Sol-Gel 1984 → 2004 (?)

Journal of Non-Crystalline Solids 73 (1985) 599-612 North-Holland, Amsterdam

599

Section VI. New glasses S O L - G E L 1984 ~ 2004 (?) Helmut DISLICH Schott Glaswerke, Hattenbergstr. 10, 6500 Mainz, Fed. Rep. Germany

The future of sol-gel will show "tailor-made" products with better-defined compositions (also stoichiometric), higher homogeneity and purity, and more controlled processes at lower temperatures. The higher the sol-gel specific advantage is, the higher the future chance. Prognosis for special ceramics should be especially good. "Heavy" industrial procedures will not be replaced. Some fields of interest are described in this sense.

1. Introduction Professor Uhlmann's kind invitation to talk on the subject of sol-gel at this colloquium in honour of Professor Norbert Kreidl on the occation of his 80th birthday is a great privilege, but this privilege also entails a certain element of danger. The danger lies in the fact that not only a state-of-the-art report is expected, but also an extrapolation, a prognosis of the anticipated progress in

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Fig. 1. Significance of sol-gel R & D. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

2004 =lOOyrs NK

600

H. Dislich / Sol-gel 1984 ~ 2004 (?)

this area up to the year 2004, when Professor Kreidl will reach his 100th birthday. This challenge is fully in accord with the nature of Professor Kreidl's lifework, in particular his characteristic scientific curiosity, but Professor Uhlmann is perfectly capable of arranging the affair in 20 years time in such a manner that today's speakers are re-invited to speak at the celebration of Professor Kreidl's 100th birthday on the subject " W h a t has come true, and what not". For today's speakers the danger in this respect increases at least in proportion to their youth. Out of respect and regard for the person at the centre of today's celebrations, however, this danger must be borne bravely. The extrapolation of a curve can be carried out with more confidence the longer the known part of the curve is, that is the more information one has on the rising tendency of the line. The sol-gel area of research is very new, and consequently the problem of extrapolation is quite a difficult one, but nevertheless an interesting challenge if bravely faced. The situation is shown in fig. 1. In my talk I will confine myself to the upper right quadrant of optimism, without wanting to define the tendency of the curve rigidly.

2. Literature on s o l - g e l s

The simplest starting point is a statistical representation of the literature on sol-gels (ca. threehundred publications and patents). Fig. 2 shows that a dramatic increase took place at the end of the seventies. It should be noted that a sol-gel synthesis was reported as early as last century [1], according to the following method: Silicic acid esters decompose in the presence of water, whereby the alcohol and silicic acid are regenerated. In each case a silicic acid jelly is produced, of similar appearance to that produced on treatment of silicates with acid. However, if the ester is simply allowed to remain in contact with a damp atmosphere, one observes that after a certain time a solid transparent mass is produced. This mass then shrinks progressively under further exposure to the damp atmosphere and after two to three months a transparent product is obtained which shows no evidence of crystallization, but which nevertheless possesses the sheen and rupture characteristics of rock crystal. The substance has become hard and will scratch glass, although with some difficulty. It possesses a specific gravity of 1.77. Sol-gel dip coatings are mentioned in the patent literature for the first time in 1939 [2], and may reports of single oxide coatings followed, till the sixties [3,4]. Around 1970 the sol-gel synthesis of defined multicomponent oxides was reported [5-9] and from this point on there followed an avalanche of publications. As a result the first workshop "Glasses and Glass Ceramics from Gels" was held in Padua in 1981, the second followed in Wi)rzburg in 1983 and the third meeting will take place in Montpellier in 1985. It is safe to assume that the sol-gel literature will continue to expand strongly, and the circle of researchers interested in this field will grow. Workers

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Fig. 2. Publications on s o l - g e l s .

in the area of glass have already responded across the board, but in future more progress may be expected in the areas of ceramic and glass-ceramic. Naturally this increase in interest is not of itself an absolute measure of achievement; what counts is: - a deeper understanding of the process and the products, leading to - new processes, new products. - a knowledge of the advantages and disadvantages. - technical products to maintain interest in the area [10].

