Consolidation of collodal suspensions

Abstract

A key step in the processing of ceramics is the consolidation of powders into engineered shapes. Colloidal processing uses solvents (usually water) and dispersants to break up powder agglomerates in suspension and thereby reduce the pore size in a consolidatd compact. However, agglomeration and particle rearrangement leading to pore enlargement can still occur during drying. Therefore, it is beneficial to consolidate the compact asdensely as possible during the suspension stage. The consolidation techniques of pressure filtration and centrifugation were studied and the results are reported in this paper. In particular, the steady-state pressure-density relationship was studied, and information was obtained regarding the consolidation process, the microstructure, and the average density profile of consolidated cakes. We found that the compaction processes in these two consolidation methods are quite different. In general, a consolidated cake is a particulate network made up of many structural units which are fractal objects formed during aggregation in the suspension. In pressure filtration, compaction is a process of breaking up the fractal structural units in the particulate network by applied pressure; the resulting partie rearrangement is produced by overcoming energetic barriers which are related to tee packing density of the compact. Recently, we performed Monte Carlo simulations on a cluster-cluster aggregation model with restructuring,1 and found the exponential relationship between pressure and density is indeed the result of the breaking up of the fractal structural units.2 On the other hand, in centrifugation, compaction involves the rearrangement of the fractal structural units without breaking them so that the self-similar nature of the aggregates is preserved. Furthermore, we calculated density profiles fom the bottom to the top of the cakes by solving the local static force balance equation n the continuum particulate network. In pressure filtration of alumina and boehmite, the cakes are predicted have uniform density. The results of γ-ray densitometry3 on a pressure-filtrated alumina cake confirmed this prediction. In contrast, in centrifugation, the density profiles are predicted to show significant variation for cakes on the order of one centimeter high. Moreover, the pressure-filtered boehmite cakes showed no cracking during drying. This indicated that pressure filtration is a good consolidation technique for nanometer-sized particles such as boehmite. The improved drying property is probably a result of a minimal shrinkage due to the formation of higher packing densities during filtration.

Aggregation Effects on the Compressive Flow Properties and Drying Behavior of Colloidal Silica Suspensions

“…In centrifugation, these properties can be assessed by two approaches: (1) measurement of sediment height during centrifugation at multiple spinning speeds, 4,20-23 or (2) measurement of the volume fraction profile during centrifugation 4,23-25 by destructive sectioning 4,23 or by ␥-ray densitometry. 24,25 (A) Sedimentation: As-prepared SiO 2 suspensions of varying solids volume fraction and solution composition were transferred to experimental cells (graduated cylinders of 2.0 cm diameter and 15.0 cm height) to a filled height of 12.8 cm. Each cylinder then was capped to minimize solvent loss during the experiment.…”

Elastic and Yield Behavior of Strongly Flocculated Colloids

“…The concentration dependence of GЈ in the solid regime has been studied extensively. 18,[20][21][22][23][24][25][26][27][28][29][30]34 GЈ has been shown to increase as a power law of concentration in the absence of extensive particulate restructuring. 18,[20][21][22][23][24][25][26] When the particulate network is treated as a spring network, the power-law dependence of GЈ directly results from the fractal structure of the clusters that pack to form the network.…”

Processing and characterization of textured mullite ceramics from phyllosilicates

“…The mean compressive pressure P m applied onto the centrifuged powder compact was described as a power-law function of the average density d m . 26,27 P m = βd n m (1) where P m is a function of the apparatus size, rotation speed and suspension height in the container. The n and β parameters depend on the suspension rheology, mean density and characteristics of particles.…”

“…The possible occurrence of aggregated particles can also influence the density of the powder compact. 26 During centrifugation, the solid content of the liquid decreases with time and the maximum pressure stress is attained at the bottom of the container at time t = 0. At equilibrium, the density of the top layer of the powder compact tends to that of the liquid.…”

“…At equilibrium, the density of the top layer of the powder compact tends to that of the liquid. The density profile d(r) of the powder compact was described as 26 :…”