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Optical transmission properties of silica aerogels prepared from polyethoxidisiloxanes
Journal of Non-Crystalline Solids 210 Ž1997. 224–231
Optical transmission properties of silica aerogels prepared from polyethoxidisiloxanes G.M. Pajonk a
a,)
, E. Elaloui a , B. Chevalier b, R. Begag
a
UniÕersite´ Claude Bernard, Laboratoire d’Application de la Chimie a` l’EnÕironnement, UMR CNRS 5634–43, Bld du 11 NoÕembre 1918, 69622 Villeurbanne, France b CSTB, 24 rue Fourier, 38000 St. Martin D’Heres, France Received 24 October 1995; revised 22 July 1996
Abstract Transparent, crack-free low density silica monolithic aerogels synthesized from a series of polyethoxidisiloxanes have been characterized in the UV-visible-NIR optical range Ž0.3–2.5 mm.. Normal hemispherical transmittances in the visible range t vnh were measured as well as extinction coefficients at 550 nm. These quantities are valuable parameters to quantify the image quality seen through a piece of aerogel by an empirical index called the transparency ratio ŽTR.. The transparency ratios depend upon catalysis, volumic percentage of solvent and chemical nature of precursors, at the sol–gel step, as well as the supercritical temperature of drying and the sample thickness. Results show that the transparency ratio of the best aerogel is relatively close to that of a simple window pane of comparable thicknesses Ž0.92 versus 0.99, respectively. while their respective extinction coefficients at 550 nm are similar Ž16.3 my1 versus 16.4 my1 , respectively..
1. Introduction It is widely accepted that silica aerogels, which are made by the sol–gel method, and supercritically dried with respect to the liquid phase, are very interesting transparent and thermal superinsulator materials for double windows w1–9x. As a solid very porous and light material, they also exhibit thermal conductivities lower than 20 mWrKrm at atmospheric pressure, but in general their transparencies, in the visible range, depend on the degree of Rayleigh
)
Corresponding author. Tel.: q33-4 72 44 82 52; fax: q33-4 78 94 19 95; e-mail: [email protected].
and surface-scattering w9x. Only rare gas can match these values at ambient pressures ŽKr, Xe and A, exhibit thermal conductivities of 8.6, 5.8 and 16.2 mWrmK, respectively., while still air has a thermal conductivity of 24.1 mWrmK under the same conditions. It is important to improve these aerogels to get closer to the transparency exhibited by a single window pane similar to that used currently in double window glazings. A recent series of papers demonstrated the advantage of making silica aerogels with a family of new prepolymerized precursors based upon the partial hydrolysis of tetraethoxisilane ŽTEOS. in the presence of sulfuric acid as catalyst w10–12x. The use of commercial prepolymers versus the corresponding monomers, presents the advan-
0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 6 . 0 0 6 0 0 - X
G.M. Pajonk et al.r Journal of Non-Crystalline Solids 210 (1997) 224–231
tages of having been partially hydrolyzed at will tailor made and they do not need the addition of water to be gelled. They allow better control of the condensation step. Physical, textural and morphological properties of these monoliths are reported in Ref. w12x, as well as their detailed method of preparation. This paper is a continuation of the description of the properties of the silica aerogels in Ref. w12x from the point of view of their optical transparency in the visible range, in relation to the parameters governing their synthesis already published, i.e., nature of the precursor, catalyst, solvent, volumic percentage of precursor and supercritical drying selected temperatures Žhigh temperatures ) 2408C or near ambient ones 35–408C.. Beck et al. w8x characterized the imaging through aerogels by measuring a modulation of the image consisting of black and white lines and expressing the corresponding modulation transfer function. Their work was made chiefly with TMOS aerogels Žtetramethoxisilane. or gels from water–glass ŽNa 2 SiO 3 .. It was shown that Naq ions exerted a strong detrimental effect upon the imaging qualities of the aerogel. Cao and Hunt disclosed that two step silica gels lead to better transparent aerogel than one step material w7x. They prepared monoliths from TEOS and measured light scattering intensities of both types of silica aerogels. Their two-step gels, which involves partially condensed silica differ from our own precursors: the former precursors were first hydrolyzed in an acidic medium using HCl and then condensed with a basic catalyst; our precursors are hydrolyzed with several water to TEOS molar ratios Žsubstoichiometric water quantities. with H 2 SO4 as catalyst and condensed either in the presence of NH 4 OH or HF and then systematically evacuated at high or low supercritical temperature conditions w12x. We made two different two-step silica gels: Acid ŽH 2 SO4 . –base ŽNH 4 OH. ones, and acid ŽH 2 SO4 . – acid ŽHF. ones. The optical properties of the aerogels obtained were measured by spectrometry using a double beam Perkin Elmer l19 apparatus equipped with an integrating sphere 16 cm in diameter. The different kinds of solar and luminous transmittances w13x allow the calculation of the transparency ratio ŽTR. defined in w10x. Comparisons are also given for a commercial silica monolithic aerogel pane made by Airglass
225
ŽStaffenstorp, Sweden. a window pane and a IEA C6 aerogel ŽJapan..
