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Recovery of superconductivity in the water degraded YBCO samples
Physica C 307 Ž1998. 51–60
Recovery of superconductivity in the water degraded YBCO samples Sandeep Rekhi ) , G.L. Bhalla, G.C. Trigunayat Department of Physics and Astrophysics, UniÕersity of Delhi, Delhi-1100 07, India Received 30 April 1998; revised 6 August 1998; accepted 14 August 1998
Abstract The recovery of superconductivity in water degraded YBa 2 Cu 3 O 7y d samples has been studied by subjecting them to heat treatments. The possible recovery of superconductivity has been estimated through structural, electrical, magnetic and morphological investigations. The results reveal the transformation of secondary phases formed during degradation into YBCO phase. The presence of some additional phases of YBCO, possibly arising from deficiency of the constituent elements resulting from degradationrlack of proper reaction between the secondary phases, is also observed. The heat treatment carried out on a sample after pulverising it led to full recovery of superconductivity in it. A possible mechanism for the observed recovery of superconductivity has been suggested. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Superconductivity; YBCO samples; Heat treatments
1. Introduction Numerous investigations have been performed on the hydrophilic nature of YBCO cuprates w1–12x under different conditions, e.g., deionized water w1,3,4x, dry and wet steam w6,11x, humidity w5,7,10x and various solvents w12x. The main conclusions drawn from the studies relate to the preferential extraction of barium from the unit cells, hydroxylation of the surface, and nucleationrgrowth of secondary products. Trolier et al. w12x proposed the possibility of dissolution mechanism, which either includes leaching of Ba ions or a congruent dissolu)
Corresponding author. Department of Earth Sciences, Mineralogy–Petrology, Villavagen 16, 752 36 Uppsala, Sweden. Tel.: q 4 6 -1 8 -4 7 1 2 5 8 7 ; F ax : q 4 6 -1 8 -4 7 1 2 5 9 1 ; E -m ail: [email protected]
tion followed by rapid precipitation of new phases. They suggested that the reprecipitation of this type could also account for the apparent non-stoichiometry in the dissolution of solvent-exposed samples. The estimation of stability of YBCO in deionized water and the reaction mechanism involved in the process have been worked out in detail w13,14x. The instability of these cuprates in a humid environment seriously comes in the way of their commercial exploitation. Various means to restrict their degradation or to eliminate the source of ingress have been suggested w15–19x. In view of the high cost of production of superconducting compounds, it is very desirable to examine the possibility of recovery of the degraded samples, so that they can be reused. It is assumed that weak attacks of a degradant only reduce the oxygen content, with the phase itself remaining intact; for severe attacks the possibility of
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 8 . 0 0 4 0 4 - 3
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S. Rekhi et al.r Physica C 307 (1998) 51–60
back conversion of the secondary phases into the original YBCO phase is unclear. It has to be determined whether the process of degradation is reversible and, if so, to further investigate how the secondary products formed during degradation are restored to their initial state. It was reported that YBCO powderrpellet on degradation with deionized water showed disappearance of the phase w20x. A full recovery was secured by reheating the sample in air at 9308C for 6 h Žthe pellet also sintered at 9508C for 12 h.. However, the results are highly suspect because several workers, e.g., Ref. w21x, have pointed out that heating of YBCO at high temperature Žabove 7508C to 9008C. in air results in loss of oxygen, leading to transformation from the superconducting orthorhombic to non-superconducting tetragonal phase. Although it is understandable that if the degraded sample is pulverised and again heated, a solid state reaction may take place to again form YBCO, the effect of heat treatment on an unpulverised sample remains to be understood. In the present work, it was planned to subject the degraded samples to a suitable heat treatment and to investigate the resulting possible recovery of their superconductivity by employing various experimental techniques, viz. XRD, oxygen content estimation, electrical and magnetic studies, and surface morphological studies through SEM. The information so gathered could be utilised for visualising the mechanism involved in the process of recovery.
2. Experimental The degradation of YBa 2 Cu 3 O 7y d Žprepared by solid state reaction technique. with deionized water for 6 h, 12 h, 18 h, 24 h, 38 h, 48 h has been reported elsewhere w13,14x. The degraded samples were simultaneously sintered and annealed in a furnace at 9508C for 12 h in oxygen with a flow rate of 1 mlrmin. They were slowly cooled down to 7508C, maintained at this temperature for 6 h, further cooled down to 5008C, at that temperature for 24 h, and finally cooled down to room temperature. The oxygen flow was maintained until the temperature dropped to 2008C. The initial high temperature Ž) 9008C. was chosen to allow the secondary phases formed during degradation to be thermally dissoci-
ated. However, for the sake of comparison the samples treated for longest time, viz. 48 h were subjected to three different types of heat treatment carried out in oxygen atmosphere: Ži. annealing of the sample as such, Žii. calcination and annealing of the sample as such, and Žiii. calcination and annealing of pulverised and palletised samples. The samples were subjected to X-ray diffraction studies on a PHILIPS X-ray diffractometer ŽModel PW 1130r00., using CuK a radiation. The 2 u values were taken from 208 to 708 with an approximate error of 0.018. The diffractograms were employed to compute the unit cell dimensions by Cohen’s method Ža least-squares refinement technique to minimise the random errors in estimating the interplanar spacing ‘d’ arising from absorption, eccentricity and beam divergence w22x.. The resistance versus temperature measurements on the samples were performed by the conventional four probe technique, using KEITHLEY instruments, and the magnetic studies were performed on a Lakeshore A.C. Susceptometer ŽModel 7000., using field of 2 Arm and ac frequency of 111.1 Hz. Both the real and imaginary parts of the susceptibility were studied. The oxygen content of the samples was determined both by iodometric titration method and from the knowledge of the unit cell c-dimension, ˚ . w23x. using the relation 7 y d s 67.108–5.161 c ŽA The morphological studies of the samples were carried out on a scanning electron microscope ŽModel JEOL-JSM 840.. A thin gold film was sputtered on the surface using an ion-sputter before employing the samples for surface investigations.
