Forming and Microstructure Control of Ceramics by Electrophoretic Deposition ( EPD ) †

Electrophoretic deposition (EPD) is one of the colloidal processes during in ceramic production and has gained significant interest because of the high versatility of its use with different materials including nanoparticles and its cost-ef fectiveness requiring simple equipment. Of the major parameters for ceramic processing involving the EPD, preparation of the suspensions and application methods of electric fields are particularly important factors that af fect the microstructure. At the beginning of this review, we introduce the fundamental aspects of the EPD processing. We then focus on the following four points: (1) the stability of the Pb(Zr,Ti)O2 /ethonol suspension by the addition of phosphate esters and its influence on the subsequent EPD process, (2) the stability of a TiO2/(2-propanal+2.4-pentanedione) suspension, which is a suspension without dispersants, (3) the film performance of the pulsed direct current EPD using an aqueous suspension, and (4) the laminated textured ceramics by EPD in a strong magnetic field.


Introduction
Electrophoretic deposition (EPD) is the process of forming deposited layers of particles on substrates.In this process material, particles such as ceramics are electrically charged and dispersed in a liquid medium.When an electric field is applied to the suspension, the charged particles migrate toward the electrode with the opposite charge and the particles are then deposited onto the substrates.This EPD technique is receiving attention as a convenient method of producing relatively uniform deposition films of particles within a short time interval and at low cost [1][2][3][4][5][6][7] . Eectrically conductive materials such as metals and graphite are generally used as substrates and as electrodes.Although the mechanism of the particle deposition has not been completely elucidated, it is generally believed that particles migrate toward the oppositely charged electrode under the effect of an electric field; when they reach the surface of the substrate, the repulsive potential between the particles is reduced, and the particles coagulate because of van der Waals attraction and are deposited on the substrate.Of the major parameters for ceramic processing involving the EPD, preparation of the suspensions and application methods of the electric fields are particularly important factors that affect the microstructure 8 -12) .In this review, after introduction to the EPD processing, we focus on the following four points: (1) the stability of the ceramic particle /ethonal suspension by the addition of phosphate esters and its influence on the subsequent EPD process, (2) the stability of the ceramic particle/(2-propanal＋2.4-pentanedione) suspension, which is a suspension without dispersants, (3) the film performance of the pulsed direct current EPD using an aqueous suspension, and (4) the laminated textured ceramics by EPD in a strong magnetic field.

