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New Anisotropic Ceramic Membranes from Chemically Fixed Dissipative Structures Ahmed A. Eljaouhari, Rainer Mu¨ller,* Matthias Kellermeier, Klaus Heckmann, and Werner Kunz Institute of Physical and Theoretical Chemistry, UniVersity of Regensburg, UniVersita¨tsstrasse 31, 93053 Regensburg, Germany ReceiVed April 28, 2006. In Final Form: August 30, 2006 The formation of highly ordered capillaries in alginate gels is due to a dissipative convective process resulting from opposing diffusion gradients and friction. Ceramic membranes with an anisotropic pore structure have been gained from this self-organization process by incorporating inorganic particles into the gel matrix, followed by subsequent ion exchange, drying, and sintering. The aim of this study was to overcome existing preparative deficiencies and to optimize the capillary structure and surface properties with respect to specific technical applications. A new method of ion exchange was introduced, and the sintering program was improved to obtain reproducible product quality. By controlling the parameters of the self-organization reaction, the overall porosity of the ceramic membranes was adjusted to selected values between 60% and 83%. Capillary sizes were varied between 8 and 50 µm. Further modification by metal plating, particle coating, or hydrophobization led to an extended spectrum of applicability of the ceramic membranes. For the first time, anisotropic capillary ceramics have been characterized in detail as to their technical use as catalyst supports, filter membranes, or other solid-fluid and solid-gas contact processes.
1. Introduction Numerous new microstructures in soft matter have been observed and exploited for material design over the last few decades. Many such “templates” are initially in thermodynamic equilibrium or at least near to it. Examples are cubic phases, hexagonal and lamellar liquid-crystalline phases, bicontinuous and reverse phase microemulsions, and liposomes.1,2 From such soft matter new nano- and microstructured materials could be obtained, e.g., nanotubes, nanocages, and others.3-5 That irreversible processes far from equilibrium can lead to ordering is well-known. One example is the Rayleigh-Be´nard heat convection that can be detected when a water pot is uniformly heated from below. The heated water of lower density flows upward, and the cold water of higher density flows downward. Above a critical temperature difference between the bottom and the top of the pot, regular macroscopic convection cells can be detected. These regular patterns are typical examples of ordered dissipative structures, but as soon as the driving force that keeps them far from equilibrium vanishes, ordering also ceases to exist and the structures disappear.6 More than 50 years ago, Thiele discovered that under certain conditions so-called ionotropic gels with hexagonally ordered capillaries can be formed when an aqueous sol of a negatively charged polysaccharide, e.g., an alginate, is brought into contact with a solution of multivalent cations.7-9 The unidirectional * To whom correspondence should be addressed. Phone: +49 941 943 4521. Fax: +49 941 943 4532. E-mail:
[email protected]. (1) Andre, P.; Filankembo, A.; Lisiecki, I.; Petit, C.; Tulik-Krzywicki, G.; Ninham, B. W.; Pileni, M. P. AdV. Mater. 2000, 12, 119-123. (2) Hyde, S. T.; Andersson, S.; Larsson, K.; Blum, Z.; Landh, T.; Lidin, S.; Ninham, B. W. The Language of Shape; Elsevier: Amsterdam, 1997. (3) Saito, Y.; Matsumoto, T. Nature (London) 1998, 392, 237. (4) Goldberger, J.; He, R.; Zhang, Y.; Lee, S.; Yan, H.; Choi H.-J.; Yang P. Nature 2003, 422, 599-602. (5) Shimizu, T.; Masuda, M.; Minamikawa H. Chem. ReV. 2005, 105, 14011443. (6) Cross, M. C.; Hohenberg, P. C. ReV. Mod. Phys. 1993, 65, 851-1112. (7) Thiele, H.; Andersen G. Kolloid-Z. 1955, 140, 76-102. (8) Thiele, H.; Hallich, K. Kolloid-Z. 1957, 151, 1-12.
