- Email: [email protected]
Ti tapes
Physica C 469 (2009) 713–716
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Properties of hot pressed MgB2/Ti tapes P. Kovácˇ a,*, I. Hušek a, T. Melíšek a, J. Fedor a, V. Cambel a, A. Morawski b, A. Kario b a b
Institute of Electrical Engineering, Slovak Academy of Sciences, Dúbravska cesta 9, 841 04 Bratislava, Slovakia Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warszawa, Poland
a r t i c l e
i n f o
Article history: Received 4 November 2008 Received in revised form 3 March 2009 Accepted 6 March 2009 Available online 18 March 2009 PACS: 74.25.Sv 74.25.Fy 74.70.Ad
a b s t r a c t Hot axial and hot isostatic pressing was applied for single-core MgB2/Ti tapes. Differences in transport current density, n-exponents and critical current anisotropy are discussed and related to the grain connectivity influenced by pressing. The magnetic Hall probe scanning measurements allowed observing the isolated regions for axially hot pressed sample attributed to the longitudinally oriented cracks introduced by pressing. The highest current densities were measured for the tape subjected to hot isostatic pressing due to improved connectivity. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: MgB2 Hot pressing Critical currents n-exponents Grain connectivity
1. Introduction MgB2 wires prepared by in situ technique using Mg + B powders mixture have usually low core density due to the volume shrinking caused by Mg + B? MgB2 conversion [1]. Recent review of Eisterer is well presenting critical currents, connectivity, current percolation and parameters influencing Jc(l0H) performance of MgB2 [2]. The method of mechanical alloying (MA) is a variant of the in situ technique. Mg and B are intensively milled in a planetary ball mill where high energy milling leads to a partial reaction to MgB2 [3–5]. This fine-particles precursor powder is highly reactive and can be processed further at relatively low temperatures [4,5]. High density MgB2 with very high current densities (104 A cm2 at 1416 T) has been presented recently for carbon doped MA powder [6,7]. High reactivity of MA powder causes the creation of Fe2B layer at the core/Fe interface even at low temperatures, which influences the phase purity (boron deficiency) and also the thermal and electrical properties of composite wire [8,9]. Recently, titanium as a sheath material has been used and no reaction or diffusion up to 900 °C was observed [10]. Effect of hot isostatic pressing has been applied for bulk samples [11] and also for wires [12]. * Corresponding author. Tel.: +421 2 5477 5853; fax: +421 2 5477 5816. E-mail address: [email protected] (P. Kovácˇ). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.03.007
The Jc improvement for ex situ MgB2 wire was attributed to the large amount of crystalline defects [12]. The aim of this contribution is to show how the applied hot axial and isostatic pressure can influence the transport current densities of MgB2 tapes made from not doped mechanically alloyed powder in chemically inert Ti sheath. 2. Experimental MA/Ti composite was prepared by two-axial rolling deformation into mono-filamentary tape of 0.35 mm in thickness and 3.3 mm in width [8]. Hot pressed sample (HP) at 80 MPa and hot isostatically pressed one (HIP) by 1 GPa were done at temperature 650 °C. Hot pressing was performed between two stainless steel anvils covered by BN powder (to protect the sample gliding) in gaseous axial press and pressing direction was applied perpendicularly to flat side of the tape. Hot isostatic pressing was done in the set composed of three stage compressor and high pressure chamber 30 mm in diameter filled by pure Ar [13]. The reference not pressed sample (N) was annealed at 650 °C in argon pressure slightly higher than atmosphere. Vickers microhardness measurements (HV 0.05–50 g) were performed in the cores cross-section of studied tapes to see the effect of densification. Critical currents (Ic at 1 lV cm1) of tapes (N, HP and HIP) were measured in paral-