2. Experimental procedures SiO 2 gels were prepared from the new polymeric precursors, commercially manufactured by PCAS Longjumeau, based upon the partial hydrolysis of TEOS with H 2 SO4 and ethanol as described in Ref. w12x. Table 1 recalls briefly their mode of synthesis. In this table, the precursors are noted Px , where x is an indication of their degrees of hydrolysis, i.e., they are hydrolyzed with a substoichiometric number n of H 2 O moles, where n is always - 2. The index x is defined by the relationship: Ž nr2. = 10 3. Two supercritical drying procedures were used: at high temperature with respect to an organic compound Žmethanol, ethanol. or with respect to liquid CO 2 Žafter a solvent exchange step. at low temperature w12,14x. Samples dried at high temperature are labelled A while those dried at low temperature are called carbogels and labelled C. All C aerogels were obtained with HF as catalyst w12,15x unless otherwise noted. All optical measurements were performed in the transmission mode and extinction coefficients E at 550 nm are imported in my1 . The Perkin Elmer l19 spectrophotometer measures the normalrhemispherical transmittance Žnrh. Ždirectional and diffuse. noted tvnh as shown in Fig. 1. The normalrdiffuse transmittance t vndf can also be obtained by eliminating the normalrdirect transmission contribution. All samples were measured as obtained from the autoclave drying treatment. Unless otherwise mentioned, their thickness was 1.00 cm. The transparency ratio TR is defined by Eq. Ž1. where t vnh and tvndf are the normalrhemispherical Table 1 Partially hydrolyzed TEOS samples. The Px series contain 28% SiO 2 in weight Samples
Mole of waterrmole of TEOS
P900 P750 P600 P400
1.8 1.5 1.2 0.8
G.M. Pajonk et al.r Journal of Non-Crystalline Solids 210 (1997) 224–231
226
Fig. 1.
and normalrdiffuse transmittances respectively, measured for the visible range Ž0.380–0.780 nm.: TR s
tvnh y tvndf tvnh
s
tvnd tvnh
.
Ž 1.
This ratio gives the normalrdirect transmittance t vnd with respect to the normalrhemispherical one. The transmittance quantities are values integrated over the visible range: 380–780 nm using a weighing function w13x. The higher the TR, the better the transparency and clarity of the image seen through the material. This ratio does not take into account the influence of light absorption. It does not permit separation of the forward scattering contribution from the isotropic one. The TR values are between 0 and 1 and are expressed in percent. They represent the influence of diffuse light on the transparency. A good sample must exhibit both a high normalrhemispherical transmittance and a high TR value. Both quantities i.e., tvnh and TR must be as high as possible, but they only represent the necessary conditions Žand not the sufficient ones. to be met, in order to obtain good imaging systems.
Optical measurements were reproducible as determined by measuring the root mean squares of two series of transmission data: one series on five identical samples, made separately, from the same sol–gel chemistry: P900 Ž60%. in ethylacetoacetate Ž40%., 2% HF as catalyst and supercritically dried with respect to CO 2 ŽC form. ŽTable 2. and the other series on the same sample Žsample 3 of Table 2. on which four transmission spectra were recorded. The second series of four optical transmission measurements expressed as tvnh ones, shows a root mean square value of 0.07 and a mean value of 91.92, respectively. The above results indicate that measurement error is only a few percent.
3. Results Results are given as spectra and gathered in tables where only t vnh and TR values are introduced, t vndf values can be easily estimated from Eq. Ž1.. Spectra are given for the full range UV–VIS–NIR 0.3 to 2.5 mm. Figs. 2–4 show the different transmission spectra obtained for a float glass of 0.185 cm thickness Ža., a sample of Airglass aerogel of 1.778 cm thickness Žb. and a IEA-C6 sample thickness 1.2 cm Žc.. Table 3 displays their tvnh , TR, and E values, respectively. In a previous paper w12x it was shown that the principal chemical parameters affecting the monolithic silica aerogels were first the nature of the partially hydrolyzed precursor Px ŽTable 1., its volumic percentage V with respect to the solvent, the
Table 2 Error estimation on five identical P900 carbogels Sample
t vnh Ž%. tenh Ž%. a TR Ž%. e Žcm. b E Žmy1 .
1 2 3 4 5
90.0 90.8 91.9 87.6 90.8
Root mean square
1.61
Mean value 90.22 a b c
88.5 89.2 90.1 86.3 90.4 1.63 88.9
Energetic transmittance. Sample thickness. Extinction coefficient.
89.6 92.7 90 89 90.8
0.989 1.04 0.97
c
21.7 16.4 19.6 28.9 13.7
1.43
5.8
90.42
20.6
Fig. 2. Transmittance with glass of 0.785 cm thickness.
G.M. Pajonk et al.r Journal of Non-Crystalline Solids 210 (1997) 224–231
Fig. 3. Transmittance with Airglass Aerogel of 1.778 cm thickness.
Fig. 4. Transmittance with IEA C-6, 1.20 cm thickness.
Table 3 Optical properties in the visible range of three reference samples. ŽTwo window panes of 0.4 cm thickness each, stuck together gave a global TR value of 0.94. Sample
t vnh Ž%.
TR Ž%.
E Žmy1 .
Float glass Airglass ŽSweden. IEA-C-6 aerogel ŽJapan.
90.1 63.9 84.2
97.9 79.5 78.5
16.4 32 30
227
Fig. 5. Transmittance of aerogels as function of precursor Žform A..
Fig. 6. Transmittance of aerogels as function of precursor Žform C..
Table 5 Influence of the volumic percentages of P900 in acetone on t vnh and TR values P900 Ž%.
t vnh
Ž%. TR Ž%.