3. Results and discussion As mentioned earlier, the heat treatment given to the degraded samples was performed at high temperature to promote decomposition of various secondary products, e.g., hydroxides, oxides and carbonates, accumulated on the hydroxylated YBCO samples. It is conjectured that some of the secondary products possibly formed during degradation, viz. BaŽOH. 2 , BaCO 3 , YŽOH. 3 , Y2 ŽCO. 3 , get thermally dissociated to yield the starting stoichiometric products, e.g., Y2 O 3 and BaO. Either single species such as BaO or complex species such as Y2 BaCuO5 then react to
S. Rekhi et al.r Physica C 307 (1998) 51–60
form the YBCO phase. The annealing of sample in oxygen atmosphere helps the so formed YBCO to attain an optimum value of the oxygen content. It also enables the unaltered material already existing in the degraded sample not to lose its oxygen at elevated temperatures. It was reported earlier w13x that after degradation the samples found to be covered with a white layer
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of certain amorphousrcrystalline secondary phases. The annealing of the samples in the present work made this white layer to disappear, leaving blackcoloured samples behind, indicating the transformation of amorphousrcrystalline secondary phases into YBCO phase. Fig. 1 depicts the X-ray diffractograms of both untreated and the oxygen-annealed samples. The pattern A pertaining to the untreated
Fig. 1. X-ray diffractograms of an untreated sample and the samples degraded for various time durations and then oxygenated wA: untreated; B: 6 hŽo.; C: 12 hŽo.; D: 18 hŽo.; E: 24 hŽo.; F: 36 hŽo.; G: 48 hŽo.x.
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Table 1 Structural parameters for the degraded and subsequently annealed samples Samples
Fresh 6 hŽo. 12 hŽo. 18 hŽo. 24 hŽo. 36 hŽo. 48 hŽo.
˚. Lattice parameters ŽA
Orthorhombicity
a
b
c
=10y3
Cell diagonal ˚. ŽA
Cell volume ˚. ŽA
3.816Ž7. 3.814Ž4. 3.811Ž5. 3.811Ž3. 3.812Ž8. 3.811Ž5. 3.814Ž5.
3.879Ž6. 3.881Ž8. 3.875Ž5. 3.872Ž7. 3.880Ž6. 3.882Ž3. 3.878Ž2.
11.685Ž9. 11.684Ž8. 11.680Ž8. 11.690Ž3. 11.689Ž9. 11.699Ž9. 11.690Ž9.
16.374 17.414 16.654 15.879 17.680 18.458 16.641
12.889Ž12. 12.888Ž11. 12.882Ž10. 12.890Ž06. 12.892Ž07. 12.901Ž11. 12.893Ž10.
172.50Ž7. 172.91Ž6. 172.45Ž6. 172.50Ž5. 172.77Ž8. 173.08Ž5. 172.88Ž5.
sample manifests the existence of single orthorhombic YBCO phase. The patterns B to E pertaining to the degraded Ž6 h to 24 h. and subsequently oxygen-annealed samples reveal relatively small contents of the secondary phases. In contrast, the patterns F and G, pertaining to 36 h and 48 h degraded and oxygen-annealed samples show much more prominent peaks of the secondary phases Že.g., the peaks of Y2 BaCuO5 and BaCO 3 . indicating a much greater presence of these phases in the samples. The unit cell dimensions calculated from these diffractograms are listed in Table 1. The variation in the values is found to be uneven as compared to that for the degraded Žbut not annealed. samples, in which a continuous decrease in the b-dimension and a continuous increase in c-dimension were observed w13x.
The variations in the values of orthorhombicity, unit cell diagonal and cell volume, calculated from the cell dimensions, are also listed in Table 1. They are also seen to be uneven. The orthorhombicities for the 36 hŽo. and 48 hŽo. samples are notably found to be more than that for the fresh samples The uneven variation in the structural parameters implies that the material formed after oxygen annealing possibly comprises, besides YBCO, some additional phases of YBCO, e.g., oxygen-deficient andror barium-deficient YBa 2 " x Cu 3 O 7y d andror yttrium deficient Ba 2Y1 " y Cu 3 O 7y d , andror copper-deficient YBa 2 Cu 3 " zO 7y d . Such additional phases may form both during the loss of various constituents of YBCO and during the partial transformation of the secondary products into YBCO. As mentioned earlier, the 48 h treated samples were subjected to two more types of treatment, viz. pulverisation and calcination, besides the usual types of oxygen annealing. Fig. 2 shows the XRD of a 48 h degraded, calcinated and oxygen-annealed sample
Fig. 2. X-ray diffractogram of a 48 h treated, calcinated and oxygen-annealed sample w48 hŽcqo.x.
Fig. 3. X-ray diffractogram of a 48 h treated, pulverised, repelletised, calcinated and oxygen-annealed sample w48 hŽpqcqo.x.
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Table 2 Structural parameter of the fresh, the degraded and subsequently annealed samples Samples
˚. Lattice parameters ŽA a b c
Orthorhombicity =10y3
Fresh 48 h 48 hŽo. 48 hŽcqo. 48Žpqcqo.
3.824Ž1. 3.822Ž4. 3.814Ž5. 3.822Ž6. 3.824Ž1.
17.369 12.739 16.641 15.834 16.083
3.891Ž3. 3.871Ž2. 3.878Ž2. 3.883Ž7. 3.886Ž4.
