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Preparation of a tubular anodic aluminum oxide membrane
j o u r n a l of MEMBRANE SCIENCE ELSEVIER
Journal of Membrane Science 117 (1996) 189-196
Preparation of a tubular anodic aluminum oxide membrane N. Itoh a,*, K. Kato b, T. Tsuji b, M. Hongo b a National Institute of Materials and Chemical Research, Tsukuba 305, Japan b Department oflndustrial Chemistry, University ofNihon, Narashino 275, Japan Received 14 November 1995; revised 7 February 1996; accepted 12 February 1996
Abstract An anodic aluminum oxide tube with perforated straight pores to use as a porous membrane or as a support for a composite membrane was prepared. An aluminum oxide layer with straight micropores closed by an oxide barrier layer was formed on the surface of an aluminum tube, 45 mm long, 0.5 mm thick and 6 mm in outer diameter, by anodic oxidation in an aqueous solution of oxalic acid. The pores were opened by dissolution of the inner aluminum and the subsequent barrier layer of aluminum oxide. The tubular alumina membrane obtained was 35-40 ~zm thick with straight micropores of 20-50 nm, and therefore showed Knudsen permselectivity for inorganic gases. It was found that the tube could withstand at least up to 4.4 atm of transmembrane pressure. Keywords: Gas separations; Membrane preparation and structure; Microporous and porous membranes; Anodic alumina; Tubular membrane
1. Introduction Since being advantageous in terms of obtaining higher membrane area per unit volume and easier installation into a membrane module container, a tubular or a hollow-fiber type of membrane is employed in most commercial polymeric membrane separation units. This would be also true in the case of ceramic membranes. However, one of the difficulties in practical use of a ceramic membrane, especially at higher temperatures, is how to house it in a membrane container, usually made of metal, where joining ceramics and metal is not easy. In this sense,
* Corresponding author. Fax: +81-298-56-8587, E-mail: nitoh@ nimc.go.jp.
a porous alumina membrane produced by means of electrochemical anodic oxidation is thought to be very attractive because it can be obtained in a form adherent to metallic aluminum, which can be used as sealing material as it is. Such a structure is also very useful as a support, on which a thin permselective layer may be formed by a s o l - g e l process [1-3], zeolite formation by hydrothermal synthesis [4-6], palladium and its alloy deposition by means of electroless- or electro-plating [7-9], etc. become possible. Composite membranes, possessing both high permeability and high selectivity, are applicable to not only membrane separators but also to membrane reactors (see the special issue of this journal [10]). Preparation of a flat type anodic aluminum oxide membrane has been tried by Smith [11] and established by Itaya et al. [12]. Their effort has triggered
0376-7388/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0376-73 8 8 ( 9 6 ) 0 0 0 6 3 - 4
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N. ltoh et al. / Journal of Membrane Science 117 (1996) 189-196
off further detailed investigations [13-17], in the background of which was a growing interest in membrane technology. Still, such a flat type has some disadvantages as mentioned in the beginning. Instead, if a tubular type of the membrane can be presented, a potential practical application will increase. No attempt, however, has been made to make such a tubular type of membrane. The aim of this study, therefore, is to find a method to prepare an anodic aluminum oxide tube with perforated straight pores, which can be used as a porous membrane or as a support for a composite membrane. This type of tubular membrane is expected to be of great use since aluminum is left on the both sides of the tube without being oxidized; i.e. the structure becomes aluminum-alumina membrane-aluminum, so that metallic aluminum can be used to easily connect to a membrane container.
2. Preparation method A tubular anodic aluminum oxide membrane was prepared by the following four procedures.
2.1. Pretreatment of the aluminum tube The aluminum tube used was 45 m m long, 0.5 mm thick and 6 m m in outer diameter. First, the tube was ultrasonically cleaned in methanol for 20 min, and was annealed in air for 2 h at 300°C in an electric furnace. A chemical polish was made with a solution of H3PO4 (85 w t % ) - H N O 3 (61 w t % ) - H 2 0 (8:1:1 by volume) for 3 min at 80-90°C. Subsequently, an electrolytic polish was made at a constant voltage of 20 V with a solution of H3PO 4 (85 w t % ) - H 2 S O 4 (98 w t % ) - H 2 0 (7:2:1 by volume), which contained 45 g/1 CrO3, for 3 rain at 80-90°C.
2.2. Anodic oxidation The central 20 m m of the tube (L a) was oxidized as an anode in an aqueous solution of 4 wt% oxalic acid at 17°C using an experimental set-up specially designed for anodizing a specimen in the tubular form (Fig. 1). Simultaneously, a change in the anodic current with time was recorded by a personal computer-aided data acquisition system at every 10
~1~°ut Coolingwaterin>
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~
Data
~
system
Siliconetube
Anodizinalaver.I PtJTinet ~ (Cathode)
~~-I~--Silicone ~ r e r
rubberstopper
Fig. 1. Schematicof the experimentalset-upfor anodicoxidation of tubularaluminum. s. The effects of the anodizing voltage in the range 40 to 60 V and oxidation time were examined.
2.3. Dissolution of the inner aluminum The central 10 m m of the inner aluminum that was not oxidized (L S) was dissolved with an aqueous solution containing 20 wt% HC1 and 0.1 mol/1 CuC12 at a room temperature, which was introduced into the inner side of the tube in the manner shown in Fig. 2. L s must be less than L a in order to support
Dissolvingagent foraluminumor barrierlaver
Ir Aluminum tube / ~ ~ Anodizedlayer I1!~ Siliconeplug o ~ Distilledwater o \ Stirrer
~La
Fig. 2. A methodto dissolvethe inneraluminumandbarrierlayer.
