Compaction Behavior of Spray‐Dried Alumina

Abstract

Compaction behavior and resultant porosity of spray-dried alumina were examined over a range of pressures from 18 to 345 MPa. The variability in pressed density and pore size was measured as a function of spray-dried granule size, binder concentration, and powder moisture content. An exponential behavior was found between pressed density and compaction pressure, density increasing with the logarithm of the compaction pressure. Pore size also displays an exponential behavior, pore diameter decreasing with the log of increasing pressure. The distribution width about the median pore size was noted to decrease approximately in a log-log response to increasing compaction pressure. The behavior of the compaction and porosity curves is related to the degree to which the polymer binder is plasticized which, in turn, affects the pressure at which the spray-dried granules begin to crush. This apparent yield pressure, although dependent on amount of binder and powder moisture, is intrinsically dependent on the waterhinder ratio. Finally, by analogy to soil mechanics, an empirical equation relating compact density, yield pressure, and slope is presented.

Processing of Alumina and Corresponding Composites

“…They have to be large (>50 mm) and strong enough for handling but soft enough to be destroyed by compaction to avoid relicts of the granules in the green body that will evolve to defects by differential sintering. In addition, they have to uniformly deform to fill interagglomerate void space during pressing (Frey & Halloran, 1984;Lin & Lin, 2008). It is necessary to avoid the movement of the binders toward the surface of the slurry droplets, in the stream of hot air during the spray drying process, because it would produce binder segregation at the surface of the granules after the liquid evaporates.…”

Effect of internal binder on microstructure in compacts made from granules

“…[1][2][3][4][5][6][7][8] The characteristics of binder must be carefully controlled, but may change easily in many systems, such as PVA. The granules with PVA are hard under glass transition temperature (∼25 • C) and become soft in high relative humidity over 80% and temperature above 20 • C. 9 Hard elastic granules tend to form large flaws in green compacts as well as in sintered ceramics.…”

X-ray tomography analysis of aluminum alloy powder compaction

“…Owing to the nonhomogeneous density distribution, geometry deviations may occur during the subsequent sintering. Many theoretical models were developed to give a mathematic relationship between applied pressure and average density of green compact [1][2][3]. However, those models cannot predict or analyze the density distribution in green compact, because pressure varies along compaction direction.…”