electrophoretic deposition of yttria-stabilised zirconia powder from aqueous suspensions for...
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Journal of the European Ceramic Society 34 (2014) 401–412
Electrophoretic deposition of yttria-stabilised zirconia powder from aqueoussuspensions for fabricating ceramics with channel-like pores
Kirsten Moritz ∗, Christos G. AnezirisTechnische Universität Bergakademie Freiberg, Institute of Ceramics, Glass and Construction Materials, Agricolastr. 17, 09596 Freiberg, Germany
Received 10 July 2013; received in revised form 9 August 2013; accepted 15 August 2013Available online 5 September 2013
bstract
lectrophoretic deposition with simultaneous gas bubble formation by electrolysis can be used for producing ceramic green bodies, typically fewillimetres in thickness, with unidirectionally aligned channel-like pores. The method is successfully applied to yttria-stabilised zirconia. Two
ypes of aqueous suspension compositions are investigated. Suspensions with acetic acid additions are particularly suitable for forming greenodies with fine pore channels. Only small amounts of acetic acid, promoting the gas evolution, are needed for this purpose. Dissolution of yttrian the acidic range has to be considered, but the required low acid concentrations do not measurably affect the yttrium content of the deposits.
ttria dissolution can be minimised by a suspension composition containing an anionic polyelectrolyte and ammonia instead of acetic acid.he ammonia concentration influences the size of the tubular pores of the deposits formed under constant-voltage conditions. Using structuredeposition electrodes, the regularity of the pore arrangement can be enhanced.2013 Elsevier Ltd. All rights reserved.
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eywords: Electrophoretic deposition; Porous ceramics; Yttria-stabilised zirco
. Introduction
Electrophoretic deposition (EPD) is known as shaping, infil-ration, or coating technique. Main advantages which are usedn ceramic processing are the applicability of this method toowders with particle sizes in the submicron or nanometreange and the dense, homogeneous particle packing which cane obtained by EPD from well-dispersed, stable suspensions.hen aqueous suspensions are used in EPD, gas bubbles are
ormed by electrolysis above the decomposition voltage ofater. Pores resulting from this gas generation have to be pre-ented in most applications by suitable approaches such ashe EPD at a membrane positioned in front of the depositionlectrode1 or the EPD in a pulsed DC2 or asymmetric AC field3
nstead of a continuous DC field. On the other side, gas bub-
le formation during electrophoretic deposition can be used forabricating porous materials, but only few publications abouthis method are known.4,5 Potential applications of ceramics∗ Corresponding author. Tel.: +49 3731 393497; fax: +49 3731 392419.E-mail addresses: [email protected] (K. Moritz),
[email protected] (C.G. Aneziris).
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955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.jeurceramsoc.2013.08.019
ttria dissolution
ith the resulting tubular or conical pores are filters,6 catalystarriers, substrates for gas sensors, or porous bioceramics. Pre-ious own publications7–9 describe the successful use of thislectrophoretic method with simultaneous gas generation forroducing zirconia ceramics with aligned pore channels fromqueous suspensions of the yttria-stabilised ZrO2 powder TZ-3YTosoh, Japan).
The commercially available powder TZ-3Y, containingpprox. 3 mol% Y2O3, is widely used for fabricating Y-TZPeramics (yttria-stabilised tetragonal zirconia polycrystals). Thetabilising oxide Y2O3 prevents the tetragonal to monoclinichase transformation, associated with volume expansion, onooling down from sintering temperature. In this way, it enablesoughening by transformation of metastable tetragonal grainso the monoclinic phase under mechanical load. The effective-ess of the stress-induced transformation toughening is mainlyontrolled by the yttria content and the grain size of the sin-ered ceramic.10 Additionally, it depends on how the yttria isistributed in the zirconia – homogeneously on an atomic scale
s in the case of the commonly used co-precipitated powders oress homogenously as, for example, in the case of ZrO2 powdersoated with Y2O3.11 Commercial Y-TZP ceramics are preparedith Y2O3 contents in the range between 1.75 and 3.5 mol%,12
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02 K. Moritz, C.G. Aneziris / Journal of the E
sually between 2 and 3 mol%.13 The 3 mol% Y2O3 composi-ion provides insurance against spontaneous transformation tohe monoclinic form and allows in comparison with lower yttriaontents a larger critical grain size to remain metastable on expo-ure to moisture at temperatures around 200 ◦C.10 As reportedy Basu et al.,11 the toughness of ceramics based on Tosoh pow-ers TZ-3Y (3 mol% Y2O3), TZ-2Y (2 mol% Y2O3), or powderixtures of TZ-0 (monoclinic ZrO2 without Y2O3) and TZ-3Y,
owever, increases with decreasing total yttria content down to mol%. It is shown in the same publication that the yttria contentust be above 1.75 wt% to inhibit spontaneous transformation
f tetragonal ZrO2 grains during cooling from sintering to roomemperature. The tetragonal-to-cubic phase ratio of commercial-TZP ceramics, which depends on composition, firing tem-erature, and holding time, lies typically in the range between00%/0% and 60%/40%.12
When aqueous suspensions of yttria-stabilised zirconia pow-ers are used in ceramic processing, it is necessary to be awaref the preferential dissolution of yttria in the acidic range.14–17
n the aforementioned investigations concerning the fabricationf porous ZrO2 ceramics by electrophoretic deposition withimultaneous gas bubble formation from suspensions of theowder TZ-3Y,7–9 small amounts of acetic acid were added tohe suspensions in order to increase and adjust the intensity oflectrolytic gas evolution.
It is important that the extent of dissolution of yttria at thearticle surfaces in the suspension has to remain moderate athe used acid concentrations to prevent a decline in mechanicalroperties of the Y-TZP ceramic.
