Preparation of Porous Glass Monoliths with an Aligned Pore System via Stretch Forming.pdf

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Preparation of Porous Glass Monoliths with an Aligned Pore System viaStretch Forming

Andrei Barascu, Jens Kullmann, Bjoern Reinhardt,, Thomas Rainer, Hans Roggendorf,

Manfred Dubiel, and Dirk Enke

Institute of Chemical Technology, University of Leipzig, Leipzig 04103, Germany

Boraident GmbH, Koethener Str. 33a, Halle 06118, Germany

Institute of Physics, University of Halle, Halle 06120, Germany

Porous glasses with an aligned pore system were prepared byusing a stretch-forming device. For the experiments a sodiumborosilicate glass with the composition 62.5 wt% SiO2,30.5 wt% B2O3 and 7 wt% Na2O was prepared. The charac-terization of this glass includes X-ray fluorescence analysis and

dilatometry. The applied stretch-forming apparatus allowed thesimultaneous phase separation and elongation of the glass sam-ples. The obtained glass templates were chemically treated withhydrochloric acid and sodium hydroxide solution to extract thesoluble sodium rich borate phase. The resulting porous glasssamples were characterized by environmental scanning electronmicroscopy and mercury intrusion. Different degrees of porealignment were realized by varying the parameters of thestretch-forming process.

I. Introduction

P OROUS glasses based on phase-separated alkali-borosili-cates have a variety of advantageous properties. Theyshow a high chemical, thermal, and mechanical stability. Por-ous glasses are also characterized by a narrow pore size dis-tribution. The pore sizes can be adjusted in the rangebetween 1 and 1000 nm selectively. In addition, due to theirsurface consisting of silanol groups, porous glasses offer thepossibility for various surface modifications. This opens up awide range of applications for these materials, e.g., in sensortechnology, chromatography, membrane technology, andhost guest chemistry.13 Another application is the use ofporous glasses as model systems in the evaluation of charac-terization techniques,4 and in modeling of diffusion in porousmaterials.5

Despite these advantageous properties, porous glasses haveone disadvantage: Due to the spontaneous spinodal decom-position mechanism, only three-dimensional disordered poresystems could be generated by now. An aligned pore struc-ture would bring substantial benefits. On the one hand themass transport properties of the material will be improvedconsiderably and on the other hand the pressure drop will bereduced, compared to monoliths with disordered pore struc-ture. Due to the pore alignment, higher throughputs at mem-brane separation processes, chromatographic separations and

catalytic processes can be achieved. This in turn increases theproductivity of the respective applications. Porous glasses arealready used in the field of sensors. With the faster transportof the detectable substances to the sensitive centers, theresponse times of the sensors can be reduced significantly.

Furthermore, a change of the optical properties (e.g., trans-parency) can be expected by the alignment of the pores.

Porous materials with an aligned pore system can be pro-duced using different ways as described in the literature. Forexample Okada et al. prepared porous alumina ceramics withuni-directionally oriented pores by extrusion method,6,7 usingcarbon fibers or a plastic substance as pore formers. Glasswith an aligned pore system can be produced from hollowglass fiber bundles by using a special drawing technology.8

Another approach was reported by Lloyd et al.9 Theyachieved an aligned pore system in polymer membranes via astretching procedure.

In this study, we report for the first time about a stretch-ing process coupled with the phase separation of alkali-boro-silicate glasses to produce porous glasses with differentdegrees of pore alignment.

II. Experimental Procedure

(1) Preparation of the Initial Glass MonolithsAn initial glass with the composition 62.5 wt% SiO2, 30.5 wt%B2O3, and 7 wt% Na2O was molten at 1773 K for 3 h andpoured into a preheated ashlar-formed casting mold with thedimensions of 22 mm 9 25 mm 9 105 mm. The solidifiedglass block was placed into an oven at 753 K to allow slowcooling to room temperature and to avoid stress in the glass.The resulting glass blocks were sawn into thin plates with thedimensions of 22 mm 9 1.2 mm 9 105 mm.

(2) Stretch-Forming Process and Phase SeparationThe stretch-forming process was performed using a glass-stretching apparatus developed by Borek10 as shown sche-matically in Fig. 1.

