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Microstructures and superconducting properties of transformed Nb3Al wire
Physica C 372–376 (2002) 1307–1310 www.elsevier.com/locate/physc
Microstructures and superconducting properties of transformed Nb3Al wire A. Kikuchi *, Y. Iijima, K. Inoue National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
Abstract According to a TEM observation study of rapidly-heating/quenching and transformation (RHQT) processed Nb3 Al wires, many stacking faults spaced 10–20 nm apart were found in the A15 phases except for the grain boundaries. The formation of Al-rich stacking faults seems to shift the composition of the A15 phase to an off-stoichiometric one. Therefore, Tc and Hc2 become rather lower than those of stoichiometric A15 phase. Moreover, we also found that nanodisk shaped Nb–Al oxides precipitate in Nb–Al bcc phase at a rather low temperature of about 400 °C. These nano-disk oxides melt away and form solid-solution with Nb–Al bcc phase at about 750 °C, which is lower than that for transformation. Oxygen contamination may also degrade the superconducting properties of RHQT processed Nb3 Al wires. On the other hand, those oxide nano-disks may behave as flux pinning centers if they remain after phase transformation. Furthermore, we studied the relation between microstructure and superconducting property of some wire specimens, which were transformed under different heat treatment conditions, and the resulting high temperature and very short time heat treatment was very effective to enhance its Tc and Hc2 . This special heating operation was dubbed double rapidly-heating/quenching. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Nb3 Al; Rapid-heating/quenching; TEM structure; Critical current density
1. Introduction The rapidly-heating/quenching and transormation (RHQT) processed Nb3 Al multifilamentary wires show the excellent superconducting performance [1]. Their critical temperature Tc of 17.8 K and upper critical magnetic field Hc2 (4.2 K) of 26 T are much higher than those of the Nb3 Al multifilamentary wires fabricated by a conventional low-temperature diffusion reaction. How-
*
Corresponding author. Fax: +81-298-59-2601. E-mail address: [email protected] (A. Kikuchi).
ever, those values are still rather lower than those of stoichiometric A15-Nb3 Al phase directly synthesized by an arc melting at very high temperature [2]. Recently, it was found that many stacking faults, showing Al-rich composition, formed in the RHQT processed Nb3 Al wire [3]. The formation of the Al rich stacking faults causes to shift the A15 phase matrix composition to the Nb-rich one. The formation of Al-rich stacking faults should be caused by the phase transformation at low temperature of about 800 °C, where off-stoichiometric A15 phase is stable. Since the stoichiometric A15 phase is only stable at 1940 °C, we tried to carry
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 1 0 1 6 - X
1308
A. Kikuchi et al. / Physica C 372–376 (2002) 1307–1310
out the pulsed heat treatment for the phase transformation around that high temperature by using ohmic resistive heating on the reel to reel apparatus, which have been named as the double rapidly-heating/quenching (DRHQ) process [4,5]. In order to get an understanding in detail of transformation mechanism from bcc to A15 phases, we directly observed the TEM microstructure of Nb–Al bcc phase under elevating temperature up to 800 °C. Moreover, we considered the relation between microstructure and superconducting properties of RHQT and DRHQ processed Nb3 Al wires.
2. Experimental procedures The Nb/Al precursor wire fabricated by a conventional jelly-roll process. Nb/Al atomic ratio in the all composite-filaments is designed as 3=1, where is the stoichiometric A15 composition. The RHQ operation for the Nb/Al precursor wire was carried out in a dynamic vacuum chamber. The precursor wire, moving with a speed of 1 m/s, was continuously heated up to 2000 °C by resistiveheating during about 0.1 s, with transporting a dc current between an electrode pully and a molten Ga bath. The wire was subsequently quenched from 2000 °C into the molten Ga bath at about 40 °C. The Nb/Al composite filaments were converted into the Nb–Al supersaturated bcc solid solution filaments by the RHQ operation. Some of wire specimens were applied to the DRHQ which have been previously reported elsewhere in detail. Additional annealing at 800 °C for 12 h was applied for the phase transformation from bcc to A15 and for the improvement of long-range-ordering of disordered A15 phases after removing the coated Ga on the surface of the wire by chemical etching. Tc and Hc2 (4.2 K) were measured by dc four-probe resistive method, and defined as the temperature and the field, respectively, where the sample shows the half value of normal-state resistance. Ic (4.2 K) was determined with the criterion of 100 lV/m in perpendicular fields up to 35 T with increasing transport current. Jc (4.2 K) were defined as Ic (4.2 K) divided by the total cross-sectional area of jellyrolled filaments in the Nb matrix, which is the
maximum expected value of the total A15 phase area formed in the wire. Thin film specimens for TEM observation were prepared from the transverse cross section of the wire, by using a focused Ga ion beam (FIB; Hitachi FB2000A) after mechanical slicing and grinding, resulting in film specimen with about 100 nm in thickness. Microstructure was observed by a field-emission-transmission electron microscopy (FE-TEM; Hitachi HF2000, operated at 300 kV) at room temperature. Direct TEM observation on microstructure of Nb–Al bcc thin films at temperatures ranges from room temperature to 800 °C were also studied by using a sample holder with W heater. Phase identification and quantitative composition analysis of crystal grains were carried out with convergent-beam electron diffraction (CBED) and energy dispersive X-ray (EDX) spectrometer. The minimum electron beam diameter was 1 nm. X-ray La and Ka peaks were used for the quantitative analysis of Nb and Al, respectively.
