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Modification of the high-Tc superconductor YBa2Cu3O7−x by particle-beam irradiation
PHYSICA
Physica C 190 (1991) 116-118 North-Holland
Modification of the high-To superconductor YBa2Cu307_x by particle-beam irradiation Takayuki Terai, Kazuyuki Kusagaya and Yoichi Takahashi Department of Nuclear Engineering, Universityof Tokyo, 7-3-1, Hongo, Tokyo 113, Japan
Some results obtained on irradiation effects on high-To superconductors YBa2Cu3OT_x (YBCO) are reviewed from the viewpoint of its modification by particle-beam irradiation. The main features of the irradiation effects are Jr enhancement by the introduction of pinning centers in low-fluence regions and degradation of To, J~ and crystal structure in high-fluence regions. The critical fluence depends strongly upon the kind and the energy of the incident particle as well as irradiation temperature. The difference in irradiation effects among particles will be related to the structure and the characteristics of the irradiation defects.
1. Introduction Partiele-beam irradiation techniques using neutrons, ions, electrons and photons have been examined to modify the properties of high-To superconductors, e.g. ( 1 ) Jc enhancement by the introduction of pinning centers, (2) a decrease of Tc and Je by heavy irradiation, and (3) the improvement of superconductivity by the rearrangement of constitutional atoms due to energy deposition. We also have been studying irradiation effects on high-To superconductors, YBazCu3OT_x (YBCO), focusing on the first two items [ 1-6]. In this paper, we review our experimental results from the viewpoint of radiation damage theory to discuss its mechanism.
2. Radiation damage theory Radiation causes several kinds of changes in solid materials. One of the most important changes is atom displacement due to knocking-on and another is electron excitation or ionization. The latter often results in chemical reactions and finally disappears, while the former results in radiationinduced defects, which change the material properties. These two changes are derived from nuclear energy loss and electric energy loss of radiation in condensed matter, and the contribution of each phenomenon differs among the kind and the energies of the incidental particles. In the case of electrons and photons, electric energy loss is dominant, while in the case of ions and neutrons, nuclear energy loss is also important. A model on the formation of radiation-induced defects is shown in fig. 1. A high-energy ion beam introduces cascade damage in a
cylindrical shape whose axis is parallel to the incidental direction. Neutron irradiation also introduces cascade damage because primary knocked-on atoms behave like ions of 1 keV-1 MeV in energy. In this case, however, the cascade damage regions have a spherical shape. On the contrary, several MeV electron irradiation generates only point defects in stead of cascade defects, because the knocked-on atoms have the kinetic energy of only 10-102 eV. I-MeV y-ray irradiation also can introduce only point defects, though the density is much smaller than electron irradiation. Such irradiation defects are also influenced strongly by the irradiation temperature. At low temperature, the defects cannot move, while, at high temperature, the defects disappear by recombination. In the middle temperature range, the defects form some kinds of complex defect clusters such as dislocation loops and stacking faults in stead of annihilation. Radiochemical effects during irradiation make the situation more complicated. Local heating effects by thermal spikes, etc., are expected to accelerate the moving of interstitial atoms and vacancies. Thus, non-superconducting regions are formed by irradiation, and their size ( 1 nm-100 nm) is dependent on the irradiation conditions. This means that we can control the microstructure of superconductor by irradiation to modify its properties. The most important points are what kind of defects are formed at a given temperature and what change is given in properties such as ~ , Jc and crystal structure by them.
3. Experimental resets To clarify the irradiation effects on To, Jc, etc., a well-char-
0921-4534/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.
T. Terai et at / Modification of YBCO by particle-beam irradiation
[,,,!ON(~MeV) I
i (Cylindrical) [ Cascade Damage (Spherical)
NEUTRON(~MeV) I Internal ion irrad. (keV~lO2keV) I
High Temp. Thermal Annealing
l
Radiochemical Effect (Thermal Spike, Local Heating etc.)
