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insulator and electrical properties
Applied Surface Science 130–132 Ž1998. 866–869
Hetero interfaces of Bi-based superconductorrinsulator and electrical properties Sunao Sugihara ) , Yukio Yutoh, Taka-aki Aoi Department of Materials Science and Ceramic Technology, Shonan Institute of Technology,1-1-25,Tsujido nishikaigan, Fujisawa, 251, Japan Received 17 October 1997; accepted 20 November 1997
Abstract A thick film of the Bi-based superconductor was fabricated onto the oxide polycrystalline substrates such as MgO, ZrO 2 and SrTiO 3 where system will be a new composite. The thickness was about 10 m m. The lowest electrical resistivity of 11 m V cm at 110 K for the isostatic pressed system of the superconductorrMgO, and 10 m V cm at 110 K for the SrTiO 3 substrate. The large grain size at the surfaces was found to be important as well as the dense surfaces. No changes of the critical temperature were found on any substrate. The interfaces between the film and the substrate can be controlled by the pressing process which is important to reduce the electrical resistivity. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Bi-based superconductor; Oxide polycrystalline; Electrical resistivity; Hetero interfaces; Film surface morphology; Lattice distortion
1. Introduction The mesoscopical physical phenomena at the interfaces were theoretically studied on the superconductorrusual conductor or semiconductor w1x and metal superconductor was also discussed by Nishino et al. w2x. These studies are related to the superconducting electron migration towards the semiconductor in the thin film of nm order. Furthermore, the enhancement of Tc was studied on the film of Bi 2 Sr2 Ca ny1Cu nO 2 nq1 prepared by sequential sputter deposition, changing the number of CuO 2 layers for the intergrowth films and they concluded that the Tc of the superstructure can be understood by the charge transfer between the neighbouring units w3x.
In the YBaCuO system, there have been many studied in the last ten years even the composites between YBaCuO thick film and other ceramics such as Ce-TZP w4x, PbŽZr,Ti.O 3 w5x, Al 2 O 3 w6x, semiconductive BaTiO 3 w7x, etc. However, there have been few study of the BiPbSrCaCuO thick film on a polycrystalline oxide. In this study, we aim at preparing the thick film of the Bi-based superconductor on the polycrystalline oxide such as MgO, ZrO 2 and SrTiO 3 ,and at investigating the interfaces between them,which are affected by the process parameters such as pressing, and effect on the electrical property of the superconducting film. 2. Experimental
)
Corresponding author. Tel.: q81-466-34-4111; fax: q81466-36-1594; e-mail: [email protected].
The powder of Bi 1.85 Pb 0.34 Sr1.90 Ca 2.05 Cu 3.05 O x ŽSeimi Chemical. was used for the film, and the
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 1 6 7 - 6
S. Sugihara et al.r Applied Surface Science 130–132 (1998) 866–869
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substrate for the film were polycrystalline MgO, ZrO 2 and SrTiO 3 . The superconducting powder was pressed at 2.94 MPa, then sintered at 1123 K for 24 h in the atmospheric pressure. The pellets were ball-milled in the organic solvent after crushing in a mortar for 30 min, then the slurry was pasted onto each substrate, which system was pressed at 49 MPa and cold isostatic press ŽCIP. at 294 MPa in order to investigate the effect of pressurization on the quality of the film. The film was analyzed with an XRD, and observed with a scanning electron microscope ŽSEM. and electron probe microanalysis ŽEPMA. for surface and cross section of the system. The superconducting properties were measured on electrical resistivity.
