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Chain-like structure of ultra-low density SiO2 sol-gel glass observed by TEM
148
Journal of Non-Crystalline Solids 82 (1986) 148-153 North-Holland, Amsterdam
C H A I N - L I K E S T R U C T U R E OF U L T R A - L O W D E N S I T Y SiO 2 S O L - G E L G L A S S O B S E R V E D BY TEM C.A.M. M U L D E R , G. van L E E U W E N - S T I E N S T R A and J.G. van L I E R O P Philips Research Laboratories, 5600 JA Eindhoven, The Netherlands
J.P. W O E R D M A N Huygens Laboratorium, Rijksuniverstiteit Leiden, 2300 RA Leiden, The Netherlands
We report a study of low density (0.03 < p < 0.2 g/cm3) porous SiO2 glass prepared by the sol-gel process. From TEM micrographs we observe that the structure of this ultra-low density sol-gel glass is characterized by SiO2 spheroids (diameter _<20 nm) arranged along branched chains. By comparing these measurements with the specific surface area of the gels we conclude that these spheroids have a rather low density.
1. Introduction The low-temperature synthesis of glass via the gel route involves first the hydrolysis and polymerization of solutions of silicon-alcoholates. After drying the (monolithic) gel is densified to highly pure fused quartz [1,2]. The microstructure of the gel and the mechanisms which operate during the gel-to-glass conversion have been inferred from physical measurements [3-5]. Until now only few direct observations by transmission electron microscopy (TEM) of gels have been reported [3,6-8]. In this study we present TEM micrographs of a series of extremely-low density silica gels. TEM reveals information about the shape of the gel constituents and about the highly branched chain structure of the porous network. This information is compared with results of specific surface area measurements.
2. Experimental Monolithic gels were prepared as described by Van Lierop et al. [9]. First tetraethylorthosilicate (TEOS) soluted in ethanol is partly hydrolysed in an acid medium at 50°C. Next polymerization is stimulated in a basic medium. The structure of the thus produced porous silica is determined by the acidity of the initial solution. The extremely-low density gels were obtained by diluting the starting solution with ethanol. Table 1 summarizes the composition of the solutions and gives some physical properties. All gels were dried by supercriti0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
C.A.M. Mulder et al. / Ultra-low density SiO 2 sol-gel glass
149
Table 1 Experimental data of gels Gel
Solution TEOS/alcohol/ water (molar)
Acid-base ratio (vol. parts)
Gel density ( g / c m 3)
Particle diameter (nm) ")
Specific surface area (m2/g)
Sphere sensity ( g / c m 3 ) b)
1 2 3
1 : 2 :2 1 : 12 : 2 1:20:2
2 : 30 3 : 29 6:26
0.15 0.05 0.03
20 10 8
520 1080 1590
0.38 0.37 0.31
a) Approximate diameter of the particles as derived from the TEM pictures. b) Calculated from the specific surface area and particle diameter measurements taking into account the degree of interconnection of the particles (see text).
cally heating in an autoclave (300°C), applying a prepressure of 80 bar nitrogen gas [9]. Afterwards the samples were dried at 700°C in an oxygen ambient. TEM samples were obtained by cutting small pieces of gel with a scalpel on a microscope slide. The gel material is crumbled between two microscope slides. A formvar prepared copper grid is slightly touched against the thinnest part of the sample on one of the microscope slides. Now the sample can be directly studied in a transmission electron microscope (Philips EM400T).
