"~ °~ Solld State Communications, Vol. 67, No. 4, pp. 359-362, 1988. • 8 - ~ Printed in Great Britain.

0038-1098/88 $3.00 + .00 Pergamon Press plc

STABILITY OF YTTRIUM BASED SUPERCONDUCTORS IN MOIST AIR

L.B. Harris and F.K. Nyang

Department of Applied Physics, University of New South Wales, P.O. Box 1, Kensington, N.S.W. 2033, Australia

(Received 10 May 1988 by C.W. McCombie)

Loss of superconductivity in YBa2Cu3OT~scaused by exposure to moisture varies with the oxygen content of the superconducting phase and with the phase mixture of the specimen, and is found to be associated with a phase transit ion to tetragonal structure. Such behaviour can be explained by rapid diffusion of water molecules along the channels used for oxygen intercalation, leading to eventual reaction of water with barium ions.

1. Introduction

Soon after the i n i t i a l discovery of high- temperature yttrium-based superconducting oxide [ I ] i t was noted that this material could lose i ts superconductivity when exposed to the ambient atmosphere, though observed behaviour was not always consistent since certain specimens could be stored for weeks without apparent change whereas other specimens, under similar conditions, lost their superconductivity overnight. More detailed observation showed that the material was sensitive to moisture, which introduced disorder into the regular lat t ice structure that was characteristic of freshly prepared specimens [2]. When YIBa2Cu30~ (the 123 superconducting phase) was placed in direct contact with water i t was observed to decompose, though such decomposition was slow unless the water was raised in temperature [3].

The decomposition process has been examined by several workers [3-5], and i t is agreed that the i n i t i a l reaction of water is with barium to produce Ba(OH) 2, which then reacts with CO z in the atmosphere to form BaC03. I t is signif icant that water decomposes both the superconducting orthorhombic 123 phase and the non-superconducting tetragonal phase into which the 123 phase trans- forms when i ts oxygen content is reduced [4]. Such an observation suggests a l ink between attack by moisture and the manner in which oxygen enters and leaves the crystal, and further suggests that differences in decomposition of the two phases might account for the dif ferent rates of loss of superconductivity in different spec- imens. I t would appear, however, that the loss of superconductivity is due to total loss of, rather than mere change in, crystal structure. To provide further information on these matters, an investigation was carried out on the degradation of a number of di f ferent ly prepared specimens.

2. Experimental

Mixtures of Y203, BaC03 and CuO, combined to give the 123 phase, were reacted in a i r at 950°C for 6h, reground, and then reacted again for 6h in air at 950°C. After further regrinding the powder was pressed to O.42GPa, sintered in

a i r at 950°C for 6h and, during cooling, was held at 550°C for period t . The sintered pellets were cut into rectangular parallelopipeds for resistance measurement in a 4-probe configuration using si lver paste electrodes, a portion of each pellet being powdered for x-ray di f f ract ion analysis

All specimens exhibited a sharp drop to zero resistance at temperatures close to 9OK, while resistance in the normal state above the transit ion varied according to the oxygen equilibrium established by holding time t. Increasing period t decreased normal res is t i v i t y and, at the same time, increased the diamagnetic susceptibi l i ty measured below the transit ion, as already reported[6]. Increasing t also increased the intensity of the x-ray di f f ract ion lines associated with the 123 phase, though the temperature and width of the res is t i v i t y transit ion showed no systematic variation.

To provide quantitative information on the perception that deterioration by the atmosphere was less apparent in specimens that were good conductors and good superconductors, a comparison was made between specimens for which t = lh and t = 32h, the former having a normal res is t i v i t y on order of magnitude higher than the res is t i v i t y of the lat ter . The specimens, each suspended over a tray of water in a sealed enclosure, were exposed to saturated water vapour at room temperature, specimen res is t i v i t y being sampled at periodic intervals. Over any period of 24h the temperature oscillated between approximately 16 ° and 24°C. Such observations were later extended to other specimens.

3. Results

The high value of normal res is t i v i t y obtained when time t was limited to lh was found to increase even further on exposure to moisture, as shown in Figure 1, where p~ is room temperature resist- i v i t y and p_ is the ~nset res is t i v i t y at the start of th~ sharp downturn of the transit ion. Resistivity values are seen to increase by a factor of almost three after 48h exposure, whereas when t was extended to 32h the res is t i v i t y showed l i t t l e variation even after 15 days exposure. In Fig. 2 there is an increase after 24h that appears to be genuine, but no subsequent change.