diffusion of the cations through the alginate sol causes orientation and densification of the polyelectrolyte chains and induces the generation of capillaries which are oriented parallel to the diffusion direction. It was shown only recently that the origin of these anisotropic structures is a convective process resulting from two opposing diffusion gradients and friction.10,11 This dissipative phenomenon is quite similar to the Rayleigh-Be´nard convection, but in contrast to this, the pattern is fixed during the crosslinking of the polyelectrolyte chains by the cations, thus forming anisotropic structured hydrogels. Several techniques have been reported for transfer of the anisotropic capillary structure to materials that exhibit higher mechanical stability than the alginate hydrogels. One strategy was to stabilize the gels by chemical cross-linking of the carbohydrate network using diisocyanates.12 Polymerizable monomers, e.g., methyl methacrylates, have been incorporated within the gel network and, after polymerization, plastic materials exhibiting an open capillary structure have been obtained.13 Additionally it was found that the formation of alginate-based anisotropic capillary gels also took place when fine particles were suspended in the polyelectrolyte sol (Figure 1); water was released into the capillary lumen again, and the suspended component accumulated in the capillary walls during gel formation.14 Drying and sintering of the gel bodies resulted in capillary-structured ceramic membranes, and the organic templating-guide substance was burnt. As exemplary solid components, fine powders of aluminum oxide, titanium oxide, or hydroxyapatite have been used.15-17 (9) Thiele, H.; Hallich, K. Z. Naturforsch. 1958, 13b, 580-588. (10) Thumbs, J.; Kohler, H.-H. Chem. Phys. 1996, 208, 9-24. (11) Treml, H.; Woelki, S.; Kohler, H.-H. Chem. Phys. 2003, 293, 341-353. (12) Thiele, H.; Wiechen, A. Z. Naturforsch. 1967, 22a, 1571-1574. (13) Thiele, H.; Hallich, K. Kolloid-Z. 1959, 163, 115-122. (14) Schu¨tt, H. Kolloid-Z. Z. Polym. 1968, 228, 78. (15) Weber, K.; Tomandl, G.; Wenger, T.; Heckmann, K. Key Eng. Mater. 1997, 132-136, 1754-1757. (16) Dittrich, R.; Tomandl, G.; Mangler, M. AdV. Eng. Mater. 2002, 4, 487490. (17) Liebig, B.; Puszynski, J. A. Ceram. Trans. (InnoVatiVe Processing and Synthesis of Ceramics, Glasses, and Composites VII) 2003, 154, 87-98.
10.1021/la061152w CCC: $33.50 © 2006 American Chemical Society Published on Web 11/11/2006
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Figure 1. Scheme of the different phases of alginate-based capillary gel formation. When an aqueous solution of sodium alginate is covered by a layer of an electrolyte solution containing a divalent cation, a membrane-like precipitate of metal alginate is formed immediately at the interface of both fluids. This so-called primary membrane allows orientated diffusion of the electrolyte ions into the solution of the polymer which causes gel formation by ionic crosslinking of the alginate moieties. The ongoing precipitation results in the generation of water-filled capillaries, while dispersed ceramic particles are incorporated into the capillary walls.
Although the principal feasibility for manufacture of anisotropic ceramics from chemically fixed dissipative structures has been outlined by the studies described above, there was still a need for improvement of the preparative techniques to obtain reproducible product quality. Some optimization of drying techniques and sintering have been reported which were found to be inadequate for producing ceramic membranes suitable for technical applications. In the present report, we describe the reproducible manufacture of disc-shaped bodies of capillarystructured alumina ceramics whose pore structures and surface properties can be well-defined by accurately adjusting the process parameters. The porous materials were modified by particle coating, nickel plating, and hydrophobic silanization in order to expand the range of potential technical applications. The resulting materials were characterized using appropriate methods and further investigated with respect to their technical applicability. 