3. Results and discussion The surface morphologies of the film were different corresponding to the substrate. As shown in Fig. 1a, the superconducting film on the ZrO 2 indicated the smaller grains as compared to those on the MgO ŽFig. 1b. although both films seemed to be porous. The high density of the film can be obtained by increasing grain size and wettability of the oxide superconductor to the substrate. The sintering temperature of the system plays an important role to increase grain size. Fig. 2a and b, for instances, indicate the different surface morphologies corresponding to the sintering temperatures such as 1093 K and 1123 K, respectively. The film density at 1113 K ŽFig. 2a. seemed to be denser and less porous as compared to those at 1093 K ŽFig. 2b.. Furthermore, the cross-sections of the film revealed the different morphologies by pressing or CIP process as shown in Fig. 3; the interfaces were not smooth, and no grain growths were observed. It seemed that nopressing process film ŽFig. 3a. looked dense. Fig. 3b showed the interfaces of the film pressed in a single mode to become denser film. Furthermore, CIP process made the film thinner by about a half. It seemed to be more in parallel to the substrate as shown in Fig. 3c. Moreover, the grain direction was parallel to the substrate comparatively. The effects of the pressing on the electrical resistivity were shown in Fig. 4 for a single mode press and CIP, and the heat processing temperature was
Fig. 1. SEM photographs of superconducting film on ZnO 2 Ž1-a., and MgO Ž1-b., after heat treatment at 1123 K.
1123 K as well. There appeared a high temperature phase and a low temperature one for both processes with the same critical temperature. However, the electrical resistivity was 35 m V cm at the off-set temperature of a high temperature phase for a single press, meanwhile it was 11 m V cm for the CIPed system at 110 K. Therefore, the CIP process attributed to reduce an electrical resistivity, due to a dense film as mentioned above. The heat processing temperature played also an important role to reduce an electrical resistivity as well as pressing process. The resistivity of the heattreated system at 1123 K on the MgO substrate showed a value of 73 m V cm at 115 K, meanwhile the resistivity was 140 m V cm on the heat-treated
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S. Sugihara et al.r Applied Surface Science 130–132 (1998) 866–869
same system at 1093 K Žwithout pressing for both system.. There were differences of electrical resistivity from the substrate; 10, 35 and 75 m V cm for SrTiO 3 , ZrO 2 and MgO, respectively. It is reasonably suggested that the surface morphologies of the film affect the electrical resistivity. The lowest resistivity was obtained for the system of the film on the SrTiO 3 substrate whose surface had the largest grains and dense morphology among other oxide. In ionic crystals such as the substrates in the present study, single electrons or holes can produce more localized lattice distortions resulting from their electrostatic interaction with neighbor ions. This distortion accompanies as it moves through the lattice. In particular, the interfaces between the film and
Fig. 3. SEM photographs of interfaces between superconducting film and MgO at heat-treatment at 1123 K. Ža. No pressing. Žb. Pressed. Žc. CIPed before heat-treatment.
insulators may produce more lattice distortion and we expected that the electrons or holes can be transferred from the insulator to the superconducting film. Therefore, the interfacial structures joined smoothly are important to reduce the electrical resistivity. The charge will polarize surrounding atoms,which will lower the energy. The approximate formula for the polarization energy given w8x, Fig. 2. SEM photographs of superconducting film on MgO at different heat-treatment temperature. Ža. 1093 K. Žb. 1123 K.
D E s ye 2r Ž 8p´ 0 r . Ž 1 y 1r´r .
Ž 1.
where r is the radius of the orbital, and ´ r the
S. Sugihara et al.r Applied Surface Science 130–132 (1998) 866–869
869
to be affected by the electrons in the substrate through the interfaces where the distortion is mostly existing.
4. Conclusion The thick film of the superconducting BiPbSrCaCuO were studied on the oxide polycrystalline such as ZrO 2 , MgO and SrTiO 3 . There appeared two phases for every substrate and the offset of the high temperature phase was 110 K without differences from that of the oxide superconductor bulk. The CIP process attributed to reduce an electrical resistivity due to a dense film. The electrical resistivities were the lowest in the system of the film on the SrTiO 3 which had the better surface morphology and it may be possible to improve superconducting properties by controlling the film surface morphology and interfaces between the film and the oxide. Fig. 4. Pressing effect on electrical resistivity of the superconducting film on MgO at the heat-treatment of 1123 K. Župper. pressed in a single mode, Žlower. CIPed before heat-treatment.
