JOURNAL OF COLLOID AND INTERFACE SCIENCE 195, 59–65 (1997)ARTICLE NO. CS975121

Sol–Gel Preparation and Electrorheological Activityof SiO2–TiO2 Composite Powders

Juan Yang,* ,1 Jose Maria F. Ferreira,* Wenjian Weng,† and Ying Tang†

*Department of Ceramics and Glass Engineering, University of Aveiro, 3810 Aveiro, Portugal; and†Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

Received March 27, 1997; accepted August 14, 1997

ena. It is well established that the particle nature can influ-The SiO2–TiO2 composite powders with various Si /Ti ratios ence the ER activity of the resulting suspension; however,

were prepared by the sol–gel processing, using tetraisopropylor- the research work on this aspect remains insufficient, espe-thotitane (Ti(OPr i )4 ) and tetraethoxysilane (Si(OEt)4) as starting cially for the effect of the surface state and porous structurematerials. The effect of the powder’s heat history on the electro-

of particles on ER activity.rheological (ER) activity of the resulting suspensions, which wereTiO2 is a good solid material for ER fluid due to its highprepared by dispersing the powders into silicon oil, was investi-

dielectric constant of around 80, but its semiconductinggated. The infrared spectra, scanning electron microscopy, X-rayproperty generally makes the resulting ER fluid broken whendiffraction, and zeta potential were used to characterize the struc-the resultant suspension is exposed to the external field. Thistural evolution of the as-prepared gel powders upon heating. The

results show a remarkable reduction in gelation time, relative to does not allow a wide use of this material in ER fluid. SiO2

the single-component systems, due to the combined effects of a is an insulate material. The addition of SiO2 into TiO2 pow-higher reactivity of Ti(OPr i )4 over Si(OEt)4 and a nucleophilic der can increase the insulating property and thus prevent theattack of the Ti atom by the as-hydrolyzed species of Si precursor ER system from breaking.to form an environment similar to alkali catalysis. It was found Sol-gel processing has been widely used to control thethat gel powders would gradually eliminate the –OH groups and surface state and the porous structure of products (9, 10).become denser in structure upon heating, leading to a decrease of

The preparations of single-component SiO2 and TiO2 films,ER activity of the resulting suspensions. However, the initial Si /monoliths, and fibers have also been extensively reported;Ti composite compositions have little effect on ER activity behav-however, the understanding of the chemical process of theior. q 1997 Academic Press

Si/Ti mixed system is still very poor.Key Words: sol-gel process; electrorheological activity; SiO2–In the present work, SiO2–TiO2 composite powders wereTiO2; powder.

prepared by the sol–gel method. The sol-gel process of theSi/Ti mixed system with various Si/Ti ratios and the effectof heat treatment of the gel powders on the resulting ER1. INTRODUCTIONactivity were investigated. Zeta potential measurement wasdeployed to evaluate the variation of the surface state of the

Electrorheological fluids (ER fluids) are suspensions of Si/Ti composition particles prepared by heating at differentcolloidal particles dispersed in dielectric insulate liquids, temperature. A correlation between Si/Ti particle structurewhich exhibit large, reversible changes in rheological prop- and the corresponding ER activity is proposed.erties when exposed to an external electric field (1) . Poten-tial applications of ER fluids in electrochemical devices have

2. EXPERIMENTALmotivated both the development of practical fluids and re-search on the mechanism of the ER effect (2) . Many kinds

2.1. Preparation of SiO2–TiO2 Composite Powders andof particulate materials, from the earliest starch and silicaER Fluidsgel (1) to recent ionic conductors (3, 4) and polymers (5) ,

have been used to prepare ER fluids. Some models suchSiO2–TiO2 composite powders were prepared by sol-gel

as fibrillation (1) , electrical double layer (6) , Maxwell–processing, using Ti(OPr i)4 and Si(OEt)4 (Merck, AR

Wagner–Sillars interfacial polarization (7) , and particle po-grade) as precursors. Ti(OPr i )4 was stabilized first by adding

larization (8) have been proposed to explain the ER phenom-acetic acid (HOAC, Aldrich, AR grade) (HOAC/Ti Å 1)to allow a more compatible hydrolysis rate between Ti(O-

1 To whom correspondence should be addressed. Pr i )4 and Si(OEt)4 . The obtained solution was mixed with

59 0021-9797/97 $25.00Copyright q 1997 by Academic Press

All rights of reproduction in any form reserved.

