Oct 11 2000, Albuquerque, New Mexico, USA
| Feasibility study of a large-scale elaboration process of transparent and monolithic silica aerogels - Presentation of the European HILIT project (Highly Insulating and LIght Transmitting aerogel glazing for windows) |
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Abstract |
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Index A.Rigacci1*, L. Gullberg2, G.Petermann2, K.I. Jensen3, J.M. Schultz3, B. Chevalier4, P. Nitz5, D. Valette6, P. Achard1, G.M. Pajonk7, M. Durant6, M. Ryden8, S. Buathier7, M.A. Einarsrud9 and E. Nilsen9 1Ecole des Mines
de Paris CENERG Sophia Antipolis B.P. 207 F-06560 Valbonne, France,
3Dep. of Buildings and Energy Tech. Univ. of Denmark Build. 118, Brovej DK-2800 Kgs. Lyngby, Denmark 4CSTB 24 rue Joseph Fourier F-38400 Saint Martin d’Hères France, 5Fraunhofer Insitute for Solar Energy Systems ISE, Oltmannsstr. 5 D-79100 Freiburg Germany, 6PCAS B.P. 111 F-91161 Longjumeau, France, 7LACE Université Claude Bernard Lyon1 43 boulevard du 11 Novembre 1918 F- 69622 Villeurbanne France, 8Air Liquide Gas AB Lundavegen 147 S-212 24 Malmoe Sweden, 9Department of Chemistry, NTNU, N-7491 Trondheim Norway,
Abstract
An experimental
feasibility study concerning large-scale elaboration of large, flat monolithic
and transparent silica aerogels for highly insulating and light transmitting
aerogel glazings for windows is described. Promising structural, thermal
and optical results were obtained on aerogels, based on polyethoxydisiloxane
silica precursors in ethylacetoacetate under HF conditions and directly
dried under supercritical CO2. The first glazing prototype is
presented. The first conclusions of the energetic interest and European
market studies are also summarized. Finally, main perspectives for the
future of the present project are underlined particularly concerning the
decrease of the drying process duration.
Keywords
: Silica aerogel, Large-scale supercritical drying, Glazing assembly, Energy
efficiency window.
Index
The present European HILIT project (EU contract JOR3-CT97-0187) concerns development and investigation of monolithic silica aerogel applied as transparent insulation for highly insulating and light transmitting aerogel glazings for windows. The main objectives are: - to make a pre-industrial elaboration process evaluation for the aerogel material, - to develop an assembly process for aerogel windows including glazing evacuation, - to estimate the energetic potential of such glazing. At the end of the project, a complete process ranging from aerogel production to glazings evacuation and assembly is expected. Aerogels were first made by Kistler in the early thirties [1]. He started with sodium silicate and hydrochloric acid and introduced supercritical drying of the alcogel to avoid cracking and shrinkage during drying. To avoid dissolution of silica during the supercritical drying, pore liquid was exchanged (mainly water) with ethanol. This time consuming exchange of the pore liquid was later avoided by developing a procedure where alcogels directly prepared from alcoxides was supercritically dried from ethanol or methanol. [2]. Due to the high supercritical temperature of alcohols, a new and safer route consisting of an exchange of the pore liquid with CO2 followed by a drying at the supercritical conditions of CO2 was developed [3]. Later, direct supercritical CO2 washing was tested to improve the diffusion in the nanoporosity of the wet gel [4]. Since then, few studies have been performed using the direct supercritical drying process [5]. Recently other innovative and competitive drying routes have been developed to avoid the supercritical step among which we can quote : - drying at atmospheric pressure based on a strengthening treatment of the wet gel [6] - a chemical surface modification inducing a gentle “springback” effect [7] - drying at sub-critical pressure using suitable solvent reaction with silica [8]. The obtained results are very promising, however, these processes have still not shown to be preferable for a production of large aerogel sheets. This is the reason why the direct supercritical CO2 drying combined with a patented gel preparation [9] and a wet gel strengthening step [10] was chosen for the work in this project. The
present paper aims at presenting the advancement of the HILIT project,
mainly emphasizing large-scale feasibility demonstration achieved during
the first half of the contract.