3. What is special about the sol-gel process? The answer to this question is the basis of any future prognosis. The Materials Research Society Symposium in Albuquerque in 1984 has chosen the title "Better Ceramics through Chemistry", and that is an excellent choice. Chemical processes give us better glass, perhaps better glass ceramics, better

602

H. Dislich / Sol-gel 1984 ~ 2004 (?)

coatings, better moulded products, possibly better fibres. Here "better" means better defined also in a stoichiometric sense, more homogeneous, purer, more controlled; in a word "tailor-made". The word "better" refers to the synthesis in two respects. Firstly, the sol-gel method makes possible the production of new products, which are practically or completely unobtainable by classical means, the properties of which are however very desirable. Secondly, the high temperature required to melt glass is drastically reduced, sometimes even by 1000°C. This immediately raises the question of whether the process is also more economical, a question which is hardly original but of prime importance in the present era of increasing energy costs. The answer simultaneously frames the future potential of sol-gels: - the sol-gel process is not economically suitable for the productions of large quantities of a product [11]. Even in the future it will not replace any "heavy" industrial procedure. - the sol-gel process is suitable, taking economic factors into account, when the typical advantages of sol-gel products or production are fulfilled [12]. - the sol-gel process is suitable when scientific progress to new products can be achieved by its application, even although economic factors initially prohibit the sol-gel production, since - the sol-gel process can be improved and simplified. These statements may sound somewhat trivial, but it required years of development to come to these conclusions. In the following, these statements will be illustrated by consideration of several representative products, giving a detailed prognosis in several distinct directions; a complete coverage is not possible in this short report.

4. S o l - g e l s , today and in the future

The presentation used here is confined to a minimum of detail and presumes a knowledge of the fundamental sol-gel principles. The latter is relatively easy to acquire, since the Journal of Non-Crystalline Solids has become the centre of sol-gel publications, and articles also appear there even when the process leads to crystalline products. The importance of sol-gel principles will be evaluated for the future and for areas into which the process could break into. Concerning the latter point one is on dangerous ground; the author recalls the comment of a business consultant: " I f it turns out that you were 51% right, then you have been useful; if you were only 49% right, then you have caused damage".

4.1. The chemistry of the sol-gel process It is characteristic of single oxides in the sol'--gel process that a portion of the cross-linking reactions (as shown in fig. 3) take place at relatively low

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-Si-OR+HO-Si=_

~

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Sol-gel 1984 ---* 2004 (?)

-Si-O-Si-+ROHt

-Si-OH*HO-Si=

- S i - O - S i - +H20 I

-Si-OR÷RO-Si_=

_--Si-O-Si- .R20 [

Fig. 3.

Network

603

formingreactions.

temperature, and often even in solution. Thus variations of the network can be introduced by controlled hydrolysis-polymerization reactions of metal alkoxides. This permits structural variation without compositional alteration. Glass formation by melting, however, does not allow a significant variation in network parameters, except by compositional changes [13]. This advantage has been recognized for a long time - i.e. variations in the indices of refraction of layers without changing the layer material [3,4,10] - but little advantage has been taken of this fact to-date; this situation will change in the future, and related advantages will also prove to be useful, among which that the porosity of catalyst supports [14] and membrane separation processes [15] can be specifically set, and that foreign bodies can be fixed to membranes [16]. Economic factors will be decisive here, since the leaching process is a worthy competitor [17]. The above advantages also apply for multicomponent oxides, but the most important fact here is that the oxide network starts even at room temperature in solution, as shown in fig. 4. This leads to well defined tailor-made multicomponent oxides, specifically composed - doping included - homogeneous right down to the molecular region. Fig. 5 shows which elements have been used to date in this way. These advantages will be increasingly applied in the future, presumably especially in the area of coatings. It is also safe to assume that progress will be made in the direction of sulfides and oxinitrides, where the first steps have already been taken [18]. In these latter cases hydrolysis is replaced by sulfidolysis and ammonolysis. From a scientific point of view, the detailed investigation of the first steps in the reaction (complexation or association, hydrolysis, condensation) will receive increased attention, since these govern the subsequent steps in the process.

