Energetic Films Research Articles

Spin coating preparation and thermal properties of metastable Al/PVDF energetic film with graphene

Spin coating is a cost-effective technique for the preparation of thin films, widely used in the fields of physics, medicine, biology and chemistry. In this paper, we applied spin coating method to prepare metastable Al/PVDF (polyvinylidene fluoride) composite films with low mass loading of aluminum. Through controlling the speed and time of spin coating, a desired composite film can be easily obtained. In order to adjust the performance of the films, coupling agent γ-methacryloxypropyl trimethoxy silane (KH-570) and graphene were added. Morphology, structure and thermal analysis were carried out by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), x-ray diffractometer (XRD) and thermogravimetry-differential scanning calorimetry (TG-DSC). The results indicate that the appropriate amount of KH-570 not only improve the dispersibility of Al in the film significantly, but also advance the temperature of the reaction between Al2O3 shell and fluorine from 372 °C to 350 °C. While graphene delay the progress of the reaction between Al2O3 shell and fluorine from 350 °C to 370 °C, and change the reaction exotherm mode (the exothermic peak changes from three to four). Kinetic analysis of exothermic peaks indicates that the apparent activation energy calculated by Kissinger method is slightly lowered after the addition of graphene. After adding graphene, the apparent activation energy corresponding to each exothermic peak changes from 244.47 kJ mol−1, 175.03 kJ mol−1, 245.90 kJ mol−1 to 213.80 kJ mol−1, 131.80 kJ mol−1, 218.50 kJ mol−1 (and 172.10 kJ mol−1).

Effects of Shear Rate during Energetic Material Processing on Reactivity

Energetic materials are often processed at high rates of deformation as colloidal slurries and then cured. The slurries are non‐Newtonian colloidal solutions that exhibit changes in microstructure with variations in applied flow. This study shows that changes to microstructure due to applied flow affect the reactivity of energetic thin films. Energetic thin films of identical composition and geometry are prepared with different applied shear rates, which produce variations in the film microstructure by segregating smaller particles toward surfaces. Results show that films exhibit significant gains in flame speed with increasing shear rate. The differences in flame speed are linked to variations in microstructure. Specifically, densification of smaller particles near a boundary promote increased flame speeds. However, when particles become segregated, larger particles tend to contribute less to the overall reaction because they burn slowly compared to the smaller particles. When segregated, the larger particles may not be adding chemical energy to the reaction front because propagation is dominated by the more ignition sensitive smaller particles. This study demonstrates explicit changes in reactivity arising from changes in processing conditions that affect microstructure. Controlling applied shear rate introduces a new approach to regulating energetic material reactivity when processed using extrusion‐based advanced manufacturing techniques.

Super-Efficient Synthesis of Mesh-like Superhydrophobic Nano-Aluminum/Iron (III) Oxide Energetic Films.

In this study, a novel superhydrophobic nano-aluminum/iron (III) oxide composite has been prepared by a facile one-step process of electrophoretic deposition, with wide potential applications. The optimal suspension included ethanol, acetyl-acetone, and the additives of fluorotriphenylsilane and perfluorodecyltriethoxysilane. The microstructure, wettability, and exothermic performance were analyzed by field emission scanning electron microcopy (FESEM), X-ray diffraction (XRD), water contact angle measurements, and the differential scanning calorimetry (DSC) technique. The water contact angle and the heat-release of the target composites could reach to ~170° and 2.67 kJ/g, and could still keep stable, after exposure for six months, showing a great stability. These results provided an exquisite synthesis of ideas, for designing other superhydrophobic energetic materials with self-cleaning properties, for real industrial application.

Highly reactive energetic films by pre-stressing nano-aluminum particles.

