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Surface resistances of 5-cm-diameter YBCO films prepared by MOD for microwave applications
Physica C 445–448 (2006) 823–827 www.elsevier.com/locate/physc
Surface resistances of 5-cm-diameter YBCO films prepared by MOD for microwave applications T. Manabe *, M. Sohma, I. Yamaguchi, K. Tsukada, W. Kondo, K. Kamiya, T. Tsuchiya, S. Mizuta, T. Kumagai National Institute of Advanced Industrial Science and Technology (AIST), Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan Available online 9 June 2006
Abstract Large-area high-Tc superconducting films with low surface resistances Rs are required for use in microwave applications such as band pass filters. In this paper, preparation of 5-cm-diameter YBCO films on LaAlO3 (LAO) and CeO2-buffered sapphire (CbS) substrates by metalorganic deposition (MOD) using a fluorine-free coating solution and their superconducting properties are described. The optimum firing conditions for YBCO films greatly depend on the substrate materials; a heating rate at ramp as high as 200 C/min is necessary for films on LAO whereas a lower heating rate, e.g., 20 C/min, is required for films on CbS. Accordingly, the suitable furnace systems for these substrates have been varied. As a result, a YBCO film with high Jc (77 K) of 2.7 MA/cm2 and a low Rs (12 GHz, 77 K) of 0.54 mX was prepared on LAO by using an infrared image furnace. On the other hand, a YBCO film with a higher Jc (77 K) of 4.0 MA/cm2 and the same Rs (12 GHz, 77 K) of 0.54 mX was prepared on CbS by using a tube furnace. 2006 Elsevier B.V. All rights reserved. PACS: 74.72.Bk; 74.76.Bz; 81.15.Lm; 81.15.Np Keywords: MOD; Large-size film; Surface resistance; YBa2Cu3O7
1. Introduction For microwave applications, high-Tc superconducting films having large area and low surface resistance (Rs) are required. Large-area YBa2Cu3O7 (YBCO) films with a size 5-cm-diameter or larger have successfully been prepared on LaAlO3 (LAO) substrates by various methods, such as vacuum evaporation [1,2], pulsed laser deposition, sputtering, and metal organic deposition (MOD) [3–7]. Among these processes, MOD is the lowest cost and non-vacuum process and, therefore, is suitable for mass production of YBCO films. In addition, the superconducting properties of MOD-derived YBCO films have remarkably been improved in the last few years. In this paper, the present status of the superconducting properties, i.e., critical current densities (Jc) and microwave *
Corresponding author. Tel.: +81 29 861 4556; fax: +81 29 861 4553. E-mail address: [email protected] (T. Manabe).
0921-4534/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.04.042
surface resistances (Rs), of 5-cm-diameter YBCO films prepared by MOD is described. The optimum condition for obtaining high-Jc YBCO films on LAO and CeO2-buffered sapphire (CbS) is different. Recently, the MOD process using a metal trifluoroacetate solution (TFA-MOD) is intensively investigated at a number of research institutes and fabrication of 5-cm-diameter YBCO films with high Jc and low Rs was reported [3,4]. Meanwhile, high Jc and low Rs YBCO films have also been prepared by MOD using a fluorine-free coating solution [5–7]. The fluorinefree MOD never produces the corrosive HF gas through the whole processing; therefore it is much more environment-compatible than TFA-MOD. 2. Experimental The substrates used were 5-cm-diameter LAO and CeO2buffered R-plane sapphire (CbS). The thickness of the CeO2buffer layer deposited by vacuum evaporation was 40 nm. A
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coating solution starting with metal acetylacetonates (Nihon Kagaku Sangyo Co., Ltd.) with a molar ratio Y:Ba:Cu = 1:2:3 [8] was adopted for the coating solution. This solution was spin-coated onto the substrates at 2000 rev./min. The coated films were heated at 500 C in air to remove most of the organic component. This coating-and-pyrolysis procedure was repeated several times to increase film thickness. The prefired films were heated at higher temperatures in an Ar–O2 mixed gas stream for crystallization of YBCO. Depending on the substrate materials, two types of furnace systems were adopted for this final heat treatment. The films on LAO were fabricated in an infrared image furnace with a quartz chamber (inner cross section: 45 · 120 mm2). This furnace provides fairly uniform temperature distribution, namely, within 5 C at 800 C over an area approximately 9-cm-B, and high irradiation power density that enables rapid thermal annealing, e.g., approximately 200 C/min without overshooting. On the other hand, the films on CbS were mainly fired in a large-diameter tube furnace (inner diameter: 135 mm). This tube furnace has much more uniform temperature zone, i.e., within 2 C at 800 C along the length 30 cm and more, although the heating rate is slower, e.g., 20 C/min. In each case, the specimen was placed horizontally in the center of the homogeneous temperature zone and heat-treated in an Ar–O2 mixed gas stream with an oxygen partial pressure p(O2) of about 10 Pa, followed by pure O2 treatment. The optimum temperature dwell and the heating rate at ramp were: 770 C and 200 C/min for the films on LAO, and 760 C and 20 C/min for the films on CbS. Spatial distribution of critical current density Jc of the 5cm-diameter films was investigated by mapping analysis of inductive-Jc measurement (THEVA Cryoscan; probe coil diameter: 5 mm). Microwave surface resistance Rs of the films was measured at 12 GHz by the dielectric resonator method using the TE011- and TE013-mode sapphire rods in a closed type resonator structure [9].
