Influence of the high energy ball milling on structure and reactivity of the Ni+Al powder mixture
Abstract Investigation of the micro- and atomic structures for the planetary ball-milled Ni + Al mixtures has revealed existence of intermediate nano-crystalline and amorphous phases, which are not detectable by XRD analysis, but can be observed by means of HRTEM. Annealing of the milled mixtures at moderate temperature, 205–280 °C, transforms the nano-phases into crystal state and makes them XRD-detectable. These nano-scale structures may serve as nucleus for the intermetallic phases produced via reaction between Ni and Al and, therefore, decrease the activation energy of this reaction and diminish the temperature of the reaction initiation. Apparently, the active nano-phases are formed du…
Structure evolution and reaction mechanism in the Ni/Al reactive multilayer nanofoils
Abstract The extremely rapid gasless reactions in binary Ni/Al reactive multilayer nanofoils (RMNFs) are investigated both experimentally and theoretically. The quenching technique is used to study the dynamics of structural transformation on the micro- and nanoscales. The experimentally obtained patterns of structural evolution related to the heterogeneous reactions are compared with the results of molecular dynamics simulations carried out for the same reactive system. Based on the obtained data, the intrinsic mechanism of the reaction in Ni/Al RMNF is considered, which adequately explains the unusual parameters of a gasless combustion wave in such a system.
Optimization of MCM-41 type silica nanoparticles for biological applications: Control of size and absence of aggregation and cell cytotoxicity
Abstract Mesoporous silica nanoparticles were synthesized at high pH using CTAB as a template and TEOS as a silica precursor. It was shown that varying the NaOH concentration between 5 and 27.5 mM allows the size, pore and silica structure of mesoporous nanoparticles to be precisely tuned. In particular, monodisperse nanoparticles with the MCM-41 structure with size ranging from 90 nm to 450 nm were obtained by increasing the NaOH concentration from 12.5 to 22.5 mM. It thus demonstrates that NaOH concentration must range between 12.5 and 15 mM in order to prepare MCM-41 silica nanoparticles with optimal size for nanovectorization. We also found that under usual conditions the aggregation of…
Combustion synthesis of MoSi2 and MoSi2–Mo5Si3 composites: Multilayer modeling and control of the microstructure
International audience; In this work, we present a multilayer modeling for the formation of molybdenum silicides in the exothermic reaction between Mo and Si under the influence of a temperature pulse. The heating rate can either be a well-controlled ramp or be generated spontaneously by the propagation of a combustion synthesis front. The model addresses the specific situation above the melting point of silicon and describes the solid–liquid reaction taking place in a single representative particle of molybdenum surrounded by the melt of silicon. We obtain a set of kinetic equations for the propagation of the interfaces between the different layers (Mo/Mo5Si3 and Mo5Si3/MoSi2) in the solid…
Models of SHS: An overview
International audience; The theoretical models of SHS based on lamellar or cellular approximations of the heterogeneous reactive media are comparatively analyzed. It is shown that the ratio of the reaction time to the characteristic time of heat transfer between particles is a decisive parameter for the combustion wave propagation. When the time of reaction is shorter than the time of heat exchange, the combustion occurs in a discrete mode; in the opposite case, a quasi-homogeneous combustion mode occurs. Development of the discrete cellular model does not discard the quasi-homogeneous approach but markedly extends the scope of combustion theory. This extension enables explanation of many o…
Explosive crystallization in amorphous CuTi thin films: a molecular dynamics study
Abstract Molecular dynamic simulation was used to study mechanism of self-propagating waves of explosive crystallization (devitrification) in the CuTi metallic glass. Processes in thin rectangular samples composed of one to two million atoms were simulated and compared with experimental data. It was shown that the nucleation of primary crystalline clusters occurs homogeneously due to spontaneous fluctuations of atomic structure; the clusters not
Hydrodynamic description of the adiabatic piston.
A closed macroscopic equation for the motion of the two-dimensional adiabatic piston is derived from standard hydrodynamics. It predicts a damped oscillatory motion of the piston towards a final rest position, which depends on the initial state. In the limit of large piston mass, the solution of this equation is in quantitative agreement with the results obtained from both hard disk molecular dynamics and hydrodynamics. The explicit forms of the basic characteristics of the piston's dynamics, such as the period of oscillations and the relaxation time, are derived. The limitations of the theory's validity, in terms of the main system parameters, are established.