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metaloxane bonds.

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4.2. Technology of the sol-gel process One important point is the drastically reduced synthesis temperature, but this is achieved at the cost of highly reactive starting materials, mostly alkoxides, which are more expensive than oxides. The recurring question of saving energy must be answered in the negative, even in the future. Without carrying out detailed calculations, it can still be estimated that the predictable application of special products defines the total economic limits on the one hand, while on the other hand the energy required for the production of the highly reactive synthetic starting materials (these do not occur in nature, since they must by definition be hydrolysable) must also be taken into account. Over and above, the use of solvents must be considered in the cost. Nevertheless this does not limit the importance of the sol-gel process for the future, which will now be shown for several examples. The intensive study of a new process always produces ingenuity and creativity when technical difficulties (or economic pressures) exert an influence. As one example, the author compressed his first sol-gel Duran glass granulate at 550°C and 2800 atm, yielding a solid borosilicate glass, in order to prove the identity with melted Duran [6] (fig. 6). More specific condensation procedures make possible a direct preparation, without pressing [19,20]. One high-point was the preparation of a communications-fibre preform from alkoxides [21] (fig. 7). This volume shrinkage by a factor of 40 reminds one of the advantages of the removal of the solvent and the side products of hydrolysis in the super-critical state, with respect to the avoidance of strain

H. Distich / Sol-gel 1984 ---, 2004 (?)

605

Fig. 6. Preparation of a borosilicate glass from hydrolysed alkoxides.

[22]. Thus SiO2-aerogels were made from alkoxides with the sensational values of D = 0.2, n D = 1.04. This is really air contained and confined in next-tonothing [23]. Nevertheless, working with alkoxides (whereby the alcohol must be split off and removed) is bothersome. The logical development was a transition to a simplified procedure, whereby a dispersion of fumed silica in water [24] or alcohol [25] was poured into a mould, gelled and fired. Thus the possibility arises that silica glass products or communications-fibre preforms could be prepared without requiring a melt phase. In this case it could even result in a saving of energy. The future will probably show that moulded products from other oxides, perhaps even glass ceramics, can be produced by this means. In this respect another advantage of sol-gels, namely the extreme purity, is also important. Sol-gel starting materials such as halides or alkoxides are obtainable in a high degree of purity. At the second International Otto-Schott-Colloquium in 1982 the question of the perspectives and limits of the development of optical glass was discussed [26]. It was determined that the deciding factor lay in the fact that in the melting of extreme glass systems, aggregation and phase-separation took place

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drying

Fig. 7. Outline of sol-gel process.

sintering

606

H. Dislich / Sol-gel 1984 --* 2004 (?)

to such an extent that the resulting glass was cloudy and thus not suitable as an optical medium. The sol-gel process does not have this problem, since no melt is involved. In this respect the further development of the nD--PD diagram via sol-gel derived glass is possible. In the present-day state of development of the direct preparation of moulded products the problem of economy remains paramount. If the optical properties are sufficiently interesting and the product is urgently needed then a solution will be found, but it will not be cheap. In the field of coatings, particularly on glass, a highly developed technology has existed for twenty-five years [10]. Major products in this respect are sun-shielding layers (TiO2), vehicle rearview mirrors (TiO2-SiO2-TiO2) and non-reflective surfaces (TiO2/SiO2-TiO2-SiO:). Aside from the homogeneity achieved over large surfaces, one further advantage lies in the fact that the dip coating technology uses up only the amount of material actually required for the useful coating, provided the dip-baths have a sufficiently long pot-life. This minimises one of the disadvantages of the sol-gel process, namely that expensive starting materials are required. Despite the fact that coating processes in recent years have tended towards vacuum sputter procedures, the sol-gel dip process has a good future. This prognosis is based on the ability to produce defined multicomponent oxides layers. Noble metals can also be incorporated in colloidal form in the oxide layers, as for instance is the case in TiO2 sun-shielding layers with palladium and gold. These were thus cermet-layers. One can imagine that other uses will be found in the future. There is a great need for heat-reflecting surfaces, because of the energy situation. This opens the way for the sol-gel process, as will be discussed below. Some special fibres have been produced by the sol-gel process [27], for example TiO2-SiO 2 fibres with a thermal expansion of practically zero, and fibres with a high zirconium content (and thus stable to alkali). Both of these types are hardly available via melts, because of the temperatures involved; this gives the sol-gel process an opportunity for the future. But where meltable fibres are concerned, the author does not see any real possibility of competing, since the deciding factor of the absence of micro-fissures (for example in the case of reinforcing fibres) is certainly more easily realised in the melt procedures. In the field of sealing radioactive waste, the sol-gel process will have a good chance of establishing itself due to the absence of dust and the low temperatures involved [28]. The same applies to the production of thin-walled glass hollow spheres [29]. Ceramics have an especially good prognosis [30,31], possibly in the direct production of moulded products, but certainly in the production of very fine powders. The special advantages of the sol-gel process are: extreme purity, since extremely pure starting materials are available perfect stoichiometry and dosability, since the bonds can be preformed in solution - very fine spherical particles (e.g. ca. 1 /~m), which can hardly be produced by milling -

