Titania and alumina ceramic membranes

Journal of Membrane Science, 39 (1988) 243-258 Elsevier Science Publishers B .V., Amsterdam - Printed in The Netherlands

243

TITANIA AND ALUMINA CERAMIC MEMBRANES

MARC A. ANDERSON*, MARY J . GIESELMANN and QUNYIN XU Water Chemistry Program, University of Wisconsin - Madison, 660 N. Park St . Madison, WI 53706 (U.S.A.) (Received July 14, 1987; accepted in revised form November 30, 1987)

Summary This paper discusses the physical-chemical properties of y-A1 2 03 and Ti02 ceramic membranes which have been prepared by sol-gel techniques from alkoxide starting materials . Mean pore size and pore size distributions are examined using N2 sorption analysis . Particle size of sols as measured by in situ quasi-elastic light scattering is compared with SEM analysis of sintered membranes . Crack-free unsupported monoliths with thicknesses up to 120 pm have been prepared of y-A12 03. Supported Ti0 2 membranes have been fabricated with thicknesses up to 0 .5 µm. The yA1203 membranes are obtained from "particulate" sols, while for Ti0 2 both "particulate" and "polymeric" sols have been used to produce membranes .

Introduction Historical use of membranes

Membranes are used primarily for separations because, compared to other processes, membranes offer several major advantages which include reduced energy requirements, high permselectivities and low-temperature operation in processing thermally labile solutes (e .g., foodstuffs and pharmaceuticals) . Applications for membrane separations include : feedwater preparation for boilers, food processing, water desalination, gas separations, treatment of municipal, mining, metal plating, pulp, paper, and other industrial waste streams, hydrocarbon extraction, and biomedical processes [ 1,2 ] . Increased use of membranes has led to an expanded knowledge base of fabrication technology, with the result that membranes have been designed with larger throughput, improved selectivity and longer stability . While most emphasis has been placed on organic membranes [3 ], ceramic membranes have several advantageous characteristics : (1) Chemical stability - ceramic membranes withstand organic solvents, chlorine and pH extremes . *Author to whom all correspondence should be sent .

0376-7388/88/$03 .50

© 1988 Elsevier Science Publishers B .V.

24 4

(2) High temperature applications - ceramic membranes are stable at very high temperatures, allowing sterilization of process equipment for food and pharmaceutical applications . (3) Stability to microbial degradation - certain organic membranes are particularly susceptible to microbial degradation . (4) Mechanical stability - organic membranes compact under high pressure, leading to lower permeabilities . (5) Cleaning conditions - items 1-3 indicate that harsher, more effective cleaning treatments could be used with ceramic membranes . In addition to these stability advantages, careful selection of the preparation conditions, including the inorganic compounds employed as precursors, can provide further control of separation processes via modification of the physical, chemical and electrical properties of the ceramic membranes . While these characteristics seem to favor inorganic membranes, they have not been used to any significant extent in commercial applications because of the difficulty in producing crack-free membranes having ultrafine pores and narrow pore size distributions . Previous work with ceramic membranes Larger-pore membranes

While Vycor-type glass membranes have been used since the mid 1940's, they have been plagued by inconsistent pore size distributions [4-6] and a tendency to dissolve in process fluids [ 7 ] . "Dynamic" inorganic membranes, formed by filtering colloidal solutions through porous filters [8-11 ], have the disadvantage that flux decline is so large after a few weeks of operation that replacement is mandatory . A third type of inorganic membrane, commonly referred to as Ucarsep [ 12 ], has been produced by Union Carbide by placing ultra-stabilized zirconia particles on a silica support . While these membranes offer a wide range of pH stability, they have fairly high molecular weight cutoff values because of their large pore size . Smaller pores through controlled processing

The sol-gel process for producing uniformly-sized powders, introduced in the early 1940's, has been used for the last 20 years to prepare uranium dioxide pellets for nuclear reactors . Barringer and Bowen [131 recently synthesized small Ti02 particles with high specific surface areas by using this technique . Other work has been reported on the preparation of titania films and porous coatings using the sol-gel process [14-181 . Yoldas [19,20] prepared porous transparent alumina from organometallic precursors and found that the amount of peptizing acid added influenced the sol-gel transformation and the ability of the gel to retain its integrity. Kaiser and Schmidt [ 21 ] used alkoxysilanes to deposit porous silica membranes (5-10 um thickness) on porous supports .

