J. Phys. Chem. B 2001, 105, 11949-11954
11949
ARTICLES Effect of Additive ZrO2 on Spinodal Phase Separation and Pore Distribution of Borosilicate Glasses Wei-Fang Du,*,† Koji Kuraoka,‡ Tomoko Akai,‡ and Tetsuo Yazawa‡ New Energy and Industrial Technology DeVelopment Organization, Midorigaoka 1-8-31, Ikeda City, Osaka 563-8577, Japan and National Institute of AdVanced International Science and Technoloy, Midorigaoka 1-8-31, Ikeda City, Osaka 563-8577, Japan ReceiVed: March 29, 2001; In Final Form: September 6, 2001
Sodium borosilicate porous glasses arising from spinodal phase separation are promising functional materials for separation membranes, enzyme and catalyst supports, and photonic materials. The present paper studied the effect of the additive of ZrO2 on the spinodal phase separation and pore distribution of the sodium borosilicate glasses using 11B nuclear magnetic resonance spectrum, 29Si nuclear magnetic resonance spectrum, mercury measurement, and nitrogen adsorption techniques. The experimental results showed that ZrO2 inhibited both the initiation process in early stage and the coarsening process in later stage of the spinodal phase separation. The pore volume was found to decrease slightly with the addition of ZrO2 at the beginning. However, when ZrO2 content > 7 mass%, the pore volume decreases dramatically with further addition of ZrO2. 65% of the pore volume in the sample without addition of ZrO2 will be lost when the addition amount of ZrO2 increases from 0 to 10 mass%. The inhibition effect on the pore volume is due to the structural change of boron network by the introduction of ZrO2. The oxygen defects, such as oxygen vacancy, which initiates the spinodal phase separation, are reduced during the transformation process from four-coordinated boron to three-coordinated boron by the introduction of ZrO2. The growth of the pore size of the sample, which is controlled by the dynamic process of coarsening in the later stage of the spinodal phase separation, is also inhibited by the introduction of ZrO2. With the addition of zirconia, the three-coordinated boron with stronger bond energy increases. This may reduce the movement of the oxygen-contained boron groups during mass transfer of the coarsening process of the spinodal phase separation, and consequently inhibit the growth of the pore size.
Sodium borosilicate is a typical spinodal phase separation glass and source materials for porous glass, which is finding widespread application in industry.1-3 By the spinodal phase separation, two interconnected phases, one rich in silica and the other rich in boric oxide, are thereby induced. These two phases differ in their solubility behavior: when treated with dilute acid, the boron-rich phase readily dissolves, leaving the silica-rich phase almost untouched. This results in a highly porous glass. On the basis of this spinodal phase separation, the porous glasses with highly connective micropore, mesopore, and macro-pore, which are promising materials for separation membranes, enzyme and catalyst supports, photonic materials, and so forth4-14 can be obtained. However, the porous glasses arisen from sodium borosilicate glasses have a fatal defect in practical application, i.e., the water and alkali resistance is insufficient. The amorphous SiO2 has a
water solubility of about 100 ppm at room temperature.15 Therefore, it is very difficult to apply the porous glasses in aqueous solution for long time. It was found that the water and alkali resistance of the porous glasses arisen from sodium borosilicate glasses could be markedly improved by the addition of ZrO2 to the glass composition. The effect of the ZrO2 on the increase of the water and alkali resistance is extensively explored in our previous work,16 whereas the effect of the addition of ZrO2 on spinodal phase separation and resultant pore distribution of the sodium borosilicate glasses is still ambiguous. From the point of the view in the design of porous glass materials, the pore volume and surface area should be as large as possible after the addition of ZrO2 to keep the high work efficiency of the porous materials in practical industrial application. However, it is still lack of experimental data about where is the optimum composition at which the maximum pore volume and surface area could be obtained after the addition of ZrO2.
* To whom correspondence should be addressed. Present address: Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario K1A 0R6, Canada. E-mail:
[email protected] † New Energy and Industrial Technology Development Organization. ‡ National Institute of Advanced International Science and Technology.