3. Experimental results The microstructure of Nb–Al supersaturated bcc solid solution was observed before transformation into A15 phase. Fig. 1 shows TEM image at about 400 °C by directly observation using the sample heating holder. Numerous precipitates
Fig. 1. TEM image of Nb–Al supersaturated bcc solid solution at about 400 °C by direct observation using a sample heating holder.
A. Kikuchi et al. / Physica C 372–376 (2002) 1307–1310
were found in the Nb–Al supersaturated bcc solid solution, and those shape seems to be like a disk with nano size, about 200 nm in diameter and about 5 nm in thickness. The EDX analysis using nano electron beam clearly indicated that those nano-disk were Nb–Al oxides. Nb–Al oxide nanodisk were precipitated in parallel with the alignment h1 1 0i of Nb–Al bcc solid solution. When the temperature goes up to about 750 °C, most of oxide nano-disk disappear and melt away into Nb–Al bcc solid solution again. These results indicate the Nb–Al supersaturated bcc solid solutions include some amount of oxygen, which may influence the final superconducting properties of A15-Nb3 Al after phase transformation. The degradation of Tc and Hc2 of RHQT processed Nb3 Al wires is considered to be essentially caused by the formation of off-stoichiometric A15 phase with Alrich stacking faults. However, the oxygen contamination may also degrade the superconducting properties of RHQT processed Nb3 Al wires. On the other hand, those oxide nano-disks may behave as flux pinning centers if those will be able to remain after phase transformation. The oxygen contamination may occur from the existence of Nb oxide and Al oxide on the surfaces of starting Nb and Al sheet materials. Fig. 2 is overall (non-Cu) Jc (4.2 K) versus magnetic field curves for the DRHQ processed Nb3 Al multifilamentary conductors and the nor-
1309
Fig. 2. Overall (non-Cu) Jc (4.2 K) versus magnetic field curves of DRHQ and normal RHQT processed Nb3 Al multifilamentary conductors. All wire specimens were additionally annealed at 800 °C for 12 h. The Nb matrix/A15 ratios are shown in parentheses.
mal RHQT processed one. The Nb matrix/A15 ratios are shown in the parenthesis. Large overall (non-Cu) Jc (4.2 K) of about 135 A/mm2 at 25 T is obtained for the DRHQ processed Nb3 Al multifilamentary conductor, and very attractive for the practical applications in high magnetic field.
Fig. 3. TEM images of A15 phases synthesized by (a) the normal RHQT process and (b) the DRHQ process.
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TEM images of A15 phases synthesized by (a) normal RHQT process and (b) DRHQ process are shown in Fig. 3, respectively. In normal RHQT processed Nb3 Al, many stacking faults including Al rich composition are clearly observed as seen in Fig. 3(a). The stacking faults have the space of 10– 20 nm each other. The stacking faults formation in the A15 grain seems to arise due to the phase transformation with short-range-diffusion from the Nb–Al bcc phase including the surplus Al atoms at the transformation temperature of 800 °C. In TEM image of the DRHQ processed A15, the stacking faults were still formed, however, their density becomes rather small compared with that of A15 grains synthesized by the normal RHQT process as shown in Fig. 3(b). The drastic improvements on the superconducting properties of the DRHQ processed Nb3 Al should be essentially caused by the formation of near-stoichiometric A15 phases. On the other hand, the small density of stacking faults in the DRHQ processed Nb3 Al may cause the relatively low Jc in low magnetic fields, because the stacking faults are considered as effective pinning centers in the Nb3 Al conductor.