245

These silica membranes had a mean pore diameter of 2 .5 nm, leading to surface areas of 200-300 m2 /g . Such ceramic coatings improved the filtration properties of a deep-bed filter . Sol-gel techniques have also been employed to synthesize zirconia [ 22 ] . Leenaars et al . [ 23-25 ] used sol-gel techniques to prepare alumina ceramic membranes and supported membranes . By controlling suspension pH, alumina concentration, and calcination conditions, they were able to construct membranes having both ultrafine pores (3-5 nm) and narrow pore size distributions . These membranes have cut-off values for molecular weight exclusion which are 10 times lower than those of Ucarsep membranes and offer greater stability than either Vycor or "dynamic" membranes . Further, their research has indicated that the molar ratio of peptizing acid to alumina, type of acid anion, size of crystallites, and sintering temperature are the most critical variables in controlling pore size distributions . All of these parameters can be optimized to increase the packing density of crystallites, ultimately decreasing the size of the micropores . Research objectives Our research program is directed toward producing new ceramic membranes with desirable pore sizes, pore size distributions, and surface properties by controlling the suspension and interfacial chemistries of these systems . To this end our two major objectives are : (1) to understand the major solution variables (pH, ionic strength, specific solute effects, etc .) controlling sol and gelation chemistries, since these sols and gels serve as precursors to ceramic membranes and (2) to study these effect of the variables as well as the sintering conditions on the final unsupported ceramic membranes . While working on these two tasks, we have also been experimenting with supporting the membranes, a necessity if the membranes are to function in a practical commercial scale unit . In order to meet these objectives, our research has focused on two areas : (1) the preparation of Ti02 and y-A1 2O3 unsupported ceramic membranes and (2) the production of both a-A1 203 and clay supports for these membranes . This paper reports on our results concerning unsupported Ti0 2 and y-A1 2 O3 ceramic membranes . Unsupported titania and alumina ceramic membranes TiO2 membranes All the gels which are precursors of Ti0 2 ceramic membranes are prepared by the controlled hydrolysis of titanium alkoxides [ 26 ], although two separate routes have been followed in synthesizing these gels . The first method utilizes hydrolysis in water, forming what we will call a "particulate" sol, followed by



246

gelation. The second method involves hydrolysis in alcohol, with a small amount of water added, to form soluble intermediate species which then condense forming inorganic polymers . We will call this a "polymeric" sol . These sols can also be gelled under certain conditions . Unfortunately, except for silicon, most metal alkoxides, including titanium alkoxides, form precipitates when contacted with water . This is precisely the reason why polymeric silica and mixtures of silica with other elements have been more extensively studied than the corresponding titania or alumina systems . Experimental methods Materials Titanium tetraisopropoxide

was obtained from Aldrich Chemical Co . Both ethyl alcohol (absolute) and isopropanol were AR grade . All chemicals were used without further purification . Water used in reactions was deionized using a Milli-Q water purification system (Millipore Corp .) . Glasses used as initial substrate materials were flat non-porous microscope slides which were degreased before use . Membranes from particulate sols Particulate sols were prepared by the hydrolysis of titanium tetraisopropoxide at room temperature : Ti(iso-OC 3 H 7 ) 4 +4H2 0

'Ti(OH) 4 +4C 3 H 7 0H

(1)