-

H. Dislich / Sol-gel 1984 ---, 2004 (?)

607

a narrow particle size distribution. These lead to advantages in the sintering to denser moulded bodies at lower temperatures. Since the description of the chemistry and technology of the sol-gel process has been presented up to this point in a general review, two specific examples will now be discussed, in which it will be shown what aims were involved, what was achieved, and what the future holds. Both examples belong to the frontiers of the sol-gel process, both from a technical and fundamental point of view. -

5. Electrically c o n d u c t i v e i n d i u m - t i n - o x i d e coatings

Transparent coatings capable of conducting electricity are required for heating windows, for anti-fogging, displays, photocells, solar cells and for many other applications. These are produced via sputter-, CVD- and spray processes, and are now also available via the sol-gel procedure [32,33]. The production is relatively simple; a solution of reactive indium compounds and reactive additive (to regulate the conductivity to the required value) is used, whereby the surface resistance may be varied upward from 20 f2/D over many orders of magnitude, depending on the additive and other parameters. Tin is often used as an additive, similar to the other processes. One can foresee a competition between the present processes and the sol-gel method developing in the future, at least for the above-mentioned uses. This competition will be decided primarily by economic factors, but homogeneity, stability, transmission, etchability, and other factors will also play a role. Highly conductive ITO sol-gel dip coatings of ca. 20 12/[] have a high degree of transparency to sunlight and a high degree of reflection of longer wavelength IR radiation. These therefore excel when incorporated in an insulating glass windows as passive collectors of solar energy, since they combine a high input of solar energy with excellent heat insulating properties. Fig. 8 shows the production process in brief, but it should be noted that the process is still in the laboratory stage. These coatings have been subjected to external weather conditions for over 3 years and have been shown to be stable, which, in combination with the physical properties listed in figs. 9 and 10 (as recorded in 1983), ensures them a place in any future competition. Thus the sol-gel process has entered the 1. Special cleaning of the float glass 2. Coating for avoiding alkali diffusion, hardening at max. 400-500°C 3. ITO-coating from a solution containing reactive In- and Sn-compounds. 4. Heating process, max. 400-500°C, using reducing atmosphere (N2, H 2, 02, and H20 ). 5. Cooling down to 200°C under reducing atmosphere.

Fig. 8. Production steps of ITO-coatings.

608

H. Distich / Sol-gel 1984 ---, 2004 (?)

Film thickness

"~ 80 - 100 nm

Carrier density

5 - 6 x1020 cm -3

Mobility

60 -70 cm2/V sec

Conductivity

5 0 0 0 - 6 0 0 0 Qcm

Sheet resistGnce

",, 25 ~'/[]

Fig. 9. Properties of an ITO-film optimized for window systems.

