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This work is aimed at quantifying mechanochemical effects in reactive solid–gas systems. It deals in particular with the kinetics of H 2 absorption processes on a Mg 2 Ni/Ni composite under thermal and mechanical activation. In a first series of experiments, powders were exposed to H 2 under static conditions for selected time intervals and successively subjected to mechanical treatment. In a second series, the latter was instead started in coincidence with the exposure of powders to H 2 . The effects of mechanical processing on the H 2 absorption processes have been quantified via the accurate description of the absorption kinetics with suitable model curves. A tentative rationalization of experimental observations is proposed.

A systematic investigation on different systems has been performed in order to ascertain the possible role of local temperature rise at impacts in the mechanism of mechanically induced phase transformation processes. High-purity commercial powders have been mechanically treated under different milling conditions to study their kinetic behavior. Phase transformations have been correlated to the minimum temperatures needed to trigger the transformation processes under thermal activation conditions. Deep contradictions have arisen in connection with the so-called “hot spots” theory.

The present investigation focuses on the kinetics of hydrogen absorption in Mg2Ni powders subjected to thermal or mechanochemical activation. The process was initially carried out under isobaric-isothermal conditions in a mechanochemical reactor at rest. Once the static hydrogen absorption process approached completion, the system was subjected to mechanical activation at constant hydrogen pressure. The mechanical treatment of powders induced further hydrogen absorption at rates depending on the processing intensity. The observed mechanochemical effect is related to the generation of reactive surfaces when mechanical loads apply on powders. The reactive surface area involved in the mechanochemical hydrogen absorption and the duration of the absorption process are roughly estimated.

We report on the structural evolution and thermodynamic features of Zr-based amorphous and nanostructured powders subjected to mechanically induced hydrogen absorption. Zr 55 Cu 30 Al 10 Ni 5 and Zr 69.5 Cu 12 Al 7.5 Ni 11 compositions were studied. The process began with a milling induction period, during which neither gaseous uptake nor structural modifications were observed, then, according to an s-shaped kinetics, sorption reached the maximum rate, approaching finally the asymptotic value of 1.5 H/M wt.%. Specific kinetic features were depending on the starting materials. As a result of the absorption, the amorphous alloys of both the compositions promoted the segregation of crystalline δ-ZrH 1.66 phase, which was paralleled by the formation of an amorphous hydride of depleted Zr composition, with relative abundance of the two hydrides 45:55. The different thermodynamic stabilities of the two hydrides were evidenced by calorimetric analyses. While desorption started below 400 K in the metastable one, conversely the crystalline δ-ZrH 1.66 decomposed only above 1070 °C. Faster absorption kinetics and different structural reconstruction path characterised the hydrogenation of the Zr 55 Cu 30 Al 10 Ni 5 nanocrystalline alloy. Starting from a mixture of two nanosized fcc phases, with similar (Cu, Al, Ni)Zr 2 composition, a single fcc H–(CuAlNi)Zr 2 phase was formed, which reversibly desorbed H 2 , restoring the two initial phases.

Hydrogen absorption and desorption properties in nanocrystalline LaMg 2 Ni are presented. Nanostructured phases have been obtained by milling grain coarse ingot and by mechanically alloying the parent elements. The structural and hydriding properties were examined by X-ray diffraction, thermal analysis and thermal desorption measurements. Ball milling and mechanical alloying give a significant refinement of the microstructure. Reactive milling has been used for hydrogen absorption experiments. Hydrogenation by means of reactive milling at 300 K under a pressure of 0.4 MPa leads to the formation of a stable La-hydride phase together with an amorphous phase. Thermal desorption up to 983 K of hydrogenated samples leads again to parent LaMg 2 Ni phase.

An equiatomic Co–Fe solid solution was synthesized by mechanical processing. A dissolution process of Co in Fe was observed. Its kinetic characterization permitted a reliable estimate of the number of Co atoms dissolved in collision events. This number was then compared with the corresponding quantity observed in Co–Fe diffusion couples under thermal activation conditions. The data support a mechanistic hypothesis for mechanochemical transformations alternative to the ones based on thermal diffusion.

In mechanical treatments by ball milling, the mechanical energy is transferred to powders when these are trapped between colliding milling tools. Under such processing conditions, the number of collisions is therefore the natural quantity to which refer the kinetics of any given transformation. A statistical model is here developed to relate the transformation rate to the amount of powder trapped at each collision. The phenomenological model is applied to mechanically induced amorphization processes of intermetallic compounds and binary mixtures. Provided that accurate experimental methodologies are employed to estimate the amorphization rate and the number of collisions, the comparison between experimental and theoretical kinetic curves permits to obtain a deeper insight into the observed kinetic behavior.

