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Jc enhancement of high density MgB2 bulk made by Premix-PICT-Diffusion method
Physica C 460–462 (2007) 581–582 www.elsevier.com/locate/physc
Jc enhancement of high density MgB2 bulk made by Premix-PICT-Diffusion method I. Iwayama *, S. Ueda, A. Yamamoto, Y. Katsura, J. Shimoyama, S. Horii, K. Kishio Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Available online 14 April 2007
Abstract A high Jc of 1.05 MA/cm2 at 20 K in self-field was achieved by a MgB2 bulk synthesized by newly developed ‘‘Premix-Powder-InClosed-Tube-Diffusion’’ (Premix-PICT-Diffusion) method starting from Mg and powder mixture of B and MgB2, which were separately packed into Nb tubes. MgB2 bulks prepared by this method are highly dense without voids and cracks, resulting in two or three times larger effective grain connecting area contributing dramatically high Jc compared with the conventional PICT-processed porous bulks. This method will be easily extended for development of superconducting current leads with large capacities. Ó 2007 Published by Elsevier B.V. Keywords: MgB2; High Jc; PICT-Diffusion method; High density
1. Introduction Since magnesium diboride (MgB2) has the highest Tc (40 K) among the metallic superconductors, numerous efforts have been made to improve its Jc properties suitable for practical use under high fields at 15–20 K. MgB2 bulks made by standard in situ method [1] are porous with 50% of theoretical density. Low bulk density directly reduces effective current path. Therefore, development of a new method to synthesize highly dense MgB2 bulks is one of the promising ways to enhance Jc. In our previous study, the Powder-In-Closed-Tube (PICT)-Diffusion method [2] was developed to synthesize highly dense MgB2 bulks. MgB2 bulks made by this method exhibit two times higher Jc than those of conventional in situ processed porous bulks due to an increase of current path. However, large cracks with a typical interval of 100 lm in these bulks prevented their application as large current conductors. Generation of the cracks were considered to originate from inhomogeneous and low filling density of boron part, which turns to dense MgB2 after *
Corresponding author. Tel.: +81 3 5841 7713; fax: +81 3 5689 0574. E-mail address: [email protected] (I. Iwayama).
0921-4534/$ - see front matter Ó 2007 Published by Elsevier B.V. doi:10.1016/j.physc.2007.04.108
diffusion of Mg. Therefore, we have attempted to develop a new method for the synthesis of high density and crack-free MgB2 bulks in the present study. Premixing of commercial MgB2 powder with the B was found to be effective to improve both the density and the homogeneity of initial B part, resulting in an elimination of cracks in dense MgB2 bulks and the record-high Jc. 2. Experimental MgB2 bulks were prepared by the newly developed ‘‘Premix-PICT-Diffusion’’ method starting from Mg (99.9% in purity, 100 mesh), B (99.95%, 325 mesh) and commercial MgB2 (99%, 100 mesh) powders. The commercial MgB2 powder was pulverized by ball-milling to decrease the grain size to 0.5–2.0 lm, prior to mixing with B powder with a molar ratio of 2.0:0.5. This mixture and Mg were separately placed in an Nb tube (2.4/3.0 mm/). The molar ratio of Mg against B was 1.1:2.0. The tubes were uniaxially pressed under 500 MPa and both ends were also closed by pressing. In order to diffuse Mg into boron part, each tube was heated at 750–900 °C for 24–100 h in an evacuated quartz ampoule. Tape-shaped MgB2 bulks were obtained by removing the Nb tube.
I. Iwayama et al. / Physica C 460–462 (2007) 581–582
Constituent phases of the samples were analyzed by the powder X-ray diffraction method. Microstructure was observed by a scanning electron microscope (KEYENCE VE-7800). Magnetization properties were measured by a SQUID magnetometer (Quantum Design MPMS-XL5s). Jc was calculated from the width of the magnetization hysteresis loops, DM, based on the extended Bean model, Jc = 20DM/(a a2/3b), where a and b are lengths of short and long sides, respectively.
Premix-PICT-Diffusion PICT
106
Jc / Acm-2
582
105
15 K
105
30 K 20 K
3. Results and discussion 103
All the samples prepared by the Premix-PICT-Diffusion method contained MgB2 as the main phase. High densities of 2.3–2.56 g/cm3 were reproducibly achieved, which correspond to 88–98% of the theoretical density. Fig. 1 shows a typical secondary electron image of a fractured surface of an MgB2 bulk prepared by the Premix-PICT-Diffusion method. The surface was highly dense and homogeneous without voids and cracks. Studies on the temperature dependence of normal state resistivity revealed that the grain connectivity was improved by approximately three times in the present highly dense MgB2 bulks compared to those of the conventional PICT-processed bulks. This indicates that the effective superconducting current path was increased by an enhancement of bulk density. The sample prepared at 800 °C for 72 h exhibited the highest Jc among samples prepared at 750–900 °C. Fig. 2 shows the Jc–H properties of a sample prepared by Premix-PICT-Diffusion method reacted at 800 °C for 72 h. High Jc’s of 1.30 MA/cm2 at 15 K, 1.05 MA/cm2 at 20 K and 0.45 MA/cm2 at 30 K were achieved in low fields. These values were three times higher than those of porous bulks prepared by conventional PICT method. Improve-
0
1
2
3
μ0H / T
4
5
Fig. 2. Jc–H properties of MgB2 bulks prepared by Premix-PICTDiffusion method reacted at 800 °C for 72 h and by conventional PICT method.
