Colloidal Silica Dispersions Research Articles

Nanoparticle depressants in froth flotation – The effect of colloidal silica with different size and surface modifications on the selective separation of semi-soluble salt type minerals

Colloidal silica has been presented as a selective depressant in froth flotation separation of scheelite and calcite by our group in 2020. Here we analyze the effects of colloidal silica dosage, specific surface area (i.e. size), and modification with microflotation on pure scheelite, calcite, fluorite, and apatite minerals. In a reagents scale-up procedure batch flotation experiments were conducted on a low-grade scheelite ore using statistical experimental design methodology. Microflotation experiments showed colloidal silica selectively depressing calcite flotation with increasing intensity with reduced size of colloidal silica. The nanoparticulate depressant did not affect the recoveries of scheelite, apatite, and fluorite – typical ore minerals that occur associated with calcite. Batch flotation of a low-grade scheelite ore confirmed the observations made at the microscale: colloidal silica significantly improves the scheelite-calcite selectivity index. Here, we observe a series of relations between colloidal silica specific surface area and modification on different process indicators such as scheelite-calcite selectivity index, recoveries, and froth properties. These relations are assumed to be due to the different aggregation mechanisms of the colloidal silica dispersions. The aluminate-modified colloidal silica, which tends to form a coherent gel network in the presence of Ca2+, shows the strongest effect on reducing calcite recovery accompanied by a significant reduction in scheelite recovery compared to the non-functionalised and silane-modified colloidal silica.

Influence of cation concentration and valence on the structure and texture of spray-dried supraparticles from colloidal silica dispersions

The structure and texture of supraparticles determine their properties and performance, thus playing a critical role in research studies as well as industrial applications. The addition of salts is a well-known strategy to manipulate the colloidal stability of nanoparticles. In this study, this approach is used to tune the structure of spray-dried supraparticles. Three different salts (NaCl, CaCl2, and AlCl3) were added to binary silica (SiO2) nanoparticle dispersions (of 40 and 400 nm in size) to change their colloidal stability by lowering the electrostatic repulsion or enhancing the cation bridging. Dependent on the cation valence of the added salt and the nanoparticle size, the critical salt concentration, which yields nanoparticle agglomeration, is reached at different salt amounts. This phenomenon is exploited to tune the final structure of supraparticles - obtained by spray-drying binary dispersions - from core-shell to Janus-like to well-mixed structures. This consequently also tunes textural properties, like surface roughness and the pore system of the obtained supraparticles. Our results provide insights for controlling the structure of spray-dried supraparticles by manipulating the stability of binary nanoparticle dispersions, and they establish a framework for composite particle design.

Novel silanized colloidal silica-MgO self-flowing dispersions with improved hydroxylation resistance

MgO-containing and colloidal silica-bonded are important classes of refractory castables of, respectively, high refractoriness and straightforward processing. On the other hand, their combination is not synergetic. Although 0.1–1 wt% additions of MgO set and harden typical colloidal silica-bonded castables in a 1-hour time frame, greater amounts gel the system instantaneously. This paper addresses the combination of fine MgO sinter with a novel aqueous dispersion of colloidal silica whose particles’ surfaces were modified with an epoxysilane-based coupling agent. Their low reactivity produced self-flowing MgO-colloidal silica suspensions of excellent workability and, after curing, generated a packed magnesium silicate hydroxide hydrate (Mg5Si8O20(OH)2.8H2O) protective coating over MgO particles, hindering their hydroxylation and preventing volumetric expansion. The cast structures attained displayed safer drying and suitable strength. During thermal treatment, silica particles softened and promoted viscous flow, favoring grains densification. Therefore, after sintering, the microstructure comprised dense MgO grains surrounded by thin layers of forsterite (Mg2SiO4).

