F. Einar Kruis

Reducing Statistical Noise and Extending the Size Spectrum by Applying Weighted Simulation Particles in Monte Carlo Simulation of Coagulation

The direct simulation Monte Carlo (DSMC) method is widely utilized to simulate microscopic dynamic processes in dispersed systems that give rise to the population balance equation. In conventional DSMC approaches, simulation particles are equally weighted, even for broad size distributions where number concentrations in different size intervals are significantly different. The resulting statistical noise and limited size spectrum severely restrict the application of these DSMC methods. This study proposes a new Monte Carlo (MC) method, the differentially weighted time-driven method, which captures the coagulation dynamics in dispersed systems with low noise and is simultaneously able to track the size distribution over the full size range. Key elements of this method include constructing a new jump Markov process based on a new coagulation rule for two differentially weighted simulation particles, and restricting the number of simulation particles in each size interval within prescribed bounds. The method is validated by using an ideal coagulation kernel with a known analytical solution and a real coagulation kernel for which an accurate solution can be found numerically (self-preserving particle size distribution in the continuum regime).

Influence of Ag particle size on ethanol sensing of SnO1.8:Ag nanoparticle films: A method to develop parts per billion level gas sensors

The influence of Ag particle size on ethanol sensing of SnO1.8:Ag films composed of size-selected nanoparticles with independently controlled size and concentration of Ag is reported in the present study. The study shows that Ag nanoparticles are acting as catalyst for chemical sensitization through a spillover effect. The catalyst particles are observed to be more active on decreasing their size, resulting into an improved sensor response. A response time of 2s for 1000ppm ethanol has been achieved. Detection of 100ppb ethanol in air has been demonstrated using this well-defined technique.

Preparation of size-classified PbS nanoparticles in the gas phase

We report on the synthesis of size-classified PbS nanocrystals by differential mobility analysis of a polydisperse aerosol formed by nucleation and aggregation processes in a furnace reactor. The sublimation temperature employed is below the stoichiometric evaporation temperature, thus enabling direct and simple synthesis by sublimation. The irregularly shaped and amorphous agglomerate particles are then sintered in a second furnace, resulting in the formation of monocrystalline and quasispherical particles with sizes adjustable between 3 and 20 nm and a standard deviation of 1.13.

A differentially weighted Monte Carlo method for two-component coagulation

The direct simulation Monte Carlo (DSMC) method for population balance modeling is capable of retaining the history of each simulation particle and is thus able to deal with multivariate properties in a simple and straightforward manner. As opposed to conventional DSMC approaches that track equally weighted simulation particles, a differentially weighted Monte Carlo method is extended to simulate two-component coagulation processes and is thereby able to simulate the micromixing of the components. A new feature of the method for this bivariate population balance modeling is that it is possible to specify how the simulation particles are distributed over the compositional axis. This allows us to obtain information about particles in those regions of the size and composition distribution functions where the non-weighted MC methods place insufficient simulation particles to obtain an inaccurate solution. The new feature results in lower statistical noise for simulating two-component coagulation, which is validated by using two-component coagulation cases for which analytical solutions exist (a discrete process with sum kernel for initial monodisperse populations and a process with constant kernel for initial polydisperse populations).

Synthesis of tailored composite nanoparticles in the gas phase

We report on a method to obtain tailored nanoparticle aggregates of two components in the gas phase. The method is based on the modification of the Brownian collision rate by charging the nanoparticles. Particles of different components are charged oppositely in order to obtain composite nanoparticle aggregates via preferential coagulation. The resulting composite aggregates are uncharged, which allows for their separation from both, charged unaggregated particles and charged aggregates of only one component. The mean size and standard deviation of each particle component can be adjusted by means of differential mobility analysis. Experimental results are presented for composites of PbS and Ag.