ment of effective current path ratio and elimination of cracks contributed the dramatically high Jc. In addition, we have succeeded in the synthesis of 50 mm long, highly dense and crack-free MgB2 bulks. This means that the Premix-PICT-Diffusion method is an excellent method to prepare MgB2 bulks applicable as for superconducting current leads with large capacities. 4. Conclusions Highly dense MgB2 bulks without large voids and cracks were successfully synthesized by the newly developed Premix-PICT-Diffusion method starting from Mg and powder mixture of B and MgB2. Effective current path was increased in the MgB2 bulks prepared by this method, resulting in very high Jc’s of 1.30 MA/cm2 at 15 K, 1.05 MA/cm2 at 20 K, and 0.45 MA/cm2 at 30 K in low fields. Further enhanced Jc will be achieved by using finer boron powder as starting material as well as by impurity doping, such as SiC and B4C [3,4]. In addition, this method was confirmed to be easily extended for development of superconducting current leads with large capacities. References
Fig. 1. Secondary electron image of the fractured surface of the MgB2 bulk prepared by the Premix-PICT-Diffusion method reacted at 800 °C for 72 h.
[1] A. Yamamoto, J. Shimoyama, S. Ueda, Y. Katsura, S. Horii, K. Kishio, Supercond. Sci. Technol. 17 (2004) 921. [2] S. Ueda, J. Shimoyama, I. Iwayama, A. Yamamoto, Y. Katsura, S. Horii, K. Kishio, Appl. Phys. Lett. 86 (2005) 222502. [3] S.X. Dou, A.V. Pan, S. Zhou, M. Ionescu, H.K. Liu, P.R. Munroe, Supercond. Sci. Technol. 15 (2002) 1587. [4] A. Yamamoto, J. Shimoyama, S. Ueda, I. Iwayama, S. Horii, K. Kishio, Supercond. Sci. Technol. 18 (2005) 1323.
Jc enhancement of high density MgB2 bulk made by Premix-PICT-Diffusion method I. Iwayama *, S. Ueda, A. Yamamoto, Y. Katsura, J. Shimoyama, S. Horii, K. Kishio Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Available online 14 April 2007
Abstract A high Jc of 1.05 MA/cm2 at 20 K in self-field was achieved by a MgB2 bulk synthesized by newly developed ‘‘Premix-Powder-InClosed-Tube-Diffusion’’ (Premix-PICT-Diffusion) method starting from Mg and powder mixture of B and MgB2, which were separately packed into Nb tubes. MgB2 bulks prepared by this method are highly dense without voids and cracks, resulting in two or three times larger effective grain connecting area contributing dramatically high Jc compared with the conventional PICT-processed porous bulks. This method will be easily extended for development of superconducting current leads with large capacities. Ó 2007 Published by Elsevier B.V. Keywords: MgB2; High Jc; PICT-Diffusion method; High density
1. Introduction Since magnesium diboride (MgB2) has the highest Tc (40 K) among the metallic superconductors, numerous efforts have been made to improve its Jc properties suitable for practical use under high fields at 15–20 K. MgB2 bulks made by standard in situ method [1] are porous with 50% of theoretical density. Low bulk density directly reduces effective current path. Therefore, development of a new method to synthesize highly dense MgB2 bulks is one of the promising ways to enhance Jc. In our previous study, the Powder-In-Closed-Tube (PICT)-Diffusion method [2] was developed to synthesize highly dense MgB2 bulks. MgB2 bulks made by this method exhibit two times higher Jc than those of conventional in situ processed porous bulks due to an increase of current path. However, large cracks with a typical interval of 100 lm in these bulks prevented their application as large current conductors. Generation of the cracks were considered to originate from inhomogeneous and low filling density of boron part, which turns to dense MgB2 after *
Corresponding author. Tel.: +81 3 5841 7713; fax: +81 3 5689 0574. E-mail address: [email protected] (I. Iwayama).