2. Experimental Section Preparation of Sols and Suspensions. Alginic acid sodium salt was obtained as Manugel DJX from Kelco Int. Ltd. (England). The average molecular weight of the polymer was 100 000 g/mol, and the content of guluronic acid moieties was about 70%. Poly(guluronic acid) sequences were 50-55%, poly(mannuronic acid) sequences were 10-14%, and alternating sequences were 31-40%. The sodium alginate was dissolved in deionized water at a concentration of 2%. Microcrystalline particles of R-aluminum oxide (Al2O3) exhibiting an average particle size of 300 nm and a specific BET-surface area of 10.4 m2/g were purchased from Alcoa Deutschland GmbH (Germany). A nanocrystalline Al2O3 powder with an average particle size of 10-20 nm and a specific BET-surface area of 104 m2/g was obtained from Alfa Aeser Johnson Matthey GmbH (Germany). The ceramic powders were dispersed at varying concentrations (4-16%) in deionized water in an ultrasonic bath using a high performance dissolver stirrer model Eurostar power of IKA Labortechnik (Germany). Finally, the alginate solutions and the particle dispersions were mixed in a 1:1 mass ratio using this stirrer to obtain homogeneously mixed sols. Preparation of Capillary Ceramic Specimens. The homogenized sols were placed in cylindrical molds made of anodized aluminum of variable diameter and covered with 1 mol/L solutions of copper or calcium nitrate (Merck, Germany) using pump spray bottles. Diffusion-controlled gelation of the mixed sols was allowed for 36
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Figure 2. Macroscopic properties of an ionotropic gel formed by superimposing a slurry containing 1% alginate and 2% alumina with 1 mol/L Cu(NO3)2. Gel thickness and capillary diameter increased, whereas pore number decreased with ongoing time of diffusion of the cross-linking copper cations. h under ambient conditionssavoiding concussion and evaporation of water. The gels obtained were cut perpendicular to the longitudinal axis of the capillaries using a cutting machine developed for the purpose. The top layer of 5 mm thickness, including the primary membrane and the zone of the capillary cones, was removed and two disks of 10 mm in thickness were prepared. The densification of the gel disks, by exchanging the metal ions for protons, was performed in two ways.16,18 Disks were soaked in hydrochloric acid (HCl) solutions of increasing concentrations (0.001, 0.01, 0.1, and 1 mol/L) for a minimum of 12 h each. Alternatively, the gel bodies were equilibrated in a 1 mol/L aqueous solution of methyl formate (Merck, Germany) at room temperature (RT) for 12 h and at 60 °C for 24 h. Water and excess acid were removed from the gel disks by bathing the specimens consecutively in acetone-water mixtures of increasing acetone content (20-100%) for a minimum of 2 h each. Organic solvent-containing gel specimens were placed between two porous clay disks and allowed to dry for several days enclosed in a glass Petri dish. The dried green bodies were burnt in a high temperature furnace HT08/16E of Nabertherm GmbH (Germany) applying the following temperature program: heating from RT to 260 °C at 2 °C/min, keeping at 260 °C for 1 h, heating to 500 °C at 2 °C/min, keeping at 500 °C for 1 h, heating to the final sintering temperature between 1200 and 1600 °C at 3 °C/min, keeping at this temperature for 2 h, and finally cooling to RT. Grinding and polishing of the raw ceramic bodies was performed using abrasive paper and diamond suspension after incorporation of molten wax which was finally removed by petroleum ether extraction and burning. Modification of Capillary Ceramics. To enlarge the specific surface area, the capillary walls were coated with highly porous alumina made from colloidal aluminum hydroxide. This so-called washcoat procedure was performed according to a previously published protocol.19 Briefly, a stable alumina sol (8%) was prepared by hydrolysis of 1 mol of aluminum tri-sec-butylate (Merck, Germany) in 100 mol of water at 80 °C and precipitation with 0.07 mol of HCl at 80 °C. The ceramic bodies were coated by immersion in the alumina sol for 1 h. The samples were drained, excess sol was removed using an air jet, and finally the ceramics were heated in an oven at 550 °C for 2 h. These steps were repeated three times. Electroless nickel plating of the capillary walls was performed in accordance with a procedure previously described,20 including sensitization, activation, and deposition processes. The sensitizing solution consists of 10 g/L tin chloride (SnCl2) and 40 mL/L concentrated HCl, both of which dissolved in deionized water (DI). Ceramic specimens were treated with this solution in an ultrasonic bath for 10 min at 60 °C. After extensive rinsing with DI and drying (18) Bergmeier, M.; Hoffmann, H.; Thunig, C. J. Phys. Chem. B 1997, 101, 5767-5771. (19) Brinker, C. J.; Scherer, G. W. Sol-Gel Science, the physics and chemistry of Sol-Gel processing; Academic Press: Boston, MA, 1990; p 68. (20) Gawrilov, G. G. Chemische (stromlose) Vernickelung, Eugen G. Leuze Verlag: Saulgau, Germany, 1974; p 46.
