Journal of Sol-GelScience and Technology 8, 17-22 (1997) 1997Kluwer Academic Publishers. Manufactured in The Netherlands. Past and Present of Sol-Gel Science and Technology JERZY ZARZYCKI University of Montpellier II, Laboratory of Science of Vitreous Materials, Place Eugene Bataillon, 34095 Montpellier Cedex 5, France Abstract. In the last twenty years sol-gel science has undergone a spectacular development. The various stages of the sol-gel process have been scrutinised in considerable detail and a sound basis for future technological developments established. In the beginning the studies centred mainly on silica and silicate glasses and were progressively extended to many ceramics and composites. A turning point was reached with the emergence of ormosils and of organic-inorganic nanocomposites. This opened the gateway to whole classes of new materials. The present studies show a definite tendency towards very specialised high-tech applications. Keywords: sol-gel science, sol-gel technology, sol-gel structure, industrial status, new materials 1. Introduction--Initial Motivations The "sol-gel" method of preparing glasses is being ac- tively studied in leading laboratories all over the world and, in the last ten years, the number of scientific pub- lications in this field shows an exponential increase. Industrial applications, however, are still scarce and it, therefore, seems worthwhile to investigate the reasons for this apparent lack of interest. The emergence of the Sol-Gel Science was at first very progressive and, curiously enough, the different initial motivations were of a practical, if not technical, nature. Thus, already in 1845 when Ebelmen reported the formation of a transparent solid by slow hydroly- sis of silicic esters [1], he wrote that "it is permitted to hope that it could be used in the construction of optical instruments". His discovery was, however, for- tuitous and not the result of a systematic search for an alternative way of producing glasses. In the 1930's Geffcken and Dislich of Schott Company searched an economic way of covering industrial glass with a thin oxide layer [2] and developed the basis of the dip- coating process which was later abandoned for eco- nomic reasons [3]. After the war the motivations came from laboratory studies. D.M. Roy and R. Roy sought a way of producing homogeneous melts for phase equi- librium studies [4, 5]. Hypercritically dried alcogels were prepared by Teichner et al. to serve as catalysts with very large specific surfaces [6]. Our own interest in the process came from a search for a method of producing high-melting homoge- neous glasses for the glass-ceramic process, difficult to obtain by fusion [7]. The renewed interest in the sol-gel route was triggered off by the hope of pro- ducing optical glass components: preforms for op- tical fibres and large elements such as lenses and mirrors by direct low temperature precision forming without the need of subsequent polishing. This stim- ulated the search for ways of obtaining monolithic pieces of gels of a sufficient size which could be sintered without the use of hot-pressing techniques [8]. Parallely, the possibility of tailoring on a nano- scale the structure of materials provided a link with the synthesis of advanced ceramics. At this time the atomic energy laboratories in the USA had al- ready largely developed the sol-gel route for produc- ing fuel powders and pellets, minimizing pollution hazards--their work, however, remained secret until the early 1970's. It may be said that, in a way, sol-gel technology pre- ceded sol-gel science, but that the fundamental studies are now considerably in advance of the practical appli- cations. Every stage of the sol-gel process was closely scrutinised in the hope that a better understanding
Transcript
Journal of Sol-Gel Science and Technology 8, 17-22 (1997) �9 1997 Kluwer Academic Publishers. Manufactured in The Netherlands.
Past and Present of Sol-Gel Science and Technology
JERZY ZARZYCKI University of Montpellier II, Laboratory of Science of Vitreous Materials,
Place Eugene Bataillon, 34095 Montpellier Cedex 5, France
Abstract. In the last twenty years sol-gel science has undergone a spectacular development. The various stages of the sol-gel process have been scrutinised in considerable detail and a sound basis for future technological developments established. In the beginning the studies centred mainly on silica and silicate glasses and were progressively extended to many ceramics and composites. A turning point was reached with the emergence of ormosils and of organic-inorganic nanocomposites. This opened the gateway to whole classes of new materials. The present studies show a definite tendency towards very specialised high-tech applications.
The "sol-gel" method of preparing glasses is being ac- tively studied in leading laboratories all over the world and, in the last ten years, the number of scientific pub- lications in this field shows an exponential increase. Industrial applications, however, are still scarce and it, therefore, seems worthwhile to investigate the reasons for this apparent lack of interest.