Novel microporous MgO-based high-temperature thermal insulator

This paper addresses the development of a novel microporous MgO-based (MPMB) thermal insulator whose temperature-resistant microporous structure was engineered for the introduction of controlled packing flaws amongst their highly asymmetric and porous particles during compacting by uniaxial pressing. Hard-burnt MgO, dolomite, and TiO2 particles were dry-mixed with expanded aluminum phyllosilicate and fibers before being sprayed with a colloidal silica dispersion. The mixture was uniaxially pressed as 400 × 100 × 20 mm boards and dried at 120 °C. Samples were thermally treated at different temperatures (120-1100 °C) for the evaluation of their physical properties (compression strength, rigidity, permanent dimensional thermal variation, solid density, total porosity, pore diameter distribution, and thermal conductivity), crystalline phases, and microstructure. The microporous microstructure generated during pressing was not significantly affected by thermal treatments up to 1100 °C and the samples’ thermal conductivity, total porosity, and compression strength varied between 0.20 and 0.14 W (m K)−1, 54–56%, and 29-15 MPa in the 200-1000 °C temperature range.

Novel colloidal silica technology for MgO-containing refractories – Part 1: Anti-hydroxylation binder

Refractory castables play major roles in high-temperature applications and two important types are MgO-containing and colloidal silica-bonded castables. Whereas the former shows high refractoriness and resistance to slag corrosion, the latter offers straightforward mixing, curing, and drying processes. However, their association in a same system has not been synergistic. During mixing, MgO particles release Mg2+ ions that trigger the gelling of colloidal silica. Therefore, small MgO additions gradually hard high-alumina colloidal silica-bonded castables; conversely, larger amounts instantly set the system, hindering homogenization and casting. Despite the challenges, there is potential for making these systems compatible towards technological application. This study successfully combined fine MgO sinter with a novel aqueous dispersion of epoxysilane-modified colloidal silica, creating self-flowing suspensions of good workability. Cured samples exhibited no signs of damage by MgO hydroxylation, sufficient strength for demolding and safer drying. Thermal treatments resulted in a dense microstructure with MgO grains surrounded by Mg2SiO4.

Rheological control on the segmentation of the mid-ocean ridges: Laboratory experiments with extension initially perpendicular to the axis

Mid-ocean ridges (MOR) axes are not straight, but segmented over scales of 10s to 100s of kilometers by several types of offsets including transform faults (TF), overlapping spreading centers (OSC) and non-transform, non-overlapping offsets (NTNOO). Variations in axial morphology and segmentation have been attributed to changes in magma supply, axial thermal structure (which depends on mantle temperature and spreading rate), and axial mechanical properties. To isolate the effect of each of these processes is difficult with field data alone. We therefore present a series of analogue experiments using colloidal silica dispersions as an Earth analogue. Diffusion of salt from saline solutions placed in contact with these fluids, causes formation of a skin, whose rheology evolves from viscous to elastic and brittle with increasing salinity. Applying a fixed spreading rate to this pre-formed, brittle plate results in cracks, faults, and ridge segments. Lithospheric thickness is varied independently by changing the surface water layer salinity. Experimental results depend on the axial failure parameter ΠF, the ratio of a mechanical length scale (Zm) and the axial elastic thickness (Zaxis), which depends on mantle temperature and spreading velocity. Slow-spreading fault-dominated, and fast-spreading fluid intrusion-dominated, ridges on Earth and in the laboratory are separated by the same critical value ΠFc±0.024, suggesting that the axial failure mode governs ridge geometry. Here, we examine ridge axis segmentation. Measurements of >4000 experimental ridge segments and offsets yield an average segment length Lm that is quasi-constant at all spreading velocities. Scaled to the Earth, Lm∼55 km, in agreement with the natural data. Experiments with low ΠF show offset size varying as dl=csteLmZaxis regardless of offset type, a correlation well explained by fracture mechanics. Finally, as on Earth, experimental ridge segments are separated by transform and non-transform discontinuities, and their nature and occurrence vary with ΠF. NTNOOs develop when ΠF<ΠFc, while OSCs develop when ΠF>ΠFc. In contrast, TF may form at any ΠF, but the proportion of TFs relative to OSCs or NTNOOs decreases when ΠF/ΠFc>> 1 or << 1, in agreement with natural MOR.