30%
40%
50%
60%
A
C
A
C
A
C
A
C
82.4 71.7
– –
85.7 71.8
90.6 67.8
83.2 75.1
84 84.5
77.2 72.9
88.7 70.9
Table 4 Optical transmission properties of aerogels, A and C forms, as a function of the degree of hydrolysis of the precursors
Table 6 Influence of the nature of the solvent on t vnh and TR values
Samples
Solvent
P600 A
t vnh
Ž%. TR Ž%.
63.8 52.6
P750 C 81.8 64.4
A 84.6 76.3
P900 C 82.1 75.8
A 83.2 75.1
C 84 84.5
t vnh
Ž%. TR Ž%.
Methanol
Ethanol
A
C
A
C
Acetone A
C
84.1 73.1
62 49
78.3 75.1
71.1 65.1
83.2 75.1
84 84.5
228
G.M. Pajonk et al.r Journal of Non-Crystalline Solids 210 (1997) 224–231
Fig. 7. Transmittance of aerogels as function of % in precursor P900 Žform A..
Fig. 8. Transmittance of aerogels as function of % in precursor P900 Žform C..
Fig. 10. Transmittance of aerogels as function of solvent with precursor P900 Žform C..
Fig. 11. Transmittance aerogels in function of catalyst with precursor P900 .
Table 7 Influence of the nature of the catalysts on the t vnh and TR values on A form aerogels P900
HF A
NH 4 F A
NH 4 OH A
t vnh Ž%. TR Ž%.
92.1 72.5
90.9 74.2
83.2 75.1
Table 8 Influence of the drying procedure ŽA and C forms. on the t vnh and TR values
Fig. 9. Transmittance of aerogels as function of solvent with precursor P900 Žform A..
Drying procedure
A
C
t vnh
92.1 72.5
84 84.5
TR
G.M. Pajonk et al.r Journal of Non-Crystalline Solids 210 (1997) 224–231
229
50% and HF as catalyst from the results given in both Tables 4 and 5, Table 6 Žfor C. and Table 7 Žfor A. respectively, and recalled in Table 8. Fig. 12 shows their transmission spectra.
4. Discussion
Fig. 12. Transmittance of aerogels in function of drying procedure.
nature of the solvent, that of the catalyst and the drying mode. These factors govern the quality of gels and the supercritical drying step can preserve the textural properties of the wet gels in the solid materials obtained. Aerogels made of P600 , P750 and P900 precursors in acetone with a volumic percentage of 50% and catalyzed by ammonia were supercritically dried with respect to ethanol, they are noted A and three other similar samples but catalyzed by HF and supercritically dried with respect to liquid CO 2 Žnoted C. were investigated for their tvnh and TR values. Table 4 gathers the corresponding values while Figs. 5 and 6 show their transmission spectra. Two P900 series ŽA and C types. with 30%, 40%, 50% and 60% percentages in acetone are described in Table 5 in terms of tvnh and TR, respectively. Figs. 7 and 8 show their corresponding transmission spectra. Two other P900 series ŽA and C forms. were prepared with a volumic percentage of 50% and three solvents: methanol, ethanol and acetone. Table 6 gives their tvnh and TR values respectively, while their transmission spectra are shown in Figs. 9 and 10. A P900 aerogel ŽA form only in this case. corresponding to a precursor volumic percentage of 50% in acetone was prepared with three catalysts: HF, NH 4 F and NH 4 OH. The t vnh and TR values are given in Table 7 and the corresponding spectra are shown in Fig. 11. The influence of drying mode is readily shown with sample P900 in acetone, volumic percentage of
Results in Tables 4–7 are strongly related to the sol to gel step combined with the supercritical drying mode. By contrast, Table 8 only deals with the drying mode since in this case the sol to gel processes are identical. For these reasons, the influences of degree of hydrolysis, volumic percentages and solvent will be considered and discussed for A and then C forms, separately. Moreover it is recalled that the supercritical drying mode involving an organic solvent, therefore high supercritical temperatures, combines both chemical and physical interactions during the drying process while the supercritical drying mode with CO 2 implies physical interactions only because of its chemical inertness coupled with the low supercritical temperature used, as mentioned earlier in a previous paper w12x. 4.1. A form aerogels From Table 4 it is seen that the degree of hydrolysis of the precursor exerts a strong influence on both tvnh and TR values when one considers P600 , P750 or P900 samples. The higher this degree the better the results. This can be explained on the basis that the three samples were obtained from sol–gels synthesized with a two-step catalysis and different amounts of water as in the case related by Cao and Hunt w7x who found that increasing the molar ratio H 2 OrTEOS, decreased the light scattering phenomenon. Their explanation, as ours, relies on the mechanistical consideration of the building of the gel from the sol and the type of catalyst used Žacid and thereafter basic. also described by Venkateswara Rao et al. w16x. The effect of the dilution Žor of the volumic percentages. summarized in Table 4 are very small for values ranging from 30 up to 50%, while when V attained 60% a noticeable decrease of tvnh as well as TR was recorded. This behavior is again on line with Cao and Hunt’s results w7x.