A phenomenological model is developed for describing the kinetics of the crystallite size refinement process of Cu powder under mechanical treatment conditions. Based on the evidence that collisions represent the elementary events of energy transfer, the rate of crystallite size decrease is related on a statistical basis to the amount of powder trapped at each collision, to the number of collisions and to the collision energy. The mathematical approach allows for identifying the approximate functional form of the kinetic curves obtained at largely different impact energies. The values of the apparent kinetic constants and of the model parameters involved can be thus estimated by fitting the model curves to the experimental data. The results obtained provide a deeper insight into the details of the crystallite size refinement process.

The present work discusses the preparation of nanostructured ceramics by mechanical processing. Coarse-grained powders of different oxides, carbides, borides and silicides have been subjected to mechanical treatment inside a commercial ball mill under controlled experimental conditions. Ceramic powders underwent a microstructural refinement process down to the nanometer range, accompanied by the accumulation of structural disorder. The analysis of X-ray diffraction patterns according to the Rietveld method permitted the estimation of both the average crystallite size and strain content as a function of the milling time. Opposite trends are observed, the decrease of the crystallite size being paralleled by the increase of the strain content. The processes of crystallite size decrease and strain accumulation were then phenomenologically characterized in terms of apparent rate constants and asymptotic final values of crystallite size and strain. It has been thus possible to point out an intimate connection between the processes of crystallite size decrease and strain accumulation.

A systematic study of the crystallite size refinement process of elemental species under mechanical processing conditions has been carried out. Elemental powders were treated in a milling device at different intensities. The dependence of the rate of size reduction as well as of the final crystallite size attained on both impact energy and collision frequency was thus investigated. The final crystallite size shows only a weak dependence on the average impact energy. Conversely, the latter greatly affects the rate of crystallite size decrease. A threshold impact energy is also observed below which no significant structural refinement occurs. The threshold energy is found to scale with the shear modulus of the chemical species.

We focus on the hydriding kinetics of Mg2Ni/Ni nanocrystalline and Zr55Cu30Al10Ni5 amorphous powders under reactive milling conditions. A methodological approach, based on mechanochemical parameters, is presented to quantify the different behaviour shown by the two systems. A decelerating course characterized the H2 absorption rate in the Mg based powders. Although they were able to absorb H2 under static conditions, a speeding up of the kinetics was observed under milling. The mechanochemical gain, g m , that is the rate increase at the milling outset, was found to depend on the specific milling intensity I M . By contrast an incubation period preceded the absorption in the case of the Zr based amorphous powders. Furthermore, S-shaped trends described the reactive behaviour in this case even if the kinetics was still dependent on I M . Irrespective of the milling intensity it was found that the same mechanical work, that is the energy dose, was required to reach the maximum hydriding rate.

The effects of mechanical processing by ball milling on powder mixtures consisting of immiscible elements were studied. Equiatomic Ag–Cu and Co–Cu mixtures were subjected to mechanical treatment at different values of milling intensity. The kinetics of structural evolution were characterized by means of X-ray diffraction analyses taking advantage of the Rietveld method. The evolution processes followed sigmoidal trends, the rate of evolution being proportional to the milling intensity and thereby connected with the impact energy. A threshold impact energy was determined below which no structural evolution is observed.

The effect of mechanical activation of reactants on the self-propagating high-temperature synthesis of TaSi 2 and Ta 5 Si 3 was investigated. SHS experiments were performed on mixtures of compositions Ta:Si = 1:2 and Ta:Si = 5:3 which had been milled for various times. For both compositions, mechanical activation had a large influence on the characteristics of the combustion process, particularly on the combustion peak temperature and wave velocity. The product composition depended on the initial stoichiometry and milling time. For the 5:3 stoichiometry, the product was single phase Ta 5 Si 3 (in its hexagonal and tetragonal forms) only when the powders were milled for sufficiently long milling times (>120 min). The 1:2 stoichiometry produced TaSi 2 in a pure form, regardless of milling conditions. A mechanism for the combustion process is proposed on the basis of the microstructural evolution in quenched reactions.

A comprehensive approach is presented for defining hydrogen activation and absorbing kinetics in heterogeneous Mg2Ni/Ni powder composites that were subjected to mechanical refinement. Hydriding tests were performed under conventional hydrogen dissolving and under reactive milling. Irrespective of the absorbing mode, the absorption kinetics is deceleratory throughout. Under conventional thermodynamic conditions, the hydriding rate depends strongly on the microstructural features of both the absorbing Mg2Ni intermetallic and the Ni phase. The latter plays an important role in the dissociative chemisorption of hydrogen. Under milling the hydrogen uptake and the hydriding kinetics also depend on the intensity of the milling processing, IM (watt g−1), with the absorption rate increasing exponentially with IM. The mechanical treatment was found effective even when thermodynamic absorption reached saturation level. Hydriding rates, mechanochemical gains, and instantaneous mechanochemical yields (mol J−1) were used to compare the processes on an absolute scale and to spotlight possible mechanisms controlling kinetics trends and absorbing features under milling.