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lel and perpendicular external magnetic field orientation to the tape side at liquid helium temperature. We have explored also the local magnetic field close to the transverse section of the above-described HP and HIP samples using high-resolution (5 lm) scanning Hall probe microscope (SHPM) [14,15]. Perpendicular magnetic field was applied to the transverse section of the sample. All scans consisted of 200 60 points with the steps of 5 lm in both x and y directions, which gave the overall scanned area 1 mm 0.3 mm. The distance of the Hall probe from the sample surface was 25 lm ± 5 lm. Both samples were placed on the same SHPM sample holder and cooled down to 20 K in zero field. Then, external magnetic field of B = +150 mT was applied. Magnetic field distribution in close proximity of the sample surface was imaged. 3. Results and discussion Fig. 1a shows the magnetic field dependence of critical current density for N, HP and HIP tape. It is apparent that Jc(l0H) is influenced by the applied pressing. The lowered Jc is measured for HP tape (104 A cm2 at 8.0 T), which correlates with the lowered core density expressed by the micro hardness data in Table 1 (HV
0.05 = 290). The highest Jc (104 A cm2 at 10.8 T) was measured for HIP sample having the highest core density (see Table 1) and consequently improved grain connectivity. The current density at 10 T is increased from 10,000 to 16,700 A cm2 by hot isostatic pressing at 1 GPa (improvement by 67%). HP and HIP tapes show also slightly less steep Jc decrease with field than N (see dotted lines in Fig. 1a). Fig. 1b presents the plot of n(l0H) values evaluated from IV curves, which correlates with Jc(l0H) data. As visible, all three n(l0H) characteristics follow well a linear dependences for semi logarithmic plot (exponential decrease) in the selected field range but n-exponents decrease less rapidly with field than Jc. While the Jc drop by one order of magnitude is observed for the field increase by 4.5 T (see Fig. 1a), 10 times decreased n occurs in the field range of 8.5 T. Apparently lowered n-exponents for HP sample can be attributed to globally worsened grain connectivity (see the core microhardness in Table 1) and consequent current redistribution inside the MgB2 core. Fig. 1c compares the anisotropy factor (ka = Ic-par/Ic-perp) of all three tapes annealed at the same temperature 650 °C, the insert shows the critical currents measured in parallel and perpendicular field for HP tape. An exponential increase of ka with field was measured for each sample, but their absolute values are different. Crit-
Fig. 1. Transport current densities measured at 4.2 K in parallel external field (a), corresponding n-exponents (b) and anisotropy factor ka = Ic-par/Ic-perp (c) as a function of field magnitude for HP, HIP compared with the reference sample N.
P. Kovácˇ et al. / Physica C 469 (2009) 713–716
715
Table 1 MgB2 core aspect ratio (width/thickness) and Vickers microhardness HV 0.05 (local positions 13 in Fig. 4 and averaged values) obtained for samples N, HP and HIP annealed at 650 °C. Sample
Pressure (MPa)
MgB2 core width/thickness
HV 0.05 pos. 1
HV 0.05 pos. 2
HV 0.05 pos. 3
Averaged HV 0.05
N HP HIP
No 80 1000
11 17.5 12
705 270 1030
703 290 1300
790 305 1500
730 290 1300
ical current anisotropy in MgB2 is generally explained by some caxis grain alignment (texture) [16]. The lowest anisotropy
Fig. 2. Magnetic field distribution obtained by scanning Hall probe microscope over the MgB2 core of HP sample at 20 K after zero field cooling in a field of +150 mT.