In this paper, we extensively studied the effect of the additive of ZrO2 on the spinodal phase separation and pore distribution of the sodium borosilicate glasses using 11B nuclear magnetic resonance spectrum, 29Si nuclear magnetic resonance spectrum, mercury measurement, and nitrogen adsorption techniques.
1. Introduction
10.1021/jp0111970 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/02/2001
11950 J. Phys. Chem. B, Vol. 105, No. 48, 2001
Du et al.
2. Experimental Section 2.1. Sample Preparation. A typical glass with the composition of 9.4Na2O-25.4B2O3-65.2SiO2 (mol %), which can exhibit spinodal phase separation by heat treatment, was used in this investigation. Sodium borosilicate glasses with the addition of 0∼10 mass% ZrO2 were prepared from reagent grade sodium carbonate, boric acid, silica, and ZrO2. First, reagent-grade chemicals were thoroughly mixed. Then the mixtures were placed in a platinum crucible and fused at 1400 °C in an electric furnace for 60 min. The melts were poured into carbon molds. The glasses were transparent and showed no sign of devitrification when checked by X-ray diffraction. The heat-treatment was carried out at 600 °C for 24 h for the development of phase separation. 2.2. NMR Measurements. (1) 11B NMR Measurement. 11B NMR measurement was performed to check the variation of the boron network structure of the borosilicate glasses with different amount of sodium oxide. The 11B NMR experiments were performed at 64.19 MHz. The spectra were obtained by Fourier transforming free-induction decay. The length of 90° pulse was set to 4.5 µs by using H3BO3 solution. For the measurement, we used a pulse as short as 1 µs to avoid the distortion of the spectra from quadrupole effects.17 (2) 29Si Magic Angle Spinning (MAS) NMR Measurement. FT-NMR techniques in conjunction with MAS were used to obtain 29Si spectra. 29Si MAS NMR spectra were collected under 4.7 T with 39.789 MHz. A pulse width of 3 µs and dead time 20 µs were used. Spinning rate was 3000 Hz and the spectrum was accumulated 3200 times. 2.3. Measurement of the Domain Size of Phase Separation. Pore size was measured after leaching out boron-rich phase in samples so as to estimate the domain size of the phase separation. The phase-separated glasses prepared by above methods were etched using an aqueous solution of 0.25N HNO3 at 371 K and were washed with water and dried. The specific surface areas and the pore size distribution of the samples were determined by nitrogen adsorption (Belsorp 28, Bel Japan Inc.) or mercury method. Prior to the measurement, the samples were evacuated at 423 K for 3 h. The specific surface areas were calculated by BET method,18 and the pore size distribution was analyzed by MP method.19 3. Results 3.1. Dependence of Pore Volume on Additive ZrO2. Figure 1 shows the dependence of pore volume on the amount of additive ZrO2. In general, the pore volume decreases with increasing content of ZrO2, showing inhibition effect of ZrO2 on porous structure. In addition, two different decrease tendency of pore volume with ZrO2 content can be observed in the curve of the pore volume vs ZrO2 content. When ZrO2 content e 7 mass%, the pore volume decreases slightly with the addition of ZrO2. The pore volume only reduces 0.17 mL/g when ZrO2 content increases from 0 into 7%. However, when ZrO2 content > 7 mass%, the pore volume begins to dramatically decrease with further addition of ZrO2. It can be seen from Figure 1 that the pore volume greatly decreases from 0.53 mL/g to 0.24 mL/g with further 2% increase in ZrO2 content. We have checked another composition with higher ZrO2 content of 12 mass%. The glass with this higher ZrO2 content of 12 mass% was prepared by fusing raw materials at 1600 °C for 90 min. After heat treatment and leaching, we nearly could not observe any peaks from the pore size distribution pattern, i.e., the pore volume and the surface area approached the value of zero.
Figure 1. Dependence of the pore volume on the amount of additive ZrO2 for the sodium borosilicate glasses after introduction of phase separation by a heat treatment under 600 °C for 24 h and the remove of boron-rich phase by acid leaching.
