Reactive Nanolaminates Research Articles

Microstructural evolution and mechanical properties of Ni/Al reactive nanolaminates with different NixAly intermetallic phases

Nanolaminates made up by alternative Ni and Al nanolayers are widely applied to solder structural materials, due to abundant heat releases when supplied with an energetic input. This work studies the microstructural evolution and mechanical properties of Ni/Al nanolaminates with a wide mole ratio of Ni (MNi, 0.18–0.53), to ascertain the role of NixAly intermetallic phases during heat-treatment. To this end, 3 nanolaminates with Al thickness fixed at ≈110 nm and Ni thickness varying 15–75 nm are fabricated and are heat-treated at 400 °C, to trigger formations of NiAl3, Ni2Al3 and NiAl intermetallic phases. The results suggest an enhanced strength of Ni/Al after heat-treatment. However, their evolution with MNi is complex. The nanolaminate with MNi ≈0.18 is the softest after heat-treatment, due to the dominant plasticity of residual Al phases. The maximized strength is achieved when MNi ≈0.40, in which a novel NiAl3/Ni2Al3 nanolaminate structure is formed. Owing to the brittleness of both phases, the nanolaminate after heat-treatment is fractured by shearing upon uniaxial compression. The best strength-ductility synergy is achieved when Ni mole ratio ≈0.53. In the heat-treated nanolaminate a homogenous nanocrystalline NiAl phase is formed. As a result, the nanolaminate exhibits excellent ductility and intermediate strength among the 3 nanolaminates.

Leveraging high heating rates to attain desirable reaction products in Al/Zr/C nanocomposites

Reactive nanolaminates are a class of energetic materials which store significant chemical energy in their heterogeneous microstructure that comprises alternating nano-scaled layers of two or more reactants which can undergo self-propagating exothermic reactions to form stable compound phases. We previously observed that the products of self-propagating formation reactions in Al/Zr/C nanolaminates differ dramatically from those obtained after heating slowly to any temperatures up to 1450 °C. Here we explore this heating-rate dependent phase formation in Al/Zr/C reactive nanolaminates through a combination of nanocalorimetry coupled with in situ synchrotron X-ray diffraction, as well as a suite of ex situ analyses. Specifically, we show that forming a cermet of ZrC + Al requires either a sufficiently high heating rate (such as is present during a self-propagating reaction) or quenching from high temperatures (≈ 1600 °C), demonstrating the utility of high heating rates to produce desirable phases.

Environmentally Friendly Chemical Time Delays Based on Interrupted Reaction of Reactive Nanolaminates

Chemical time delays, devices that burn over specific times from under a millisecond to several seconds, are widely used in mining as well as civilian and military pyrotechnics and generally are composed of environmentally hazardous materials. Reactive nanolaminates are energetic materials composed of two or more reactants organized in an alternating layered structure, which may be fabricated with environmentally friendly components. Many traits of reactive nanolaminates are desirable for time delay applications, including their long shelf life and their highly repeatable and finely tunable reaction velocities. However, their reaction velocities are generally too high to be broadly applicable in chemical time delays. In this work, we use polymer mesh substrates to produce a constrained network of reactive particles, which reduce the reaction velocity of Al/Ni multilayers approximately 100× from a range of 2.8 to 7 m s–¹ to a range of 7 to 90 mm s–¹. This is accomplished via an interrupted reaction mechanism wherein individual particles on the coated mesh react rapidly but exhibit a delay before igniting neighboring particles due to the required heat transfer at the particles’ interfaces. We present the macroscale reaction velocities as a function of substrate composition and size, as well as reactive coating thickness and bilayer spacing. We employ high-speed videography to probe the ignition delays and finite element analysis simulations to aid in explaining the observed trends. Through selection of substrate and coating properties, time delays spanning a large range of delay times can be packaged into standard form factors.

Probing the Reaction Zone of Nanolaminates at ∼μs Time and ∼μm Spatial Resolution

Reactive nanolaminates are a high-energy-density configuration for energetics that have been widely studied for their tunable energy release rates. In this study, we characterized Al/CuO nanolamina...

