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Improvement of HTSC-materials by treatment with laser
Physica C 372–376 (2002) 1195–1199 www.elsevier.com/locate/physc
Improvement of HTSC-materials by treatment with laser R. Lutciv a, J. Plewa b,c,*, M. Vasyuk a, I. Solski d, V. Vashook e, O. Tolochko e, N. Munser c, H. Altenburg b,c b
a Lviv State University, 290005 Lviv, Ukraine Steinfurter Initiative f€ur Materialforschung, SIMa, Stegerwaldstrasse 39, 48565 Steinfurt, Germany c Fachhochschule M€unster/University of Applied Sciences, 48565 Steinfurt, Germany d Scientific Research Company CARAT, 290031 Lviv, Ukraine e National Academy of Sciences of Belarus, 229972 Minsk, Belarus
Abstract The influence of different kinds (Nd, ruby and CO2 ) of laser radiation on HTSC (Y-123, Bi-2212, Bi-2223) ceramics are shown. Irradiation of HTSC leads to modification of the near-surface layers, by processes such as ceramic densification, granular melting and the appearance of continuous melting with increasing laser irradiation energy. On the whole, the density and smoothness of the surface were improved and content of carbon was minimised with the help of radiation and subsequent thermal treatment. The current density has higher values (>500 A/cm2 ) than those which were achieved before the laser treatment (20–340 A/cm2 ). Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: HTSC-materials; Laser radiation; Current density
1. Introduction Application of massive HTSC-materials is limited by low volume of critical currents. During investigation of granular HTS-materials it is necessary to take into account that these ceramics consist of weakly connected crystals. Such ceramics have a critical current density of about 10–103 A/cm2 , which is in some cases lower than inside the grains [1]. The change of the inter-grain environment by melting and elimination of other phases * Corresponding author. Address: Steinfurter Initiative f€ ur Materialforschung, SIMa, Stegerwaldstrasse 39, 48565 Steinfurt, Germany. Tel.: +49-2551-962-412; fax: +49-2551-952-169. E-mail address: [email protected] (J. Plewa).
from grain boundaries and the texturation should actively influence the critical currents of HTSCceramic [1–3]. Because of this, the most important problem is the search for ways to increase jC . It can be solved by using laser radiation to alter a ceramic material. This leads to densification of the ceramic due to the melting of the near-surface layers melting and their re-crystallisation with preserved HTSC-structure. The influence of laser radiation on the characteristics of HTSC was studied in a number of publications [4–11]. However their results are ambiguous and inconsistent. In general, degradation of the HTSC-material is observed, but there is also data showing an increase in the critical current [5–11].
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 0 9 7 1 - 1
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2. Experimental The lasers used to irradiate the HTSC were the CO2 -laser (k ¼ 10:6 lm, s 0:3 s, P ¼ 300–500 W/cm2 ), pulsed solid state ruby laser (k ¼ 0:679 lm, s ¼ 1:5 ms, E 102 J/cm2 ), and pulsed millisecond Nd-laser (k ¼ 1:06 lm, s 103 s, E ¼ 0:1–10 J/cm2 ). Samples for measurement of critical currents, laser processing, research of microstructure and other properties of near-surface layers were made from the tablets prepared using solid state ceramic methods. Parameters of initial samples: Bi1:65 Pb0:35 Sr1:8 Ca2:2 Cu3 O10d (Bi-2223, TC ¼ 109 1 K; jC ¼ 65 A/cm2 ); Bi1:7 Pb0:3 Sr2:1 Ca0:9 Cu2 O8d (Bi-2212, TC ¼ 79 1 K; jC ¼ 16:5 A/cm2 ); YBa2 Cu3 O7d (Y-123, TC ¼ 91 1 K; jC ¼ 342 A/cm2 ). The irradiation was carried out in air with and without the presence of additional heating. The energies of pulsed and continued irradiation were sufficient for homogeneous melting of surface layer. The temperature of additional heating during a laser irradiation was about 400–750 °C. For restoration of superconducting properties after laser processing, additional thermal annealing was carried out in O2 . The contact platforms for electrical measurements were made with silver pastes or liquid solders with the use of ultrasonic heaters. The transport critical current was measured with the four-point method at a constant current. The method of a magnetic susceptibility was also applied.