The hydroxide precipitate was peptized with appropriate amounts of HN0 3 to form a highly dispersed, stable colloidal solution . This suspension was then gelled by water evaporation under constant relative humidity at room temperature. Both unsupported and supported titania membranes can be prepared in this manner . A detailed procedure is illustrated in Fig . 1 . Transparent titania gel membranes obtained by this method were fired at a temperature of 400 ° C to form titanium dioxide membranes and remove all water, alcohol, and nitrate . In order to prepare titania membranes on ceramic or glass supports, two methods can be used : dipping for thinner films (about 0 .02-0.05 ym thickness) and spreading for thicker films (about 0 .5 ,um) . Repeated operations give even thicker membranes (greater than 1 pm) . Membranes from polymeric sols Polymeric sols were prepared as illustrated in Fig. 1 . Titanium tetraisopropoxide was hydrolyzed using a small quantity of water in alcohol (either ethanol or isopropanol) while controlling the pH to prevent the forming Ti0 2 precipitates . The reaction in aqueous isopropanol can be described by the following equation : Ti(OR) 4 +xH2 O

->Ti(OH) x (OR) 4 - x +xHOR

(2)

where : x c 4 . Partially hydrolyzed products which are soluble in the alcoholic medium immediately condense to form polymeric chains through condensation of oxygen bridges :



24 7

M(OR)4dissolved in alcohol added at 2500 with high speed stirring small quantity of water dissolved in alcohol

4,

I

clear solution

water

hydroxide precipitation stirring at 25 °C for 0 .5 hour

I

polymeric sol

suspension

HN03

12 hours heating 85-95°C dipping colloidal suspension supported membrane

cool to roo temperature

controlled rel hum clear supported gel membrane

gelling

firing

400-500°C

ceramic membrane

stable colloidal sol

pouring

sol in plastic container controlled rel hum transparent unsupported membrane

gelling

Fig. 1 . Preparation scheme for particulate and polymeric ceramic membranes .

Ti (OR)y(OH), TiOZ (OR),,1 (OH),1 + (y-y1 ) ROH

(3)

where x 1
acid added to an uncharged particle system peptizes the larger particles, producing smaller ones which become positively charged through proton adsorption . Since our Ti (OH ) 4 particles in aqueous suspensions have been shown to be isoelectric at pH = 6 .8, lower pH values promote this charging process . The charged particles repel each other, thus developing stable sols . Figure 2 illustrates that particle size in the suspension goes through a minimum with increasing acid concentration and reaches a critical range where stable sols exist . From the experimental data shown in Table 1, it is evident that the size of sol particles, estimated from quasi-elastic light scattering measurements, var-



24 8

t W N N W J

U

I

a a z N Q W it U Z

Peptized Stable Sols ACID ADDITION

4

Fig. 2 . Effect of acid concentration on particle size . TABLE 1 TiO2-particulate sol composition and gel properties Stability Features Weight loss Group No. H2 O/Ti H+/Ti Ti02 (mole ratio) (mole ratio) (Wt. %) of soles of solid gel in gelation b (%) A

B

1 2 3 4 5 6

200 200 200 200 200 200

0 .08 0 .2 0 .4 0 .7 1 .0 1 .2

2 .0 2 .0 2 .0 2 .0 2 .0 2 .0

NP S S S S NS

good good good crack crack

1 2 3 4

300 300 300 300

0.2 0.5 1 .0 1 .2

1 .3 1 .3 1 .3 1 .3

S S S S

good good good good

97.66 97.62 97.61 97 .60

Particle size` (nm) 87 82 96 139

79

'S=stable, NS = not stable, NP = not peptized completely. `'Weight loss from original sol to solid gel, given as a percentage of the original sol weight . `Average effective diameter measured by light scattering. ies with acid concentration in a similar manner to that shown in Fig . 2 . Table 1 also shows that stable sols can be achieved if the molar ratio of H+ (from the acid) to Ti is between 0 .1 and 1 .0 . This range can be expanded only in dilute sol solutions, possibly because the increased interparticle distances make aggregation more difficult than in concentrated sots . Only stable sols could then be transformed into coherent transparent gels and finally into coherent oxide membranes by pyrolysis . Inorganic acid concentration also affects the gelling volume, an observation which has been reported for alumina gels [ 19 ] . It was found that the gelling