11.678Ž5. 11.722Ž5. 11.690Ž9. 11.693Ž4. 11.691Ž3.
w48 hŽc q o.x. The peaks due to the secondary phases still exist, although with the reduced intensity Žcf. Fig. 1, pattern G.. It follows that the calcination of the sample does not eliminate the secondary phases but only reduces their content; like the 48 hŽo. samples, the 48 hŽc q o. samples may also consist of additional YBCO phases, along with the secondary phases. Fig. 3 depicts the XRD of a 48 h degraded, pulverised, calcinated and oxygen annealed sample w48 hŽp q c q o.x. The secondary phases are now found to be eliminated altogether and the orthorhombic YBCO phase is found to have recovered. Table 2 lists the unit cell dimensions for the various samples. Maximum recovery is noted for the 48 hŽp q c q o. sample. The changes observed in the unit cell dimensions of the 48 hŽc q o. samples are attributed to the calcination of the samples, which leads to the loss of carbonates and solid state reaction between various constituents to yield YBCO. The pulverisation of the samples further facilitates such a solid state reaction, through improved mixing of the constituents, which accounts for the observed further recovery in the values of the lattice parameters for 48 hŽp q c q o. samples, such that they are now found to be very
Fig. 4. Resistance versus temperature characteristics for various degraded and subsequently oxygen-annealed samples.
close to the lattice parameters of the fresh sample. A small difference between the two sets of values may arise from the partial loss of certain constituent elements during degradation, leading to a slight change in the stoichiometry of the compound. Earlier, for the degraded Žbut not annealed. samples the value of the oxygen content was found to decrease continuously with increase in the time of reaction w14x, but in the present case the oxygen content of the annealed samples was found to increase Ždetermined by iodometric titration as well as from the knowledge of unit cell c-dimension.. Table 3 lists the values of the oxygen content and the copper valence state calculated therefrom. The values determined from the XRD data are generally higher than those determined from the other technique by 1.5%. Prima facie, the values listed in the table show a somewhat irregular variation. However,
Table 3 Oxygen content and copper valence state of degraded and subsequently annealed samples Samples
c-parameter ˚. ŽA
Oxygen content
Copper valence
From c-values
Iodometrically
From c-values
Iodometrically
Fresh 6 hŽo. 12 hŽo. 18 hŽo. 24 hŽo. 36 hŽo. 48 hŽo.
11.685Ž9. 11.684Ž8. 11.680Ž8. 11.690Ž3. 11.689Ž3. 11.699Ž9. 11.690Ž9.
6.80Ž4. 6.81Ž4. 6.83Ž4. 6.77Ž1. 6.78Ž1. 6.73Ž4. 6.77Ž4.
6.918Ž2. 6.898Ž1. 6.942Ž1. 6.850Ž3. 6.881Ž2. 6.803Ž4. 6.875Ž2.
2.201 2.204 2.220 2.184 2.187 2.152 2.184
2.278 2.265 2.294 2.233 2.254 2.202 2.250
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Table 4 Electrical and magnetic parameters for the degraded and subsequently annealed samples Samples
Fresh 6 hŽo. 12 hŽo. 18 hŽo. 24 hŽo. 36 hŽo. 48 hŽo. a
x –T studies
R–T studies Tc Ž R s 0. ŽK.
Tc Žonset. ŽK.
DTc ŽK.
Tc Žonset. ŽK.
Phase fraction Ž%.
92.3 87.0 89.8 87.0 89.7 89.0
95.0 97.2 95.0 95.0 107.0 98.5 96.0
2.7 10.3 5.2 8.0 17.3 9.5 –
92.5 92.0 92.5 92.0 93.0 93.0 93.0
100 70 80 64 73 47 45
a
Residual resistance of 2 m V at 77 K.
a closer look reveals that within the computed error of determination, the oxygen content shows a downward trend as the degradation time increases.
The resistance versus temperature characteristics of the samples are depicted in Fig. 4. The values of Tc Žonset. vary within a range of 3.5 K, except for
X
Fig. 5. The plot of real part of susceptibility, x , as a function of temperature.
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the 24 hŽo. sample, which has an apparent Tc Žonset. as 107 K and an exceptionally high value DTc s 17.3 K. The value of Tc Ž R s 0. lies between 87 K and 89.8 K ŽTable 4.. It was reported earlier w13x that the values of Tc Ž R s 0. for the samples degraded up to 18 h showed a progressive reduction with the time of degradation and the 24 h treated samples showed a semiconducting behaviour with a transition and a residual resistance of 1200 m V at 77 K. In the present case of recovered samples, except for the 48 hŽo. samples, the Tc Ž R s 0. values for the samples vary unevenly. The exceptional 48 hŽo. sample exhibits a slight semiconducting behaviour, up to 50 m V at nearly 150 K, followed by as many as four transitions, and finally a residual resistance of 2 m V at 77 K. The transition width of all the samples is found to be appreciably high, which can be attributed to an appreciable deviation in stoichiometry of the material. Resistive tailing observed at the end of the transition may be indicative of weak intergrain
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linksrpresence of polyphases of YBCO in the material. The results of R–T measurements were further substantiated by the magnetic measurements. Fig. 5 depicts the plots of real part of susceptibility Ž x X versus T . for the recovered samples. The superconducting phase fraction of the samples Žat ; 75 K. is found to have an uneven variation ŽTable 4. and its values are less than that for the fresh sample. The undulations seen in the transition part of the curve imply that the transition occurs in steps, possibly due to the presence of one or more than one additional phases Žpolyphases. in the samples. It seems that during the oxygen annealing treatment of the samples the initially present tetragonal and other secondary phases, after conversion into YBCO, are not able to acquire the oxygen content Žcorresponding to that for the fresh sample. and form one or more than one oxygen-deficient phases. It is also possible that one or more barium-, yttrium-, copper-deficient phases are also formed w24,25x. The Tc Žonset. values
X
Fig. 6. The plot of imaginary part of susceptibility, x , as a function of temperature.