2.
Experimental
2.1.
Brief overview of the laboratory-scale elaboration
The wet gel synthesis process was presented by Pajonk et al. [9]. At laboratory scale, the gelation process is based on the gelation of prepolymerized TEOS precursors, called polyethoxydisiloxane (PEDS-Px), under acidic conditions (2vol% HF 21N) in ethylacetoacetate (etac) at room temperature. The PEDS-Px (PCAS, France) were synthesized by prepolymerization of TEOS monomers under acidic conditions (H2SO4) in ethanol :
Precursors were produced at large scale in a plant with good batch reproducibility. In the present study we focus on gels elaborated with n*=1.5 and a volume percentage of precursor in etac equal to 50%. In
parallel, washing and aging studies of wet gels, respectively in ethanol/water
and PEDS-P750/ethanol, are performed to increase the wet gels mechanical
strength and stiffness as well as the permeability [10]. For clarity reasons,
the results presented in this paper are only dealing with samples strengthened
by aging in mother liquor (i.e. stored in pure etac bath (48-72 hours)
before supercritical drying). At lab-scale, wet gels were directly washed
and dried with supercritical CO2 in a 19 liter-autoclave. Numerous
10x10x1 cm3 monolithic flat and transparent aerogels were obtained
with high reproducibility [11].
2.2.
Brief overview of the large-scale elaboration process
A method
has been developed at AIRGLASS AB based on the laboratory scale experience
to mix, mould, handle and store silica wet gel sheets of size 57x57cm2
with a thickness of 1-2 cm, producing flat monolithic sheets with smooth
surfaces. The corresponding developed equipment is capable of deliver sheets
for the direct supercritical CO2 drying process in AIRGLASS
autoclave system. Gelation times between half an hour to about one hour
have been used (instead of 30 minutes at mid-scale [12]). After gelation
the sheets are stored in pure etac for about seven to ten days. A shrinkage
of around 7% to 8% is observed (instead of 5% at mid-scale).
Modifications (including process controlling hardware and software due to the change from working with supercritical methanol [13]) have been done on the existing supercritical drying loop[14] to suite the direct supercritical carbon dioxide drying process for producing silica aerogel sheets of size 53x53cm2 (Figs. 1 and 2). A special inner container of 0,5 m3 has been developed and built for the already existing autoclave of 3 m3. The purpose of this inner container is to get the most efficient direct washing with supercritical carbon dioxide. The possibility to get turbulent drying with carbon dioxide is also available but will be tested in the near future of the project. The drying of the wet gel has so far been performed in an alternating way, i.e. with consecutive dynamic filling and static diffusion phases, which has resulted in quite rather long processing time. In the near future a continuous drying method will be implemented according to mass-transfer simulation results (Fig. 6). To
prevent large amounts of CO2 to be used during the extraction
and of environmental reason, a separation and recovery loop system is under
construction. The mixture of CO2
and etac from the outlet will
be separated in a high pressure vessel. The gaseous phase in the upper
part of the vessel will be transferred to a low pressure vessel with a
built in cooling unit so the gaseous CO2 can be liquefied and
repumped into the system. Etac will be drained from the bottom of the separation
vessel.
2.3.
Brief overview of the glazing assembly
Up to the current project, the prototypes of aerogel glazings were first assembled and then evacuated though e.g. a tube in the rim seal (Fig. 3). The main disadvantage of this method is the long evacuation time. Therefore another method was developed [15], where the aerogel is evacuated over the entire surface. Hereby, the evacuation time is dependent on the thickness of the aerogel instead of the distance from the rim seal to the centre of the aerogel. The
key element of the method is a vacuum chamber in which the aerogel glazing
is placed prior to mounting of the last glass pane. This glass pane is
fixed a few centimetres above the rest of the glazing while the chamber
is evacuated. When the desired gas pressure is reached within the aerogel
the glazing is assembled and the chamber is ventilated. Due to the fast
process, it can be considered as semi-online, and especially the capital
cost is significantly lower for this method in comparison with a true online
process.