3.2. Pore Size Distribution with Different Amount of Additive Zr2O. Figure 2 shows pore size distribution with different amount of additive ZrO2 after 24 h of heat treatment at 600 °C. The peaks of the pore size distribution shifts to low value with the increase of the additive amount of ZrO2. Figure 3 shows the dependence of the average pore diameter on ZrO2 concentration. The results exhibit that ZrO2 has great inhibition effect on the pore size growth. The borosilicate glass without ZrO2 addition has a porous structure with an average size of 52.7 nm after phase separation and acid leaching. However, The porous structure in the borosilicate glass with 10 mass% ZrO2 can only reach an average pore size of 29.9 nm after phase separation and acid leaching under the same condition. The pore diameter decreases with the increase of the amount of ZrO2 additive. 3.3. Dependence of Surface Area on Additive ZrO2. Figure 4 shows the dependence of surface area on the amount of additive ZrO2. The surface area increases with the addition of ZrO2 at beginning. However, when ZrO2 content > 7 mass%, the surface area dramatically decreases with the further addition of ZrO2. The variation of the surface area depends on the pore diameter and pore volume. The surface area of the porous glass (Sa) increases with pore volume (Pv), whereas decreases with pore diameter (D); that is
Sa ∝ Pv/D
(1)
The result of the measurement shows that, in the beginning, pore size is a control factor for determining the surface area. Therefore, the surface area decreases with the addition of ZrO2. When ZrO2 content > 7 mass%, the pore volume begin to dramatically decrease and become a control factor for determining the surface area. Thus, the surface area of the porous glass decreases with the further addition of ZrO2. 4. Discussion 4.1. Inhibition Effect of ZrO2 on the Initiation of Spinodal Phase Separation. The dependence of pore volume on the addition of ZrO2 indicates that ZrO2 has an inhibition effect on initiation of spinodal phase separation. The propagation of spinodal phase separation includes two stages, the initial stage and the later stage. In the initial stage, usually within minutes of heat treatment, boron-rich phase will completely separate out through matrix. This initial stage is characterized with the
Effect of Additive ZrO2 on Borosilicate Glasses
J. Phys. Chem. B, Vol. 105, No. 48, 2001 11951
Figure 2. Pore size distribution with the different amount of additive ZrO2 for the sodium borosilicate glasses after introduction of phase separation by a heat treatment under 600 °C for 24 h and the remove of boron-rich phase by acid leaching.
Figure 3. Dependence of the average pore diameter on the amount of ZrO2.
continuous growing of the concentration of boron in the boronrich phase. Whereas in the later stage, as the phase-separated region reaches saturation, the main characteristics in this stage is the growth of the size of the boron-rich phase by coarsening, i.e., by the transfer of materials from one particle to another one.20-21 As the volume ratios of the coexisting phases in glasses always approach an equilibrium value in the later stage, the volume of the boron-rich phase does not change with further heat treatment. Thus, the pore volume and surface area obtained after 24-h heat treatment in our experiments should correspond to the maximum volume of boron-rich phase, which could be separated from matrix in initial stage. This suggests that the lager the pore volume, the more boron-rich phase could separate from matrix in the initial stage.
Figure 4. Dependence of surface area on the amount of additive ZrO2 for the sodium borosilicate glasses after introduction of phase separation by a heat treatment under 600 °C for 24 h and the remove of boronrich phase by acid leaching.
As shown in Figure 1, the pore volume decreases with the increase of ZrO2. This confirms the inhibition effect of ZrO2 on the initiation of spinodal phase separation. Increasing the amount of ZrO2 leads to smaller boron oxides separating from matrix. As the spinodal phase separation is essentially caused by the rearrangement and aggregation of boron and silicon network, to further clarification of the mechanism of the inhibition effect of ZrO2 on the initiation of the spinodal phase separation, we also checked the variation of coordinated numbers of B and Si in the glass network with the addition of ZrO2 by the spectrum of 11B NMR and 29Si NMR. The typical 11B and 29Si NMR spectra of the melt-quenched samples are shown in Figure 5 and Figure 6, respectively.
11952 J. Phys. Chem. B, Vol. 105, No. 48, 2001
Du et al.
Figure 5. Typical 11B NMR spectra of samples with different amount of ZrO2.