The surface condition was observed by optical and electronic microscanning and also by X-ray spectroscopy.
3. Results and discussion In Fig. 1 the microstructures of different ceramics that were melted by Nd-laser surface irradiation are shown. After processing the Y-123 with the Nd-laser, changes were observed in its surface layer. The surface was more homogeneous with melted inter-grain zones (Fig. 2). Distribution of intensity of the basic component Ka (Cu), La (Ba) along a zone of laser influence of the Nd-laser on the basis of the X-ray microanalysis testifies to the more homogeneous distribution of the components after laser influence in comparison with the non-irradiated surface (Fig. 3). In addition to the basic component peaks in a spectrum of the non-irradiated material, a peak of carbon was also found by a OJE-spectroscopy method. In Fig. 4 a portion of a OJE-electron spectrum is shown. Carbon which was observed in initial ceramics was absorbed under influence of laser radiation and promoted formation of more dense ceramics. Depending on the intensity and duration of laser application, the surface layers of the investigated HTSC melted with subsequent recrystallisation with a complex surface structure for different HTSC. The melted zone can be up to 80 lm thick. An increase of microhardness (by 3–8
Fig. 1. Surfaces of the HTS-samples after Nd-laser treatment with different yield: (a) Bi-2223, E ¼ 4:5 J/cm2 , (b) Bi-2212, E ¼ 4:5 J/cm2 , (c) Y-123, E ¼ 7:9 J/cm2 (light zone ¼ laser-irradiated zone).
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Fig. 2. Microstructure of the Y-123 sample before (a) and after (b) Nd-laser treatment.
Fig. 3. Distribution of Ba and Cu on Y-123 sample after laser radiation.
times, due to the influence of the CO2 -laser on Bi2223 and Bi-2212) was simultaneously observed, but due to the fast heating and cooling there is a grid of non-oriented microcracks (Fig. 2b) and degradation of the material (Fig. 5a and b). Comparative X-ray diffractograms from before and after the laser irradiation are shown in Fig. 5a for Bi-2212 heated with a ruby laser, and in Fig. 5b, for Bi-2223 heated with a CO2 -laser. As it can be seen from data of X-ray analysis (Fig. 5), additional heat treatment in O2 for reproduction of
Fig. 4. Auger spectra of Y-123 sample: (a) initial material, (b) at the centre of the laser-irradiated zone, (c) on the border of the laser-irradiated zone.
superconducting properties is required. The absorption of carbon (Fig. 4) and re-crystallisation of HTSC-ceramic under the influence of laser irradiation and subsequent heat treatment are the factors that form a laser-irradiated superconductor layer that has increased density, improved
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Fig. 5. X-rays of Bi-2212 sample (a) and Bi-2223 sample (b) before and after laser radiation.
inter-grain zones, and, as a consequence, increased critical parameters. The dependence of changes in the critical current in Y-123 ceramic on irradiation with Nd-laser and subsequent heat treatment was shown [10]. Directly after laser irradiation a decrease of the critical current was observed, and for the parameters E ¼ 0:15 J and D ¼ 0:2 cm, it decreased by about 1.5 times. Further heat treatment in an oxygen atmosphere promotes an increase of a critical current up to 600 A/cm2 . According to the dependence of critical parameters on heat treatment, the following profile of heat treatment is offered (Fig. 6). The first part of the treatment is the laser irradiation of the sample which has been previously heated to 750 °C. This reduces the number of cracks after laser treatment, minimises the deviation on phase structure, and
creates a homogeneous distribution in the component with partial texturing. After such laser processing, reduction of critical current is observed when compared with the initial sample. The subsequent heating of the sample to 920 °C and annealing for 20 h was used for solid phase growth of the grains. The short-term heating for about 1 h at 940 °C influenced inter-grain regions and optimised the connections between the grains. The homogenisation processing at 400 °C formed an oxygen index in Y-123, which caused the critical current to increase by 2.5–3 times in comparison with the critical current of the initial samples. As it can be seen from the temperature dependence of magnetic susceptibilities of Y-123 sample (Fig. 7), directly after laser irradiation, extension of superconducting transition and small changes of the TC are observed. The offered temperature
Fig. 6. Temperature–time schedule for laser and furnace treatment.