3. Results and discussion
A typical TEM micrograph of gel 1 (density 0.15 g / c m 3) is shown in fig. 1. Part of the three-dimensional structure of the gel is projected in the plane of the photograph. Small globular particles can be seen which are in contact with each other leaving little pore space. The totally interconnected particle structure is lost due to the sample preparation. Only a carbon replica from a freshly fractured surface of the gel reveals the completely entangled network structure (see fig. 2). A more detailed micrograph of gel 1 is depicted in fig. 3 in a stereo combination; in going from one to the other image the sample was tilted by 3 ° about an axis vertical to the electron beam. Due to particle stacking in the electron beam direction, an appreciable fraction of the particles on these high magnification TEM photographs is out of focus. The formvar film is chosen to be in focus for all micrographs, such that an increasing width of the (first) dark Fresnel fringe around a particle indicates that it, geometrically, lies further away from the focal plane. Viewing fig. 3 in stereo by looking with each eye at the appropriate photograph, shows the local three-dimensional structure of the gel. Figs. 4 and 5 show part of the microstructure of the ultra-low density gels 2 (density 0.05 g / c m 3) and 3 (density 0.03 g/cm3), respectively.
150
C.A.M. Mulder et al. / Ultra-low density SiO 2 sol-gel glass
~
~ 200nm
Fig. 1. Electron micrograph of silica gel 1 described in table 1.
200 Fig. 2. Electron micrograph of a carbon replica of a freshly fractured surface of gel 1.
nm
C.A.M. Mulder et al. / Ultra-low density SiO 2 sol-gel glass
151
Fig. 3. Stereo-pair of electron micrographs of gel 1.
F r o m the T E M micrographs it can be observed that the gels consist of small spheroidal particles arranged along branched chains: It is noted that all particles, if not darkened by another one display a homogeneous contrast. The diameter of the particles ranges from 20 nm for gel 1 to about 10 n m and 8 nm for gels 2 and 3, respectively (see table 1). The chain-like structure is especially
Fig. 4. Electron micrograph of silica gel 2 described in table 1.
152
CA.M. Mulder et al. / Ultra-low density SiO 2 sol-gel glass
50nm i
J
Fig. 5. Electron mlcrograph of silica gel 3 described in table 1.
evident for gel 1, whereas the ultra-low density gels 2 and 3 exhibit a more compact structure due to extensive branching. In addition, the pore space between the particles for the latter samples appears to be less than for the higher density gels. This is the more peculiar if one has to imagine an almost completely filled gel structure of a density more than 70 times smaller than that of bulk fused silica. We discuss this in the next paragraph. In table 1 results of specific (BET) surface area measurements of the gels accounting for the surface accessible to N 2 only are included. From the specific surface area A s and the diameter d of the particles derived from the TEM micrographs, one can calculate the apparent density of the material in the spherical particles, Psph, according to the formula,
p,ph = C / ( . 4 , d ). For free spheres C = 6, if A s is expressed in m2/kg, d in m and p in k g / m 3. Due to the interconnection of the spheroidal particles, the value of C decreases for decreasing effective surface area. From our TEM micrographs we estimate C - 4. Then the apparent density of the particles for the studied ranges of the three gels varies from 0.4 to 0.3 g / c m 3 (see table 1). So, the spherical particles must be very porous structures themselves. Such fine porosity would not be probed by the N 2 BET measurements (lower detection limit - 1.5 nm). For the ultra-low density gels 2 and 3 the gel density is a factor of about 10 smaller than the apparent particle density. As the electron microscope typically reveals
C.A.M. Mulder et al. / Ultra-low density SiO 2 sol-gel glass
153
an isotropic contrast within the particles of the gel, these spheres must consist of extremely small SiO 2 entities. Such entities would be discernible in the TEM if they were larger than, say, 0.5 nm. The above results should be compared to those of Brinker et al. [3] reporting on the sol-gel transition in simple silicates, who observed gels to consist of coarse particles (90 nm) which in turn were made up of much smaller ones ( < 10 nm). This was interpreted as a hierarchical random close packing of dense spheres (Brinker and Scherer, in ref. 8).
