Dimension Of Agglomerates Research Articles

Impact of nanoparticle agglomeration on thermal conductivity of molten salt based nanofluids: Insights from molecular dynamics and lattice Boltzmann methods

The nanoparticle aggregation has significant influence on heat transfer properties. The contradictions between different researches exist and the underlying mechanisms of heat conduction remain unclear. To address these issues, the thermal conductivity of solar salt-SiO2 nanofluids in the non-aggregated and aggregated states was investigated, based on the combination of the advantages of molecular dynamics (MD) in revealing physical insights and lattice Boltzmann method (LBM) in simulating properties for complex structures. LBM models were simplified on the basic of MD results. Different heat transfer physics in nanofluids at nanoscale and mesoscale were compared. The results show that the thermal conductivity of aggregated nanofluid is higher than the non-aggregated nanofluid. By analyzing the diffusion coefficient, number density and vibrational density of states, we found that the previous hypotheses (such as Brownian motion, interfacial layer and interfacial thermal resistance) cannot explain the enhanced thermal conductivity induced by the nanoparticle agglomeration. The heat transfer mechanisms of aggregated nanofluid were further analyzed from new perspectives, i.e., the contributions of different material components and fluctuation modes to heat flux. It is found that the nanoparticle aggregation leads to the nanoparticle contribution increase, indicating the formation of low thermal resistance channels within the aggregates. The heat conduction contributed by collision decreases in the aggregated state, while that contributed by potential energy increases. Moreover, the thermal conductivity of aggregated nanofluid is negatively correlated with temperature, aggregate size and fractal dimension of agglomerates, and positively correlated with mass fraction of nanoparticles, degree of agglomeration, and backbone length of each aggregate.

Langevin dynamics simulation and collision frequency modification in population balance model of nanoparticle coagulation during simultaneous agglomeration and sintering

Coagulation of nanoparticles from the free-molecular to the transition regime under a high-temperature environment is investigated by coupling Langevin Dynamics (LD) model with agglomeration and finite-rate sintering. The effect of temperature on the evolution of particle size and structure is investigated, variations in agglomeration-sintering dynamics are obtained. The fractal dimension of agglomerates at different temperatures can be expressed as a function of the ratio between the characteristic collision time and sintering time. The overall collision frequency (β) is enhanced by the polydispersity and fractal properties of agglomerates compared to the monodisperse collision kernel functions. Finally, the classic function of β is modified based on the LD simulations and used in a one-dimensional population balance equation, which reproduces a better prediction of the evolution of particle size distribution during particle coagulation with agglomeration and sintering.

Three-dimension soft X-ray tomographic reconstruction of particulates emitted from a diesel engine

The diesel particulate matter (PM) is typically agglomerates of soot coated with condensed organic compounds. Three-dimensional (3D) measurement of PM particle structures is currently not undertaken as established techniques usually damage the structure of PM particles by dissociating or evaporating the condensed organic compounds absorbed on the soot. In this work, PM particles emitted from a diesel engine were sampled by a micro-orifice uniform deposit impactor and their soot content was measured by a thermo-gravimetric analyzer. The morphology and microstructure of individual PM particle were non-destructively studied using synchrotron soft X-ray tomography (SXT). The 3D tomographic reconstruction from the aligned tilt-series images yielded useful information about PM volumetric characters. Fractal-like agglomerate of soot and the condensed organic compounds were identified. The results showed that the agglomerates are constituted by loosely attached aggregates which are composed of spherules holding tightly to each other. Aggregate size distributions exhibited no specific trend with respect to the engine load and PM size in this work, while soot volume fractions calculated based on the 3D images of PM particles, decreased with increasing load and PM size. The fractal dimensions of agglomerates and PM particles were for the first time presented and calculated by image analysis. The essential properties of PM particles, agglomerates and aggregates are characterized. The population-averaged fractal dimension of agglomerates was smaller and more sensitive to engine load and PM size than that of aggregates, indicating that agglomerates was less compact than aggregates. This work demonstrated the significant advantages of SXT in the description of actual PM morphologies with high accuracy.

