Electrokinetic Characterization of Ceramic Suspensions Biščan J.1, Rudjer Boškovič Institute, Zagreb M. Kosec, Jožef Štefan Institute, Ljubljana Electrokinetic ineasurements, such as microelectrophoresis, are often in use for investigating optimal conditions for the preparatien pf single-component or complex ceramic suspensions, as well as for the control of final ceramic povvders. Very often, particularly in the čase of apueous suspensions, the surface electric charge of soiid particles and the electrokinetic (zeta) potential is determined by the pH of the dispersing medium. A high, stable electrokinetic potential is in most cases a guarantee for the colloidal stability of the suspension. On the contrary, the so called isoelectric conditions (zero electrcphpretic mobility) usually coincide with the agglomeration of particles. One should avoid this, m order to meet the high guaiity standards of final ceramic bodies. In this work the isoelectric conditions of single oxides: PbO, ZrO and Ti O . and of complex oxide of general formula PbZr0 52Ti048O3 (PZT) are discussed vvith respect te the composition (chloride. nitrate, acetate) and the pH of the surroundmg medium. The influence of the synthesis temperature and non~stcichiometry of PZT on the electrokinetic data are also discussed. Key vvords: electrokinetic characterizatiop: stablility of suspensions: suspensions of ceramic oxides; isoelectric points: lead-zirconate-titanate (PZT) complex oxides Introduction In the processing of high performance ceramics. one of the critical steps is the preparation of stable suspensions of fine raw povvders. In order to meet the high quality standards of final ceramic bodies. one has to avoid the uncontrolled formation ot ag-gregates due to either clectrostatic or non-electrostatic interac-tions betvveen the particles in ceramic suspensions. Usuully, in the multicomponent systems, such as ceramic suspensions, the aetion of electrical forces betvv een charged particles of equal oropposite polaritv is the most critical one for the stabililv of the dispersion, especiallv for aqueous svstems. The DLVO theorv quantifies vvell the total interaetion cnergv betvveen the particles of equal polarity. According to this theory the repulsive cnergv component is due to overlapping of the electrical ilouble lavers around the approaching particles and the at-traetive component i-- due to Van der Waals forces. An cnergv barrier vvhich prevents the strong association and agglomeration of particles is an important characteristic of the net cnergv of interaetion. In the čase of surface charge compensation the cnergv barrier decreases or even vanishes and, in the absenee of any other repulsion meehanism. the particles aggregate in a so called primarv minimum1 Jasenka lilŠCAN. dipl. in/. Rnder Boškm ic iiiMitule Biicnička 54. 41110(1 Zagreb Pivsenl address: di lasenka li 1-..111 iu/enjering Biš, an. Rudarska draga s. 4143(1 Samnhor. Croalia Since the charge and surface potential of pure oxides are pri-niarilv pH dependent it is possible. in some cases. to adjust the pH conditions of the mixture (slurrv) in such a vvav that ali the componenls exhibil the same charge polaritv. Hovvever. the problem of different charge densitv and surface potential stili re-mains and mav cause the aggregation4 \ Sometimes it is not even possible to obtain the same polaritv of ali dispersion components bv a simple adjustment of the pH. l! is due to an e.