Fluidized bed atomic layer deposition (FB-ALD) has emerged as a scalable technique for surface coating on nanoparticles with subnanometer precision. Nevertheless, nanoparticles, due to strong cohesive forces, tend to form polydisperse agglomerates during fluidization, complicating mass transfer and reaction processes. In this study, a two-stage diffusion-reaction model for nanoparticle agglomerates is developed and coupled with a two-fluid model–population balance model (TFM-PBM), which effectively simulates fluidized bed reactors while considering fragmentation and reassembly of agglomerates during fluidization. The effects of particle cohesion and fluidizing gas velocity on ALD of nanoparticles in a fluidized bed reactor are investigated. The diffusion-reaction model can capture diffusion resistances at two scales: primary and complex agglomerates of nanoparticles and describe how the diffusion resistance increases with agglomerate size. The main factors affecting precursor conversion are the average size of complex agglomerates and the precursor residence time. The smaller the agglomerate size, the smaller the diffusion resistance; the longer the precursor residence time, the more sufficient contact with substrate particles. The precursor utilization is optimal when the agglomerate size is small and the gas velocity is just sufficient for fluidization.

The growing need for advanced materials with tunable properties has triggered an increasing interest in innovative surface modification techniques. Fluidized-bed atomic layer deposition (FB-ALD) offers a powerful solution for surface engineering and functionalization of powder-based materials for a variety of applications. By relying on its capability for controlling uniformity and conformality of the coatings precisely at the atomic scale, ALD can effectively modify surface characteristics to improve the functionality and durability of the materials. In this review, we will provide comprehensive fundamentals and strategies to improve the fluidization of nanopowders and reveal the potential of FR-ALD in two emerging applications. The first application is in energy devices, where FB-ALD is employed to develop Pt-based electrocatalysts for fuel cells and other catalytic reactions. We demonstrate that FB-ALD enables precise control of size, composition, and dispersion of Pt nanoparticles over the support surface, resulting in a strong enhancement in catalytic performance. We additionally discuss the application of FB-ALD in boosting the stability and durability of catalysts by surface engineering with ultrathin films and ultrasmall nanoparticles without compromising their activity. These capabilities open new avenues for the development of high-performance and durable catalysts for energy applications. The second application is in pharmaceutical research, where FB-ALD is employed to coat active pharmaceutical ingredients with thin films of biocompatible materials, such as Al₂O₃, ZnO, SiO₂, and TiO₂, to control their release profiles and improve their physical properties, such as wettability, dispersibility, flowability, and solubility, which are essential for enhancing therapeutic efficacy and patient compliance. The versatility and precision of FB-ALD position it as a key technology for the development of next-generation materials, addressing the critical challenges of performance, stability, and functionality of powder-based materials for different fields.

Nanoparticles are usually fluidized as agglomerates, which are in dynamic states of agglomeration and fragmentation. It is critical to consider the size distribution of agglomerates in modeling of the fluidization of nanoparticle agglomerates. In this article, the fluidization behavior of nanoparticle agglomerates is investigated using a two-fluid model─population balance model. The model includes the agglomeration and breakage kernel functions based on the continuum theory of cohesive particles developed by Kellogg et al. (J. Fluid Mech. 2017;832:345-382). The ratio of the critical breakage velocity to the critical agglomeration velocity is defined to represent the cohesion of nanoparticles. The predictions of bed pressure drop, bed expansion ratio, and bed collapse curves agree well with those of experiments. By changing the critical agglomeration velocity and the ratio between the critical velocities, the transition from almost defluidization to uniform fluidization is predicted. Finally, the model’s ability to simulate the fluidization of fine particles with a few micrometers is also shown. This study provides a practical tool for simulating the fluidization of nanoparticle agglomerates.