The present paper describes the electrokinetic and rheolo-ical behaviour of the TZ-3Y suspensions as a function ofhe acetic acid concentration and shows examples for poretructures obtained by the EPD/electrolysis method. Partic-lar emphasis is laid on yttria dissolution under conditionselevant for the EPD experiments in order to prove the appli-ability of these suspensions with respect to the requiredttria content of the ceramic. The yttria content must not sig-ificantly decrease. For minimising the dissolution of yttria,uspensions containing an anionic polyelectrolyte and ammoniaolution were examined as alternative composition. The suit-bility of this suspension type for fabricating similar poroustructures as obtained using the suspensions with acetic acid wasnvestigated.
. Materials and methods
The used Y2O3-stabilised ZrO2 powder TZ-3Y fromosoh/Japan had a specific surface area of 15.3 m2/g and a crys-
allite size of 260 A (manufacturer’s certificate of analysis). The2O3 content was 5.2% by weight.For preparing the suspensions, the powder was dispersed in
ure deionised water or deionised water containing acetic acidfirst suspension type) or the commercially available polyelec-
rolyte solution Dolapix PC 21 and ammonia (second suspensionype). The suspensions were stirred for 10 min and then sub-ected to ultrasonic agitation for 3 min (ultrasonic homogeniserSonopuls HD 2200”, Bandelin, Berlin, Germany) in order toTtpp
ean Ceramic Society 34 (2014) 401–412
estroy agglomerates of the fine particles. Heating during ultra-onication was avoided by means of a cooling bath. After thisonication, the temperature of the suspensions was adjusted to1 ◦C.
The influence of glacial acetic acid (100% CH3COOH,erck, Germany) on the electrokinetic behaviour of a 40 wt%
olids content suspension was investigated by measurements ofhe electrokinetic sonic amplitude (ESA) using the ESA 8000ystem with SP-80 probe (Matec Instruments/Hopkinton, MA,SA). ESA signal (which is a measure for the zeta potential and
he electrophoretic mobility) and specific electrical conductiv-ty were measured as a function of the acetic acid concentration.he glacial acetic acid was diluted with deionised water in a
atio of 1:1 for this purpose. This solution was added in stepsf 0.001 ml to the suspension (initial suspension volume: 70 ml)y an automatic titrator. (In the diagram, see Section 3.1.1, theSA signal was plotted against the corresponding amount ofndiluted glacial acetic acid per g of powder.)
The same solids content as in the above-mentioned previ-us own work on electrophoretic deposition from suspensionsontaining acetic acid,7–9 40% by weight, was used for rheolo-ical measurements, investigation of yttria dissolution, and EPDxperiments. In addition, a 65 wt% suspension was investigatedecause increased solids contents provide higher electrophoreticeposition rates.
Shear stress and dynamic viscosity of suspensions with dif-erent concentrations of acetic acid were measured as a functionf shear rate by means of a rotational rheometer RheoStressS 150 (Haake, Karlsruhe, Germany) with double-slit measur-
ng cell DG 41 Ti. The shear rate was increased from 0 s−1 to00 s−1 in 300 s, kept at 800 s−1 for 90 s and then decreased to
s−1 in 300 s.The amount of yttria dissolved from the powder surface
fter 1.5 h, a typical time of contact between powder andiquid in the EPD experiments (including suspension prepara-ion), was investigated in centrifugation experiments. Using thexample of 40 wt% suspensions, the influence of an increasedontact time was also shown. In this case, the suspensionas divided in half: one half was stirred for 1.5 h and thether one for 24 h by means of a magnetic stirrer. Afterhe contact time of 1.5 or 24 h, the solid was separatedrom the liquid by centrifugation at 5000 rpm for 1 h. Theentrifugates (supernatants) were removed by a pipette andentrifuged again (at 5500 rpm for 1.5 h) to remove remain-ng particles. The pH values of the suspensions and of theentrifugates at 21 ◦C were measured using a pH meter “pH5” (wtw, Weilheim, Germany). The yttrium content in theentrifugates was analysed by ICP-OES (inductively coupledlasma optical emission spectrometry) using an INTEGRAM Sequential ICP-OES Spectrometer (GBC Scientific Equip-ent).Rectangular planar green bodies were produced by elec-
rophoretic deposition at constant DC voltage (5, 10, or 15 V).
he deposition time, chosen depending on the suspension andhe applied voltage, was 30, 35, or 60 min. Platinum foil orlatinum gauze (225 meshes per cm2, wire diameter: 0.12 mm)ositioned on platinum foil was used as deposition electrode,
uropean Ceramic Society 34 (2014) 401–412 403
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Fig. 1. Electrokinetic sonic amplitude (ESA) and specific electrical conductivityof a 40 wt% ZrO2 TZ-3Y suspension as functions of the added amount of glacialacetic acid; suspensions of the compositions marked by the circles on the abscissawere investigated by rheological measurements (see Fig. 2). Filled symbolssu
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K. Moritz, C.G. Aneziris / Journal of the E
latinum foil as counter electrode. The rectangular electrodes27 mm × 26 mm) were horizontally arranged at a distance of0 mm, the lower one being the deposition electrode (i.e., oppo-itely charged to the particles).
The pore structure of the green bodies was investigatedy optical microscopy using a stereomicroscope “Technival”JENOPTIC, Jena, Germany). The average pore channel diam-ters at the surface of the deposits, shown in the opticalicrographs, were measured by means of the image analysis
oftware “Olympus stream”.The yttrium content of samples taken from the upper and the
ower side of the deposits by a razor blade was analysed by ICP-ES. For this purpose, the powdery samples were converted
nto solution using K2S2O7 in deionised H2O with H2SO4. Twoolutions per sample were prepared.