Single Charging of Nanoparticles by UV Photoionization at High Flow Rates

The feasibility of UV photoionization for single unipolar charging of nanoparticles at flow rates up to 100 l· min −1 is demonstrated. The charging level of the aerosol particles can be varied by adjusting the intensity of the UV radiation. The suitability of a UV photocharger followed by a DMA to deliver monodisperse nanoparticles at high aerosol flow rates has been assessed experimentally in comparison to a radioactive bipolar charger ( 85 Kr, 10 mCi). Monodisperse aerosols with particle sizes in the range of 5 to 25 nm and number concentrations between 10 4 and 10 5 cm −3 have been obtained at flow rates up to 100 l· min −1 with the two aerosol chargers. In terms of output particle concentration, the UV photoionizer performs better than the radioactive ionizer with increasing aerosol flow rate. Aerosol charging in the UV photoionizer is described by means of a photoelectric charging model that relies on an empirical parameter and of a diffusion charging model based on the Fuchs theory. The UV photocharger behaved as a quasi-unipolar charger for polydisperse aerosols with particles sizes less than 30 nm and number concentrations ∼10 7 cm −3 . Much reduced diffusion charging was observed in the experiments, with respect to the calculations, likely due to ion losses onto the walls caused by unsteady electric fields in the irradiation region.

Vibrational and defect states in SnOx nanoparticles

We have studied SnOx nanoparticles fabricated by gas-phase condensation and in-flight sintering using Raman and photoluminescence (PL) spectroscopy. We are able to identify various vibrational states of the rutile phase of the SnOx crystal. By thorough analysis of the vibrational modes, we are able to determine the bond lengths of the O–O and Sn–O bonds for the substoichiometric SnO1.5, leading, together with x-ray diffraction data, to a full characterization of the SnO1.5 lattice. In absorption and photoluminescence spectra, we observe a finite density of states inside the band gap due to oxygen vacancies, giving rise to a midgap luminescence peak. Our results suggest that the defect related luminescence efficiency is limited by nonradiative recombination processes and by the oxygen vacancy density. We therefore conclude that the PL intensity has a maximum around a stoichiometry of SnO1.7.

Mixing selectivity in bicomponent, bipolar aggregation

Abstract The selectivity of aggregation in mixtures of two charged aerosols containing chemically dissimilar nanoparticles is studied by means of a newly developed direct simulation Monte Carlo method. This method allows to trace changes in complex multidimensional systems, in this case describing particle size, charge and aggregate composition. A new procedure was developed for estimating the effective collision diameter of an aggregate composed of primary particles of any size. Three model systems were studied: polydisperse aerosols with initially bipolar charge distribution, unipolarly charged polydisperse aerosols and quasi-monodisperse oppositely charged aerosols. The study is focused on the aggregate compositions dependence on the initial size and charge distribution. It was found that the use of bipolarly charged aerosols does not increase the selectivity of mixing whereas unipolarly, oppositely charged aerosols reach more rapidly a more homogeneous distribution of components within the aggregates. In the last case, the addition of one more elementary charge to the particles roughly doubles the fraction of bicomponent, 1 : 1 mixed nanoaggregates and accelerates the process.

Nanoparticle design and handling — challenges for engineers and particle technologists

In this paper, the opportunities and challenges which future applications based on nanoparticles offer to engineers are described. New high-added value products induce us to rethink ways to control nanoparticle design and handling. Several challenges are described and typical solutions are given: (1) finding relevant engineering tasks outside traditional process industry; (2) the importance of mixing reactants; (3) obtaining monodisperse particles; (4) gaining efficient control via electric forces and (5) developing multi-step processes which allow more control over particle properties.

Feasibility study of nanoparticle synthesis from powders of compounds with incongruent sublimation behavior by the evaporation/ condensation method

Abstract In this paper we investigate the feasibility of a fabrication route to produce nanocrystals of compound material with incongruent sublimation behavior via the simple evaporation of the powder of the compound. The generation of stoichiometric particles would only be possible if the particle formation occurs at temperatures below the incongruent sublimation point. Our experiments, done on three different III–V compounds, show that the simple evaporation of the powder of those materials to obtain stoichiometric particles is not possible. Particle formation does not start at temperatures below the incongruent sublimation point. Particles synthesized consisted not of the compound but almost entirely of the more volatile group-V element, leading to a change in the composition of the source material and thus to irreproducible behavior.