The furnace (2) moves precisely in a low speed rangebetween 10 and 3000 lm/min over the glass template. Twoclamps (1 and 4) are used to fix the glass plate (3). The rightclamp (4) is movably mounted on a guidance (5) to whichvariable weights (6) may be attached to induce differenttensile forces. The glass template was clamped into the appa-ratus and deformed at different temperatures and tensileloads while simultaneously a phase separation occurs. Anoverview of the experimental conditions is shown in Table I.The tensile forces or the weights, respectively, were kept

constant. The glass plates were stretched until breaking. For

J. Maurocontributing editor

Manuscript No. 31677. Received June 30, 2012; approved July 09, 2012.Author to whom correspondence should be addressed. e-mail: bjoern.reinhardt@

uni-leipzig.de

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J. Am. Ceram. Soc., 95 [10] 30133015 (2012)

DOI: 10.1111/j.1551-2916.2012.05394.x

2012 The American Ceramic Society

Journal Rapid Communication


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characterization, only the stretched area of the glass plates wasused. The samples undergo an acid (HCl, 3 N, 363 K, 4 h)

and an alkaline (NaOH, 0.5 N, 2 h, room temperature) treat-ment to remove the sodium-borate phase and the colloidaldeposit. After each extraction the samples were washed withdistilled water. The experimental steps are shown in Fig. 2.

(3) CharacterizationThe initial non-porous sodium borosilicate glass was charac-terized using X-ray fluorescence analysis to determine thechemical composition and using dilatometry (Netzsch DIL402C; NETZSCH-Gera tebau GmbH, Selb, Germany) to iden-tify the glass transition temperature and the softening point.

The pore size distribution was determined by mercury intru-sion using the porosimeter Pascal 440 (Porotec GmbH, Hof-heim/Ts., Germany). A mercury contact angle was assumed to

be 141.3. The pore diameter was calculated by applying theWashburn equation and a cylindrical pore model.The specific surface areas were calculated from the nitro-

gen sorption using the BET-model. The measurements wereperformed using a Sorptomatic 1900 (Porotec). The sampleswere heated at 393 K for 24 h under vacuum to remove theadsorbed water from the pores.

The microstructure of the resulting materials was charac-terized by environmental scanning electron microscopy(ESEM XL 30 FEG; Philips Electronic Instruments,Mahwah, NJ). The samples were provided with a fresh frac-ture surface and investigated in a water vapor atmosphere ata pressure of 160 Pa.

III. Results and Discussion

(1) Characterization of the Initial Glass

The composition of the prepared glass was determined byX-ray fluorescence analysis and found with 62.50 wt% SiO2,30.47 wt% B2O3, and 7.03 wt% Na2O. The glass was theninvestigated using dilatometry and characterized by a transi-tion temperature of 736 K and a softening point of 918 K.These values specify the approximate operating range of theglass. Previous studies have shown that a phase separationabove the softening point was possible and, thus, the glasswas suitable for following deforming experiments. For acomparative illustration the initial glass was phase separatedwithout deforming at 923 K for 2400 min followed by a cou-pled acidic-alkaline leaching treatment. The obtained averagepore diameter was 108 nm and the specific pore volume was0.48 cm3/g. The pore size distribution is shown in Fig. 3(A).

(2) Stretch FormingBecause the samples were stretched, the varying times untilfracture are a function of the stretching temperature. Thiscan be explained with a decrease of the viscosity of the glasswith an increasing temperature. That means the higher thestretching temperature the easier is the stretching of the glass,but the faster it will break even.

The three samples (B, C, and D) were treated at differenttemperatures and times and exhibit various degrees of poreorientation as shown in Fig. 3. It is important to recognizethat the higher temperatures of stretching and, accordingly,the shorter the period of times of the stretching process,lower the achieved degree of pore alignment.

Fig. 2. Different steps of the deforming procedure: (A) initial glassblock, (B) sawn glass plate, (C) and (D) stretched glass plates.

Table I. Prepared Samples

Sample m (kg) T (K) t (min)

A 923 2400B 1.5 923 3000C 1.5 943 280D 1.5 953 80

(A) (B)

(C) (D)

Fig. 3. SEM-images of porous glass with different degrees of poreorientation: (A) non-stretched, (B), (C), and (D) stretched atdifferent temperatures.