A nanocalorimetric study of the effect of composition gradients on crystallization in amorphous Cu-Zr thin films

Nucleation, the initial formation of a new phase from a parent phase, plays an important role in the eventual microstructure and properties of materials. Theories and models of nucleation have been integral to materials science for close to a century. These models assume that the parent material is compositionally homogeneous on length-scales relevant to nucleation. However, in certain materials – such as thin films or reactive nanolaminates – sharp gradients in the composition may influence nucleation. Models and theories exploring these impacts are based on little direct experimental data. Here we present means of producing and characterizing samples with composition gradients to measure the impacts of gradients on nucleation. We fabricate amorphous Cu-Zr films with known composition gradients through their thicknesses; we perform isochronal nanocalorimetry to measure the impact of the gradients on nucleation and growth; and we characterize the samples before and after reaction. We see evidence of phase separation of the vapor-quenched Cu-Zr amorphous films. While we measure differences between the samples with gradients and those without, the gradients relax sufficiently during heating such that nucleation (the onset of crystallization) occurs at the same temperatures. For both sets of samples we find three distinct regions of heat release: the first we attribute to local ordering, the second to extended phase separation and interdiffusion, and the third to nucleation and growth of the Cu10Zr7 crystalline phase. This work represents a first step towards investigating the impact of gradients on nucleation, as well as growth.

Mechanisms of oxide growth during the combustion of Al:Zr nanolaminate foils

Reactive metal nanolaminates, most notably aluminum/zirconium composites, have been developed as fuels to aid combustion in explosive formulations. Thus far, however, their energy density is limited by incomplete oxidation. An in situ x-ray diffraction (XRD) study was performed on a 40 µm thick Al:Zr (atomic ratio 1:1) multilayer foil to track the growth of reaction products during ignition, combustion, and cooling (over approximately 5 s) to determine the mechanisms that prevent complete combustion from occurring. Simultaneous pyrometry provides the ability to relate the observed phase progression to the foil temperature throughout the reaction, and post-reaction cross-sectional electron microprobe and transmission electron microscopy (TEM) of the combusted foils identify the location, composition, and microstructure of each product phase.We have used the combined results to develop an understanding of the growth mechanisms at play during the rapid reactions. The most significant finding is that the primary combustion product, orthorhombic ZrO2, grows linearly throughout the first 1.3 s of the reaction, indicating interface-limited growth, but then switches to slower diffusion-controlled growth. The transition in growth rate coincides with the abrupt end of a temperature plateau that is associated with self-sustained combustion. A thick (≈8 µm) Al2Zr layer is observed beneath the oxidized exterior in cross-sections of the reacted foil and is likely responsible for the reaction “turning off” before complete combustion is achieved. This underlying Al-rich intermetallic layer prevents the diffusion of aluminum away from the oxide interface, causing an increasing proportion of aluminum to oxidize. The resulting alumina creates a barrier to oxygen diffusion and is responsible for the incomplete combustion in air.

High Heating Rate Reaction Dynamics of Al/CuO Nanolaminates by Nanocalorimetry-Coupled Time-of-Flight Mass Spectrometry

Highly tunable reactive nanolaminates have been of recent interest for various “on chip” energetic applications. The reaction dynamics of Al/CuO nanolaminates were investigated by nanocalorimetry-coupled time-of-flight mass spectrometry, capable of simultaneous measurement of temporal thermal dynamics and detection of evolved gas phase species at heating rates up to ∼106 K/s. The nanolaminates were synthesized by alternately sputtering Al and CuO onto the heater of nanocalorimeter sensors. For thin films of 80 nm with one bilayer, the stoichiometric ratio of fuel to oxidizer significantly affected the reaction mechanism: initial reactions occurred between 300 and 400 °C, and main reactions varied based on stoichiometry. For thicker films of 199 and 266 nm, a series of samples with varying bilayer numbers were analyzed to determine the effect of diffusion distance and interfacial area. Only one reaction step was observed for a sample with a bilayer thickness of 33 nm. A two-step reaction mechanism is obser...

Self-Organized Al2Cu Nanocrystals at the Interface of Aluminum-Based Reactive Nanolaminates to Lower Reaction Onset Temperature.