Study on the fractal structure of carbon nanomaterials’ smokescreen

In the diffusion of carbon nanomaterials’ smokescreen, the condensates generated by the collision of particles have fractal structures. In order to research this process, this paper is based on DLA condensation model programmed by MATLAB. Using DLA model, the geometrical structure of agglomerate formed by spherical nanoparticles is simulated and the fractal dimensions of agglomerates were computed by using the cyclotron radius method. Large numbers of simulations indicated the fractal dimension of agglomerates on two-dimensional plane distributed between 1.4 and 1.7. And with the increasing number of the particles, the fractal dimension displayed in intensive range. The research in this paper has laid a foundation for the study of the aerodynamic characteristics of condensed particles in the next stage.

A Guide through the Dental Dimethacrylate Polymer Network Structural Characterization and Interpretation of Physico-Mechanical Properties.

Material characterization by the determination of relationships between structure and properties at different scales is essential for contemporary material engineering. This review article provides a summary of such studies on dimethacrylate polymer networks. These polymers serve as photocuring organic matrices in the composite dental restorative materials. The polymer network structure was discussed from the perspective of the following three aspects: the chemical structure, molecular structure (characterized by the degree of conversion and crosslink density (chemical as well as physical)), and supramolecular structure (characterized by the microgel agglomerate dimensions). Instrumental techniques and methodologies currently used for the determination of particular structural parameters were summarized. The influence of those parameters as well as the role of hydrogen bonding on basic mechanical properties of dimethacrylate polymer networks were finally demonstrated. Mechanical strength, modulus of elasticity, hardness, and impact resistance were discussed. The issue of the relationship between chemical structure and water sorption was also addressed.

Tattoo pigment agglomerates measured in commercial ink stock products by computerised light microscopy.

The incidence of clinical complications such as granuloma formation and sarcoidosis is often seen in black tattoos and may be associated with agglomeration of black pigment. To measure count and dimensions of agglomerates in black tattoo inks vs red inks and to compare old inks and new inks of identical brands. Examination was performed by light microscopy (Olympus BX51™ ) with magnification 40X, immersion oil. Photographs (Jenoptik Gryphax RGB camera) were taken of each ink sample and analysed by ImageJ software; count, area, width, height, circumference and circularity index were measured. Agglomerates were defined as width and height of objects above 800nm. Twenty-one new unopened black inks and 17 new unopened red inks were compared. Furthermore, five old black inks and five old red inks, that had been opened and stocked for over 2years, were compared with new products of the same brands. Black agglomerates were area wise and with respect to width, height and circumference significantly larger compared with red agglomerates and more circularly shaped. Count of agglomerates was lower in black inks than in red inks, in accordance with bigger dimensions of black agglomerates. Comparison of old and new inks indicated old inks have larger agglomerates but variable bottle size and storage conditions may have confounded results. Pigment agglomerates in black tattoo ink stock products were sized larger than agglomerates in red inks. Agglomerates found directly in black inks may predispose to granuloma formation in black tattoos causing sarcoid reaction.

An improved model for determining fractal structure of nano‐agglomerates

AbstractEffects of operating conditions on fractal structures of three nanoparticle agglomerates (SiO2, TiO2, and ZnO) in a vibro‐fluidized bed were investigated. An improved model is proposed by combining the fractal relation, Richardson‐Zaki equation, and mass balance. This model was used to predict substantial properties of nanoparticle agglomerates, such as fractal dimension and size of the agglomerate particulate fluidization. It was shown that with increasing vibration intensity, the fractal dimension of agglomerates decreases slightly, while the number of primary particles in the agglomerate decreases significantly. It was found that the fractal dimension of nanoparticle agglomerates is in the range of 2.63–2.78, and the number of primary particles in the agglomerate is in the order of 1010. Agglomerate size reaches a constant value at high vibration frequency. Calculation results indicated that the vibration frequency has a more important role than its amplitude in reducing agglomerate size. Minimum fluidization velocity was calculated by applying estimated agglomerate sizes, and it was found that the results were in close agreement with the experimental data.