359

360 STABILITY OF YTTRIUM BASED SUPERCONDUCTORS IN MOIST AIR Vol. 67, No. 4

The increase in resistance shown in Fig.1 evantual ly resulted in an incomplete t rans i t ion in which resistance fa i l ed to drop to zero, the r e s i s t i v i t y at temperatures below such a t rans i t ion observed a f te r 48h exposure to moisture being 2.4 m~cm, which is higher than the onset r e s i s t i v i t y in Fig. 2. Absence of a state of zero resistance means that the continuous super- conducting path has been los t , but i t was also found to be accompanied by a change of greater s igni f icance in the x-ray spectrum. Powder d i f f r ac t i on analysis of f reshly prepared specimens for which t = lh and t = 32h showed the presence of the 123 orthorhombic phase in both samples, the only di f ference being that l ine in tens i ty was stronger for the larger value of t . The powder was then subjected to the same moist environment as the bulk specimens, and a f te r 15 days exposure the x-ray spectrum when t = 32h showed an almost unchanged orthorhombic pattern. In the case of powder for which t = lh, however, the re la t i ve in tens i ty of the two prominent l ines in the range 32°<2@<33 ° was reversed, as shown in Fig. 3, such reversal being well-known as a signature for the presence of the tetragonal phase [7] . Many superconducting specimens have been prepared in this laboratory, including some that were furnace-cooled to room temperature without any pause at 550°C, but the only occasion on which the tetragonal phase was observed was when superconductivity was los t by exposure to moisture.

Exposure to humid a i r appeared to produce a small but systematic increase in t rans i t ion temperature, which in Fig. 4 is seen to increase from 89K to 95.5K over 15 days. This trend must eventual ly disappear as the specimen begins to decompose, and th is may have happened with the more rap id ly degraded specimen of Fig. i , since the t rans i t ion temperature went from 93 to 97K over 24h and f e l l back to 93K a f te r a fur ther 24h caused loss of the zero resistance state.

75

E 50

E

25

Fig. 1.

m

0 24 /.8 Time (h)

Ambient r e s i s t i v i t y Pa and onset r e s i s t i v i t y Pn versus time of exposure to moisture f~r specimen for which t = lh. Specimen fa i l ed to show zero resistance a f te r 48h.

3

E v

o_ 1

- -O

& o O h

Po X

0 I I

5 10 Time (days)

X

I

15

Fig. 2. Ambient r e s i s t i v i t y p. and onset Pn versus time of exposure to moistur~ for specimen for which t = 32h.

I t has been observed in this laboratory that poorly superconducting specimens often have a s l i g h t l y higher value of T c as well as much higher resistance.

4. 235 specimens

In addit ion to a di f ference in s t a b i l i t y determined by oxygen content of specimens, there is a l i ke l ihood that multiphase specimens w i l l lose the i r superconductivity more rapid ly than single phase ones. This poss ib i l i t y was tested on the 235 (Y2Ba3CusO,) composition, which has been studied before[8~ and which, with a super- conduct iv i ty s imi lar to that of the 123 phase, has the advantage of being harder and more dense than the 123 phase but the disadvantage of containing other phases. X-ray d i f f r ac t i on spectra of our own 235 specimens showed that they could also contain CuO and Y2BaCuOs.

A comparison was made between specimens annealed in a i r and in oxygen when holding time at 550°C was kept at t = 12h. As before, the inf luence of moisture was obtained by determining the time taken for each specimen to lose i ts state of zero resistance below the t r a i ns i t i on temperature. For a high resistance specimen (p~ = 24m~cm, p~ = 15m~cm) this time was 24h, wh~lefor a low ~esistance specimen (Pa = 2.gmqcm, Pn = 1.3m~cm) the period was 12 days. These tTmes are shorter than those for 123 specimens of comparable r e s i s t i v i t y , such as shown in Figs. 1 & 2, and are thus part of the evidence that the s t a b i l i t y of multiphase specimens is i n f e r i o r to that of single-phase superconductors. I t is thus possible to account for the varying a b i l i t y of d i f fe ren t specimens to res is t atmospheric degradation. Good qua l i t y single-phase 123 specimens are most stable, while the more rapid

Vol, 67, No. 4 STABILITY OF YTTRIUM BASED SUPERCONDUCTORS IN MOIST AIR 361

(b)

>, . m

E e e -

0 20

100!

95 A

~_~

9O

I I I I 30 40

(a)

B

o o

I I I

0 5 10 15 Time (doys)

0 20 30 40

ze

Fig. 3. X-ray di f f ract ion pattern of specimen used for Fig. I , showing intensity reversal of (013), (110) lines at 2@ : 32.5 ° and 32.7°: (a) freshly prepared, (b) 48h exposure to moisture.

rate of degradation in other specimens is determined by phase admixture and by the degree of oxygen non-stoichiometry in the super- conducting 123 phase.