Nanoenergetic materials are beginning to play an important role in part because they are being considered as energetic components for materials, chemical, and biochemical communities (e.g., microthermal sources, microactuators, in situ welding and soldering, local enhancement of chemical reactions, nanosterilization, and controlled cell apoptosis) and because their fabrication/synthesis raises fundamental challenges that are pushing the engineering and scientific frontiers. One such challenge is the development of processes to control and enhance the reactivity of materials such as energetics of nanolaminates, and the understanding of associated mechanisms. We present here a new method to substantially decrease the reaction onset temperature and in consequence the reactivity of nanolaminates based on the incorporation of a Cu nanolayer at the interfaces of Al/CuO nanolaminates. We further demonstrate that control of its thickness allows accurate tuning of both the thermal transport and energetic properties of the system. Using high resolution transmission electron microscopy, X-ray diffraction, and differential scanning calorimetry to analyze the physical, chemical and thermal characteristics of the resulting Al/CuO + interfacial Cu nanolaminates, we find that the incorporation of 5 nm Cu at both Al/CuO and CuO/Al interfaces lowers the onset temperature from 550 to 475 °C because of the lower-temperature formation of Al-Cu intermetallic phases and alloying. Cu intermixing is different in the CuO/Cu/Al and Al/Cu/CuO interfaces and independent of total Cu thickness: Cu readily penetrates into Al grains upon annealing to 300 °C, leading to Al/Cu phase transformations, while Al does not penetrate into Cu. Importantly, θ-Al2Cu nanocrystals are created below 63% wt Cu/Al, and coexist with the Al solid solution phase. These well-defined θ-Al2Cu nanocrystals seem to act as embedded Al+CuO energetic reaction triggers that lower the onset temperature. We show that ∼10 nm thick Cu at Al/CuO interfaces constitutes the optimum amount to increase both reactivity and overall heat of reaction by a factor of ∼20%. Above this amount, there is a rapid decrease of the heat of reaction.

First-Principles Analysis on the Catalytic Role of Additives in Low-Temperature Synthesis of Transition Metal Diborides Using Nanolaminates.

Carbon is known to significantly accelerate the formation reaction of zirconium diboride in reactive nanolaminates, although the detailed mechanism remains unclear. Here we investigate the catalytic effect of both C and N on the synthesis of ZrB2 using a first-principles theoretical approach. We show that the strong interactions of C and N with Zr at the B/Zr interfaces of the nanolaminate enhance the solid-state amorphization of the Zr lattice. Amorphization of the Zr, in turn, accelerates intermixing of the constituent layers of the reactive nanolaminate. On the basis of these results we propose that the addition of elements with strong binding energies to transition metals may facilitate low-temperature synthesis of transition metal diborides using reactive nanolaminates.

Kinetic Role of Carbon in Solid-State Synthesis of Zirconium Diboride using Nanolaminates: Nanocalorimetry Experiments and First-Principles Calculations.

Reactive nanolaminates afford a promising route for the low-temperature synthesis of zirconium diboride, an ultrahigh-temperature ceramic with metallic properties. Although the addition of carbon is known to facilitate sintering of ZrB2, its effect on the kinetics of the formation reaction has not been elucidated. We have employed a combined approach of nanocalorimetry and first-principles theoretical studies to investigate the kinetic role of carbon in the synthesis of ZrB2 using B4C/Zr reactive nanolaminates. Structural characterization of the laminates by XRD and TEM reveal that the reaction proceeds via interdiffusion of the B4C and Zr layers, which produces an amorphous Zr3B4C alloy. This amorphous alloy then crystallizes to form a supersaturated ZrB2(C) compound. A kinetic analysis shows that carbon lowers the energy barriers for both interdiffusion and crystallization by more than 20%. Energetic calculations based on first-principles modeling suggest that the reduction of the diffusion barrier may be attributed to the stronger bonding between Zr and C as compared to the bonding between Zr and B.

First-Principles Theoretical Studies and Nanocalorimetry Experiments on Solid-State Alloying of Zr-B.

The thermodynamics and kinetics of the solid-state alloying of Zr-B, underlying a variety of synthesis processes of the ultrahigh-temperature ceramic ZrB2, are widely unknown. We investigate the energetics, diffusion kinetics, and structural evolution of this system using first-principles computational methods. We identify the diffusion pathways in the interpenetrating network of interstitial sites for a single B atom and demonstrate a dominant rate-controlling step from the octahedral to the crowdion site that is distinct from the conventional mechanism of octahedral-tetrahedral transition in hexagonal close-packed structures. In the intermediate compounds ZrBx, 0 < x ≤ 2, the diffusivity of B is highly dependent on the composition while reaching a minimum for ZrB. The activation barrier of diffusion in ZrB2 is in good agreement with nanocalorimetry measurements performed on Zr/B reactive nanolaminates.

