The adhesion of fine particles plays a significant role in the performance of particulate processes and in the quality of particulate products. The extent of adsorption of water on the particles from the surrounding atmosphere is governed directly by the relative humidity of the air. Published evidence suggests that changes in relative humidity of the air can have a profound influence on the adhesion of individual particles. However, there are numerous conflicting reports in the literature suggesting that adhesion can increase, decrease or pass through a maximum as the relative humidity increases. A thorough review of the relevant literature is presented in which the experimental evidence relating to the influence of relative humidity on particle adhesion is gathered and discussed. Apart from the amount of water adsorbed on the surface, it is clear that the adhesion depends upon the surface roughness which prevents the formation of a complete capillary meniscus at the particle contact point until a critical relative humidity is reached. At this point, sufficient water is adsorbed to engulf the asperities and a marked increase in adhesion is noted. Many surfaces undergo some physical or chemical change in the presence of adsorbed water, e.g. solubility, softening, or phase change. This can lead to complex adhesion behaviour. Hysteresis of adhesion with increase/decrease of relative humidity is commonly observed. Theoretical approaches correctly recognise the role of the Laplace pressure in the capillary bridge as being dominant in controlling adhesion. However, evidence suggests that contributions from the solid-solid interaction, surface tension of the bridging film and dis-joining pressure can also be important under certain circumstances. As an illustration of complex behaviour, original data for the adhesion of a glass microsphere on a flat glass surface are presented. The data were obtained using a custom-built AFM instrument. Upon desorption, a critical relative humidity lying between 30% and 40% was observed. At this point, a singular peak in adhesion occurred which was accompanied by long-range repulsion between the surfaces prior to contact. The long-range repulsion was found to be an exponential function of separation distance. This phenomenon has been attributed to the spontaneous formation of glass corrosion products from the liquid layer on desorption when the two surfaces approach each other. According to published literature, these corrosion products can take the form of needle-like or dendritic structures. The resulting steric repulsion is observed to be exponential with respect to distance, in agreement with the established trend. Once contact is made between the surfaces, the strong adhesion may be due to either sintering or entanglement of the corrosion products. Further work is required to determine the precise chemistry of the process. At high humidities, the force-separation curves exhibit an extended pull-off region, suggesting that the adsorbed film is mobile. This effectively reduces the adhesion by increasing the volume of the liquid bridge as the surfaces are pulled apart. This may explain why some reports in the literature show decreases in adhesion at high humidity.
A microcapsule is a tiny capsule and its preparation procedure, called microencapsulation, can endow various traits to the core material in order to add secondary functions and/or compensate for shortcomings. This paper first gives a general overview of microencapsulation technology and subsequent sections cover topics of microencapsulation research, including application of microcapsules to information and image technologies for microparticle-based paper-like display systems, a use that has attracted considerable attention recently. The research results of the project conducted by the Japan Chemical Innovation Institute under the commission of NEDO, as one of the projects of the METI Nanotechnology Program to develop microparticle-based paper-like display systems, are also introduced.
Methods currently in use for the in-vitro measurement of the particle size distribution of aerosols from medical inhalers are reviewed with emphasis on their applicability both for product development and quality control testing and for simulation of likely performance in clinical use. Key attributes and limitations of the various techniques are identified, and consideration is also given both to likely developments to improve the capabilities of these analyzers as well as to the procedures for their use.
We present a fluidized bed apparatus that enables us to test the bulk mechanical properties such as the yield stresses and compressibility of fine cohesive powders. Every measurement is preceded by driving the powder into the bubbling regime in which the material loses memory of its previous history. Then the gas flow is set to a given value to take the powder into a well defined and reproducible initial state of low consolidation. Reverse flow is used to exert high compressive stress. A cornerstone of our technique is that the procedure is automatized, thus making results operator insensitive. Besides being a practical tool to diagnose the flowability of experimental powders, the Sevilla Powder Tester (SPT) also provides us with a powerful technique to research fundamental problems in powder mechanics.
A computer model for particle packing is of importance in both theories and applications. By taking a very different approach from existing packing algorithms, our digital packing algorithm – called DigiPac – is able to avoid many of the difficulties normally encountered by the conventional algorithms in dealing with non-spherical particles. Using the digital approach, it is easy to pack particles of arbitrary shapes and sizes into a container of any geometry. This paper briefly describes the digital packing algorithm, but the focus is on validation of the DigiPac model through several case studies involving mono-sized non-spherical particles and also powders with different size distributions. Packing densities from DigiPac simulations are compared with those measured experimentally by ourselves in some cases and in others with data published in the literature using other models. The results show a good agreement in all the cases, which enhances our confidence in DigiPac that despite being a geometrical packing algorithm with no explicit consideration of particle interactions, it is able to predict quite accurately the packing structure of particulates whose shapes are commonly encountered in both industry and everyday life.
