Bubble-Free Aqueous Electrophoretic Deposition (EPD) by Pulse-Potential Application

Bubble-Free Aqueous Electrophoretic Deposition (EPD) by Pulse-Potential Application

Laxmidhar Besra, Tetsuo Uchikoshi,w Tohru S. Suzuki,* Yoshio Sakka*

Nano Ceramics Center, Fine Particle Processing Group (WPI Center Initiative for Materials Nanoarchitechtronics),National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0047, Japan

A novel method based on the application of a square-wave pulsepotential of 50% duty cycle has been demonstrated to obtaindense bubble-free deposits of alumina by constant voltageelectrophoretic deposition (EPD) from an aqueous suspension.Application of continuous dc voltage invariably resulted in theincorporation of bubbles in the deposits. Bubbles in the depositdecreased progressively with decrease in the size of pulse widthduring the pulse potential EPD. A unique and narrow band ofpulse width exists for each voltage within which a bubble-freedeposit is obtained. The band is wider at low applied voltagesthan at higher voltages. Such bands of pulse width were found tobe independent of substrate material and occurred at the samerange for stainless steel and nickel substrates, suggesting thatthe process may be generic and applicable to any conductivesubstrate. The green density of deposits obtained by pulse EPDhas been found to be the same as those obtained by continuousdc voltage EPD. The cathodic pulse EPD produced uniform andhomogeneous deposits and was more convenient and amenableto better control than anodic pulse EPD.

I. Introduction

ELECTROPHORETIC deposition (EPD), a colloidal processingtechnique, has recently gained considerable interest in tra-

ditional ceramics as well as in a wide range of novel applicationsin advanced ceramic materials and coatings to fabricate thinfilms, multilayered composites, functionally graded materials,hybrid materials, micropatterned colloidal assemblies, andnanotechnology.1–6 In this method, the powder materials dis-persed in a liquid medium, on application of certain electricalpotential, are attracted and migrate toward the oppositelycharged electrode on which they eventually get deposited, form-ing a relatively dense and homogeneously compact film. Themajor advantages and attractiveness of EPD is its simple appa-ratus, short formation time, little restriction in the shape of sub-strate, suitability for mass production, and no requirement forbinder burnout as the green coating contains few or no organics.Compared with other advanced ceramic processing techniques,the EPD process is very versatile because it can be modifiedeasily for specific applications. In particular, despite being a wetprocess, EPD offers easy control of the thickness and morphol-ogy of a deposited film through simple adjustment of the appliedpotential and deposition time.

In general, organic liquids are more popularly used as thesuspending medium in EPD.7 While the generally low dielectricconstants of organic liquids limit the charge on the particles as a

result of lower dissociating power, much higher field strengthscan be applied because the problems of electrolytic gas evolu-tion, joule heating, and electrochemical attack of the electrodesare greatly reduced or nonexistent in organic liquids. The pref-erence of organic liquids is also due to their higher density, goodchemical stability, and low conductivity. But the use of aqueoussystems has important advantages because they need muchlower voltage to be applied and the environmental problemsassociated with organics are avoided.8 Obviously, the use ofwater also implies advantages such as higher temperature con-trol during the process and faster kinetics, in addition to im-portant health benefits, benign environment, and low cost.9–11

These advantages have promoted interest to develop water-based EPD to process technical ceramics.12–14 The water-basedsuspension however causes a number of problems in electroph-oretic forming.15 First and foremost, the electrolysis of waterstarts to occur at very low voltage, and gas evolution at theelectrodes is inevitable at field strengths high enough to enablereasonable kinetics of deposition during EPD. Hydrogen andoxygen gases are evolved at the cathode and anode, respectively,in accordance with the following decomposition reactions:

