Theory analysis on shock-induced chemical reaction of reactive metal

In this paper, the energy release mechanism of shock-induced chemical reaction (SICR) of Al/Ni composites was investigated. The Al/Ni composites with two additives, namely Teflon (PTFE) and copper (Cu), were considered in both theoretical calculations and experiments to investigate their influence on SICR characteristics of Al/Ni composites system. Assuming the SICR process is controlled by shock temperature rising in the materials, the reaction efficiency of Al/Ni composites was calculated by Arrhenius reaction rate and Avrami–Erofeev reaction models. Hugoniot curves and the temperature rise under shock compression were calculated to analyze the mechanism of the influence on SICR characteristics by additives. Impact-initiated experiments of Al/Ni composites were carried out to study SICR characters at various impact velocities. The parameters of SICR model were determined by using corresponding experimental results. The calculation results showed that for the SICR process of Al/Ni, the critical shock tem...

3 - Thermochemical modeling on shock-induced chemical reaction of MESMs

Multifunctional energetic structural materials (MESMs) are a new class of energetic materials which release energy due to exothermic chemical reactions initiated under shock loading conditions. In order to analyze shock-induced chemical reactions (SICR) for MESMs, theoretical models have been developed to calculate the Hugoniot data which include the heat released by shock temperature controlled reactions. The temperature rise of porous materials due to shock compression is first calculated using a constant volume and pressure adjustment. Then the Arrhenius reaction rate and Avrami-Erofeev kinetic models are used to calculate the extent of reaction of MESMs under shock compression. Thermochemical models for shock-induced reactions, in which the reaction efficiency is considered, are given by combining the shock temperature rise with the chemical reaction kinetics. The Hugoniot relations and temperatures are calculated by using the proposed method. The models developed have been validated against the experimental SICR data involving Fe2O3/Al, Al/Ni, and Ti/Ni mixtures. It has been shown that the theoretical calculations correlate reasonably well with the corresponding experimental and simulation results. The models presented can be used to predict the reaction results of MESMs over a wide range of pressure satisfactorily.

Multifunctional energetic structural materials (MESMs) are a new class of energetic materials, which release energy due to exothermic chemical reactions initiated under shock loading conditions. In order to analyze the impact-initiated process of MESMs, a quasi-sealed test chamber, which was originally developed by Ames [“Vented chamber calorimetry for impact-initiated energetic materials,” in AIAA (American Institute of Aeronautics and Astronautics, 2005), p. 279], is used to study on shock-induced chemical reaction characters at various impact velocities. The impact initiated experiments are involving two typical MESMs, Al/PTFE (polytetrafluoroethylene), W/Zr and inert 2024 Al fragment. The video frames recorded from reactive and inert material impact events have shown the process of late-time after burn phenomena. The total pressure and shock wave reflection at the wall of the test chamber are measured using high frequency gauges. The quasi-pressures inside the test chamber, which is fitting from the total pressure curves, are used to determine the impact initiated reaction efficiencies of MESMs at different impact velocities. A thermochemical model for shock-induced reactions, in which the reaction efficiency is considered, is validated against the experimental data from impact initiation. The results show that the impact velocity plays a significant role in chemical reaction and the energy release characteristics of MESMs. The theoretical calculations correlate reasonably well to the corresponding experimental results, which can be used to predict the reaction results of MESMs over a wide range of pressure satisfactorily.

Shock synthesis of niobium silicide (Nb5Si3) via the flyer plate impact technique with high impact velocities

Reactive metals (RMs) are a new class of material that can withstand mechanical loads and chemically react to release large amounts of heat under strong impact loading. They are gradually becoming widely used in defense and military fields, including for high-efficiency warheads and reactive armor. For the numerical simulation method considering the combined mechanical-thermo-chemical process for the impact energy release behavior of the RMs, the Al/Ni-based RMs were investigated in this work by combining experiments, theoretical calculations and a numerical simulation. Three kinds of Al/Ni-based RMs (Al-Ni, Al-Ni-CuO and Al-Ni-MoO3), were prepared using the hot-pressing forming process. Firstly, the compressive behavior and the parameters of the Johnson-Cook constitutive model were obtained using a mechanical testing machine and split Hopkinson pressure bars (SHPB). Secondly, the parameters of the equation of state (EOS) under the medium and low pressure conditions of the Al/Ni-based RMs, which were was seen as porous mixtures with high theoretical material density percentages (TMD%), were calculated based on the cold-energy superposition theory and the Wu-Jing method. Third, the impact energy release behaviors of the three RMs were studied with direct ballistic tests. The shock temperatures at different impact velocities were calculated based on the existing shock-induced chemical reaction thermo-chemical model while considering the chemical reaction efficiency, the relationship between the shock temperature and the extent of the chemical reaction was established, and the parameters of the relevant chemical kinetic equations were fitted. Finally, the user’s subroutines defining the material model were implemented to update the stresses in the solids elements in LS-DYNA. The model was based on the Johnson-Cook constitutive model with consideration of the mechanical-thermo-chemical coupling effect, which was verified by the experimental results. The results show that the constitutive model developed in this work can describe the impact energy release behavior of the Al/Ni-based RMs.