230
G.M. Pajonk et al.r Journal of Non-Crystalline Solids 210 (1997) 224–231
The results of Table 6 clearly show that methanol, as solvent, contributes the most to light scattering since it increases tvnh but decreases the TR values. This is perhaps due to some transesterification reactions during the sol–gel process, resulting in the raise of reactivity of the building blocks containing a few methoxy groups instead of ethoxy ones, as already reported by Pajonk et al. w12x and Bernards et al. w17x. Venkateswara Rao et al. w16x also found that methanol gave the better transparency for TEOS Žone step. monolithic aerogels. Acid catalyst like HF Žsee Table 7. led to higher transparency Žtvnh . than NH 4 F and NH 4 OH respectively, while it is the reverse ranking when one considers the TR values. The same results were also registered for TEOS, one step catalysis aerogels, described by Venkateswara Rao et al. w16x for their respective global transparencies. Therefore, the higher TR values observed when NH 4 OH and NH 4 F are the catalysts, indicate a better homogeneity in the monolithic aerogel in relation with the homogeneity of the corresponding gel itself. 4.2. C form aerogels From Table 4, the same behavior appears in function of the degree of hydrolysis. The main difference is that the TR indexes increased more for the C than for the A forms. This behavior can be attributed to the mild drying conditions of the CO 2 supercritical drying and the acidic two step catalysis which is characteristic of that kind of silica monoliths at the sol–gel stage w12,15x. The influence of the volumic percentage on tvnh and the TR results is more complicated than for A samples. A minimum of tvnh is observed together with a maximum TR index value at V s 50% ŽTable 5.. In both forms, A and C, a maximum TR index is recorded at V s 50%, which indicates that such a value gives more homogeneous samples. Tables 2 and 6 show the particularly strong influence of the solvent on the transmission properties. Both tvnh and TR increase when going from methanol to ethanol and acetone and finally ethyl acetoacetate. Considering that the best results are obtained with ethyl acetoacetate, one hypothesis consists in taking into account the very strong interaction already observed with ketolization reactions, which however
are developing more slowly when the sol–gel catalyst is an acid than when it is a base ŽA form. w12x. The fact that both tvnh and TR index evolutions are parallel, contrary to the A form aerogels, underlines the differences between the low temperature ŽCO 2 . and high temperatures Žorganics. supercritical drying processes. Optical qualities of the C form aerogels are very sensitive to the nature ŽpH. of the catalysts as shown in Table 7 where it is also seen that Naq ions are very detrimental towards transparency. A similar result is reported in the literature by Emmerling et al. w9x who doped TMOS aerogels by Na 2 SO4 and observed a very poor transparency, similar to that of aerogels made from waterglass. These results demonstrate that no matter the supercritical drying mode selected, low or high temperature conditions, waterglass ŽNa 2 SiO 3 . cannot be chosen as a silicon precursor in order to make transparent SiO 2 monolithic aerogels, unless one omits Naq in the gel. It is well documented that the BASF aerogel technique gives only translucent silica w9,18x. The last parameter studied in this report is the influence of the drying conditions as indicated by Fig. 12 and Table 8 respectively. It is evident from these data that sample C is much less diffusing than its counterpart, and therefore it yields a higher TR value. Sample A is however characterized by a better tvnh factor than sample C. The only difference between these two aerogels is the fact that the A form has been subject to a kind of a ‘temperature programmed thermal ageing’ in the autoclave, at a rate of 0.3 Krmin, in presence of an organic compound exhibiting some chemical reactivities in these conditions.
5. Conclusions A form aerogels generally lead to higher transmittances tvnh in the visible range, than the C form aerogels. The reverse is true at transparency ratios TR, which means that taking into account the Rayleigh visible light scattering, the A forms contain more inhomogeneities than the C ones. This is a problem which can be solved at the sol–gel stage. Surface scattering of our samples cannot be treated by chemical means at the sol–gel stage. Surface
G.M. Pajonk et al.r Journal of Non-Crystalline Solids 210 (1997) 224–231
scattering can be decreased by improving the smoothness of the interface between the gel and the mould walls. The best result in tvnh and TR was obtained with a P900 , V s 60% in ethyl acetoacetate, catalyzed by HF gel, dried with respect to liquid CO 2 ŽC form..
Acknowledgements The French ADEME agency is gratefully thanked for its financial supports through several grants, as well as the European Union for its Joule CT 92 contract.
References w1x M. Rubin and C.M. Lampert, Solar Energy Mater. 7 Ž1983. 393. w2x P.H. Tervari, A.J. Hunt and K.D. Lofftus, in: Aerogels, ed. J. Fricke ŽSpringer, Berlin, 1986. p. 31; Mater. Lett. 3 Ž1985. 363. w3x K.I. Jensen, J. Non-Cryst. Solids 145 Ž1992. 237. w4x S. Swendsen, J. Non-Cryst. Solids 145 Ž1992. 240.