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Figure 3. Optical micrographs of copper cross-linked gels (1% alginate, 2% alumina, 1 mol/L Cu(NO3)2) in cross-section and longitudinal section. Images A and B represent the gel body cut about 5 mm from the top of the gel close to the primary membrane. Images C and D represent the gel body cut about 15 mm from the top of the gel. The capillaries are empty, and alumina particles are completely trapped within the capillary walls. in air stream, the ceramic bodies were soaked in the activator solution (1 g/L palladium chloride (PdCl2) and 10 mL/L concentrated HCl in DI) in an ultrasonic bath for 10 min at 60 °C. The specimens were again rinsed extensively and dried before treatment with the plating solution (24 g/L nickel chloride (NiCl2‚6H2O), 17 g/L sodium hypophosphite (NaH2PO2‚H2O), 15 g/L orthoboric acid (H3BO3), and 5 g/L sodium fluoride (NaF)). After plating for 5 min at 60 °C, the ceramic bodies were transferred into a flow cell designed to allow electrolyte flow through the capillaries, and the plating process was continued for 30 min at 60 °C. The metallization process was completed after extensive rinsing and drying of the specimens. Hydrophobizing of the alumina ceramics was performed by heating the specimens in anhydrous toluene containing 2% (heptadecafluoro1,1,2,2-tetrahydrodecyl)triethoxysilane (ABCR GmbH, Germany) for 12 h at 110 °C. After excess silane had been removed from the samples by extensive rinsing with pure toluene in an ultrasonic bath, the ceramic samples were heated for 3 h at 150 °C to complete condensation reactions. Characterization of Ceramic Specimens. For physical characterization of the capillary ceramics, disk-shaped specimens with 20 mm diameter and 2.5 mm thickness have been prepared. The pore structures of the gels and the ceramic samples were viewed either by light microscopy using a Leitz Dialux 20 microscope or by scanning electron microscopy (SEM) using a Zeiss DSM 940 microscope. The overall porosity of the ceramic specimens was determined with a Carlo Erba mercury porosimeter. The air permeability of the porous ceramics was measured with a Coulter porometer type II. To measure water permeability, ceramic specimens were mounted tightly into glass tubes which were connected with one end to the water conduit via a pressure reducing regulator and to a flow control unit on the other end. Two pressure sensors, one of which was placed in front of the ceramic specimen and the other located behind the ceramic disk, recorded the pressure difference over the sample as a function of the water flow. BET surface area was determined with a Micromeritics accelerated surface area and porosimetry system ASAP 2010 using nitrogen gas. The mechanical stability of the metallized samples was investigated by a crushing test measuring the diametrical tensile strength of the ceramics. The samples were exposed to a mechanical load on an Instron 4301 material testing machine until deterioration occurred; tensile strength was calculated from the force applied and the geometrical data of the ceramic disks. Fluorocarbon silane-modified ceramics were characterized by X-ray photoelectron spectroscopy (XPS) using a Physical Electronics PHI 5700 by application of monochromatic Al KR radiation at an analytical angle of 45°. The distribution of the elements at the surfaces was quantified by considering all signals identified and linear deduction of the background. Water contact angles (θa) on the modified ceramics were measured using the sessile drop method on
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Figure 4. Al2O3 particle-containing bodies obtained after exchange of copper ions for protons in alginate gels (1% alginate, 2% alumina, cross-linked with 1 mol/L Cu(NO3)2). Planar bodies having regular pores were produced by gradient-free interaction with formic acid, generated in situ by temperature-induced hydrolysis of methyl formate. Ion exchange by immersing the gels in solutions of increasing hydrochloric acid concentrations resulted in the formation of vaulted ceramic specimens and irregular capillaries. a P1 goniometer from Erna Inc. Droplets of 2 µL were advanced toward the samples by a syringe tip until they contacted the surfaces. Contact angles were read on one side of five droplets. To test the hydrophobized ceramic membranes concerning their ability to break oil-in-water emulsions, a formulation based on dehydrogenated castor oil (7.5%), stabilized with 1,6-hexenediol (5%), and containing further odorous compounds was used which had been stable for over 2 years. Fifty milliliters of this emulsion placed on top of a horizontally fixed ceramic disk were allowed to pass through the capillary system without application of any additional pressure. Changes in turbidity were determined by measuring the optical density of the emulsion using a Perkin-Elmer Lambda-18 spectrophotometer. Particle size analysis was performed using a Malvern Instruments Zetasizer 3000 equipped with a He-Ne laser (λ ) 633 nm) and operating at a scattering angle of 90°.