ELECTRON(~MeV) Internal ion irrad. (10eV~102eV)
117
Middle Temp. Dislocation Loop, Defect Cluster etc. Low Temp. No Further Change
~-RAY(~MeV) Internal electron irrad. (< MeV)
Non-superconducting region (rim "~ #m t
I
To, Jr, Crystal Structure ? ] Fig. 1. A model for the formation of radiation-induced defects by several kinds of particle beams. acterized sample (if possible a single crystal) is promising. We utilized c-axis oriented YBCO thin films grown on SrTiO3( 1 0 0) substrates (Tc=88 K, 200 nm in thickness)
Table l Critical fluence giving a 1% T:decrease in a YBCO thin film by irradiation at room temperature
for this purpose. A typical example of the changes of Tc and Jc by irradiation is shown in fig. 2 [4]. This is the result of the enhancement ratios of T~ and Jc plotted against the fluence of 1 MeV Ar + ion irradiation. In a low-fluence region, Jc is enhanced by irradiation, while Tc does not change. On the other hand, in a high-fluence region, both T¢ and J¢ decrease to zero with increasing fluence. Changes in the crystal structure such as increases in d-spacing along the c-axis and in FWHM of XRD peaks are observed in the same fluence region. Enhancement
Sort
Energy (MeV)
Critical fluence (cm -2)
Reference
Ni2+ Ar + O+ He + n e-
2.5 1.0 2.5 2.5 > 0.1 3
1x 1013 1× 10~3 1X 1014 1 X l 015 5 X 1017 3 × 10is
[5] [4] [6] [5] [7 ] [ 8]a~
I
I
I
'°"-x.
10
I
• :L/T~0
11
12 Iog(¢t/ions'em -2)
13
14
Fig. 2. Enhancement ratios of T, and Jc by 1 MeV Ar ÷ ion irradiation on YBa2Cu307_x thin film. The values of J~ are defined as the current which gives the voltage of 10 gV cm- ~at 80 K and 0T.
a~ Sintered polycrystalline sample. of J¢ is also observed by 28 MeV electron irradiation [ 3 ]. This means that defects dusters may act as pinning centers in stead of point defects because of the size of effective pinning centers is of the same order as the coherence length. Such degraded properties are partially recovered by annealing the sample at 400°C in air, and perfectly by annealing at 900 ° C [ 2 ]. This suggests that the radiation damage mainly consists of interstitial atoms and vacancies of oxygen, which can migrate at 400°C, and a small portion of those of other metallic elements. The critical fluence, which is defined as the minimum fluence giving 1% decrease in To, is listed in table 1 for several kinds of incidental particles. Apart from energy, critical fluence increases with decreasing mass of the particles, which suggests that the collision of the particles to constitutional atoms of YBCO has a great influence on the change of the macroscopic properties of the compounds. It will be our next subject to identify the size, the density and the characteristic
I 18
T. Terai et al. / Modification o f YBCO by particle-beam irradiation
of the defects introduced by irradiation followed by thermal annealing.
Acknowledgement A part of this study was financially supported by a Grantin-Aid for Scientific Research on Chemistry of New Superconductors (no. 03211207 ).
References [ t ] T. Masegi, T. Terai, Y. Takahashi, Y. Enomoto and S. Kubo, Jpn. J. Appl. Phys. 28 (1989) L 1521.
[ 2 ] T. Terai, T. Masegi, Y. Takahashi, Y. Enomoto and S. Kubo, Mol. Cryst. Liq. Cryst. 184 (1990) t 65. [ 3 ] I". Terai, T. Masegi, Y. Takahashi, Y. Enomoto and S. Kubo, Jpn. J. Appl. Phys. 29 (1990) L2053. [ 4 ] T. Terai, T. Furuta, T. Masegi, K. Kusagaya and Y. Takahashi, Jpn. J. Appl. Phys. 30 ( 1991 ) L728. [5 ] T. Terai, K. Kusagaya, T, Furuta, Y. Takahashi, Y. Enomoto and S. Kubo, presented at M2S-HTSC III ( 1991 ). [6] K. Kusagaya, T. Terai, Y. Takahashi, S. Kubo and Y. Enomoto, Jpn. J. Appl. Phys., to be submitted. [7 ] A. Umezawa, G.W. Crabtree, J.Z. Liu, H.W. Weber, W.K. Kwok and LH. Nunez, Phys. Rev. B36 (1987) 7151. [8] K. Shiraishi, T. Kato and J. Kuniya, Jpn. J. Appl. Phys. 27 (1988) L2339.