191
N. ltoh et aL / Journal of Membrane Science 117 (1996) 189-196
the anodized alumina and achieve a larger adherence between the alumina and the aluminum tubes. 2.4. Dissolution o f the inner alumina barrier
Subsequently, in the same manner as above, the thin barrier layer of aluminum oxide, formed at the bottom of the pores, was dissolved with an aqueous solution of H3PO 4 at different temperatures and time.
permeation cell with a union connector. The permeation rate was measured with varying transmembrane pressure, regulated with a pressure regulator and a needle valve, while the permeate side was kept at atmospheric pressure. The temperature dependence of the permeability was also examined. Furthermore, the breaking pressure was examined to assess the durability of the membrane tube against external pressure.
4. Results and discussion 3. Measurement of morphological parameters and gas permeability The diameter and density of the pores of the anodic oxide tube prepared were determined by scanning electron microscopy (SEM), which was a very helpful way to determine the morphology of the porous membrane. The thickness was measured with a micrometer (Digital gauge, model D-10SS, Ozaki MFG CO.). Gas permeabilities of inorganic gases (H 2, He, N2, 0 2, Ar and CO 2) were measured with the apparatus shown in Fig. 3(a). The details of the permeation cell are shown in Fig. 3(b), where the membrane tube joined with metallic aluminum tubes on both sides could be easily plugged with a cap on one side and then connected on the other side to the
Pressure Pressure ~ g e ~ Regulator W.) ~ ~L, ~
l
U
4.1. Preparation o f the tubular anodic aluminum oxide membrane
In Fig. 4, the process of preparing a tubular membrane is photographed in sequence: (a) after polishing, (b) after anodic oxidation and (c) after perforation. Anodic oxidation is considered to be the key process for the successful preparation because the pores are formed in this process. The magnitude of the voltage applied during anodic oxidation, an important factor affecting the oxide structure, was found to be especially sensitive; that is, a higher voltage was apt to cause longitudinal cracks. On the other hand, the oxidation time is closely related to the thickness of the membrane, and is generally propor-
Flow_Meter H
t
Out
In • ~Membrane I1~~
Tube
GasCylinder (a) Schematicdiagram
(b) Membranecell
Fig. 3. Schematicflow diagramof the gas permeationmeasurement(a) and the detailed permeationcell (b).
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N. ltoh et al. / Journal of Membrane Science 117 (1996) 189-196
|_
Fig. 4. Outside appearance of the tube with progress of the preparation step: (a) after electrolytic polish, (b) after anodic oxidation, and (c) after dissolution of the inner aluminum and barrier layer.
tional to the time. From the standpoint of mechanical strength, a thickness of about 3 0 - 5 0 p~m was appropriate under the conditions investigated. The dissolution of the aluminum on the inner tube was continued until the objective part became visually transparent. Usually, it took about 30 min. The dissolution of the alumina barrier was the last stage before obtaining an alumina tube with perforated pores. Since the solvent can chemically dissolve not only the barrier but also the wall of pore, the dissolving time and temperature must be controlled. When the time was too short or the temperature was too low, only a part of the barrier was dissolved as demonstrated in Fig. 5. On the other hand, too long a time or too high a temperature
Fig. 5. Partly dissolved barrier layer.
resulted in enlarging the diameter of pore. Thus, this can be utilized to regulate the pore diameter. Experimentally, it became clear that a temperature higher than 17°C and a time of more than 120 min were required when a 4 wt% H3PO 4 aqueous solution was used for the dissolution. Thus, the following conditions were employed: 1. Anodic oxidation: 40 V, 180 min. 2. Dissolution of the inner aluminum: room temperature, 30 min. 3. Dissolution of the barrier: 17°C, > 60 min with 6 wt% H3PO 4.
Fig. 6. SEM images of the perforated pores with varying anodizing voltage: (a) 40 V, (b) 50 V, and (c) 60 V.
193
N. hoh et al. / Journal of Membrane Science 117 (1996) 189-196
30 . ]
4.2. Determination of pore density and thickness The pore density of the alumina membrane prepared was determined based on SEM observation. Fig. 6 shows SEM images of the pores taken from the inner side when the anodizing voltage was changed. It is clearly seen that the pore density decreases with an increase in the voltage whereas the pore diameter increases. Using the number of pores counted in the SEM image, the pore density, N / m 2, was determined. The pore density is plotted against voltage in Fig. 7, where the results for a fiat-type membrane estimated by means of small angle X-ray scattering by Itaya et al. [12] is also presented for comparison. The thickness can be determined by knowing the total current consumed during the anodic oxidation. If this method is applicable, it would be very convenient because it is not necessary to break the tubular specimen into small pieces for direct measurement with a micrometer, which is the case with a flat membrane. Fig. 8 shows an example of the current change with time. When the current supplied, I (A or C / s ) is consumed by the formation of alumina (A1203) and the dissolution of aluminum, the total
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.....................................................
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.
.
.
.
.
.
.
.
small-angle X-ray scattering technique 4 Xl013
35
40
45
i
,
i
50
55
60
65
Anodizing Voltage [ V ] Fig. 7. Relation between pore density and anodizing voltage.
.
.
.
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. . . . . . . . . . . .
2O
..........................
. 300 T°tal Char e / 226~ . . . . . . . . . . . .