249

volume goes through a minimum when the molar ratio of acid to titanium was near 0 .4 . This also corresponds to the minimum particle size observed for these systems by light scattering . All of the sols lost at least 4 .5% of their original weight (depending on the electrolyte concentration) to arrive at the gelling point. Furthermore, they lost more than 90% of their original weight to form a final solid gel (xerogel) . (This loss is independent of electrolyte concentration.) Heating the final gels resulted in further weight loss (13 .5%) without destroying the internal structure . In firing the xerogel to produce the final membrane materials, we have investigated the effects of temperature on surface area, on porosity, and on crystalline character . Although these results will be reported in a future paper, we summarize here that the Ti0 2 membranes when fired at 400°C were 30% porous, had narrow pore size distributions, had mean pore diameters of 38 A, and were predominantly composed of 20 nm anatase particles . While only unsupported membranes have been discussed, we have also tried to coat these gels onto glass supports . A transparent membrane coating of up to 0.5 µm in thickness has been obtained without cracking . We are now in the process of preparing Ti0 2 -coated clay systems for use in filtration processes . Polymeric sols Gels can be prepared from polymeric sols by further chemical polymerization, forming an oxide network . However, difficulties arise when making these gels because the alkoxide precursors hydrolyze rapidly upon the addition of water . This hydrolysis produces titanium hydroxide precipitates, which do not polymerize any further . Efforts in our laboratory to circumvent this problem have led to protocols for the preparation of clear polymeric sols without precipitates, using initial water concentrations up to 16 mole per mole of titanium . Analysis of the experimental data shown in Table 2 indicates that the water concentration in the initial solution controls the viscosity of the sol : the more water, the more viscous the solution . Water also affects the sol-gel transformation . Transparent gels can be obtained only when the molar ratio of H20 to titanium is < 4 . The influence of water concentration upon gelling volume and gelling time is shown in Table 3 . Since gelling volumes reflect the degree and type of polymerization, these results indicate that no obvious changes occur with increasing water content . When these results are compared to those of silica systems [27], it appears that a higher degree of polymerization can be achieved in Ti0 2 systems even though this polymerization takes place in a system having lower water content . This hypothesis seems to be verified by the fact that no cracks have been detected during pyrolysis up to 500'C, a result which indicates that these Ti02 systems consist of a strong oxide network with high levels of oxygen bridging . The type of alcoholic solvent which dilutes the reacting species is another important factor because the solvent affects the rates of hydrolysis and polymerization . Ethyl alcohol and isopropanol are preferred, and the former may

250 TABLE 2 Ti02-polymeric sol composition and gel properties C2HSOH/Ti (mole ratio)

Gel features

3 3 3 3 3

50 50 50 50 50

0.08 0.08

12 12

8 8

1

0.025

12

8

clear gel clear gel clear gel opaque gel too viscous to form gel clear gel too viscous to form gel clear gel

1

0.08

12

8a

opaque gel

Group

no.

H2O/Ti (mole ratio)

H+/Ti (mole ratio)

E

1 2 3 4 5

1 2 4 8 16

0 .08 0 .08 0 .08 0 .08 0 .08

6 7

1 2

8 1

P

Ti0 2 (Wt. %)

a Ratio of iso-C 3H,OH to Ti . TABLE 3 Influence of water concentration on gelation Group

no.

H 2O/Ti (mole ratio)

H+/Ti (mole ratio)

C3H,OH/Ti (mole ratio)

Gelling time (hr)

Gelling volume a (cm)

G

1 2 3 4 5 6 7

1 1 .5 2 2 .5 3 3.5 4

0 .025 0 .025 0 .025 0 .025 0 .025 0.025 0.025

28 28 28 28 28 28 28

19.5 14.5 11 .0 5 .0 4 .0 3 .5 1 .5

1 .80 1 .85 1 .90 1 .95 2.00 2.00 2 .00

'Gelling volume is represented by the height of sample in the vial at the gelling point . The initial height was 2 .05 cm for every sample .