3. Results and Discussion Ionotropic Gel Formation. Anisotropically structured, particlecomprising alginate gels were obtained after copper or calcium ion-driven ionotropic gel formation, induced by superimposing alginate/particle slurries with electrolyte solutions. Ion diffusion time could be correlated with the thickness of the resulting gel, the capillary diameter, and the density of the capillaries. Figure 2 illustrates the results obtained for the system 1% alginate, 2% alumina, and 1 mol/L Cu(NO3)2. The displacement of the front of gel formation (zR), which is equal to the gel thickness, was found to scale linearly with the square root of the time zR ) 2(γt)1/2.11 The displacement parameter γ of this system was calculated to be 20.7 × 10-10 m2/ s. The value of 20.8 × 10-10 m2/s has been calculated for a nonparticle-comprising gel of the same alginate concentration. The number of capillaries decreased and the pore diameter increased with growing gel thicknesssevidence that the capillaries exhibited some conic geometry. The capillary structure was maintained up to a distance of 25 mm from the primary membrane, corresponding to 20.4 h of ion diffusion. As ion diffusion continued further, only unstructured hydrogels were created. For the preparation of capillary ceramics, two disk-shaped bodies, each 55 mm diameter and 10 mm thick, were cut from each cross-linked gel. These were characterized by light microscopy (Figure 3). The gels were traversed by capillaries arranged almost hexagonally in cross-section and showed almost
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Table 1. Change of the Physical Properties of the Material Bodies during Processing from Cross-Linked Alginate Gels to Moisture- and Organic-Free Ceramic Bodiesa
materials
volume [cm3]
density [g/cm3]
pore diameter [µm]
copper alginate gel alginic acid gel dried green body sintered ceramic body
14.3 6.7 2.1 0.8
0.05 0.20 0.45 0.95
90 65 43 28
a
Gels were prepared from 1% alginate sols containing 2% alumina, cross-linked with 1 mol/L Cu(NO3)2, and treated with 1 mol/L methyl formate.
parallel alignment in longitudinal section. The capillaries exhibited free lumens, and the alumina particles were dispersed completely within the capillary walls. The micrographs clearly show the increase in capillary diameter and the decrease in the overall pore number from the top to the bottom of the gel. Transformation of Ionotropic Alginate Gels to Capillary Ceramics. The process steps necessary for transforming the gels into ceramic bodies included densification of the alumina particles by ion exchange, drying of the alginate gels, and sintering of the resulting green bodies. The exchange of the cross-linking metal ions for protons induced densification of the gels by transforming the polyelectrolyte salt into nonsoluble alginic acid. This processing step is necessary as sintering of dried gel disks without ion exchange causes complete disintegration of the ceramic bodies. Previous work had shown that a proton concentration of 1 mol/L is required to remove the metal ions from the gel:16,17 hitherto the way of acidifying the gels had not been considered to be very critical. It was proposed to leach the gel bodies in hydrochloric acid solutions either directly in a solution of high acid concentration or stepwise in solutions of increasing acid concentrations. We found that both procedures caused distortions of the capillary structures, or even of the whole gel bodies due to the occurrence of high pH gradients and osmotic forces. Although the deformed ceramic bodies could be ground and polished, the pore structure remained irregular and a high variety in pore size and shape was observed (Figure 4). A second consequence was that the thickness of the ceramic specimen became highly reduced. By contrast, gel densification could be carried out much more homogeneously if a neutral precursor molecule from which an acidifying agent could be generated by a controlled change of a specific parameter was used.18 This was achieved by equilibrating the cut gel bodies in aqueous solutions of methyl formate, an ester which hydrolyzes to formic acid and methanol upon increasing the temperature to 60 °C. Gels condensed in this way displayed an overall intact capillary structure (Figure 4). The variety of procedures necessary to transform the compacted hydrogels to burnable green bodies has been discussed earlier in detail.15,21 It was found that the drying processes, combining good efficiency in compacting the inorganic particles and low tendency for crack development during sintering, were solventexchange processes, e.g., for acetone or ethanol, freeze-drying, or supercritical drying. We optimized the drying procedure of organic-solvent-containing specimens by placing them between two porous tray disks. In this way, the speed of volatilization of the organic solvent and the occurrence of irregularities in the microstructure were reduced. Sintering of the dried disks was optimized with respect to residual moisture and to burn out the organic material completely. (21) Liebig, B.; Puszynski, J. A. Ceram. Trans. (InnoVatiVe Processing and Synthesis of Ceramics, Glasses, and Composites VII) 2003, 154, 87-98.
Figure 5. Optical micrographs of sintered (1400 °C) Al2O3 capillary ceramics prepared from gel bodies of 1% alginate containing different amounts of dispersed alumina particles, cross-linked with 1 mol/L Cu(NO3)2, and treated with 1 mol/L methyl formate. (A) Thin capillary walls and high porosity were found for ceramics prepared from sols of 2% alumina content; (B) thicker capillary walls and a lower overall porosity appeared in ceramics stemming from sols of 6% alumina content.
