Effect of particle type on the mechanisms of break up of nanoscale particle clusters (original) (raw)

Break‐Up of Nanoparticle Agglomerates by Hydrodynamically Limited Processes

Journal of Dispersion Science and Technology, 2008

When dry nano-particulate powders are first added into a liquid, clusters as large as hundreds of microns can be formed. In this study, high shear impellers, such as the sawtooth Ekatomizer and rotor-stator impellers were used to suspend and break-up these agglomerates in a stirred vessel. The high local energy dissipation rates generated by these impeller could slowly break up clusters to sub-micron sizes by an erosional mechanism. In comparison, single and multiple passes through a valve homogeniser could quickly break the nano-particle clusters to sub-micron sizes; single pass operation had the highest breakage efficiency for a given specific energy input. For both equipment types, the rate of fines generation was found to be controlled by the maximum energy dissipation rate. However, the size of the fine aggregates produced was a constant and was not a function of the energy dissipation rate.

Dispersion of clusters of nanoscale silica particles using batch rotor-stators

Advanced Powder Technology, 2017

Nanoparticle powders added into a liquid medium form structures which are much larger than the primary particle size (aggregates and agglomerates)-typically of the order of 10's of microns. An important process step is therefore the deagglomeration of these clusters to achieve as fine a dispersion as possible. This paper reports the findings of a study on the dispersion of hydrophilic fumed silica nanoparticle clusters, Aerosil 200V, in water using two batch rotor-stators: MICCRA D-9 and VMI. The MICCRA D-9 head consists of a set of teeth for the stator and another for the rotor, whereas the VMI has a stator with slots and a rotor which consists of a 4-bladed impeller attached to an outer set of teeth. The dispersion process, studied at different power input values and over a range of concentrations (1, 5, 10 wt.%), was monitored through the evolution of PSD. Erosion was found to be the dominant breakage mechanism irrespective of operating conditions or rotor-stator type. The smallest attainable size was also found to be independent of the power input or the design of the rotor-stator. Break up kinetics increased upon the increase of power input, and this also depended on the rotor-stator design. With MICCRA D-9 which has smaller openings on both the stator and rotor, the break up rate was faster. Increasing the particle concentration decreased break up kinetics. It could also be shown that operating at high concentrations can still be beneficial as the break up rate is higher when assessed on the basis of specific power input per mass of solids.

Breakup of nanoparticle clusters using Microfluidizer M110-P

Chemical Engineering Research and Design, 2018

A commercial design, bench scale microfluidic processor, Microfluidics M110-P, was used to study the deagglomeration of clusters of nanosized silica particles. Breakup kinetics, mechanisms and the smallest attainable size were determined over a range of particle concentrations of up to 17% wt. in water and liquid viscosities of up to 0.09 Pa s at 1% wt. particle concentration. The device was found to be effective in achieving complete breakup of agglomerates into submicron size aggregates of around 150 nm over the range covered. A single pass was sufficient to achieve this at a low particle concentration and liquid viscosity. As the particle concentration or continuous phase viscosity was increased, either a higher number of passes or a higher power input (for the same number of passes) was required to obtain a dispersion with a size distribution in the submicron range. Breakup took place through erosion resulting in a dispersion of a given mean diameter range regardless of the operating condition. This is in line with results obtained using rotor-stators. Breakup kinetics compared on the basis of energy density indicated that whilst Microfluidizer M110-P and an in-line rotor-stator equipped with the emulsor screen are of similar performance at a viscosity of 0.01 Pa s, fines volume fraction achieved with the Microfluidizer was much higher at a viscosity of 0.09 Pa s.

Break up of nano-particle clusters in high-shear devices

Chemical Engineering and Processing: Process Intensification, 2007

Formulation of stable nano-suspensions by breaking up nano-particle clusters is considered. Two devices of practical importance are investigated: the Silverson 150/250MS rotor-stator mixer and the high-pressure nozzle disintegrator. The main part of the work is related to model formulation and simulation of the processes of disintegration of Aerosil 200 V agglomerates in both systems. The population balance modelling is applied to account for effects of breakage and restructuring of aggregates on their size distribution. Effects of resulting structure of aggregated suspension on its rheology and details of the flow are simulated as well. Effects of the flow on creation of local stresses include hydrodynamic stresses and stresses generated by cavitation. Population balances are solved using the QMOM that is linked to the CFD code FLUENT. Results of numerical simulations show that the high-pressure system is more efficient than the rotor-stator device; one pass through the high-pressure system gives better disintegration than several passes through the rotor-stator. This may result from the fact that in the high-pressure system disintegration results from both: hydrodynamic stresses and effects of cavitation, whereas, in the case of the rotor-stator mixer only hydrodynamic stresses are active. Simulations are based on models validated earlier by comparison with experimental data.