AID JCIS 5121 / 6g33$$$301 11-18-97 12:05:32 coida

60 YANG ET AL.

Si(OEt)4 in different Si/Ti proportions of 1:7, 1:3, 2:3, 1:1,2:1, 3:1, and 5:1, using absolute alcohol (Aldrich, AR grade)as solvent. The final sum concentration of Si and Ti precur-sors was fixed at 0.5 M for all samples. The hydrolysis wascarried out by adding a definite amount of deionized waterto the above mixture. The resultant gels were first maintainedat room temperature for 1 week and then dried in an ovenat 607C for 1 day. The xerogels were ground and then cal-cined at temperatures of 600, 800, and 9507C for 2 h at aheating rate of 57C/min with an intermediate soaking time of2 h at 4507C. ER suspensions were obtained by ultrasonicallydispersing the composite powders into silicon oil. All pow-ders were dried at 1507C for 48 h to remove the physicallyabsorbed water before use. The selected powders used toprepare ER suspensions are shown in Table 1.

2.2. Characterization

During the gelation process, the viscosity of the solutiongradually increases until the surface becomes immovable.The gelation time in the present work was defined as thetime period between formation of the initial solution and thetime at which its fluidity was lost. IR spectra of the powderswere scanned in a spectrophotometer (Shimadzu IR 425,Japan) with a wavenumber resolution of {0.2 cm01 , usingthe KBr pellet method. XRD patterns of the powders afterheat treatment were determined in a X-ray diffractometer(D/MAX-C, Rigaku, Japan), using CuKa radiation. Theparticle morphology was observed under scanning electronmicroscopy (Hitachi S-4100, Japan). Malvern Mastersize(Malvern Instrument, UK) was used to measure the particlesize. Zeta potential measurement was carried out in a Mal-vern AZ 6004 Zetasizer (Malvern Instruments, UK) to deter-mine the change of the surface structure ( –OH groups)/ ofthe SiO2–TiO2 powders upon heating. An electrolyte solu-tion (0.001 M KCl) was used to disperse the powders tomaintain an almost constant ionic strength. The ER effectof the suspensions was evaluated by using a modified Rhe- FIG. 1. Changes of gelation time with the molar ratio of Ti/(Ti / Si)

and the amount of water. The sum concentration of Si/Ti components wasometer 2 (Rotatronsviskosimeter, Germany) at 207C underfixed to 0.5 M, and H2O/(Si / Ti) Å 2.0, 2.5, and 3.0 for a, b, and c,a DC field. The gap between two cylinders was 1.2 mm.respectively.

3. RESULTS

Figure 1 shows the relationship between gelation time andgelation time decreased with Ti/(Si / Ti) and reached athe molar ratio of Ti/(Si / Ti) under different amounts ofminimum near the Ti/(Si / Ti) range of 0.5–0.75, whereH2O. It can be seen that the gelation time decreased withthe gels could be quickly formed. Further increases in theincreasing contents of H2O. For a given amount of H2O, theTi/(Si / Ti) ratio caused the gelation time to increase.

