Here, we report on the direct sequential imaging of laser-induced cavitation of micron and nanoscale bubbles using Movie-Mode Dynamic Transmission Electron Microscopy (MM-DTEM). A 532 nm laser pulse (∼12 ns) was used to excite gold nanoparticles inside a ∼1.2 μm layer of water, and the resulting bubbles were observed with a series of nine electron pulses (∼10 ns) separated by as little as 40 ns peak to peak. Isolated nanobubbles were observed to collapse in less than 50 ns, while larger (∼2-3 μm) bubbles were observed to grow and collapse in less than 200 ns. Temporal profiles were generally asymmetric, possibly indicating faster growth than collapse dynamics, and the collapse time scale was found to be consistent with modeling and literature data from other techniques. More complex behavior was also observed for bubbles within proximity to each other, with interaction leading to longer lifetimes and more likely rebounding after collapse.

In addition to its ability to produce geometrically complex parts, additive manufacturing offers a unique opportunity to collect data about a component while it is being fabricated. However, there has only been limited effort to characterize parts morphologically and compositionally in situ. In this article, we present a layer-by-layer, laser profilometry-based in situ characterization technique as a method to digitally reconstruct a multi-material part. Data collected by the laser profilometer yields height maps and grayscale images which are voxelized using purpose-built software to volumetrically reconstruct the part. The same part was also analyzed using X-ray computed tomography (CT) which was not able to resolve the different compositional regions within the part, but captured the filament morphology. The part was then bisected to compare the digital reconstruction to the actual part morphology and composition. Overall, the digital reconstruction was in good agreement with both the CT and bisected images. Deviations between the digital reconstruction and the CT/bisected images are likely the result of image segmentation settings or material shifts after data was collected. The in situ characterization method demonstrated here sets the stage for real time process monitoring and paves the way for additively manufactured parts that are “born qualified.”

Here we present analysis of a novel reactive material system that employs dinitrogen tetroxide (N2O4) as a liquid oxidizer with metal powder fuels. The oxidizer was added to micron scale aluminum and zirconium powders by a remote injection system. When ignited with a high voltage spark, the mixtures were observed to possess reactivity comparable to nanocomposite reactive materials, with open-tube flame expansion velocities from 500 to 1400 m/s depending on fuel/oxidizer ratio and tube diameter. Temperatures were observed to range from 300 0 to 3500 K as measured with gray body fits to 16-channel time-resolved pyrometer and time-integrated spectrometer data. These values were significantly below calculated adiabatic flame temperatures, which we attribute to local deviations from stoichiometry and kinetic/energetic limitations similar to those observed in studies of particles burning in high pressures gaseous oxidizers. Al/N2O4 reactivity was found to be most likely limited by the vaporization of the metal from the particle surface and Zr/N2O4 was limited by slower burning and complex interactions involving the solubility of nitrogen and oxygen in the molten Zr. We also discuss the potential for these materials to be used to create an "on/off" reactive material, since N2O4 can be added remotely and driven to evaporate via vacuum or purge of inert gases to return it to a safe condition. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The ignition of sputter deposited nanolaminate foils comprising alternating Co and Al layers results in rapid, self-propagating formation reactions. The propagating waves present after ignition of 150 nm-thick foils are characterized in movie mode dynamic transmission electron microscopy where these are found to have reaction speeds and wave morphology that vary with bilayer thickness. High speed videography reveals different bilayer thickness-wave character relationships in 750 nm-thick and 7500 nm-thick Co/Al foils. The reaction speed dependencies on bilayer thickness are calculated for each total thickness by treating the effect of radiation loss as a perturbation from an analytical model described by the difference in the heat of reaction measured in calorimetry and the adiabatic heat of product formation. From this model, an effective activation energy, diffusion constant, and flame temperatures are obtained, which allows for an interpretation of the reaction phase variations with laminate design and their effects on the propagating wave morphology.

We report a novel mechanism for explosive crystallization in amorphous germanium (a-Ge), which operates through liquid-mediated nucleation occurring under extreme thermal gradient conditions. The crystallization kinetics of sputter-deposited films with thicknesses ranging from 30 to 150 nm were characterized using in situ movie-mode dynamic transmission electron microscopy (MM-DTEM). After localized heating from a short laser pulse, explosive liquid phase nucleation (LPN) was observed to occur during the early stage (<2 μs) of crystallization in the thicker (>50 nm) films deposited on silicon nitride substrates. The crystallization front propagated at ∼12–15 m/s and produced nanocrystalline microstructure with ∼50 nm grains. A mechanism involving the existence of a relatively thick (>100 nm) transient liquid layer and a high nucleation rate is proposed to explain the behavior. The key thermodynamic and kinetic features as well as the feasibility of the mechanism are further explored by employing parametric and systematic phase-field modeling and simulations.