Fig. 7. Magnetic susceptibilities of Y-123 sample after laser irradiation.
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regime can be carried out by overlapping laser processing with the subsequent heating to 920–940 °C in an atmosphere of oxygen, without cooling the sample to room temperature after laser influence.
Forschung und Technologie under the contract no. 13N7325 and by the Ministerium f€ ur Schule, Wissenschaft und Forschung von NordrheinWestfalen.
4. Conclusions
References
The formation of Y-123 with increased critical current density occurs, first of all, in samples with the lowest initial critical current density. With an increase of laser irradiation intensity the melting of the inter-grain regions increases along with the elimination of carbon groups from the borders of the grains. The following heat treatment is necessary for the reproduction of the oxygen index and for improvement of contacts between grains. The laser-irradiated layer consists of Y-123 and also has phase inclusions (such as Y2 O3 , Y-211). It is a partially textured layer with high density and increased jC . The structural modification of HTS-ceramic surface with application of laser irradiation and furnace treatment, both its subsequent re-crystallisation and heat treatment in an atmosphere of oxygen allows the critical current to increase 2.5–3 times with a minimal deviation of superconducting transition temperature of 1–2 K and DTC ¼ 5 K. Acknowledgements This research was supported by the German Bundesministerium f€ ur Bildung, Wissenschaft;
[1] A.M. Campbel, Physica B 216 (1996) 266. [2] R. Adam, W. Kula, R. Sobolewski, Appl. Phys. Lett. 67 (1995) 3801. [3] D.I. Pantelis, G. Pantazopoulos, Physica C 265 (1996) 341. [4] M. Okutomi, H. Nomura, T. Tsukamoto, N. Dahorte, H. Shen, Nucl. Instr. Meth. Res. B 169 (2000) 6. [5] H. Dyck, N. Munser, W. Jaszczuk, C. Seega, H. Altenburg, J. Plewa, I. Kononyuk, V. Vashook, S. Tolochko, A. Shkadarevitch, A. Sabasnov, in: 6. Statussem.: Supraleitung & Tieftemperaturtechnik, Gelsenkirchen, Tagungsband, Okt., 1998, p. 479. [6] V.A. Lomonosov, I.F. Kononyuk, S.P. Tolochko, V.V. Vashook, H. Dyck, H. Altenburg, in: Second International Conference on Inorganic Materials, vol. 44, Santa Barbara, USA, September, 2000. [7] M.M. Vasyuk, R.V. Lutciv, in: Second International Symposium on High Tc Superconduction and Tunnel Phenomena, vol. 169, Slavyanogorsk, Donetsk, Ukraine, 1995. [8] R. Lutciv, J. Plewa, I. Solski, V. Vashook, O. Tolochko, N. Munser, H. Altenburg, in: 7. Statussem.; Supraleitung & Tieftemperaturtechnik, Garmischparten-Kirchen, Tagungsband, Dez., 2000, p. 542. [9] A.L. Mikhailchenko, G.N. Michailova, A.M. Prokhorov, A.S. Seferov, A.V. Trocki, A.O. Mednikov, B.L. Mikhailov, G.S. Burshanov, I.E. Lapshina, Quant. Electron. 23 (1996) 715. [10] M.M. Vasyuk, I.V. Lazaryuk, M.V. Matviyiv, H. Altenburg, J. Plewa, Visnyk Lviv Univ. Ser. Phys. 33 (2000) 173. [11] J.C. Diez, L.A. Angurel, H. Miao, J.M. Fernandez, G.F. de la Fuente, Supercond. Sci. Technol. 11 (1998) 101.