The aim of this work was, on the one hand, to gain a better understanding of the effect of flow additive content on the powder flowability, and on the other hand, to point out the most suitable tests to characterize the flow properties of industrial powder paints used in automotive industries. The flow properties of 5 powder coatings, containing 0, 0.12, 0.30, 0.53 and 0.96 w/w%, respectively, of a flow additive and an industrial batch, were tested using both conventional and novel characterization techniques. The lubricant used was a silica powder. Test methods employed were a packing test, a circular shear cell (Peschl), a powder rheometer and a fluidization/de-aeration test. The flowability of powder batches is significantly improved with increasing lubricant content up to an optimal value of about 0.53%. SEM images of different powder samples showed that the optimal point corresponds to a critical additive content where the amount of additive is high enough to form a continuous film around the particles. Beyond this critical content, the particle-lubricant contacts are replaced by lubricant-lubricant contacts. This phenomenon leads to a degradation of flowability due to a higher cohesivity of additive particles.
Aerosol technology is the key process for large-scale production of nano-structured materials such as carbon black, titania and silica. The understanding of gas-phase synthesis was transferred successfully from classic vapor-fed flames to liquid-fed aerosol reactors, enabling now also the one-step production of demanding and highly functional products. Such aerosol-derived nano-structured metal oxides, mixed metal oxides, and metals on metal oxides find application in the fields of catalysis, sensors, fillers, and electronics, and have advanced the research and development of these reactors in recent years. The four main spray methods include spray pyrolysis in a tubular reactor (SP), spray pyrolysis using a vapor flame reactor (VFSP), the emulsion combustion method (ECM) and flame spray pyrolysis (FSP). These methods are discussed and key concepts are compared such as the energy source driving the solvent/fuel evaporation and precursor reaction, final particle formation. Advances in fundamental understanding, scaling and simulation are highlighted. Specific strategies for the production of homogeneous products are presented in context with existing methods and specific applications. Finally, research needs are discussed with respect to new flame-made materials, instrumental strategies for their production and process optimization, including diagnostic techniques and process simulation.
Pull-off forces were measured using the vibration method at 30–50% relative humidity for glass and tin spheres on a variety of substrates. The results were compared with those obtained through the colloidal probe technique. Both methods show good agreement for small particle sizes. Since the vibration method causes sinusoidally alternating stresses, the method yields detachment and contact forces between particles and substrate of the same order of magnitude. Alternating contact forces of the vibration method can cause an increase in the adhesion force through flattening of asperities, which also depend on the surface roughness and the mechanical properties of particle and substrate. Pull-off force measurements with the colloidal probe technique and special attention to the influence of the contact force also show an adhesion force intensification with increasing contact forces depending on the surface roughness. No significant adhesion force intensification caused by increasing contact time to 30s at several contact forces was observed. For theoretical predictions based on van der Waals adhesion, an approach presented by Rabinovich and approximations of plastic micro-asperity flattening were combined.
The processing characteristics of agglomerates of silica aerogel particles (surface treated to possess either hydrophilic or hydrophobic nature) have been examined. Although these agglomerates were purported to be highly uniform in terms of internal structure, fluid infiltration studies performed via direct visualization revealed significant heterogeneities. Dispersion studies performed under both steady and time-varying flow conditions, revealed additional non-uniformities among the population of agglomerates. Despite these difficulties, discernable trends linked to the duration of time the agglomerates were allowed to soak in the processing medium prior to the application of the shear flows were discovered. Additional experiments, in which the agglomerates were presoaked with a different, low viscosity fluid prior to shearing, revealed that dispersion could be promoted through this processing strategy. The experimental approaches and analyses presented in this study should be of interest to particle technologists faced with understanding and characterizing dispersion phenomena when the agglomerates being dispersed exhibit significant variability in properties.
The homogeneous distribution of the reinforcement phase is a prime requisite for a composite material to present its superior performance. Powder metallurgy can produce composite materials in the whole range of matrix reinforcement composition, without the segregation typical of the casting process, and mechanical alloying serves to optimise the particle mixing stage, enhancing the reinforcement distribution. This work investigates the use of mechanical alloying plus hot extrusion to obtain Al6061 matrix composites reinforced with Si3N4, AlN and ZrB2, and compares the result with the same composite materials obtained by more conventional powder metallurgy techniques. The incorporation of the reinforcement does not suffice to produce a significant improvement of the mechanical properties of the conventional powder metallurgy composites. Mechanical alloying breaks the reinforcement particle clusters, eliminates most of the defects present in these particles, decreases their size and enhances their distribution, which together with the metallurgical phenomena that change the metallic matrix, such as work hardening and oxide and carbide dispersion, produces an increase of about 150% in the hardness of the powder, when compared with the hardness of the as-received, non-reinforced aluminium powder alloy; and of 100% in the hardness and ultimate tensile strength of the consolidated materials, when compared with material of same composition processed by conventional powder metallurgy.
A molecular modeling based approach to design/selection of dispersants for colloidal processing of ceramics powders is presented. The validity and the utility of the proposed approach are illustrated with two case studies. The relative strength of interaction of dispersants belonging to two known family of dispersants namely, hydroxy aromatics (phenol derivatives) and aliphatic/aromatic carboxylic acids with alumina are found to correlate well with the experimental trends reported in literature. The dispersibility of BaTiO3 nanoparticles as determined by the sediment density in different organic liquids are observed to match well with the corresponding interaction energies computed theoretically.
