak profiles to the experimental ones in order to establish a correlation between the powder microstructure and the ball milling parameters. It was shown that the simulated X-ray diffraction peak profiles agree satisfactorily with the experimental ones taking into account transmission electronic microscopy observations where twin boundaries and dislocations were observed.

This paper presents experimental data on solid-phase synthesis in double-layer thin-film systems. The rule of the first phase formed at the interface of film reagents at elevated temperatures of annealing is formulated for solid-phase reactions determined by martensite transformations. The temperature at which synthesis is initiated in Ni-Ti, Cd-Au, and Al-Ni films coincides with the temperature of the back martensite transition in NiTi, AuCd, and AlNi alloys so that martensite phases are formed in the reaction products. The first phase rule was also verified for solid-phase synthesis in Cd-Ag, Ni-Mn, FeMn, and Au-Mn systems. In thin films, low-energy processes initiate solid-phase reactions associated with martensite transformations, mass transfer of reagents being up to 200 nm. The martensite model of atomic transfer through the reaction product during solid-phase synthesis in thin films is considered. Martensite shifts can play the dominant role in mass transfer of reagents through the reaction product. It is assumed that the first phase initially formed at the interface of film reagents is formed irrespective of the mode of solid-phase synthesis initiation.

Mechanical energy released during stirred grinding promotes the polymerization reactions of styrene monomers on calcium carbonate and those of polystyrene on titanium dioxide surfaces and makes the surface modification by polymer grafting more efficient and with less initiating agent. Polymer grafting modification activated by mechanochemical effects improves the compatibility of calcium carbonate filler with the polymer matrix and the fluidity of alkyd varnish with modified titanium dioxide as the paint material. The process of polymer grafting modification activated by mechanochemical action and the changes of application properties of materials filled with modified powders are described, the interactions between the modifying agents and the surface of the powders is discussed.

Mechanochemical reactions for the synthesis of cluster compounds of higher boranes, carboranes, and transition metal complexes with structural metal chalcogenide cluster fragments containing [Nb₂], [W₃] and [Mo₃] are reviewed. These are typical systems in cluster chemistry. Reactions were performed by mechanical activation (MA) of mixtures of starting crystalline substances in hermetically sealed vibratory ball mills. A brief overview of basic definitions and concepts of cluster chemistry and mechanochemistry is presented. The results of studies on six types of mechanochemical reaction on cluster compounds are summarized. Published data on the experimental technique for reactions performed under conditions of MA are given. MA can change the structure or composition of the cluster nucleus, or lead to elimination of structural cluster fragments from coordination polymers; it can also lead to exchange of the ligand surroundings of the nucleus-forming atoms, and to formation of discrete molecules or ionic saltlike compounds containing clusters. Solid-phase redox reactions on transition metal atoms can take place during formation of heteroatomic metallocarboranes.

The effect of mechanical treatment on complex systems containing a solid phase and the components of casing-head gas is considered. The process is carried out in planetary ball mills with the acceleration of milling bodies (steel balls) up to 600 m/s². An intense mechanical action on the material of balls and mill walls causes the formation of active centres initiating transformations of the gaseous components. The possibility to obtain hydrogen and methane from hydrocarbon gases by means of mechanochemically initiated destruction was demonstrated. The presence of a solid phase, i.e. crystal quartz, which efficiently generates active centres of radical nature during mechanical treatment, increases the transformation degree of initial components.

Nanostructured molybdenum disilicide (MoSi₂) compound was synthesized using an alternative route called MASHS (Mechanically Activated Self-propagating High-temperature Synthesis). This original process combines a short-duration ball milling with a self-sustaining combustion. These two steps were investigated in this paper. The microstructure evolution of powder mixture during mechanical activation was monitored using XRD profiles analysis and TEM investigations. Short-duration ball milling of (Mo() powders results in Mo and Si nanocrystallites in micrometric particles. It was demonstrated that a pure α-MoSi₂ with nanometric structure (D_MoSi2 = 88nm) could be produced via a very fast combustion front in contrast with the classical SHS process.