Energetic films were synthesized using stress altered nano-aluminum particles (nAl). The nAl powder was pre-stressed to examine how modified mechanical properties of the fuel particles influenced film reactivity. Pre-stressing conditions varied by quenching rate. Slow and rapid quenching rates induced elevated dilatational strain within the nAl particles that was measured using synchrotron X-ray diffraction (XRD). An analytical model for stress and strain in a nAl core–Al2O3 shell particle that includes creep in the shell and delamination at the core–shell boundary, was developed and used for interpretation of strain measurements. Results show rapid quenching induced 81% delamination at the particle core–shell interface also observed with Transmission Electron Microscopy (TEM). Slower quenching elevated dilatational strain without delamination. All films were prepared at approximately a 75 : 25 Al : poly(vinylidene fluoride) PVDF weight ratio and were 1 mm thick. A drop weight impact test was performed to assess ignition sensitivity and combustion. Stress altered nAl exhibited greater energy release rates and more complete combustion than untreated nAl, but reaction dynamics and kinetics proceeded in two different ways depending on the nAl quenching rate during pre-stressing.

In situ synthesis of energetic metal–organic frameworks [Cd5(Mtta)9]n film exhibiting excellent ignition capability

Energetic thin films have been widely used in the field of microscale energy-demanding systems. In this work, a novel energetic metal–organic frameworks [Cd5(Mtta)9]n film is fabricated on the copper substrate through an in situ synthesis method for the first time. A 5-methyl tetrazole/copper complex is deposited to modify the copper substrate through an electrochemical-assisted strategy. The [Cd5(Mtta)9]n film is then integrated on the surface of modified copper substrate by an in situ reaction of acetonitrile with sodium azide and cadmium nitrate. The morphological, structural and compositional information of the [Cd5(Mtta)9]n film is characterized by field emission scanning electron microscopy, X-ray diffraction and infrared spectrum. The thermostabilities of the [Cd5(Mtta)9]n film are studied by differential scanning calorimetry and thermogravimetry. In addition, the preliminary laser ignition test is conducted by using pulsed Nd:YAG laser. The results show that the as-prepared [Cd5(Mtta)9]n film possesses a rod-like topography. The flame height and ignition duration of [Cd5(Mtta)9]n film reach 9.0 mm and 220 µs, respectively, revealing that [Cd5(Mtta)9]n film is an excellent ignition material. This research opens up a new avenue for the preparation of novel energetic films, which provides potential applications on microelectromechanical systems to achieve functional nanoenergetics-on-a-chip.

Integrating micro-ignitors with Al/Bi2O3/graphene oxide composite energetic films to realize tunable ignition performance

The integration of composite energetic films (CEFs) with various types of initiators can effectively adjust their performance and represents potential applications in microscale energy-demanding systems. In this study, the Al/Bi2O3/graphene oxide (GO) CEFs were successfully integrated into copper micro-ignitors by electrophoretic deposition, a low-cost and time-saving method. The effects of the Al/Bi2O3/GO CEFs with different GO contents on exothermic performance and ignition properties of micro-ignitors were then systematically investigated in terms of heat release, activation energy, ignition duration, the maximum height of the ignition product, and ignition delay time. The results showed that the addition of GO promoted more heat releases and higher activation energies of Al/Bi2O3/GO CEFs. The addition of ≤3.5 wt. % GO prolonged the ignition duration from 450 μs to 950 μs and increased the maximum height of the ignition product from about 40 mm to 60 mm. However, the micro-ignitors with more than 3.5 wt. % GO cannot be ignited, which suggested that GO played a contradictory role in the ignition properties of micro-ignitors and the controlled GO content was a prerequisite for improved ignition performance. The ignition delay time gradually extended from 10.7 μs to 27.6 μs with increases in the GO contents of Al/Bi2O3 CEFs, revealing that an increase in the weight ratio of GO leads to lower ignition sensitivity of micro-ignitors.

Back Cover: Synthesis and Characterization of Novel Azido Polymers: Hyperbranched Poly-3-azidomethyl-3-hydroxymethyl Oxetane (Prop., Explos., Pyrotech. 9/2017)

The back cover picture shows that we synthesized a series of novel hyperbranched azido polymers namely poly-3-azidomethyl-3-hydroxymethyl oxetanes (HBPAMHMOs) using cationic ring opening polymerization. GPC, 13C NMR and MALDI-TOF results indicate these polymers have controllable molecular weight and degree of branching, with narrow molecular weight distributions. To take a step forward, we prepared HBPAMHMO-based energetic binder film. Compared to its linear analogues, HBPAMHMO shows lower glass transition temperature, sensitivity, higher combustion heat and mechanical properties, suggesting a great prospect for numerous applications. More details can be found in the Full Paper by Dr. Chi Zhang et al. on page 1037.