The emergence of the Sol-Gel Science was at first very progressive and, curiously enough, the different initial motivations were of a practical, if not technical, nature. Thus, already in 1845 when Ebelmen reported the formation of a transparent solid by slow hydroly- sis of silicic esters [1], he wrote that "it is permitted to hope that it could be used in the construction of optical instruments". His discovery was, however, for- tuitous and not the result of a systematic search for an alternative way of producing glasses. In the 1930's Geffcken and Dislich of Schott Company searched an economic way of covering industrial glass with a thin oxide layer [2] and developed the basis of the dip- coating process which was later abandoned for eco- nomic reasons [3]. After the war the motivations came from laboratory studies. D.M. Roy and R. Roy sought a way of producing homogeneous melts for phase equi- librium studies [4, 5]. Hypercritically dried alcogels
were prepared by Teichner et al. to serve as catalysts with very large specific surfaces [6].
Our own interest in the process came from a search for a method of producing high-melting homoge- neous glasses for the glass-ceramic process, difficult to obtain by fusion [7]. The renewed interest in the sol-gel route was triggered off by the hope of pro- ducing optical glass components: preforms for op- tical fibres and large elements such as lenses and mirrors by direct low temperature precision forming without the need of subsequent polishing. This stim- ulated the search for ways of obtaining monolithic pieces of gels of a sufficient size which could be sintered without the use of hot-pressing techniques [8]. Parallely, the possibility of tailoring on a nano- scale the structure of materials provided a link with the synthesis of advanced ceramics. At this time the atomic energy laboratories in the USA had al- ready largely developed the sol-gel route for produc- ing fuel powders and pellets, minimizing pollution hazards--their work, however, remained secret until the early 1970's.
It may be said that, in a way, sol-gel technology pre- ceded sol-gel science, but that the fundamental studies are now considerably in advance of the practical appli- cations. Every stage of the sol-gel process was closely scrutinised in the hope that a better understanding
18 Zarzycki
would lead to optimised results and, perhaps, to new materials or products.
2. Advantages and Disadvantages of the Sol-Gel Process
The sol-gel process appears attractive because it offers in principle several obvious advantages [9]:
1. Lower processing temperature. 2. High homogeneity and purity of resulting materials. 3. Possibility of various forming processes.
In the case of glasses the process is essentially at- tractive for the production of those compositions which require high melting temperatures. As the sintering op- eration has to be carried out at temperatures much lower than those required for the melting of glass-forming components, practically in the vicinity of the glass tran- sition temperature, the gain may be quite substantial. For example, silica glass may be obtained in the vicin- ity of 1000~ rather than in the 2000~ range. Be- sides pure Si t2 glass, which was the object of the great majority of studies, other high-melting systems, e.g., SiO2-TiO2, SIO2-B203, SiO2-ZrO2 and SIO2-P205 as well as ternaries combining preceding systems with A1203, alkali, alkaline earth or rare earth oxides have been investigated.
Another important characteristic of the process is that final homogeneity is directly obtained in solution on a molecular scale. In the sol-gel route the wet gel may in principle be prepared in stoichiometric condi- tions and with a degree of purity which depends only on the starting ingredients. The composition of the final glass will mainly depend on the sintering process which is performed at lower temperatures and thus reduces the risks of contamination and loss of more volatile components. This can be compared to the difficul- ties of preparing homogeneous glasses in the classic way, particularly when the components differ greatly in volatility (e.g., Si t2 and B203 or P205) and where the resulting melts possess a high viscosity which hin- ders efficient mixing of the constituents. In some cases it is then necessary to remelt the original batch several times to reach the required compositional uniformity. This in turn increases the likelihood of contamination from crucible walls, particularly at high temperatures or during repeated crushing procedures.
