HISTORY OF SOL-GEL SCIENCE AND TECHNOLOGY (REVIEW)

Ivanova, R.and Iordanova Journal of the UniversityY.ofDimitriev, ChemicalY.Technology Metallurgy, 43, 2, 2008, 181-192

HISTORY OF SOL-GEL SCIENCE AND TECHNOLOGY (REVIEW) Y. Dimitriev, Y. Ivanova, R. Iordanova*

University of Chemical Technology and Metallurgy 8 Kl. Ohridski, 1756 Sofia, Bulgaria E-mail: [email protected] * Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

Received 05 March 2008 Accepted 27 May 2008

ABSTRACT The objective of this review is to summarize some of the most significant research achievements in the sol-gel science and technology. Comprehensive review papers have been made regularly in the past three decades by outstanding scientists such as Mackenzie, Sakka, Zarzycki, Schmidt, Uhlmann, Livage, Ulrich, Dislich. The main sources for the scientific information are the Journal of Sol-Gel Technology started in 1993 and the International Sol-Gel Workshops periodically held from 1981 up to now. In the beginning the studies have been centered mainly on silica and silicate glasses but they progressively have been extended to many other oxide and non-oxide ceramics and composites. It has been shown that the sol-gel methods have great potential in producing important materials in forms of bulk, fibers, sheets, coating films, and particles at relatively low temperatures. It is also possible to produce materials of new compositions with high purity, high homogeneity, and to control particle size distributions in a nano-scale level. One of the most important advances of sol-gel science is the preparation of inorganic-organic hybrids. They started with the development of ormocers based on the formation of chemical bonds between the constituents and nanocomposites, containing organic molecules incorporated into porous gel matrix. That is why the sol-gel method is a representative nanotechnology. Specific examples are presented that have been carried out in leading universities and laboratories. Some our results in this field are also discussed briefly. A classification was made of sol-gel derived materials according to their functions: optical (solar collectors, fibers, waveguide), electronic (piezoelectric transducers, non-volatile memory, solid electrolytes), thermal (refractorys and low expansion ceramics, aerogels), chemical (catalysts, membranes, corrosion protection), biomedical (entrapment of enzyme and living tissue, implants). It was emphasized the crucial role of starting precursors and the processing routes to the final structure and the impact on the technological functions. Keywords: sol-gel technology, sol-gel materials, hydrolysis, condensation, precursors. INTRODUCTION The preparation of inorganic refractory materials follows well elaborated technological schemes which

comprise the proceeding of high-temperature solid state reactions. Ceramic and glassy materials have been obtained in this way for thousands of years on the basis of natural raw materials. Irrespective of the specific tech-

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Journal of the University of Chemical Technology and Metallurgy, 43, 2, 2008

nological conditions of the different schemes used in industry one of the main factors in each case is the temperature of synthesis, sintering and melting. The sol gel technologies were developed during the past 40 years as an alternative for the preparation of glasses and ceramics at considerably lower temperatures. The initial system represents a solution where different polymerization and polycondensation processes lead to the gradual formation of the solid phase network. The sol formed is at first subjected to a series of operations: gelling, drying, pressing, drawing and casting, which results in various structural and phase transformations. This permits formation of powders, fibers, coatings, bulk monolithic products, etc. from the same initial composition. The synthesis schemes mostly used can be divided, depending on the initial precursors, into three groups: (i) aqueous solutions of metal salts; (ii) metal alkoxide solutions; (iii) mixed organic and inorganic precursors. The purpose of the present review is to summarize the main achievements in this field using results of the sol gel technology and some reviews published at different times. Special attention is paid to study associated with the utilization of alkoxide precursors which contribute to the successful development and popularization of the sol gel technology in various areas of application. HISTORY OF THE PROBLEM The development of the sol gel science and technology is impressing and instructive, the more so as at present they show an intense development accompanied by important applications in the practice. According to Sakka [1] who is one of the pioneers in this new trend of technological development, the sol-gel technology is a typical nanotechnology because all gel products may contain nanoparticles or are nanocomposites. In this sense it plays a principal role in the development of modern nanotechnology for the preparation of new materials. Ebelman was the first to report (1845) the formation of a transparent material as a result of slow hydrolysis of an ester of the silicic acid. In 1930 Geffcen and Berger from Shott Company established the way of achieving a sol-gel process for oxide layers on industrial glasses using combinations of metal-containing precur-