Sintering was performed in air at 1450 ◦C (2 h holding time)ith a 2-h intermediate step at 600 ◦C for burning out residualrganic additive. Cross sections of the sintered deposits wererepared and mechanically and chemically polished in order toxamine the microstructure of the skeleton structure by fieldmission scanning electron microscopy (FESEM; DSM 982emini, Zeiss, Oberkochen, Germany).A suspension composition containing the commercial defloc-
ulant Dolapix PC 21 (Zschimmer & Schwarz, Lahnstein,ermany) and aqueous ammonia solution (25% NH3, BDHrolabo®, VWR International, Darmstadt, Germany) was testeds alternative to the suspensions with acetic acid. Dolapix PC 21s an aqueous solution of an alkali-free, synthetic polyelectrolyteconcentration of active ingredient: approx. 25%, pH value ofhe solution: approx. 9).
The electrokinetic sonic amplitude of a 40 wt% suspensionas measured as a function of the amount of PC 21, which was
dded in steps of 0.008 ml into 70 ml of the suspension. Fur-hermore, the ESA signal of a 40 wt% suspension containing 2 gf PC 21 per 100 g of powder was recorded as a function of themount of ammonia solution added in steps of 0.008 ml (startinguspension volume: 70 ml). Rheological measurements, cen-rifugation experiments, and electrophoretic deposition wereonducted in the same manner as described above for the sus-ensions with acetic acid.
. Results and discussion
.1. Suspensions with acetic acid
.1.1. Electrokinetic and rheological behaviourFig. 1 shows the influence of acetic acid on the electrokinetic
onic amplitude (ESA) and the specific electrical conductivityf a 40 wt% suspension of ZrO2 TZ-3Y.
ESA curves reflect the electrophoretic mobility of the par-icles and allow conclusions about double layer repulsion as aunction of the additive concentration. Glacial acetic acid addi-ions up to approximately 2 × 10−3 ml/g of powder led to a slight
+
ncrease of the ESA signal due to the adsorption of H (in otherords: by a Brønsted acid–base reaction between surface groupsf the particles and CH3COOH). The slight decrease of the ESAignal at glacial acetic acid additions above 2.5 × 10−3 ml/g ofmsct
how the acetic acid concentrations preferred for fabricating green bodies withnidirectionally aligned pore channels by EPD.
owder (see Fig. 1) can be attributed to the compression of thelectric double layer by an increased concentration of counterons.
The results indicate that acetic acid had only a minor effect onhe electrokinetic behaviour. As shown in a former publication,18
n adequate electrophoretic mobility and stability of ZrO2 TZ-Y suspensions can also be obtained without any additives. Theain purpose of the acetic acid addition was not to influence
he electrokinetic behaviour and the repulsive forces betweenhe particles but to promote the gas generation by electrolysisuring the EPD.
According to Faraday’s first law of electrolysis, the quantitiesf substances involved in the chemical change are proportionalo the quantity of electricity passing through the electrolyte.19
or a given suspension composition and electrolyte, the rate ofas evolution at a given voltage increases with increasing electri-al conductivity adjusted by the electrolyte concentration. Thelectrical conductivity of the 40 wt% TZ-3Y suspension was sig-ificantly enhanced by the acetic acid (Fig. 1). The conductivityurve shows a steep slope up to glacial acetic acid additions ofpproximately 5 × 10−3 ml/g of powder; at higher CH3COOHoncentrations the graph gets flatter. Small CH3COOH additionsf only 0.9 × 10−3 or 1.8 × 10−3 ml/g of powder were sufficiento provide an adequate gas bubble formation for fabricatingeposits with unidirectionally aligned pore channels as pre-ented in previous papers.7–9 It should be mentioned that in thease of one powder batch of TZ-3Y (not the powder batch usedere) porous green bodies were obtained at the solids content of0 wt% also without additives, but usually the small acetic aciddditions are needed. (As found in preliminary investigations,he concentration of ions dissolved from the powder surfacend, consequently, the electrical conductivity of the additive-freeuspension may differ depending on the used powder batch.)
Suspension compositions chosen for rheological measure-ents are marked in Fig. 1 by circles on the abscissa. The
uspensions already proven to be suitable for fabricating porouseramic structures by EPD (filled circles) were compared withhe additive-free slurry and with a suspension which contains
404 K. Moritz, C.G. Aneziris / Journal of the European Ceramic Society 34 (2014) 401–412
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f glacial acetic acid per g of powder); shear stress and viscosity as a functionf ascending shear rate.
n increased amount of acetic acid. Consistent with the ESAurve, the rheological curves of the suspensions containing no,.9 × 10−3, or 1.8 × 10−3 ml of CH3COOH per g of powderere nearly identical (Fig. 2). Only values above shear ratesf 60 s−1 are plotted in the flow and viscosity curves becausef the increased error of measurement at low shear rates whichecomes especially noticeable at low viscosities. The suspensionith increased additive content (17.9 × 10−3 ml of glacial acetic
cid per g of powder) showed slightly enhanced viscosity val-es and shear-thinning behaviour indicating slightly increasedgglomeration, but, nonetheless, the viscosity and the degreef shear thinning were still low at the solids content of 40 wt%.one of the suspensions showed time-dependent flow behaviour.herefore, it was considered sufficient to plot in Fig. 2 only theurves measured as a function of ascending shear rate in ordero reduce the number of graphs.
Increasing the solids content to 65 wt% resulted, as expected,n higher viscosity and increased shear-thinning behaviour.ig. 3 compares the flow and viscosity curves of additive-freeuspensions at 40 wt% and 65 wt% solids content. Furthermore,his figure shows the rheological behaviour of a 65 wt% suspen-ion containing 0.45 × 10−3 ml of CH3COOH per g of powder.his small additive concentration was chosen by EPD experi-ents using 65 wt% ZrO2 TZ-3Y suspensions. It was found to
e suitable for fabricating green bodies with regularly arranged
ore channels by EPD at a gauze electrode as will be shownn Section 3.1.3. The rheological behaviour at the glacial aceticcid concentration of 0.45 × 10−3 ml/g of powder was similar toala
uspension; shear stress and viscosity as a function of ascending and descendinghear rate.
hat of the additive-free suspension at the same solids loading,ut the viscosity was a little lower. There were only negligibleifferences between the curves measured as a function of ascend-ng and descending shear velocities (filled and open symbols,espectively) also at this increased solids content.