231
w5x V. Wittwer, J. Non-Cryst. Solids 145 Ž1992. 233. w6x P. Wan, W. Korner, A. Emmerling, A. Beck, J. Kuhn and J. Fricke, J. Non-Cryst. Solids 145 Ž1992. 141. w7x W. Cao and A.J. Hunt, J. Non-Cryst. Solids 176 Ž1994. 18. w8x A. Beck, J. Linsmeier, W. Korner, H. Scheller and J. Fricke, J. Non-Cryst. Solids 186 Ž1995. 232. w9x A. Emmerling, R. Petricevic, A. Beck, P. Wang, H. Scheller and J. Fricke, J. Non-Cryst. Solids 185 Ž1995. 240. w10x E. Elaloui, P. Achard, B. Chevalier, J.L. Chevalier, M. Durant and G.M. Pajonk, SPIE 1727 Ž1992. 402. w11x G.M. Pajonk, E. Elaloui, M. Durant, J.L. Chevalier, B. Chevalier and P. Achard, in: Sol–gel Processing and Applications, ed. Y.A. Attia ŽPlenum, New York, 1994. p. 267. w12x G.M. Pajonk, E. Elaloui, P. Achard, B. Chevalier, J.L. Chevalier and M. Durant, J. Non-Cryst. Solids 186 Ž1995. 1. w 1 3 x E u r o p e a n C o m m itte e f o r S ta n d a r d iz a tio n CENrTCr129rWG9rN4E Rev. 7 July Ž1992., final draft prEN 410. w14x G.M. Pajonk, in: Sol–gel Processing and Applications, ed. Y.A. Attia ŽPlenum, New York, 1994. p. 201. w15x E. Elaloui, PhD dissertation, Universite´ Claude Bernard Lyon I, No. 47–94 Ž1994.. w16x A. Venkateswara Rao, G.M. Pajonk, N.N. Parvathy and E. Elaloui, in: Sol–gel Processing and Applications, ed: Y.A. Attia ŽPlenum, New York, 1994. p. 237. w17x T.M.N Bernards, M.J. van Bommel and A.H. Boonstra, J. Non-Cryst. Solids 134 Ž1991. 1. w18x G. Herrmann, R. Iden, M. Mielke, F. Teich and B. Ziegler, J. Non-Cryst. Solids 186 Ž1995. 380.
Optical transmission properties of silica aerogels prepared from polyethoxidisiloxanes G.M. Pajonk a
a,)
, E. Elaloui a , B. Chevalier b, R. Begag
a
UniÕersite´ Claude Bernard, Laboratoire d’Application de la Chimie a` l’EnÕironnement, UMR CNRS 5634–43, Bld du 11 NoÕembre 1918, 69622 Villeurbanne, France b CSTB, 24 rue Fourier, 38000 St. Martin D’Heres, France Received 24 October 1995; revised 22 July 1996
Abstract Transparent, crack-free low density silica monolithic aerogels synthesized from a series of polyethoxidisiloxanes have been characterized in the UV-visible-NIR optical range Ž0.3–2.5 mm.. Normal hemispherical transmittances in the visible range t vnh were measured as well as extinction coefficients at 550 nm. These quantities are valuable parameters to quantify the image quality seen through a piece of aerogel by an empirical index called the transparency ratio ŽTR.. The transparency ratios depend upon catalysis, volumic percentage of solvent and chemical nature of precursors, at the sol–gel step, as well as the supercritical temperature of drying and the sample thickness. Results show that the transparency ratio of the best aerogel is relatively close to that of a simple window pane of comparable thicknesses Ž0.92 versus 0.99, respectively. while their respective extinction coefficients at 550 nm are similar Ž16.3 my1 versus 16.4 my1 , respectively..
1. Introduction It is widely accepted that silica aerogels, which are made by the sol–gel method, and supercritically dried with respect to the liquid phase, are very interesting transparent and thermal superinsulator materials for double windows w1–9x. As a solid very porous and light material, they also exhibit thermal conductivities lower than 20 mWrKrm at atmospheric pressure, but in general their transparencies, in the visible range, depend on the degree of Rayleigh
)
Corresponding author. Tel.: q33-4 72 44 82 52; fax: q33-4 78 94 19 95; e-mail: [email protected].
and surface-scattering w9x. Only rare gas can match these values at ambient pressures ŽKr, Xe and A, exhibit thermal conductivities of 8.6, 5.8 and 16.2 mWrmK, respectively., while still air has a thermal conductivity of 24.1 mWrmK under the same conditions. It is important to improve these aerogels to get closer to the transparency exhibited by a single window pane similar to that used currently in double window glazings. A recent series of papers demonstrated the advantage of making silica aerogels with a family of new prepolymerized precursors based upon the partial hydrolysis of tetraethoxisilane ŽTEOS. in the presence of sulfuric acid as catalyst w10–12x. The use of commercial prepolymers versus the corresponding monomers, presents the advan-
0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 6 . 0 0 6 0 0 - X
G.M. Pajonk et al.r Journal of Non-Crystalline Solids 210 (1997) 224–231
tages of having been partially hydrolyzed at will tailor made and they do not need the addition of water to be gelled. They allow better control of the condensation step. Physical, textural and morphological properties of these monoliths are reported in Ref. w12x, as well as their detailed method of preparation. This paper is a continuation of the description of the properties of the silica aerogels in Ref. w12x from the point of view of their optical transparency in the visible range, in relation to the parameters governing their synthesis already published, i.e., nature of the precursor, catalyst, solvent, volumic percentage of precursor and supercritical drying selected temperatures Žhigh temperatures ) 2408C or near ambient ones 35–408C.. Beck et al. w8x characterized the imaging through aerogels by measuring a modulation of the image consisting of black and white lines and expressing the corresponding modulation transfer function. Their work was made chiefly with TMOS aerogels Žtetramethoxisilane. or gels from water–glass ŽNa 2 SiO 3 .. It was shown that Naq ions exerted a strong detrimental effect upon the imaging qualities of the aerogel. Cao and Hunt disclosed that two step silica gels lead to better transparent aerogel than one step material w7x. They prepared monoliths from TEOS and measured light scattering intensities of both types of silica aerogels. Their two-step gels, which involves partially condensed silica differ from our own precursors: the former precursors were first hydrolyzed in an acidic medium using HCl and then condensed with a basic catalyst; our precursors are hydrolyzed with several water to TEOS molar ratios Žsubstoichiometric water quantities. with H 2 SO4 as catalyst and condensed either in the presence of NH 4 OH or HF and then systematically evacuated at high or low supercritical temperature conditions w12x. We made two different two-step silica gels: Acid ŽH 2 SO4 . –base ŽNH 4 OH. ones, and acid ŽH 2 SO4 . – acid ŽHF. ones. The optical properties of the aerogels obtained were measured by spectrometry using a double beam Perkin Elmer l19 apparatus equipped with an integrating sphere 16 cm in diameter. The different kinds of solar and luminous transmittances w13x allow the calculation of the transparency ratio ŽTR. defined in w10x. Comparisons are also given for a commercial silica monolithic aerogel pane made by Airglass
225
ŽStaffenstorp, Sweden. a window pane and a IEA C6 aerogel ŽJapan..
