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Abstract
The rheological and flow behaviour of ceramic pastes with varying solids loadings (solids volume fraction) has been studied. The pastes were shown to exhibit power law slip flow at both low and high solids loadings, with no slip yield stress. As would be expected, the extrusion rheometry data showed an increase in the die entry extrusion pressures with solids loading, in a trend similar to that of the Dougherty–Krieger equation. The die land flow however was shown to exhibit, only within a narrow range of solids loadings of approximately 53 to 56 v/v %, a trend in the die land extrusion pressure that was relatively independent of solids loading. Outside of this range the die land extrusion pressures increased significantly with solids loading in a trend similar to that of the die entry pressures. Using the Mooney analysis method, this was shown to be due to the development of slip flow with solids loading, as is consistent with other studies into the flow behaviour of concentrated suspensions. The slip velocity was also shown to be related to the wall shear stress by way of power law relationship also consistent with results from previous studies of concentrated suspensions using shear thinning liquid phases. The results presented in this report show that by using paste formulations that lie within said range of solids loadings, improved plug flow can be achieved in the die land without a significant change in the die land extrusion pressure at a given flow rate. This offers an advantage in the co-extrusion of ceramic products such as micro-tubular solid oxide fuel cells, as a uniform velocity profile results in improved control of the laminate structure.
Highlights
The co-extrusion of multiple pastes into a single extrudate requires the fine tuning and unification of the paste rheologies in order to prevent the formation of flow defects in a continuous or semi-continuous extrusion process
Formulating a paste within the range of solids loadings where slip flow is dominant provides a benefit in the co-extrusion of pastes such as in the manufacture of micro-tubular solid oxide fuel cells [6]
The differences in slip velocity at a given wall shear stress cannot be explained in terms of solid loading by application of the relationship proposed by Lam et al [10]
Summary
The co-extrusion of multiple pastes into a single extrudate requires the fine tuning and unification of the paste rheologies in order to prevent the formation of flow defects in a continuous or semi-continuous extrusion process. This requires the formulation of individual pastes with consideration of effects due to particle size, size distribution, morphology and surface chemistry, as well as pore size, distribution and shape. The tubes were relatively large, with layer thicknesses being in the order of 1 mm, a limitation which could be overcome by a change in design of the co-extrusion process and equipment Some of these authors reported the formation of delamination defects, which again can be overcome by improved design of the co-extruder [5,6]. This paper follows on from two previous publications by the same authors [5,6] which presented the results of an investigation into
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