Thermal and Combustion Properties of Energetic Thin Films with Carbon Nanotubes

The effect of varying carbon nanotube concentration on ignition delay, flame speed, and electrical and thermal conductivity of three-dimensional printable energetic thin films made of magnesium and manganese oxide was investigated. Polyvinylidene fluoride was used as the binder for depositing the stoichiometric mixture into thin films with an average thickness of using an extrusion-based blade-casting method. Four films with a 0, 0.5, 1.0, and 1.5 wt % carbon nanotube were prepared. The ignition delay and flame speed of the films were measured using high-speed imaging techniques. Electrical and thermal conductivities were also measured. Results show that the inclusion of a 1.5 wt % carbon nanotube in the /polyvinylidene fluoride films improved their flame speed by 440% and electrical conductance by two orders of magnitude (from 4.23 to 655.33 nS) and decreased ignition delay by 87.2%. Interconnectivity of carbon nanotubes in the films was estimated using a basic percolation model. Films with a 1.5 wt % carbon nanotube demonstrated the highest interconnectivity, which aided improved thermal and electrical energy transport, thereby improving combustion performance. The development of three-dimensional extruded thin films with tailorable thermal and combustion parameters is a precursor for the additive manufacturing of energetic materials.

3D processing and characterization of acrylonitrile butadiene styrene (ABS) energetic thin films

This study examined processing and characterization of an energetic material synthesized with a binder that is extensively used in existing additive manufacturing methods. Aluminum (Al), molybdenum trioxide (MoO3), and potassium perchlorate (KClO4) are suspended in a solvent–binder system composed of acetone solvent and acrylonitrile butadiene styrene (ABS) binder. The concentration of ABS is varied from 10 to 50 wt% and the mass of acetone is correspondingly varied to ensure a slurry with constant volume percent solids. Three-dimensional films are cast with 1 mm thickness for all ABS concentrations tested. Rheological results show that all slurries exhibit non-Newtonian shear thinning behavior for viscosity as a function of shear rate such that negligible extrudate swell (i.e., die swell) is produced. Investigation into the viscoelastic properties of the slurries revealed them to be highly elastic such that thin sections in the film that cause crack formation can be induced. These results combine to show that 20 wt% ABS is a minimum threshold for ABS to provide a matrix capable of supporting the energetic materials and the elastic nature of the slurry contributes to crack formation below this threshold. Flame speed characterization showed that at and above 40 wt% ABS, the volatiles produced from the thermal degradation of the polymer cause burning above the surface of the film excluding the energetic materials from the reaction. Polymer concentrations of 20 wt% (i.e., 80 wt% energetic material loading) exhibited the highest flame speeds of 1.21 cm/s with an energy density estimated as 5737 kJ/kg. Optimal energetic composites processed using additive manufacturing are achieved if binder concentration can be minimized, as shown here.

Sensitiveness of Porous Silicon‐Based Nano‐Energetic Films

AbstractNanoporous silicon (pSi) films on a silicon wafer were loaded with sodium perchlorate and perfluoropolyether (PFPE) oxidizing agents. Sensitiveness to impact, friction and electrostatic discharge (ESD) of the resulting energetic thin films were investigated. It was observed that pSi loaded with perchlorate was sensitive at the lowest limit of detection for the available equipment (<4.9 J impact energy, <5 N friction force, and <45 mJ ESD spark energy). When loaded with PFPE the material was very sensitive to impact (<4.9 J), moderately sensitive to ESD (between 45 and 100 mJ) and insensitive to friction (>360 N). pSi loaded with either perchlorate or PFPE displayed behavior during sensitiveness testing similar to other primary explosive materials.

Probing the Reaction Mechanism of Aluminum/Poly(vinylidene fluoride) Composites.