Figure 5 shows that the melt-quenched sample with deferent amount of ZrO2 additive possesses two strong peaks from threecoordinated boron, and a weak peak from four-coordinated boron. Bray et al. have extensively studied the structure of the homogeneous glass containing boron by NMR.22-31 It has been shown that boron atoms exist as either 3-fold or 4-fold coordination with network oxygen. By Figure 5, it is apparent that the boron network in the sample without ZrO2 additive is mainly composed of three-coordinated boron. Quantitative calculation shows the boron network of the sample without ZrO2 is composed of 71% of three-coordinated boron (N3) and 29% of four-coordinated boron (N4). With the addition of ZrO2, the peaks from three-coordinated boron in the 11B NMR spectra become stronger, which displaying that some of the boron network changed from four-coordinated boron into threecoordinated boron by the introduction of ZrO2. The dependence of the fraction of four-coordinated boron on the amount of ZrO2 additive is shown in Figure 7, exhibiting the decrease tendency of the fraction of the four-coordinated boron with the addition of ZrO2. The fraction of the four-coordinated boron decreases from 29.08% to 13.18% when the addition of ZrO2 increases from 0 to 10 mass%.
The structural change of the four-coordinated boron into the three-coordinated boron by the introduction of ZrO2 has a close relation with the spinodal phase separation, and finally, it will affect the pore volume we can get after acid leaching. The study of the mechanism of spinodal phase separation in the initial stage in the sodium borosilicate glasses has showed that the spinodal phase separation is controlled by oxygen diffusion, which results from the structural defects of oxygen atoms such as oxygen vacancies. There are some evidences that these structural defects of oxygen atoms are very sensitive to the transformation process between the structure of three-coordinated boron and the structure of four-coordinated boron6,33-35. Prabakar and Rao22 have found that at some topologically feasible positions in sodium borate binary glasses, threecoordinated borons and two-coordinated oxygens get coupled forming BO4 and BO3 units, resulting in the formation of the over-coordination of oxygen atoms during the transformation process between three-coordinated boron and four-coordinated boron. From the point of the view in topochemistry, the formation of the over-coordination will cause a staggering of B-O chains. The over-coordinated oxygen atoms where they coordinated trigonal borons are positioned in other layers where
Effect of Additive ZrO2 on Borosilicate Glasses
J. Phys. Chem. B, Vol. 105, No. 48, 2001 11953
Figure 8. Removal of the oxygen vacancy by the introduction ZrO2 during the transformation process from four-coordinated boron to threecoordinated boron.
Figure 6. Typical 29Si NMR spectra of samples with different amount of ZrO2.
Figure 7. Dependence of the fraction of four-coordinated boron on the amount of ZrO2 additive.
The status of Si network with the addition of ZrO2 can be seen from 29Si NMR spectra. As shown in Figure 6, the 29Si NMR spectra remains unchangeable when the addition amount of ZrO2 increases from 0 to 10 mass%. This indicates that Si network are not affected by the addition of ZrO2. The results further support the above analyses about the mechanism that the spinodal phase separation in the initial stage is inhibited mainly by the transformation from four-coordinated boron to three-coordinated boron in boron network. 4.2. Inhibition Effect of Zr2O on the Coarsening Process of Spinodal Phase Separation in Later Stage. The variation of pore size, as shown in Figure 2-3, is controlled by the dynamic process of coarsening of the boron-rich phase in the later stage of the spinodal phase separation, which is the growth of large inhomogeneity regions by the combination of smaller ones. The decrease of the system’s free energy by decrease of interface area is the driving force of this process. The essential description of the dynamic process of the coarsening in the later stage can be found in Langer’s papers. The mean radius of the inhomogeneity regions at this stage increases with interdiffusion coefficient and heat treatment time according to following law
r ∼ Dt1/3 tetrahedral borons are simultaneously coordinated by the overcoordinated oxygen atoms. The staggering of the B-O chains caused by such over-coordination of oxygen atoms can be thought as in an active state of structural defects and is very easy to finally transfer into three-coordinated boron by the introduction of ZrO2. As shown in Figure 8, the structural defects of oxygen vacancies, which initiate phase separation, are thus removed by the introduction of ZrO2. Therefore, the process of the transformation from the four-coordinated boron into threecoordinated boron by the introduction of zirconia may also inhibit the spinpodal phase separation in the initial stage. In addition, the bond energy of the three-coordinated boron, which is 119 kcal/mol, is stronger than that of the four-coordinated boron, which is 89 kcal/mol. Thus, the -B-O-Si- bond with three-coordinated boron is more difficult to break than that with four-coordinated boron. This can lead to more difficult aggregation of boron group, which may also be one of the causes that the increase of three-coordinated boron by the introduction of ZrO2 inhibits the spinodal phase separation in the initial stage.
