. Corni et al Jounal of the European Ceramic Sociery 28(2008)1353-1367 1355 this mechanism cannot clarify depositions carried out for longer LYOSPHERE DISTORTION BY EPD times, or for processes in which the particle-electrode contact is not permitted, for example when the deposition occurs on a semi-permeable membrane placed between the electrodes 2.1.3. Electrochemical particle coagulation mechanism This mechanism implies the reduction of the repulsive forces between the particles in suspension. Koelmans calculated the rise of the ionic strength close to the electrode when a difference LOCAL LYOSPHERE THINNING of potential was applied. This behaviour was due to an increase of the electrolyte concentration around the particles. He discov ered that the value of ionic strength was similar to that required to flocculate a suspension. Therefore, Koelmans' proposed a mechanism based on the fact that an increase of the electrolyte oncentration produces a decrease of the repulsion between the 澜一: particles close to the electrode(lower -potential) and conse- quently the particles coagulate. Considering that a finite time is needed for the increase of the electrolyte concentration next to the electrode, it can be concluded that a certain time has to pass COAGULATION in order to have deposition. This time is inversely proportional to the square of the applied voltage(to 1/E), i. e. the higher the applied potential the shorter the time required for deposi tion. This mechanism is plausible when the electrode reactions OH ions, e.g., suspensions containing water, but it when there is no increase of electrolyte concentration Fig. 3. Schematic representation of the deposition mechanism due to elec trical double layer distortion and thinning. I(Reproduced with permission of 2.1.4. Electrical double layer(edl) distortion and Blackwell Publishing. Sarkar and Nicholson proposed a model mainly based on the distortion of the particle double layer to explain the invalida- 2.2. Novel theories and models tion of the electrochemical coagulation mechanism when there is no increase of electrolyte concentration near the electrode. Studies of electrodynamic particle tion during They noted that when a positive particle and its shell are moving EPD have been carried out under steady and alternating towards the cathode, the double layer is distorted( thinner ahead electric fields. These models produced equations for the time and wider behind), as shown in Fig 3, due to fluid dynamics and evolution of the probability of separation between deposited to the effect of the applied electric field. As a result the counter particles in different conditions. These equations are able to ions(negative)in the extended tail experience a smaller coulom- explain the experimentally observed clustering of colloidal bic attraction to the positively charged particle and can more particles deposited near an electrode in a DC electric field easily react with other cations moving towards the cathode. This by considering convection by electro-osmotic flow about the process reduces the thickness of the double layer and therefore, particles. Numerical simulations have been also employe hen another particle with a thin double layer is approaching, to a limited extent to model the accumulation of charged the two particles come close enough to interact through London particles on an electrode during EPD. 21,22 These studies are Van der Waal attractive forces and coagulate. This mechanism of fundamental and practical interest to describe the local is plausible considering a high concentration of particles close variations of particle interaction during deposition, which can to the electrode (or high collision frequency). This mechanism be used to optimize the EPd technique works also for incoming particles with thin double layer heads, Regarding the growth of colloidal films during EPD, Sarkar coagulating with particles already in the deposit. et al. provided another fundamental study observing the depo- Subsequently Nicholson et al. showed that the model pre- sition of silica particles on silicon wafers as a function of viously proposed by Sarkar and Nicholson was not complete deposition time. They compared the nucleation and growth and proposed a new theory based on a decrease of the concen- of the silica particle layer with that of atomic film growth tration of H at the cathode due to particle discharge or other via molecular-beam epitaxy and noticed a prominent similar- hemical reactions. Therefore the local pH increases towards ity between the two processes. From this observation a new the isoelectric point (iep), s-potential decreases and the parti- direction for further research could follow in order to optimize cles coagulate. This mechanism is general for all suspensions the microstructure of EPD films. Theoretical work was also car containing hydrogen ions ried out by Van der Biest et al.24-27 who produced a modelI. Corni et al. / Journal of the European Ceramic Society 28 (2008) 1353–1367 1355 this mechanism cannot clarify depositions carried out for longer times, or for processes in which the particle-electrode contact is not permitted, for example when the deposition occurs on a semi-permeable membrane placed between the electrodes. 