Chemistry of High-Temperature Superconductors - ACS Publications

Chapter 12

Superconductivity in ΥΒa Cu Ο for x Greater Than 7.0 2

3

χ

Steven W. Keller , Kevin J. Leary , Tanya A. Fattens , James N. Michaels , 1'2

2,3

and Angelica M .

1,2

Stacy

2,3

1,2

Department of Chemistry, University of California—Berkeley, Berkeley, CA 94720 Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory,

1

2

Berkeley, CA 94720 Department of Chemical Engineering, University of California—Berkeley, Berkeley, CA 94720

Downloaded by UNIV OF NEW ENGLAND on February 9, 2017 | http://pubs.acs.org Publication Date: August 28, 1987 | doi: 10.1021/bk-1987-0351.ch012

3

The effect of oxygen stoichiometry on the magnetic properties of Y B a C u O has been studied for χ ≥7.0. The oxygen content of the samples was determined using temperature-programmed desorption (TPD) and temperature-programmed reduction (TPR). For χ = 7.1, the material exhibited a sharp diamagnetic transition with an onset temperature of 90 Κ and a transition width of 10 Κ (10% - 90%). The magnitude of the Meissner effect for the YBa2Cu3O7.1 sample was 28 % at 5 K. When χ was increaseato 7.2, the diamagnetic transition became broader and the magnitude of the Meissner effect decreased, but the onset temperature remained constant. 2

3

x

The recent discovery of superconductivity above 90 Κ in samples of yttrium-barium-copper oxides has generated much enthusiasm among scientists and engineers in a variety of disciplines (1). The 90 Κ superconductor has been identified as Y B a C u O where x s 6.9 - 7.0 (2-5). Several studies on this material have focussed on the effect of oxygen stoichiometry on the superconducting properties; in particular, for ranges of χ between 6.5 and 7.0 (6-8). In this study, we show that samples with χ between 7.0 and 7.5 can be prepared. The value of χ was measured using temperature-programmed desorption (TPD) and temperature-programmed reduction (TPR). We found that as χ was increased above 7.0, the onset temperature remained constant, but the width of the superconducting transition increased and the magnitude of the Meissner effect decreased. 2

3

x

EXPERIMENTAL MATERIALS. A Y B a C u O sample was prepared by reacting stoichiometric ratios of Y 0 (Alpha Products), B a C 0 (Fisher Scientific), and CuO (Fisher Scientific). An intimate mixture of finely divided particles was obtained by dissolving the reactants in concentrated nitric acid and evaporating the solution to dryness at 120 C. The nitrates were decomposed at 500 C for 4 h in air, ground, and reacted further at 750 C for a few days in flowing 0 with several intermediate grindings. Finally, the 2

2

3

x

3

3

2

0097-6156/87/0351-0114$06.00/0 ©

1987 A m e r i c a n C h e m i c a l Society

Nelson et al.; Chemistry of High-Temperature Superconductors ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

12.

Superconductivity in

KELLER E T AL.

YBa Cu O 2

3

x

115

sample was fired in flowing O for 12 h at 950 C, annealed in oxygen for 12 h at 500 C, and slow cooled to room temperature. Powder X-ray diffraction of the material showed a single phase, and the oxygen content was determined as shown below to be χ = 7.1. Three other samples of varying oxygen content were prepared using the Y B a C u 0 sample as a starting material. In each case, a portion of the Y B a C u 0 ' sample was first reduced by heating in helium at 830 C (1100 K) for 5 min. Then by annealing the reduced material in oxygen under different conditions, the oxygen content was varied. The three different annealing conditions used were: 300 C for 14 h, 400 C for 30 min, and 400 C for 2 h. 2