Physica C 190 (1991) 116-118 North-Holland
Modification of the high-To superconductor YBa2Cu307_x by particle-beam irradiation Takayuki Terai, Kazuyuki Kusagaya and Yoichi Takahashi Department of Nuclear Engineering, Universityof Tokyo, 7-3-1, Hongo, Tokyo 113, Japan
Some results obtained on irradiation effects on high-To superconductors YBa2Cu3OT_x (YBCO) are reviewed from the viewpoint of its modification by particle-beam irradiation. The main features of the irradiation effects are Jr enhancement by the introduction of pinning centers in low-fluence regions and degradation of To, J~ and crystal structure in high-fluence regions. The critical fluence depends strongly upon the kind and the energy of the incident particle as well as irradiation temperature. The difference in irradiation effects among particles will be related to the structure and the characteristics of the irradiation defects.
1. Introduction Partiele-beam irradiation techniques using neutrons, ions, electrons and photons have been examined to modify the properties of high-To superconductors, e.g. ( 1 ) Jc enhancement by the introduction of pinning centers, (2) a decrease of Tc and Je by heavy irradiation, and (3) the improvement of superconductivity by the rearrangement of constitutional atoms due to energy deposition. We also have been studying irradiation effects on high-To superconductors, YBazCu3OT_x (YBCO), focusing on the first two items [ 1-6]. In this paper, we review our experimental results from the viewpoint of radiation damage theory to discuss its mechanism.
2. Radiation damage theory Radiation causes several kinds of changes in solid materials. One of the most important changes is atom displacement due to knocking-on and another is electron excitation or ionization. The latter often results in chemical reactions and finally disappears, while the former results in radiationinduced defects, which change the material properties. These two changes are derived from nuclear energy loss and electric energy loss of radiation in condensed matter, and the contribution of each phenomenon differs among the kind and the energies of the incidental particles. In the case of electrons and photons, electric energy loss is dominant, while in the case of ions and neutrons, nuclear energy loss is also important. A model on the formation of radiation-induced defects is shown in fig. 1. A high-energy ion beam introduces cascade damage in a
cylindrical shape whose axis is parallel to the incidental direction. Neutron irradiation also introduces cascade damage because primary knocked-on atoms behave like ions of 1 keV-1 MeV in energy. In this case, however, the cascade damage regions have a spherical shape. On the contrary, several MeV electron irradiation generates only point defects in stead of cascade defects, because the knocked-on atoms have the kinetic energy of only 10-102 eV. I-MeV y-ray irradiation also can introduce only point defects, though the density is much smaller than electron irradiation. Such irradiation defects are also influenced strongly by the irradiation temperature. At low temperature, the defects cannot move, while, at high temperature, the defects disappear by recombination. In the middle temperature range, the defects form some kinds of complex defect clusters such as dislocation loops and stacking faults in stead of annihilation. Radiochemical effects during irradiation make the situation more complicated. Local heating effects by thermal spikes, etc., are expected to accelerate the moving of interstitial atoms and vacancies. Thus, non-superconducting regions are formed by irradiation, and their size ( 1 nm-100 nm) is dependent on the irradiation conditions. This means that we can control the microstructure of superconductor by irradiation to modify its properties. The most important points are what kind of defects are formed at a given temperature and what change is given in properties such as ~ , Jc and crystal structure by them.
3. Experimental resets To clarify the irradiation effects on To, Jc, etc., a well-char-
0921-4534/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.
T. Terai et at / Modification of YBCO by particle-beam irradiation
[,,,!ON(~MeV) I
i (Cylindrical) [ Cascade Damage (Spherical)
NEUTRON(~MeV) I Internal ion irrad. (keV~lO2keV) I
High Temp. Thermal Annealing
l
Radiochemical Effect (Thermal Spike, Local Heating etc.)