Highly versatile laboratory X-ray scattering instrument enabling (nano-)material structure analysis on multiple length scales by covering a scattering vector range of almost five decades.

A compact laboratory X-ray scattering platform that uniquely enables for high-performance ultra-small-angle X-ray scattering (USAXS), small- and wide-angle X-ray scattering (SAXS/WAXS), and total scattering (atomic pair distribution function analysis; PDF) experiments was developed. It covers Bragg spacings from sub-Angstroms to 1.7 μm, thus allowing the analysis of dimensions and complex structures in (nano-)materials on multiple length scales. The accessible scattering vector q-range spans over almost five decades (qmin = 0.0036 nm-1, qmax = 215 nm-1), without any gaps. Whereas SAXS is suitable to characterize materials on a length scale of 1-100 nm, with USAXS, this range can be significantly extended to the micrometer range. On the other end, from WAXS and particularly from PDF measurements, information about the local atomic order and disorder can be obtained. The high performance, exceptional versatility, and ease-of-use of the instrument are enabled by a high-resolution 2-circle goniometer with kinematic mounts, a modular concept based on prealigned, quickly interchangeable X-ray components, and advanced detector technology. For USAXS measurements, a modified Bonse-Hart experimental setup with single crystal collimator and analyzer optics is used. SAXS/WAXS measurements are enabled by focusing optics, an evacuated beam path, and a 2D detector. For total scattering experiments, a high-energy X-ray source is used in combination with a hybrid pixel array detector that is based on a CdTe sensor for the highest counting efficiency. To ensure high resolution and sensitivity in these various applications, special care is taken to suppress any type of background scattering signal. The high resolution that can be achieved with the USAXS collimation system is demonstrated on a set of monodisperse, colloidal silica dispersions and derived colloidal crystals, with particle diameters in the range of hundreds of nanometers up to 1.6 µm. USAXS and SAXS results are shown to be consistent with those obtained by static light scattering (SLS) and dynamic light scattering. It is demonstrated that the obtainable USAXS data bridge the gap in q between SAXS and SLS. The capabilities of the instrument to acquire high-quality total scattering data for PDF analysis are demonstrated on amorphous SiO2 nanoparticles as well as on NaYF4 upconversion nanocrystals. To the best of our knowledge, it is for the first time that we present a single laboratory instrument that enables measurements of high-quality X-ray scattering data within such a wide q-range, by combining four complementary elastic X-ray scattering techniques. The modular design concept of the instrument allows for incremental improvements as well as to add more applications in the future.

Closure to “Analytical Model for Transport of Dilute Colloidal Silica Dispersions through Porous Media” by J. Richard Weggel, Patricia Gallagher, and Yuanzhi Lin

Closure to “Analytical Model for Transport of Dilute Colloidal Silica Dispersions through Porous Media” by J. Richard Weggel, Patricia Gallagher, and Yuanzhi Lin

Discussion of “Analytical Model for Transport of Dilute Colloidal Silica Dispersions through Porous Media” by J. Richard Weggel, Patricia Gallagher, and Yuanzhi Lin

Discussion of “Analytical Model for Transport of Dilute Colloidal Silica Dispersions through Porous Media” by J. Richard Weggel, Patricia Gallagher, and Yuanzhi Lin

Structuration of the Surface Layer during Drying of Colloidal Dispersions.