Anode : 4OH�ðaqÞ ! O2ðgÞ þ 2H2OðlÞ þ 4e�

Cathode : 4H2OðlÞ þ 4e� ! 2H2ðgÞ þ 4OH�ðaqÞ

The overall cell reaction is

2H2OðlÞ ! 2H2ðgÞ þO2ðgÞ

In the above reactions, the volume of H2 gas produced istwice the volume of O2 gas. The bubbles generated at the elec-trode surface disturb the electrophoresis of the particles andsuppress their deposition. Moreover, the bubbles also get en-trapped within the deposit, leading to the formation of a porousdeposit. To date, the challenge for aqueous EPD suspension isto inhibit the water electrolysis that interferes with the depositlayer. Several approaches have been investigated in the litera-ture. The simplest method is to conduct EPD experiments atvoltages lower than the decomposition voltage of water (1.23 Vat 251C), but the deposition rate is negligible and not practical.Ryan and colleagues16,17 investigated the use of porous mold toseparate and suppress bubble contamination in the deposit butfound it to be ineffective. Clasen18,19 was successful in obtainingbubble-free deposits on a microporous membrane placed infront of the electrode, but the green density of the deposit wascomparably low and high shrinkage (up to 30%) occurs duringdrying and sintering. Other studies have involved anodic depo-sition of the negatively charged particles on easily oxidizableanodes such as Zn, but the drawback of anodic deposition is therelease of metal cations from the anodic material into the sus-pension, which contaminates the deposit.20,21 Winkle22 sug-gested a method for eliminating or reducing defects inpolymeric films deposited at the cathode by electrodeposition.The method involves decreasing the evolution of hydrogen gas

W. Lee—contributing editor

*Member, The American Ceramic Society.This study was supported in part by the Grant-in Aid for Scientific Research of the JSPS

and by World Premier International Research Center Initiative (WPI) on Materials Nano-architechtronics, MEXT, Japan.

wAuthor to whom correspondence should be addressed. e-mail: [email protected]

Manuscript No. 24334. Received February 19, 2008; approved June 2, 2008.

Journal

J. Am. Ceram. Soc., 91 [10] 3154–3159 (2008)

DOI: 10.1111/j.1551-2916.2008.02591.x

r 2008 The American Ceramic Society

3154

i:/BWUS/JACE/02591/[email protected]

i:/BWUS/JACE/02591/[email protected]

at the cathode by adding a compound to the polymer emulsionsolution. This compound is reduced by the hydrogen producedat the cathode during the electrodeposition. The hydrogen reactswith this nongaseous compound rather than becoming hydro-gen gas and forms bubbles, which leads to pinhole defects.Wang et al.23 designed an electrochemical apparatus that in-cludes a physical barrier that prevents any trapped gas or gasgenerated during processing from residing in areas that cancause defects on the substrate. The apparatus contains two ad-jacent chambers and two separate electrolyte-containing fluidsseparated by a membrane that prevents the gas from beingtransferred into the opposing chamber. The method is howevercomplicated and expensive. Sakurada et al.24 obtained bubble-free deposits of zirconia on anodic substrates of palladium orstainless steel by adding hydroquinone (HQ) to the alkalineaqueous suspension during EPD. The oxygen produced by elec-trolysis of water during EPD was believed to be consumed bythe chemical oxidation of HQ to quinone at high pH in alkalinesolution, enabling a bubble-free deposit on the anode substrate.Our recent studies have shown that the deposition on palladiumcathodic substrate can produce bubble-free deposits with highdensity because palladium readily absorbs the hydrogen gasgenerated at the cathode by electrolysis of water.11,25–27 Wepresent here a novel method based on the application of pulsevoltage for obtaining dense and bubble-free deposits by EPDfrom aqueous suspensions.

The application of pulse current (PC) is not new and has beenused extensively for electrodeposition of metals from their inor-ganic salt solutions.28–34 It has been recognized that applicationof PC in a specific pulse condition permits higher current den-sities than the limiting direct current density, thus enabling de-posits of fine-grained microstructure.35,36 But to the best of ourknowledge, the use of pulse voltage has not been investigated forEPD of powder in aqueous suspension. Although the basictechniques involved in electrophoretic and electrolytic deposi-tion are similar, the associated mechanisms of deposition areuniquely different. Hence, it is worthwhile to investigate the ap-plication of pulse voltage on EPD. With this background, thepresent study was aimed at demonstrating the applicability ofpulse voltage on EPD using an aqueous alumina suspension.