Tracking the progression of dispersion of graphite nanoplates in a polypropylene matrix by melt mixing

This work reports the dispersion of graphite nanoplates (GnP) in polypropylene by melt mixing using a small‐scale continuous prototype mixer. The focus is to study the progression of the dispersion of GnP, a process that is not well understood. Composites were prepared with 2 and 4 wt% of GnP, as‐produced and functionalized by grafting polypropylene modified with maleic anhydride to the GnP surface. The processability of the composites was similar to that of polypropylene, as assessed by capillary rheometry. The nanoparticle dispersion was analyzed by optical microscopy. The progression of the number of agglomerates and their size, for pristine and functionalized nanoparticles, was observed along the mixer. The final agglomerate dimensions were found to be similar for pristine and functionalized GnP, and indicated that agglomerate dispersion progressed toward individual nanoparticles. The interface improvement of functionalized GnP with polypropylene was observed by scanning electron microscopy. POLYM. COMPOS., 38:947–954, 2017. © 2015 Society of Plastics Engineers

Effect of sintering on the fractal prefactor of agglomerates

In a previous study different agglomerates composed by a variable number of primary particles corresponding to extreme and intermediate values of fractal dimension (Df=1, 2 and 3) were characterized to be used as reference shapes in a method for the determination of the fractal dimension of actual agglomerates. In that work, primary particles were assumed to be spherical and with no sintering or necking effects. In this work, the geometrical effect of processes such as coalescence, sintering, flattening, or surface growth is considered to evaluate the impact of these phenomena in the morphological parameters of fractal-like agglomerates. A sintering coefficient is firstly defined. Afterwards, the moment of inertia and radius of gyration of the agglomerate are determined and finally the prefactor of the power–law relationship is obtained as a function of the number of primary particles and the sintering coefficient. The equations proposed are used in a geometrical method to determine the fractal dimension of soot agglomerates, showing that sintering increases the prefactor significantly, which leads to a slight decrease in the fractal dimension of the agglomerates.

A polycaprolactone‐based compatibilization treatment to improve dispersion and interphase structure of silica polyurethane composites

Silica nanoparticles (SNs) were grafted with ε‐caprolactone using an environmentally friendly approach. By using tartaric acid as a catalyst and the silanol groups as initiators, grafted nanoparticles (GNs) with organic weight fractions (wof) within the range 0–46 wt% were synthesized. Thermogravimetric (TGA) and infrared analysis were used to measure the wof and to corroborate the covalent bond between the SN and the caprolactone monomer. Transmission electron micrographs of the polyurethane (PU) nanocomposites based on the SN and the GN revealed that the interfacial area of the GN‐based PU increased by the reduction of agglomerate dimensions from 10 µm to around 0.1 µm. Dynamic mechanical analysis showed that the GN nanocomposites improved the storage shear modulus from 616±11 to 849±8 MPa for a GN with wof = 16.7% and 3 wt% filler concentration. In addition, the GN particles prevented a relevant decrease of the transition temperature (Tg). Differential scanning calorimetry corroborated that GN increased the enthalpic energy associated to the physical crosslinking of the hard segments (HS). Wide‐angle X‐ray diffraction proved that the GN formed a HS structure with improved crystallinity. The thermal stability of the GN‐based PU a nanocomposite was improved by an increase of the thermal stability of the castor oil soft segments. POLYM. ENG. SCI., 54:1817–1826, 2014. © 2013 Society of Plastics Engineers

Electrochemical growth of ZnO nanowires on atomic layer deposition coated polystyrene sphere templates

We report on the electrochemical growth of n-type ZnO NWs on thin films of atomic layer deposited ZnO on polystyrene spheres. The development of crystal agglomerates during ALD is shown to control the diameter of nanowires obtained by electrochemical deposition. Increasing the thickness of the ALD coating from 20nm to 100nm increases the crystallite size from about 10nm to 22nm on the polystyrene spheres as well as the maximum agglomerate size from 80nm to 190nm of the coatings. Subsequent electrochemical deposition with identical parameters showed that the resulting ZnO nanowire diameter is almost 1:1 related to the agglomerate dimensions of the ALD coating. This effect offers another degree of freedom in synthesizing transparent, high surface, tailored urchin-inspired building blocks having applications in solar cells and low wetting coatings.

Formation of ZnS nano- and microparticles from thiourea solutions

Effects of temperature, concentration of thiourea and reaction conditions (thermal heating and microwave irradiation) on morphological properties of ZnS particles obtained by sedimentation from 0.01, 0.1 and 1М zinc nitrate solutions are studied. It is found that ZnS particles of two shapes are formed from thiourea solutions at a thermal heating (70–90°С): agglomerated particles of the spherical shape with the dimensions of agglomerates 50–100nm (particle size in agglomerates of 3–5nm) and hexagonal columns in length up to 2μm and diameter of 80–100nm. At the microwave heating conditions (90°С), irrespective of thiourea concentration formation of spherical ZnS particles with the size 0.6–1.2μm is observed.

