We report a method for fabricating optical quality silica and silica-titania glasses by additive manufacturing, or 3D printing. Key to this success was the combination of sol-gel derived silica and silica-titania colloidal feedstocks, 3D direct ink writing (DIW) technology, and conventional glass thermal processing methods. Printable silica and silica-titania sol inks were prepared directly from molecular precursors by a simple one-pot method, which was optimized to yield viscous, shear-thinning colloidal suspensions with tuned rheology ideal for DIW. After printing, the parts were 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 This article is protected by copyright. All rights reserved. 2 equivalent to commercial optical fused silica (Corning ® 7980) and silica-titania glasses (Corning ULE ® 7972). More specifically, they exhibit comparable chemical composition, SiO 2 network structure, refractive index, dispersion, optical transmission, and coefficient of thermal expansion. 3D printed silica and silica-titania glasses also exhibited 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. † denotes value determined by LA-ICP-MS; a-SiO 2 used to represent amorphous SiO 2
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.
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 word “nanoparticle” nominally elicits a vision of an isolated sphere; however, the vast bulk of nanoparticulate material exists in an aggregated state. This can have significant implications for applications such as combustion, catalysis, and optical excitation, where particles are exposed to high temperature and rapid heating conditions. In such environments, particles become susceptible to morphological changes which can reduce surface area, often to the detriment of functionality. Here, we report on thermally-induced coalescence which can occur in aluminum nanoparticle aggregates subjected to rapid heating (106–1011 K/s). Using dynamic transmission electron microscopy, we observed morphological changes in nanoparticle aggregates occurring in as little as a few nanoseconds after the onset of heating. The time-resolved probes reveal that the morphological changes initiate within 15 ns and are completed in less than 50 ns. The morphological changes were found to have a threshold temperature of about 1300 ± 50 K, as determined by millisecond-scale experiments with a calibrated heating stage. The temperature distribution of aggregates during laser heating was modeled with various simulation approaches. The results indicate that, under rapid heating conditions, coalescence occurs at an intermediate temperature between the melting points of aluminum and the aluminum oxide shell, and proceeds rapidly once this threshold temperature is reached.
Al/CuO reactive nanolaminate ignition was studied using temperature jump (T-Jump) heating for rates greater than 105 K/s. Multilayer 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.
The reaction between metallic fuel and oxygen carriers produced by the laser heating of aluminum and copper oxide (CuO) nanoparticles (NPs) was investigated (NPs) using movie mode dynamic transmission electron microscopy (MM-DTEM), which enables multiframe imaging with nanometer spatial and nanosecond temporal resolution. Nanothermite materials heated in situ at ∼10 11 K/s showed significant morphological changes on time scales of 1− 5 μs. The resulting structures were typically phase-separated into adjoining spheroids. Further analysis with energy dispersive spectroscopy (EDS) and selected area electron diffraction (SAED) was used to determine the extent of reaction. Bulk scale reaction experiments using temperature jump wire heating (∼10 5 K/s) revealed that both the reaction products and general processes were comparable to the reactions driven by the DTEM laser heating. These results indicate that condensed phase and interfacial reactions are fast and dominant mechanisms in nanothermite combustion.
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