\eessive span betvveen their isoelectric points. In that ease the surface charge and surface potential can bc modified bv the controlled and se-lective adsorption of various organic substances (polvelec-trolvtes, fatty acids. fish oil. etc.) as commonh used in ceramic technologv'' . Adsorption of these compounds can eompensate the surface charge of particles, suppressing the eleetrostatic in-teractions betvveen the original particles*. The DLVO theorv re-vised to include the repulsive cnergv due to steric effects of the adsorbed polymers accuratelv quantifies particle-particle inter-actions in most colloidal dispersions. Thus. either the eleetrostatic or the steric repulsion controls the stabilitv of the dispersion. The letter becomes important if the adsorbed organic polvmers are interaeting betvveen the partiele surfaces. Both mechanisms are possible. either in aqueous or nonaqueous svstems. Hovvever. the steric repulsion predominates in the organic dispersion medium. vvhereas in aqueous svstems the eleetrostatic repulsion controls the dispersion stabilitv". The present vvork illustrates the importance of the isoelectric conditions for the colloidal stabililv of ceramic oxides' suspensions in sodium chloride (NaCl). ammonium acetate (CH,COONH4) and potassium nitrate (KNO:) as supporting electrolvtcs. We assume that the electrophoretic mobility or a calculated electrokinetic (zeta) potential reflects the electrostat-ic energv barrier between the particles. Specificallv. this study deals vvith complex oxides of formula PbZr„vTi„ wO,(P/T) produced bv conventional solid state reaction at 9()0°C'"'" and with single oxides. PbO. ZrO,. TiO, as commercial raw c.omponents. Furthermore. the investigation includes the non-stoichiometric modification of PZT of general formula Pb,., x7.r)(«.Ti„ hO, (x: 0: -0.01; 1.025). In order to investigate the inlluence of the svnthe-sis temperature, the PZT svnthesized al the temperature of 800°C was compared to standard sample produced at 900°C. Experimental The follovving chemicals were used for PZT solid state syn-thcsis: PbO (Ventron 99.9r/f p.a.). TiO, i Fluka >99%). and ZrO, (Ventron 99c/< p.a). These materials were used vvithout further purification or treatment. Oxide mixtures were prepared according to the formula PhZr„,_.Ti,.jyO:. Typically, 50 g of oxide mixture was homogeni/cd with 25 cm of acetone in a 50 cm' zirconia container (Fritsch Pulverisette 5) loaded up to 3091 volume vvith zirconia balls (diameter K) mm). The slurries vvere dried at 100 C. The syn-theses vvere performed in covered alumina crucibles at various temperatures ranging from 800C to 900 C. The time for synthe-sis vv as about tvvo hours. For the present vvork the sample prepared at 900°C vvas analyzed. After svnthcses the povvders (pellets) vvere crushed in an agate mortar and passed through a 500 um sieve. Mineralogical analvses vvere made vvith a Phillips 1710 X-ray diffractometer using Cu Ka irradiation. The morphologv of the samples vvas examined by a Leitz AMR 1600T scanning electron microscope. The measurements of specific surface area vvere made using a FlovvSorb II 2300 Micromeritics. Norcross. Georgia, USA. The so called neutral or indifferent electrolvtcs vvere aque-ous solutions of sodium chloride. ammonium acetate, and potassium nitrate. A vveighed amount of povvder (0.005 to 0.1 g) vvas dispersed in 100 cm' of the electrolvte solution and exposed to ultrasound for 5 min. The pH of the suspcnsions vvas adjusted ei-ther b\ addition ofHCl and NaOH (in the čase of sodium chloride solution). vv ith CH.COOH and NH4OH (in the čase of ammonium acetate) and vvith KOH and HNO. (in the čase of potassium nitrate). An automatic microelectrophoretic instrument, type S3000. from Pen Kem, Bedford Hills, N.Y.. USA, combined vv ith an automatic titrator, ABU93 and sample changer, SAC80, from Radiometer. Copenhagen. Denmark, vvas used for the electrokinetic measurements and titrations. Zeta potential (č) vvas calculated using Henry's equation12, assuming Ka»l vvhere K is the reciprocal of the double laver thickness and a is the particle diameter. Results Characteristics ofthe o.