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TEM characterization of nanoscale YBCO particles
Materials Letters 57 (2003) 3869 – 3873 www.elsevier.com/locate/matlet
TEM characterization of nanoscale YBCO particles H.Y. Pan a,b,*, X.L. Xu b, J.D. Guo b a
Electron Microscopy Laboratory, Department of Physics, Physics Building, Peking University, Beijing 100871, PR China b Department of Physics and Mesoscopic Physics National Laboratory, Peking University, Beijing 100871, PR China Received 20 June 2002; received in revised form 25 March 2003; accepted 25 March 2003
Abstract The nanoscale YBa2Cu3O7 x (YBCO) particles, which are synthesized by a citrate pyrolysis technique [Physica C, 371 (2002) 129], are heated at 850 jC under O2 atmosphere for 0, 0.5, 2, 8, 20 h, respectively. The size and crystalline quality of the YBCO nanoscale particles after annealing are studied by transmission electron microscopy (TEM). D 2003 Elsevier Science B.V. All rights reserved. Keywords: Transmission electron microscopy (TEM); Annealing; Nanoscale YBCO superconductors
1. Introduction The third generation of high-Tc superconducting tapes could be produced by coating soft metal substrate with nanoscale YBa2Cu3O7 x (YBCO) powders. By using a roll technique, YBCO superfine grains coated on the soft metal substrate can be connected to each other directly to form melting texture. This technique also overcomes many difficulties in the traditional method of fabricating long superconducting tapes. Its achievement could therefore induce a technical revolution in the production of superconducting tapes and take great effect on the extensive application of high-Tc superconducting materials. By citrate pyrolysis technique [1], the nanoscale superconducting YBa2Cu3O7 x is synthesized. This * Corresponding author. Electron Microscopy Laboratory, Department of Physics, Physics Building, Peking University, Beijing 100871, PR China. E-mail addresses: [email protected], [email protected] (H.Y. Pan).
process is different from the conventional solid phase ways of bulk superconductor preparation owing to its success in producing superfine crystalline materials. The grain size and structure of superconducting particles have great effect on the quality of the coated types as well as its Jc and Tc. Therefore, it is very important to study the microstructure of superconducting particles. Since Hiraga et al. [2] did research on the high resolution transmission microscope (HRTEM) image of YBCO superconductor, many transmission electron microscopy (TEM) studies on YBCO has been focused on the bulk materials and thin film materials [3]; only a few exceptions report the TEM study concerning the nanoscale YBCO particles [4]. By using a citrate pyrolysis technique, Xu et al. [1] have successfully synthesized the YBCO superconducting nanoscale particles and made the X-ray diffraction and SEM analysis. The results showed that the obtained YBCO superfine particles were unsuperconducting tetragonal phase and have a mean size of 40– 60 nm. The crystallite size can also be determined by X-ray diffraction spectrum using the Scherrer
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00219-2
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H.Y. Pan et al. / Materials Letters 57 (2003) 3869–3873
3. Result and discussion The size distribution is first analyzed. The No. 1 sample is not annealed under O2 atmosphere. Its structure proves to be unsuperconductivity tetragonal phase by X-ray diffraction [1]. The size distribution
Fig. 1. The distribution curve of grain sizes for Sample No. 1.
equation from the width of the selected peak at halfheight. It has been calculated that the crystalline size of samples range 18 –20 nm, smaller than that shown in SEM images. The obtained fine powders were then annealed under O2 atmosphere in order to acquire the desired superconducting phase, i.e., orthorhombic phase. It is found that the minimum transition temperature from tetragonal phase to orthorhombic phase is 850 jC. In this temperature, the fine particles begin to aggregate. However, at a lower temperature the orthorhombic phase cannot be formed and the aggregation of tetragonal phase can occur. For the sake of both lessening the aggregation and obtaining the orthorhombic phase, different annealing time was attempted under the minimum phase transition temperature of 850 jC and standard atmosphere. Just because of the key role of annealing time in affecting the particles aggregation, the paper mainly focuses on the TEM study of nanoscale YBCO particles aggregation with different annealing time.
