- Email: [email protected]
A comparative study of YBa2Cu4O8 (Y-124) superconductors prepared by a sol–gel method
Chemical Physics 327 (2006) 220–228 www.elsevier.com/locate/chemphys
A comparative study of YBa2Cu4O8 (Y-124) superconductors prepared by a sol–gel method A. Zalga a, J. Reklaitis b, E. Norkus c, A. Beganskiene a, A. Kareiva
a,*
a
c
Department of General and Inorganic Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania b Lithuanian Customs Laboratory, Akademijos 7, LT- 08412 Vilnius, Lithuania Department of Chemical Kinetics and Catalysis, Institute of Chemistry, A. Gostauto 9, LT-01108 Vilnius, Lithuania Received 11 January 2006; accepted 19 April 2006 Available online 26 April 2006
Abstract The aqueous sol–gel synthesis technique for the preparation of YBa2Cu4O8 (Y-124) superconductor was studied using two different complexing agents with the same chemical composition, namely L-(+)-tartaric (natural) and DL-tartaric (synthetic) acids. The characterization of Y–Ba–Cu–O gels by XRD and TGA/DTA measurements showed the individuality of precursor gels and a high level of homogeneity. On the other hand, the morphological features of the gels observed by SEM measurements were slightly different. The ceramic samples calcined for 30 h at 780 C in a flowing oxygen atmosphere contained homogeneous Y-124 crystallites as a major phase, as was shown by XRD analysis. The TC (onset) observed by resistivity and magnetic susceptibility measurements was approximately the same for the both samples (78–80 K). However, the temperature–resistivity and temperature–magnetic susceptibility dependences were found to be different for the Y-124 samples synthesized by different sol–gel procedures. Such behaviour was discussed in correlation with the morphological properties of the prepared samples. 2006 Elsevier B.V. All rights reserved. Keywords: Superconductors; YBa2Cu4O8; Sol–gel processes; Complexing agent; Physical properties
1. Introduction The YBa2Cu3O7x [1,2], often called ‘‘Y-123’’ or ‘‘123’’, belongs to a family which can be described with the general formula Y2Ba4Cu6+nO14+n (n = 0, 1, 2) [3–5]. High TC superconducting YBa2Cu4O8 (Y-124 or 124) is the n = 2 member of the above homologous series of compounds. The first synthesis of YBa2Cu4O8 in bulk phase and subsequent preparations were carried out under high oxygen pressure [6,7]. The 124 superconductor exhibits a transition temperature, TC at around 80 K, and unlike the 123 phase its oxygen content has excellent thermal stability. The characteristic properties make the 124 phase very interesting from a theoretical point of view as a slightly different model
*
Corresponding author. Tel.: +370 5 2336214; fax: +370 5 2330987. E-mail address: [email protected] (A. Kareiva).
0301-0104/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2006.04.007
system for testing the general trends of high-TC materials and also in view of possible applications [8–12]. Over the last few decades, sol–gel techniques have been used to prepare variety mixed-metal oxides. In these sol–gel processes, good quality of the oxide products was expected primarily due to the purity of the precursor materials used and chemical homogeneity obtained from the synthesis route [13–18]. The molecular level mixing and the tendency of partially hydrolysed species to form extended networks facilitate the structure evolution thereby lowering the crystallisation temperature of multicomponent metal oxide ceramics. Recently we described the use of an aqueous sol–gel method for the synthesis of monophasic non-substituted YBa2Cu4O8, as well as superconducting Y-124 samples substituted in the yttrium, barium and copper positions by other elements [19–22]. Also, it was shown the importance of nature of the complexing agent in the sol–gel process in order to obtain monophasic supercon-
A. Zalga et al. / Chemical Physics 327 (2006) 220–228
ducting Y-124 oxide [23]. The aim of this study was to investigate the influence of two complexing agents having the same chemical composition, namely L-(+)-tartaric (natural) and DL-tartaric (synthetic) acids, on the morphological characteristics and superconducting properties of YBa2Cu4O8 superconductor prepared by an aqueous acetate–tartrate sol–gel route. In addition, the purpose of this study was to show that adjusting the nature of complexing agent during sol–gel preparation of superconducting cuprate can be used to control the physical properties of the end product. 2. Experimental The YBa2Cu4O8 superconducting samples were prepared by two acetate–tartrate sol–gel routes. As starting compounds stoichiometric amounts of Y2O3, Cu(CH3COO)2 Æ H2O, and Ba(CH3COO)2, all of them analytical grade, were used. In the sol–gel process Y2O3 first was dissolved in 0.2 M acetic acid at 55–60 C. Next, Ba(CH3COO)2 and Cu(CH3COO)2 Æ H2O, both of them dissolved in a small amount of distilled water, were added with intermediate stirring during several hours at the same temperature. Finally, solution of tartaric acid ((a) L-(+)-tartaric acid; synthesis route NSG and (b) DL-tartaric acid; synthesis route SSG) in water was added to prevent crystallization of metal acetates or hydroxides during gelation. The obtained solutions were concentrated during about 8 h at 60–65 C in an open beaker. Under continuous stirring the transparent blue gels have formed. After further drying in an oven at 80 C fine grained blue powders were obtained. The precursor powders were calcined for 10 h at 780 C in flowing oxygen, reground in an agate mortar, pelletized and again heated for 20 h at 780 C in a flowing oxygen atmosphere at ambient pressure with intermediate regrinding and repelletizing. The synthesized Y-124 oxides were characterized by means of powder X-ray diffraction analysis performed with a Philips MPD 1880 diffractometer, using Cu Ka1 radiation. Scanning electron microscopes (SEM) CAM SCAN S4 and JEOL 820 were used to study the morphology and microstructure of the gel precursor and ceramic samples. The cation content in the obtained ceramics was analyzed by energy-dispersive spectrometry (EDS) in a JEOL 820 scanning electron microscope, using L, K lines. The TGA/DTA measurements were performed on a STA 490 analyzer (Netzsch) in oxygen at a heating rate of 5 C/ min (sample weight 25–30 mg). A standard four-probe technique was used for measuring the temperature dependence of the resistivity in the range 20–300 K. The diamagnetic susceptibility was determined with a Lake Shore 7000 AC susceptometer. The distribution of Cu(II), Y(III) and Ba(II) among the acetate–tartrate complexes and the concentration of free (uncomplexed) metal ions were calculated by solving the system of non-linear equations in terms of the Newton iteration method [24]. The values of equilibrium constants
221
Table 1 Equilibrium constants for system Ba(II), Cu(II), Y(III) – acetate, tartrate Reaction
