Effect of cation non-stoichiometry and of process parameters on properties of YBCO

Materials Science and Engineering, A 109 (1988) 299- 305

299

Effect of Cation Non-stoichiometry and of Process Parameters on Properties of YBCO* H. J. SCHEELt

Physics Department, University of Geneva, CH-121I Geneva 4 (Switzerland) F. LICCI, T. BESAGNI, F. BOLZONI and S. CATTANI

MASPEC Institute of C.N.R., 1-43100, Parma (Italy) D. ECKERT

Physics Department, University of Geneva, CH-1211 Geneva 4 (Switzerland) G. SALVIATI

MASPEC Institute of C.N.R., 1-43100, Parma (Italy) (Received June 2, 1988)

Abstract

The role of small deviations from the 1Y:2Ba:3Cu ratio of YBCO and of various processing parameters has been studied by systematic characterization of the ceramics obtained. Significant effects on density, resistivity, To, ATc, Meissner and a.c. susceptibility were observed. Low-temperature specific-heat measurements of stoichiometric samples with A T~ of less than 1 K showed very low y values. The results couM be interpreted to some extent by scanning electron microscopy photographs and by traces of secondary phases detected by X-ray diffraction. 1. Introduction

High-temperature superconductivity may be characterized by a fascinating phenomenon which is not well understood, and by a variety of theories attempting to explain contradictory and non-reproducible measurements on samples which are insufficiently characterized. In view of the numerous parameters which influence the ceramic, single crystal and thin film fabrication and the characteristics of the samples, repro*Paper presented at the Symposium on Ceramic Materials Research at the E-MRS Spring Meeting, Strasbourg, May 31-June 2, 1988. tPresent address: Institute of Micro- and Optoelectronics, Physics Department, Swiss Federal Institute of Technology, CH- 1015 Lausanne, Switzerland. 0921-5093/89/$3.50

ducibility has never been achieved so far in practice. In this work an attempt is made to study the effect of selected parameters in ceramic fabrication which often are overlooked and which are expected to play a significant role on superconducting properties. Since high-purity chemicals frequently contain appreciable amounts of volatile compounds such as water or organic matter or even other compounds of the respective cation, stoichiometry generally is not achieved unless specific care is taken. Stoichiometric samples of YBCO as well as YBCO with small excesses of the individual starting chemicals were prepared as ceramic pellets by using identical preparation and annealing conditions. The characterization of the samples firstly for traces of secondary phases and secondly for their superconducting properties revealed a systematic dependence on non-stoichiometry and also on a correlation with the preparation conditions (firing temperatures and times, number of cycles). In the next section the preparation procedures and the characteristics of the samples are described. A systematic study of the structure of the ceramic samples by scanning electron microscopy (SEM) follows in Section 3. The SEM photographs could explain several phenomena and properties discussed in Sections 2, 4 and 5 and increase the consistency between the results. In Section 4 the superconducting properties are presented and correlated with non-stoichio© Elsevier Sequoia/Printed in The Netherlands

300

metry and preparation condmons. The evaluation of the results in the concluding section give helpful clues for increasing the reproducibility in ceramic fabrication processes and hopefully for improving the performance (i.e. current density) of high- Tc superconducting ceramics.

2. Ceramic preparation and characterization The starting chemicals were checked for volatile compounds, and in the case of CuO for full oxidation, by 15 h annealing at 600°C in oxygen: in all cases weight losses were observed: BaCO3 (Merck p.a.>99%) ().17 wt.%, CuO (Merck p.a.>99%) 0.61 wt.%, and Y203 (Goldschmidt>99.9%) 0.48 wt.%. In order to achieve the defined stoichiometry of YBCO (and specific deviations from stoichiometry) all chemicals were heat treated for 15 h at 600 °C in flowing oxygen. After cooling the furnace, the chemicals were kept in sealed containers in desiccators until used. In most cases the above starting chemicals were carefully and efficiently weighed and ground in a ball mill in agate containers with the addition of ethanol for 3-5 h. In three cases (specified as P.R.), prereacted YBCO powder was used which was prepared by firing the homogenized mixture for 27 h at 900°C followed by grinding and repeated firing at 900°C for 12 h. The mixed chemicals and the prereacted YBCO powder were pressed to pellets 1 cm in diameter and typically 1.5-2 mm thick at 2500 kg cm -2 pressure. All sintering steps were carried out in flowing oxygen and with the pellets kept in narrow TABLE 1 Compositions of samples and X-ray detection of traces (T) or faint traces (FT) of secondary phase (GP = Green phase Y2BaCuOs, B = BaCuO2, CuO) of samples treated for 32 h at 900 °C and 92 h at 480 °C in oxygen