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DETERMINATION OF OXYGEN CONTENT. The oxygen content of the samples was determined using temperature programmed desorption (TPD) and temperature programmed reduction (TPR). By measuring the amount of oxygen which desorbed when the sample was heated in helium and the amount of water which desorbed when the sample was reduced subsequently in hydrogen, the oxygen content of each of the samples was determined. The apparatus used for these experiments has been described previously (10). In a typical TPD experiment, 25 mg of sample were placed in a quartz microreactor which was mounted inside a furnace. Following evacuation for 1 h at room temperature, helium was flowed over the sample at a rate of 100 cc/min (STP) and the temperature was raised at 0.5 K/s. During heating, the desorption products were swept from the reactor by the helium stream and monitored downstream with a UTI Model 100 C quadrupole mass spectrometer. Upon completion of each TPD experiment, the mass spectrometer was calibrated for oxygen as described below, and then a TPR experiment was performed using a hydrogen flow rate of 200 cc/min (STP). After each TPR experiment, the mass spectrometer calibration was repeated. To calibrate the mass spectrometer for oxygen, known quantities of oxygen were pulsed into a 100 cc/min (STP) helium stream and the oxygen concentration in the pulse was monitored with the mass spectrometer. The integrated area of the oxygen signal as a function of time for each pulse varied linearly with the amount of oxygen pulsed. Therefore a calibration factor was obtained by dividing the amount of oxygen in a pulse by the integrated area. This factor was multiplied by the area under the oxygen desorption curve in each TPD spectrum to determine the amount of oxygen which desorbed. The accuracy of the calibration was estimated to be better than ± 2 %. The mass spectrometer was calibrated for water by performing TPR experiments with known quantities of CuO. In each case, a hydrogen flow rate of 200 cc/min was used. During TPR, the CuO was completey reduced to copper metal. The area under the TPR spectrum was found to vary linearly with the amount of CuO, allowing a calibration factor for water to be determined. Following each TPR of the CuO samples, the mass spectrometer was calibrated for oxygen in the manner described above. From these results, the relative sensitivity of the mass spectrometer to water compared to oxygen was determined. Since we found that the relative sensitivity remained constant, it was only necessary to calibrate the mass spectrometer for oxygen after each experiment. From the CuO experiments, the accuracy of this calibration was determined to be ±3 %.


116

CHEMISTRY OF HIGH-TEMPERATURE SUPERCONDUCTORS

RESULTS Figure 1 shows the TPD spectrum obtained on a sample of Y B a C u 0 sample which had been exposed to the air for one week. During exposure to the air, these materials incorporate a significant amount of water, CO, and C 0 (9). As shown in Figure 1, these impurities desorb over the entire temperature range. Figure 1 also shows that oxygen desorbs during heating in helium in a large peak centered at approximately 815 K, and in a second much smaller peak at 1035 K. By integrating the area under the oxygen desorption spectrum, we found that 0.6 ± .01 moles of oxygen atoms (0.3 moles of 0 ) were removed from the sample per mole of starting material. Upon completion of the TPD experiment and calibration of the mass spectrometer, the sample was heated in hydrogen to 1200 K. Under these conditions, the sample was reduced to Y 0 , BaO, and Cu (2). The TPR spectrum is shown in Figure 2. The amount of water produced was determined by integrating the TPR spectrum. This amount corresponded to the removal of 3.0 ± .09 moles of oxygen atoms per mole of starting material. Adding this to the 0.6 moles removed by TPD and the 3.5 moles of oxygen in the reduction products, we obtain a stoichiometry of the starting material of Y B a C u 0 The uncertainty in the value of χ is ± 0.1. The oxygen contents of the other three samples were determined in the same manner. The sample which was prepared by heating Y B a ^ C u ^ j in helium at 1100 Κ for 5 min and annealing in oxygen at 300 C for 14 h was found to have an oxygen stoichiometry of χ = 7.0. The samples which were annealed at 400 C for 30 min and 2 h had oxygen contents of χ = 7.2 and 7.5, respectively. We should emphasize that the exact times and temperatures needed to vary the oxygen content are sample dependent. In each case, the differences in oxygen content were reflected in the differences in the amount of oxygen which desorbed during the TPD experiment. The TPR spectra of all the samples looked very similar to the one shown in Figure 2, although the peak positions and the relative heights of the peaks varied slightly from sample to sample. Thus, heating in helium to 1100 Κ reduced the samples to Y B a C u 0 . From these results, we conclude that for single phase Y B a C u O v samples with χ 6.5, the oxygen stoichiometry can be determined by TPD alone. Further reduction of the samples in hydrogen is unnecessary. One advantage of using TPD to determine the oxygen content of the samples is that TPD is also useful for characterizing these materials. The effect of oxygen content on the oxygen desorption spectrum of Y B a C u O is shown in Figure 3 for values of χ between 7.0 and 7.5. The spectrum for χ = 7.0 contains a large desorption peak near 860 Κ and a much smaller peak near 1015 K. This spectrum looks similar to that shown in Figure 1 for χ = 7.1, and appears to be characteristic of a material with a sharp diamagnetic transition. As χ increases to 7.2, a third small peak begins to grow in as a shoulder on the low temperature peak between 900 and 950 K. This shoulder is never observed on samples with sharp diamagnetic transitions. As χ increases still futher to 7.5, the shoulder does not increase in size, but the size of the high temperature peak between 1000 and 1050 Κ increases dramatically. The magnetic susceptibilities of the starting material Y B a C u 0 and a sample which was determined to have the stoichiometry YBa Cu' 0 were measured as a function of temperature, and the results are shown in Figure 4. The magnetic measurements were made using a SHE SQUID magnetometer in a field of 12 Gauss; the Meissner effect was determined by 2

