ICMCTF2011 Session TS5: Energetic Materials and Micro-Structures for Nanomanufacturing

Due to its favorable combination of properties the B2 intermetallic RuAl is superior to many other B2 aluminides. It combines an extraordinary high melting point of 2333 K, a high heat of formation and an unusual high room temperature toughness compared to other intermetallics. This property profile makes that intermetallic compound interesting for high as well as room temperature thin film applications. Among the room temperature applications, nanometric Ru/Al multilayers are very promising as a new kind of energetic material system. Typical applications may be e.g. the future use for reactive bonding purposes. These applications take advantage of the released exothermic reaction heat of a self-propagating reaction in a metallic multilayer. Hence, the fundamental understanding of self-propagating reactions in Ru/Al multilayers which are characterized by reaction front velocities up to several m/s is essential. High-speed methods are used to measure e.g. reaction velocities, temperatures as well as phase transformations in Ru/Al multilayers. Ignition thresholds are determined. Based on these results a reaction mechanism for the self-propagating reaction in Ru/Al multilayers to RuAl is proposed. Microstructural analysis of the final RuAl thin films is presented. Lastly, the findings for RuAl are compared to similar systems to discuss the potential of RuAl in the mentioned applications.

Vapor-deposited, metal-metal multilayers are an ideal class of materials for systematic, detailed investigations of reactive material properties. Created in a pristine vacuum environment by sputter deposition, these high-purity materials have uniform reactant layer thicknesses between 1 and 1000 nm, minimal void density and intimate contact between layers. These key compositional and geometrical characteristics generally lead to reproducible reaction behaviors.

With this presentation, we describe the effects of test environment on the self-propagating synthesis of sputter-deposited, equiatomic Ni/Ti and Sc/Cu multilayers. By conducting reactions in air and at different vacuum pressures, we evaluate how surrounding gaseous environment affects (i) the propagating reaction wave dynamics and (ii) the final phase formation of these different multilayers. First, we show that the surrounding environment can affect the average propagation speed of a reactive multilayer with increased speeds observed at atmospheric pressure. For Ni/Ti multilayers the differences in speed manifest in the nucleation rate of transverse reaction bands that originate at the edge of specimens. For Sc/Cu we show evidence of a second propagating oxidation wave that trails the intermetallic reaction front when tested in air. High-speed microscopy demonstrates how this second oxidation wave can interfere with the intermetallic reaction front giving rise to a new form of oscillatory combustion. Reactions in air yield a mixture of intermetallic compounds and metal oxide for both Ni/Ti and Sc/Cu. As indicated by X-ray diffraction Ni/Ti multilayers reacted in air show evidence of both rutile and anatase forms of TiO₂ along with various Ni_xTi_y phases. Scandium/copper multilayers form Sc₂O₃ in addition to ScCu and ScCu₂ when reacted in air. Reaction of Ni/Ti and Sc/Cu in vacuum (1 mTorr) leads to the formation of intermetallic compounds with no evidence of crystalline oxide phases.

Reactive nanoscale multilayer foils represent a relatively new class of materials which have recently received considerable attention for use in joining applications. Other possible applications include targeted heat sources for micro scale manufacturing processes, biomedical applications, micro actuators and energy sources for autonomous micro devices. However, due to the fast reaction rates in these foils (between 1 and ~ 20 m/s for the Ni/Al system, depending on the individual bilayer thickness), much of the understanding of the reactions is derived from comparing before and after states of the samples. Only very recently different methods for analysis during reaction such as Dynamic Transmission Electron Microscopy (DTEM) [1] or fast X-Ray Diffraction (XRD) [2] have become available.

Here, we present a method for measuring the core temperature of multilayer foils during reaction based on lattice parameter measurements by fast high resolution in-situ XRD. Ni(V)/Al and Ni/Al multilayer foils have been analyzed using this method, allowing the identification of reaction stages during SPER on nanoscale structures, as well as Energy Filtered Transmission Electron Microscopy (EFTEM) Scanning Electron Microscopy (SEM), Electron Energy Loss Spectroscopy (EELS) and Differential Scanning Calorimetry (DSC). The results highlight the influence of intermixing and deposition parameters on the reaction kinetics, dictating the flame front velocity. Furthermore, it is shown that foils with different amounts of intermixing can reach the adiabatic formation temperature of B2-NiAl. These experimental results are important for further validation of numerical models regarding temperature and structure development in Ni–Al multilayer foils, as well as other metallic reactive systems. The new method significantly expands the characterization possibilities in this field.

[2] J. C. Trenkle, L. J. Koerner, M. W. Tate, S. M. Gruner, T. P. Weihs and T. C. Hufnagel, Appl. Phys. Lett. 93, 081903 (2008)

The nickel-aluminum (Ni/Al) intermetallic system is useful for a variety of reactive material applications, including the initiation of subsequent reactions, thermal batteries, and the localized heating required by many welding and joining applications. Reaction characteristics are well studied at the normal self-heating rates of 10³–10⁶ K/s. We recently reported on new experiments where we electrically heated patterned Ni/Al bridges at 10¹¹–10¹² K/s, effectively forcing the reaction to occur at much higher rates. Such studies are important for future nanomanufacturing techniques, where it may be necessary to control spatial thermal distributions much more precisely by careful control of the timescales over which these reactions take place.