Dynamical and statistical properties of high-temperature self-propagating fronts: An experimental study
International audience; We present a detailed experimental study of high-temperature self-propagating fronts using image processing techniques. The intrinsic features of the wave propagation are investigated as a function of the combustion temperature TC for a model system made of titanium and silicon powders. Different front behavior is realized by changing the molar ratio x of the mixture Ti+xSi. Outside the range x=[0.3,1.5], no thermal front is propagating while inside, three regimes are observed: steady-state combustion which is characterized by a flat front propagating at constant velocity and two unsteady regimes. The combustion temperature (or the corresponding ratio x) is thus play…
Molecular dynamics simulations of nanometric metallic multilayers: Reactivity of the Ni-Al system
The reactivity of a layered Ni-Al-Ni system is studied by means of molecular dynamics simulations, using an embedded-atom method type potential. The system, made of an fcc-Al layer embedded in fcc-Ni, is initially thermalized at the fixed temperature of 600 K. The early interdiffusion of Ni and Al at interfaces is followed by the massive diffusion of Ni in the Al layer and by the spontaneous phase formation of $B2$-NiAl. The solid-state reaction is associated with a rapid system heating, which further enhances the diffusion processes. For longer times, the system may partly lose some its $B2$-NiAl microstructure in favor of the formation of $L{1}_{2}$-${\mathrm{Ni}}_{3}\mathrm{Al}$. This st…
Macrokinetics of combustion in heterogeneous media: comparative analysis of homogeneous, layered and cellular models
Experimental data on intrinsic features of the combustion wave propagation in heterogeneous reactive mixtures published during last decade have initiated development of several novel theoretical models of the process. In this paper these models are overviewed, analyzed and compared. The analysis shows that ratio of the characteristic reaction time to the time of heat transfer between particles of the mixture is a key parameter for the mode of combustion wave propagation. When the reaction time is much smaller than the time of heat exchange, the combustion occurs in so-called discrete mode; in the opposite case quasi-homogeneous combustion mode takes place. It is emphasized that the developm…
FLUCTUATION-INDUCED LOCAL OSCILLATIONS AND FRACTAL PATTERNS IN THE LATTICE LIMIT CYCLE MODEL
The fractal properties of the Lattice Limit Cycle model are explored when the process is realized on a 2-dimensional square lattice support via Monte Carlo Simulations. It is shown that the structure of the steady state presents inhomogeneous fluctuations in the form of domains of identical particles. The various domains compete with one another via their borders which have self-similar, fractal structure. The fractality is more prominent, (fractal dimensions df < 2), when the parameter values are near the critical point where the Hopf bifurcation occurs. As the distance from the Hopf bifurcation increases in the parameter space the system becomes more homogeneous and the fractal dimens…
Gasless Combustion Regimes Near the Concentration Limits of Extinction
International audience
Molecular-dynamics studies of annihilation reactions
The validity of the reaction-diffusion formulation of annihilation kinetics, with randomly distributed initial conditions, is investigated by molecular-dynamics simulations of dense hard-disk fluids. For the reaction A + B → C + C quantitative agreement is found. Yet, this proves not to be the case for the reaction A + A → C + C, where major discrepancies are observed. For this latter reaction, more sophisticated theories predict a logarithmic decay law of the form ln (t)/t. The microscopic simulations essentially confirm this prediction.
Study of the reactive dynamics of nanometric metallic multilayers using Molecular Dynamics: the Al−Ni system
A molecular dynamics study of a layered Ni-Al-Ni system is developed using an embedded atom method potential. The specific geometry is designed to model a Ni-Al nanometric metallic multilayer. The system is initially thermalized at the fixed temperature of 600 K. We first observe the interdiffusion of Ni and Al at the interfaces, which is followed by the spontaneous phase formation of B2-NiAl in the Al layer. The solid-state reaction is associated with a rapid system's heating which further enhances the diffusion processes. NiAl phase is organized in small regions separated by grain boundaries. This study confirms the hypothesis of a layer-by-layer development of the new phase. For longer t…
Determination of transport and kinetic properties in self-propagating high-temperature synthesis
International audience; Exothermic reactions in solid powders are analyzed using the usual macroscopic modeling based on the heat transfer equation coupled to an Arrhenius type of dynamics. This problem have important applications in the synthesis of intermetallics and ceramic materials which occur when a high temperature reaction wave propagates throughout the system. Understanding the mechanism of such processes are thus crucial in mastering real laboratory experiments. We first analyze the model, both theoretically and numerically, for a set of representative parameters. We then use traditional data analyses procedures to estimate from the temperature profiles the same set of representat…
Modeling self-sustaining waves of exothermic dissolution in nanometric Ni-Al multilayers
Abstract The self-sustained propagating reaction occurring in nanometric metallic multilayers was studied by means of molecular dynamics (MD) and numerical modeling. We focused on the phenomenon of the exothermic dissolution of one metallic reactant into the less refractory one, such as Ni into liquid Al. The exothermic character is directly related to a negative enthalpy of mixing. An analytical model based on the diffusion-limited dissolution [1] coupled with heat transfer was derived to account for the main aspects of the process. Together, several microscopic simulations were carried out. The first series were set up to obtain all the parameters governing the process, including the heat…
Microstructure development during NiAl intermetallic synthesis in reactive Ni–Al nanolayers: Numerical investigations vs. TEM observations
Abstract Heterogeneous reactions leading to the formation of intermetallic compounds in nanometric Ni–Al multilayer system are examined both numerically and experimentally. On the numerical side, the reactivity of a layered Ni–Al–Ni system is studied by means of molecular dynamics simulations, using an embedded-atom method (EAM) potential. The mechanism of nucleation and growth of the intermetallic phase is determined. Four main stages in the reactive process, which lead to the formation of rounded shape grains of intermetallic phase at the Ni–Al interface, are delineated. On the experimental side, TEM imaging of quenched samples revealed the behavior of the Ni–Al reactive foils and showed …
A multilayer model for self-propagating high-temperature synthesis of inter-metallic compounds
International audience; Self-propagating high-temperature synthesis of intermetallic compounds is of wide interest. We consider reactions in a binary system in which the rise and fall of the temperature during the reaction is such that one of the reacting metals melts but not the other. For such a system, using the phase diagram of the binary system, we present a general theory that describes the reaction taking place in a single solid particle of one component surrounded by the melt of the second component. The theory gives us a set of kinetic equations that describe the propagation of the phase interfaces in the solid particle and the change in composition of the melt that surrounds it. I…