Acoustophoretic studies of aqueous suspensions of alumina and 8 mol% yttria stabilised zirconia powders

The iso electric points (IEPS) of four alumina powders and three 8 mol% yttria stabilised zirconia powders were investigated using acoustophoresis to measure the zeta potential as a function of pH. All the powders were characterised by X-ray fluorescence spectroscopy for elemental composition and BET nitrogen adsorption for surface area. Initial experiments revealed that the IEPS of the alumina powders were independent of the volume fraction of the alumina suspensions. Further experiments investigated the effect of adjusting the pH with different bases, i.e. NaOH, KOH, and NH4OH, on the IEPS of the alumina powders. The IEP of an individual powder was independent of the base used, however, the degree of hysteresis varied depending on the polarisability of the cation. The smallest degree of hysteresis was recorded with additions of NaOH. The differences in IEP between the alumina powders were attributed to the different surface chemistries of the powders. For the zirconia powders the IEP of each suspension was identified using HCl and NaOH to adjust the pH. Again the differences in the IEP were attributed to different surface chemistries of the powders. Two of the powders demonstrated an IEP of approximately 6.5; however the Tosoh powder suspension demonstrated considerable hysteresis and an IEP of 9.3. This was due to its natural pH being an approximate pH unit lower than the other two powders. At this lower natural pH, yttria dissolved out of the Tosoh powder, generating yttrium ions in solution. These precipitated onto the powder surface and altered the surface chemistry with time; i.e increased the zeta potential. Atomic absorption experiments on the supernatant confirmed the dissolution of the yttria from the Tosoh powder.