Figure 6. Overall porosity and density of Al2O3 capillary ceramics as functions of the applied sintering temperature. The ceramics were prepared from gel bodies of 1% alginate containing 4% alumina, cross-linked with 1 mol/L Cu(NO3)2, and treated with 1 mol/L methyl formate.
Sintering of the dried disks resulted in hard and brittle ceramic specimens free of moisture and organic material. Each process step caused a significant decrease of the overall body volume and an increase in material density. Although there were obvious changes in the macroscopic properties of the samples, the capillary microstructures remained intact disclosing only a decrease in capillary diameter and wall thickness. Table 1 comprises relevant parameters obtained for the system 1% alginate containing 2% alumina, which was cross-linked with 1 mol/L Cu(NO3)2 and treated with 1 mol/L methyl formate. Properties of Anisotropic Capillary Ceramics. The most important properties of the anisotropic capillary ceramics with respect to technical applicability are their relative robustness and their open porous structure. Changing of process parameters allows distinct variations of material properties concerning porosity and pore diameter. Among these parameters, the type of the cross-linking cation, the concentration of inorganic particles in the sol, the distance from the primary membrane at which the disklike specimen is cut from the cross-linked gel, and the sintering temperature are specified as follows. Figure 5 gives examples of structures of sintered Al2O3 capillary ceramics prepared from alginate solutions containing varying amounts of dispersed alumina particles. Increasing the particle concentration led to an increase in capillary wall thickness and to a decrease of the overall porosity. The capillary diameters, however, did not change noticeably. In particular, the overall porosity decreased from 83%, when sols of 2% particle content were used, to 76% for 4% alumina, to 63% for 6% alumina, and to 56% when sols containing 8% inorganic particles were employed. The influence of the sintering temperature on the overall porosity and density of capillary ceramics is illustrated
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Table 2. Macroscopic Properties of Ceramic Disks Obtained after Variation of the Cross-Linking Metal Ion and Variation of the Distance of the Cutting Plane from the Top of the Gel Bodya Cu2+ b porosity [%] dE [µm] dS [µm] NE [105/cm2] NS [105/cm2] conicity [%]
Ca2+ b
5 mmc
15 mmc
5 mmc
15 mmc
78 8(1 13 ( 2 5.5 ( 0.5 3.0 ( 0 0.22
75 17 ( 2 30 ( 5 3.25 ( 0.25 1.25 ( 0.25 0.50
63 24 ( 2 38 ( 1 0.95 ( 0.05 0.4 ( 0 0.53
60 37 ( 3 47 ( 2 0.45 ( 0.05 0.25 ( 0.05 0.34
a Disks were prepared from 10 mm thick gel bodies of 1% alginate containing 2% alumina, cross-linked with 1 mol/L electrolyte solution, treated with 1 mol/L methyl formate, and sintered at 1500 °C. b Complexing cation. c Cutting height.