be better than the latter for making transparent gels . However, it was found that acid did not significantly affect the properties of polymeric gels but only acted as a catalyst, providing the low pH condition necessary to prevent precipitation during hydrolysis . In terms of the basic gelation process, if polymeric titania sols are placed in a closed bottle and aged for weeks, a monolithic crack-free gel forms . This gel then expels liquid from its structure and shrinks with time . This phenomenon occurs because polymerization continues in the liquid system . Network for-



25 1 TABLE 4 Summary of hydrolysis conditions and appearance of resulting boehmite sols and xerogels H

[AlIT (M)

[H+ ]a (M)

2 3 4 5 6 7` 8 9 10 11 12 13

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0 .5 0 .5 0 .5 0 .5 1 .0

0 .024 0 .048 0 .045 0 .037 0 .035 0 .035 0 .035 0 .035 0 .035 0 .015

[NaNO 3 ] (M) 0.01 0.10 0.001 0.10

0 .030

[ H 3P04 ] (M)

0 .001 0 .00001 -

Sol clarity'

Xerogel appearance

3 4 2 2 2 3 2 2 2 3 opaque 5

monolith monolith monolith monolith monolith cracked monolith monolith monolith cracked cracked monolith

This value refers to the initial acid concentration on addition . The final or equilibrium concentration is much lower since H+ reacts with the boehmite particles . bl=clear; 5 =very cloudy . `Gel formed before completion of hydrolysis step . a

mation achieved by expelling organic groups from the structure leads to a gel state without solvent evaporation [ 27 ] . Upon exposure to air, the alcohol solvent evaporates quickly from the gel causing cracking to occur as a result of rapid shrinkage . y-A1203 membranes Experimental method

Unsupported alumina membranes are made from particulate sols prepared from aluminum tri-sec-butoxide (ATSB) (Alfa Chemical Co .) using the following procedure . ATSB is first diluted in 2-butanol (2-BuOH) to a concentration of approximately 2M to facilitate handling the otherwise viscous material . This mixture is then quickly dripped into water at 80-85'C with vigorous stirring . (Heating the water above 80°C ensures the formation of boehmite (y-A100H), which is a stable crystalline compound . At lower temperatures, an unstable amorphous monohydroxide forms, which undergoes a slow spontaneous transformation to bayerite (fl-Al (OH) 3 ) , which does not gel [ 28,29 ] .) The hydrolysis and polycondensation reactions are allowed to continue for 2 hr . An aliquot of 1 .6 M HN0 3 is then added to peptize the precipitate and stabilize the resulting suspension . Heating and stirring are maintained for another hour, after which the reaction vessel is opened and the temperature is increased to remove the alcohol .



2 52 Sorption Isotherm Gel

6

Pore Size Distribution 20 15-

4-

v Sr 10-

Nsp 2 1 2

SSA=245 m /g P=31 .9 %

0

I

•

I

i

50

2

SSA=215 m /g P=49 .2 %

0 0

.2 .4 .6 Relative pressure

20 ' 40 Tp

60

(A)

Fig. 3. N Z sorption isotherms and pore size distributions (calculated from the adsorption branch) of a typical boehmite xerogel and y-A1 2 03 ceramic obtained after firing to 500'C . Specific surface area (SSA) and porosity (P) are indicated . The temperature climbs to 87 0 C, the boiling point of the 2-BuOH-water azeotrope, and later to 100'C when almost no 2-BuOH is left . A condenser is then positioned above the reaction vessel and the contents are refluxed overnight . The flask is then cooled to room temperature, and water is added to replace that lost by evaporation . The sol is transferred to a glass bottle and stored in the dark at 35-40 ° C until needed . In contrast to the procedure of Leenaars et al . [23-25 ], our gels are prepared by transferring aliquots of the sol to disposable polystyrene weighing dishes or flat pieces of polystyrene or Teflon . Water is allowed to evaporate at ca . 30% rh at room temperature until first a hydrogel and then a xerogel is formed . Gels up to 120 um thick have been prepared without evidence of cracking, and unsupported membranes of y-Al 203 are produced when these xerogels are fired at . 500°C Table 4 is a summary of conditions for 12 hydrolyses . In most cases, the total aluminum concentration, [All T, was maintained at 0 .5M while the initial [ H+ ] was varied between 0 .015 and 0 .048M. In three of the reactions, different amounts of NaNO 3 were added as an inert electrolyte, and H 3PO4 was added to two others as a possible bridging anion . The resulting sols varied in cloudiness, an indication of variations in average particle size . However, all sols were