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show a slight variation Žbetween 92 K and 93 K.. The Tc Ž R s 0. could not be estimated for any sample as the curves did not saturate even up to nearly 75 K. However, the steepness of the curves appreciably reduces, implying a high value of transition width for the samples. Fig. 6 depicts x Y Žimaginary part of susceptibility. peaks for the samples. The peaks for the degraded and subsequently oxygen-annealed samples are seen to be suppressed and generally broader than the corresponding peaks for the fresh sample. The broadening of the peaks implies that after the heat treatment of the degraded material the grains become finer and the material acquires a high degree of
porosity which will render the intergranular couplings weak. This is further substantiated through surface morphology studies ŽSEM., described in the following. Scanning electron microscopy was employed to examine the surface morphology of the basal surface of the samples after oxygen annealing. The micrograph 7a, pertaining to the fresh samples shows exclusive presence of YBCO grains on the surface of the sample. The surfaces of the degraded and subsequently annealed samples manifest a high degree of porosity, which could possibly be attributed to the evolution of water vapours and other gases during desorption and disintegration of waterrhydroxidesr
Fig. 7. Ža. A SEM micrograph of the surface of a fresh sample, depicting exclusive presence of the YBCO grains Ž=3500.. Žb. A SEM micrograph of a 12 hŽo. sample Ž=5000.. Žc. A SEM micrograph of a 36 hŽo. sample Ž=5000.. Žd. A SEM micrograph of a 48 hŽo. sample. The appearance of a large number of randomly scattered small round spot possibly indicate the formation of a secondary phase Ž=4500..
S. Rekhi et al.r Physica C 307 (1998) 51–60
oxides or carbonates wsee micrographs 7b and 7c for 12 hŽo. and 36 hŽo. samples, respectivelyx. Unlike the degraded Žbut not annealed. samples w14x, the presence of any secondary phases is hardly indicated in the micrographs except for the 48 hŽo. samples ŽFig. 7d.. In all the annealed samples the YBCO grains are seen to be coagulated to form clusters, possibly resulting from an initial decomposition followed by a solid state reaction of the resulting products.
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for their assistance in X-ray diffractometry and microscopy studies, respectively. Thanks are also due to Dr. S. K. Agarwal ŽNPL, Delhi. for his kind help in susceptibility measurements. This work has been carried out under a research project financed by Department of Science and Technology, Govt. of India.
References 4. Conclusions The main conclusions drawn from the above study are as follows. The results obtained from XRD analysis, oxygen content estimation, electrical and magnetic measurements, and surface morphology study are in mutual agreement. They indicate the formation of some additional phases of YBCO, along with the decomposition of the secondary phases formed during degradation. The R–T and x –T measurements clearly indicate the presence of the additional phases while the X-ray diffractometry study indicates the absence of impurity phases for less degraded and oxygen-annealed samples. The impurities like Y2 BaCuO5 and BaCO 3 are prominently present in prolonged degraded and oxygen-annealed samples. The recovery of superconductivity is also indicated through a marked increase in the values of oxygen content of heat treated samples, as compared to the degraded samples. A high degree of porosity along with coagulation of grains has been noticed, possibly arising from the desorption of water vapour and other gases during the disintegration of impurities formed during degradation. No impurities or surface alterations are noticed, as found on degraded sample’s surface. A pulverised, calcinated and oxygen annealed sample has shown marked recovery of superconductivity.
Acknowledgements The authors are grateful to Mr. P.C. Padmakishan ŽDept. of Geology, D.U.., Dr. Mehra ŽUSIC, D.U..
w1x N.P. Bansal, A.L. SandKuhl, Appl. Phys. Lett. 53 Ž4. Ž1988. 323. w2x M.F. Yan, R.L. Barns, H.M. O’Bryan Jr., P.K. Gallagher, R.C. Sherwood, S. Jin, Appl. Phys. Lett. 51 Ž1987. 532. w3x D.R. Riley, D. Jurbergs, J.P. Zhou, J. Zhao, J.T. McDevitt, Solid State Commun. 88 Ž6. Ž1993. 431. w4x Z. Dexin, X. Mingshan, Z. Ziquing, Y. Shubin, Z. Huansui, S. Shuxia, Solid State Commun. 65 Ž5. Ž1988. 339. w5x J.P. Zhou, J.T. McDevitt, Solid State Commun. 86 Ž1. Ž1993. 11. w6x D.K. Pham, R.P. Zhao, P.E. Fielding, S. Myhra, P.S. Turner, J. Mater. Res. 6 Ž6. Ž1991. 1148. w7x D.R. Riley, J.P. Zhou, A. Manthiram, D. Jurbergs, J. Zhao, J.T. McDevitt, in: C.W. Chu et al. ŽEds.., Proc. TCSUC workshop, World Scientific Singapore, 1992, 199. w8x J.P. Zhao, J.T. McDevitt, in: C.W. Chu et al. ŽEds.., Proc. TCSUC Workshop, World-Scientific Singapore, 1992, p. 377. w9x R. Zhao, S. Myhra, Physica C 230 Ž1994. 75. w10x R. Zhao, M.J. Goringe, S. Myhra, P.S. Turner, Philos. Mag. A 66 Ž1992. 491. w11x L.D. Fitch, V.L. Burdick, J. Creamic Soc. 72 Ž10. Ž1989. 2020. w12x S.E. Trolier, S.D. Atkinson, P.A. Fuierer, J.H. Adair, R.E. Newnham, Am. Ceram. Soc. Bull. 67 Ž4. Ž1988. 759. w13x S. Rekhi, G.L. Bhalla, G.C. Trigunayat, Physica C 231 Ž1994. 277. w14x S. Rekhi, G.L. Bhalla, G.C. Trigunayat, accepted in Indian J. of Cryogenics. w15x J.P. Zhou, S.M. Savoy, R.K. Lo, J. Zhao, M. Arendt, Y.T. Zhu, A. Manthiram, J.T. McDevitt, Appl. Phys. Lett. 66 Ž21. Ž1995. 2900. w16x C.A. Chang, Appl. Phys. Lett. 53 Ž12. Ž1988. 1113. w17x D. Veretnik, S. Reich, Appl. Phys. Lett. 56 Ž21. Ž1990. 2150. w18x G.L. Bhalla, A. Malik, A. Kumar, B. Kumar, G.C. Trigunayat, Physica C 213 Ž1993. 78. w19x G.L. Bhalla, B. Kumar, A. Malik, V.K. Agarwal, G.C. Trigunayat, A.V. Narlikar, Phys. Stat. Sol. ŽA. 143 Ž1994. 131. w20x N.M. Hwang, G.W. Bahng, Y.K. Park, J.C. Park, H.G. Moon, J. Mat. Sci. Lett. 8 Ž1989. 517.