Results of electrophoretic deposition will be shown in Section.1.3 for following suspension compositions
40 wt% TZ-3Y, 1.8 × 10−3 ml of CH3COOH per g of powder,representing a successful suspension for producing ceramicswith channel like pores by the EPD/electrolysis method.
40 wt% TZ-3Y, 17.9 × 10−3 ml of CH3COOH per g of powderin order to show the influence of an increased acid content forthe purpose of comparison, and
65 wt% TZ-3Y, 0.45 × 10−3 ml of CH3COOH per g of pow-der.
The degree of yttria dissolution in these suspensions, inves-igated by centrifugation experiments, is reported in the nextection.
.1.2. Dissolution of yttriaIt is well known that under acidic conditions yttria is leached
ut from the surface of Y2O3-stabilised zirconia because of
n increased solubility, whereas the dissolution of zirconia isow.14–17 The pH values of the investigated suspensions withcetic acid after their preparation and after the contact time of
K. Moritz, C.G. Aneziris / Journal of the European Ceramic Society 34 (2014) 401–412 405
Table 1Centrifugation experiments for investigating yttria dissolution in suspensions containing acetic acid.
Solids content ofthe suspension[wt%]
Amount of glacialacetic acid [ml/gof powder]
Time from suspensionpreparation tocentrifugation [h]
pH value Yttrium content inthe centrifugate
Suspension Centrifugate [mg/l] [mg/100 g of powder]
15 min 1.5 h 24 h
40 1.8 × 10−3 1.5 5.1 5.3 – 5.2 120.0 18.024 – 5.4 5.2 126.9 19.4
40 17.9 × 10−3 1.5 3.2 3.4 – 3.5 689.6 103.424 – 3.6 3.6 936.0 140.4
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bupdrwtghgas formation. There was no noticeable influence of the voltageon the porous structure in this example. (Larger differences inthe applied voltage, however, can have a significant effect.7,8)
5 0.45 × 10−3 1.5 5.9
.5 or 24 h, the pH values of the centrifugates, and the yttriumontents measured in the centrifugates are given in Table 1.
When the pH value of suspensions is measured by poten-iometric method, often the so-called “suspension effect” isbserved. “Suspension effect” means differences between theon activities measured in suspensions and in their separatedupernatants, filtrates, or centrifugates (for more informationee Ref. 20). However, the deviations between the measured pHalues of the suspensions at the end of the contact time and ofheir centrifugates were small.
Significant concentrations of dissolved yttrium ions wereound in the centrifugates of all three suspensions. As expected,onsiderably more yttria was dissolved at 40 wt% solids con-ent when the amount of CH3COOH was increased from.8 × 10−3 ml to 17.9 × 10−3 ml/g of powder. The rate of dis-olution decreases with time as shown by comparison of thettrium contents in the centrifugates after 1.5 h and 24 h of pow-er/liquid contact.
The lowest amount of dissolved yttrium ions was found inhe case of the 65 wt% solids content suspension prepared with.45 × 10−3 ml of CH3COOH per g.
In order to illustrate the degree of yttria dissolution in relationo the Y or Y2O3 content of the raw material, the yttrium concen-rations measured in the centrifugates were also converted intoemaining yttrium and yttria contents of the powder. It shoulde noted that the temperature rise during centrifugation, whichannot be avoided in the case of the used centrifuge, may resultn higher yttria dissolution than in the electrophoretic depositionxperiments.
The yttrium content of the as-received powder was found to bepprox. 4.10 wt% (equivalent to 2.91 mol% Y2O3). The yttriumyttria) content can be assumed to remain nearly unchangedithin the investigated period of 1.5 h in the case of the 65 wt%
uspension with 0.45 × 10−3 ml of CH3COOH per g. A slightecrease to 4.08 wt% Y (2.90 mol% Y2O3) at both powder/liquidontact times, 1.5 h and 24 h, was calculated for the 40 wt% sus-ension containing 1.8 × 10−3 ml of CH3COOH per g of pow-er. For the suspension with increased glacial acetic acid con-entration, a decrease to 4.00 wt% Y (2.84 mol% Y O ) in 1.5 h
2 3nd to 3.96 wt% Y (2.81 mol% Y2O3) in 24 h was estimated.Remaining yttrium contents were directly measured in thease of electrophoretically deposited green bodies by ICP-OES
Faad
5.9 – 6.0 69.8 3.8
nalysis of samples taken from the surface and the bottom sidef the deposits (see next section).
.1.3. EPD resultsAs already shown by the ESA measurements, the particle
harge in the used suspensions was positive meaning that thearticles were deposited at the cathode. Pores were formed as aesult of the evolution of hydrogen at the deposition electrode.
Figs. 4 and 5 show optical micrographs of typical greenodies produced by electrophoretic deposition on platinum foilsing the 40 wt% suspension with 1.8 × 10−3 ml of CH3COOHer g of powder. The EPD parameters and the thicknesses of theeposits are given in Table 2. The applied voltage was 5 and 15 V,espectively. When the higher voltage was applied, a shorter timeas chosen because of the increased deposition rate. Generally,
he ratio between the rates of electrophoretic deposition andas generation by electrolysis influences the pore structure. Aigher voltage increases both the deposition rate and the rate of
ig. 4. Optical micrograph of the surface of a green body produced by EPD from 40 wt% ZrO2 TZ-3Y suspension containing 1.8 × 10−3 ml of glacial aceticcid per g of powder (=typical CH3COOH content); EPD at 5 V for 60 min;eposition electrode: platinum foil.
406 K. Moritz, C.G. Aneziris / Journal of the European Ceramic Society 34 (2014) 401–412
Table 2Yttrium content at the upper and the lower side of the deposits from suspensions with acetic acid.