New nanostructural biomaterials based on active silicate systems and hydroxyapatite: characterization and genotoxicity in human peripheral blood lymphocytes

To characterize and investigate the genotoxic effect of a new endodontic cement based on dicalcium- and tricalcium-silicate (CS) with hydroxyapatite (HA) on human lymphocytes. Hydrothermal treatment was applied for synthesis of CS and HA. The final mixture HA-CS, with potential to be used in endodontic practice, is composed of CS (34%) and HA (66%). Human lymphocytes were incubated with HA, HA-CS and CS for 1 h, at 37 °C and 5% CO2. Cell viability was determined using the trypan blue exclusion assay. To evaluate the level of DNA damage comet assay (single cell gel electrophoresis) was performed. For the statistical analysis anova and Duncan's Post Hoc Test were used. The SEM analysis indicated that CS consisted mostly of agglomerates of several micrometers in size, built up from smaller particles, with dimensions between 117 and 477 nm. This is promising because dimensions of agglomerates are not comparable with channels inside the cell membranes, whereas their nano-elements provide evident activity, important for faster setting of these mixtures compared to MTA. Values of DNA damage obtained in the comet assay indicated low genotoxic risk of the new endodontic materials. The significantly improved setting characteristics and low genotoxic risk of the new material support further research.

Coagulation Coefficient of Agglomerates with Different Fractal Dimensions

A coagulation coefficient of agglomerates with different fractal dimensions has not been considered in the past, even though there is a possibility of variations in the fractal dimension of agglomerates at any instant. In this study, a Brownian dynamics simulation was performed with simultaneous collision and sintering, and variations in the fractal dimension of agglomerates were observed. A coagulation coefficient expression for agglomerates with two different fractal dimensions was proposed. The coagulation coefficient based on the different fractal dimensions was at most 140% higher than that based on the average fractal dimension. To determine an accurate coagulation coefficient of agglomerates, the fractal dimension of each agglomerate has to be considered.

Challenges in Exposure Modeling of Nanoparticles in Aquatic Environments

ABSTRACT Managing the potential environmental risks of nanoparticles requires methods to link nanoparticle properties with macro-scale risks. This study outlines challenges in exposure modeling of nanoparticles in aquatic environments, such as the role of natural organic matter, natural colloids, fractal dimensions of agglomerates, coatings and doping of particles, and uncertainties regarding nanoparticle emissions to aquatic environments. The pros and cons of the exposure indicators mass concentration, particle number concentration, and surface area are discussed. By applying colloid chemistry kinetic equations describing particle agglomeration and sedimentation for the case of titanium dioxide nanoparticles, a limited exposure assessment including some of the factors mentioned is conducted with particle number concentration as the exposure indicator. The results of the modeling indicate that sedimentation, shear flows, and settling are of less importance with regard to particle number based predicted environmental concentrations. The inflow of nanoparticles to the water compartment had a significant impact in the model, and the collision efficiency (which is affected by natural organic matter) was shown to greatly affect model output. Implications for exposure modeling, regulation, and science are discussed. A broad spectrum of scientific disciplines must be engaged in the development of exposure models where nano-level properties are linked to macro-scale risk.

A TEM-based method as an alternative to the BET method for measuring off-line the specific surface area of nanoaerosols

At the present time, no stabilised method exists allowing an estimation of the specific surface area for airborne nanostructured particles (nanoaerosols). Recent toxicological studies have, however, revealed biological effects linked to the surface area of these particles. Only the BET method, which can determine the specific mass surface area of a powder, constitutes a reference both in toxicology and in the materials domain. However, this technique is not applicable to nanostructured aerosols given the mass quantities of particles required (between approximately some mg to hundreds of mg taking into account the limit of quantification of existing BET instruments). To characterise the specific surface area of airborne nanostructured particles, a method based on analysing transmission electron microscopy (TEM) images is proposed. This has recourse in particular to previous work carried out in the area of nanoparticles originating from combustion (soot), and takes into account structural parameters of nanostructured particles including the number distribution of primary particles, their overlap coefficient and the fractal dimension of agglomerates and aggregates. The approach proposed in this work was applied to five commercially-available nanostructured powders of differing natures (SiO 2, ZrO 2, Al 2O 3, Fe 2O 3 and Fe 3O 4). This first involved their prior analysis by the BET method and then being placed in suspension in aerosol form using a vortex-type shaker system. The procedure to calculate the specific surface area using image analysis was then applied to the sampled aerosols and compared to the BET measurements. The experimental results obtained on the five nanostructured powders cover a range of specific surface areas from 20 to 200 m 2/g, the primary particles having mean diameters varying from 7 to 47 nm. Close agreement was observed between the two approaches which, taking into account measurement uncertainties, are statistically equivalent at significance level α = 0.05.

