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Effects of sintering on properties of alumina microfiltration membranes
Journal of Membrane Science 155 (1999) 309±314
Short communication
Effects of sintering on properties of alumina micro®ltration membranes Pei Wanga, Pei Huanga, Nanping Xua,*, Jun Shia, Y.S. Linb a
Membrane Science and Technology Research Center, Nanjing University of Chemical Technology, no. 5 Xin Mofan Road, Nanjing 210009, China b Department of Chemical Engineering, University of Cincinnati, OH 45221, USA Received 19 January 1998; received in revised form 12 June 1998; accepted 11 September 1998
Abstract The transformation of membrane channels during the sintering process conforms to the Rhines' topological decay model of intermediate sintering stage. The pore size of the membranes enlarges with the increase of sintering temperature. The pore size increment caused by the increase of sintering temperature is more obvious for thin membranes than for thick membranes. With the increase of sintering temperature, the water permeance of membranes increases at ®rst and then decreases after a turning point of sintering temperature. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Ceramic membranes; Sintering; Micro®ltration; Membrane preparation and structure
1. Introduction Sintering is a key step in the preparation of alumina micro®ltration (MF) membranes. Some material researchers have studied the effect of sintering on the pore size of the alumina compacts. Hillman et al. [1] found that the mean pore size of compacts increased with the increase of sintering temperature; Page et al. [2] discovered that the pore size remained constant during the sintering process; Fang et al. [3] pointed out that the transition of pore size during the sintering process depended on the porosity of the green compacts. Lin et al. [4] found that the pore size of the g-alumina membrane increased sharply at a temperature around 9008C. Leavanen et al. [5] found *Corresponding author.
that both the pore size and water permeance increase with the increase of sintering temperature in the range 1100±13008C. This work reports an extensive research work on the in¯uence of sintering on the changes of pore size distribution (PSD) and water permeance of alumina MF membranes. Such a study will also help to identify the optimum sintering conditions of the membrane preparation. 2. Experimental Two kinds of alpha alumina powder with mean particle size of 0.5 and 2.8 mm (Aloca, USA) were used. The typical chemical analysis and the particle size distribution were shown in Tables 1 and 2, respec-
0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00297-X
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P. Wang et al. / Journal of Membrane Science 155 (1999) 309±314
Table 1 Typical chemical analysis of the powder used in this paper (wt%) Diameter (mm)
Al2O3
Na2O
SiO2
Fe2O3
CaO
B2O3
MgO
0.5 2.8
99.8 99.9
0.07 0.05
0.03 0.01
0.02 0.02
0.02 0.04
0.0001 0.02
0.04
Table 2 Particle size distributions of the powder used in this paper Diameter (mm)
D90 (mm)
D50 (mm)
D10 (mm)
0.5 2.8
1.7 5.1
0.5 2.8
0.2 1.1
tively. Their purity was both more than 99%. Porous alpha alumina tubes were used as the supports of the membrane, the inner diameter of the tubes was 8 mm, the thickness was 2 mm, and the length was 100 mm. The porosity of the tubes is 41%, and the mean pore size is 8 mm by mercury intrusion porosimetry (MIP). Other raw materials included were deionized water, binder and dispersant. The supported membranes were prepared by the following steps: the alpha alumina powders, dispersant and binder were added to the deionized water in a certain order with rapid stir for 1 h to get stable suspension. The membrane layer was formed on the inner surface of the dried tubes by a slip-casting method in a home made apparatus. The membranes were dried in air at ®rst, and were further dried and sintered by a program controlled SX2 Box Electric Furnace (Wuxi Universal Furnace Engineering, China) at 38C/min to the required temperature. Two-layer composite membranes comprised the support and a layer of membrane, the mean diameter of the particles used for preparing the membrane layer is 2.8 mm. The two-layer composite membrane tubes were used as the supports of three-layer composite membranes with top layers formed by alumina particles of 0.5 mm mean diameter. Nonsupported membranes were prepared by almost the same process using plaster support (separated from the membranes before sintering). The PSD of the supported membranes was measured by the gas bubble pressure (GBP) method according to ASTM F316-86 [6] on a home-made apparatus. MIP was adopted to determine the PSD of