Several mechanochemical reactions, involving the formation of amorphous phases and extended solid solutions as well as the spontaneous spreading of self-propagating reactions, were investigated under controlled milling conditions. The work was aimed to identify possible connections among milling parameters and conversion mechanisms. Mechanochemical transformations are shown to proceed according to two different kinetics, characterized by continuously decelerating and sigmoidal conversion trends. Experimental findings are rationalized on the basis of invariant quantities which were found to characterize the observed reactions. Such quantities also provide a means for the absolute comparison of the reactions. It is demonstrated that a major role should be attributed to the mechanical work done on the powders during the course of the mechanical treatment. The degree of conversion is found to depend only on the energy dose transferred to reactants, independently of the milling intensity.

A complex methodology has been developed in order to quantify the conditions of mechanical treatment in laboratory and commercial ball mills. The experimental determination of the collision frequency and the impact energy has been paralleled by an accurate numerical modelling of the balls and powder dynamics. The obtained results have been further confirmed by a direct inspection of the motion of balls and powder inside a transparent quartz reactor by means of high-speed video recording. The temperature of mechanochemical reactors has been also continuously monitored during the course of mechanical treatments. The methodology has been systematically applied to characterise on absolute basis the kinetic behaviour of a large variety of mechanically-induced structural and chemical transformations. In particular, systematic investigations have been performed on the amorphisation of intermetallic and binary systems, on the formation of extended solid solutions in immiscible systems, on the ignition of combustion-like reactions in metal-metalloid mixtures and on the hydrogen storage in nanocrystalline and amorphous multicomponent metallic alloys. Mechanochemical transformations appeared to be basically ruled by two different mechanisms characterised by simple asymptotic trends and sigmoidal curves respectively. An attempt to rationalise the observed mechanisms on a phenomenological basis has been made.

Transparent quartz vials were employed for a direct viewing of ignition and propagation phenomena in combustion-like reactions under milling. Combustion temperatures were measured from infrared light emission by two-colours pyrometers. Such a methodological approach allowed one the in situ characterisation of the self-propagating behaviour of a variety of carbides and silicides. Experimental combustion temperatures close to the theoretically calculated adiabatic ones were observed for the reaction which goes to completion within millisecond period.

Mechanically-induced combustion reactions were carried out in a transparent quartz vial. The combustion processes were followed by high-speed video recording and optical pyrometers. Depending on the vial dynamics, the powder particles were seen to gather in a periodic collective motion, behaving like a granular gas characterised by subsequent changes in its density. After ignition, a considerable increase of brightness was observed, due to the extremely rapid propagation of the high-temperature reaction to the whole mass of powder. Experimental observations find a very good correlation if compared to the results of numerical calculations simulating the particle system dynamics. Balls and powder dynamics were reproduced according to a well-known visco-elastic force scheme, while chemical conversion of each powder particle was described by a conventional nucleation-and-growth kinetics. The propagation mechanism of the reaction was studied and related to the dynamic features of the whole system.

Mechanically alloyed Mg2Ni/Ni nano-composite and Zr55Cu30Ni5Al10 amorphous powders were hydrogen charged at constant pressure under ball milling. While the Mg-based samples were able to absorb large amount of hydrogen under static conditions, and from the very onset of the mechanical treatment, an incubation milling period was needed for the hydrogen activation in the case of the amorphous sample. A progressively decreasing rate characterizes the process kinetics in the case of Mg2Ni/Ni, whereas a self increasing sigmoid trend was observed for the multicomponent Zr alloy, which showed structural demixing phenomena in the absorption course. Irrespective of the system, the maximum absorption rate was strongly dependent on the milling intensity (Watt·g-1). The hydriding rate (mole·g-1s-1) and the mechanochemical yield (mole·Joule-1) were used to compare the processes on an absolute scale. In the case of Mg-based alloy, comparison was also made between mechanically induced and thermally (static) activated hydriding processes. Attempt was also made to work out the hydrogen atoms absorbed at the ball-hit event.