The great majority of studies concerned pure S i te gels and here the results obtained are indeed excellent. When, however, multicomponent gels are considered,
the situation is less favourable. The difference in the speed of hydrolysis of various alkoxide precursors may introduce microheterogeneity. For glasses where sev- eral glass-formers are present, mixed bond formation may be absent or delayed until the sintering stage. When silica is introduced in the form of alkoxides or colloids and the other compounds as inorganic salts (e.g., nitrates, acetates) the final reactions occur only after the thermal decomposition of the salts. Composi- tional variations sometimes occur during the drying and sintering stages, especially during hypercritical evacu- ation of the solvent.
Originally used for thin film deposition and the pro- duction of powders, the sol-gel process was shown to be also applicable to the production of fibres and even of bulk glass objects although at the cost of increased processing complexity.
The lower elaboration temperature in the sol-gel route from which energy saving might be expected is, however, largely offset by the high cost of the initial in- gredients necessary for making the gel. Organometal- lic precursors are not always available at present for the more exotic cations that might be required in some cases, and the initial formulation of the solution leading to a proper gel (without flocculation) can be a difficult task indeed. The subsequent treatment of the gel, the drying-curing and sintering stages, are also, in prac- tice, more complicated and time-consuming than direct melting and fining in classical glass-practice. They are, furthermore, specific to a given composition and the process has to be "tailored" for each new glass which requires a complete preliminary study in each case. For fibre spinning, for example, it is necessary to maintain constant the viscosity of the solution in spite of its nat- ural tendency to increase rapidly in the vicinity of the gelling point.
An intensive research effort was needed to significantly improve the different stages of sol-gel technology. This constituted an occasion for more ba- sic studies of the structure of gels, the mechanisms of gelling, drying and sintering in the best sense of Materi- als Science. The interaction between these two aspects was constantly present during the last decades.
3. Advances in the Processing Techniques
3.1. Initial Sol Formation and Gelling
Differences of reactivity of various components which lead to local heterogeneities were attenuated either by
Past and Present of Sol-Gel Science and Technology 19
pre-reacting the slower component or by chelating the fast component; in some cases, however, the use of polyatomic alkoxides is still indispensable.
The addition of an alcoholic solvent to suppress im- miscibility of alkoxide-water mixtures was shown to be avoidable by the use of ultrasonic irradiation. The "sonogels" obtained in this way were seen to possess greater density and improved processing characteris- tics. The ultrasonic insonation improves the uniformity of polycomponent systems and accelerates the gelling rate [10].
Structure control of the sol enabled spinnable sols to be obtained for fibering [l 1].
3.2. Drying
The drying stage is critical, especially when monolithic pieces of dry gel are required for the preparation of bulk glass and intensive research was carried out on this point.
Hypercritical drying, previously used to produce porous substrates for catalysis (where, however, mono- lithicity was not at stake), was improved in order to rou- tinely obtain crack-free (monolithic) objects ready for sintering [8]. The discovery of efficient Drying Con- trol Chemical Additives (DCCA) opened up new ways of producing large monoliths without the complication of the preceding autoclave process [12]. The "double processing" method was used to increase the pore size of gels which prevents cracking but at the cost of higher sintering temperature [13].
3.3. Curing
The use of various chlorination treatments enabled OH- free gels to be prepared for optical fibre preforms, and to avoid foaming of gel-prepared glasses during sub- sequent high-temperature forming operations. Highly complicated step-wise thermal treatments in oxygen at- mosphere, adapted for specific cases, were designed to permit total elimination of unwanted organic residues.
3.4. Sintering
Precise heat-treatment programs were needed in or- der to ensure complete sintering without parasitic crystallisation. It was shown that Time-Temperature- Transformation (TTI') diagrams may be adapted to take into account the variable OH-content of the gels. They
constitute a precious guide to following the compe- tition between sintering and devitrification phenom- ena and to designing appropriate thermal sintering treatments with or without external pressure applica- tion [14].
4. Fundamental Aspects of the Process
From a purely fundamental standpoint the study of the sol-gel process provided the occasion to test the gelling theories (e.g., Flory's approach and percolation theo- ries). The possibility of applying fractal concepts con- stituted a large part of recent motivation and resulted in a great number of studies using small-angle X-ray (SAXS) or neutron (SANS) scattering methods [15].
