ICMCTF2015 Session TS3: Energetic Materials and Micro-Structures for Nanomanufacturing

It is well recognized that initiation of self-sustained exothermic reaction in micrometer-scale gasless reactive mixtures (e.g. Ni+Al, Ti+C) is correlated to the formation of a liquid phase at the melting point of the less refractory reactant or at the lowest eutectic temperature of the system. In some cases (e.g. Ta+C; Mo-B) the impurity gases adsorbed on the surface of the precursors may also significantly accelerate mass transport. In recent studies [1-5] we have shown that short-term (minutes) high energy ball milling (HEBM) of such powder mixtures leads to a significant (on hundreds or even thousands of degrees) decrease of the self-ignition temperatures (T_ig).

The relatively low values of ignition temperature, which are well below the melting points of all reagents, indicate that solely solid-sate mass transport is responsible for the reaction self-initiation in the mechanically activated medium. In this work we discuss the phenomenon, which takes place during energetic milling of different binary reactive gasless systems, such as Ni-Al (ductile-ductile), Ti-C (ductile-brittle), Si-C (brittle-brittle). To understand the influence of HEBM on enhancement of the system’s reactivity, a comprehensive analysis of the mechanically-induced composite structures for such systems has been performed. The relationship between the microstructures and reactivity of such materials are discussed.

Our results indicate that, during early stages of HEBM, the initially micrometer-scale heterogeneous mixtures transform into nanostructured composite particles. For example, it was shown in the Ni+Al (ductile-ductile) system that Ni/Al composite particles are formed by cold welding within the first 2 min of milling. Further mechanical treatment leads to a significant refinement of inner structure of such composite particles, which results in the formation of intermixed Ni/Al layered nanostructure with average layer size less than 50nm. It was experimentally proven that such nano-laminated structures are responsible for the self-ignition of HEBM-Ni/Al composite particles at temperature as low as 500K [1-3]. During HEBM of Ti + graphite and Ta + graphite mixtures (ductile-brittle) the crystalline graphite flakes rapidly (1 min) transforms to amorphous carbon followed by the Me/C composite particles formation. Microstructural refinement of the inner structure of such composite particles leads to nano-mixing of amorphous carbon and the metal, which simultaneous results in the formation of tiny (1-2 nm) nuclei of carbide phases (e.g. TiC_x) [4]. Such mechanically-induced particles react through a solely solid-state mechanism. During HEBM of the Si + graphite (brittle-brittle) mixture, nano-scaled (80-300 nm) composite particles form, which consist of amorphous carbon and crystalline silicon. Such structure allows for direct synthesis of SiC nanopowders in the combustion wave [5].

In general it was concluded that the following main factors are responsible for the reactivity enhancement in the mechanically-fabricated gasless high energy density systems:

- formation of oxygen-free high surface area contacts between the reagents;

- mixing of the reactants on the nano-scale level;

- formation of a sold solution or nucleation of the product phases;

- amorphization of the reagents

References:

1. A.S. Mukasyan, B. B. Khina, R.V. Reeves, S.F. Son, Chem. Eng. J., 174, 77 (2011).

2. K.V. Manukyan, B.A. Mason, L.J. Groven, Y.-C. Lin, et al., J. Phys. Chem. C, 116, 21027 (2012).

3. A.S. Rogachev, N.F Shkodich, S.G Vadchenko, F. Baras, et al., J. Alloys & Compounds, 577, 600 (2013).

4. K.V. Manukyan, Y.-C. Lin, S. Rouvimov, P.J. McGinn, A.S. Mukasyan, J. Appl. Phys, 113, 024302 (2013).

The steel to aluminum weld-brazing process is recently investigated by automotive manufacturers in joining functional hybrid metallic structures for car weight reduction. To achieve good mechanical strength of these join structures and to be able to industrialize this process, manufacturers are currently facing different issues. One of them is the formation of active micro/nano-intermetallic phases between the seam and the steel part leading to generally to brittle fracture. This phenomenon depends mostly on the weld-brazing process parameters optimization and particularly on the composition of the wire. In this study several wires with different composition are used and their influences on the mechanical strength response of the welded join are analyzed. Moreover, the influence of the wire composition on the micro-nano intermetallic structures formation is discussed. The results are hence cross-checked and classified according to the process parameters optimization to establish an efficient weld-brazing method at full scale.

