ON Ba2REHf05.5 (RE = La and Pr) SUBSTRATES

Transcription

1 CHAPTER 5 SUPERCONDUCTING Bi-2223 THICK FILMS (Tc(0) = 110 K) ON Ba2REHf05.5 (RE = La and Pr) SUBSTRATES 5.1 Introduction The discovery of the superconducting phenomenon in the bismuth- strontium-calcium-copper-oxide (BSCCO) compound has generated an enormous amount of activities in the field of high temperature superconductivity. Three phases have been identiikd in this system namely, Bi2Sr2Cu10x (Bi-2201) with a T,(O) of 22 K, Bi2Sr2CalCu20x having a T,(O) of 85 K (the low T, phase or Bi-2212) and the Bi2Sr2Ca2Cu3Qx showing a superconducting transition temperature of 110 K (the high T, phase or Bi-2223) (1). However, the synthesis of the high Tc phase with the nominal composition Bi2Sr2Ca2Cu30, has been very difficult and now it is established that the partial substitution of bismuth by lead enhances the formation of the Bi-2223 phase (2-8). As in the case of the Y BazCu307.s (YBCO) superconductor, the immediate applications of BSCCO superconductors are mostly in the form of films in microelectronic devices such as superconducting quantum interference devices (SQUID), high Q resonators, sensitive infrared sensors etc. (9-12). The particular advantages of thick film route are simplicity and low cost of process and the ability to apply coating on large areas and on curved surfaces. Similar to the case of YBCO systems, the substrate plays a vital role in the preparation of high quality Bi-cuprate films (1 3-16). The commercially available substrates such as Si, SiOz, A1203, etc, react with BSCCO superconducting fiims at the processing temperature, thereby reducing the transition temperature of the film drastically (17-18). MgO is the most widely reported conventional substrate material used for both YBCO and BSCCO superconductors in microwave applications. However, even on MgO substrate, the BSCCO films developed always contained both the low Tc (Bi- 2212) and high T, (Bi-2223) phases (19-20). From our detailed studies it was

2 found that among the newly developed barium rare earth hafnate compounds, Ra2LaHf05.5 and Ba2PrHf05,5 are chemically non-reacting with Bi-2223 superconductor even under extreme processing conditions. Our studies on the chemical compatibility of Ba2R~HfD5,q5 (RE = La, Pr, Nd and Eu) ceramic compounds with Bi-2223 superconductor and the development of superconducting Bi-2223 thick films by dip-coating on sintered polycrystalline BazLaHfOsms (BLHO) and Ba2PrHf05,5 (BPHO) substrates are presented in this chapter. 5.2 Synthesis of Bi-2223 Superconductor In the present study, single phase bismuth strontium calcium copper oxide superconductor (Bi-2223) powder was prepared from high purity (99.9%) Bi203, PbO, SrC03, CaC03 and CuO by the conventional solid state reaction method (2 1). The initial reactants were weighed in the precise stoichiometric ratio of (BiI,sPbo,5)SrzCa2Cu30, and the precursor powder was thoroughly wet mixed in an agate mortar with acetone as the wetting medium. The mixhue was then dried in an oven and the dried powder was calcined in air at 800 C for 36 h with three intermediate grindings, and cooled slowly at a rate of 2'Clmin. to room temperature. The finely ground calcined powder was pressed in the form of circular discs with dimensions of 13 mm diameter and mrn thickness by applying a pressure of 300 MPa. These discs were then sintered in air at 850 C for 200 h continuously, and the sintered samples were then cooled slowly at a rate of 1 Clmin from the sintering temperature to 800 C, and finally furnace cooled to room temperature. Figure 5.1 shows the XRD pattern of the phase pure Bi-2223 superconductor sintered at 850 C for 200 h. All the peaks in the figure correspond to the Bi-2223 (high T, phase) phase of the BSCCO superconductor. The temperature vs. resistivity curve of a bulk Bi-2223 superconducting sample with a superconducting transition temperature [T,(O)] of 110 K is shown in figure 5.2. Table 5.1 gives the detailed x-ray diffraction data of a sintered Bi-2223 superconductor.