ELECTRON(~MeV) Internal ion irrad. (10eV~102eV)
117
Middle Temp. Dislocation Loop, Defect Cluster etc. Low Temp. No Further Change
~-RAY(~MeV) Internal electron irrad. (< MeV)
Non-superconducting region (rim "~ #m t
I
To, Jr, Crystal Structure ? ] Fig. 1. A model for the formation of radiation-induced defects by several kinds of particle beams. acterized sample (if possible a single crystal) is promising. We utilized c-axis oriented YBCO thin films grown on SrTiO3( 1 0 0) substrates (Tc=88 K, 200 nm in thickness)
Table l Critical fluence giving a 1% T:decrease in a YBCO thin film by irradiation at room temperature
for this purpose. A typical example of the changes of Tc and Jc by irradiation is shown in fig. 2 [4]. This is the result of the enhancement ratios of T~ and Jc plotted against the fluence of 1 MeV Ar + ion irradiation. In a low-fluence region, Jc is enhanced by irradiation, while Tc does not change. On the other hand, in a high-fluence region, both T¢ and J¢ decrease to zero with increasing fluence. Changes in the crystal structure such as increases in d-spacing along the c-axis and in FWHM of XRD peaks are observed in the same fluence region. Enhancement
Sort
Energy (MeV)
Critical fluence (cm -2)
Reference
Ni2+ Ar + O+ He + n e-
2.5 1.0 2.5 2.5 > 0.1 3
1x 1013 1× 10~3 1X 1014 1 X l 015 5 X 1017 3 × 10is
[5] [4] [6] [5] [7 ] [ 8]a~
I
I
I
'°"-x.
10
I
• :L/T~0
11
12 Iog(¢t/ions'em -2)
13
14
Fig. 2. Enhancement ratios of T, and Jc by 1 MeV Ar ÷ ion irradiation on YBa2Cu307_x thin film. The values of J~ are defined as the current which gives the voltage of 10 gV cm- ~at 80 K and 0T.
a~ Sintered polycrystalline sample. of J¢ is also observed by 28 MeV electron irradiation [ 3 ]. This means that defects dusters may act as pinning centers in stead of point defects because of the size of effective pinning centers is of the same order as the coherence length. Such degraded properties are partially recovered by annealing the sample at 400°C in air, and perfectly by annealing at 900 ° C [ 2 ]. This suggests that the radiation damage mainly consists of interstitial atoms and vacancies of oxygen, which can migrate at 400°C, and a small portion of those of other metallic elements. The critical fluence, which is defined as the minimum fluence giving 1% decrease in To, is listed in table 1 for several kinds of incidental particles. Apart from energy, critical fluence increases with decreasing mass of the particles, which suggests that the collision of the particles to constitutional atoms of YBCO has a great influence on the change of the macroscopic properties of the compounds. It will be our next subject to identify the size, the density and the characteristic
I 18
T. Terai et al. / Modification o f YBCO by particle-beam irradiation
of the defects introduced by irradiation followed by thermal annealing.
Acknowledgement A part of this study was financially supported by a Grantin-Aid for Scientific Research on Chemistry of New Superconductors (no. 03211207 ).
References [ t ] T. Masegi, T. Terai, Y. Takahashi, Y. Enomoto and S. Kubo, Jpn. J. Appl. Phys. 28 (1989) L 1521.
[ 2 ] T. Terai, T. Masegi, Y. Takahashi, Y. Enomoto and S. Kubo, Mol. Cryst. Liq. Cryst. 184 (1990) t 65. [ 3 ] I". Terai, T. Masegi, Y. Takahashi, Y. Enomoto and S. Kubo, Jpn. J. Appl. Phys. 29 (1990) L2053. [ 4 ] T. Terai, T. Furuta, T. Masegi, K. Kusagaya and Y. Takahashi, Jpn. J. Appl. Phys. 30 ( 1991 ) L728. [5 ] T. Terai, K. Kusagaya, T, Furuta, Y. Takahashi, Y. Enomoto and S. Kubo, presented at M2S-HTSC III ( 1991 ). [6] K. Kusagaya, T. Terai, Y. Takahashi, S. Kubo and Y. Enomoto, Jpn. J. Appl. Phys., to be submitted. [7 ] A. Umezawa, G.W. Crabtree, J.Z. Liu, H.W. Weber, W.K. Kwok and LH. Nunez, Phys. Rev. B36 (1987) 7151. [8] K. Shiraishi, T. Kato and J. Kuniya, Jpn. J. Appl. Phys. 27 (1988) L2339.