Reactive multilayers grown by sputter deposition have recently attracted interest for emerging applications including material joining (soldering, brazing) and energy sources. For these applications, a metal-metal multilayer is typically designed to have a composition that corresponds to the peak enthalpy for a given material system as this approach maximizes heat output or design targets a particular intermetallic compound with optimal mechanical properties. With the focus on a single composition, little is known about the range of composition that gives rise to self-sustained, high temperature synthesis (SHS) reactions for a given reactive metal pair.

With this presentation, we describe the compositional limits of reactive Al/Pt multilayers. For multilayers having a total thickness of 1.6 microns, self-sustained, high temperature reactions occur when the net multilayer composition is in the range of Al_0.25Pt_0.75 to Al_0.75Pt_0.25. Multilayers having a net composition of Al_0.2Pt_0.8 and Al_0.8 Pt _0.2 do not react when stimulated at a point.

Results are correlated with measurements of heat of reaction as determined by differential scanning calorimetry (DSC). Equiatomic Al/Pt multilayers have the maximum heat of reaction of all films investigated in this study, consistent with the maximum reaction velocity. Attempts are made to extract heats of formation for the various compositions studied. Reaction speeds are also reported for the various multilayers as a function of composition and multilayer periodicity (bilayer thickness).

_{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.}

The interface between an oxidizer and a reducing agent such as in Al-based reactive thin film nanolaminates (e.g. CuO/Al, ZnO/Al) has been shown to impact drastically their thermalproperties. Despite promising studies of the formation mechanism of interfaces and the role of surface engineering to improve their performance ¹, the main difficulty in deriving fundamental chemical mechanisms is the intrinsic roughness of metal oxide films. For instance, sputter deposited CuO or ZnO films exhibit a columnar growth with surface roughness exceeding 10 nm.

The synthesis of thin model oxides on atomically controlled substrates would greatly facilitate the interpretation of surface characterization techniques such as Fourier transform infrared spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS) and low energy ion scattering (LEIS) ¹. We present here an attempt to grow ZnO thin film on atomically flat surfaces using atomic layer deposition (ALD). ZnO is chosen to build the model system because deposited layer is crystalline at low deposition temperature and the roughness can be well controlled with ALD. Two atomically flat model-surfaces are used. The first is a monohydride-terminated Si(111) surface, obtained by aqueous NH₄F etching ², and the second is a nano-patterned OH-terminated Si(111) surface (~30% Si-OH and ~70% Si-H), obtained by methanol treatment of previous H/Si(111) surface ³. The ALD process is based on diethylzinc (DEZ) and water precursors alternated exposures ⁴. The growth conditions are set to be reactant exposure times of 1 s for each precursor and purge times of 5 s between each precursor pulse.

Using in situ IR spectroscopy in an ultra-high vacuum environment, we find that DEZ is reactive with the surface OH groups, providing the initial grafting for ZnO growth on the nanopatterned surfaces.

The interaction of TMA on such ultra-thin ZnO surfaces is then examined using a combination of in-situ studies including FTIR, LEIS and XPS, and compared to that on CuO surfaces [1]. Experimental findings will be confronted to Density Functional calculations to shed light on molecule/surface basic chemical mechanisms. Finally, the grow ZnO films using Pulsed Laser Deposition is attempted, and found to give rough films due to columnar growth, just as in the case of sputter-deposited films.

¹ ACS Appl Mater Interfaces. 3, 605-13 (2013)

² Appl. Phys. Lett ., 56, 656-658 (1990)

³ Nature Mater. 9, 266–271 (2010)

A new method for synthesizing flexible free standing energetic material coatings and the characterization of their reactivity in terms of energy propagation is presented. Aluminum (Al) and molybdenum trioxide (MoO₃) were mixed with potassium perchlorate (KClO₄) additive and combined with a silicone/xylene binder/solvent system. The formulation was tape cast to form a free standing flexible energetic film and could also be used as a coating. The concentration of KClO₄ was varied to understand the role of this additive on energy propagation of the tape case film. All films were cast to a thickness of 2.5 mm with a constant volume percent solids to ensure rheology of the samples remained constant, which has been previously shown to affect homogeneity of reactants as well as combustion. The films were ignited and analyzed using a high speed camera aligned perpendicular to the direction of energy propagation. The results show that films synthesized with this new method are capable of supporting their own weight as well as retaining a high degree of flexibility. The sample containing a higher mass concentration of KClO₄ exhibited increased flame speeds when compared to the lower KClO₄ concentration sample. Also shown is that the release agent used to aid sample delamination from the substrate does not influence energy propagation. Thermochemical simulations and thermal gravimetric analysis reveal negligible gas generation resulting from the KClO₄ additive such that the additive facilitates the diffusion of energy during propagation. These newly synthesized flexible free standing energetic materials are gaining increased attention due to the desire for a stable, easy to store, and safe method of producing energetic composites.

