세관형 무기막표면의 성능개선을 위한 나노구조박막의 제조

김영석, 양승만

한국과학기술원 화학공학과

Preparation of nano-structured thin film on a hallow fiber silica membrane

Young-Seok Kim, Seung-Man Yang

Department of Chemical Engineering, KAIST, Taejon 305-701, Korea

Introduction

Microporous inorganic membranes have been increasingly used in separation process due to their excellent thermal and chemical stability. They offer considerable promise in applications such as gas separation, removal of VOCs and NOx, hydrogen recovery and membrane reactor [1]. The microporous membranes possess much higher permeance and relatively lower selectivity than the polymeric membranes. The mechanism of gas separation by a porous membrane is closely related to the membrane pore size relative to the permeate molecule size. It is thus important to control the pore size uniformly to nanometer scale without cracks or pinholes [2]. However, the improvement of membrane performance (flux and selectivity), processibility, and stability remains a challenge.

It has been known that the quality of the underlying support determines, to a high degree, the properties and the quality of the selective microporous silica membrane. Rough or large-pore support surfaces would cause cracking of overlying sol-gel derived membranes due to stress development on film coating. Therefore, it is important to develop a simple and effective way to modify the support surface to facilitate deposition of a defect-free overlying microporous membrane [3,4].

Here we report a sol-gel dip-coating process to form crack or pinhole free microporous silica membranes with an advanced preparation method. A thin film layer have been synthesized on a □-alumina hallow fiber support tube using sol-gel process based on the self-organization between surfactant and silicate [5]. It is the shrinkage of the solid network at the critical point that establishes the final pore volume, pore size, and surface area of the dry gel or xerogel. The extent of drying shrinkage depends on the balance between the magnitude of the capillary pressure that collapses the gel and the stiffness of the gel that provides resistance to collapse [6]. Therefore, silicon oil was introduced to minimize surface tension when the drying process and to prevent a deposited microporous layer from penetrating into a support structure.

Experimental

Tetraethylorthosilicate (TEOS) was dissolved with cetylpyridinium chloride (CpCl), distilled water and a small amount of hydrochloric acid in an ethanol, giving a clear solution with the typical molar ratios of TEOS:CpCl:H2O:HCl:EtOH= 5:1:15:0.027:50 and stirred for 24 hours at room temperature. The TEOS in the solution was partially hydrolyzed by a substoichiometric amount of water (H2O/TEOS<4). Then, the support tube (O.D.:2.1mm, I.O.:1.9mm, porosity:0.39, average pore size:150nm) was dipped in a prepared sol for several minutes and pulled out at the rate of around 10cm/min.

Silicon oil was used to control the processibility of dip-coating method. First, thin film was coated on a support and directly dipped into silicon oil before the gelation occurred. Second, before coating process, a support tube was wetted with silicon oil to fill the rough surface structure and make a smooth interface. For the drying of inner coated layer, N2 was flowed into tube side for a day. After drying process, silicon oil was removed from a membrane using hexane. Unsupported silica was prepared by casting a sol on a glass plate and then allowing it to dry overnight in ambient air. The prepared membranes and materials were then calcined in air flow at 773K for 1 hour with a increasing rate of 1K/min. The pore size distribution of the silica powders was determined from Ar sorption isotherm at 87K using BJH (Barrett-Joyner-Halenda) method.

Results and Discussion

The XRD patterns of the calcined mesoporous silica samples are shown in Figure 1. This disordered or partially ordered structure can be indexed to the hexagonal structure consistent with the TEM image (Figure 2). The mesoporous channels have the same diameter as ordered structure, but the channels in the disordered material are interconnected in a three-dimensional disordered way. Ar sorption isotherm showed a narrow pore size distribution around 1.6nm.

Due to preferential alcohol evaporation during the thin dip-coating process, water is the dominant solvent during the final stage of drying where the pore size is established. Within seconds the coated sol is concentrated and dried to form a thin film (typically 10 to 300nm). This short characteristic time of the thin film deposition process represents the time for continued condensation reactions to occur. Thus, compared to bulk gels, films are less highly condensed prior to the critical point and hence suffer greater collapse during drying. Consequently, films are characterized by lower pore sizes, pore volumes and surface areas than their bulk counterparts. To prevent top layer of coating film from fracturing of gel by shrinkage, silicon oil, which is immiscible with sol, was coated on sol coated layer and dried from the bottom side of support tube. Figure 3 shows SEM images of film layer coated on support, which dipped into silicon oil after sol coated. Crack and pinhole free thin film (~500nm) was successfully formed by one step coating process. Figure 4 shows SEM images of film layer coated on support, which dipped into silicon oil before sol coated. It is clear, the sol did not penetrate into the porous support structure and the thin film (about 1□m) was formed regardless the nature of surface of support tube.

Conclusion

Microporous silica membranes were prepared by sol-gel dip-coating method. Crack and pinhole free thin film was successfully formed on a porous support tube with preventing coating film from fracturing of gel by shrinkage, using immiscible silicon oil during drying process.

References

[1] R.W. Spillman, Chem. Eng. Prog. 85(1), 41 (1989)

[2] Y.S. Kim et al., Korean J. Chem. Eng., 18(1), 106 (2001)

[3] C.Y. Tsai, J. Membr. Sci., 169, 255, (2000)

[4] Y. Lu et al., Nature, 389, 364 (1997)

[5] R. Ryoo et al., J. Phys. Chem. B, 101, 10610, (1997)

[6] C.J. Brinker et al., J. Membr. Sci., 94, 85, (1994)

Figure 1. X-ray diffraction patterns after calcination of prepared silica powder.

Figure 2. The transmission electron micrograph of calcined mesoporous silica.

Figure 3. SEM images of film layer coated on support, which dipped into silicon oil after sol coated.

Figure 4. SEM images of film layer coated on support, which dipped into silicon oil before sol coated.