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important arena of energy conservation at a time of energy crisis, with the necessary rider that this process is still at the laboratory level. In the future there will be a hard level of competition among various processes in the field of windows used as passive collectors of solar energy. At the moment the sputter process is in a good position. The process of direct coating on a moving floatbelt using CVD- or spray procedures has been under intensive study for years [34].

6. Organically modified silicates The sol-gel process uses organic solvents and benefits from the use of the high reactivity of metallo-organic compounds (usually in the form of

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Sol-gel 1984 ~ 2004

(b

609

Si(OR}t +2H2 0 temp... SiO2+ LROH 1

Fig. 11. Synthesisof SiO2 from Si(OR)4.

CH3[

~ H3

O ... c H3Si (OR) 3 H20 . . . . . O - - S i - - O - - S i - I I 0 0 : I . O - - S i - - o .... I CH3

Fig. 12. Production of methylpolysiloxane. m e t a l - O - C ) , but concentrates on the goal of inorganic (mostly oxide) products, such as glass. The organic groups are completely removed during the process (fig. 11). If an organic group is attached to a silicon atom by a non-hydrolysable bond, the group survives the reaction without change while everything else proceeds analogously. Thus UV-transparent, low-refracting, clear-as-glass optical coatings of methyl polysiloxane were produced from methyl trialkoxysilane and bonded to silica glass fibres to produce fibre optics for UV light [35], as shown in fig. 12. A decisive concept, one of the most interesting in the whole area of sol-gels, was now to combine the definite combination of various metal oxides to silicates with the retention of organic groups in the product. This led to the hetero-organo-polysiloxanes [36], which were known, but in a way which made a specified structure possible, and thus the tailoring of various properties [37]. These products are in essence organic-inorganic plastics, often aimed-for and prepared but never before attained in a simple manner. The synthetic procedure will not be dealt with here [15,16,38], but instead fig. 13 shows schematically what can be "made-to-order".

I CH3 O C6H~ I I I --O--Si--O--Ti--O--Si--O--AI--O-I I I I 0 O I I - O -- S i - CH2-CH2--CH2--Si--O -I I

C6H5

O I

Fig. 13. Illustrativeexamplesof organicallymodified silicates.

610

H. Distich / Sol-gel 1984 --* 2004 (?)

As long as one uses Mackenzie's definition, that "glass is a non-crystalline solid", one can still formally speak of a glass in this case. On the other hand, according to the ASTM definition, that "glass is an inorganic product of fusion, which has cooled to a solid state without crystallizing", no sol-gel product is a glass. The organically modified silicates have produced the following products: - a coating or powder for coupling antibodies using amino-anilino or aldehyde groups - a material for contact lenses with a high oxygen permeability, with hydrophilic properties (wettable) - an abrasive material with a specific hardness for the treatment of the skin disorder acne - heat-sealing materials and membranes. The fact that we are formally diverging further and further from the subject of sol-gel derived glasses actually demonstrates the strength of the sol-gel process.

7. Looking to the future The chemistry and technology of the sol-gel process are inseparable and mutually dependent. Both are still in the stage of fundamental development,

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REA C TIVE POWDERS

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GLASS/ CERAMICS MONOLITHS. WITHOUT MELTING

ABSORBENTS OR MOLECULAR SHIEVES

CATALYSr q

NUCLEAR WASTE FIXATI0 N

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Fig. 14. Sol-gel technology.

H. Dislich / Sol-gel 1984 ~ 2004 (?)

611

w h i c h has n e v e r t h e l e s s a l r e a d y p r o d u c e d results w h i c h are a h e a d o f t h e s t a t e - o f - t h e - a r t t e c h n o l o g y . P r o c e s s e s a n d p r o d u c t s w i t h typical s o l - g e l adv a n t a g e s h a v e a d i s t i n c t c h a n c e of s u r v i v a l in the future, b u t t h o s e w h i c h m e r e l y c o m p e t e w i t h w h a t is a l r e a d y a v a i l a b l e c a n h a r d l y b e e x p e c t e d to be successful. S u c h c o a t i n g s o n glass w h i c h h a d d i s t i n c t a d v a n t a g e s w e r e forer u n n e r s of the s o l - g e l p r o c e s s e v e n b e f o r e this t e r m was c o i n e d . T h e a u t h o r was i m p r e s s e d b y the s o l - g e l tree f r o m a poll b y B a t t e l l e C o l u m b u s L a b o r a t o r i e s , s h o w n in fig. 14, m o s t l y d u e to its s o m e w h a t d i f f e r e n t s t a n d p o i n t . T h i s tree n o w closes this p r e s e n t a t i o n , w i t h the wish that " 1 0 0 0 f l o w e r s m a y b l o o m " in t h e s o l - g e l area. W h e t h e r the y e a r 2004 finds us in a f o r e s t d e p e n d s o n us all.