Multifunctional energetic structural materials (MESMs) are usually granular mixtures, which release energy due to exothermic chemical reaction initiated under shock loading conditions. The mesostructure, in terms of the size, shape, and distribution of granular mixture, plays a significant role in chemical reaction and the energy release characteristics of MESMs. However, it is difficult to model such a complex process involving thermal-mechanical-chemical responses, especially the effects of the initial mesostructures. In this paper, a multiscale modelling approach is proposed to simulate the chemical reaction of MESMs under a shock compression. The thermal-mechanical response of MESMs is first obtained from mesoscale simulations. Then, the macroscale thermochemical model for a shock-induced chemical reaction is given, in which the extent of reaction is considered. Finally, the spatial profiles of temperature and pressure from the mesoscale heterogeneous simulation are homogenized into cells as an initial state for chemical reaction and further combined with the thermochemical model in macroscale. Hence this provides insight into thermal-mechanical-chemical responses based on the initial mesostructures. Aluminum/Tungsten/Polytetrafluoroethylene granular mixture is selected to demonstrate the method and the effects of volume fraction and impact velocity on the shock-induced chemical reaction. The multiscale approach developed, which combines the mesoscale simulation and macroscale thermochemical modelling, can be used to predict the shock-induced chemical reaction of MESMs with different mesoscale characteristics over a wide range of impact velocities.

Flow Chemistry & Catalysis – Where do we stand and where do we need to go?

A new method for the shock consolidation of hard metallic powders has been successfully tested. This method extends the process developed by Sawaoka and Akashi for the processing of ceramics (U.S. Patent 4,655,830) to metallic powders. Shock-activated reactions between elemental mixtures of niobium and aluminum powders were used to chemically induce bonding between difficult-to-consolidate intermetallic TiAl compound powder particles. The highly exothermic reactions activated by the passage of shock waves form an intermetallic binder phase which assists in the consolidation of the very hard TiAl alloy powders. Shock impact experiments were carried out utilizing a twelve-capsule shock recovery system in which a plane wave generating lens is used for accelerating a flyer plate to velocities of 1.7 and 2.3 km/s. With these impact velocities, sufficient shock pressures are generated in the powders, contained in capsules, to result in shock-induced reactions between the elemental powders of the mix. Fully dense compacts were successfully recovered and were subsequently characterized by optical, transmission, and scanning electron microscopy, x-ray diffraction, and microhardness testing. Transmission electron microscopy revealed both microcrystalline and amorphous regions in the reaction zone. In one instance, the amorphous material crystallized under the heating effect of the electron beam. Shock induced reaction between elemental powders and with the TiAl powders, producing ternary compounds, was also observed.

Shock experiments with an l-alanine solution and implications for the cometary delivery of amino acids and dimers with enantiomeric excesses to the early Earth

Influence of additives on microstructures, mechanical properties and shock-induced reaction characteristics of Al/Ni composites

The shock-compression response of as-blended and ball-milled elemental Ni and Ti powder mixtures is investigated in this study. An 80-mm diameter single-stage gas-gun was employed to perform time-resolved measurements using piezoelectric stress gauges to monitor the stress wave profiles at the front (input) and back (propagated) surfaces of the ∼50% dense Ni+Ti powder mixture compacts. Shock-compression experiments performed on as-blended Ni+Ti powder mixtures in the range of 522m∕sto1046m∕s impact velocities, showed characteristics of powder densification at measured input stress of 1.12GPa, and shock-induced chemical reaction indicated by volume expansion and wave speed increase at measured input stress of 3.2GPa. Ball-milled powder mixtures also showed similar evidence of shock-induced chemical reaction at stresses exceeding 3GPa, with the degree of expansion depending on the energy release associated with the reaction in the powder mixtures ball-milled for various times. The measured high-pressure expanded state was observed to correspond to that calculated using a thermodynamic “ballotechnic” model for shock-induced formation of reaction products in as-blended and ball-milled powder mixtures. The results of instrumented experiments provide clear evidence of shock-induced chemical reactions occurring in Ni+Ti powder mixtures (following densification at stresses exceeding the crush strength) as evidenced by increased shock velocity and generation of an expanded state of products resulting from the associated heat of reaction.