• ....j2,o
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__
,,08
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o
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i
i
r
50
100
150
i
200
Anodizing time [ min ] Fig. 8. Current change during anodic oxidation.
volume of the porous alumina layer, v (cm3), can be calculated from the following equation.
8Md fo,Zdt v = ZF-----
(])
where 8 is a correction factor defined as the ratio of current due to the oxide layer formation to the total current including that due to dissolution of A1 into the solution, M is the molecular weight of A1203, Z is the valence, F is the Faraday constant, and p is the density of alumina. In this study, 8 and p are given as 0.6925 and 2.78 g / c m 3, respectively, where such a lower density compared with that of yalumina, 3.5-3.9 g / c m 3, was because the alumina layer formed was amorphous and additionally contains some anions originating from the anodizing solution [18]. Therefore, the thickness, t m (mm), is calculated by solving the following arithmetic equation.
v = rr(r2o - r?)L 6 X l 0 la ................................................................................................. Itaya et.al "
.
i
= 7r(r o - r i ) ( r o + ri)L = Trtm(Zro - tm)L
(2)
where ro and r i are the outer and inner radius of the membrane tube, and L is the length of the tube. In this equation, since u, r o and L are known, t m c a n be estimated. In the case of the example taken in Fig. 8, a graphical integration of the current was 226 C, which corresponds to a 9.92 mm 3 oxide layer.
194
N. ltoh et al. / Journal of Membrane Science 117 (1996) 189-196
45
~6o'c
18x10 ql
zot
Box: , 49"c
o 14x10 -11
~
E
~
-6 E
40
He
10x10-11
55 E
F-
6x10.11 ....................
Q-
35 35
40 Thickness (by micrometer) [ N.m ]
2x10 ql 0.045
45
................................................
I
i
0.05
0.055
N 2 .......
0.06
1/qT [ 1/VK]
Fig. 9. Comparison of thicknesses between those directly measured and the estimated values.
Fig. 11. Plots of gas permeabilities against 1/~/T.
4.3. Gas permeability
of the square root of the molecular weight in Fig. 10, which passes through the origin. In Fig. 11, temperature dependency of the permeabilities is shown. The permeabilities of all gases tested increase with an increase of 1/~/T. From such results, it is clear that the gas permeation through the tubular alumina membrane obeys Kunudsen flow. This can be applied as a method of estimating the pore diameter. The permeation rate of gas through straight pores, Q ( m o l / s ) , is formulated as follows.
Measured gas permeabilities of H 2, He, N 2, 02, Ar and CO 2 at 40°C are plotted against the inverse
Q = -~wNdpV
In Fig. 9, a comparison of the thicknesses thus calculated with those directly measured is made. The accord between them seems satisfactory, and therefore the cumulative current method is useful, especially with a tubular anodic oxide membrane because it is thus possible to determine the thickness without breaking the tube.
I
1
3/
2
Ap
wMRT
tm
(3)
where dp is the pore diameter (m), R is the gas constant (m 3 P a / m o l K), T is the absolute tempera-
i
16 X 10"11
a.t~
.....313KT..............................................;~.......~....H ....2.. ...............
,,.p E
100
--E'~..-!..~....!.".. ......
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o
................................ ...............
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.............
................
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< 10
15
.....'~............. ....
._~
_ /P~"'o=A:' ~ o
................. ; ...................
20
1/,JM [ 1H(kg/mol)]
Fig. l 0. Plots of gas permeabilities against 1A/M.
25
)4 Time~required to dissolve barrier Layer i i i 50 100 150 200 Dissolving Time
250
[ min ]
Fig. 12. Relationships between pore diameter and dissolving time of the barrier layer at 17 and 25°C.
N. Itoh et al. / Journal of Membrane Science 117 (1996) 189-196
ture (K), and A p is the transmembrane pressure (Pa). In Eq. (3), since N and t m are already determined, dp c a n be obtained. In Fig. 12, t h e dp values thus calculated are correlated against the dissolving time of the barrier layer. This plot shows that the pore wall itself is also dissolved with the dissolving solution of barrier layer and therefore the diameter increases with time. The reason why a logarithmic plot is made is based on a simple mathematical consideration as follows. Assuming that the dissolution reaction of the alumina wall of a cylindrical pore is the rate-determining step, the following equation can be derived.
where d o is the initial pore diameter, k ( m 3 / ( m o l min)) is the rate constant of dissolution, C a ( m o l / m 3) is the concentration of the dissolving agent, and t (min) is the dissolving time. C a may be assumed to be constant since the dissolving agent flows continuously and the dissolving rate is comparatively slow. In Fig. 12, a larger slope at 25°C means that the dissolving rate becomes larger as the temperature rises and that a higher temperature of the dissolution process results in a larger pore size in a shorter time. Fig. 13 represents that a higher concentration of dissolving agent and, as expected from Eq. (4), results in a higher dissolving rate. The pressure durability of the membrane tube was 1 O0
iiiiiiiiiiiiiiiiiiiiill......................i........................i............................................ ..........17°C
........... i........................i...............................................
E r-
E "O
......................t.....................~
O
[4wt% H3PO4
10
0
50
100
150
Dissolving Time
200
250
[ rain ]
Fig. 13. Relationships between pore diameter and dissolving time of the barrier layer when 4 and 6 wt% aqueous solutions of H3PO4 were used.