The effect of the mechanical activation of the reactants on the self-propagating high-temperature synthesis (SHS) of titanium carbide was investigated. The SHS experiments were performed on two compositions, Ti50C50 and Ti70C30, which define the homogeneity range of the TiC equilibrium carbide. Milling times were progressively increased up to the time at which a combustion-like process ignites spontaneously under milling. The combustion peak temperature, wave velocity, and ignition temperature were markedly influenced by the degree of mechanical activation of the reactants. In particular the ignition temperature was observed to decrease from a temperature corresponding to the melting point of Ti to 500°C. The apparent activation energy for propagation of the combustion wave was also determined (∼100 kJ·mol−1) and was found to be independent of both the degree of mechanical activation and the composition of the starting mixture.

The reactivity of Ta-C mixtures with a composition of Ta:C=50:50 was investigated under different mechanical processing conditions. The specific milling intensity was changed by varying the average impact energy and the collision frequency. These quantities were proven to be the key parameters for a quantitative description of the chemical reaction. The gradual formation of the non-stoichiometric carbide phase TaC was observed either at very low or at very high impact energies. At intermediate values, the mechanical processing induced the spontaneous spreading out of the reaction. Ignition times were found to depend on the impact energy and the shortest activation period was registered for the highest impact energy. Results prove that the mechanical work required for the combustion is independent of the milling intensity and is a characteristic quantity of the system.

We report on the reactive behaviour of Ti–C powder system under mechanical work by milling. A gas-less combustion range was found under constant milling intensity extending from Ti 73 C 27 to Ti 30 C 70 . By increasing the impact energy the ignition time decreases exponentially revealing the existence of a threshold energy below which the spontaneous spreading out of the reaction cannot take place. Mechanical pre-treatments were shown to have a large influence on conventional SHS experiments as far as the combustion temperature and propagation velocity of the reaction front are concerned. However, the apparent activation energy for the overall combustion process was found independent on the degree of mechanical activation of the reactants.

We report on the mechanical hydriding of amorphous and nanostructured Zr-based alloys. The absorption process induces a demixing of the crystalline ZrH1.66 from the amorphous matrix with the concurrent formation of a less stable amorphous hydride. This phase releases the absorbed hydrogen at lower temperatures. No phase separation was observed in the nanostructured powders, which absorb hydrogen more quickly with respect to the amorphous structure through high diffusivity paths such as grain boundaries and free surfaces. Irrespective of the structural characteristics and the composition of the starting alloys, the absorption kinetics follows in both cases a well-defined sigmoidal trend. The conversion rate increases with the intensity of the milling treatment.

The influence of mechanical activation on the characteristics and mechanism of ignition of self-propagating high-temperature synthesis processes of different silicides in the systems Me–Si (Me =Ti, Nb, Mo) was investigated. The results show that mechanical activation does not alter the mechanism involved but influences significantly the ignition characteristics. The influence, however, strongly depends on the stoichiometry of the mixtures. The composition Ti:Si = 1:2 shows the largest influence, with the ignition temperatures decreasing from 1400 °C for unmilled powders to about 600 °C for powders milled for several hours. The compositionsTi:Si = 5:3, Nb:Si = 1:2 show less pronounced decreases, while the compositionMo:Si = 1:2 shows no decrease. These differences are discussed in terms of the role of microstructure in the reaction mechanism and the different response of the systems to contamination, particularly from oxygen. The results suggest that for these systems self-ignition processes during milling cannot be explained only on the basis of the decrease in the ignition temperature.

The direct observation of the self-combustion of structurally co-deformed elemental powders, in the course of the ball milling treatment, was possible by employing quartz transparent reaction vessels and a high-speed image acquiring system. The synthesis of a titanium–silicon system was chosen as a case study. Whereas the primary ignition appears to be linked to an impact event, the reaction spreads out because of the violent and continuous throwing out of burning particles inside the milling vial. The reaction reaches completion within milliseconds. The use of radiation pyrometers enabled us to register the combustion temperature, which reached the melting temperature of the reaction product and approached the adiabatic temperature of the process. Results indicate that the underlying reaction mechanism relies on liquid–solid diffusion as in the conventional self-sustaining high-temperature synthesis process. Important information of the ball–vial dynamics was also obtained by tracking the trajectory of the ball, which, due to being heated up to luminosity, became visible for several cycles of the reciprocating milling machine.

It is well-known that the mechanochemical processing of binary mixtures of transition metals can induce the formation of nanostructured and amorphous phases. Previous kinetic studies have shown that the phase transformation in binary mixtures follows sigmoidal conversion curves irrespective of the intensity of mechanical processing. The rate of such transformations is instead strongly influenced by both the intensity of the mechanical treatment and the physical properties of the reactant powders. In the present work, we give additional information on the amorphisation kinetics of binary mixtures by exploring the effect of composition on the transformation kinetics. Cu–Ti and Fe–Ti mixtures with composition within the amorphisation ranges have been subjected to mechanical processing at fixed intensity. The kinetics of phase transformation has been followed by quantitative X-ray diffraction. The experimental data obtained demonstrate that the amorphisation rate has a strong dependence on the product of the mass fractions of the single reactants.

