Synthesis and Characterization of α-alumina Membrane Support and Effects of Binders

Transcription

1 Synthesis and Characterization of α-alumina Membrane Support and Effects of Binders M. Khalili a,*, S. Sabbaghi a, H. Daneshmand a, M.M. Zerafat b a Faculty of Nanotechnology, Nano Chemical Engineering Department, Shiraz University, Shiraz, Iran b Faculty of Engineering, Chemical Engineering Department, Tehran university, Tehran, Iran * Corresponding author m.kh.nano@gmail.com Abstract: In this paper, macroporous (pore diameter larger than 50nm) symmetric ceramic membranes were prepared by dry pressing of α- alumina powder with the addition of various binders. After burning away the organic material, the compact is sintered and the porosity can be adjusted by the sintered temperature. The results show that the enhancement of sintering temperature leads to creation of compact membrane supports without any crack on the surface also it decreases the size of grains. Moreover the use of different binders and additives create various membrane supports different in the size of pores and porosity. These membranes can be used for microfiltration at elevated temperature and under extreme environmental conditions. They can be used as porous support for the production of composite-asymmetric nanofiltration membrane. Keywords: Ceramic membrane support; α-alumina powder; Dry pressing; Binder; Sintering temperature; Moulding pressure. 1. Introduction Membranes can be generally classified into two major groups according to their material properties: organic (polymeric) membranes and inorganic membranes. Inorganic membranes are also known as ceramic membrane. They are versatile and can be operated at elevated temperatures, high-pressure gas separation, treatment of corrosive liquids and gases. These advantages make porous ceramic membranes particularly suitable for food, biotechnology and pharmaceutical application in which membranes require repeated steam sterilization and cleaning with aggressive solutions [1, 2, 3]. Ceramic membranes are usually composite ones consisting of several layers of one or more different ceramic materials. They generally have a macroporous support, one or two mesoporous intermediate layers and a microporous (or a dense) top layer. The bottom layer provides mechanical support, while the middle layers bridge the pore size differences between the support layer and the top layer where the actual separation takes place. Permeability and separation factor of a ceramic membrane are the two most important performance indicators [4]. For a porous ceramic membrane, they are typically governed by thickness, pore size and surface porosity of the membrane [4, 5]. On the other hand, the porosity in each membrane layer controlling the permeability depends strongly on the microstructure of the porous ceramic support [6]. One of the most important in the preparation of ceramic membranes should be considered extremely, preparation of these membranes without any cracks on the surface. The size of defects considered to be significant depends on the function of the membrane layer (support for MF, UF and NF). As a result, defects in a support layer will transfer if they are the same size or thickness as the next layer. The optimization of the fabrication process and the careful selection of raw materials can have a significant effecton the defect formation [7, 8]. The quality of the support (or supporting layer) beneath the separation layer is critical for the quality of the membrane itself. Consequently high quality supports should be smooth, have constant and homogeneous surface characteristics (wettability) and preferably have a relatively narrow pore size distribution. They should have sufficient mechanical strength which does not age with time [8]. Ceramic membranes are manufactured by a number of methods. Among the various methods, pressing is a commonly used method for preparation of disk inorganic membranes supports. A special press machine is used to apply more than 100 MPa pressure to press powders into a compacted disk [4]. After the powder compaction, membranes supports are sintered at high temperatures in order to make a rigid and resistant structure (mechanically). To make these disk inorganic membranes supports, some kinds of additives which are called binder should be added to the initial powder. The role of these binders is to keep together the particles of this initial powder and to get defect-free membranes[9]. The parameters that affected the porosity and pore size of membranes supports

2 including: the type of binder, milling time of powder and binder, moulding pressure and sintering temperature. In this paper, preparation of the alumina supports was studied by the pressing process. The α-alumina powder was mixed with organic binders and pressed in order to form discs membranes supports. These discs, then, were sintered at high temperatures. The porosity and pore size distribution of pressed alumina powder were evaluated. 2. Materials and Method The starting material for preparation of membranes supports was α-alumina (with the average size of 0.3 µm, presi). For preparing disk-like supports by the pressing process, two kinds of binders, that be ethanol and PVA (polyvinyl alcohol), was added to the starting α-alumina powder. For preparing membranes supports, automatic powder pressing system was used. Microstructures and pore size distribution was characterized by scanning electron microscope (SEM). For determining porosity, the method taken by Zheng et al. [10] was used Ethanol as a binder In this method, first a solution of α-alumina and ethanol (merck) was prepared. This mixture was prepared in a ratio that 30% of its volume was α-alumina. Then the prepared solution was put in a shaker at the temperature of 50 ºC for 1 hours, in order to be mixed mechanically in a complete way and α-alumina powder be mixed uniformly in ethanol. After this, the mixed solution was put in oven at the temperature of 1100 ºC for 24 hours in order to be dried completely. Then the prepared powder was milled by a mortar. These powders were pressed at different pressures of 70 and 90 kn in the form of discs supports. These discs were sintered at the temperatures of 1100 and 1200 ºC with the heating rate of 5 ºC/min for 1 hour PVA as a binder In this method, first a solution with a ratio of 10% of PVA was prepared. To do this, fully hydrolyzed PVA with the molecular weight of (merck) was used. Then, 8 wt.% of this solution was mixed with α-alumina powder. The mixture was milled completely for 3 hours in order to be prepared for pressing. Similar to the method related to ethanol, the milled powders were pressed at the pressure of 70 and 90 kn, and then the prepared discs were sintered at the temperatures of 1100 and 1200 ºC with the heating rate of 5 ºC/min for 1 hour. binders of ethanol and PVA, respectively. These micrographs show that the discs membranes supports possessed regular and smooth surface crack free moulding pressure effect Figures 1(a) and 1(b) show the SEM of the discs membranes supports, in making of which ethanol has been used, at sintering temperature of 1400 C and moulding pressures of 70 and 90 kn. The values of the porosity and pore size of these membranes supports are given in table 1. These values show that the porosity and pore size decrease with the increasing of moulding pressure. Actually, further moulding pressure produced dense membranes with undesirable pore size distribution and increased strength bonds between particles. The SEM micrograph of the discs membranes supports (figures 2(a) and 2(b)), in making of which PVA has been used, and the values of table 2 show the same results. (a) (b) 3. Results and discussion As mentioned, permeability and separation factor are the most important performance parameters of ceramic membranes. Besides, these parameters are dependent on the pore size, porosity and surface membrane characteristics. Figures (1) and (2) show the SEM micrographs of discs membranes supports at different sintering temperatures and moulding pressures for the 328 (c)