the nonsupported membranes using an autoscan porosimeter (Quanta Chrome, USA). The microstructure of the membranes was observed by a JSM-6300 scanning electronic microscope (JEOL, Japan). 3. Results and discussion 3.1. Pore size distribution The PSD (by MIP) of the nonsupported membranes sintered at different temperatures is shown in Fig. 1. It can be observed that the PSD remains essentially the same despite the different sintering temperatures. This can be explained by the Rhines' topological decay model for intermediate sintering stage [7]. By this model, the porosity of the porous material is composed of many nodes and channels, which form an interconnected pore net work. During sintering process, the pore net work decays in a stable manner, and the ratio of pore volume fraction to pore surface area remains constant from different sintering temperatures. Since the channel radius is proportional to this ratio, it follows that the channel radius must remain constant also. Densi®cation, thus, proceeds by a reduction in the total length and number of channels without any reduction in their diameter, and the number of the nodes will also decrease. Similar observations of constant channel radius have been obtained for alpha alumina compacts with both MIP by Occhinonero et al. [8] and small angle scattering techniques by Page et al. [2]. DeHoff et al. [9] reported the same observations for copper powder compacts. The PSD (by GBP method) results of the supported membranes sintered at different temperatures are shown in Fig. 2. It can be observed that the mean pore size of the membranes enlarges with the increase of sintering temperature, and the PSD broadens at the same time. Fig. 3 shows the relationship between the mean pore size and sintering temperature of
P. Wang et al. / Journal of Membrane Science 155 (1999) 309±314
311
Fig. 1. PSD of nonsupported membranes sintered at different temperatures (by MIP): (a) mean particle size of powder 0.5 mm: (&) 1373 K, (*) 1473 K, (~) 1573 K, and (b) mean particle size of powder 2.8 mm: (&) 1373 K, (*) 1473 K, (~) 1573 K.
Fig. 2. PSD of supported membranes sintered at different temperatures (by GBP method): (a) three layer membranes (top layer thickness 10 mm): (~) 1573 K, (&) 1373 K, (*) 1273 K, and (b) two layer membranes (top layer thickness 45 mm): (&) 1723 K, (}) 1673 K, (~) 1623 K, (*) 1273 K.
membranes with different thickness. It can be found that the pore size increment of the thick membrane is much smaller than that of the thin membranes. It is also found that the pore size of thick membranes is much smaller than that of the thin membranes. There is obvious difference between the results of the supported and nonsupported membranes. In order
to explain the above phenomena, the membrane thickness should be considered. Wang et al. [10] found that the membrane thickness has important in¯uence on the membrane PSD measured by GBP method. With the increase of membrane thickness the mean pore size of the membranes decreases and the PSD becomes narrower. Also the decrease in speed of the membrane
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P. Wang et al. / Journal of Membrane Science 155 (1999) 309±314
branes, since the tested sample particle size is in the range of millimeter, much larger than the thickness of the supported membranes. Fig. 4 is the surface SEM photos of membranes sintered at different temperatures. It can be observed that the small particles disappear and the large particles get connected with each other with increasing sintering temperature. The average size of the particles (particle number average) sintered at 1273, 1673 and 1773 K is about 0.52, 0.82 and 1.44 mm, respectively. The number of pores in the membrane surface decreases sharply with the increase of sintering temperature, but there is no obvious increase of the pore size. These observations conform to the topological decay model of sintering. 3.2. Permeance Fig. 3. Mean pore size vs. sintering temperature of the supported two layer membranes with different thickness: (~) 20 mm, (&) 10 mm, and (*) 5 mm.