Improvement of HTSC-materials by treatment with laser R. Lutciv a, J. Plewa b,c,*, M. Vasyuk a, I. Solski d, V. Vashook e, O. Tolochko e, N. Munser c, H. Altenburg b,c b
a Lviv State University, 290005 Lviv, Ukraine Steinfurter Initiative f€ur Materialforschung, SIMa, Stegerwaldstrasse 39, 48565 Steinfurt, Germany c Fachhochschule M€unster/University of Applied Sciences, 48565 Steinfurt, Germany d Scientific Research Company CARAT, 290031 Lviv, Ukraine e National Academy of Sciences of Belarus, 229972 Minsk, Belarus
Abstract The influence of different kinds (Nd, ruby and CO2 ) of laser radiation on HTSC (Y-123, Bi-2212, Bi-2223) ceramics are shown. Irradiation of HTSC leads to modification of the near-surface layers, by processes such as ceramic densification, granular melting and the appearance of continuous melting with increasing laser irradiation energy. On the whole, the density and smoothness of the surface were improved and content of carbon was minimised with the help of radiation and subsequent thermal treatment. The current density has higher values (>500 A/cm2 ) than those which were achieved before the laser treatment (20–340 A/cm2 ). Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: HTSC-materials; Laser radiation; Current density
1. Introduction Application of massive HTSC-materials is limited by low volume of critical currents. During investigation of granular HTS-materials it is necessary to take into account that these ceramics consist of weakly connected crystals. Such ceramics have a critical current density of about 10–103 A/cm2 , which is in some cases lower than inside the grains [1]. The change of the inter-grain environment by melting and elimination of other phases * Corresponding author. Address: Steinfurter Initiative f€ ur Materialforschung, SIMa, Stegerwaldstrasse 39, 48565 Steinfurt, Germany. Tel.: +49-2551-962-412; fax: +49-2551-952-169. E-mail address: [email protected] (J. Plewa).
from grain boundaries and the texturation should actively influence the critical currents of HTSCceramic [1–3]. Because of this, the most important problem is the search for ways to increase jC . It can be solved by using laser radiation to alter a ceramic material. This leads to densification of the ceramic due to the melting of the near-surface layers melting and their re-crystallisation with preserved HTSC-structure. The influence of laser radiation on the characteristics of HTSC was studied in a number of publications [4–11]. However their results are ambiguous and inconsistent. In general, degradation of the HTSC-material is observed, but there is also data showing an increase in the critical current [5–11].
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 0 9 7 1 - 1
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2. Experimental The lasers used to irradiate the HTSC were the CO2 -laser (k ¼ 10:6 lm, s 0:3 s, P ¼ 300–500 W/cm2 ), pulsed solid state ruby laser (k ¼ 0:679 lm, s ¼ 1:5 ms, E 102 J/cm2 ), and pulsed millisecond Nd-laser (k ¼ 1:06 lm, s 103 s, E ¼ 0:1–10 J/cm2 ). Samples for measurement of critical currents, laser processing, research of microstructure and other properties of near-surface layers were made from the tablets prepared using solid state ceramic methods. Parameters of initial samples: Bi1:65 Pb0:35 Sr1:8 Ca2:2 Cu3 O10d (Bi-2223, TC ¼ 109 1 K; jC ¼ 65 A/cm2 ); Bi1:7 Pb0:3 Sr2:1 Ca0:9 Cu2 O8d (Bi-2212, TC ¼ 79 1 K; jC ¼ 16:5 A/cm2 ); YBa2 Cu3 O7d (Y-123, TC ¼ 91 1 K; jC ¼ 342 A/cm2 ). The irradiation was carried out in air with and without the presence of additional heating. The energies of pulsed and continued irradiation were sufficient for homogeneous melting of surface layer. The temperature of additional heating during a laser irradiation was about 400–750 °C. For restoration of superconducting properties after laser processing, additional thermal annealing was carried out in O2 . The contact platforms for electrical measurements were made with silver pastes or liquid solders with the use of ultrasonic heaters. The transport critical current was measured with the four-point method at a constant current. The method of a magnetic susceptibility was also applied.