Two sol-gel methods for the preparation of WO₃ and MoO₃ nanopowders were used in this work: i) an ion-exchange reaction and ii) an oxidizing reaction (M+H₂O₂). The phase and structural transformations undergone by colloidal solutions of tungsten acid (i) and peroxotungsten and peroxomolybdic acids (ii) as a function of thermal treatment were investigated by X-ray diffraction (XRD) and infrared spectroscopy (IR). Depending on the methods used, different phases were obtained: crystalline hydrates, amorphous and nanocrystalline products. The tungsten trioxide hydrates were prepared by the ion exchange method, crystallization in m-WO₃ occurring above 300^oC. The tungsten sample being formed in the oxidizing reaction remained amorphous up to 300^oC; above 300^oC, m-WO₃ crystallized. The particle size of m-WO₃ was 15nm irrespective of the methods applied. IR analysis showed that amorphous tungsten network was built by distorted WO₆ units without participation of peroxo groups (O₂^2-). The preparation of MoO₃ nanopowders by an oxidizing reaction was also studied. Crystallization of MoO₃ was found to start earlier (200^oC), leading to completely crystallized o-MoO₃ at 300 ^oC. The amorphous state of the product was detected at 100^oC only. Comparative analysis of the methods applied showed the oxidizing method to be more suitable for obtaining nanoparticles.

The processes involved in mechanical alloying (MA) of the mixture of Fe and Mg non-dissolving in equilibrium at the atomic ratio of 93 : 7 are studied by means of X-ray diffraction, Mössbauer spectroscopy, Auger electron and secondary ion mass spectrometry, chemical analysis of the composition. Surface enrichment of the powder particles with magnesium is observed at the first stage of MA. It is concluded that Mg gets segregated not only on the surface of particles but also within α-Fe grains. With a decrease in crystallite size (to ~10 nm), supersaturated solid solution of Mg in a-Fe is formed in the particles of the latter component, which is observed as an increase (to 0.288 nm) in the parameter of the bcc lattice and the appearance of additional components in the Mossbauer spectrum. It was established on the basis of model thermodynamic calculations that the driving force of the formation of solid solution in the system Fe-Mg may be the accumulated excess energy of coherent interphase boundaries of the Fe/Mg nanocomposite formed at the initial step of MA.

The reactivity of the mixture Mo-10 % B is investigated, along with the effect of the parameters of mechanical activation on heat evolution, specific surface, structure of powders, activation energy of combustion process, heat evolution rate, temperature and rate of combustion. It is shown that activation in the optimal regimes increases the reactivity of low-exothermal mixture Mo-10 % B. Using the technology of force compaction, a multilayer composite target for magnetron sputtering was obtained. It was established that the density and hardness of the synthesized material increase in the case if preliminary mechanical activation is applied.

Titanium alloys are typically brazed in the USA and Russia with two powder filler metals of Ticuni® family having compositions of 70Ti-15Cu-15Ni and 60Ti-15Cu-25Ni, several pre-alloyed filler metals of Ti-Zr-Cu-Ni system such as VPr16 and VPr28 powders, and amorphous foils of Stemet® family. All these brazing materials are characterized by high price that may be attributed to both the cost of the components and atomization process of Ti- and Zr-based alloys or with the production of amorphous tapes. The technology of producing mechanically-alloyed filler metals from elemental metal powders or hydrides was developed to cut manufacturing expenses by 40-50%. Some of these alloys such as TiBraze®375 (Ti-37.5Zr-15Cu-10Ni), TiBrazeâ260 (Ti-26Zr-14Cu-14Ni-0.5Mo), and TiBrazeâ15-15 (Ti-15Cu-15Ni) were successfully tested and accepted by the USA industry. These mechanically-alloyed filler metals are characterized by low erosion of base materials, tensile strength of Ti-6Al-4V brazed joints in the range of 670-740 MPa depending on the brazing gap and temperature, shear strength of joints 520-580 MPa, and by relatively low brazing temperature in the range of 850-890^oC that allows to perform the brazing process below b-transus temperature of most titanium base alloys. Solid-state synthesis of Ti-Zr-Cu-Ni alloys was investigated by varying the time of high-energy ball milling. The products were studied by DTA, EDS analysis, and scanning electron microscopy. Clear evidences of solid-state reactions obtained in this study confirm that the resulting alloys are partially pre-alloyed and comprise Cu and Ni dispersed throughout the Ti and Zr phases. DTA results displayed a decrease in the liquidus temperature. The notable effect of milling is the induced exothermic effect prior to melting of mechanically-alloyed brazing alloys.

