B. Jachimska, Z Adamczyk /Joumal of the European Ceramic Society 27(2007)2209-2215 10000 10000 (a) Size [nm (b) Size [nm] of the size 0.40 um. (b)Z O2 with PSS at pH 10. 1=5 x 10-M, averaged particle size 0.56um, standard deviation of the size 0. 25ul um standard deviation Fig.I. Particle size distribution of zirconia determined by the PCS method (a)Z O2 at pH 10. 1=5 x 10-3M, averaged particle size 0.6 diameter was 0.64 um with the standard deviation of 0.40 um. However, a prerequisite of a correct interpretation of viscos A similar but more narrow size distribution was obtained for zir- ity measurements is the knowledge of the volume fraction of conia suspension in PSS solution at pH 10 and 1=5 x 10-M the solid in the suspensions v. This quality can only be cal- as can be seen in Fig. lb. The averaged particle diameter was culated if the real density of particles in suspension is known 0.56 um with the standard deviation of 0. 25 um. The density was determined by the dilution method, i.e., from The size of primary particle of zirconia was determined by the dependence of the suspension density on the weight frac the scanning electron microscopy(SEM). From the SEM images tion of zirconia w= p/msus, where p is the particle mass (shown in Fig. 2)it was estimated that the size of primary parti cles was 0.2 um. It is interesting re this value with the BETdata, giving specific surface area S=5.0 m-/g as mentioned previously. By assuming a spherical shape of the particles and using the formula d=6/ppS(where Pp is the specific density of the solid zirconia, equal 5.79 g/cm, as specified by the pro- ducer)one obtains d=0. 21 um. As can be noticed this is similar to the primary particle size derived from the SEM observations 3. Result and discussion It is known that the stability and aggregation degree of oxides, in particular zirconia, depends to a critical extent on pH value that governs the number of hydroxyl groups increasing the neg- Ace.V Spot Magn Del WD H ative charge of particles. The increase in the surface char 150k2020000xSE111Z02 usually increases the absolute value of the zeta potential of par ticles(the electric potential in the slip plane)which results in an increased stability of suspensions. 2 The zeta potential is a quantity well accessible experimentally, e. g, by using the micro- electropheretic method as done in this work. The dependence of the zeta potential of ZrO2 suspensionon on the ph determined for a fixed ionic strength of 10-3M is shown in Fig 3.As can be noticed, for pH 4.7, the zeta potential of the suspension of pure zirconia was close to zero. This pH value is referred to as the isoelectric point. In this respect our data agree well with that reported by Ewais et al. and Crucean et al. 3,4 On the other hand,Fengqiuet al. reported pH 6 as the isoelectric point.Gen- ally, there is a rather large spread in the reported data, e.g Johnson et al. reported pH 7.5 as the isoelectric point of zirco- nia. This issue is discussed at length in the book of kosmulski. 14 These high values of zeta potential(in absolute terms)are expected to stabilize of the zirconia suspension. However, the dispersion of aggregates seems rather unlikely. This hypothe Fig 2. SEM image of zirconia powder deposited on mica surface: (a) ZrO2and sis was tested by using the dynamic viscosity measurements. (b)ZrO2 with PSSB. Jachimska, Z. Adamczyk / Journal of the European Ceramic Society 27 (2007) 2209–2215 2211 Fig. 1. Particle size distribution of zirconia determined by the PCS method. (a) ZrO2 at pH 10, I = 5 × 10−3 M, averaged particle size 0.64m, standard deviation of the size 0.40m. (b) ZrO2 with PSS at pH 10, I = 5 × 10−3 M, averaged particle size 0.56m, standard deviation of the size 0.25m. diameter was 0.64m with the standard deviation of 0.40m. A similar but more narrow size distribution was obtained for zir￾conia suspension in PSS solution at pH 10 and I = 5 × 10−3 M as can be seen in Fig. 1b. The averaged particle diameter was 0.56m with the standard deviation of 0.25 m. The size of primary particle of zirconia was determined by the scanning electron microscopy (SEM). From the SEM images (shown in Fig. 2) it was estimated that the size of primary parti￾cles was 0.2m. It is interesting to compare this value with the BET data, giving specific surface area S = 5.0 m2/g as mentioned previously. By assuming a spherical shape of the particles and using the formula d = 6/ρpS (where ρp is the specific density of the solid zirconia, equal 5.79 g/cm3, as specified by the pro￾ducer) one obtains d = 0.21m. As can be noticed this is similar to the primary particle size derived from the SEM observations. 3. Result and discussion It is known that the stability and aggregation degree of oxides, in particular zirconia, depends to a critical extent on pH value that governs the number of hydroxyl groups increasing the neg￾ative charge of particles. The increase in the surface charge usually increases the absolute value of the zeta potential of par￾ticles (the electric potential in the slip plane) which results in an increased stability of suspensions.12 The zeta potential is a quantity well accessible experimentally, e.g., by using the micro￾electropheretic method as done in this work. The dependence of the zeta potential of ZrO2 suspensionon on the pH determined for a fixed ionic strength of 10−3 M is shown in Fig. 3. As can be noticed, for pH 4.7, the zeta potential of the suspension of pure zirconia was close to zero. This pH value is referred to as the isoelectric point. In this respect our data agree well with that reported by Ewais et al.3 and Crucean et al.13,14 On the other hand, Fengqiu et al.5 reported pH 6 as the isoelectric point. Gen￾erally, there is a rather large spread in the reported data, e.g., Johnson et al.6 reported pH 7.5 as the isoelectric point of zirco￾nia. This issue is discussed at length in the book of Kosmulski.14 These high values of zeta potential (in absolute terms) are expected to stabilize of the zirconia suspension. However, the dispersion of aggregates seems rather unlikely. This hypothe￾sis was tested by using the dynamic viscosity measurements. However, a prerequisite of a correct interpretation of viscos￾ity measurements is the knowledge of the volume fraction of the solid in the suspensions ΦV. This quality can only be cal￾culated if the real density of particles in suspension is known. The density was determined by the dilution method, i.e., from the dependence of the suspension density on the weight frac￾tion of zirconia w = mp/msus, where mp is the particle mass Fig. 2. SEM image of zirconia powder deposited on mica surface: (a) ZrO2 and (b) ZrO2 with PSS.