(2)
where r is the mean radius of the inhomogeneity regions or the mean radius of pore after leaching; D is the interdiffusion coefficient; and t is the heat treatment time. The interdiffusion coefficient is determined by the slowest moving species, which is most likely oxygen.36-38 Although the oxygen diffusion coefficient is strongly affected by viscosity in the molten state far from glass transition temperature, some investigation confirms that there is little effect of the viscosity on oxygen diffusion when temperature approaches or is below the glass transition temperature,39-40 as that in our experiment. Therefore, the diffusion process including mass transfer in the coarsening stage of the boron-rich phase is mostly possible to be also controlled by the coordination structure of the oxygencontained boron groups. With the addition of zirconia, the fourcoordinated boron decreases and three-coordinated boron increases, as showed in Figure 5 and Figure 7. Because the bond energy of the three-coordinated boron is stronger than that of the four-coordinated boron, the movability of the oxygencontained boron groups with three-coordination is weaker than
11954 J. Phys. Chem. B, Vol. 105, No. 48, 2001 that with four-coordination during mass transfer. Consequently, the coarsen process is inhibited by the addition of zirconia. Thus, the pore size continuously decreases with the additive of zirconia. 5. Conclusions ZrO2 shows inhibition effect on both the initiation process in early stage and the coarsening process in later stage of the spinodal phase separation. (1) The pore volume is found to decrease slightly with the addition of ZrO2 at the beginning. However, when ZrO2 content > 7 mass%, the pore volume decrease dramatically with further addition of ZrO2. 65% of the pore volume in the sample without addition of ZrO2 will be lost when the addition amount of ZrO2 increases from 0 to 10 mass%. The inhibition effect on the pore volume is due to the structural change of boron network by the introduction of ZrO2. The oxygen defects such as oxygen vacancy, which initiates the spinodal phase separation, are reduced during the transformation process from four-coordinated boron to three-coordinated boron by the introduction of ZrO2. (2) The pore size of the sample, which is controlled by the dynamic process of coarsening in the later stage of the spinodal phase separation, is also inhibited by the introduction of ZrO2. With the addition of zirconia, the three-coordinated boron with stronger bond energy increases. This may reduce the movement of the oxygen-contained boron groups during mass transfer of the coarsening process of the spinodal phase separation, and consequently inhibit the growth of the pore size. (3) The surface area is mainly affected by the pore size variation at beginning, and thus increases with the addition of ZrO2 when ZrO2 e 7 mass%. However, when ZrO2 content > 7 mass%, the pore volume begin to dramatically decrease and become a control factor for determining the surface area. Thus, the surface area of the porous glass decreases with the further addition of ZrO2. References and Notes (1) Yazawa, T.; Key Eng. Mater. 1996, 15, 125. (2) Kokubu, T.; Yamane, M. J. Mater. Sci. 1985, 20, 4309. (3) Maekawa, H.; Maekawa, T.; Kawamura, K.; Yokokawa, T. J. NonCryst. Solids, 1991, 127, 53. (4) Du, W. F.; Kuraoka, K.;.Yazawa, T. J. Mater. Chem. 1999, 9, 2723-2725. (5) Yazawa, T.; Kuraoka, K.; Du, W. F. J. Phys. Chem. B, 1999, 103, 9841-9845. (6) Yazawa, T.; Kuraoka, K.; Akai, T.; Umesaki, N.; DU, W. F. J. Phys. Chem. B, 2000, 104, 2109-2116.