(ka = 10 at 8.4 T) was measured for not pressed tape N. Slightly increased anisotropy of HIP tape indicates only small effect of isostatic pressing on the texture (core aspect ratio was increased from 11 to 12, Table 1). The highest ka values (10 at 6.3 T) as well as higher in field increase was measured for HP tape. Core aspect ratio of HP tape is increased from 11 to 17.5 (see Table 1). It is expected that apparent core widening has influenced also the c-axis grain alignment of MgB2. Increased current anisotropy by texture was also observed for Ti sheathed MgB2 tape subjected to cold rolling, which was confirmed by X-ray analysis (by texture parameters (Fa, Fc) and by (0 0 2) pole figure) [17]. Figs. 2 and 3, including the 3D plots and the contour plots, present the magnetic field distribution obtained by SHPM over the MgB2 cores of HP and HIP tapes. The scanned sample area 1 mm 0.3 mm is marked by dotted line in Fig. 4. For HP sample the currents at the field of 150 mT (Fig. 2) almost fully compensate for the external field only at coordinates x = 150 lm, y = 800 lm (local minimum), and its large part is penetrated by the external magnetic field. Induced supercurrents are probably limited by cracks located along the short side of the sample (parallel with the x axis) they divide the sample into isolated parts in which the induced currents are strongly limited. To model roughly, how is the magnetic field distribution, generated by the external field and by the induced current, we assume Bean model for hard superconductors. The Maxwell equation rot H = Jc can be rewritten for the current components Jx and Jy in x and y directions, respectively, as follows:
Jx ¼
1 dBz
l0 d y
;
Jy ¼
1 dBz
l0 d x
ð1Þ
The equations can be approximated by the relations
Jx
Fig. 3. Magnetic field distribution obtained by scanning Hall probe microscope over the MgB2 core of HIP sample at 20 K after zero field cooling in a field of 150 mT.
1 DB z
l 0 Dy
;
Jy
1 DBz
l 0 Dx
ð2Þ
where l0 is the permeability of vacuum, DBz is the local change of the magnetic field on a distance Dx or Dy. These field changes define the local value of the critical current density. Applying the relations to the field close to the local minima (x = 150 lm, y = 800 lm) mentioned above (Fig. 2, contour plot), one achieves roughly |Jy| 1.5 105 A cm2, so the critical current density is rather high at this locality. The high-quality superconductor is located also at other local minima, and it corresponds to parts of the sample having different core density (see Table 1) that are divided probably by the cracks. The situation is quite different in the case of the HIP sample. In this case the superconducting currents almost fully compensate for the external field of 150 mT in the central part of the sample (see
Fig. 4. Partial cross-section of HIP tape with remarked area of scanned magnetic field (by solid line) and the places of local HV 0.05 measurements (1–3), corresponding hardness are given by Table 1.
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P. Kovácˇ et al. / Physica C 469 (2009) 713–716
of cracks inclined by 45° corresponding to gliding planes, see Fig. 5b. These cracks are dividing the MgB2 core longitudinally and consequently lowered Jc (Fig. 1a), decreased n-exponents (Fig. 1b) and not uniform current flow are obtained for HP. The size of core elements divided by cracks (see Fig. 5b) correlate with the scanned field minima or maxima placed close to the core centre (Fig. 2). 4. Conclusion
Fig. 5. Outer (a) and central part (b) of HP tape showing different core uniformity and local region with cracks inclined by 45°.
Fig. 3). The distribution of the magnetic field observed is evidently generated by rather homogeneous currents that flow around the whole sample, which is not divided into isolated parts. This feature differs much from the HP sample shown in Fig. 2. The critical current density can be again very roughly estimated and it reads Jy 3 105 A cm2. Fig. 4 shows the partial transversal sections of HIP tape in which the scanned area and positions of local microhardness measurements (1–3) are marked. Table 1 presents the core aspect ratio (width/thickness), local and averaged microhardness values for all pressing modes. While HIP increases the core aspect ratio only slightly, HP does it much more due to an apparent tape widening during hot axial pressing. HV 0.05 between 700 and 800 was measured for as rolled tape annealed at ambient argon pressure – tape N. Apparent difference in averaged microhardness (290–1300) is attributed to the applied pressing mode. HP and HIP tapes have similar hardness gradients: the smallest at the core edge (position 1) and the highest in the core centre (position 3), but one can see that considerably increased density for HIP sample (doubled HV 0.05 in comparison to N) as a consequence of high isostatic pressure by 1 GPa. Core density has a direct effect to grain connectivity and consequently improved critical current density (see Fig. 1a) and well uniformity (see Figs. 2 and 3) are measured. While low core density and no cracks structure (HV 0.05 270) was measured at the core edges of HP tape (Fig. 5a), the central part is more dense (HV 0.05 305) but full
