Control of the morphology of nanostructured particles prepared by the spray drying of a nanoparticle sol
Introduction
The production of nanostructured particles via spray drying is of interest due to the fact that the prepared particles exhibit unique properties that permit their use in various fields, such as catalysts, chromatography, fillers, pigments, cosmetics, and photoluminescence materials. Spray drying is an established method that is initiated by atomizing/spraying suspensions into droplets followed by a drying process, resulting in solid particles [1]. This method is widely used because of its many advantages, including the fact that it is a simple system, its cost effectiveness, and the fact that it can be scaled up to ton quantities. The fabrication of micrometer-sized particles via spray drying has been reported previously by several authors [1], [2], [3]. We used a spray drying method to produce ordered porous particles by a very simple and rapid route [4].
Although the morphology of the prepared particles is generally spherical, the presence of other morphologies such as ellipsoids and toroids has been reported in previous publications [2], [3], [4]. Iskandar et al. [4] assumed that the oscillation of the droplet shape, resulting in deviation from the spherical shape, is a basic mechanism for the formation of toroid particles, similar to the formation of toroid particles by growing colloidal crystals in aqueous droplets suspended on fluorinated oil [5]. However, a detailed report regarding the control of particle morphology during spray drying appears to be unavailable in the literature.
In this paper we present a theoretical explanation for the control of particle morphology via a spray drying method. The structural stability of the droplet and hydrodynamic effects during the spray drying process appear to have important effects on particle morphology. The effects of the size of the solid particle, the concentration of the precursor used, the furnace temperature, and flow rate on the morphology of a prepared particle were investigated experimentally. A silica colloidal suspension was used as a precursor to produce nanostructured particles because their physical and chemical properties are well known. Colloidal silica is a good precursor for spray drying because of its stability, its ease of dispersion in water, the fact that it can be obtained in a highly pure state, and the fact that its size can be controlled within the nanometer scale.
Section snippets
Hypothetical effects influencing structural destabilization of the two-phase droplets
Figure 1 shows the scheme used in the preparation of submicrometer-sized nanostructured particles using the spray drying method. The precursor is atomized to form spherical droplets with a diameter dd, which contains nanometer-sized solid particles (sol) with a diameter dp. Droplets with diameters in the range between 1 and 100 μm are usually produced by an atomizer such as an ultrasonic nebulizer. The droplets are carried into a tubular reactor by a carrier gas for drying. The dispersion water
Experimental
Figure 6 shows the experimental setup used to produce the nanostructured silica particles from colloidal suspensions. It consists of three sections: (i) a spray generator, (ii) a vertical tubular furnace, and (iii) a particle collector. Nanosized silica colloidal suspensions were used as the precursor for producing submicrometer sized of silica particles. The spray generator used was an ultrasonic spray nebulizer with a 1.75 MHz resonator (Omron Corp.). The atomized droplet size (from water as
Results and discussion
Figure 8 shows SEM photographs of silica particles prepared from a silica primary particle with diameter dp=25 nm, flow rate Q=2 l/min, mass fraction of silica particles ϕ=1 wt%, and drying temperature T=600 °C. It can be seen that there are many spherical particles and also some toroid particles formed from the fraction of the larger droplets produced by the ultrasonic nebulizer. As shown in Eq. (1), the Bond number is proportional to the square of the droplet size; if the droplet size becomes
Summary
The control of nanostructured particle morphology prepared by the spray drying of a nanoparticle sol was investigated experimentally and the results are explained qualitatively based on available theory. The structural stability of a droplet and hydrodynamic effects during the drying process play important roles in controlling the morphology of particles. The sol size in the droplet, the droplet size, the viscosity of droplet, the drying temperature, the gas flow rate, and the addition of
Acknowledgements
This work was supported in part by the New Energy and Industrial Technology Development Organization (NEDO)'s Nanotechnology Materials Program—Nanotechnology Particle Project based on funds provided by the Ministry of Economy, Trade, and Industry, Japan (METI).
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