4. Conclusions We obtained the following conclusions. 1. In the RHQT processed Nb3 Al, many stacking faults having the space of 10–20 nm were found in the A15 phases except for the grain boundaries. The Al-rich composition of the stacking faults seems to shifts the composition of A15 phase to an off-stoichiometric Al-poor one. Therefore, Tc and Hc2 become rather lower than those of stoichiometric A15 phase. 2. We found that Nb–Al oxides having shape of nano-disk precipitate in Nb–Al bcc phase at about 400 °C with elevating temperature. These
nano-disk oxides melt away into Nb–Al bcc phase at about 750 °C before the transformation to A15 phase begin. However, the oxygen contamination may also degrade the superconducting properties of RHQT processed Nb3 Al wires. On the other hand, those oxide nanodisks may behave as flux pinning centers if those will be able to remain after phase transformation. 3. The DRHQ processed Nb3 Al multifilamentary conductors show the maximum values of Tc and Hc2 (4.2 K); about 18.4 K and 30 T, respectively. 4. Large overall (non-Cu) Jc (4.2 K) of about 135 A/mm2 at 25 T can be obtained in the DRHQ processed Nb3 Al tape-shaped multifilamentary conductor with Nb matrix/A15 ratio of 0.45. It seems to be possible to fabricate the superconducting magnet, which can generate high fields over 25 T at 4.2 K due to the very high overall Jc . 5. According to TEM observation, the stacking faults were still observed in the DRHQ processed A15 grains, but their density is small compared with that in the normal RHQT processed A15 grains. The DRHQ operation apparently prevents the formation of Al-rich stacking faults in the A15 superconducting grains. References [1] Y. Iijima, M. Kosuge, T. Takeuchi, K. Inoue, Adv. Cryog. Eng. 40 (1994) 899. [2] S. Foner, E.J. Mcniff Jr., B.T. Matthias, T.H. Geballe, R.H. Willens, E. Corenzwit, Phys. Lett. 31A (1970) 349. [3] A. Kikuchi, Y. Iijima, K. Inoue, IEEE Trans. Appl. Supercond. 11 (2001) 3615. [4] A. Kikuchi, Y. Iijima, K. Inoue, IEEE Trans. Appl. Supercond. 11 (2001) 3968. [5] A. Kikuchi, Y. Iijima, K. Inoue, M. Kosuge, Adv. Cryog. Eng. 47–48 (2002), in press.
Microstructures and superconducting properties of transformed Nb3Al wire A. Kikuchi *, Y. Iijima, K. Inoue National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
Abstract According to a TEM observation study of rapidly-heating/quenching and transformation (RHQT) processed Nb3 Al wires, many stacking faults spaced 10–20 nm apart were found in the A15 phases except for the grain boundaries. The formation of Al-rich stacking faults seems to shift the composition of the A15 phase to an off-stoichiometric one. Therefore, Tc and Hc2 become rather lower than those of stoichiometric A15 phase. Moreover, we also found that nanodisk shaped Nb–Al oxides precipitate in Nb–Al bcc phase at a rather low temperature of about 400 °C. These nano-disk oxides melt away and form solid-solution with Nb–Al bcc phase at about 750 °C, which is lower than that for transformation. Oxygen contamination may also degrade the superconducting properties of RHQT processed Nb3 Al wires. On the other hand, those oxide nano-disks may behave as flux pinning centers if they remain after phase transformation. Furthermore, we studied the relation between microstructure and superconducting property of some wire specimens, which were transformed under different heat treatment conditions, and the resulting high temperature and very short time heat treatment was very effective to enhance its Tc and Hc2 . This special heating operation was dubbed double rapidly-heating/quenching. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Nb3 Al; Rapid-heating/quenching; TEM structure; Critical current density
1. Introduction The rapidly-heating/quenching and transormation (RHQT) processed Nb3 Al multifilamentary wires show the excellent superconducting performance [1]. Their critical temperature Tc of 17.8 K and upper critical magnetic field Hc2 (4.2 K) of 26 T are much higher than those of the Nb3 Al multifilamentary wires fabricated by a conventional low-temperature diffusion reaction. How-
*
Corresponding author. Fax: +81-298-59-2601. E-mail address: [email protected] (A. Kikuchi).