Equilibrium constant
Ba2+ + CH3COO ¢ Ba(CH3COO)+ Ba2+ + Tart2 ¢ BaTart Cu2+ + CH3COO ¢ Cu(CH3COO)+ Cu2+ + 2CH3COO ¢ Cu(CH3COO)2 Cu2+ + Tart2 ¢ CuTart Cu2þ þ 2Tart2 ¢ CuTart2 2 Y3+ + CH3COO ¢ Y(CH3COO)2+ Y3þ þ 2CH3 COO ¢ YðCH3 COOÞþ 2 Y3+ + 3CH3COO ¢ Y(CH3COO)3 Y3+ + Tart2 ¢ YTart+ Y3þ þ 2Tart2 ¢ YTart 2 Y3+ + OH ¢ Y(OH)2+ þ 3þ Y þ 2OH ¢ YðOHÞ2 Y3+ + Tart2 + OH ¢ YTart(OH) Y3þ þ 2Cu2þ þ 3Tart2 þ 5OH ¢ Cu2 YTart3 ðOHÞ4 5 CH3COOH ¢ CH3COO + H+ + H2Tart ¢ HTart + H HTart ¢ Tart2 + H+ Cu2+ + 2OH ¢ Cu(OH)2 Y3+ + 3OH ¢ Y(OH)3
log b1 = 1.15 log b1 = 1.68 log b1 = 2.23 log b2 = 3.63 log b1 = 3.25 log b2 = 4.90 log b1 = 1.53 log b2 = 2.66 log b3 = 3.4 log b1 = 4.07 log b2 = 6.89 log b1 = 10.5 log b2 = 19.8 log b = 11.7 log b = 54.4 log Ka = 4.56 log Ka1 = 2.89 log Ka = 4.52 log Ks0 = 18.2 log Ks0 = 24.5
used were taken from [25–29] and are listed in Table 1. The following conditions were used for the speciation analysis in the pH range from 2 to 7: (i) Y(III) – 0.03125 M; Ba – 0.0625 M, Cu(II) – 0.125 M, acetate – 0.575 M, tartrate – 0.0625 M; (ii) Y(III) – 0.3125 M; Ba – 0.625 M, Cu(II) – 1.25 M, acetate – 5.75 M, tartrate – 0.625 M. 3. Results and discussion 3.1. Distribution of physico-chemical forms The calculations showed that the distribution of Y(III), Ba(II) and Cu(II) among the complexes in the acetate– tartrate solutions depends on pH of solution (Figs. 1–3). The distribution of physico-chemical forms of Y(III) in acetate–tartrate solutions is shown in Fig. 1. In the range of pH from 2 up to 5.5 yttrium tartrates are predominating species in the system. Yttrium acetates and Y(III) hydroxycomplexes predominate at higher pH. Ba(II) ions exist in three forms in the solutions. Depending on the total concentration, Ba(II) aqua-ions prevail till pH 3–4, and again Ba(II)–monoacetate complex predominates with further increase in pH (see Fig. 2). However, a part of barium exists in the form of barium tartrate in the whole investigated range of pH. It is evident that Cu(II) acetate and tartrate complexes prevail at pH over 2.5 independent on the total concentration of copper. The calculations of free metal ions concentration and its comparison with data about insoluble hydroxides formation are presented in Fig. 4. As seen, the formation of Cu(OH)2 precipitate during gelation process in the acetate–tartrate system possibly could start at pH over 7.5, while Y(OH)3 precipitation was not expected in the investigated pH range.
222
A. Zalga et al. / Chemical Physics 327 (2006) 220–228
Fig. 1. Distribution of Y(III) among the complexes in acetate–tartrate containing solutions. Solution composition (M): (a) [CH3COOH]0 – 0.575, [tartaric acid]0 – 0.0625, [Cu(II)]0 – 0.125, [Ba(II)]0 – 0.0625, [Y(III)]0 – 0.03125; (b) [CH3COOH]0 – 5.75, [tartaric acid]0 – 0.625, [Cu(II)]0 – 1.25, [Ba(II)]0 – 0.625, [Y(III)]0 – 0.3125.
Fig. 2. Distribution of Ba(II) among the complexes in acetate and tartrate containing solutions. Solution composition (M): (a) [CH3COOH]0 – 0.575, [tartaric acid]0 – 0.0625, [Cu(II)]0 – 0.125, [Ba(II)]0 – 0.0625, [Y(III)]0 – 0.03125; (b) [CH3COOH]0 – 5.75, [tartaric acid]0 – 0.625, [Cu(II)]0 – 1.25, [Ba(II)]0 – 0.625, [Y(III)]0 – 0.3125.
Fig. 3. Distribution of Cu(II) among the complexes in acetate and tartrate containing solutions. Solution composition (M): (a) [CH3COOH]0 – 0.575, [tartaric acid]0 – 0.0625, [Cu(II)]0 – 0.125, [Ba(II)]0 – 0.0625, [Y(III)]0 – 0.03125; (b) [CH3COOH]0 – 5.75, [tartaric acid]0 – 0.625, [Cu(II)]0 – 1.25, [Ba(II)]0 – 0.625, [Y(III)]0 – 0.3125.
The speciation analysis clearly showed that all three metals in the range of pH 4–6 should be coordinated by different ligands. Since the stability constants of metal complexes with both L-(+)- and DL-tartaric acids are the same, theoretically in the sol–gel process these complexing agents should act during hydrolysis and condensation reac-
tions in the same manner. Therefore, the homogeneous Y– Ba–Cu–O products should be obtained during both NSG and SSG synthesis routes. However, the experimental results were not completely consistent with theoretical assumption made from the calculation of distribution of physico-chemical forms.
A. Zalga et al. / Chemical Physics 327 (2006) 220–228
223
Fig. 4. Dependence of concentration of free (uncomplexed) Cu2+ (curves 1 and 2, a) and Y3+ (curves 1 and 2, b) ions on pH in acetate and tartrate containing solutions. Solution composition (M): for curves 1: [CH3COOH]0 – 0.575, [tartaric acid]0 – 0.0625, [Cu(II)]0 – 0.125, [Ba(II)]0 – 0.0625, [Y(III)]0 – 0.03125; for curves 2: [CH3COOH]0 – 5.75, [tartaric acid]0 – 0.625, [Cu(II)]0 – 1.25, [Ba(II)]0 – 0.625, [Y(III)]0 – 0.3125. The dashed lines represent precipitation of Cu(OH)2 (curve 3, a) and Y(OH)3 (curve 3, b), respectively.
3.2. Characterization of Y–Ba–Cu–O acetate–tartrate precursor gels The main requirement for the sol–gel approach is to achieve very high level of precursor homogeneity. This feature leads to excellent homogeneity in the final ceramic material. In aqueous sol–gel synthetic approaches the use of complexing agents is necessary to control the processes of crystallization. These anions compete with aquo ligands for coordination to the metal centers, and in many cases, the nature of anions strongly affects the evolving particle morphology and stability [13,14,16,30–35]. In order to examine whether the mixed-metal acetate–tartrate precursors were formed in the reaction mixture and to eliminate the presence of unreacted individual metal salts, special attention was paid to the powder X-ray diffraction studies of the precursor gels. The XRD patterns of the Y–Ba–Cu– O acetate–tartrate gels obtained from NSG and SSG synthesis routes showed broad peaks due to the amorphous
character of the powders. No peaks due to insignificant crystallization of metal acetates or tartrates, or crystallization of any undesired or contaminating phase could be identified in both XRD patterns. These data confirm the individuality, consequently sufficiently good quality of the synthesized precursors by both NSG and SSG synthesis routes. It is well known that thermal characterization of synthesized samples is important both for the control of the reaction process and for the properties of materials obtained. The TGA/DTA curves for Y–Ba–Cu–O acetate–tartrate gels are shown in Figs. 5 and 6. Evidently, the TGA/ DTA curves recorded for two precursor samples are very similar. In general three main weight losses in the temperature ranges 20–200 C (4.5%), 200–260 C (22.0%) and 260–395 C (13.5%) could be observed. The weight loss below 175 C is due to the evaporation of water and solvent molecules. The two significant decomposition steps observed as exothermic features can be attributed to the
Fig. 5. TGA/DTA profiles of the Y–Ba–Cu–O precursor acetate–tartrate gel obtained using natural L-(+)-tartaric acid as complexing agent in the sol–gel process.
