Powders

Powder coatings are a green alternative to conventional solvent-borne liquid coatings, but they have the intrinsic drawback of color-matching and adjustment in production with the conventional extrusion process. In this study, an industrially applicable approach to formulate color powder coatings utilizing ultrafine powders, i.e., a powdery blending and pressing method, was invented. This novel method was validated by comparing samples prepared by the Method 1 conventional extrusion method with an extra ultramarine pigment at 3%; Method 2 powdery blending and pressing of the original coatings and the same coating with 6% ultramarine pigment utilizing regular (coarse) powder coatings; and Method 3 utilizing ultrafine powder coatings for the two coatings with the same formulations as Method 2. The coating powders were prepared to have similar particle sizes and particle size distributions, with three commonly used coating binders, namely polyester-epoxy hybrid, polyester/TGIC (triglycidyl isocyanurate), and polyurethane (PU). The powders prepared by Methods 1 and 3 had similar flow abilities in terms of angle of repose (AOR) and avalanche angle (AVA). The performance of the new coatings by Method 3 was close to or better than the ones prepared by Method 1 in terms of the specular gloss, DOI (distinctness-of-image), reflection haze and color values, being superior to Method 2. The coatings via ultrafine powders also exhibited a comparable ultramarine particle distribution in the coating cross-sections as the conventional ones, whereas the ones via regular powders had an inferior pigment dispersion. The new method can greatly enhance the production efficiency and reduce the cost of powder coatings with compound colors, especially for small batch manufacturing. Full article

Additive manufacturing (AM) as a disruptive technique has offered great potential to design and fabricate many metallic components for aerospace, medical, nuclear, and energy applications where parts have complex geometry. However, a limited number of materials suitable for the AM process is one of the shortcomings of this technique, in particular laser AM of copper (Cu) is challenging due to its high thermal conductivity and optical reflectivity, which requires higher heat input to melt powders. Fabrication of composites using AM is also very challenging and not easily achievable using the current powder bed technologies. Here, the feasibility to fabricate pure copper and copper-carbon nanotube (Cu-CNT) composites was investigated using laser powder bed fusion additive manufacturing (LPBF-AM), and 10 × 10 × 10 mm³ cubes of Cu and Cu-CNTs were made by applying a Design of Experiment (DoE) varying three parameters: laser power, laser speed, and hatch spacing at three levels. For both Cu and Cu-CNT samples, relative density above 90% and 80% were achieved, respectively. Density measurement was carried out three times for each sample, and the error was found to be less than 0.1%. Roughness measurement was performed on a 5 mm length of the sample to obtain statistically significant results. As-built Cu showed average surface roughness (R_a) below 20 µm; however, the surface of AM Cu-CNT samples showed roughness values as large as 1 mm. Due to its porous structure, the as-built Cu showed thermal conductivity of ~108 W/m·K and electrical conductivity of ~20% IACS (International Annealed Copper Standard) at room temperature, ~70% and ~80% lower than those of conventionally fabricated bulk Cu. Thermal conductivity and electrical conductivity were ~85 W/m·K and ~10% IACS for as-built Cu-CNT composites at room temperature. As-built Cu-CNTs showed higher thermal conductivity as compared to as-built Cu at a temperature range from 373 K to 873 K. Because of their large surface area, light weight, and large energy absorbing behavior, porous Cu and Cu-CNT materials can be used in electrodes, catalysts and their carriers, capacitors, heat exchangers, and heat and impact absorption. Full article