Fig. 2 shows inductive-Jc mapping and a Jc chart along line A–B for one of the best YBCO films on LAO, fired at 770 C with the heating rate of 200 C/min in the infrared image furnace. The film thickness was 0.70 lm. A high Jc value of 2.7 MA/cm2 at 77 K was obtained for the center position of the film, and the fluctuation of Jc was relatively small. High Jc properties of the 5-cm-diameter MODYBCO films on LAO were also confirmed by the transport method in our previous paper [10]. Microwave surface resistance Rs of the film was measured at 12 GHz using the dielectric resonator method as shown in Fig. 3. The Rs values were 0.54 mX and 0.28 mX at 77 K and 50 K, respectively. These Rs values were comparable to the reported Rs values for a YBCO film on LAO prepared by TFA-MOD process [4]. These
Fig. 1. The optimum annealing schedule of (a) temperature and (b) p(O2) for YBCO films on LaAlO3 (LAO).
3. Results and discussion 3.1. YBCO films on LaAlO3 The heating rate at ramp in the final heat treatment is one of the most important parameters for the growth of high-Jc films on LAO. The a-axis-oriented YBCO phase is likely to occur at lower temperatures during the heating ramp, resulting in the mixture of c- and a-axis-oriented grains with lower Jc properties. A heating rate as high as 100 C/min and more is required for obtaining purely c-axis-oriented YBCO films on LAO substrates. Therefore, an infrared image furnace is suitable for such purpose because it has a high irradiation power density that enables rapid thermal annealing, e.g., approximately 200 C/min without overshooting. The optimum temperature dwell and heating rate at ramp were 770 C and 200 C/min, respectively, for the films on LAO. The annealing schedule of temperature and p(O2) is schematically shown in Fig. 1.
Fig. 2. Inductive Jc-mapping at 77.3 K with the Jc chart along a diameter (A–B) for a 5-cm-B YBCO film on LaAlO3 (LAO), annealed in an infrared image furnace.
T. Manabe et al. / Physica C 445–448 (2006) 823–827
Fig. 3. Temperature dependence of Rs at 12 GHz for a 5-cm-B YBCO film on LaAlO3 (LAO).
results are satisfactory if one considers its low cost of processing, although the Rs values are slightly higher than the best values for the films on MgO prepared by evaporation [2] or by PLD [4]. 3.2. YBCO films on CeO2/Al2O3 The superconducting properties of YBCO films on CbS markedly depend on the quality of the CeO2 buffer layer. It was found that not only the in-plane alignment but also the surface morphology of CeO2 layer affect the Jc properties of MOD-YBCO films [11]. In optimizing the annealing conditions for MODderived large-area YBCO films on CbS, the uniformity of temperature over the whole specimen is one of the most important parameters in the final heat treatment. The phase BaCeO3 tends to grow at higher temperature as a reaction product of YBCO with CeO2 during the heat treatment. Thus, a slightly lower annealing temperature, e.g., 760 C, has to be adopted for growing the films on CbS. The processing window for annealing temperature for the films on CbS is narrower than that for the films on LAO. Fig. 4 shows inductive-Jc mapping for a typical YBCO film on CbS, fired at 750 C with a heating rate of 200 C/min in the infrared image furnace. The film thickness was 0.20 lm. A high Jc value of 2.8 MA/cm2 at 77 K was obtained at the center of the film, however, the fluctuation of Jc was rather large. This fluctuation is considered to be derived from the temperature distribution in the image furnace. On the other hand, formation of the a-axis-oriented YBCO phase during the heating ramp was not so significant in the films on CbS. Purely c-axis-oriented YBCO films were found to be obtained by the heat treatment with a lower heating rate such as 20 C/min. This arises from the fact that the crystallization of YBCO films on CbS is slower than on LAO in the fluorine-free MOD process.