Hot axial and hot isostatic pressing were applied for single-core MgB2/Ti tapes made of not doped mechanically alloyed powder. It was shown that current densities and n-exponents are influenced by the applied pressing mode. Hot isostatic pressing by 1 GPa has increased Jc (10 T) by 67% due to effectively improved grain connectivity. Hot axial pressing with much lower pressure (80 MPa) caused an apparent MgB2 core widening and longitudinal cracks generation resulting in lowered Jc and n-exponents. On the other side, hot axial pressing is increasing Ic anisotropy, which can be attributed to improved texture. Further Jc improvement can be reached by a proper combination of hot isostatic pressing with optimized carbon substitution (e.g. SiC addition). Acknowledgements This work was partially supported by the Slovak Scientific Agency APVV-0398-07 and VEGA 2/0037/09. Authors would like to thank to W. Haessler and M. Herrmann from IFW Dresden for mechanically alloyed powder. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
P. Kovácˇ et al., Supercond. Sci. Technol. 20 (2007) 105. M. Eisterer, Supercond. Sci. Technol. 20 (2007) R47. O. Perner et al., Supercond. Sci. Technol. 17 (2004) 1148. W. Häßler et al., Supercond. Sci. Technol. 16 (2003) 281. C. Fischer et al., Appl. Phys. Lett. 83 (2003) 1. M. Herrmann et al., Appl. Phys. Lett. 91 (2007) 082507. W. Häßler et al., Supercond. Sci. Technol. 21 (2008) 062001. P. Kovácˇ et al., Supercond. Sci. Technol. 19 (2006) 998. J.C. Grivel et al., IOP J. Phys.: Conf. Ser. 43 (2006) 123. M. Alessandrini et al., Supercond. Sci. Technol. 19 (2006) 179. C.Q. Jin, S.C. Zhu, F.Y. Li, R.C. Yu, J. Mater. Res. 17 (2002) 525. A. Serquis et al., Appl. Phys. Lett. 82 (2003) 2847.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Properties of hot pressed MgB2/Ti tapes P. Kovácˇ a,*, I. Hušek a, T. Melíšek a, J. Fedor a, V. Cambel a, A. Morawski b, A. Kario b a b
Institute of Electrical Engineering, Slovak Academy of Sciences, Dúbravska cesta 9, 841 04 Bratislava, Slovakia Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warszawa, Poland
a r t i c l e
i n f o
Article history: Received 4 November 2008 Received in revised form 3 March 2009 Accepted 6 March 2009 Available online 18 March 2009 PACS: 74.25.Sv 74.25.Fy 74.70.Ad
a b s t r a c t Hot axial and hot isostatic pressing was applied for single-core MgB2/Ti tapes. Differences in transport current density, n-exponents and critical current anisotropy are discussed and related to the grain connectivity influenced by pressing. The magnetic Hall probe scanning measurements allowed observing the isolated regions for axially hot pressed sample attributed to the longitudinally oriented cracks introduced by pressing. The highest current densities were measured for the tape subjected to hot isostatic pressing due to improved connectivity. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: MgB2 Hot pressing Critical currents n-exponents Grain connectivity
1. Introduction MgB2 wires prepared by in situ technique using Mg + B powders mixture have usually low core density due to the volume shrinking caused by Mg + B? MgB2 conversion [1]. Recent review of Eisterer is well presenting critical currents, connectivity, current percolation and parameters influencing Jc(l0H) performance of MgB2 [2]. The method of mechanical alloying (MA) is a variant of the in situ technique. Mg and B are intensively milled in a planetary ball mill where high energy milling leads to a partial reaction to MgB2 [3–5]. This fine-particles precursor powder is highly reactive and can be processed further at relatively low temperatures [4,5]. High density MgB2 with very high current densities (104 A cm2 at 1416 T) has been presented recently for carbon doped MA powder [6,7]. High reactivity of MA powder causes the creation of Fe2B layer at the core/Fe interface even at low temperatures, which influences the phase purity (boron deficiency) and also the thermal and electrical properties of composite wire [8,9]. Recently, titanium as a sheath material has been used and no reaction or diffusion up to 900 °C was observed [10]. Effect of hot isostatic pressing has been applied for bulk samples [11] and also for wires [12]. * Corresponding author. Tel.: +421 2 5477 5853; fax: +421 2 5477 5816. E-mail address: [email protected] (P. Kovácˇ). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.03.007