\tde constituents and PZT The main characteristics ofthe oxide constituents, PbO, TiO, and ZrO.. and of the complex oxide PbZr(1 s,Tin4K0, (PZT) are shovv n in Table I. The crvstal strueture of PZT is a funetion of the Zr/Ti ratio. The investigated material of formula PbZrliXTi04,O, is characterized bv a morphotropic phase bound-ary, and is tvpicallv a mixture of Ti-rich tetragonal and Zr-rich rhombohedral phases '. as found also in the present čase. Table 1: Characteristics of Simple and Complev Ovidcs Composition Synthesis Crvstal temperature strueture (°C) Particle Agglomerate size size (lini) (Lini) PbO litharge massicot -i 10-25 TiO, anatase 0.2 10-20 ZrO, baddelevite 0.4 10-15 PTZ(,()0 900 PZT (t.r) 1 10-15 P rzs()() 800 PZT (t.r) 1 15 P......ZT 900 PTZ (t.t) 1 40-60 P /1 900 PZT (t.r) 0.5-3 10-20 PbO massicot t-tetragonal, r-rhombohedral In the PZT sample svnthesized under conditions of an excess of PbO. free PbO phase (massicot) vvas dctected bv X-ray diffraction. As shovvn in the next seetion, electrokinetic measurements on the same sample clcarlv indicate the presence of unre-acted PbO by a significant shift ofthe isoelectric point in acetate medium. The particle size (diameter) ofthe oxide constituents vvas in the range of 0.2-0.5 um. vvhereas the size ofthe PZT grains vvas I to severa! um. The agglomcrates present in the final product vvere tvpicallv of 10-15 um. The specific surface areas vvere 0.44 m/g for PbO. 3.4 nr/g for ZrO . 8.4 nr/g for TiO, and 0.30 nr/g for PZT svnthesi/ed al 400 C. X Q 0.40 0.60 0.80 y . g/dm3 0.00 0.40 0.60 0.80 r . g/dm3 1.00 1.20 Figure 1: The effect of mass concentration v on the pH (upper) and zeta potential (lovver) ofthe suspcnsions: (Filled svmbols and tuli lines denote sodium chloride; open symbols and broken lines denote ammonium acetate: □. PbO; O. Zr02: A. TiO. Snspension concentration effect In the microelectrophorelic measurements performed, the effect of the suspcnsion concentration (7) in the range 0.05-1 g/dm' was analvzcd vvith respect to the pH and zeta potcntial. Tite results are shown in Figure 1. As shown in the upper part of Figure 1 the pH of the PbO suspcnsion changed from about pH 7 at the lovvest concentration of 0.06 g/dm' to about pH 10 at the concentration 0.5 g/dm' and higher. The same effect was observed for both sodium chloride and ammoniunt acetate.The pH of other two oxides, ZrO, and TiO,. did not change signit"icantly over the entire suspcnsion concentration range. I11 chloride solution. at the highest suspcnsion concentration, the pH values vvere 6.5 for ZrO, and 6.3 for TiO,. The corresponding values in acetate medium vvere 6.2 for both oxides. The pH values of PZT suspensions. vvhich are not shovvn for the sake of claritv of the figure, vvere constant in both electrolv tes, shovving pl I 6.3 in chloride and pH 6.4 in acetate solution. The lovver part of Figure 1 shovvs the change of the zeta potential as an effect of the suspcnsion concentration. A small positive zeta potential at the lovvest concentration of PbO raises to high positive zeta potential of about +50 mV at the concentration 0.25 g/dm and remaining constant at higher concen- trations. The zeta potential of ZrO, depends significantly on the suspcnsion concentration. changing from negative to positive values vv ith an interseetion point at 0.25 g/dm3. The zeta potential of TiO, at the lovv suspcnsion concentration changes from small lo higher negative values and reaching at the suspen-sion concentration of 0.25 g/dm' about -40 m V in chloride and -50 m V in acetate medium. Generalk, the zeta potential vvas constant for ali compounds at the concentrations higher than ap-proximately 0.25 g/dm'. These. stable values of zeta potential are +50 to +60 m V for PbO. +I6±1 m V for ZrO, and -40 to -50 m V for TiO,. For PbO and TiO, the lovver values are for the chloride and higher for the acetate solution. Isoelectric conditions Figure 2a and 2b shovv the zeta potential data for PbO. ZrO: and TiO, and for the complex oxide (PZT) in the pH range 3-12 in 1()4 M chloride and acetate solutions. The isoelectric points vvere at pil 6.8 for ZrO, and at pH 3.5 for TiO; in both electrolv tes. The isoelectric point of PZT at pH 6.6+0.2 roughlv co-incides vvith one for ZrO,. I11 ammonium acetate there is a shift of the isoelectric point to pH 6.3. The eleetrokinetic data for PbO are quite different compared to the other tvvo oxides. One can observe a pH region of stable. highlv positively charged PbO betvveen pH 6 and pH 10 in 1() 4 mol/dm1 NaCI and betvveen pH 7 and 9 in acetate medium. The results for PbO in these tvvo electrolv tes indicate the isoelectric point betvveen pH 10.6 and 1 1.2. At higher pH values negative > E c (D O a -O. -50 sodium chloride: 10 *M 70 -50 pH PH ammonium acetate: 10~"M > E c © o a Figure 3: Zeta potential of PbO as a funetion of pH of the suspensions in sodium chloride at various electrolv te concentrations: +. I04 mol/dm : A. 10 ' mol/dm': •. I()- mol/dm' > E o Q 10 11 12 Figure 2: Zeta potential of suspensions in K) 1 mol/dm sodium chloride (upper) and ammonium acetate (lovver) for PbO (■). ZrO, (•), TiO, (Al and PZT (+) Figure 4: Zeta potential of PbO as a funetion of pH of the suspensions in ammonium acetate at various electrolyte concentrations: +. 10 ' mol/dm : A. 10 ; mol/dm': •. 10: mol/dm' 50 40 > h 30 20 C 0) -t-' o 10 Q 1 0 -10 -20 2 pH Figure 5: Zeta potential of PbO as a function ol" pH ol' the suspension in potassium nitrate at various electrolvte concentrations: +. K)4 mol/dm': A. 10 ' mol/dm!; •. II)' mol/dm1 Pb1JZr0,2T,a48)O3 sodium chloride: 10""V1 PH ammonium acetate: 10"*M pH Figure 6: Influence of the stoichiometrv of PZT 011 the zeta potential in 10 ' mol/dni' sodium chloride (upper) and ammonium acetate llouen: ■ PZT: • P......ZT: A. P /I /da potential and further on an agglomeration was observed. Another. low-pH isoelectric point was at pil 4.5 in acetate medi-uni. At higher electrolyte concentralion in sucli a lovv pH region either precipitation lin the čase of chloride) or dissolution (in the čase of acetate) occurs. Due to peculiar behaviour of PbO the electrophoretic measurements have been extended to three electrolytes (chloride, acetate. nitrate) and three concentrations (K)-. I()! and 1()4 M). The results are shown in Figures 3-5. In K)4 and 10 5 M sodium chloride solutions, in the pH region 7 to 9, the constant high pos-itive values of zeta potential are persisting. In 10: \1 electrolyte fast agglomeration occurs making the measurements hardly re-producible. For ali three electrolytes one can observe high posi-tive constant values of zeta potential betvveen pH 7 and 9. At the same time the svstem maintains colloidal stability during 4,S hours. At higher pH values. a steep deerease of zeta potential occurs betvveen pH 10 and I 1. At pH<7 there is a slovv deerease of zeta potential in the nitrate medium, vvhile in the other tvvo electrolytes the abrupt changes occur, accompanied by either precipitation (in chloride) or bv dissolution (in acetate). For ali three electrolvtes the high-pH isoelectric point is al 10.5 ± 0.2. Onlv in the čase of acetate the isoelectric points appear in the range of pH 9.6-10.3. The specific reactivitv of acetate vvith PbO vvas used to de-tect the free PbO phase in nonstoichiometric PZT. vvhere an ex-ccss of PbO vvas added to the reaction mixture. In Figure 6 the zeta potential data are shovvn for stoichiometric PZT of the formula PbZrll5:Ti04SO, and compared vvith those for non-stoichio- metric PZT. either vv ith a PbO deficiency as in Pb......