L Besra, M. Liu/ Progress in Materials Science 52(2007)1-61 epulsion due to high particle charge is required to avoid particle agglomeration. The par- ticle charge also affects the green density of the deposit. During formation of the deposit the particles become closer to each other and with increasing attraction force. If the par ticle charge is low, the particles would coagulate even for relative large inter-particle distances, leading to porous, sponge-like deposits. On the contrary, if the particles have a high surface charge during deposition they will repulse each other, occupying positions which will lead to high particle packing density [36]. It is therefore very important to con trol the solids loading and concentration of solvents and additives in the EPd suspension in order to reach the highest possible green density of the deposit. The zeta potential can be controlled by a variety of charging agents such as acids, bases and specifically adsorbed ions or polyelectrolytes, to the suspension [37]. Thus there exists a variety of additives that affect the charge magnitude and its polarity. These additives act by different mechanisms The main criteria for selection of a charging agent are the preferred polarity and deposi- tion rate of the particles Chen et al. [38] found that the stability and deposition rates of alumina from its suspen- on in ethanol was maximum at pH value of 2.2 at which the positive zeta potential of alumina was maximum(Fig 3). However, under higher pH value of ll, the suspensions were less stable. This can be explained based on a charging mechanism recently proposed by Wang et al. [39]on the alumina surface 1OH, AIOH = AlO+H,0 Under basic conditions such as phll, AlOH tends to form alo however, the presence of water is prone to bring the above reaction towards the formation of AlOH,, rather than the formation of AlO, resulting in an absolute value of the zeta potential greater pH 2 than at pH ll. This led to high stability of suspension at lower pH tha higher pH conditions AL,o, powder dispered in ethanol (1.5 vol%6) 0 oelectric point (EP pH value Fig 3. Zeta potential of Al2O3 powder in ethanol [38]repulsion due to high particle charge is required to avoid particle agglomeration. The par￾ticle charge also affects the green density of the deposit. During formation of the deposit, the particles become closer to each other and with increasing attraction force. If the par￾ticle charge is low, the particles would coagulate even for relative large inter-particle distances, leading to porous, sponge-like deposits. On the contrary, if the particles have a high surface charge during deposition they will repulse each other, occupying positions which will lead to high particle packing density [36]. It is therefore very important to con￾trol the solids loading and concentration of solvents and additives in the EPD suspension in order to reach the highest possible green density of the deposit. The zeta potential can be controlled by a variety of charging agents such as acids, bases and specifically adsorbed ions or polyelectrolytes, to the suspension [37]. Thus there exists a variety of additives that affect the charge magnitude and its polarity. These additives act by different mechanisms. The main criteria for selection of a charging agent are the preferred polarity and deposi￾tion rate of the particles. Chen et al. [38] found that the stability and deposition rates of alumina from its suspen￾sion in ethanol was maximum at pH value of 2.2 at which the positive zeta potential of alumina was maximum (Fig. 3). However, under higher pH value of 11, the suspensions were less stable. This can be explained based on a charging mechanism recently proposed by Wang et al. [39] on the alumina surface AlOHþ 2 ( Hþ AlOH ) OH AlO þ H2O ð5Þ Under basic conditions such as pH 11, AlOH tends to form AlO; however, the presence of water is prone to bring the above reaction towards the formation of AlOHþ 2 , rather than the formation of AlO, resulting in an absolute value of the zeta potential greater at pH 2 than at pH 11. This led to high stability of suspension at lower pH than at higher pH conditions. Fig. 3. Zeta potential of Al2O3 powder in ethanol [38]. L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61 9