195
examined with increasing applied pressure using argon while the permeate side (inner side of the tube) was kept at atmospheric pressure. As a result, the permeability could be measured up to 4.4 atm of absolute pressure without being broken. A breaking strength, ~rb ( k g f / c m 2) working on the tube surface at the breaking pressure, Pb (kgf/cm2), is given as follows [19]. ~b = r o P b / t m
(5)
In this case, r o = 3 mm, t m = 34 p~m and P b = 4.4 k g f / c m z, so that o"b is 388 k g f / c m 2. As is easily understood from Eq. (5), P b is reversibly proportional to r o, so that further efforts to prepare a finer tube should be necessary.
5. Conclusion
A tubular anodic aluminum oxide membrane with straight pores was successfully prepared. Different from a flat type of membrane, already established, the preparation of a tubular type was accompanied with some difficulties; that is, unclear condition of the anodizing voltage, how to dissolve the inner aluminum and barrier layer and so on. As a result, it was found that a comparatively lower voltage is preferable for the successful preparation of a tubular membrane. The two dissolution steps could be made easier by devising flow lines of the dissolving solutions. According to the morphological study, the tubular membranes prepared in this study were found to have a thickness of 3 5 - 4 5 txm, a pore diameter of 2 0 - 5 0 nm and 6 × 1013-11 )< 1013 straight pores per square meter. The mechanism of gas permeation through the porous membrane could be explained by Knudsen flow. Further, the membrane tube was able to tolerate about 4.4 k g f / c m 2 external pressure; this corresponded to a breaking strength of 388 k g f / c m 2.
References
[1] A.F. Leenaars and A.J. Burggraaf, The preparation and characterization of alumina membranes with ultrafine pores. Part 2. The formation of supported membranes, J. Colloid Interface Sci., 105 (1985) 27.
196
N. ltoh et al. / Journal of Membrane Science 117 (1996) 189-196
[2] M. Asaeda and L.D. Du, Separation of alcohol/water gaseous mixtures by a thin ceramic membrane, J. Chem. Eng. Jpn., 19 (1986) 72. [3] A, Larbot, J.P. Fabre, C. Guizard and L. Cot, Inorganic membranes obtained by sol-gel techniques, J. Membrane Sci., 39 (1988) 203. [4] A. Ishikawa, T.H. Chiang and F. Toda, Separation of water-alcohol mixtures by permeation through a zeolite membrane on porous glass, J. Chem. Soc., Chem. Commun., (1989) 764. [5] M.D. Jia, K.V. Peinemann and R.D. Behling, Ceramic zeolite membranes, J. Membrane Sci., 82 (1993) 15. [6] M.D. Jia, B. Chen, R.D. Noble and J.L. Falconer, Ceramiczeolite composite membranes and their application for separation of vapor/gas mixtures, J. Membrane Sci., 90 (1994) 1. [7] S. Uemiya, Y. Kude, K. Sugino, N. Sato, T. Matsuda and E. Kikuchi, A palladium/porous-glass composite membrane for hydrogen separation, Chem. Lett., (1988) 1687. [8] J. Shu, B.P.A. Grandjean, E. Ghali and S. Kaliaguine, Simultaneous deposition of Pd and Ag on porous stainless steel by electroless plating, J. Membrane Sci., 77 (1993) 181. [9] N. Itoh, K. Haraya and Y. Shindo, Jpn. Pat., 1828312 (1993). [10] R.D. Noble and J.N. Armor (Eds.), Special issue on Membrane Catalysis, J. Membrane Sci., 77 (1993) 137. [11] A.W. Smith, Porous anodic aluminum oxide membrane, J. Electrochem. Soc., 120 (1973) 1068. [12] K. Itaya, S. Sugawara, K. Arai and S. Saito, Properties of
[13]
[14]
[15]
[16] [17]
[18]
[19]
porous anodic aluminum oxide films as membranes, J. Chem. Eng. Jpn., 17 (1984) 514. R.C. Furneaux, W.R. Rigby and A.P. Davidson, The formation of controlled-porosity membranes from anodically oxidized aluminum, Nature, 337 (1989) 147. S. Ono, K. Wada, T. Yoshino, N. Baba and K. Wada, Structure and formation behavior of anodic oxide films on aluminum prepared as gas separation membranes having ultrafine pores, Hyomen Gijutsu (J. Surf. Finishing Soc., Jpn.), 40 (1989) 1381. K. Wada, S. Ono, K. Wada, T. Yoshino, N. Baba, K. Kuroda, O. Takahashi, Z. Kawahara and S. Yabusita, Adsorption spectra and gas permeabilities of porous anodic aluminum oxide films, Hyomen Gijutsu (J. Surf. Finishing Soc., Jpn.), 40 (1989) 1388. S.K. Dalvie and R.E. Baltus, Transport studies with porous alumina membranes, J. Membrane Sci., 71 (1992) 247. P.P. Mardilovich, A.N. Govyadinov, N.I. Mukhurov, A.M. Rzhevskii and R. Paterson, New and modified anodic alumina membranes. Part I. Thermotreatment of anodic alumina membranes, J. Membrane Sci., 98 (1995) 131. M. Nagayama, H. Takahashi and M. Koda, Formation and dissolution behavior of anodic oxide films on aluminum, Kinzoku Hyomen Gijutsu (J. Metal Finishing Soc., Jpn.), 30 (1979) 438. H. Nanjo, Handbook of Machine Elements, 3rd ed., 3-2, Kyoritsu, Tokyo, 1956.