ever, those values are still rather lower than those of stoichiometric A15-Nb3 Al phase directly synthesized by an arc melting at very high temperature [2]. Recently, it was found that many stacking faults, showing Al-rich composition, formed in the RHQT processed Nb3 Al wire [3]. The formation of the Al rich stacking faults causes to shift the A15 phase matrix composition to the Nb-rich one. The formation of Al-rich stacking faults should be caused by the phase transformation at low temperature of about 800 °C, where off-stoichiometric A15 phase is stable. Since the stoichiometric A15 phase is only stable at 1940 °C, we tried to carry
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 1 0 1 6 - X
1308
A. Kikuchi et al. / Physica C 372–376 (2002) 1307–1310
out the pulsed heat treatment for the phase transformation around that high temperature by using ohmic resistive heating on the reel to reel apparatus, which have been named as the double rapidly-heating/quenching (DRHQ) process [4,5]. In order to get an understanding in detail of transformation mechanism from bcc to A15 phases, we directly observed the TEM microstructure of Nb–Al bcc phase under elevating temperature up to 800 °C. Moreover, we considered the relation between microstructure and superconducting properties of RHQT and DRHQ processed Nb3 Al wires.
2. Experimental procedures The Nb/Al precursor wire fabricated by a conventional jelly-roll process. Nb/Al atomic ratio in the all composite-filaments is designed as 3=1, where is the stoichiometric A15 composition. The RHQ operation for the Nb/Al precursor wire was carried out in a dynamic vacuum chamber. The precursor wire, moving with a speed of 1 m/s, was continuously heated up to 2000 °C by resistiveheating during about 0.1 s, with transporting a dc current between an electrode pully and a molten Ga bath. The wire was subsequently quenched from 2000 °C into the molten Ga bath at about 40 °C. The Nb/Al composite filaments were converted into the Nb–Al supersaturated bcc solid solution filaments by the RHQ operation. Some of wire specimens were applied to the DRHQ which have been previously reported elsewhere in detail. Additional annealing at 800 °C for 12 h was applied for the phase transformation from bcc to A15 and for the improvement of long-range-ordering of disordered A15 phases after removing the coated Ga on the surface of the wire by chemical etching. Tc and Hc2 (4.2 K) were measured by dc four-probe resistive method, and defined as the temperature and the field, respectively, where the sample shows the half value of normal-state resistance. Ic (4.2 K) was determined with the criterion of 100 lV/m in perpendicular fields up to 35 T with increasing transport current. Jc (4.2 K) were defined as Ic (4.2 K) divided by the total cross-sectional area of jellyrolled filaments in the Nb matrix, which is the
maximum expected value of the total A15 phase area formed in the wire. Thin film specimens for TEM observation were prepared from the transverse cross section of the wire, by using a focused Ga ion beam (FIB; Hitachi FB2000A) after mechanical slicing and grinding, resulting in film specimen with about 100 nm in thickness. Microstructure was observed by a field-emission-transmission electron microscopy (FE-TEM; Hitachi HF2000, operated at 300 kV) at room temperature. Direct TEM observation on microstructure of Nb–Al bcc thin films at temperatures ranges from room temperature to 800 °C were also studied by using a sample holder with W heater. Phase identification and quantitative composition analysis of crystal grains were carried out with convergent-beam electron diffraction (CBED) and energy dispersive X-ray (EDX) spectrometer. The minimum electron beam diameter was 1 nm. X-ray La and Ka peaks were used for the quantitative analysis of Nb and Al, respectively.
3. Experimental results The microstructure of Nb–Al supersaturated bcc solid solution was observed before transformation into A15 phase. Fig. 1 shows TEM image at about 400 °C by directly observation using the sample heating holder. Numerous precipitates
Fig. 1. TEM image of Nb–Al supersaturated bcc solid solution at about 400 °C by direct observation using a sample heating holder.
A. Kikuchi et al. / Physica C 372–376 (2002) 1307–1310
were found in the Nb–Al supersaturated bcc solid solution, and those shape seems to be like a disk with nano size, about 200 nm in diameter and about 5 nm in thickness. The EDX analysis using nano electron beam clearly indicated that those nano-disk were Nb–Al oxides. Nb–Al oxide nanodisk were precipitated in parallel with the alignment h1 1 0i of Nb–Al bcc solid solution. When the temperature goes up to about 750 °C, most of oxide nano-disk disappear and melt away into Nb–Al bcc solid solution again. These results indicate the Nb–Al supersaturated bcc solid solutions include some amount of oxygen, which may influence the final superconducting properties of A15-Nb3 Al after phase transformation. The degradation of Tc and Hc2 of RHQT processed Nb3 Al wires is considered to be essentially caused by the formation of off-stoichiometric A15 phase with Alrich stacking faults. However, the oxygen contamination may also degrade the superconducting properties of RHQT processed Nb3 Al wires. On the other hand, those oxide nano-disks may behave as flux pinning centers if those will be able to remain after phase transformation. The oxygen contamination may occur from the existence of Nb oxide and Al oxide on the surfaces of starting Nb and Al sheet materials. Fig. 2 is overall (non-Cu) Jc (4.2 K) versus magnetic field curves for the DRHQ processed Nb3 Al multifilamentary conductors and the nor-
1309
Fig. 2. Overall (non-Cu) Jc (4.2 K) versus magnetic field curves of DRHQ and normal RHQT processed Nb3 Al multifilamentary conductors. All wire specimens were additionally annealed at 800 °C for 12 h. The Nb matrix/A15 ratios are shown in parentheses.