224
A. Zalga et al. / Chemical Physics 327 (2006) 220–228
Fig. 6. TGA/DTA profiles of the Y–Ba–Cu–O precursor acetate–tartrate gel obtained using synthetic DL-tartaric acid as complexing agent in the sol–gel process.
pyrolysis of organic compounds and the degradation of intermediate species formed during the gelation process. In the absence of any structural evidence it is difficult to state whether one single species was formed, however, the results obtained suggest that mixed-metal species (based on acetate and tartrate ligands) with rather complex structure were formed during the gelation process. The thermal decomposition behaviour is associated with endothermic and exothermic effects in the DTA curves. The first decomposition step assignable to removal of adsorbed and chemisorbed water is indicated by a broad endothermic peaks around 90–100 C on the DTA curves. Exothermic peaks around 200–400 C in the DTA curves are due to the pyrolysis processes occurring during further heating of the gels. The scanning electron micrographs indicate the formation of monolithic gels in the both synthesis routes (Fig. 7). However, the morphology observed is not completely the same in NSG and SSG derived gels. Evidently, the surface of Y–Ba–Cu–O precursor acetate–tartrate gel obtained using natural L-(+)-tartaric acid as complexing agent in the sol–gel process (Fig. 7, top) is not so smooth and contains much larger and deeper cracks.
Resistivity vs. temperature measurements on asprepared YBa2Cu4O8 samples are shown in Figs. 9 and 10. Critical temperature of superconductivity TC (onset)
3.3. Characterization of Y-124 superconductors The X-ray diffraction patterns of both ceramic samples presented in Fig. 8 show the formation of single Y-124 phase [19,21]. No evidence for the formation of impurity phases like orthorhombic or tetragonal Y-123 or Y-247 could be observed. However, the diffraction peaks for the Y-124 sample derived from SSG route are sharper and much more intensive (see Fig. 8b). Consequently, the level of crystallinity of the YBa2Cu4O8 sample obtained when synthetic DL-tartaric acid was used as complexing agent in the sol–gel process expected to be higher.
Fig. 7. Scanning electron micrographs of the Y–Ba–Cu–O precursor acetate–tartrate gels obtained using natural L-(+)-tartaric acid (top) and synthetic DL-tartaric acid (bottom) as complexing agents in the sol–gel process. Magnification 200·.
A. Zalga et al. / Chemical Physics 327 (2006) 220–228
Fig. 8. Powder X-ray diffraction patterns of the superconducting YBa2Cu4O8 samples prepared using natural (a) and synthetic (b) tartaric acids as complexing agents in the sol–gel process.
of Y-124 sample prepared by NSG route was found to be 79 K and TC (zero) = 50 K. Thus, the sample could be characterized as having rather broad transition width (29 K). The Y-124 sample prepared by SSG route, however, showed a sharp superconducting transition with TC (onset) = 80 K and TC (zero) = 67 K. The superconducting properties of the Y-124 cuprates were also estimated from the magnetic susceptibility measurements. The curves presented in Figs. 11 and 12 show that a NSG synthesis product has slightly higher diamagnetic volume fraction. Besides, it is evident that both superconductors have very close onset transition temperatures (about 80 K). However,
225
some difference in the behaviour of magnetic susceptibility curves could be also detected. For instance, the magnetic susceptibility vs. temperature curve for the superconducting YBa2Cu4O8 prepared using natural tartaric acid as complexing agent shows a broad transition with two obvious steps (Fig. 11). The elemental composition of YBa2Cu4O8 prepared by two different routes was calculated from EDS analyses data. The EDS analysis performed at several spots corresponds to the stoichiometry that required for single-phase Y-124 particles, and no metal-rich, or Y-123, or Y-247 phases could be observed. The textural properties of the calcined powders were also investigated by SEM. Fig. 13 shows the SEM micrographs of YBa2Cu4O8 samples synthesized by NSG and SSG routes. The SEM micrograph suggests that the Y-124 solids prepared by NSG route (Fig. 13, top) are composed of irregular sub-micron grains with an average grain size of less than 1000 nm. Moreover, it was clear that micrograin network was also formed [36,37]. Besides, the SEM image of Y-124 material prepared by NSG route exhibits more dense packing of smaller individual particles. In the case of YBa2Cu4O8 sample synthesized by SSG route (Fig. 13, bottom), the formation of monodispersed spherically shaped and plate-like ultrafine crystallites with an average grain size of 1–5 lm is evident. The Y-124 compound prepared by the SSG synthesis route is composed of larger crystallites with grains of fairly uniform size. As seen, the adjacent grains tend to fuse and microscopic crystal growth on each grain begins to occur. Apparently, the superconducting properties of YBa2Cu4O8 superconductor prepared at slightly different sol–gel synthetic conditions correlate with characteristic morphological features [37,38]. Thus, we can conclude that initial stage of preparation of superconducting oxides is extremely important processing parameter
Fig. 9. Resistivity vs. temperature for the superconducting YBa2Cu4O8 prepared using natural tartaric acid as complexing agent in the sol–gel process.
226
A. Zalga et al. / Chemical Physics 327 (2006) 220–228
Fig. 10. Resistivity vs. temperature for the superconducting YBa2Cu4O8 prepared using synthetic tartaric acid as complexing agent in the sol–gel process.
Fig. 11. Magnetic susceptibility vs. temperature for the superconducting YBa2Cu4O8 prepared using natural tartaric acid as complexing agent in the sol– gel process.
Fig. 12. Magnetic susceptibility vs. temperature for the superconducting YBa2Cu4O8 prepared using synthetic tartaric acid as complexing agent in the sol– gel process.
