Table1_Dry powder coating in additive manufacturing.DOCX

Dry powder coating is used in many industries to tailor the bulk solid characteristics of cohesive powders. Within this paper, the state of the art of dry coating of feedstock materials for powder based additive manufacturing (AM) processes will be reviewed. The focus is on feedstock materials for powder bed fusion AM processes, such as powder bed fusion of polymers with a laser beam and powder bed fusion of metals with lasers or an electron beam. Powders of several microns to several ten microns in size are used and the feedstock’s bulk solid properties, especially the flowability and packing density are of immanent importance in different process steps in particular for powder dosing and spreading of powder layers onto the building area. All these properties can be tuned by dry particle coating. Moreover, possibilities to improve AM processability and to manipulate the resulting microstructure (c.f. grain refinement, dispersion strengthening) by adhering nanoparticles on the powders will be discussed. The effect of dry coating on the obtained powder properties along the whole AM process chain and the resulting part properties is assessed. Moreover, appropriate characterization methods for bulk solid properties of dry-coated AM powders are critically discussed.</p

Surface Charging and Interfacial Water Structure of Amphoteric Colloidal Particles

Colloidal stability and adsorption processes at particle surfaces are controlled by the generation of surface charge and the molecular structure of the resulting electrical double layers. We have applied second-harmonic light scattering (SHS) to address double-layer charging and the orientation of water molecules at surfaces of amphoteric particles in unprecedented detail. For that purpose we have performed the first SHS experiments of neat polystyrene particles with a mixture of sulfate (R–O–SO₃^–) and amino (R–NH₂) surface functional groups. Changing the pH has a dramatic effect on the surface potential which can be tuned from positive to negative values and has allowed us to record changes in SHS signal while the isoelectric point of the particles is being crossed. The SHS signal is a function of two contributions which are directly related to the first molecular layer, its orientation, and to a second electric field induced contribution of the unidirectional field within the interfacial electric double layer. Through additional charge screening experiments we measured the surface charge density for positively as well as for negatively charged particle surfaces and confirmed the isoelectric point, where an increase in ionic strength had little effect on the SHS intensity. Furthermore, we have determined the net orientation of water molecules directly adsorbed to the particle surface from pH-dependent changes in the relative phase of the two SHS contributions

Green and Scalable Fractionation of Gold Nanoclusters by Anion Exchange Chromatography: Proof of Principle and Scale-Up

The chromatographic separation of atomically precise glutathione-stabilized gold nanocluster (AuNC) mixtures is demonstrated for purely aqueous anion exchange chromatography (AEX) and compared to the results of hydrophilic interaction liquid chromatography (HILIC), which requires mixtures of water and acetonitrile. For isocratic HILIC separations, the influence of the mobile phase composition with respect to ionic strength and acetonitrile content is investigated. Individual AuNCs are identified by their optical properties directly from chromatograms and by comprehensive electrospray ionization differential mobility mass spectrometry. Separations by AEX are performed in aqueous mobile phases and are thus considered as a green and sustainable technique for the efficient fractionation of AuNCs. Henry coefficients are determined, which describe the retention behavior of single AuNCs and thus form the basis for modeling, optimization, and scale-up of the separation. The separation by AEX is optimized by linear salt gradient experiments. The scalability of the separation is demonstrated by increasing the AuNC feed concentration up to 64 g/L. Moreover, we quantify the amounts of individual AuNCs by inductively coupled plasma–optical emission spectroscopy after separation and close the mass balance demonstrating the reversible adsorption from the stationary phase. Our AEX approach is superior to HILIC since it offers a higher resolution, provides access to larger clusters, is readily scalable, and is purely water-based. AEX is considered as a sustainable, scalable, and widely applicable technique for the isolation of all types of water-soluble single cluster species. Therefore, we anticipate applications for instance for highly specialized nanocatalysts, sensors, or in medicine

Enhanced Nucleation of Lysozyme Using Inorganic Silica Seed Particles of Different Sizes