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sors by deep-coating. After that D. Roy and R. Roy proposed a method for preparation of more homogeneous melts and glasses using a sol-gel process. In the early 60s H. Schroeder deposited transparent coatings on glass surfaces in order to correct the refractive index using titanium butoxide. At the same time, in the nuclear laboratories in USA one has worked on large scale projects based on sol gel processes in association with the production of nuclear fuel, as a result of which material of a higher density has been obtained. However, these results [2, 3] have to remain secret for a long time. Other episodical investigations [4, 5] concerning low-temperature synthesis of ceramic material and quartz glass from gel precursors have been published during the same period of time. A serious success are the studies of Dislich [6, 7] which aroused great interest among the scientists. Dislich synthesized boronsilicate glass by heating bathes of oxide powders obtained by a low temperature sol-gel process. The results obtained initiated systematic investigations by a lot of research teams and contributed very much to the development and popularization of the sol-gel technology within a short time. Two reviews [8, 9] contained analyses of the first practical achievements associated with the preparation of fibers, coatings and monolithic products. They also presented important prognostications concerning the synthesis of new glasses and ceramics. According to Dislich, the number of publications at the beginning of the 80s of the 20th century shows a sustainable exponential growth. The first International Conference on Glasses and Glass Ceramics Obtained from Gels was held in Padova, Italy in 1981 [10]. This significant event marks the actual modern development of sol-gel science. Ever since then periodic similar conferences [11-20] demonstrating the advance of the studies have taken place. The interest in organizing such events is still great. The last two conferences were held in Australia (2003) and in USA (2005). The timely publication of the conference materials is the main source of new technological and basic knowledge and a stimulus for extending the studies in various directions. Another important factor for exchange of information is the specialized international Journal of Sol-Gel Science and Technology which has been issued regularly since 1993 (Editor-in-chief, S. Sakka). It presents a wide range of investigations comprising all nuances in the correspond-

Y. Dimitriev, Y. Ivanova, R. Iordanova

ing field. In each volume there are original scientific papers, reviews on topical themes and special issues on symposia and conferences. A considerable number of the reports at international congresses on glasses held during the past 20 years were dedicated to sol-gel technologies and were mostly presented at special sessions. The publications presented at the 17th Congress on Glass held in 1995 and the 20th Congress in Kyoto [21, 22] were significant in number. The huge and steadily and quickly increasing amount of publications makes rapid and adequate analysis of the achievements difficult. Professional reviews by renowned scientists are very helpful. Irrespective of when they were published, they continue to be a topical source of knowledge and to generate new ideas. The sol-gel synthesis methods and the products obtained by them were summarized by Sakka in 1985 and 1987 [23, 24]. Already at that time the author paid special attention to the practical applications of the fibres and coatings of TiO2, TiO2-SiO2, ferroelectrics, aerogels and hybrid products. It is worth noting that during the past years the interest of leading industrial firms in the application of the low-temperature methods is obvious. The most active among them are some companies in Japan: Nippon, Sheet Glass, Asahi Glass Company, and Central Glass Company, which have introduced photocatalyst gels, coatings against mist, hydrophilic aerogels, coatings for cars and for construction classes, for TV sets etc [25]. On the basis of a wide inquiry among specialists Uhlman [26] pointed to the real possibilities of definite applications in catalysis, in the preparation of microfilters and sensors. The first results on the preparation of solid electrolytes by sol gel methods were summarized by Livage [27]. He considered the electrochemical properties of gels comprising V2O5.nH2O, WO3.nH2O, TiO2-CeO2. In the papers of the International Conference on Glass Science in 2004 held in 1984 and dedicated to the 70th anniversary of Professor Kraidel, important and true prognoses on the future development of the fundamental and applied sol-gel research were made [27-33]. It was pointed out that one of the most important and promising results was the development of amorphous materials containing organic and inorganic components in their structure (organically modified silicates). Ten years later Zarzycky [34] noted again the increasing number of investigations on the organo-inorganic hybrid materials.