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Magnetron Sputtered Al‐CuO Nanolaminates: Effect of Stoichiometry and Layers Thickness on Energy Release and Burning Rate

AbstractThis paper reports on the reaction characteristic of Al/CuO reactive nanolaminates for different stoichiometries and bilayer thicknesses. Al/CuO nanolaminates are deposited by a DC reactive magnetron sputtering method. Pure Al and Cu targets are used in argon‐oxygen gas mixture plasma and an oxygen partial pressure of 0.13 Pa. This process produces low stress multilayered materials, each layer being in the range of 25 nanometers to one micrometer. Their structural, morphological, and chemical properties were characterized by high resolution transmission electron microscopy (HR‐TEM), X‐ray Diffraction (XRD), and X‐ray photoelectron spectroscopy (XPS). The heat of reaction and onset temperature were measured using differential scanning calorimetry (DSC). Under stoichiometric conditions, the reactivity quickly increases with the decrease of Al/CuO bilayer thickness. The burning rate is 2 m s−1 for bilayer thickness of 1.5 μm and reaches 80 m s−1 for bilayer thickness of 150 nm. At constant heating rate, the Al/CuO heat of reaction depends on both stoichiometry and bilayer thickness. When the bilayer thickness exceeds 300 nm, the heat of reaction decreases; it seems that only the region near the interface reacts. The best nanolaminate configuration was obtained for Al/CuO bilayer thickness of 150 nm.

Reaction instabilities in Co/Al nanolaminates due to chemical kinetics variation over micron-scales

The reaction front dynamics of Co/Al reactive nanolaminates were studied as a function of the initial temperature of the unreacted material. Sample geometries that exhibit stable reaction fronts as well as geometries that present “spinning” reaction front instabilities were investigated at initial temperatures ranging from room temperature to 200 °C. It was found that reactions in samples with small reactant periodicities (&amp;lt;66.4 nm) were stable at all temperatures, reaction in large periodicity samples (≥100 nm) were unstable at all temperatures, and reactions in samples with intermediate periodicities transitioned from unstable behavior to stable behavior with increasing initial temperature. The results suggest that behaviors typical of two types of reaction kinetics are present in unstable reaction fronts: slow, diffusion-limited kinetics in the regions between transverse reaction bands, and a faster mechanism at the leading edge of the transverse bands.

A generalized reduced model of uniform and self-propagating reactions in reactive nanolaminates

A multiscale inference analysis is conducted in order to infer intermixing rates prevailing during different reaction regimes in Ni/Al nanolaminates. The analysis combines the results of molecular dynamics (MD) simulations, used in conjunction with a mixing measure theory to characterize intermixing rates under adiabatic conditions. When incorporated into reduced reaction models, however, information extracted from MD computations leads to front propagation velocities that conflict with experimental observations, and the discrepancies indicate that our MD simulations over-estimate the atomic intermixing rates. Thus, using only insights gained from MD computations, a generalized diffusivity law is developed that exhibits a sharp rise near the melting temperature of Al. By calibrating the intermixing rates at high temperatures from experimental observations of self-propagating fronts, and inferring the intermixing rates at low and intermediate temperatures from ignition and nanocalorimetry experiments, the dependence of the diffusivity on temperature is inferred in a suitably wide temperature range. Using this generalized diffusivity law, one obtains a generalized reduced model that, for the first time, enables us to reproduce measurements of low-temperature ignition, homogeneous reactions at intermediate temperatures, as well as the dependence of the velocity of self-propagating reaction fronts on microstructural parameters.

Effect of thermal properties on self-propagating fronts in reactive nanolaminates

The effects of thermal diffusion on flame front dynamics in a (1:1) Ni/Al multilayered system are computationally investigated. A systematic refinement of the thermal conductivity model is performed, namely by incorporating the effects of concentration, direction, and temperature dependence. The resulting thermal conductivity models are incoporated into the reduced reaction formalism developed by Salloum and Knio [Combust. Flame 157(6),1154 (2010]). Computations using constant and variable conductivity models are contrasted with each other, for axial and normal front propagation. Notable differences between the predictions of the various conductivity models are observed, particularly concerning the thermal and reaction widths. Differences in the average front propagation velocity are, unexpectedly, less pronounced. Brief computational experiments are finally conducted for 3D front propagation using constant and variable thermal conductivity models. The 3D variable-conductivity computations reveal the occurrence of transient, spinlike reactions that appear to be consistent with recent experimental observations, whereas stable front behavior is observed when a constant-conductivity model is used. Thus, the present experiences suggest that thermo-diffusive instabilities are likely to play a role in the onset and manifestation of some of the experimentally-observed transient front propagation regimes.