Solids content [wt%] Amount of glacial acetic acid [ml/g of powder] EPD parameter Sample thickness [mm] Yttrium content [wt%]
Upper side Lower side
40 1.8 × 10−3 5 V, 60 min 2.1 4.10 4.1115 V, 35 min 2.9 4.12 4.15
40 17.9 × 10−3 5 V, 35 min Approx. 4.0 4.06 4.145 V, 60 min Approx. 5.2 4.05 4.20
65 0.45 × 10−3 10 V, 30 min 2.7 4.12 4.11
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or comparison: 2.5 mol% and 3.0 mol% Y2O3, which are typical Y2O3 conten
he deposits in Figs. 4 and 5 had small tubular pores. The aver-ge diameter of the visible pore openings at the sample surfaceas about 190 �m in both cases, but it has to be noted that some
f the fine pores were covered with a thin layer of suspensionetting the sample surface.ig. 5. Optical micrograph of the surface (upper side, (a)) and the fracture surfacecross section, (b)) of a green body produced by EPD from a 40 wt% ZrO2 TZ-Y suspension containing 1.8 × 10−3 ml of glacial acetic acid per g of powder=typical CH3COOH content); EPD at 15 V for 35 min; deposition electrode:latinum foil.
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Y-TZP ceramics, correspond to 3.53 wt% and 4.22 wt% Y, respectively.
A different pore structure was obtained at the increasedlacial acetic acid amount of 17.9 × 10−3 ml/g in consequencef the higher rate of gas evolution (Fig. 6). Small pores formedt the beginning of the EPD were merging to large ones in theurther course of the deposition. At the higher acetic acid con-entration only an applied voltage of 5 V was used because ofhe otherwise too intensive gas generation. The sample shownn Fig. 6 was significantly thicker than the green body depositedrom the suspension with 1.8 × 10−3 ml of CH3COOH per g ofowder under the same experimental conditions (5 V, 60 min),ee Table 2. The corresponding weights of the deposits per unitrea confirmed the higher deposition rate of the particles inhe suspension containing more acetic acid. They were approx..44 g/cm2 at 1.8 × 10−3 ml of CH3COOH per g but about.8 g/cm2 at 17.9 × 10−3 ml of CH3COOH per g of powder. Inddition to the deposition time of 60 min, chosen for reasons ofomparability, a shorter time was used for the electrophoreticeposition from the suspension with the higher acetic acid con-ent (Table 2). The different deposition rates depending on thecetic acid concentration are not caused by differences in the
lectrophoretic mobility (see the results of the ESA measure-ent in Section 3.1.1). As examined by sedimentation tests inhe EPD cell without applied electric field, no sedimentation
ig. 6. Optical micrograph of the surface of a green body produced by EPD from 40 wt% ZrO2 TZ-3Y suspension with increased glacial acetic acid contentf 17.9 × 10−3 ml/g of powder; EPD at 5 V for 60 min; deposition electrode:latinum foil.
K. Moritz, C.G. Aneziris / Journal of the Europ
Fig. 7. Optical micrograph of the surface of a green body produced by EPD froma 65 wt% ZrO2 TZ-3Y suspension containing 0.45 × 10−3 ml of glacial aceticag
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rali
acrylates are well-known dispersing agents for suspensions of
cid per g of powder; EPD at 10 V for 30 min; deposition electrode: platinumauze on platinum foil.
ccurred during the periods of time used for the electrophoreticeposition. Additional experiments, not being reported hereecause this is not the object of the present work, showed thathe higher deposition rate at the increased acetic acid concen-ration can be attributed to a slower decrease in the effectiveeld strength during the electrophoretic deposition. The effec-
ive field strength, the driving force for the particle movement,s the field strength across the suspension and can be estimatedy the quotient of current density in the EPD cell and specificlectric conductivity of the suspension.21 Its decrease with timen constant-voltage EPD, caused by the increasing potential dif-erence across the deposit, is well-known (see Refs. 22–24 forore information).When gauze alone or on foil is used instead of an unstructured
eposition electrode, a very regular arrangement of pore chan-els corresponding to the structure of the gauze can be obtaineds already shown by EPD experiments using 40 wt% suspen-ions with 0.9 × 10−3 ml or 1.8 × 10−3 ml of CH3COOH per
of powder.7–9 In the present work, planar green bodies withuch a regular pore arrangement were successfully produced at5 wt% solids content of the suspension. The intensity of gaseneration had to be matched with the deposition rate of theowder particles in order to obtain the desired regular poroustructure. A small glacial acetic acid amount of 0.45 × 10−3 ml/gf powder was found to be sufficient (Fig. 7). The average porehannel diameter at the surface of the deposit was approximately60 �m. For the deposition on platinum foil, however, a highermount of CH3COOH would be necessary because otherwise,s shown by experiments, the EPD results in a deposit withore channels of such small diameters that most of the pores areovered by a thin layer of suspension.
The yttrium contents at the upper and the lower side of thereen bodies are listed in Table 2. For interpreting the results,
he analysis error has to be considered. As already mentionedn Section 2, two separate solutions were prepared per powderyample. Each solution was analysed in triplicate. The valuesmls
ean Ceramic Society 34 (2014) 401–412 407
iven in Table 2 are the arithmetic means of the yttrium contentseasured on the two separately prepared solutions. The devia-
ion between both values was in each case ≤0.06 wt% Y.As expected on the basis of the centrifugation experiments,
he yttrium content of the deposit produced by EPD from the5 wt% suspension was not lower than that of the raw material.he yttrium contents of the deposits from the 40 wt% suspensionith 1.8 × 10−3 ml of CH3COOH per g of powder did not fallelow that of the powder either. Differences between surfacend bottom side were within the range of the analysis error.
Differences of 0.08 and 0.15 wt% Y were found betweenhe upper and the lower side of the two deposits which wereabricated using the 40 wt% suspension with increased aceticcid concentration (17.9 × 10−3 ml/g).
