Physica B 165&166 (1990) 1499-1500North-Holland

ANNEALING EFFECTS ON GROWTH OF HIGH T c PHASE IN BiSrCaCuO SCREEN PRINTEDFILM

Neeraj KHARE, Sangeeta CHAUDHRY, A.K. GUPTA, V.S. TOMAR, V.N. OJHANational Physical Laboratory, Dr K.S. Krishnan Road, New Delhi, 110012, India

Annealing effects of two step heat treatment have been studied on BiSrCaCuO films made by screenprinting. The volume fraction of high Tc phase is sensitive to the temperature of first step. The methodhas proved effective in obtaining large volume fraction of high Tc phase.

1 Introduction

Fig.!. Normalised resistance vs temperature curves for,mples 1 to 3.

300250100 150 200

TEMPERATURE (K)

0.0f---~­

50

1.0s....~ 08

~enl:! 06o....<J)

:J 0.4<t

~z 0.2

2 Experimental

Although screen printing techniques basically yield thick films,however it has been found effective in attaining a large vol­ume fraction of high Tc phase in BiSrCaCuO superconductorsby subjecting to proper annealing treatment [1]. This paperdescribes the study of the effect of annealing on the growthof high Tc phase in screen printed films of BiSrCaCuO.

High purity powders of BiSrCaCuO are made by mixingBi203 , SrC03 , CaC03 and CuO in atomic ratio of 1112. Themixture is thoroughly ground and calcined twice at 820°Cfor 12 hours, and later cooled slowly to room temperature.The resultant compound is pulverised to a fine powder andpassed through 350 mesh screen. The powder is subsequentlyconverted to a paste by thoroughly mixing in cyclohexanolfor screen printing of the films on (100) MgO substrates. Thefilms are subjected to two step heat treatment. In the firststep the films are heated to a temperature (T1) ranging be­tween 870 and 900°C for 45 minutes followed by cooiing at arate of 2°C per minute up to 864°C. In the second step, thefilms are maintained at a temperatre (T2) equal to 864°C for80 hours followed by cooling at a rate of 2°C per minute.

3 Results and discussion

The R-T curve for representative samples 1 to 3 are givenin Fig.!. Table 1 summarises the effects of T1 on samples 1to 6. Tc offset or Tc(O) are found to vary from 71 to lOOK,The XRD spectra for samples 1 and 3 are illustrated in Fig.2with peaks indexed as' given there. The volume fraction ofhigh Tc phase is defined by

1= (002)HI1(002)H +1(002)£ (1)

sample no. Tl(°C) Tc(O)(K) 1(%)1 874 82 602 878 88 593 880 100 744 885 85 385 895 77 66 898 71 0

Table 1: Effect of T1 on growth of high Tc phase.

where 1(002)H and 1(002)L represent intensities of 002 peakfor high and low Tc phases. It represents the volume fractionpercent of high Tc phase present in the sample. Fig.3 shows

1990 - Elsevier Science Publishers B.V. (North-Holland)

1500 N. Khare, S. Chaudhry, A.K. Gupta, V.S. Tomar, V.N. Ojha

o 0870 875 88J 885 890 895 900

ANNEAUNG TEMPERATURE ('I;)

100

80

60

"! 401;

o-U

20

,--,-----jp<;:-------;,--.-----r---,IOO

l80 8

x....60 §

,,-40~!

ON-0-0

20 ::

Fig.2. XRD patterns of sample 1 and 3 (o-high Tc phase,o-Iow Tcphase, l'I-lowest Tc phase, O-impurities).

the variation of I and Tc(O) of the films with T1. Both pa­rameters vary with T1 in similar fashion with maxima for T1being 880°C for Tc (0)=100K and volume fraction being 74%.The contribution of T1 in annealing of BiSrCaCuO films isto impart partial melting of the sample in the first step itself.High Tc phases never crystallises in the first instance. Partialmelting of 1112 composition promotes growth oflow Tc phasealong with a small fraction of calcium copper oxide. High Tc

phase crystallises eutectically by conversion of low Tc phaseduring long annealing at T2. It also promotes diffusion of Caand Cu in the lattice of (2212) low Tc phase. The growth ki­netics depend intimately on the value of T1. If melting doesnot occur in the first step, there would be no crystallisationand thus the amount of the high Tc phase grown is also negli­gibly small even after long annealing in the second step. Forannealing below 870°C where no melting occurs, films showa drop in resistance but show no zero resistivity. XRD ofsuch films shows growth of unoriented 2212 phase. It is onlyafter the partial melting that growth of the high Tc phasetakes place. T1 of 880°C is the optimum temperature neces­sary for melting of the film which give rise to the growth ofmaximum volume fraction of high Tc phase during annealingat T2. Fig.4 shows the scanning electron micrograph of sam­ple 3 for 880°C where large needle (platelets) shaped crystalsare clearly seen. This also suggests that at 880°C, sufficientmelting occurs which gives rise to the growth of the largergrains and also enhanced volume fraction of high Tc phaseduring the second step of annealing. Above 880°C, both theTc(O) and the volume fraction of the high Tc phase decline.XRD of such films also show the presence of 2201 phase.Characteristic (002) peak of 2201 phase at 2(} =7.1° becomesmore evident as the temperature is higher than 880°C. It isalso expected that when T1 is higher than 885°C, loss of el­ements takes place which changes the stoichiometry of thefilm. Above 880°C the appearence of 2201 phase and loss ofelement are responsible for the decline in values of Tc (0) andthe volume fraction of the high Tc phase.

Fig.3. Variation ofTc(O) and I with annealing at T1. I(002)Hand I(002)L are intensities of 002 peaks of high and low Tc

phase. I=I(002)Hj[I(002)H+I(002)L].

Fig.4. Electron micrograph of sample 3.

4 Conclusion

Volume fraction of high Tc phase in BiSrCaCuO films havebeen increased by following two step annealing treatment.

REFERENCES[1] S.Chaudhry, Neeraj Khare, A.K. Gupta, K.C. Nagpal,V.N. Ojha, G.S.N. Reddy and V.S. Tomar. Proc. Int. Conf.on Superconductivity, Bangalore, India. (1990).[2] D.Shi, M. Tang, K. Vandervoort and H. Claus, Phy. Rev.B39 (1989) 9091.