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12.

Superconductivity in

KELLER E T AL.

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YBa2Cu O

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900

1100

Temperature (K)

Figure 1 : TPD spectrum of Y B a C u 0 for a helium flow rate of 100 cc/min (STP) and a heating rate of 0.5 K/s. 2

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1000

t

800 h

400

600 800 1000 Temperature (K)

1200

Figure 2: TPR spectrum of Y B a C u 0 for a hydrogen flow rate of 200 cc/min (STP) and a heating rate of 0.5 K/s. 2

3

7 1



400

Figure 3:

600 800 1000 Temperature (K)

Effect of oxygen content on the oxygen desorption spectrum of Y B a C u O for values of χ between 7.0 and 7.5. In each case the material is reduced to Y B a C u 0 . 2

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T E M P E R A T U R E (Κ)

Figure 4:

Magnetic susceptibility vs. temperature curves for (•) Y B a C u 0 and ( Θ ) Y B a C u 0 . 2

3

7 Λ

2

3

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12.

KELLER ET AL.

Superconductivity in YBa2Cu O 3

119

x

measuring the magnetization as the samples were cooled from 110 K. For both samples, the onset of diamagnetism occured at 90 K. The Y B a C u 0 sample had a sharp transition and a transition width of 10 Κ as snown m Figure 4. A 28 % Meissner effect was calculated for this sample using the measured density of 3.92 g/cc. For samples with χ > 7.1, the transitions were always broader, but the Meissner effect varied depending on the length of time the samples were exposed to air. The magnetic data for the Y B a C u 0 sample after exposure to the air for one month is shown in Figure 4. The results in Figure 4 suggest that the magnitude of the Meissner effect at 5 Κ decreases when χ is increased from 7.1 to 7.2. Further experiments are in progress to quantify these results. 2