The Jc improvement for ex situ MgB2 wire was attributed to the large amount of crystalline defects [12]. The aim of this contribution is to show how the applied hot axial and isostatic pressure can influence the transport current densities of MgB2 tapes made from not doped mechanically alloyed powder in chemically inert Ti sheath. 2. Experimental MA/Ti composite was prepared by two-axial rolling deformation into mono-filamentary tape of 0.35 mm in thickness and 3.3 mm in width [8]. Hot pressed sample (HP) at 80 MPa and hot isostatically pressed one (HIP) by 1 GPa were done at temperature 650 °C. Hot pressing was performed between two stainless steel anvils covered by BN powder (to protect the sample gliding) in gaseous axial press and pressing direction was applied perpendicularly to flat side of the tape. Hot isostatic pressing was done in the set composed of three stage compressor and high pressure chamber 30 mm in diameter filled by pure Ar [13]. The reference not pressed sample (N) was annealed at 650 °C in argon pressure slightly higher than atmosphere. Vickers microhardness measurements (HV 0.05–50 g) were performed in the cores cross-section of studied tapes to see the effect of densification. Critical currents (Ic at 1 lV cm1) of tapes (N, HP and HIP) were measured in paral-
714
P. Kovácˇ et al. / Physica C 469 (2009) 713–716
lel and perpendicular external magnetic field orientation to the tape side at liquid helium temperature. We have explored also the local magnetic field close to the transverse section of the above-described HP and HIP samples using high-resolution (5 lm) scanning Hall probe microscope (SHPM) [14,15]. Perpendicular magnetic field was applied to the transverse section of the sample. All scans consisted of 200 60 points with the steps of 5 lm in both x and y directions, which gave the overall scanned area 1 mm 0.3 mm. The distance of the Hall probe from the sample surface was 25 lm ± 5 lm. Both samples were placed on the same SHPM sample holder and cooled down to 20 K in zero field. Then, external magnetic field of B = +150 mT was applied. Magnetic field distribution in close proximity of the sample surface was imaged. 3. Results and discussion Fig. 1a shows the magnetic field dependence of critical current density for N, HP and HIP tape. It is apparent that Jc(l0H) is influenced by the applied pressing. The lowered Jc is measured for HP tape (104 A cm2 at 8.0 T), which correlates with the lowered core density expressed by the micro hardness data in Table 1 (HV
0.05 = 290). The highest Jc (104 A cm2 at 10.8 T) was measured for HIP sample having the highest core density (see Table 1) and consequently improved grain connectivity. The current density at 10 T is increased from 10,000 to 16,700 A cm2 by hot isostatic pressing at 1 GPa (improvement by 67%). HP and HIP tapes show also slightly less steep Jc decrease with field than N (see dotted lines in Fig. 1a). Fig. 1b presents the plot of n(l0H) values evaluated from IV curves, which correlates with Jc(l0H) data. As visible, all three n(l0H) characteristics follow well a linear dependences for semi logarithmic plot (exponential decrease) in the selected field range but n-exponents decrease less rapidly with field than Jc. While the Jc drop by one order of magnitude is observed for the field increase by 4.5 T (see Fig. 1a), 10 times decreased n occurs in the field range of 8.5 T. Apparently lowered n-exponents for HP sample can be attributed to globally worsened grain connectivity (see the core microhardness in Table 1) and consequent current redistribution inside the MgB2 core. Fig. 1c compares the anisotropy factor (ka = Ic-par/Ic-perp) of all three tapes annealed at the same temperature 650 °C, the insert shows the critical currents measured in parallel and perpendicular field for HP tape. An exponential increase of ka with field was measured for each sample, but their absolute values are different. Crit-
Fig. 1. Transport current densities measured at 4.2 K in parallel external field (a), corresponding n-exponents (b) and anisotropy factor ka = Ic-par/Ic-perp (c) as a function of field magnitude for HP, HIP compared with the reference sample N.