Here we report on rapid Ni/Al reactions observed by streak camera spectroscopy, with an optical resolution of 2.0 nm resolved temporally over 350 ns. We measured non-reactive Ni, Al, and reactive Ni/Al samples, and each exhibited both similar and distinct spectroscopic features. For example, both Al and Ni/Al samples produced peaks associated with atomic Al at 396 nm. However, peaks were not present at expected wavelengths for ionic Al, suggesting that Al atoms retained their electrons long enough to participate in intermetallic reactions with Ni. In separate experiments probing the kinetic energy of material ejected from the reaction zone, we measured a 10% increase in kinetic energy over non-reactive control samples, with the magnitude of this increase being equivalent to the energy expected from a Ni/Al reaction.

All samples exhibited an initial period of nearly broadband emission, with superimposed spectral peaks which grew more intense at later times. This emission was especially obvious in reactive Ni/Al samples, persisting much longer and at higher intensities than either the Al-only or Ni-only samples. By comparing this broadband emission to expected blackbody radiation curves given by Plank’s Law, we deduced temperature values which peaked at 8180 K for Ni/Al and at 5230 K for Al, and which decayed initially with similar characteristic time constants of 198 and 175 ns, respectively. These higher Ni/Al temperatures validate our past measurements of increased kinetic energies.

The final paper will include temperatures calculated from other spectroscopic methods, as well as predicted temperatures based on measured electrical power delivered to each sample. These additions will help validate the significant result that Ni/Al formation reactions can proceed at much higher rates, leading to a variety of new reactive material applications.

This talk will present research efforts at NJ Institute of Technology aimed to develop new reactive materials for various applications. Specifically, a broad range of reactive nanocomposite powders is prepared using Arrested Reactive Milling, a ball-milling technique customized for work with material components capable of highly exothermic chemical reaction. Starting materials are mixed and ball milled with or without process control agents and the milling is interrupted just before the chemical reaction can be triggered mechanically. Produced powders typically comprise micron-sized particles with relatively broad particle size distribution while each particle has nanocomposite structure. Typically, a more ductile material component (such as Al for thermite compositions) forms a matrix while the second, more brittle material is contained as inclusions. The inclusion sizes vary from 10 to 1000 nm. The reactivity of such materials is defined by the interface area between the inclusions and matrix. Because of versatility of mechanical milling as a materials processing technique, there is practically no limitations on the compositions that can be prepared; the materials prepared to date include thermite (metal - metal oxide systems), metal-metalloid, and metal-metal composites. In addition to binary materials, more complex ternary compositions are also prepared.

Characterization of the new materials relies on a number of experimental methods, from thermo-analytical measurements, electron microscopy, and x-ray diffraction to custom ignition and combustion measurements for powder clouds and individual particles. The work focused on quantitative description of the thermal initiation mechanisms and reactions responsible for ignition of new materials at different heating rates. The role of oxidation reactions as well as various phase changes occurring in the prepared materials upon their heating is explored and will be discussed.

Nanoscale materials that exhibit Self-Propagating Exothermic Reactions (SPER) are promising energy sources for thermal manufacturing, owing to their ability to provide intense localized heat. These materials are mainly manufactured in the form of bimetallic multilayer foils using magnetron sputtering. One alternative processing route is mechanical alloying in the form of ball milling (BM), in which powder mixtures are subjected to repeated plastic deformation. The idea is to generate lamellar structures within the powders to imitate sputtered multilayers, which have faster reaction rates owing to the high contact area between reactants. Although mechanical alloying has been carried out to produce materials with superior high-temperature mechanical properties, very little has been reported on synthesized powder mixtures that exhibit SPER. Most studies concentrate on reactions during processing, instead of external ignition of the milled powders to study the exothermic reactions and evaluate the heat released.

In this work, we show that BM is an alternative viable method for the generation of micro and nanoscale heat sources for thermal manufacturing. Al and Ni powders with a molar ratio of 1:3 were ball milled for different time durations in order to synthesize powder mixtures, which were then compacted into pellets and subsequently ignited using an external heat source. In order to evaluate the microstructure after powder mixing, Scanning- and Back Scatter Electron Microscopy images were studied, while X-Ray Diffraction analysis indicated potential formation of intermetallic phases during milling that may hinder ignition. Additionally, thermal evolution of ignited powder consolidates and the propagation speed of the heat wave-front were studied using high speed and infrared cameras. Finally, Differential Scanning Calorimetry analysis was used to characterize the thermal stability of the compacts.

Results revealed that after 7 h of interrupted and 3 h of continuous BM an aluminum/nickel lamellar structure forms at the particle boundaries for the selected parameters. The characteristic lamella dimension reduces with increasing milling time down to approximately 200 nm after 10 h and 4 h of interrupted and continuous BM, respectively. The ignition conditions for compacts of milled powders indicate that samples with a higher volume fraction of the lamellae possess the shortest ignition time.