5. Discuss ion

The fact that the loss of superconductivity caused by moisture is accompanied by the tetragonal phase change that is otherwise normally attributed to oxygen intercalation suggests that the mechanism of the change is similar for the two different species. Ideal oxygen content for superconductivity has a formula value of 07 , at which composition there is one oxygen vacahcy per unit cell associated with the Cu-0 a-b planes lying between the two adjacent Ba-0 planes [9]. As shown in Fig. 5

Fig. 4. Variation of c r i t i ca l temperature with time of exposure to moisture ,~r specimen used to obtain Fig. 2.

these vacancies are located at (½ 0 0) coordinates, while oxygens remain at (0 ½ 0) sites. These oxygens move out of the lat t ice when the ortho- hombic phase becomes tetragonal, the phase being tetragonal from ? to 0~, in which state there is a disordered ~ overall s ta t i s t i ca l l y symmet- r ica l ) distr ibution of oxygens over the (½ 0 0) and (0 ½ 0) sites in the Cu-0 planes. A tetragonal phase in the presence of moisture becomes possible when water molecules, oxygens and vacancies are symmetrically distributed over the same sites. This requires water molecules to transverse the same paths as the intercalating oxygen ions, which is feasible since a water molecule is basically the same as an oxygen ion, but with two added protons that give i t a f lex ib le polar character.

The water molecules appear to f i t snugly in the crystal la t t ice since they produce only a modest increase in T~ (Fig. 4), whereas ambient ~as that enters the 123 lat t ice has been reported L10] to exert internal pressure that produces a marked increase in c r i t i ca l temperature. I t is thus expected that cell parameters of the tetra- gonal phase in the presence of moisture w i l l d i f fer but l i t t l e from the cell parameters of the moisture-free tetragonal phase.

Water molecules in Cu-0 planes find them- selves close to both barium and oxygen ions (Fig.5) and thus the ingredients for the formation of barium hydroxide are al l assembled close together. The suscept ibi l i ty of the 123 compound to moisture is, in the f i r s t instance, to be attributed to the a f f i n i t y of barium ion compounds for water, and for this reason decomposition takes place by disruption within the crystal l ine lat t ice [2] and not merely by dissolution over the interfaces of the granular 123 phase.

Variation of oxygen content in the orthor- hombic phase is accompanied by a variation in lat t ice cell parameters a and b. The values

362 STABILITY OF YTTRIUM BASED SUPERCONDUCTORS IN MOIST AIR Vol. 67, No. 4

0 Cu 0

I J

Ba~-O Cu-O

i>

\

Ba-O

Fig. 5. The Cu-O plane at an oxygen content of 07(orthorhombic phase) containing vacancies V. At oxygen content of 06 (tetragonal phase) a l l oxygen sites in this phase are vacant.

obtained in this work for t = lh, corresponding to Fig. 1, were a = 3.853A, b = 3.892A, while for t = 32h, corresponding to Fig.2, a = 3.828A, b = 3.896A. From these values i t is deduced [9] that oxygen content approaches 07 when t = 32h but is below 06 ~ when t = lh. The measurements made in tMis work thus show that the larger number of vacancies associated with a lower oxygen content allows easier access of water and more rapid degradation of the specimen.

The observed rate of deter iorat ion of specimens requires a high rate of water d i f fus ion. In order to obtain an estimate of the d i f fus ion coef f i c ien t of water in the 123 compound, an order of magnitude analysis may be applied to

values quoted for the loss of x-ray in tens i ty within a sintered disk penetrated by water [4] . X-ray in tens i ty is proport ional to the percentage volume of the appropriate phase in a mixture, while the d i f fus ion length x of la species is given approximately by x = (Dt) 2, where D is d i f fus ion coe f f i c ien t , t is d i f fus ion time, and the average distance t rave l led by a species d i f fus ing from a constant composition source is obtained where d i f fus ing composition C is 16% of surface composition c n. Figures quoted indicate that when the disk wa~ exposed to high humidity for 90 mins. at 85°C, the volume of crystal lost by water degradation was 23% at 320um penetration. From these f igures a d i f fus ion coe f f i c ien t D for water ma~ be estimated as approximately 3 x I0" m2s - I at 85°C, so that D at room temp- erature is of the order of i01~n2~ I . This value is comparable with the d i f fus ion coe f f i c ien t of water in the a l ka l i halides [ i i ] and, though high, is consistant with jumps of defect species between l a t t i c e si tes.

There is s im i l a r i t y between penetration of water into the 123 phase, followed by formation of barium compound, and d i f fus ion of water along grain boundaries in NaCI b icrysta ls , followed by formation of calcium prec ip i ta te [ I i ] . Penetration is rapid in both and is accompanied by reaction with a deliquescent a lka l ine earth ion. In the case of NaCI grain boundaries, where a small amount of calcium was present as segregated residual impurity, the process was revers ib le, a dry environment leading to removal of water and shrinkage of precip i tates, In the case of the 123 superconductor, where barium content is large and extensive, i t is doubtful whether r e v e r s i b i l i t y , with restorat ion of superconductivity but without substantial loss of crystal structure, is possible. What is clear is that the presence of a regular and extensive array of deliquescent barium ions, f ree ly accessible by continuous open channels, ensures that the 123 phase w i l l always be susceptible to moisture. Increase of oxygen content can slow degradation but cannot prevent ult imate decomposition as long as a lka l ine earth ions remain an essential component of the superconducting structure.

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