II. Experimental Procedure

Alumina powder (Sumitomo AKP-50, average particle size of0.20 mm) was dispersed in ultrapure distilled water by ultrasoni-cation at 160 kW for 10 min to break up the agglomerates. Ni-tric acid and sodium hydroxide were used for adjusting the pHof the suspension. The z potential of the suspension was mea-sured by a laser electrophoresis z-potential analyzer (LEZA-600,Otsuka Electronics Co., Osaka, Japan). Figure 1 presents thez potential of alumina suspension as a function of pH. The iso-electric point (IEP) of alumina powder is at pH 7.9. The aluminapowder surface is positively charged at a pH below 7.9 and isnegatively charged at a pH above 7.9.

EPD experiments were carried out on stainless steel (316 L)plates of 2 cm� 5 cm� 0.4 mm dimension, hung from the elec-trode holders into the suspension in a glass beaker. A stainlesssteel plate of the same dimension, hung parallel and facing thedeposition electrode at a distance of 2 cm, formed the counter-electrode. Deposition was carried out on an area of 2 cm� 2 cm.

Pulse EPD was conducted at constant voltage mode by ap-plication of a series of pulses of dc voltage of equal amplitudeand duration in the same direction, separated by periods of zerovoltage, using a system source meter (Model 2611, Keithley In-struments Inc. Cleveland, Ohio). A simple square-wave pulse asshown in Fig. 2 was used. The duty cycle (DC) of the pulse [i.e.,DC5Ton/(Ton1Toff)] was set constant at a fixed value of 50%.The RC charging time (the response time of the circuit when astep voltage or signal is applied) of the source meter iso100 ms.The height of the pulse in Fig. 2 represents the magnitude of theapplied potential. Ton (also called the pulse width) represents theportion of the cycle for which the voltage is ON, and Toff is theportion of the cycle for which the voltage is OFF. By changingTon and Toff we could change the frequency of pulse applicationat constant voltage and DC. Unless and otherwise mentioned,deposition was carried out for a total pulse voltage ON time(Ton) of 3 min. It must be noted that the actual time of exper-iment was much more than 3 min and it increased with decreasein pulse width.

The obtained deposits were dried overnight in air at roomtemperature and weighed together with the substrates to deter-mine the deposit weight. After drying, the deposits were peeledoff the substrate surface and their thickness was measured with adigital micrometer caliper. The green densities were measured bythe Archimedes method using kerosene as the liquid. The de-posit quality was examined macroscopically by optical photo-graphs recorded using a stereomicroscope.

III. Results and Discussion

(1) EPD by Continuous dc Voltage

Deposition characteristics of the alumina suspension under acontinuous dc voltage were determined first, because theyformed the reference against which the subsequent pulse EPDresults were compared. Figure 3 presents the deposited weightand density of deposits on stainless steel as a function of pH at acontinuous dc voltage of 20 V applied for 3 min to 5 vol% al-umina suspension.

It must be noted that because the IEP of alumina is at a pHvalue of 7.9, the deposits were obtained at the cathode belowthis pH and at the anode at higher pH. It is evident from Fig. 3that the deposited amount increased as the pH was increasedfrom 3 to 7.5, but with a decrease in adhesiveness to the sub-strate. Stronger adhesion to substrate was observed at pH valuesaway from the IEP for both cathodic and anodic EPD. At pHvalues very near the IEP (as shown by the hatched area inFig. 3), thick deposits formed under the suspension, but theiradhesion to the substrate was very weak and fell off when takenout from the suspension. This may be because of spontaneousflocculation of the particles near the IEP. It is well known that inthe absence of electrostatic repulsive forces near the IEP, the vander Waals attractive forces between the particles are strong. Thegravity effect of the aggregate thus formed at the electrode isperhaps higher than the electrophoretic effect because of theFig. 1. z potential of alumina as a function of pH.

Toff

Ton

t

V

Fig. 2. Constant voltage pulse of 50% duty cycle.