Energetic thin films with high mass loadings of nanosized components have been recently fabricated using electrospray deposition. These films are composed of aluminum nanoparticles (nAl) homogeneously dispersed in an energetic fluoropolymer binder, poly(vinylidene fluoride) (PVDF). The nascent oxide shell of the nAl has been previously shown to undergo a preignition reaction (PIR) with fluoropolymers such as polytetrafluoroethylene (PTFE). This work examines the PIR between alumina and PVDF to further explain the reaction mechanism of the Al/PVDF system. Temperature jump (T-jump) ignition experiments in air, argon, and vacuum environments showed that the nAl is fluorinated by gas phase species due to a decrease in reactivity in a vacuum. Thermogravimetric analysis coupled with differential scanning calorimetry (TGA/DSC) was used to confirm the occurrence of a PIR, and gas phase products during the PIR and fluorination of nAl were investigated with temperature jump time-of-flight mass spectrometry (T-jump TOFMS). Results show a direct correlation between the amount of alumina in the PVDF film and the relative signal intensity of hydrogen fluoride release (HF). Although the PIR between alumina and PVDF plays an important role in the Al/PVDF reaction mechanism, burn speeds of Al/PVDF films containing additional pure alumina particles showed no burn speed enhancement.

Development of flexible, free-standing, thin films for additive manufacturing and localized energy generation

Film energetics are becoming increasingly popular because a variety of technologies are driving a need for localized energy generation in a stable, safe and flexible form. Aluminum (Al) and molybdenum trioxide (MoO3) composites were mixed into a silicon binder and extruded using a blade casting technique to form flexible free-standing films ideal for localized energy generation. Since this material can be extruded onto a surface it is well suited to additive manufacturing applications. This study examines the influence of 0-35% by mass potassium perchlorate (KClO4) additive on the combustion behavior of these energetic films. Without KClO4 the film exhibits thermal instabilities that produce unsteady energy propagation upon reaction. All films were cast at a thickness of 1 mm with constant volume percent solids to ensure consistent rheological properties. The films were ignited and flame propagation was measured. The results show that as the mass percent KClO4 increased, the flame speed increased and peaked at 0.43 cm/s and 30 wt% KClO4. Thermochemical equilibrium simulations show that the heat of combustion increases with increasing KClO4 concentration up to a maximum at 20 wt% when the heat of combustion plateaus, indicating that the increased chemical energy liberated by the additional KClO4 promotes stable energy propagation. Differential scanning calorimeter and thermogravimetric analysis show that the silicone binder participates as a fuel and reacts with KClO4 adding energy to the reaction and promoting propagation.

Synthesis and characterization of flexible, free-standing, energetic thin films

Abstract This study uses blade casting methods for the synthesis of flexible, free-standing energetic films. Specifically, films include aluminum (Al) and (MoO 3 ) powder thermites combined with potassium perchlorate (KClO 4 ) and silicone binder. In addition to this base composite, carbon fiber fabric reinforcement fabric has been incorporated to improve the structural integrity of the film. All films were cast at 1 mm thickness with constant percent solids to ensure consistent rheological properties. The films were ignited and flame propagation was recorded with a high speed camera. The results show that the energy propagation of the films increases with increasing mass percent KClO 4 . The inclusion of carbon fiber fabric reinforcement fabric in the energetic film decreased the flame speed by 30% but maintained stable and steady energy propagation. The strengths of the films were tested to determine the effects of the carbon fiber fabric reinforcement fabric on the mechanical properties of the films. The non-reinforced film, failed upon initial loading of approximately 2.27 kg while the reinforced film maintained a load of 72.3 kg. While this method of synthesis allows manufacture of a flexible free-standing energetic film, the composition and rheology of the mixed slurry have potential as an extrusion cast energetic for additive manufacturing of energetic materials.