2. Experimental procedures SiO 2 gels were prepared from the new polymeric precursors, commercially manufactured by PCAS Longjumeau, based upon the partial hydrolysis of TEOS with H 2 SO4 and ethanol as described in Ref. w12x. Table 1 recalls briefly their mode of synthesis. In this table, the precursors are noted Px , where x is an indication of their degrees of hydrolysis, i.e., they are hydrolyzed with a substoichiometric number n of H 2 O moles, where n is always - 2. The index x is defined by the relationship: Ž nr2. = 10 3. Two supercritical drying procedures were used: at high temperature with respect to an organic compound Žmethanol, ethanol. or with respect to liquid CO 2 Žafter a solvent exchange step. at low temperature w12,14x. Samples dried at high temperature are labelled A while those dried at low temperature are called carbogels and labelled C. All C aerogels were obtained with HF as catalyst w12,15x unless otherwise noted. All optical measurements were performed in the transmission mode and extinction coefficients E at 550 nm are imported in my1 . The Perkin Elmer l19 spectrophotometer measures the normalrhemispherical transmittance Žnrh. Ždirectional and diffuse. noted tvnh as shown in Fig. 1. The normalrdiffuse transmittance t vndf can also be obtained by eliminating the normalrdirect transmission contribution. All samples were measured as obtained from the autoclave drying treatment. Unless otherwise mentioned, their thickness was 1.00 cm. The transparency ratio TR is defined by Eq. Ž1. where t vnh and tvndf are the normalrhemispherical Table 1 Partially hydrolyzed TEOS samples. The Px series contain 28% SiO 2 in weight Samples
Mole of waterrmole of TEOS
P900 P750 P600 P400
1.8 1.5 1.2 0.8
G.M. Pajonk et al.r Journal of Non-Crystalline Solids 210 (1997) 224–231
226
Fig. 1.
and normalrdiffuse transmittances respectively, measured for the visible range Ž0.380–0.780 nm.: TR s
tvnh y tvndf tvnh
s
tvnd tvnh
.
Ž 1.
This ratio gives the normalrdirect transmittance t vnd with respect to the normalrhemispherical one. The transmittance quantities are values integrated over the visible range: 380–780 nm using a weighing function w13x. The higher the TR, the better the transparency and clarity of the image seen through the material. This ratio does not take into account the influence of light absorption. It does not permit separation of the forward scattering contribution from the isotropic one. The TR values are between 0 and 1 and are expressed in percent. They represent the influence of diffuse light on the transparency. A good sample must exhibit both a high normalrhemispherical transmittance and a high TR value. Both quantities i.e., tvnh and TR must be as high as possible, but they only represent the necessary conditions Žand not the sufficient ones. to be met, in order to obtain good imaging systems.
Optical measurements were reproducible as determined by measuring the root mean squares of two series of transmission data: one series on five identical samples, made separately, from the same sol–gel chemistry: P900 Ž60%. in ethylacetoacetate Ž40%., 2% HF as catalyst and supercritically dried with respect to CO 2 ŽC form. ŽTable 2. and the other series on the same sample Žsample 3 of Table 2. on which four transmission spectra were recorded. The second series of four optical transmission measurements expressed as tvnh ones, shows a root mean square value of 0.07 and a mean value of 91.92, respectively. The above results indicate that measurement error is only a few percent.
3. Results Results are given as spectra and gathered in tables where only t vnh and TR values are introduced, t vndf values can be easily estimated from Eq. Ž1.. Spectra are given for the full range UV–VIS–NIR 0.3 to 2.5 mm. Figs. 2–4 show the different transmission spectra obtained for a float glass of 0.185 cm thickness Ža., a sample of Airglass aerogel of 1.778 cm thickness Žb. and a IEA-C6 sample thickness 1.2 cm Žc.. Table 3 displays their tvnh , TR, and E values, respectively. In a previous paper w12x it was shown that the principal chemical parameters affecting the monolithic silica aerogels were first the nature of the partially hydrolyzed precursor Px ŽTable 1., its volumic percentage V with respect to the solvent, the
Table 2 Error estimation on five identical P900 carbogels Sample
t vnh Ž%. tenh Ž%. a TR Ž%. e Žcm. b E Žmy1 .
1 2 3 4 5
90.0 90.8 91.9 87.6 90.8
Root mean square
1.61
Mean value 90.22 a b c
88.5 89.2 90.1 86.3 90.4 1.63 88.9
Energetic transmittance. Sample thickness. Extinction coefficient.