October 2008 EPD by Pulse-Potential Application 3155

small magnitude of z potential near the IEP. The green densityof the deposit, on the other hand, was found to be high at pHvalues away from the IEP, which decreases and tends to reach aminimum near the IEP. This is expected because the particles arewell dispersed away from the IEP because of electrostatic repul-sion on account of high z potential. Hence, they form deposits ofwell-packed compacts by electrophoresis compared with thatnear the IEP. It was also observed during the investigation thatmore bubbles are formed in the deposit away from the IEP. Thenumber of bubbles in the deposits was found to decrease as thepH approached IEP.

(2) EPD by Pulse Voltage

Figure 4 presents the deposition characteristics of alumina bycathodic pulse EPD at pH values 4.5 and 6.0. The pulse width of180 s shown in Fig. 4 is equivalent to continuous dc voltageapplied for 3 min. It is evident from Fig. 4 that the amount de-posited by continuous dc voltage is highest for each pH inves-tigated. Application of pulse voltage invariably decreases thedeposited amount. The deposited amount was found to decrease

with decrease in pulse width. This is contrary to the observationin pulse plating of metals from their salt solution in which ahigher deposit amount has been reported by pulsing comparedwith continuous dc plating.29–31 It has been advocated that therelaxation period in pulse plating enables a larger current to beapplied during the transient period. Further, the population ofadatoms on the surface during pulse deposition is higher due tothe increased nucleation rate, resulting in the deposition of filmswith higher thickness. This is possible during deposition of met-als or alloys because the nucleation sites are progressivelyformed during the deposition on the electrically conductive de-posit surface. On the other hand, the number of sites for depo-sition on the substrate surface during EPD of ceramic particlesfrom suspension is fixed and diminishes as deposition pro-gresses.

Figure 5 shows the surface photographs of deposits obtainedby a total pulse voltage ON time (Ton) of 3 min for differentpulse widths to 5 vol% alumina suspension maintained at pH4.5. Deposits obtained by continuous dc voltage EPD showedthe presence of a large amount of bubbles on both stainless steeland nickel substrates at all applied voltages. The amount ofbubbles on the deposit decreased on application of pulse poten-tial for each of the cases. In general, more bubbles are found ondeposits formed at high pulse widths than those formed at lowpulse widths. There seems to exist a critical pulse width for eachapplied potential, below which the deposit is free from any mac-robubbles. It should be noted that the present investigation wasaimed solely at studying the influence of pulse voltage applica-tion on controlling bubbles during EPD. Because we did not useany additives such as dispersants or binders, the samples exhib-ited cracks on drying. The influence of additives on pulse EPDwill be reported in subsequent communications.

A series of experiments were conducted at different pulsewidths at close ranges for each applied voltage to determine therange within which macroscopically bubble-free deposits areobtained. Figure 6 presents a comprehensive plot of such arange of pulse widths for each applied voltage for 5 vol% al-umina deposited on steel substrates at pH 4.5. For each appliedvoltage, it indicates the existence of a window in the plot with alower limit, below which deposition does not occur. There alsoexists an upper threshold limit of pulse width, above which de-posits invariably contained bubbles. Homogeneous and bubble-free deposits are obtained within this window of pulse width.The lower limit of pulse width for 20 V applied potential is 0.015s and the upper limit is 0.02 s for both stainless steel and nickelsubstrate. Such a window of pulse width was found to bebroader at low voltages compared with the very narrow pulsewidth at higher applied voltages. Hence, it is very critical to setthe pulse width carefully, especially at high voltages. It is mucheasier to control bubble suppression at low voltages because ofthe broader window of pulse width. But because the deposityield is lower at low voltages, optimization is necessary depend-ing on the application. Experiments were also conducted at pHvalues higher than the i.e.p of pH 7.9 at which the negativelycharged alumina particles deposited on the anode (i.e., anodicpulse EPD). Because the volume of oxygen gas evolved at theanode is half the hydrogen gas evolution at the cathode, a bettersuppression of bubbles in the deposit was expected by anodicpulse EPD. On the contrary, the quality of deposit in terms ofhomogeneity and uniformity was apparently found to be betterfor cathodic pulse EPD than for anodic pulse EPD. The reasonfor such a phenomenon is not known at present and warrantsfurther investigation. A probable reason for the formation ofinhomogeneous deposits with more bubbles for anodic deposi-tion compared with the cathodic ones may be explained on thebasis of the differential solubility of hydrogen and oxygen inwater. The solubility of hydrogen in water at 251C and 1 barpressure is about 1617.6 mg/L whereas that of oxygen is onlyabout 43.3 mg/L.37,38 Therefore, the hydrogen gases might bedissolved and/or diffused away more easily from the cathodesurface compared with dissolution and diffusion of oxygen fromthe anode. This could result in a high residence time of oxygen