Effects of rheological properties on reactivity of energetic thin films

Magnesium (Mg) and manganese dioxide (MnO2) powders were mixed with polyvinylidene fluoride (PVDF) binder and n-methyl pyrrolidone (NMP) solvent and blade cast onto stainless steel foil. The rheological properties of these mixtures were investigated to quantify the mixing condition. Parameters including wet film thickness, equivalence ratio and solids loading were varied. Flame speed and calorific output were investigated for each of these parameters. Results show energy propagation rates increased as a function of dry film thickness, although calorific output remained relatively constant. Stoichiometrically fuel rich compositions were self-quenching, demonstrating the necessity of available oxygen for reaction propagation. A 0.45 solids–liquid mixing ratio resulted in up to an order of magnitude higher energy propagation rate for both open and confined configurations. Rheometry measurements and physical characterizations of the films reveal that the solids loadings resulting in the most stable suspensions also produced the highest energy propagation. Changing solids loading affects the density of the film, which in turn affects energy propagation. Capillary drying forces at high liquid loadings result in higher porosity leading to reduced deposition density and thickness. Very high solids loading results in films with dilatant properties and poor mixing. These results show solids loading affects mixing and energy propagation and could impact slurry cast energetic materials as in additive manufacturing processes.

Novel approach to the preparation of organic energetic film for microelectromechanical systems and microactuator applications.

An activated RDX-Fe2O3 xerogel in a Si-microchannel plate (MCP) has been successfully prepared by a novel propylene epoxide-mediated sol-gel method. A decrease of nearly 40 °C in decomposition temperature has been observed compared with the original cyclotrimethylene trinitramine (RDX). The RDX-Fe2O3 xerogel can release gas and solid matter simultaneously, and the ratio of gas to solid can be tailored easily by changing the initial proportions of RDX and FeCl3·6H2O, which significantly enhances the explosive and propulsion effects and is of great benefit to the applications. The approach, which is simple, safe, and fully compatible with MEMS technology, opens a new route to the introduction of organic energetic materials to a silicon substrate.

Nano-scale energetic films by superfluid helium droplet assembly

The helium droplet technique has been utilized to study a wide range of topics in chemical physics such as atomic scale manifestations of superfluidity, chemistry at ultra-low temperatures, and the assembly of exotic Van der Waals complexes. Our efforts have transitioned the helium droplet methodology from a cryogenic nano-scale matrix to a film deposition technique capable of creating nano-structured films of composite metal-based energetic materials (EM). Such materials are ideal candidates to study propagation of reactions at small scale, and could be alternatives to organic based EM due to their higher energy densities. The helium droplet methodology may also provide a solution overcoming issues of reaction-limiting effects such as the formation of oxide layers by exploiting 'magic-number' cluster sizes and core-shell cluster mechanisms. This proceeding describes our previously reported efforts to model and characterize the deposition of magnesium clusters by superfluid helium droplet assembly, our efforts to produce magnesium-Fomblin core-shell EM cluster-based films, and our early attempts at making intermetallic cluster-based films.

Deposition and characterization of energetic thin films

A new approach for depositing thin energetic films is introduced using doctor blade casting. Magnesium (Mg) and manganese dioxide (MnO2) is mixed with a solvent that includes a binder and is blade cast onto a foil substrate. This study investigated the effect of binder chemistry and concentration on combustion behavior. The Mg–MnO2 system was studied in the following binder–solvent systems: Polyvinylidene Fluoride (PVDF) – Methyl Pyrrolidone (NMP); Viton® fluoroelastomer (Viton A) – acetone; and, paraffin–xylene. Films were cast onto substrates to approximately 100μm thickness. Calorific output and flame velocity were measured for varying binder concentration. Calorific output increased with increasing binder concentration, to a maximum of 4.0kJ/g, suggesting participation of the binder in the exothermic reaction. Flame velocity decreased with increasing binder concentration, with a maximum of 0.14m/s. Binders are less conductive than metals and metal oxides thereby hindering the energy propagation with increasing binder content. Confined flame propagation tests were also conducted for the NMP/Mg–MnO2–PVDF system, with a maximum recorded flame velocity of 3.5m/s. High velocity imaging shows considerable differences in flame front, which may suggest a transition in propagation mechanism accounting for the observed increase in flame velocity.