(Zr. TilO or vvith an excessof PbO in Pb, u2, (Zr, Ti)0„ In sodium chloride medium there is a common isoelectric point for ali three samples. vvhereas in acetate medium it vvas shovvn that an excess of PbO caused a shift from pHkp 6.3 to pH„n 4.7. vvhich reflects a strong interaetion of acetate ions vvith PbO. Furthermore, the shift of the isoelectric point from about pH 6.5 to 3.5 vvas detected in the čase of PZT svnthesized at 800°C instead of 900 C. This results are shovvn in Figure 7. F igure 7: Influence ol' the svnthesis temperature of PZT on the zeta potential in 10 4 mol/dm1 sodium chloride: 800 C: ■ 900 'C Interpretation of the data and discussion Suspension eoncentration For the most oxide/aqueous solution svstems the generation ol surface charge is due to amphoteric surface reactions general-ly deserihed by equations (1) and (2). s t III • II • • M)H' (It s(S-OH + OH <-> sS-0 + H:0 (2) where =S OH are the surface hydroxyl groups. In the čase of high mass concentration (more than 30 g/dm) and 111 ahsence of any specific adsorption of foreign ions. the in-herent pH would be identični to the isoelectric point1'. In present work the suspension concentrations are 10 to 100 times lower. Therefore. in some cases the suspension concentration was not sufficient to establish an equilibrium. That is in the čase of ZrO, vvhere its inherent pH is onlv slightlv lower than the isoelectric point. Therefore a reversal of charge oecurs on each minute change in the suspension, including the mass concentration, vvhich might influence the equilibrium. as shovvn in Figure 1. The results for ZrO. and TiO. can be simplv explained con-sidering the amphoteric equilibria. For example. if the inherent pH values lbrZr02and TiO. are at pil 6.5 and 6.2 in chloride solution and their isoelectric points (as found bv this vvork) are at pH 6.S and pH 3.5. then the expected electrokinetic result vvould give small positive zeta potential for ZrO, and comparativelv high negative /eta potential for TiO,. Indeed. it vv as observed in the region of constant pH of the suspension at mass concentration >0.25 g/dm' (Figure 1). Lovv isoelectric point for TiO, at pH 3.5 vv as not often found bv other authors. Hovvever, there is a literature evidence of the isoelectric point of TiO, at the pH 2'\ It obviouslv depends on Figure 8: Distribution diagram for lead hydroxv and lcad chloro speeies (Redravvn from reference 17) If other speeies are included in surface charge formation as probablv in the čase of the PbO/electrolv te interface then a sim-ple interpretation such as surface amphoteric ioni/ation is not sufficient. In that čase a large number of hvdrolvtic processes and a v arietv of ionic euuilibria determine the speeies in the solution1". Each of those speeies could be adsorbed at the solid surface depending on the pH and the composition and concentration of the electrolv te. Figure 8 shovv s the distribution of lead speeies in aqueous solutions. It is clear that in the pH range 7-9 vvhere our results shovved a constant. highlv positive zeta potential. the dontinating speeies are the PbOH+ ions. These speeies are possiblv adsorbed at the surface and thev determine the over-all surface charge and /eta potential. At higher pH the dominat-ing speeies are Pb(OH), vvhich could precipitate giving a new solid phase. Pb(OH),(sr Raising the pH. the negative speeies. such as Pb(OH), dominate and determine the overall charge. On the other side. at pH <7. the Pb:' speeies dominate. lovvering slovvlv the zeta potential due to double laver compression untd either precipitation (in chloride) or dissolution (in acetate) be-gins. The isoelectric point for PbO at the pH 10.6 ± 0.2. found in our vvork, coincides excellent vvith the pH of the transition from positive. over neutral. to negative lead hydroxo speeies. as shovvn in Figure 8. If other foreign ions are present such as O . Ae or NO,. one mav expect even more contplicate distribution of speeies. Hovv ev er. the nitrate medium seems to be indifferent tovvard PbO compared to the other tvvo electrolytes. From the results in Figure 1 - Figure 8 vve mav conclude that in the pH range 7-0 established bv oxide