In this work we investigate the impact of differently sized plain silica nanoparticles (NPs) between 10 and 200 nm on the crystallization of lysozyme (LSZ). In the first part of our work we investigate the electrostatic interactions between LSZ and NPs by zeta potential measurements and place special emphasis on the adsorption behavior of LSZ@SiO₂. The determined adsorption isothermsderived from UV–vis spectroscopyindicate that with increasing particle size more LSZ is adsorbed per NP surface area probably due to a size-dependent surface chemistry and the variation of surface curvature. Second, seeded crystallization experiments both at the microliter and milliliter scale and thus close to a technically relevant scale were performed. A clearly extended crystallization window upon the addition of seed particles toward lower protein and salt concentrations was found. Moreover, induction times of crystal formation and crystallization times were considerably reduced. These effects were intensified with the addition of larger seed particles. In general, with the addition of silica seed particles, a shift of the final crystal size distribution to larger structures is observed

Advanced Multiwavelength Detection in Analytical Ultracentrifugation

This work highlights significant advancements in detector hardware and software for multiwavelength analytical ultracentrifugation (MWL-AUC) experiments, demonstrating improvement in both the spectral performance and UV capabilities of the instrument. The hardware is an extension of the Open AUC MWL detector developed in academia and first introduced in 2006 by Bhattacharya et al. Additional modifications as well as new analytical methods available for MWL data have since been reported. The present work describes new and continuing improvements to the MWL detector, including mirror source and imaging optics, UV sensitive acquisition modes and revised data acquisition software. The marked improvement of experimental data promises to provide access to increasingly complex systems, especially semiconductor nanoparticles, synthetic polymers, biopolymers, and other chromophores absorbing in the UV. Details of the detection system and components are examined to reveal the influences on data quality and to guide further developments. The benchmark comparisons of data quality across platforms will also serve as a reference guide for evaluation of forthcoming commercial absorbance optics

Lubrication of Individual Microcontacts by a Self-Assembled Alkyl Phosphonic Acid Monolayer on α‑Al₂O₃(0001)

We report on the tribological behavior of a self-assembled alkyl phosphonic acid monolayer on the microscale using the colloidal probe technique. Friction–load data and adhesion forces were measured with borosilicate glass particles on uncoated and octadecylphosphonic acid (ODPA) coated α-Al₂O₃(0001) surfaces. A significant decrease in friction force was observed after surface coating, while the adhesion force was only moderately reduced. We assume the lubrication effect of the ODPA self-assembled monolayer (SAM) to be close to the maximum obtainable of alkyl phosphonic acids in the studied system due to the high molecular order which was confirmed by vibrational sum-frequency generation. At small loads, a nonlinear dependence of friction force to load was maintained after surface coating. However, a shift from a contact behavior well described by the DMT model toward the JKR model occurred that is possibly related to the altered elastic properties of the coated surface. With increasing load, a linear friction–load behavior was observed on the coated samples. Molecular plowing and adhesive interactions were identified as responsible mechanisms. In all friction experiments, we could not detect any wear neither of the colloidal probes nor at the surfaces of uncoated and coated samples. This proves the high wear resistivity of the studied ODPA SAM

Self-Assembled Monolayers Get Their Final Finish via a Quasi-Langmuir–Blodgett Transfer

The growth of self-assembled monolayers (SAMs) of octadecylphosphonic acid (ODPA) molecules on α-Al₂O₃(0001) and subsequent dewetting of the SAMs were studied with a combination of in situ sum-frequency generation (SFG) and molecular dynamics (MD) simulations. Although SAM growth after deposition times >8 h reduces to nearly negligible values, the resultant ODPA SAMs in solution are still not in a well-ordered state with the alkyl chains in all-trans configurations. In fact, in situ SFG spectroscopy revealed a comparatively high concentration of gauche defects of the SAM in the ODPA 2-propanol solution even after a growth time of 16 h. Here, results of the MD simulations strongly suggest that defects can be caused by ODPA molecules which are not attached to the substrate but are incorporated into the SAM layer with the polar headgroup oriented into the 2-propanol solvent. This inverted adsorption geometry of additional ODPA molecules blocks adsorption sites and thus stabilizes the SAM without improving ordering to an extent that all molecules are in the all-trans configuration. While persistent in solution, the observed defects can be healed out when the SAMs are transferred from the solvent to a gas phase. During this process, a quasi-Langmuir–Blodgett transfer of molecules takes place which drives the SAM into a higher conformational state and significantly improves its quality