During evaporative drying of a colloidal dispersion, the structural behavior at the air-dispersion interface is of particular relevance to the understanding of the consolidation mechanism and the final structural and mechanical properties of the porous media. The drying interface constitutes the region of initial drying stress that, when accumulated over a critical thickness, leads to crack formation. This work presents an experimental study of top-down drying of colloidal silica dispersions with three different sizes (radius 5, 8, and 13 nm). Using specular neutron reflectivity, we focus on the structural evolution at the free drying front of the dispersion with a macroscopic drying surface and demonstrate the existence of a thick concentrated surface layer induced by heterogeneous evaporation. The reflectivity profile contains a strong structure peak due to scattering from particles in the interfacial region, from which the interparticle distance is deduced. A notable advantage of these measurements is the direct extraction of the corresponding dispersion concentration from the critical total reflection edge, providing a straightforward access to a structure-concentration relation during the drying process. The bulk reservoir of this experimental configuration renders it possible to verify the evaporation-diffusion balance to construct the surface layer and also to check reversibility of particle ordering. We follow the structural evolution of this surface layer from a sol to a soft wet-gel that is the precursor of a fragile skin and the onset of significant particle aggregation that precedes formation of the wet-crust. Separate complementary measurements on the structural evolution in the bulk dispersion are also carried out by small-angle neutron scattering, where the particle concentration is also extracted directly from the experimental curves. The two sets of data reveal similar structural evolution with concentration at the interface and in the bulk and an increase in the degree of ordering with the particle size.

Analytical Model for Transport of Dilute Colloidal Silica Dispersions through Porous Media

AbstractThe purpose of this paper is to present a simple analytical model to describe the flow of a dilute colloidal silica dispersion through a porous medium column. The model presents the movemen...

Preparation of macroporous scaffolds with holes in pore walls and pressure driven flows through them.

Controlling the pore architecture in macroporous scaffolds has important implications for their use as reactor packings and as catalyst supports. We report the preparation of a macroporous structure, where the pore walls are perforated by holes. These materials are prepared by modification of the ice-templating protocol developed in our group. We freeze a dispersion of colloidal silica, polymer and cross-linker in a water/acetonitrile medium and allow crosslinking to proceed in the frozen state. The presence of a small fraction of acetonitrile (varying between 1.6% to 6.4%) results in the formation of holes in the pore walls. Increasing the acetonitrile concentration changes the pore size distribution, and produces smaller pores on average. This also results in an increasing fraction of the wall area being covered by small pores, of the order of a few microns in size. Perforation of the walls by pores does not change the overall porosity or modulus of the scaffolds. However, the introduction of pores leads to a drastic reduction in the pressure drop required to pump liquid through the scaffolds. The observed residence time distribution (RTD) in the scaffolds is represented by two plug flow reactors (PFRs) in parallel. The RTD results indicate that increasing the hole fraction in the pore walls results in increased channelling which explains the aforementioned decreased pressure drop during pressure driven flow.

Effect of particle/polymer number ratio on the structure and dynamics of complex between large polymer and nanoparticle

The effect of particle/polymer number ratio on the structure and dynamics of particle-polymer complexes at a fixed polymer concentration has been investigated. Mixtures of poly(ethylene oxide) (PEO, Mv=1,000,000 gmol−1, Rh ∼39nm) aqueous solution and colloidal silica dispersion (Rh ∼10nm) were used as models. By adding silica particles, both zero-shear viscosity and Rh of the silica-PEO complexes increased with particle/polymer number ratio. The maximum zero-shear viscosity was reached when the PEO coils were saturated by silica particles. After that, a decrease in zero-shear viscosity and Rh was observed. SAXS results showed that PEO had a great effect on the inter-particle correlations before the PEO chains were saturated with silica particles, however, this effect diminished when even more silica particles were introduced. The reversible bridging interaction and inter-particle electrostatic repulsion were identified as the major factors accounting for the change in zero-shear viscosity and hydrodynamic size, and a tentative mechanism was proposed based on the spatial distribution of silica particles in the mixtures.

Influence of colloidal silica dispersion on the decrease of roughness in silicon chemical mechanical polishing

Colloidal silica with different dispersion states during silicon chemical mechanical polishing (CMP) are investigated in detail. As the dispersion of colloidal silica is improved, the roughness (Ra ) decreases gradually during polishing. The silicon surface has a minimum Ra of 0.118 nm when the polydispersity index of particles is 0.078. Comparing with surface polished by colloidal silica in poor dispersion, the surface quality has great improvement. The important role of dispersant is presented by the results of investigations. The research results indicate that dispersant makes aggregate micelles dispersed into many smaller uniform colloidal silica particles. The uniformly dispersed colloidal silica forms a thin film between the surfaces of silicon and polishing pad during CMP, which is beneficial for the stability of friction, and therefore greatly decreases roughness of silicon surface after polishing.