2. Experimental The nanoscale YBCO particle samples (No. 1 – 5), which are synthesized by a citrate pyrolysis technique [1], are heated under O2 atmosphere at 850 jC for 0, 0.5, 2, 8, 20 h, respectively. The particle samples are supersonicly vibrated in the ethanol solution; then carbon mesh samples for TEM observation are made. The TEM observation is taken by Hitachi H9000NAR HRTEM at 300 kV (with point resolution of 0.18 nm) and JEOL-200CX at 200 kV.
Fig. 2. (a) The low magnification HRTEM image of Sample No. 1. (b) The high magnification HRTEM image corresponding to the area A in (a).
H.Y. Pan et al. / Materials Letters 57 (2003) 3869–3873
curve of this sample is acquired through TEM observation and statistical analysis, as shown in Fig. 1. The size of YBCO particles ranges 8 –30 nm, most of all between 14 and 16 nm, just as shown in Fig. 2. The result well tallies with the X-ray diffraction analysis [1]. The No. 2 sample is annealed under O2 atmosphere for 0.5 h. By X-ray diffraction analysis [1], the emergence of the orthorhombic superconductivity phase is proved. It is found that there are some big particles which sizes are between 0.5 and 1 Am in the No. 2 sample (shown in Fig. 3a), but most particles
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are still small nanoscale particles as shown in Fig. 3b. Fig. 3c shows that the lattice of nanometer particles begins to bend, and the interface of particles becomes ambiguous. The particles will step into a state of recrystallization. The No. 3 sample is annealed under O2 atmosphere for 2 h. The appearance of the orthorhombic superconductivity phase is also proved by X-ray diffraction [1]. Compared with No. 2 sample, big particles in No. 3 sample increase, while small particles decrease. Under the similar observation and analysis with TEM and X-ray diffraction, the same trend of size
Fig. 3. (a) The TEM image of big particles from Sample No. 2; the size of big particles is between 0.5 and 1 Am. (b) The HRTEM image of small size particles from Sample No. 2. (c) The high magnification HRTEM image from A area in (b), the lattice of nanometer particle begins to bend, and the interface of particles becomes ambiguous, and the particles begin to aggregate.
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H.Y. Pan et al. / Materials Letters 57 (2003) 3869–3873
distribution is shown in samples No. 4 (annealed for 8 h) and No. 5 (annealed for 20 h) whose main structure are both orthorhombic superconducting phase, namely, the more annealing time given, the more big particles and the less small particles appear. But the size of the big particle changes nothing between 0.5 and 1 Am, shown in Fig. 4. Secondly, the quality of big particles with the orthorhombic phase structure is analyzed. In most cases, the separate big particles are single crystals as shown in Fig. 5. But the quality of single crystalline is not perfect, a lot of stack faults is observed. Thus it can be concluded that the big particle is not formed by simply connecting many small polycrystalline interfaces, but through the recrystallizing of many small particles. From the abovementioned, it can be found that during the annealing with different time (0 –20 h) at 850 jC under O2 atmosphere the more time given, the more big particles and the less small particles appear. The recrystalline phase transition, which is analogous to energy ‘quantum’ transition process, is not a uniform process, i.e., not all of the particles grow bigger simultaneously; some particles grow bigger preferentially. The residual energy and substance are not
Fig. 5. (a) The TEM image of one big particle from Sample No. 3; the particle is a single crystalline which contains a lot of stack faults. (b) The selected area electron diffraction pattern of one big particle corresponding to (a); it consists of one set of electron diffraction pattern [001] axis.
Fig. 4. The TEM image of big particles from Sample No. 5; the size of big particles is between 0.5 and 1 Am.
transferred to other particles until the thermal dynamics symmetries are established in big particles. In the process of annealing, the transition from the tetragonal phase to the orthorhombic superconducting phase occurs, i.e., small particles with tetragonal phase structure recrystallize into big particles with orthorhombic superconducting phase structure. However, the size of the big particles remains unchanged between 0.5 and 1 Am. It shows that the size of the big particles has no clear reliance on time within the range of 0– 20 h, but its size is probably related to the
H.Y. Pan et al. / Materials Letters 57 (2003) 3869–3873
heat treatment temperature and O2 pressure which affect thermodynamics energy of the system. So at this temperature, instead of acquiring nanoscale YBCO particles, only micron scale YBCO superconducting particles can be formed.
