S Novak et al. Journal of the European Ceramic Society 28 (2008)2801-2807 2803 (a) Table I Compositions and properties of the characteristic suspensions used in the EPD experiments and corresponding properties of wet deposits 0000 Suspension Deposit H Conductivity ZP wt %e -CTAB R 2.80.78 △PE|+HCI 3 CTAB 3.90.24 O CA+PEI 5(PED+CA 0.75+2280.86 Dolapix+ NaoH -60 6(PED+HCI 542 00.1020.3040.5060.708091 2.60.7 surfactant(wt % 8 (CA)+PEI 0.5+0.33.30.3 15 CTAB 15 12 (DCE64)+NaOH 0.5 -4962 EE至号 Fig. 2a also shows that the addition of citric acid to SiC suspension has a minor effect on the ZP, but it sigi icantly increases its conductivity. However, the conductivity decreased considerably with further additions of pEl, while the ZP increased from 15 to 40mV. This observation agrees with previous reports,23 which showed that the surfactant-addition 00.10.2030405060.70809 sequence has a large effect on the stability of the suspensions. surfactant (wt % The addition of the anionic deflocculant Dolapix CE64 increased the absolute value of the ZP up to -24 mv, which Fig. 2. The ZP(a)and conductivity change(b) due to the addition of different was further increased to-49 mV by adjusting the pHto 9: how surfactants to SiC suspensions(solids content: 25 wt %). The arrows show the ever, the subsequent conductivity increase was relatively higl effect of an additional pH change for a particular surfactant addition. (see Fig. l a and b) The above results are summarised in Table l and indicate that very low. This implies that a ph decrease will probably not high, positive zP values can be obtained with the addition of be sufficient for EPD and hence, to increase the positive net- 0.4% CTAB, 0.8% of PEI at pH& or through combined addition surface charge, the addition of a cationic surfactant is require of 0.5% CA and 0.3%o Pel, while high, negative values can be In contrast, the pH increase with TMAH or NaOH resulted in achieved by a pH increase to 9 or with the addition of Dolapix a much more pronounced effect on the ZP, giving a value of CE64 at pH 9. However, so slightly composition with characteristic ZP values ton these results, 50mV at pH 9. However, a further increase in the ph only strong increase in the conductivity too. Based increased the conductivity, whereas the ZP began to decrease in afurther step, we performed EPD experiments using a selected mne ig 2a and b illustrates the effect of the different surfactants the ZP and the conductivity of the SiC suspension. It is clear 3. 2. EPD experiments that the addition of the cationic surfactant ctab increased the ZP up to 48 mV without a significant change in the conductivity, The EPD experiments were performed by using suspensions while the addition of PEl resulted in a smaller increase in the with 25 wt %o of solids at selected characteristic points in the ZP ZP, reaching a maximum value of 35 mV with 0.8% PEL. As VS pH diagrams. In order to evaluate the quality of the deposits reported by Zhang et al. 21, 22 due to the low dissociation of steel electrodes were used in the first trials. The suspension PEI in the alkaline region, PEI does not stabilise the alkaline compositions and properties, related to the final result of the to, according to their reports, a significant increase in the ZP 21.22 First, an EPD experiment was performed using a susper In our measurements the ZP reached its highest value(44 mv) sion without any additive. As expected, due to the absence of only after the subsequent addition of 2 wt. of citric acid (at charged particles in the Sic suspension at its natural pH of 4.1 pH 2.8), whereas the conductivity was already very high. a no deposit was observed after 5 min at an applied voltage up better result was obtained when the pH of the PEl-containing to 60 V. When the pH of the suspension was adjusted to 2.8, suspension was adjusted with HCl: at a pH of 8 the ZP reached where the ZP was 1l mv, a very loose deposit was formed on 52 mv(the effect of the pHchanges are indicated with the dashed the cathode, but it slid from the electrode during removal from arrows in Fig. 2a and b). theS. Novak et al. / Journal of the European Ceramic Society 28 (2008) 2801–2807 2803 Fig. 2. The ZP (a) and conductivity change (b) due to the addition of different surfactants to SiC suspensions (solids content: 25 wt.