2.4.
Thermal conductivity and optical transparency
The
equivalent thermal conductivity (leq) was measured at room temperature
under atmospheric air pressure using a transient method (hot-wire
technique [16]) and two steady-state techniques (micro-fluxmeter and guarded
hot-plate techniques[17]). The transmittance ratio (%TR) and the extinction
coefficient (E) at 550 nm were deduced from spectral optical transmittance
measurements at normal incidence, realised with a double beam spectrophotometer
(Perkin Elmer l19) [12].
2.5.
Characterisation
Apparent
density was estimated byweight and dimension measurements and by the Gladstone
law after measurements of refractive index. The aerogels were further characterized
by Small-Angle X-ray Scattering (SAXS) [18], non-intrusive mercury porosimetry
[19] and nitrogen adsorption [12].
3.
Results
3.1. Aerogel properties Technology
transfer from laboratory-scale to large-scale has been achieved with success.
monolithic, flat and crack-free samples (53x53 cm2) were obtained
with good reproducibility (Fig. 4).No significant structural and physical
properties dispersion was observed. The transparent samples elaborated
were light fractal mesoporous materials, with a wide pore size distribution
(Fig. 5) and consisting of a silica network formed by clusters
of smooth nanosized particles [18], as listed in Table 1.
3.2.
Process Characteristics
At the wet gel step, slight differences between laboratory and large-scale are observed. Due to boundary effect on sol-gel kinetic [20], gelation process appears longer. Furthermore, as expected from literature [21], syneresis phenomena are also slower because of more difficult liquid expulsion through large porous matrices. To reach sufficient strength of the wet gel – to withstand the drying stresses [22] - longer aging timein pure etac bath were necessary for the larger samples. Until now, direct supercritical drying process has been performed with overestimated drying parameters (autoclave filling and depressurization, supercritical washing duration) to obtain reproducible production of monolithic, flat and transparent large materials. Consequently, the present discontinuous process induces artificial long drying times. In
parallel, diffusion simulations, based on Wawrzyniack et al. mass-transfer
approach [23], have shown that shorter washing times can be expected by
proceeding a continuous washing process (Fig. 6). At present, it is important
to note that the model does not use experimental effective diffusion coefficient
but only diffusion coefficient based on Wilke and Chang approach [24].
According to previous literature on ethanol-supercritical (or liquid) CO2
system [23,25], this represents a useful estimation.
3.3.
Glazing results
One
aerogel glazing prototype (18 mm-thick aerogel) was produced by means of
the evacuation and assembly apparatus elaborated through the present project.
Two 4 mm low iron contant glass panes (60x60 cm2) were used.
The rim seal consisted of a non-metal, laminated plastic foil and butyl
sealant. At a Danish location, an outdoor test showed a direct-hemispherical
solar transmittance of 72% for the aerogel glazing. It is expected, that
by means of a surface treatment of the glass panes, this value can be increased
by 2-3%.
4.
Discussion
4.1
Structural changes due to direct supercritical CO2 process
Lighter
materials were obtained applying the direct supercritical CO2
route comparing to liquid CO2 way [18]. This decrease in density
suggests presence of smaller mechanical stresses during direct supercritical
drying step, partly because of negligible interfacial tension between liquid
etac and supercritical CO2 [24]. Comparing with previous results
using the liquid CO2 route does not show any large structural
differences (Fig. 5) [12,18]. Structural modification, reflected in the
density decrease, might appear at larger scales. Because macroporous structures
are known to affect significantly optical [26] and thermal [27] properties,
macroporosity and cluster aggregates must be studied soon with appropriate
techniques.