It was initially expected that the low-temperature ap- proach would enable glasses with a lower fictive tem- perature, a modified structure or presenting anisotropy effects, to be produced. In systems presenting unmix- ing phenomena, the low temperature approach was also hoped to extend the miscibility interval. These ex- pectations were not fulfilled, and it was demonstrated that, when all the possible compositional differences are taken into account, the sol-gel produced glasses are indistinguishable from the classically obtained prod- ucts. This is due to the fact that at sintering tempera- tures the relaxation processes are sufficiently rapid to obliterate the structural differences present in the xero- or aerogel [16]. The deviations observed can all be ascribed to compositional variations and, in particular, to the presence of residual --OH groups. By-passing unmixing also proved impossible.
No glasses have been prepared by the sol-gel method which could not be obtained by other methods, e.g., liq- uid quench or vapour deposition. Some of the systems are indeed easier to synthesise by the sol-gel route: this is particularly the case with glasses from glass- formers, e.g., SiOz-GeO2, SiO2-TiO2, SiOz-B203 and also SiOe-ZrO2. The addition of modifiers, however, increases the likelihood of devitrification--in particu- lar, gels with alkali-oxides are very difficult to obtain free of crystalline phases. Thus common glasses which are easily obtained by quench are often very difficult to prepare by the sol-gel route. Precise kinetic measure- ments have shown the decisive role of residual --OH groups and also of the large interface of the dry gel in promoting crystallisation [14]. It seems that the sol-gel process, far from extending the glass-forming regions, rather restricts them as a result of heteroge- neously catalysed nucleation.
20 Zarzycki
5. Present Industrial Status
What is the outcome of this tremendous effort which gave birth to several series of colloquia and mono- graphs [17-19]?
Generally speaking, industrial objectives fall into one of the following three groups:
1. Improving existing fabrication processes in order to obtain better and/or cheaper products (optimisa- tion).
2. Conferring an added value to existing products, e.g., by diversifying their functionality (valorisation).
3. Creating entirely new, unique products or services impossible to achieve in other ways (innovation).
As far as glasses are concerned, elementary eco- nomic considerations clearly show that the sol-gel processes are too expensive and complicated to com- pete with existing current technology by which sheet glass, hollow ware or fibres are manufactured.
In the field of optical glasses the considerable effort of L. Hench's group on a semi-industrial scale to obtain moulded optical SiO2 lenses of superior quality with- out polishing is, to my knowledge, the only example of a far-reaching tentative [20]. The method of preparing SiO2 tubing or plates from SiO2-SiO2 composites [21] which combines an alkoxy-derived SiO2 matrix with particulate SiO2 filler is not yet applied industriallly. The added value is possible in the field of functionally modified flat glass. The application of thin coatings by the sol-gel process can be competitive with other means (e.g., sputtering) as it is simple and requires less costly equipment. The present industrial applications of the sol-gel process are indeed concentrated in this field (e.g., Schott's IROX ~ and other products which confer a large diversification to the base glass. The production of fibres by the sol-gel route is attractive for refractory compositions. The mechanical properties are, how- ever, inferior to those of the melt-drawn products and the transversal section of the fibres is not circular but el- liptic. They may be heat-treated to produce crystallised products. At present, fibres of the A1203-SiO2-B203 system are commercially available.
6. Sol-Gel Method- -Gateway to New Materials
The preceding considerations show that, in practice, the sol-gel method can rarely compete with the more
classic techniques where bulk glass is the final aim. However, the very complexity of the process, where many intermediate stages are present, provides an enor- mous potential for producing a whole array of new types of materials with specific properties, difficult or impossible to obtain by other ways. In these cases the sintering stage is not reached and the interesting prop- erties arise from the diphasic character of the gels.
A gel is basically a two-phase system consisting of a backbone and an interstitial liquid which, as already shown by Kistler [22] are largely independent. Each of these phases may be processed separately by chemical or physical treatments far below the sintering temper- ature.
6.1. Influencing the Interstitial Liquid Phase
The interstitial liquid which is water in the case of aquagels and an alcohol + water mixture in alcogels is normally eliminated during the drying stage. The re- sulting porosity which is a typical characteristic of dry gels and which normally disappears during sintering is in itself a precious asset which can be used as such or manipulated further. The extreme fineness of the pores which can reach nanometer scale results in gels which may look completely transparent without any scatter- ing effects in the visible range. The resulting decrease in the refractive index can be used for antireflective purposes [23].