Fluidization of nanoparticle agglomerates is a promising technique to process nanoparticles. However, possible elutriation of small agglomerates may cause significant loss of bed material. To obtain the elutriation behavior under stable operation, in this study the elutriation fraction of silica nanoparticle agglomerates is measured in a vibro-fluidized bed, which is operated for several hours. Among conditions with different fluidizing gas velocities, U_g, and vibration strength, Λ, the lowest elutriation fraction measured is around 5% after 7-hour fluidization. The elutriation fraction increases significantly with U_g, while varies slightly with Λ. To help elucidating the elutriation behavior, the agglomerates at three different locations (bed surface, splash zone, and bed outlet) are sampled and their size distributions are determined. The elutriation rate constant is found to be much smaller than the literature results for ordinary particles, and the reasons are discussed in detail. Finally, an empirical correlation considering size distribution is proposed to fit the elutriation rate constant for the conditions in this study.

Bubbles and slugs coexist in a fluidized bed with strong inter-particle forces. However, the effects bubbles and slugs impose over fluidizing stability, and chemical reaction efficiency are different. The present work distinguished bubbles and slugs with the aid of X-ray tomography, and the evolution of their properties with varying inter-particle force in a fluidized bed were studied. The results show that the gas-holding capacity of the emulsion phase improves as inter-particle forces initially increase. In this process, the bubble frequency decreases noticeably, and the bubble size slightly increases. Bubbles rise slower than theoretically predicted because inter-particle force reduces the particle flow. The frequency and the size of slugs increase with inter-particle forces. As forces initially increases, the slug size increases with measurement height. An opposite trend is observed when inter-particle forces continues to strengthen. Near the gas distributor, the slug rising velocity is independent of size. In contrast, large slugs have a higher velocity than small slugs near the bed surface. Finally, three slugging patterns are distinguished, and the influence they impose on particle flow is analyzed.

The increase of inter-particle cohesive force greatly changes the fluidization dynamics, finally leading to the partial or complete failure of fluidization. However, few studies concern such transition process. This paper investigates the fluidization dynamics of Geldart B particles with a wide-range of cohesive force by analyzing the in-bed pressure fluctuation signals. Combining the bubble information reported in Part I, the local and global fluidization dynamics under different cohesive forces were discussed. The results show that bulk bubble dynamics is weaken with the presence of inter-particle cohesive force. As the force increases, fluidization changes from multi-bubbling regime to single-bubble regime and the factor governing the pressure fluctuation changes from bubble formation to bubble eruption. When the cohesive force is strong, slugging appears near the bed surface, then gradually extends toward the bottom bed by capturing freely fluidizing particles, and finally develops into the whole-bed slugging. At this time, regular fluidization turns into an alternative process between whole-bed slugging and regular status, corresponding to two distinct peaks in power spectral density of pressure signals at 0.1 Hz and 1 Hz respectively. The size of gas slug decreases with the elevation of measurement height. Basically, any operations that promote bubble growth will also facilitate the appearance of whole-bed slugging under strong cohesive force. Reducing the static bed height is a preferable approach to weaken, or even avoid the defluidization of whole-bed slugging, without changing other operational parameters.

Due to the presence of inter-particle cohesive force, cohesive particles reveal totally different fluidization behaviors as compared to the non-cohesive system. This paper studies the fluidization dynamics of Geldart B particles with varying thermal-induced cohesive forces. Multi-source X-ray tomography was applied to reconstruct 3D temporal images of bubbles, based on which, various bubble properties were extracted. The results show that increasing cohesive force will decrease bubble number while increase bubble size, implying that the presence of cohesive force facilitates bubble coalescence. By examining the bubble size distribution, cohesive force is found to have no effect on the number of median bubbles but greatly influence small and large bubbles. When the cohesive force is strong, the bubbles grow to a considerable size similar with bed dimension, giving rise to slugging near bed surface. With the action of inter-particle cohesive force, particle slug gradually grows by capturing other freely fluidizing particles, finally inducing “whole-bed” slugging. The particle slug may rupture in the rising process, and the bed turns back to normal fluidization. In comparison to normal bubbles, the gas slug has much larger size but far smaller frequency. The rise velocity of gas slug is also very low due to the particle-wall friction and gas-solid momentum dissipation. Therefore, the averaged values of bubble properties dramatically changed as bed temperature exceeds 35 °C. When the temperature attains 45 °C, the cohesive force is so strong that the fluidization completely fails in terms of stable whole-bed slugging.