A. Zalga et al. / Chemical Physics 327 (2006) 220–228
227
SEM measurements were slightly different. The surface of Y–Ba–Cu–O acetate–tartrate precursor gel obtained using natural L-(+)-tartaric acid as complexing agent in the sol–gel process was less smooth and contained much larger and deeper cracks. Two ceramic samples calcined for 30 h at 780 C in a flowing oxygen atmosphere contained homogeneous Y124 crystallites as a major phase, as was shown by XRD analysis. The TC (onset) observed by resistivity and magnetic susceptibility measurements was approximately the same for both samples (78–80 K). However, the NSG derived sample showed rather broad transition width (29 K) with two steps determined from resistivity and magnetic susceptibility measurements. The Y-124 solids prepared by NSG route were composed of irregular grains with an average grain size of less than 1 lm and showed dense microstructure. On the other hand, the highly crystalline nature of the SSG derived Y-124 powders was established. The Y-124 compound prepared by the SSG synthesis route was composed of larger crystallites (1– 5 lm) with grains of fairly uniform size. Acknowledgements
Fig. 13. Scanning electron micrographs of the superconducting YBa2Cu4O8 prepared using natural L-(+)-tartaric acid (top) and synthetic DLtartaric acid (bottom) as complexing agents in the sol–gel process. Magnification 2500·.
substantially influencing the physical properties of the final material. 4. Conclusions The aqueous sol–gel synthesis technique for the preparation of YBa2Cu4O8 superconductor was studied using two different complexing agents with the same chemical composition, namely L-(+)-tartaric (natural) acid and DL-tartaric (synthetic) acid (NSG and SSG synthesis routes, respectively). The use of L-(+)-tartaric (natural) acid as complexing agent in a sol–gel process has been described earlier, but the successful use of DL-tartaric (synthetic) acid has not been previously reported, to our knowledge. According to the speciation analysis data, the character of distribution of the physico-chemical forms of yttrium, barium and copper in acetate–tartrate solutions was the same for both complexing agents. Nevertheless, these two complexing agents were found to influence significantly some characteristics of both the precursors and the obtained superconducting materials. The SEM micrographs of the Y–Ba–Cu–O acetate–tartrate gels indicated the formation of monolithic gels during both synthesis routes. However, some morphological features of the gels observed by
A.Z. is thankful for Lithuanian Science and Studies Foundation Scholarship. The authors from Vilnius University gratefully acknowledge the financial support from the Lithuanian State Science and Studies Foundation under project MODELITA (No. C-03048). References [1] C.W. Chu, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang, Phys. Rev. Lett. 58 (1987) 405. [2] M.K. Wu, J.R. Ashburn, P.H. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang, C.W. Chu, Phys. Rev. Lett. 58 (1987) 908. [3] N.M. Plakida, High-Temperature Superconductivity. Experiment and Theory, Springer, Berlin, 1995. [4] C. Park, R.L. Snyder, J. Am. Ceram. Soc. 78 (1995) 3171. [5] R.J. Cava, J. Am. Ceram. Soc. 83 (2000) 5. [6] D.E. Morris, J.H. Nickel, J.Y.T. Wei, N.G. Asmar, J.S. Scott, U.M. Scheven, C.T. Hultgren, A.G. Markelz, J.E. Post, P.J. Heaney, D.R. Veblen, R.M. Hazen, Phys. Rev. B 39 (1989) 7347. [7] S. Adachi, N. Watanabe, N. Seiji, N. Koshizuka, H. Yamauchi, Physica C 207 (1993) 127. [8] H. Schwer, J. Karpinski, E. Kaldis, G.I. Meijer, C. Rossel, M. Mali, Physica C 267 (1996) 113. [9] M. Hagiwara, T. Yamao, T. Shima, H. Deguchi, M. Matsuura, Physica C 412–414 (2004) 94. [10] T. Machi, N. Watanabe, Y. Itoh, N. Koshizuka, Physica C 412–414 (2004) 342. [11] O.F. Schilling, Supercond. Sci. Technol. 17 (2004) L17. [12] C.H. Chin, H.-C.I. Kao, C.M. Wang, Mater. Chem. Phys. 89 (2005) 143. [13] J. Livage, M. Henry, C. Sanchez, Progr. Solid State Chem. 18 (1988) 259. [14] C.J. Brinker, G.W. Scherrer, Sol–Gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic Press, San Diego, 1990. [15] A. Baranauskas, D. Jasaitis, A. Kareiva, Vibr. Spectrosc. 28 (2002) 263. [16] B.L. Cushing, V.L. Kolesnichenko, C.J. O’Connor, Chem. Rev. 104 (2004) 3893.
228
A. Zalga et al. / Chemical Physics 327 (2006) 220–228
[17] A. Leleckaite, A. Kareiva, H. Bettentrup, T. Justel, H.-J. Meyer, Z. Anorg. Allg. Chem. 631 (2005) 2987. [18] M. Chroma, J. Pinkas, I. Pakutinskiene, A. Beganskiene, A. Kareiva, Ceram. Int. 31 (2005) 1123. [19] A. Kareiva, M. Karppinen, L. Niinisto, J. Mater. Chem. 4 (1994) 1267. [20] A. Baranauskas, D. Jasaitis, A. Kareiva, R. Haberkorn, H.P. Beck, J. Eur. Ceram. Soc. 21 (2001) 399. [21] A. Kareiva, S. Mathur, J.-E. Jørgensen, S. Tautkus, Philos. Mag. 83 (2003) 1917. [22] G. Nenartaviciene, D. Jasaitis, A. Kareiva, Acta Chim. Slov. 51 (2004) 661. [23] A. Kareiva, I. Bryntse, M. Karppinen, L. Niinisto, J. Solid State Chem. 121 (1996) 356. [24] K. Ebert, H. Ederer, Computeranwendung in der Chemie, VCH, Weinheim, 1985. [25] L.G. Sillen, A.E. Martell (Eds.), Stability Constants of Metal–Ion Complexes, Special Publication 25, Supplement No. 1, Chemical Society, London, 1971. [26] E. Ho¨gfeld (Ed.), Stability Constants of Metal–Ion Complexes. Part A: Inorganic Ligands, Pergamon Press, Oxford, 1982.
[27] A.E. Martell, R.M. Smith (Eds.), Critical Stability Constants, vol. 3, Plenum Press, New York, 1977. [28] D.D. Perrin (Ed.), Stability Constants of Metal–Ion Complexes. Part B: Organic Ligands, Pergamon Press, Oxford, 1979. [29] E.P. Serjeant, B. Dempsey (Eds.), Ionisation Constants of Organic Acids in Aqueous Solutions, Pergamon Press, Oxford, 1979. [30] J. Livage, P. Barboux, F. Taulelle, J. Non-Cryst. Solids 147/148 (1992) 18. [31] J. Zarzycki, J. Sol–Gel Sci. Technol. 8 (1997) 17. [32] D.R. Uhlmann, G.J. Teowee, J. Sol–Gel Sci. Technol. 13 (1998) 153. [33] C. Sanchez, F. Ribot, B. Lebeau, J. Mater. Chem. 9 (1999) 35. [34] C. Sanchez, B. Lebeau, F. Ribot, M. In, J. Sol–Gel Sci. Technol. 19 (2000) 31. [35] C. Sanchez, G.J.D.A.A. Soler-Illia, F. Ribot, D. Grosso, C. R. Chimie 6 (2003) 1131. [36] T. Yamao, M. Hagiwara, T. Shima, M. Matsuura, Physica C 412–414 (2004) 98. [37] L.C. Pathak, S.K. Mishra, Supercond. Sci. Technol. 18 (2005) R67. [38] T. Mouganie, M.A. Moram, J. Sumner, B.A. Glowacki, B. Schoofs, I. Van Driessche, S. Hoste, J. Sol–Gel Sci. Technol. 36 (2005) 87.
