E. Laarz et al / Journal of the European Ceramic Society 21(2001)1027-1035 The solubility of TiC and Tin powders in water is neg- for TiN and TiC powders are somewhat less consistent gible (<6 ppm Ti) over practically the entire pH range: with our results: different grades of Tin powders only under very acidic conditions (pH=0. 8)significant showed pHiep in the range pH=3-5. 8,9, 13, 14and oncentrations of dissolved Ti (100 ppm) could be pHiep 2 has been found for TiC >However, since non- observed. The surface oxides passivate the TiN and Tic oxide ceramic powders are thermodynamically unstable materials, protecting them from further oxidation inin air and water, these differences are not unexpected aqueous environments. and can be attributed mainly to different degrees of surface oxidation. For example, it has been shown ear- 3.2. Dispersion in aqueous medium lier that the synthesis route and post-synthesis treatment of Si3N4 powders strongly affect the surface oxygen The oxidized TiC and tin powders exhibited iden content and the correlated area density of potential ical isoelectric points at pHicp 4.3 in NaCl electrolyte determining surface groups. 16, 17 Accordingly, increasing solutions(Fig. 2). The isoelectric point of TiC whiskers surface oxidation will shift the isoelectric point of the the isoelectric point of the a-alumina powder is much the value for the less acidic oxide TiO2(pHiep o/s ards was determined to be pHiepa43 as well. In comparison, acidic TiN and Tic powders more and more tow higher(pHiep9. 2). However, as shown in Fig. 2, upon Based on our studies of the surface chemistry of TiN addition of 0.5 wt. PAA to the alumina suspension and TiC, we developed the processing scheme depicted the pHiep shifts to acidic values(pHiep 3)due to poly- in Fig. 3, which can be employed for preparation of electrolyte adsorption 10-12 Measurements at various Al O -TiN/TiC composite suspensions and sintered ionic strengths confirmed that NaCl acts as an indiffer- bodies. As shown in detail in previous work, 8-l0 well- ent electrolyte in all cases(not shown). The zeta-poten- dispersed a-Al2O3 suspensions with an inherent suspen tial values for Al2O3 suspensions compare very well with sion pH value of A9 are obtained by simply mixing previous results. Previously reported pHicp-values deionized water, powder, and >0.2 wt. ammonium alt of PAA. according to our processing Table 2 done in the first milling step and in the next milling step Particle size distribution and specific surface area of the as-received tin or tic is added. In this manner heterocoagulation TiC and TiN powders is prevented, as can be seen from steady-shear measure TiN ments on suspensions with 20 vol. solids loading Fig. 4). Before and after adding tin or TiC, the sus- article size distribution(um) pensions show almost no shear thinning, thus indicating a well-dispersed suspension. However, if addition of PAA is omitted, a flocculated strongly shear-thinning ET surface area(m-g-) suspension is obtained(Fig 4). Clearly, the absence of BET equivalent spherical 0.45 0.026 diameter(um) flocculation is related to the electrostatic repulsion between alumina particles with adsorbed PAA layers e-TiC TiC(Whisker) Al O -o AL o,(0.5 wt% PAA) H Fig. 2. Zeta potentials of the powders used in 0.01 M NaCl electrolyte solution. )  
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