Recent advances in studies of shock-induced chemistry in reactive solids have led to the recognition of a new class of energetic materials which are unique in their response to shock waves. Experimental work has shown that chemical energy can be released on a time scale shorter than shock-transit times in laboratory samples. However, for many compositions, the reaction products remain in the condensed state upon release from high pressure, and no sudden expansion takes place. Nevertheless, if such a reaction is sufficiently rapid, it can be modeled as a type of detonation, termed ‘‘heat detonation’’ in the present paper. It is shown that unlike an explosive detonation, an unsupported heat detonation will decay to zero unless certain conditions are met. An example of such a reaction is Fe2 O3 +2Al+shock→Al2 O3 +2Fe (the standard thermite reaction). A shock-wave equation of state is determined from a mixture theory for reacted and unreacted porous thermite. The calculated shock temperatures are compared to experimentally measured shock temperatures, demonstrating that a shock-induced reaction takes place. Interpretation of the measured temperature history in the context of the thermochemical model implies that the principal rate-controlling kinetic mechanism is dynamic mixing at the shock front. Despite the similarity in thermochemical modeling of heat detonations to explosive detonations, the two processes are qualitatively very different in reaction mechanism as well as in the form the energy takes upon release, with explosives producing mostly work and heat detonations producing mostly heat.

The initiation of chemical reaction in cold-rolled Ni/Al multilayered composites by shock compression is investigated numerically. A simplified approach is adopted that exploits the disparity between the reaction and shock loading timescales. The impact of shock compression is modeled using CTH simulations that yield pressure, strain, and temperature distributions within the composites due to the shock propagation. The resulting temperature distribution is then used as initial condition to simulate the evolution of the subsequent shock-induced mixing and chemical reaction. To this end, a reduced reaction model is used that expresses the local atomic mixing and heat release rates in terms of an evolution equation for a dimensionless time scale reflecting the age of the mixed layer. The computations are used to assess the effect of bilayer thickness on the reaction, as well as the impact of shock velocity and orientation with respect to the layering. Computed results indicate that initiation and evolution of the reaction are substantially affected by both the shock velocity and the bilayer thickness. In particular, at low impact velocity, Ni/Al multilayered composites with thick bilayers react completely in 100 ms while at high impact velocity and thin bilayers, reaction time was less than 100 μs. Quantitative trends for the dependence of the reaction time on the shock velocity are also determined, for different bilayer thickness and shock orientation.

Shock compaction of reactive powder mixtures to synthesize new materials is one of the oldest material processing techniques and has been studied extensively by several researchers over the past few decades. The quantitative connection between the shock energy imparted and the extent of reaction that can be completed in the small time window associated with the passage of the shock wave is complicated and depends on a large variety of parameters. In particular, our understanding of the complex interplay between the thermo-elasto-viscoplastic behaviour of the granular constituents and their temperature dependent, diffusion-limited reaction mechanism may be enriched through careful numerical simulations. A robust numerical model should be able to handle extremely large deformations coupled with diffusion mediated fast reaction kinetics. In this work, a meshfree discrete particle numerical method based on smoothed particle hydrodynamics (SPH) to simulate shock-induced chemical reactions (SICR) in reactive powder mixtures is proposed. We present a numerical strategy to carry out reactions between reactant powder particles and partition the obtained products between the particles in a manner that accounts for the requirement that the total mass of the entire system remains constant as the reactions occur. Instead of solving the reaction–diffusion problem, we propose a ‘pseudo-diffusion’ model in which a distance dependent reaction rate constant is defined to carry out chemical reaction kinetics. This approach mimics the actual reaction–diffusion process at short times. Our numerical model is demonstrated for the well-studied reaction system Nb + 2Si NbSi2. The predicted mass fractions of the product obtained from the simulations are in agreement with experimental observations. Finally, the effects of impact speed, particle arrangement and mixing ratio on the predicted product mass fractions are discussed.