4.2.
Expected process improvement
The present process is time-consuming, however improvements are expected during the second half of HILIT project. A continuous drying process is planned at AIRGLASS using etac/CO2 separation and CO2 regaining loops. This might decrease the washing time without consuming large CO2 amounts. In parallel, precise determination of end-of-washing criteria (by etac profile concentration measurement with online Gaseous Phase Chromatography) is also planned and will permit to decrease duration of the direct supercritical CO2 washing step. Finally,
if their feasibility is demonstrated at large-scale, wet gel strengthening
washing and aging treatments will be applied to decrease also depressurization
duration [10].
4.3.
Markets and areas
In
order to identify the market potential for aerogel windows in Europe a
feasibility study has been performed. Different types of aerogel glazing
have been considered ranging from glass covers of tempered ordinary glass
(present solution) to non-tempered low iron glass covers. The aerogel glazings
have been evaluated against common low energy glazings for use in retrofit
and new built residential and office buildings located in Denmark, Germany
and Italy. The energetic study is performed by means of the building simulation
program TSBI3 [28]. The general result is that aerogel windows are most
profitable compared to common double pane low energy glazings on the market
for retrofit of buildings in northern countries. The simple payback period
is in all retrofit cases less than the lifetime of the aerogel glazing.
Based on a rough price estimate of 20 Euro/m2 aerogel, the optimised
aerogel glazing will for Danish conditions in all cases be more profitable
than a double paned super low energy glazing. For the German climate this
will only be true for retrofitting and for the Italian climate aerogel
windows will not be profitable at all.
5.
Conclusion
Large-scale feasibility elaboration of flat transparent monolithic silica aerogels based on polyethoxydisiloxane silica precursors and direct supercritical CO2 drying have been demonstrated during the first half of the European HILIT project. Promising preliminary optical result have been obtained with the first glazing assembly prototype. A market research had underlined the huge interest of the aerogel glazing in Northern Europe. At
present, the process is till too time-consuming however realistic improvements
are planned for the second year of the project to overcome those hardships.
This
work is a part of the HILIT EU JOULE III Program under the contract JOR3-CT97-0187.
European financial support is highly acknowledged. We wish thanks R. Pirard
(Laboratoires de Génie Chimique et d’Etudes Physique des Matériaux,
Liège University, Belgium) for non intrusive mercury porosimetry
measurements. We are also grateful to the ESRF, Grenoble,
for access to the French CRG beamline BM2. Our warmest thanks are extended
to, F. Bley, E. Geissler, F. Livet, F. Ehrburger-Dolle and C. Rochas for
their technical advice and for enlightening discussions.
References