Electrophoretic Process as Consolidation and Formation Technology for Particles
The electrophoretic process, as a colloidal process, can easily accommodate denser green bodies as compared to dry processes.Fig. 1 shows a comparison of the sintering behavior in air of the following samples: (1) zirconia (3YTZ) nanoparticle green body formed by uniaxial pressing (UP) at 100 MPa, (2) zirconia nanoparticle green body formed by UP at 100 MPa and subsequent cold isostatic pressing (CIP) at 200 or 400 MPa, and (3) zirconia nanoparticle green body formed by EPD.The green body formed by EPD exhibited a degree of sintering comparable to or better than the degree of sintering of the green body formed by CIP at 200 MPa In the casting method such as slip casting in which a solvent moves through the pore channels consolidated by the par ticles, the channel size becomes narrower and the casting rate declines as the particle size decreases 13,14) .However, during the electrophoretic process in which particles themselves move, the deposition rate does not depend on the particle size and is extremely fast.Based on this point, EPD is a suitable method for consolidating nanoparticles.As compared with tape casting, EPD is also suitable for the production of thickness-controlled laminates with good adherence between the layers. 15)Therefore, recently EPD has recently been applied in many fields, such as dielectric films 16 -19) , electrochromic films 20) , lithium batteries 21) , fiber-reinforced composites 22) , solid oxide fuel cell laminates 23 -26) , phosphor films 27,28) , etc.
A significant advantage of using electric fields is that the charged particles in solution do not directly migrate to the electrodes the shortest distance away, but rather migrate along the electric potential gradient in a space filled with solution and then deposit.Fig. 2 shows a schematic representation of the electric field lines and migration of particles between two cylindrical electrodes.Unlike during vapor deposition and sputtering, particles migrating between the electrodes along the electric field lines can reach the backside of the electrodes, and thus, relatively uniform deposition layers can be easily formed on substrates with curved or uneven surfaces.Therefore, by three-dimensionally controlling the polarity and spatial position of the electrodes and electric field potential, particles can be led to a specific location on a substrate and deposited there. 29)Such particle assembling by precise control of the electric fields is considered a great advantage of the electrophoretic process.Fig. 3 shows an example of a deposition pattern formed with zirconia (3Y-SZ) nanoparticles on a conducting polymer (polypyrrole; PPy) pattern electrode formed on silica glass. 30,31) tention is being focused on a process of producing ceramics onto which the contours and patterns of the substrates are transferred.In this process, by using the capability of EPD to produce dense green bodies, as shown in Fig. 1, and to form deposition layers on substrates with cur ved or uneven surfaces, particles are deposited on a substrate that has been previously processed into the desired form or  impressed with complex patterns in advance.The obtained deposition layer and the substrate, which is also the electrode, are then sintered together.Fig. 4 shows a cap-like sintered body that is an example of such products.Bimodal alumina powders (particle size: 0.6μm + 30 nm) are deposited by the EPD on a ceramic substrate coated with PPy, and the deposited layer and the coated substrate are then dried and sintered together in air.Because the PPy layers are then decomposed during sintering in air, the use of ceramic substrates that have been previously processed into the desired shape in advance produces ceramic sintered bodies to which the shape of the substrates are clearly transferred.Fig. 5 shows an example of a free-standing alumina films with the surface pattern of a coin produced by this method.An elaborate pattern has been accurately copied onto the film. 31)ig. 6 shows a schematic representation of the migration and deposition of particles and ions in a suspension in an electric field.Particles having an electric double layer migrate in a medium.When they reach the substrate, they gradually lose the electric double layer; subsequently, they coagulate and are deposited on the substrate.During this process, the particles are pressed onto the substrate due to the effect of the electric field.Because the cohesive power becomes weak if the repulsive potential does not significantly decrease, only low-density green  bodies may be obtained or particles may slip off the substrate when the electricity is turned off.According to Hamaker's mass balance law, the weight of particles deposited by the electrophoretic process W (g) can be expressed by the following equation: 32) where t is the deposition time (s), μ is the electrophoretic mobility (m 2 V -1 s -1 ), E is the electric potential gradient (V/m), C is the amount of the solid phase in the suspension (g/m 3 ), S is the electrode area (m 2 ), and f is the adhesion probability of particles reaching the substrate (0 ≦ f ≦ 1).The electric potential gradient E varies during the particle deposition, and the following equation holds between the current in the circuit I (A) and the electric conductivity Λ (S/m): The potential applied to the circuit Va decreases due to potential drops at the anode and cathode, and also by any ohmic loss in the suspension and the solidified layer.Therefore, the following equation holds among them: where ΔΦanode and ΔΦcathode are the potential drops at the anode and cathode, respectively; Rsus and Rs are the values of the apparent resistance (Ω/m) of the suspension and solidified layer, respectively; d is the distance between the electrodes; and ds is the thickness of the solidified layer.When the electrophoretic process is performed in a constant voltage mode, the decrease in the current of the circuit with time indirectly indicates the IR-drop, that is, the progress of the particle deposition.In the constant current mode, the increase in voltage with time indicates the progress of the particle deposition.

Suspensions Suitable for EPD
Difficulties in the EPD process can be classified into the following four categories: particles do not deposit; layers do not thicken; the quality of layers is poor; and the layers crack.These are mostly caused by problems with the method used for preparing the suspensions.Generally, suspensions suitable for the EPD process require the following: • The particle surfaces are charged.(If the particles are not charged, they do not migrate by the applied electric field.) • Particles are well dispersed.(Coagulated suspensions result in low-density deposited bodies.) • Suspensions contain excess ions.(Ions other than the charged particles lower the transport number of the par ticles.Compression of the electric double layer reduces the stability of the slurry.) • Particles have a high adhesion ability.(Binders can be added as necessary.)Normal colloidal processes often involve aqueous solvents because of their lower cost and environmental load, and ease in controlling dispersion and coagulation 33 -40) .Although aqueous solvents can be used in the electrophoretic process, non-aqueous solvents are often selected because aqueous solvents evolve gases, which cause the formation of pores in deposited bodies due to the electrolysis of water.As stated above, the electrophoretic process requires particles that have surfaces charged in a liquid.In a few cases, stable suspensions with high zeta potentials can be prepared by simply adding particles to the solvents and stirring.However, in general, to positively or negatively charge particles, a proper amount of acid, base, or polyelectrolytes having functional groups such as amino groups and carboxylic groups on the side chains must be added to a solution to which powders are added.Ceramic particles in solution usually adsorb hydroxyl groups on their surfaces.When an acid or base is added to the solution, the hydroxyl groups on the surfaces are protonated or deprotonated, as shown in the following formulas.Thus, the particle surfaces are charged positively or negatively.
Polyelectrolytes are adsorbed onto the particle surfaces, and the side chains of the polyelectrolytes are positively or negatively charged according to the functional group.Consequently, the particle surfaces modified by the polyelectrolytes can be considered to have positive or negative charges corresponding to the functional group.While acid, base, and polyelectrolytes have the advantage of producing positive or negative charges (which are necessary to electrophoresis) to the particle surfaces, they are disadvantageous because the unadsorbed ions compress the electric double layers; in this case, the dispersiveness of the suspensions becomes worse if they are added in excess.Furthermore, excess free ions become the majority charge carriers in solution and the transport number of the charged particles significantly drops.Therefore, special care must be taken not to add excess ions because this may prevent particle deposition and thickening of the layers even though the EPD is conducted for a long time.Cationic, anionic, or nonionic polymers are ofen used as binders.Because the excess addition of binders also causes the poor deposition of particles, a decrease in the layer density, and lack of uniformity of layers, the optimum amount of binders must be determined by adding them in small increments and carefully observing the results.