relative dielectric constant of the solid, and ´ 0 the dielectric permittivity of vacuum Ž8.854 = 10y1 2 Fmy1 .. Since SrTiO 3 possesses the largest ´ r , D E is suggested to be the largest, according to that the reported electronic polarizabilities of ions are 0.094, 0.37 and 0.86 for Mg 2q, Zr 4q and Sr 2q, respectively w9x. The lowest resistivity, discussed above, on the SrTiO 3 substrate whose electronic polarizability was the largest among those oxides. The interaction between the electron and the lattice distortion is known as a polaron, which electron at the interfaces comes from the substrate. Therefore, the electrical properties of the superconducting film is postulated
References w1x P.G. DeGennes, Rev. Mod. Phys. 36 Ž1965. 225. w2x T. Nishino, K. Yamada, U. Kawabe, Phys. Rev. B 33 Ž1986. 2042. w3x T. Hatano, K. Nakamura, in: H. Maeda, K. Togano ŽEds.. Bismuth-based high-temperature superconductors, Marcel Dekker, New York, 1996, p. 545. w4x E. Saiz, M.I. Osendi, P. Miranzo, J. Requena, J.S. Moya, J. Less-Common Metals 164 Ž1990. 458. w5x S. Sugihara, H. Fujitani, J. Eur. Ceram. Soc. 15 Ž1995. 1043. w6x D. Duran, J. Taraj, J.F. Fernandez, C. Moure, Ferroelectrics 128 Ž1992. 231. w7x S. Sugihara, K. Fukushima, J. Jpn. Soc. Powder Powder Met. 40 Ž1993. 1092, Žin Japanese.. w8x P.A. Cox, The Electronic Structure and Chemistry of Solids, Oxford Science Publications, Oxford, 1987, p. 179. w9x C. Kittel, Introduction to Solid State Physics, Wiley, New York, 1956, p. 165.
Hetero interfaces of Bi-based superconductorrinsulator and electrical properties Sunao Sugihara ) , Yukio Yutoh, Taka-aki Aoi Department of Materials Science and Ceramic Technology, Shonan Institute of Technology,1-1-25,Tsujido nishikaigan, Fujisawa, 251, Japan Received 17 October 1997; accepted 20 November 1997
Abstract A thick film of the Bi-based superconductor was fabricated onto the oxide polycrystalline substrates such as MgO, ZrO 2 and SrTiO 3 where system will be a new composite. The thickness was about 10 m m. The lowest electrical resistivity of 11 m V cm at 110 K for the isostatic pressed system of the superconductorrMgO, and 10 m V cm at 110 K for the SrTiO 3 substrate. The large grain size at the surfaces was found to be important as well as the dense surfaces. No changes of the critical temperature were found on any substrate. The interfaces between the film and the substrate can be controlled by the pressing process which is important to reduce the electrical resistivity. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Bi-based superconductor; Oxide polycrystalline; Electrical resistivity; Hetero interfaces; Film surface morphology; Lattice distortion
1. Introduction The mesoscopical physical phenomena at the interfaces were theoretically studied on the superconductorrusual conductor or semiconductor w1x and metal superconductor was also discussed by Nishino et al. w2x. These studies are related to the superconducting electron migration towards the semiconductor in the thin film of nm order. Furthermore, the enhancement of Tc was studied on the film of Bi 2 Sr2 Ca ny1Cu nO 2 nq1 prepared by sequential sputter deposition, changing the number of CuO 2 layers for the intergrowth films and they concluded that the Tc of the superstructure can be understood by the charge transfer between the neighbouring units w3x.