A detailed study on the disordering of the equilibrium compound NiTi 2 under mechanical processing conditions is reported. The characterization of the milling regime, together with the application of refined X-rays analysis techniques, allowed for the quantitative kinetic description of the amorphization process as a function of the number of impacts. Experimental findings show that the phase transformation follows first-order kinetics. Complete disordering of the crystalline lattice required about ten millions of impacts.

The effect of the mechanical activation of the reactants on the self-propagating high-temperature synthesis (SHS) of niobium silicides was investigated. SHS experiments were performed on reactant powder blends of composition Nb:Si = 1:2 and Nb:Si = 5:3 pretreated for selected milling times. A self-sustaining reaction could be initiated when a sufficiently long milling time was employed. At short milling times, the reactions self-extinguished or propagated in an unsteady mode. Combustion peak temperature, wave velocity, and product composition were markedly influenced by the length of the milling treatment. Single-phase products could be obtained for sufficiently long milling times. Observation of microstructural evolution in quenched reactions together with isothermal experiments allowed clarification of the mechanism of the combustion process and the role played by the mechanical activation of the reactants.

High-energy ball milling was applied to prepare powder samples by treating copper or copper oxides with magnesium or magnesium oxide. The five samples thus prepared were characterized by physical methods. According to X-ray diffraction the mechanical treatment resulted in the formation of nanostructured powders. Carbon and oxygen impurities and Mg (mainly as oxide) were detected by X-ray photoelectron spectroscopy of the as-milled samples. Four samples, namely, nanocomposite materials prepared by the self-sustaining reaction of copper oxides and magnesium applied in stoichiometric amounts [(Cu 2 O)Mg and (CuO)Mg] and samples produced by mechanical treatment of copper oxides and magnesium oxide [(Cu 2 O) 7 (MgO) 93 and (CuO) 13 (MgO) 87 ] also had copper on the surface. The latter specimens and 3% Cu+MgO were unique in their characteristics to have, after a short hydrogen treatment or after catalytic application, an unusually large concentration of Cu 0 measured by the N 2 O titration method. Temperature-programmed reduction showed, in each case, the existence of reducible copper species on the surface. The quantity of reducible copper was quite low for (Cu 2 O)Mg, (CuO)Mg, and 3% Cu+MgO, whereas almost the total amount of copper of (Cu 2 O) 7 (MgO) 93 and (CuO) 13 (MgO) 87 was available for reduction. Two types of copper species have been detected: bulk CuO with a reduction temperature of about 260°C [characteristic of (CuO)Mg and also found in (Cu 2 O)Mg] and species strongly interacting with the support with a reduction temperature of about 350°C [3% Cu+MgO, (Cu 2 O) 7 (MgO) 93 , (CuO) 13 (MgO) 87 , and also detected in (Cu 2 O)Mg]. The 3% Cu+MgO, exhibited the highest basicity measured by the decomposition of 2-methyl-3-butyn-2-ol and the dimerization of acetone. The samples showed high activity in the dehydrogenation of 2-propanol, whereas their activity in the one-step synthesis of methyl isobutyl ketone from acetone was moderate. Methyl isobutyl ketone (MIBK) was formed in low selectivity over the samples prepared by a self-sustaining reaction and 3% Cu+MgO, whereas very high MIBK selectivities were observed over the mixed oxides. These features were correlated with the relative concentration of active sites capable of hydrogenating the carbon–oxygen and carbon–carbon double bond.

Several Ti-, Zr-, Hf- and Nb-based alloys were synthesized using differing milling regimes. Metastable phases, either crystalline or amorphous, develop from the parent elements according to a general sigmoid-shaped behaviour ruled by an interface-controlled kinetic mechanism. The extent of the alloying reactions was related to the operating variables, experimentally determined in the course of the process. Although the transformation rates depended on the milling intensity, that is on the impact energy times the impact frequency, it was found that the reaction yield, defined by the ratio of the transformed fraction to the specific energy dose, is an invariant quantity characteristic of each system. The specific energy dose defines the mechanical work done on the system per mass unit of the reactants. A rationale for the observed behaviours was provided by the energy needed to reach a given level of the reactant dispersion. Ruling the total extent of the grain boundary area, and the overall kineti...