3 (d) (d) Figure 1.SEM micrographs of membranes supports, in making of which ethanol has been used, (a) 70 kn and 1100 ºC (b) 90 kn and 1100 ºC (c) 70 kn and 1200 ºC (d) 90 kn and 1200 ºC. (a) Figure 2.SEM micrographs of membranes supports, in making of which PVA has been used, (a) 70 kn and 1100 ºC (b) 90 kn and 1100 ºC (c) 70 kn and 1200 ºC (d) 90 kn and 1200 ºC sintering temperature effect Figures 1(a) and 1(c) show the SEM of the discs membranes supports, in making of which ethanol has been used, at moulding pressure of 70 kn and sintering temperatures of 1400 and 1500 C. As for the values of porosity and pore size of these membranes in table 1, with the increasing of sintering temperatures, porosity and pore size distribution are decreased. But, the size of the grains increases with sintering temperatures. The SEM micrograph of the discs membranes supports (figures 2(a) and 2(c)), in making of which PVA has been used, and the values of table 2 show the same results. In fact, with the increasing of sintering temperatures, alumina particles stick together, therefore the grains become greater, and porosity and pore size are reduced. (b) (c) 3.3. Binder effect As can be seen from the SEM micrographs ( figures 1 and 2), the surface of membranes, in making of which ethanol has been used, possessed further uniformity (in terms of grain) and a more regular surface than PVA binder. But, the values of tables 1 and 2 show that membranes, in making of which PVA has been used, possessed narrower pore size distribution and lower porosity. The surface uniformity of the membranes, in making of which ethanol has been used, is due to the methods of powder preparation for the pressing process. In the method of preparation by ethanol binder, initially alumina powder is dissolved in ethanol and the solution is mixed well by shaker. This operation causes the powder to be mixed well with ethanol. So the surface of the membrane would have a better quality. Of course the membranes, in making of which PVA has been used, also had a suitable surface, which can be bettered by suitable milling. Since PVA is a binder of good quality and has a suitable softening feature, the membranes, in making of which this binder has been used, have a denser structure with a smaller pore size, narrower pore size distribution and lower porosity in comparison with the membranes, in making of which ethanol has been used. However, according to the suitable softening feature of these alcohols, the discs supports did not have any cracks. 329

4 Table 1. characteristics of discs membranes supports related to ethanol binder Moulding pressure (kn) Sintering temperature (ºC) Heating rate (ºC/min)/hour Porosity (%) Pore size distribution (µm) Average pore size (µm) / / / / Table 2. characteristics of discs membranes supports related to PVA binder Moulding pressure (kn) Sintering temperature (ºC) Heating rate (ºC/min)/hour Porosity (%) Pore size distribution (µm) Average pore size (µm) / / / / Conclusion Alumina powder is a non-plastic substance. In order to make discs membranes supports out of this powder, there is required additives such as binder. In this study, the two binders of ethanol and PVA were used, and also different sintering temperatures and moulding pressures were used for converting powder into membranes supports. The results showed that the increase in moulding pressure and sintering temperature leads to a decrease in porosity and pore size and an increase in grain size (both for ethanol and PVA). Also was shown that using ethanol prepares membranes with better surface properties in comparison with PVA. The amounts of porosity and pore size distribution for the membranes, in making of which ethanol has been used, were within the range of 36-46% and 2-13 µm, respectively. Also these parameters for the membranes, in making of which PVA has been used, were 34-42% and 1-8 µm, respectively. These amounts show that the pore size distribution is narrower for membranes, in making of which PVA has been used, in comparison with the membranes, in making of which ethanol has been used, which is an important factor in supports membranes. Also it came up that membranes, in making of which PVA has been used, had a lower porosity, which was due to the high adhesion of PVA. These amounts make the prepared discs membranes supports for microfiltration applications. Acknowledgement The authors are indebted to Kamal Abbasi for helping a lot during preparing this article, and also thanks goes to Ali Tahari and Ali Kalateh for fascillating the laboratory conditions. REFERENCES [1] K. Nath, Membranes Separation Processes, PHI Learning Pvt. Ltd. New Delhi, pp , [2] N. N. Li, A. G. Fane, W. S. Winston Ho, T. Matsuura, Advanced Membrane Technology and Applications, John Wiley & Sons, Inc. New Jersey, pp ,

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