%). The arrows show the effect of an additional pH change for a particular surfactant addition. very low. This implies that a pH decrease will probably not be sufficient for EPD and hence, to increase the positive net￾surface charge, the addition of a cationic surfactant is required. In contrast, the pH increase with TMAH or NaOH resulted in a much more pronounced effect on the ZP, giving a value of −50 mV at pH 9. However, a further increase in the pH only increased the conductivity, whereas the ZP began to decrease slightly. Fig. 2a and b illustrates the effect of the different surfactants on the ZP and the conductivity of the SiC suspension. It is clear that the addition of the cationic surfactant CTAB increased the ZP up to 48 mV without a significant change in the conductivity, while the addition of PEI resulted in a smaller increase in the ZP, reaching a maximum value of 35 mV with 0.8% PEI. As reported by Zhang et al.,21,22 due to the low dissociation of PEI in the alkaline region, PEI does not stabilise the alkaline suspension, while any further addition of citric acid should lead to, according to their reports, a significant increase in the ZP.21,22 In our measurements the ZP reached its highest value (44 mV) only after the subsequent addition of 2 wt.% of citric acid (at pH 2.8), whereas the conductivity was already very high. A better result was obtained when the pH of the PEI-containing suspension was adjusted with HCl: at a pH of 8 the ZP reached 52 mV (the effect of the pH changes are indicated with the dashed arrows in Fig. 2a and b). Table 1 Compositions and properties of the characteristic suspensions used in the EPD experiments and corresponding properties of wet deposits No. Composition Suspension Deposit Additive wt.% pH Conductivity ZP wt.% solids 1 0 4 0.08 −3 / 2 HCl 2.8 0.78 14 / 3 CTAB 0.4 3.9 0.24 48 67 4 PEI 0.8 8 0.12 35 / 5 (PEI) + CA 0.75 + 2 2.8 0.86 44 54 6 (PEI) + HCl 0.8 8 0.3 52 64 7 CA 0.5 2.6 0.7 15 / 8 (CA) + PEI 0.5 + 0.3 3.3 0.3 40 59 9 NaOH 9 0.15 −54 66 10 TMAH 9 0.15 −50 67 11 DCE64 0.5 6.3 0.5 −24 / 12 (DCE64) + NaOH 0.5 9 0.74 −49 62 Fig. 2a also shows that the addition of citric acid to the SiC suspension has a minor effect on the ZP, but it signif￾icantly increases its conductivity. However, the conductivity decreased considerably with further additions of PEI, while the ZP increased from 15 to 40 mV. This observation agrees with previous reports22,23 which showed that the surfactant-addition sequence has a large effect on the stability of the suspensions. The addition of the anionic deflocculant Dolapix CE64 increased the absolute value of the ZP up to −24 mV, which was further increased to −49 mV by adjusting the pH to 9; how￾ever, the subsequent conductivity increase was relatively high (see Fig. 1a and b). The above results are summarised in Table 1 and indicate that high, positive ZP values can be obtained with the addition of 0.4% CTAB, 0.8% of PEI at pH 8 or through combined addition of 0.5% CA and 0.3% PEI, while high, negative values can be achieved by a pH increase to 9 or with the addition of Dolapix CE64 at pH 9. However, some of these compositions show a strong increase in the conductivity too. Based on these results, in a further step, we performed EPD experiments using a selected composition with characteristic ZP values. 3.2. EPD experiments The EPD experiments were performed by using suspensions with 25 wt.% of solids at selected characteristic points in the ZP vs. pH diagrams. In order to evaluate the quality of the deposits, steel electrodes were used in the first trials. The suspensions’ compositions and properties, related to the final result of the EPD experiments, are listed in Table 1. First, an EPD experiment was performed using a suspen￾sion without any additive. As expected, due to the absence of charged particles in the SiC suspension at its natural pH of 4.1, no deposit was observed after 5 min at an applied voltage up to 60 V. When the pH of the suspension was adjusted to 2.8, where the ZP was 11 mV, a very loose deposit was formed on the cathode, but it slid from the electrode during removal from the suspension