Fig. 3. Yield and relative green density of alumina deposited on stain-less steel as a function of pH by continuous dc voltage electrophoreticdeposition (Suspension: 5 vol%; interelectrode distance: 20 mm; depo-sition area: 20 mm� 20 mm; applied potential: 20 V; deposition time:3 min).

Fig. 4. Deposit yield as a function of pulse width on stainless steelsubstrate (Suspension: 5 vol %, pH 4.5; applied potential: 20 V; inter-electrode distance: 20 mm; deposition area: 20 mm� 20 mm). A pulsewidth of 180 s is equivalent to continuous dc voltage application for3 min.

3156 Journal of the American Ceramic Society—Besra et al. Vol. 91, No. 10

gases near the anode compared with the residence time of hy-drogen near the cathode. The presence of more oxygen gas nearthe anode will adversely influence and limit the mass transportof particles to the electrode surface. Further, incorporation ofthis oxygen gas will result in a more inhomogeneous morphol-ogy of deposits during anodic EPD.

Considering the distinct difference in the surface morphologyof deposits obtained by pulse EPD compared with that by con-tinuous dc voltage application, we anticipated an increase ingreen density as well. Figure 7 presents the green density esti-mated by the Archimedes method. The deposit obtained bycontinuous dc voltage EPD from 5 vol% alumina suspension atpH 4.5 exhibited a relative green density of 62.0%. It decreasedto 54.0% when the pH increased to 6.0. This was obviously be-cause of the better dispersion of the suspension at pH 4.5 than atpH 6.0. It is now well established in the literature that the dis-persion and stability of colloidal suspensions can be effectivelycontrolled by changing the solution pH, which in turn changesthe ionic strength of the suspension.14,39 The electrical-double-layer thickness of particles in suspension is a sensitive functionof the ionic strength of the dispersing medium. At pH valuescloser to the IEP, the ion conductivity of the solvent is generallylower than that at more acidic pH.13 Therefore, most of the ap-

ContinuousDC voltage

Pulse width:1.0 sec

Pulse width:0.05 sec

(a)





Pulse width:1.0 sec

Pulse width:0.5 sec

(b)

Pulse width:0.1 sec





Pulse width:1.0 sec

Pulse width:0.5 sec

(c)

Pulse width:0.1 sec

Pulse width:0.2 sec




1 mm 1 mm 1 mm 1 mm 1 mm 1 mm

Fig. 5. Photograph of the top surface of as-deposited alumina obtained by pulse electrophoretic deposition from a 5 vol% suspension at pH 4.5 on (a)stainless steel (316 L) substrate (applied potential540 V), (b) stainless steel (316 L) substrate (applied potential5 20 V); (c) nickel substrate (appliedpotential5 20 V).

Fig. 6. Pulse width versus applied voltage diagram showing regions ofbubble-free deposition of AKP-50 alumina suspension on stainless steel(316 L) substrate (powder concentration: 5 vol%, pH 4.5; pulse ON time(Ton): 3 min).


plied current is expected to be carried by particles at pH closer toIEP. Hence, comparatively, the deposit yield is higher at pH 6.0than at pH 4.5 (Fig. 4). But the z potential at pH 6.0 is lowerthan that at pH 4.5. Under such conditions, the electrostaticforces at pH 6.0 are of insufficient magnitude to overcome thelarge van der Waals attraction forces for which the particlesform bigger agglomerates and loosely packed deposits com-pared with those at pH 4.5. The deposit formed by the moredispersed suspension at pH 4.5 was more densely packed be-cause of the smaller agglomerate size.