pore size will become slow with the increase of membrane thickness. By virtue of these conclusions, the membrane thickness should be ®xed, when the effects of sintering are studied. A pore in the membrane is composed of many connected channels which form a run-through passage in the membrane. According to the topological decay model of sintering, the actual radii of the channels maintain essentially constant during the sintering process, but the total length and number of the channels decrease. Though the membrane thickness keeps constant during the sintering process, the number of channels in one pore and their total length (pore length) is reduced by sintering. Thus increasing sintering temperature has the same effects as reducing the thickness of the membrane. By the discoveries of Wang et al. [10], for the membranes with the same thickness the decrease of pore length will enlarge the membrane pore size and broaden the PSD during the sintering process. For the membranes with different thickness, the pore size increment caused by sintering is relatively small for the thick membrane, since the increase in speed of the pore size caused by the shortening of pore length will become slow with the thickening of membrane. The PSD measured by MIP is somewhat like that of the very thick mem-
Fig. 5 shows water permeance of membranes sintered at different temperatures (for 1 h). It can be found that water permeance increases at ®rst and decreases after a turning point of temperature with increasing sintering temperature. This result is different from that of Levanen et al. [5]. According to Poiseuille equation, the water ¯ux of a single pore in the membrane can be shown as follows: F
Pr 4 ; 8L
(1)
where r is the mean pore radius, P the transmembrane pressure, L the pore length, and m is the viscosity of the water. The number of the pores in the membrane is not included in Eq. (1), which is determined by connectivity and porosity of the membrane. Thus, the permeance is actually determined by the pore size, pore length and pore number; among the three factors pore size seems more effective according to Eq. (1). The pore size of the membranes enlarges with the increase of sintering temperature, and the pore number and pore length reduce due to the decrease of porosity and the connectivity. According to the results of Fig. 5, there exists a turning point of sintering temperature (more exactly, porosity) for a certain kind of membranes. Under this temperature the water permeance ascends with increasing sintering temperature because the increment of pore size and the decrease of pore length result in a sharp reduction in the resistance
P. Wang et al. / Journal of Membrane Science 155 (1999) 309±314
313
Fig. 4. Surface SEM photos of supported membranes with different sintering temperatures: (a) 1273 K, (b) 1673 K, and (c) 1773 K.
of ¯ow. Above this temperature the water permeance descends drastically with increasing sintering temperature, since the effect of pore number decrease surpasses the effects of the two former factors. Since the decrease curve of porosity vs. sintering temperature of membranes is relatively smooth, it can be concluded that the pore connectivity reduction makes more contribution to the decrease of the pore number than porosity decrease. 4. Conclusions The transformation of membrane channels during the sintering process conforms to the Rhines' topological decay model of intermediate sintering stage. With the increase of sintering temperature the actual
channel radii remain constant while the number and total length of the channels decrease. The mean pore size of the membranes enlarges with the increase of sintering temperature because of the decrease of pore length. The pore size increment caused by the increase of sintering temperature is more obvious for the thin membranes than that of the thick membranes. The water permeance of alumina MF membranes is determined by the joint effects of pore size, pore length and pore number. With increasing sintering temperature the water permeance ascends at ®rst and then descends after a turning point of sintering temperature. Acknowledgements This work was supported by the Commission of Science and Technology of China and the Ministry of
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Fig. 5. Water permeance and mean pore size vs. sintering temperature of the supported membranes: (a) three layer membranes (top layer thickness 10 mm): (~) permeance, (*) mean pore size, and (b) two layer membranes (top layer thickness 45 mm): (~) permeance, (*) mean pore size,
Chemical Industry of China. YSL acknowledges the support of US NSF for the collaboration. References [1] S.H. Hillman, R.M. German, Constant heating rate analysis of simultaneous sintering mechanics in alumina, J. Mater. Sci. 27 (1992) 2641. [2] R.A. Page, Y.M. Pan, Microstructure evolution during sintering, Mater. Res. Soc. Symp. Proc. 249 (1992) 449. [3] T.T. Fang, H. Palmour III, , Useful extensions of the statistical theory of sintering, Ceram. Int. 15 (1989) 329. [4] Y.S. Lin, K.J. de Vries, A.J. Burggraaf, Thermal stability and its improvement of the alumina membrane toplayers prepared by sol±gel methods, J. Mater. Sci. 26 (1991) 715.
[5] E. Levanen, M. Kolari, T. Mantyla, Preparation of sprayed alumina microfiltration membranes, The symposium of the ICIM'3, Worcestor, USA, 1994, pp. 549±552. [6] ASTM F316-86, Standard test method for pore size characteristics of membrane filters by bubble point and mean flow pore test, ASTM Committee on Standards, USA. [7] F.N. Rhines, R.T. Dehoff, Channel network decay in sintering, Mater. Sci. Res. 16 (1984) 49. [8] M.A. Occhionero, J.W. Halloran, The influence of green density upon sintering, Mater. Sci. Res. 16 (1984) 89. [9] R.T. DeHoff, R.A. Rummel, H.P. Labuff, F.N. Rhines, in: Morden Developments in Powder Metallurgy, vol. 1, H.H. Hausner Press, New York, 1966, 310 pp. [10] Pei Wang, Nanping Xu, Jun shi, Pore Size Control of Alumina Microfiltration Membranes, J. Chem. Eng. Chinese Universities (Chinese), in press.