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Fig. 4. Inductive Jc-mapping at 77.3 K with the Jc chart along a diameter (A–B) for a 5-cm-B YBCO film on CeO2-buffered sapphire (CbS), annealed in an infrared image furnace.
Therefore, the tube furnace is more favorable than infrared image furnace for annealing the films on CbS, because it has much more uniform temperature zone although the heating rate is lower. The optimum temperature dwell and heating rate at ramp were found to be 760 C and 20 C/min, respectively, for the films on CbS. The optimum annealing schedule of temperature and p(O2) is schematically shown in Fig. 5. Fig. 6 shows inductive-Jc mapping for one of the best YBCO films on CbS annealed in the tube furnace. The film thickness was 0.21 lm. A high Jc value of 4.0 MA/cm2 at 77 K was obtained at the center of the film. The fluctuation of Jc was small and Jc values above 3 MA/cm2 were observed at almost all the measurement points.
Fig. 5. The optimum annealing schedule of (a) temperature and (b) p(O2) for YBCO films on CeO2-buffered sapphire (CbS).
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3.3. Relationships between Jc and Rs
Fig. 6. Inductive Jc-mapping at 77.3 K with the Jc chart along a diameter (A–B) for a 5-cm-B YBCO film on CeO2-buffered sapphire (CbS), annealed in a tube furnace.
Microwave surface resistance Rs of the film was measured at 12 GHz using the dielectric resonator method as shown in Fig. 7. The Rs values were 0.54 mX and 0.37 mX at 77 K and 50 K, respectively. These Rs values were comparable to those for the YBCO film on LAO shown in Fig. 3. For comparison, surface resistance of a commercially available YBCO film on CbS (film thickness: 0.30 lm, Jc: 2.8 MA/cm2) prepared by evaporation process was measured at the same apparatus. The Rs values of 0.46 mX and 0.15 mX at 77 K and 50 K, respectively, were obtained for the evaporated YBCO films. Although the Rs values of the MOD-YBCO film are slightly higher than the evaporated film especially at low temperature region, the difference of Rs at high temperature as 77 K is small.
Fig. 7. Temperature dependence of Rs at 12 GHz for a 5-cm-B YBCO film on CeO2-buffered sapphire (CbS).
The Rs values measured for YBCO films on LAO and CbS are plotted against the inductive-Jc value at the center of each film in Fig. 8, in which those of other MOD-derived YBCO films as well as of an evaporated YBCO film on CbS, for comparison, are plotted. As clearly seen from this figure, there is some strong correlation between Rs and Jc for the MOD-derived YBCO films on LAO and CbS; namely, Rs is generally inversely proportional to Jc. This result is in good agreement with that reported for sputtered-YBCO films on BaSnO3-buffered MgO substrates, in which the Rs and Jc values were measured by the sapphire rod resonator method and the magnetic measurement, respectively [12]. As for the difference between YBCO films on CbS and those on LAO, the data points for the films on CbS locate slightly upper side in this figure. One possible reason for this is that the thickness of the films on CbS (200–300 nm) is thinner than those for the films on LAO (700 nm) and is thinner than the magnetic penetration length k of YBCO at 77 K [13]. 3.4. Double-sided YBCO films The double-sided and large-area superconducting films are required for use in microwave applications such as microstrip band-pass filters [14], which consist of a patterned superconductor strip (resonator) on one side and of a superconductor ground plane on the other side of a dielectric substrate. MOD is also applicable to prepare double-sided YBCO films. The double-sided YBCO films were prepared through the following steps: firstly, the spin-coating and pyrolysis procedure is carried out on one side of a substrate; secondly, the same spin-coating and pyrolysis proce-
Fig. 8. Relationship between Jc and Rs for MOD-derived YBCO films on LaAlO3 (LAO) and CeO2-buffered sapphire (CbS). For comparison, the result of an evaporated YBCO film on CbS is included.