mal RHQT processed one. The Nb matrix/A15 ratios are shown in the parenthesis. Large overall (non-Cu) Jc (4.2 K) of about 135 A/mm2 at 25 T is obtained for the DRHQ processed Nb3 Al multifilamentary conductor, and very attractive for the practical applications in high magnetic field.
Fig. 3. TEM images of A15 phases synthesized by (a) the normal RHQT process and (b) the DRHQ process.
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A. Kikuchi et al. / Physica C 372–376 (2002) 1307–1310
TEM images of A15 phases synthesized by (a) normal RHQT process and (b) DRHQ process are shown in Fig. 3, respectively. In normal RHQT processed Nb3 Al, many stacking faults including Al rich composition are clearly observed as seen in Fig. 3(a). The stacking faults have the space of 10– 20 nm each other. The stacking faults formation in the A15 grain seems to arise due to the phase transformation with short-range-diffusion from the Nb–Al bcc phase including the surplus Al atoms at the transformation temperature of 800 °C. In TEM image of the DRHQ processed A15, the stacking faults were still formed, however, their density becomes rather small compared with that of A15 grains synthesized by the normal RHQT process as shown in Fig. 3(b). The drastic improvements on the superconducting properties of the DRHQ processed Nb3 Al should be essentially caused by the formation of near-stoichiometric A15 phases. On the other hand, the small density of stacking faults in the DRHQ processed Nb3 Al may cause the relatively low Jc in low magnetic fields, because the stacking faults are considered as effective pinning centers in the Nb3 Al conductor.
4. Conclusions We obtained the following conclusions. 1. In the RHQT processed Nb3 Al, many stacking faults having the space of 10–20 nm were found in the A15 phases except for the grain boundaries. The Al-rich composition of the stacking faults seems to shifts the composition of A15 phase to an off-stoichiometric Al-poor one. Therefore, Tc and Hc2 become rather lower than those of stoichiometric A15 phase. 2. We found that Nb–Al oxides having shape of nano-disk precipitate in Nb–Al bcc phase at about 400 °C with elevating temperature. These
nano-disk oxides melt away into Nb–Al bcc phase at about 750 °C before the transformation to A15 phase begin. However, the oxygen contamination may also degrade the superconducting properties of RHQT processed Nb3 Al wires. On the other hand, those oxide nanodisks may behave as flux pinning centers if those will be able to remain after phase transformation. 3. The DRHQ processed Nb3 Al multifilamentary conductors show the maximum values of Tc and Hc2 (4.2 K); about 18.4 K and 30 T, respectively. 4. Large overall (non-Cu) Jc (4.2 K) of about 135 A/mm2 at 25 T can be obtained in the DRHQ processed Nb3 Al tape-shaped multifilamentary conductor with Nb matrix/A15 ratio of 0.45. It seems to be possible to fabricate the superconducting magnet, which can generate high fields over 25 T at 4.2 K due to the very high overall Jc . 5. According to TEM observation, the stacking faults were still observed in the DRHQ processed A15 grains, but their density is small compared with that in the normal RHQT processed A15 grains. The DRHQ operation apparently prevents the formation of Al-rich stacking faults in the A15 superconducting grains. References [1] Y. Iijima, M. Kosuge, T. Takeuchi, K. Inoue, Adv. Cryog. Eng. 40 (1994) 899. [2] S. Foner, E.J. Mcniff Jr., B.T. Matthias, T.H. Geballe, R.H. Willens, E. Corenzwit, Phys. Lett. 31A (1970) 349. [3] A. Kikuchi, Y. Iijima, K. Inoue, IEEE Trans. Appl. Supercond. 11 (2001) 3615. [4] A. Kikuchi, Y. Iijima, K. Inoue, IEEE Trans. Appl. Supercond. 11 (2001) 3968. [5] A. Kikuchi, Y. Iijima, K. Inoue, M. Kosuge, Adv. Cryog. Eng. 47–48 (2002), in press.