89.6 92.7 90 89 90.8
0.989 1.04 0.97
c
21.7 16.4 19.6 28.9 13.7
1.43
5.8
90.42
20.6
Fig. 2. Transmittance with glass of 0.785 cm thickness.
G.M. Pajonk et al.r Journal of Non-Crystalline Solids 210 (1997) 224–231
Fig. 3. Transmittance with Airglass Aerogel of 1.778 cm thickness.
Fig. 4. Transmittance with IEA C-6, 1.20 cm thickness.
Table 3 Optical properties in the visible range of three reference samples. ŽTwo window panes of 0.4 cm thickness each, stuck together gave a global TR value of 0.94. Sample
t vnh Ž%.
TR Ž%.
E Žmy1 .
Float glass Airglass ŽSweden. IEA-C-6 aerogel ŽJapan.
90.1 63.9 84.2
97.9 79.5 78.5
16.4 32 30
227
Fig. 5. Transmittance of aerogels as function of precursor Žform A..
Fig. 6. Transmittance of aerogels as function of precursor Žform C..
Table 5 Influence of the volumic percentages of P900 in acetone on t vnh and TR values P900 Ž%.
t vnh
Ž%. TR Ž%.
30%
40%
50%
60%
A
C
A
C
A
C
A
C
82.4 71.7
– –
85.7 71.8
90.6 67.8
83.2 75.1
84 84.5
77.2 72.9
88.7 70.9
Table 4 Optical transmission properties of aerogels, A and C forms, as a function of the degree of hydrolysis of the precursors
Table 6 Influence of the nature of the solvent on t vnh and TR values
Samples
Solvent
P600 A
t vnh
Ž%. TR Ž%.
63.8 52.6
P750 C 81.8 64.4
A 84.6 76.3
P900 C 82.1 75.8
A 83.2 75.1
C 84 84.5
t vnh
Ž%. TR Ž%.
Methanol
Ethanol
A
C
A
C
Acetone A
C
84.1 73.1
62 49
78.3 75.1
71.1 65.1
83.2 75.1
84 84.5
228
G.M. Pajonk et al.r Journal of Non-Crystalline Solids 210 (1997) 224–231
Fig. 7. Transmittance of aerogels as function of % in precursor P900 Žform A..
Fig. 8. Transmittance of aerogels as function of % in precursor P900 Žform C..
Fig. 10. Transmittance of aerogels as function of solvent with precursor P900 Žform C..
Fig. 11. Transmittance aerogels in function of catalyst with precursor P900 .
Table 7 Influence of the nature of the catalysts on the t vnh and TR values on A form aerogels P900
HF A
NH 4 F A
NH 4 OH A
t vnh Ž%. TR Ž%.
92.1 72.5
90.9 74.2
83.2 75.1
Table 8 Influence of the drying procedure ŽA and C forms. on the t vnh and TR values
Fig. 9. Transmittance of aerogels as function of solvent with precursor P900 Žform A..
Drying procedure
A
C
t vnh
92.1 72.5
84 84.5
TR
G.M. Pajonk et al.r Journal of Non-Crystalline Solids 210 (1997) 224–231
229
50% and HF as catalyst from the results given in both Tables 4 and 5, Table 6 Žfor C. and Table 7 Žfor A. respectively, and recalled in Table 8. Fig. 12 shows their transmission spectra.
4. Discussion
Fig. 12. Transmittance of aerogels in function of drying procedure.
nature of the solvent, that of the catalyst and the drying mode. These factors govern the quality of gels and the supercritical drying step can preserve the textural properties of the wet gels in the solid materials obtained. Aerogels made of P600 , P750 and P900 precursors in acetone with a volumic percentage of 50% and catalyzed by ammonia were supercritically dried with respect to ethanol, they are noted A and three other similar samples but catalyzed by HF and supercritically dried with respect to liquid CO 2 Žnoted C. were investigated for their tvnh and TR values. Table 4 gathers the corresponding values while Figs. 5 and 6 show their transmission spectra. Two P900 series ŽA and C types. with 30%, 40%, 50% and 60% percentages in acetone are described in Table 5 in terms of tvnh and TR, respectively. Figs. 7 and 8 show their corresponding transmission spectra. Two other P900 series ŽA and C forms. were prepared with a volumic percentage of 50% and three solvents: methanol, ethanol and acetone. Table 6 gives their tvnh and TR values respectively, while their transmission spectra are shown in Figs. 9 and 10. A P900 aerogel ŽA form only in this case. corresponding to a precursor volumic percentage of 50% in acetone was prepared with three catalysts: HF, NH 4 F and NH 4 OH. The t vnh and TR values are given in Table 7 and the corresponding spectra are shown in Fig. 11. The influence of drying mode is readily shown with sample P900 in acetone, volumic percentage of
Results in Tables 4–7 are strongly related to the sol to gel step combined with the supercritical drying mode. By contrast, Table 8 only deals with the drying mode since in this case the sol to gel processes are identical. For these reasons, the influences of degree of hydrolysis, volumic percentages and solvent will be considered and discussed for A and then C forms, separately. Moreover it is recalled that the supercritical drying mode involving an organic solvent, therefore high supercritical temperatures, combines both chemical and physical interactions during the drying process while the supercritical drying mode with CO 2 implies physical interactions only because of its chemical inertness coupled with the low supercritical temperature used, as mentioned earlier in a previous paper w12x. 4.1. A form aerogels From Table 4 it is seen that the degree of hydrolysis of the precursor exerts a strong influence on both tvnh and TR values when one considers P600 , P750 or P900 samples. The higher this degree the better the results. This can be explained on the basis that the three samples were obtained from sol–gels synthesized with a two-step catalysis and different amounts of water as in the case related by Cao and Hunt w7x who found that increasing the molar ratio H 2 OrTEOS, decreased the light scattering phenomenon. Their explanation, as ours, relies on the mechanistical consideration of the building of the gel from the sol and the type of catalyst used Žacid and thereafter basic. also described by Venkateswara Rao et al. w16x. The effect of the dilution Žor of the volumic percentages. summarized in Table 4 are very small for values ranging from 30 up to 50%, while when V attained 60% a noticeable decrease of tvnh as well as TR was recorded. This behavior is again on line with Cao and Hunt’s results w7x.