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dure was carried out on the other side; and finally, the double-sided precursor films were annealed at high temperature for obtaining the double-sided YBCO films. In this procedure, crystallization of YBCO films on both sides simultaneously occurs during the final step. The properties of double-sided 5 cm-B YBCO films on LAO were reported in our previous paper [5]; high Jc values above 1 MA/cm2 were observed for the both sides. In addition, we fabricated a 5-pole microwave filter and confirmed its low insertion loss, namely, less than 0.1 dB at 70 K, using a double-sided 2-cm-sq. YBCO film [15]. Recently, the double-sided 5-cm-B YBCO films were successfully prepared on a sapphire substrate with doublesided CeO2-buffer layers; low Rs (12 GHz, 77 K) below 1 mX was observed for the both sides [16]. 4. Summary The 5-cm-diameter YBCO films were prepared on LAO and CbS substrates by MOD using a fluorine-free coating solution. The optimum firing conditions for YBCO films depend on the substrate materials and the suitable furnace systems for these substrates are accordingly different. For the films on LAO, a heating rate at ramp as high as 200 C/min is necessary so that an infrared image furnace is suitable for the final heat treatment. A YBCO film with high Jc (77 K) of 2.7 MA/cm2 and low Rs (12 GHz, 77 K) of 0.52 mX was prepared on LAO by using the infrared image furnace. For the films on CbS, on the other hand, a lower heating rate at ramp as 20 C/min and the uniformity of temperature over the whole specimen are required. Therefore, a tube furnace is suitable for their final heat treatment. A YBCO film with high Jc (77 K) of 4.0 MA/cm2 and low Rs (12 GHz, 77 K) of 0.52 mX was
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prepared on CbS. MOD is also applicable to prepare the double-sided YBCO films. References [1] H. Kinder, P. Berberich, W. Prusseit, S.R. Zecha, R. Semerad, B. Utz, Physica C 282–287 (1997) 107. [2] W. Prusseit, R. Semerad, K. Irgmaier, G. Sigl, Physica C 392–396 (2003) 1225. [3] T. Araki, K. Yamagiwa, I. Hirabayashi, K. Suzuki, S. Tanaka, Supercond. Sci. Technol. 14 (2001) L21. [4] K. Suzuki, T. Araki, T. Konno, T. Suzuki, I. Hirabayashi, Y. Enomoto, Physica C 372–376 (2002) 623. [5] T. Kumagai, T. Manabe, I. Yamaguchi, W. Kondo, F. Imai, K. Murayama, S. Mizuta, Physica C 357–360 (2001) 1346. [6] T. Kumagai, T. Manabe, W. Kondo, K. Murayama, T. Hashimoto, Y. Kobayashi, I. Yamaguchi, M. Sohma, T. Tsuchiya, K. Tsukada, S. Mizuta, Physica C 378–381 (2002) 1236. [7] T. Manabe, W. Kondo, I. Yamaguchi, M. Sohma, T. Tsuchiya, K. Tsukada, S. Mizuta, T. Kumagai, Physica C 417 (2005) 987. [8] T. Kumagai, H. Yamasaki, K. Endo, T. Manabe, H. Niino, T. Tsunoda, W. Kondo, S. Mizuta, Jpn. J. Appl. Phys. 32 (1993) L1602. [9] Y. Kobayashi, H. Tamura, IEICE Trans. Electron. E77-C (1994) 882. [10] S. Torii, S. Akita, T. Manabe, T. Kumagai, K. Inoue, Physica C 392– 396 (2003) 932. [11] M. Sohma, I. Yamaguchi, K. Tsukada, W. Kondo, S. Mizuta, T. Manabe, T. Kumagai, Physica C 412–414 (2004) 1326. [12] S. Ohshima, S. Oikawa, T. Noguchi, M. Inadomaru, M. Kusunoki, M. Mukaida, H. Yamasaki, Y. Nakagawa, Physica C 372–376 (2002) 671. [13] A. Mogro-Campero, L.G. Turner, A.M. Kadin, D.S. Mallory, J. Appl. Phys. 73 (1993) 5295. [14] T. Kinpara, M. Kusunoki, M. Mukaida, S. Ohshima, Physica C 357– 360 (2001) 1503. [15] T. Kumagai, T. Manabe, I. Yamaguchi, S. Nakamura, W. Kondo, S. Mizuta, F. Imai, K. Murayama, A. Shimokobe, Y.M. Kang, in: T. Yamashita, K. Tanabe (Eds.), Adv. Supercond. XII, SpringerVerlag, Tokyo, 2000, p. 927. [16] M. Sohma, K. Kamiya, K. Tsukada, I. Yamaguchi, W. Kondo, S. Mizuta, T. Manabe, T. Kumagai, IEICE Trans. Electron. E89-C (2006) 182.
