Factors Influencing Anisotropic Sintering Shrinkage in Tape‐Cast Alumina: Effect of Processing Variables

Abstract

The effect of the processing variables shear rate, solids loading, and sintering temperature on the anisotropy of sintering shrinkage of aqueous tape-cast alumina was studied. Higher shear rates and higher solids loading resulted in higher in-plane shrinkage anisotropy, whereas the shrinkage anisotropy in the thickness direction was higher for low solids loadings. The in-plane shrinkage anisotropy was found to be fairly constant above a certain critical shear rate (B100 s À1 ) independent of the solids loading. The shrinkage anisotropy through the thickness was higher than in-plane directions. A higher thickness direction sintering rate was observed and attributed to a greater number of interparticle necks in the thickness direction because of the platy nature of alumina particles and the greater thickness direction strains associated with binder removal. The binder did not significantly affect the in-plane sintering shrinkage but significantly affected the shrinkage in the thickness direction. It was suggested that emulsion binder particles occupy sites in between layers of particles in the thickness direction. The degree of anisotropic shrinkage was quantified using edge orientation polarograms and a direct correlation was obtained between the processing variables, shrinkage anisotropy, and the edge orientation index.

Effect of powder, binder and process parameters on anisotropic shrinkage in tape cast ceramic products

“…Numerous studies come to the same conclusion. [8][9][10]22,26 Regardless of the very low absolute shrinkage values of the sintered alumina tapes, the anisotropy occurs clearly, because it develops already in the initial stage of sintering as demonstrated by Raj and Cannon. 18 The shrinkage behaviour can be influenced by particle shape and process parameters.…”

“…[8][9][10][11]22,26 Particle orientation can also be analyzed in terms of the voids between the particles. Nonspherical particles lead to non-isometric pore geometries and aligned particles possess different contact points or rather different contact areas and radii to the particles nearby in x-and y-direction.…”

“…[5][6][7] It is caused by the formation of an anisotropic microstructure of the green tapes. [8][9][10][11][12][13][14][15] During casting, the slurry is sheared while passing the blade of the casting head. Anisotropic particles and binder molecules are supposed to be oriented in the shear field.…”

Low toxicity binder systems for tape cast Ce0.9Gd0.1O1.95 laminates

“…5). Anisotropic inplane shrinkage is often seen when tape casting anisotropic powder due to alignment of the long axis of the particles in the casting direction, or in some cases, due to preferred crystallographic alignment [25,26]. The absence of anisotropic inplane shrinkage for the present tapes is believed due to the CGO powder being mainly isotropic or with disc-shaped geometry (cf.…”

“…In some cases, the anisotropy has been reported to decrease with increasing solid loadings, and correspondingly lower binder contents, and be a result of the preferential distribution of the polymer phase in the planes normal to z [25]. Thus, anisotropic z-shrinkage is typically related to the de-binding and not the sintering itself.…”