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w21x B. Raveau, C. Michel, M. Hervieu, D. Groult, in: R.M. Osgood et al. ŽEds.., Crystal Chemistry of High Tc Superconducting Copper Oxide, Springer, New York, 1991, pp. 34, 35, 48. w22x H.P. Klug, L.E. Alexander, X-ray Diffraction procedures, Wiley, New York, 1974, p. 597.
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Recovery of superconductivity in the water degraded YBCO samples Sandeep Rekhi ) , G.L. Bhalla, G.C. Trigunayat Department of Physics and Astrophysics, UniÕersity of Delhi, Delhi-1100 07, India Received 30 April 1998; revised 6 August 1998; accepted 14 August 1998
Abstract The recovery of superconductivity in water degraded YBa 2 Cu 3 O 7y d samples has been studied by subjecting them to heat treatments. The possible recovery of superconductivity has been estimated through structural, electrical, magnetic and morphological investigations. The results reveal the transformation of secondary phases formed during degradation into YBCO phase. The presence of some additional phases of YBCO, possibly arising from deficiency of the constituent elements resulting from degradationrlack of proper reaction between the secondary phases, is also observed. The heat treatment carried out on a sample after pulverising it led to full recovery of superconductivity in it. A possible mechanism for the observed recovery of superconductivity has been suggested. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Superconductivity; YBCO samples; Heat treatments
1. Introduction Numerous investigations have been performed on the hydrophilic nature of YBCO cuprates w1–12x under different conditions, e.g., deionized water w1,3,4x, dry and wet steam w6,11x, humidity w5,7,10x and various solvents w12x. The main conclusions drawn from the studies relate to the preferential extraction of barium from the unit cells, hydroxylation of the surface, and nucleationrgrowth of secondary products. Trolier et al. w12x proposed the possibility of dissolution mechanism, which either includes leaching of Ba ions or a congruent dissolu)
Corresponding author. Department of Earth Sciences, Mineralogy–Petrology, Villavagen 16, 752 36 Uppsala, Sweden. Tel.: q 4 6 -1 8 -4 7 1 2 5 8 7 ; F ax : q 4 6 -1 8 -4 7 1 2 5 9 1 ; E -m ail: [email protected]
tion followed by rapid precipitation of new phases. They suggested that the reprecipitation of this type could also account for the apparent non-stoichiometry in the dissolution of solvent-exposed samples. The estimation of stability of YBCO in deionized water and the reaction mechanism involved in the process have been worked out in detail w13,14x. The instability of these cuprates in a humid environment seriously comes in the way of their commercial exploitation. Various means to restrict their degradation or to eliminate the source of ingress have been suggested w15–19x. In view of the high cost of production of superconducting compounds, it is very desirable to examine the possibility of recovery of the degraded samples, so that they can be reused. It is assumed that weak attacks of a degradant only reduce the oxygen content, with the phase itself remaining intact; for severe attacks the possibility of
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 8 . 0 0 4 0 4 - 3
52
S. Rekhi et al.r Physica C 307 (1998) 51–60
back conversion of the secondary phases into the original YBCO phase is unclear. It has to be determined whether the process of degradation is reversible and, if so, to further investigate how the secondary products formed during degradation are restored to their initial state. It was reported that YBCO powderrpellet on degradation with deionized water showed disappearance of the phase w20x. A full recovery was secured by reheating the sample in air at 9308C for 6 h Žthe pellet also sintered at 9508C for 12 h.. However, the results are highly suspect because several workers, e.g., Ref. w21x, have pointed out that heating of YBCO at high temperature Žabove 7508C to 9008C. in air results in loss of oxygen, leading to transformation from the superconducting orthorhombic to non-superconducting tetragonal phase. Although it is understandable that if the degraded sample is pulverised and again heated, a solid state reaction may take place to again form YBCO, the effect of heat treatment on an unpulverised sample remains to be understood. In the present work, it was planned to subject the degraded samples to a suitable heat treatment and to investigate the resulting possible recovery of their superconductivity by employing various experimental techniques, viz. XRD, oxygen content estimation, electrical and magnetic studies, and surface morphological studies through SEM. The information so gathered could be utilised for visualising the mechanism involved in the process of recovery.
