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Preparation of TeO2-based materials by Sol-Gel process by Sandrine Coste, Philippe Thomas, André Lecomte, Jean-Claude Champarnaud-Mesjard, Thérèse Merle-Méjean, René Guinebretière and Alain Dauger. SPCTS, UMR 6638 CNRS, Faculté des Sciences de Limoges, 123 av. A Thomas,87060 Limoges Cedex, France |
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1. Introduction Recent studies on TeO2-based glasses have shown that their non linear refractive indices could be up to 100 times as large as that of SiO2 (1). Such materials are so very promising for nonlinear optical device technology. Among such materials, pure tellurium oxide glass could present the highest nonlinear index. However, this glass is very difficult to obtain under conventional process of melting and air-quenching. Only small amounts of pure TeO2 glass (powder samples) have been successfully prepared using melting at 800°C and then quenching in a freezing mixture, consisting of ice, ethanol and NaCl kept at » –11°C (2). Therefore the sol-gel process has been investigated in order to produce at low temperature, and especially without melting, pure TeO2 amorphous materials. Until now, only few investigations have been done on such materials using this technique. Pierre et al (3) have determined the conditions to produce TeO2 powders from tellurium alkoxides by precipitation. More recently, elaboration of TeO2 thin films has been studied (4-6). The different conditions that allow the preparation of either a precipitate, a sol, or a gel of pure TeO2 from tellurium isopropoxide (Te(OC3H7)4) will be presented in this paper. The structural evolution of the different materials with temperature and the transformation of the microstructure from sol to gel have been followed. 2. Experimental Tellurium isopropoxide (Te(OCH(CH3)2)4,
purity>99.9%, Alfa-Aesar) is highly reactive towards hydrolysis and
condensation. Its chemical reactivity has to be decreased to avoid uncontrolled
precipitation. Therefore the effect of different chemical modifiers (acetic acid
(>99%, Prolabo), hydrochloric acid (37-39%, Prolabo), 2-methoxyethanol (99%,
Aldrich) and acetylacetone (Prolabo)) has been studied in order to moderate the
alkoxide reactivity. In a glove box under dry air, these modifiers were added (with
a R molar ratio of modifier/Te(OCH(CH3)2)4)
to a solution of ((Te(OCH(CH3)2)4) in
dehydrated 2-propanol (Prolabo) such as the final alkoxide concentration (Cf)
was reached at the end of the synthesis. Then a mixture of water and 2-propanol
was added with a W molar ratio of water/alkoxide. Whatever the nature of the
modifier and the values of R and Cf, addition of water leads to a white
precipitate. Without addition of water (W=0), either stable sols or gels have
been produced in the tellurium isopropoxide – 2-propanol - acetic acid and
tellurium isopropoxide – 2-methoxyethanol – acetic acid systems. The structural evolution of either sols or gels, previously dried first at 60°C and then at 100°C, has been followed by using thermal X-ray diffraction (D5000 Siemens diffractometer (q /q mode, Cu Ka radiation), Anton Paar HTK10 furnace, Elphyse sensitive position detector (14° aperture)). Microstructural modifications have been determined by using Small Angle X-ray Scattering (SAXS) (7).
3. Results and discussion 3.1. Tellurium isopropoxide, 2-propanol, acetic acid system At 60°C, with a molar ratio R = 4 and a final alkoxide concentration Cf = 0.1-0.2 a gel is obtained whereas a sol is observed with Cf = 0.05. At 20°C or at 60°C a gel is obtained with R = 65 and Cf = 0.1-0.2 then a precipitation occurs during ageing. 3.1.1. With R = 4 The sols and gels have been dried at 100°C and the powders such obtained are amorphous up to about 280°C-300°C: crystallization of the metastable g -TeO2 (8) and a -TeO2 compounds occurs (see figure 1). These crystallizations are observed more and more simultaneously according to the final concentration Cf. These phases coexist until g –TeO2 transforms into a -TeO2 at temperatures ranging from 410°C to 430°C.
3.1.2. With R = 65 The xerogels obtained by drying the gels are amorphous. We first observe at about 300°C the crystallization of the g -TeO2 compound and then, at about 300-350°C, the formation of the a -TeO2 polymorph. The progressive transformation of g -TeO2 into a -TeO2 occurred at temperature ranging from 360°C to 460°C.Just after drying at 60°C, the precipitates, which form during ageing of the gels, are well- crystallized (figure 2). The phases a -TeO2, b-TeO2 and g -TeO2 are observed according to the final concentration Cf. The phases b-TeO2 and g -TeO2 transform into a -TeO2 at temperatures ranging from 400°C to 500°C.
3.2. Tellurium isopropoxide, 2-methoxyethanol, acetic acid system At 60°C, with R = 4 and Cf = 0.1, a sol that never gels is obtained. With Cf = 0.5, a gel forms. With R = 65 and Cf = 0.1-0.2, gels which precipitate during ageing, are produced. 3.2.1. With R = 4 The products obtained after drying at 100°C of the sols and gels are amorphous. As previously evidenced in the system tellurium isopropoxide -2-propanol - acetic acid, the phase g -TeO2 crystallizes at about 280-310°C. Then, the phase a -TeO2 is observed at about 320-340°C and coexists with g -TeO2 until this phase transforms progressively into a -TeO2 in the temperature range of 350-440°C.3.2.2. With R = 65 The gels precipitate during drying at 60°C or 100°C. The precipitates are crystallized and the phase a -TeO2 and g -TeO2 are observed according to Cf. The phase g -TeO2 transforms into a -TeO2 at temperatures ranging from 350°C to 410°C.3.3. Microstructure as determined by SAXS The sol to gel transformation has been determined by SAXS measurements. In the system tellurium isopropoxide - 2-propanol - acetic acid, whatever the concentration Cf or modifier ratio R were, elementary particles (~2-4 nm) are readily formed. They rapidly stick together to form aggregates which mean diameter is at least 50 nm. According to the increase of R, the compactness of agglomerates decreases. With R=65, during the gel ageing, the chemical reactions go on and the size of elementary particles growths to reach about 12 nm. This growth leads to a cluster restructuring which explain the observed precipitation. In the system tellurium isopropoxide-2-methoxyethanol-acetic acid, the elementary particles are not observable and further SAXS measurements are in progress.
4. Conclusion Tellurium alkoxides are very reactive towards hydrolysis and condensation but sols and gels have been successfully prepared in the tellurium isopropoxide, 2-propanol, acetic acid and tellurium isopropoxide, 2-methoxyethanol, acetic acid systems. As for rich-TeO2 glasses obtained by conventional process of melting and quenching, the crystallisation of the metastable g -TeO2 and stable a -TeO2 polymorphs has been observed. However the b -TeO2 compound, that have never been obtained during the crystallisation of tellurite glasses, has been evidenced when some precipitates are formed during ageing of a gel.
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