The IR spectra of the powders with Si/Ti ratios of 2, 3,and 5 during heat treatment are presented in Figs. 2–4,TABLE 1respectively. The absorption bands at 3300 cm01 (OH), 1630Powders Selected To Prepare ER Suspensionscm01 (H2O), and 1420 and 1530 cm01 (COO0) are observed

Si/Ti molar ratio 2 5 5 in the spectra of gel powders. When the temperature is in-Heat treatment (7C) No No No 600 950 creased, the intensity of the two COO0 bands graduallySolid loading (vol %) 2.5 5.0 7.5 5.0 7.5 10.0 5.0

decreased and completely disappeared until 6007C, but the

AID JCIS 5121 / 6g33$$$301 11-18-97 12:05:32 coida

61SiO2–TiO2 COMPOSITE POWDERS

FIG. 4. IR spectra of powders after calcination at various temperaturesFIG. 2. IR spectra of powders after calcination at various temperaturesfor the composition Si/Ti Å 5; (a) dried at 607C, (b) 6007C, (c) 8007C,for the composition Si/Ti Å 2; (a) dried at 607C, (b) 6007C, (c) 8007C,and (d) 9507C.and (d) 9507C.

other two bands of 3300 cm01 (OH) and 1630 cm01 (H2O) tase crystallized from the amorphous powder with Si/Ti Ådecreased in intensity very slowly and did not disappear at 1

3 after calcination at 6007C. For the powders with Si/Ti Åall even after heating the powders at 9507C. It is interesting 1 and 3, the temperatures at which anatase crystallized fromto note that a broad band centered at 600 cm01 corresponding the amorphous state increased to 800 and 9507C, respec-to Ti–O–Ti bond vibration appeared after heat treatment, tively. This indicates that the crystallization temperature ofbut the temperature at which it appeared increased with an anatase is dependent on the Ti content in the compositeincrease of the Si/Ti ratio, namely, 600, 800, and 9507C for powders. However, no SiO2 crystalline phase occurs afterSi/Ti ratios of 2, 3, and 5, respectively. The absorption calcined at 9507C, which is not high enough to cause SiO2

bands at 1100 and 800 cm01 , corresponding to the Si–O– to crystallize from amorphous state.Si vibration, and the band at 950 cm01 , due to Si–O–Ti The average size of the particle with Si/Ti Å 5 dried atvibrations, can be seen in all samples. 607C was 6.5 mm. After the particle was calcined at 9507C

Figure 5 gives some XRD results of powders with differ- for 2 h, the average size increased to 7.2 mm. However, theent Si/Ti ratio after calcination at various temperatures. All variation in particle size before and after heat treatment isgel powders are amorphous (XRD results not shown). Ana-

FIG. 3. IR spectra of powders after calcination at various temperaturesfor the composition Si/Ti Å 3; (a) dried at 607C, (b) 6007C, (c) 8007C,and (d) 9507C. FIG. 5. XRD patterns of the various powders.

AID JCIS 5121 / 6g33$$$301 11-18-97 12:05:32 coida

62 YANG ET AL.

FIG. 6. Plots of zeta potential versus pH for the Si/Ti Å 1 compositepowders dried or calcined at different temperatures.

FIG. 8. Effect of solid loading on ER activity of dried powder(Si/Ti Å 2).

limited, and therefore it would not significantly affect theelectrorheological activity or electrophoretic behavior.

Figure 6 presents changes of the zeta potential of Si/Ti seen that the porous agglomerates, which consist of manytiny particles with a size of about 50 nm, are observed incomposite particles in aqueous suspensions as a function

of pH. It can be seen that the electrophoretic behavior is the dried powder. After heat treatment, the agglomeratesbecome denser, seemingly losing some porous structure.considerably affected by the sample heat treatment history.