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1.2 BaCO 3 1.2 BaCO 3 0.33 Y203 0.6 BaCO 3 0.36 CuO 0.72 CuO 0.72 CuO 0 0 0

T:B+GP T:B+GP T:GP/FT:CuO T:B + G P FT:CuO T:CuO/FT:B + GP FT:B + GP FT: GP FT: B + GP n.m?

alumina boats in order to achieve minimum contact area between YBCO and alumina. The first sintering step consisted of 8 h at 900 °C followed by furnace cooling. A second sample series was prepared by firing for 32 h at 900 °C followed by 60 h annealing at 420 °C (samples for density and Meissner measurements) or 92 h at 480°C (for Tc, A Tc, a.c. susceptibility, X-ray and specific heat measurements). The initial characterization of the ceramic pellets consisted of density determination (from the geometry and weight) and of trace determination of secondary phases by means of a focussing Guinier-de-Wolff camera (Nonius) using Cu K~ radiation and applying long exposure times (6 h). Table 1 shows the starting compositions for the series of samples and the results of the X-ray trace analyses where traces are expected to be of the order of 0.5-1 wt.% and faint traces to be below 0.5 wt.%.

3. Scanning electron microscopy investigation The morphology of the ceramic samples (pellet surfaces) was investigated by SEM (Stereoscan Cambridge 250) using an accelerating voltage of about 19-20 kV. Figure 1 shows stoichiometric YBCO compared with Y203-, BaO-, CuO- and BaO-CuOdoped samples which were annealed at 900 °C for 32 h. In the BaO sample grain growth is inhibited to a size below 1/~m, whereas the other samples show grains in the range 1-4/~m and significant coalescence. The largest degree of liquid-assisted growth is found in the BaO-CuO sample. The effect of sintering time on grain growth is shown in Fig. 2 where the Y203-doped sample in Fig. 2(a) was annealed at 900°C for 8 h and that in Fig. 2(b) for 32 h. Grain growth is accompanied by coalescence and facet development. The effect of prereacted YBCO starting material for pellets annealed for 32 h at 900 °C is shown in Fig. 3(a)-(c). In the stoichiometric sample (Fig. 3(a)) the grain size reaches the 5-15 ~m range, and the grains are strongly intergrown. The BaOdoped sample (Fig. 3(b)) shows moderate grain growth to about 0.5-2 /~m, whereas the CuOdoped sample lies between these two extremes. These significant morphological differences of the various non-stoichiometric samples along with knowledge about secondary phases will assist in explaining the differences in superconducting properties.

301

Fig. 1. Scanning electron micrographs of various doped YBCO samples annealed for 32 h at 900 °C: (a) stoichiometric, (b) Y203doped, (c) BaO-doped, (d) CuO-doped and (e) BaO + CuO-doped YBCO.

4. Superconducting properties The superconducting transition temperatures and the transition width were determined by the a.c. susceptibility measurements (slow heating from 4 K to room temperature) as shown in Figs. 4 and 5. These show significant differences in the transition width between the stoichiometric and

CuO-doped samples (Fig. 4) with narrow A Tc and the broad transitions of the samples with excess BaCO3 (Fig. 5). The transition temperatures (defined as midpoints between the temperatures corresponding to 10% and 90% of the susceptibility) and the A Tc values for the various samples are shown in Fig. 6.

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Fig. 2. SEM of Y203-doped YBCO annealed for (a) 8 h and (b) 32 h at 900 °C in oxygen.