An important peculiarity of the sol gel technologies is the possibility to control the mechanism and kinetics of the proceeding chemical reactions. In other words, controlling each step of the sol gel process one may affect the final structure of the materials and the modification of the processes. Some of these principal processes are subjected to detailed analysis by Livage et al. [35] and by Yoldas [36]. The structure and phase changes accompanying the development of a definite sol gel process can be followed by different physical experimental techniques, which are constantly improving. Among them the spectral methods proved very appropriate due to the relatively easy processing of the results and their explicit interpretation in the case of both amorphous and crystal state of the gel products. Combining several structural methods adequate information have been obtained for sol-gel materials in the system TiO2-SiO2 [37]. During the 90s of the past century a series of reviews [38-50] were dedicated to the synthesis of various materials by sol gel methods and most of them are still attracting. Important information on the synthesis of ferroelectric polycrystalline and amorphous coatings of alkoxides with the participation of LiNbO3, KNbO3 and BaTiO3 and the possibility of their application as condensers, waveguide, etc. was summarized [39]. At the same time, the successful development of nonlinear optical devices with the participation of gold particles [51] and CdS in a silicate matrix [44] were reported. The development of a method for preparing optical fibres [47] and the wide application of the sol-gel processes to optical telecommunication technologies at present show a high degree of realization on an industrial level [50]. Kakihana [49] made an exclusively detailed analysis of the applicability of the different variants of sol gel technologies for the preparation of complex multicomponent materials such as ceramic superconductors using a wide range of precursors. Gugliemi [41] and Mackenzi [45] analysed the advantages of the sol-gel coatings for protecting the metals from oxidation, thermal and chemical corrosion and especially for improving their strength. An important problem concerning the density of gel coatings was solved successfully by Scherer [42]. He showed that sintering of the amorphous gel particles at high temperatures preceding the crystallization processes and realized by suitable

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heating rates is important for improving the quality of the coatings. At the end of the 20th century increasing attention was paid to the organo-inorganic hybrids and their relation to the sol-gel processes [52, 53]. This problem is the subject of several reviews. BASIC AND APPLIED RESEARCH Mackenzie in his short but very informative review considered [54] the principal achievements associated with the sol gel processes and the products obtained by them. In historical aspect he notices two periods of studies. According to him, the first generation of sol-gel processes and the resulting materials are dedicated to various reactions with the participation, above all, of alkoxides, while the obtaining of organic-inorganic hybrids belongs to the second period. Following the sequence proposed by him, we shall mention briefly some of the important achievements, which are expected to continue developing in the future. Above all these are the studies of Livage et al. [35, 55] dealing with the physical chemistry of sol-gel solutions, on the basis of which a particle charge model has been elaborated with a view to predicting and controlling the reactivities of the alkoxides and the formation of stable metal oxide clusters. More recently it was found that attaining a high homogeneity of the solutions did not lead to homogeneous structure of the corresponding solid phases because the structural units may be transformed with time before being frozen in a solid phase matrix [56]. Nakanishi [57] was the first to achieve phase separation in gel solutions, which permitted successful applications to the production of porous solid materials with a design microstructure. The theoretical analysis of Scherer [33, 58] concerning the drying mechanism and the experimental techniques he proposed represent an essential contribution to the efforts to prevent cracking and uncontrolled destruction of monolithic gels during the thermal treatment. One of the first applications of the sol-gel technology is the production of glass fibres by lowtemperature sol-gel synthesis [23,59]. A simple and effective method for drawing amorphous fibres from viscose sols by thermal treatment at temperatures much lower than the melting temperature of SiO2 was developed. These investigations are the basis for obtaining the other new kinds of optical fibres mentioned above [47, 50]. A typical

