Nanoparticles are raw materials for many kinds of new functional materials, from nanodevices to hybrid composite materials. For commercial application and processing, it is necessary to develop and characterize nanoparticle dispersions and suspensions without aggregation, and to control for instance the application behavior. Using various Anton Paar high-precision instruments, we study, e.g. shear-induced orientational effects in cellulose solutions, the viscosity of graphene coatings with nanomaterial content, the application behavior of paints and coatings based on inorganic nanoparticle powder, or the viscosity of CMP slurries.
The concentration of gold and silver particles in graphene coatings for electronics is crucial in terms of their conductivity. Together with knowing the ideal particle size, the final viscosity of the product needs to be adjusted properly in order to obtain a consistent coating of the electronics or on metal surfaces.
A concentric cylinder measuring geometry was filled with 11 mL sample volume. Then the spindle was inserted into the measuring geometry, filled with the sample, and mounted on the rotational viscometer. To precisely control the temperature during the measurement to +23 °C a Peltier temperature device was used. The T-Ready function of the temperature device ensured that the temperature of the sample was adjusted to the test temperature prior to starting the measurement. A linear speed ramp from low (1 rpm) to high (100 rpm) speed with 11 measurement points was performed. The measurement point duration for one measurement point was set to 30 sec.
The flow behavior of the graphene coating can be analyzed through viscosity determinations at multiple speeds. The coating showed a shear-thinning flow behavior. Shear thinning means that the viscosity decreases with increasing speed. The most significant change in viscosity takes place at low speeds. The degree of shear thinning can be calculated automatically using the mathematical model “shear thinning index”. Here, the dynamic viscosity at a low rotational speed was divided by the viscosity at the highest speed. Higher ratios indicate a greater shear thinning effect. For the graphene coating a shear thinning index of 26.1575 (speeds: 1 rpm and 100 rpm) was calculated. Having the proper shear-thinning flow behavior (and adjusting ingredients accordingly) results in a consistent coating of the final material.
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The concentration of inorganic nanoparticle powder in the final product is crucial in terms of its application behavior. These samples are non-Newtonian and time-dependent by their nature, meaning the viscosity depends on the shear rate applied, and on the measurement time (application time). The viscosity is therefore studied to allow adjustment of the final product’s characteristics and guarantee proper application behavior.
A concentric cylinder measuring geometry was filled with approx. 19 mL sample volume. Then the bob was inserted into the measuring geometry, filled with sample, and mounted on the rotational rheometer. To precisely control the temperature during the measurement to +20 °C, a Peltier temperature device was used. After adjusting the temperature of the sample to the test temperature a 3-interval thixotropy test (3ITT) was performed.
The third interval of the 3ITT test was used to analyze the thixotropic behavior of the sample. In Interval 3, after 120 sec, the structure and the viscosity of sample A reached 82 % of the initial value. It can therefore be concluded that the sample almost completely regenerated its structure after shearing. However, after 120 sec, the viscosity value of sample B reached only 62 % of the initial value. Sample A, with its rapid structural regeneration, showed less sagging but inadequate leveling, as its structure regenerated relatively quickly after structural processing. Sample B, however, with its slow structural regeneration, exhibited better leveling behavior. Due to the strong sag tendency, it was expected that sample B would produce an inadequate layer thickness. The structural regeneration must not be too fast, in order to allow for good leveling of the paint or coating, and it must not be too slow, in order to prevent sagging and to ensure a sufficient wet-layer thickness.
3-interval thixotropy test as a rotational test with 3 intervals. Measurement of the time dependent viscosity: before, during and after shearing, using two textured coatings as an example.
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Printing inks have come a long way and are used for far more than just texts. Recent developments of so called “conductive inks”, which contain silver nanoparticles, enable the utilization of the ink as a conductive film for fabrication of electrodes [1]. The film is typically formed upon treating the ink with heat or UV light, providing a very thin and smooth layer. When it comes to the printing process, viscosity is an important factor. It influences flow behavior as well as droplet size and formation, greatly affecting the quality of the process.
Measurements were performed on a measuring system containing a rolling-ball viscometer and a digital density meter or density and sound velocity analyzer. For most low-viscous inks, a capillary of 1.59 mm diameter with a steel ball was utilized; viscosities are determined at 20 °C. As inks typically show slight shear-thinning behavior, so-called “zero-shear viscosity” is determined by performing several measurements at varying shear rates – in this instance, this was achieved by varying the inclination angle of the rolling ball viscometer and then performing the extrapolation.
The measurement of viscosity enables the fine-tuning of the ink properties during manufacturing and for the printing process. Most crucially, the zero-shear viscosity – as it is independent of shear rate effects – provides valuable insight into the behavior of the ink during printing. Low-viscous conductive inks are typically in the range from 4 mPa·s to 16 mPa·s [1-3], which makes them ideal for inkjet printing. The added benefit of determining the density and sound velocity is that the purity of the material can be assessed.