Du et al. (7) Yazawa, T.; Tanaka, H.; Eguchi, K.; Yokoyama, S.; Arai, T. J. Mater. Sci. Lett., 1993, 12, 263. (8) Yazawa, T.; Tanaka, H.; Eguchi, K. J. Mater. Sci. Lett. 1993, 13, 494. (9) Yazawa, T.; Miyake, A.; Tanaka, H. J. Ceram. Soc. Jpn. 1991, 99, 1094. (10) Yazawa, T.; Tanaka, H.; Nakamichi, H.; J. Ceram. Soc. Jpn. 1991, 99, 1271. (11) Yazawa, T.; Tanaka, H.; Nakamichi, H.; Eguchi, K. J. Ceram. Soc. Jpn. 1988, 96, 630. (12) Yazawa, T.; Tanaka, H.; Nakamichi, H.; Eguchi, K. J. Ceram. Soc. Jpn. 1988, 96, 18. (13) Kuraoka, K.; Tanaka, H.; Yazawa, T. J. Mater. Sci. Lett. 1996, 15, 1. (14) Kuraoka, K.; Qun, Z.; Kushibe, K.; Yazawa, T. Sep. Sci. Technol. 1998, 33, 297. (15) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1976; p.31. (16) Yazawa, T.; Tanaka, H.; Eguchi, K.; Yokoyama, S. J. Mater. Sci., 1994, 29, 3433. (17) Fenzke, D.; Freude, D.; Frolich, T.; Hasse, J. Chem. Phys. Lett. 1984, 111, 171. (18) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (19) Mikhail, R. S.; Brunauer, S.; Bodor, E. E. J. Colloid Interface Sci. 1968, 26, 45. (20) M. B. Volf Chemical Approach to Glass; Elsevier Science Publisher: New York, 1984. (21) A. Paul Chemistry of Glassesl; Chapman and Hall, Ltd.: London, 1982. (22) Yun, Y. H.; Bray, P. J. J. Noncrystalline Solids 1978, 27, 363. (23) Gresch, R.; Muller-Warmuth, W.; Dutz, H. J. Noncrystalline Solids 1976, 21, 31. (24) Milberg, M. E.; O’Keefe, J. G.; Verhelst, R. A.; Hooper, H. O. Phys. Chem. Glasses 1972, 13, 79. (25) Beekenkamp, P. Phys. Chem. Glasses 1968, 9, 14. (26) Bishop, S. G.; Bray, P. J. Phys. Chem. Glasses 1966, 7, 73. (27) Rhee, C.; Bray, P. J. Phys. Chem. Glasses 1971, 12, 165. (28) Park, M. J.; Bray, P. J. Phys. Chem. Glasses 1972, 13, 50. (29) Kim, K. S.; Bray, P. J. J. Nonmetal. 1974, 2, 95. (30) Kim, K. S.; Bray, P. J. Phys. Chem. Glasses 1974, 15, 47. (31) Kim, K. S.; Bray, P. J.; Merrin, S. J. Chem. Phys. 1976, 64, 4459. (32) Kriz, H. M.; Park, M. J.; Bray, P. J. Phys. Chem. Glasses 1971, 12, 45. (33) DU, W. F.; Kuraoka, K.; Akai, T.; Yazawa, T. J. Mater. Sci. 2000, 35, 3913. (34) Prabakar, S.; Rao, K. J.; Rao, C. N. R. Proc. R. Soc. Lond. A 1990, 429, 1. (35) Mazurin, O. V.; Porai-Koshits, E. A. Phase Separation in Glass; North-Holland: New York 1984. (36) Tomozawa, M.; MacCrone, R. K.; Herman, H. Phys. Chem. Glasses 1970, 11, 136. (37) Burnett, D. G.; Douglas, R. W. Phys. Chem. Glasses 1970, 11, 125. (38) Hammel, J. J. J. Chem. Phys. 1967, 46, 2234. (39) Milne, A. J. J. Soc. Glass Tech. 1952, 36, 275. (40) Oishi, Y.; Ueda, H.; Terai, R. 10th Int. Congr. Glass; Kyoto 1974; pp 8-30.