Surface resistances of 5-cm-diameter YBCO films prepared by MOD for microwave applications T. Manabe *, M. Sohma, I. Yamaguchi, K. Tsukada, W. Kondo, K. Kamiya, T. Tsuchiya, S. Mizuta, T. Kumagai National Institute of Advanced Industrial Science and Technology (AIST), Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan Available online 9 June 2006
Abstract Large-area high-Tc superconducting films with low surface resistances Rs are required for use in microwave applications such as band pass filters. In this paper, preparation of 5-cm-diameter YBCO films on LaAlO3 (LAO) and CeO2-buffered sapphire (CbS) substrates by metalorganic deposition (MOD) using a fluorine-free coating solution and their superconducting properties are described. The optimum firing conditions for YBCO films greatly depend on the substrate materials; a heating rate at ramp as high as 200 C/min is necessary for films on LAO whereas a lower heating rate, e.g., 20 C/min, is required for films on CbS. Accordingly, the suitable furnace systems for these substrates have been varied. As a result, a YBCO film with high Jc (77 K) of 2.7 MA/cm2 and a low Rs (12 GHz, 77 K) of 0.54 mX was prepared on LAO by using an infrared image furnace. On the other hand, a YBCO film with a higher Jc (77 K) of 4.0 MA/cm2 and the same Rs (12 GHz, 77 K) of 0.54 mX was prepared on CbS by using a tube furnace. 2006 Elsevier B.V. All rights reserved. PACS: 74.72.Bk; 74.76.Bz; 81.15.Lm; 81.15.Np Keywords: MOD; Large-size film; Surface resistance; YBa2Cu3O7
1. Introduction For microwave applications, high-Tc superconducting films having large area and low surface resistance (Rs) are required. Large-area YBa2Cu3O7 (YBCO) films with a size 5-cm-diameter or larger have successfully been prepared on LaAlO3 (LAO) substrates by various methods, such as vacuum evaporation [1,2], pulsed laser deposition, sputtering, and metal organic deposition (MOD) [3–7]. Among these processes, MOD is the lowest cost and non-vacuum process and, therefore, is suitable for mass production of YBCO films. In addition, the superconducting properties of MOD-derived YBCO films have remarkably been improved in the last few years. In this paper, the present status of the superconducting properties, i.e., critical current densities (Jc) and microwave *
Corresponding author. Tel.: +81 29 861 4556; fax: +81 29 861 4553. E-mail address: [email protected] (T. Manabe).
0921-4534/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.04.042
surface resistances (Rs), of 5-cm-diameter YBCO films prepared by MOD is described. The optimum condition for obtaining high-Jc YBCO films on LAO and CeO2-buffered sapphire (CbS) is different. Recently, the MOD process using a metal trifluoroacetate solution (TFA-MOD) is intensively investigated at a number of research institutes and fabrication of 5-cm-diameter YBCO films with high Jc and low Rs was reported [3,4]. Meanwhile, high Jc and low Rs YBCO films have also been prepared by MOD using a fluorine-free coating solution [5–7]. The fluorinefree MOD never produces the corrosive HF gas through the whole processing; therefore it is much more environment-compatible than TFA-MOD. 2. Experimental The substrates used were 5-cm-diameter LAO and CeO2buffered R-plane sapphire (CbS). The thickness of the CeO2buffer layer deposited by vacuum evaporation was 40 nm. A
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coating solution starting with metal acetylacetonates (Nihon Kagaku Sangyo Co., Ltd.) with a molar ratio Y:Ba:Cu = 1:2:3 [8] was adopted for the coating solution. This solution was spin-coated onto the substrates at 2000 rev./min. The coated films were heated at 500 C in air to remove most of the organic component. This coating-and-pyrolysis procedure was repeated several times to increase film thickness. The prefired films were heated at higher temperatures in an Ar–O2 mixed gas stream for crystallization of YBCO. Depending on the substrate materials, two types of furnace systems were adopted for this final heat treatment. The films on LAO were fabricated in an infrared image furnace with a quartz chamber (inner cross section: 45 · 120 mm2). This furnace provides fairly uniform temperature distribution, namely, within 5 C at 800 C over an area approximately 9-cm-B, and high irradiation power density that enables rapid thermal annealing, e.g., approximately 200 C/min without overshooting. On the other hand, the films on CbS were mainly fired in a large-diameter tube furnace (inner diameter: 135 mm). This tube furnace has much more uniform temperature zone, i.e., within 2 C at 800 C along the length 30 cm and more, although the heating rate is slower, e.g., 20 C/min. In each case, the specimen was placed horizontally in the center of the homogeneous temperature zone and heat-treated in an Ar–O2 mixed gas stream with an oxygen partial pressure p(O2) of about 10 Pa, followed by pure O2 treatment. The optimum temperature dwell and the heating rate at ramp were: 770 C and 200 C/min for the films on LAO, and 760 C and 20 C/min for the films on CbS. Spatial distribution of critical current density Jc of the 5cm-diameter films was investigated by mapping analysis of inductive-Jc measurement (THEVA Cryoscan; probe coil diameter: 5 mm). Microwave surface resistance Rs of the films was measured at 12 GHz by the dielectric resonator method using the TE011- and TE013-mode sapphire rods in a closed type resonator structure [9].