The air quality in a hospital’s underground car park is a concern because diesel fumes from cars impact upon vulnerable people attending medical consultations. This research aims to quantify the potential health risk associated with a particular hospital car park. Particulate matter was evaluated in the area with direct reading devices for particle numbers and mass concentrations (CPC 3007, EEPS 3090, Trolex Air XD, Nanozen, and Grimm 1109). Elemental and total carbon concentrations were measured following the NIOSH 5040 method, while volatile organic compounds (VOCs), and polycyclic aromatic hydrocarbons (PAHs) were measured through laboratory analysis and Scanning Electron Microscopy and Energy Dispersive using X-Ray Analysis SEM-EDX microscopy. The nanoparticle levels reached over 80,000 nanoparticles/cm³ (double the German Institut für Arbeitsschutz (IFA) benchmark levels). Diesel particulate matter levels measured as elemental carbon were around 35% of the occupational limit, and from the 49 VOCs analyzed only 13 were detected in quantities below the 0.1% of the occupational limit, while levels of the 13 PAHs analyzed, were below the laboratory limit of quantification. The study concludes that particulate matter in the underground car park can easily exceed nanoparticles benchmark levels and could be harmful, mainly to vulnerable people. It is therefore recommended that they use the outdoor car park or minimize their time in the underground one. Full article

Laser powder bed fusion is an attractive manufacturing technology promising novel components for the aircraft, automobile, and medical industries. However, depending on the material, some defects in the parts, especially pores or microcracks, cannot be avoided in the LPBF process. To achieve a part with low defect density, the optimal parameter sets must be determined. Many investigations have focused on how laser speed and laser power influence the melting process and the relative density of as-built parts. In this study, we considered laser and heated powder beds as two energy input sources, represented by volume energy density and preheating temperature, respectively. The interaction of these two energy inputs for the fabrication of AISI H13 was investigated. It was found that high preheating temperatures shifted the optimal parameter sets from the low energy density area to the high energy density area. In addition, high preheating also led to hot cracking, which was confirmed with Scheil solidification simulations. Full article

Heterogeneous catalysis plays a central role in the chemical and energy fields, owing to the high and tunable activities of solid catalysts that are essential to achieve the favorable reaction process efficiency, and their ease of recycle and reuse. Numerous research efforts have been focused on the synthesis of solid catalysts towards obtaining the desired structure, property and catalytic performance. The emergence and development of microfluidic reactor technology provide a new and attractive platform for the controllable synthesis of solid catalysts, primarily because of its superior mixing performance and high heat/mass transfer efficiency. In this review, the recent research progress on the synthesis of solid catalysts based on microfluidic reactor technology is summarized. The first section deals with the synthesis strategies for solid catalysts, including conventional methods in batch reactors and microfluidic alternatives (based on single- and two-phase flow processing). Then, different kinds of solid catalysts synthesized in microflow are discussed, especially with regard to the catalyst type, synthetic process, structure and property, and catalytic performance. Finally, challenges in the microreactor operation and scale-up, as well as future perspectives in terms of the synthesis of more types of catalysts, catalyst performance improvement, and the combination of catalyst synthesis process and catalytic reaction in microreactors, are provided. Full article

Due to the processes solid-state nature, cold gas-dynamic spray metal additive manufacturing may be considered microstructurally and micromechanically retentive, such that properties of the feedstock material are refined and partially retained, influencing component performance. As a result, cold spray processing enables unique freedoms regarding feedstock, which can be pre-processed using chemical, thermal, and mechanical treatments to produce powder properties that achieve finely controlled consolidations with application-specific behaviors. Given such features of the cold spray process, the present review article is concerned with the through-process integration of mechanically and microstructurally characterized feedstocks for optimizable cold spray metal additive manufacturing. Therefore, in this paper, we consider how nanoindentation (dynamic, static, and quasi-static) was coupled with microstructural characterization for experimental feedstock evaluation, testing, and characterization. Atomized aluminum alloys, atomized stainless steel, and copper feedstocks, among others, were considered. Accordingly, the review validates how microparticle feedstock pre-processing and characterization in cold spray metal additive manufacturing and processing lead to controllable component performance and properties. Full article