P. Kovácˇ et al. / Physica C 469 (2009) 713–716
715
Table 1 MgB2 core aspect ratio (width/thickness) and Vickers microhardness HV 0.05 (local positions 13 in Fig. 4 and averaged values) obtained for samples N, HP and HIP annealed at 650 °C. Sample
Pressure (MPa)
MgB2 core width/thickness
HV 0.05 pos. 1
HV 0.05 pos. 2
HV 0.05 pos. 3
Averaged HV 0.05
N HP HIP
No 80 1000
11 17.5 12
705 270 1030
703 290 1300
790 305 1500
730 290 1300
ical current anisotropy in MgB2 is generally explained by some caxis grain alignment (texture) [16]. The lowest anisotropy
Fig. 2. Magnetic field distribution obtained by scanning Hall probe microscope over the MgB2 core of HP sample at 20 K after zero field cooling in a field of +150 mT.
(ka = 10 at 8.4 T) was measured for not pressed tape N. Slightly increased anisotropy of HIP tape indicates only small effect of isostatic pressing on the texture (core aspect ratio was increased from 11 to 12, Table 1). The highest ka values (10 at 6.3 T) as well as higher in field increase was measured for HP tape. Core aspect ratio of HP tape is increased from 11 to 17.5 (see Table 1). It is expected that apparent core widening has influenced also the c-axis grain alignment of MgB2. Increased current anisotropy by texture was also observed for Ti sheathed MgB2 tape subjected to cold rolling, which was confirmed by X-ray analysis (by texture parameters (Fa, Fc) and by (0 0 2) pole figure) [17]. Figs. 2 and 3, including the 3D plots and the contour plots, present the magnetic field distribution obtained by SHPM over the MgB2 cores of HP and HIP tapes. The scanned sample area 1 mm 0.3 mm is marked by dotted line in Fig. 4. For HP sample the currents at the field of 150 mT (Fig. 2) almost fully compensate for the external field only at coordinates x = 150 lm, y = 800 lm (local minimum), and its large part is penetrated by the external magnetic field. Induced supercurrents are probably limited by cracks located along the short side of the sample (parallel with the x axis) they divide the sample into isolated parts in which the induced currents are strongly limited. To model roughly, how is the magnetic field distribution, generated by the external field and by the induced current, we assume Bean model for hard superconductors. The Maxwell equation rot H = Jc can be rewritten for the current components Jx and Jy in x and y directions, respectively, as follows:
Jx ¼
1 dBz
l0 d y
;
Jy ¼
1 dBz
l0 d x
ð1Þ
The equations can be approximated by the relations
Jx
Fig. 3. Magnetic field distribution obtained by scanning Hall probe microscope over the MgB2 core of HIP sample at 20 K after zero field cooling in a field of 150 mT.
1 DB z
l 0 Dy
;
Jy
1 DBz
l 0 Dx
ð2Þ
where l0 is the permeability of vacuum, DBz is the local change of the magnetic field on a distance Dx or Dy. These field changes define the local value of the critical current density. Applying the relations to the field close to the local minima (x = 150 lm, y = 800 lm) mentioned above (Fig. 2, contour plot), one achieves roughly |Jy| 1.5 105 A cm2, so the critical current density is rather high at this locality. The high-quality superconductor is located also at other local minima, and it corresponds to parts of the sample having different core density (see Table 1) that are divided probably by the cracks. The situation is quite different in the case of the HIP sample. In this case the superconducting currents almost fully compensate for the external field of 150 mT in the central part of the sample (see
Fig. 4. Partial cross-section of HIP tape with remarked area of scanned magnetic field (by solid line) and the places of local HV 0.05 measurements (1–3), corresponding hardness are given by Table 1.
