Powders, Volume 4, Issue 3 (September 2025) – 7 articles

The addition of hard particles such as borides to a ductile stainless steel matrix can be very efficient for improving mechanical properties. Powder metallurgy represents a suitable route for developing these material modifications, combining high reproducibility and cost-effectiveness. The present research investigated the effect of sintering time on an atomized, pre-alloyed 2205 stainless steel with 2.5 wt.% boron, using two different powder size distributions: fine (<45 µm) and coarse (250–500 µm). Cold uniaxial compaction was conducted using a cylindrical closed die. Sintering was carried out at 1200 °C with a dwell time of 2 and 4 h in argon atmosphere. Microstructural investigation showed that borides were formed in the powder’s atomization step and presented a small size with different morphologies. The borides significantly improved the hardness and compression strength. Compared to the reference 2205 stainless steel, specimens prepared with the fine powder size distribution achieved a twofold enhancement in yield stress, while hardness increased by 26%. Full article

Electrostatic precipitators (ESPs) are widely applied to reduce particle concentrations. However, the performance of ESPs is impaired in the nanosized diameter range due to the difficulty in electrically charging these particles. The present work evaluated the inclusion of a wire screen, perpendicular to the airflow, as an additional collecting electrode of a single-stage wire-plate ESP containing two collecting plates and a single discharge wire. ESP performance was evaluated in terms of voltage, air velocity and electrode positioning in relation to the beginning of the collecting plate (inlet spacings of 1.5, 10 and 23 cm). When compared to theoretical prediction, the penetration results presented a decay for larger particles not predicted by the diffusion battery model. It was observed that the inclusion of the wire screen increased the removal of ultrafine particles and that the overall collection efficiencies increased up to 70% in the operating conditions evaluated. Moreover, the central positioning of the electrodes (inlet spacing of 10 cm) achieved the highest collection efficiencies at high voltages, but the final positioning (inlet spacing of 23 cm) presented a better performance at higher air velocities. Therefore, the wire screen can be an alternative to enhance nanoparticle collection. Full article

Thermal wick-debinding, commonly used in low-pressure injection molding, remains challenging due to complex interactions between binder transport, capillary forces, and thermal effects. This study presents a numerical simulation of binder removal kinetics by coupling Darcy’s law with the Phase Transport in Porous Media interface in COMSOL Multiphysics. The model was validated and subsequently used to study the influence of key debinding parameters. Contrary to the Level Set method, which predicts isolated binder clusters, the Multiphase Flow in Porous Media method proposed in this work more accurately reflects the physical behavior of the process, capturing a continuous binder extraction throughout the green part and a uniform binder distribution within the wicking medium. The model successfully predicted the experimentally observed decrease in binder saturation with increasing debinding temperature or time, with deviation limited 3–10 vol. % (attributed to a mandatory brushing operation, which may underestimate the residual binder mass). The model was then used to optimize the debinding process: for a temperature of 100 °C and an inter-part gap distance of 5 mm, the debinding time was minimized to 7 h. These findings highlight the model’s practical utility for process design, offering a valuable tool for determining optimal debinding parameters and improving productivity. Full article

Smokeless powders are a commonly used low explosive within the ammunition industry. Their ease of purchase has allowed criminals to use these products to build improvised explosive devices. Canines have become a vital tool in locating such improvised devices. With differing fabrication processes, one of the most difficult challenges for canine handlers is the optimal selection of training aids to choose as odor targets to allow for broad generalization. Several studies have been underway to understand the chemical odor characterization of smokeless powders, which can help provide canine teams with essential information to understand odor signatures from powder varieties. In this study, a SPME method optimization was conducted using unburned smokeless powders to provide a chemical odor profile assessment. Concurrently, statistical analysis using PCA and Spearman’s rank correlations was performed to explore whether odor volatile composition depicted associations between and within powder brands. The results showed that a longer extraction time (24 h) was optimal across all powders, as this yielded higher compound abundance and number of extracted odor volatiles. The optimal SPME fiber varied per powder, depicting the complexity of powder composition. There were 66 highly frequent compounds among the 18 powders, including 2-ethyl-1-hexanol, diphenylamine (DPA), and dibutyl phthalate. Principal component analysis (PCA) showed that while powders may be of the same type (single/double base), they can still portray clustering differences across and within brands. The Spearman’s rank correlation within powder type suggested that the double-base powders had a slightly higher similarity index when compared with the single-base powder types. Understanding the volatile odor profiles of various smokeless powders can enhance canine training by informing the selection of effective training aids and supporting odor generalization. Full article

