JOURNAL OF MATERIALS SCIENCE LETTERS18 (1999 )1367– 1369

Synthesis and microstructure of silica-doped alumina composite

membrane by sol-gel process

JIN-HA LEE, SUNG-CHURL CHOIDepartment of Ceramic Engineering, Hanyang University, Seongdong-Gu, Seoul 133-791, South KoreaE-mail: leejinha@hanimail.comE-mail: Choi0505@email.hanyang.ac.kr

DONG-SIK BAE, KYONG-SOP HANDivision of Ceramics, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang,Seoul 136-791, South Korea

The preparation and application of ceramic membraneshas received much attention in the past few years [1].Ceramic membranes are technically important in sepa-ration and filtration as well as in catalytic reactions, be-cause of their high thermal and chemical stability, longlife time and good defouling properties in comparisonwith polymeric membranes [2, 3]. Many researchershave reported on the synthesis and microstructure de-velopment of ceramic membranes, and their application[2, 4–8]. Nowadays, commercial applications of alu-mina membranes can be found in wine and beer clarifi-cation as well as the pharmaceutical industry [4]. Of thevarious methods used for inorganic membrane prepara-tion, sol-gel process is considered the most practical forceramic membrane synthesis because of the advantagesof being able to make micro-scale thin membrane toplayers with nanoscale pore diameter and narrow poresize distribution [2]. In sol-gel membrane synthesis, thepore size is determined by the primary particle size inthe sol [4]. The sol is deposited on the porous body bya dipping or a slip casting method. After drying andheat treatment, the thin film forms the smallest poresize at a relatively low temperature. In most cases, thepore size of thin film increases with the heat treatmenttemperature.

There are many reports referring to the preparationof alumina membrane by the sol-gel process [2, 4, 5].Alumina membranes are usually synthesized with theirtransition forms (e.g.γ -, δ-, θ -Al2O3). Because of theirfine particle size, high surface area, and catalytic activ-ity of their surfaces, the transition alumina (especiallytheγ form) find application in industry as absorbents,catalysts or catalyst carriers, coatings and soft abra-sives [9]. But they are transformed at high tempera-tures (usually about 1000–1100◦C) and develop ab-normal grain growth (vermicular structure). At above800◦C, the BET surface area of pure alumina mem-branes start to decrease and at above 1000◦C the porestructure which is preferable for membrane application,is destroyed [4]. So their high temperature applicationis limited. Lin and Burggraff [2] investigated the ef-fects lanthanum’s presence has on the thermal stabilityof alumina membranes. According to this report, up to1200◦C, they were thermally stabilized, e.g. the porecharacteristics were maintained up to 1200◦C.

In this study, silica doped alumina membranes wereprepared by the sol-gel method for gas separation ap-plication. Transition alumina shows relatively weakchemical durability than other oxide, and silica hasexcellent chemical stability except against hydrogenfluoride (HF). It is expected that silica-doping couldimprove the chemical stability of alumina membraneand delay the surface diffusion in the sintering pro-cess, resulting in pore characteristics improvement. Themicrostructure change of silica-doped alumina mem-brane as a function of heat treatment temperature wasexamined.

The alumina sol was prepared by the Yoldas process[10] and the ceramic membranes were coated by thesealed dipping method. The alumina sol (0.2 mol l−1)was prepared by adding aluminum-tri-sec-butoxide(ATSB, Aldrich) to distilled water. The sol was vigor-ously stirred for complete alkoxide hydrolysis and waspeptized with hydrogen chloride (HCl) at 75–85◦C.This solution was stirred on a hot plate for over 24 h toensure complete mixing and hydrolysis. HCl was addedat 75–80◦C for peptization of the sol (molar ratio;0.07). A polyvinyl alcohol (PVA) solution, prepared bydissolving 10 g of PVA (Aldrich, MW=8000–10000)in 90 g of distilled water, was used as a drying controlchemical additive (DCCA) for making supported ce-ramic membrane. A silica sol (5 wt % sol, particle size;several nm, Seok-Keyng Chemical Co.) was purchased.The Al2O3 sol and the SiO2 sol were mixed in a molarratio of 5 to 15 mol % per alumina. For complete mix-ing, the mixed sol was stirred vigorously on a magneticplate for over 10 min.