2 53

12-

15

H/A1= 0 .030

Sv 10 Sr 5

V

0 H/A1= 0 .048

10 Sv 11 5 0 15 100

DESORPTION BRANCH

H/Al = 0 .074

, v 10 Sr 5

Sv

0

&50

H/Al = 0 .090

10

V

Sr 5

4

60

0 0

10

20

40

30

r,

50

60

(A)

Fig. 4 . Pore size distributions calculated from the same isotherm as in Fig . 3 using both the adsorption and desorption branches . Fig. 5. Pore size distributions of y-A1 2 0 3 , showing the effect of increasing acid concentration .

stable, i.e. none formed a precipitate . All gelled readily, although three (H7, H11 and H12) cracked under the stresses of drying . None of the others cracked, and in the table, they are denoted by the term "monolith" . The microstructures of the membranes resulting from these preparations have been characterized by N2 sorption isotherms and scanning electron microscopy (SEM) . Results and discussion N2 sorption isotherms Figure 3 shows two sets of data, the sorption isotherms

and pore size distributions (PSD ), for both the xerogel and the ceramic membrane produced in preparation H4 . The gel shows primarily microporosity. Heating to 500'C enlarges the pores to an approximate modal size of 4 nm . This increase is not surprising at this temperature because the material undergoes a calcination reaction but no real sintering or densification occurs . (A membrane prepared from H5 was also analyzed by mercury porosimetry by Dr . M .W. Shafer, IBM Labs, Yorktown Heights, NY . The modal pore size was found to be about 4 .5 nm, a result which is in good agreement with our findings .) The PSD calculations shown in Fig . 3 were performed on the adsorption branch of the isotherm . Figure 4 shows the PSD's resulting from both the adsorption and desorption branches of the N 2 sorption isotherm for the ceramic membrane shown in Fig . 3 . Distributions from desorption branches are always much narrower than those from adsorption branches . However, the desorption



2 54

15 1

(H3P04)=0M (NaN03)= OM

Sv 1 Sr

Sv 10 Sr

0 15

1 (NaNO3)=0 .001M Sv 1 Sr

( (H3P04)=10-5M Sv 10 Sr

1

(NaNO3)=0 .01 M

Sv 1 Sr

15

(NaN03)=O .1 M

Sv Sr

O V 7,

(H3P04)= 10-3M

Sv 10 Sr

0

(A)

, 10

I 30

~ 20 r,

40

50

60

(A)

Fig . 6 . Pore size distributions of y-A1203 showing the effect of increasing NaN03 concentration . Fig . 7 . Pore size distributions of y-A120 ;3 showing the effect of increasing H3P04 concentration .

branch overemphasizes the neck or narrow regions of the pore structure while the adsorption branch better represents the PSD of the entire sample . Since solutes pass through the filter via the largest channels available, it is necessary to know if large pores are present . In order to use calculated PSD's to ascertain if such pores are present, it is preferable to perform the calculations using data from the adsorption branch of the isotherms . Unfortunately, this procedure results in distributions that are much broader than those generally reported by investigators who use the desorption branches . Figure 5 shows the effect of the initial acid concentration on the pore structure . The top PSD (sample H11) was obtained from the sample prepared with the least amount of acid that could be used to obtain a stable sol . This sample had a very good total porosity (58%) but the PSD is very broad and the gel cracked while drying . When additional acid was added, membranes could be produced which remained whole during drying (samples H2, H4, and H5) . These membranes exhibited somewhat narrower PSD's and retained good porosities of about 50% . Figure 6 shows the effect of increasing the concentration of the inert electrolyte, NaNO3i from 0 to 0 .1M . The PSD's for samples H5, H6, and H8 are similar . These gels formed membranes which remained intact and had porosities of about 50% . For sample H7, shown on the bottom of Fig . 6, the PSD has