Comparative investigation of the process of mechanochemical preparation of supersaturated solid solutions is carried out with the composite powders (Co-P)_100-x/Cu_x, which are fine particles with the nucleus composed of amorphous Co₈₈P₁₂ alloy or crystal alloy Co₉₅P₅ coated with a copper layer, and a mixture of Co-P and Cu powders. The changes in structural and phase state of the powders during mechanical alloying are investigated by means of electron microscopy, X-ray diffraction, magnetic measurements and NMR spectroscopy. It is shown that mechanical alloying (MA) of the powder of composite particles with amorphous nuclei proceeds much more rapidly than with a similar powder with crystalline nuclei or mechanical mixtures of Co and Cu powders. It is established that MA in Co-P/Cu system with the mass concentration of copper 20 % (a thin Cu coating) passes through amorphization stage. In the case of composite powders (Co₈₈P₁₂)₅₀/Cu₅₀ (with an increased thickness of Cu coating) crystallization of the Co-P nucleus is observed at the start of MA process, then the formation of equimolar supersaturated solid solution occurs.

The effect of mechanical activation of quartz (separate and joint with aluminium) and abrasion-reaction wear of the material of milling bodies on the induction period and temperature of thermite process of obtaining sintered materials based on quartz is investigated.

The solid phase synthesis of Bi₂Mo₃O₁₂ is investigated. The mechanochemical activation is used in order to reduce the synthesis times and temperatures. A 5 h mechanical treatment leads to a drastic decrease in the temperature of classical solid state synthesis of Bi₂Mo₃O₁₂ (from 600 to 350^oC). The product obtained is single-phase, consisting of even size distributed submicronic particles. The synthesis of Bi₂Mo₃O₁₂ was monitored by X-ray diffraction (XRD) and infrared spectroscopy (IR). Shape and size changes of reagent particles after mechanical treatment and morphological peculiarities of the products were studied by SEM.

The effect of mechanical activation on the synthesis of Cr₂(MoO₄)₃ is studied. Its formation was monitored by X-ray diffraction (XRD) and infrared spectroscopy (IR). Shape and size of the product particles were studied by scanning electron microscopy (SEM). The compound studied presents a special interest as a catalyst for mild oxidation of alcohols. A stoichiometric mixture of Cr(OH)₃ and MoO₃ in a 2:3 molar ratio was subjected to intense mechanical treatment in a planetary ball mill (Fritsch-7) for different periods of time, followed by calcination at various temperatures. The 2.5 h milling produced broadening and decrease in the intensity of the MoO₃ XRD peaks, while the principle peaks of Cr₂O₃ appeared. Further milling (5 h) resulted in complete amorphization of the reagent mixture. IR spectroscopic analysis indicated occurrence of structural transformation in the mixture of solid reagents during the mechanical activation. These phase and structural changes are a precondition for a drastic decrease in the temperature of classical solid state synthesis of Cr₂(MoO₄)₃ from 700 to 400^oC.