Journal of Membrane Science 117 (1996) 189-196
Preparation of a tubular anodic aluminum oxide membrane N. Itoh a,*, K. Kato b, T. Tsuji b, M. Hongo b a National Institute of Materials and Chemical Research, Tsukuba 305, Japan b Department oflndustrial Chemistry, University ofNihon, Narashino 275, Japan Received 14 November 1995; revised 7 February 1996; accepted 12 February 1996
Abstract An anodic aluminum oxide tube with perforated straight pores to use as a porous membrane or as a support for a composite membrane was prepared. An aluminum oxide layer with straight micropores closed by an oxide barrier layer was formed on the surface of an aluminum tube, 45 mm long, 0.5 mm thick and 6 mm in outer diameter, by anodic oxidation in an aqueous solution of oxalic acid. The pores were opened by dissolution of the inner aluminum and the subsequent barrier layer of aluminum oxide. The tubular alumina membrane obtained was 35-40 ~zm thick with straight micropores of 20-50 nm, and therefore showed Knudsen permselectivity for inorganic gases. It was found that the tube could withstand at least up to 4.4 atm of transmembrane pressure. Keywords: Gas separations; Membrane preparation and structure; Microporous and porous membranes; Anodic alumina; Tubular membrane
1. Introduction Since being advantageous in terms of obtaining higher membrane area per unit volume and easier installation into a membrane module container, a tubular or a hollow-fiber type of membrane is employed in most commercial polymeric membrane separation units. This would be also true in the case of ceramic membranes. However, one of the difficulties in practical use of a ceramic membrane, especially at higher temperatures, is how to house it in a membrane container, usually made of metal, where joining ceramics and metal is not easy. In this sense,
* Corresponding author. Fax: +81-298-56-8587, E-mail: nitoh@ nimc.go.jp.
a porous alumina membrane produced by means of electrochemical anodic oxidation is thought to be very attractive because it can be obtained in a form adherent to metallic aluminum, which can be used as sealing material as it is. Such a structure is also very useful as a support, on which a thin permselective layer may be formed by a s o l - g e l process [1-3], zeolite formation by hydrothermal synthesis [4-6], palladium and its alloy deposition by means of electroless- or electro-plating [7-9], etc. become possible. Composite membranes, possessing both high permeability and high selectivity, are applicable to not only membrane separators but also to membrane reactors (see the special issue of this journal [10]). Preparation of a flat type anodic aluminum oxide membrane has been tried by Smith [11] and established by Itaya et al. [12]. Their effort has triggered
0376-7388/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0376-73 8 8 ( 9 6 ) 0 0 0 6 3 - 4
190
N. ltoh et al. / Journal of Membrane Science 117 (1996) 189-196
off further detailed investigations [13-17], in the background of which was a growing interest in membrane technology. Still, such a flat type has some disadvantages as mentioned in the beginning. Instead, if a tubular type of the membrane can be presented, a potential practical application will increase. No attempt, however, has been made to make such a tubular type of membrane. The aim of this study, therefore, is to find a method to prepare an anodic aluminum oxide tube with perforated straight pores, which can be used as a porous membrane or as a support for a composite membrane. This type of tubular membrane is expected to be of great use since aluminum is left on the both sides of the tube without being oxidized; i.e. the structure becomes aluminum-alumina membrane-aluminum, so that metallic aluminum can be used to easily connect to a membrane container.
2. Preparation method A tubular anodic aluminum oxide membrane was prepared by the following four procedures.
2.1. Pretreatment of the aluminum tube The aluminum tube used was 45 m m long, 0.5 mm thick and 6 m m in outer diameter. First, the tube was ultrasonically cleaned in methanol for 20 min, and was annealed in air for 2 h at 300°C in an electric furnace. A chemical polish was made with a solution of H3PO4 (85 w t % ) - H N O 3 (61 w t % ) - H 2 0 (8:1:1 by volume) for 3 min at 80-90°C. Subsequently, an electrolytic polish was made at a constant voltage of 20 V with a solution of H3PO 4 (85 w t % ) - H 2 S O 4 (98 w t % ) - H 2 0 (7:2:1 by volume), which contained 45 g/1 CrO3, for 3 rain at 80-90°C.
2.2. Anodic oxidation The central 20 m m of the tube (L a) was oxidized as an anode in an aqueous solution of 4 wt% oxalic acid at 17°C using an experimental set-up specially designed for anodizing a specimen in the tubular form (Fig. 1). Simultaneously, a change in the anodic current with time was recorded by a personal computer-aided data acquisition system at every 10
~1~°ut Coolingwaterin>
D.C.PowerSupply
- - i ~
~
!WII~ acquisition ~
Aluminumtube.I
~
Data
~
system
Siliconetube
Anodizinalaver.I PtJTinet ~ (Cathode)
~~-I~--Silicone ~ r e r
rubberstopper
Fig. 1. Schematicof the experimentalset-upfor anodicoxidation of tubularaluminum. s. The effects of the anodizing voltage in the range 40 to 60 V and oxidation time were examined.
2.3. Dissolution of the inner aluminum The central 10 m m of the inner aluminum that was not oxidized (L S) was dissolved with an aqueous solution containing 20 wt% HC1 and 0.1 mol/1 CuC12 at a room temperature, which was introduced into the inner side of the tube in the manner shown in Fig. 2. L s must be less than L a in order to support
Dissolvingagent foraluminumor barrierlaver
Ir Aluminum tube / ~ ~ Anodizedlayer I1!~ Siliconeplug o ~ Distilledwater o \ Stirrer
~La
Fig. 2. A methodto dissolvethe inneraluminumandbarrierlayer.