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4 Temperature (K) Fig. 5.2 Temperature-resistivity curve of a sintered Bi-2223 sample 5.3 Chemical Compatibility between BazREHf05m5 and Bi-2223 In order to determine whether the newly developed Ba2REHf05,, ceramic materials Me chemically compatible with the high T, phase of the BSCCO superconductor, the chemical reactivity of Ba2R~Hf05,5 ceramics with Bi-2223 was studied upto a temperature of 850 C. The Ba2R~Hf05,5 compounds and the Bi-2223 powder were mixed thoroughly in a 1:l volume ratio and the composite powder was pressed in the form of circular discs with dimensions of 11 mm diameter and - 2 mm thickness by applying a pressure of 300 MPa. These circular discs were then annealed in air at 850 C for 10 h and slow cooled at a rate of 1 C/min. up to 800 C and then furnace cooled to room temperature. The chemical reactivity of the BazREHf05+5 ceramics with Bi-2223 superconductor at the processing temperature of 850 C was studied by the powder x-ray diffraction technique. The XRD pattern of the Ba2R~Hf05+5-(Bi-2223) composites annealed at 850 C for 10 h is shown in fig.5.3 (B-E). The XRD pattern of the annealed Ba2REHf05.5- (Bi-2223) samples is compared with those of pure Bi-2223 [fig. 5.3(A)] and the respective Ba2REHf05.5 compounds. It is clear from the figure that except in the case of the Ba2LaHf05,5 -(Bi-2223) and Ba2PrHf05,5-(Bi-2223)

5 composites [Fig. 5.3 (B) and (C)] for all other composites decomposition of the Bi-2223 superconducting phase has taken place. In the case of the BafldHfOs.5- (Bi-2223) and BaZEuHf05,5-(Bi-2223) composites [Fig.5.3(D) and (E)] the Bi phase has decomposed into the low T, (Bi-2212) and high T, (Bi-2223) phases. Therefore the BafidHf05,5 and Ba2E~HfU5,5 ceramic materials could be used as suitable substrates for the Bi superconductor. Hence, among the four newly developed BazREHfOs,5 ceramic compounds, Ba2LaHf05,5 and Ba2PrHfOs,5 do not show any chemical reaction with the Bi-2223 superconductor even under extreme processing conditions and hence can be used as substrates for Bi-2223 superconducting films. Fig. 5.3 XRD Patterns of (A) pure Bi-2223, (B) 1 : 1 volume mixture of Bi-2223 and Ba2LaHfD5 5 (C) 1 : I volume mixture of Bi-2223 and Ba2PrHP05,5, (D) I : 1 volume mixture of Si-2223 and BalNdHfOs,5 and (E) 1 : 1 volume mixture of Bi-2223 and BalEuHf05 5 (All annealed at 850 C for 10 h)

6 The effect of BazREHf05,5 ceramic compounds on the superconducting property of Bi-2223 was studied by temperature resistance measurements of the Ba2REHf05,5-(Bi-2223) composites following the standard four probe technique. A Keithley current source model 220 and a Keithley nanovoltmeter model 181 were used for resistance measurements. The temperature of the composite samples was measured by a calibrated copper constantan thermocouple. Figure 5.4 (A-C) shows the temperature-resistivity curves for the Ba2REHfQ5-(Bi-2223) composites containing 20 vol % of Ba2REHf05,5 annealed at 850 C for 10 h. A zero-resistivity superconducting transition temperature [T,(O)] of 110 K was obtained in our temperature-resistance measurements of the Ba2LaHfQ5-(Bi-2223) and Ba2PrHf05,5-(Bi-2223) composites (Figure 5.4 A and B), indicating that the addition of a substantial amount of these ceramic compounds did not have any detrimental effect on the superconducting transition temperature of Bi-2223 even after severe heat treatment. However, a zero resistivity superconducting transition temperature [Tc(0)] of 108 K was obtained for the Ba2EuHf05,5-(Bi-2223) composite. In the case of the Ba2NdHf05,5-(Bi-2223) composite, no superconducting transition temperature was observed up to 77 K. Temperature (K) Fig. 5.4 Temperature-resistivity curves for Bi BazREHtOs.s composites containing 20 volume % of (A) Ba2LaHf05,5, (B) Ba2PrHfOs.s and (C) B~?EUH~O~.~ (all annealed at 850 C for 10 h)

