Susumu Inasawa’s research while affiliated with Tokyo University of Agriculture and Technology and other places

Chemical reaction formula does not provide information on size and morphology of formed solids. In this talk, we introduce our recent results on process-dependent changes in size and morphology of silicon solids produced via zinc reduction of SiCl4. Silicon whiskers, microparticles and nanowires form in the reaction, in which zinc vapor reduces SiCl4 and zinc droplets catalyse crystallization of silicon. Flow of gases in a reactor and temperature of substrate affect solidification processes of silicon. Effects of there factors on the formation of silicon whiskers and nanowires are discussed using both experimental results and numerical analysis.

Drying of suspensions has been widely used in industrial processes. Involatile components are concentrated during evaporation of solvents, which induces precipitation of solutes or packing of suspended particles. Evaporation also causes deformation of liquid-air or liquid-liquid interfaces and flows in drying liquid films. They are correlated to others in a complicated manner and fundamental understanding is still necessary. In this manuscript, we provide a short review on recent studies on drying-induced phenomena, especially on drying suspensions.

Wrinkles formed during the drying of polymer films by hot-air flow but not when drying by infrared irradiation, even though drying rates were comparable. The top surface was hotter than the bottom during hot-air drying. Conversely, the temperatures of the top and bottom surfaces were very similar when drying by infrared irradiation. We also examined uniform heating with faster and slower drying rates using drying with combined infrared irradiation and air flow (hot or room temperature). Wrinkles formed in both cases. This demonstrated the negative effect of air flow. Features and mechanism of each drying method are discussed.

Drying of volatile oil droplets immersed in a continuous water phase was observed and analysed. Drying sample solutions were sandwiched between two glass plates and the water and oil phases were observed by confocal microscopy. In the initial stage of drying, evaporation of water was dominant and drying of the oil droplets was negligible. However, the rate of water evaporation decreased when the oil droplets were compressed. Comparison of experimental data with a diffusion model of water vapour showed that the decline in drying rates occurred earlier in the experiment than in the theoretical prediction. This implies that compression and narrowing of water paths caused the decline in the rate of water evaporation. After most water had evaporated, evaporation of the oil droplets occurred. The oil droplets did not shrink isotropically and the air–liquid interface invaded into the drying oil droplets. Cross-sectional observation by z-scanning revealed direct exposure of the oil droplets and they were pinned by the residual water phase. Water network between oil droplets collapsed after the oil droplets had evaporated. Correlation between changes in structures and drying kinetics in both liquid phases was discussed.

A gravity effect was demonstrated for 10-nm particles drying in colloidal suspensions. The particles were well-dispersed and did not sediment. However, when a suspension was dried on a tilted directional cell, a clear downward flow of particles was observed around the packing front, which was the boundary between the packed particles layer and the suspension. Three particle sizes (10–110 nm) were examined, with the most pronounced effect being on the 10-nm particles. The primary origin of the downflow was attributed to condensation of particles near the packing front and the subsequent increase in the overall density of the condensed layer. Because of the flow, the packing front was not parallel to the drying interface and tilted cracks formed in the packed layer. A mathematical model was proposed that considered conservation of the suspended particles in the condensed layer. Three competing factors of particle transport (advection, particle consumption by packing, and particle transport by the downward flow) were used to explain the experimental results. Overall, the results suggested that simple substrate tilting would be useful to evaluate whether suspended particles are easily packed or not during drying.

In this study, we demonstrate that thick silicon whiskers (wires) formed on a sufficiently cooled substrate. We used the gas phase zinc reduction reaction of SiCl4 at a gas reaction temperature of 910 °C. Wires formed via the tip-growth mode in the vapor–liquid–solid mechanism. When the substrate was the same temperature as the gas phase, the formation of multiple wires from a single point was observed. Conversely, this multiple formation was not observed, and thicker wires formed, on the sufficiently cooled substrate. We propose that splitting of a zinc catalytic droplet into several smaller ones is one of the main reasons for the formation of multiple wires. Suppression of this splitting on the cooled substrate was considered to be a main reason for the formation of thicker wires. We conducted a numerical approach to predict temperature profiles along a growing wire, because the growth rate of wires was about 5 mm min⁻¹ and heating owing to the release of the latent heat for solidification was not negligible. Three processes, the conduction in a wire, the convection to the surrounding gas and the radiation via the surface of wire, were considered. We also considered the heating of the growing tip due to solidification as well. Our simulation revealed that the tip temperature of a rapidly growing wire increased and was higher than that of the substrate. The effect of substrate temperature and subsequent temperature profile for a growing wire was discussed.

We investigated the effects of polymer addition on crack formation in particulate films of colloidal silica. Two types of polyvinyl alcohol (PVA) with different degrees of saponification were compared. Partially saponified PVA (PVA-1) suppressed crack formation. In contrast, weaker suppression was observed when we used fully saponified PVA (PVA-2). Centrifugation experiments of colloid–polymer suspensions revealed that PVA-1 adsorbed on particles well whereas PVA-2 was not adsorbed effectively, suggesting that the polymer–particle interactions strongly depended on the content of polymer functional groups. In addition, we observed segregation of PVA-2 between packed particles, which contributed to a sparse packing structure. We did not observe such segregation of PVA-1 and particles were packed more tightly than for PVA-2. These results clearly showed that the affinity between polymer molecules and suspended particles affected both crack formation and packing structures. We also found that adsorption of polymer molecules did not necessarily suppress crack formation. When we added polyvinyl pyrrolidone (PVP) to silica suspensions, we observed much more cracks than in the sample without polymer even though PVP molecules adsorbed on the silica particles. We conclude that adsorption of polymer molecules on suspended particles is preferable, but does not necessarily suppress crack formation. Miscibility between polymer molecules and particles would be a key issue in crack formation and packing structure of colloid-polymer films.

We evaluate an enhancement of the drying flux from a narrow drying interface. We examine drying of colloidal suspensions in a directional drying cell with various combinations of width and height to change the area of the drying interface. The drying flux changes depending on the height or width of the drying interface. A simple scaling law describes the experimental data quantitatively. Using this effect, we systematically change the drying flux and analyze the formation kinetics of particulate films. There is a threshold drying flux for immediate film formation after the beginning of drying. In addition, the threshold drying flux depends on the initial volume fraction of particles. A kinetics model based on the conservation of particles explains the threshold well.

We focus on the zinc reduction method of SiCl4. Previous studies revealed that SiNWs formed as a mat at high temperature around 980˚C in the reaction. However the detailed mechanism of growing SiNWs and other preferable conditions have not been well understood, therefore my research focuses on the elucidation of preferable reactor/reaction conditions for the formation of a mat of SiNWs. In this poster presentation, effects of gas flow on the formation of SiNWs is mainly explained.