We have prepared a Pd-MgO sample by self-sustaining reaction between Mg and PdO, and two Pd-Mg bicomponent alloys by mechanochemical synthesis induced by ball milling They were characterized by XRD, DSC and XPS techniques Further characterization was carried out following controlled oxidation which was necessary to enhance the catalytic performance in the synthesis of methyl isobutyl ketone. Complex transformations characterized in most cases by he formation of Pd-Mg intermetallics are observed by XRD. A common feature of all samples is the surprisingly low concentration of the metallic components due to oxygen and carbon impurities and the lack of palladium on the surface High catalytic activities were achieved in catalytic studies but overhydrogenation due to the high hydrogenating ability of Pd resulted in decreased selectivities.

Pd25Zr75, Pd22Cu10Zr68, and Pd72Cu10Si18 alloy ribbons prepared by the rapid quenching technique, and Pd25Zr75, Pd25Cu10Zr65, Pd80Si20, and Pd72Cu10Si18 alloy powders made by mechanical alloying proved to be amorphous by X-ray diffraction and differential scanning calorimetry. The powder samples after HF treatment have been found to exhibit high selectivity in the semihydrogenation of 1,3-cyclooctadiene: the pronounced drop in the hydrogen uptake after the complete consumption of the diene allows the selective synthesis of the corresponding monoene.

Cu 15 Mg 85 and Cu 3 Mg 97 alloy powders, prepared by mechanical alloying of the constituent elements and Cu 78 Mg 22 and Cu 14.5 Mg 85.5 ribbons made by rapid quenching were characterized by physical methods (DSC, XRD, and XPS). Carbon and oxygen impurities and Mg (mainly as oxide) were detected by XPS of the as-received samples, and only the Cu 78 Mg 22 ribbon was shown to have copper on the surface. The powders, in turn, are the only samples possessing Cu 0 measured by the N 2 O titration method. The catalytic properties of the specimens were tested in the dehydrogenation of 2-propanol and in the one-step synthesis of methyl isobutyl ketone (MIBK) from acetone. The Cu 15 Mg 85 powder, is the only alloy exhibiting satisfactory catalytic performance: MIBK could be isolated in a yield of 35%. The data indicate that copper species determined by N 2 O titration are involved in the catalytic reactions.

The mechanochemical behavior of the Ti–Si system was investigated across the whole composition range at a constant milling intensity. At low Si content the mechanical alloying process leads progressively to an amorphous structure. In the central range of the equilibrium diagram, crystalline intermetallic compounds form with a combustion-like behavior. A nanostructured composite of TiSi2 gradually evolves above the Ti25Si75 stoichiometry. Transformation behaviors relate to the thermodynamic and thermochemical properties of the tested mixtures as well as with their mechanical features and structural refinement, which change drastically within the explored composition range.

In this paper, we deal with the oxygen displacement reactions in the Cu 2 O+M, and CuO+M (M=Ti, Zr, Hf) systems under self propagating regimes. Mechanically induced Self-sustaining Reactions (MSR) and conventional Self-sustaining High-temperature Synthesis (SHS) methods were employed. In the MSR processes, heats of reaction and adiabatic temperatures were experimentally determined. CuO based systems showed shorter incubation periods than the Cu 2 O based systems, and Zr, Ti and Hf in the order, required longer milling treatment to ignite. The use of appropriate methodologies, to evaluate the milling dynamics parameters, permitted to relate the observed features to the intrinsic reactivity of the components and to the role of interfacial conditions and deformation mixing. Similar reactivity behaviours were observed under SHS conditions.

Some potential uses of red mud as a raw component in clay mixtures for ceramic bodies production are presented. The influence of increasing amounts of red mud on the forming procedure, sintering and final properties was analyzed. Samples were produced by uniaxial pressing and slip casting. Two different clays are used as basic materials, the former being currently employed for the production of bricks by extrusion, the second — almost pure Kaolin — for high quality ceramic manufacturing. In both cases the addition of red mud led to more deflocculated solid–water systems and an increase of the critical moisture content. Mixtures prepared with the first clay and red mud loads up to 50% were fired at 850°C. The red mud content did not influence the sample porosity while determining a strength decrease attributed to the inertness of red mud at the working temperature. Samples produced using the second clay and red mud (0 – 20%) were fired at 950 and 1050°C. The addition of red mud determined increases of density and flexural strength which can be accounted for by the formation of a larger amount of glassy phase at higher red mud contents. The results of this work indicate excellent perspectives for using ‘red mud’ as raw material in mixtures with clay for the production of ceramic bodies.