191
N. ltoh et aL / Journal of Membrane Science 117 (1996) 189-196
the anodized alumina and achieve a larger adherence between the alumina and the aluminum tubes. 2.4. Dissolution o f the inner alumina barrier
Subsequently, in the same manner as above, the thin barrier layer of aluminum oxide, formed at the bottom of the pores, was dissolved with an aqueous solution of H3PO 4 at different temperatures and time.
permeation cell with a union connector. The permeation rate was measured with varying transmembrane pressure, regulated with a pressure regulator and a needle valve, while the permeate side was kept at atmospheric pressure. The temperature dependence of the permeability was also examined. Furthermore, the breaking pressure was examined to assess the durability of the membrane tube against external pressure.
4. Results and discussion 3. Measurement of morphological parameters and gas permeability The diameter and density of the pores of the anodic oxide tube prepared were determined by scanning electron microscopy (SEM), which was a very helpful way to determine the morphology of the porous membrane. The thickness was measured with a micrometer (Digital gauge, model D-10SS, Ozaki MFG CO.). Gas permeabilities of inorganic gases (H 2, He, N2, 0 2, Ar and CO 2) were measured with the apparatus shown in Fig. 3(a). The details of the permeation cell are shown in Fig. 3(b), where the membrane tube joined with metallic aluminum tubes on both sides could be easily plugged with a cap on one side and then connected on the other side to the
Pressure Pressure ~ g e ~ Regulator W.) ~ ~L, ~
l
U
4.1. Preparation o f the tubular anodic aluminum oxide membrane
In Fig. 4, the process of preparing a tubular membrane is photographed in sequence: (a) after polishing, (b) after anodic oxidation and (c) after perforation. Anodic oxidation is considered to be the key process for the successful preparation because the pores are formed in this process. The magnitude of the voltage applied during anodic oxidation, an important factor affecting the oxide structure, was found to be especially sensitive; that is, a higher voltage was apt to cause longitudinal cracks. On the other hand, the oxidation time is closely related to the thickness of the membrane, and is generally propor-
Flow_Meter H
t
Out
In • ~Membrane I1~~
Tube
GasCylinder (a) Schematicdiagram
(b) Membranecell
Fig. 3. Schematicflow diagramof the gas permeationmeasurement(a) and the detailed permeationcell (b).
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|_
Fig. 4. Outside appearance of the tube with progress of the preparation step: (a) after electrolytic polish, (b) after anodic oxidation, and (c) after dissolution of the inner aluminum and barrier layer.
tional to the time. From the standpoint of mechanical strength, a thickness of about 3 0 - 5 0 p~m was appropriate under the conditions investigated. The dissolution of the aluminum on the inner tube was continued until the objective part became visually transparent. Usually, it took about 30 min. The dissolution of the alumina barrier was the last stage before obtaining an alumina tube with perforated pores. Since the solvent can chemically dissolve not only the barrier but also the wall of pore, the dissolving time and temperature must be controlled. When the time was too short or the temperature was too low, only a part of the barrier was dissolved as demonstrated in Fig. 5. On the other hand, too long a time or too high a temperature
Fig. 5. Partly dissolved barrier layer.
resulted in enlarging the diameter of pore. Thus, this can be utilized to regulate the pore diameter. Experimentally, it became clear that a temperature higher than 17°C and a time of more than 120 min were required when a 4 wt% H3PO 4 aqueous solution was used for the dissolution. Thus, the following conditions were employed: 1. Anodic oxidation: 40 V, 180 min. 2. Dissolution of the inner aluminum: room temperature, 30 min. 3. Dissolution of the barrier: 17°C, > 60 min with 6 wt% H3PO 4.
Fig. 6. SEM images of the perforated pores with varying anodizing voltage: (a) 40 V, (b) 50 V, and (c) 60 V.
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N. hoh et al. / Journal of Membrane Science 117 (1996) 189-196
30 . ]
4.2. Determination of pore density and thickness The pore density of the alumina membrane prepared was determined based on SEM observation. Fig. 6 shows SEM images of the pores taken from the inner side when the anodizing voltage was changed. It is clearly seen that the pore density decreases with an increase in the voltage whereas the pore diameter increases. Using the number of pores counted in the SEM image, the pore density, N / m 2, was determined. The pore density is plotted against voltage in Fig. 7, where the results for a fiat-type membrane estimated by means of small angle X-ray scattering by Itaya et al. [12] is also presented for comparison. The thickness can be determined by knowing the total current consumed during the anodic oxidation. If this method is applicable, it would be very convenient because it is not necessary to break the tubular specimen into small pieces for direct measurement with a micrometer, which is the case with a flat membrane. Fig. 8 shows an example of the current change with time. When the current supplied, I (A or C / s ) is consumed by the formation of alumina (A1203) and the dissolution of aluminum, the total
12 X1013 \ !
!
\
'
'
i
\ 17°C 10X101a ................................. ~ - ......... ..........
.....................................................
. . . . . . . . . .
.........
.
.
.
.
.
.
.
.
small-angle X-ray scattering technique 4 Xl013
35
40
45
i
,
i
50
55
60
65
Anodizing Voltage [ V ] Fig. 7. Relation between pore density and anodizing voltage.
.
.
.
'
2,
. . . . . . . . . . . .
2O
..........................
. 300 T°tal Char e / 226~ . . . . . . . . . . . .
• ....j2,o
. . . .
