Michael J. Carrier

MOLECULAR DYNAMICS COMPUTATION OF GAS-PHASE NANOPARTICLE SINTERING: A COMPARISON WITH PHENOMENOLOGICAL MODELS

Abstract The mechanism and kinetics of the growth of silicon nanoparticles via particle–particle interactions has been investigated through the use of classical molecular dynamics (MD) trajectory calculations. Computations over a broad range of temperatures and particle sizes have shown that particle sintering is very dependent on size and temperature when solid-like, and considerably less sensitive when liquid-like. These atomistic computations have been used for the first time to validate previously postulated phenomenological mechanisms/models for both solid and liquid particle coalescence. The results have shown that solid-like particles sinter by a solid-state diffusion mechanism while liquid-like particles sinter by a viscous flow mechanism.

Particle shape effects on subvisible particle sizing measurements.

Particle analysis tools for the subvisible (<100 μm) size range, such as light obscuration, flow imaging (FI), and electrical sensing zone (ESZ), often produce results that do not agree with one another, despite their general agreement when characterizing polystyrene latex spheres of different sizes. To include the effect of shape in comparison studies, we have used the methods of photolithography to create rods and disks. Although the rods are highly monodisperse, the instruments produce broadened peaks and report mean size parameters that are different for different instruments. We have fabricated a microfluidic device that simultaneously performs ESZ and FI measurements on each particle to elucidate the causes of discrepancies and broadening. Alignment of the rods with flow causes an oversizing by FI and undersizing by ESZ. FI also oversizes rods because of the incorrect edge definition that results from diffraction and imperfect focus. We present an improved correction algorithm for this effect that reduces discrepancies for rod-shaped particles. Tumbling of particles is observed in the microfluidic ESZ/FI and results in particle oversizing and breadth of size distribution for the monodisperse rods.

Variable Threshold Method for Determining the Boundaries of Imaged Subvisible Particles

An accurate assessment of particle characteristics and concentrations in pharmaceutical products by flow imaging requires accurate particle sizing and morphological analysis. Analysis of images begins with the definition of particle boundaries. Commonly a single threshold defines the level for a pixel in the image to be included in the detection of particles, but depending on the threshold level, this results in either missing translucent particles or oversizing of less transparent particles due to the halos and gradients in intensity near the particle boundaries. We have developed an imaging analysis algorithm that sets the threshold for a particle based on the maximum gray value of the particle. We show that this results in tighter boundaries for particles with high contrast, while conserving the number of highly translucent particles detected. The method is implemented as a plugin for FIJI, an open-source image analysis software. The method is tested for calibration beads in water and glycerol/water solutions, a suspension of microfabricated rods, and stir-stressed aggregates made from IgG. The result is that appropriate thresholds are automatically set for solutions with a range of particle properties, and that improved boundaries will allow for more accurate sizing results and potentially improved particle classification studies.

Rapid evaporation at the superheat limit of methanol, ethanol, butanol and n-heptane on platinum films supported by low-stress SiN membranes

The bubble nucleation temperatures of several organic liquids (methanol, ethanol, butanol, n-heptane) on stress-minimized platinum (Pt) films supported by SiN membranes is examined by pulse-heating the membranes for times ranging from 1 µs to 10 µs. The results show that the nucleation temperatures increase as the heating rates of the Pt films increase. Measured nucleation temperatures approach predicted superheat limits for the smallest pulse times which correspond to heating rates over 108 K/s, while nucleation temperatures are significantly lower for the longest pulse times. The microheater membranes were found to be robust for millions of pulse cycles, which suggests their potential in applications for moving fluids on the microscale and for more fundamental studies of phase transitions of metastable liquids.

SIZE DEPENDENT PROPERTIES OF NANOSCALE PARTICLES (SILICA)

The properties of silica clusters at temperatures of 1500 to 2800 K have been investigated using classical molecular dynamics simulations for particles containing up to 1152 atoms. We found that the atoms in the cluster were arranged in a shell-like structure at low temperatures and that the radial density profile peaked near the outer edge of the particle. Smaller clusters have much higher pressures with the magnitudes corresponding quite well to those obtained from the Laplace-Young equation, when evaluated in a self-consistent manner using our derived surface tension. Our computed surface tension did not show any significant size dependent behavior in contrast to the decreasing surface tension observed for Lennard-Jones liquid clusters.

Nanoscale SiO2 Particles at High Temperatures: Size Dependent Properties

The properties of silica clusters at temperatures of 1500 to 2800 K have been investigated using classical molecular dynamics simulations for particles containing up to 1152 atoms. We found that the atoms in the cluster were arranged in a shell like structure at low temperatures and that the radial density profile peaked near the outer‐edge of the particle. Our computed surface tension did not show any significant size dependent behavior. Finally our computed diffusion coefficients in the liquid state are larger than bulk computed diffusivities. Smaller clusters have much higher pressures and lower the temperature of melting.