E. Laarz et al /Journal of the European Ceramic Society 21(2001)1027-1035 and TIN or TiC particles which, according to Fig. 2, are not change the flow behavior compared to suspensions also highly negatively charged at pH=9 containing 25 vol. micron-sized TiN. a slight viscos- Viscosity measurements on 20 vol. TiC suspensions ity increase at 10 vol. TINnano concentration is obser Fig. 5)suggest that the negative surface charge is high vable, and even higher concentrations lead to nough at pH-77 to prevent homocoagulation. At substantial shear-thinning(Fig. 6). This effect might be lower pH the surface charge is apparently too low, related to a crowding effect or an induced aggregation though, and flocculation takes place. In comparison, the due to changes in suspension pH with addition of H of Tin samples must be increased to 10 to achieve a TiNnano. The pH of an alumina suspension dispersed well-dispersed suspension with a Newtonian shear with 0.5 wt. PAA decreases from 9.1 to 8.2 upon response(Fig. 5). Fig. 6 shows the flow curves of com- addition of 20 vol. TiNnano which can result in posite suspensions containing TINnano. USing nano- enhanced homocoagulation of the TiN particles(Fig. 5) sized TiN up to 5 vol. of the total solids loading does Another factor contributing to the high viscosity might be the interaction between TiNnano particles and PAA macromolecules. Previous results on the electrokinetics Mixing and Dispersing(I) Alos and rheology of TiNnano particles dispersed with poly Ho+ PAA acrylic-type dispersants 9. 4+ imply that PAA can be H 9 adsorbed on titanium nitride. These studies suggested that adsorption occurs even at a ph where both parti- Mixing and Dispersing(Il) cles and polymer carry negative charges. Hence, it is Addition of tintic possible that in our system weak adsorption ( 1.e adsorption below saturation level) of PAA on TiN causes bridging flocculation at high TiNnano particle Freeze Granulation and concentrations and, thus, a higher suspension viscosity Freeze Drying Summarizing the results of our dispersion experi ments, we can state that well-dispersed, low-viscous composite suspensions with 20-25 vol. solids loading (minimum viscosities of the composite suspensions were Sieving Granules in the range 3-4 mPa s)can easily be produced with all investigated alumina-based composite systems by using the same processing scheme. An additional advantage Hot Pressing in Ar of the chosen processing scheme is its tolerance towards at1700°C,28MPa,90 variations in degree of surface oxidation of the non- oxide components, which can be difficult to control in Fig 3. Processing scheme for composite preparation industrial practice A2O325%TN(no。PAA) 102[△42°325%TNQ5w%PA 103E°425%T(a5w%PA) a888. 0.5 wt% PAA 10 000 Rate [s" Fig 4. Steady-shear flow curves of 20 vol. suspensions at pHR9   
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