4. Conclusion
Ultra-low density porous SiO2 gels are obtained by using strongly diluted initial solutions. After supercritical drying in a prepressurized autoclave, the structure of these gels is characterized in the TEM as a highly branched network of low density globular SiO2 particles. Very high values for the specific surface area (up to 1600 m2/g for a gel of density 0.03 g / c m 3) indicate that the spheroidal particles must be very porous structures themselves, consisting of extremely small SiO 2 entities. A detailed TEM study of the densification of low density gels to glass is presented at this conference [10].
References [1] [2] [3] [4] [5] [6] [7] [8]
[9] [10]
H. Dislich, Angew. Chem. Inst. Ed. 10 (1971) 363. B.E. Yoldas, J. Mater. Sci. 12 (1977) 1203. C.J. Brinker, K.D. Keefer, D.W. Schaefer and C.S. Ashley, J. Non-Cryst. Solids 48 (1982) 47. C.J. Brinker, K.D. Keefer, D.W. Schaefer, R.A. Assink, B.D. Kay and C.S. Ashley, J. Non-Cryst. Solids 63 (1984) 45. J. Zarzycki, in: Glas...Current Issues, eds. A.F. Wright, J. Dupuy (Nijhoff, Dordrecht, The Netherlands, 1985) p. 203. C.R. Veale, Fine Powders: Preparation, Properties and Uses (Applied Science Publ., London, 1972). M. Prassas, J. Phalippou and J. Zarzycki, J. Mater. Sci. 19 (1984) 1656. C.J. Brinker and G.W. Scherer, J. Non-Cryst. Solids 70 (1985) 301; C.J. Brinker, G.W. Scherer and E.P. Roth. J. Non-Cryst. Solids 72 (1985) 345; G.W. Scherer, C.J. Brinker and E.P. Roth, J. Non-Cryst. Solids 72 (1985) 369. J.G. van Lierop, A. Huizing, W.C.P.M. Meerman and C.A.M. Mulder, these Proceedings (Gels '85) J. Non-Cryst. Solids 82 (1986) 265. C.A.M. Mulder, J.G. van Lierop and G. Frens, these Proceedings (Gels '85) 82 (1986) 92.
Journal of Non-Crystalline Solids 82 (1986) 148-153 North-Holland, Amsterdam
C H A I N - L I K E S T R U C T U R E OF U L T R A - L O W D E N S I T Y SiO 2 S O L - G E L G L A S S O B S E R V E D BY TEM C.A.M. M U L D E R , G. van L E E U W E N - S T I E N S T R A and J.G. van L I E R O P Philips Research Laboratories, 5600 JA Eindhoven, The Netherlands
J.P. W O E R D M A N Huygens Laboratorium, Rijksuniverstiteit Leiden, 2300 RA Leiden, The Netherlands
We report a study of low density (0.03 < p < 0.2 g/cm3) porous SiO2 glass prepared by the sol-gel process. From TEM micrographs we observe that the structure of this ultra-low density sol-gel glass is characterized by SiO2 spheroids (diameter _<20 nm) arranged along branched chains. By comparing these measurements with the specific surface area of the gels we conclude that these spheroids have a rather low density.
1. Introduction The low-temperature synthesis of glass via the gel route involves first the hydrolysis and polymerization of solutions of silicon-alcoholates. After drying the (monolithic) gel is densified to highly pure fused quartz [1,2]. The microstructure of the gel and the mechanisms which operate during the gel-to-glass conversion have been inferred from physical measurements [3-5]. Until now only few direct observations by transmission electron microscopy (TEM) of gels have been reported [3,6-8]. In this study we present TEM micrographs of a series of extremely-low density silica gels. TEM reveals information about the shape of the gel constituents and about the highly branched chain structure of the porous network. This information is compared with results of specific surface area measurements.