Short communication
Effects of sintering on properties of alumina micro®ltration membranes Pei Wanga, Pei Huanga, Nanping Xua,*, Jun Shia, Y.S. Linb a
Membrane Science and Technology Research Center, Nanjing University of Chemical Technology, no. 5 Xin Mofan Road, Nanjing 210009, China b Department of Chemical Engineering, University of Cincinnati, OH 45221, USA Received 19 January 1998; received in revised form 12 June 1998; accepted 11 September 1998
Abstract The transformation of membrane channels during the sintering process conforms to the Rhines' topological decay model of intermediate sintering stage. The pore size of the membranes enlarges with the increase of sintering temperature. The pore size increment caused by the increase of sintering temperature is more obvious for thin membranes than for thick membranes. With the increase of sintering temperature, the water permeance of membranes increases at ®rst and then decreases after a turning point of sintering temperature. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Ceramic membranes; Sintering; Micro®ltration; Membrane preparation and structure
1. Introduction Sintering is a key step in the preparation of alumina micro®ltration (MF) membranes. Some material researchers have studied the effect of sintering on the pore size of the alumina compacts. Hillman et al. [1] found that the mean pore size of compacts increased with the increase of sintering temperature; Page et al. [2] discovered that the pore size remained constant during the sintering process; Fang et al. [3] pointed out that the transition of pore size during the sintering process depended on the porosity of the green compacts. Lin et al. [4] found that the pore size of the g-alumina membrane increased sharply at a temperature around 9008C. Leavanen et al. [5] found *Corresponding author.
that both the pore size and water permeance increase with the increase of sintering temperature in the range 1100±13008C. This work reports an extensive research work on the in¯uence of sintering on the changes of pore size distribution (PSD) and water permeance of alumina MF membranes. Such a study will also help to identify the optimum sintering conditions of the membrane preparation. 2. Experimental Two kinds of alpha alumina powder with mean particle size of 0.5 and 2.8 mm (Aloca, USA) were used. The typical chemical analysis and the particle size distribution were shown in Tables 1 and 2, respec-
0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00297-X
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P. Wang et al. / Journal of Membrane Science 155 (1999) 309±314
Table 1 Typical chemical analysis of the powder used in this paper (wt%) Diameter (mm)
Al2O3
Na2O
SiO2
Fe2O3
CaO
B2O3
MgO
0.5 2.8
99.8 99.9
0.07 0.05
0.03 0.01
0.02 0.02
0.02 0.04
0.0001 0.02
0.04
Table 2 Particle size distributions of the powder used in this paper Diameter (mm)
D90 (mm)
D50 (mm)
D10 (mm)
0.5 2.8
1.7 5.1
0.5 2.8
0.2 1.1
tively. Their purity was both more than 99%. Porous alpha alumina tubes were used as the supports of the membrane, the inner diameter of the tubes was 8 mm, the thickness was 2 mm, and the length was 100 mm. The porosity of the tubes is 41%, and the mean pore size is 8 mm by mercury intrusion porosimetry (MIP). Other raw materials included were deionized water, binder and dispersant. The supported membranes were prepared by the following steps: the alpha alumina powders, dispersant and binder were added to the deionized water in a certain order with rapid stir for 1 h to get stable suspension. The membrane layer was formed on the inner surface of the dried tubes by a slip-casting method in a home made apparatus. The membranes were dried in air at ®rst, and were further dried and sintered by a program controlled SX2 Box Electric Furnace (Wuxi Universal Furnace Engineering, China) at 38C/min to the required temperature. Two-layer composite membranes comprised the support and a layer of membrane, the mean diameter of the particles used for preparing the membrane layer is 2.8 mm. The two-layer composite membrane tubes were used as the supports of three-layer composite membranes with top layers formed by alumina particles of 0.5 mm mean diameter. Nonsupported membranes were prepared by almost the same process using plaster support (separated from the membranes before sintering). The PSD of the supported membranes was measured by the gas bubble pressure (GBP) method according to ASTM F316-86 [6] on a home-made apparatus. MIP was adopted to determine the PSD of