The dried powder exhibits an isoelectric point (IEP) at about Figures 8 and 9 give the ER activity for the powders withSi/Ti Å 2 and 5, respectively. Shear stress increased withpH 3.8. After calcination at 6007C, the whole electrophoretic

curve was shifted toward the acidic direction with the nega- increase of external field strength (E) in all samples. For agiven E, the shear stress increases with increasing the solidtive zeta potential increasing for a given pH. This effect is

even stronger for the powder calcined at 9507C. The corre- concentration in the ER suspension. Comparing these two fig-ures, it can be observed that the initial Si/Ti ratio in thesponding IEPs are shifted to about pH 2.1 and 1.8 for the

powders calcined at 600 and 9507C, respectively. composition powders has no great influence on the ER activity.Figure 10 presents the effect of heat treatment of the Si/Figure 7 shows the SEM photographs of composite pow-

ders before and after heat treatment at 9507C. It could be Ti Å 5 powder on ER activity. The solid loadings of all the

FIG. 7. SEM morphology of powders with Si/Ti Å 1; (a) dried; (b) calcined at 9507C.

AID JCIS 5121 / 6g33$$$301 11-18-97 12:05:32 coida

63SiO2–TiO2 COMPOSITE POWDERS

catalysts to promote the formation of the gel network inthe present case. The gel network was formed therefore byheterocondensation of Si and Ti hydrolyzed species to formthe Si–O–Ti bond, which was confirmed by IR spectra inFigs. 3–5. Both effects are responsible for the reduction ingelation time with increasing Ti/(Si / Ti) ratio up to about0.75. With further increases in the Ti/(Si / Ti) ratio, how-ever, the possibility of Ti atoms being attacked by Si(OH)decreases since the amount of hydrolyzed species of Si(OH)decreases. Although more Ti nucleation sites are expectedin this composition region, its effect on the gelation timebecomes less important due to the decrease of relativeSi(OH) amount in the composite system, and therefore anincrease of gelation time was observed.

The same phenomenon of reduction in gelation time hasalso been observed in the Ba–Ti system (12). It can bededuced that the rate of hydrolysis and condensation of amulticomponent precursor could be increased because theFIG. 9. Effect of solid loading on ER activity of dried powder

(Si/Ti Å 5). hydrolysis product of one precursor can act as alkaline catal-ysis and the resultant gel will be formed more easily, as ifthe process was catalyzed by an extra added alkaline.

suspensions are 5.0 vol%. It can be seen that with increasingthe calcination temperature, the ER effect gradually de- 4.2. Evolution of Powder Structure During Heatcreased and almost disappeared after the sample was heated Treatmentat 9507C.

It could be expected that a large amount of hydroxyl ( –OH)groups still stay on the surfaces of a sol–gel-derived dried4. DISCUSSIONpowder. During heat treatment, these –OH groups on thepowder surfaces will gradually be lost. This causes changes4.1. Sol–Gel Process of the Si/Ti Mixed Systemof particle nature. Electrophoretic measurements can provide

It is well known that many processing factors such as complementary information about the changes of the particleconcentration of precursor, water amount, temperature, pH, surface state (13). As reported by Pasks (14), heat treatmentcatalysts, and so on will influence the sol–gel process. In would shift IEP to a more acidic region due to the bulk orthe present Si/Ti system, the addition of a small amount surface dehydration. The change of IEP during dehydrationof Ti(OPr i )4 into the Si(OEt)4 solution exhibited a shortergelation time than the pure Si precursor system. The samephenomenon was observed in the Ti(OPr i )4 side. The mini-mum gelation time occurred in the composition range of Ti/(Ti / Si) between 0.5 and 0.75. This could be attributed toa higher hydrolysis reactivity of the Ti precursor than thatof Si(OEt)4 and to the nucleophilic attack of Ti atom bythe hydrolyzed species of Si(OEt)4 (Si(OH)).