Hydrated yttrium cations (Y3+) and soluble hydroxy com-lexes such as Y(OH)2+ are the ionic species resulting from theissolution of yttria in the acidic range.25,26 Yttrium cations andationic hydroxyl complexes move in the electric field appliedo the suspension in the same direction as the positively chargedarticles. This can be assumed to attenuate the decrease inttrium content of the deposits compared with that of the rawaterial. Because of their smaller size, the yttrium cations are
resumed to have a higher velocity than the particles. This wouldxplain higher yttrium contents at the lower side of the deposits.n addition, yttrium ions can move towards the cathode alsoithin the deposit.Based on the findings, it can be concluded that additions of
cetic acid in the low concentrations necessary for preparingeramic structures with unidirectionally aligned pore channelshich are small in diameter did not cause a decrease in thettrium content of the deposits in comparison with the raw mate-ial. At increased acid concentration, used in these investigationsor the purpose of comparison, the remaining yttria content doesot decrease below the level required for stabilising the tetrago-al phase (it is clearly above this level, see also the results ofhe centrifugation experiments in Section 3.1.2), but differencesetween the yttrium contents of the upper and the lower side ofhe deposits can occur.
The FESEM image in Fig. 8 shows the typical denseicrostructure of the deposits after sintering. The high densityithin the skeleton of the porous ceramic was due to the dense
nd homogenous particle packing in the pore walls of the greenodies which was indicated by the pore size distribution (seeef. 8).
.2. Suspensions with anionic polyelectrolyte and ammonia
Although, as shown above, the low acetic acid concentrationsequired for the formation of fine tubular pores have no measur-ble impact on the yttrium content of the deposits, it seems to beogical to use a suspension composition out of the acidic rangen order to minimise yttria dissolution.
Anionic polyelectrolytes such as polyacrylic acid and poly-
etal oxides. Generally, polyelectrolytes induce both double-ayer and steric repulsion contributing to the stabilisation of theuspension at adequate amounts of the deflocculant (so-called
408 K. Moritz, C.G. Aneziris / Journal of the European Ceramic Society 34 (2014) 401–412
dy; E
esTpwwuwmtPao2niw
Fs
Tna
1tT(tb
ob
Fig. 8. FESEM micrograph of a polished section of a sintered bo
lectrosteric stabilisation). For example, Hashiba et al. stabiliseduspensions of the zirconia powders TZ-0 and TZ-3Y fromosoh by adding ammonium polyacrylate.27 The influence of theolyelectrolyte solution Dolapix PC 21, also used in the presentork, on the electrophoretic behaviour of a TZ-3Y suspensionas already investigated by Börner and Herbig, but the authorssed a lower solids content (2% by volume).28 In the presentork, the ESA signal depending on the amount of PC 21 waseasured at a solids content of 40 wt%, which was the same as in
he EPD experiments. As shown in Fig. 9, the addition of DolapixC 21 caused a charge reversal from positive to negative valuest 0.95 g of PC 21 per 100 g of powder because of the adsorptionf polyanions at the surface of the zirconia particles. Above PC1 amounts of 1.5 g per 100 g of powder, the negative ESA sig-
al reached a plateau and showed only a small further increasen its absolute value. The absolute ESA value at the plateau levelas marginally higher than the initial positive ESA signal.-25
-20
-15
-10
-5
0
5
10
15
20
25
0 0. 5 1 1. 5 2 2. 5 3
ES
A [
mP
a*m
/V]
Dolapix PC 21 [g per 100 g of powder]
ig. 9. Electrokinetic sonic amplitude (ESA) of a 40 wt% ZrO2 TZ-3Y suspen-ion as a function of the added amount of Dolapix PC 21.
(sfeiabapfp
settbta
PD at 5 V; 1.8 × 10−3 ml of glacial acetic acid per g of powder.
When PC 21 is stepwise added to the 40 wt% suspension ofZ-3Y, thickening in the range around the point of zero ESA sig-al can be observed; re-deflocculation becomes clearly visiblebove approximately 1.4 g of PC 21 per 100 g of powder.
The rheological behaviour of 40 wt% suspensions containing.6 or 2.0 g of PC 21 per 100 g of powder was similar to that ofhe additive-free suspension at the same solids content (Fig. 10).he viscosity of the suspensions with PC 21 was slightly lower
mainly due to the additional steric stabilisation by the polyelec-rolyte and also consistent with the slightly higher ESA signal),ut the difference was negligible.
Unfortunately, suspensions containing Dolapix PC 21 as thenly additive were not suitable for fabricating porous ceramicsy the EPD/electrolysis method. No pore channels were formedonly in some cases, under some conditions, pores at the bottomide which did not reach up to the surface). The deposits slidrom the electrode when the electrode was removed from thelectrophoresis cell. Varied suspension compositions were testedn further EPD experiments. The combined addition of PC 21nd of sufficient amounts of ammonia solution was found toe successful as will be shown below. The purpose of addingmmonia was not to influence the electrokinetic and rheologicalroperties, which seemed to be appropriate, but to enable theormation of deposits with pore channels and a dense particleacking in the skeleton structure.