The relationships for irreversible flows of matter, vacancies, surfaces, and heat are derived with the allowance for various cross effects on the basis of the model of a medium with additional parameters that determine structural features and availability of vacancies and internal surfaces. The generalized linearized relationships for the rates of chemical reactions, for the sources of vacancies and surfaces, as well as a generalization of the law of internal friction have been obtained. Analysis of various types of diffusive flows has been conducted for the diffusion that proceeds through the replacement mechanism. It was revealed, in particular, that the coefficient of volumetric diffusion in a polycrystalline material is other than the coefficient of self-diffusion for a single crystal that should be taken into account when interpreting experimental data for diffusion in fine structured materials.

Effect of inorganic salts NaF, NaCl, MgF₂, CrCl₃, and (NH₄)₂CO₃ additives on the processes of hydrogenation and dehydrogenation of magnesium mechanical alloys has been studied. It has been demonstrated that mechanical alloying of magnesium with a series of inorganic salts not only facilitates the grinding of metal, but also modifies its surface that leads to an acceleration of hydrogenation and dehydrogenation reactions and to rather high hydrogen content of samples (up to 5.5-6 mass % with the use of NaF, NaCl, or CrCl₃). It has been found that salts modify differentially the metal surface that is reflected in the kinetic curves, especially, during the first hydrogenation cycle.

Mechanoactivation method has been applied to an officinal medical preparation of calcium gluconate for obtaining its modified form. It was found by X-ray diffraction and microscopic investigation techniques that the mechanoactivated powder constitutes X-ray amorphous, nanodispersed product with particle size from 50 to 500 nm. X-ray electron spectral analysis revealed no basic changes in chemical composition of calcium gluconate upon mechanoactivation.

Electrochemical properties and ionic conductivity of LiTi₂(PO₄)₃ (space group R-3c) synthesized using mechanical activation (MA) are investigated. It is established that MA decreases synthesis temperature and time allowing one to obtain the products of more homogeneous phase composition. The cycling ability of the samples is investigated with the help of a galvanostatic set-up in the electrochemical cell LiTi₂(PO₄)₃ (C)/(LiPF₆+EK+DMK/Li. It is established that within the potential range 2.0-3.5V the capacity of the samples obtained with the help of MA is higher than that of the ceramic samples by about 20%. Reversible cycling ability of LiTi₂(PO₄)₃, NaTi₂(PO₄)₃ and KTi₂(PO₄)₃ within the range 0.2-1.5V was discovered. It may be due changes in the chemical composition of the electrode as a result of irreversible redox processes participated by PO₄ groups. It is shown that the conductivity of LiTi₂(PO₄)₃ and Li_1.3Al_0.3Ti_1.7(PO₄)₃ obtained using MA is 2-3 orders of magnitude higher than the conductivity of ceramic samples due to a decrease in resistance of the grain boundaries.

Selection of conditions is performed for mechanochemical activation of Siberian larch wood and for aqueous extraction that provide the maximum yield of arabinogalactan (AG) and dihydroquercetin (DHQ). The possibility of processing an extracted wood of larch to produce microcrystalline cellulose (MCC) by the use of the organic solvent delignification process with acetic acid-water-hydrogen peroxide mixture in the presence of sulphate catalyst was shown. A diagram for comprehensive processing of larch wood to yield AG, DHQ, MCC, and low-molecular lignin is suggested.

The mechanisms of synthesis processes and mechanochemical reactions in the dynamically loaded layer of a powder mixture able to undergo gas-free self-propagating high-temperature synthesis (SHS) are investigated by means of computer simulation. Within the model of reacting powder medium, the related problems of shock compaction, establishment of thermal balance, filtration of a melt of the low-melting component, and macrokinetics of chemical transformations are considered. We take into account the macroscopic structure of concentration non-uniformity of the initial powder mixture, the possibility of mechanical activation of the reacting components during shock compression, and modification of the parameters of state and structure at all the stages of synthesis. The plastic deformation of crystal structure and destruction of the surface layers of powder particles are considered to be the decisive factors of mechanical activation. The model of reaction cell of Arrhenius type with variable macrokinetic parameters is used. The activation energy of the reacting mixture is considered to be a linear function of the work of plastic deformation and the work of destruction of the surface layers of particles. The pre-exponential factor agrees with the power-behaved reaction diffusion. The powder body is represented by a model system of powder components with the given structural parameters, physical and chemical characteristics; the structure of the powder layer is represented by the model regular structure of the cells of concentration non-uniformity. Powder mixtures of Ti-Al and Ti-C types preliminarily pressed to the required mean porosity are considered in the computational experiment. The factors determining changes in the reactivity of the reacting powder mixture during mechanical load are considered, as well the effect of the formation of a structure with nanometer-scale morphological elements are investigated.

