Boiling heat transfer intensification is of significant relevance to energy conversion and various cooling processes. This study aimed to enhance the saturated pool boiling of FC-72 (a dielectric liquid) by surface modifications and explore mechanisms of the enhancement. Specifically, circular and square micro pin fins were fabricated on silicon surfaces by dry etching and then copper nanoparticles were deposited on the micro-pin-fin surfaces by electrostatic deposition. Experimental results indicated that compared with a smooth surface, the micro pin fins increased the heat transfer coefficient and the critical heat flux by more than 200 and 65–83%, respectively, which were further enhanced by the nanoparticles up to 24% and more than 20%, respectively. Correspondingly, the enhancement mechanism was carefully explored by high-speed bubble visualizations, surface wickability measurements, and model analysis. It was quantitatively found that small bubble departure diameters with high bubble departure frequencies promoted high heat transfer coefficients. The wickability, which characterizes the ability of a liquid to rewet a surface, played an important role in determining the critical heat flux, but further analyses indicated that evaporation beneath bubbles was also essential and competition between the wicking and the evaporation finally triggered the critical heat flux.

There exists a demand for production of metal nanoparticles for today's emerging nanotechnology. Aerosol-generated metal nanoparticles can oxidize during particle formation due to impurities in the carrier gas. One method to produce unoxidized metal nanoparticles is to first generate metal oxides and then reduce them during sintering. Here, we propose to instead prevent oxidation by introducing the reducing agent already at particle formation. We show that by mixing 5% hydrogen into the nitrogen carrier gas, we can generate single crystalline metal nanoparticles by spark discharge from gold, cobalt, bismuth, and tin electrodes. The non-noble nanoparticles exhibit signs of surface oxidation likely formed post-deposition when exposed to air. Nanoparticles generated without hydrogen are found to be primarily polycrystalline and oxidized. To demonstrate the advantages of supplying the reducing agent at generation, we compare to nanoparticles that are generated in nitrogen and sintered in a hydrogen mixture. For bismuth and tin, the crystal quality of the particles after sintering is considerably higher when hydrogen is introduced at particle generation compared to at sintering, whereas for cobalt it is equally effective to only add hydrogen at sintering. We propose that hydrogen present at particle generation prevents the formation of oxide primary particles, thus improving the ability to sinter the nanoparticles to compact and single crystals of metal. This method is general and can be applied to other aerosol generation systems, to improve the generation of size-controlled nanoparticles of non-noble metals with a suitable reducing agent. Copyright © 2018 American Association for Aerosol Research

Directed self-assembly of nanoparticles (NPs) is a promising strategy for bottom-up fabrication of nanostructured materials with tailored composition and morphology. Here, we present a simple and highly flexible method where charged magnetic aerosolized (i.e. suspended in a gas) NPs with tunable size and composition are self-assembled into nanostructures using combined electric and magnetic fields. Size-selected Co, Ni, and Fe NPs have been generated by spark ablation, and self-assembled into different structures, ranging from one-dimensional nanochains to macroscopic three-dimensional networks. By comparing the resulting structures with simulations, we can conclude that the magnetization of the NPs governs the self-assembly through interparticle magnetic dipole−dipole interactions. We also show how the orientation of the external magnetic field directs the self-assembly into differently aligned nano- and macroscopic structures. These results demonstrate how aerosol deposition in a combined electric and magnetic field can be used for directed bottom-up self-assembly of nanostructures with specialized composition and morphology.

Nanoparticle generation by aerosol methods, particularly spark ablation, has high potential for creating new material combinations with tailored magnetic properties. By combining elements into complex alloyed nanoparticles and controlling their size and structure, different magnetic properties can be obtained. In combination with controlled deposition, to ensure nanoparticle separation, it is possible to minimize interparticle interactions and measure the intrinsic magnetic property of the nanoparticles. Most magnetic materials are highly sensitive to oxygen and it is therefore crucial to both understand and control the oxidation of magnetic nanoparticles. In this study we have successfully generated oxidized, bimetallic FeCr and FeMn nanoparticles by spark ablation in combination with a compaction step, and thoroughly characterized individual particles with aerosol instruments, transmission electron microscopy (TEM) and synchrotron-based X-ray photoelectron spectroscopy (XPS). The generated nanoparticles had almost identical transition metal ratio as the electrodes used as seed material. Further, we demonstrate how the carrier gas can be used to dictate the oxidation and how to alternate between self-passivated and entirely oxidized nanoparticles. We also discuss the complexity of compacting alloyed nanoparticles consisting of elements with different vapor pressures and how this will affect the composition. This knowledge will further the understanding of design and generation of complex alloyed nanoparticles based on transition metals using aerosol methods, especially for the size regime where a compaction step is needed. As a proof of concept, measurements using a magnetometer equipped with a superconducting quantum interference device (SQUID) were performed on samples with different particle coverages. These measurements show that the magnetic properties could be explored for both high and low surface coverages which open up for studies where interparticle interactions can be systematically controlled.

An in-depth understanding of the reaction mechanism is required for the further development of Mo-based catalysts for biobased feedstocks. However, fundamental studies of industrial catalysts are challenging, and simplified systems are often used without direct comparison to their industrial counterparts. Here, we report on size-selected bimetallic NiMo nanoparticles as a candidate for a model catalyst that is directly compared to the industrial system to evaluate their industrial relevance. Both the nanoparticles and industrial supported NiMo catalysts were characterized using surface- and bulk-sensitive techniques. We found that the active Ni and Mo metals in the industrial catalyst are well dispersed and well mixed on the support, and that the interaction between Ni and Mo promotes the reduction of the Mo oxide. We successfully produced 25 nm NiMo alloyed nanoparticles with a narrow size distribution. Characterization of the nanoparticles showed that they have a metallic core with a native oxide shell with a high potential for use as a model system for fundamental studies of hydrotreating catalysts for biobased feedstocks.