The surface condition was observed by optical and electronic microscanning and also by X-ray spectroscopy.
3. Results and discussion In Fig. 1 the microstructures of different ceramics that were melted by Nd-laser surface irradiation are shown. After processing the Y-123 with the Nd-laser, changes were observed in its surface layer. The surface was more homogeneous with melted inter-grain zones (Fig. 2). Distribution of intensity of the basic component Ka (Cu), La (Ba) along a zone of laser influence of the Nd-laser on the basis of the X-ray microanalysis testifies to the more homogeneous distribution of the components after laser influence in comparison with the non-irradiated surface (Fig. 3). In addition to the basic component peaks in a spectrum of the non-irradiated material, a peak of carbon was also found by a OJE-spectroscopy method. In Fig. 4 a portion of a OJE-electron spectrum is shown. Carbon which was observed in initial ceramics was absorbed under influence of laser radiation and promoted formation of more dense ceramics. Depending on the intensity and duration of laser application, the surface layers of the investigated HTSC melted with subsequent recrystallisation with a complex surface structure for different HTSC. The melted zone can be up to 80 lm thick. An increase of microhardness (by 3–8
Fig. 1. Surfaces of the HTS-samples after Nd-laser treatment with different yield: (a) Bi-2223, E ¼ 4:5 J/cm2 , (b) Bi-2212, E ¼ 4:5 J/cm2 , (c) Y-123, E ¼ 7:9 J/cm2 (light zone ¼ laser-irradiated zone).
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Fig. 2. Microstructure of the Y-123 sample before (a) and after (b) Nd-laser treatment.
Fig. 3. Distribution of Ba and Cu on Y-123 sample after laser radiation.
times, due to the influence of the CO2 -laser on Bi2223 and Bi-2212) was simultaneously observed, but due to the fast heating and cooling there is a grid of non-oriented microcracks (Fig. 2b) and degradation of the material (Fig. 5a and b). Comparative X-ray diffractograms from before and after the laser irradiation are shown in Fig. 5a for Bi-2212 heated with a ruby laser, and in Fig. 5b, for Bi-2223 heated with a CO2 -laser. As it can be seen from data of X-ray analysis (Fig. 5), additional heat treatment in O2 for reproduction of
Fig. 4. Auger spectra of Y-123 sample: (a) initial material, (b) at the centre of the laser-irradiated zone, (c) on the border of the laser-irradiated zone.
superconducting properties is required. The absorption of carbon (Fig. 4) and re-crystallisation of HTSC-ceramic under the influence of laser irradiation and subsequent heat treatment are the factors that form a laser-irradiated superconductor layer that has increased density, improved
1198
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Fig. 5. X-rays of Bi-2212 sample (a) and Bi-2223 sample (b) before and after laser radiation.
inter-grain zones, and, as a consequence, increased critical parameters. The dependence of changes in the critical current in Y-123 ceramic on irradiation with Nd-laser and subsequent heat treatment was shown [10]. Directly after laser irradiation a decrease of the critical current was observed, and for the parameters E ¼ 0:15 J and D ¼ 0:2 cm, it decreased by about 1.5 times. Further heat treatment in an oxygen atmosphere promotes an increase of a critical current up to 600 A/cm2 . According to the dependence of critical parameters on heat treatment, the following profile of heat treatment is offered (Fig. 6). The first part of the treatment is the laser irradiation of the sample which has been previously heated to 750 °C. This reduces the number of cracks after laser treatment, minimises the deviation on phase structure, and
creates a homogeneous distribution in the component with partial texturing. After such laser processing, reduction of critical current is observed when compared with the initial sample. The subsequent heating of the sample to 920 °C and annealing for 20 h was used for solid phase growth of the grains. The short-term heating for about 1 h at 940 °C influenced inter-grain regions and optimised the connections between the grains. The homogenisation processing at 400 °C formed an oxygen index in Y-123, which caused the critical current to increase by 2.5–3 times in comparison with the critical current of the initial samples. As it can be seen from the temperature dependence of magnetic susceptibilities of Y-123 sample (Fig. 7), directly after laser irradiation, extension of superconducting transition and small changes of the TC are observed. The offered temperature
Fig. 6. Temperature–time schedule for laser and furnace treatment.