230
G.M. Pajonk et al.r Journal of Non-Crystalline Solids 210 (1997) 224–231
The results of Table 6 clearly show that methanol, as solvent, contributes the most to light scattering since it increases tvnh but decreases the TR values. This is perhaps due to some transesterification reactions during the sol–gel process, resulting in the raise of reactivity of the building blocks containing a few methoxy groups instead of ethoxy ones, as already reported by Pajonk et al. w12x and Bernards et al. w17x. Venkateswara Rao et al. w16x also found that methanol gave the better transparency for TEOS Žone step. monolithic aerogels. Acid catalyst like HF Žsee Table 7. led to higher transparency Žtvnh . than NH 4 F and NH 4 OH respectively, while it is the reverse ranking when one considers the TR values. The same results were also registered for TEOS, one step catalysis aerogels, described by Venkateswara Rao et al. w16x for their respective global transparencies. Therefore, the higher TR values observed when NH 4 OH and NH 4 F are the catalysts, indicate a better homogeneity in the monolithic aerogel in relation with the homogeneity of the corresponding gel itself. 4.2. C form aerogels From Table 4, the same behavior appears in function of the degree of hydrolysis. The main difference is that the TR indexes increased more for the C than for the A forms. This behavior can be attributed to the mild drying conditions of the CO 2 supercritical drying and the acidic two step catalysis which is characteristic of that kind of silica monoliths at the sol–gel stage w12,15x. The influence of the volumic percentage on tvnh and the TR results is more complicated than for A samples. A minimum of tvnh is observed together with a maximum TR index value at V s 50% ŽTable 5.. In both forms, A and C, a maximum TR index is recorded at V s 50%, which indicates that such a value gives more homogeneous samples. Tables 2 and 6 show the particularly strong influence of the solvent on the transmission properties. Both tvnh and TR increase when going from methanol to ethanol and acetone and finally ethyl acetoacetate. Considering that the best results are obtained with ethyl acetoacetate, one hypothesis consists in taking into account the very strong interaction already observed with ketolization reactions, which however
are developing more slowly when the sol–gel catalyst is an acid than when it is a base ŽA form. w12x. The fact that both tvnh and TR index evolutions are parallel, contrary to the A form aerogels, underlines the differences between the low temperature ŽCO 2 . and high temperatures Žorganics. supercritical drying processes. Optical qualities of the C form aerogels are very sensitive to the nature ŽpH. of the catalysts as shown in Table 7 where it is also seen that Naq ions are very detrimental towards transparency. A similar result is reported in the literature by Emmerling et al. w9x who doped TMOS aerogels by Na 2 SO4 and observed a very poor transparency, similar to that of aerogels made from waterglass. These results demonstrate that no matter the supercritical drying mode selected, low or high temperature conditions, waterglass ŽNa 2 SiO 3 . cannot be chosen as a silicon precursor in order to make transparent SiO 2 monolithic aerogels, unless one omits Naq in the gel. It is well documented that the BASF aerogel technique gives only translucent silica w9,18x. The last parameter studied in this report is the influence of the drying conditions as indicated by Fig. 12 and Table 8 respectively. It is evident from these data that sample C is much less diffusing than its counterpart, and therefore it yields a higher TR value. Sample A is however characterized by a better tvnh factor than sample C. The only difference between these two aerogels is the fact that the A form has been subject to a kind of a ‘temperature programmed thermal ageing’ in the autoclave, at a rate of 0.3 Krmin, in presence of an organic compound exhibiting some chemical reactivities in these conditions.
5. Conclusions A form aerogels generally lead to higher transmittances tvnh in the visible range, than the C form aerogels. The reverse is true at transparency ratios TR, which means that taking into account the Rayleigh visible light scattering, the A forms contain more inhomogeneities than the C ones. This is a problem which can be solved at the sol–gel stage. Surface scattering of our samples cannot be treated by chemical means at the sol–gel stage. Surface
G.M. Pajonk et al.r Journal of Non-Crystalline Solids 210 (1997) 224–231
scattering can be decreased by improving the smoothness of the interface between the gel and the mould walls. The best result in tvnh and TR was obtained with a P900 , V s 60% in ethyl acetoacetate, catalyzed by HF gel, dried with respect to liquid CO 2 ŽC form..
Acknowledgements The French ADEME agency is gratefully thanked for its financial supports through several grants, as well as the European Union for its Joule CT 92 contract.
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