0921-4534/$ - see front matter Ó 2007 Published by Elsevier B.V. doi:10.1016/j.physc.2007.04.108
diffusion of Mg. Therefore, we have attempted to develop a new method for the synthesis of high density and crack-free MgB2 bulks in the present study. Premixing of commercial MgB2 powder with the B was found to be effective to improve both the density and the homogeneity of initial B part, resulting in an elimination of cracks in dense MgB2 bulks and the record-high Jc. 2. Experimental MgB2 bulks were prepared by the newly developed ‘‘Premix-PICT-Diffusion’’ method starting from Mg (99.9% in purity, 100 mesh), B (99.95%, 325 mesh) and commercial MgB2 (99%, 100 mesh) powders. The commercial MgB2 powder was pulverized by ball-milling to decrease the grain size to 0.5–2.0 lm, prior to mixing with B powder with a molar ratio of 2.0:0.5. This mixture and Mg were separately placed in an Nb tube (2.4/3.0 mm/). The molar ratio of Mg against B was 1.1:2.0. The tubes were uniaxially pressed under 500 MPa and both ends were also closed by pressing. In order to diffuse Mg into boron part, each tube was heated at 750–900 °C for 24–100 h in an evacuated quartz ampoule. Tape-shaped MgB2 bulks were obtained by removing the Nb tube.
I. Iwayama et al. / Physica C 460–462 (2007) 581–582
Constituent phases of the samples were analyzed by the powder X-ray diffraction method. Microstructure was observed by a scanning electron microscope (KEYENCE VE-7800). Magnetization properties were measured by a SQUID magnetometer (Quantum Design MPMS-XL5s). Jc was calculated from the width of the magnetization hysteresis loops, DM, based on the extended Bean model, Jc = 20DM/(a a2/3b), where a and b are lengths of short and long sides, respectively.
Premix-PICT-Diffusion PICT
106
Jc / Acm-2
582
105
15 K
105
30 K 20 K
3. Results and discussion 103
All the samples prepared by the Premix-PICT-Diffusion method contained MgB2 as the main phase. High densities of 2.3–2.56 g/cm3 were reproducibly achieved, which correspond to 88–98% of the theoretical density. Fig. 1 shows a typical secondary electron image of a fractured surface of an MgB2 bulk prepared by the Premix-PICT-Diffusion method. The surface was highly dense and homogeneous without voids and cracks. Studies on the temperature dependence of normal state resistivity revealed that the grain connectivity was improved by approximately three times in the present highly dense MgB2 bulks compared to those of the conventional PICT-processed bulks. This indicates that the effective superconducting current path was increased by an enhancement of bulk density. The sample prepared at 800 °C for 72 h exhibited the highest Jc among samples prepared at 750–900 °C. Fig. 2 shows the Jc–H properties of a sample prepared by Premix-PICT-Diffusion method reacted at 800 °C for 72 h. High Jc’s of 1.30 MA/cm2 at 15 K, 1.05 MA/cm2 at 20 K and 0.45 MA/cm2 at 30 K were achieved in low fields. These values were three times higher than those of porous bulks prepared by conventional PICT method. Improve-
0
1
2
3
μ0H / T
4
5
Fig. 2. Jc–H properties of MgB2 bulks prepared by Premix-PICTDiffusion method reacted at 800 °C for 72 h and by conventional PICT method.
ment of effective current path ratio and elimination of cracks contributed the dramatically high Jc. In addition, we have succeeded in the synthesis of 50 mm long, highly dense and crack-free MgB2 bulks. This means that the Premix-PICT-Diffusion method is an excellent method to prepare MgB2 bulks applicable as for superconducting current leads with large capacities. 4. Conclusions Highly dense MgB2 bulks without large voids and cracks were successfully synthesized by the newly developed Premix-PICT-Diffusion method starting from Mg and powder mixture of B and MgB2. Effective current path was increased in the MgB2 bulks prepared by this method, resulting in very high Jc’s of 1.30 MA/cm2 at 15 K, 1.05 MA/cm2 at 20 K, and 0.45 MA/cm2 at 30 K in low fields. Further enhanced Jc will be achieved by using finer boron powder as starting material as well as by impurity doping, such as SiC and B4C [3,4]. In addition, this method was confirmed to be easily extended for development of superconducting current leads with large capacities. References
Fig. 1. Secondary electron image of the fractured surface of the MgB2 bulk prepared by the Premix-PICT-Diffusion method reacted at 800 °C for 72 h.
[1] A. Yamamoto, J. Shimoyama, S. Ueda, Y. Katsura, S. Horii, K. Kishio, Supercond. Sci. Technol. 17 (2004) 921. [2] S. Ueda, J. Shimoyama, I. Iwayama, A. Yamamoto, Y. Katsura, S. Horii, K. Kishio, Appl. Phys. Lett. 86 (2005) 222502. [3] S.X. Dou, A.V. Pan, S. Zhou, M. Ionescu, H.K. Liu, P.R. Munroe, Supercond. Sci. Technol. 15 (2002) 1587. [4] A. Yamamoto, J. Shimoyama, S. Ueda, I. Iwayama, S. Horii, K. Kishio, Supercond. Sci. Technol. 18 (2005) 1323.