A comparative study of YBa2Cu4O8 (Y-124) superconductors prepared by a sol–gel method A. Zalga a, J. Reklaitis b, E. Norkus c, A. Beganskiene a, A. Kareiva
a,*
a
c
Department of General and Inorganic Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania b Lithuanian Customs Laboratory, Akademijos 7, LT- 08412 Vilnius, Lithuania Department of Chemical Kinetics and Catalysis, Institute of Chemistry, A. Gostauto 9, LT-01108 Vilnius, Lithuania Received 11 January 2006; accepted 19 April 2006 Available online 26 April 2006
Abstract The aqueous sol–gel synthesis technique for the preparation of YBa2Cu4O8 (Y-124) superconductor was studied using two different complexing agents with the same chemical composition, namely L-(+)-tartaric (natural) and DL-tartaric (synthetic) acids. The characterization of Y–Ba–Cu–O gels by XRD and TGA/DTA measurements showed the individuality of precursor gels and a high level of homogeneity. On the other hand, the morphological features of the gels observed by SEM measurements were slightly different. The ceramic samples calcined for 30 h at 780 C in a flowing oxygen atmosphere contained homogeneous Y-124 crystallites as a major phase, as was shown by XRD analysis. The TC (onset) observed by resistivity and magnetic susceptibility measurements was approximately the same for the both samples (78–80 K). However, the temperature–resistivity and temperature–magnetic susceptibility dependences were found to be different for the Y-124 samples synthesized by different sol–gel procedures. Such behaviour was discussed in correlation with the morphological properties of the prepared samples. 2006 Elsevier B.V. All rights reserved. Keywords: Superconductors; YBa2Cu4O8; Sol–gel processes; Complexing agent; Physical properties
1. Introduction The YBa2Cu3O7x [1,2], often called ‘‘Y-123’’ or ‘‘123’’, belongs to a family which can be described with the general formula Y2Ba4Cu6+nO14+n (n = 0, 1, 2) [3–5]. High TC superconducting YBa2Cu4O8 (Y-124 or 124) is the n = 2 member of the above homologous series of compounds. The first synthesis of YBa2Cu4O8 in bulk phase and subsequent preparations were carried out under high oxygen pressure [6,7]. The 124 superconductor exhibits a transition temperature, TC at around 80 K, and unlike the 123 phase its oxygen content has excellent thermal stability. The characteristic properties make the 124 phase very interesting from a theoretical point of view as a slightly different model
*
Corresponding author. Tel.: +370 5 2336214; fax: +370 5 2330987. E-mail address: [email protected] (A. Kareiva).
0301-0104/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2006.04.007
system for testing the general trends of high-TC materials and also in view of possible applications [8–12]. Over the last few decades, sol–gel techniques have been used to prepare variety mixed-metal oxides. In these sol–gel processes, good quality of the oxide products was expected primarily due to the purity of the precursor materials used and chemical homogeneity obtained from the synthesis route [13–18]. The molecular level mixing and the tendency of partially hydrolysed species to form extended networks facilitate the structure evolution thereby lowering the crystallisation temperature of multicomponent metal oxide ceramics. Recently we described the use of an aqueous sol–gel method for the synthesis of monophasic non-substituted YBa2Cu4O8, as well as superconducting Y-124 samples substituted in the yttrium, barium and copper positions by other elements [19–22]. Also, it was shown the importance of nature of the complexing agent in the sol–gel process in order to obtain monophasic supercon-
A. Zalga et al. / Chemical Physics 327 (2006) 220–228
ducting Y-124 oxide [23]. The aim of this study was to investigate the influence of two complexing agents having the same chemical composition, namely L-(+)-tartaric (natural) and DL-tartaric (synthetic) acids, on the morphological characteristics and superconducting properties of YBa2Cu4O8 superconductor prepared by an aqueous acetate–tartrate sol–gel route. In addition, the purpose of this study was to show that adjusting the nature of complexing agent during sol–gel preparation of superconducting cuprate can be used to control the physical properties of the end product. 2. Experimental The YBa2Cu4O8 superconducting samples were prepared by two acetate–tartrate sol–gel routes. As starting compounds stoichiometric amounts of Y2O3, Cu(CH3COO)2 Æ H2O, and Ba(CH3COO)2, all of them analytical grade, were used. In the sol–gel process Y2O3 first was dissolved in 0.2 M acetic acid at 55–60 C. Next, Ba(CH3COO)2 and Cu(CH3COO)2 Æ H2O, both of them dissolved in a small amount of distilled water, were added with intermediate stirring during several hours at the same temperature. Finally, solution of tartaric acid ((a) L-(+)-tartaric acid; synthesis route NSG and (b) DL-tartaric acid; synthesis route SSG) in water was added to prevent crystallization of metal acetates or hydroxides during gelation. The obtained solutions were concentrated during about 8 h at 60–65 C in an open beaker. Under continuous stirring the transparent blue gels have formed. After further drying in an oven at 80 C fine grained blue powders were obtained. The precursor powders were calcined for 10 h at 780 C in flowing oxygen, reground in an agate mortar, pelletized and again heated for 20 h at 780 C in a flowing oxygen atmosphere at ambient pressure with intermediate regrinding and repelletizing. The synthesized Y-124 oxides were characterized by means of powder X-ray diffraction analysis performed with a Philips MPD 1880 diffractometer, using Cu Ka1 radiation. Scanning electron microscopes (SEM) CAM SCAN S4 and JEOL 820 were used to study the morphology and microstructure of the gel precursor and ceramic samples. The cation content in the obtained ceramics was analyzed by energy-dispersive spectrometry (EDS) in a JEOL 820 scanning electron microscope, using L, K lines. The TGA/DTA measurements were performed on a STA 490 analyzer (Netzsch) in oxygen at a heating rate of 5 C/ min (sample weight 25–30 mg). A standard four-probe technique was used for measuring the temperature dependence of the resistivity in the range 20–300 K. The diamagnetic susceptibility was determined with a Lake Shore 7000 AC susceptometer. The distribution of Cu(II), Y(III) and Ba(II) among the acetate–tartrate complexes and the concentration of free (uncomplexed) metal ions were calculated by solving the system of non-linear equations in terms of the Newton iteration method [24]. The values of equilibrium constants
221
Table 1 Equilibrium constants for system Ba(II), Cu(II), Y(III) – acetate, tartrate Reaction
Equilibrium constant