Dispersity, structure, phase composition and magnetic properties of the powder obtained by grinding the alloy of iron with silicon (with the atomic concentration of Si equal to 20 %) in liquid hydrocarbons (heptane, heptane with oleic acid as an additive) in a ball planetary mill were investigated by means of X-ray structural analysis, Mössbauer and Auger spectroscopy, secondary ion mass spectrometry, magnetic measurements. It was shown that during grinding the alloys get saturated with the products of destruction of the organic liquid (C, O, H) with the formation of metastable amorphous and carbide phases. With an increase in grinding time, the particle size decreases to 0.1-0.2 mm. Isochronous (500 ^oC, 1 h) annealing leads to the crystallization of the amorphous phase with the formation of ferric silicocarbide Fe₈Si₂C, Fe₃Si and C in the case of grinding in heptane, and Fe₃C with SiO₂ in the case of grinding with oleic acid added. The magnetic characteristics of the powder depend on the time and medium of grinding, and also on annealing temperature. An increase in coercivity from 10 to 180 A/cm correlates with the amount of the formed silicocarbide. The resulting silicocarbide has the composition Fe₈Si₂C; it has a triclinic lattice with the parameters: a=6.401Å, b=6.434Å, c=9.884 Å, α=83.590^o, β=99.343^o, γ=120.924^o; it is a ferromagnetic with Curie temperature T_C equal to 788 K, stable up to 870 K.

It was established by comparing the results of synthesis of peroxides using mechanical activation of the initial reagent mixture in a disintegrator and controllable precipitation both these procedures of preparation of the initial components lead to the same result: a decrease in synthesis temperature by 100^oC. It was demonstrated that in the case of mechanical activation the factor facilitating the course of the solid-phase reaction is excess enthalpy brought into the mixture by the shock action. In the case of chemical activation by the sol-gel procedure, the reaction course is facilitated due to efficient mixing. Simplicity and the possibility of rapid preparation of the initial mixture for the reaction in the solid phase make mechanical activation preferable.

Solid solutions of the LiFe_1-yTi_1-xMn_x+yO₄ (0 ≤ x ≤ 1; 0 ≤ y ≤ 1) composition are investigated; their physicochemical properties are studied with the help of X-ray diffraction and electrical measurements. It is shown that the substitution of manganese cations for titanium and iron in the spinel structure results in a linear decrease in the lattice parameter. Electrical conduction increases monotonously with an increase in manganese concentration. Conductivity is electronic; activation energy decreases monotonously with an increase in (x + y) from 0.50 to 0.33 eV. Quenched samples possess higher conductivity and larger lattice parameter, which may be due to non-stoichiometry in the oxygen sublattice.

The results of investigation of the effect of mechanochemical treatment of coal under the shock-shear action on coal liquefaction process in a hydrogen-donor solvent are presented. In order to explain the choice of coal raw material most easily subjected to mechanical activation and thermal dissolution, the investigation was carried out with vitrinite concentrates of brown and black coal samples with different degree of metamorphism of technological grades D, G, Zh, K (R₀ = 0.4-1.25 %). It was shown that liquefaction of mechanochemically activated coal not only results in an increase in the yield of liquid products of the transformation of the organic matter of coal samples but also in substantial changes in their qualitative composition.

