The aqueous sol-gel process in the Y-Ba-Cu-O acetate-tartrate system has been monitored using IR spectroscopy. Precursor gel samples for YBa2Cu4O8 superconductor were prepared under different temperatures (55-80 °C), and durations (1-10 h) of an initial stage of synthesis. The increasing of temperature during sol-gel process did not reflect on the IR spectra of gels obtained. However, the general trend of increasing the absorption in OH stretching frequency region with increasing synthesis time up to 5 hours was observed. Therefore, the observed spectroscopic feature suggested that the number of “ol” bridges between metal atoms increases with increasing of synthesis time, consequently the condensation in Y-Ba-Cu-O aqueous acetate-tartrate solution occurs via olation mechanism.
Precursor gel samples were also prepared using different evaporation regimes during gelation process (evaporation temperature, 55-95 °C; evaporation time, ~12-3 h). The IR spectroscopic and thermogravimetric analyses revealed that the gel samples prepared at high temperatures (85-95 °C), and short time of evaporation (about ~5-3 h), were inhomogeneous, indicating the partial crystallization of initial metal salts. Consequently, the YBa2Cu4O8 samples prepared from these precursors were nonsuperconducting and exhibited semiconducting behaviour. This suggests that gelation time is critical factor determining the nature of the gel, which is of crucial importance for the good properties of oxide superconductor.
We have reported on the preparation of gel precursors in the Y(Sc)-Ba-Cu-O and Y(Ga)-Ba-Cu-O acetate/tartrate systems. A systematic characterization of precursor gels let us to predict the approximate composition and involved chemical reactions during gelation. Livage and Sanchez have first demonstrated that the sol-gel synthesis of metal oxides is based on the polymerization (polycondensation) of molecular precursors and low-valent cations (z < +4) give rise to aquo hydroxo and/or hydroxo complexes over the whole range of pH during hydrolysis and condensation processes. Thus, the process of gelation in the system investigated probably occurs via the olation mechanism. Basically it corresponds to a nucleophilic substitution in which M-OH or complexing ligands are nucleophiles and H2O is a leaving group. From the qualitative point of view, judging from the IR data and the above assumptions the different ligands, such as C4H4O62-, CHO64-, CH3COO–, OH–, and H2O are present in the coordination sphere of metals. According to charge-balance considerations, the analyzed content of metals in the gels, and total weight loss during thermal decomposition, the possible compositions of the gels can be written as follows:
Apparently the influence of substituents (scandium and gallium) on the total weight loss of the precursor material is negligible. Thus, for simplicity the compositions of the clusters I-V are written without these elements.
As already mentioned, the amount of metals in the Y-Ba-Cu-O acetate-tartrate precursor gels was found to be close to the starting composition (± 0.4%). The elemental analysis data for Y-Ba-Cu-O acetate-tartrate gel powders are following (mass %): (a) found, C, 19,96; H, 2,83, N, 0,00, and (b) calculated, C, 16,15; H, 2,35 (I); C, 17,62; H, 2,60 (II); C, 19,30; H, 2,75 (III); C, 16,02; H, 2,34 (IV); C, 19,23; H, 3,14 (V). The chemical analysis data clearly show that selected compositions I, II and IV do not correspond the nominal composition of the precursor. On the contrary, the calculated compositions for III and V are very closed to the experimental data. However according to the initial weight loss which is due to the evaporation of water (~4.8-5.1%) detected from the TG curves the gel might be composed only of [YBa2Cu4(C4H4O6)2(CH3COO)8(OH)3(H2O)3] (III). In the absence of any structural evidence it is difficult to state whether one single species is formed. For example, the formation of two species such as [Cu4(C4H4O6)(CH3COO)4(OH)2(H2O)2] (individual homopolynuclear gel) and [YBa2(C4H4O6)(CH3COO)4(OH)(H2O)] (double heteropolynuclear gel) also is possible.
However, the results obtained suggest that mixed-metal species (based on acetate and tartrate ligands) with rather complex structure very closed to the composition (III) are formed during the gelation process. The heterometallic gels, which offer the possibility of assembling two or more different metal atoms in one single molecule are an attractive choice for suitable precursors for high purity ceramic materials.