Ba2+ + CH3COO ¢ Ba(CH3COO)+ Ba2+ + Tart2 ¢ BaTart Cu2+ + CH3COO ¢ Cu(CH3COO)+ Cu2+ + 2CH3COO ¢ Cu(CH3COO)2 Cu2+ + Tart2 ¢ CuTart Cu2þ þ 2Tart2 ¢ CuTart2 2 Y3+ + CH3COO ¢ Y(CH3COO)2+ Y3þ þ 2CH3 COO ¢ YðCH3 COOÞþ 2 Y3+ + 3CH3COO ¢ Y(CH3COO)3 Y3+ + Tart2 ¢ YTart+ Y3þ þ 2Tart2 ¢ YTart 2 Y3+ + OH ¢ Y(OH)2+ þ 3þ Y þ 2OH ¢ YðOHÞ2 Y3+ + Tart2 + OH ¢ YTart(OH) Y3þ þ 2Cu2þ þ 3Tart2 þ 5OH ¢ Cu2 YTart3 ðOHÞ4 5 CH3COOH ¢ CH3COO + H+ + H2Tart ¢ HTart + H HTart ¢ Tart2 + H+ Cu2+ + 2OH ¢ Cu(OH)2 Y3+ + 3OH ¢ Y(OH)3
log b1 = 1.15 log b1 = 1.68 log b1 = 2.23 log b2 = 3.63 log b1 = 3.25 log b2 = 4.90 log b1 = 1.53 log b2 = 2.66 log b3 = 3.4 log b1 = 4.07 log b2 = 6.89 log b1 = 10.5 log b2 = 19.8 log b = 11.7 log b = 54.4 log Ka = 4.56 log Ka1 = 2.89 log Ka = 4.52 log Ks0 = 18.2 log Ks0 = 24.5
used were taken from [25–29] and are listed in Table 1. The following conditions were used for the speciation analysis in the pH range from 2 to 7: (i) Y(III) – 0.03125 M; Ba – 0.0625 M, Cu(II) – 0.125 M, acetate – 0.575 M, tartrate – 0.0625 M; (ii) Y(III) – 0.3125 M; Ba – 0.625 M, Cu(II) – 1.25 M, acetate – 5.75 M, tartrate – 0.625 M. 3. Results and discussion 3.1. Distribution of physico-chemical forms The calculations showed that the distribution of Y(III), Ba(II) and Cu(II) among the complexes in the acetate– tartrate solutions depends on pH of solution (Figs. 1–3). The distribution of physico-chemical forms of Y(III) in acetate–tartrate solutions is shown in Fig. 1. In the range of pH from 2 up to 5.5 yttrium tartrates are predominating species in the system. Yttrium acetates and Y(III) hydroxycomplexes predominate at higher pH. Ba(II) ions exist in three forms in the solutions. Depending on the total concentration, Ba(II) aqua-ions prevail till pH 3–4, and again Ba(II)–monoacetate complex predominates with further increase in pH (see Fig. 2). However, a part of barium exists in the form of barium tartrate in the whole investigated range of pH. It is evident that Cu(II) acetate and tartrate complexes prevail at pH over 2.5 independent on the total concentration of copper. The calculations of free metal ions concentration and its comparison with data about insoluble hydroxides formation are presented in Fig. 4. As seen, the formation of Cu(OH)2 precipitate during gelation process in the acetate–tartrate system possibly could start at pH over 7.5, while Y(OH)3 precipitation was not expected in the investigated pH range.
222
A. Zalga et al. / Chemical Physics 327 (2006) 220–228
Fig. 1. Distribution of Y(III) among the complexes in acetate–tartrate containing solutions. Solution composition (M): (a) [CH3COOH]0 – 0.575, [tartaric acid]0 – 0.0625, [Cu(II)]0 – 0.125, [Ba(II)]0 – 0.0625, [Y(III)]0 – 0.03125; (b) [CH3COOH]0 – 5.75, [tartaric acid]0 – 0.625, [Cu(II)]0 – 1.25, [Ba(II)]0 – 0.625, [Y(III)]0 – 0.3125.
Fig. 2. Distribution of Ba(II) among the complexes in acetate and tartrate containing solutions. Solution composition (M): (a) [CH3COOH]0 – 0.575, [tartaric acid]0 – 0.0625, [Cu(II)]0 – 0.125, [Ba(II)]0 – 0.0625, [Y(III)]0 – 0.03125; (b) [CH3COOH]0 – 5.75, [tartaric acid]0 – 0.625, [Cu(II)]0 – 1.25, [Ba(II)]0 – 0.625, [Y(III)]0 – 0.3125.
Fig. 3. Distribution of Cu(II) among the complexes in acetate and tartrate containing solutions. Solution composition (M): (a) [CH3COOH]0 – 0.575, [tartaric acid]0 – 0.0625, [Cu(II)]0 – 0.125, [Ba(II)]0 – 0.0625, [Y(III)]0 – 0.03125; (b) [CH3COOH]0 – 5.75, [tartaric acid]0 – 0.625, [Cu(II)]0 – 1.25, [Ba(II)]0 – 0.625, [Y(III)]0 – 0.3125.
The speciation analysis clearly showed that all three metals in the range of pH 4–6 should be coordinated by different ligands. Since the stability constants of metal complexes with both L-(+)- and DL-tartaric acids are the same, theoretically in the sol–gel process these complexing agents should act during hydrolysis and condensation reac-
tions in the same manner. Therefore, the homogeneous Y– Ba–Cu–O products should be obtained during both NSG and SSG synthesis routes. However, the experimental results were not completely consistent with theoretical assumption made from the calculation of distribution of physico-chemical forms.
A. Zalga et al. / Chemical Physics 327 (2006) 220–228
223
Fig. 4. Dependence of concentration of free (uncomplexed) Cu2+ (curves 1 and 2, a) and Y3+ (curves 1 and 2, b) ions on pH in acetate and tartrate containing solutions. Solution composition (M): for curves 1: [CH3COOH]0 – 0.575, [tartaric acid]0 – 0.0625, [Cu(II)]0 – 0.125, [Ba(II)]0 – 0.0625, [Y(III)]0 – 0.03125; for curves 2: [CH3COOH]0 – 5.75, [tartaric acid]0 – 0.625, [Cu(II)]0 – 1.25, [Ba(II)]0 – 0.625, [Y(III)]0 – 0.3125. The dashed lines represent precipitation of Cu(OH)2 (curve 3, a) and Y(OH)3 (curve 3, b), respectively.
3.2. Characterization of Y–Ba–Cu–O acetate–tartrate precursor gels The main requirement for the sol–gel approach is to achieve very high level of precursor homogeneity. This feature leads to excellent homogeneity in the final ceramic material. In aqueous sol–gel synthetic approaches the use of complexing agents is necessary to control the processes of crystallization. These anions compete with aquo ligands for coordination to the metal centers, and in many cases, the nature of anions strongly affects the evolving particle morphology and stability [13,14,16,30–35]. In order to examine whether the mixed-metal acetate–tartrate precursors were formed in the reaction mixture and to eliminate the presence of unreacted individual metal salts, special attention was paid to the powder X-ray diffraction studies of the precursor gels. The XRD patterns of the Y–Ba–Cu– O acetate–tartrate gels obtained from NSG and SSG synthesis routes showed broad peaks due to the amorphous
character of the powders. No peaks due to insignificant crystallization of metal acetates or tartrates, or crystallization of any undesired or contaminating phase could be identified in both XRD patterns. These data confirm the individuality, consequently sufficiently good quality of the synthesized precursors by both NSG and SSG synthesis routes. It is well known that thermal characterization of synthesized samples is important both for the control of the reaction process and for the properties of materials obtained. The TGA/DTA curves for Y–Ba–Cu–O acetate–tartrate gels are shown in Figs. 5 and 6. Evidently, the TGA/ DTA curves recorded for two precursor samples are very similar. In general three main weight losses in the temperature ranges 20–200 C (4.5%), 200–260 C (22.0%) and 260–395 C (13.5%) could be observed. The weight loss below 175 C is due to the evaporation of water and solvent molecules. The two significant decomposition steps observed as exothermic features can be attributed to the
Fig. 5. TGA/DTA profiles of the Y–Ba–Cu–O precursor acetate–tartrate gel obtained using natural L-(+)-tartaric acid as complexing agent in the sol–gel process.
