Electrostatic Attraction Enables Precise Control in Thin-Film Assembly
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for about 20 h. Afterwards, stirring the slurry in a vacuum desiccator helped to eliminate any trapped air. Then the slurry passed to a cylindrical container specially designed for this experiment. This container had one end immersed in ethanol cooled at -50°C. They fabricated this section of the container with a metallic material of high conductivity. For the sides of the container, they used a resin. The other end of the container remained open to the air allowing growth of frozen ice in a vertical direction. Drying of the ice thus formed took one day in conditions of reduced pressure. The sintering conditions of each sample were 1400°C and 1550°C, both for 2 h . Two slurries were prepared at different concentrations. Measurements of their rheological properties revealed a uniform distribution of the alumina powders. The freeze-dried samples showed large pores aligned with the growth direction. After freeze-drying, the volume change measured was almost negligible. Pore-size distributions measured after sintering have two peaks at 0.1 µm and 10 µm for the samples sintered at 1400°C. On the other hand, the pore-size distributions of the samples sintered at 1550°C only have one peak at around 10 µm. Inspection of these samples using a scanning electron microscope allowed the detection of a dendritic structure inside the large pores for all samples. In the case of the sample sintered at 1400°C, inside the dendritic structures there were small pores. These pores correspond to the peak at 0.1 µm observed in the pore-size distribution. It was also possible to detect a decrease in the pore size with increasing concentration of the slurry. Furthermore, the researchers’ current results indicate that by changing the freezing temperature, control of the macroscopic pore size is possible. Since the pores appear before sintering, this method is applicable to any ceramic. Recently, the researchers obtained porous silicon nitride after application of this technique. SIARI S. SOSA
Electrostatic Attraction Enables Precise Control in Thin-Film Assembly A research group at the University of Massachusetts—Lowell has prepared a thin film consisting of oppositely charged alternating layers of organic laser dye molecules intercalated into an aluminosilicate lattice and polycation spacer layers. The components of the film were sequentially deposited onto a glass substrate and allowed to assemble by electrostatic attraction. This method allowed for
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precise control of film thickness and dye molecule orientation in the film. The dye molecules in the assembly were more thermally stable than free dye and exhibited a high degree of order. As reported in the March issue of Chemistry of Materials, the dye molecules (coumarin 1 or 7-diethylamino-4-methyl coumarin) were first protonated by hydrochloric acid treatment. A suspension of the aluminosilicate hectorite was then added to the protonated dye solution and ion exchange was allowed to proceed for 24 h. The resulting host/guest composite was separated by centrifugation, washed with wate
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