4. Conclusions 1. The size of YBCO particles in No. 1 sample, which is not annealed, is between 8 and 30 nm, most between 14 and 16 nm. 2. The more annealing time given, the more big particles and the less small particles appear, but the size of the big particles remains unchanged between 0.5 and 1 Am and has no clear reliance on time within the range of 0– 20 h. 3. The transition from the tetragonal phase to the orthorhombic superconducting phase is a process of recrystallization. The big particle with the orthorhombic phase structure is a single crystalline, which is not formed by simply connecting many small polycrystalline interfaces, but through the recrystallizing of many small particles. The quality of the single crystalline is not perfect. Besides, the crystalline contains a lot of stack faults. To sum up, under the condition of appropriate heat treatment, the nanoscale YBCO particles can be trans-
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formed into superconductivity orthorhombic phase. This is inspiring for the manufacturing of the third generation superconducting tapes. By coating soft metal substrate with nanoscale YBCO particle materials, using roll technique and then annealing at appropriate temperature under O 2 atmosphere, YBCO superconducting tapes could be produced. This work on the preparation of long superconducting tapes with nanoscale YBCO powders is in progress and we look forward to future good results.
Acknowledgements The work is supported by China Post-doctor Foundation, School Measurement Foundation of Peking University and China National Natural Science Foundation No. 10004001.
References [1] X.L. Xu, J.D. Guo, Y.Z. Wang, A. Suzzi, Physica, C 371 (2) (2002) 129 – 132. [2] K. Hiraga, D. Shindo, M. Hirabayashi, et al., Jpn. J. Appl. J. Electron Microsc. 36 (1987) 261. [3] L. Li, Materials Science and Engineering, Report: a Review Journal, 29 (2000) 153 – 181. [4] H. Kezuka, Z. Xi, Q. Zhang, Physica, C 282 – 287 (1997) 523 – 524.
TEM characterization of nanoscale YBCO particles H.Y. Pan a,b,*, X.L. Xu b, J.D. Guo b a
Electron Microscopy Laboratory, Department of Physics, Physics Building, Peking University, Beijing 100871, PR China b Department of Physics and Mesoscopic Physics National Laboratory, Peking University, Beijing 100871, PR China Received 20 June 2002; received in revised form 25 March 2003; accepted 25 March 2003
Abstract The nanoscale YBa2Cu3O7 x (YBCO) particles, which are synthesized by a citrate pyrolysis technique [Physica C, 371 (2002) 129], are heated at 850 jC under O2 atmosphere for 0, 0.5, 2, 8, 20 h, respectively. The size and crystalline quality of the YBCO nanoscale particles after annealing are studied by transmission electron microscopy (TEM). D 2003 Elsevier Science B.V. All rights reserved. Keywords: Transmission electron microscopy (TEM); Annealing; Nanoscale YBCO superconductors
1. Introduction The third generation of high-Tc superconducting tapes could be produced by coating soft metal substrate with nanoscale YBa2Cu3O7 x (YBCO) powders. By using a roll technique, YBCO superfine grains coated on the soft metal substrate can be connected to each other directly to form melting texture. This technique also overcomes many difficulties in the traditional method of fabricating long superconducting tapes. Its achievement could therefore induce a technical revolution in the production of superconducting tapes and take great effect on the extensive application of high-Tc superconducting materials. By citrate pyrolysis technique [1], the nanoscale superconducting YBa2Cu3O7 x is synthesized. This * Corresponding author. Electron Microscopy Laboratory, Department of Physics, Physics Building, Peking University, Beijing 100871, PR China. E-mail addresses: [email protected], [email protected] (H.Y. Pan).