__
,,08
a5
o
lO
0
i
0
i
i
r
50
100
150
i
200
Anodizing time [ min ] Fig. 8. Current change during anodic oxidation.
volume of the porous alumina layer, v (cm3), can be calculated from the following equation.
8Md fo,Zdt v = ZF-----
(])
where 8 is a correction factor defined as the ratio of current due to the oxide layer formation to the total current including that due to dissolution of A1 into the solution, M is the molecular weight of A1203, Z is the valence, F is the Faraday constant, and p is the density of alumina. In this study, 8 and p are given as 0.6925 and 2.78 g / c m 3, respectively, where such a lower density compared with that of yalumina, 3.5-3.9 g / c m 3, was because the alumina layer formed was amorphous and additionally contains some anions originating from the anodizing solution [18]. Therefore, the thickness, t m (mm), is calculated by solving the following arithmetic equation.
v = rr(r2o - r?)L 6 X l 0 la ................................................................................................. Itaya et.al "
.
i
= 7r(r o - r i ) ( r o + ri)L = Trtm(Zro - tm)L
(2)
where ro and r i are the outer and inner radius of the membrane tube, and L is the length of the tube. In this equation, since u, r o and L are known, t m c a n be estimated. In the case of the example taken in Fig. 8, a graphical integration of the current was 226 C, which corresponds to a 9.92 mm 3 oxide layer.
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N. ltoh et al. / Journal of Membrane Science 117 (1996) 189-196
45
~6o'c
18x10 ql
zot
Box: , 49"c
o 14x10 -11
~
E
~
-6 E
40
He
10x10-11
55 E
F-
6x10.11 ....................
Q-
35 35
40 Thickness (by micrometer) [ N.m ]
2x10 ql 0.045
45
................................................
I
i
0.05
0.055
N 2 .......
0.06
1/qT [ 1/VK]
Fig. 9. Comparison of thicknesses between those directly measured and the estimated values.
Fig. 11. Plots of gas permeabilities against 1/~/T.
4.3. Gas permeability
of the square root of the molecular weight in Fig. 10, which passes through the origin. In Fig. 11, temperature dependency of the permeabilities is shown. The permeabilities of all gases tested increase with an increase of 1/~/T. From such results, it is clear that the gas permeation through the tubular alumina membrane obeys Kunudsen flow. This can be applied as a method of estimating the pore diameter. The permeation rate of gas through straight pores, Q ( m o l / s ) , is formulated as follows.
Measured gas permeabilities of H 2, He, N 2, 02, Ar and CO 2 at 40°C are plotted against the inverse
Q = -~wNdpV
In Fig. 9, a comparison of the thicknesses thus calculated with those directly measured is made. The accord between them seems satisfactory, and therefore the cumulative current method is useful, especially with a tubular anodic oxide membrane because it is thus possible to determine the thickness without breaking the tube.
I
1
3/
2
Ap
wMRT
tm
(3)
where dp is the pore diameter (m), R is the gas constant (m 3 P a / m o l K), T is the absolute tempera-
i
16 X 10"11
a.t~
.....313KT..............................................;~.......~....H ....2.. ...............
,,.p E
100
--E'~..-!..~....!.".. ......
5 E
12 X 10-H
"6
8 X 1041
E 4X1041
o
................................ ...............
i ............................
.............
................
....
/ ?co: 5
lO
"(3
(3..
< 10
15
.....'~............. ....
._~
_ /P~"'o=A:' ~ o
................. ; ...................
20
1/,JM [ 1H(kg/mol)]
Fig. l 0. Plots of gas permeabilities against 1A/M.
25
)4 Time~required to dissolve barrier Layer i i i 50 100 150 200 Dissolving Time
250
[ min ]
Fig. 12. Relationships between pore diameter and dissolving time of the barrier layer at 17 and 25°C.
N. Itoh et al. / Journal of Membrane Science 117 (1996) 189-196
ture (K), and A p is the transmembrane pressure (Pa). In Eq. (3), since N and t m are already determined, dp c a n be obtained. In Fig. 12, t h e dp values thus calculated are correlated against the dissolving time of the barrier layer. This plot shows that the pore wall itself is also dissolved with the dissolving solution of barrier layer and therefore the diameter increases with time. The reason why a logarithmic plot is made is based on a simple mathematical consideration as follows. Assuming that the dissolution reaction of the alumina wall of a cylindrical pore is the rate-determining step, the following equation can be derived.
where d o is the initial pore diameter, k ( m 3 / ( m o l min)) is the rate constant of dissolution, C a ( m o l / m 3) is the concentration of the dissolving agent, and t (min) is the dissolving time. C a may be assumed to be constant since the dissolving agent flows continuously and the dissolving rate is comparatively slow. In Fig. 12, a larger slope at 25°C means that the dissolving rate becomes larger as the temperature rises and that a higher temperature of the dissolution process results in a larger pore size in a shorter time. Fig. 13 represents that a higher concentration of dissolving agent and, as expected from Eq. (4), results in a higher dissolving rate. The pressure durability of the membrane tube was 1 O0
iiiiiiiiiiiiiiiiiiiiill......................i........................i............................................ ..........17°C
........... i........................i...............................................
E r-
E "O
......................t.....................~
O
[4wt% H3PO4
10
0
50
100
150
Dissolving Time
200
250
[ rain ]
Fig. 13. Relationships between pore diameter and dissolving time of the barrier layer when 4 and 6 wt% aqueous solutions of H3PO4 were used.