Dispersion of Nano-Particle Clusters Using Mixed Flow and High Shear Impellers in Stirred Tanks

Chemical Engineering Research and Design, 2007

When dry nano-particulate powders are first added into a liquid, clusters as large as hundreds of microns can be formed. These tend to float at the liquid surface, either due to their poor wettability, or because air is trapped inside. Many formulation processes require suspension of these particle clusters in the liquid, followed by breakage or de-agglomeration, ideally down to the primary nano-particle size. Stirred tanks are often used for this wetting and suspension stage and for the generation of a coarse dispersion. In this study, a variety of impeller types, with different power and flow characteristics, were used to suspend and break-up these agglomerates in a stirred vessel. Initially, the operating window of rotational speeds for each impeller type was determined from (1) the minimum speed for off-bottom suspension of nano-particle agglomerates and (2) the maximum speed, above which surface aeration occurred. The effects of different impeller types on the rate of particle draw-down and wetting was also investigated and subsequently, the break up process was studied. Impellers characterized by a high local energy dissipation rate, such as the Ekatomizer sawtooth impeller, or the in-tank rotor-stator, could break up nano-particle clusters to submicron sizes by an erosional mechanism. The rate of fines generation within the de-agglomeration process was found to be controlled by the maximum energy dissipation rates produced by the mixing device. However, the size of the fine aggregates produced was a constant and not a function of the energy dissipation rate.

Determination of Agglomeration Kinetics in Nanoparticle Dispersions

Industrial & Engineering …, 2011

The direct application of nanoparticles as nonsupported adsorbents and catalysts is of high interest since they offer high surface areas with reduced mass transfer limitations. However, the natural tendency of these materials to aggregate, even faster when at high temperatures, makes the agglomeration process an important phenomenon to be studied, understood and, eventually controlled. A method to obtain the kinetics of nanoparticle agglomeration processes is presented in this paper. This analysis was based on the change of particle diameter during aggregation. The kinetic expression was validated with a series of experiments where the growth of Fe 2 O 3 nanoparticles immersed in base oil was followed at different times, temperatures, and particle concentrations. Results revealed the nature of the particle agglomeration process in the ranges of the experimental conditions; they indicated that physical adhesion, more than chemical binding, is the determining mechanism for agglomeration of Fe 2 O 3 nanoparticles immersed in base oil.

De-agglomeration of nanoparticles in a jet impactor-assisted fluidized bed

Powder Technology, 2017

Nanoparticles in agglomerated state lose their outstanding properties; hence, it is essential to break them up prior to use and prevent the re-agglomeration. Even though there are several dry techniques to disperse nanopowders, none of them have been able to produce truly nanoscale aerosols so far. Here, we study deagglomeration of dry silica nanopowder via a Jet Impactor-assisted Fluidized Bed (JIAFB). The particle size distribution of fragmented powders was characterized by in-line Scanning Mobility Particle Spectrometry (SMPS) and offline Transmission Electron Microscopy (TEM). In order to ascertain the jet length and that the kinetic energy of particles is sufficient for de-agglomeration, a CFD simulation was carried out. Both SMPS and TEM measurements imply that at certain fluidization velocity, increasing the jet velocity shifts the particle size distribution towards smaller diameters, and at higher velocities the mode value reduced from 113-130 to 55-60 nm. However, the geometric standard deviation or degree of polydispersity rises from 1.5 to 2.0 by increasing the jet velocity up to 197 m/s, as it will increase the total superficial velocity and consequently entrainment of larger particles from the bed. In addition, the TEM results indicate that the range of individual particle size in supplied nanopowder is wide; hence, increasing the geometric standard deviation can be an indicator of higher level of agglomerate dispersion.

Impact fragmentation of nanoparticle agglomerates

Journal of Aerosol Science, 2003

A method of fragmenting nanoparticle agglomerates by impaction from the aerosol phase in a single-stage impactor is described. The degree of fragmentation as a function of impaction velocity is determined by TEM image analysis of impacted agglomerates. Images of unfragmented particles are obtained by di usional deposition in the same apparatus but operated in a di erent way. As an illustration, the method was applied to aerosols of silver, nickel and titanium dioxide agglomerates with primary particle diameters of between 3 and 8:3 nm (Ag), 4 nm (Ni) and 95 nm (TiO 2), respectively, for which we determined the in uence of impact energy and primary particle size on fragmentation. For silver, the degree of fragmentation at a given impact energy decreased with primary particle size; at 3 nm hardly any fragmentation was observed. At a given primary particle size, however, the range of energies required for complete fragmentation was found to be relatively narrow, indicating a fairly uniform bond structure. These trends are re ected by a model that was devised to calculate bond strength distributions between polydisperse primary particles assuming there are only van-der-Waals interactions. Some measurements were also carried out to investigate potential e ects of impact energy on primary particle size.

Cavitation Disintegration of Microparticles and Nanoparticles in Dense Liquid Dispersions

With the current rapid development of nanotechnology, top-down preparation of nanoparticles by various disintegration techniques became an important industrial sector. In addition to the presently common high speed bead mills working with particles in liquid dispersions, jet mills are also frequently used. They operate in the dry disintegration mode of extreme aerosol flow with cyclonic separation of the fine fraction. The common characteristic of these both mill types is the submicron size of disintegrated output particles. Very promising alternative to these techniques is the use of the disintegrating effect of cavitation implosion. In this work, we present experimental results and technical experience with a new experimental device for cavitation disintegration of microparticles and nanoparticles in dense liquid dispersions. In first test experiments, the mean size of the primary silicon particles of 140 nm was reduced to 40 nm by the cavitation disintegration lasting 200 minutes.