We demonstrate that the critical size cluster concept, commonly used in a nucleation theory, should be given some further attention. It has been implied that the supercritical cluster (size larger than critical) can grow via condensation. However, as we show, there is a size range, where the arrival of a vapor molecule onto a cluster surface leads to such a heating of the supercritical cluster that, due to possible evaporation, makes it unstable and, therefore, disables its condensation growth. The described phenomenon leads to substantial accumulation of certain size clusters in the system, which is clearly evident from our experimental investigation. The found suppression of the nucleus growth within the certain size range (exceeding critical) has fundamental implications for many systems where the generation of nanoparticles occur at high temperatures.

In the last two decades, a series of novel nanostructured carbon materials have been synthesized in the laboratory, [1] including carbon nanotubes, [2] carbon onions, [3] and carbon nanocapsules. [4,5] Intensive studies of nanostructured carbon synthesis are motivated by potential applications such as intercalation materials for Li batteries, [6] gas-storage media, cold-electron field emitters, [7] etc. Consisting of bent graphene layers (GLs), these nanomaterials possess unique electric properties due to their finite characteristic size, making them attractive for such applications. Depending on the curvature of the graphene sheets, carbon nanomaterials can demonstrate metallic or semiconducting behavior. [8] As has been shown in our recent paper, [9] alternating metallic and semiconducting regions within carbon nanomaterials produces high levels of cold-electron field emission (FE) from nanostructured carbon materials. Therefore, there is a strong need to find an industrially attractive pathway to synthesize carbon nanoparticles with this structure. It is worth noting that although a variety of methods have been developed to produce carbon nanomaterials, these methods generate only a raw product requiring either further purification (e.g., after laser ablation or arc discharge) or the removal of catalysts (after chemical vapor deposition). Herein, we report a new synthesis pathway for generating onionlike shell-shaped carbon nanoparticles (SCNPs) which is a one-step process. Our SCNPs are highly crystalline and not mixed with other types of carbon, and therefore do not need further purification. Furthermore, our method does not use any catalysts. We show that a transparent acetylene flow can produce onion-like SCNPs (which have continuous bent GLs) in bulk quantities when it is exposed to external irradiation from a continuous-wave (CW) infrared CO 2 laser. The striking feature of this process is that it is launched only above a critical threshold of laser irradiance. Below the irradiance threshold, the flame generates carbon soot. It is this critical threshold phenomenon that distinguishes our work from any other work in the field of flame-formed carbon nanoparticles. At the same time, we demonstrate that the formation of hollow SCNPs in our experiment has nothing to do with soot restructuring due to the ordering of basic structural units (BSUs), which has been recently reported in a hydrocarbon flame under pulsed irradiation from a Nd:YAG (YAG: yttrium aluminum garnet) laser.[10] The generation of SCNPs is governed by the direct growth of graphene sheets from precipitating acetylene molecules. The demonstration of the possibility of this latter process should be a significant contribution to the field of flame-formed carbon nanoparticles. We also show that our SCNPs exhibit FE performance comparable to that of carbon nanotubes. Experiments have been carried out with the setup used successfully in our previous work.[11±15] The multi-nozzle-type burner allowed us to supply gases through different annular coaxia...

The influence of particle shape on filtration processes was investigated. Two types of particles, including spherical polystyrene latex (PSL) and iron oxide, and perfect cubes of magnesium oxide, were examined. It was found that the removal efficiency of spherical particles on fibrous filters is very similar for corresponding sizes within the range of 50-300 nm, regardless of the fact that the densities of PSL and iron oxide differ by a factor of five. On the other hand, the removal efficiency of magnesium oxide cubic particles was measured, and found to be much lower than the removal efficiency for the aerodynamically similar spheres. Such disparity was ascribed to the different nature of the motion of the spherical and cubic particles along the fiber surface, following the initial collision. After touching the fiber surface and before coming to rest, the spherical particles could either slide or roll compared to the cubic ones, which could either slide or tumble. During tumbling, the area of contact between the particle and the fiber changes significantly, thus affecting the bounce probability, whilst for the spheres, the area of contact remains the same for any point of the particle trajectory. The extra probability of particle bounce by the cubes was derived from the experimental data. The particle kinetic energy was proposed to be responsible for the difference in removal efficiency of particles with alternative shapes, if all other process parameters remain the same. The increase in kinetic energy is shown to favor the increase of the bounce probability.

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The light absorption of silica nanoparticles generated in a flame has been measured in visible light. It was revealed that the frequency dependence of the light absorption coefficient is nonexponential. Its absolute value in the forbidden band was found to be unusually high. This behavior of light absorption is ascribed to a narrowing of the energy gap due to a high concentration of structural defects that appear during particle generation and growth at high temperatures.

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