Surface modified alumina powder that has photocatalytic activity was studied. To control the amount of Ti introduced on the surface at the molecular level, the powder was prepared by chemical surface modification with a monofunctional titanate-based coupling agent, followed by partial oxidation of the modification group. The surface density of Ti introduced onto the surface was quantitatively increased with the concentration of the modifying reagent, and/or repetition of the modification. Furthermore, the relationship between the surface structure and the occurrence of photocatalytic activity was examined. The surface structure was measured by XPS and UV-Visible absorption spectrometry. The photocatalytic activity was determined by measuring the degradation of Methylene Blue molecules in water solution with UV irradiation. Photocatalytic activity was realized at a Ti surface density of 3.7 nm–2. The optical absorption edge in UV of the sample shifted to longer wavelengths. The ability as a photocatalyst increased with the amount of Ti-O-Ti. Photocatalytic activity was recognized to be related to the amount of Ti-O-Ti on the alumina surface and the ability of UV absorption.
†This report was originally printed in J. Soc. Powder Technology, Japan 39, 102-107 (2002) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
Tribo-electrification between aluminum target and grass beads has been studied experimentally based on the current generated by the electrification of grass beads. Effect of tangential velocity of impact on the current is investigated. As far as the velocity is low enough, the current shows positive value. However, for higher velocity, the current changes from positive to negative as the time elapses. Sudden increase or decrease in the velocity gives a respective current corresponding to each velocity. The fact means that the polarity of the current is solely determined by the tangential velocity as far as the target is not exchanged. Preservation conditions of the metal target affect on the polarity of the current. If the target is preserved in atmospheric condition for a day, positive current is generated firstly, decreased with time elapsed and finally negative current is generated. Two different experiments support that the polarity change found in this study is not related to the surface oxidization. Effect of water molecules adsorbed on the target surface has been investigated through heat treatment of the target. Higher temperature of the treatment causes the sharper decrease in the initial current, which suggests that the water molecules adsorbed on the target surface causes the change in the polarity of the tribo-electrification. Contact potential difference between particles and target also changes by heat treatment of the target.
†This report was originally printed in J. Soc. Powder Technology, Japan, 40, 860-867 (2003) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
We investigated the sol-gel synthesis process of barium titanate ultrafine particles with using barium hydroxide aqueous solution and titanium alkoxide solution. Emphasis was placed on the relationship between the mixing condition of both solutions and the properties of the particles. A static mixer was used as reactor and the mixing condition was quantitatively characterized by the f low rate of solutions in a static mixer. Mean particle size and the width of size distribution of cubic barium titanate ultrafine particles decreased with increasing f low rate, because the initial concentration of nuclei increased by rapidly mixing the solutions. Since the density of the hydroxyl group in the lattice of cubic barium titanate increased with increasing f low rate, barium titanate had a lattice structure of expanded cubic perovskite.
†This report was originally printed in J. Ceramic Society of Japan, 111(1), 67-72 (2003) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of The Ceramic Society of Japan.
We have prepared the thermoelectric materials, where the small particles are dispersed in the matrix, by mechanical grinding (MG) or alloying (MA). Si and C powders were added to the p-type Fe0.92Mn0.08Si2 or n-type Fe0.98Co0.02Si2 powders, and they were mechanically milled followed by being hot-pressed. The amounts of the β-phase, ε-phase, α-SiC and C in the sintered samples depend on the amount of the added (Si+C) and Si/C molar ratio, which affect the values of the Seebeck coefficient and electrical resistivity of the samples. A lot of fine α-SiC particles around 20nm were dispersed in the samples, which decreases the thermal conductivity of the samples. The maximum figure of merit values appear at Si/C=1.5 for p-type Fe0.92Mn0.08Si2 containing 4 mass% (Si+C) and Si/C=1.75 for n-type Fe0.98Co0.02Si2. We have also prepared the CoSb3-FeSb2 composite where the FeSb2 particles are dispersed in the CoSb3 matrix. The dispersed FeSb2 particles contributed to decreasing the thermal conductivity and maintaining the moderate Seebeck coefficient and electrical resistivity of the samples. As a result, the composite whose molar ratio CoSb3/FeSb2 is 0.7/0.3 and milling time is 25h has a maximum value of figure of merit of 6.1×10−4 K−1 at 756K, which is much larger than that of 3.2×10−4 K−1 for CoSb3.
†This report was originally printed in J. of the Japan Soc. of Powder and Powder Metallurgy, 50, 451-457 (2003) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Japan Society of Powder and Powder Metallurgy.
Barium titanate (BT) nano-particles with different diameters were prepared by Sol-Gel method. The particle diameter was determined by XRD, and the lattice constants of nano-particles were also determined by XRD. We also measured the lattice vibration for BT nano-particles by Raman scattering to discuss on the size effect. As a result, we successfully calculated the intrinsic dielectric constants for BT nano-particles by analyzing the lattice vibration. Moreover, we confirmed the shift of Curie temperature by the size effect.
†This report was originally printed in J. Soc. Powder Technology, Japan, 41(2), 86-91 (2004) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.