ICMCTF2014 Session TS3: Energetic Materials and Micro-structures for Nanomanufacturing

Here we report the use of two additive micro-manufacturing techniques: 1) Electrophoretic deposition (EPD) and 2) Direct ink writing (DIW) as a means to prepare thin films and three-dimensional structures of well-mixed copper (II) oxide/ aluminum (CuO/Al) binary particulate composites. Films were deposited onto patterned electrodes, with very fine feature sizes, which were used for mechanistic investigations of the ignition and combustion. The EPD films were examined using electron microscopy, and their combustion characteristics were analyzed with high-speed videos. The results show that films prepared by EPD show a large enhancement in the combustion speed with total film thickness. These films are also particularly useful for developing thermites for micro-energetic applications. In further studies, the DIW method was successfully used in combination with EPD as a means to synthesize various architectures of energetic materials. Patterns of electrodes are written onto an arbitrary substrate, and EPD is then used to deposit thermite materials directly onto the patterns. This combination of techniques allows for investigations of the characteristic fuel / oxidizer ignition and reaction in engineered three-dimensional microstructures. The results thus far suggest that, in addition to good fuel/oxidizer mixing, other design criteria can be utilized to tailor the reactivity of composite thermites. This includes using large enough features to allow gas trapping and pressure unloading, and also designing microstructures which enable more directed transport of hot gases and particles in the desired propagation direction. The combination of EPD and DIW allows for the synthesis of such architectures, and this scalable capability will allow for a bottom-up development of energetic systems with micro-engineered control.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Most processes in materials naturally occur in conditions far-from-equilibrium having transient states that evolve on short time scales. Due to the resolution limitations of conventional analytical techniques, we are typically confined to conduct experiments near equilibrium or observe the material post process. Though these observations have allowed useful insights about the material’s behavior, attempts to understand the coupled and convoluted events in complex processes have been hampered by the difficulty of capturing the events in detail as they unfold on nanosecond and microsecond timescales, requiring a technique with improved temporal resolution. To meet this need, the dynamic transmission electron microscope (DTEM) at Lawrence Livermore National Laboratory was developed which can capture diffraction patterns or images of a fast-evolving material process.

Prior DTEM hardware only allowed single-pump/single-probe operation, building up a process's typical time history by repeating an experiment with varying time delays at different sample locations. The Movie Mode (MM) DTEM upgrade now enables single-pump/multi-probe operation and comprises two unique technologies, a cathode laser system that allows nanosecond pulse shaping for producing electron pulse trains and a high-speed electrostatic deflector array that directs each electron pulse (image) to a separate patch on a CCD camera. At the end of the experiment, the CCD image is read-out and segmented into a time-ordered series of images, i.e., a movie.

These technical improvements allow us to track the creation, motion, and interaction of defects, phase fronts, and chemical reactions, providing invaluable information of the chemical, microstructural and atomic level features that influence the dynamics and kinetics of rapid material processes. In particular, we have used the new MM-DTEM capability to study reaction dynamics in Ti-B and Co-Al based reactive nanolaminates. By tracking the position of the reaction front with multiple image acquisition, we have precisely measured the front velocities as a function of composition, bilayer content and thickness. We have also quantified the phase evolution behind the reaction front through a MM-DTEM obtained sequence of diffraction patterns, allowing us to determine the evolution and rapid kinetics of the exothermic reactions. This presentation will discuss new insights gained about the dynamics of reactive nanolaminates using this novel MM-DTEM capability. This work was performed under the auspices of the U.S. Department of Energy, Contract No. DE-AC52-07NA27344, and supported by DOE-BES, Division of Materials Science and Engineering.

In order to develop low cost, energetic, thin film heat sources, the mixing condition of the energetic thin film needs to be understood. In this work, magnesium and manganese oxide powders were mixed with Polyninylidene Fluoride (PVDF) with a Methyl Pyrrolidone (NMP) solvent and blade cast onto stainless steel foil and glass substrates. The rheological properties of these mixtures were investigated to quantify the mixing condition. Solids content, equivalence ratio and dry film thickness were varied and open flame propagation speed and calorific output was investigated for each mixture ratio. A 0.45 solids-liquid ratio resulted in significantly higher flame propagation rates for both open and confined configurations. Rheometry measurements and physical characterizations of the films reveal that this solids-liquid ratio produced the most homogeneous mixtures that then resulted in the highest flame speeds. On a scale beyond thin films, these results imply that for any thermite or energetic composite, changing the solids loading can affect mixing and impact the final flame speed and energy propagation mechanism. It was found that flame speed increased as a function of dry film thickness, although calorific output stayed constant. This indicates that flame speed is increased by greater available thermal energy per inch provided by a thicker film, but no additional thermal energy is available. For varying equivalence ratio, flame speeds were highest for an equivalence ratio of 1.0. Flame speed decreased for lower equivalence ratios, but would not propagate for higher equivalence ratios. Calorific output increased as equivalence ratio approached 1.0, but remained relatively constant as it increased past that, indicating that net benefit to combustion was negligible for increased equivalence ratio.