in Figure 6; porosity decreases and density increases with rise of the sintering temperature. Products of varying capillary diameter could be obtained by changing the cross-linking cation as well as by changing the distance of the cutting position from the primary membrane. The smallest capillary diameter we obtained was about 5 µm. For the largest we measured about 50 µm. Ceramic-traversing capillaries showed a conicity of 0.2-0.5%, whereby the tube diameter increased from the electrolyte side (dE, top) to the sol side (dS, bottom) in the direction of capillary growth. Correspondingly, the pore density decreased from the electrolyte side (NE) to the sol side (NS). Table 2 summarizes the macroscopic properties of four different ceramics obtained by varying the cross-linking cation and the distance of the cutting plane from the top of the gel. The porosity of the ceramic body, defined as the ratio of free lumen against the lumen of the whole material, decreased by changing copper for calcium or by cutting the gel at a lower level. Air and water flow through the ceramic disks depended mainly on the capillary size and the direction of the flow, but not on the overall porosity (Figure 7). Disks with large capillary diameters allowed significantly higher gas and liquid flow rates compared with those with smaller pores, although the latter exhibited a free lumen which was 18% higher than that of the former. From the measured mass flow rates, the velocity in the capillaries is calculated to be 0.07 m/s for an average diameter of 8 µm and 1.34 m/s for an average diameter of 37 µm, the pressure of the water column being 1 bar. In all experiments, the mass flow through diverging capillaries was higher than flow through converging capillaries, in agreement with well-established hydrodynamic models of conical tubes.22 Improving the quality of these ceramics simultaneously widens the spectrum of technically realizable products. Having the new technique at hand, we were able to produce ceramic disks with a diameter of about 10 cm, 10 times larger than those described by others.16 To our best knowledge, there is no other technically feasible way currently available for production of highly porous capillary ceramics with hexagonal structures other than those we describe in the present work. Extrusion, injection molding, slip casting, or the use of pre-designed structures represent stateof-the-art techniques which are, however, hardly suitable for manufacturing porous devices with pore diameters below 50 µm. Interestingly, the anodic oxidation of aluminum in acidic electrolytes creates structures of hexagonally arranged capillaries which have diameters smaller than 300 nm.23,24 Anodic alumina can be used as an etching mask for semiconductor substrates, as template for the creation of nanoscaled metallic pins or carbon (22) Bond, W. N. Proc. Phys. Soc. 1924, 36, 367-378. (23) Jessensky, O.; Mu¨ller, F.; Go¨sele, U. Appl. Phys. Lett. 1998, 72, 11731175. (24) Li, A. P.; Mu¨ller, F.; Birner, A.; Nielsch, K.; Go¨sele, U. J. Appl. Phys. 1998, 84, 6023-6026.
Figure 7. Permeability of Al2O3 capillary ceramics specified in Table 2. Water flow (A) and air flow (B) were significantly influenced by the capillary diameters. Filled symbols represent flow through diverging capillaries, open symbols describe flow through converging capillaries.
tubes,25 or as support for the generation of nano-black lipid membranes.26 However, one limitation for direct applicability of such porous alumina is the small specimen maximum thickness of only 200 µm. By contrast, the ceramic bodies described in this paper can be manufactured up to a thickness of 5 mm. Modification of Anisotropic Capillary Ceramics for Technical Applications. Materials employed as catalyst supports require large surface areas to achieve rapid reaction rates. Depending on the particle size of the ceramic raw material used, variations in the surface area of the capillary walls have been obtained. The BET surface area of ceramic bodies sintered at 1400 °C increased from 1.8 to 3.0 m2/g when the particle size of the Al2O3 powder was decreased from 300 to 10-20 nm; scanning electron microscopy (SEM) images of these two capillary ceramics support this. The surface of the capillary walls consisting of the smaller particles is clearly rougher than that of the walls made up of the larger particles (Figure 8). Additional coating of the capillary walls with alumina from colloidal (25) Shingubara, S. J. Nanopart. Res. 2003, 5, 17-30. (26) Ro¨mer, W.; Steinem, C. Biophys. J. 2004, 86, 955-965.
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Figure 8. SEM images of capillary ceramics. Reduction of the particle size of the ceramic raw material from 300 nm (A) to 10-20 nm (B) increased the surface roughness of the capillary walls. The coating layer applied by the washcoat procedure (C) smoothened the surface of the capillary walls, leaving the capillaries open.
Figure 9. Permeability of capillary ceramics before and after nickel plating. Water flow was reduced due to decreased capillary diameters in the course of metal plating. Filled symbols represent flow through diverging capillaries, open symbols describe flow through converging capillaries.
aluminum hydroxide produced materials with a BET surface area of about 20.0 m2/g. In the SEM image, this coating was seen as a thin layer that smoothens the surface of the capillary walls. The capillaries remained open after application of the washcoat procedure (Figure 8). This procedure is a state-of-the-art technique which increases surface areas of supporting materials for catalysis, e.g., monoliths or ceramic foams.27,28 The range of porosity of the anisotropic capillary ceramics manufactured in this study is similar to that of ceramic foams and monoliths.29,30 By controlling the content of solids and the sintering temperature, porosity could be modulated between 45% and 85%. This kind of pore modulation can provide material for use in various applications, e.g., filters or supports in bioreactors.31 Applications in filter technology often require a high mechanical strength and low brittleness of the filter membranes, not achievable with ceramic materials. One way to enhance the stability of ceramic membranes against mechanical loading is to cover the surface with a thin ductile metallic layer. For instance, the tensile strength of capillary ceramic disks was raised from 4.7 ( 0.7 to 26.0 ( 4.2 MPa upon plating the surfaces with nickel. These plated ceramics kept their open porous structure, as seen from the flow characteristics in Figure 9. However, the water flow was reduced, indicating a decrease of the capillary diameters. Hydrophobic porous membranes consisting of fluorocarbon polymers can be used for phase separation processes such as the breaking of oil/water emulsions. One interesting example concerns the phase separation that is necessary at the end of solvent (27) Cybulski, A.; Moulijn, J. A. Catal. ReV. 1994, 36, 179-270. (28) Richardson, J. T.; Garrait, M.; Hung, J.-K. Appl. Catal., A: Gen. 2003, 255, 69-82. (29) Richardson, J. T.; Peng, Y.; Remue, D. Appl. Catal., A: Gen. 2000, 204, 19-32. (30) Stankiewicz, A. Chem. Eng. Sci.. 2001, 56, 359-364. (31) Zeschky, J.; Hoefner, T.; Arnold, C.; Weissmann, R.; Bahloul-Hourlier, D.; Scheffler, M.; Greil P. Acta Mater. 2005, 53, 927-937.
