J Mater sci(2006041:8093-8100 conductivity, was negligible(Fig. 3f). This suggests a 18 minor contribution of the charged um-SiC particles to H17 wt the overall conductivity. Furthermore, no current drop during the deposition was observed 8wt.% The above results reveal that the consistency of the deposit, evaluated in this study on the basis of its density and the solids content, is governed primarily by the surface charge on the particles. The deposits with the highest density were obtained by depositing from the acidic suspension in which the Sic particles carried F limited value by keeping the concentration of ions low c the highest charge, while the current was kept at a (small electrolyte additions ). The density as well as the deposition rate increased with the solids content in the starting suspensions. It should also be pointed out that a minor current drop during the epd was observed when a deposit with high solids content (72.3 wt % was formed, i.e. at pH 2.8. In contrast, a significant current drop was observed at pH 5 and 11 where deposits with lower densities were formed. This is not time [min] in agreement with the assumption that the current drop caused by the inhibited transport of charge carriers Fig. 4 Electrical current during the deposition of nano-SiC ue to the accumulation of the deposit at the electrode. suspension as a function of solids content in the starting Further,according to the results presented in Figs. 3e suspensions(60 V, steel electrode) and f, the major charge carrier in the analysed suspensions was the ions, while the charged particles contribute to the conductivity of the suspensions make a minor contribution hence. it is reasonable to however a lower concentration of silanol ions in the assume that a large particle separation in the deposits diluted suspensions might also have the same effect. containing up to 81 wt. solids, i.e. 52 vol %(see Furthermore, during the deposition a significant cur- Fig 3b, f)allows the ions, as the proposed major rent drop was also observed harge carriers, to move towards the electrode through The addition of the AP changed not only the zP vs the deposit. pH relationship, but also the course of the deposition. Additional EPD experiments were performed with At the natural pH of the um-SiC suspension with the the nano-SiC powder. Due to a high apparent viscosity AP addition, i.e. pH 2, the particles carry a low of the suspensions, resulting from a high speci ositive net charge, as presented in Fig. 1. and there surface area of the powder, a suspension with a fore they collect at the cathode. Figure 5 shows that maximum of 17 wt. was used for the EPD. As a the addition of the AP significantly increased the initial result, in accordance with the above-presented depen- current, most probably due to a high concentration of dence of deposition rate on the solids content in the free phosphate ions in the suspensions. The deposition suspensions, the deposition rate was only 0. 25 g/min. rate was 0.3 g/min, which was much lower than for the The deposit was very loose, and after 10 min of suspension with a similar ph but without the additive deposition it slipped from the electrode. As presented (see Fig. 3c). As presented in Fig. 5, increasing the pl in Fig 4, the initial current was much higher than that to 10, where the ZP was observed to have high observed for all the analysed um-SiC suspensions This negative value, resulted in a decreased initial current could be explained by the presence of surface silanol and an increase in the deposition rate to 0.8 g/min. The groups losing the proton in water containing solutions decreased conductivity suggests a decreased concen- that reflects also in the natural acidity of the suspension tration of free ions due to a chemical reaction of (pH <2). Furthermore, in contrast to the um-Sic phosphate ions with the added ammonium hydroxide powder, where a negligible effect of the particles In contrast, the deposition from the suspension of concentration on the conductivity was observed (see nano-Sic powder with the AP additive was only Fig 3f), the dilution of the suspension caused a successful in acidic suspensions. However, even in this decrease in the initial current, suggesting that the case the deposition was slow, and the deposits small and numerous nano-SiC particles might also tained bubbles. The initial current was very 2 Springconductivity, was negligible (Fig. 3f). This suggests a minor contribution of the charged lm-SiC particles to the overall conductivity. Furthermore, no current drop during the deposition was observed. The above results reveal that the consistency of the deposit, evaluated in this study on the basis of its density and the solids content, is governed primarily by the surface charge on the particles. The deposits with the highest density were obtained by depositing from the acidic suspension in which the SiC particles carried the highest charge, while the current was kept at a limited value by keeping the concentration of ions low (small electrolyte additions). The density as well as the deposition rate increased with the solids content in the starting suspensions. It should also be pointed out that a minor current drop during the EPD was observed when a deposit with high solids content (72.3 wt.%) was formed, i.e. at pH 2.8. In contrast, a significant current drop was observed at pH 5 and 11 where deposits with lower densities were formed. This is not in agreement with the assumption that the current drop is caused by the inhibited transport of charge carriers due to the accumulation of the deposit at the electrode. Further, according to the results presented in Figs. 3e and f, the major charge carrier in the analysed suspensions was the ions, while the charged particles make a minor contribution. Hence, it is reasonable to assume that a large particle separation in the deposits containing up to 81 wt.% solids, i.e. 52 vol.% (see Fig. 3b, f) allows the ions, as the proposed major charge carriers, to move towards the electrode through the deposit. Additional EPD experiments were performed with the nano-SiC powder. Due to a high apparent viscosity of the suspensions, resulting from a high specific surface area of the powder, a suspension with a maximum of 17 wt.% was used for the EPD. As a result, in accordance with the above-presented depen￾dence of deposition rate on the solids content in the suspensions, the deposition rate was only 0.25 g/min. The deposit was very loose, and after 10 min of deposition it slipped from the electrode. As presented in Fig. 4, the initial current was much higher than that observed for all the analysed lm-SiC suspensions. This could be explained by the presence of surface silanol groups losing the proton in water containing solutions that reflects also in the natural acidity of the suspension (pH < 2). Furthermore, in contrast to the lm-SiC powder, where a negligible effect of the particles concentration on the conductivity was observed (see Fig. 3f), the dilution of the suspension caused a decrease in the initial current, suggesting that the small and numerous nano-SiC particles might also contribute to the conductivity of the suspensions. However, a lower concentration of silanol ions in the diluted suspensions might also have the same effect. Furthermore, during the deposition a significant cur￾rent drop was also observed. The addition of the AP changed not only the ZP vs. pH relationship, but also the course of the deposition. At the natural pH of the lm-SiC suspension with the AP addition, i.e. pH 2, the particles carry a low positive net charge, as presented in Fig. 1, and there￾fore they collect at the cathode. Figure 5 shows that the addition of the AP significantly increased the initial current, most probably due to a high concentration of free phosphate ions in the suspensions. The deposition rate was 0.3 g/min, which was much lower than for the suspension with a similar pH but without the additive (see Fig. 3c). As presented in Fig. 5, increasing the pH to 10, where the ZP was observed to have high negative value, resulted in a decreased initial current and an increase in the deposition rate to 0.8 g/min. The decreased conductivity suggests a decreased concen￾tration of free ions due to a chemical reaction of phosphate ions with the added ammonium hydroxide. In contrast, the deposition from the suspension of nano-SiC powder with the AP additive was only successful in acidic suspensions. However, even in this case the deposition was slow, and the deposits con￾tained bubbles. The initial current was very high, 0 2 4 6 8 10 12 14 16 18 0 1 3 4 6 7 8 91 2 5 0 time [min] Current [mA] 17 wt. % 8 wt. % Fig. 4 Electrical current during the deposition of nano-SiC suspension as a function of solids content in the starting suspensions (60 V, steel electrode) J Mater Sci (2006) 41:8093–8100 8097 123