process is different from the conventional solid phase ways of bulk superconductor preparation owing to its success in producing superfine crystalline materials. The grain size and structure of superconducting particles have great effect on the quality of the coated types as well as its Jc and Tc. Therefore, it is very important to study the microstructure of superconducting particles. Since Hiraga et al. [2] did research on the high resolution transmission microscope (HRTEM) image of YBCO superconductor, many transmission electron microscopy (TEM) studies on YBCO has been focused on the bulk materials and thin film materials [3]; only a few exceptions report the TEM study concerning the nanoscale YBCO particles [4]. By using a citrate pyrolysis technique, Xu et al. [1] have successfully synthesized the YBCO superconducting nanoscale particles and made the X-ray diffraction and SEM analysis. The results showed that the obtained YBCO superfine particles were unsuperconducting tetragonal phase and have a mean size of 40– 60 nm. The crystallite size can also be determined by X-ray diffraction spectrum using the Scherrer
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00219-2
3870
H.Y. Pan et al. / Materials Letters 57 (2003) 3869–3873
3. Result and discussion The size distribution is first analyzed. The No. 1 sample is not annealed under O2 atmosphere. Its structure proves to be unsuperconductivity tetragonal phase by X-ray diffraction [1]. The size distribution
Fig. 1. The distribution curve of grain sizes for Sample No. 1.
equation from the width of the selected peak at halfheight. It has been calculated that the crystalline size of samples range 18 –20 nm, smaller than that shown in SEM images. The obtained fine powders were then annealed under O2 atmosphere in order to acquire the desired superconducting phase, i.e., orthorhombic phase. It is found that the minimum transition temperature from tetragonal phase to orthorhombic phase is 850 jC. In this temperature, the fine particles begin to aggregate. However, at a lower temperature the orthorhombic phase cannot be formed and the aggregation of tetragonal phase can occur. For the sake of both lessening the aggregation and obtaining the orthorhombic phase, different annealing time was attempted under the minimum phase transition temperature of 850 jC and standard atmosphere. Just because of the key role of annealing time in affecting the particles aggregation, the paper mainly focuses on the TEM study of nanoscale YBCO particles aggregation with different annealing time.
2. Experimental The nanoscale YBCO particle samples (No. 1 – 5), which are synthesized by a citrate pyrolysis technique [1], are heated under O2 atmosphere at 850 jC for 0, 0.5, 2, 8, 20 h, respectively. The particle samples are supersonicly vibrated in the ethanol solution; then carbon mesh samples for TEM observation are made. The TEM observation is taken by Hitachi H9000NAR HRTEM at 300 kV (with point resolution of 0.18 nm) and JEOL-200CX at 200 kV.
Fig. 2. (a) The low magnification HRTEM image of Sample No. 1. (b) The high magnification HRTEM image corresponding to the area A in (a).
H.Y. Pan et al. / Materials Letters 57 (2003) 3869–3873
curve of this sample is acquired through TEM observation and statistical analysis, as shown in Fig. 1. The size of YBCO particles ranges 8 –30 nm, most of all between 14 and 16 nm, just as shown in Fig. 2. The result well tallies with the X-ray diffraction analysis [1]. The No. 2 sample is annealed under O2 atmosphere for 0.5 h. By X-ray diffraction analysis [1], the emergence of the orthorhombic superconductivity phase is proved. It is found that there are some big particles which sizes are between 0.5 and 1 Am in the No. 2 sample (shown in Fig. 3a), but most particles
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are still small nanoscale particles as shown in Fig. 3b. Fig. 3c shows that the lattice of nanometer particles begins to bend, and the interface of particles becomes ambiguous. The particles will step into a state of recrystallization. The No. 3 sample is annealed under O2 atmosphere for 2 h. The appearance of the orthorhombic superconductivity phase is also proved by X-ray diffraction [1]. Compared with No. 2 sample, big particles in No. 3 sample increase, while small particles decrease. Under the similar observation and analysis with TEM and X-ray diffraction, the same trend of size
Fig. 3. (a) The TEM image of big particles from Sample No. 2; the size of big particles is between 0.5 and 1 Am. (b) The HRTEM image of small size particles from Sample No. 2. (c) The high magnification HRTEM image from A area in (b), the lattice of nanometer particle begins to bend, and the interface of particles becomes ambiguous, and the particles begin to aggregate.