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example is the preparation of self-cleaning surfaces based on the photocatalytic effect of TiO 2 (anatase nanoparticles) for decomposing organic compounds and obtaining a hydrophilic surface under the effect of UV radiation [60-64]. Coatings of that kind for motor vehicles and architecture glasses are obtained. Recently the research team of Minami [66-67] obtained a new type of hybrid structures with the participation of TiO2 and Al2O3 which have unique properties as coatings of glass. Depending on their structure and composition, they can possess both superhydrophilic and hydrophobic properties. It can be said that the pioneer works of H. Schmidt [29, 53, 68] on the preparation and investigation of hybrid structures mark the beginning of the second important period in the development of the sol-gel science and technology. A new type of noncrystalline materials (called at first ormosil and ormoker) with strong chemical bonds between organic and inorganic components were obtained. Avnir et al. [69] laid the beginnings of investigations on another kind of hybrid materials obtained according to a sol gel method and incorporating organic dye in oxide gels. Since the chemical bonds between the organic molecules and the inorganic matrix in these structures are insignificant, these materials are called nanocomposites. On this basis, compositions containing organic luminescent dyes and SiO2 have been successfully developed for new type of self-tuning lasers [70, 71]. Other promising applications of these hybrids are associated with the production of solar collectors, elements for nonlinear optics, sensors, biological markers, materials for fluorescence diagnostics and photodynamic therapy in medicine. The successful development of those applications requires selection of a suitable active component and elucidation of the character of their interaction with the matrix [72-74]. Especially successful is the development of hybrid organo-inorganic structures associated with the biological and medical nanocomposites [75, 76] which, according to E. Poppe [77], can be added to the so-called live ceramic. Addition of biopolymers (cytosam, gelatine, alginita) [78,79] to an inorganic (silicate or oxide) matrix has led to improvement of the biocompatibility with the live tissue and the immobilization of enzymes, and cells is facilitated. Another direction of hybrid structure development is associated with their use in the synthesis of refrac-


tory ceramic materials. As a result of the efforts of a lot of research teams [80-83], ceramics (fibres, coatings and monolithic products) of the system Si-C-O have been obtained as well as more complex combinations with the participation of Ti, B and Zr. The most usual initial precursors are alkyl, aryl or phenyl substituted alkoxosilanes, which, following an appropriate scheme (hydrolysis, condensation and pyrolysis in inert medium) are transformed into inorganic carbon-containing materials (black glass) or into polycrystalline ceramics after heating above 14000C. An analogous scheme has led to oxynitride glass and ceramics (Si-N-O) after treatment in ammonia atmosphere [84]. The preparation of nonoxide ceramics (C-Si-N, C-Si-N-Ti-B, etc) develops independently by the so-called polymeric methods [85-87]. Choosing specific precursors (polycarboxylan, polyvinylsilazan, polysilazan, polyborosilazan, polymethylvinylsilane), one obtained spatial networks called pre-ceramic polymers [88-91]. As a result of pyrolysis processes in a suitable gaseous medium, ceramization of the polymers (organicinorganic transition) occurred and the so-called polymeric amorphous ceramics were obtained [92, 93]. Depending on the chosen thermal regime, these materials can be preserved in the amorphous state or transformed into nanocrystalline structures as coatings, powders, fibres or bulk solid products. The successful development of modern ceramic technology associated with the synthesis of ceramic materials is, to a large extent, due to the application of different variants of the sol-gel technologies to the synthesis of submicron powders. The achievements in this respect are summarized in the review of Ganguli [94]. Ceramic powders obtained by the sol gel technology are characterized by lower sintering temperatures, improved stability with respect to grain growth (recrystallization) and, finally, formation of dense ceramics. A wide application during the last decade was attained by the modern version of the method of Pechini [49, 95, 96]. This method is based on esterification processes between chelate complexes of metal ions (soluble metal salts, nitrates, acetates, etc. combined with a chelate agent, citric acid or EDTA) and ethylene glycol. The resin obtained in this way is transformed, after thermal treatment, into a nanosized powder in which the particles are distributed within very narrow limits (monodispersity) and the reactivity is enhanced. The