224
A. Zalga et al. / Chemical Physics 327 (2006) 220–228
Fig. 6. TGA/DTA profiles of the Y–Ba–Cu–O precursor acetate–tartrate gel obtained using synthetic DL-tartaric acid as complexing agent in the sol–gel process.
pyrolysis of organic compounds and the degradation of intermediate species formed during the gelation process. In the absence of any structural evidence it is difficult to state whether one single species was formed, however, the results obtained suggest that mixed-metal species (based on acetate and tartrate ligands) with rather complex structure were formed during the gelation process. The thermal decomposition behaviour is associated with endothermic and exothermic effects in the DTA curves. The first decomposition step assignable to removal of adsorbed and chemisorbed water is indicated by a broad endothermic peaks around 90–100 C on the DTA curves. Exothermic peaks around 200–400 C in the DTA curves are due to the pyrolysis processes occurring during further heating of the gels. The scanning electron micrographs indicate the formation of monolithic gels in the both synthesis routes (Fig. 7). However, the morphology observed is not completely the same in NSG and SSG derived gels. Evidently, the surface of Y–Ba–Cu–O precursor acetate–tartrate gel obtained using natural L-(+)-tartaric acid as complexing agent in the sol–gel process (Fig. 7, top) is not so smooth and contains much larger and deeper cracks.
Resistivity vs. temperature measurements on asprepared YBa2Cu4O8 samples are shown in Figs. 9 and 10. Critical temperature of superconductivity TC (onset)
3.3. Characterization of Y-124 superconductors The X-ray diffraction patterns of both ceramic samples presented in Fig. 8 show the formation of single Y-124 phase [19,21]. No evidence for the formation of impurity phases like orthorhombic or tetragonal Y-123 or Y-247 could be observed. However, the diffraction peaks for the Y-124 sample derived from SSG route are sharper and much more intensive (see Fig. 8b). Consequently, the level of crystallinity of the YBa2Cu4O8 sample obtained when synthetic DL-tartaric acid was used as complexing agent in the sol–gel process expected to be higher.
Fig. 7. Scanning electron micrographs of the Y–Ba–Cu–O precursor acetate–tartrate gels obtained using natural L-(+)-tartaric acid (top) and synthetic DL-tartaric acid (bottom) as complexing agents in the sol–gel process. Magnification 200·.
A. Zalga et al. / Chemical Physics 327 (2006) 220–228
Fig. 8. Powder X-ray diffraction patterns of the superconducting YBa2Cu4O8 samples prepared using natural (a) and synthetic (b) tartaric acids as complexing agents in the sol–gel process.
of Y-124 sample prepared by NSG route was found to be 79 K and TC (zero) = 50 K. Thus, the sample could be characterized as having rather broad transition width (29 K). The Y-124 sample prepared by SSG route, however, showed a sharp superconducting transition with TC (onset) = 80 K and TC (zero) = 67 K. The superconducting properties of the Y-124 cuprates were also estimated from the magnetic susceptibility measurements. The curves presented in Figs. 11 and 12 show that a NSG synthesis product has slightly higher diamagnetic volume fraction. Besides, it is evident that both superconductors have very close onset transition temperatures (about 80 K). However,
225
some difference in the behaviour of magnetic susceptibility curves could be also detected. For instance, the magnetic susceptibility vs. temperature curve for the superconducting YBa2Cu4O8 prepared using natural tartaric acid as complexing agent shows a broad transition with two obvious steps (Fig. 11). The elemental composition of YBa2Cu4O8 prepared by two different routes was calculated from EDS analyses data. The EDS analysis performed at several spots corresponds to the stoichiometry that required for single-phase Y-124 particles, and no metal-rich, or Y-123, or Y-247 phases could be observed. The textural properties of the calcined powders were also investigated by SEM. Fig. 13 shows the SEM micrographs of YBa2Cu4O8 samples synthesized by NSG and SSG routes. The SEM micrograph suggests that the Y-124 solids prepared by NSG route (Fig. 13, top) are composed of irregular sub-micron grains with an average grain size of less than 1000 nm. Moreover, it was clear that micrograin network was also formed [36,37]. Besides, the SEM image of Y-124 material prepared by NSG route exhibits more dense packing of smaller individual particles. In the case of YBa2Cu4O8 sample synthesized by SSG route (Fig. 13, bottom), the formation of monodispersed spherically shaped and plate-like ultrafine crystallites with an average grain size of 1–5 lm is evident. The Y-124 compound prepared by the SSG synthesis route is composed of larger crystallites with grains of fairly uniform size. As seen, the adjacent grains tend to fuse and microscopic crystal growth on each grain begins to occur. Apparently, the superconducting properties of YBa2Cu4O8 superconductor prepared at slightly different sol–gel synthetic conditions correlate with characteristic morphological features [37,38]. Thus, we can conclude that initial stage of preparation of superconducting oxides is extremely important processing parameter
Fig. 9. Resistivity vs. temperature for the superconducting YBa2Cu4O8 prepared using natural tartaric acid as complexing agent in the sol–gel process.
226
A. Zalga et al. / Chemical Physics 327 (2006) 220–228
Fig. 10. Resistivity vs. temperature for the superconducting YBa2Cu4O8 prepared using synthetic tartaric acid as complexing agent in the sol–gel process.
Fig. 11. Magnetic susceptibility vs. temperature for the superconducting YBa2Cu4O8 prepared using natural tartaric acid as complexing agent in the sol– gel process.
Fig. 12. Magnetic susceptibility vs. temperature for the superconducting YBa2Cu4O8 prepared using synthetic tartaric acid as complexing agent in the sol– gel process.