Preparation of suspensions using acidic phosphoesters
Phosphate esters are formed by dehydration condensation between phosphoric acid (O=P(OH)3) and an alcohol (ROH), and all or part of the three hydrogen atoms in the phosphoric acid are substituted by organic groups R. Phosphate esters having one, two, and three substituents are called monophosphate esters, dishosphate esters, and triphosphate esters, respectively.Of these, the monophosphate esters and dishosphate esters are also called acidic phosphate esters because they easily lose protons due to the dissociation of the O-H bond of the hydroxyl groups bounded to the phosphorus; conseqyently, they act as relatively strong acids even in organic solvents 30,31) .
In suspensions to which acidic phosphate esters are added, dissociated protons H + in organic solvents are adsorbed onto the particle surfaces to produce the positive charges.When the adsorbed amount of protons becomes saturated and the concentration of protons in the solution becomes excessive, the degree of dissociation declines and an excessive increase in the ionic strength of the solution can be avoided.Fig. 7 shows the relationship between the amount of butyl acid phosphate (JP-504; Johoku Chemical Co., Ltd.) added and the pH and electric conductivity of a suspension of PZT (average particle size: 93 nm; Sakai Chemical Industry Co., Ltd.) 28) .At first, the pH and electric conductivity significantly change with the addition of the phosphate ester, but the changes slow down when the added amount reaches about 0.4wt%.The zeta potential at this level is shown in Fig. 8.It represents the zeta potential that reaches a peak when the added amount reaches about 0.4wt%.A comparison showed that the adsorption equilibrium can be easily determined by monitoring the changes the pH and electric conductivity of the slurry when adding the phosphate esters.Fig. 9 shows the relationship between the deposited weight of the PZT particles on a 304 stainless steel substrate and the amount of added phosphate ester (Experimental conditions: concentration, 1.3 vol; applied voltage, 100 V; area of a substrate, 4cm 2 ; and energized time, 4min).The deposited weight of the particles reaches a local maximum at 0.4wt% of the amount of added phosphate ester and then decreases once, but increases again with the addition of excess phosphate ester.When the slurry to which the added phosphate ester in excess was used in an experiment, the adhesiveness of the particles to the stainles steel substrate was so strong that the deposited layers could not be easily wiped off with a Kimwipe®wipe paper.This may be because phosphate esters have the effect of enhancing the affinity to metals.A dense PZT ceramics was obtained after sintering at 1250℃ and the fol-   lowing properties were observed; the remanent polarization, Pr, was 38mC/cm 2 ; and the coercive field, Ec, was 20kV/cm.No influence of residual phosphorous impurities (0.41wt% of phosphate ester addition) was observed 28) .