Hiding in Plain View: Colloidal Self-Assembly from Polydisperse Populations.

We report small-angle x-ray scattering experiments on aqueous dispersions of colloidal silica with a broad monomodal size distribution (polydispersity, 14%; size, 8nm). Over a range of volume fractions, the silica particles segregate to build first one, then two distinct sets of colloidal crystals. These dispersions thus demonstrate fractional crystallization and multiple-phase (bcc, Laves AB_{2}, liquid) coexistence. Their remarkable ability to build complex crystal structures from a polydisperse population originates from the intermediate-range nature of interparticle forces, and it suggests routes for designing self-assembling colloidal crystals from the bottom up.

Dish-like drying patterns of the water-soluble gelatin sheet wetted by an aqueous droplet

Drying dissipative patterns of the water-soluble gelatin sheet wetted by an aqueous droplet were observed as a function of time elapsed. The arrayed clusters of dishes formed at the broad ring area and grew outward toward multiple arrays with time. The drying patterns formed by the cooperative contribution of wetting, swelling, dissolving, evaporative, convectional, sedimentary and solidifying processes. Drying patterns were studied also for aqueous ethanol, aqueous NaCl solutions and dispersions of colloidal silica and poly(methyl methacrylate) spheres. The dish patterns were observed for the water-soluble substrates, for the first time, in this work.

Two-dimensional DPPC based emulsion-like structures stabilized by silica nanoparticles.

We studied the mechanical and structural properties of mixed surface layers composed by 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and silica nanoparticles (NPs). These layers are obtained by spreading a DPPC Langmuir monolayer on a colloidal silica dispersion. The transfer/incorporation of NPs into the DPPC monolayer, driven by electrostatic interactions, alters the molecular orientation, the mechanisms of domain formation, and consequently the phase behavior of the surface layer during compression. The investigation of these systems by means of complementary techniques (Langmuir trough, fluorescence microscopy, ellipsometry, and scanning electron microscopy (SEM)) shows that the incorporated NPs preferentially distribute along the liquid expanded phase of DPPC. The layer assumes the stable and homogeneous bidimensional structure of a two-dimensional (2D) analogue of a Pickering emulsion. In fact, the presence of particles provides a circular shape to the DPPC domains and stabilizes them against growth and coalescence during the monolayer compression.

Particle shape dependence of rheological behavior for colloid-polymer mixtures

The effect of particle shape on the rheological behavior of small particle-large polymer chain mixture solutions has been investigated with two model colloidal silica dispersions, one of which is ellipsoidal (BINDZIL20/440) and the other is spherical (TM40). It was found that BINDZIL20/440 series showed shear-thickening at lower shear rates and had a lower upper limit in PEO concentration to demonstrate shear-thickening phenomena. The particle shape was identified as the major factor accounting for these differences. This work enables one to control the rheological behavior of colloid-polymer mixture through simply changing particle geometry instead of performing surface modifications, which could be especially useful in cases where only certain chemicals are allowed, for example in vivo applications.