1. S.S. Kistler, J. Phys. Chem. 63 (1932) 52. 2. G.A. Nicoaon and S.J. Teichner, US Patent 3,672,833, 1972. 3. P.H. Tewari, A.J. Hunt, K.D. Lofftus, in Aerogels, Ed. J. Fricke (Springler-Verlag), N.Y., (1986) 31. 4. M.J. Van Bommel, A.B. de Haan, J. of Non-Cryst. Solids186 (1995) 78 5. Z. Knez, Z. Novak, in Proceedings of the 5th Meeting on Supercritical Fluids, Vol. 1, Nice (1998) 13. 6. S. Haereid, M.A. Einarsrud, G.W. Scherer, J. of Sol-Gel Science and Technology, 3, (1994) 199. 7. D.M. Smith, D. Stein, J.M. Anderson, W. Ackerman, J. of Non-Cryst. Solids186 (1995) 104. 8. F. Kirkbir, H. Murata, D. Meyers, S. Ray Chaudhuri, J. of Non-Cryst. Solids 225 (1998) 14. 9. G. Pajonk, E. Elaloui, R. Begag, M. Durant, B. Chevalier, J.L.Chevalier, P. Achard, US Patent n°5795557 (1998). 10. M.A. Einarsrud, E. Nilsen, A. Rigacci, G.M. Pajonk, S. Buathier, D. Valette, M. Durant, B. Chevalier, P. Nitz, F. Ehrburger-Dolle, J. of Non-Cryst. Solids, this volume, Submitted. 11. A. Rigacci, P. Ilbizian, P. Achard, in Proceedings of the 6th Meeting on Supercritical Fluids, Nottingham (1999) 23. 12. R. Begag, G.M. Pajonk, E. Elaloui, B. Chevalier, Materials Chemistry and Physics 58 (1999) 256. 13. K.I. Jensen, Evacuation and assembly of aerogel glazings (in danish), Report SR9923, Department of Buildings and Energy, Technical University of Denmark, Copenhagen (1999). 14. S. Henning in Aerogels, Ed. J. Fricke (Springler-Verlag), N.Y., (1986),38. 15. K.I. Jensen, J.M. Schultz, S. Svendsen, Eu Contract JOU2-CT92-0192 Final Report (1995). 16. D. Quénard, H. Sallée, Cahiers du CSTB n°2295 (1988). 17. H. Sallee, Br. Chevalier, CSTB Grenoble, private communication (1999) 18. A. Rigacci, F. Ehrburger-Dolle, E. Geissler, B. Chevalier, H. Sallée, P. Achard, O. Barbieri, S. Berthon, F. Bley, F. Livet, G.M. Pajonk, N. Pinto, C. Rochas, J. of Non-Cryst. Solids, this volume, submitted. 19. R. Pirard, thesis, University of Liège, Belgium, 2000. 20. E. Anglaret, A. Hasmy, R. Jullien, Phys. Rev. Letters, 27 (1995) 4059. 21. G.W. Scherer, J. of Non-Cryst. Solids 108 (1989) 28. 22. T.Woignier, G.W. Scherer, and A. Alaoui, J. Sol-Gel Sci. Tech. 3 (1994) 141. 23. P. Wawrzyniack, G. Rogacki, J. Pruba, Z. Bartczak, J. of Non-Cryst. Solids 225 (1998) 86. 24. R.C. Reid, J.M. Praustnitz, B.E. Poling, The properties of gas and liquids, 4th Ed., MacGraw-Hill (1987). 25. G. Rogacki, P. Wawrzyniack, J. of Non-Cryst. Solids 186 (1995) 73. 26. W. Cao, A.J. Hunt, J. Non-Cryst. Solids 176 (1994) 18. 27. J. Fricke, E. Hümmer, H.J. Morper, P. Scheuerpflug, Revue de Physique Appliquée Colloque C4 supplément au n°4 tome 24 (1989) 87. 28. K.
Johnsen,
J.E. Christensen, K. Grau, TSBI3 users manual Danish Building
Research Institute. Copenhagen (1991).
Table
1. Average structural (density r, specific surface area SBET,
mean mesopore size diameter dHg, cluster and particle diameter
dc and dp, considered as spherical objects, and mass
fractal exponent Dm), optical transmission ratio %TR and specific
extinction coefficient at 500 nm, E, and equivalent thermal conductivity
(leq) results obtained on samples based on P750-precursor aged
in etac for 7-10 days.
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(where
)
(1)
Figures
Index
 Figure
1. Outside CO2 installation at AIRGLASS
(CO2 cryotank, bunker and AIR LIQUIDE separation sphere) |
 Figure
2. Inside the autoclave bunker
(autoclave and CO2 high-pressure connections) |
 Figure3.Sketch
of cross section of the cylindrical vacuum chamber for evacuation and assembly
of aerogel glazings.
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 |
 |
 Figure
6. Time evolution of the etac mass concentration in autoclave during direct
supercritical CO2 washing (Cmetac). Comparison between
experimental alternating drying and the corresponding simulated continuous
one (with two 53x53x1.5 cm3 gels washed at 35°C and 85 bars
with 100 to 150 kg /h CO2 flow in the 0.5 m3 inner
container)
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