It has also been shown by earlier investigators that a pH gra-dient is established at the cathode surface during EPD becauseof charge transport by electrochemical reactions; i.e., protonsgets transported from bulk to the electrode surface where theyget discharged.39,40 If the charge transfer is in equilibrium, thediffusion rate of protons from the bulk will control the process.In this way, concentration overpotential is established. But it hasalso been shown that a potential drop is present over the depositand that no significant polarization is present at the electrode.The deposit formed on the electrode allows only ions and liquidto pass through it, while the powder particles get deposited asporous compact at the growth front. It has been suggested that acomplex mechanism involving electrostatic interactions betweenthe charged deposit and the charged solution species (ions) mayplay a role. The retention of ions in the deposit, which causes areduction in mass transport through the deposit, is dependentnot only on the pore size but also on the surface charge andDebye screening length of the deposited particles forming thepore wall. The ionic strength generally increases and the Debyescreening length decreases with increasing acid concentration(low pH). Hence, in the present investigation, the suspension isexpected to have higher ion conductivity at pH 4.5, which mayresult in higher pH gradient and high retention of ions in thedeposit compared with that at pH 6.0. This will result in com-paratively more reduction in mass transport at pH 4.5 than atpH 6.0, leading to decreased yield (Fig. 4). Figure 7 also showsthe green density of deposits obtained by pulse EPD. Contraryto our expectation, the green density remained constant for eachof the pulse widths and was the same as the density of contin-uous dc voltage EPD for the corresponding pH of the suspen-sion. This implies that the green density is determined solely bythe state of dispersion of the suspension and is not influenced bypulse voltage. The better the dispersion, the higher the density ofthe deposit. Because the density of deposits obtained by thepulse method is the same as those obtained by continuous dc, itmight appear that the regions devoid of bubbles are of higherdensity. However, this is not true, and the density of the bubble-free regions might be the same because the amplitude of the

applied potential, which serves as the driving force of the EPDprocess, in both the cases is maintained constant. At the sametime, because the hydrogen gas when generated must escapethrough the deposit, it is likely that the deposits will contain onlyopen pores and are devoid of any closed pores. The keroseneused for density measurement in the Archimedes method caneasily enter the open pores and will not contribute to any differ-ence in the measured density. Density would be different only ifclosed pores were present in the deposit.

The reason why bubbles get suppressed on application ofpulse voltage during EPD is not yet understood clearly. It isperceived that when continuous dc voltage is applied, the masstransport of particles due to the electrophoretic effect and theirdeposition at the electrode is a continuous process in addition tothe electrolysis of water. Hence, the hydrogen or oxygen gener-ated at the electrodes by electrolysis continuously gets incorpo-rated into the deposits resulting in porous deposits. It is knownthat the electrolysis of water molecules into hydrogen andoxygen occurs through a process that involves several steps.41