The support used was tubular typeα-Al2O3 whichwas fabricated at Dong-Su Co. The mean pore size ofthe support is about 0.1µm and the porosity is about30%. The porosity of the support was determined by theArchimedes method. The support has desirable prop-erties for inorganic composite membrane as reportedearlier [3]. The support was coated with the mixed solby the sealed dipping method for 45 s and dried in the at-mosphere at room temperature. The dried supports werecalcined from 700 to 1300◦C (heating rate; 1◦C min−1)for 1 h. This sealed dip coating method is the one thatcontrols the coating thickness with ease. The surface ofsupport that is not to be coated is sealed, and the sealed

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Figure 1 Microstructures of Al2O3 composite membranes (a) sintered at 800◦C and not doped, (b) sintered at 800◦C and 15 mol % SiO2 doped,(c) sintered at 1100◦C and not doped, (d) sintered at 1100◦C and 15 mol % SiO2 doped. The bar in the photograph represents 300 nm.

support is dip-coated. This method prevents gel layerfrom thickening in the early stage of dip coating.

The particle size of hydrolyzed alumina sol wasexamined by dynamic light scattering (Ar laser,Nicomp 370). The surface morphology and the thick-ness of the membrane were observed by SEM (Hitachi,S-4200). The variation of surface pore size was ana-lyzed by Image Analyzer (Leica, Quantimet 520).

In order to uniform pore size distribution of the mem-brane, the sol should contain uni-modal particle sizedistribution. The mean particle size of the sols wasabout 11 nm, and the particles were well dispersed inthe mixed solution.

In order to improve formation of thin film, PVA wasadded to the mixed solution. PVA increased the vis-cosity of the mixed sol. In the preliminary experiment,the pure sol penetrated into the support because of itslow viscosity. PVA addition to mixed solution makesthin film formation possible on the support, and to pre-vent crack formation in the top layer during the dryingstage [4]. The forming mechanism of the supported thinfilm could be described by a slip casting model [11].Due to the capillary forces, water is sucked into the poreof the support. The concentration of sol at the entranceof the pore increases and gelation of the sol occurs. Thecalcined thickness of the top layer of the membrane wasabout 1.5µm.

The surface microstructures of the membranes wereinvestigated by SEM (Fig. 1). The surfaces of the mem-branes were smooth and cracks had not occurred. The

alumina membranes were stable up to 1000◦C be-cause the particle morphologies of the membranes werenot changed up to 1000◦C. There was no evidence ofdevelopment of vermicular structure up to this tem-perature. But the heat treatment temperature of themembrane increased up to 1100◦C, the particles whichconsist of the membrane tend to grow directionallyand become the vermicular structures at 1200◦C. It isshown that alumina membrane has limited applicationfor gas separation.

Otherwise, the particle morphology of the silica-doped membrane was not changed up to 1100◦C. Thevariations of surface pore size with heat treatment tem-perature are shown in Fig. 2. The determined pore sizes

Figure 2 The surface pore size variation as a function of sintering tem-peratures.

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are the mean equivalent circle diameters. It is not theactual pore size of the top layer that shows the changetrend of pore size.

The average pore diameter of Al2O3 membrane wasmore increased than that of SiO2-doped Al2O3 mem-brane with the heat treatment temperature increased upto 1100◦C. It is shown that the SiO2 introduction in thematrix delayed the growth of Al2O3 particles. It is re-ported that the particles on the surface ofγ -Al2O3 low-ered the driving force for sintering and thus reducedthe sintering rate and extent [12, 13]. This could alsoexplain the retardation of the pore growth for the silica-doped membranes in this study. It is presumed that thesurface energy of Al2O3 particles was reduced with theSiO2 particles introduced into the surface of Al2O3 par-ticles. This surface energy reduction of the Al2O3 parti-cles would be inhibited by the surface diffusion duringthe sintering process. It is known that the phase trans-formation fromγ -Al2O3 toα-Al2O3 proceeds via a nu-cleation and growth mechanism with one nucleus beingformed per crystallite [14]. The presence of the silicaon theγ -Al2O3 crystallite surface may reduce the pos-sibility of nucleation ofα-Al2O3, thus raising the phasetransformation temperature. The introduction of SiO2into Al2O3 membrane enhanced the pore characteris-tics and retarded the increase of the pore size of pureAl2O3 membrane up to 1100◦C. The heat treatmenttemperature of the membrane was above at 1100◦C,the morphology of the top layer was transformed intoa needle-like crystalline phase. It is shown that silicadoped membrane could not apply for gas separationabove this temperature. Therefore, it is expected that

the thermal stability of the Al2O3 membrane would beimproved by a SiO2-doping up to 1100◦C.

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Received 12 Marchand accepted 27 April 1999

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