Nanocrystalline Ni coatings strengthened with ultrafine particles

In this study, nanocrystalline Ni powders and thermally sprayed coatings, containing ultrafine AlN particles, were synthesized and characterized. The results indicated that the presence of AlN particles in the powders drastically decreased the dimension of agglomerates formed by cryomilling and increased the surface roughness of the agglomerates. The AlN phase was broken down into ultrafine particles of approximately 30 nm in size. These particles were dispersed in the Ni matrix and enhanced the development of a nanocrystalline structure in the Ni matrix during cryomilling. Selected-area diffraction patterns, obtained from transmission electron microscopy (TEM) and X-ray mapping with scanning electron microscopy (SEM), confirmed the presence of AlN particles in the coatings. The presence of AlN particles also led to an increase in the amount of NiO phase that was distributed in the coating, in the form of ultrafine, round particles. AlN particles increased the microhardness of the Ni coating by approximately 60 pct. Indentation-fracture results also indicated that the fine, dispersed AlN particles raised the apparent toughness of the Ni coating. The synthesized Ni coatings containing ultrafine AlN particles were characterized as equiaxed nanocrystalline grains with an average size of 24 nm, in which twins were observed. The increase in microhardness resulted from both grain refinement and the presence of ultrafine particles. The latter played the primary role in strengthening.

Morphology of soot collected in microgravity droplet flames

Measurements of the primary particle size, radius of gyration, fractal dimension, and the mass fractal prefactor were measured for soot sampled in n-heptane droplet flames under microgravity conditions. These represent the first such measurements obtained for spherically symmetric droplet flames. Experiments were performed in the NASA-Glenn Research Center in Cleveland, OH. Soot was sampled at 0.2 and 0.5 s after ignition for a 2.1 mm droplet burning in atmospheric pressure air. The measured primary particle sizes were significantly larger under microgravity conditions compared to normal gravity and were found to increase with residence time. The average radius of gyrations were also larger for soot collected in microgravity conditions. Differences in the particle and agglomerate dimensions are believed to be caused by the longer residence times. The fractal dimensions were found to be nearly constant for all experiments while the prefactor term was found to increase slightly with residence time.

Percolation in carbon black filled epoxy resin

AbstractThe conditions to form a conductive network in carbon black filled epoxy resin were examined. It was found that the formation is controlled by particle‐particle interaction which can be influenced by shear forces or by the ionic concentration of the resin. Therefore the network morphology and thus the percolation threshold depends strongly upon the processing route. On best conditions the percolation threshold was reduced to 0.06 vol.‐%. That a continuous network can form with such a low volume fraction can be explained by the fractal dimension of agglomerates the network is made of.

In situ measurement of size and density of submicron aerosol particles

The density of nanometer sized particles of different materials (Pd, Ag, Co, Carbon, oil droplets and oil burner exhaust) suspended in a carrier gas is determined in situ as function of particle radius via the measurement of particle mobility and inertia using a differential mobility analyzer and a low pressure impactor. From the density and size of agglomerated particles, information on the structure of the agglomerates can be obtained; in particular a fractal-like dimension can be determined. Whereas the densities of carbon particles and of particles in oil burner exhaust have the expected values, the densities of the metal particles are too low compared to the bulk values. The results for the fractal-like dimensions of the agglomerates can be explained by assuming a mixture between monomer-cluster and cluster-cluster agglomeration in the free-molecular regime. The fractal-like dimension of ferromagnetic Cobalt agglomerates provides evidence that particles consisting of magnetic materials tend to form chainlike agglomerates.