Aem Lect8

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Aem Lect8

  1. 1. Compaction: General Pressing operation 1. Filling the mold or die with powder - free-flowing spherical granulated powder prepared by spray drying are generally used to optimize the die filling - typical granules ( range: ~44 to 400 µm, avg. ~100 to 200 µm) - typical granule density : 45-55% - typical die fill density before pressing: 25-35% 2. Compacting the powder to a specific size and shape - consolidation by granule rearrangement - granule deformation - increase in particle-coordination number, green density, and compact strength - typical compaction pressure : 35- 104 MPa 3. Ejecting the compact from die - elastic springback (the expansion of a compact upon ejection from the die should be minimized. S. J. Glass and K. G. Ewsuk, MRS Bulletin, Dec., 24 (1997) Advanced Electronic Ceramics I (2004) Spray drying Spray drying : the process that granulates the fine primary particles into spherical secondary spheres in order to increase the flowability of the powder via the spray of the mixture between powder, additive, and solvent The additive for spray drying 1. Binder : strength 2. Plasticizer : deformable granule 3. Lubricant : reduce the frictional effect S. J. Glass and K. G. Ewsuk, MRS Bulletin, Dec., 24 (1997) Advanced Electronic Ceramics I (2004)
  2. 2. Possible problems in compaction End capping - body with poor green strength and high springback - High die-wall friction and high pressing pressure Ring capping - poor tolerances between the punch and die body - excessive differential springback Lamination - poor green strength - pressing at excessively high pressures - high springback or die fraction Vertical cracks - excessive springback S. J. Glass and K. G. Ewsuk, MRS Bulletin, Dec., 24 (1997) Advanced Electronic Ceramics I (2004) Consideration of Die Narrower diameter die usually results lower packing density because the packing density at the die wall is lower than that at the bulk of the compact. High aspect-ratio (height/diameter) are undesirable because the packing density can vary with height In order to reduce the die-wall effect - Use the smaller particle or large die recommended compact diameter and thickness 1. Diameter: ~X250 of the average agglomerate size of powder 2. Thickness: ~X40 of the average agglomerate size of powder S. J. Glass and K. G. Ewsuk, MRS Bulletin, Dec., 24 (1997) Advanced Electronic Ceramics I (2004)
  3. 3. Dual-action and Single-action Pressings Density gradients predicted by the FE compaction model in a quarter section of a 94 wt% alumina tube. The 25,4 mm long tubes were compacted uniaxially at 103.4 Mpa by: a) dual-action pressing and b) single- action pressing from the top. Density changes with color from blue to green, yellow, orange, and red. K.G. Ewsuk et al., Am.Ceram.Soc.Bull., 80(1), 53 (2001) Advanced Electronic Ceramics I (2004) Green-Density Gradient along the pressing axis The density gradients through the cross section of a cylindrical powder compact of 94wt.% alumina after uniaxial pressing at 69 MPa: a) measured by ultrasound velocity measurements completed on a bisque-fired part, and b) predicted using the FE compaction model assuming a die wall friction coefficient of 0.25. K.G. Ewsuk et al., Am.Ceram.Soc.Bull., 80(1), 53 (2001) Advanced Electronic Ceramics I (2004)
  4. 4. Compaction curve S. J. Glass and K. G. Ewsuk, MRS Bulletin, Dec., 24 (1997) Advanced Electronic Ceramics I (2004) Compaction: the effect of granules Easy rearrangement No rearrangement proper rearrangement No deformation deformation at center proper deformation S. J. Glass and K. G. Ewsuk, MRS Bulletin, Dec., 24 (1997) Advanced Electronic Ceramics I (2004)
  5. 5. The properties for the granules 1. Hard enough to rearrange during die filling and low-pressure compaction 2. Soft enough to completely deform under maximum pressing pressure S. J. Glass and K. G. Ewsuk, MRS Bulletin, Dec., 24 (1997) Advanced Electronic Ceramics I (2004) Compaction S. J. Glass and K. G. Ewsuk, MRS Bulletin, Dec., 24 (1997) Advanced Electronic Ceramics I (2004)

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