Particle deposition is a phenomenon that occurs in many natural and industrial systems. Nevertheless, the modelling and understanding of such processes are still quite a big challenge. This study uses a discrete phase model (DPM) to determine the deposition constant for the particles in a liquid phase flowing in a horizontal pipe. This study also develops a steady-state population balance equation (PBE) for the particles in the flow involving deposition and aggregation and an unsteady-state PBE for particles depositing on the wall. This establishes a mathematical relationship between the deposition constant and velocity. An industrial setting of a 1000 m long pipe of 0.5 m in diameter was used for the population balance modelling (PBM). Based on the extracted deposition constant from the DPM, it was found that the particle deposition velocity increases with the continuous flow velocity. However, the number and volume of the deposit particles on the wall reduce with the increase of the continuous flow velocity. The deposition was found mainly taking place in the inlet region and reduces significantly towards the pipe outlet. The deposition was also found driven by advection of particles. Calculated deposit thickness showed that increasing the continuous flow velocity from 1 m s⁻¹ to 5 m s⁻¹, the thickness at the inlet would reduce to nearly 1/40th. With a 10 m s⁻¹ flow, this would be 1/80th. Full article

The separation of particles smaller than 1 µm either by composition or by size is still a challenge. For the separation of SiO₂ and SnO₂, the creation of a selective separation feature and the specific adsorption of salts and surfactants were investigated. The adsorption of various salts, e.g., AlCl₃, ZnCl₂, MnCl₂ and MgCl₂ were therefore analyzed, and the necessary concentration for the charge reversal of the material was determined. It was noticed that the investigated materials differ in their isoelectric point (IEP) and therefore in their adsorption behavior because only ZnCl₂ and MgCl₂ are suitable for a charge reversal of both metal oxides. The phase transfer of the pure material at different pH values with ZnCl₂ or MgCl₂ and sodium dodecyl sulfate (SDS) revealed that the adsorption behavior of the particle has an influence on the phase transfer. As a result, the phase transfer of SiO₂ is pH dependent, whereas the phase transfer of SnO₂ operates over a wider pH range. This allowed the separation of SiO₂ and SnO₂ to be controlled by the salt and surfactant concentration as well as pH. The separation of SiO₂ and SnO₂ was investigated for various parameters such as salt and surfactant concentration, particle concentration and composition of the mixture. Also, pH 8, where a selective phase transfer for SiO₂ occurs, and pH 6, where the greatest difference between the materials exists, were also investigated. By comparing the parameters, it was found that the combination of ZnCl₂/SDS and MgCl₂/SDS enables a selective separation of the materials. Furthermore, it was also found that the concentration of SDS has a significant effect on the separation, as the formation of a bilayer structure is important for the separation, and therefore, higher SDS concentrations are required at higher particle concentrations to increase the separation efficiency. Full article

The effect of induction heating on alumina ceramics and alumina ceramic composites based on α-Al₂O₃ nanopowders (additives: SiC, Si₃N₄, SiO₂, ZrO₂) has been examined. Various factors such as the structure, grain size, distribution of elements, hardness, fracture toughness, and wear rate of hot-pressed ceramic materials were assessed. Despite achieving improved densification of alumina ceramics at a higher temperature of 1720 °C, there is a consistent trend toward a decline in hardness and fracture toughness. Heating at lower temperatures of 1300–1500 °C results in the development of a strengthened surface layer with a fine-grained structure enriched with carbon. Therefore, the wear rate behavior of such ceramics differs from the behavior of samples made at higher temperatures of 1600–1720 °C. This fact indicates the presence of a non-thermal microwave effect of induction heating. The incorporation of additives to alumina leads to the formation of novel structures with altered crack propagation patterns. The optimal ceramic composite, containing 5 wt. % SiC, displayed superior hardness and the lowest wear rate when compared to pure alumina ceramics. Across all investigated composites, a short dwell time at 1700 °C results in an enhancement of the mechanical properties. Full article