As shown in Fig. 11, the absolute value of the negative ESAignal at 2.0 g of PC 21 per 100 g of powder was only marginallynhanced by the addition of ammonia solution. It can be assumedhat the maximum degree of dissociation of the anionic polyelec-rolyte is nearly achieved in the suspension without additional
ase. (The pH value of this suspension was 8.8.) Ammonia solu-ion was added during the ESA measurement up to a maximalmount of 0.1 ml/g of powder. This maximal amount was chosen
K. Moritz, C.G. Aneziris / Journal of the European Ceramic Society 34 (2014) 401–412 409
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
10008006004002000
Sh
ea
r s
tre
ss
[P
a]
Shear rate [1/s ]
additive-free
1.6 g of PC 21 per 10 0 g of pow der
2.0 g of PC 21 per 10 0 g of pow der
0
0.5
1
1.5
2
2.5
3
3.5
4
10008006004002000
Dyn
am
ic v
isc
os
ity [
mP
a*s
]
Shear rate [1/s ]
additive-f ree
1.6 g of PC 21 per 100 g of po wder
2.0 g of PC 21 per 100 g of po wder
Fig. 10. Rheological behaviour of 40 wt% ZrO TZ-3Y suspensions containingDs
ildus
cead
mt
Fpt
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 200 40 0 600 80 0 10 00
Sh
ea
r s
tre
ss [
Pa
]
Shea r rate [1 /s]
without ammonia
0.045 ml/g
0.180 ml/g
suspensions contain 2.0 g of PC 21 per 100 g of powder
ammonia solu tion:
0
0.5
1
1.5
2
2.5
3
3.5
4
0 200 40 0 60 0 800 100 0D
yn
am
ic v
isc
os
ity [
mP
a*s
]
Shear rate [1/s ]
without amm onia
0.045 ml/ g
0.180 ml/ g
suspens ions contain 2.0 g of PC 21 per 100 g of powder
ammonia solution :
Fig. 12. Rheological behaviour of 40 wt% ZrO2 TZ-3Y suspensions containingDs
eo2ltdPast
2
olapix PC 21 in comparison with the additive-free 40 wt% suspension; sheartress and viscosity as a function of ascending shear rate.
n order to limit the decrease in solids content because the abso-ute value of the ESA signal decreases when the suspension isiluted (see, for example, Ref. 29). Adding ammonia solutionp to the endpoint of the ESA curve resulted in a decrease of theolids content to 94% of the initial solids volume fraction.
The solids content of the suspensions for rheologi-al investigations, conductivity measurements, centrifugationxperiments, and electrophoretic deposition was in all casesdjusted to 40 wt% by reducing the amount of deionised waterepending on the amount of ammonia solution.
Similar to the acetic acid concentration in the EPD experi-ents described in Section 3.1.3, the amount of ammonia solu-
ion influenced the porous structure as shown in the following
-25
-20
-15
-10
-5
0
0 0.02 0.04 0.06 0.08 0.1 0.12
ES
A [
mP
a*m
/V]
Ammonia solut ion 25% [m l per g of powder ]
suspensio n contains 2.0 g of PC 21 per 100 g of p owd er
ig. 11. Electrokinetic Sonic Amplitude (ESA) of a 40 wt% ZrO2 TZ-3Y sus-ension containing Dolapix PC 21 (2.0 g per 100 g of powder) as a function ofhe added amount of ammonia solution.
tabwttlst
TIs
A(
000
olapix PC 21 (2.0 g per 100 g of powder) at different amounts of ammoniaolution; shear stress and viscosity as a function of ascending shear rate.
xample by two suspension compositions: 0.045 ml and 0.18 mlf ammonia solution per g of powder. The amount of PC 21 was.0 g per 100 g of powder in both cases. Fig. 12 shows the rheo-ogical behaviour of these suspensions. The measured curves ofhe suspension with 0.045 ml of ammonia solution per g of pow-er were nearly identical with those of the suspension containingC 21 without ammonia; the viscosity at the increased amount ofmmonia solution (0.18 ml/g) was only slightly higher. Table 3hows the increase in electrical conductivity due to the addi-ion of ammonia solution. The porous structures fabricated fromhese suspensions by electrophoretic deposition at constant volt-ge are shown in Fig. 13. A low voltage of 5 V was chosenecause higher field strength led to less homogeneous structuresith very large pores besides smaller ones. The suspension con-
aining 0.045 ml of ammonia solution per g of powder was foundo be suitable for fabricating deposits with relatively small tubu-
ar pores (Fig. 13a). The average pore channel diameter at theample surface was about 270 �m. When the ammonia concen-ration was further reduced, almost pore-free deposits slidingable 3nfluence of ammonia addition on the specific electrical conductivity of 40 wt%olids content suspensions containing 2 g of Dolapix PC 21 per 100 g of powder.
mount of ammonia solution25%) [ml/g of powder]
Specific electrical conductivity at21 ◦C [mS/cm]
1.64.045 2.16.18 2.47
410 K. Moritz, C.G. Aneziris / Journal of the European Ceramic Society 34 (2014) 401–412
Fig. 13. Optical micrographs of the surface of green bodies produced by EPDfrom 40 wt% ZrO2 TZ-3Y suspensions containing Dolapix PC 21 (2.0 g per100 g of powder) and ammonia solution; EPD at 5 V for 35 min; depositionelectrode: platinum foil; the influence of the amount of ammonia solution iss
fdsspagwadsifiqitvbd
Fig. 14. Optical micrograph of the surface of a green body produced by EPDfrom a 40 wt% ZrO2 TZ-3Y suspension containing Dolapix PC 21 (2.0 g per1f
to
scbnwatem
asoaTarsotc
fataabtamentioned unavoidable temperature rise of the liquid. However,
hown: (a) 0.045 and (b) 0.18 ml/g of powder.
rom the electrode on removal were obtained (similar to theeposits from suspensions with PC 21 but without ammoniaolution). Larger pore diameters of approximately 435 �m at theample surface were obtained at 0.18 ml of ammonia solutioner g of powder because of the increased rate of gas evolutiont the higher electrolyte content (Fig. 13b). The thickness of thereen bodies was 2.70 and 2.71 mm, respectively. In comparisonith the suspension containing 1.8 × 10−3 ml of glacial acetic
cid per g of powder at the same solids content (Table 2), theeposition rate was higher. This can be attributed not only to alightly increased electrophoretic mobility (see the ESA curvesn Figs. 1 and 11), but mainly to a smaller decrease in effectiveeld strength during the electrophoretic deposition. Using theuotient of current and specific electrical conductivity accord-ng to,21 the effective field strength at the end of the depositionime (35 min) was estimated to be 48% and 58% of the initialalue, respectively, in the case of the deposition of the green
odies shown in Fig. 13a and b. By contrast, it was found to beecreased to about 11% at the same applied voltage and afterts
00 g of powder) and 0.07 ml of ammonia solution per g of powder; EPD at 5 Vor 35 min; deposition electrode: platinum gauze on platinum foil.
he same time when the 40 wt% suspension with 1.8 × 10−3 mlf glacial acetic acid per g of powder was used.