2. Experimental Monolithic gels were prepared as described by Van Lierop et al. [9]. First tetraethylorthosilicate (TEOS) soluted in ethanol is partly hydrolysed in an acid medium at 50°C. Next polymerization is stimulated in a basic medium. The structure of the thus produced porous silica is determined by the acidity of the initial solution. The extremely-low density gels were obtained by diluting the starting solution with ethanol. Table 1 summarizes the composition of the solutions and gives some physical properties. All gels were dried by supercriti0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
C.A.M. Mulder et al. / Ultra-low density SiO 2 sol-gel glass
149
Table 1 Experimental data of gels Gel
Solution TEOS/alcohol/ water (molar)
Acid-base ratio (vol. parts)
Gel density ( g / c m 3)
Particle diameter (nm) ")
Specific surface area (m2/g)
Sphere sensity ( g / c m 3 ) b)
1 2 3
1 : 2 :2 1 : 12 : 2 1:20:2
2 : 30 3 : 29 6:26
0.15 0.05 0.03
20 10 8
520 1080 1590
0.38 0.37 0.31
a) Approximate diameter of the particles as derived from the TEM pictures. b) Calculated from the specific surface area and particle diameter measurements taking into account the degree of interconnection of the particles (see text).
cally heating in an autoclave (300°C), applying a prepressure of 80 bar nitrogen gas [9]. Afterwards the samples were dried at 700°C in an oxygen ambient. TEM samples were obtained by cutting small pieces of gel with a scalpel on a microscope slide. The gel material is crumbled between two microscope slides. A formvar prepared copper grid is slightly touched against the thinnest part of the sample on one of the microscope slides. Now the sample can be directly studied in a transmission electron microscope (Philips EM400T).
3. Results and discussion
A typical TEM micrograph of gel 1 (density 0.15 g / c m 3) is shown in fig. 1. Part of the three-dimensional structure of the gel is projected in the plane of the photograph. Small globular particles can be seen which are in contact with each other leaving little pore space. The totally interconnected particle structure is lost due to the sample preparation. Only a carbon replica from a freshly fractured surface of the gel reveals the completely entangled network structure (see fig. 2). A more detailed micrograph of gel 1 is depicted in fig. 3 in a stereo combination; in going from one to the other image the sample was tilted by 3 ° about an axis vertical to the electron beam. Due to particle stacking in the electron beam direction, an appreciable fraction of the particles on these high magnification TEM photographs is out of focus. The formvar film is chosen to be in focus for all micrographs, such that an increasing width of the (first) dark Fresnel fringe around a particle indicates that it, geometrically, lies further away from the focal plane. Viewing fig. 3 in stereo by looking with each eye at the appropriate photograph, shows the local three-dimensional structure of the gel. Figs. 4 and 5 show part of the microstructure of the ultra-low density gels 2 (density 0.05 g / c m 3) and 3 (density 0.03 g/cm3), respectively.
150
C.A.M. Mulder et al. / Ultra-low density SiO 2 sol-gel glass
~
~ 200nm
Fig. 1. Electron micrograph of silica gel 1 described in table 1.
200 Fig. 2. Electron micrograph of a carbon replica of a freshly fractured surface of gel 1.
nm
C.A.M. Mulder et al. / Ultra-low density SiO 2 sol-gel glass
151
Fig. 3. Stereo-pair of electron micrographs of gel 1.
F r o m the T E M micrographs it can be observed that the gels consist of small spheroidal particles arranged along branched chains: It is noted that all particles, if not darkened by another one display a homogeneous contrast. The diameter of the particles ranges from 20 nm for gel 1 to about 10 n m and 8 nm for gels 2 and 3, respectively (see table 1). The chain-like structure is especially
Fig. 4. Electron micrograph of silica gel 2 described in table 1.