the nonsupported membranes using an autoscan porosimeter (Quanta Chrome, USA). The microstructure of the membranes was observed by a JSM-6300 scanning electronic microscope (JEOL, Japan). 3. Results and discussion 3.1. Pore size distribution The PSD (by MIP) of the nonsupported membranes sintered at different temperatures is shown in Fig. 1. It can be observed that the PSD remains essentially the same despite the different sintering temperatures. This can be explained by the Rhines' topological decay model for intermediate sintering stage [7]. By this model, the porosity of the porous material is composed of many nodes and channels, which form an interconnected pore net work. During sintering process, the pore net work decays in a stable manner, and the ratio of pore volume fraction to pore surface area remains constant from different sintering temperatures. Since the channel radius is proportional to this ratio, it follows that the channel radius must remain constant also. Densi®cation, thus, proceeds by a reduction in the total length and number of channels without any reduction in their diameter, and the number of the nodes will also decrease. Similar observations of constant channel radius have been obtained for alpha alumina compacts with both MIP by Occhinonero et al. [8] and small angle scattering techniques by Page et al. [2]. DeHoff et al. [9] reported the same observations for copper powder compacts. The PSD (by GBP method) results of the supported membranes sintered at different temperatures are shown in Fig. 2. It can be observed that the mean pore size of the membranes enlarges with the increase of sintering temperature, and the PSD broadens at the same time. Fig. 3 shows the relationship between the mean pore size and sintering temperature of
P. Wang et al. / Journal of Membrane Science 155 (1999) 309±314
311
Fig. 1. PSD of nonsupported membranes sintered at different temperatures (by MIP): (a) mean particle size of powder 0.5 mm: (&) 1373 K, (*) 1473 K, (~) 1573 K, and (b) mean particle size of powder 2.8 mm: (&) 1373 K, (*) 1473 K, (~) 1573 K.
Fig. 2. PSD of supported membranes sintered at different temperatures (by GBP method): (a) three layer membranes (top layer thickness 10 mm): (~) 1573 K, (&) 1373 K, (*) 1273 K, and (b) two layer membranes (top layer thickness 45 mm): (&) 1723 K, (}) 1673 K, (~) 1623 K, (*) 1273 K.
membranes with different thickness. It can be found that the pore size increment of the thick membrane is much smaller than that of the thin membranes. It is also found that the pore size of thick membranes is much smaller than that of the thin membranes. There is obvious difference between the results of the supported and nonsupported membranes. In order
to explain the above phenomena, the membrane thickness should be considered. Wang et al. [10] found that the membrane thickness has important in¯uence on the membrane PSD measured by GBP method. With the increase of membrane thickness the mean pore size of the membranes decreases and the PSD becomes narrower. Also the decrease in speed of the membrane
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P. Wang et al. / Journal of Membrane Science 155 (1999) 309±314
branes, since the tested sample particle size is in the range of millimeter, much larger than the thickness of the supported membranes. Fig. 4 is the surface SEM photos of membranes sintered at different temperatures. It can be observed that the small particles disappear and the large particles get connected with each other with increasing sintering temperature. The average size of the particles (particle number average) sintered at 1273, 1673 and 1773 K is about 0.52, 0.82 and 1.44 mm, respectively. The number of pores in the membrane surface decreases sharply with the increase of sintering temperature, but there is no obvious increase of the pore size. These observations conform to the topological decay model of sintering. 3.2. Permeance Fig. 3. Mean pore size vs. sintering temperature of the supported two layer membranes with different thickness: (~) 20 mm, (&) 10 mm, and (*) 5 mm.