2. Experimental The degradation of YBa 2 Cu 3 O 7y d Žprepared by solid state reaction technique. with deionized water for 6 h, 12 h, 18 h, 24 h, 38 h, 48 h has been reported elsewhere w13,14x. The degraded samples were simultaneously sintered and annealed in a furnace at 9508C for 12 h in oxygen with a flow rate of 1 mlrmin. They were slowly cooled down to 7508C, maintained at this temperature for 6 h, further cooled down to 5008C, at that temperature for 24 h, and finally cooled down to room temperature. The oxygen flow was maintained until the temperature dropped to 2008C. The initial high temperature Ž) 9008C. was chosen to allow the secondary phases formed during degradation to be thermally dissoci-
ated. However, for the sake of comparison the samples treated for longest time, viz. 48 h were subjected to three different types of heat treatment carried out in oxygen atmosphere: Ži. annealing of the sample as such, Žii. calcination and annealing of the sample as such, and Žiii. calcination and annealing of pulverised and palletised samples. The samples were subjected to X-ray diffraction studies on a PHILIPS X-ray diffractometer ŽModel PW 1130r00., using CuK a radiation. The 2 u values were taken from 208 to 708 with an approximate error of 0.018. The diffractograms were employed to compute the unit cell dimensions by Cohen’s method Ža least-squares refinement technique to minimise the random errors in estimating the interplanar spacing ‘d’ arising from absorption, eccentricity and beam divergence w22x.. The resistance versus temperature measurements on the samples were performed by the conventional four probe technique, using KEITHLEY instruments, and the magnetic studies were performed on a Lakeshore A.C. Susceptometer ŽModel 7000., using field of 2 Arm and ac frequency of 111.1 Hz. Both the real and imaginary parts of the susceptibility were studied. The oxygen content of the samples was determined both by iodometric titration method and from the knowledge of the unit cell c-dimension, ˚ . w23x. using the relation 7 y d s 67.108–5.161 c ŽA The morphological studies of the samples were carried out on a scanning electron microscope ŽModel JEOL-JSM 840.. A thin gold film was sputtered on the surface using an ion-sputter before employing the samples for surface investigations.
3. Results and discussion As mentioned earlier, the heat treatment given to the degraded samples was performed at high temperature to promote decomposition of various secondary products, e.g., hydroxides, oxides and carbonates, accumulated on the hydroxylated YBCO samples. It is conjectured that some of the secondary products possibly formed during degradation, viz. BaŽOH. 2 , BaCO 3 , YŽOH. 3 , Y2 ŽCO. 3 , get thermally dissociated to yield the starting stoichiometric products, e.g., Y2 O 3 and BaO. Either single species such as BaO or complex species such as Y2 BaCuO5 then react to
S. Rekhi et al.r Physica C 307 (1998) 51–60
form the YBCO phase. The annealing of sample in oxygen atmosphere helps the so formed YBCO to attain an optimum value of the oxygen content. It also enables the unaltered material already existing in the degraded sample not to lose its oxygen at elevated temperatures. It was reported earlier w13x that after degradation the samples found to be covered with a white layer
53
of certain amorphousrcrystalline secondary phases. The annealing of the samples in the present work made this white layer to disappear, leaving blackcoloured samples behind, indicating the transformation of amorphousrcrystalline secondary phases into YBCO phase. Fig. 1 depicts the X-ray diffractograms of both untreated and the oxygen-annealed samples. The pattern A pertaining to the untreated
Fig. 1. X-ray diffractograms of an untreated sample and the samples degraded for various time durations and then oxygenated wA: untreated; B: 6 hŽo.; C: 12 hŽo.; D: 18 hŽo.; E: 24 hŽo.; F: 36 hŽo.; G: 48 hŽo.x.
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Table 1 Structural parameters for the degraded and subsequently annealed samples Samples
Fresh 6 hŽo. 12 hŽo. 18 hŽo. 24 hŽo. 36 hŽo. 48 hŽo.
˚. Lattice parameters ŽA
Orthorhombicity
a
b
c
=10y3
Cell diagonal ˚. ŽA
Cell volume ˚. ŽA
3.816Ž7. 3.814Ž4. 3.811Ž5. 3.811Ž3. 3.812Ž8. 3.811Ž5. 3.814Ž5.
3.879Ž6. 3.881Ž8. 3.875Ž5. 3.872Ž7. 3.880Ž6. 3.882Ž3. 3.878Ž2.
11.685Ž9. 11.684Ž8. 11.680Ž8. 11.690Ž3. 11.689Ž9. 11.699Ž9. 11.690Ž9.
16.374 17.414 16.654 15.879 17.680 18.458 16.641
12.889Ž12. 12.888Ž11. 12.882Ž10. 12.890Ž06. 12.892Ž07. 12.901Ž11. 12.893Ž10.
172.50Ž7. 172.91Ž6. 172.45Ž6. 172.50Ž5. 172.77Ž8. 173.08Ž5. 172.88Ž5.
sample manifests the existence of single orthorhombic YBCO phase. The patterns B to E pertaining to the degraded Ž6 h to 24 h. and subsequently oxygen-annealed samples reveal relatively small contents of the secondary phases. In contrast, the patterns F and G, pertaining to 36 h and 48 h degraded and oxygen-annealed samples show much more prominent peaks of the secondary phases Že.g., the peaks of Y2 BaCuO5 and BaCO 3 . indicating a much greater presence of these phases in the samples. The unit cell dimensions calculated from these diffractograms are listed in Table 1. The variation in the values is found to be uneven as compared to that for the degraded Žbut not annealed. samples, in which a continuous decrease in the b-dimension and a continuous increase in c-dimension were observed w13x.