Samples of red mud, by-products of alumina production from bauxite, are studied in the 120–1400°C interval. An extensive characterization was performed by thermal and X-ray diffraction analyses. The identification of gaseous species released upon heating was carried out by coupling the thermal analizer with a gas-chromatographic/mass spectrometer. Density evolution was also determined as a function of the heat treatment. Results indicate primary H2O release from aluminium hydroxides, followed by carbonate decomposition with CO2 evolution below 900°C. Alkaline oxides, mainly CaO and Na2O, lead to the formation of Ca3Al2O6 and NaAlSiO4 between 900 and 1100°C. At the highest temperatures, reduction of Fe3+ to Fe2+, involving O2 release, promotes the formation of Fe2TiO4, with the disappearance of the rutile-TiO2 phase. The various solid state reactions, ascertained at different stages of the heating process, and possible mass balances are discussed with reference to the state diagrams of principal red mud components.

It has been shown that structural evolution occurring in powder mixtures subjected to mechanical treatment by milling follow well-defined conversion trends as a function of milling time. Sigmoidal curves were observed in the case of the mechanical alloying of transition metal mixtures, whereas a simpler kinetic course with a progressively decreasing transformation rate was found to characterize the disordering process of intermetallic equilibrium compounds by mechanical milling. Under the stipulation that collisions are the dominant energy transfer events, a kinetic model is developed to relate the observed macrokinetic features to the discrete powder fractions, which transform at each impact. Because of its intrinsic qualities, the milling process was regarded as discrete processing. A statistical approach was followed to work out a set of differential equations, solutions of which provide a sound description of the transformation kinetics in terms of conventional rate expressions. The model allows one to reproduce the different kinetic behaviors by means of a single, unifying mathematical formalism. Furthermore, quantifying the structural evolution rate by suitable kinetic constants permits the exploration of the reactive behavior of a system treated under different milling regimes or to compare, on an absolute basis, different systems processed under similar conditions.

In this work, we report on the self-propagating reaction in Ti–Si blends, observed by SHS and MASHS (mechanical activated SHS) techniques. In spite of the differences between the two reacting methods, correlations were found between the key parameters of the two modes of activation. Moreover, this comparative study enabled us to gain some hints on the reaction mechanism. The combustive behavior of powder mixtures with stoichiometries corresponding to the intermetallics present in the Ti–Si phase diagram (TiSi2, TiSi, Ti5Si4, and Ti5Si3) was studied. The SHS characteristics, such as combustion temperature, propagation rate, and ignition temperature was strongly dependent on both the initial stoichiometry and milling time. Particular attention was paid to the influence of the initial stoichiometry and milling conditions on the reaction mechanism. A single-step dissolution-precipitation mechanism was found for the composition Ti : Si = 5 : 3. On the other hand, at the composition Ti : Si = 1 : 2, the mechanism shows two steps, the first, active at the leading front of the combustion front, involving only solid phases, and the second, active in the afterburn region, involving solid–liquid interaction.

This paper deals with the mechanochemical degradation of hexachlorobenzene over Li, Na, K, Mg, Ca, Ba, Ti, and W. Apart from the mixtures containing Ti and W, the other systems react in a combustive mode leading to the chlorides and the carbides of the inorganic substrates. Reaction stoichiometries and heats were experimentally determined. The activation time required to ignite the reaction decreases from Li to Mg and correlates with mechanical properties, such as the shear modulus and the mineralogical hardness, of the reducing agents. Physicochemical characteristics and electronic properties of the metallic substrates were then considered. A close correlation was observed when the ignition time of alkaline and alkaline earth metals was plotted as a function of the ionization energy. It was suggested that the reaction can spread out only when a given deformation mixing level and sufficient interfacial area have been reached due to milling. This condition depends on the reducing capability of the metallic substrate and on the mechanical qualities of the reacting blend.

Amorphous powders prepared by mechanical alloying were tested as heterogeneous catalysts in the 4-methylpentan-2-ol conversion into its dehydration and dehydrogenation products. The reaction was performed both in a conventional fixed-bed flow microreactor and in a modified milling vial used as a mechanochemical batch reactor. In the thermal catalytic tests several amorphous alloys, CuM (M = Ti, Zr, Hf), PdZr, and CuPdZr were employed as catalysts. Pretreatment conditions and catalytic runs induced different structural evolution in the tested systems. A correspondence was observed between structural modifications and catalytic properties. The mechanochemical trials were performed over a-Cu40Hf60 powders. Conversion and selectivity results were found to depend on the impact energy. The determination of the milling dynamics parameters allowed evaluation of the mechanochemical activity and comparison of, on an absolute basis, the results of the catalytic trials performed by the two techniques.