195
examined with increasing applied pressure using argon while the permeate side (inner side of the tube) was kept at atmospheric pressure. As a result, the permeability could be measured up to 4.4 atm of absolute pressure without being broken. A breaking strength, ~rb ( k g f / c m 2) working on the tube surface at the breaking pressure, Pb (kgf/cm2), is given as follows [19]. ~b = r o P b / t m
(5)
In this case, r o = 3 mm, t m = 34 p~m and P b = 4.4 k g f / c m z, so that o"b is 388 k g f / c m 2. As is easily understood from Eq. (5), P b is reversibly proportional to r o, so that further efforts to prepare a finer tube should be necessary.
5. Conclusion
A tubular anodic aluminum oxide membrane with straight pores was successfully prepared. Different from a flat type of membrane, already established, the preparation of a tubular type was accompanied with some difficulties; that is, unclear condition of the anodizing voltage, how to dissolve the inner aluminum and barrier layer and so on. As a result, it was found that a comparatively lower voltage is preferable for the successful preparation of a tubular membrane. The two dissolution steps could be made easier by devising flow lines of the dissolving solutions. According to the morphological study, the tubular membranes prepared in this study were found to have a thickness of 3 5 - 4 5 txm, a pore diameter of 2 0 - 5 0 nm and 6 × 1013-11 )< 1013 straight pores per square meter. The mechanism of gas permeation through the porous membrane could be explained by Knudsen flow. Further, the membrane tube was able to tolerate about 4.4 k g f / c m 2 external pressure; this corresponded to a breaking strength of 388 k g f / c m 2.
References
[1] A.F. Leenaars and A.J. Burggraaf, The preparation and characterization of alumina membranes with ultrafine pores. Part 2. The formation of supported membranes, J. Colloid Interface Sci., 105 (1985) 27.
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N. ltoh et al. / Journal of Membrane Science 117 (1996) 189-196
[2] M. Asaeda and L.D. Du, Separation of alcohol/water gaseous mixtures by a thin ceramic membrane, J. Chem. Eng. Jpn., 19 (1986) 72. [3] A, Larbot, J.P. Fabre, C. Guizard and L. Cot, Inorganic membranes obtained by sol-gel techniques, J. Membrane Sci., 39 (1988) 203. [4] A. Ishikawa, T.H. Chiang and F. Toda, Separation of water-alcohol mixtures by permeation through a zeolite membrane on porous glass, J. Chem. Soc., Chem. Commun., (1989) 764. [5] M.D. Jia, K.V. Peinemann and R.D. Behling, Ceramic zeolite membranes, J. Membrane Sci., 82 (1993) 15. [6] M.D. Jia, B. Chen, R.D. Noble and J.L. Falconer, Ceramiczeolite composite membranes and their application for separation of vapor/gas mixtures, J. Membrane Sci., 90 (1994) 1. [7] S. Uemiya, Y. Kude, K. Sugino, N. Sato, T. Matsuda and E. Kikuchi, A palladium/porous-glass composite membrane for hydrogen separation, Chem. Lett., (1988) 1687. [8] J. Shu, B.P.A. Grandjean, E. Ghali and S. Kaliaguine, Simultaneous deposition of Pd and Ag on porous stainless steel by electroless plating, J. Membrane Sci., 77 (1993) 181. [9] N. Itoh, K. Haraya and Y. Shindo, Jpn. Pat., 1828312 (1993). [10] R.D. Noble and J.N. Armor (Eds.), Special issue on Membrane Catalysis, J. Membrane Sci., 77 (1993) 137. [11] A.W. Smith, Porous anodic aluminum oxide membrane, J. Electrochem. Soc., 120 (1973) 1068. [12] K. Itaya, S. Sugawara, K. Arai and S. Saito, Properties of
[13]
[14]
[15]
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[18]
[19]
porous anodic aluminum oxide films as membranes, J. Chem. Eng. Jpn., 17 (1984) 514. R.C. Furneaux, W.R. Rigby and A.P. Davidson, The formation of controlled-porosity membranes from anodically oxidized aluminum, Nature, 337 (1989) 147. S. Ono, K. Wada, T. Yoshino, N. Baba and K. Wada, Structure and formation behavior of anodic oxide films on aluminum prepared as gas separation membranes having ultrafine pores, Hyomen Gijutsu (J. Surf. Finishing Soc., Jpn.), 40 (1989) 1381. K. Wada, S. Ono, K. Wada, T. Yoshino, N. Baba, K. Kuroda, O. Takahashi, Z. Kawahara and S. Yabusita, Adsorption spectra and gas permeabilities of porous anodic aluminum oxide films, Hyomen Gijutsu (J. Surf. Finishing Soc., Jpn.), 40 (1989) 1388. S.K. Dalvie and R.E. Baltus, Transport studies with porous alumina membranes, J. Membrane Sci., 71 (1992) 247. P.P. Mardilovich, A.N. Govyadinov, N.I. Mukhurov, A.M. Rzhevskii and R. Paterson, New and modified anodic alumina membranes. Part I. Thermotreatment of anodic alumina membranes, J. Membrane Sci., 98 (1995) 131. M. Nagayama, H. Takahashi and M. Koda, Formation and dissolution behavior of anodic oxide films on aluminum, Kinzoku Hyomen Gijutsu (J. Metal Finishing Soc., Jpn.), 30 (1979) 438. H. Nanjo, Handbook of Machine Elements, 3rd ed., 3-2, Kyoritsu, Tokyo, 1956.