Fig. 2 shows inductive-Jc mapping and a Jc chart along line A–B for one of the best YBCO films on LAO, fired at 770 C with the heating rate of 200 C/min in the infrared image furnace. The film thickness was 0.70 lm. A high Jc value of 2.7 MA/cm2 at 77 K was obtained for the center position of the film, and the fluctuation of Jc was relatively small. High Jc properties of the 5-cm-diameter MODYBCO films on LAO were also confirmed by the transport method in our previous paper [10]. Microwave surface resistance Rs of the film was measured at 12 GHz using the dielectric resonator method as shown in Fig. 3. The Rs values were 0.54 mX and 0.28 mX at 77 K and 50 K, respectively. These Rs values were comparable to the reported Rs values for a YBCO film on LAO prepared by TFA-MOD process [4]. These
Fig. 1. The optimum annealing schedule of (a) temperature and (b) p(O2) for YBCO films on LaAlO3 (LAO).
3. Results and discussion 3.1. YBCO films on LaAlO3 The heating rate at ramp in the final heat treatment is one of the most important parameters for the growth of high-Jc films on LAO. The a-axis-oriented YBCO phase is likely to occur at lower temperatures during the heating ramp, resulting in the mixture of c- and a-axis-oriented grains with lower Jc properties. A heating rate as high as 100 C/min and more is required for obtaining purely c-axis-oriented YBCO films on LAO substrates. Therefore, an infrared image furnace is suitable for such purpose because it has a high irradiation power density that enables rapid thermal annealing, e.g., approximately 200 C/min without overshooting. The optimum temperature dwell and heating rate at ramp were 770 C and 200 C/min, respectively, for the films on LAO. The annealing schedule of temperature and p(O2) is schematically shown in Fig. 1.
Fig. 2. Inductive Jc-mapping at 77.3 K with the Jc chart along a diameter (A–B) for a 5-cm-B YBCO film on LaAlO3 (LAO), annealed in an infrared image furnace.
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Fig. 3. Temperature dependence of Rs at 12 GHz for a 5-cm-B YBCO film on LaAlO3 (LAO).
results are satisfactory if one considers its low cost of processing, although the Rs values are slightly higher than the best values for the films on MgO prepared by evaporation [2] or by PLD [4]. 3.2. YBCO films on CeO2/Al2O3 The superconducting properties of YBCO films on CbS markedly depend on the quality of the CeO2 buffer layer. It was found that not only the in-plane alignment but also the surface morphology of CeO2 layer affect the Jc properties of MOD-YBCO films [11]. In optimizing the annealing conditions for MODderived large-area YBCO films on CbS, the uniformity of temperature over the whole specimen is one of the most important parameters in the final heat treatment. The phase BaCeO3 tends to grow at higher temperature as a reaction product of YBCO with CeO2 during the heat treatment. Thus, a slightly lower annealing temperature, e.g., 760 C, has to be adopted for growing the films on CbS. The processing window for annealing temperature for the films on CbS is narrower than that for the films on LAO. Fig. 4 shows inductive-Jc mapping for a typical YBCO film on CbS, fired at 750 C with a heating rate of 200 C/min in the infrared image furnace. The film thickness was 0.20 lm. A high Jc value of 2.8 MA/cm2 at 77 K was obtained at the center of the film, however, the fluctuation of Jc was rather large. This fluctuation is considered to be derived from the temperature distribution in the image furnace. On the other hand, formation of the a-axis-oriented YBCO phase during the heating ramp was not so significant in the films on CbS. Purely c-axis-oriented YBCO films were found to be obtained by the heat treatment with a lower heating rate such as 20 C/min. This arises from the fact that the crystallization of YBCO films on CbS is slower than on LAO in the fluorine-free MOD process.