In the YBaCuO system, there have been many studied in the last ten years even the composites between YBaCuO thick film and other ceramics such as Ce-TZP w4x, PbŽZr,Ti.O 3 w5x, Al 2 O 3 w6x, semiconductive BaTiO 3 w7x, etc. However, there have been few study of the BiPbSrCaCuO thick film on a polycrystalline oxide. In this study, we aim at preparing the thick film of the Bi-based superconductor on the polycrystalline oxide such as MgO, ZrO 2 and SrTiO 3 ,and at investigating the interfaces between them,which are affected by the process parameters such as pressing, and effect on the electrical property of the superconducting film. 2. Experimental
)
Corresponding author. Tel.: q81-466-34-4111; fax: q81466-36-1594; e-mail: [email protected].
The powder of Bi 1.85 Pb 0.34 Sr1.90 Ca 2.05 Cu 3.05 O x ŽSeimi Chemical. was used for the film, and the
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 1 6 7 - 6
S. Sugihara et al.r Applied Surface Science 130–132 (1998) 866–869
867
substrate for the film were polycrystalline MgO, ZrO 2 and SrTiO 3 . The superconducting powder was pressed at 2.94 MPa, then sintered at 1123 K for 24 h in the atmospheric pressure. The pellets were ball-milled in the organic solvent after crushing in a mortar for 30 min, then the slurry was pasted onto each substrate, which system was pressed at 49 MPa and cold isostatic press ŽCIP. at 294 MPa in order to investigate the effect of pressurization on the quality of the film. The film was analyzed with an XRD, and observed with a scanning electron microscope ŽSEM. and electron probe microanalysis ŽEPMA. for surface and cross section of the system. The superconducting properties were measured on electrical resistivity.
3. Results and discussion The surface morphologies of the film were different corresponding to the substrate. As shown in Fig. 1a, the superconducting film on the ZrO 2 indicated the smaller grains as compared to those on the MgO ŽFig. 1b. although both films seemed to be porous. The high density of the film can be obtained by increasing grain size and wettability of the oxide superconductor to the substrate. The sintering temperature of the system plays an important role to increase grain size. Fig. 2a and b, for instances, indicate the different surface morphologies corresponding to the sintering temperatures such as 1093 K and 1123 K, respectively. The film density at 1113 K ŽFig. 2a. seemed to be denser and less porous as compared to those at 1093 K ŽFig. 2b.. Furthermore, the cross-sections of the film revealed the different morphologies by pressing or CIP process as shown in Fig. 3; the interfaces were not smooth, and no grain growths were observed. It seemed that nopressing process film ŽFig. 3a. looked dense. Fig. 3b showed the interfaces of the film pressed in a single mode to become denser film. Furthermore, CIP process made the film thinner by about a half. It seemed to be more in parallel to the substrate as shown in Fig. 3c. Moreover, the grain direction was parallel to the substrate comparatively. The effects of the pressing on the electrical resistivity were shown in Fig. 4 for a single mode press and CIP, and the heat processing temperature was
Fig. 1. SEM photographs of superconducting film on ZnO 2 Ž1-a., and MgO Ž1-b., after heat treatment at 1123 K.
1123 K as well. There appeared a high temperature phase and a low temperature one for both processes with the same critical temperature. However, the electrical resistivity was 35 m V cm at the off-set temperature of a high temperature phase for a single press, meanwhile it was 11 m V cm for the CIPed system at 110 K. Therefore, the CIP process attributed to reduce an electrical resistivity, due to a dense film as mentioned above. The heat processing temperature played also an important role to reduce an electrical resistivity as well as pressing process. The resistivity of the heattreated system at 1123 K on the MgO substrate showed a value of 73 m V cm at 115 K, meanwhile the resistivity was 140 m V cm on the heat-treated
868
S. Sugihara et al.r Applied Surface Science 130–132 (1998) 866–869
same system at 1093 K Žwithout pressing for both system.. There were differences of electrical resistivity from the substrate; 10, 35 and 75 m V cm for SrTiO 3 , ZrO 2 and MgO, respectively. It is reasonably suggested that the surface morphologies of the film affect the electrical resistivity. The lowest resistivity was obtained for the system of the film on the SrTiO 3 substrate whose surface had the largest grains and dense morphology among other oxide. In ionic crystals such as the substrates in the present study, single electrons or holes can produce more localized lattice distortions resulting from their electrostatic interaction with neighbor ions. This distortion accompanies as it moves through the lattice. In particular, the interfaces between the film and
Fig. 3. SEM photographs of interfaces between superconducting film and MgO at heat-treatment at 1123 K. Ža. No pressing. Žb. Pressed. Žc. CIPed before heat-treatment.