255

Fig. 8. Scanning electron micrographs of the top surface of a) unsupported and b) supported yA1203 membranes . The precursor gels were formed by slow evaporation in a) and by slipcasting onto a porous clay support in b) . Size bars are 1 um in a) and 10 um in b) .

broadened considerably and the total porosity is only 17% . This gel also cracked while drying . The last factor investigated to date is the effect of phosphoric acid concentration (between 0 and 10 -3 M) . The PSD's of the membranes in question (samples H5, H9, and H10) are shown in Fig . 7. All three of these gels remained intact and had porosities of about 50% . While the PSD results indicate that, in concentrations up to 10 -3 M, phosphate additions do not seem to have any effect, these results do not necessarily rule out the possibility that phosphate may cause ordering in these systems or may affect the pore structure at higher concentrations . Further experiments are needed in this area . Scanning electron microscopy A micrograph of a typical top surface of an

256

Fig. 9. Scanning electron micrographs at two magnifications of the fracture edge of an unsupported y-A1,03 membrane. Size bars are 1 um in each image . unsupported membrane is shown in Fig . 8a. "Dimples" (or "craters"), which form during the drying process as the water evaporates, would act as stress concentrators in the final product . However, when the sol is slipcast onto a porous support no dimples form (Fig . 8b), although other structural features are present . Apparently, the mechanism for solvent removal during slipcasting to prepare supported membranes (where water is absorbed into the pores of the support by capillary action relatively quickly) is sufficiently different from that involved in preparation of the unsupported membranes (where the water evaporates over a period of 8-10 days) to prevent the forming of dimples on the surface . Unfortunately, the support used had a very rough surface containing both "pits" and "boulders" which appear in the micrograph . It is a distinct possibility that supports may alter surface aggregation processes [ 30 ] . Typical electron photomicrographs of the fracture edges of unsupported membranes are shown in Fig . 9 . These show no voids or defects in the bulk

25 7

membrane and indicate a reasonably constant particle size (ca . 50 nm) and shape . This result is in good agreement with an independent determination of particle size in the sol by quasi-elastic light scattering, which yielded an average particle size of 44 nm . Although it is still not possible to say if these particles are aggregates or primary particles, the agreement in size between these techniques indicates that the particles imaged in SEM are the same ones present and measured in the suspension after sonication for 20 min . Our initial results with supported membranes look very promising in that we have been able to obtain very good permeate rejection with reasonable flow . Further work on supported alumina membranes is in progress, and we will report on these results in a forthcoming publication . Conclusion Our initial efforts in the area of ceramic membranes have focused on studying the solution variables that control the characteristics of the precursor sols, gels, and ultimately, the ceramic membranes . We have succeeded in producing crack-free Ti02 and y-A12O3 unsupported ceramic membranes . The y-A1 2 O3 membranes are prepared from particulate sols while the Ti02 unsupported membranes are prepared from both polymeric and particulate sols . These membranes have pore diameters in the 18-50 A range . Major variables in both cases include the alkoxide to water ratio, acid concentration and the initial reaction temperature. Ionic strength seems to be of lesser importance, most likely because the suspensions are peptized at a low pH, which implies that the system already has high ionic strength. Sintering conditions are also important in controlling the pore size . We are just beginning to use supported membranes in separation processes . Our long range research goals are to tie modifications in surface chemistry to permselectivity of the resulting membranes . Acknowledgement The authors gratefully acknowledge the financial support of the Department of Energy - Office of Industrial Programs under Contract No . DE-AS07861D12626 .

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