152
CA.M. Mulder et al. / Ultra-low density SiO 2 sol-gel glass
50nm i
J
Fig. 5. Electron mlcrograph of silica gel 3 described in table 1.
evident for gel 1, whereas the ultra-low density gels 2 and 3 exhibit a more compact structure due to extensive branching. In addition, the pore space between the particles for the latter samples appears to be less than for the higher density gels. This is the more peculiar if one has to imagine an almost completely filled gel structure of a density more than 70 times smaller than that of bulk fused silica. We discuss this in the next paragraph. In table 1 results of specific (BET) surface area measurements of the gels accounting for the surface accessible to N 2 only are included. From the specific surface area A s and the diameter d of the particles derived from the TEM micrographs, one can calculate the apparent density of the material in the spherical particles, Psph, according to the formula,
p,ph = C / ( . 4 , d ). For free spheres C = 6, if A s is expressed in m2/kg, d in m and p in k g / m 3. Due to the interconnection of the spheroidal particles, the value of C decreases for decreasing effective surface area. From our TEM micrographs we estimate C - 4. Then the apparent density of the particles for the studied ranges of the three gels varies from 0.4 to 0.3 g / c m 3 (see table 1). So, the spherical particles must be very porous structures themselves. Such fine porosity would not be probed by the N 2 BET measurements (lower detection limit - 1.5 nm). For the ultra-low density gels 2 and 3 the gel density is a factor of about 10 smaller than the apparent particle density. As the electron microscope typically reveals
C.A.M. Mulder et al. / Ultra-low density SiO 2 sol-gel glass
153
an isotropic contrast within the particles of the gel, these spheres must consist of extremely small SiO 2 entities. Such entities would be discernible in the TEM if they were larger than, say, 0.5 nm. The above results should be compared to those of Brinker et al. [3] reporting on the sol-gel transition in simple silicates, who observed gels to consist of coarse particles (90 nm) which in turn were made up of much smaller ones ( < 10 nm). This was interpreted as a hierarchical random close packing of dense spheres (Brinker and Scherer, in ref. 8).
4. Conclusion
Ultra-low density porous SiO2 gels are obtained by using strongly diluted initial solutions. After supercritical drying in a prepressurized autoclave, the structure of these gels is characterized in the TEM as a highly branched network of low density globular SiO2 particles. Very high values for the specific surface area (up to 1600 m2/g for a gel of density 0.03 g / c m 3) indicate that the spheroidal particles must be very porous structures themselves, consisting of extremely small SiO 2 entities. A detailed TEM study of the densification of low density gels to glass is presented at this conference [10].
References [1] [2] [3] [4] [5] [6] [7] [8]
[9] [10]
H. Dislich, Angew. Chem. Inst. Ed. 10 (1971) 363. B.E. Yoldas, J. Mater. Sci. 12 (1977) 1203. C.J. Brinker, K.D. Keefer, D.W. Schaefer and C.S. Ashley, J. Non-Cryst. Solids 48 (1982) 47. C.J. Brinker, K.D. Keefer, D.W. Schaefer, R.A. Assink, B.D. Kay and C.S. Ashley, J. Non-Cryst. Solids 63 (1984) 45. J. Zarzycki, in: Glas...Current Issues, eds. A.F. Wright, J. Dupuy (Nijhoff, Dordrecht, The Netherlands, 1985) p. 203. C.R. Veale, Fine Powders: Preparation, Properties and Uses (Applied Science Publ., London, 1972). M. Prassas, J. Phalippou and J. Zarzycki, J. Mater. Sci. 19 (1984) 1656. C.J. Brinker and G.W. Scherer, J. Non-Cryst. Solids 70 (1985) 301; C.J. Brinker, G.W. Scherer and E.P. Roth. J. Non-Cryst. Solids 72 (1985) 345; G.W. Scherer, C.J. Brinker and E.P. Roth, J. Non-Cryst. Solids 72 (1985) 369. J.G. van Lierop, A. Huizing, W.C.P.M. Meerman and C.A.M. Mulder, these Proceedings (Gels '85) J. Non-Cryst. Solids 82 (1986) 265. C.A.M. Mulder, J.G. van Lierop and G. Frens, these Proceedings (Gels '85) 82 (1986) 92.