References [1] Ebelmen, Ann. 57 (1846) 319. [2] W. Geffcken and E. Berger, Dtsch, Reichspatent 736 411 (1939), Jenaer Glaswerk Schott & Gen., Jena. [3] H. Schroeder, Optica Acta 9 (3) (1962) 249. [4] H. Schroeder, Phys. Thin Films (1969) 87. [5] H. Dislich, P. Hinz and R. Kaufmann, FRG Patent 19 41 191 (1969) Jenaer Glaswerk Schott & Gen., Mainz, FRG. [6] H. Dislich, Angew. Chem. Int. Ed. Engl. 10 (6) (1971) 363. [7] L. Levene and I.M. Thomas, DAS 20 09 653 (1969) Owens-Illinois, Inc., USA. [8] R. Roy, J. Amer. Ceram. Soc. 52 (1969) 344. [9] K.S. Mazdiyasni, R.T. Dollof and J.S. Smith 11, J. Amer. Ceram. Soc. 52 (1969) 513; 53 (1970) 91. [10] H. Dislich and E. Hussmann, Thin Solid Films 77 (1969) 129. [11] J.D. Mackenzie, J. Non-Crystalline Solids 48 (1982) 1. [12] H. Dislich, J. Non-Crystalline Solids 57 (1983) 371. [13] B.E. Yoldas, J. Non-Crystalline Solids 51 (1982) 105. [14] G. Carturan, G. Facchin, V. Gottardi, M. Guglielmi and G. Navazio, J. Non-Crystalline Solids 48 (1982) 219. [15] H. Scholze and H. Schmidt, DOS 29 25 969, (1979) Fraunhofer Gesellschaft zur F6rderung der angewandten Forschung e.V., Miinchen, FRG. [16] H. Schmidt and H. Scholze, DOS 27 58 414 (1977), Fraunhofer Gesellschaft zur FOrderung der angewandten Forschung e.V., Mianchen, FRG. [17] L.M. Cock, K.H. Mader and R. Schnabel, US-Patent Application, Ser. No. 309 149 (1981). [18] C.J. Brinker, Commun. Am. Ceram. Soc. (1982) C-4. [19] J. Zarzycki, M. Prassas and J. Phalippou, J. Mater. Sci. 17 (1982) 3371. [20] B.E. Yoldas, J. Mater. Sci. 14 (1979) 1843. [21] I. Matsuyama, K. Sagamihara, K. Susa, S. Satoh, S. lruma, T. Suyanuma and S. Torkorozawa, DOS 31 16 883 (1981) Hitachi Ltd, Tokyo, Japan. [22] M. Prassas, J. Phalippou and J. Zarzycki, J. Mater. Sci. (1984) to be published. [23] S. Henning and L. Svensson, Phys. Scripta 23 (1981) 697. [24] E.M. Rabinovich, D.W. Johnson Jr, J.B. Macchesney and E.M. Vogel, J. Non-Crystalline Solids 47 (1982) 435. [25] G.W. Scherer, Glastechn. Ber. 56, Sonderband Kongrel3vortr~ge Band 2 (1983) 834. [26] W. Vogel, Wissensch. Z. F, Schiller Universit~t 32, Heft 2/3 (1983) 495. [27] K. Kamiya and S. Sakka, Yogyo-Kyokai-Shi 85 [6] (1977) 308.

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H. Distich / Sol-gel 1984 ---, 2004 (?)

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