Silk fibroin has emerged as a leading biomaterial for biomedical applications. 3D printing has been successfully used for printing with silk fibroin, albeit in the form of a bioink, in direct-write 3D printers. However, in the form of bioinks, stability and mechanical attributes of silk are lost. An innovative alternative to producing 3D printed solid silk constructs is silk milled into powder for printing in a binder jetting printer. In this work, we focus on characteristics of silk powder to determine suitability for use in 3D printing. Two different silk powders are compared with hydroxyapatite powder, a known biomaterial for biomedical constructs. We have investigated powder size and shape by Camsizer X2 and Scanning Electron Microscope and bulk behaviour, dynamic flow behaviour, and shear behaviour by FT4 powder rheometer. Preliminary printing tests were conducted in an in-house custom-built printer designed for silk powder. It was found that silk powder has low flowability and stability. Therefore, to print solely out of silk powder, a 3D printer design will need sophisticated techniques to produce flow to ensure even distribution and consistent thickness of powder layers during the printing process. It was also found that high concentrations of formic acid (>75 to 99 wt.%) can fuse particles and therefore be used as a binder ink for 3D printing. The printer design challenges for silk powder are discussed. Full article

The advantage of nanoparticles to improve bioavailability of poorly soluble drugs is well known. However, the higher-energy state of nanoparticles beneficial for bioavailability presents challenges for both the stability of nanosuspensions and preventing irreversible aggregation if isolated as dry solids. The aim of this study is to explore the feasibility of an evaporation isolation route for converting wet media milled nanosuspensions into high drug-loaded nanocomposites that exhibit fast redispersion in aqueous media, ideally fully restoring the particle size distribution of the starting suspension. Optimization of this approach is presented, starting from nanomilling conditions and formulation composition to achieve physical stability post milling, followed by novel evaporative drying conditions coupled with various dispersant types/loadings. Ultimately, isolated nanocomposite particles reaching 55–75% drug load were achieved, which delivered fast redispersion and immediate release of nanoparticles when the rotary evaporator drying approach was coupled with higher concentration of hydrophilic polymers/excipients. This bench-scale rotary evaporation approach serves to identify optimal nanoparticle compositions and has a line of sight to larger scale evaporative isolation processes for preparation of solid nanocomposites particles. Full article

A gas phase, probe molecule doser was fabricated and connected to a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reaction chamber to study the reactions and stability of two organosilanes with the surfaces of metallic aluminum and boehmite powders in situ. Two metallic aluminum powder surfaces were studied, including an as-received, native oxide layer surface, and a laboratory prepared, boehmite-like surface. Neat boehmite powder was also used for reference and comparison to the laboratory prepared surface. We found that the metalloxane bond (Al-O-Si) was observed in the 1100–950 cm⁻¹ region for all surfaces, which indicates chemisorption between the adsorbate and available surface hydroxyls. We were also able to draw correlations between the loss of surface –OH and the subsequent growth of –CH for additional confirmation of adsorbate retention. Hydrothermal stability was probed through intentional exposure to water after chlorotrimethyl silane dosing, which showed adsorbate loss through fractional decreases in intensity of the –CH stretches. These results provide clear evidence of metalloxane bonds formed on aluminum powder and insight into their stability, supporting the identification of these bonds on bulk scale silane treated powders. Full article

Stellite-6 powders were sprayed on Ni-Al bronze in order to produce coatings via high-velocity oxygen fuel (HVOF). The microstructural observations revealed the main mechanisms taking place for the substrate–coating adhesion. It was revealed that tungsten-rich particles are very active in improving the coating adhesion as well as the mechanical properties. The X-ray diffraction analysis of the coating material showed pronounced peak broadening, revealing high residual stresses related to excellent bonding to the substrate. As expected, the coating procedure led to an increase in surface hardness. The surface properties of the coatings were evaluated through cyclic three-point bending tests at different maximum loads. It was demonstrated that the main part of the fatigue life is spent in the crack initiation stage, with a short propagation stage. Obviously, this behavior decreases as the maximum cyclic stress increases. The micro-mechanisms taking place during cyclic loading were evaluated through fracture surface observations via scanning electron microscopy. Full article