Initially, the randomly oriented water molecules spin and orientthemselves toward positive and negative poles of the voltagefields applied. The positively charged hydrogen atoms are at-tracted to a negative voltage field (cathode), while at the sametime the negatively charged oxygen atoms are attracted to thepositive voltage field (anode), resulting in the development of apolar charge alignment/distribution within the molecule andelongation of the water molecule. When the applied potentialis high enough such that the attraction force from the voltagefields exceeds the force of covalent bonds between the atoms ofthe water molecule, the hydrogen and oxygen atoms get sepa-rated as charged ions in addition to release of electrons in theprocess. The released electrons either complete the external cir-cuit or are carried by the particles present in the suspension.During continuous dc voltage EPD, electrons are also continu-ously introduced into the water bath by the circuit; hence,hydrogen ions reaching the cathode become neutralized and lib-erated from water as gas bubbles. On the other hand, for pulsevoltage EPD, the duration of voltage interruption (Toff) is equalto the time during which no current flows between the electrodesand the electrolysis process is interrupted. Hence, by varyingthe pulse size and amplitude, the amount of gas evolved at theelectrodes can be controlled. With this argument, the bubblegeneration and incorporation in the deposit for the case of con-tinuous dc voltage EPD are expected to be the maximum andhave been indeed experimentally found to be true. With de-creasing size of pulse width, the amount of bubble incorporationhas been found to decrease because of the decrease in electrolysisat lower pulse sizes. Secondly, the hydrogen or oxygen emittedat the electrode interface might be partly diffused away from thesubstrate in the duration of voltage interruption (Toff) and sup-pressed from being incorporated in the deposit during pulseEPD.42,43 At the same time, the population of adparticles nearthe deposition electrode surface during pulse EPD is still high.This is because the mass transport of the particles from bulksuspension toward the electrode surface is likely to continueeven during the voltage interruption due to their inertia of mo-bility caused by the effect of preceding voltage ON duration(Ton). We had perceived that the incorporation of gas bubbles inthe deposit could be reduced if the applied current density isdecreased on application of pulse voltage. So, we monitored thechange in current with time during deposition for each pulsewidth. For the same applied voltage of 20 V to 5 vol% suspen-sion at pH 4.5, we observed a very marginal decrease in initialcurrent density (which fluctuated between 0.0017 A to 0.0013 A)when the mode of voltage application was changed from con-tinuous dc to 0.01 s pulsed voltage. Such a small change in cur-rent density is insignificant to cause a practical change in theextent of electrolysis per pulse. Alternatively, we believe that thegas evolution and dissipation in the case of pulse voltage elec-trolysis is a dynamic process in which the gas generation sites onthe electrode surface change after each voltage interruption time(Toff) and may be different from the preceding and succeeding

Fig. 7. Green density of deposit on stainless steel (316 L) substrate as afunction of pulse width (suspension: 5 vol%; applied potential: 20 V;interelectrode distance: 20 mm; deposition area: 20 mm� 20 mm). Apulse width of 180 s is equivalent to continuous dc voltage applicationfor 3 min.

3158 Journal of the American Ceramic Society—Besra et al. Vol. 91, No. 10

ones. Such evolution of gas at different sites for each pulse ONtime (Ton) will lead to the production of separate micro- andnanosized bubbles uniformly distributed throughout. Becausethe coalescence of such small bubbles is energetically unfavor-able, their incorporation will not produce any macrobubbles inthe deposit.

IV. Conclusions

Based on the above investigation on EPD of aqueous suspen-sion, it can be concluded that application of pulse potential ofsuitable width enables obtaining smooth, bubble-free deposit.The yield of deposits obtained by pulse EPD is generally lessthan that obtained by EPD using continuous dc voltage. Thedeposited weight decreases progressively with decrease in pulsewidth. Application of continuous dc produced deposits withmaximum thickness and incorporated bubbles in the deposit.The film thickness and bubble incorporation decreased with de-crease in pulse width for the same applied voltage. The greendensity of deposits obtained by pulse voltage EPD was found tobe the same as those obtained by continuous dc voltage EPD.There exists a narrow band of pulse width within which bubble-free deposit is possible by pulse EPD. There is no deposition atpulse voltages smaller than the lower limit of pulse width. Atlarger pulse width, the deposits invariably contained bubbles inthem. The band of pulse width for obtaining bubble-free depositis wider and lies at a higher value of pulse for lower appliedpotential compared with that for higher applied potential. Theupper and lower limits of pulse tend to merge with each other athigh applied voltages. This suggests that it is moreconvenient and practicable for better control of pulse EPD toobtain bubble-free deposit at lower applied potential, but at theexpense of deposit throughput. Apparently the cathodic pulseEPD produced better quality deposit and is amenable to bettercontrol of process parameters than the anodic pulse EPD.

Acknowledgments

One of the authors (L. B.) is thankful to the National Institute for MaterialsScience (NIMS) for the postdoctoral fellowship. The authors also wish to thankMs. Ayako Miki for assistance in z potential measurement.

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Bubble-Free Aqueous Electrophoretic Deposition (EPD) by Pulse-Potential Application