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Fig. 4. Inductive Jc-mapping at 77.3 K with the Jc chart along a diameter (A–B) for a 5-cm-B YBCO film on CeO2-buffered sapphire (CbS), annealed in an infrared image furnace.
Therefore, the tube furnace is more favorable than infrared image furnace for annealing the films on CbS, because it has much more uniform temperature zone although the heating rate is lower. The optimum temperature dwell and heating rate at ramp were found to be 760 C and 20 C/min, respectively, for the films on CbS. The optimum annealing schedule of temperature and p(O2) is schematically shown in Fig. 5. Fig. 6 shows inductive-Jc mapping for one of the best YBCO films on CbS annealed in the tube furnace. The film thickness was 0.21 lm. A high Jc value of 4.0 MA/cm2 at 77 K was obtained at the center of the film. The fluctuation of Jc was small and Jc values above 3 MA/cm2 were observed at almost all the measurement points.
Fig. 5. The optimum annealing schedule of (a) temperature and (b) p(O2) for YBCO films on CeO2-buffered sapphire (CbS).
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3.3. Relationships between Jc and Rs
Fig. 6. Inductive Jc-mapping at 77.3 K with the Jc chart along a diameter (A–B) for a 5-cm-B YBCO film on CeO2-buffered sapphire (CbS), annealed in a tube furnace.
Microwave surface resistance Rs of the film was measured at 12 GHz using the dielectric resonator method as shown in Fig. 7. The Rs values were 0.54 mX and 0.37 mX at 77 K and 50 K, respectively. These Rs values were comparable to those for the YBCO film on LAO shown in Fig. 3. For comparison, surface resistance of a commercially available YBCO film on CbS (film thickness: 0.30 lm, Jc: 2.8 MA/cm2) prepared by evaporation process was measured at the same apparatus. The Rs values of 0.46 mX and 0.15 mX at 77 K and 50 K, respectively, were obtained for the evaporated YBCO films. Although the Rs values of the MOD-YBCO film are slightly higher than the evaporated film especially at low temperature region, the difference of Rs at high temperature as 77 K is small.
Fig. 7. Temperature dependence of Rs at 12 GHz for a 5-cm-B YBCO film on CeO2-buffered sapphire (CbS).
The Rs values measured for YBCO films on LAO and CbS are plotted against the inductive-Jc value at the center of each film in Fig. 8, in which those of other MOD-derived YBCO films as well as of an evaporated YBCO film on CbS, for comparison, are plotted. As clearly seen from this figure, there is some strong correlation between Rs and Jc for the MOD-derived YBCO films on LAO and CbS; namely, Rs is generally inversely proportional to Jc. This result is in good agreement with that reported for sputtered-YBCO films on BaSnO3-buffered MgO substrates, in which the Rs and Jc values were measured by the sapphire rod resonator method and the magnetic measurement, respectively [12]. As for the difference between YBCO films on CbS and those on LAO, the data points for the films on CbS locate slightly upper side in this figure. One possible reason for this is that the thickness of the films on CbS (200–300 nm) is thinner than those for the films on LAO (700 nm) and is thinner than the magnetic penetration length k of YBCO at 77 K [13]. 3.4. Double-sided YBCO films The double-sided and large-area superconducting films are required for use in microwave applications such as microstrip band-pass filters [14], which consist of a patterned superconductor strip (resonator) on one side and of a superconductor ground plane on the other side of a dielectric substrate. MOD is also applicable to prepare double-sided YBCO films. The double-sided YBCO films were prepared through the following steps: firstly, the spin-coating and pyrolysis procedure is carried out on one side of a substrate; secondly, the same spin-coating and pyrolysis proce-
Fig. 8. Relationship between Jc and Rs for MOD-derived YBCO films on LaAlO3 (LAO) and CeO2-buffered sapphire (CbS). For comparison, the result of an evaporated YBCO film on CbS is included.
