Knowledge of the structure of granules and compacts of ceramic products is crucial to enhancing their performance, according to Keizo Uematsu, head of a research team at Nagaoka University of Technology (Nagaoka, Japan). "I think that ceramics can be made 10 times stronger if almost all defects are eliminated," Uematsu says. "And in our laboratory, we are studying the origin of such defects." Uematsu and graduate assistant Yutaka Saito have developed a way to look at ceramic structures in three dimensions using a confocal laser scanning fluorescence microscope and immersing samples of compacted material in a solution containing fluorescent dye.
Internal structure of green compacts pressed at various pressures. Each compact was pressed at 25°C, 70% RH.
Their experiments began with the preparation of green (unsintered) bodies with artificial pores. They used high-purity commercial alumina powder consisting of particles an average of 0.5 µm in diameter. A slurry of 100 parts powder, 0.1 parts binder and 0.6 parts dispersant was ball-milled for 24 hours and treated in a vacuum for 30 minutes to de-air it. Then they added polystyrene spheres, and filtered and dried the slurry to form green bodies called compacts.
They also prepared green compacts from granules of industrial-grade commercial alumina, with polyvinyl alcohol as a binder, mixed with distilled water. A spray dryer was used to prepare the granules, which were pressed with steel dies at 50 to 200 MPa. The compacts were heated to 1000°C at a rate of 5°/min. and kept one hour in air to remove the binder. looking at the structures
All the specimens were thinned to a precise 0.2 mm with sandpaper; then they were immersed in a solution of a fluorescent reagent mixed with methylene iodide. Next, the specimens were evacuated for five minutes and placed on the observation plate of a confocal laser scanning fluorescence microscope. Using a 488-nm wavelength argon ion laser, the team observed the internal structure of the ceramic specimens.
First, they examined the green-body specimens at different focal depths, ranging in 5-µm steps from 5 µm to 75 µm. The team observed bright, round features that indicated the fluorescent solution had penetrated the artificial pores introduced with polystyrene spheres. Because adjusting the depth of focus of the scanning laser bisects the specimen in much the same way a CT scanner bisects a body, the group was able to reconstruct a three-dimensional structure from the results. This reconstruction not only verified the artificial pores in the green-body specimens, but also showed many filament-like cracks. "We think the cracks may have been formed during the drying process," says Saito.
The figures show scans of alumina compacts done 10 µm below the surface. The team compressed the compacts at different pressures and captured the images for 50 MPa, 100 MPa, and 200 MPa before binder removal. "Notice the dark network in this image," says Saito (see figure on page 9). "That's the binder, which does not allow the fluorescent dye to penetrate. The bubble-like regions the dark network surrounds are granules. Note that they deform more as greater pressure is applied. Any bright spots are intergranule pores or low-density regions."
The images show that pores tend to decrease with increasing pressure yet remain to a certain extent, even at 200 MPa. The final image shows the internal structure of an alumina compact, pressed at 100 MPa, after the binder has been removed. "You can see the bright spots clearly," Saito explains. "This indicated reduced density at the boundaries of the granules. In other words, the packing density of the powder particles was low in the boundary region of the granules." The brighter the area, the lower the density, says Saito.
The Nagaoka University of Technology asserts that this is a powerful characterization technique for ceramic green microstructures. It allows the direct observation of the internal structure and can be used to examine how binder detrimentally affects compaction processing of ceramics.
"We find that both green bodies and sintered bodies produced in summer have fewer defects than those produced in winter," says researcher Nobuhiro Shinohara of Asahi Glass Co. Ltd. (Tokyo, Japan). "The Nagaoka University of Technology technique may help us determine why."