A. Zalga et al. / Chemical Physics 327 (2006) 220–228
227
SEM measurements were slightly different. The surface of Y–Ba–Cu–O acetate–tartrate precursor gel obtained using natural L-(+)-tartaric acid as complexing agent in the sol–gel process was less smooth and contained much larger and deeper cracks. Two ceramic samples calcined for 30 h at 780 C in a flowing oxygen atmosphere contained homogeneous Y124 crystallites as a major phase, as was shown by XRD analysis. The TC (onset) observed by resistivity and magnetic susceptibility measurements was approximately the same for both samples (78–80 K). However, the NSG derived sample showed rather broad transition width (29 K) with two steps determined from resistivity and magnetic susceptibility measurements. The Y-124 solids prepared by NSG route were composed of irregular grains with an average grain size of less than 1 lm and showed dense microstructure. On the other hand, the highly crystalline nature of the SSG derived Y-124 powders was established. The Y-124 compound prepared by the SSG synthesis route was composed of larger crystallites (1– 5 lm) with grains of fairly uniform size. Acknowledgements
Fig. 13. Scanning electron micrographs of the superconducting YBa2Cu4O8 prepared using natural L-(+)-tartaric acid (top) and synthetic DLtartaric acid (bottom) as complexing agents in the sol–gel process. Magnification 2500·.
substantially influencing the physical properties of the final material. 4. Conclusions The aqueous sol–gel synthesis technique for the preparation of YBa2Cu4O8 superconductor was studied using two different complexing agents with the same chemical composition, namely L-(+)-tartaric (natural) acid and DL-tartaric (synthetic) acid (NSG and SSG synthesis routes, respectively). The use of L-(+)-tartaric (natural) acid as complexing agent in a sol–gel process has been described earlier, but the successful use of DL-tartaric (synthetic) acid has not been previously reported, to our knowledge. According to the speciation analysis data, the character of distribution of the physico-chemical forms of yttrium, barium and copper in acetate–tartrate solutions was the same for both complexing agents. Nevertheless, these two complexing agents were found to influence significantly some characteristics of both the precursors and the obtained superconducting materials. The SEM micrographs of the Y–Ba–Cu–O acetate–tartrate gels indicated the formation of monolithic gels during both synthesis routes. However, some morphological features of the gels observed by
A.Z. is thankful for Lithuanian Science and Studies Foundation Scholarship. The authors from Vilnius University gratefully acknowledge the financial support from the Lithuanian State Science and Studies Foundation under project MODELITA (No. C-03048). References [1] C.W. Chu, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang, Phys. Rev. Lett. 58 (1987) 405. [2] M.K. Wu, J.R. Ashburn, P.H. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang, C.W. Chu, Phys. Rev. Lett. 58 (1987) 908. [3] N.M. Plakida, High-Temperature Superconductivity. Experiment and Theory, Springer, Berlin, 1995. [4] C. Park, R.L. Snyder, J. Am. Ceram. Soc. 78 (1995) 3171. [5] R.J. Cava, J. Am. Ceram. Soc. 83 (2000) 5. [6] D.E. Morris, J.H. Nickel, J.Y.T. Wei, N.G. Asmar, J.S. Scott, U.M. Scheven, C.T. Hultgren, A.G. Markelz, J.E. Post, P.J. Heaney, D.R. Veblen, R.M. Hazen, Phys. Rev. B 39 (1989) 7347. [7] S. Adachi, N. Watanabe, N. Seiji, N. Koshizuka, H. Yamauchi, Physica C 207 (1993) 127. [8] H. Schwer, J. Karpinski, E. Kaldis, G.I. Meijer, C. Rossel, M. Mali, Physica C 267 (1996) 113. [9] M. Hagiwara, T. Yamao, T. Shima, H. Deguchi, M. Matsuura, Physica C 412–414 (2004) 94. [10] T. Machi, N. Watanabe, Y. Itoh, N. Koshizuka, Physica C 412–414 (2004) 342. [11] O.F. Schilling, Supercond. Sci. Technol. 17 (2004) L17. [12] C.H. Chin, H.-C.I. Kao, C.M. Wang, Mater. Chem. Phys. 89 (2005) 143. [13] J. Livage, M. Henry, C. Sanchez, Progr. Solid State Chem. 18 (1988) 259. [14] C.J. Brinker, G.W. Scherrer, Sol–Gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic Press, San Diego, 1990. [15] A. Baranauskas, D. Jasaitis, A. Kareiva, Vibr. Spectrosc. 28 (2002) 263. [16] B.L. Cushing, V.L. Kolesnichenko, C.J. O’Connor, Chem. Rev. 104 (2004) 3893.
228
A. Zalga et al. / Chemical Physics 327 (2006) 220–228
[17] A. Leleckaite, A. Kareiva, H. Bettentrup, T. Justel, H.-J. Meyer, Z. Anorg. Allg. Chem. 631 (2005) 2987. [18] M. Chroma, J. Pinkas, I. Pakutinskiene, A. Beganskiene, A. Kareiva, Ceram. Int. 31 (2005) 1123. [19] A. Kareiva, M. Karppinen, L. Niinisto, J. Mater. Chem. 4 (1994) 1267. [20] A. Baranauskas, D. Jasaitis, A. Kareiva, R. Haberkorn, H.P. Beck, J. Eur. Ceram. Soc. 21 (2001) 399. [21] A. Kareiva, S. Mathur, J.-E. Jørgensen, S. Tautkus, Philos. Mag. 83 (2003) 1917. [22] G. Nenartaviciene, D. Jasaitis, A. Kareiva, Acta Chim. Slov. 51 (2004) 661. [23] A. Kareiva, I. Bryntse, M. Karppinen, L. Niinisto, J. Solid State Chem. 121 (1996) 356. [24] K. Ebert, H. Ederer, Computeranwendung in der Chemie, VCH, Weinheim, 1985. [25] L.G. Sillen, A.E. Martell (Eds.), Stability Constants of Metal–Ion Complexes, Special Publication 25, Supplement No. 1, Chemical Society, London, 1971. [26] E. Ho¨gfeld (Ed.), Stability Constants of Metal–Ion Complexes. Part A: Inorganic Ligands, Pergamon Press, Oxford, 1982.
[27] A.E. Martell, R.M. Smith (Eds.), Critical Stability Constants, vol. 3, Plenum Press, New York, 1977. [28] D.D. Perrin (Ed.), Stability Constants of Metal–Ion Complexes. Part B: Organic Ligands, Pergamon Press, Oxford, 1979. [29] E.P. Serjeant, B. Dempsey (Eds.), Ionisation Constants of Organic Acids in Aqueous Solutions, Pergamon Press, Oxford, 1979. [30] J. Livage, P. Barboux, F. Taulelle, J. Non-Cryst. Solids 147/148 (1992) 18. [31] J. Zarzycki, J. Sol–Gel Sci. Technol. 8 (1997) 17. [32] D.R. Uhlmann, G.J. Teowee, J. Sol–Gel Sci. Technol. 13 (1998) 153. [33] C. Sanchez, F. Ribot, B. Lebeau, J. Mater. Chem. 9 (1999) 35. [34] C. Sanchez, B. Lebeau, F. Ribot, M. In, J. Sol–Gel Sci. Technol. 19 (2000) 31. [35] C. Sanchez, G.J.D.A.A. Soler-Illia, F. Ribot, D. Grosso, C. R. Chimie 6 (2003) 1131. [36] T. Yamao, M. Hagiwara, T. Shima, M. Matsuura, Physica C 412–414 (2004) 98. [37] L.C. Pathak, S.K. Mishra, Supercond. Sci. Technol. 18 (2005) R67. [38] T. Mouganie, M.A. Moram, J. Sumner, B.A. Glowacki, B. Schoofs, I. Van Driessche, S. Hoste, J. Sol–Gel Sci. Technol. 36 (2005) 87.