The extraordinary light materials obtained by hyper- critical evacuation, aerogels, possess very interesting characteristics. They are the best thermal insulators known, which may be at the same time transparent or translucent. Their index of refraction close to unity also permits their application in solid state Cherenkov detectors in high energy particle physics [24].
Impregnation of the porous gels by plastics poly- merised in situ permitted a range of composites to be obtained with new mechanical properties [25].
The addition of colourants before gelling or impreg- nation, after settings with suitable organic dyes, is the basis of new non-linear optical materials for solid-state accordable lasers or optical memories.
Reactions within the interstitial phase between orig- inally contained components or brought in by external diffusion opened up the way to producing nano-scale precipitates. Glasses with non-linear optical proper- ties based on quantum confinement phenomena could be prepared by sol-gel methods.
Past and Present of Sol-Gel Science and Technology 21
6.2. Influencing the Nature o f the Backbone
The most successful way so far consists in produc- ing hybrid organic-inorganic backbones where the or- ganic groupings are not destined to be subsequently eliminated but to form the integral part of the struc- ture. This is the case of Organically Modified Silicates (ORMOSILS), also known as ORMOCERS or CER- AMERS [26]. Combining the powerful synthesising capacity of organic polymer chemistry with the inor- ganic sol-gel techniques opens up an unlimited field of new materials which, in the case of SiO2-based gels, are intermediate between silica and silicones.
7. Present Research
Recent colloquia [27, 28] and reviews [29-31] clearly show the following trends in sol-gel research:
- - t h e diversification of applications; - - t h e increasing part played by hybrid, inorganic-
organic materials; - - t h e preponderance of thin film geometry to the
detriment of monoliths.
Thin films for planar waveguides, films with non- linear optical, electrooptic and electrochromic prop- erties are currently developed for high technology applications. Entire elaborate systems for photocon- version or photochromic applications were devised around these materials. Initially pure oxide systems were used; at present, the hybrid organic-inorganic ORMOSILS are gaining in importance.
In some recent applications the gel tends more and more to be used merely as a host matrix containing active components: laser dyes, photochromic dyes, non-linear optical dyes, liquid crystals, magnetic fer- rofluids, etc. All sorts of materials were tested: even enzymes and biomolecules for various sensors (e.g., immunosensors) were entrapped in the gels thanks to a flexible solution chemistry involving little or no heat- ing. Thin film geometry is most suitable for the ma- jority of practical applications which again favours the sol-gel techniques.
The study of the encapsulation of some of these func- tional molecules bridges the chemistry of inorganics with biochemistry and tends to establish a link between inanimate and animate worlds.
The nature of the problems in sol-gel research has been shifted: it is now necessary to study the best way
of incorporating the second phase into the gel without denaturating the encapsulated reactants. In some cases the pore-size of the gel has to be tailored in order to match larger molecules, etc. As the final product is no longer a stable, refractory, inorganic material but often a fragile, labile system, supported by the gel substrate, the protection of the contents against aging processes which condition the shelf life of the material becomes of primary importance.
It is in the field of these entirely new products for high technology applications that the future of the sol-gel process lies. Sol-gel science should make a significant contribution in the vital areas of in- formation technology, environment technology and biotechnology.
References
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363-370 (1971). 4. D.M. Roy and R. Roy, Am. Mineralogist 39, 957-975 (1954). 5. R. Roy, J. Am. Cer. Soc. 39, 145-146 (1956). 6. S.J. Teichner, G.A. Nicolaon, M.A. Vicarini, and G.E.E. Gardes,
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13. E.M. Rabinovich, in Ref. [17], pp. 260-294. 14. J. Zarzycki, in Advances in Ceramics 4, 204-217 (1982). 15. J. Zarzycki, J. Non-Cryst. Sol. 95/96, 173-184 (1987). 16. A.R. Cooper, in Better Ceramics through Chemistry, edited by
C.J. Brinker, D.E. Clark, and D.R. Ulrich, Mat. Res. Soc. (Pitts- burgh, Pa, Vol. II, 1989) pp. 421-430.
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