Synthesis of Goethite α‑FeOOH Particles by Air Oxidation of Ferrous Hydroxide Fe(OH)₂ Suspensions: Insight on the Formation Mechanism

Iron oxide and iron oxyhydroxide particles, particularly, the goethite α-FeOOH phase, are environmentally friendly materials and are used in various technological applications as adsorbents, precursors of Fe powders for magnetic recording media, and pigments. In this work, the formation process of α-FeOOH by air oxidation of Fe­(OH)₂ suspensions has been studied. The effects of the air flow rate, as well as of the reactant concentration ratio, <i>R</i> (=[(OH)⁻]/[Fe­(II)]), on the reaction product were analyzed. It has been found that the morphology and the size of the α-FeOOH particles can be modified by means of the air flow rate. Furthermore, by performing a detailed microscopic analysis of the morphology of the initial, intermediate, and final reaction products, we have obtained evidence of epitaxial growth of α-FeOOH on the Fe­(OH)₂ substrate. It is suggested that the similarity between the anion arrangements in both phases facilitates this process. Based on these results, a pathway for the formation of α-FeOOH in highly alkaline medium is proposed in which the size and shape of the initial Fe­(OH)₂ particles plays a significant role in the formation the α-FeOOH particles obtained upon completion of the oxidation process

<i>In Situ</i> Study on the Evolution of Multimodal Particle Size Distributions of ZnO Quantum Dots: Some General Rules for the Occurrence of Multimodalities

Properties of small semiconductor nanoparticles (NPs) are strongly governed by their size. Precise characterization is a key requirement for tailored dispersities and thus for high-quality devices. Results of a careful analysis of particle size distributions (PSDs) of ZnO are presented combining advantages of UV/vis absorption spectroscopy, analytical ultracentrifugation, and small-angle X-ray scattering (SAXS). Our study reveals that careful cross-validation of these different methods is mandatory to end up with reliable resolution. PSDs of ZnO NPs are multimodal on a size range of 2–8 nm, a finding that is not yet sufficiently addressed. In the second part of our work the evolution of PSDs was studied using <i>in situ</i> SAXS. General principles for the appearance of multimodalities covering a temperature range between 15 and 45 °C were found which are solely determined by the aging state indicated by the size of the medium-sized fraction. Whenever this fraction exceeds a critical diameter, a new multimodality is identified, independent of the particular time–temperature combination. A fraction of larger particles aggregates first before a fraction of smaller particles is detected. Fixed multimodalities have not yet been addressed adequately and could only be evidenced due to careful size analysis

Enhanced Crystallization of Lysozyme Mediated by the Aggregation of Inorganic Seed Particles

We show that aggregation plays a major role in seeded growth of protein crystals. The seeded batch approach provides the opportunity to set the starting conditions for protein crystallization by adding a defined amount of well-characterized seed particles. The experimental observations for tetragonal hen egg-white lysozyme (LSZ) confirm the concept of the oriented aggregation of larger building blocks to form a protein crystal. It was shown that the aggregation of the seed particles/bioconjugates is advantageous for the product quality in terms of larger and more defined LSZ crystals and in terms of accelerated reaction kinetics. We present a population balance (PB) model for the seeded batch crystallization of LSZ considering the aggregation of growth units to form protein crystals. For the modeling of crystal growth, evolving particle size distributions (PSDs) of agglomerating LSZ molecules were measured by dynamic light scattering (DLS). Moreover, the aggregation of seed particles in LSZ solutions under crystallization conditions was investigated by DLS. In line with our expectations, the number of seeds was found to be important as it strongly affects the collision frequency in the aggregation term of our PB model. Finally, the applied model gives trends of the supersaturation depletion curves and orders of magnitude of the measured CSDs in particle size correctly, ranging from only a few nanometers up to micrometer-sized particles/crystals. Thus, by the combination of PB modeling and experimentally determined crystallization parameters, insights into the crystal formation mechanism were obtained. To the best of our knowledge, this is the first attempt to model growth within a crystal population by an aggregation mechanism induced by seeding with foreign particles