The variations in the values of orthorhombicity, unit cell diagonal and cell volume, calculated from the cell dimensions, are also listed in Table 1. They are also seen to be uneven. The orthorhombicities for the 36 hŽo. and 48 hŽo. samples are notably found to be more than that for the fresh samples The uneven variation in the structural parameters implies that the material formed after oxygen annealing possibly comprises, besides YBCO, some additional phases of YBCO, e.g., oxygen-deficient andror barium-deficient YBa 2 " x Cu 3 O 7y d andror yttrium deficient Ba 2Y1 " y Cu 3 O 7y d , andror copper-deficient YBa 2 Cu 3 " zO 7y d . Such additional phases may form both during the loss of various constituents of YBCO and during the partial transformation of the secondary products into YBCO. As mentioned earlier, the 48 h treated samples were subjected to two more types of treatment, viz. pulverisation and calcination, besides the usual types of oxygen annealing. Fig. 2 shows the XRD of a 48 h degraded, calcinated and oxygen-annealed sample
Fig. 2. X-ray diffractogram of a 48 h treated, calcinated and oxygen-annealed sample w48 hŽcqo.x.
Fig. 3. X-ray diffractogram of a 48 h treated, pulverised, repelletised, calcinated and oxygen-annealed sample w48 hŽpqcqo.x.
S. Rekhi et al.r Physica C 307 (1998) 51–60
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Table 2 Structural parameter of the fresh, the degraded and subsequently annealed samples Samples
˚. Lattice parameters ŽA a b c
Orthorhombicity =10y3
Fresh 48 h 48 hŽo. 48 hŽcqo. 48Žpqcqo.
3.824Ž1. 3.822Ž4. 3.814Ž5. 3.822Ž6. 3.824Ž1.
17.369 12.739 16.641 15.834 16.083
3.891Ž3. 3.871Ž2. 3.878Ž2. 3.883Ž7. 3.886Ž4.
11.678Ž5. 11.722Ž5. 11.690Ž9. 11.693Ž4. 11.691Ž3.
w48 hŽc q o.x. The peaks due to the secondary phases still exist, although with the reduced intensity Žcf. Fig. 1, pattern G.. It follows that the calcination of the sample does not eliminate the secondary phases but only reduces their content; like the 48 hŽo. samples, the 48 hŽc q o. samples may also consist of additional YBCO phases, along with the secondary phases. Fig. 3 depicts the XRD of a 48 h degraded, pulverised, calcinated and oxygen annealed sample w48 hŽp q c q o.x. The secondary phases are now found to be eliminated altogether and the orthorhombic YBCO phase is found to have recovered. Table 2 lists the unit cell dimensions for the various samples. Maximum recovery is noted for the 48 hŽp q c q o. sample. The changes observed in the unit cell dimensions of the 48 hŽc q o. samples are attributed to the calcination of the samples, which leads to the loss of carbonates and solid state reaction between various constituents to yield YBCO. The pulverisation of the samples further facilitates such a solid state reaction, through improved mixing of the constituents, which accounts for the observed further recovery in the values of the lattice parameters for 48 hŽp q c q o. samples, such that they are now found to be very
Fig. 4. Resistance versus temperature characteristics for various degraded and subsequently oxygen-annealed samples.
close to the lattice parameters of the fresh sample. A small difference between the two sets of values may arise from the partial loss of certain constituent elements during degradation, leading to a slight change in the stoichiometry of the compound. Earlier, for the degraded Žbut not annealed. samples the value of the oxygen content was found to decrease continuously with increase in the time of reaction w14x, but in the present case the oxygen content of the annealed samples was found to increase Ždetermined by iodometric titration as well as from the knowledge of unit cell c-dimension.. Table 3 lists the values of the oxygen content and the copper valence state calculated therefrom. The values determined from the XRD data are generally higher than those determined from the other technique by 1.5%. Prima facie, the values listed in the table show a somewhat irregular variation. However,
Table 3 Oxygen content and copper valence state of degraded and subsequently annealed samples Samples
c-parameter ˚. ŽA
Oxygen content
Copper valence
From c-values
Iodometrically
From c-values
Iodometrically
Fresh 6 hŽo. 12 hŽo. 18 hŽo. 24 hŽo. 36 hŽo. 48 hŽo.
11.685Ž9. 11.684Ž8. 11.680Ž8. 11.690Ž3. 11.689Ž3. 11.699Ž9. 11.690Ž9.
6.80Ž4. 6.81Ž4. 6.83Ž4. 6.77Ž1. 6.78Ž1. 6.73Ž4. 6.77Ž4.
6.918Ž2. 6.898Ž1. 6.942Ž1. 6.850Ž3. 6.881Ž2. 6.803Ž4. 6.875Ž2.
2.201 2.204 2.220 2.184 2.187 2.152 2.184
2.278 2.265 2.294 2.233 2.254 2.202 2.250
S. Rekhi et al.r Physica C 307 (1998) 51–60
56
Table 4 Electrical and magnetic parameters for the degraded and subsequently annealed samples Samples
Fresh 6 hŽo. 12 hŽo. 18 hŽo. 24 hŽo. 36 hŽo. 48 hŽo. a
x –T studies
R–T studies Tc Ž R s 0. ŽK.
Tc Žonset. ŽK.
DTc ŽK.
Tc Žonset. ŽK.
Phase fraction Ž%.
92.3 87.0 89.8 87.0 89.7 89.0
95.0 97.2 95.0 95.0 107.0 98.5 96.0
2.7 10.3 5.2 8.0 17.3 9.5 –
92.5 92.0 92.5 92.0 93.0 93.0 93.0
100 70 80 64 73 47 45
a
Residual resistance of 2 m V at 77 K.
a closer look reveals that within the computed error of determination, the oxygen content shows a downward trend as the degradation time increases.
The resistance versus temperature characteristics of the samples are depicted in Fig. 4. The values of Tc Žonset. vary within a range of 3.5 K, except for
X
Fig. 5. The plot of real part of susceptibility, x , as a function of temperature.