An experimental series was undertaken to recover and characterize the reaction products formed during detonation of Composition B under conditions varying from C-J detonation to overdriven shocks. Overdriven conditions were produced utilizing a two-stage gas gun, to provide a supported shock at a continuous pressure for the entirety of the detonation event. Input pressures ranged from 31.3 GPa to 55.0 GPa, and the results were compared to a standard detonation of Composition B. In all tests, the amount of post-detonation products, gaseous and condensed phase, were quantified. The morphology and phase of the carbon-rich soot was analyzed by electron microscopy and diffraction. Several trends were identified. The gas generation rate generally increased with input pressure. For condensed phase products, results suggest that higher shock pressures lead to the formation of more amorphous and larger structures.

In article 1700323, Rebecca Dylla-Spears and co-workers form optical quality silica and silica-titania glasses by three-dimensional (3D) printing silica inks formulated directly from molecular precursors, followed by heat treatment and polishing. This method demonstrates the potential of sol-gel chemistry for creating alternate glass compositions for optical 3D printing applications, which may lead to chemical and structural tuning of optical components.

A method for fabricating optical quality silica and silica–titania glasses by three‐dimensional (3D) printing is reported. Key to this success is the combination of sol–gel derived silica and silica–titania colloidal feedstocks, direct ink writing (DIW) technology, and conventional glass thermal processing methods. Printable silica and silica–titania sol inks are prepared directly from molecular precursors by a simple one‐pot method, which is optimized to yield viscous, shear‐thinning colloidal suspensions with tuned rheology ideal for DIW. After printing, the parts are dried and sintered under optimized thermal conditions to ensure complete organic removal and uniform densification without crystallization. Characterizations of the 3D‐printed pure silica and silica–titania glasses show that they are equivalent to commercial optical fused silica and silica–titania glasses. More specifically, they exhibit comparable chemical composition, SiO2 network structure, refractive index, dispersion, optical transmission, and coefficient of thermal expansion. 3D‐printed silica and silica–titania glasses also exhibit comparable polished surface roughness and meet refractive index homogeneity standards within range of commercial optical grade glasses. This method establishes 3D printing as a viable tool to create optical glasses with compositional and geometric configurations that are inaccessible by conventional optical fabrication methods.

The unsteady propagation mechanism for the crystallization of amorphous germanium (a-Ge) was studied with in situ movie-mode dynamic transmission electron microscopy (MM-DTEM). Short laser pulses were used to heat sputter-deposited a-Ge films and the resulting crystallization process was imaged with up to 16 sequential 50 ns long electron pulses separated by a controlled delay that was varied between 0.5 and 5 mu s The unsteady crystallization in the radial, net-growth direction was observed to occur at a decreasing rate of similar to 1.5-0.2 m/s through a mechanism involving the formation of discrete similar to 1.1 mu m wide bands that grew with velocities of 9-12 m/s perpendicular to the radial direction and along the perimeter of the crystallized area. The crystallization rate and resulting microstructure were consistent with a liquid-mediated growth mechanism, which suggests that locally the band front reaches the amorphous melting temperature of Ge. A mechanism based on the notion of a critical temperature is proposed to explain the unsteady, banded behavior. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Reduced diffusion length scales and increased specific surface areas of nanosized metal fuels have recently demonstrated increased reaction rates for these systems, increasing their relevance in a wide variety of applications. The most commonly employed metal fuel, aluminum, tends to oxidize rapidly near its melting point (660 °C) in addition to undergoing a phase change of the nascent oxide shell. To further expand on the understanding of nanosized metal fuel oxidation, tantalum nanoparticles were studied due to their high melting point (3017 °C) in comparison to aluminum. Both traditional slow heating rate and in-situ high heating rate techniques were used to probe the oxidation of tantalum nanoparticles in oxygen containing environments in addition to nanothermite mixtures. When oxidized by gas phase oxygen, the oxide shell of the tantalum nanoparticles rapidly crystallized creating cracks that may attribute to enhanced oxygen diffusion into the particle. In the case of tantalum based nanothermites, oxide shell crystallization was shown to induce reactive sintering with the metal oxide resulting in a narrow range of ignition temperatures independent of the metal oxide used. The oxidation mechanism was modeled using the Deal-Grove model to extract rate parameters, and theoretical burn times for tantalum based nanocomposites were calculated.

Nanothermites offer high energy density and high burn rates, but are mechanistically only now being understood.

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.