Fig. 1. Schematic assembling of the glass-stretching apparatusdeveloped by Borek.10

Table II. Textural Properties of the Prepared Samples

Sample dpA (nm) dpM (nm) Vp (cm3/g) Os (m

2/g)

A 108 111 0.48 B 249 304 0.42 43C 160 171 0.54 39D 122 111 0.64 37

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Due to the very different times of stretching and conse-quently, the time to phase separate, the samples are ofvarying average pore diameter. It is known that the tem-perature and time on the phase separation interdepend.Same pore sizes can be achieved either by annealing athigh temperature for a short time or at a lower tempera-ture for a longer period. In the case of our experiments,the temperature varies only slightly in a range between 923and 953 K. The time varied between 80 min and up to50 h. Thus, the strong deviation of the pore distribution ofsamples can be explained. The textural properties of thesamples are summarized in Table II, the intrusion curvesand pore size distributions from the mercury intrusion areshown in Fig. 4.

IV. Conclusions

Porous glasses with an aligned pore system and variousdegrees of pore orientation were successfully prepared forthe first time via a combination of phase separation andstretch-forming process. For that purpose, an initialsodium borosilicate glass was molten whose softeningpoint is in the same temperature range as the phase sepa-ration. The stretch-forming process was performed with aspecially developed glass-extending machine at differenttemperatures and stretching times. These varied parame-ters resulted in different degrees of deformation and porealignment.

References

1F. Janowski and D. Enke, Porous Glasses; pp. 1432542 in Handbook ofPorous Solids, Vol. 3, Edited by F. Schu th, K. S. W. Sing and J. Weitkamp.Wiley-VCH, Weinheim, 2002.

2D. Enke, F. Janowski, and W. Schwieger, Porous Glasses in the 21stCentury A Short Review, Microporous Mesoporous Mater., 60, 1930 (2003).

3M. Beiner, G. T. Rengarajan, S. Pankaj, D. Enke, and M. Steinhart,Manipulating the Crystalline State of Pharmaceuticals by Nanoconfinement,Nano Lett., 7 [5] 13815 (2007).

4J. Kullmann, D. Enke, S. Thraenert, R. Krause-Rehberg, and A. Inayat,

Characterization of Nanoporous Monoliths Using Nitrogen Adsorption andPositronium Annihilation Lifetime Spectroscopy, Colloids Surf. A, 357, 1720(2010).

5C. Chmelik, D. Enke, P. Galvosas, O. Gobin, A. Jentys, H. Jobic, J. Kaer-ger, C. B. Krause, J. Kullmann, and J. Lercher, Nanoporous Glass as aModel System for a Consistency Check of the Different Techniques of Diffu-sion Measurement, Chem. Phys. Chem., 12 [6] 11304 (2011).

6T. Isobe, Y. Kameshima, A. Nakajima, and K. Okada, Preparation andProperties of Porous Alumina Ceramics With Uni-Directionally OrientedPores by Extrusion Method Using a Plastic Substance as a Pore Former,J. Eur. Ceram. Soc., 27, 616 (2007).

7T. Isobe, T. Tomita, Y. Kameshima, A. Nakajima, and K. Okada, Prepa-ration and Properties of Porous Alumina Ceramics With Oriented CylindricalPores Produced by an Extrusion Method, J. Eur. Ceram. Soc., 26, 95760(2006).

8A. Hesse, S. Mrotzek, D. Hu lsenberg, and E. Ra dlein, High Aspect RatioGlass Structures Produced by Means of the Drawing Technology, Microsyst.Technol., 14, 15414 (2008).

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J. A. Morehouse, L. S. Worrel, D. L. Taylor, D. R. Lloyd, B. D. Freeman,and D. F. Lawler, The Effect of uni-Axial Orientation on MacroporousMembrane Structure, J. Porous Mater., 13, 6172 (2006).

10R. Borek, K.-J. Berg, and G. Berg, Low-Temperature Tensile Deforma-tion of Flat Glass Containing Metal Particles to Generate Dichroism, GlassSci. Technol., 71 [12] 3529 (1998). h

(A) (B)

(C) (D)

Fig. 4. Mercury intrusion curves and pore size distribution of the prepared samples.

October 2012 Rapid Communications of the American Ceramic Society 3015

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Preparation of Porous Glass Monoliths with an Aligned Pore System via Stretch Forming.pdf