The mechanical alloying behavior of transition metal-based mixtures and the amorphization of equilibrium intermetallic compounds by milling were characterized on a quantitative basis as a function of the processing parameters. Different kinetic mechanisms rule the two processes. The former giving rise to a sigmoidal-shaped trend, while the latter is characterized by a continuous, gradual decrease of the conversion rate. It was found that the extent of the microstructure refinement determines the overall transformation rates and that the mechanical energy dose required to transform a given amount of the parent phases is a characteristic invariant quantity of the system undergoing amorphization. The atom fluxes at the impact, the sublayer distance affected, and the dimension of the amorphous domains formed were also determined. The emerged framework supports the view that diffusive phenomena and chemical driving forces play a minor role in the mechanical alloying processes. Conversely, the thermodynamic qualities of the reacting systems and the enthalpy gain remain the main factors in mechanically induced combustion reactions, provided a critical dispersion of the reactants is reached under milling.

A phenomenological approach is presented to analyse the reaction kinetics and compare the efficiency of processes driven by mechanochemical and thermal activation. Solid phase systems as well as gas–solid processes are examined: the synthesis of equiatomic CoFe nanostructured solid solution, the hydrogen absorption by Mg2Ni alloy and the CO hydrogenation over supported CoFe powder catalyst. In spite of the low efficiency of the mechanical processing technique, related to the discontinuous treatment, however, when elementary impact events are taken into account, mechanically activated processes show remarkably higher conversion rate values than their thermal counterparts. Such result can be related to the occurrence of mechanochemical effects.

We report on the highly exothermic solid-state reaction between hexachlorobenzene and calcium hydride. Once ignited by a thermal spike, the reaction displays a self-sustaining character in the CaH2/C6Cl6 molar ratio of 3:18. The high temperatures reached, i.e., 2550−2900 K, ensure a complete breakdown of the aromatic molecule and of undesired chloro-organic congeners, with only inorganic halide salts being found among the end-products. Combustive-like reactions were also observed when reactant powders were subjected to intensive mechanical treatment by ball milling. The combustive range of mechanically driven processes falls within that found in the true self-sustaining regime even if the activation and the extinction of the reaction were ruled by completely different mechanisms. A neat correlation was worked out relating the temperatures at the combustion front to the total heat evolved in the reaction carried out in the mechanochemical mode. The same end-products were also found. The practical exploitat...

Cu40Mg60 amorphous alloy powders, prepared by mechanical alloying of the constituent elements, and Cu–MgO samples, made by the self-sustaining reaction between Cu2O and Mg in stoichiometric amounts and additional ball milling, were characterized by physical methods (DSC, XRD, and XPS). The catalytic properties of the specimens were tested in the dehydrogenation of 2-propanol and in the one-step synthesis of methyl isobutyl ketone from acetone. The samples exhibited high activities and selectivities in the dehydrogenation of 2-propanol. In contrast, their overall performance in the synthesis of methyl isobutyl ketone is inferior to that of Cu-on-MgO prepared by conventional methods. The catalytic performances of the samples are interpreted by the structural and surface changes induced by the catalytic reactions.

Selected results concerning the acetone conversion to methyl isobutyl ketone promoted by mechanical energy are presented. Cu–MgO composites at different Cu contents and amorphous Cu 40 Mg 60 , prepared by different ball milling techniques, were employed as solid substrates for the mechanochemical process. The conversion and selectivity values of the acetone transformation were followed under Ar and H 2 reaction atmosphere and as a function of the impact energy. The mechanical treatment under H 2 resulted mainly in acetone hydrogenation leading to 2-propanol, while under Ar, the formation of diacetone alcohol, mesityl oxide and methyl isobutyl ketone (MIBK) was observed. Conversion changes and selectivity shifts were observed depending on the substrates employed, and the largest amount of MIBK was obtained when amorphous Cu 40 Mg 60 was used. An important structural transformation occurred during the mechanochemical runs.

Some selected results are reported from an ongoing study concerning mechanochemical dehalogenation reactions carried out under autopropagating high-temperature regimes. The work shows that instantaneous burning reactions can be ignited in highly exothermic systems involving chloroorganics when subjected to intensive mechanical treatments. A reference trial using calcium hydride-hexachlorobenzene is first presented. No residual traces of the organohalide compound were found in the reacted powders or in the gaseous phase sampled from the headspace of the milling reactor. The solid products of the reaction were restricted to CaHCl and CaCl[sub 2] salts and graphite. Similar combustion-like phenomena were observed when contaminated mud samples were added to initial reactants. The contaminated mud was obtained from a waste site and contained traces of polychlorodibenzodioxins and polychlorodibenzofurans. It was observed that an effective chemical oven was created which caused the thermochemical breakdown of the hazardous compounds. A transformation yield greater than 99.6% was obtained with a decrease of the toxic equivalent concentration from 2,492 to 0.788 [micro]g/kg. Unlike conventional incineration methods, the mechanically induced burst processes run under a strictly confined environment and controlled conditions.