Application of Fluorescence Correlation Spectroscopy in the Characterization of Particle Size Distributions of Colloidal Silica Abrasives Used in Chemical-Mechanical Planarization

Recent studies have demonstrated that fluorescence correlation spectroscopy (FCS) can be used to analyze the adsorption of dissolved chemical compounds onto the dispersed abrasive particles in a CMP slurry (1, 2). The technique exploits competitive adsorption at the particle’s surface between a fluorophore and CMP-relevant, dissolved chemical compounds. As chemical compounds in a slurry adsorb to a fluorescent abrasive particle and de-sorb the fluorophore from the particle, FCS-measured diffusion coefficients of the fluorophores will indicate increasing concentration of the free fluorophore in solution as the concentration of adsorbed chemical compounds increases.In the present study, the FCS method is extended for the analysis of particle size distributions of smaller-sized colloidal silica CMP abrasives. This new application of the FCS technique is motivated by the commercial need to further reduce wafer defect formation in conjunction with increased removal of slurry and wafer particulates produced during CMP processes. A key analytical capability gating this processing requirement is the necessity to analyze, quantitatively, the smallest abrasive particles in a CMP slurry which are present at extreme dilution. Established methods for particle size analysis, such as disc centrifugation and dynamic light scattering, typically lack the sensitivity required to analyze a minute fraction of abrasive particles, such as colloidal silica, with hydrodynamic diameters less than 20 nm in a CMP slurry. Because FCS has sizing capability for small molecules equivalent in hydrodynamic diameter (d h) to fluorescent dyes (d h = 1.0 to 1.5 nm) coupled with its analytical sensitivity in the nanomolar range, particle size distribution analysis of silica abrasive particles with mean diameters less than 10 nm, is potentially accessible via FCS.Representative particle size distributions determined by FCS are shown in Figure 1 for a pair of related fluorescently-labeled silica dispersions. The data overlay in the figure compares the particle size distributions of a parent silica dispersion (solid line, mean hydrodynamic diameter = 45 nm) and that of the isolated supernatant of the dispersion obtained by ultracentrifugation of the parent dispersion in order to isolate the smallest particles (dashed line, mean hydrodynamic diameter = 11 nm) in the dispersion. These data demonstrate the ability of the FCS technique to detect and quantity the fraction of smallest particles in abrasive colloidal silica dispersions. Additional examples illustrating the range and applicability of the technique in the quantitative analysis of particle size distributions for representative silica particles used commonly in CMP abrasives are discussed in this presentation.1. Moinpour, M.; Wayman, A.; Rawat, R.; Carver, C. T.; Remsen, E. E. Surface Adsorption of CMP Slurry Additives on Abrasive Particles. Electrochem. Soc. Trans. 2013, 52,489-494.2. England, A.N.; Rawat, A.; Moinpour, M.; Remsen, E.E. Additive/Abrasive Interactions in Solution: Investigation of the Surface Chemistry and Adsorption Behaviour of CMP Abrasives. Proc. Int. Conf. Planar. Tech. 2012, 1-6.

Recrystallization and Zone Melting of Charged Colloids by Thermally Induced Crystallization

We examined the application of recrystallization and zone-melting crystallization methods, which have been used widely to fabricate large, high-purity crystals of atomic and molecular systems, to charged colloidal crystals. Our samples were aqueous dispersions of colloidal silica (with particle diameters of d = 108 or 121 nm and particle volume fractions of ϕ = 0.035-0.05) containing the weak base pyridine. The samples crystallized upon heating because of increases in the particle charge numbers, and they melted reversibly on cooling. During the recrystallization experiments, the polycrystalline colloids were partially melted in a Peltier cooling device and then were crystallized by stopping the cooling and allowing the system to return to ambient temperature. The zone-melting crystallization was carried out by melting a narrow zone (millimeter-sized in width) of the polycrystalline colloid samples and then moving the sample slowly over a cooling device to recrystallize the molten region. Using both methods, we fabricated a few centimeter-sized crystals, starting from millimeter-sized original polycrystals when the crystallization rates were sufficiently slow (33 μm/s). Furthermore, the optical quality of the colloidal crystals, such as the half-band widths of the diffraction peaks, was significantly improved. These methods were also useful for refining. Small amounts of impurity particles (fluorescent polystyrene particles, d = 333 nm, ϕ = 5 × 10(-5)), added to the colloidal crystals, were excluded from the crystals when the crystallization rates were sufficiently slow (∼0.1 μm/s). We expect that the present findings will be useful for fabricating large, high-purity colloidal crystals.