insulators may produce more lattice distortion and we expected that the electrons or holes can be transferred from the insulator to the superconducting film. Therefore, the interfacial structures joined smoothly are important to reduce the electrical resistivity. The charge will polarize surrounding atoms,which will lower the energy. The approximate formula for the polarization energy given w8x, Fig. 2. SEM photographs of superconducting film on MgO at different heat-treatment temperature. Ža. 1093 K. Žb. 1123 K.
D E s ye 2r Ž 8p´ 0 r . Ž 1 y 1r´r .
Ž 1.
where r is the radius of the orbital, and ´ r the
S. Sugihara et al.r Applied Surface Science 130–132 (1998) 866–869
869
to be affected by the electrons in the substrate through the interfaces where the distortion is mostly existing.
4. Conclusion The thick film of the superconducting BiPbSrCaCuO were studied on the oxide polycrystalline such as ZrO 2 , MgO and SrTiO 3 . There appeared two phases for every substrate and the offset of the high temperature phase was 110 K without differences from that of the oxide superconductor bulk. The CIP process attributed to reduce an electrical resistivity due to a dense film. The electrical resistivities were the lowest in the system of the film on the SrTiO 3 which had the better surface morphology and it may be possible to improve superconducting properties by controlling the film surface morphology and interfaces between the film and the oxide. Fig. 4. Pressing effect on electrical resistivity of the superconducting film on MgO at the heat-treatment of 1123 K. Župper. pressed in a single mode, Žlower. CIPed before heat-treatment.
relative dielectric constant of the solid, and ´ 0 the dielectric permittivity of vacuum Ž8.854 = 10y1 2 Fmy1 .. Since SrTiO 3 possesses the largest ´ r , D E is suggested to be the largest, according to that the reported electronic polarizabilities of ions are 0.094, 0.37 and 0.86 for Mg 2q, Zr 4q and Sr 2q, respectively w9x. The lowest resistivity, discussed above, on the SrTiO 3 substrate whose electronic polarizability was the largest among those oxides. The interaction between the electron and the lattice distortion is known as a polaron, which electron at the interfaces comes from the substrate. Therefore, the electrical properties of the superconducting film is postulated
References w1x P.G. DeGennes, Rev. Mod. Phys. 36 Ž1965. 225. w2x T. Nishino, K. Yamada, U. Kawabe, Phys. Rev. B 33 Ž1986. 2042. w3x T. Hatano, K. Nakamura, in: H. Maeda, K. Togano ŽEds.. Bismuth-based high-temperature superconductors, Marcel Dekker, New York, 1996, p. 545. w4x E. Saiz, M.I. Osendi, P. Miranzo, J. Requena, J.S. Moya, J. Less-Common Metals 164 Ž1990. 458. w5x S. Sugihara, H. Fujitani, J. Eur. Ceram. Soc. 15 Ž1995. 1043. w6x D. Duran, J. Taraj, J.F. Fernandez, C. Moure, Ferroelectrics 128 Ž1992. 231. w7x S. Sugihara, K. Fukushima, J. Jpn. Soc. Powder Powder Met. 40 Ž1993. 1092, Žin Japanese.. w8x P.A. Cox, The Electronic Structure and Chemistry of Solids, Oxford Science Publications, Oxford, 1987, p. 179. w9x C. Kittel, Introduction to Solid State Physics, Wiley, New York, 1956, p. 165.