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P. Kovácˇ et al. / Physica C 469 (2009) 713–716
of cracks inclined by 45° corresponding to gliding planes, see Fig. 5b. These cracks are dividing the MgB2 core longitudinally and consequently lowered Jc (Fig. 1a), decreased n-exponents (Fig. 1b) and not uniform current flow are obtained for HP. The size of core elements divided by cracks (see Fig. 5b) correlate with the scanned field minima or maxima placed close to the core centre (Fig. 2). 4. Conclusion
Fig. 5. Outer (a) and central part (b) of HP tape showing different core uniformity and local region with cracks inclined by 45°.
Fig. 3). The distribution of the magnetic field observed is evidently generated by rather homogeneous currents that flow around the whole sample, which is not divided into isolated parts. This feature differs much from the HP sample shown in Fig. 2. The critical current density can be again very roughly estimated and it reads Jy 3 105 A cm2. Fig. 4 shows the partial transversal sections of HIP tape in which the scanned area and positions of local microhardness measurements (1–3) are marked. Table 1 presents the core aspect ratio (width/thickness), local and averaged microhardness values for all pressing modes. While HIP increases the core aspect ratio only slightly, HP does it much more due to an apparent tape widening during hot axial pressing. HV 0.05 between 700 and 800 was measured for as rolled tape annealed at ambient argon pressure – tape N. Apparent difference in averaged microhardness (290–1300) is attributed to the applied pressing mode. HP and HIP tapes have similar hardness gradients: the smallest at the core edge (position 1) and the highest in the core centre (position 3), but one can see that considerably increased density for HIP sample (doubled HV 0.05 in comparison to N) as a consequence of high isostatic pressure by 1 GPa. Core density has a direct effect to grain connectivity and consequently improved critical current density (see Fig. 1a) and well uniformity (see Figs. 2 and 3) are measured. While low core density and no cracks structure (HV 0.05 270) was measured at the core edges of HP tape (Fig. 5a), the central part is more dense (HV 0.05 305) but full
Hot axial and hot isostatic pressing were applied for single-core MgB2/Ti tapes made of not doped mechanically alloyed powder. It was shown that current densities and n-exponents are influenced by the applied pressing mode. Hot isostatic pressing by 1 GPa has increased Jc (10 T) by 67% due to effectively improved grain connectivity. Hot axial pressing with much lower pressure (80 MPa) caused an apparent MgB2 core widening and longitudinal cracks generation resulting in lowered Jc and n-exponents. On the other side, hot axial pressing is increasing Ic anisotropy, which can be attributed to improved texture. Further Jc improvement can be reached by a proper combination of hot isostatic pressing with optimized carbon substitution (e.g. SiC addition). Acknowledgements This work was partially supported by the Slovak Scientific Agency APVV-0398-07 and VEGA 2/0037/09. Authors would like to thank to W. Haessler and M. Herrmann from IFW Dresden for mechanically alloyed powder. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
P. Kovácˇ et al., Supercond. Sci. Technol. 20 (2007) 105. M. Eisterer, Supercond. Sci. Technol. 20 (2007) R47. O. Perner et al., Supercond. Sci. Technol. 17 (2004) 1148. W. Häßler et al., Supercond. Sci. Technol. 16 (2003) 281. C. Fischer et al., Appl. Phys. Lett. 83 (2003) 1. M. Herrmann et al., Appl. Phys. Lett. 91 (2007) 082507. W. Häßler et al., Supercond. Sci. Technol. 21 (2008) 062001. P. Kovácˇ et al., Supercond. Sci. Technol. 19 (2006) 998. J.C. Grivel et al., IOP J. Phys.: Conf. Ser. 43 (2006) 123. M. Alessandrini et al., Supercond. Sci. Technol. 19 (2006) 179. C.Q. Jin, S.C. Zhu, F.Y. Li, R.C. Yu, J. Mater. Res. 17 (2002) 525. A. Serquis et al., Appl. Phys. Lett. 82 (2003) 2847.