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dure was carried out on the other side; and finally, the double-sided precursor films were annealed at high temperature for obtaining the double-sided YBCO films. In this procedure, crystallization of YBCO films on both sides simultaneously occurs during the final step. The properties of double-sided 5 cm-B YBCO films on LAO were reported in our previous paper [5]; high Jc values above 1 MA/cm2 were observed for the both sides. In addition, we fabricated a 5-pole microwave filter and confirmed its low insertion loss, namely, less than 0.1 dB at 70 K, using a double-sided 2-cm-sq. YBCO film [15]. Recently, the double-sided 5-cm-B YBCO films were successfully prepared on a sapphire substrate with doublesided CeO2-buffer layers; low Rs (12 GHz, 77 K) below 1 mX was observed for the both sides [16]. 4. Summary The 5-cm-diameter YBCO films were prepared on LAO and CbS substrates by MOD using a fluorine-free coating solution. The optimum firing conditions for YBCO films depend on the substrate materials and the suitable furnace systems for these substrates are accordingly different. For the films on LAO, a heating rate at ramp as high as 200 C/min is necessary so that an infrared image furnace is suitable for the final heat treatment. A YBCO film with high Jc (77 K) of 2.7 MA/cm2 and low Rs (12 GHz, 77 K) of 0.52 mX was prepared on LAO by using the infrared image furnace. For the films on CbS, on the other hand, a lower heating rate at ramp as 20 C/min and the uniformity of temperature over the whole specimen are required. Therefore, a tube furnace is suitable for their final heat treatment. A YBCO film with high Jc (77 K) of 4.0 MA/cm2 and low Rs (12 GHz, 77 K) of 0.52 mX was
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prepared on CbS. MOD is also applicable to prepare the double-sided YBCO films. References [1] H. Kinder, P. Berberich, W. Prusseit, S.R. Zecha, R. Semerad, B. Utz, Physica C 282–287 (1997) 107. [2] W. Prusseit, R. Semerad, K. Irgmaier, G. Sigl, Physica C 392–396 (2003) 1225. [3] T. Araki, K. Yamagiwa, I. Hirabayashi, K. Suzuki, S. Tanaka, Supercond. Sci. Technol. 14 (2001) L21. [4] K. Suzuki, T. Araki, T. Konno, T. Suzuki, I. Hirabayashi, Y. Enomoto, Physica C 372–376 (2002) 623. [5] T. Kumagai, T. Manabe, I. Yamaguchi, W. Kondo, F. Imai, K. Murayama, S. Mizuta, Physica C 357–360 (2001) 1346. [6] T. Kumagai, T. Manabe, W. Kondo, K. Murayama, T. Hashimoto, Y. Kobayashi, I. Yamaguchi, M. Sohma, T. Tsuchiya, K. Tsukada, S. Mizuta, Physica C 378–381 (2002) 1236. [7] T. Manabe, W. Kondo, I. Yamaguchi, M. Sohma, T. Tsuchiya, K. Tsukada, S. Mizuta, T. Kumagai, Physica C 417 (2005) 987. [8] T. Kumagai, H. Yamasaki, K. Endo, T. Manabe, H. Niino, T. Tsunoda, W. Kondo, S. Mizuta, Jpn. J. Appl. Phys. 32 (1993) L1602. [9] Y. Kobayashi, H. Tamura, IEICE Trans. Electron. E77-C (1994) 882. [10] S. Torii, S. Akita, T. Manabe, T. Kumagai, K. Inoue, Physica C 392– 396 (2003) 932. [11] M. Sohma, I. Yamaguchi, K. Tsukada, W. Kondo, S. Mizuta, T. Manabe, T. Kumagai, Physica C 412–414 (2004) 1326. [12] S. Ohshima, S. Oikawa, T. Noguchi, M. Inadomaru, M. Kusunoki, M. Mukaida, H. Yamasaki, Y. Nakagawa, Physica C 372–376 (2002) 671. [13] A. Mogro-Campero, L.G. Turner, A.M. Kadin, D.S. Mallory, J. Appl. Phys. 73 (1993) 5295. [14] T. Kinpara, M. Kusunoki, M. Mukaida, S. Ohshima, Physica C 357– 360 (2001) 1503. [15] T. Kumagai, T. Manabe, I. Yamaguchi, S. Nakamura, W. Kondo, S. Mizuta, F. Imai, K. Murayama, A. Shimokobe, Y.M. Kang, in: T. Yamashita, K. Tanabe (Eds.), Adv. Supercond. XII, SpringerVerlag, Tokyo, 2000, p. 927. [16] M. Sohma, K. Kamiya, K. Tsukada, I. Yamaguchi, W. Kondo, S. Mizuta, T. Manabe, T. Kumagai, IEICE Trans. Electron. E89-C (2006) 182.