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2

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DISCUSSION Most of the studies on the effect of oxygen stoichiometry on the superconducting properties of Y B a C u O have concentrated on ranaes of χ between 6.0 and 7.0. It has been shown that the reduction of Y B a C u 0 to Y B a C u 0 corresponds to the removal of half of the oxygen atoms in the chains oetween the planes of CuO (11). However, single crystal X-ray analysis has shown that the oxygen content can be reduced further to YBa2Cu30g (13). Our results suggest that the first 0.5 moles of oxygen atoms are removed rapidly but that further reduction requires longer times and/or higher temperatures. We postulate that the change in the kinetics of oxygen removal may be associated with the orthorhombic to tetragonal phase transition; the mobility of oxygen in the orthorhombic phase is substantially higher than in the tetragonal modification. Because these materials are reduced reproducibly to Y B a C u 0 by heating in helium to 1100 K, the oxygen content can be determined by TPD alone. One advantage of using TPD to characterize Y B a C u O samples is that it is very sensitive to minor differences between samples. For example, differences between the Y B a C u 0 and Y B a C u 0 samples were clearly evident in the oxygen desorption spectra shown in Figure 3. Another advantage is that impurities such as absorbed water and carbon oxides can be detected easily as shown in Figure 1. These impurities can not be detected directly with a technique like thermal gravimetric analysis (TGA). Therefore if TGA is used to measure oxygen addition and removal from these materials, significant errors can be made unless care is taken to first remove these impurities (9). These impurities also may have an effect on the electronic and magnetic properties of these materials. It has been reported that water has a detrimental effect on the superconductive properties of lanthanum-strontium- copper oxides (11). The effects of these impurities on Y B a C u O samples is under further investigation. In summary, we have shown that samples of Y B a C u O can be prepared with χ > 7.0. For χ = 7.1, the material exhibits a sharp diamagnetic transition with an onset temperature of 90 Κ and a transition width of 10 Κ (10% - 90%). When χ is increased to 7.2, the transition is broadened substantially, and the magnitude of the Meissner effect at 5 Κ is decreased. 2

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ACKNOWLEDGMENTS The authors would like to thank William K. Ham and Hans-Conrad zur Loye for help with sample preparation. This research was supported by a Presidential Young Investigator Award from the National Science Foundation to AMS with matching funds from E.I. DuPont de Nemours and


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Company and from Dow Chemical Company, and by the Director, Office of Basic Energy Sciences, Materials Science Division and U.S. Department of Energy under contract No. DE-AC03-76SF0098.

LITERATURE CITED 1.


2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Wu, M.K.; Ashburn, J.R.; Torng, C.J.; Hor, P.H.; Meng, R.L.; Gao, L.; Huang, Z.J.; Wang, Y.Q.; Chu, C.W. Phys. Rev. Lett., 1987, 58, 908. Cava, R.J.; Batlogg, B.; VanDover, R.B.; Murphy, D.W.; Sunshine, S.;Siegrist, T.; Remeika, J.P.; Rietman, E.A.; Zahurak, S.; Espinosa, G.P. Phys. Rev. Lett., 1987, 58, 1676. Beyers, R.; Lim, G.; Engler, E.M.; Savoy, R.J.; Shaw, T.M.; Dinger, T.R.; Gallagher, W.J.; Sandstrom, R.L. Appl. Phys. Lett., submitted for publication. Le Page, Y.; McKinnon, W.R.; Tarascon, J.M.; Greene, L.H.; Phys. Rev. Lett., submitted for publication. Okamura, F.P.; Sueno, S.; Nakai, I.; Ono, Α.; submitted for publication. Swinnea, J.S.; Steinfink, H.; preprint. Morris, D.E.; Scheven, U.M.; Bourne, L.C.; Cohen, M.L.; Crommie, M.F.; Zettl, Α.; Proceedings from the Materials Research Conference, Symposium S. Anaheim, CA. April, 1987, in press. Tarascon, J.M., Proceedings from the International Conference on Novel Mechanisms of Superconductivity, Berkeley, CA, June, 1987, to be published. Keller, S.W.; Leary, K.J.; Stacy, A.M.; Michaels, J.N.; Matt. Lett., in press. Leary, K.J.; Michaels, J.N.; Stacy, A.M.; J. Catal., 1986, 101, 301. Beno, M.A.; Soderholm, L.; Capone, D.W. II; Hinks, D.G.; Jorgensen, J.D., Schuller, I.K.; Segre, C.U.; Zhang, K.; Grace, J.D. submitted to Appl. Phys. Lett., 1987. Kisho, K.; Sugii, N.; Kitazawa, K.; Fueki, K.; Jpn. J. Appl. Phys., 1987, 26, L466. Steinfink, H; Swinnea, J.S.; Sui, Z.T.; Hsu, H.M.; and Goodenough, J.B.; J. Amer. Chem. Soc.; in press.

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Chemistry of High-Temperature Superconductors - ACS Publications

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