Cobalt/aluminum multilayers made by vapor deposition undergo high temperature, self-propagating formation reactions but exhibit several different reaction modes depending on multilayer design and environmental conditions. With this presentation, we describe the reaction front dynamics of Co/Al reactive nanolaminates 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^oC. It was found that reactions in samples with small reactant periodicities (<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 reaction ‘spin’ bands and a faster mechanism at the leading edge of the bands.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Reactive nanolaminates are characterized by a strong chemical reactivity of their interacting individual components (metal/metal or metal/ oxide alternating layers) that causes local release of large amount of chemical energy. Today, the relation between the energetic performances with their composition and micro/nanostructure is a major challenge to produce tunable and controlled material that can be integrated into MEMS chip [1] to produce local pressure, temperature or specific gas species. Along this line, the relations between detailed atomic interface arrangement and aging/initiation/self-sustained combustion remain elusive. From a modelling standpoint, Molecular Dynamics has been applied to the simulation of very early stages of energetic materials initiation on model interfaces built from crystallographic data of bulk materials. In this paper, we propose a new methodology enabling to simulate the Al-oxyde interface formation and predict its structure taking into account real experimental deposition processing conditions. To this end a multi-level approach is presented that combines DFT-based calculations (Density Functional Theory) with Kinetic Monte Carlo (KMC) techniques. In this DFT/KMC frame, we will show how to extend the kinetic-limited conventional KMC to take into account exothermic reactions and associated extra hyperthermal atomic motions beyond kinetic-based hopping from one energetical minimum to another. Presented results will include Al/oxide nanolaminates surface chemical mechanisms via DFT calculations with a specific attention on Aluminum oxidation, KMC simulations of early stages of oxide PVD deposition onto Al(111), hyperthermal surface motions of deposited oxygen atoms through a “hot atom” derived KMC.

[1] J. Micromech. Microeng. 23 (2013) 105009.

Interface layers in Al-based reactive thin film play a crucial role in the energetic properties and reactivity of such materials [1]. The composition and related microstructure of the interface of Al-based reactive films can greatly influence the ignition temperature, reaction kinetics and even the stability at low temperature. This works aims at developing an understanding of the interface formation processes between Al and reactive oxides and investigating its role in the reaction kinetics.

Two types of oxides (CuO and ZnO) are synthesized by Atomic Layer Deposition to obtain high-quality, thin and chemically-controlled model oxide surfaces. Al deposition is then performed by e-beam evaporation in high vacuum (10^-9 Torr) and the Al-(Cu,Zn)-O interfaces probed as a function of deposition rate (0.5A-5A/min). Specifically, extensive characterization, combining in-situ IR, XPS and low energy ion scattering (LEIS) with ex-situ XRD and AFM are used to characterize the nature of both the thin oxide films (the crystalline ZnO, polycrystalline CuO) as well as the formation of an amorphous interfacial layer prior to pure Al deposition. The mechanism of interface formation upon Al deposition is elucidated by first principles calculations, in particular the propensity for Al atoms to penetrate into the oxide films.

Once the Al/(Cu,Zn)O nanolaminates are grown, a quantitative investigation of the interface formation and evolution at low temperature is also performed by nanocalorimetry. To that end, we use a nanocalorimeter based on a micromachined hotplate platform that has been designed, fabricated and characterized with different Al-based thin films to screen the reaction kinetics during interface formation.

[1] Appl. Mater. Interfaces 2013, 5, 605−613

Low-energy ball milling of aluminum and nickel particles with an overall composition corresponding to the NiAl intermetallic phase was performed up to milling durations of 13 hours. Results show that microstructural refinement with increasing milling times increases reactivity, where the intermetallic formation temperatures reduce to those of sputtered nanostructured multilayer foils with a similar phase formation sequence during differential scanning calorimetry (DSC) analysis. Furthermore, loose NiAl particle (milled for 11-12 hours) piles, NiAl particles cold-compacted into pellets or pressed/rolled between thin metal overlayers (e.g. Al) into thin sandwich structures, and NiAl particles mixed with additional Al or coated with an Al shell, could all be ignited with a low-energy spark from a 9 V battery. Results from high-speed optical and infra-red imaging suggest that the thermal front velocities and maximum temperatures strongly depend on the produced structures/shapes. Al overlayers from the sandwich structures provide extra cooling and reduce energy density, resulting in quenching of the reactions at a sandwich thickness of 400 µm, and decreasing the maximum temperature and velocity to approximately 1300ºC and 0.16 m/s (compared to approximately 1500ºC and 0.2 m/s for 800 µm thick structures), respectively. Additional bonding experiments using the sandwiched structures as reactive inserts was successful, indicating that they can be used in a similar fashion to sputtered nanostructured foils.