above examples from various scientific regions confirm once more that the sol-gel methods are among the main routes of obtaining hybrid and nanostructured materials. Several specialized reviews [52, 97-99] have been dedicated to this problem. Sanchez [97] demonstrated the possibilities of different chemical strategies for preparation of hybrid structures based on sol-gel processes combined with controlled growth upon suitable templers. The networks generated by this research team may contain hierarchically ordered polyfunctional clusters or nanostructured blocks. Portier et al [99] showed how combinations of various polymers and inorganic compounds may be included into hybrid structures (ormokers ionomers, polymeric salts, complexes). The purpose of these studies is associated with finding new applications of the hybrids as nanocomposites, nanosized powders, electrolytes, membranes, sensors, etc. The experience accumulated by several generations of scientists in the field of the sol-gel processes is summarized in a series of monographs and topical proceedings [100-106]. The monograph of Brinker and Scherer [104] is exclusively popular. The last fundamental edition is the three-volume collection edited by Sakka [106] with the participation of world-known researchers where the most recent achievements and the classical pioneer studies are summarized. The main kinds of precursors, synthesis methods, structure and phase analysis as well as the most important practical applications are described. This monograph can be helpful for a wide round of specialists. SOL-GEL STUDIES AT THE DEPARTMENT OF “TECHNOLOGY OF SILICATES” OF THE UNIVERSITY OF CHEMICAL TECHNOLOGY AND METALLURGY, SOFIA The investigations on the sol-gel processes at the Department Technology of Silicates at the University of Chemical Technology and Metallurgy in Sofia began as a new direction of studies affected, on one hand, by the investigations of the pioneers in this field, and, on the other, by personal contacts with them and their visits to Bulgaria. In 1987, at the Conference on Glass and Ceramics in Varna, Sakka [107] presented a large plenary lecture on the state problem and the future of the sol-gel method. He received 17 years later a

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Fig. 1. The award of prof. S. Sakka by the academic council of UCTM, 2005.

Honnory plaque of UCTM for his outstanding contributions to the sol-gel science (Fig. 1). E. Zarzycky [108, 109] was an active participant in the work of the International School on Condensed Matter Physics in 1988 [108] and in the Conference on Glass and Ceramics in 1993 [109]. He read reviews on the fractal structure of amorphous gels, peculiarities of preparation of monodisperse powders and the role of different precursors with respect to the formation of gel materials (Fig. 2). Dislich [110, 111] also contributed very much to the Bulgarian scientists joining in this field by their participation in the conferences on Glass and Ceramics in Varna in 1993 and 1996 (Fig. 3). In an extensive report in 1996 Samuneva [112] summarized the first investigations performed in our country on the use of low-temperature sol-gel methods for synthesis of glasses, glass-ceramic coatings, ceramic powders, pigments and glazing. In this analysis it was pointed out that the researchers of the Department were interested in developing and applying new combinations of precursors. In the middle of the 90s of the 20th century, several teams were formed at the Department, which extended the investigations on new sol-gel products.

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Coatings based on silicates, phosphates and oxides were obtained [113-119]. Ceramic powders containing spinel, garnet and perovskite phases compositions for glazing

Fig. 2. A plenary lecture of prof. Zarzycky at the Conference on Glass and Ceramics in Varna, 1993.


sol-gel synthesis was realized during the preparation of perovskite phases which could be used as membranes in heating elements [146]. Multicomponent fire-resistant ceramic materials were synthesized in mixed oxide-nonoxide systems containing Si, C, Ti, B, O, N [147-156] on the basis of hybrid structures using original precursors. CONCLUSIONS

Fig. 3. Dr. Dislich at the Conference on Glass and Ceramics, Varna, 1996.

The based on the data concerning the development of sol gel technologies, in summary one can say that a very important moment is the choice of appropriate precursors. Most often these are alkoxides, soluble metal salts, polymers, colloids, which, depending on their nature, may be combined with suitable solvents, and the aggregation processes stimulating solid phase formation can be controlled. Fig. 4 shows schematically the most used variant of the sol-gel process. We accepted a more general interpretation proposed by Kachichana [49] according to which a chemical process starting from solutions and leading to a solid phase without a precipitate is a sol-gel process even if the system does not represent an infinite solid network. One of the methods leading to colloid dispersions (sols) is based on inorganic salts, water and occurrence of hydrolysis processes at a definite pH. A classical example is the formation of a gel of SiO2 whose detailed description is given in the monograph of Iler [157]. This technique was not a subject of the present review. The non-hydrolytic sol-gel method (without participation of water) is also promising, especially with respect to transition metal