Figure 10. Turbidity of a surfactant-stabilized oil-in-water emulsion as measured by optical density as a function of the numbers of passages through a highly hydrophobic fluorinated capillary membrane.
extraction processes or in the field of metal ion extraction.32 For such use, we have coated the capillary walls of our anisotropic ceramics with a highly hydrophobic fluorocarbon silane. Elemental analysis of the ceramic surface revealed the introduction of 27% fluorine; the water contact angle was increased from below 10 up to 130°. A surfactant-stabilized oil-in-water emulsion was passed through the hydrophobic capillaries by gravitational force only, i.e., without application of additional pressure. The membrane dwell time per passage was calculated to be 1.5 s. As depicted in Figure 10, the turbidity of the emulsion decreased with increasing numbers of passages through the ceramic. After the 24th passage, the liquid was effectively clarified. Particle size analysis revealed that the average diameter of the oil drops in the emulsion was reduced during the procedure from 74.7 ( 1.4 nm in the original emulsion to 25.1 ( 1.8 nm after 24 passages through the membrane. We therefore believe that our hydrophobized ceramics act as “filters”, rather than as coalescers, for the breaking O/W emulsions; trapping oil (preferably larger drops) within the membrane. Due to the pore size and the slight pore conicity, the presented products could also be interesting materials for the fabrication of surface burners. In typical ceramic plate burners, the gas flows through multiple channels or pores and combustion occurs on the top surface of the plate. Materials typically used for this application are cordierite plates with honeycomb-type pores with an average pore diameter of 100 µm and a porosity of about 50%.33 A higher porosity, as it is displayed by the ceramic bodies presented here, should enhance the efficacy of surface combustion. In the field of combustion technology, anisotropic ceramics might also be valuable devices in pore burners, where ceramic foams with an isotropic pore structure are used nowadays.34 (32) Daiminger, U.; Nitsch, W.; Plucinski, P.; Hoffmann, S. J. Membr. Sci. 1995, 99, 197-203. (33) Garcia, E.; Osendi, M. I.; Miranzo, P. J. Appl. Phys. 2002, 92, 23462349. (34) Durst, F.; Weclas, M. Proc.-Inst. Mech. Eng. D 2001, 215, 63-81.
Anisotropic Ceramic Membranes
4. Conclusion It can be concluded from these studies that dissipative structures can be used as templates for the design of new anisotropic materials. Because all mechanisms involved are now understood, strategies for detailed material design have become possible. We have presented the methods for production and modification of anisotropic capillary ceramics, which could be applied technically in areas such as filtration and catalysis. Also, because alginates are nontoxic, we can consider their use as possible guiding structures in live matter, e.g., in the regeneration of injured nerve axons.35 Acknowledgment. The authors thank J. Vancea for performing XPS measurements, D. Touraud for preparing the oil-in-
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water emulsion, C. Becker and B. Bartel for performing SEM measurements, R. Dittrich and G. Tomandl for assistance with mechanical testing and porosity measurements, and J. B. Gill for reviewing the manuscript. We gratefully acknowledge Deutsche Forschungsgemeinschaft (DFG, HE 378/24) for support of parts of this work. LA061152W
(35) Prang, P.; Mu¨ller, R.; Eljaouhari, A. A.; Heckmann, K.; Kunz, W.; Weber, T.; Faber, C.; Vroemen, M.; Bogdahn, U.; Weidner, N. Biomaterials 2006, 27, 3560-3569.