Simulation of reactive nanolaminates using reduced models: III. Ingredients for a general multidimensional formulation

A transient multidimensional reduced model is constructed for the simulation of reaction fronts in Ni/Al multilayers. The formulation is based on the generalization of earlier methodologies developed for quasi-1D axial and normal propagation, specifically by adapting the reduced formalism for atomic mixing and heat release. This approach enables us to focus on resolving the thermal front structure, whose evolution is governed by thermal diffusion and heat release. A mixed integration scheme is used for this purpose, combining an extended-stability, Runge–Kutta–Chebychev (RKC) integration of the diffusion term with exact treatment of the chemical source term. Thus, a detailed description of atomic mixing within individual layers is avoided, which enables transient modeling of the reduced equations of motion in multiple dimensions. Two-dimensional simulations are first conducted of front propagation in composites combining two bilayer periods. Results are compared with the experimental measurements of Knepper et al. [ 22], which reveal that the reaction velocity can depend significantly on layering frequency. The comparison indicates that, using a concentration-dependent conductivity model, the transient 2D computations can reasonably reproduce the experimental behavior. Additional tests are performed based on 3D computations of surface initiated reactions. Comparison of computed predictions with laser ignition measurements indicates that the computations provide reasonable estimates of ignition thresholds. A detailed discussion is finally provided of potential generalizations and associated hurdles.

Simulation of reactive nanolaminates using reduced models: II. Normal propagation

Transient normal flame propagation in reactive Ni/Al multilayers is analyzed computationally. Two approaches are implemented, based on generalization of earlier methodology developed for axial propagation, and on extension of the model reduction formalism introduced in Part I. In both cases, the formulation accommodates non-uniform layering as well as the presence of inert layers. The equations of motion for the reactive system are integrated using a specially-tailored integration scheme, that combines extended-stability, Runge–Kutta–Chebychev (RKC) integration of diffusion terms with exact treatment of the chemical source term. The detailed and reduced models are first applied to the analysis of self-propagating fronts in uniformly-layered materials. Results indicate that both the front velocities and the ignition threshold are comparable for normal and axial propagation. Attention is then focused on analyzing the effect of a gap composed of inert material on reaction propagation. In particular, the impacts of gap width and thermal conductivity are briefly addressed. Finally, an example is considered illustrating reaction propagation in reactive composites combining regions corresponding to two bilayer widths. This setup is used to analyze the effect of the layering frequency on the velocity of the corresponding reaction fronts. In all cases considered, good agreement is observed between the predictions of the detailed model and the reduced model, which provides further support for adoption of the latter.

Simulation of reactive nanolaminates using reduced models: I. Basic formulation

Development and testing are described of a reduced model for the simulation of transient reactions in Ni/Al nanolaminates. The model relies on a simplified description of local atomic mixing rates, expressed in terms of an evolution equation for a dimensionless time scale that reflects the age of the mixed layer. The latter is obtained from a theoretical analysis of the quasi-1D evolution equation of a conserved scalar, which also yields canonical curves for the mean composition and its relevant moments as function of the local bilayer age. The reduced model is tested against results of the detailed model of Besnoin et al. [E. Besnoin, S. Cerutti, O.M. Knio, T.P. Weihs, J. Appl. Phys. 92 (2002) 5474–5481] for different scenarios, including self-propagating reactions, and ignition thresholds. Results show that the reduced model predictions are in good agreement with those obtained using the detailed model. By achieving order-of-magnitude increase in computation efficiency, the present model reduction formalism offers a path towards modeling transient reaction fronts in multiple space dimensions.

Imaging of Transient Structures Using Nanosecond in Situ TEM

The microstructure and properties of a material depend on dynamic processes such as defect motion, nucleation and growth, and phase transitions. Transmission electron microscopy (TEM) can spatially resolve these nanoscale phenomena but lacks the time resolution for direct observation. We used a photoemitted electron pulse to probe dynamic events with "snapshot" diffraction and imaging at 15-nanosecond resolution inside of a dynamic TEM. With the use of this capability, the moving reaction front of reactive nanolaminates is observed in situ. Time-resolved images and diffraction show a transient cellular morphology in a dynamically mixing, self-propagating reaction front, revealing brief phase separation during cooling, and thus provide insights into the mechanisms driving the self-propagating high-temperature synthesis.