Interface layers in Al-based reactive thin films play a crucial role in the energetic properties and reactivity of such materials¹. 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 work aims at developing an understanding of the interface formation processes between Al and metal based oxidizer and investigating the role of interface treatment in the reaction kinetics.

Al/CuO nanolaminates represent a good model system for studying the reactive interfaces because they have a high energy density and are particularly sensitive to the nature of the Al_xO_y interface created during deposition. In this work, we combine three techniques to obtain bonding information (IR spectroscopy), oxidation state (XPS) and atomic position (Low Energy Ion Scattering) during interface formation, all located within the same ultra-high vacuum cluster system. These results are completed with Density Functional calculations for allowing spectra assignation or unraveling chemical kinetic information.

Similarly to a previous study¹, the bonding configuration and evolution of Al atoms is examined upon exposure of trimethylaluminum (TMA) of the CuO surface, prior to Al deposition. As expected for vapor phase exposure typically used in ALD, the surface reaction and coverage is conformal and complete (i.e. saturated as evidenced by the lack of more adsorption with multiple exposures). TMA is found to be adsorbed by shedding one of its methyl ligand onto a surface Cu atom and forming (CuO)–Al(CH₃)₂ eventually leading to (CuO)–Al(CH₃)–O–CH₃ by reducing CuO¹. Once saturated with TMA, the surface is exposed to atomic Al by physical evaporation and the interface composition monitored. In particular, the role of TMA to stabilize the interface (i.e. slow the Al penetration into the CuO substrate) is examined by comparing the Al position as a function of Al coverage with and without TMA treatment.

A similar study with Cu physical evaporation on sputter deposited CuO will be carried out prior to Al deposition. Preliminary data indicates that a thin Cu layer dramatically increases the ignition rate. Similarly, a thin Cu layer will be deposited on Al film prior to CuO deposition. The focus here also will be on examining Cu atom diffusion and the surface composition at the interface of both CuO and Al.

This work will shed light on the interface formation mechanisms and composition of Al and Cu based nanolaminates which will be particularly important in engineering Al/CuO based new energetic materials.

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

Recent discoveries on xenon flash ignition of nanoscale aluminum (nAl), carbon nanotubes and mechanically activated Al-PMF composite particles sparked interest on electromagnetically assisted ignition of reactive materials. We performed flash and laser ignition experiments on microscale ball-milled particles of Al-Sucrose and Ti-Sucrose. Bare and soot covered thin thermocouples were used to measure maximum temperatures for comparison. The results show that regular heating alone is not sufficient and an amplifying effect is necessary to explain composite particle ignitions, for example through localized surface plasmon resonance (LSPR) in nanoparticles of aluminum (nAl). It is possible that the incorporation of dialectric material through milling enhances LSPR among nanoscale Al lamella that normally occurs due to air gaps in nAl. The wavelength-particle size dependence due to LSPR can possibly be explored with different milling conditions for tuning the wavelength response of nanostructured microscale particles.

Nanolaminated materials made of nanometer-thick layers are nowadays facing the technological challenge of mastering interfacial layers, inevitably formed during technological fabrication processes. At these dimensions, interfacial layers become preponderant and play a determining role on the macroscopic properties of designed materials.

In nanolaminated energetic materials, more precisely multilayered Al/CuO nanothermites, in which intimacy of Al and CuO components must be maximized, it has been proven that interfacial layers, still unknown in composition and not well controlled during processing, impact final performances of achieved materials, such as stability, reactivity and energy release. In this field, master of interfaces during their fabrication is a major issue since it offers the new opportunity to increase and tune their performances as for example in the perspective of their integration into MEMS technologies.

Actually, multilayered nanothermites suffer from a lack of knowledge of the structure and composition of both the deposited layers and interfaces, with an atomic scale precision, in relation with the deposition process parameters (temperature, partial pressures …).

To push forward their nanoscale–controlled fabrication, predictive atomic scale modeling of the deposition process can help, by providing a fundamental description on how they are achieved. Simulations appear here as a powerful tool for the characterization of phenomena occurring at the nanoscale as a deep dive into the matter, to guide the technologists toward the development of advanced nanostructured materials. In our approach, we propose a model based on a kinetic Monte Carlo (KMC) method to simulate multilayered Al/CuO materials deposition, as PVD process, in order to get clues, depict and predict a realistic interfacial formation. Here, Density Functional Theory-based (DFT) calculations are used to identify and characterize, kinetically and thermodynamically, atomic scale events. Then these DFT data are used as input parameters to parametrize a KMC model. Presented results will include CuO chemistry on the Al surface, and the early stages of PVD deposition will be examined up to the complete passivation of the Al surface. This methodology offers the possibility to access to the exact structure of the material as a function of the manufacturing process and thus to access to the detailed composition, that depends on the conditions in which it was synthesized. We except to propose microscopic elements that could guide the technologist to improve processing and improve the properties of the operating material.