Preparation of binder-free suspensions
If binders are added to the suspensions, in some cases they must be removed during the subsequent heating or sintering process.Although a normal binder-removal treatment usually involves heating at 773 K or higher, the heat treatment must be avoided in some cases.For example, in the case of photoelectrodes for dye-sensitized solar cells (DSSCs), which are formed by the deposition of titania (anatase) onto transparent indium tin oxide (ITO) transparent electrodes, the heat treatment temperature is restricted to 673 K or less to prevent a drastic drop in the electric conductivity of the ITO.Such cases require binder-free suspensions having a good stability and adherence 41,42) .A mixed solvent system of 2,4-pentanedione (acetylacetone) and 2-propanol is a dispersion medium that allows control of the particle dispersion by simply varying the mixing ratio.Fig. 10 shows the appearances of suspensions having various solvent mixing ratios, in which titania nanoparticles (TiO2, average particle size: 30 nm; NanoTek) were dispersed using an ultrasonic homogenizer.The solutions stood for two weeks after the dispersion.The figure indicates that the suspensions become stable when the suspensions contain 20 to 50 vol% of 2-propanol.Fig. 11 shows the relationship between the electrophoretic mobility of these nanoparticles the mixed solvents of 2,4-pentanedione and 2-propanol, and the mixing ratio of the solvents.The electrophoretic mobility shows the highest value at the mixing ratio of 50:50, suggesting that the surface charges of titania reach a peak at that ratio.The amount of protons generated by the keto-enol tautomerism of 2,4-pentanedione (represented in the following equation) and the amount of protons adsorbed onto the titania particles was balanced 43) . ( Measurement of the electric conductivity demonstrated that there are a few excess ions when the ratio of 2-propanol is lower than the mixing ratio of 50:50, and excess ions in the solution increase when the ratio of 2-propanol becomes higher.Fig. 12 is an example of a uniform titania nanoparticle membrane formed on an ITO substrate using a suspension prepared with a solvent whose mixing ratio is 50:50.Even though binders were not added to the suspension, this membrane did not detach from the substrate even after heat treatment at 673 K, and it had a good adherence.

Use of Modulated Electric Fields
Although normal EPD processes involve a continuous direct current (DC), microstructure control of the deposited layers by using modulated electric fields such as pulsed DC electric fields 44 -47) and asymmetric alternating current (AC) electric fields 48) has recently been receiving attention.This study investigated the influence of applying of a square wave and a continuous wave, as shown in Fig. 13, on deposited layers using a Source Meter (Model 2611; Keithley Instruments).Fig. 14 shows the deposited layers formed from an aqueous suspension (pH 4.5) of alumina (AKP-50, average particle size: 0.2μm; Sumitomo Chemical Co., Ltd.) on SUS316L cathode substrates by applying a square-wave current having 20 V/0 V of ON/OFF voltages and the same pulse width of the ON/OFF time for 3 min in total 44) .While a continuous current without a pulse caused forma-tion of an unlimited number of bubbles on the surface of the deposited layers due to the electrolysis of water, the formation of bubbles was inhibited as the pulse frequency increased.The formation of bubbles was completely inhibited at a certain pulse frequency and a smooth deposited layer was obtained.When the pulse frequency was further increased, the particles did not deposit.In other words, the optimum values of the pulse frequency were within a certain range.When an increasing voltage was applied, the optimum values shifted to higher frequencies and the suitable range became narrower.The same results were obtained in a constant-current condition.Fig. 15 shows the conditions where good-quality deposited layers having no bubbles were obtained when applying DC pulses in the constant-current mode.Alter natively, when par ticles negatively charged by the addition of the anionic dispersants were deposited onto the anode, the formation of bubbles could be inhibited by optimizing the pulse frequency.Although this mechanism is still under study, it is believed that the evolution of hydrogen due to the dissociation of water (H2O→H + +OH − ) is prevented under the conditions depicted in the diagonally shaded area of Fig. 15 by fast switching between ON and OFF.Thus, particles continue to migrate during the OFF time due to an inertial force, and reach the substrate, resulting in formation of deposited layers having no bubbles.This suggests that the use of modulated electric fields may become more important in order to convert conventional nonaqueoussolvent processes to aqueous-solvent processes that have a lower environmental load 46),49) .