S. Rekhi et al.r Physica C 307 (1998) 51–60
the 24 hŽo. sample, which has an apparent Tc Žonset. as 107 K and an exceptionally high value DTc s 17.3 K. The value of Tc Ž R s 0. lies between 87 K and 89.8 K ŽTable 4.. It was reported earlier w13x that the values of Tc Ž R s 0. for the samples degraded up to 18 h showed a progressive reduction with the time of degradation and the 24 h treated samples showed a semiconducting behaviour with a transition and a residual resistance of 1200 m V at 77 K. In the present case of recovered samples, except for the 48 hŽo. samples, the Tc Ž R s 0. values for the samples vary unevenly. The exceptional 48 hŽo. sample exhibits a slight semiconducting behaviour, up to 50 m V at nearly 150 K, followed by as many as four transitions, and finally a residual resistance of 2 m V at 77 K. The transition width of all the samples is found to be appreciably high, which can be attributed to an appreciable deviation in stoichiometry of the material. Resistive tailing observed at the end of the transition may be indicative of weak intergrain
57
linksrpresence of polyphases of YBCO in the material. The results of R–T measurements were further substantiated by the magnetic measurements. Fig. 5 depicts the plots of real part of susceptibility Ž x X versus T . for the recovered samples. The superconducting phase fraction of the samples Žat ; 75 K. is found to have an uneven variation ŽTable 4. and its values are less than that for the fresh sample. The undulations seen in the transition part of the curve imply that the transition occurs in steps, possibly due to the presence of one or more than one additional phases Žpolyphases. in the samples. It seems that during the oxygen annealing treatment of the samples the initially present tetragonal and other secondary phases, after conversion into YBCO, are not able to acquire the oxygen content Žcorresponding to that for the fresh sample. and form one or more than one oxygen-deficient phases. It is also possible that one or more barium-, yttrium-, copper-deficient phases are also formed w24,25x. The Tc Žonset. values
X
Fig. 6. The plot of imaginary part of susceptibility, x , as a function of temperature.
58
S. Rekhi et al.r Physica C 307 (1998) 51–60
show a slight variation Žbetween 92 K and 93 K.. The Tc Ž R s 0. could not be estimated for any sample as the curves did not saturate even up to nearly 75 K. However, the steepness of the curves appreciably reduces, implying a high value of transition width for the samples. Fig. 6 depicts x Y Žimaginary part of susceptibility. peaks for the samples. The peaks for the degraded and subsequently oxygen-annealed samples are seen to be suppressed and generally broader than the corresponding peaks for the fresh sample. The broadening of the peaks implies that after the heat treatment of the degraded material the grains become finer and the material acquires a high degree of
porosity which will render the intergranular couplings weak. This is further substantiated through surface morphology studies ŽSEM., described in the following. Scanning electron microscopy was employed to examine the surface morphology of the basal surface of the samples after oxygen annealing. The micrograph 7a, pertaining to the fresh samples shows exclusive presence of YBCO grains on the surface of the sample. The surfaces of the degraded and subsequently annealed samples manifest a high degree of porosity, which could possibly be attributed to the evolution of water vapours and other gases during desorption and disintegration of waterrhydroxidesr
Fig. 7. Ža. A SEM micrograph of the surface of a fresh sample, depicting exclusive presence of the YBCO grains Ž=3500.. Žb. A SEM micrograph of a 12 hŽo. sample Ž=5000.. Žc. A SEM micrograph of a 36 hŽo. sample Ž=5000.. Žd. A SEM micrograph of a 48 hŽo. sample. The appearance of a large number of randomly scattered small round spot possibly indicate the formation of a secondary phase Ž=4500..
S. Rekhi et al.r Physica C 307 (1998) 51–60
oxides or carbonates wsee micrographs 7b and 7c for 12 hŽo. and 36 hŽo. samples, respectivelyx. Unlike the degraded Žbut not annealed. samples w14x, the presence of any secondary phases is hardly indicated in the micrographs except for the 48 hŽo. samples ŽFig. 7d.. In all the annealed samples the YBCO grains are seen to be coagulated to form clusters, possibly resulting from an initial decomposition followed by a solid state reaction of the resulting products.
59
for their assistance in X-ray diffractometry and microscopy studies, respectively. Thanks are also due to Dr. S. K. Agarwal ŽNPL, Delhi. for his kind help in susceptibility measurements. This work has been carried out under a research project financed by Department of Science and Technology, Govt. of India.
References 4. Conclusions The main conclusions drawn from the above study are as follows. The results obtained from XRD analysis, oxygen content estimation, electrical and magnetic measurements, and surface morphology study are in mutual agreement. They indicate the formation of some additional phases of YBCO, along with the decomposition of the secondary phases formed during degradation. The R–T and x –T measurements clearly indicate the presence of the additional phases while the X-ray diffractometry study indicates the absence of impurity phases for less degraded and oxygen-annealed samples. The impurities like Y2 BaCuO5 and BaCO 3 are prominently present in prolonged degraded and oxygen-annealed samples. The recovery of superconductivity is also indicated through a marked increase in the values of oxygen content of heat treated samples, as compared to the degraded samples. A high degree of porosity along with coagulation of grains has been noticed, possibly arising from the desorption of water vapour and other gases during the disintegration of impurities formed during degradation. No impurities or surface alterations are noticed, as found on degraded sample’s surface. A pulverised, calcinated and oxygen annealed sample has shown marked recovery of superconductivity.
Acknowledgements The authors are grateful to Mr. P.C. Padmakishan ŽDept. of Geology, D.U.., Dr. Mehra ŽUSIC, D.U..
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