and pigments were obtained from precursors containing organic and inorganic components [120-126]. The preparation of bioceramic materials is a permanently topic for the researchers of the Department. The sol-gel methods are used for investigating silicate and phosphate systems containing fluoroappatite, Hydrolysis and condensation tricalcium phosphate as well as gels with antiof metal alkoxides bacterial effect [127-134]. A series of organoinorganic systems for immobilization of cells Colloidal dispersion [135-138] can be attached among this group of materials. Incorporation of rare-earth ions into SolSol-gel processing amorphous gel silicate matrices stable at high temperatures without metastable phase separation, was achieved. Multicomponent ceramic nanopowders containing rare-earth manganites Inorganic-organic hybrids were obtained [139-141]. Specific sol gel methPolymer pyrolisis ods (peroxy method, ion exchange) were used to obtain nanostructured transition metal oxides (V2O5, MoO3, WO3) [142-145]. Successful Fig. 4. Different routes of the sol-gel processing.

Non-hydrolytic sol-gel reactions

Pechini gel methods (chelate polyesterification)

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Applications of sol-gel method

Optical and photonic functions

Fluorescence solar collector, solar cell; Laser element, light guide; optical switching, light amplification, antireflecting coatings; non-linear optical effect (second generation).

Electronic functions (ferrolectricity electronic and ionic conduction) Capacitor, piezoelectric transfer; Non-volatile memory, transparent semiconductors; Solid electrolyte (battery, fuel cell);

Chemical functions Mechanical functions

Protection with hard coat, strong ceramics abrasive

Catalyst, membrane, gas barrier, repellent film

Thermal function

Refractory ceramics, fibers wood, aerogels; Low expansion ceramics;

Biomedical functions

-Entrapment of enzyme, cell, coated implant, medical test

Fig. 5. Applications of sol gel method according to S. Sakka [106].

oxides. This concerns mainly reactions associated to chloride hydrolysis with metal alkoxides [158, 159]. The most used is the so-called Pechini method [49, 95] which was applied successfully to the preparation of finely dispersed powders in systems with complex compositions. Pyrolysis of polymers and their transformation into polymer ceramics is developing successfully regardless of the classical sol-gel technologies but is genetically connected with them. Thus, the transformation of polysilans and polycarbosilans into inorganic ceramics includes formation of hybrid organic-inorganic nanostructures which, at high temperatures, can be transformed into powders, coatings, fibres or monolithic products [87]. As it was pointed out in the foregoing, during the past 30 years the methods associated with the participation of alkoxides, alcohols and water showed the most intense development. The occurring hydrolysis-conden-

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sation processes depend on a series of parameters and the successful synthesis depends on the quantitative ratio of the alkoxy groups with respect to water, the kind of catalyst, solvent, temperature, medium pH, component concentrations in the initial mixtures, etc. [160]. The chemical reactivity of the metal alkoxides depends on the type of functional groups, the oxidation degree of the metal ions, the ionic radius and the coordination number with respect to the anion [35]. As a rule, heterometal alkoxides are very often more appropriate because already at a molecular level strong chemical bonds are formed [161]. It is worth noting that there is a principal difference between the solgel processes occurring in a silicate matrix and processes which can take place in oxide systems especially with the participation of transition metal ions. In the latter case there is a much stronger trend to clustering, coordination number change and oxidation degree.


The sol-gel technology is very efficient in producing various functional materials in which particle size, porosity, thin layer thickness, separation of particles with different compositions and structures may be controlled and successful applications have been achieved. Following the analysis of Sakka [106] they may be summarized in a scheme (Fig. 5) where examples of materials applied in electrotechnics, electronics, optics, photonics, high-temperature technologies, chemical technologies, biochemistry and medicine are given. The history of the sol-gel technologies and the development of materials by using them is far from ending. The possibility of varying both parameters and precursors is endless. A typical example is the development of organic-inorganic hybrid and nanostructured materials on which the attention of modern scientists is focused. Acnowledgemens This work was financially supported by the Nacional Science Fund, Ministry of Education and Science of Bulgaria, contract No BY-TH 102 (No 806, UCTM, Sofia).

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