Suspensions of the polyelectrolyte/ammonia type with aolids content of 65 wt% were also tested, but high ammonia con-entrations would be necessary for the pore channel formationecause of the increased deposition velocity. When the ammo-ia content was too low, the rate of gas evolution in comparisonith the deposition rate of the particles was insufficient. Higher
mmonia concentrations, however, cause shear-thinning andhixotropic rheological behaviour indicating increased agglom-ration of the particles. Thus, the solids content of 40 wt% wasore suitable in this case.As shown in Fig. 14, regular pore arrangements can be
chieved by electrophoretic deposition using a 40 wt% suspen-ion with Dolapix PC 21 and 0.07 ml of ammonia solution per gf powder, an applied voltage of 5 V, and platinum gauze on foils deposition electrode. The deposit was 3.14 mm in thickness.he pore channel diameters at the surface of the sample averagedpproximately 450 �m. The ammonia content was crucial for theegularity of the porous structure. A lower amount of ammoniaolution (0.045 ml/g of powder) led to a random arrangementf fine pore channels similar to the deposition on an unstruc-ured electrode, whereas the regularity at too high electrolyteoncentrations was decreased by the formation of large pores.
As expected, the concentrations of dissolved yttrium ionsound in the centrifugates of the suspensions containing PC 21nd ammonia (Table 4) were much lower than in the case ofhe suspensions with the same solids content but acetic aciddditions (Table 1). Besides the pH values out of the acidic range,
protection of the powder surface by the adsorbed polymer cane assumed to contribute to the lower yttria dissolution. It haso be taken into account that during centrifugation a part of themmonia probably volatilised, especially because of the already
he influence on the analytical results was estimated to be rathermall.
K. Moritz, C.G. Aneziris / Journal of the European Ceramic Society 34 (2014) 401–412 411
Table 4Centrifugation experiments for investigating yttria dissolution in 40 wt% solids content suspensions containing Dolapix PC 21 and ammonia solution; time fromsuspension preparation to centrifugation: 1.5 h.
Amount of Dolapix PC 21[g/100 g of powder]
Amount of ammonia solution(25%) [ml/g of powder]
pH value Yttrium content in thecentrifugate [mg/l]
Suspension Centrifugate
15 min 1.5 h
2 10.42 11.0
4
awroflscta
tadafibstbicpd
wsreetatoafegopAss
icrgsiass
A
(M
AgR
R
.0 0.045
.0 0.180
. Conclusions
Porous ceramic structures can be produced by a promisingnd simple method combining the electrophoretic depositionith a desired gas bubble formation by electrolysis. This method
equires no expensive equipment and relatively low contents ofrganic additives. It is suitable for fabricating ceramic parts,ew millimetres in thickness, with unidirectionally aligned tubu-ar pores. Applied to yttria-stabilised zirconia, two types ofuspension compositions were successfully used: suspensionsontaining small amounts of acetic acid (positively charged par-icles) and suspensions containing an anionic polyelectrolytend ammonia (negatively charged particles).
An advantage of the suspensions with acetic acid is thathe electrophoretic mobility and rheological properties, beinglready adequate in additive-free suspensions of the used pow-er TZ-3Y, are relatively insensitive to the amount of acetic acid,t least within the relevant range of concentrations. This simpli-es the adjustment of the appropriate intensity of gas evolutiony the electrolyte content in order to obtain the desired poretructure. However, yttria dissolution in the acidic range haso be considered. Therefore, the amount of added acid shoulde as small as possible. The suspension type with acetic acids particularly suitable for producing deposits with fine porehannels. The low acetic acid concentrations required for thisurpose do not measurably affect the yttrium content of theeposits.
Although small additions of acetic acid are unproblematicith regard to the undesirable yttria dissolution, preference
hould be given to suspension compositions out of the acidicange. For this reason, the suspensions with anionic poly-lectrolyte and ammonia were investigated. According toxpectations, the dissolution of yttria can be minimised usinghese additives instead of acetic acid. Electrophoretic mobilitynd rheological properties depend on the polyelectrolyte concen-ration. The suitable polyelectrolyte content can be determinedn the basis of electrokinetic and rheological measurements. Theddition of a sufficient amount of ammonia solution is necessaryor producing green bodies with channel-like pores. Thus, thelectrolyte content, controlling the intensity of gas formation at aiven voltage, must not be reduced below the required amountsf polyelectrolyte and ammonia. Nevertheless, relatively fineorous structures can be obtained under suitable conditions.
bove the required minimum ammonia concentration, the poreize can be easily “tailored” by the added amount of ammoniaolution.
10.4 10.1 2.9 10.9 10.4 6.9
Normally, unstructured deposition electrodes are used, result-ng in a random arrangement of the unidirectionally aligned porehannels. When gauzes are placed on the deposition electrode,egular arrangements of the tubular pores corresponding to theauze structure can be achieved. The suitability of the suspen-ion type with acetic acid for this purpose was already shownn previous and confirmed in the present work. Using suitablemmonia concentrations and EPD parameters, such regular poretructures can also be formed by electrophoretic deposition fromuspensions of the polyelectrolyte/ammonia type.
cknowledgements
Financial support by the German Research FoundationDeutsche Forschungsgemeinschaft, DFG) under Grant nos.
O 1404/1-1 and MO 1404/1-2 is gratefully acknowledged.We would like to thank the group “Ceramography/Phase
nalysis” at the Fraunhofer IKTS Dresden for FESEM investi-ations. Special thank goes to Dr.-Ing. Sabine Hönig and Angelaeichel, TU Bergakademie Freiberg, for ICP-OES analysis.
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