7 5.4 Development of Bi-2223 Thick Films on Ba2REHf05.5 In order to conh the suitability of the BqLaHfQ, (BLHO) md Ba2PrHf05.5 (BPHO) ceramic materials as substrates for the Bi-2223 superconductor, thick films of Bi-2223 superconductor were fabricated on we11 sintered and polished polyctystalline Ba2LaHfD5.5 and Ba2PrHfU5,5 samples by the dip-coating technique followed by melt texturing. The Bi-2223 suspension for dip- coating was prepared by thoroughly mixing fine superconducting powder with isopropyl alcohol or n-butanol and the viscosity of the suspension was controlled by the addition of commercially available fish oil. The polished and cleaned substrates were then dipped in the suspension and dried. This step was repeated till a required thickness is attained. The dip-coated films were then dried in an oven and heated in a programmable hrnace at a rate of S0C/min in air up to 880UC. The films were annealed at 880 C for 2 min and then cooled at a rate of l0c/min to 850T and then annealed at this temperature for 6 h. The films were cooled at a rate of I0C/min to 800 C fiom the annealing temperature of 850 c, and then furnace cooled to room temperature. The heating and cooling schedule follou7ed for the preparation of the superconducting Bi-2223 thick films on polycrystalline Ba2R~HfQ5 substrates is shown in figure 5.5. Processing time (rnin) Fig. 5.5 Heating and cooling schedule adopted for the fabrication of Be Bi-2223 thick films on Ba2REHPD5,5 subssates

8 Fig.5.6 XRD Pattern of dip-coated Bi-2223 thick film on polycrystalline Ba?L~iHf0~,~ substrate (substrate peaks are marked '0') The structure of the Bi-2223 thick films prepared on the BLHO and BPHO substrates by dip-coating were characterized by XRD. The XRD patterns of the Bi-2223 thick film fabricated on the polycrystalline substrates are shown in figure 5.6 and 5.7. It may be noted from the figures that except for the characteristic peaks of the respective substrates, all other peaks are those of the Bi-2223 superconductor. Fig. 5.7 XRD pattern of a dip coated Bi-2223 thick film on polycrystalline Ba2PrHPDS5 substrate (substrate peaks are marked '0')

9 The phase purity of the Bi-2223 superconducting phase was fbrther established by calculating the volume fraction of Bi-2223 from the XRD data of the Bi-2223 thick films given in table 5.2 and table 5.3, using the empirical relation (22) given as where V is the volume fraction of Bi-2223 and Y is the 20 for the peak appearing between and 29.07, caused by the interplay of reflection from the crystal planes 0010 of the Bi phase and 0012 of the Bi-2223 phase. The calculation using the above empirical relation shows that the volume fiaction of Bi-2223 in the dip-coated and melt textured Bi-2223 thick films on the polycrystalline BLHO and BPHO substrates is 100%. SI. No. Table 5.2 X-ray diffraction data of dip-coated Bi-2223 thick film on the sintered polycrystalline Ba2LaHf05,5 substrate hkl * Substrate peak 108

10 Table 5.3 X-ray diffraction data of dip-coated Bi-2223 thick film on the sintered polycrystalline BazPrHfOs substrate SI. No. 2e(deg) la hkl * Substrate peak I OOU OOU * I I u * OOB The superconductivity of the Bi-2223 thick films developed on the polycrystaliine BLHO and BPHO substrates were studied by temperatureresistance measurements using the standard four probe technique. A Keithley current source model 22 1 and a KeithIey nanovoltmeter model 181 were used for the resistance measurements. The temperature of the samples were measured by a calibrated copper-constantan thermocouple with an accuracy of + 2 K. Figures5.8 and 5.9 show the temperature vs resistance curve of the Bi-2223 thick films developed on polycrystalline BLHO and BPHO substrates respectively. The

11 films showed a metallic behavior in the normal state and gave a zero-resistivity transition temperature of 110 K with a superconducting transition width of - 2 K. Fig. 5.8 temperature resistance Curve of a dip-coated Bi-2223 thick film on polycrystalline Ba2LaHfQ5 substrate Temperature (K) Fig. 5.9 Temperature resistance curve of a dip-coated Bi-2223 thick film on polycrystalline Ba2PrHf05,5 substrate The critical current density (Jc) of the superconducting Bi-2223 films was measured on thick films coated on the sintered polycrystalline substrates by the standard four probe method using the 1 pv criterion. The dip-coated Bi-2223 thick films had a critical current density (Jc) of - 3 x lo3 A/CIII~ at 77 K under