One of these promising applications of Metastable Intermolecular Composites (MIC) is reactive bonding [1], which utilizes heat produce from the thermite reaction for joining, welding, soldering and brazing. Thanks to the dominant solid-state chemical reactions and localized heat and mass transport phenomena, successful MIC-based joining processes are developed for components with different material and thermal properties such as electrical conductivities and thermal expansion coefficients. This work aims at developing effective electrophoretic deposition (EPD) to fabricate MIC foils and wires with tunable ignition and combustion properties. As demonstrated early [2] EPD is a very flexible deposition technique that can be arranged with simple apparatus, and its rate of deposition and foil microstructures can be effectively controlled by adjusting the applied voltage, colloid concentrations, pH value and operating temperature, etc. [3].

In this study the thermite components, fuel (aluminum nanoparticles of 130 nm and 60-80 nm) and a variety of oxidizers (CuO, NiO and Fe2O3 nanoparticles), are deposited separately or in a mixed way onto interested substrates. The EPD suspensions are prepared with HPLC grade ethanol (≥99.8%). The colloidal particle concentrations range from 0.25% to 1.5% (by weight) and the operating temperature and pH values are monitored and controlled. The MICs and as-produced thermite foils/wires are characterized by Thermal Gravimetric Analysis (TGA)/ Differential Scanning Calorimetry (DSC), SEM and Flame propagation experiments. In flame propagation experiments different ignition methods and these substrates with various thermal properties are used. Gas/vapor generation, flame propagation rate and product compositions are investigated.

References:

[1] X. Zhou, M. Torabi, J. Lu, R. Shen and K. Zhang (2014) “Nanostructured energetic composites: synthesis, ignition/combustion modeling and applications”, ACS Applied Materials and Interfaces 6: 3058-3074.

[2] K.T. Sullivan, M.A. Worsley, J.D.d Kuntz and A.E. Gash (2012) "Electrophoretic deposition of binary energetic composites," Combustion and Flame 159: 2210-2218.

[3] J.J. Moore, J.H. Kang, S.H. Jayaram and J.Z. Wen (2012) “Performance of nanotube-based electrodes from temperature-controlled electrophoretic deposition”, Journal of Applied Electrochemistry 42: 501-508.

Reactive multilayered foils, wherein two or more layers of different materials are stacked on top of each other, represent an interesting class of nanostructured energetic materials. These multilayered structures can sustain a self-propagating exothermic reaction once it is thermally by a local perturbation (e.g. a spark). Most studies of multilayered reactive materials have been focused on Al/Ni systems, motivated by both fundamental science and varioius potential applications such as bonding, brazing and sealing. Thermite-based multilayered foils have received comparatively less attention despite their higher energy densities and reactivity compared to intermetallic systems. We present here a new growth sequence of Al/CuO nanothermites, in which intermediate pure metallic or pure organometallic exposures transforms the response of the system previously limited by poorly controlled interfacial alumina-based barrier layers formed during standard fabrication. All layers, except the organometallic treatment, were deposited by DC magnetron sputtering. A comparison study was performed with conventionally processed multilayered material, i.e. with formation of alumina-based barrier layers composed of 8 layers of 100 nm of Al and 7 layers of 200 nm of CuO. The heat of reaction and onset temperature were measured using differential scanning calorimetry (DSC). As expected from previous results, the heat of reaction is around 1.6 KJ/g, and the burning rate is around 60 m/s. For alternative barrier layers, with an intermetallic layer between Al and CuO, an increase of the heat of reaction of more than 50% attributed to the absence of interfacial Al₂O₃ layer. This relatively modest improvement increases the burning rate by a decade. First Principles calculations and advanced characterization including in situ IR, XPS and LEIS are beginning to shed light onto the basic mechanisms of interface formation, and adsorptions and penetration bulk diffusion of atoms deposited at the surface. .

The ability to tune the interfaces of nanothermites, along with other attributes such as high volumetric energy density and the capability to produce environmentally benign products, make reactive multilayered foils a very attractive nanoenergetic material for wide applications, including environmentally clean primers, detonator, explosives, and in situ welding.