Textured Ceramics
The controlled development of a texture is one of the ways for effectively improving various properties 50 -52) .Textured ceramics have been produced by a variety of techniques; such as tape casting, hot forging and templated or seeded grain growth. 50)Recently, we have demonstrated that the crystal orientation of feeble magnetic materials such as alumina can be controlled by colloidal processing in a high magnetic field. 52 -54)he principle of the process is that a crystal with an anisotropic magnetic susceptibility will rotate to an angle minimizing the system energy when placed in a magnetic field.The magnetic torque T attributed to the interaction between the anisotropic susceptibility and a magnetic field is estimated by Eq. ( 9) 52) .
where Δ χ, (= l χ a,b− χ c l ) is the anisotropy of the magnetic susceptibilities which are measured for the a,b-axis ( χ a,b) and c-axis ( χ c ), V is the volume of the materials, B is the applied magnetic field, ɵ is the angle between the easy magnetization axis in a crystal and imposed magnetic field direction, and μ0 is the permeability in a vacuum.This magnetic torque is the driving force for the magnetic alignment.
To obtain the oriented materials with feeble magnetic susceptibilities, the following conditions are necessary: 52) 1) the crystal structure should be noncubic in order to apply the anisotropic magnetic susceptibilities, 2) the particle should be a single crystal and well dispersed, 3) the viscosity of the suspension should be low enough to rotate the particles with a low energy, and 4) grain growth is necessar y to obtain a highly oriented structure especially when a spherical particle is used.We have fabricated many kinds of oriented ceramics by the slip casting in a high magnetic field followed by sintering, such as Al2O3, TiO2, ZnO, AlN, SiC, Si3N4, etc., 52 -64) their composites, 60,65) and multi-component ceramics by reaction sintering 66) .However, when we use whisker or plate-like particles, special attention is necessary due to the effect of gravity energy 67,68) that is the highest energy in the colloidal dispersion system 69) .The effect also strongly depends on the easy magnetization angle, which has been described elsewhere. 68)e have demonstrated that EPD in a high magnetic field is an excellent method to fabricate thick cr ystalline textured ceramic bodies. 70 -78)The direction of the electric field relative to the magnetic field (the angle between the vector E and B) was altered (0, 45, 90°) to control the dominant crystal faces as is shown in Fig. 16.structure for the improvement of the mechanical, dielectric, and thermo-electric properties, etc. 74 -76)

Summar y
Recently EPD has received a significant amount of attention for fabricating highly structured controlled ceramics resulting in advanced ceramics.The process is a simple, easy to use and cost-eefective method of deposition.Here, the preparation of suspensions suitable for EPS is very important.In this review we have introduced our recent results; (1) the stable Pb(Zr,Ti)O2 /ethanol suspension is prepared by the addition of phosphate esters and a good film is fabricated by the subsequent EPD process, (2) the stable TiO2/(2-propanol +2.4-pentanedione acetylacetone) suspension is prepared without dispersants and a good film is also fabricated by the subsequent EPD process , (3) a film without voids is prepared by the appropriate tpulsed DC EPD using an aqueous suspension, and (4) the laminated textured ceramics is fabricated by the EPD in a strong magnetic field.These techniques are expected to be applicable for various fields.

Fig. 1
Fig. 1 Difference in sintering density of zirconia nanoparticles depending on preparation methods.

Fig. 2 Fig. 3
Fig. 2 Electric field lines and migration of particles between two cylindrical electrodes.

Fig. 4
Fig. 4 Alumina deposition layer formed onto the ceramic substrate coated with conducting polymer: (a) appearance before sintering, (b) surface of deposition layer before sintering, and (c) and (d) appearance after sintering.

Fig. 5
Fig.5Free-standing alumina film (after sintering) onto which the pattern of a 100-yen coin was copied.

Fig. 6
Fig. 6 Migration and deposition of particles and ions in suspension.

Fig. 7
Fig. 7 Relationship between the amount of added phosphoester and the pH and electric conductivity of PZT suspension (left).

Fig. 8 2 Fig. 9
Fig. 8 Relationship between zeta potential of PZT and the amount of added phosphoester (right).

Fig. 12
Fig. 12 Appearance and cross section of titania membrane formed on ITO glass (sintered at 673 K for 2 h).

Fig. 13
Fig.13 Waveforms of modulated electric fields used in this study (left).

Fig. 14
Fig. 14 Relationship between pulse width and deposited layers formed by pulsed DC EPD from an aqueous suspension of alumina on an SUS substrate (ON/OFF voltage = 20 V/0 V, 3 min in total) (right).

Fig. 15
Fig.15 Relationships between applied current and pulse width in pulsed DC EPD (in constant-current mode) and the quality of the membrane.

Fig. 17
Fig.17shows the variation in the XRD patterns of the top planes with the angle between the directions of B and E, where the α-alumina(particle size of 0.2μm) was deposited in 10 T followed by sintering at 1873 K.The Φηkl of the appeared peaks are also shown in the figure.When the E is parallel to B (φB −E=0°)), the diffraction peaks of the planes at low interplanar angles (Φηkl is close to 0°) are predominant.When the Φηkl is changed to 45°, the dominant diffraction peaks change to the planes of the intermediate interplanar angles (φB−E ιs close to 45°).When E is perpendicular to B (φB−E =90°)), the dominant

Fig. 17
Fig. 17 Changes in the XRD patterns of the top planes with the angle between the direction of B and E.

Fig. 18
Fig.18 Schematic illustration and the cross-sectional microstructure of a crystalline-textured alumina/alumina laminated composite prepared by electrophoretic deposition in a strong magnetic field (12 T), in which the direction of the electric current relative to the magnetic field is alternately changed from 0 to 90 , layer by layer.