12 zero applied magnetic field. The high transition temperature obtained for the superconducting Bi-2223 thick films could be attributed to the chemical nonreactivity of Bi-2223 with these substrates even at the high processing temperature. The dip-coated Bi-2223 thick films had excellent adhesion on the sintered polycrystalline BLHO and BPHO substrates. 5.5 Conclusions In the present study, it was found that among the newly synthesised Ba2R~Hf& ceramic compounds, BalLaHf05.5 and BazPrHf05.5 are non- reacting with Bi-2223 superconductor even at the extreme processing conditions and an addition of 20 vol % of these compounds in the Bi-2223 superconductor did not show any detrimental effect on the superconducting transition temperature of Bi On the other hand, the x-ray diffraction studies of the Ba2R~Hf05,5 (RE = Nd and Eu) - (Bi-2223) composites annealed at 850 C showed that the presence of the ceramic compound has affected the structure of the Bi-2223 compound. The Bi-2212 phase of the BSCCO superconductor was present along with the Bi-2223 phase in the Ba2NdHf05,5-(Bi-2223) and Ba2EuHf05,5-(Bi-2223) composites indicating that these materials can be used as substrate materials for Bi-2212 superconducting films. The suitability of the Ba2LaHf05,5 and Ba2PrHf05.5 compounds as substrates for the Bi-2223 superconductor was confirmed by dip-coating superconducting Bi-2223 thick films on these substrates. The films showed a zero resistivity transition temperature [T,(O)] at 110 K and gave a critical current density [J,(0)] of -3 x lo3 ~ icrn~ at 77 K under zero applied magnetic field. The films have excellent adhesion to the substrates and are highly stable under atmospheric conditions.

13 REFERENCES 1. H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano, (1988), Jpn. J. Appl. Phys., 27, L A.Oota,Y. SasakiandA.Kirihigashi,(1988),Jp~z. J Appl. Phys.,27,1, E. Yanagisawa, D. R. Dietderich, H. Kumakura, K. Togano, H. Maeda and K. Takahashi, (1988), Jpn. AppE. Phys., 27, L N. Murayama, E. Suda, M. Acvano, K. Kani and Y. Torii, (1988), Jpn. J. Appl. Phys., 27, L M. Matsuda, Y. Iwai, M. Takata, M. Ishii, T. Yamashita and H. Koinuma, (1988), Jpn. J. Appl. Phys., 27, L A. Oota, A. Kirihigashi, Y. Sasaki and K. Ohba, (1988), Jpn. J. Appl. Phys., 27, L H. K. Liu, S. X Dou, N. Savvides, J. P Zhou, N. X. Tan, A. J. Bourdillon, M. Kuiz and C. C. Sorsell, (1989), Physica C, 157, B. Zhu, L. Lei, S. L. Yuan, S. B. Tang, W. Wang, G. G. Zhing, W. Y. Guan and J. Q. Zheng, (1989), Physica C, 157, N. McN Alford, T. W. Button and D. Opie, (1991), Supercond Sci Technol. 4, N. McN. Alford, S. J. Penn and T. W. Button, (1997), Supercond. Sci. Technol., 10, C. L. Bohn, J. R.Delayen, U. Balachandran and M. T. Lanagan, (19891, Appl. Phys. Lett., 55, J. Talvacchio, G.R. Wagner and S.1-I. Talisa, ( ), Microwave J, July, H..I. Scheel, M. Berkowski and B. Chabot, (1991), Physica C, 2095, J. M. Phillips, (1 996), J Appl. Phys., 79, R. G. Humphreys, J.S. Satchell, N.G.Chew, J.A.Edwards, S.W.Goodyear, S.E.Blenkinsop, 0.D.Dosser and A.G.Cullis, (1 9901, Supercond. Sci Technol., 3, 38.

14 16. J-Koshy, J.Kurian, J.K.Thomas, Y.P.Yadava amd A.D.Damodaran, (1994), Jpn. J. Appl. Phys., 33, C. T. Cheung and E. Ruekenstein, (1 9891, f. Mater. Res. 4, S. X. Dou, H. K. Liu, S. J. Guo, K. E. Easterling and J. Mikael, (1989), Supercond. Sci. Techno/., 2, W. C. McGinnis and J. J. Briggs, (1992), J Mat. Res., 7, A. Agrawal, R.P. Gupta, W. S. Khokale, K. D. Kundra, P. R. Deshmuleh, M. Singh and P. D. Vyas, (19931, S~percond, Sci. Technol., 6, J. Koshy. K. S. Kumar, Y. P. Yadava and A. D. Damodaran, (1994), JI Mater. Sci. Left., 13, F.H. Chen, H.S. Koo and T,Y. Tseng, (1992), J. Am. Ceram Soc., 75,96.