pore size will become slow with the increase of membrane thickness. By virtue of these conclusions, the membrane thickness should be ®xed, when the effects of sintering are studied. A pore in the membrane is composed of many connected channels which form a run-through passage in the membrane. According to the topological decay model of sintering, the actual radii of the channels maintain essentially constant during the sintering process, but the total length and number of the channels decrease. Though the membrane thickness keeps constant during the sintering process, the number of channels in one pore and their total length (pore length) is reduced by sintering. Thus increasing sintering temperature has the same effects as reducing the thickness of the membrane. By the discoveries of Wang et al. [10], for the membranes with the same thickness the decrease of pore length will enlarge the membrane pore size and broaden the PSD during the sintering process. For the membranes with different thickness, the pore size increment caused by sintering is relatively small for the thick membrane, since the increase in speed of the pore size caused by the shortening of pore length will become slow with the thickening of membrane. The PSD measured by MIP is somewhat like that of the very thick mem-
Fig. 5 shows water permeance of membranes sintered at different temperatures (for 1 h). It can be found that water permeance increases at ®rst and decreases after a turning point of temperature with increasing sintering temperature. This result is different from that of Levanen et al. [5]. According to Poiseuille equation, the water ¯ux of a single pore in the membrane can be shown as follows: F
Pr 4 ; 8L
(1)
where r is the mean pore radius, P the transmembrane pressure, L the pore length, and m is the viscosity of the water. The number of the pores in the membrane is not included in Eq. (1), which is determined by connectivity and porosity of the membrane. Thus, the permeance is actually determined by the pore size, pore length and pore number; among the three factors pore size seems more effective according to Eq. (1). The pore size of the membranes enlarges with the increase of sintering temperature, and the pore number and pore length reduce due to the decrease of porosity and the connectivity. According to the results of Fig. 5, there exists a turning point of sintering temperature (more exactly, porosity) for a certain kind of membranes. Under this temperature the water permeance ascends with increasing sintering temperature because the increment of pore size and the decrease of pore length result in a sharp reduction in the resistance
P. Wang et al. / Journal of Membrane Science 155 (1999) 309±314
313
Fig. 4. Surface SEM photos of supported membranes with different sintering temperatures: (a) 1273 K, (b) 1673 K, and (c) 1773 K.
of ¯ow. Above this temperature the water permeance descends drastically with increasing sintering temperature, since the effect of pore number decrease surpasses the effects of the two former factors. Since the decrease curve of porosity vs. sintering temperature of membranes is relatively smooth, it can be concluded that the pore connectivity reduction makes more contribution to the decrease of the pore number than porosity decrease. 4. Conclusions The transformation of membrane channels during the sintering process conforms to the Rhines' topological decay model of intermediate sintering stage. With the increase of sintering temperature the actual
channel radii remain constant while the number and total length of the channels decrease. The mean pore size of the membranes enlarges with the increase of sintering temperature because of the decrease of pore length. The pore size increment caused by the increase of sintering temperature is more obvious for the thin membranes than that of the thick membranes. The water permeance of alumina MF membranes is determined by the joint effects of pore size, pore length and pore number. With increasing sintering temperature the water permeance ascends at ®rst and then descends after a turning point of sintering temperature. Acknowledgements This work was supported by the Commission of Science and Technology of China and the Ministry of
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P. Wang et al. / Journal of Membrane Science 155 (1999) 309±314
Fig. 5. Water permeance and mean pore size vs. sintering temperature of the supported membranes: (a) three layer membranes (top layer thickness 10 mm): (~) permeance, (*) mean pore size, and (b) two layer membranes (top layer thickness 45 mm): (~) permeance, (*) mean pore size,
Chemical Industry of China. YSL acknowledges the support of US NSF for the collaboration. References [1] S.H. Hillman, R.M. German, Constant heating rate analysis of simultaneous sintering mechanics in alumina, J. Mater. Sci. 27 (1992) 2641. [2] R.A. Page, Y.M. Pan, Microstructure evolution during sintering, Mater. Res. Soc. Symp. Proc. 249 (1992) 449. [3] T.T. Fang, H. Palmour III, , Useful extensions of the statistical theory of sintering, Ceram. Int. 15 (1989) 329. [4] Y.S. Lin, K.J. de Vries, A.J. Burggraaf, Thermal stability and its improvement of the alumina membrane toplayers prepared by sol±gel methods, J. Mater. Sci. 26 (1991) 715.
[5] E. Levanen, M. Kolari, T. Mantyla, Preparation of sprayed alumina microfiltration membranes, The symposium of the ICIM'3, Worcestor, USA, 1994, pp. 549±552. [6] ASTM F316-86, Standard test method for pore size characteristics of membrane filters by bubble point and mean flow pore test, ASTM Committee on Standards, USA. [7] F.N. Rhines, R.T. Dehoff, Channel network decay in sintering, Mater. Sci. Res. 16 (1984) 49. [8] M.A. Occhionero, J.W. Halloran, The influence of green density upon sintering, Mater. Sci. Res. 16 (1984) 89. [9] R.T. DeHoff, R.A. Rummel, H.P. Labuff, F.N. Rhines, in: Morden Developments in Powder Metallurgy, vol. 1, H.H. Hausner Press, New York, 1966, 310 pp. [10] Pei Wang, Nanping Xu, Jun shi, Pore Size Control of Alumina Microfiltration Membranes, J. Chem. Eng. Chinese Universities (Chinese), in press.