Fig. 7. Magnetic susceptibilities of Y-123 sample after laser irradiation.
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regime can be carried out by overlapping laser processing with the subsequent heating to 920–940 °C in an atmosphere of oxygen, without cooling the sample to room temperature after laser influence.
Forschung und Technologie under the contract no. 13N7325 and by the Ministerium f€ ur Schule, Wissenschaft und Forschung von NordrheinWestfalen.
4. Conclusions
References
The formation of Y-123 with increased critical current density occurs, first of all, in samples with the lowest initial critical current density. With an increase of laser irradiation intensity the melting of the inter-grain regions increases along with the elimination of carbon groups from the borders of the grains. The following heat treatment is necessary for the reproduction of the oxygen index and for improvement of contacts between grains. The laser-irradiated layer consists of Y-123 and also has phase inclusions (such as Y2 O3 , Y-211). It is a partially textured layer with high density and increased jC . The structural modification of HTS-ceramic surface with application of laser irradiation and furnace treatment, both its subsequent re-crystallisation and heat treatment in an atmosphere of oxygen allows the critical current to increase 2.5–3 times with a minimal deviation of superconducting transition temperature of 1–2 K and DTC ¼ 5 K. Acknowledgements This research was supported by the German Bundesministerium f€ ur Bildung, Wissenschaft;
[1] A.M. Campbel, Physica B 216 (1996) 266. [2] R. Adam, W. Kula, R. Sobolewski, Appl. Phys. Lett. 67 (1995) 3801. [3] D.I. Pantelis, G. Pantazopoulos, Physica C 265 (1996) 341. [4] M. Okutomi, H. Nomura, T. Tsukamoto, N. Dahorte, H. Shen, Nucl. Instr. Meth. Res. B 169 (2000) 6. [5] H. Dyck, N. Munser, W. Jaszczuk, C. Seega, H. Altenburg, J. Plewa, I. Kononyuk, V. Vashook, S. Tolochko, A. Shkadarevitch, A. Sabasnov, in: 6. Statussem.: Supraleitung & Tieftemperaturtechnik, Gelsenkirchen, Tagungsband, Okt., 1998, p. 479. [6] V.A. Lomonosov, I.F. Kononyuk, S.P. Tolochko, V.V. Vashook, H. Dyck, H. Altenburg, in: Second International Conference on Inorganic Materials, vol. 44, Santa Barbara, USA, September, 2000. [7] M.M. Vasyuk, R.V. Lutciv, in: Second International Symposium on High Tc Superconduction and Tunnel Phenomena, vol. 169, Slavyanogorsk, Donetsk, Ukraine, 1995. [8] R. Lutciv, J. Plewa, I. Solski, V. Vashook, O. Tolochko, N. Munser, H. Altenburg, in: 7. Statussem.; Supraleitung & Tieftemperaturtechnik, Garmischparten-Kirchen, Tagungsband, Dez., 2000, p. 542. [9] A.L. Mikhailchenko, G.N. Michailova, A.M. Prokhorov, A.S. Seferov, A.V. Trocki, A.O. Mednikov, B.L. Mikhailov, G.S. Burshanov, I.E. Lapshina, Quant. Electron. 23 (1996) 715. [10] M.M. Vasyuk, I.V. Lazaryuk, M.V. Matviyiv, H. Altenburg, J. Plewa, Visnyk Lviv Univ. Ser. Phys. 33 (2000) 173. [11] J.C. Diez, L.A. Angurel, H. Miao, J.M. Fernandez, G.F. de la Fuente, Supercond. Sci. Technol. 11 (1998) 101.