The importance of the oxidation state of an oxidizer and its impact on gaseous oxygen and total gas production in nanocomposite thermite combustion was investigated by probing the reaction and ignition properties of aluminum nanoparticles (Al-NPs) with both cupric oxide (CuO) and cuprous oxide (Cu2O) nanoparticles. The gas release and ignition behavior of these materials were tested with >10(5) K/s temperature jump (T-jump) heating pulses in a high temporal resolution time-of-flight mass spectrometer (ToF-MS) as well as in an argon environment. Reactivity was tested using a constant volume combustion cell with simultaneous pressure and optical measurements. A variety of Cu2O particle sizes ranging from 200 to 1500 nm were synthesized and found to release oxygen at similar to 1200 K, which is higher than the values found for a variety of CuO particle sizes (similar to 1000 K). Both oxides were found to ignite around 1000 K, which implies a consistent ignition mechanism for both through a condensed phase pathway. The higher oxidation state (CuO) thermites were found to react faster and produce higher pressures by several orders of magnitude, which implies that gaseous species play a critical role in the combustion process. Differences in reactivity between argon and vacuum environments and the use of Cu diluent to simulate Cu2O suggest that it is the intermediate product gas, O-2 that plays the most significant role in combustion as an enabler of heat transfer and a secondary oxidizer. The lack of any oxidizer size dependence on ignition is suggestive of rapid sintering that wipes out the effect of enhanced interfacial contact area for smaller oxidizers.

Nanoparticles hosted in conductive matrices are ubiquitous in electrochemical energy storage, catalysis and energetic devices. However, agglomeration and surface oxidation remain as two major challenges towards their ultimate utility, especially for highly reactive materials. Here we report uniformly distributed nanoparticles with diameters around 10 nm can be self-assembled within a reduced graphene oxide matrix in 10 ms. Microsized particles in reduced graphene oxide are Joule heated to high temperature (∼1,700 K) and rapidly quenched to preserve the resultant nano-architecture. A possible formation mechanism is that microsized particles melt under high temperature, are separated by defects in reduced graphene oxide and self-assemble into nanoparticles on cooling. The ultra-fast manufacturing approach can be applied to a wide range of materials, including aluminium, silicon, tin and so on. One unique application of this technique is the stabilization of aluminium nanoparticles in reduced graphene oxide film, which we demonstrate to have excellent performance as a switchable energetic material.

Aluminum (Al) nanopowders are of significant interest for many propellant, pyrotechnic, and explosive applications due to their high energy density and fast reaction kinetics. The interaction between the Al core (MP 660 C) and the Al2O3 shell (MP 2072 C) of nano-Al is critical to understanding the initiation mechanism for oxidation, which is currently under debate [1]. Various mechanisms for the oxidation of Al in nanothermites have been proposed that detail the diffusion of oxidizer and fuel through the shell [2], with experimental results showing ignition occurring near the melting point of Al [3]. The crystallization of the oxide shell is an important part of the Al ignition mechanism [4], but is a difficult process to investigate because oxide crystallization occurs at a temperature very close to the melting point of Al. To examine the oxidation mechanisms of nanosized metal fuels, tantalum (Ta) was chosen due to its high melting point in comparison to Al (3017 C vs. 660 C). This research examines the oxidation mechanism of nano-Ta by replicating an ultra-fast heating environment in an electron microscope as well as in an oxygenated atmosphere.

Over the past two decades there has been significant growth in our understanding of nanothermites and other nanoenergetic material. However, there is still much that is unknown about the physics and processes that control these highly exothermic reactions. In this chapter, we provide an overview of the three main components to the combustion of aluminum nanoparticle-based energetics: heat transfer mechanism, effect of the oxide shell, and reaction pathway. Each section contains a discussion of the general theories and processes involved, as well as the latest evidence. Combining all the ideas and most likely mechanisms, we finally discuss how an understanding of these mechanisms can shape and guide the future development of nanoenergetic materials.

Al/CuO reactive nanolaminate ignition was studied using temperature jump (T-Jump) heating for rates greater than 10(5) K/s. Multi layer samples were sputter deposited onto thin platinum filaments in alternating layers of Al and CuO. The filaments were resistively heated in a time-of-flight mass spectrometer (ToF-MS), while ignition and reaction were observed with high-speed video. A total deposited thickness of 1800 nm was maintained for all samples, while the number of bilayers was varied from 1 to 12. Increasing this value decreased the diffusion distances and increased the amount of interfacial area across which reaction could occur, while keeping the overall energy of the system constant. From 2 to 6 bilayers, the ignition temperature decreased from 1250 to 670 K and the overall reactivity increased. Past 6 bilayers, the ignition temperature only decreased slightly and there was little impact on the overall reactivity. This behavior is consistent with a mass-transport model where the predominant diffusing species exhibits a low activation energy (50 kJ/mol). Ignition temperature, which depends upon bilayer thickness, is found to be a good predictor of flame speed.