2.1.3. Electrochemical particle coagulation mechanism This mechanism implies the reduction of the repulsive forces between the particles in suspension. Koelmans17 calculated the rise of the ionic strength close to the electrode when a difference of potential was applied. This behaviour was due to an increase of the electrolyte concentration around the particles. He discov￾ered that the value of ionic strength was similar to that required to flocculate a suspension. Therefore, Koelmans17 proposed a mechanism based on the fact that an increase of the electrolyte concentration produces a decrease of the repulsion between the particles close to the electrode (lower -potential) and conse￾quently the particles coagulate. Considering that a finite time is needed for the increase of the electrolyte concentration next to the electrode, it can be concluded that a certain time has to pass in order to have deposition. This time is inversely proportional to the square of the applied voltage (t ∝ 1/E2), i.e. the higher the applied potential the shorter the time required for deposi￾tion. This mechanism is plausible when the electrode reactions generate OH− ions, e.g., suspensions containing water, but it is invalid when there is no increase of electrolyte concentration near the electrode. 2.1.4. Electrical double layer (EDL) distortion and thinning mechanism Sarkar and Nicholson1 proposed a model mainly based on the distortion of the particle double layer to explain the invalida￾tion of the electrochemical coagulation mechanism when there is no increase of electrolyte concentration near the electrode. They noted that when a positive particle and its shell are moving towards the cathode, the double layer is distorted (thinner ahead and wider behind), as shown in Fig. 3, due to fluid dynamics and to the effect of the applied electric field. As a result the counter ions (negative) in the extended tail experience a smaller coulom￾bic attraction to the positively charged particle and can more easily react with other cations moving towards the cathode. This process reduces the thickness of the double layer and therefore, when another particle with a thin double layer is approaching, the two particles come close enough to interact through London Van der Waal attractive forces and coagulate. This mechanism is plausible considering a high concentration of particles close to the electrode (or high collision frequency). This mechanism works also for incoming particles with thin double layer heads, coagulating with particles already in the deposit. Subsequently Nicholson et al.12 showed that the model pre￾viously proposed by Sarkar and Nicholson1 was not complete and proposed a new theory based on a decrease of the concen￾tration of H+ at the cathode due to particle discharge or other chemical reactions. Therefore the local pH increases towards the isoelectric point (iep), -potential decreases and the parti￾cles coagulate. This mechanism is general for all suspensions containing hydrogen ions. Fig. 3. Schematic representation of the deposition mechanism due to elec￾trical double layer distortion and thinning.1 (Reproduced with permission of Blackwell Publishing.) 2.2. Novel theories and models Studies of electrodynamic particle aggregation during EPD have been carried out under steady18 and alternating electric fields.19 These models produced equations for the time evolution of the probability of separation between deposited particles in different conditions. These equations are able to explain the experimentally observed clustering of colloidal particles deposited near an electrode in a DC electric field by considering convection by electro-osmotic flow about the particles.20 Numerical simulations have been also employed to a limited extent to model the accumulation of charged particles on an electrode during EPD.21,22 These studies are of fundamental and practical interest to describe the local variations of particle interaction during deposition, which can be used to optimize the EPD technique. Regarding the growth of colloidal films during EPD, Sarkar et al.23 provided another fundamental study observing the depo￾sition of silica particles on silicon wafers as a function of deposition time. They compared the nucleation and growth of the silica particle layer with that of atomic film growth via molecular-beam epitaxy and noticed a prominent similar￾ity between the two processes. From this observation a new direction for further research could follow in order to optimize the microstructure of EPD films. Theoretical work was also car￾ried out by Van der Biest et al.24–27 who produced a model to