In this work, we study three aluminum oxides (alpha, gamma, boehmite) and various oxidized metallic aluminum powders to observe their dehydration and decomposition behavior using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and scanning electron microscopy (SEM). We find that a temperature increase to the aluminum oxides (aluminas) reduces physically adsorbed water molecules to reveal the presence of hydroxyl groups. All three aluminas contained bridged hydroxyls located at 3670 cm⁻¹; we found additional surface hydroxyls, which varied based on the oxidation state of the aluminum atom. Oxidized metallic aluminum powders that were aged resulted in similar behavior; however, the results differed depending on the method of aging. We find that naturally aged aluminum (NA-Al) powders with heavy oxidation in the form of the tri-hydroxide decomposed and did not reveal any detectable surface hydroxyl peaks. When aged using artificial methods (AA-Al), we find both surface hydroxyls, including bridged hydroxyls at 3670, 3700, and 3730 cm⁻¹, and a remaining boehmite-like surface. These results show that metallic aluminum powders can be tailored for specific applications, regardless of age. It also elucidates different ways to pre-process the powders to control the surface oxide layer, corroborated by comparison with the models oxides studied herein. Full article

Snow is a sintered matrix of ice, the strength of which is determined by the number and size of bonds between ice grains. However, because snow is a thermally unstable material, it is problematic to transport and store samples for accurate ex situ testing of mechanical behaviour. As an analogue for snow, we examined the sintering behaviour of different types of granular sugar at different humidities over different temporal periods and then assessed the extent of sintering and resistance to penetration of these samples. Like snow, increased sintering occurs in sugar over time. Sintering extent and rate are affected by the humidity environment and penetration resistance generally increases after increased sintering time. This preliminary examination suggests that in the absence of snow testing facilities, humidity-controlled sintered sugar may serve as a valuable proxy for examining the temporal variation of penetration resistance in snow. Full article

The nanosized silicon powder has been produced by reduction of silica with magnesium in an argon medium using both the mechanically activated self-propagating high-temperature synthesis and the direct mechanochemical synthesis and has been investigated by X-ray phase analysis, Infrared spectroscopy, electron scanning microscopy, and energy dispersive X-ray spectroscopy. The optimal Mg:SiO₂ ratio has been found to provide the minimum content of contaminant impurities of magnesium silicide and silicate in mechanically activated self-propagating high-temperature synthesis. For the first time, direct mechanochemical synthesis of Si via reduction of silica with magnesium has been implemented. Optimal component ratio and mechanical activation parameters have been determined, yielding Si/MgO composites without impurity phases (magnesium silicide and silicate). A purification procedure has been proposed for separating silicon obtained from magnesium oxide and other impurity phases. The ratio of initial components has been determined, at which purified silicon has the least amount of impurities. The particle size of silicon powder obtained was 50–80 nm for the mechanically activated self-propagating high-temperature synthesis, and 30–50 nm for the direct mechanochemical synthesis. Full article

The development of a Ti-34Nb-6Sn alloy by the powder metallurgy method, employing two different compaction conditions, A (100 MPa) and B (200 MPa), was carried out. To evaluate the feasibility of the Ti-34Nb-6Sn alloy as an implant biomaterial, microstructural and mechanical characterizations, as well as corrosion susceptibility and ion release tests, were performed. Results indicated microstructures dominated by the presence of β-Ti phase and a lower percentage of α-Ti and Nb phases. The porosity percentage decreased when the compaction pressure increased. Both conditions presented a good match between the elastic moduli of the alloy (14.0 to 18.8 GPa) and that reported for the bone tissue. The Ti, Nb and Sn ions released for both compaction conditions were within the acceptable ranges for the human body. Condition B showed higher corrosion resistance in comparison with condition A. Based on the obtained results, the produced porous Ti-34Nb-6Sn alloys are feasible materials for orthopedic implant applications. Full article