Viscosity of nanosized colloidal silica suspensions, used as binders in the investment casting, was determined as a function of their weight fraction reaching 50%. A new capillary viscometer was used whose construction eliminated the suspension sedimentation effects. The influence of suspension ionic strength at fixed pH equal 9,5 was systematically studied in order to assess the magnitude of electroviscous effects. It was found that for the low silica concentration range suspension viscosity increased much more rapidly than the prediction of Einstein's or Betchelor's theories. This discrepancy was accounted for by postulating a loose, gel-like structure of colloid silicas used in experiments. Hence, the apparent hydrodynamic radius of silica clusters was considerably larger than the primary particle size in accordance with direct microscope observations of the structure of silica sols. Based on this postulate an apparent density of the silica sols was found to be 1,2 g/cm³ instead of 2,3 g/cm³ as determined from the suspension dilution method. Also for higher concentration range experimentally determined viscosity increased more rapidly with increased sol concentration than predicted by the Dougherty-Krieger model derived for hard particles. The deviation was attributed to the secondary electroviscous effect stemming from the electrostatic interactions among silica particles in sheared suspensions. This effect has quantitatively been interpreted in terms of the Russel's theory. On the other hand, for the very high concentration reaching 50% the experimental results were accounted for by using the effective hard particle concept. Exploiting our experimental findings a sensitive method of determining the structure and apparent density of silica sols in aqueous media was proposed. Simultaneously with the investigations mentioned above, the mechanical properties of thin layers of ceramic containing colloidal silica suspension were investigated.

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