Design of ceramic paste formulations for co-extrusion

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.