Synthesis of nickel picrate energetic film in a 3D ordered silicon microchannel plate through an in situ chemical reaction

Micro-energetic devices with energetic and functional diversity have attracted interest from scientific communities, through features such as the integration of energetic materials into micro-electro-mechanical systems (MEMS). In this study, a method for the preparation of nickel picrate energetic films on the sidewalls of a silicon microchannel plate (Si-MCP) is presented. The Si-MCP was produced by a photoelectrochemical process and a thin film of nickel (Ni) was synthesized by electroless plating of Ni on the sidewalls of the Si-MCP. The thin film of nickel picrate was successfully produced via an in situ chemical reaction method by introducing picric acid into the 3D ordered nickel/silicon microchannel plate (Ni/Si-MCP). Field emission scanning electron microscopy, Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy were used to study the morphological and structural properties of the thin film. The results demonstrate that picric acid reacted with Ni to form a nickel picrate thin film. Also, differential scanning calorimetry and thermogravimetric analysis were employed to characterize the thermal decomposition of the energetic film. The approach can solve the problem of integrating organic energetic materials with MEMS devices. Also, nickel picrate can release a mass of energy and gas simultaneously, which further enhances the functional diversity of MEMS devices.

Fabrication and performance characterization of Al/Ni multilayer energetic films

Al/Ni multilayer bridge films, which were composed of alternate Al and Ni layers with bilayer thicknesses of 50, 100 and 200 nm, were prepared by RF magnetron sputtering. In each bilayer, the thickness ratio of Al to Ni was maintained at 3:2 to obtain an overall 1:1 atomic composition. The total thickness of Al/Ni multilayer films was 2 μm. XRD measurements show that the compound of AlNi is the final product of the exothermic reactions. DSC curves show that the values of heat release in Al/Ni multilayer films with bilayer thicknesses of 50, 100 and 200 nm are 389.43, 396.69 and 409.92 J g−1, respectively. The temperatures of Al/Ni multilayer films were obviously higher than those of Al bridge film and Ni bridge film. Al/Ni multilayer films with modulation of 50 nm had the highest electrical explosion temperature of 7000 K. The exothermic reaction in Al/Ni multilayer films leads to a more intense electric explosion. Al/Ni multilayer bridge films with modulation period of 50 nm explode more rapidly and intensely than other bridge films because decreasing the bilayer thickness results in an increased reaction velocity.

TRIBOLOGY OF BIO-INSPIRED NANOWRINKLED FILMS ON ULTRASOFT SUBSTRATES

Biomimetic design of new materials uses nature as antetype, learning from billions of years of evolution. This work emphasizes the mechanical and tribological properties of skin, combining both hardness and wear resistance of its surface (the stratum corneum) with high elasticity of the bulk (epidermis, dermis, hypodermis). The key for combination of such opposite properties is wrinkling, being consequence of intrinsic stresses in the bulk (soft tissue): Tribological contact to counterparts below the stress threshold for tissue trauma occurs on the thick hard stratum corneum layer pads, while tensile loads smooth out wrinkles in between these pads. Similar mechanism offers high tribological resistance to hard films on soft, flexible polymers, which is shown for diamond-like carbon (DLC) and titanium nitride thin films on ultrasoft polyurethane and harder polycarbonate substrates. The choice of these two compared substrate materials will show that ultra-soft substrate materials are decisive for the distinct tribological material. Hierarchical wrinkled structures of films on these substrates are due to high intrinsic compressive stress, which evolves during high energetic film growth. Incremental relaxation of these stresses occurs by compound deformation of film and elastic substrate surface, appearing in hierarchical nano-wrinkles. Nano-wrinkled topographies enable high elastic deformability of thin hard films, while overstressing results in zigzag film fracture along larger hierarchical wrinkle structures. Tribologically, these fracture mechanisms are highly important for ploughing and sliding of sharp and flat counterparts on hard-coated ultra-soft substrates like polyurethane. Concentration of polyurethane deformation under the applied normal loads occurs below these zigzag cracks. Unloading closes these cracks again. Even cyclic testing do not lead to film delamination and retain low friction behavior, if the adhesion to the substrate is high and the initial friction coefficient of the film against the sliding counterpart low, e.g. found for DLC.