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H.Y. Pan et al. / Materials Letters 57 (2003) 3869–3873
distribution is shown in samples No. 4 (annealed for 8 h) and No. 5 (annealed for 20 h) whose main structure are both orthorhombic superconducting phase, namely, the more annealing time given, the more big particles and the less small particles appear. But the size of the big particle changes nothing between 0.5 and 1 Am, shown in Fig. 4. Secondly, the quality of big particles with the orthorhombic phase structure is analyzed. In most cases, the separate big particles are single crystals as shown in Fig. 5. But the quality of single crystalline is not perfect, a lot of stack faults is observed. Thus it can be concluded that the big particle is not formed by simply connecting many small polycrystalline interfaces, but through the recrystallizing of many small particles. From the abovementioned, it can be found that during the annealing with different time (0 –20 h) at 850 jC under O2 atmosphere the more time given, the more big particles and the less small particles appear. The recrystalline phase transition, which is analogous to energy ‘quantum’ transition process, is not a uniform process, i.e., not all of the particles grow bigger simultaneously; some particles grow bigger preferentially. The residual energy and substance are not
Fig. 5. (a) The TEM image of one big particle from Sample No. 3; the particle is a single crystalline which contains a lot of stack faults. (b) The selected area electron diffraction pattern of one big particle corresponding to (a); it consists of one set of electron diffraction pattern [001] axis.
Fig. 4. The TEM image of big particles from Sample No. 5; the size of big particles is between 0.5 and 1 Am.
transferred to other particles until the thermal dynamics symmetries are established in big particles. In the process of annealing, the transition from the tetragonal phase to the orthorhombic superconducting phase occurs, i.e., small particles with tetragonal phase structure recrystallize into big particles with orthorhombic superconducting phase structure. However, the size of the big particles remains unchanged between 0.5 and 1 Am. It shows that the size of the big particles has no clear reliance on time within the range of 0– 20 h, but its size is probably related to the
H.Y. Pan et al. / Materials Letters 57 (2003) 3869–3873
heat treatment temperature and O2 pressure which affect thermodynamics energy of the system. So at this temperature, instead of acquiring nanoscale YBCO particles, only micron scale YBCO superconducting particles can be formed.
4. Conclusions 1. The size of YBCO particles in No. 1 sample, which is not annealed, is between 8 and 30 nm, most between 14 and 16 nm. 2. The more annealing time given, the more big particles and the less small particles appear, but the size of the big particles remains unchanged between 0.5 and 1 Am and has no clear reliance on time within the range of 0– 20 h. 3. The transition from the tetragonal phase to the orthorhombic superconducting phase is a process of recrystallization. The big particle with the orthorhombic phase structure is a single crystalline, which is not formed by simply connecting many small polycrystalline interfaces, but through the recrystallizing of many small particles. The quality of the single crystalline is not perfect. Besides, the crystalline contains a lot of stack faults. To sum up, under the condition of appropriate heat treatment, the nanoscale YBCO particles can be trans-
3873
formed into superconductivity orthorhombic phase. This is inspiring for the manufacturing of the third generation superconducting tapes. By coating soft metal substrate with nanoscale YBCO particle materials, using roll technique and then annealing at appropriate temperature under O 2 atmosphere, YBCO superconducting tapes could be produced. This work on the preparation of long superconducting tapes with nanoscale YBCO powders is in progress and we look forward to future good results.
Acknowledgements The work is supported by China Post-doctor Foundation, School Measurement Foundation of Peking University and China National Natural Science Foundation No. 10004001.
References [1] X.L. Xu, J.D. Guo, Y.Z. Wang, A. Suzzi, Physica, C 371 (2) (2002) 129 – 132. [2] K. Hiraga, D. Shindo, M. Hirabayashi, et al., Jpn. J. Appl. J. Electron Microsc. 36 (1987) 261. [3] L. Li, Materials Science and Engineering, Report: a Review Journal, 29 (2000) 153 – 181. [4] H. Kezuka, Z. Xi, Q. Zhang, Physica, C 282 – 287 (1997) 523 – 524.