At the Si(OEt)4-rich region (left side) , as the Ti/(Ti /Si) ratio increases, more Ti nucleation sites become availablein a short time due to the high hydrolysis rate of Ti precursor,which facilitates the condensation of the mixed solution andthus reduces the gelation time. On the other hand, the Tiatom in the Ti(OPr i )4 has a higher partial positive chargethan that of Si in Si(OEt)4 according to the partial chargemodel (11), making it more eligible to be nucleophilicallyattacked by the –OH groups of the hydrolyzed speciesSi(OH). The resultant system seemed to undergo hydrolysisand condensation under alkaline catalysis, even though nofree OH0 groups are available in this system. The hy- FIG. 10. Effect of heat treatment on the ER activity of Si/Ti Å 5

powders.drolyzed species of Si(OEt)4 can probably act similarly as

AID JCIS 5121 / 6g33$$$301 11-18-97 12:05:32 coida

64 YANG ET AL.

process was interpreted in terms of dependence of the acidic face –OH groups and the particle structure can considerablyinfluence the ER activity. The alteration of the compositestrength of the surface MOH groups on the O20 /OH0 ratio.

O20 has a larger negative partial charge than O atom in OH0 composition, however, will not significantly change the sur-face state and the particle structure of the dried gel powderand thus is easier to attract H/ . Therefore, the IEP of the

powder with –OH groups attached at its surface must be and thus will have little effect on the ER activity.Since the –OH groups are unipolar, particles containinghigher than that of the powder after dehydration. The zeta

potential results in Fig. 6 showed that the IEP of the samples a high amount of –OH groups can be easily polarized toform particle chains between the electrode’s gaps and thuswith increasing calcination temperature moved to the lower

pH area, suggesting that the amount of the –OH groups re- exhibit strong ER activity even under a low electric fieldstrength. On the other hand, a porous particle structure canmaining on the Si/Ti dried composite powder surfaces would

decrease with increasing temperature. increase the wettability between the particles and the liquidby forming a high solid–liquid interface and therefore canIR spectra results show that the appearing temperature of

Ti–O–Ti bond vibration in the composite powder decreased create a high interface for polarization. Clearly, both effectscan enhance the ER activity. This is evidenced by our results.for the powder with a higher Ti content. This temperature

is close to the formation temperature of anatase, as shownin Fig. 5. The higher anatase crystallization temperature in 5. CONCLUSIONSthe powder with a higher Si content can be explained as dueto the stronger inhibitory effect of SiO2 particles on the In the sol–gel process of Si/Ti composite system, thecrystallization of anatase with increasing of Si content (15). mixed solution of Si(OEt)4 with addition of a little amountPasks reported that the anatase crystalline phase usually ex- of Ti(OPr i )4 demonstrated a reduction of gelation time com-hibits an IEP at around pH 6 (14). Note that the IEPs of pared to the single system of Si(OEt)4 . This is the samethe calcined composite powders in the present work are lo- case for Ti(OPr i )4 . The observed reduction in gelation timecated at the very low pH value of about 2, suggesting that could be ascribed to a higher reactivity of Ti precursor thana silica-like surface in the calcined powders was probably that of Si(OEt)4 and to the easier nucleophilic attack of theformed, leading to a shift of IEPs to a more acidic region Ti atom by –OH groups in the hydrolyzed species of Si-when it was exposed to the aqueous solution (14, 16). The (OEt)4 (Si(OH)), which produce an environment like basereason that SiO2 prefers to stay on the powder surfaces after catalysis.heat treatment is still uncertain, and more work is needed. Zeta potential measurement is a useful technique to probe

Heat treatment could also generate a change in SiO2–TiO2 the variation of –OH groups in the powder surfaces. Duringcomposite powder structure. SEM observation of the gel heat treatment, gel powders gradually lose –OH groups andpowders in Fig. 7 exhibits that the agglomerate particle con- became denser in structure.sists of many tiny primary particles of about 50 nm. Such ER activity strongly depends on the amount of –OHa particle morphology generally exhibits a porous character- groups as well as the porous nature of the powder particles.istic (17), which would favor its ER activity as will be The –OH groups on the powder surfaces can easily be polar-discussed later. After heat treatment, however, the tiny parti- ized to form a chain-like structure and therefore can demon-cles will grow to a large size due to their high reactivity, strate high ER activity under the electric field. A powderyielding a denser structure comparing with the gel powder, with a porous structure can enhance the wettability betweenas is observed in Fig. 7. Such a powder structure will show powders and the silicon oil, encouraging the particle polar-a less porous characteristic and reduce its ER activity. ization and consequently increasing the ER activity. How-