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Chemical mechanical polishing (CMP) slurries are indispensable in the semiconductor industry. They are colloidal formulations, containing abrasive particles as well as various chemical etching agents, utilized for polishing wafer surfaces.[1] The viscosity of the CMP slurry influences the polishing results[2], material removal rates, and the interactions between the polishing pad and the wafer.[3] Density is also a powerful tool for monitoring the quality of slurries. In fact, variations of density are signals of inhomogeneous slurry in case of formation of agglomerate. Those imperfections lead to changes in removal rate and defects formation on the wafer.[4] Therefore, assessing the viscosity is an important step in the manufacturing quality control process.
Measurements were performed on a measuring system consisting of a rolling-ball viscometer and a digital density meter. The dynamic viscosity of the samples was determined at 25 °C. 2 capillaries with diameters of 1.59 mm and 1.8 mm, respectively, as well as steel balls were used.
The viscosity of CMP slurries is strongly dependent on the composition (abrasives, additives, etching agents) and can vary considerably. As several important performance parameters of CMP slurries are strongly affected by viscosity, determining this crucial parameter facilitates the manufacturing of high-quality products.
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Dispersions are multi-phase substances comprising at least one liquid solvent and one solid, particulate component. Their manifold applications include paints & coatings, food and energy materials, where nano-sized particles yield advantageous properties over larger, µm-sized particulates (e. g. Bapat et al. 2020) – in addition to solid fraction, particle size (distribution), shape and density control flow, form stability, sedimentation and (de)agglomeration behavior. The dispersing process requires a high amount of energy to homogenize the solid fraction with the solvent, and may also involve reduction of particle size. The case study presented here demonstrates how decreasing particle size accompanies fundamental changes in rheological properties, and the possible implications for printing ink applications.
Printing ink nano-dispersions with identical solid fraction were produced from a base ink by varying the energy input from 50 kWh/t, over 100 kWh/t, to 150 kWh/t. Rheological measurements were performed with an air-bearing Modular Compact Rheometer in concentric cylinder geometry to investigate flow behavior, gel structure, sedimentation stability and structural recovery. Using a concentric cylinder geometry allows the counteracting of fast sample drying and surface oxidation, by overfilling with a protective oil. Particle size distribution was determined by laser diffraction measurements.
Starting from the base ink, where particle size distribution peaks at 10 µm to 100 µm, the dispersion process systematically decreased the particle size, and peaked at 100 nm to 1000 nm for the highest dispersion energy input (150 kWh/t). The reduction of the particle size results in changes of (shear-thinning) viscosity, shear moduli and ability for structural recovery. With nano-sized particles dominating, the printing inks are more viscous, and have higher form stability and a higher percentage of elasticity regarding visco-elastic behavior. Even though all inks show form stability, only those with nano-sized particles recover gel structure after being subjected to high shear load, representing typical application processes. Similarly, sedimentation stability is only observed for dispersions with nano-sized particles.
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The flow behavior of complex fluids, as described by rheology, is determined by their underlying structure. While micrometer-size components can be easily monitored by rheo-optical microscopy, more complex methods, like rheo x-ray or neutron scattering are required for nanomaterials. Nevertheless, since the rheological behavior of materials is often determined by the deformation and orientation of particles, molecules, droplets or polymer chains under shear or stress conditions, such effects can be well visualized by polarized light imaging (PLI), mentioned in literature also as SIPLI (shear-induced polarized light imaging). Additional to the observation of orientational effects, the method is also suitable for following crystallization processes of polymer melts and solutions.
Sample: Gel-like cellulose nanocrystals dispersion under shear Characterization technique: Different from other optical methods, the Polarized Light Imaging enables observation of the large sample area (25 mm diameter) while sheared in the rheometer; hence, the integrated behavior of macromolecules and nanoparticles can be detected. Usually, due to orientation and deformation of structures in the shear flow, the liquid crystals, particle dispersions, polymer solutions and melts become birefringent (optically anisotropic). The PLI method is based on the phase difference of light passing through such optical active materials, provides information on optical path boundaries between sample structures of differing refractive indices, and distinguishes between isotropic and anisotropic materials.
Figure 1. The polarized imaging option enables monitoring of the sample using polarized light, a telecentric optics module and a CCD camera.
The sample was subjected to shear rates varied from 1s-1 to 1000 s-1 by using a parallel-plate system (Fig. 2).
At low shear rates, the cellulose nanocrystals are non-oriented within the flow; when the shear rate is increased, the viscosity decreases, due to increasing orientation of the cellulose nanocrystals. The Maltese cross in the PLI images indicates a parallel orientation of one of the main optical axes of a birefringent structure in the plane of polarization of the incident light.
Consequently, the linearly polarized light passing through will not change its polarization resulting in dark regions. A relative orientation to either polarizer or analyzer filter results in bright regions. The Maltese cross becomes more pronounced with increasing shear rate, i.e. with stronger orientation of the nanocrystals in the flow.
Figure 2. Viscosity curve of the cellulose solution with SIPLI images indicating orientation of the chains in the flow.
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