| Popis: |
Aqueous alumina slurry was prepared with a commercial powder of elongated particles, which has the aspect ratio ranging from 1 to 3.5 with the mean of 1.6, to examine the effect of forming conditions on the particle alignment in green tapes. The slurry appeared pseudoplastic with a yield stress, but showed no thixotropic behavior. Its flow curve fitted very well to the Herschel–Bulkley model approximation, which suggested shear-thinning constant of 0.54. Polarized microscopy with the liquid immersion technique was applied to examine the particle orientation through the direction along the tape thickness. In the absence of coquette flow, randomly oriented particles were noted in the tape. At the top surface, particles were aligned with their long-axes ( a -axis) along the casting direction. The variation in the degree of orientation was 6.8 ± 1.2. In the area near the Mylar carrier, a -axis of particle made an angle to the carrier surface with the degree of orientation about 5.8 ± 1.0. As the combination of pressure flow and coquette flow, tape cast with casting velocity of 2.5 and 91.5 cm/min, which respectively resulted in shear rate of 1.38 and 50.8 s −1 , were observed. The orientation was significant near the top surface and was higher than that above the carrier surface. The a -axis of particles above the carrier surface was inclined to the surface at low shear rate (1.38 s −1 ), but was nearly parallel at high shear rate (50.8 s −1 ). Nevertheless, the orientation varies with the location in the tape prepared at the shear rate of 50.8 s −1 . |
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