ever, the initial Si /Ti ratios have little effect on the formation4.3. ER Effect of –OH groups and porosity of the powders, and thus

showed no great influence on ER activity.The ER activity measurements showed that the composite

composition of particles has little influence on the ER activ-ACKNOWLEDGMENTity of its suspensions. In contrast, there is a remarkable dif-

ference in ER activity between ER fluids prepared fromThe authors express thanks to Prof. L. W. Zhou and Dr. C. Z. Li for their

particles with and without heat treatment. After heat treat- help with ER activity measurement and to Dr. Y. X. Huang for his usefulment, the ER activity of the corresponding powder is gradu- discussions. The first author is also grateful to Foundation Oriental for the

grant.ally lost. This is related to the changes of the surface stateand the porous structure during heat treatment, as discussedabove. The powders after heat treatment lose –OH groups REFERENCESand porous nature, and thus the ER effect is reduced. Thelarge difference of the ER effect in the samples with or 1. Winslow, W. M., J. Appl. Phys. 20, 1137 (1949).

2. Williams, H. A., New Scientist 17, 37 (1990).without heat treatment also indicates that the amount of sur-

AID JCIS 5121 / 6g33$$$301 11-18-97 12:05:32 coida

65SiO2–TiO2 COMPOSITE POWDERS

3. Conrad, H., Sprecher, A. F., Choi Y., and Chen, Y., J. Rheol. 35, 1393 11. Livage, J., in ‘‘Chemical Processing of Ceramics’’ (B. I. Lee andE. J. A. Pope, Eds.) , p. 23. Marcel Dekker, Inc., New York, 1994.(1991).

12. Yang, J., Ph.D. Dissertation, Zhejiang University, China, 1996. [In4. Ginder, J. M., and Ceccio, S. L., J. Rheol. 39, 211 (1995).Chinese]5. Tse, K. L., and Shine, A. D., J. Rheol. 39, 1021 (1995).

13. Pugh, R. J., in ‘‘Surface and Colloid Chemistry in Advanced Ceramics6. Klass, D. L., and Martinek, T. W., J. Appl. Phys. 38, 67 (1967).Processing’’ (R. J. Pugh and L. Bergstrom, Eds.) , p. 127. Marcel

7. Uejima, H., Jpn. J. Appl. Phys. 11, 319 (1972).Dekker, Inc., New York, 1994.

8. Klingenberg, D. J., Swol, F. V., and Zukoski, C. F., J. Chem. Phys. 94, 14. Pasks, G. A., Chem. Rev. 65, 177 (1965).6160 (1991). 15. Prison, A., Mohsine A., Marchot P., Michaux, B., Cantfort, O. V.,

9. Brinker, C. J., and Scherer, G. W., ‘‘Sol-Gel Science: The Physics and Pirard, J. P., and Lecloux, A. J., J. Sol-Gel Sci. Technol. 4, 179 (1995).Chemistry of Sol-Gel Processing.’’ Academic Press, San Diego, CA 16. Okada, K., and Otsuka, N., J. Am. Ceram. Soc. 69, 652 (1986).1990. 17. Sakka, S., Kozuka, H., and Adachi, T., in ‘‘Porous Materials, Ceramic

10. Fricke, J., and Gross, J., in ‘‘Chemical Processing of Ceramics’’ (B. I. Transitions’’ (K. Ishizaki, L. Sheppard, S. Okada, T. Hamasaki, andLee and E. J. A. Pope, Eds.) , p. 311. Marcel Dekker, Inc., New York, B. Huybrechts, Eds.) , Vol. 31, p. 27. The American Ceramic Society,

Inc., Westerville, OH, 1993.1994.

AID JCIS 5121 / 6g33$$$301 11-18-97 12:05:32 coida