In situ imaging of ultra-fast loss of nanostructure in nanoparticle aggregates

Abstract

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.

Comment on “In situ imaging of ultra-fast loss of nanostructure in nanoparticle aggregates” [J. Appl. Phys. 115, 084903 (2014)]

“…Thus, "larger aggregates were far more likely to coalesce than small (<10 particles) aggregates" 1 and "superficially similar aggregates behaving differently when exposed to identical laser pulses." 1 In the flame tube, there is no significant scatter in the flame rate, i.e., there were no cases that flame rates for the same Al particles and conditions correspond to the MDM predictions in some experiments and do not correspond in others. Thus, while there are particles with various defects (e.g., pre-sintered, with non-homogeneous shell width, or with defective shell), the major portion of particles satisfy MDM conditions.…”

“…1 for the agglomeration and sintering of Al nanoparticles and in the following paper 2 for reactions within the nanoscale Al and CuO mixture for heating rate 10 11 K/s. The main conclusion is that both Al and CuO nanoscale particles first agglomerate and sinter in a larger scale Al and CuO contacting aggregate, which then react through a condensed state reaction.…”

High‐Energy Pollen‐Like Porous Fe2O3/Al Thermite: Synthesis and Properties

“…Based on the research results from Zachariah'sg roup [19,[29][30][31],s intering process occurs early in the redoxr eaction on af ast timescale, resultingi nr apid melting and coalescenceo fa ggregated particles. The initial size and morphologyo ft he particles were dramatically changedb efore the remainder of the materialb urns, suggesting ap ossible reason why nanostructured particlesc an not react as fast as expected [19].T herefore, the Al-NPs in our work may also highly agglomerate before the reaction with Fe 2 O 3 and the possible pre-combustion sintering process in sample Fe-Al-1 and Fe-Al-4 is displayedi nF igure 7.…”

A new strategy for the fabrication of high performance reactive microspheres via energetic polyelectrolyte assembly

“…Although the NEMs are comprised of particles with primary sizes <100 nm, the question is, do nanoenergetic particles remain nano-sized during combustion or reaction? [3][4][5] Michael R. Zachariah mainly proposed that under high heating rates, NPs in NEMs would sinter into structures at larger size levels, before the bulk of the combustion can take place, which is also referred to as nanostructure loss, and is conrmed by theory and experiment. [3][4][5] Even so, increasing the interfacial contact between the metal and oxidizer NPs to a large extent still plays a decisive role in producing NEMs with high performance.…”

“…[3][4][5] Michael R. Zachariah mainly proposed that under high heating rates, NPs in NEMs would sinter into structures at larger size levels, before the bulk of the combustion can take place, which is also referred to as nanostructure loss, and is conrmed by theory and experiment. [3][4][5] Even so, increasing the interfacial contact between the metal and oxidizer NPs to a large extent still plays a decisive role in producing NEMs with high performance. From the view point of the condensed phase interfacial reaction mechanism, 6,7 it is assumed that when the sintering is occurring between the metal and oxidizer NPs, it is an effective process; otherwise it is an unfavorable process.…”