They reveal an increase of Tc and a decrease of A Tc in the sequence Ba-, Y-, Ba + Cu- and Cuexcess to stoichiometric YBCO prepared by a conventional ceramic process and by a citrate precursor process [1]. Pellets formed from prereacted YBCO show slightly better values than pellets prepared from mixed chemicals. The low values of A Tc of 0.6, 0.85 and 1 K for stoichiometric and CuO-doped YBCO ceramics are remarkable. Resistivity measurements were performed by using a four-point probe with 1 mm copper wires pressed onto the ceramic and giving a contact resistance of about 1Q. A closed-cycle helium cryostat allowed measurements in the range 10-300 K, and a pulsed d.c. current of 2 mA was applied. The Tc and A Tc values obtained from these resistivity measurements were in good agreement (1 K) with results from a.c. suscepti-

Fig. 3. SEM of prereacted YBCO samples annealed for 32 h at 900°C in oxygen: (a) Stoichiometric, (b) BaO-doped and (c) CuO-doped YBCO.

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bilities. Resistivity plots revealed a transition from semiconducting to metallic behaviour above Tc as a function of annealing time which is shown with a BaO-doped prereacted sample in Fig. 7 as a typical example. Meissner effects were measured in a vibratingsample magnetometer in an applied field of about 20 Oe (lower than//el) by slow cooling from 300 to 77 K. Due to the irregular shape of the samples the corrections for the demagnetizing field were not taken into account so that the Meissner values given may be up to about 20% higher than the real values. These measurements confirmed the sequence of BaO-doped to stoichiometric YBCO with respect to increasing quality as shown in Figs. 8 and 9. In Fig. 8 the Meissner values for two different annealing times demonstrate the importance of sufficient sintering time for achieving a high percentage of superconducting material. Figure 9 shows Meissner values and densities of the sample series for different heat treatments. Prereacted samples are better than ceramics prepared from powder mixtures. A further improve-

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ment of the Meissner effect up to 55% was achieved with a prereacted stoichiometric sample (full rhombus) which was annealed for 25 h at 900°C in nitrogen followed by l l 0 h oxygen annealing at 480 °C. The low-temperature specific heat is very sensitive to homogeneity and purity of YBCO ceramics. This is shown in C / T vs. T 2 plots where a low y (the linear term extrapolated to T = 0 K) and a small upturn of the curve below 10 K indicate high quality and low BaCuO 2 concentration [2]. In view of published 7 values of between 3.9 and 14 mJ K -2 mol-1, the ? values for two stoichiometric YBCO samples of 4.1 and 4.9 (compare Fig. 10) indicate very small amounts of secondary phases. 5. Discussion and conclusions

Small deviations from cation stoichiometry (which easily originate from volatile components

in the starting materials) have significant effects on the superconducting properties of YBCO ceramics. These effects are caused (a) by the sensitivity of the sintering process to impurities as revealed by the SEM investigations and (b) by the presence of secondary phases as revealed by high-sensitivity X-ray methods. Despite taking care of these aspects, all samples in this investigation still contained faint traces of secondary phases. This is shown by the y value and the slight upturn in the low-temperature specific-heat measurements which, although comparable with the lowest published y value, confirm the presence of traces of secondary phases. These traces have also been found in recent transmission electron microscopy investigations [3]. More work is required to obtain really single-phase material of a ceramic structure which is optimized with respect to obtaining improved current densities. The highest densities and optimized magnetic and electrical characteristics were found in stoichiometric YBCO samples. Increasing deterioration was observed by doping with CuO, B a O + CuO, 5(203 and BaO. The latter had a specially negative effect, probably due to its inhibition of grain growth and coalescence in the ceramics combined with the formation of a thin non-superconducting layer around the superconducting grains. It is felt that even when all the parameters in this work have been optimized there is still room for improvement, with respect to current densities, by looking into more subtle aspects such as texture and twin densities in the YBCO ceramics.

305

Acknowledgments

References

The authors are grateful for contributions from the Physics Department DPMC of the University of Geneva, especially Prof. O. Fischer and Dr. A. Junod for helpful discussions, Mr. Bouvier for a.c. susceptibility measurements, and Mr. D. Cattani for a preliminary critical-current measurement.

1 F. Licci and T. Besagni, ItaL Pat. AppL 20697,4/87 (1987). 2 D. Eckert, A. Junod, T. Graf and J. Muller, Physica C, 153-155 (1988) 1038. 3 H. W. Zandbergen, R. Gronsky, G. Thomas, Physica C, 153-155 (1988) 1002.