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J. Götz, L. Rewesa, M. Walch, A. Geissler
Influence of an Ultrasonic Treatment on the Structure and Flow Behaviour of Oxide Ceramic Masses
Appl. Rheol. 15:4 (2005) 204-217
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Oxide ceramic masses are used for catalysts and catalyst carriers. For a reliable processing hydrocolloids (e. g.
cellulose) are usually added in order to decelerate demixing phenomena. Oxide ceramic masses react to simple
shearing with hardening (peptisation: increase of the shear stress with the shear deformation) [1]. The present
study analyses, if an ultrasonic treatment has also an impact on the structure, the shelf-life (in the green state),
the correlated flow behaviour of oxide ceramic masses and presumably (not tested) the mechanical properties
in the hardened, sintered state. The idea of using ultrasonic treatment is to change the microstructure (see
below) and, therefore, to minimise or even give up the standard addition of stabilizers to minimize demixing in
aqueous oxide ceramic suspensions. Besides the additional costs of an extra process unit, stabilisers cause often
deteriorated mechanical properties (porosity, crack behaviour) of the ceramics in the hardened state after the
sintering. Therefore, pump experiments (apparent viscosity), oscillatory (G´ and G´´) and steady shear experiments
(h), particle-size analysis (particle-size distribution, agglomerate strength), light microscopy, decanting
experiments and pH-determinations have been performed. The obtained results show, that the hardening of
the apparent viscosity (derived from the flow) during pump experiments with simultaneous ultrasonic treatment
in a flow cell is combined with an increase of the fine fraction, the formation of enlarged, but smoother
agglomerates, the change of the pH-value and the evolution of a three-dimensional network (gelling). All these
processes increase both the amount of bound/immobilised (chemically or physically bound by or onto the solidsurfaces)
and of retained water (interior of agglomerates and/or the pores of the flowand ultrasonic-induced
network). This means that the volume fraction of the rheologically "free" water decreases and simultaneously
the effective solid volume fraction increases. With respect to the concept of the rheologically effective solid fraction
this is combined with an increasing viscosity. At the same time the tendency of demixing decreases significantly.
Thus, by an appropriate combination of shear flow and ultrasonic treatment, the aqueous oxide ceramic
suspensions are stabilised and a reliable processing of the initially problematic solid/fluid mixtures can be
realised without stabilisers (eluding their negative consequences with respect to the quality of the sintered state).
► Cite this publication as follows:
Gotz J, Rewese L, Walch M, Geissler A: Influence of an Ultrasonic Treatment on the Structure and Flow Behaviour of Oxide Ceramic Masses, Appl. Rheol. 15 (2005) 204.
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