povvders suspended in aqueous solutions. PbO particles are highlv positivelv charged, in contrast to TiO, and ZrO. vvhich are negativ elv charged. In such a čase the eleetrostatic repulsion barrier betvveen PbO and TiO, and ZrO, particles is absent and strong interaetion betvveen them can be expected. Il should be tested practically vvhether such interactions lead to unfavorable hard agglomeration or to formation of relativelv stable suspensions of microaggregates betvveen PbO particles and TiO, or ZrO: particles. For PZT the surface charge in NaCl solutions is probablv determined bv zirconia. as one can conclude from the contmon isoelectric points for PZT and ZrO,. In ammonium acetate there is a slight shift in the isoelectric point of PZT from pH 6.8 to pH 6.3. vvhich is probablv due lo the strong affinitv of acetate ions for the lead oxide components in PZT. For the same reason a sig-nificant shift in the isoelectric point of PZT svnthesized under conditions of excess PbO vvas observed. This could serve as a quick qualitative analvsis of tree PbO phase in PZT. vvhich mav have significant practical implications. For that purpose the svs-tematic analyses of the isoelectric points of PZT samples having different percentage of free PbO vvould be necessary. The isoelectric point for PZT sample svnthesized at S00 C found at the pH 3.5 indicates some difference betvveen the samples svnthesized at 800 C and 900 C or higher (Figure 7). It is possible that the process of mixed crvstal phases formation is not completed al the lovver temperature. It seems that the Ti-rich phase determine the surface charge and the zeta potential in thi-, čase. Hovvever, at present, this is a speculation rather than proved finding. Conclusion Electrokinetic results suggest that commercial raw materials TiO, and ZrO, used in this vvork behav e as tv pieal acidic and amphoteric oxides having isoelectric points at pH 3.5 and pH 6.8. Lead oxide. PbO. positiv elv charged at eleetrolvte concentration lovver than 10' M vvithin the limited pil range of about 7-9. behav es as a reactive eonstituent vvhich practical I v deter- mineš the overall behavior of the suspension. Due to soluhilitv of PbO and hvdrohsis o!' Pb:" a number of hydroxo spccies can be formed. In chloride and acetate media several additional spccies are possible. Small pH changes. particularly between pil 9 and 10 !PbO suspension inherent pil), and the presence of foreign ions. such as chloride or acetate, is the reason for poor stability of lead-based oxide aqueous suspensions. In practice. either the additives are used to improve the colloidal štabi 1 it> or. instead of aqueous. the non-aqueous medium is applied. However. such a solution is not al\vays eonvenient. either for the reason of econo-mv. or ecologv or the quality of the final product. Lead oxide shovved particularlv strong interaction uitli acetate. This effect v.as used to dcteiTnine the Irec PbO phase in PZT. The isoeleetric point analvsis shovved the difference betvveen the samples svnthesized at X0()°C and 900°C. This is ascribed to a non-completed mixed phascs structuring. vvhere Ti-rich phase determine the zeta potential. This preliminary conciusion has to be proved to be valid. Acknovvltdgemenl This vvork vvas performed vvithin the Project NISTA« 1-31. The financial support of the National Institute for Standards and Technologv. USA. the Ministrv of Science and Technologv of Croatia (Project 1-07-162) and the Ministrv of Science and Technologv of Slovenia is gratefullv acknovvledged. References " B. V. De i jaguin and L. D. Landau, Actu Plnsic ochim. URSS. 14. 1941.633 E. J. Vervvcv and J. Th. G. Overbeek, Thcorv of the stabilitv of Lvophobic Colloids. Elsevier. Amsterdam, 1948. 205 A. Bleier. Colloids and Surfaces, 66, 1992, 157 J. H. Adair and R. V. Linhart. 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