The possibilities to utilize pyrite concentrates from Chelopech Region (Bulgaria) were investigated using mechanochemical and thermal methods to obtain soluble iron-containing compounds and to extract useful microelements form the polymetallic ore. Mechanochemical activation was carried out in a planetary mill for 10 h; the resulting solid-phase samples were heated under isothermal conditions at 573 K for 4 h. To estimate mechano and thermochemical effects, Mössbauer spectroscopy, X-ray phase analysis and thermal methods of analysis in the oxidative gas medium were used. It was proved by means of physicochemical methods that phase transformation of pyrite to FeSO₄ and a decrease in its ignition temperature by 120 K are possible after triboactivation of pyrite samples.

The effect of grinding method on the size of particles of the mixture components and on the parameters of the solid-phase synthesis of beryllium indialite from a mixture of dry oxides BeO, MgO, Al₂O₃, SiO₂ is investigated. It is demonstrated that with the use of mechanical activation the formation of intermediate phases starts at a temperature of 1100 ^oC, while beryllium indialite starts to form at 1200 ^oC. The content of beryllium indialite in the activated mixture at the moment when the temperature of 1340 ^oC is achieved is ~90 %, while without mechanical activation it is only ~50 %.

The results of the treatment of titanium-containing raw material using the complex procedure including mechanochemical, coal chemical and sulphuric treatment are reported. Regularities significant both from the theoretical viewpoint (development of research in the area of mechanochemical intensification of technological processes) and practical application (choice of optimal parameters of the consumption of raw material and energy) are revealed.

Theoretical and experimental investigation of the solid-phase mechanosynthesis of nano-sized target product on the basis of dilution of the initial powder mixture of reagents with another product of the exchange reaction was carried out. Optimal molar ratios of the mixture components providing shock-friction contacts of reagent particles and excluding aggregation of nano-sized particles of the target reaction product were calculated basing on the three-modal particle size distribution. A deduction of the kinetic equation for the mechanosynthesis of nanoparticles by means of dilution is presented. The necessary time of mechanical activation (MA) for the exchange reactions to process till completion is calculated using this equation. The obtained theoretical results agree with the available literature data and with the results of experimental investigation of the kinetics of mechanosynthesis of nano-sized target product TlCl according to the exchange reaction 2NaCl + Tl₂SO₄ + zNa₂SO₄ = (z + 1)Na₂SO₄ + 2TlCl for the optimal value of dilution parameter z= z₁* = 11.25. A number of specific features of this reaction are established. Comparing the kinetic parameters obtained for mechanosynthesis of TlCl with those for theoretically investigated exchange reaction KBr + TlCl + zKCl = (z + 1)KCl + TlBr for z = z₁* = 13.5, experimental estimation of the mass transfer coefficient was carried out for the first time and the dynamics of changes in the size of nanoparticles depending on the time of MA was established for a ball planetary mill AGO-2.

Mathematical models of connecting materials using a mixture of exothermal composition with inert filler in the regime of thermal explosion and in the regime of combustion are proposed. The first problem is formulated as thermal inflammation in a vessel with thick walls, the second one as the propagation of the front of exothermal reaction in a slit between two thick plates. In general, the addition of an inert filler to the connecting mixture causes an increase in inflammation time, a decrease in temperature and rate of reaction, which coincides with the known experimental data. Within a definite region of the model parameters, numerical investigation of the inflammation problem revealed different regimes of inflammation: thermal explosion and a slow transformation under weakly changing temperature. When investigating the problem of reaction propagation in a slit, also different regimes of transformation with variations of thermal properties of the substances were discovered. The region of parameters in which the self-sustaining regime of transformation occurs was found. For real technologies, low-temperature synthesis regimes are of interest, because in this case thermal strain decreases.

The effect of mechanochemical activation (MCA) on the structure and properties of Syrian phosphorite is investigated. The character of structural changes and their effect on the solubility of activated Syrian phosphorite are studied with the help of X-ray phase analysis, infrared and electron spectroscopy, and thermal analysis. It is established that MCA causes substantial structural perturbations in the initial phosphorite; they result in an increase in the reactivity and solubility of mechanochemically activated phosphorite samples.